![](media/image1.jpeg){width="7.79375in" height="10.75in"}**Solar
Dynamics**

**Observatory**

***"...to understand the nature and source***

***of the solar variations that affect***

***life and society."***

**Report of the Science Definition Team**

**Solar Dynamics Observatory Science Definition Team**

+-----------------------+---------------------+------------------------+
| David Hathaway        | John W. Harvey      | K. D. Leka             |
|                       |                     |                        |
| Chairman              | National Solar      | Colorado Research      |
|                       | Observatory         | Division               |
| Code SD50             |                     |                        |
|                       | P.O. Box 26732      | Northwest Research     |
| NASA/MSFC             |                     | Assoc.                 |
|                       | Tucson, AZ 85726    |                        |
| Huntsville, AL 35812  |                     | 3380 Mitchell Lane     |
|                       |                     |                        |
|                       |                     | Boulder, CO 80301      |
+-----------------------+---------------------+------------------------+
| Spiro Antiochos       | Donald M. Hassler   | David Rust             |
|                       |                     |                        |
| Code 7675             | Southwest Research  | Applied Physics        |
|                       | Institute           | Laboratory             |
| Naval Research        |                     |                        |
| Laboratory            | 1050 Walnut St.,    | Johns Hopkins          |
|                       | Suite 426           | University             |
| Washington, DC 20375  |                     |                        |
|                       | Boulder, Colorado   | Laurel, MD 20723       |
|                       | 80302               |                        |
+-----------------------+---------------------+------------------------+
| Thomas Bogdan         | J. Todd Hoeksema    | Philip Scherrer        |
|                       |                     |                        |
| High Altitude         | Code S              | HEPL Annex B211        |
| Observatory           |                     |                        |
|                       | NASA/Headquarters   | Stanford University    |
| P. O. Box 3000        |                     |                        |
|                       | Washington, DC      | Stanford, CA 94305     |
| Boulder, CO 80307     | 20546               |                        |
+-----------------------+---------------------+------------------------+
| Joseph Davila         | Jeffrey Kuhn        | Rainer Schwenn         |
|                       |                     |                        |
| Code 682              | Institute for       | Max-Planck-Institut    |
|                       | Astronomy           | für Aeronomie          |
| NASA/GSFC             |                     |                        |
|                       | University of       | Max Planck Str. 2      |
| Greenbelt, MD 20771   | Hawaii              |                        |
|                       |                     | Katlenburg-Lindau      |
|                       | 2680 Woodlawn Drive |                        |
|                       |                     | D37191 GERMANY         |
|                       | Honolulu, HI 96822  |                        |
+-----------------------+---------------------+------------------------+
| Kenneth Dere          | Barry LaBonte       | Leonard Strachan       |
|                       |                     |                        |
| Code 4163             | Institute for       | Harvard-Smithsonian    |
|                       | Astronomy           |                        |
| Naval Research        |                     | Center for             |
| Laboratory            | University of       | Astrophysics           |
|                       | Hawaii              |                        |
| Washington, DC 20375  |                     | 60 Garden Street       |
|                       | 2680 Woodlawn Drive |                        |
|                       |                     | Cambridge, MA 02138    |
|                       | Honolulu, HI 96822  |                        |
+-----------------------+---------------------+------------------------+
| Bernhard Fleck        | Judith Lean         | Alan Title             |
|                       |                     |                        |
| ESA Space Science     | Code 7673L          | Lockheed Martin Corp.  |
| Dept.                 |                     |                        |
|                       | Naval Research      | 3251 Hanover Street    |
| c/o NASA/GSFC         | Laboratory          |                        |
|                       |                     | Palo Alto, CA 94304    |
| Code 682.3            | Washington, DC      |                        |
|                       | 20375               |                        |
| Greenbelt, MD 20771   |                     |                        |
+-----------------------+---------------------+------------------------+
| Richard Harrison      | John Leibacher      | Roger Ulrich           |
|                       |                     |                        |
| CCLRC                 | National Solar      | Department of          |
|                       | Observatory         | Astronomy              |
| Chilton, Didcot       |                     |                        |
|                       | P.O. Box 26732      | UCLA, 9831 MSB         |
| Oxfordshire OX11 0QX  |                     |                        |
|                       | Tucson, AZ 85726    | Los Angeles, CA 90024  |
| UNITED KINGDOM        |                     |                        |
+-----------------------+---------------------+------------------------+
|                       | Barbara Thompson    |                        |
|                       |                     |                        |
|                       | Project Scientist   |                        |
|                       |                     |                        |
|                       | Code 682            |                        |
|                       |                     |                        |
|                       | NASA/GSFC           |                        |
|                       |                     |                        |
|                       | Greenbelt, MD 20771 |                        |
+-----------------------+---------------------+------------------------+

Table of Contents

**1** **Executive Summary** 1

**2** **Overview** [3](#overview)

**3** **Current Scientific Understanding and Outstanding Questions**
[8](#current-scientific-understanding-and-outstanding-questions)

3.1 Solar Influences on Global Change and Space Weather
[8](#solar-influences-on-global-change-and-space-weather)

3.1.1 Irradiance Variations [8](#irradiance-variations)

3.1.2 Energetic Particles from Flares and CMEs
[13](#flares-and-cmes-as-the-source-of-energetic-particles)

3.1.3 Coronal Structure and Solar Wind Variations
[17](#coronal-structure-and-solar-wind-variations)

3.2 Mechanisms of Solar Variability
[20](#mechanisms-of-solar-variability)

3.2.1 The Solar Cycle [21](#the-solar-cycle)

3.2.2 Active Region Evolution [24](#active-region-evolution)

3.2.3 Small-Scale Magnetic Structure
[27](#small-scale-magnetic-structure)

**4** **Required Observations** [31](#required-observations)

4.1 Helioseismic Images [31](#helioseismic-images)

4.2 Longitudinal Magnetograms [32](#longitudinal-magnetograms)

4.3 Atmospheric Images [33](#atmospheric-images)

4.4 EUV Spectral Irradiance [34](#euv-spectral-irradiance)

4.5 Photometric Images 35

4.6 Vector Magnetograms 36

4.7 UV/EUV Spectra [37](#uveuv-spectra)

4.8 Coronagraphic Images [38](#coronagraphic-images)

4.9 Total Irradiance [39](#total-irradiance)

4.10 Coronal Spectroscopy [40](#coronal-spectroscopy)

4.11 Heliometry [41](#heliometry)

**5** **Potential Instruments and Allocation of Resources**
[41](#potential-instruments-and-allocation-of-resources)

**6** **Mission Concept** [42](#mission-concept)

6.1 Orbit Selection [43](#orbit-selection)

6.2 Attitude Control System [44](#attitude-control-system)

6.3 Data and Communication System [45](#data-and-communication-system)

6.4 Spacecraft Power [46](#spacecraft-power)

6.5 Instrument Module [46](#instrument-module)

6.6 Ground System [46](#ground-system)

6.7 Mission and Science Operations [47](#mission-and-science-operations)

**7** **Concurrent Observations** [48](#concurrent-observations)

7.1 STEREO [48](#stereo)

7.2 Solar-B 49

7.3 Solar Probe 49

7.4 SORCE [50](#sorce)

7.5 GOES/NPOESS [51](#goesnpoess)

7.6 SOLIS [52](#solis)

7.7 ATST [52](#atst)

7.8 Solar Sentinels [53](#solar-sentinels)

7.9 Solar Orbiter [53](#solar-orbiter)

7.10 FASR [54](#fasr)

**8** **Acknowledgements** 54

# **Executive Summary**

The Solar Dynamics Observatory (SDO) is a cornerstone mission within the
Living With a Star (LWS) program. SDO's mission is to understand the
nature and source of the solar variability that affects life and
society. As such, its principal functions are two-fold. First, it must
make accurate measurements of those solar parameters that are necessary
to provide a deeper physical understanding of the mechanisms that
underlie the Sun's variability on timescales ranging from seconds to
centuries. Second, through remote sensing, it must monitor and record
those aspects of the Sun's variable radiative, particulate, and magnetic
plasma outputs that have the greatest impact on the terrestrial
environment and the surrounding heliosphere.

Our Sun is an active star. This activity impacts planet Earth and human
society in numerous ways. Terrestrial climate, ozone concentrations in
the stratosphere, and atmospheric drag on satellites all respond to
variations in the Sun's radiative output. Astronauts, airline
passengers, and satellite electronics are all imperiled by the energetic
particles produced in solar flares and coronal mass ejections (CMEs).
Electrical power to our homes and businesses, communications, and
navigation systems can all be interrupted by geomagnetic storms driven
by blasts in the solar wind. SDO will study the mechanisms of solar
variability -- through a broad spectrum of temporal, spatial, and
energetic scales -- to provide the tools and scientific understanding
that will enable us to improve the quality of forecasts of solar
activity. SDO will also provide the measurements that are critical as
input to studies of the geospace environment at their point of origin in
the Sun-Earth system in order to quantify the Sun's influence on global
change and improve our characterizations and forecasts of space weather.

In this report we discuss several key science questions that derive from
our present incomplete knowledge of the physical underpinnings of the
Sun's variability. They are selected for the promise they hold in
catalyzing a significant advance in modeling, quantifying, and perhaps
eventually predicting solar variability over the relevant timescales.
Variable solar outputs necessarily impact parallel efforts aimed at
understanding, modeling, and predicting the behavior of the Sun-Earth
system. Accordingly, we also speak to the scientific issues and problems
that arise from inadequate knowledge of solar radiative, particulate,
and magnetic plasma outputs.

Together, the solar and terrestrial science questions and ongoing
research activities point to specific observables that must be supplied
by SDO for the successful operation of the LWS science program. This
document identifies a set of required observables that best addresses
the dual objectives of SDO. In so doing, it provides the basic arguments
and the supporting evidence that leads to the identification of the
essential observations. The nature of the required measurements in turn
drives the choice of a geosynchronous (GEO) orbit for the spacecraft. A
potential suite of instruments is described which is in principle
capable of acquiring the required observables, while at the same time
satisfying the logistical constraints imposed on the SDO.

The overarching science questions to be addressed by the SDO are:

-   What mechanisms drive the quasi-periodic 11-year cycle of solar
    activity?

-   How is active region magnetic flux synthesized, concentrated, and
    dispersed across the solar surface?

-   How does magnetic reconnection on small scales reorganize the
    large-scale field topology and current systems? How significant is
    it in heating the corona and accelerating the solar wind?

-   Where do the observed variations in the Sun's total and spectral
    irradiance arise, and how do they relate to the magnetic activity
    cycles?

-   What magnetic field configurations lead to the CMEs, filament
    eruptions, and flares that produce energetic particles and
    radiation?

-   Can the structure and dynamics of the solar wind near Earth be
    determined from the magnetic field configuration and atmospheric
    structure near the solar surface?

-   When will activity occur, and is it possible to make accurate and
    reliable forecasts of space weather and climate?

The answers to these pressing science questions are to be found in
direct observations of the relevant solar activity and the
interpretation of these data. To this end, the complement of SDO
instruments should supply the following basic data types (ordered from
solar interior outward, not by priority).

-   Full-disk dopplergrams of appropriate spatial and temporal
    resolution, duration and continuity to permit accurate
    helioseismological inferences of conditions in the solar interior.

-   Full-disk magnetograms capable of characterizing the surface
    magnetic field and its evolution, and monitoring the emergence and
    processing of magnetic flux.

-   Full-disk precise photometric images to explore the temporal and
    spatial variability of the solar irradiance and determine couplings
    with the magnetic structures.

-   Full-disk filtergrams recorded simultaneously in a variety of
    visible and EUV bandpasses to assess the dynamics and energetics of
    the solar atmosphere on global and active region scales.

-   Sun-as-a-star EUV spectral irradiance measurements to monitor and
    record temporal variations of radiative outputs crucial for gauging
    ionospheric, mesospheric and thermospheric responses to solar
    forcing.

-   Restricted field of view UV/EUV slit spectra to make precise
    diagnoses of plasma dynamics and energetics.

-   White-light polarization brightness images of the solar corona to
    record and monitor coronal evolution and re-structuring important
    for generating geoeffective interplanetary disturbances.

To provide a context and a guide for proposals submitted in response to
the forthcoming AO associated with SDO, we have devised a potential
suite of generic instruments. The instruments listed below satisfy the
data requirements stated above while remaining within the financial and
logistical limitations placed upon the SDO mission.

-   Helioseismic/Magnetic Imager

-   Atmospheric Imaging Array

-   EUV Spectral Irradiance Monitor

-   Coronagraph

-   Photometric Mapper

-   UV/EUV Spectrometer

-   Vector Magnetograph

The first three instruments are of highest priority. SDO must include
instruments such as these to fulfill its mission. All three provide data
with proven value that are not likely to be supplied through other
programs. The final four instruments are of high priority. The data they
obtain are needed for the SDO mission but they may be supplied in some
other, albeit compromised, form by other programs, may be more
speculative in nature, or may tax the SDO resources. A Total Solar
Irradiance Monitor is also considered to be of highest priority, but two
TSI Monitors are expected to fly on other platforms concurrently with
SDO.

The remainder of this document amplifies on the considerations that went
into this selection of questions, critical data, and instrument
complement. It provides the traceability from the scientific questions
to the instrument requirements and mission design.

# **Overview**

The primary goal of SDO is to understand the nature and source of the
solar variations that affect life and society. This broad goal leads to
two objectives. One objective is to understand the mechanisms of solar
variability as characterized by three processes that operate on three
different timescales -- the solar cycle (months to centuries), active
region evolution (hours to months), and small-scale magnetic element
interactions (seconds to hours). The second objective is to understand
the solar influences on global change and space weather as characterized
by three different sources -- irradiance variations, energetic particles
from flares and CMEs, and plasma disturbances from solar wind
structures.

With the exception of the slow evolutionary changes in solar structure
over the last 4.5 billion years, all solar variability is magnetic in
origin. The solar cycle is a magnetic cycle in which the Sun's magnetic
poles reverse with a periodicity of approximately 11 years and intense
magnetic fields erupt through the surface in sunspots whose numbers wax
and wane with the cycle. Solar flares and CMEs occur when magnetic
fields are stressed beyond their limits. The very structure of the
corona and the solar wind is determined by the structure of the magnetic
field. The heating of the Sun's corona and the acceleration of the solar
wind are thought to be due to interactions between small-scale magnetic
elements. SDO will help us to understand the mechanisms of solar
variability by observing how the magnetic field is generated and
structured and how this stored magnetic energy is released into the
heliosphere and geospace. Our current scientific understanding leads to
a series of outstanding questions that must be addressed by SDO. These
questions lead to observations, potential instruments, and a mission
design.

![](media/image2.png){width="3.0in" height="3.0in"}

**Figure 2.1.** Recent solar cycle variability. Sunspot number and solar
10.7 cm radio flux (top two panels) are well-correlated indicators of
solar variability. Solar flares (third panel) and total solar irradiance
(fourth panel) generally follow the solar cycle. Geomagnetic variability
(bottom panel) has a component in phase with the cycle but also shows
considerable activity near solar minimum attributed to the effects of
high-speed solar wind streams.

**What mechanisms drive the quasi-periodic 11-year cycle of solar
activity?**

The solar cycle controls the long-term behavior of solar activity and
the resultant modulation of the Sun's electromagnetic, particulate and
magnetic plasma emissions that affect the Earth. Solar magnetic fields
with their associated forces and electric currents are recognized as
being responsible for the Sun\'s activity, but the underlying processes
which create and then dissipate these fields in an 11-year cycle are
poorly understood. Although helioseismology has revealed flows and
thermal structures related to the magnetic variability, present
theoretical models based on these observations can only broadly
reproduce the observed magnetic evolution and are far from having
predictive capability. Historical records suggest that the strength of
the cyclic magnetic variations may have been different from what is
observed today and that there may have been associated terrestrial
climate changes. Furthermore, sun-like stars are observed to have a
wider range of activity than is seen in the Sun, suggesting that current
solar behavior could be misleadingly steady.

SDO will examine the processes that control the solar cycle. The SDO
Magnetographs will measure the structure and evolution of the magnetic
field itself. The Helioseismograph will measure the relevant fluid flows
at levels within the Sun from the surface to the deep interior with
unprecedented resolution and coverage. By extending the volume of the
solar interior accessible to helioseismic probing and increasing its
sensitivity in the crucial regions, SDO will provide data critical for
understanding the 11-year activity cycle. The Photometric Mapper will
measure temperature and radiance variations on the solar surface that
are associated with the deep-seated drivers of the solar cycle while the
EUV Spectral Irradiance Monitor measures the dramatic variations in the
EUV.

**How is active region magnetic flux synthesized, concentrated, and then
dispersed across the solar surface?**

The evolution of active regions controls the behavior of solar activity
on timescales from hours to months. Sunspots cause decreases of a few
tenths percent in total in solar irradiance. Energetic particles are
released through magnetic reconnection associated with the evolution of
active regions. Although simple isolated regions rarely produce flares
or CMEs, complex sunspot groups with complicated and stressed magnetic
field elements frequently do produce eruptive events. Bright active
regions -- evident as faculae in the photosphere, plage in the
chromosphere and large loop structures in the corona -- alter
electromagnetic radiation at all wavelengths. The evolution of active
regions depends upon the structure of the emerging magnetic flux, the
local flow patterns, and the magnetic connections in the solar
atmosphere.

SDO will have the capability to study active regions and their evolution
by obtaining measurements unavailable from other missions or
observatories. The Helioseismograph will measure the local flow patterns
using observations with unique spatial resolution and coverage. It will
have the ability to "see" active regions as they develop on the far side
of the Sun and will resolve structures just below the surface. It may
also be able to detect magnetic structures before they emerge at the
surface, a capability shared with the Photometric Mapper. The
Magnetographs and Atmospheric Imaging Array will provide critical
information on the complexity of the magnetic structures in active
regions. The resultant fluctuations recorded by the EUV Spectral
Irradiance Monitor will capture the disk-integrated effect of the
emergence, evolution and rotation of all active regions.

**How does magnetic reconnection of solar magnetic fields on small
spatial scales relate to coronal heating, solar wind acceleration and
the transformation of the large-scale field topology?**

The small-scale magnetic elements control solar activity on short
timescales. As these elements erupt through the photosphere they
interact with each other and with larger extant magnetic structures such
as those associated with active regions. The magneto-dynamic nature of
small-scale magnetic flux may be the basis for short-term solar
variability. They may provide the triggers for eruptive events and their
constant interactions may be a key source of coronal heating and solar
wind acceleration. They may also contribute to irradiance variations in
the form of enhanced network emission.

SDO will make critical observations of the small-scale magnetic
elements, their interactions, and the resulting transformation of the
large-scale field topology. The Magnetographs will have sufficient
spatial and temporal resolution and coverage to follow the evolution of
these elements. The Helioseismograph will determine the nature of the
photospheric and sub-photospheric flows that control their motions. The
Atmospheric Imaging Array will follow the magnetic connections within
the atmosphere by simultaneously providing images of coronal loops at a
series of different temperatures. The EUV Imaging Spectrometer will
determine the physical conditions associated with these features
including temperature and bulk velocity. The Coronagraph will provide
information on the large-scale field topology and the associated solar
wind.

