Extension of the SOHO Mission

# **EXECUTIVE SUMMARY**

*SOHO*, together with *Cluster*, forms the Solar Terrestrial Science
Programme (STSP), the first cornerstone in ESA's Horizons 2000
programme. Since its launch on 2 December 1995, *SOHO* has provided an
unparalleled breadth and depth of information about the Sun, from its
interior, through the hot and dynamic atmosphere, to the solar wind and
its interaction with the interstellar medium. Research using *SOHO*
observations has revolutionized our understanding of the Sun, and
science teams from around the world have made great strides towards a
better understanding of "the big three" areas of research that *SOHO*
set out to tackle: the structure and dynamics of the solar interior, the
heating of the solar corona, and the acceleration of the solar wind --
but much remains to be done. The findings have been documented in an
impressive body of scientific literature and popular articles whose
number continues to grow. At the same time, *SOHO*'s easily accessible,
spectacular data and basic science results have captured the imagination
of the space science community and the general public alike.

Here we propose a mission extension of four years, covering the period
from April 2003 through March 2007, allowing *SOHO* to observe an entire
11-year solar cycle.

*SOHO*, together with *Cluster*, provides an unprecedented opportunity
to understand the changing behaviour of the Sun over an entire solar
cycle, as well as its influence on the Earth, one of the four major
scientific quests of ESA's long-term space science strategy. A mission
extension will give us new and deeper insights into subjects as diverse
as variations in the solar tachocline and zonal flows, the physics of
coronal mass ejections, the acceleration of the solar wind in a variety
of coronal conditions, the evolution of the solar and heliospheric
magnetic field, and comets. The spacecraft and payload are in good
health and there are no technical limitations which should prevent
*SOHO* from observing for an entire solar cycle.

*SOHO* was the highest rated project among extended missions in the 2001
NASA Senior Review of the Sun-Earth Connection Mission Operations and
Data Analysis Programs, and funding was approved for the four-year
review period FY 2002 through FY 2005. NASA also indicated the intention
to continue supporting *SOHO* through the initial operations of the
Solar Dynamics Observatory, currently scheduled for launch in 2007.
*SOHO* is operated from the SOHO Experiment Operations Facility at NASA
Goddard Space Flight Center, and NASA covers the major part of the
operating costs. The additional cost to ESA at this stage in the mission
represents excellent value-for-money in return for a significant
enhancement of the scientific harvest from the *SOHO* spacecraft.

**[1. SOHO and ESA's Strategic Goals]{.underline}**

The first major theme of the scientific quest put forward in ESA's
Science Programme Space Strategy[^1] is "the influence of our Sun on the
Earth and the behavior of our star". This is precisely what *SOHO* has
been addressing for the last six years, and what we here propose to
continue for another four. NASA's Senior Review Board called *SOHO* "the
flagship mission for solar investigations". It has invigorated all areas
of solar research and an extended mission will continue to stimulate and
give impetus to the community for many years to come. The following
sections discuss the scientific merits of the extension proposal in more
depth.

Another important, yet recent element in ESA's space science strategy is
communication, education and public outreach, "to exploit the excitement
of space science as a unifying theme to make scientific education more
attractive to the youth and science in general attractive to the less
young"[^2]. Here, too, we believe that *SOHO* has made - and will
continue to make - outstanding contributions. We have shared the
excitement of space science discoveries with the public through
immensely popular, continuously updated web pages, numerous press
releases, interactive CDs, travelling museum exhibits, a large number of
public talks, and the ubiquitous adoption of time-lapse movies of *SOHO*
images by TV and internet media. Movies from *SOHO,* used in the awesome
vision of the SOLARMAX film, showcase images of astounding aesthetic
value and visualise the origins and effects of space weather in a way no
other medium could, reaching large audiences worldwide. These activities
will be continued as an integral part of an extended mission.

