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

Chairman

Code SD50

NASA/MSFC

Huntsville, AL 35812

John W. Harvey

National Solar Observatory

P.O. Box 26732

Tucson, AZ 85726

K. D. Leka

Colorado Research Division

Northwest Research Assoc.

3380 Mitchell Lane

Boulder, CO 80301

Spiro Antiochos

Code 7675

Naval Research Laboratory

Washington, DC 20375

Donald M. Hassler

Southwest Research Institute

1050 Walnut St., Suite 426

Boulder, Colorado 80302

David Rust

Applied Physics Laboratory

Johns Hopkins University

Laurel, MD 20723

Thomas Bogdan

High Altitude Observatory

P. O. Box 3000

Boulder, CO 80307

J. Todd Hoeksema

Code S

NASA/Headquarters

Washington, DC 20546

Philip Scherrer

HEPL Annex B211

Stanford University

Stanford, CA 94305

Joseph Davila

Code 682

NASA/GSFC

Greenbelt, MD 20771

Jeffrey Kuhn

Institute for Astronomy

University of Hawaii

2680 Woodlawn Drive

Honolulu, HI 96822

Rainer Schwenn

Max-Planck-Institut für Aeronomie

Max Planck Str. 2

Katlenburg-Lindau

D37191 GERMANY

Kenneth Dere

Code 4163

Naval Research Laboratory

Washington, DC 20375

Barry LaBonte

Institute for Astronomy

University of Hawaii

2680 Woodlawn Drive

Honolulu, HI 96822

Leonard Strachan

Harvard-Smithsonian

Center for Astrophysics

60 Garden Street

Cambridge, MA 02138

Bernhard Fleck

ESA Space Science Dept.

c/o NASA/GSFC

Code 682.3

Greenbelt, MD 20771

Judith Lean

Code 7673L

Naval Research Laboratory

Washington, DC 20375

Alan Title

Lockheed Martin Corp.

3251 Hanover Street

Palo Alto, CA 94304

Richard Harrison

CCLRC

Chilton, Didcot

Oxfordshire OX11 0QX

UNITED KINGDOM

John Leibacher

National Solar Observatory

P.O. Box 26732

Tucson, AZ 85726

Roger Ulrich

Department of Astronomy

UCLA, 9831 MSB

Los Angeles, CA 90024

Barbara Thompson

Project Scientist

Code 682

NASA/GSFC

Greenbelt, MD 20771
Table of Contents

1 Executive Summary.................................................................................................1

2 Overview........................................................................................................................3

3 Current Scientific Understanding and Outstanding Questions .........8

3.1 Solar Influences on Global Change and Space Weather ...............................................8

3.1.1 Irradiance Variations............................................................................................8

3.1.2 Energetic Particles from Flares and CMEs.........................................................13

3.1.3 Coronal Structure and Solar Wind Variations.....................................................17

3.2 Mechanisms of Solar Variability................................................................................20

3.2.1 The Solar Cycle .................................................................................................21

3.2.2 Active Region Evolution....................................................................................24

3.2.3 Small-Scale Magnetic Structure .........................................................................27

4 Required Observations ........................................................................................32

4.1 Helioseismic Images..................................................................................................32

4.2 Longitudinal Magnetograms......................................................................................33

4.3 Atmospheric Images..................................................................................................34

4.4 EUV Spectral Irradiance............................................................................................35

4.5 Photometric Images...................................................................................................35

4.6 Vector Magnetograms ...............................................................................................36

4.7 UV/EUV Spectra.......................................................................................................38

4.8 Coronagraphic Images...............................................................................................39

4.9 Total Irradiance .........................................................................................................40

4.10 Coronal Spectroscopy................................................................................................41

4.11 Heliometry ................................................................................................................42

5 Potential Instruments and Allocation of Resources..............................42

6 Mission Concept......................................................................................................43

6.1 Orbit Selection...........................................................................................................44

6.2 Attitude Control System ............................................................................................45

6.3 Data and Communication System ..............................................................................46

6.4 Spacecraft Power.......................................................................................................47

6.5 Instrument Module ....................................................................................................47

6.6 Ground System..........................................................................................................47

6.7 Mission and Science Operations................................................................................48

7 Concurrent Observations....................................................................................49

7.1 STEREO ...................................................................................................................49

7.2 Solar-B......................................................................................................................49

7.3 Solar Probe................................................................................................................49

7.4 SORCE .....................................................................................................................51

7.5 GOES/NPOESS ........................................................................................................52

7.6 SOLIS .......................................................................................................................53

7.7 ATST ........................................................................................................................53

7.8 Solar Sentinels...........................................................................................................54

7.9 Solar Orbiter..............................................................................................................54

7.10 FASR ........................................................................................................................55

8 Acknowledgements ...............................................................................................54
1 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
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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.
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2 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.