**Where do the observed variations in the Sun's total and spectral
irradiance arise, and how do they relate to the magnetic activity
cycles?**

The Sun's electromagnetic radiation is the primary energy input to the
Earth. This radiation varies at all wavelengths, and on all timescales
observed thus far. Total solar irradiance varied by about 0.1% during
recent 11-year sunspot cycles. While these variations are thought to be
too small to have a dominant impact on climate, there is nevertheless
considerable evidence in both contemporary and paleo-climate records of
apparent solar-related variability. Variations in the solar UV are
larger and have a direct and significant effect on stratospheric ozone
concentrations. Still larger variations in the Sun's EUV radiation cause
dramatic fluctuations in the density of the Earth's outermost
atmospheric layers and the electron density in the ionosphere, affecting
the control and operation of earth-orbiting spacecraft, and
communication and navigation systems. Magnetic features -- sunspots,
active regions, network -- that alter the temperature and composition of
the solar atmosphere are primary sources of solar irradiance
variability.

SDO will make direct measurements of the solar irradiance, map the
sources of the irradiance variations, and provide observations of the
physical characteristics of these sources. The SDO EUV Spectral
Irradiance Monitor will provide the first continuous, high time
resolution measurements of the EUV irradiance variations that are
critically important for changes in the Earth's upper atmosphere and
ionosphere. The measurements will be made on a timescale of seconds
while the SDO mission provides coverage over the solar cycle. The
Photometric Mapper will provide unique imaging capability in bolometric
intensity and in visible and IR bandpasses that allow for the positive
identification of the solar sources of the total irradiance variations.
The Atmospheric Imaging Array will provide coincident images of the full
solar disk made in EUV radiation at selected emission temperatures so as
to identify the sources of the observed EUV variations. SDO will also
provide measurements to aid in our understanding of the sources of the
irradiance variations. The EUV Imaging Spectrometer will examine the
physical conditions of key features. Magnetograms will provide
information on the magnetic nature of the features and their
interactions while Helioseismic images provide information on related
sub-surface structures and flows.

**What magnetic field configurations lead to the CMEs, filament
eruptions, and flares that produce energetic particles?**

The Sun emits energetic particles during solar flares and CMEs. The
energetic particle flux in a large proton flare can be lethal to an
astronaut outside the protective envelope of Earth's magnetosphere.
Significant particle fluxes penetrate to aircraft altitudes where they
pose a health risk to passengers and crew. Showers of these energetic
particles degrade and destroy electronic components on commercial,
military, and research satellites. They also cause short-term depletions
of the ozone layer, especially in polar regions. The solar eruptions
that produce large fluxes of energetic particles are also magnetic in
nature, frequently resulting from the reorganization of magnetic fields
in the outer solar atmosphere.

SDO will provide continuous and improved observations of the solar
conditions that lead to eruptive events such as flares and CMEs.
Magnetographs will provide images of the underlying magnetic field. The
Atmospheric Imaging Array will reveal the resulting structures in the
chromosphere and corona at the spatial and temporal resolution of the
TRACE imager but with the full-disk coverage of the SOHO/EIT instrument.
The Coronagraph will detail the structure and evolution of the corona
and CMEs while the EUV Imaging Spectrometer examines the physical
conditions at key positions associated with the energetic particle
events.

**Can the structure and dynamics of the solar wind be determined from
the magnetic field configuration and atmospheric structure near the
solar surface?**

Disturbances in the solar wind rack the Earth's magnetosphere and drive
geomagnetic storms. These storms enhance and reorganize populations of
energetic particles in the Earth's magnetosphere, radiation belts, and
ionosphere. Storm-induced electric currents can flow through power
lines, overpower circuit breakers and transformers, and ultimately
disrupt power distribution. Sudden changes in the electron density and
structure of the ionosphere can hamper global positioning systems and
radio communications. During maxima of the solar cycle CMEs are the
primary contributors to these solar wind disturbances. Near minima the
disturbances are dominated by variations in the structure of the solar
wind itself. Solar wind structures also modulate the flux of galactic
cosmic rays, allowing a larger flux of cosmic rays at the Earth during
periods of lower solar activity. The resultant cosmogenic isotopes --
^14^C and ^10^Be -- when stored in tree rings and ice cores produce
unique archives of long-term solar variability that are crucial to
untangling solar influences on Earth's past climate. There is even
speculation that galactic cosmic rays may alter climate by creating
cloud condensation nuclei.

SDO will observe the structure and dynamics of both the corona and the
source region of the solar wind. The Coronagraph will observe both CMEs
and solar wind structures. The Magnetographs and Atmospheric Imaging
Array will reveal the related magnetic structures closer to the surface.
The EUV Imaging Spectrometer will examine the physical conditions that
accompany the acceleration of the solar wind, the heating of the corona,
and the initiation of CMEs. Coronal spectroscopy would be an important
tool for detailed measurements of velocity, composition, and
temperature.

**When will activity occur, and is it possible to make accurate and
reliable forecasts of space weather and climate?**

Within the LWS Program SDO must provide the observations that are
crucial for understanding and predicting solar variability on all
timescales. SDO must provide the data that serves as inputs to, or
boundary conditions for, the other systems within the LWS program. The
extent to which space weather and climate can be predicted will be
recognized only after we gain a better understanding of the sources and
nature of solar variability by addressing the preceding questions.

**SDO Science**. In the following section (Section 3) we outline our
current understanding of solar variability itself -- irradiance
variations, energetic particle emission by flares and CMEs, and solar
wind variations -- along with the mechanisms that drive this variability
-- the solar cycle, active region evolution, and small-scale magnetic
element interactions.

**SDO Instruments**. The outstanding scientific questions that arise can
be addressed by a set of observations as described in Section 4. A
potential set of instruments to acquire these observations on SDO
includes:

-   Helioseismic/Magnetic Imager

-   Atmospheric Imaging Array

-   EUV Spectral Irradiance Monitor

-   Coronagraph

-   Photometric Mapper

-   UV/EUV Imaging Spectrometer

-   Vector Magnetograph

A Total Solar Irradiance (TSI) Monitor should also be included if
redundant observations are not available concurrently with SDO. The
detailed instrument capabilities are traced back to the scientific
questions in the following sections. However, a brief description of the
instruments should provide some initial guidance.

The **Helioseismic/Magnetic Imager** would extend the capabilities of
the SOHO/MDI instrument by going to higher spatial and temporal
resolution with continuous full-disk coverage. The **Atmospheric Imaging
Array** would be similar to the SOHO/EIT and TRACE instruments but with
several independent telescopes providing simultaneous observations with
the spatial resolution of TRACE, an order of magnitude increase in
temporal resolution and the full-disk coverage of EIT. The **EUV
Spectral Irradiance Monitor** would provide long-term, continuous
measurements of the solar irradiance from 1 nm to 120 nm. The
**Coronagraph** would be similar to the SOHO/LASCO instrument but with
far better spatial and temporal resolution along with extended spatial
coverage from 1.1 RSun to 15 RSun. The **Photometric Mapper** would be
unique to SDO. It would provide photometric and bolometric images of the
solar radiance. The **UV/EUV Imaging Spectrometer** would be a high
resolution, high time cadence imaging spectrometer designed to
complement the Atmospheric Imaging Array and Magnetographs. The **Vector
Magnetograph** would provide both the strength and direction of the
magnetic flux across the full solar disk at 5-minute intervals.

**SDO Mission Concept**. The high resolution, rapid cadence, and
continuous coverage required for the observations lead to an SDO Mission
design that places the satellite into a geosynchronous orbit. This
allows for continuous contact at high data rate with a dedicated ground
station. A geosynchronous orbit can also be used to minimize the data
interruptions caused by the satellite passing through the Earth's and
the Moon's shadows. The **SDO Mission Concept** (Section 6) calls for a
launch in late 2006 or early 2007 on a Delta booster. The mission would
be for a nominal 5 years with an additional 5-year extended mission. The
SDO mission will help us make great strides toward understanding the
solar variability that affects life and society. The instruments will
provide the observations that lead to a more complete understanding of
the solar dynamics that drive variability in the Earth's environment.

# **Current Scientific Understanding and Outstanding Questions**

We have learned much about the Sun and solar variability in the last few
years. The scientific discoveries from missions such as SOHO and TRACE
make headlines in the popular press and are occasionally featured on the
nightly news. The very nature of scientific inquiry, however, teaches us
that the more we learn the more we find that we don't understand. We
have learned much about the solar influences on global change and space
weather, but we have also found that we do not fully understand all
sources of the irradiance variations and we can't reliably predict
energetic particle eruptions or solar wind variations. Likewise, we have
learned much about the structure and dynamics of the solar interior and
the evolution of active region magnetic fields, but we still don't
understand the solar dynamo and can't reliably predict the size of the
next solar cycle or the emergence of the next active region. The
understanding of the mechanisms of solar variability that we have gained
from previous missions, ground-based observations, and theoretical
studies leads us to additional questions that require new observations.
In the following subsections we describe our current scientific
understanding and list some of the outstanding questions that must be
addressed by SDO.

We have divided the science into two broad areas with three subdivisions
each. The solar influences on global change and space weather represent
one broad area. There are three significant types of solar influences:
photons (irradiance), energetic particles (associated with flares and
CMEs), and plasma (coronal structure and solar wind variability). The
mechanisms of solar variability represent the other broad area. While
these mechanisms are all magnetic in nature, they can be neatly
separated according to the timescales associated with each: long-term
(the solar cycle), mid-term (active region evolution), and short-term
(small-scale magnetic element interactions). SDO must address **all**
these areas to advance our understanding of the nature and source of the
solar variations that affect life and society.

## Solar Influences on Global Change and Space Weather

Understanding the solar influences on global change and space weather is
increasingly important as society becomes more reliant on technology.
Global warming and ozone depletion can be influenced by solar irradiance
variations. The health and safety of satellites and astronauts can be
influenced by energetic particles from solar flares and CMEs. Electrical
power distribution to our cities can be interrupted by geomagnetic
storms induced by variations in the solar wind. While our understanding
of these effects and their solar origins has improved greatly over the
past decade, many outstanding questions remain.

### Irradiance Variations

The radiation that the Sun provides to the Earth varies at all
wavelengths and on all timescales. While the Sun\'s fundamental energy
source is the nuclear conversion of hydrogen to helium in the core, the
immediate radiative energy source for the Earth's surface and lower
atmosphere is the solar photosphere. The chromosphere and corona produce
the UV, EUV and X-ray radiations that are the primary source of energy
for the Earth's upper atmosphere and ionosphere. With million-year
photon diffusion times in the core, the emergent luminosity at its outer
boundary should be effectively constant on any total solar irradiance
measurement timescale. Given the observed variability, it follows that
the mechanisms that deliver energy to the photosphere must be variable.

![](media/image3.png){width="2.9916666666666667in"
height="1.8131944444444446in"}

**Figure 3.1.** Solar spectral irradiance and its solar cycle
variability as functions of wavelength. The solar cycle variability at
wavelengths shorter than about 100 nm is typically greater than 100%.

The magnitude of the irradiance variability depends on the timescale and
the wavelength of the radiation (Fig. 3.1). Most variable is EUV
radiation at the shortest wavelengths. Emitted from outer layers of the
solar atmosphere, this radiation exhibits rapid short-term
order-of-magnitude fluctuations during solar flares and more gradual
variations of a few orders of magnitude during the solar cycle. Least
variable is the near infrared spectrum, emitted from the deepest layers
of the photosphere, where flare-related variability is negligible and
solar cycle changes are thought to be a few tenths of a percent, at
most.

![](media/image4.png){width="2.9923611111111112in" height="1.05625in"}

**Figure 3.2.** Total solar irradiance (W/m^2^) as a function of time.
Total solar irradiance varies by about 0.1% over a solar cycle and by
several times that on shorter (solar rotation) timescales.

The spectrally integrated 'total' irradiance, of relevance to climate on
Earth, varies by a few parts per million in association with p-mode
oscillations, a few tenths percent during solar rotation, about 0.1% in
recent solar cycles (Fig. 3.2), and possibly by a few tenths percent on
longer timescales. The UV radiation from 160 to 300 nm that produces
stratospheric ozone and modulates the middle atmosphere, varies by 1 to
20% during a solar rotation, about twice that during the solar cycle,
and possibly more on longer timescales. Although the radiative output at
ultraviolet wavelengths less than 400 nm constitutes only 8% of the
total radiative output, the stronger variability at these shorter
wavelengths makes a significant contribution (30%) to the total solar
irradiance variations. The EUV radiation from 1 to 120 nm that produces
the ionosphere and is the primary heat source for the thermosphere
varies by 50% to factors of two or more during solar rotations and the
solar cycle. Significant variations are expected during solar flares,
but are presently unknown.

####  Irradiance Variations -- Current Understanding

Magnetized features in the solar atmosphere are associated with solar
irradiance variations across the entire electromagnetic spectrum.
Magnetic fields in sunspots redirect the upward flow of energy from the
convection zone and affect the emergent radiation locally in the
photosphere. In bright magnetic regions, specifically active region
plage, faculae and dispersed network, irradiance is locally enhanced.
The interplay of locally "dark and bright" features causes variations in
photospheric radiation (wavelengths \> 200 nm) on timescales ranging
from days to a solar cycle. Bright, magnetized features largely control
the variability of chromospheric emission that includes some of the
strongest EUV emission lines. In the outer solar atmosphere, magnetic
fields in active regions expand to form coronal loop structures that at
times cover a significant fraction of the global solar atmosphere.
Density and temperature enhancements alter the emission in coronal
magnetic loops and their presence causes significant increases in X-ray
and extreme ultraviolet lines, especially during flares.

Fluctuations in total (spectrally integrated) irradiance are traceable
to both sunspot and facular sources on short (solar rotation)
timescales. The movement across the visible solar disk of a large
sunspot group can cause the total irradiance to decrease by a few tenths
of a percent. These short-term variations are superimposed on an overall
total solar irradiance increase from sunspot minimum to maximum.
Empirical models of sunspot darkening and facular brightening account
for recent solar irradiance cycles, but the models require a factor of
two more facular brightening than sunspot darkening during maximum. It
is not clear that all the variation can be found in the faculae. Models
for the flow of heat through the convection zone suggest that heat flow
blockage by sunspots should restructure the convection zone and produce
bright rings around active regions. Recent observations of bright rings
around active regions and changes in the solar radius add to the
controversy.

Ground-based measurements of the solar radius exist over the last 300
years, but these results are controversial and inconsistent. Historical
measurements show that the Sun\'s radius may even have been larger
during the Maunder Minimum - during extremely cold periods in Europe and
the Atlantic regions. The French CERGA radius measurements detected a
larger solar radius during solar minimum. Others have found the opposite
positive correlation between apparent radius changes and the solar
activity cycle. There are also hints of periodic solar radius variations
over timescales of 1,000 days to 80 years, but taken as a whole these
measurements generally are neither consistent nor conclusive. It is
apparent that the results to date have been severely limited by the
atmosphere.

Because the nuclear energy generation rate is effectively constant on
solar cycle time-scales the Sun must be storing a fraction of this
energy flux in thermal, gravitational, or magnetic forms in order for
its net luminosity to vary. Each of these mechanisms leads to distinct
perturbations in the equilibrium stellar structure and potentially
detectable changes in the solar diameter. It follows that a sensitive
determination of the solar radius fluctuations can help reveal the cause
of the solar cycle changes.

Quantitative understanding of the magnetic sources of EUV irradiance
variations is far less developed. Until recently, the only models of EUV
irradiance variability were simply parameterizations of the 10.7 cm
radio flux. In reality the database of reliable EUV irradiance
observations is simply too limited to realistically constrain even the
most simple of empirical models. This has lead to a new approach of
using differential emission measures (combined with atomic parameters of
the emitting species) of specific magnetic features (active regions,
coronal holes) and the quiet background Sun, to construct full-disk EUV
irradiance. Solar images (e.g., Ca K, SOHO/EIT, Yohkoh/ SXT) provide
estimates of the fractional volume covered by the respective features at
any time during the solar cycle. This approach depends on having
reliable emission measure distributions. Unfortunately, we have only a
small number of suitable, calibrated, high spectral and spatial
resolution observations covering the needed wide range of emission
temperatures. Nevertheless, the new approach to quantifying magnetic
sources of EUV irradiance variability from a more physical understanding
of the temperature, density and atomic parameters is promising. The
major challenge here is to obtain better data in the EUV lines of
coronal and transition region origin. Many of these lines arise in
structures whose areas are poorly estimated by traditional chromospheric
indices such as Ca K and Mg II.

####  Irradiance Variations -- Outstanding Questions

**Where do the observed variations in the Sun's total and spectral
irradiance arise, and how do they relate to the magnetic activity
cycles?** This is the overarching question for the solar irradiance
variations. It leads to a series of more focused questions in specific
areas.

**What are the sources of the total irradiance variations?** The
projected areas of sunspots and faculae are well correlated with the
total solar irradiance variations. However, alternative sources such as
luminosity changes due to radius variations associated with sunspots and
faculae can also give this correlation. Present photometry of spots and
faculae is not accurate enough to demonstrate that the irradiance
effects are equal to -- rather than merely proportional to -- the total
irradiance variations attributed to them. An accurate (\~10%) equality
would be an important test of the models. Equality of the year-to-year
photometric and radiometric contributions probably offers the cleanest
discriminator between the enhanced network, and other explanations of
the 11-year component of irradiance variation. Merely showing
proportionality leaves open the possibility that more global changes
also proportional to activity level might be playing a significant role.

\[Text highlighted in gray provides the traceability between the
scientific goals, the measurement characteristics, and the requirements
on potential instruments for SDO.\]

The **total solar irradiance** must continue to be monitored to address
these questions. Efforts toward identifying the sources of these
variations are of little use without knowledge of the variations
themselves.

**Photometric images**, photometrically accurate narrowband images and
broadband bolometric images, are needed to determine the actual sources
of the irradiance variations. These images must have the spatial
resolution to resolve sunspots and faculae and the accuracy and
precision to determine their contribution to the total irradiance.

**Magnetograms** with comparable spatial resolution are also needed in
order to discriminate between the magnetic and the non-magnetic
irradiance sources.

**Helioseismic images** will aid in measuring variations in the size of
the Sun that should be related to luminosity variations.

**Heliometry** -- precise measurements of the size and shape of the Sun
-- will also aid in measuring variations in the size of the Sun that
should be related to luminosity variations.

**How does the EUV spectral irradiance change on timescales of seconds
to decades?** There is a severe lack of knowledge of solar spectral
irradiance variability at wavelengths shorter than 120 nm, where the
database is characterized by intermittent observations made with poorly
calibrated instruments. Solar cycle EUV irradiance variability
amplitudes are uncertain by 50% or more, and variations associated with
flares are essentially unknown. Only for the total irradiance and for
the spectral irradiance from 120 to 400 nm do continuous,
well-calibrated databases exist. NASA's Office of Earth Science (OES)
has made these latter observations during the past decade at longer
wavelengths for understanding natural variations in climate and ozone.

The **EUV spectral irradiance** shortward of 120 nm must be monitored.
The observations should cover the neglected wavelength range from 1 nm
to 120 nm with enough spectral resolution to be useful to solar, upper
atmospheric and ionospheric studies. The temporal resolution should be
short enough to follow flares, and the temporal coverage should extend
through a solar cycle. These measurements should have sufficient
short-term precision and long-term stability to monitor the EUV
variations on timescales of flares to the solar cycle.