**[2. Some Scientific Highlights from 6 Years of SOHO]{.underline}**

*SOHO* enjoys a remarkable "market share" in the worldwide solar physics
community: over 1100 papers in refereed journals and over 1500 papers in
conference proceedings and other publications, representing the work of
over 1500 scientists. A searchable *SOHO* publications database[^3] is
available under the *SOHO* home page. It is not too much of an
exaggeration to say that virtually every living solar physicist has had
access to *SOHO* data.

We can assert this with confidence because all of the *SOHO* experiments
make all their data available, online, on the Web, through the *SOHO*
archives and PI sites. The *SOHO* master archive has been developed at
GSFC by the ESA *SOHO* Project Scientist team with significant
contributions from European partners. Three copies of this archive have
been established in Europe (IAS, France; RAL, UK; Univ. Torino, Italy).

In addition to professional access, amateurs routinely download LASCO
images to search for new comets. As a result, over 360 of the 1300
comets for which orbital elements have been determined since 1761 were
discovered by *SOHO*, and *more than 60% of those by amateurs accessing
LASCO data via the Web*. Similarly, over 400,000 people across the world
routinely download LASCO, EIT, and MDI images several times daily as
their *SOHO* screensavers update themselves.

It is not possible in this report to cover all the findings that have
come out of the *SOHO* mission during the 6 years since launch. The
following is a brief summary of some of the achievements:

**Solar Interior**

-   Global helioseismology using data from *SOHO* have given a number of
    new and improved results, like:

```{=html}
<!-- -->
```
-   GOLF, VIRGO and MDI giving the most precise measurements to date of
    the sound speed profile and the (differential) rotation profile
    within the Sun.

-   MDI data revealing "zonal bands" in the northern and southern
    hemispheres where currents flow at different speeds (\~ 5 m/s)
    relative to each other. The zonal bands were found to migrate
    towards the equator with time, and they were shown to penetrate to a
    depth of at least 56 Mm.

-   Data from MDI and the GONG network of ground based observatories
    showing quasiperiodic changes in the rotational contrast near the
    "tachocline" -- the interface between differentially and uniformly
    rotating layers of the Sun. This is at the base of the convection
    zone, and the presumed location of the solar dynamo. The observed
    periods (15-16 months in equatorial regions) are in stark contrast
    to the 11-year period of the sunspot cycle, thought to be driven by
    dynamo processes in the same region.

```{=html}
<!-- -->
```
-   A new technique called "time-distance helioseismology" or "solar
    tomography", applied almost exclusively to the high-quality,
    high-resolution MDI data, has proved to be one of the most exciting
    and promising techniques for probing 3D structures and flows below
    the solar surface. With this technique, MDI data has:

```{=html}
<!-- -->
```
-   Provided the first ever image of the convection zone of a star,
    showing the first maps of vertical and horizontal flows as well as
    sound speed variations just below the visible surface.

-   Shown that the entire outer layer of the Sun, to a depth of at least
    25,000 km, is slowly but steadily flowing from the equator to the
    poles. The polar flow rate is fast enough to transport an object
    from the equator to the pole in a bit more than a year.

-   Offered the possibility of studying the birth and evolution of
    active regions below the Sun\'s surface, revealing the complicated
    structure of emerging regions in the solar interior that travelled
    very rapidly through the upper 18 Mm of the convection zone.

-   Revealed sunspot \"fingers\" - long, narrow structures at a depth of
    about 4 Mm connecting the sunspot with surrounding pores of the same
    polarity. Pores with the opposite polarity on the other hand are not
    connected to the spot.

-   Detected strong inward flows towards sunspots, solving the
    long-standing riddle of why sunspots last so long. Such inward flows
    are required in theoretical models of sunspots to stabilize the
    structure, even though surface material is often observed flowing
    out of the spots.

-   Revealed that sunspots are very shallow, changing from cooler to
    hotter than the surroundings only 5000 km below the surface -- i.e.
    the cool part of a sunspot is more similar to a stack of two Euro
    coins than a tree trunk.

    -   High precision MDI measurements of the Sun\'s shape and
        brightness obtained during two special 360 degree roll
        manoeuvres of the *SOHO* spacecraft, producing the most precise
        determinations of solar oblateness ever. There is *no excess
        oblateness*, unambiguously ruling out the possibility of a
        rapidly rotating core.