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 quasiperiodic 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 pre-
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dictive 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 magnetodynamic nature of small-scale magnetic flux

may be the basis for short-term solar variability. They may provide the triggers for
5

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.
6

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 – 14C and 10Be – 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?
7

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 fulldisk 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 fulldisk coverage of EIT. The EUV Spectral

Irradiance Monitor would provide longterm, 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
8

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.

3 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.

3.1 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.

3.1.1 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 pho-
9

tosphere. 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.

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.

Figure 3.2. Total solar irradiance (W/m2) 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 pmode 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.

3.1.1.1 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 convec-
10

tion 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 timescales 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, co-
11

ronal 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.

3.1.1.2 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 nonmagnetic 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
12

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, wellcalibrated 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 fulldisk 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 104K

to a few 106K 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 mod-
13

els. 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.

3.1.2 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.

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.

3.1.2.1 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 de-
14

rived 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 Sshaped, 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.

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

(1016 g) and total energies (1032 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 104 to 107 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
15

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.

Figure 3.5. Helical structure in a CME observed with

LASCO on June 2, 1998.

3.1.2.2 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.
16

Vector 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 righthelical 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?

Vector 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
17

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.

3.1.3 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 highspeed 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.

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.

3.1.3.1 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 physi-
18

cal 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.

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).

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 reconnec-
19

tion 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-tocharge 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.

3.1.3.2 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 forcefree state, are required to provide a more

accurate determination.
20

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.

3.2 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 ac-
21

tivity 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.

3.2.1 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.

3.2.1.1 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 bipolar 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.
22

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.

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
23

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.

3.2.1.2 The Solar Cycle –

Outstanding Questions

What mechanisms drive the quasiperiodic 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 meridi-
24

onal 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 threedimensional 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.

3.2.2 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.

3.2.2.1 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
25

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 105 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 timedistance 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 highresolution 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.
26

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.

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.

3.2.2.2 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.
27

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 nonpotential 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 threedimensional 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.

3.2.3 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
28

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.

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.

3.2.3.1 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 kiloGaussstrength 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 in-
29

sights 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.

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 arcminutes (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 1023 Mx/day, which is equivalent to

10 to 20 large active regions. This rate is
30

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

107 ergs/cm2/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 highresolution 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

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. Highresolution vector magnetograms show that

many magnetic elements seen in longitudinal magnetograms may just be intersections
31

of undulating flux ropes with the photo-

sphere.

Magnetic Images are required to identify

and to follow the evolution of these ele-

ments. Full-disk observations with enough

spatial resolution (~1) to identify them

will be needed to determine their character-

istics. The temporal cadence should be fast

enough to follow their evolution and inter-

actions (~ 5 min is probably the best that can

be achieved). Longitudinal magnetograms

should be sufficient for most studies in-

volving the small-scale magnetic elements

but vector magnetograms will be required to

understand the flux ropes forming the smallscale elements.

Helioseismic Images are needed to deter-

mine the nature of the fluid flows that sur-

round 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 deter-

mine the effects of these elements on coro-

nal and chromospheric heating. These im-

ages should also have the spatial resolution

(~1) to identify the elements and the tem-

poral 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 in-

formation on the physical characteristics of

atmospheric structures above these smallscale magnetic elements. These measurements should provide important information

on events associated with coronal heating by

these magnetic elements.
32

4 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.