**What are the sources of the spectral irradiance variations?** Although
magnetic regions are understood to be the primary cause of EUV and X-ray
irradiance variations, the details of this association have yet to be
determined. This is because of the complete absence of simultaneous
well-calibrated full-disk images (that resolve the spatial features with
adequate spectral resolution) and independent, spectrally-compatible,
EUV irradiance observations (to specify the actual variations and to
verify the understanding developed from analysis of the images).

**Atmospheric images** like those obtained with SOHO/EIT and Yohkoh/SXT
will aid in the identification and quantification of the spectral
irradiance variations. Full-disk, narrowband EUV images at a series of
EUV wavelengths should be taken simultaneously with the EUV spectral
irradiance measurements. These images should have the spatial resolution
to identify the nature of the features and the temporal resolution to
follow the rapid variations in flares. The measurements must be made in
a sound radiometric way that eliminates spurious instrumental or
spacecraft effects. The spectral bands should be chosen such that
emission measures spanning a range of temperatures from 10^4^K to a few
10^6^K can be constructed for individual or classes of magnetic
elements.

**UV/EUV spectra** are needed to provide detailed information about the
density and temperature of the plasma in individual magnetic regions on
the Sun\'s disk. It is evident that a major contributor to irradiance
changes are the variations in UV/EUV emission lines associated with
regions of strong magnetic fields in the photosphere. Without sufficient
spectral and spatial resolution it is not possible to understand the
physical mechanisms responsible for the variations. High spectral
resolution would provide significantly more reliable emission measure
determinations, and the coordination with the full-disk images would
provide global context for these spatially resolved observations.

**Were solar irradiance variations larger in the historical past?**
Present specifications of solar total and spectral irradiance
variability for terrestrial research are based largely on
multi-component empirical models. Although these models can be quite
complex, they represent a very simplified view of the structure of the
solar atmosphere.

The ability to understand solar variations outside of the current era
requires the development of physical models based on observations of the
solar sources of the total and spectral irradiance variations at the
necessary spectral and spatial resolution.

### Flares and CMEs as the Source of Energetic Particles

Flares and CMEs are the primary sources of energetic particles from the
Sun. Solar flares and coronal mass ejections are the largest explosions
in the solar system, and are the root cause of much of the Earth's
geomagnetic and ionospheric disturbances. The energetic particles
produced in these explosions can be harmful to humans and electronic
instruments in space. Figure 3.3 shows LASCO images of the solar corona
before and after the flare of July 14, 2000. The energetic particles
produced in this event showered the LASCO detector and made observations
impossible for some time. The fundamental physics of these events
remains elusive and predictions must become more reliable. These
extremely complex phenomena require the explosive conversion of magnetic
energy into accelerated particles, bulk mass motions, light, and heat.
SDO must examine the evolving magnetic configurations associated with
these events and identify their precursors.

![](media/image5.jpeg){width="2.9965277777777777in"
height="1.8402777777777777in"}

**Figure 3.3**. LASCO images of the solar corona before (left) and just
after (right) the large flare of July 14, 2000 (the Bastille Day event).
Energetic particles from the flare eventually overwhelmed the detector
and made observations impossible for some time.

####  Flares and CMEs -- Current Understanding

Solar flares have long been recognized as a key component of solar
variability on short timescales. The advent of space-based observations
has also added CMEs and shocks in the solar wind as significant
components. Much has been learned about these phenomena and about how
they accelerate particles to cosmic ray energies. Our understanding and
predictive abilities have improved but not to the point where we can
reliably predict the occurrence and geoeffectiveness of these events.

Solar flares can be detected at virtually all wavelengths. The NOAA GOES
satellites include solar X-ray monitors that are used to detect and
classify solar flares. The occurrence of flares is well correlated with
the magnetic complexity of active regions. Delta spots, sunspots with
opposite magnetic polarities included within a common penumbra, often
produce flares. Detailed measurements of the magnetic field also reveal
stressed magnetic field configurations in flaring active regions. Figure
3.4 shows a flaring active region along with the observed magnetic field
and the stress-free field derived from the observed line-of-sight field
component. The magnetic field vectors are twisted or sheared in the
region where this flare occurred. Other indicators of stressed field
configurations include sigmoid, or S-shaped, coronal loops observed in
images obtained in the UV/EUV. While these magnetic structures indicate
the likelihood of a flare, they are not highly reliable and cannot
indicate when a flare will occur or how strong it will be. Detailed
determinations of the magnetic configurations in many flaring and
non-flaring active regions are needed for systematic studies of these
and other flare indicators.

![](media/image6.jpeg){width="3.0in" height="4.972222222222222in"}

**Figure 3.4**. Stressed magnetic field configuration in a flaring
active region (off-band Hα - top panel). The observed magnetic field
vectors (center panel) are twisted relative to the stress-free field
vectors (bottom panel) in the region of the flare.

Traditionally CMEs have been observed with white light coronagraphs
using polarizers and broadband filters. With the launch of SOHO,
considerable progress toward a detailed description of CMEs is being
made through a combination of observations with the EIT, LASCO and UVCS
instruments. SOHO observations with the LASCO coronagraphs show that
CMEs have a wide range of speeds from 100 km/s to greater than 1000
km/s. EIT observations show that a large CME can empty a substantial
fraction of the corona. We know typical masses (10^16^ g) and total
energies (10^32^ ergs) of CMEs, but not what causes the variations of
these parameters from one CME to another. We need to know how the
magnetic field energizes and accelerates CME material.

There is evidence that initially slower-speed CMEs that tend to have
higher accelerations are related to prominence eruptions while the
faster CMEs are associated with flare events but the correlation is not
exact. SMM observations show that the range of CME speeds does not
appear to be related to the amount of thermal input into the CME, so the
variation must be related to the nonthermal energy from the magnetic
field. UVCS observations indicate ionization states formed at
temperatures over a broad range (from 10^4^ to 10^7^ K) and heat input
comparable to the kinetic energy in the ejected material.

By their explosive nature, CMEs also reflect short-term changes in the
configuration of the coronal magnetic field. On short timescales,
magnetic reconnection of field lines is often seen just before and
sometimes after a CME has erupted, but whether reconnection is a cause
or an effect of the CME is still debated. There has been some success in
using the presence of sigmoid magnetic fields in the low corona as a
precursor for CME eruptions but it is not yet possible to predict which
sigmoid structures will erupt or when.

The geoeffectiveness of CMEs that strike the Earth is known to depend,
in part, on the shock strength and on the direction of the magnetic
field. Those CMEs that have southward directed interplanetary magnetic
fields are known to produce the largest geomagnetic storms. Studies of
the relationship between the direction of the magnetic field in CMEs and
their geoeffectiveness conclude that accurate storm forecasting requires
a means of predicting the magnetic field configuration in CMEs.

The process of magnetic reconnection is thought to be the principle
mechanism by which magnetic energy is converted into thermal energy in
flares and CMEs. A primary result of magnetic reconnection is the
creation of new field line connectivities. On large spatial scales, the
new plasma loops are often seen to connect pre-existing photospheric
flux elements with newly emerged photospheric flux elements. The rate at
which these new loops appear is on the order of 0.1% to 10% of the
Alfvén time, indicating that this occurs in the regime of fast
reconnection. The high electrical conductivity of the coronal plasma
implies that small spatial scales are often needed in order for magnetic
reconnection to occur at a reasonable rate. These spatial scales are
below the resolution limit of current instrumentation.

Besides providing clues about the storage and release of magnetic
energy, CMEs provide a major avenue (in addition to the fast and slow
speed wind) for coupling the Sun\'s atmosphere to the more extended
heliosphere. CMEs may be the fundamental means by which the Sun sheds
both magnetic flux and helicity (Fig. 3.5) over the 11-year solar cycle.
The rate of free energy built up over the solar cycle (in the form of
increasing helicity) may be about the same as the amount of free energy
removed convectively by CMEs during a solar cycle. While there is good
evidence for the expected flux and helicity transport in the past three
decades of solar wind data, quantitative inferences about magnetic
helicity from solar observations have been difficult to obtain.

![](media/image7.jpeg){width="2.9972222222222222in" height="2.075in"}

**Figure 3.5**. Helical structure in a CME observed with LASCO on June
2, 1998.

####  Flares and CMEs -- Out standing Questions

**What magnetic field configurations lead to the CMEs, filament
eruptions, and flares that produce energetic particles?** Our current
understanding of flares and CMEs suggests that the magnetic field
configuration and its evolution determine the development of these
explosive events. The photospheric magnetic fields often indicate the
existence of stressed magnetic field configurations. However, the lack
of significant changes in the photospheric fields after an eruption
suggests that the actual restructuring of the field occurs high in the
solar atmosphere where the magnetic forces dominate. Significant
restructuring of the corona is often observed before, during, and after
these events.

V**ector magnetic flux** measurements are required to determine the
configuration of the surface magnetic fields. While **longitudinal
magnetograms** provide some indication of the field\'s complexity,
vector magnetograms in the photosphere (and ideally the chromosphere
where the fields are closer to a force-free state), are required to
determine the presence and magnitude of the magnetic stresses associated
with these eruptions. Useful observations require adequate spatial
resolution (\~1 arcsec) to resolve the small-scale elements that may
trigger the eruptions, adequate temporal resolution to follow the
evolution (minutes) and the spatial and temporal coverage to capture all
events and to determine long-range interconnections between magnetic
regions.

**Atmospheric images** are required to determine the configuration of
the magnetic fields in the lower corona associated with flares and CMEs.
These images provide direct evidence of the occurrence of these events
and provide critical information on their spatial, temporal, and
physical characteristics.

**Coronagraphic images** are required to determine the configuration of
the magnetic fields in the outer corona associated with flares and CMEs.
These images also provide direct evidence of the occurrence of these
events and provide critical information on their spatial and temporal
characteristics.

**UV/EUV spectra** are required to determine the physical conditions at
the site of the eruption -- both before and after the events themselves.
These measurements may help to identify the triggering events for these
eruptions.

**Helioseismic images** are required to measure the surface and
sub-surface flows associated with the magnetic field pattern evolution.

**Is the magnetic helicity removed from the Sun by CMEs consistent with
the rate of helicity generation under the surface and with the rate of
helicity transport in the solar wind?** The Sun has a weak preference
for left-helical fields in the north and right-helical fields in the
south but the cause of this hemispheric pattern is not yet understood.
Measurements of the patterns and the net amount of helicity generation
would place important constraints on solar dynamo models. From a global
point of view, CMEs are an inevitable response to helicity generation,
but we do not know whether the helicity is primarily generated by the
dynamo, by convection, or by shearing of coronal fields. A better
understanding of helicity generation and removal is crucial to
understanding CMEs.

**Can the magnetic field direction and twist of CMEs be related to their
geoeffectiveness?** The direction of the interplanetary magnetic field
in the solar wind is known to be a good indicator of the
geoeffectiveness of the disturbance. To what extent can this field
component be determined from observations of CMEs close to the Sun?

V**ector magnetic flux** measurements are required to determine the
helicity of the surface magnetic fields.

**Atmospheric imaging** and **UV/EUV spectroscopy** can provide
information on the sign of the magnetic helicity, and the field geometry
for the twisted magnetic fields in CMEs.

**Where do CMEs form shock waves, and how are coronal shocks related to
the fluxes of energetic particles at the Earth?** Energetic particles
are accelerated in shocks associated with CMEs. The locations and
development of these shocks must be studied in more detail.

**EUV coronal spectroscopy** would provide important information on the
physical conditions inside of CMEs as they evolve during their transit
through the outer corona. Velocity distributions, line of sight
velocities, and the detection of high temperature spectral lines can be
used to determine shocks and current sheets within CME structures.

**What is the role of magnetic reconnection in the heating and
acceleration of CMEs?** Some models of CMEs give a prominent role to a
reconnection current sheet connecting the CME flux rope to a post-CME
flare arcade.

**UV/EUV spectra** can provide evidence of extremely hot structures at
the reconnection sites with observations of high temperature spectral
lines and measurements of the plasma temperature, density, and heating
rate. The presence of predicted MHD waves created at reconnection sites
can be detected by their effects on observed spectral line profiles for
comparison with models of the reconnection process.

### Coronal Structure and Solar Wind Variations

Solar wind variations also produce geomagnetic disturbances. This is
particularly evident late in each solar cycle when flares and CMEs are
less likely and the Sun's global field geometry is more stable. As the
Sun's rotation brings around the sources of high-speed streams we see
recurring geomagnetic disturbances at \~27-day intervals (Fig. 3.6).
Corotating Interaction Regions (CIRs) form within the heliosphere when
such streams interact with ambient slow solar wind. These interactions
eventually include the formation of shocks that accelerate particles
which contribute to the space weather environment.

![](media/image8.png){width="2.933333333333333in"
height="1.1340277777777779in"}

**Figure 3.6.** Geomagnetic fluctuations during the declining phase of
Cycle 22. Disturbances recurring at 27-day intervals are due to
high-speed streams in the solar wind.

####  Coronal Structure and Solar Wind Variations -- Current Understanding

In the high-temperature, low-density extended corona the plasma beta
(ratio of gas pressure to magnetic field energy density) is low enough
that motions of the coronal plasma are usually dominated by the magnetic
field configuration. We know that the high-speed wind originates in the
open magnetic field regions in coronal holes and that the slow-speed
wind emanates from above streamers.

Magnetic field extrapolation models, using photospheric magnetic fields
as input, can now fairly reliably reproduce the large-scale structure of
the inner corona (Fig. 3.7). These models have become more sophisticated
by including non-potential fields and MHD effects. Maps of regions where
the magnetic field is opened to the heliosphere correspond well to
coronal holes and the source of high-speed wind streams. Yet, the
physics involved in the heating of the corona and the acceleration of
the solar wind is still hotly debated.

While much has been learned in recent years about coronal heating and
solar wind variability, no unifying theory about the physical mechanisms
responsible has emerged. Theories of coronal heating and solar wind
acceleration generally fall into two broad categories: (1) heating and
acceleration by Alfvén or other MHD waves and (2) heating and
acceleration by the impulsive release of energy at sites of magnetic
reconnection. Observations from current solar missions (e.g. SOHO,
*Yohkoh*, and TRACE) have provided evidence for both types of
mechanisms.

![](media/image9.jpeg){width="2.8875in" height="5.7625in"}

**Figure 3.7.** Coronal structures derived from an MHD field
extrapolation model (top and middle panel, SAIC) for the August 11, 1999
eclipse (bottom panel). There are good agreements with many structures
in spite of the lack of complete (back side) magnetic field information.

While Alfvén waves with frequencies high enough to probe cyclotron
resonances (10 to 10,000 Hz) have not yet been observed directly in the
solar wind or corona, UVCS coronal hole observations have given rise to
renewed interest in the theoretical investigation of resonant wave
dissipation. Spectral line widths indicating large anisotropic ion
velocities have been observed. It is not known if the fluctuations
originate primarily at the coronal base or are generated continuously in
the acceleration region of the wind (via turbulent cascade or plasma
instabilities).

![](media/image10.jpeg){width="2.9972222222222222in"
height="2.8541666666666665in"}

**Figure 3.8.** Coronal structures from 1.0 to \> 5 R (August 11, 1999
LASCO C2 image and total eclipse photo from S. Koutchmy). Solar wind
speed can be measured using moving features in the outer corona but much
of the restructuring takes place much closer to the surface.

SOHO/MDI magnetogram observations suggest that a "magnetic carpet" of
twisted magnetic fields that reconfigure on timescales of minutes may be
responsible for heating the quiet corona. Thin current sheets are
generated between the twisted and braided field lines, and magnetic
reconnection occurs stochastically to relax the system to a lower-energy
state (with net particle heating). "Microflares" from the reconnection
sites on small loops seen in *Yohkoh*/SXT images and vector magnetograms
may provide enough energy to heat the corona at least locally in active
regions. Whether these local heating processes can heat the extended
corona above a few tenths of a solar radius is still an open question.

Recent progress has also been made on the slow speed wind and its
relation to coronal streamers. UVCS observations of high temperature
ions high in streamers suggest preferential heating of ions with high
mass-to-charge ratios. These results, while not conclusive, are
consistent with ion cyclotron resonance absorption that could lead to a
steady expansion of material into the slow speed wind. Other streamer
observations made with LASCO show moving "blobs" or density enhancements
that appear to emerge from the tips of streamers. This suggests that
some of the mass in the slow speed wind is released by intermittent
reconnections of the open and closed magnetic field lines in streamers.
This view is consistent with the results of earlier investigations that
link the origin of the slow speed wind to the streamer belt. However
coronal abundance measurements show that most of the slow speed wind
probably originates from the streamer edges, since these regions have
similar depletions of high First Ionization Potential (FIP) elements
that are found in the slow speed wind. More detailed measurements are
needed to establish, with confidence, what fraction of the slow speed
wind comes from streamer edges versus the closed-field regions of
streamers.

####  Coronal Structure and Solar Wind Variations -- Outstanding Questions

**Can the structure and dynamics of the solar wind be determined from
the magnetic field configuration and atmospheric structure near the
solar surface?** The temperatures and outflow speeds of the large polar
coronal holes at solar minimum have been shown to differ from those of
the coronal holes that appear primarily at low latitudes at solar
maximum. For the low-speed wind, the large equatorial streamer belt at
solar minimum seems to be hotter than the more sporadic streamers that
form at all latitudes at solar maximum. Precisely how these thermal
changes depend on the coronal field morphology is not well understood.
The solar corona as a whole is brighter in its EUV emission at solar
maximum than it is at solar minimum. Whether this arises because of an
overall density increase, a change in the radial and latitudinal
distribution of coronal heating, or is related to photospheric
brightness changes, is not yet known.

**Magnetograms** are required to determine the configuration of the
surface magnetic fields associated with the coronal and solar wind
structures. Magnetic measurements at the boundaries between streamers
and coronal holes will elucidate the role of open and closed fields in
producing the slow speed wind. Longitudinal magnetograms are the minimum
required observation, and to fully support the science outlined above
vector photospheric magnetic flux measurements are required. While
longitudinal magnetograms provide some indication of the field
configuration, vector magnetograms in the photosphere, and ideally in
the chromosphere where the fields are closer to a force-free state, are
required to provide a more accurate determination.

**Atmospheric images** are required to reveal the location, morphology,
and thermodynamic characteristics of atmospheric features associated
with the outer coronal structures. The locations of polar plumes and the
edges of streamers can be identified in such images. They also assist in
the determination of the magnetic field configuration.

**Coronagraphic images** are required to determine the positions of
streamers and coronal holes, the solar wind speeds associated with them,
and to follow their evolution.

**UV/EUV spectra** are required to determine the physical conditions
associated with the solar wind structures. Temperatures, densities, flow
speeds, and the presence of waves and turbulence need to be determined
within these structures. Spectroscopic measurements of bulk coronal
outflow velocities at the boundaries between streamers and coronal holes
will elucidate the role of open and closed fields in producing the slow
speed wind.

**EUV coronal spectroscopy** would also provide important information on
solar wind structures. Velocity distributions of ions and electrons
could be measured to determine the spectrum of waves and turbulence in
coronal structures. Doppler shifts and ion abundances could be measured
to characterize the source regions of the high-speed and low-speed solar
wind structures.

**Helioseismic images** are required to measure the surface and
sub-surface flows. These flows control, to some extent, the magnetic
field pattern evolution and the subsequent restructuring of the corona
and solar wind.

**What are the fundamental processes responsible for the heating and
acceleration of the high-speed and low-speed solar wind?** Solar wind
variations have a direct impact on space weather. We must better
understand the processes involved in producing these variations. Heating
and acceleration of the corona is ultimately related to the magnetic
field. Observations are needed to differentiate between the alternative
mechanisms of MHD wave absorption and impulsive Joule heating. Different
proposed mechanisms of wave generation (i.e., turbulent cascade or local
micro-instabilities) depend in different ways on the evolving coronal
flux tube area, photospheric field strength, and plasma beta.

**UV/EUV Spectra** are needed to characterize the density, bulk
velocity, energy state, ionization, and elemental abundance of solar
wind source regions. A key means of identifying the dominant physical
processes will be to observe how the measured plasma parameters and
inferred wave properties evolve on timescales from days to solar cycles.
Reconnection sites on the boundaries of streamers can be identified by
measuring spectral line widths in these regions. Magnetic reconnection
sites should also produce ions in highly excited charge states and their
characteristic spectra.

## Mechanisms of Solar Variability

Magnetic structure is produced in the Sun through a broad range of
spatial and temporal scales. The global field reverses on a solar cycle
timescale, while magnetic elements at the limit of resolution evolve on
timescales of minutes or seconds. Ultimately, the source of this
magnetic activity can be traced to the heat released by nuclear
reactions in the solar core. However, the mechanisms by which the
activity is produced involve the fluid flows driven by this heating and
the magnetic field itself. Our ability to understand and ultimately
predict solar activity hinges on our understanding of how these magnetic
structures are produced. Predictions of the size and timing of the next
solar cycle will not be reliable until we understand how the cycle is
produced. Predictions of flares and CMEs will not be reliable until we
understand how active regions form and evolve. Predictions of solar wind
variations will not be reliable until we understand how the wind is
accelerated. SDO will provide crucial observations needed to improve our
understanding of the mechanisms that produce solar magnetism.

### The Solar Cycle

The solar cycle has proven notoriously difficult to predict. Once a
cycle is well underway its smoothed behavior can be predicted with some
reliability using statistical models for the shape of the cycle.
Predictions prior to the start of a cycle are, however, much less
reliable and longer range predictions are virtually useless. Currently,
all methods of cycle prediction are empirical in nature. While we
understand many of the processes involved in producing the solar cycle
we do not have a physical model that will take initial conditions and
predict future behavior. Long-range predictions of the solar cycle (as
well as extensions backward in time) are important for understanding the
climate connection, for predicting satellite drag, and for predicting
the frequency of eruptive events such as flares and CMEs. SDO will
improve our understanding of the solar cycle by providing continuous
observations and extended spatial coverage of the two main components of
the solar cycle -- the magnetic field and the fluid flows within the
Sun.

####  The Solar Cycle -- Current Understanding

The solar cycle is a magnetic cycle in which the polarity of sunspots
and the Sun's global magnetic field reverse approximately every 11 years
(Fig. 3.9). The source of this magnetic cycle is widely believed to be a
magnetohydrodynamic dynamo seated within the solar interior. Magnetic
fields are effectively "frozen" into the highly conducting ionized
plasma within the Sun. Shearing motions within the plasma can amplify
the embedded magnetic field. Regions with strong magnetic field become
buoyant in the convection zone and rise rapidly to the surface. Magnetic
loops emerge at the surface to form bi-polar active regions with a
characteristic tilt imposed by the solar rotation that leaves the
following polarity closer to the poles than the leading polarity. At the
surface a meridional flow slowly transports magnetic elements from the
active latitudes to the poles while cellular flows like supergranules
disperse them randomly across the surface.

The cycle is completed through magnetic reconnection between elements of
opposing polarities. These ingredients (rotation, shear flow, meridional
flow, cellular flows, rising tilted loops, and reconnection) are thought
to be critical components of the dynamo. Observations and theoretical
models exist for each of these but conflicts and questions remain and a
comprehensive model containing all these ingredients is yet to be
formulated.

![](media/image11.jpeg){width="2.932638888888889in"
height="1.5222222222222221in"}

**Figure 3.9.** Longitudinally averaged magnetic field as a function of
latitude and time. Active region magnetic flux is evident in the
"butterfly" pattern within about 50° of the equator. The reversal of the
magnetic polarities from cycle-to-cycle is evident at the poles and in
the equatorward edges of the butterfly patterns.

The well-known differential rotation seen on the surface of the Sun is
one example of a shear flow -- latitudinal shear. Radial shear has long
been thought to be the principal driver for the dynamo. Early kinematic
dynamos for the Sun required a radial shear from a rotation rate that
increased towards the interior while hydrodynamic models of the
convection zone gave a rotation rate that decreased. Both of these
models were shown to have problems when helioseismic observations
revealed the true nature of the internal rotation profile. The rotation
rate is nearly constant across the bulk of the convection zone with the
shear layers at the top and bottom. The region of high shear found at
the base of the convection zone (Fig. 3.10) is thought to be the source
of the solar cycle. The rate of shear across this "tachocline" can wrap
a magnetic field line once around the Sun's equator every 16 months.
Recent observations suggest that there is a periodic variation in the
rotation rate at the tachocline with a timescale of 1.3 to 1.8 years.
Others analyzing the MDI and GONG data only find evidence for chaotic
variations. Should there be additional time periods associated with the
solar cycle, the constraints on models will tighten.

![](media/image12.jpeg){width="2.9305555555555554in"
height="2.254861111111111in"}

**Figure 3.10.** Internal rotation rate as a function of latitude and
depth. The tachocline (dashed line - where the rotation velocity changes
rapidly) is thought to be the location of the magnetic dynamo associated
with the 11-year cycle.

The torsional oscillation pattern represents a shear flow on a smaller
scale. Bands of enhanced rotation rate appear near the poles and migrate
toward the equator by the middle of each cycle. This torsional pattern
produces enhanced latitudinal shear in the active latitudes. The
detection of the torsional pattern in the outer 5 percent of the Sun\'s
radius by helioseismic probing makes it evident that the structure is
more than a superficial process. In layers below this outer 5 percent,
the nature of the velocity structures becomes more obscure.

A meridional flow is observed both on the surface and in the interior.
The strength of this flow, its latitudinal structure, and its variations
in time are still somewhat uncertain. Tracking features on the surface
gives a flow speed about half that given by the directly observed
Doppler velocities. Helioseismic studies also give higher flow speeds
and show that the poleward flow extends well into the convection zone.
"Flux transport" models for the large-scale surface magnetic field have
been developed over the last 40 years. These models do a remarkable job
of reproducing the magnetic flux distribution using only surface flows
and the eruption of magnetic flux in active regions. However, the
meridional flows employed in these models are quite different from the
observed flows in both strength and structure. There are serious
concerns about why the models of the polar cap size and field strength
work as well as they do given that the poleward flow systems vary as
much as is observed.

The hierarchy of velocity structures in the Sun begins with differential
rotation and meridional circulation at the largest scale. At smaller
scales is the torsional oscillation pattern. Superposed on the torsional
oscillations is a persistent cell-like or wave-like pattern with a
geometric scale of roughly 10 percent the solar diameter. At still
smaller scale are supergranules. The supergranule pattern dominates the
smaller scale distribution of the magnetic field and clearly plays a
role in moving magnetic flux across the surface.

####  The Solar Cycle -- Outstanding Questions

What mechanisms drive the quasi-periodic 11-year cycle of solar
activity? This key question leads to others directly concerning the
relevant processes themselves -- differential rotation, meridional
circulation, torsional oscillations, and cellular flows.

How are variations in the solar cycle related to the internal flows and
surface magnetic field? The amplitude of the solar cycle can vary
dramatically from one cycle to the next. Can we identify an early
indicator of solar cycle magnitude? We expect these variations to be
related to differences in the internal flows (e. g., differential
rotation and meridional flow) and the magnetic field itself (e. g. polar
field strength).

**How is the differential rotation produced?** The processes that
produce the differential rotation are still uncertain. Hydrodynamical
models for the solar convection zone reproduce the latitudinal
differential rotation but not its variation with depth or time. Within
these models the differential rotation is maintained by angular momentum
fluxes driven by large-scale convective flows. The detection of these
cellular flows and confirmation of the presence of the momentum flux
would help our understanding of the solar cycle.

**What is the structure of the meridional flow and how does it vary?**
The meridional flow itself has been exceedingly difficult to measure.
Its form, in both latitude and radius, is still quite uncertain and its
time dependence is unknown. The poleward surface flow extends well into
the interior but the equatorward return flow has not been detected.
Better characterizations of the meridional circulation will aid our
understanding of the solar cycle.

**What role does the torsional pattern play in the solar dynamo?** The
extension of the torsional oscillation pattern well below the surface
suggests that this phenomenon is more than a small perturbation in
rotation due to the presence of active region magnetic fields at the
surface. While it is probably not the driving force behind the dynamo,
it may very well play a significant role in that process and may signal
variations in the behavior of the solar cycle.

**Are the small latitudinal surface brightness variations related to the
torsional pattern or the meridional flow?** Latitudinal bands of
enhanced surface brightness are seen in MDI data and the extreme polar
regions appear darker. These features may be a thermal signature related
to the meridional circulation or the torsional pattern. Changes in their
structure and position may hold important clues to understanding the
solar cycle.

**Magnetic Images** are needed to follow the evolution of the surface
magnetic field. The magnetograms themselves should cover the visible
disk to allow mapping the field to all latitudes and longitudes. The
sensitivity should be sufficient to follow changes in quiet areas as
well as in active regions and to accurately measure the polar flux.

**Helioseismic Images** are required for studies of the structures and
flows in the solar interior. Helioseismic probing of the critical near
surface layers (a region that all acoustic waves travel through and,
because of the lower sound speed, spend much of their time in) requires
high spatial resolution and rapid cadence observations. Probing of the
three-dimensional structures deep in the convection zone requires good
spatial resolution near the limbs to allow for widely separated
measurement areas.

**Atmospheric Images** are needed in conjunction with the magnetograms
to determine the large-scale magnetic field configuration associated
with the solar cycle.

**Coronagraphic Images** are also needed in conjunction with the
magnetograms to determine the large-scale magnetic field configuration
associated with the solar cycle.

**Photometric Images** are needed to determine the actual source of the
solar cycle variations in irradiance. These images may also reveal
thermal structures associated with the solar dynamo process.

### Active Region Evolution

Active regions are the most visible evidence of solar variability. The
intense magnetic fields in active regions are the source of flares and
most CMEs. The chromospheric and coronal structures that accompany
active regions are the source of irradiance variations in the UV, EUV,
and X-rays. Understanding and predicting the structure and evolution of
active regions is key to our understanding and predictions of explosive
events and spectral irradiance variations. Detection of emerging
magnetic structures in the interior and understanding their physical
properties can make important contributions to space weather
forecasting.

####  Active Region Evolution -- Current Understanding

The probability of the appearance of an active region can be given as a
function of the size of the region for a given latitude and the phase
and amplitude of the solar cycle. The number distribution of active
region flux has been well measured through the solar cycle. It is an
exponential function of the flux that does not change slope over the
solar cycle. The amplitude of the function, and hence the total active
region flux, changes by about a factor of eight from solar minimum to
maximum depending upon the amplitude of the cycle.

Once an active region emerges, there is a high probability that
additional eruptions of flux will occur in the neighborhood, or even in
the same region (activity nests, active longitudes). There is some weak
evidence that activity tends to concentrate around the edges of giant
cellular patterns. Once an active region starts to become visible, its
ultimate size and complexity can be estimated by the rate of flux growth
and the complexity of its early appearance. However, none of these
predictions are based on physical models for the emergence of active
region flux. There is a clear need for a better understanding of active
region formation.

The source of the active region flux is thought to be in the tachocline,
the shear layer just below the convection zone. Differential rotation
should amplify the field until the magnetic energy density is so large
that it becomes buoyant and rises to the surface. The magnetic fields
must be amplified to about 10^5^ Gauss to become sufficiently buoyant to
begin their rise. Typically, active region magnetic flux emerges through
the surface in the form of sunspot groups and plage. The spots grow in
size over the course of days while the preceding and following spots
drift apart in longitude. Active regions have characteristic tilts with
respect to lines of constant latitude and asymmetries in the morphology
of the preceding and following spots.

Models of buoyant flux tubes have been constructed to follow their rise
from the bottom of the convection zone to within about 10,000 km of the
solar surface. Above that height the rate of change of pressure is so
great that current computers are not sufficient for the calculations
required. (Six of the fifteen scale heights between the bottom and top
of the convection zone occur in the top 10,000 km.) These models show
how the effects of solar rotation can produce active regions with both
tilt and morphological asymmetry between preceding and following spots.
However, these models neglect the turbulent motions within the
convection zone. Even neglecting turbulence it is difficult to create a
stable rising flux bundle.

Observations of the interior rotation (Fig. 3.10) show that there is a
sharp decline in the angular rotation rate over the top 10,000 km of the
convection zone. Given the high shear between the surface and the deeper
convection zone it is hard to understand how the active regions that
result from flux emergence can long remain connected to their origin in
the tachocline.

MDI high-resolution measurements have demonstrated that it is possible,
using time-distance helioseismology and acoustic tomography, to form
images of the critical top 10,000 km. Time sequences of such images
should do much to increase our understanding of active region flux
emergence and the subsequent detachment of this flux from the deep
convection zone. The behavior of the flux in the top 10,000 km of the
Sun may be key to the understanding of the solar dynamo process that
generates the active region flux.

As to the prediction of specific active regions at specific times and
places, very little can be done at present. Searches for thermal shadow
precursor signals in advance of the eruption of an active region have
been inconclusive using currently available observations. Helioseismic
tomography can be used to probe the flow field beneath active regions
(Fig. 3.11). These techniques might detect emerging active structures in
the deep interior before they are visible. However, a clear case of a
precursor signal has not been observed. Initial results using the MDI
high-resolution data indicate that the flux propagates rapidly, in less
than 8 hours, through the top 10,000 km of the convection zone. One
could speculate that this rise time is sufficiently rapid and the
cadence of existing observations is too slow to detect a precursor
signal. Alternatively, the precursor signals may simply be too weak to
detect. Surface Doppler measurements do, however, show persistent
convergence at the sites of active regions prior to their emergence.
These observational techniques provide great promise for furthering our
understanding of active regions.

![](media/image13.jpeg){width="2.9944444444444445in"
height="1.5270833333333333in"}

**Figure 3.11.** The material flows at the depth of 0-3 Mm (a) and at
6-9 Mm (b). The outline of the sunspot umbra and penumbra is shown. The
colors are vertical motion with positive values (red) corresponding to
downflow. The arrows are horizontal motion. Measurements like these may
reveal active regions before they emerge.

Another recent success is the new ability to make crude images of sound
speed anomalies near the surface on the far side of the Sun (Fig. 3.12).
Thus, it has become possible, in principle, to predict that a large
active region will rotate onto the visible disk days to weeks before
this happens.

![](media/image14.jpeg){width="2.995833333333333in"
height="4.597916666666666in"}

**Figure 3.12.** Helioseismic maps of the Sun. This set of three maps
constructed 13.5 days apart shows that the "big spot" (AR9393) was seen
on the far side before it was seen on the Earth side.

####  Active Region Evolution -- Outstanding Questions

**How is magnetic flux synthesized, concentrated, and transported to the
solar surface where it emerges in the form of evolving active regions?**

**Can active region magnetic flux be observed before it erupts through
the surface?** Helioseismic studies can see changes in the sound speed
below active regions but have not seen these variations before they
erupt through the surface.

**To what extent are the appearances of active regions predictable?** An
important goal for SDO is to develop an understanding of the locations
and times of magnetic region eruption that goes beyond simple
statistics. Can we go beyond empirically identifying the locations of
activity?

**What roles do local flows play in active region evolution?** Some
aspects of active region evolution are obviously dependent upon the
characteristics of the emerging flux itself. Other aspects depend upon
the surrounding flows that are themselves influenced by the presence of
the active region.

**Magnetic Images** are required to determine the configuration of the
surface magnetic fields associated with active regions. Vector
magnetograms are required to provide a realistic assessment and to
indicate the presence of electrical current systems and non-potential
magnetic field systems. Vector magnetograms will also give evidence as
to whether flux leaves the surface predominantly as bubbles, or whether
it is principally the outcome of local annihilation of fields of
opposing polarity. High cadence MDI observations have shown that the
magnetic configuration changes in intervals as short as minutes.
Therefore the fields must be measured on a comparable cadence in order
to separate temporal and spatial variations.

**Helioseismic Images** are needed to construct subsurface maps with
sensitivity and turnaround time superior to present capabilities. We
also need helioseismic mapping of the far side of Sun with fast
turnaround time. Helioseismic probing of the critical near surface
layers (a region that all acoustic waves travel through and, because of
the lower sound speed, spend much of their time in) requires high
spatial resolution and rapid cadence observations. Probing of the
three-dimensional structures deep in the convection zone requires good
spatial resolution near the limbs to allow for widely separated
measurement areas.

**Atmospheric Images** are needed in conjunction with the magnetograms
to determine the magnetic field configuration associated with active
regions and their evolution. Simultaneous observations in several
temperature regimes are needed to follow the evolution of the overlying
loop systems. The rapid evolution (seconds) of these loops requires
rapid cadence.

**Coronagraphic Images** are also needed in conjunction with the
magnetograms to determine the magnetic field configuration associated
with active regions and their evolution.

**Photometric Images** may reveal thermal structures associated with the
active regions before they emerge.

**UV/EUV Spectra** are needed to provide information on the physical
characteristics of atmospheric structures within active regions. These
measurements may provide important information on reconnection events
associated with the evolution of active region magnetic fields.

### Small-Scale Magnetic Structure

While the large-scale distribution of magnetic flux on the Sun
determines the coupling from the Sun to the heliosphere, the small-scale
mixed polarity flux may provide most of the energy that heats the
corona. In order to understand the distribution of fields on the Sun we
must identify the types of sources of magnetic flux, the mechanisms for
the spreading the flux, and the manner in which flux disappears from the
surface. The various mechanisms of flux disappearance are key to
understanding the local heating of the outer atmosphere, the
acceleration of the solar wind, and the contribution these elements make
to the solar irradiance variations.

![](media/image15.jpeg){width="3.0in" height="2.0in"}

**Figure 3.13.** Small-scale magnetic elements (red and blue contours)
and the coronal network bright points (SOHO/EIT image). The bright
coronal features are well correlated with magnetic elements in general
and magnetic dipoles in particular.

####  Small-Scale Magnetic Structure -- Current Understanding

Away from active regions, most of the photospheric magnetic flux appears
to exist in the form of flux tubes. These structures have complex
dynamics. They form when a sufficient amount of flux is aggregated by
local velocity flows to affect the transport of thermal energy. A
cooling and subsequent collapse of the mass in the tube leads to a
compression of the field into a kiloGauss-strength configuration. This
structure persists until torn apart by convection.

An unknown amount of flux exists outside of active regions and the
strong flux tubes. Some estimates indicate that the total flux may be as
much as or more than that contained in active regions over the solar
cycle. This pattern of mixed polarity has been characterized as a "salt
and pepper" pattern or the "magnetic carpet". How it is created, how it
evolves, and how it is destroyed are all uncertain. Presumably it is a
manifestation of a local dynamo at work in the upper photosphere, but
its relation to other scales of magnetic activity is not known.