4.1 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
33

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 106 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

4.2 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 nonmagnetic 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 tem-
34

poral 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

4.3 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. Fulldisk 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

(103 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 shortterm 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.
35

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 103 – 105

Time Cadence 10 sec

Spatial Resolution 1.2 arcsec

Field of View 40 x 40 arcsec2

Spectral Resolution λ/Δλ ~ 20

Temperature Range 0.02 – 4 MK

4.4 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, tem-
36

peratures) 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 103

Time Cadence 10 sec

Spatial Resolution None

Field of View 1°

Spectral Resolution Δλ ~ 0.1 nm

Spectral Range 1 – 120 nm

4.5 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.
37

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

Intensity

Bolometric

Intensity

Precision 0.1 % 3 %

Dynamic

Range

>10330

Time Cadence 1 min 1 min

Spatial

Resolution

1 arcsec 10 arcsec

Field of View Full Disk Full Disk

Spectral

Resolution

Narrow

band

Broad

band

Completeness 95% 95%

4.6 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
38

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

4.7 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 106 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
39

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 103

 to 105 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 103 – 105

Time Cadence 10 sec

Spatial Resolution ~1 arcsec

Field of View 16 to 34 arcmin

Spectral Resolution λ/Δλ ~ 30,000

Temperature Range 0.02 – 4 MK

4.8 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 de-
40

sired 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

103 captures most of the observed variations

in the inner corona. A somewhat wider dynamic range of about 104

 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 finescale 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 103104

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

4.9 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
41

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°

4.10 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 RSun

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 RSun), 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 coronagraphspectrometer 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 RSun. Such an instrument would be capable of characterizing sites of magnetic reconnection and shock formation in CMEs by

observing high charge state ions and nonthermal 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
42

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 105

Time Cadence 10 min

Spatial Resolution 4”

Field of View 4~30” × 2000”

Range of View 1.1-10 RSun

Spectral Resolution λ/Δλ ~ 10,000

Spectral Range 28-140 nm

4.11Heliometry

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.

5 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.
43

Table 5. Allocation of Resources

Instrument Mass

(kg)

Data Rate

(Mbps)

Power

(W)

Volume

(cm3)

HMI 40 25 60 90x40x25

Atmospheric

Imaging Array

40-70 20-50 45 100x15x30

100x45x60

EUV SIM 20-30 <1 45 44x48x21

Coronagraph 30 1 35 135x17x17

UV/EUV

Spectrometer

40-60 15-30 40 160x60x30

Photometric

Mapper

30 5 50 100x30x30

Vector

Magnetograph

10+ 5 20+ 90x40x40

Total 210-270 62-117 295

6 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.
44

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

6.1 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.

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
45

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.

Figure 6.2: Annual periods of Earth and lunar

shadow (minutes per day).

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.

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.

6.2 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 accumu-
46

lated 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 zeromomentum 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.

6.3 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,
47

and the low rate housekeeping data will be

continuously maintained between the ground

and the spacecraft.

Figure 6.5: SDO CD&H system.

6.4 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

6.5 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.

Figure 6.6: SDO instrument module launch configuration

6.6 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. Xband 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
48

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.

6.7 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 nearreal-time commanding, daily command

loads compiled by the PIs will be sent by the

Mission Operations Team.

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.
49

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.

7 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

Coronal Images

2006-

2011

Solar-B Atmospheric Images

Magnetograms

UV/EUV Spectra

2005-

Solar

Probe

Atmospheric Images

Coronal Images

Magnetograms

2010

2015

SORCE Total Irradiance

Spectral Irradiance

2002-

2007

GOES/

NPOESS

Total Irradiance

Spectral Irradiance

2010-

SOLIS Magnetograms

Spectra

2001-

2025

ATST Magnetograms N/A

Solar

Sentinels

Atmospheric Images

Helioseismic Images

N/A

Solar

Orbiter

Atmospheric Images

Coronal Images

Magnetograms

UV/EUV Spectra

2009-

FASR Coronal

Magnetograms

N/A

7.1 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 Earthbound vantage point and by using two

spacecraft to provide information on threedimensional structure. Both spacecraft carry

a suite of instruments including: two coronagraphs (covering the range 1.25 - 4

RSun, and 2-15 RSun), an extreme ultraviolet

imager (full-disk, 1 arcsec pixels), a heliospheric imager (an externally occulted coronagraph that can image the heliosphere

from 12 RSun to beyond Earth’s orbit), an

interplanetary radio burst tracker, in situ

particle and field detectors, and a plasma

composition experiment.
50

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.

7.2 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 105 to 107 K but with only one strong

emission line below 106 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.

7.3 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
51

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.

7.4 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 longterm 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
52

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 followon 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).

7.5 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, thinfilm 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.
53

7.6 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 highresolution spectrograph. It can provide fulldisk 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.

7.7 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
54

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.

7.8 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 leadtime 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.

7.9 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
55

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.

7.10 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.

8 Acknowledgements

The Science Definition Team would like to

acknowledge the following people for their
56

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.