Observations during the last few years have established that a
significant fraction of the observed flux on the solar surface emerges
in ephemeral regions -- bipolar regions with lifetimes of hours. The
ephemeral region number distribution, like the active region number
distribution, is also exponential. Although it has not been as well
measured as the active region flux over the cycle, there is strong
indication that the amplitude of the cyclic variation is a factor of two
or less. Consequentially, the ratio of total active region to ephemeral
region flux varies considerably during the cycle. Another component of
the flux is the inter-network field. This component has not yet been
well characterized. There is evidence that the field strength of the
inter-network field is 500 Gauss or less, while the active region and
ephemeral region magnetic structures, excluding sunspots, have field
strengths of 1100 ±100 Gauss.

When active region flux emerges it forms plage as well as sunspots.
Plage are regions in which the average magnetic flux is 125 ± 25 Gauss.
Since the typical magnetic elements in plage have field strengths of
about a kilogauss, the magnetic filling factor for plage is about 12%.
Quite remarkably, plage maintain their average field strength as the
active region grows and finally disappears. Outside plage, in the quiet
Sun, the average field flux is about 5 Gauss. Because about 90% of the
flux in the quiet Sun also has field strength of a kilogauss, the quiet
Sun filling factor is about 0.5%. Subsurface imaging should collect data
that will allow insights into the structure of the flow and magnetic
fields below the plage that give raise to the remarkable bimodal
character of plage and quiet Sun.

![](media/image16.jpeg){width="2.995833333333333in"
height="2.902083333333333in"}

**Figure 3.14.** Histogram for the number of magnetic regions as a
function of size (area). Ephemeral regions (open circles) appear as an
extension of the distribution function for active regions (closed
circles).

Once field elements leave the plage, they are dispersed over the surface
by the diffusive action of convection, by the differential rotation, and
by meridional flows. Linear models of flux distribution by diffusion,
rotation, and flows have been developed over the past 40 years and have
done an excellent job of predicting the mean characteristics of the
surface flux distribution. In particular, given the observed plage
strength, the mean field with a spatial resolution of a few arc-minutes
(see Fig. 3.9) and the mean field at the pole can be predicted. However,
these models do not properly reproduce the number distribution of flux,
the total flux with resolutions of a few arcseconds, or the spatial
distribution of flux over the surface.

By proper choice of the value of the diffusion coefficient, the speed of
the meridional flow, and the latitude that the meridional flow
terminates, these linear models produce remarkable agreement with
low-resolution observations of the flux distribution over the surface.
Unfortunately, the value of the diffusion coefficient that is required
by the models is higher by a factor two than the measured values
reported in the literature. This is not very serious because the
majority of the measurement techniques for determining the diffusion
constant strongly weight high concentrations of flux. Recent MDI
observations have shown that the dispersal of flux is a function of the
magnitude of the size of the flux concentration and that larger flux
concentrations disperse more slowly than small ones. (Of course, these
results mean that the uses of a fixed diffusion constant and
consequently linear model of the surface spreading of magnetic field are
not sufficient for an accurate description of flux dispersal.) Much more
serious is that both the strength and the latitudinal structure of the
meridional flow used in the models are not consistent with the
observations. The poleward flow speed and the latitude of the upper
cutoff control both the diameter and the magnetic flux of the polar cap
field. There are strong indications from both ground and MDI
measurements that these poleward current systems may also vary both in
longitude and time.

Kinematic modeling that includes the supergranulation flow shows that
there must be a constant eruption of new flux to maintain the measured
total flux observed with a resolution of a few arcseconds. Constant
eruption of flux is also required to maintain the observed quiet Sun
network. Simulations show that the magnetic network would disappear in
days without continuous replacement. The most recent measurements of the
ephemeral flux emergence rate yields about 5 10^23^ Mx/day, which is
equivalent to 10 to 20 large active regions. This rate is sufficient to
replace the quiet Sun network in less than a day and the flux in an
active region, if the emergence rate is the same as in quiet Sun, in a
few weeks. Because the rate of emergence of ephemeral flux is so high,
10 to 30% of the quiet Sun flux may reside in the cell centers just
because the travel time to the boundary is only a small fraction of the
total flux lifetime on the surface.

Ephemeral region flux emerges inside supergranulation cells and is
transported to the cell boundary where is cancels with existing flux in
the magnetic network. This means that ephemeral flux is not just the
result of magnetic loops emerging through and then sinking back below
the solar surface. Rather ephemeral regions must disappear by some type
of reconnection process. There are a number of methods for estimating
the energy released in the reconnection. But using the initial field
strength and the foot point separation of the emerging loop to estimate
a magnetic volume energy, the result is 3x 10^7^ ergs/cm^2^/sec. This
energy emergence rate is two orders of magnitude more than necessary to
heat the corona, but most of the energy maybe dissipated in the
chromosphere and transition region.

There is clear evidence from very high-resolution filter magnetograms
that magnetic field emerges on the scale of granulation.
Spectropolarimetry indicates that there is a component of the field with
a strength that is probably less than 500 Gauss, and that there is a
significant amount of weak horizontal field in quiet Sun. Taken together
these observations constitute a strong suggestion that there are surface
magnetic fields associated with every form of convection. Numerical
simulations indicated that a weak seed field is amplified sufficiently
by convective flows to generate a mixed polarity field that contains 25%
of the kinetic energy originally in the convection.

Local dynamo action is extremely interesting as a process that has the
capacity to continuously generate energy that can then be used to heat
the chromosphere, transition region and corona. In addition models
suggest that local and global dynamos can couple and thus produce
variations in the length and amplitude of the solar cycle.

#### 3.2.3.2 Small-Scale Magnetic Structure -- Outstanding Questions {#small-scale-magnetic-structure-outstanding-questions .unnumbered}

**How does magnetic reconnection of solar magnetic fields on small
spatial scales relate to coronal heating, solar wind acceleration and
the transformation of the large-scale field topology?** There are
indications that the interactions between opposite polarity elements
contribute to coronal heating. These interactions may also provide the
triggers for eruptive events that restructure the large-scale field.

**How are the small-scale magnetic features produced?** There is now
good evidence for magnetic dynamo activity in the thin layer just below
the photosphere. There is strong shear across this layer and the rate at
which magnetic flux appears and disappears suggests local generation.

**What are the dynamics of magnetic structures on very small spatial
scales?** Several mechanisms may be involved in the appearance,
disappearance, and interactions between these small magnetic structures.
High-resolution vector magnetograms show that many magnetic elements
seen in longitudinal magnetograms may just be intersections of
undulating flux ropes with the photosphere.

**Magnetic Images** are required to identify and to follow the evolution
of these elements. Full-disk observations with enough spatial resolution
(\~1″) to identify them will be needed to determine their
characteristics. The temporal cadence should be fast enough to follow
their evolution and interactions (\~ 5 min is probably the best that can
be achieved). Longitudinal magnetograms should be sufficient for most
studies involving the small-scale magnetic elements but vector
magnetograms will be required to understand the flux ropes forming the
small-scale elements.

**Helioseismic Images** are needed to determine the nature of the fluid
flows that surround these elements and to examine their sub-surface
structure. Since flows close to the surface will be the most
significant, the oscillation images will need the spatial (\~1″) and
temporal (\<50 sec) resolution to resolve them and their temporal
evolution.

**Atmospheric Images** are needed to determine the effects of these
elements on coronal and chromospheric heating. These images should also
have the spatial resolution (\~1″) to identify the elements and the
temporal resolution (\~10 sec) to follow their evolution. These images
need to cover a wide range of temperatures to determine the effects of
these elements on atmospheric heating and dynamics.

**UV/EUV Spectra** are needed to provide information on the physical
characteristics of atmospheric structures above these small-scale
magnetic elements. These measurements should provide important
information on events associated with coronal heating by these magnetic
elements.

# **Required Observations**

An array of observations is needed to address the scientific questions
posed for SDO. Each of these observations and their characteristics can
be traced back to one or more of the outstanding questions stated in the
previous section. (Note that all six of the scientific areas discussed
in the previous Section are deemed critical to the SDO Mission. The
scientific links among the different areas suggest that all would suffer
from the omission of any one.) Since single instruments might be
designed to provide more than one measurement (e.g., helioseismic images
and magnetograms), we present the requirements for the measurements
themselves in the following sub-sections. Potential instruments and the
possible allocation of resources are given in Section 5.

All of these observations are important for one or more of the
scientific areas to be addressed by SDO. However, some are deemed to be
of higher priority than others for a variety of reasons. Some of these
observations are either unique to SDO or have requirements that can be
uniquely met by SDO (e.g. full-disk spatial coverage, continuous time
coverage over several years, rapid cadence, high bandwidth). Other
observations may be provided by other programs, may be more speculative
in nature, or may be more difficult to obtain with current technology
and resources. We recognize the need for setting priorities for SDO and
have done so by grouping the observations and their respective
instruments into three priority categories: **highest priority** -- SDO
must make these observations; **high priority** -- due effort should be
made to include these observations but for one reason or another (as
discussed in the following sub-sections) the mission could go on without
them; **important** -- these would enhance the scientific return from
SDO but should not be included at the expense of higher priority
observations.

The highest priority observations include (from the interior outward):

-   Helioseismic Images

-   Longitudinal Magnetograms

-   Atmospheric Images

-   EUV Spectral Irradiance

-   Total Solar Irradiance (should concurrent measurements not be
    available from other platforms)

The high priority observations include:

-   Photometric Images

-   Vector Magnetograms

-   UV/EUV Spectra

-   Coronagraphic Images

The important observations include:

-   Coronal Spectroscopy

-   Heliometry

Further refinement of priorities within these groups is unproductive at
this phase of the mission definition. The actual resources required to
obtain the measurements, the actual instruments to be flown on other
missions, and other programmatic considerations could easily change
those priorities.

## Helioseismic Images

Helioseismic images are required for studies of the Sun's internal flows
and structures. This impacts all aspects of SDO science. Helioseismic
images provide information on solar radius changes for irradiance
studies (Section 3.1.1.2) and on sub-surface flows that drive magnetic
field evolution associated with flares and CMEs (Section 3.1.2.2) and
with coronal structures and solar wind variations (Section 3.1.3.2).
Helioseismic images provide critical measurements of the internal flows
that drive the solar cycle (Section 3.2.1.2), active region evolution
(Section 3.2.2.2), and the dynamics of the small-scale magnetic elements
(Section 3.2.3.2). One of the highest priorities for SDO is to obtain
the helioseismic observations required to provide a detailed view of the
upper 10-20 Mm of the convection zone. These measurements are of highest
priority for SDO. They will not be available with the required spatial
resolution and full-disk coverage from any other source.

**Accuracy.** Signals near the base of the convection zone likely
generate wave travel time variations as small as 0.01 second. The total
travel time approaches 2 hours so the timing accuracy must be on the
order of a part in 10^6^ over intervals of hours to days.

**Dynamic Range.** Previous experience shows that the best
signal-to-noise can be obtained with Doppler observations as compared to
brightness observations in the 1 to 5 mHz region. Therefore dynamic
range is discussed here in terms of velocity. The instrument must have a
velocity dynamic range large enough to accommodate both the spacecraft
orbit and the Sun's rotation. The orbit will have a range of about 6
km/s and the Sun adds about 7 km/s from East to West limbs giving a
dynamic range of about 13 km/s.

**Time Cadence.** There is useful wave power up to about 8 mHz and
measurable power to at least 12 mHz. A sample with a 10 mHz Nyquist
frequency leaves the spectrum clean up to 8 mHz. This gives a time
cadence of about 45 seconds per measurement.

**Spatial Resolution.** A reasonable goal is to sample the wave-speed
profile and flows with about 1 Mm depth resolution in the upper 10-20 Mm
of the convection zone. This requires sample points 3 Mm apart. At 75°
from disk center, this corresponds to about 1 arcsec resolution. This
can be provided with 0.5 arcsec detector pixels.

**Field Of View.** In order to detect fields and flows nearer the base
of the convection zone, we need simultaneous observations across the
full disk (again 75° from disk center is a reasonable goal, this is 0.97
in units of the radius).

**Completeness.** Gaps in the data stream compromise the helioseismic
objectives. Experience with MDI and GONG indicate that nearly complete
coverage (more than 95% of the time) is needed.

**Table 4.1: Measurement Characteristics for the Helioseismic Imager**

  ---------------------------------- ------------------------------------
  Observable                         Osc. Time Series

  Accuracy (Clock)                   10^-6^

  Dynamic Range                      13 km/s

  Time Cadence                       \< 50 sec

  Spatial Resolution                 1 arcsec

  Field of View                      Full Disk

  Duration                           10 years

  Completeness                       99.99% coverage 95% of time
  ---------------------------------- ------------------------------------

## Longitudinal Magnetograms

Magnetic images are critical for all of the scientific areas to be
addressed by SDO. Longitudinal magnetograms are needed: to distinguish
between magnetic and non-magnetic sources of the irradiance variations
(Section 3.1.1.2); to determine the configuration of the magnetic fields
associated with flares and CMEs (Section 3.1.2.2) and solar wind
variations (Section 3.1.3.2); to follow the evolution of the magnetic
field over the solar cycle (Section 3.2.1.2) and as active regions
evolve (Section 3.2.2.2); and to identify and follow the small magnetic
elements (Section 3.2.3.2). Longitudinal magnetograms are of highest
priority to SDO. Full-disk, high-resolution magnetograms will not be
obtained with the required temporal resolution and coverage by any other
means.

**Precision.** A pixel-by-pixel precision between 5 and 50 G over a 5
minute integration is needed to follow the evolution of the small-scale
magnetic elements.

**Accuracy.** The disk integrated magnetic flux should be measured with
an accuracy of about 0.1 G. This level of accuracy allows for studies of
active region flux imbalances and determinations of the global field
configuration.

**Dynamic Range.** Sunspot umbrae harbor the strongest magnetic fields
with peak values of about 3000 G. A dynamic range of 6-7 kG would
capture even the strongest flux concentrations.

**Time Cadence.** Individual magnetograms should be obtained at a
cadence of at least 1 minute to account for the Doppler shifts due to
the 5-min oscillations.

**Spatial Resolution.** High spatial resolution (\~1 arcsec) is required
to identify the small intranetwork magnetic elements.

**Field Of View.** Full-disk measurements are required to provide the
global field configuration, to monitor all active regions, and to
provide complete coverage for studies of irradiance and small magnetic
features.

**Table 4.2: Measurement Characteristics for the Longitudinal
Magnetograph**

  --------------------------------------- -------------------------------
  Observable                              Longitudinal B

  Precision                               5-50 G / 5 min

  Accuracy                                0.1 G

  Dynamic Range                           Several kG

  Time Cadence                            \~ 1 min

  Spatial Resolution                      1 arcsec

  Field of View                           Full Disk

  Duration                                10 years
  --------------------------------------- -------------------------------

## Atmospheric Images

Atmospheric images are required for all aspects of the SDO mission. They
indicate the sources of the EUV irradiance variations (Section 3.1.1.2),
the occurrence and nature of flares and CMEs (Section 3.1.2.2), and the
structure and dynamics of the low corona (Section 3.2.3.2). They reveal
the magnetic structures associated with the solar cycle (Section
3.2.1.2), active region evolution (Section 3.2.2.2), and small-scale
magnetic elements (Section 3.2.3.2). These observations are of highest
priority to SDO. Full-disk images at the required spatial and temporal
resolution will not be obtained by any other means.

**Precision/Accuracy.** A photometric accuracy of about 10% is needed
for these images to be useful as a tool for the determining the sources
of the spectral irradiance. Similar accuracy is needed in making useful
ratios of the images to determine physical characteristics of the
atmospheric features. These imagers must be properly calibrated to
insure the stability and repeatability of the measurements at the 10%
level.

**Dynamic Range.** A high dynamic range (10^3^ or greater) is needed to
provide observations with sufficient signal to noise and to span the
range of emission features from intra-network loops to bright active
regions.

**Time Cadence.** An important diagnostic of coronal activity may lie
with transient flow patterns that are perhaps created by short-term
heating events. The flow velocities on the order of 100 km/s, near the
sound speed, are often observed. A cadence of 10 seconds would be
sufficient to track these flows from resolution element to resolution
element between consecutive exposures.

**Spatial Resolution.** We know from measurements with HRTS and SUMER
that the atmospheric structures are highly filamentary. TRACE
observations show that with 1 arcsec spatial resolution, solar
structures are recorded with sufficient clarity that much of the
confusion produced by the fine structure is overcome. CCDs with a
4096x4096 format will soon become available and would provide a 1.2
arcsec resolution over the 40 arcmin field of view.

**Field Of View.** Full-disk images are required to observe flares and
CMEs and to identify sources of EUV irradiance variability. CMEs are
often large-scale events that can involve a significant portion of the
solar disk. In addition, it would be useful to observe the initial
acceleration of CMEs near the solar limb with some overlap with the
coronagraph. Consequently, a field of view (FOV) of 40 arc-min (1.25
RSun) is specified.

**Spectral Resolution.** Relatively high spectral purity (resolving
power of 20 or so) is needed so that the individual images refer to only
a small temperature range.

**Temperature Range.** Solar activity often involves the nearly
simultaneous rapid evolution of plasmas over a wide range of
temperatures, such as hot flare plasmas, the eruption of cool prominence
material and the reconfiguration of coronal loops during a flare or CME
event. To understand the dynamics of the coronal field, we must be able
to trace the loops as they evolve in temperature. That requires narrow
pass bands because only those allow the clear identification of loops
while minimizing the line of sight confusion. The ability of the
Atmospheric Imagers to observe plasmas over a wide range of temperature
(0.02 -- 4 MK) will help untangle the relevant physical processes. The
number of image bandpasses needed to effectively span this range is
uncertain. The minimum is probably four but seven would be far more
useful.

**Table 4.3: Measurement Characteristics for the Atmospheric Imagers**

  --------------------------------------- -------------------------------
  Observable                              Intensity

  Precision/Accuracy                      10 %

  Dynamic Range                           10^3^ -- 10^5^

  Time Cadence                            10 sec

  Spatial Resolution                      1.2 arcsec

  Field of View                           40 x 40 arcsec^2^

  Spectral Resolution                     / \~ 20

  Temperature Range                       0.02 -- 4 MK
  --------------------------------------- -------------------------------

## EUV Spectral Irradiance

The EUV irradiance and its variations influence ionospheric and
thermospheric composition, density, and temperature. The need for
continuous observations of these variations (Section 3.1.1.2) drives the
SDO requirements for an EUV Spectral Irradiance Monitor. These
observations are of highest priority to SDO. While EUV spectral
irradiance observations will be obtained by the SEE instrument on the
TIMED mission, these observations will not be concurrent with the SDO
mission and are of lower spectral and temporal resolution. The SDO EUV
observations play a critical role in helping us to understand how solar
variability influences the ionosphere and thermosphere. Tracking the
solar cycle variations, and the possibility of longer-term secular
variations is desirable. The observations should be obtained
continuously to maintain a calibrated time series, and to provide
continuous inputs for geophysical studies undertaken elsewhere within
the LWS program.

**Precision/Accuracy.** The absolute uncertainty (accuracy) translates
into uncertainties in geophysical parameters (densities, temperatures)
calculated by geophysical models that use these inputs, so the goal is
maximum accuracy. Accuracies of 10% reflect the state of the art
irradiance calibrations using synchrotron irradiance standards.
Degradation of the optics and detectors forces a requirement for some
on-board calibration as well. The measurements should have long-term
stability on the order of 5% for useful solar cycle studies.

**Dynamic Range.** The solar cycle variability ranges from 50 percent to
two orders of magnitude over this spectral range, with even larger
variations (a factor of up to 1000 in some lines) occurring during
flares. A dynamic range of about 1000 can be obtained with a combination
of detector dynamic range and different exposures.

**Time Cadence.** Synchronization with the SDO Atmospheric Imagers is
desirable for tracing the flux to the magnetic (and possibly other)
sources of its variability and for formulating physical mechanisms. This
indicates a similar time cadence of about 10 sec for the EUV irradiance
measurements.

**Field Of View.** A slightly larger FOV than the solar disk will
minimize pointing error problems. The FOV should be sufficient to
capture off-limb radiation that may contribute to the flux of some hot
coronal lines.

**Spectral Resolution.** Solar emission lines that are close in
wavelength can nevertheless have quite different variabilities if they
have different emission temperatures i.e., sources in different solar
atmospheric regions. At wavelengths longer than 10 nm, line and
continuum energy inputs to the upper atmosphere and ionosphere must be
separately measured and distinguished to achieve the required
geophysical understanding. AURIC (Atmospheric Ultraviolet Radiation and
Ionization Code), which is used to calculate photoelectron production
and heating, requires input spectra with 0.1 nm resolution.

**Spectral Range.** Understanding ionospheric and neutral density
responses to solar variability requires knowledge of the solar energy
inputs over a spectral range from 1-120 nm in both lines and continua.

**Table 4.4: Measurement Characteristics for the EUV Spectral Irradiance
Monitor**

  ------------------------------------ ----------------------------------
  Observable                           Spectral Irradiance

  Precision/Accuracy                   10 %

  Dynamic Range                        10^3^

  Time Cadence                         10 sec

  Spatial Resolution                   None

  Field of View                        1°

  Spectral Resolution                   \~ 0.1 nm

  Spectral Range                       1 -- 120 nm
  ------------------------------------ ----------------------------------

## Photometric Images

Photometric image data is essential to finding the origins of solar
irradiance and luminosity variability (Section 3.1.1.2). These images
may also reveal thermal structures associated with the solar cycle
(Section 3.2.1.2) and active region evolution (Section 3.2.2.2). These
observations are of high priority for SDO, as they address three
important areas. Data of high photometric sensitivity or extended
bandwidth will only be obtained from space and would be unique to SDO.
However, empirical modeling of the sources of the irradiance variations
suggests that the sources are well understood and helioseismic
determinations of solar radius variations indicate that these variations
probably play only a minor role in producing the irradiance variations.
The ability to image thermal perturbations associated with active
regions before they emerge is somewhat speculative but may prove to be
revolutionary.

It is likely that the photometric mapper (PM) will operate with two
channels. In its first channel it will achieve the highest possible
photometric accuracy and spatial resolution. The second channel captures
the widest possible spectral coverage (approaching total solar
irradiance bolometric spectral coverage) with sufficient spatial
resolution to identify photospheric magnetic contributions as small as
faculae.

**Precision.** In its first channel the PM must have photometric
stability and sensitivity sufficient to detect faint diffuse brightness
changes associated with surface magnetic features like sunspot bright
rings, facular shadows and the network irradiance perturbations.
Detecting these subtle features will require a precision of about 0.1%.
In its second channel the PM must have the ability to tally the
contributions to the irradiance variations from the various photospheric
features. A precision of 3% at each pixel should be sufficient for the
number of pixels given by the spatial resolution requirement.

**Dynamic Range.** The dynamic range at each pixel should be sufficient
to capture the range of intensities from the darkest sunspot umbrae to
the brightest plage and faculae.

**Time Cadence.** The images should be obtained at about 1 minute
intervals to resolve the *p*-mode oscillation component of the
irradiance variations.

**Spatial Resolution.** Photospheric faculae should be resolved in these
images. A spatial resolution between 1 arcsec and 10 arcsec should be
sufficient.

**Field Of View.** The full photospheric disk must be imaged for the
mapper to provide the observations needed to determine the sources of
the irradiance variations.

Table 4.5 Photometric Mapper Characteristics

+-------------------------+----------------------+--------------------+
| Channel                 | Photometric          | Bolometric         |
+-------------------------+----------------------+--------------------+
| Observable              | Surface              | Bolometric         |
|                         |                      |                    |
|                         | Intensity            | Intensity          |
+-------------------------+----------------------+--------------------+
| Precision               | 0.1 %                | 3 %                |
+-------------------------+----------------------+--------------------+
| Dynamic Range           | \>10^3^              | 30                 |
+-------------------------+----------------------+--------------------+
| Time Cadence            | 1 min                | 1 min              |
+-------------------------+----------------------+--------------------+
| Spatial                 | 1 arcsec             | 10 arcsec          |
|                         |                      |                    |
| Resolution              |                      |                    |
+-------------------------+----------------------+--------------------+
| Field of View           | Full Disk            | Full Disk          |
+-------------------------+----------------------+--------------------+
| Spectral                | Narrow               | Broad              |
|                         |                      |                    |
| Resolution              | band                 | band               |
+-------------------------+----------------------+--------------------+
| Completeness            | 95%                  | 95%                |
+-------------------------+----------------------+--------------------+

## Vector Magnetograms

Vector magnetograms are needed to determine: the magnetic stresses and
current systems associated with flares and CMEs (Section 3.1.2.2); the
nonpotentiality of coronal magnetic fields (Section 3.1.3.2); and the
large-scale magnetic field pattern evolution associated with the solar
cycle (Section 3.2.1.2) and active region evolution (Section 3.2.2.2).
These are high priority observations for SDO. They address critical
needs in several areas. However, full-disk vector magnetograms with the
required spatial resolution may be difficult to obtain with current
technology and mission resources. Limited field of view observations at
higher resolution will be acquired with instruments on Solar-B (Section
7.2). Full-disk measurements from the ground will be acquired with the
SOLIS instruments (Section 7.6). These alternative observations do not,
however, fully satisfy the SDO requirements. Solar-B has a very limited
field of view and SOLIS will have poorer spatial resolution and temporal
coverage.

**Precision.** The precision of the transverse field direction
measurements should be a few degrees. This translates into a
polarization precision of \~10^-4^. The precision of the field strength
measurements should be better than 50 G for a 10-minute integration
time.

**Accuracy.** The vector field measurements should yield the correct
vector field direction in each resolution element to within five
degrees. The longitudinal component accuracy should be consistent with a
disk integrated flux accuracy of about 0.1 G.

**Dynamic Range.** Sunspot umbrae harbor the strongest magnetic fields
with peak values of about 3000 G. A dynamic range of 6-7 kG would
capture even the strongest flux concentrations.

**Time Cadence.** Polarimetric scans should be obtained at a cadence
that allows the Doppler shifts due to the 5-minute oscillations to be
removed or compensated for. Full-disk vector magnetograms should be
obtained at a cadence of about 6 per hour.

**Spatial Resolution.** High spatial resolution (\~1 arcsec) is required
to identify the small magnetic elements.

**Field Of View.** Full-disk measurements are required to provide the
global field configuration, to monitor all active regions, and to
provide complete coverage for studies of irradiance and small magnetic
features.

**Table 4.6: Measurement Characteristics for the Vector Magnetograph**

  --------------------------------------- -------------------------------
  Observable                              Vector B

  Transverse Precision                    50 G (\~3°)

  Polarimetric Precision                  \~10^-4^

  Dynamic Range                           Several kG

  Time Cadence                            \~ 10 min

  Spatial Resolution                      1 arcsec

  Field of View                           Full Disk

  Duration                                10 years
  --------------------------------------- -------------------------------

## UV/EUV Spectra

The UV/EUV Imaging Spectrometer will provide spectral images and
measurements that reveal quantitatively the dynamics of the solar
atmosphere, from the photosphere, through the transition region to the
corona. The UV/EUV Imaging Spectrometer will provide quantitative
constraints on the physical mechanisms associated with: the sources of
the spectral irradiance variations (Section 3.1.1.2): the impulsive
release of energy that results in flares and CMEs (Section 3.1.2.2); and
the sources of high-speed and low-speed solar wind (Section 3.1.3.2).
UV/EUV spectra are also needed to provide information on the physical
characteristics of atmospheric structures in active regions (Section
3.2.2.2) and above small-scale magnetic elements (Section 3.2.3.2).
These observations are of high priority to SDO. They are required to
address several outstanding questions. Although there is an EUV
Spectrometer planned for the upcoming Solar-B mission (Section 7.2), it
is not ideally suited to the SDO scientific objectives for two reasons.
First, the Solar-B EIS is primarily a coronal spectrometer, with only
one strong emission line below 10^6^ K. Second, the estimated count
rates of the Solar-B EIS are too low to match the temporal cadence of
the Atmospheric Imager Array. The SDO UV/EUV Imaging Spectrometer nicely
complements the Atmospheric Imaging because it provides quantitative
observations with the spatial and temporal resolution necessary to
resolve ambiguities seen in the imager observations.

**Precision/Accuracy.** Spectral intensity should be measured to within
10% to contribute to the spectral irradiance studies. Line widths should
be measured to within about 10% to provide useful information on
non-thermal broadening associated with physical mechanisms in the
relevant features. Doppler velocities should be measured to within 1-5
km/s to determine the nature of the dynamic events.

**Dynamic Range.** Spectral intensity varies widely from one spectral
feature to another and from quiet sun to active plage. A dynamic range
of 10^3^ to 10^5^ should capture all these features.

**Time Cadence.** Ideally, the time cadence of the UV/EUV Imaging
Spectrometer should match the 10 sec cadence of the Atmospheric Imaging
Array. This can be accomplished with limited FOV raster motions about a
given location (e.g. central meridian) or target (e.g. active region),
repeated throughout the majority of the observing period. Synoptic
observing programs providing raster images of the entire disk may also
be performed periodically.

**Spatial resolution.** A spatial resolution of 1.3 arcsec should be
sufficient to resolve the facular elements associated with the spectral
irradiance variations.

**Field Of View.** The routine raster FOV will be limited by the
necessity to match the time cadence of the Atmospheric Imaging Array.
However, it will also be necessary to observe the full solar disk and
inner corona by slit rasters or other techniques.

**Spectral Resolution.** A spectral resolution (resolving power) of
30000 should be sufficient to measure the required line profile
information.

**Temperature Range.** The measurements should include spectral features
that cover the temperature range of the Atmospheric Imaging Array --
0.02 to 4 MK.

**Table 4.7: Measurement Characteristics for the UV/EUV Imaging
Spectrometer**

+--------------------------------------+-------------------------------+
| Observable                           | Line Profiles                 |
+--------------------------------------+-------------------------------+
| Precision/Accuracy                   | Intensity 10 %                |
|                                      |                               |
|                                      | Width 10%                     |
|                                      |                               |
|                                      | Velocity 1-5 km/s             |
+--------------------------------------+-------------------------------+
| Dynamic Range                        | 10^3^ -- 10^5^                |
+--------------------------------------+-------------------------------+
| Time Cadence                         | 10 sec                        |
+--------------------------------------+-------------------------------+
| Spatial Resolution                   | \~1 arcsec                    |
+--------------------------------------+-------------------------------+
| Field of View                        | 16 to 34 arcmin               |
+--------------------------------------+-------------------------------+
| Spectral Resolution                  | / \~ 30,000                |
+--------------------------------------+-------------------------------+
| Temperature Range                    | 0.02 -- 4 MK                  |
+--------------------------------------+-------------------------------+

## Coronagraphic Images

Coronagraphic images are important for many aspects of the SDO Mission.
These images indicate the occurrence of CMEs (Section 3.1.2.2) and
reveal the structure of the corona and presence of solar wind variations
(Section 3.1.3.2). They also provide important information on changes in
the magnetic configuration of the corona associated with the solar cycle
(Section 3.2.1.2) and active region evolution (Section 3.2.2.2). The
coronagraphic images are of high priority to SDO. They provide critical
information on events and processes associated with several outstanding
questions. However, the STEREO mission (Section 7.1) is designed to
address many of these questions. Nonetheless, coronagraphic images from
SDO will be needed to span the duration of the mission and provide
information on the occurrence of CMEs and solar wind variations.

The SDO coronagraphic images will probably require two separate channels
in order to map the structure of the corona from 1.1 RSun to 15 RSun.
The stray light suppression requirements are such that it is not
practical for a single coronagraph channel to be used for the entire
range of heights. Further, a desired spatial resolution of 12 arcsec or
better at the inner edge of the FOV (1.1 RSun) requires an internally
occulted coronagraph system. The channel for the outer corona should
provide overlapping coverage with the other coronagraph channel. That
implies that this channel has to be an externally occulted system. The
requirement for halo CME detection is a high priority for the LWS
mission, in general, and SDO, in particular since these are CMEs that
affect Earth and have their origins from regions directly observed on
the disk with the Atmospheric Imaging instruments.

**Precision.** The polarization brightness should be measured with a
precision of about 10% to provide useful information on coronal
structures and their variations.

**Dynamic Range.** A dynamic range of about 10^3^ captures most of the
observed variations in the inner corona. A somewhat wider dynamic range
of about 10^4^ captures most of the observed variations in the outer
corona.

**Field Of View.** An inner height of 1.1 RSun is required to overlap
the FOV of the Atmospheric Imaging instruments and thus allow for
continuous tracking of events like CMEs from their initiation in disk
imagers through their development in the extended corona. An outer
height of about 15 RSun is needed in order to allow detection of halo
CMEs that are directed toward the Earth. Experience from LASCO
observations shows that beyond 15 RSun the propagation properties of
CMEs don't change much and so this outer limit would be acceptable.

**Time Cadence.** High time cadence will be required to study the
dynamics of coronal disturbances (CMEs, streamer blowouts, eruptive
prominences, etc.). SOHO/LASCO observations have shown that 1-min and
5-min cadences, for the inner and outer channels respectively, are
required and sufficient for accurate determinations of liftoff times,
helical motions, and speed profiles for different parts of coronal mass
ejections.

**Spatial Resolution.** Observations of CMEs should have a high enough
spatial resolution to discern their small-scale structures. Plumes and
polar rays are on the order of a few arc-minutes in width but they have
fine-scale structure on the sub arc-minute level that is probably
related to their magnetic field configurations. A 2048 x 2048 CCD
detector would give a spatial resolution of 6" for the inner coronagraph
and 30 arcsec for the outer coronagraph. This would give SDO twice the
resolution of LASCO C1 and a comparable resolution to C2; it is more
than adequate to meet the SDO science requirements.

**Table 4.8: Measurement Characteristics for the Coronagraph**

  ------------------------------- ------------------- -------------------
  Channel                         Inner               Outer

  Observable                      Polarized Intensity Polarized Intensity

  Precision                       10 %                10 %

  Dynamic Range                   10^3^               10^4^

  Time Cadence                    1 min               5 min

  Spatial Resolution              6 arcsec            30 arcsec

  Field of View                   1.1-3. RSun         2.5-15 RSun

  Spectral Range                  400-700 nm          400-700 nm
  ------------------------------- ------------------- -------------------

## Total Irradiance

The total solar irradiance must be accurately and precisely monitored to
determine the nature and source of the irradiance variations (Section
3.1.1.2). These observations are of highest priority to SDO but will
likely be obtained from both SORCE (Section 7.4) and GOES/NPOESS
(Section 7.5) during the SDO mission and are therefore not included as
part of SDO. If, however, it appears that these observations will not be
provided by these alternative sources then a Total Solar Irradiance
Monitor should be placed on SDO.

**Precision/Accuracy.** The absolute uncertainty (accuracy) translates
into uncertainties in the energy input to the terrestrial climate
system. The goal is maximum accuracy to minimize uncertainties in
climate models, and also to ensure that the instrument is properly
characterized to achieve the needed high repeatability. The actual solar
energy input to the radiometer depends on the entrance aperture, but is
typically of order 100 milliWatt. The ability to measure the change in
this signal due to changes in total solar irradiance depends, in part,
on the noise floor of the radiometer electronics. This ratio sets the
dynamic range and it must be sufficient to enable the required
repeatability and uncertainty, at a cadence of 1 observation per minute.

**Time Cadence.** The 5-minute oscillations affect total solar
irradiance, and the instrument must be capable of resolving these in
time.

**Duration.** Tracking the solar cycle variations, and the possibility
of longer term secular variations, is desirable.

**Completeness.** The instrument should be operated continuously to
maintain a calibrated time series, and to provide continuous inputs for
geophysical studies undertaken elsewhere within LWS.

**Field Of View.** A FOV slightly larger than the solar disk will
minimize pointing error problems.

**Table 4.9: Measurement Characteristics for a Total Solar Irradiance
Monitor**

  --------------------------------------- -------------------------------
  Observable                              Total Irradiance

  Precision/Accuracy                      0.01%

  Repeatability                           0.001% per year

  Time Cadence                            1 min

  Duration                                Solar Cycle

  Completeness                            continuous

  Field of View                           2°
  --------------------------------------- -------------------------------

##  Coronal Spectroscopy

Spectroscopic measurements in the extended corona are needed to for
characterizing the mechanisms that accelerate CMEs (Section 3.1.2.2) and
the solar wind (Section 3.1.3.2). These observations are important for
SDO but are limited in scope and would require significant resources.
Spectroscopic measurements in the corona up to about 2 R~Sun~ can be
accomplished with either an internally occulted coronagraph design (see,
e.g. LASCO-C1) or a very sensitive "wide angle" EUV spectrograph.
However spectroscopic measurements in the extended corona (beyond 1.5
R~Sun~), require an externally occulted telescope (see, e.g. SOHO/UVCS)
due to the rapid decrease with height of coronal emission line
intensities. A large aperture, externally occulted
coronagraph-spectrometer system would have the proper stray light
suppression and sensitivity to allow for the measurement of dozens of
faint coronal lines out to heliocentric distances of 10 R~Sun~. Such an
instrument would be capable of characterizing sites of magnetic
reconnection and shock formation in CMEs by observing high charge state
ions and non-thermal line broadening. It will also measure velocity
distributions of H, He, electrons, and heavy ions to determine the power
spectrum of resonant MHD waves that may be responsible for heating and
acceleration. Helical 3D velocities can be determined from Doppler
shifts and Doppler dimming. Coronal source regions for CME and solar
wind plasma can be determined from abundance determinations.

While advanced coronagraph-spectrometers would be desirable, we felt
that such instruments could not be accommodated with the present SDO
spacecraft resources. However, if an opportunity does arise for flying
this type of instruments it would be a valuable complement to the SDO
Mission.

**Table 4.10: Measurement Characteristics for a UV Coronagraph
Spectrometer**

+-----------------------------------+----------------------------------+
| Observable                        | Line Profiles                    |
+-----------------------------------+----------------------------------+
| Precision/Accuracy                | Intensity 15 %                   |
|                                   |                                  |
|                                   | Width 10%                        |
|                                   |                                  |
|                                   | Velocity 5 km/s                  |
+-----------------------------------+----------------------------------+
| Dynamic Range                     | 10^5^                            |
+-----------------------------------+----------------------------------+
| Time Cadence                      | 10 min                           |
+-----------------------------------+----------------------------------+
| Spatial Resolution                | 4"                               |
+-----------------------------------+----------------------------------+
| Field of View                     | 4\~30"  2000"                   |
+-----------------------------------+----------------------------------+
| Range of View                     | 1.1-10 RSun                      |
+-----------------------------------+----------------------------------+
| Spectral Resolution               | / \~ 10,000                   |
+-----------------------------------+----------------------------------+
| Spectral Range                    | 28-140 nm                        |
+-----------------------------------+----------------------------------+

## Heliometry

Measurements of small changes in the solar radius and limb shape
(heliometry) are needed to determine how and where the emergent solar
luminosity is gated and stored (Section 3.1.1.2). These measurements are
important for SDO but should not require the resources of an additional
instrument. The Helioseismic Images from SDO will be important for
observing solar "acoustic" radius fluctuations. The Photometric Images
may be obtained in a manner that also allows for heliometry. The
magnitude of the radius fluctuation, compared to the irradiance change
during a solar cycle contains important information on where and how
energy is stored. If *W* is the ratio of relative radius and irradiance
changes, then physical models predict a wide range of values for *W*.
Depending on the mechanism and depth of the interior perturbation
estimates of *W* range from 2×10^-4^ to 7.5×10^-2^. Given a solar cycle
irradiance change of about 10^-3^, a desirable goal for SDO is to
achieve a radius sensitivity of at least 10 milliarcsec on solar
rotation time-scales. Such measurements will allow us to clearly
discriminate between competing solar cycle luminosity variation models.

# **Potential Instruments and Allocation of Resources**

We have examined several generic instruments for inclusion in the SDO
payload to estimate the total mass, data rate, power and volume required
to accommodate them. These estimates are included in Table 5. Several
assumptions have been made in arriving at these estimates. The masses do
not include electronics boxes, mounting, radiators, etc. The data rates
assume the use of image compression. The range of masses, data rate and
volume for the Atmospheric Imaging Array is due to the number of
possible telescope tubes.

![](media/image17.png){width="6.25in" height="0.10416666666666667in"}

**Table 5.** **Allocation of Resources**

+---------------+-----------+---------------+---------+----------------+
| Instrument    | Mass      | Data Rate     | Power   | Volume         |
|               |           |               |         |                |
|               | (kg)      | (Mbps)        | \(W\)   | (cm^3^)        |
+---------------+-----------+---------------+---------+----------------+
| HMI           | 40        | 25            | 60      | 90x40x25       |
+---------------+-----------+---------------+---------+----------------+
| Atmospheric   | 40-70     | 20-50         | 45      | 100x15x30      |
|               |           |               |         |                |
| Imaging Array |           |               |         | 100x45x60      |
+---------------+-----------+---------------+---------+----------------+
| EUV SIM       | 20-30     | \<1           | 45      | 44x48x21       |
+---------------+-----------+---------------+---------+----------------+
| Coronagraph   | 30        | 1             | 35      | 135x17x17      |
+---------------+-----------+---------------+---------+----------------+
| UV/EUV        | 40-60     | 15-30         | 40      | 160x60x30      |
|               |           |               |         |                |
| Spectrometer  |           |               |         |                |
+---------------+-----------+---------------+---------+----------------+
| Photometric   | 30        | 5             | 50      | 100x30x30      |
|               |           |               |         |                |
| Mapper        |           |               |         |                |
+---------------+-----------+---------------+---------+----------------+
| Vector        | 10+       | 5             | 20+     | 90x40x40       |
|               |           |               |         |                |
| Magnetograph  |           |               |         |                |
+---------------+-----------+---------------+---------+----------------+
| Total         | 210-270   | 62-117        | 295     |                |
+---------------+-----------+---------------+---------+----------------+

![](media/image17.png){width="6.25in" height="0.10416666666666667in"}

# **Mission Concept**

This section details the spacecraft, launch vehicle, ground system and
data system for SDO. These items have been discussed as being sufficient
to support the science as defined in this Science Definition Team
report. Future adjustments and variations will undoubtedly occur, as the
mission concept matures into the design and development phase.

The science of the Solar Dynamics Observatory optimally will be
performed on a spacecraft that allows nearly continuous observations of
the Sun and a scientific data rate well in excess of 100 Megabits per
second. These two requirements drive the orbit and spacecraft
specification and the definition of the SDO Mission. Nearly continuous
observations can be obtained from other orbits, such as a low Earth
orbit (LEO), but a LEO orbit would require on-board storage of large
volumes of scientific data pending downlink. The large data rate, along
with the strict limitations on on-board storage capacity, result in an
effective requirement of continuous contact. An inclined geosynchronous
(GEO) orbit will allow nearly continuous observation of the Sun, and can
downlink data to a single dedicated ground station.

The mission will launch into a geosynchronous transfer orbit (GTO) and
then use an apogee kick motor (AKM) or other orbital transfer mechanism
to boost the spacecraft into geosynchronous orbit. The spacecraft will
be three-axis stabilized and will maintain solar pointing with
occasional maneuvers to unload the momentum accumulated in the reaction
wheels. Twice a year SDO will undergo "eclipse seasons," which will last
2-3 weeks, with a maximum Earth eclipse period of approximately 70
minutes. Spacecraft maneuvers, eclipses, and occasional ground system
outages will interrupt the scientific observations.

Missions of the Living With a Star program are designed to perform
investigations of the long-term variations of the Sun-Earth connected
system. The SDO mission will be designed for a 5-year baseline with
expendables to last an additional five years of an extended mission.

Table 6.1 illustrates the mass breakdown used in a preliminary
definition study for SDO. Compared are the mass estimates for a
custom-built spacecraft and a spacecraft from the Rapid Spacecraft
Development Office (RSDO) catalog. An instrument module mass of 200 kg,
an Apogee Kick Motor (AKM) mass of 668 kg, and the RSDO catalog mass
estimates were used in the study calculations. Modifications of a
custom-built bus would include a lighter battery, a lighter structural
composition of the bus, and adjustments to the propulsion and
communication systems.

Table 6.1: SDO Preliminary Study Bus and Payload Mass Breakdown (kg)

  ----------------------- --------------------- --------------------------
                          Custom                RSDO

  ACS                     54                    42

  Power                   60                    92

  Harness                 15                    19

  RF comm.                35                    72 w/ C&DH

  C&DH                    11                    

  Propulsion              10                    61

  Hydrazine               20                    20

  AKM Stage               10                    8

  Balance Wt.             10                    7

  Thermal                 60                    

  Mechanical              34                    114 w/ therm

  Subtotal                319                   434

                                                

  Instruments             200                   200

  AKM                     668                   668

  Total                   1187                  1302
  ----------------------- --------------------- --------------------------

## Orbit Selection

The orbit of the Solar Dynamics Observatory will allow a high science
data rate (160 Mbps) and nearly continuous contact via a single
dedicated ground station. This ground station can be built and operated
at a fraction of the cost of a mission that would rely on existing
ground contact networks. The ground track of a geosynchronous orbit with
an inclination of 28.5 degrees orbit is shown in Figure 6.1, projected
onto 102 degrees Earth longitude.

![](media/image18.jpeg){width="2.9965277777777777in"
height="3.1354166666666665in"}

**Figure 6.1**: Ground track of geosynchronous orbit with an inclination
of 28.5 degrees.

The disadvantages of this orbit include launch and orbit acquisition
costs (relative to LEO) and eclipse (Earth shadow) seasons twice
annually. During these 2-3 week eclipse periods, SDO will experience a
daily interruption of solar observations. The maximum duration of these
interruptions is 70 minutes, during which solar observations will be
interrupted. The spacecraft attitude control system (ACS) and power
system must recover from each of these eclipse periods, which may
involve a longer duration of the interruption of the scientific
observations. Three lunar shadow events also occur annually, with
durations of approximately 30 minutes. The total duration of these
interruptions is approximately 45 hours annually. Eclipse and shadow
periods are shown in Figure 6.2.

![](media/image19.jpeg){width="2.995833333333333in"
height="1.8194444444444444in"}

**Figure 6.2**: Annual periods of Earth and lunar shadow (minutes per
day).

![](media/image20.jpeg){width="2.9965277777777777in"
height="2.526388888888889in"}

**Figure 6.3**: Illustration of a launch to geosynchronous transfer
orbit (GTO) and orbit circularization with an Apogee Kick Motor.

SDO\'s orbit can be achieved with a launch into a geosynchronous
transfer orbit (GTO), with an apogee kick motor (AKM) to circularize the
orbit at geosynchronous altitude (Figure 6.3. The total mass to be
lifted to GTO includes the spacecraft and instruments as well as the
AKM. Figure 6.4 compares the mass capabilities to GTO of various launch
vehicles, using an AKM such as the Star-30E and a spacecraft mass of 634
kg. The Delta 2925 meets the launch mass criteria, but other vehicles
such as the Delta Lite and the Taurus would require use of a much
lighter spacecraft and AKM, or would require the consideration of an
alternate orbit profile. During the design of the SDO mission,
contingency and mass margins as well as mass and spin balance
requirements must be taken into consideration; these items will
constrain the total mass able to be accommodated.

![](media/image21.jpeg){width="2.9972222222222222in" height="2.875in"}

**Figure 6.4**: Mass margins for various launch vehicles. The total mass
of the spacecraft and AKM is shown in gray. The launch vehicle
capabilities (in mass to GTO) are shown in blue. A Delta 2925 has a
margin of nearly 500 kg.

## Attitude Control System

Most of the requirements of the SDO mission can be met by a
geosynchronous spacecraft from the RSDO catalog, with some necessary
adjustments. The standard geosynchronous ACS would include a
startracker, course and fine sun sensors, gyroscopic sensors and
reaction wheels. Four reaction wheels in a pyramid configuration will
allow efficient unloading of accumulated momentum and will provide
redundancy for a long-term mission design. Attitude control system
considerations include the adaptation of a geosynchronous spacecraft to
maintain a sensitive three-axis stabilized solar pointing. These
modifications would include a measurement from a guidescope in the
instrument package which will link to the ACS to meet the stability
requirements required by the instrument package, and the use of
low-jitter reaction wheels (such as the SMEX Lite IRWA) to restrict the
jitter introduced to the system.

The spacecraft ACS requirements which were discussed were as follows:

**Jitter:** The instruments with higher resolution place restrictions on
the amount the pointing can vary during the collection of data. The HMI
and AIA require that the jitter in both pitch, yaw and roll not vary by
more than a fraction of a pixel over the interval of time required to
collect the data. Therefore, the image must be stabilized to .25 arcsec
(3 sigma) over a few seconds (determined by the image collection time)
in pitch and yaw. It is likely that the spacecraft ACS will not be able
to satisfy this requirement, and will have a pitch/yaw jitter near 5
arcsec over 45 seconds. The instruments would incorporate image
stabilization using data from the guidescope. Because the image
stabilization system cannot correct for jitter in the roll axis, the
roll jitter must be held to within 50 arcsec (3 sigma) over a period of
45 seconds.

**Accuracy/Stability:** It is estimated that an image stabilization
system cannot function properly at angles greater than 10-15 arcsec;
therefore, if the spacecraft introduces 5 arcsec jitter, the greatest
deviation from the overall pointing must be within 5-10 arcsec (3 sigma)
in pitch and yaw. A coronagraph requires a similar accuracy in pitch and
yaw; based on the field of view and the design of the coronagraph, the
Sun must be centered behind the occulting disk to within a desirable
tolerance. The accuracy and stability of the roll of the spacecraft is
specified in relative terms; the HMI requires a stable viewing angle,
though the angle does not necessarily have to be aligned at solar North.

**Attitude Knowledge:** Because of its restricted field of view, the
spectrometer would require a knowledge of a fraction of the instrument
resolution. Therefore, the knowledge in pitch and yaw would be set at a
fraction of an arc second (which can be provided by the ACS or by the
instrument package). The HMI requires a roll knowledge of 30-60 arcsec
(3 sigma), corresponding to a fraction of a pixel at the solar limb.

SDO will have several modes of operation including: **Science mode** -
3-axis zero-momentum control, pointing roll axis toward the Sun, run
wheels in bias speed, using measurement from guide telescope for pitch
and yaw and star tracker measurement for roll; **Calibration mode** -
includes possible offsets and maneuvers for instrument calibration;
**Safehold mode -** uses CSS and wheel to point solar array normal to
the Sun, similar to Sun acquisition; **Delta V mode -** for orbit
maintenance; and **Delta H mode** - to unload momentum using propulsion

These operational modes may need to include periods of recovery from
Earth shadow periods, and maneuvers to assist in instrument calibration.

## Data and Communication System

Commands to the science instruments will be scheduled for 1 contact per
day. The high rate science data downlink, at 160 Mbits/sec, and the low
rate housekeeping data will be continuously maintained between the
ground and the spacecraft.

![](media/image22.jpeg){width="3.0in" height="2.25in"}

**Figure 6.5**: SDO CD&H system.

## Spacecraft Power

The instruments were assumed to require a combined power of 250 Watts,
with 50 Watts for survival. (The estimates were based on 24% efficient
Triple Junction GaAs cells, with 2 spacecraft wings, each with solar
array area of 3.2 square meters, including losses from UV and energetic
particle radiation, thermal cycling, assembly losses, and losses from SA
to battery, battery to load, & SA to load.) The estimates of the
spacecraft power requirements are listed in Table 6.2

**Table 6.2. Spacecraft bus eclipse power (W)**

  ----------------------- --------------------- --------------------------
                          Custom                RSDO

  ACS                     84                    53

  Power                   21                    52

  RF comm.                100                   41

  C&DH                    35                    12

  Propulsion              5                     12

  Thermal                 50                    91

  Harness                 23                    

  Total                   295                   284
  ----------------------- --------------------- --------------------------

## Instrument Module

An instrument module must accommodate the SDO instruments and provide an
acceptable launch environment. The instrument module consists of a
mounting structure, instrumentation, cables, heaters and radiators. The
mounting structure shall be designed to serve as an optical bench to
allow for mounting and alignment of the sensitive SDO instruments.
Figure 6.6 shows an illustration of the launch configuration of SDO,
including the instrument module, AKM, and solar panels in a 9.5-foot
Delta fairing.

![](media/image23.jpeg){width="2.9944444444444445in" height="1.85in"}

**Figure 6.6**: SDO instrument module launch configuration

## Ground System

The SDO mission utilizes an inclined geosynchronous orbit to take
advantage of a single dedicated ground station. This ground station
would ideally be located in a location with minimal effects of rain
attenuation. X-band frequencies are becoming less available to missions
in the space sciences; it is likely that the primary downlink of SDO
data will use Ka- or Ku-band frequencies. For space research 37 GHz is
currently allocated, with a possibility of allocation at 21 or 27 GHz.
The Ku and Ka frequencies suffer greater rain attenuation than X-band,
but the antennae are much smaller and can be built inexpensively, and
backup stations can be considered to meet data completeness
requirements.

Data latency (including delivery to final users) requirements may drive
ground system considerations; several of the LWS partners may require
latency significantly less than an hour. Additionally, the high science
data rate indicates that only the health and safety data would be able
to be stored on board in the event of a contact interruption. The data
gathering architecture will be selected to maximize long continuous
streams of valid data. The high data completeness requirement will also
be a design driver for the ground system and the downlink margins.

During the nominal mode of spacecraft operation, the ground system can
operate as a semi-autonomous system, allowing standard operations to
proceed on a 40 hour per week schedule. The geosynchronous orbit and
onboard safing systems make the use of a semi-autonomous ground system a
low risk and low cost way of meeting the mission requirements.

## Mission and Science Operations

Most of the SDO planned instruments are full-disk instruments and the
observations of the mission are not "event-driven," (e.g. responses to
flares or CMEs). There is considerable value in collecting observations
as routinely, and under as stable observing conditions and repeatable
operation scenarios, as possible. Thus, the nominal science mode of
operation can be accomplished with daily command loads similar to those
used by the TRACE mission. Instead of near-real-time commanding, daily
command loads compiled by the PIs will be sent by the Mission Operations
Team.

![](media/image24.jpeg){width="3.0in" height="2.25in"}

**Figure 6.7**: SDO Mission Operations and Ground System Network

The Mission Operations Center, Science Operations Center and Ground
System could be distributed or centralized. Centralization requires less
transport of data over the Internet and greater communication between
the mission operations and science teams. However, the cost of data
transport at the time of the SDO mission is unknown, and the cost
advantage gained from less transport requires further study. Moreover,
the cost of temporary storage of SDO data at the ground system site (to
compensate for temporary network and distribution failures) is dependent
on the future price of bulk storage.

The ground station will be able to strip the science data packets from
the downlink stream and send them to the storage sites, either at PI
institutions or at a centralized data service. The packets will be
assembled into Level Zero data sets and may receive additional
processing prior to being made available for distribution.

![](media/image25.jpeg){width="2.9930555555555554in"
height="2.0131944444444443in"}

**Figure 6.8**: SDO Distributed Data System

Data generated by LWS missions are to be free and publicly available for
analysis. Proper support of an open data policy requires the provision
of the development of software and analysis tools and calibration
algorithms by the instrument team. These services will be provided
through the Principal Investigations.

# **Concurrent Observations**

Several space-based and ground-based instruments will be operational at
times during the SDO mission. Observations from some of these
instruments \[particularly those on other LWS missions\] will, at the
very least, complement those from the SDO instruments. In some cases
(e.g. total solar irradiance) these observations may replace those that
otherwise would need to be obtained with SDO. The following sub-sections
describe the instruments as they were specified at the time of this
report. Future changes in these specifications may influence the final
choices for SDO instruments. Table 7 lists the facilities along with
their anticipated dates of operation and key measurements.

**Table 7: Concurrent Observations**

+-----------------+----------------------------------+-----------------+
| **Facility**    | **Measurements**                 | **Date**        |
+-----------------+----------------------------------+-----------------+
| STEREO          | Atmospheric Images               | 2006-           |
|                 |                                  |                 |
|                 | Coronal Images                   | 2011            |
+-----------------+----------------------------------+-----------------+
| Solar-B         | Atmospheric Images               | 2005-           |
|                 |                                  |                 |
|                 | Magnetograms                     |                 |
|                 |                                  |                 |
|                 | UV/EUV Spectra                   |                 |
+-----------------+----------------------------------+-----------------+
| Solar           | Atmospheric Images               | 2010            |
|                 |                                  |                 |
| Probe           | Coronal Images                   | 2015            |
|                 |                                  |                 |
|                 | Magnetograms                     |                 |
+-----------------+----------------------------------+-----------------+
| SORCE           | Total Irradiance                 | 2002-           |
|                 |                                  |                 |
|                 | Spectral Irradiance              | 2007            |
+-----------------+----------------------------------+-----------------+
| GOES/           | Total Irradiance                 | 2010-           |
|                 |                                  |                 |
| NPOESS          | Spectral Irradiance              |                 |
+-----------------+----------------------------------+-----------------+
| SOLIS           | Magnetograms                     | 2001-           |
|                 |                                  |                 |
|                 | Spectra                          | 2025            |
+-----------------+----------------------------------+-----------------+
| ATST            | Magnetograms                     | N/A             |
+-----------------+----------------------------------+-----------------+
| Solar           | Atmospheric Images               | N/A             |
|                 |                                  |                 |
| Sentinels       | Helioseismic Images              |                 |
+-----------------+----------------------------------+-----------------+
| Solar           | Atmospheric Images               | 2009-           |
|                 |                                  |                 |
| Orbiter         | Coronal Images                   |                 |
|                 |                                  |                 |
|                 | Magnetograms                     |                 |
|                 |                                  |                 |
|                 | UV/EUV Spectra                   |                 |
+-----------------+----------------------------------+-----------------+
| FASR            | Coronal                          | N/A             |
|                 |                                  |                 |
|                 | Magnetograms                     |                 |
+-----------------+----------------------------------+-----------------+

## STEREO

STEREO (Solar-TErrestrial RElations Observatory) will provide new
perspectives on the structure of the solar corona and CMEs by moving
away from our customary Earth-bound vantage point and by using two
spacecraft to provide information on three-dimensional structure. Both
spacecraft carry a suite of instruments including: two coronagraphs
(covering the range 1.25 - 4 R~Sun~, and 2-15 R~Sun~), an extreme
ultraviolet imager (full-disk, 1 arcsec pixels), a heliospheric imager
(an externally occulted coronagraph that can image the heliosphere from
12 R~Sun~ to beyond Earth's orbit), an interplanetary radio burst
tracker, *in situ* particle and field detectors, and a plasma
composition experiment.

This mission is designed: to further our understanding of the origins
and consequences of CMEs, to determine the processes that control CME
evolution in the heliosphere, to discover the mechanisms and sites of
solar energetic particle acceleration, to determine the 3D structure and
dynamics of coronal and interplanetary plasmas and magnetic fields, and
to probe the solar dynamo through its effects on the corona and the
heliosphere.

The STEREO Mission spacecraft are expected to be launched in December
2005. The two spacecraft will slowly drift apart in ecliptic longitude
with a total separation of 45° after the first year and 90° after the
second year. The mission is expected to last two years at minimum and,
more likely, five years.

STEREO will overlap with SDO and provide coronagraphic images like those
needed for SDO. STEREO's lifetime, however, is shorter than SDO's and
during parts of the STEREO mission the STEREO instruments will be
viewing solar regions far removed from the SDO observations. The STEREO
coronagraphs will complement the measurements made from SDO but cannot
be used to replace them.

## Solar-B

Solar-B is a Japanese mission proposed as a follow-on to the highly
successful Japan/US/UK *Yohkoh* (Solar-A) collaboration. The mission
consists of a coordinated set of optical, EUV and X-ray instruments that
will investigate the interaction between the Sun\'s magnetic field and
its corona. The result will be an improved understanding of the
mechanisms that give rise to solar magnetic variability and how this
variability modulates the total solar output and creates the driving
force behind space weather.

 

The spacecraft will accommodate three major instruments: a large solar
optical telescope (SOT), an X-ray telescope (XRT), and an EUV Imaging
Spectrograph (EIS). All three major instruments will give extremely high
resolution observations of a field of view on the Sun that is restricted
to an active region spatial scale. The SOT includes focal plane
instruments that will provide longitudinal magnetograms with 1-5G
sensitivity and vector magnetograms with 30-50G sensitivity to
transverse fields. The XRT provides atmospheric images at wavelengths
from 2 to 6 nm. The EIS provides UV/EUV spectra with a resolving power
of about 10,000 over wavelength ranges from 17-21 nm and 25-29 nm
(covering temperatures from 10^5^ to 10^7^ K but with only one strong
emission line below 10^6^ K).

The Solar-B spacecraft is scheduled for launch in the fall of 2005. It
will be placed in a polar, sun-synchronous orbit about the Earth. This
will keep the instruments in continuous sunlight, with no day/night
cycling for nine months each year. Solar-B will address some of the same
problems that SDO will address but with higher spatial resolution and a
smaller field of view. The combination of vector magnetograms,
atmospheric images, and UV/EUV spectra should provide the measurements
needed to answer many of the outstanding questions concerning the
initiation of flares and CMEs.

## Solar Probe

The Solar Probe mission is an unprecedented exploration of the inner
heliosphere, which will achieve unique science by flying over the pole
of the Sun and as close to the Sun\'s surface, through the solar corona,
as is technologically feasible today. It will first travel to Jupiter
for a gravity assist, leave the ecliptic plane, fly over the Sun\'s
poles to within 8 solar radii, and reach perihelion over the equator at
4 solar radii. A unique aspect of the Solar probe orbit is that the
trajectory is orthogonal to the Sun-Earth line during perihelion passage
so that there is continuous radio contact throughout the flyby. Two
perihelion passes are planned, the first near the 2010 solar maximum and
the second near the 2015 solar minimum. This orbit ensures that the
mission will probe both the high speed solar wind streams and the
equatorial low-speed streams.

The results from SOHO and Ulysses have focused our understanding of the
solar corona to the point where *in situ* measurements are now necessary
for further progress. Both imaging and *in situ* measurements will
provide the first three-dimensional view of the corona, high spatial and
temporal resolution measurements of the plasma and magnetic fields, as
well as helioseisology and magnetic field observations of the solar
pole.

The Solar Probe Nadir-Viewing Imagers will provide the only full view of
the Sun\'s Poles, imaging both the North and South Pole within one day.
The Solar Probe Magnetograph/Helioseismograph will provide
out-of-the-ecliptic observations of the polar magnetic field and polar
sub-surface flow patterns essential to answering fundamental questions
related to the solar dynamo and the origins of the solar cycle.

The observations of the solar corona and solar wind that Solar Probe
will provide are critical to understanding fundamental processes that
can be obtained in no other way, and will provide a set of measurements
that complement, but are distinct from, the SDO observations. Finally,
the Solar Probe observations of the polar magnetic field and polar
sub-surface flow patterns are essential to answering fundamental
questions related to the solar dynamo and the origins of the solar
cycle, which are central to the SDO primary scientific objectives, but
which cannot be obtained with the SDO spacecraft.

## SORCE

SORCE (SOlar Radiation and Climate Experiment) is a program within
NASA's Office of Earth Science (OES) for measuring solar irradiance. The
solar-pointed SORCE spacecraft carries five instruments to measure both
total and spectral solar irradiances. These are the Total Irradiance
Monitor (TIM), Spectral Irradiance Monitor (SIM), two identical Solar
Stellar Irradiance Comparison Experiments (SOLSTICE) and the X-ray
Photometer System (XPS).

The TIM measures total solar irradiance using electrical substitution
radiometers. The observations have an uncertainty goal of 0.01% and a
long-term repeatability of 0.001% per year.

The SIM measures solar UV, visible and IR spectral irradiance from 200
nm to 2000 nm with spectral resolution ranging from a few nm at UV
wavelengths to tens of nm at IR wavelengths. It uses a prism for
wavelength dispersion and a bolometer and diodes for signal detection.
The SIM spectral irradiances have uncertainties of 0.03% and long-term
repeatabilities of 0.01% per year.

The two SOLSTICE instruments measure the solar UV spectral irradiance
from 120 to 300 nm with spectral resolution of about 0.1 nm, using
grating spectrometers that have the capability also to observe bright
blue stars for calibration tracking. The SOLSTICE UV spectral irradiance
uncertainties are in the range 3% to 6% and repeatabilities are 0.5%.

The XPS is a bank of broadband X-ray photometers in the range 1 -- 31 nm
with spectral bands 5 to 10 nm, uncertainty of 12% and repeatability of
3% per year.

SORCE will be launched in mid 2002, with expected mission duration of 5
years. It will overlap with ACRIMSAT, which has provided total solar
irradiance data since 2000, thereby extending the continuous record of
total solar irradiance that commenced in late 1978. NASA OES plans to
continue the solar irradiance measurements with a follow-on solar
irradiance mission in the time frame of 2006-2011 that measures total
solar irradiance (e.g., TIM) and spectral irradiance from 200 to 2000 nm
(e.g., SIM).

## GOES/NPOESS

GOES (Geosynchronous Operational Environmental Satellites) and NPOESS
(National Polar-orbiting Operational Environmental Satellite System) are
satellite systems deployed by NOAA to monitor the environment.

Solar irradiance will be monitored by NPOESS. Measurements of total
solar irradiance and of the solar spectral irradiance in the wavelength
range from 200 to 2000 nm with accuracies and precisions similar to
those of the TIM and SIM instruments on SORCE are specified. The solar
irradiance measurements are designated for flight on one of the three
NPOESS spacecraft. Since the priority for the solar measurements is
relatively low among other NPOESS measurements, failure of the
spacecraft would not trigger an immediate replacement of the solar
instruments. This means that gaps may occur in the total solar
irradiance measured by NPOESS, expected to commence around 2010,
depending on existing resources.

Measurements of solar EUV radiation in five broad bands in the range 10
to 130 nm are planned to be made from future GOES platforms commencing
in late 2002, depending on the health and status of existing GOES
spacecraft. The EUV broadband fluxes are recorded every 10 seconds, with
an uncertainty of 10% and a repeatability of 5% over 7 years. The
instrument uses diffraction gratings to disperse the light, thin-film
filters to further remove unwanted wavelengths, silicon diode detectors
to collect the light and tantalum shielding to eliminate effects from
radiation. Two GOES spacecraft are planned to be operational at any one
time providing redundancy and cross-calibration for the EUV sensor. With
the launch of each new GOES EUV Sensor every (2-5 years), a new
calibration will be applied to the data set.

In addition to these major resources, at least two European programs are
planning to measure solar irradiance during the next decade. Three solar
instruments (ACES, SOVIM and SOLSPEC) will measure the total solar
irradiance and the solar spectrum from the EUV to the IR. The solar
package is presently scheduled as part of the *Columbus* module of the
International Space Station, to be launched during 2004-2005 with a
priority of solar minimum observations. PICARD, a small spacecraft
carrying instruments to measure the total solar irradiance and solar
diameter, is under development in France, for launch by CNES around 2006
with a 2-3 year mission duration.

These measurements may facilitate the calibration of the SDO EUV
Spectral Irradiance measurements but will not have the spectral
resolution or coverage to replace them.

## SOLIS

SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a
ground-based project of NSO to provide regular observations of the Sun
for at least 25 years. It will replace many of NSO\'s current synoptic
facilities. The primary science objectives are to increase understanding
of solar activity and its effects on earth by means of observations of
the Sun\'s full vector magnetic field and the dynamics and evolution of
solar changes related to the magnetic field. Aside from basic research
on the solar activity cycle, the observations will also be used as
inputs to test models that are alleged to be able to forecast activity.

The first SOLIS facility is nearing completion and the recent NAS/NRC
report "Astronomy and Astrophysics in the New Millennium" recommends
building two additional systems to be located at longitudes different
from the US. This would increase the average 24-hour duty cycle from
about 30% to about 80%. The US system includes three instruments: a
vector spectromagnetograph (VSM), a full-disk monochromatic imager, and
an integrated sunlight spectrometer for sun-as-a-star spectroscopy.

The VSM is the most unique instrument and would be the common network
instrument. The VSM is a 50-cm telescope and a high-resolution
spectrograph. It can provide full-disk vector magnetograms in about 15
minutes with a polarimetric noise level of about 3 x10^-4^ using one
arcsecond pixels. It will also provide high-sensitivity photospheric and
chromospheric line-of-sight component magnetograms as well as He I
dynamics images.

The monochromatic imager provides intensity and Doppler images in a
number of solar spectrum lines using one arcsecond pixels. Quick-look
SOLIS data will be available on the Web within 30 minutes and more
accurately reduced data within 24 hours. Special campaigns and
user-proposed programs can be interleaved with the regular synoptic
observations. The synoptic data will be openly available.

SOLIS and SDO will complement each other in a number of ways.
Lightweight space magnetographs will be filter based and it will be very
useful to compare results from such an instrument with the higher
spectral resolution SOLIS measurements to seek out systematic errors in
both types of observations. Similarly, the higher spatial resolution
space observations will permit a study of how ground-based observations
are degraded by terrestrial seeing. In case a vector magnetograph is not
flown on SDO, the SOLIS vector observations will provide an obvious
enhancement. The same holds true for the chromospheric magnetograms.
SOLIS will provide regular monochromatic measurements of intensity and
dynamics in the cool solar atmosphere that will be valuable for use with
the SDO high temperature measurements. Perhaps the best complementarity
would be one that we cannot predict, namely, some SDO discovery that
stimulates follow-up observations with SOLIS, or vice versa.

## ATST

ATST (Advanced Technology Solar Telescope) is a proposed ground-based
telescope facility designed for high-resolution studies of the Sun. The
proposal is for a 4-meter telescope operating in the visible and
infrared (0.3 to 35 microns) with very high resolution (0.1arcsec or
better) and a large photon flux for sensitive polarimetry. The facility
will have the spatial resolution and sensitivity to study the ubiquitous
weak magnetic field elements in the photosphere and measure magnetic
fields in the corona. It will have the capability to examine waves in
magnetic flux tubes to test models of chromospheric and coronal heating.
It will have a 5 arcmin field of view that will allow studies of active
region evolution and the initiation of flares and CMEs.

ATST will address some of the same problems that SDO will address (e.g.
small-scale magnetic elements) but with higher spatial resolution,
smaller field of view, and less complete coverage. The proposal suggests
that operations begin in about 2008.

## Solar Sentinels

The Solar Sentinels will consist of a fleet of spacecraft distributed
throughout the heliosphere. They will help to improve the accuracy of
models of CMEs and other solar wind transients. They will resolve
geoeffective solar wind structures and map them back to solar features.
They will search for the locations and mechanisms of energetic particle
acceleration, and provide tomographic images of the Sun. When fully
deployed, the Sentinels will increase the lead-time and accuracy of
geospace forecasts.

The Inner Heliospheric Sentinels will make *in situ* observations of the
heliospheric vector magnetic field, the solar wind plasma properties,
and the spectrum of high-energy particles. They will make remote
measurements of the propagation of interplanetary shocks by tracking
radio bursts. A Farside Sentinel will also provide EUV images of the
solar corona along with photospheric magnetograms. Radio occultations
will be employed along with these images and those from SDO to identify
the birthplace of transients. Helioseismic measurements will be made in
conjunction with those from SDO to follow the development of structures
within the Sun.

The fleet will consist of four Inner Heliospheric Sentinels in
heliocentric orbits ranging between 0.5 and 0.95 AU, a FarSide Sentinel
in a 1 AU orbit opposite Earth, on the far side of the Sun, and a single
L1 Sentinel to provide solar wind input information to the geospace
components. These elements will work together to track solar
disturbances as they evolve and transit the inner heliosphere. The inner
heliospheric sentinels are spinning satellites. The FarSide Sentinel is
three-axis stabilized.

Solar Sentinels will supplement SDO observations to provide continuous
and whole surface imaging of the photosphere and solar corona, hence
allow the study of evolution of solar active regions. The observations
complement those from SDO but with little or no redundancy.

## Solar Orbiter

The Solar Orbiter was selected by ESA at the end of 2000 as a "flexi"-
mission, for launch in the 2008-2013 time frame. The key mission
objectives of the Solar Orbiter are: (a) to study the Sun from close-up
(45 solar radii, or 0.21 AU), and (b) to provide images of the Sun\'s
polar regions from heliographic latitudes as high as 22 degrees during
the nominal mission, and over 30 degrees during the extended mission.

Solar Orbiter's unique heliosynchronous, near-Sun trajectory will allow,
in conjunction with concurrent high-resolution remote sensing
observations, *in situ* investigations of the energetic particle
environment in close proximity to different source regions, such as
active regions, flare locations, CMEs and associated shocks. Solar
Orbiter's unique high-latitude trajectory will allow it to determine the
longitudinal extent of CMEs and provide, in conjunction with SDO and
ground-based observatories, full coverage of the entire Sun over 360º in
longitude.

The potential payload includes two instrument packages: the Heliospheric
*in situ* instrument package and the Solar Remote sensing instrument
package. The Heliospheric *in situ* instruments include: a solar wind
analyzer, radio and plasma wave analyzers, a magnetometer, energetic
particle detectors, an interplanetary dust detector, a neutral particle
detector, and a solar neutron detector. The Solar remote sensing
instruments include: an EUV full-Sun and high resolution imager, a
high-resolution EUV spectrometer, a high-resolution visible-light
telescope and magnetograph, an EUV and visible-light coronagraph, and a
radiometer.

We hope that the interested parties within ESA will do everything
possible to ensure a launch in a timely manner, i.e. in 2009 or soon
after. From recent solar and heliospheric physics missions, e.g. SOHO,
*Yohkoh*, TRACE, and *Ulysses*, we have learned that by far the best
scientific return from missions is through efficient coordination. A
launch of Solar Orbiter in 2009, or soon after, would provide a
significant overlap with SDO. The combination of Earth-orbit
high-resolution observations from SDO with the close-encounter and polar
observations from Solar Orbiter would allow an invaluable, thorough
analysis of many aspects of solar activity and its influence on the
Earth.

##  FASR

FASR (Frequency Agile Solar Radiotelescope) will provide radio
observations of coronal magnetic fields that complement optical/IR
measurements of photospheric and chromospheric fields, and in this
regard promises to be an important supporting instrument for SDO studies
of the magnetic field in the corona. FASR has been rated highly by the
NRC panel on the future of ground-based solar astronomy, and recommended
as a moderate-sized initiative by the decadal NRC Astronomy and
Astrophysics Survey Committee. The telescope will be a solar-dedicated
instrument providing excellent images of the full Sun with arcsecond
spatial resolution at a wide range of frequencies nearly simultaneously,
with both targeted research and synoptic capabilities.

Radio observations are able to measure magnetic fields due to the
gyroresonance effect: electrons spiraling in the coronal magnetic fields
provide opacity at radio wavelengths at low harmonics of the electron
gyrofrequency, 2.8  10^-3^ *B* GHz where *B* is measured in Gauss. A
given observing frequency and sense of circular polarization is
sensitive to a single value of magnetic field strength; by changing
frequencies FASR will be sensitive to magnetic fields in the range 100 -
2000 G. Surfaces of constant magnetic field strength show up in radio
maps at the appropriate frequency as bright regions: they have coronal
brightness temperatures because gyroresonance opacity makes them
optically thick, whereas the surrounding atmosphere with lower magnetic
field strength is optically thin and has much lower brightness
temperature. The dependence of opacity on viewing angle introduces some
complications but the theory is well understood, and radio data have
proven to be excellent diagnostics for testing extrapolations of
photospheric fields into the corona.

# **Acknowledgements**

The Science Definition Team would like to acknowledge the following
people for their assistance in the production of this report: John Leon,
Paul Caruso, Sahag Dardarian and Ron Miller from the GSFC Project team
for their support during the discussions of mission requirements; Art
Poland, Dick Fisher, Karel Schrijver and the LWS Science Architecture
Team for LWS program science support; Members of the SDO Mission Science
Preformulation Team: Leon Golub, Russ Howard, Steve Kahler, Dana
Longcope and Vic Pizzo; Military Space Weather advisers during
preformulation: Lt. Col. Michael Bonnadonna, Maj. Peter Engelmann, Capt.
Riley D. Jay, Maj. Phyllis Kampmeyer and Lt. Col. Erwin Williams; Joe
Gurman and Terry Kucera at GSFC for mission development; Jennifer
Rumburg for information systems; Scientists who assisted in the
instrument study and discussions: Rock Bush, Darrell Judge, Don
McMullin, Jesper Schou, Jake Wolfson and Tom Woods.