    -   Using "helioseismic holography", a technique that has been known
        in theory for over a decade, researchers using high quality MDI
        observations of the near side of the Sun were for the first time
        able to *image an active region on the far side*. In this
        process, the Sun acts like an acoustic lens, revealing the
        active region on the far side through a slight decrease (about
        12 seconds) in the total roundtrip sound travel time of about 6
        hours. Since spring 2001, this technique is being applied
        routinely to MDI data, and space weather forecasters now have
        almost real-time access to far side images of the Sun.

**Solar Atmosphere**

-   EIT, SUMER and CDS observations clearly demonstrate that the solar
    transition region and corona is extremely dynamic and time-variable
    in nature. In sum, *SOHO* instruments have highlighted the many
    shortcomings of current static models of the solar atmosphere, as
    they are simply not able to reproduce the observed flows and
    variability.

-   Coordinated measurements by SUMER and CDS have provided new
    observational data concerning the temperature structure in coronal
    holes, commonly believed to be the origin of the high-speed wind
    streams. The temperature measured starts at about 1 MK at the base,
    a higher value than anticipated and of the same order as for closed
    field regions. However, the temperature drops much more rapidly with
    height than had been expected, to only 0.3 MK at 1.3 solar radii.
    This clearly rules out a thermally driven fast solar wind.

-   The two spectrometers SUMER and CDS observe extensive evidence of
    explosive events in transition-region lines. These could be the
    signature of magnetic reconnection events associated with localized
    small magnetic dipoles that appear in magnetogrammes obtained by MDI
    near the base of many of the explosive events studied.

-   A new class of solar variability dubbed \"blinkers\" has been
    identified by CDS in quiet Sun regions. They appear as EUV intensity
    enhancements of about 10-40% with a typical duration of about 15
    minutes.

-   The UVCS coronagraph has revealed dramatic differences in the line
    widths of the strong hydrogen Lyman-alpha line, and even more so in
    the resonance lines of O VI, in coronal streamers at heliocentric
    heights from 1.25 out to 3.5 solar radii. While the protons are only
    mildly anisotropic above 2-3 solar radii and never exceed 3 MK, the
    O VI ions are strongly anisotropic at these heights, with
    perpendicular kinetic temperatures approaching 200 MK. An
    ion-cyclotron resonance with Alfven waves affords one possible
    explanation for generating the higher observed velocities in the O
    VI ions.

-   A far-ultraviolet and extreme-ultraviolet (FUV, EUV) spectral atlas
    of the Sun between 670 Å and 1609 Å, derived from observations
    obtained with the SUMER spectrograph, identifies over 1100 distinct
    emission lines, of which more than 150 had not been recorded or
    identified before. The atlas contains spectra of the average quiet
    Sun, a coronal hole and a sunspot on the disk, providing a rich
    source of new diagnostic tools to study the physical parameters in
    the chromosphere, the transition region and the corona. In
    particular, the wavelength range below 1100 Å as observed by SUMER
    represents a vast improvement over the spectra produced in the past.

-   LASCO has discovered infalling matter at coronal heights. These
    inflows show a strong correlation with nonpolar coronal holes and
    the Sun\'s nonaxisymmetric open flux, but only a broad long-term
    correlation with conventional indicators of solar activity.

-   The VIRGO instrument measures both total solar irradiance (TSI) and
    spectral irradiance, the latter in three bands (centered at 4020,
    5000 and 8620 Å). VIRGO extends the record of TSI measurements into
    cycle 23.

-   EUV irradiance measurements are being made on a regular basis by the
    CELIAS/SEM and CDS instruments. Due to its influence on the
    terrestrial atmosphere, the solar variability may constitute
    significant sources of climate changes. Tracking the long term
    variations of the solar spectral irradiance is therefore of critical
    importance.

**Solar Wind**

-   SUMER observations of coronal hole outflow velocities show a clear
    relationship between coronal hole outflow velocity and the
    chromospheric network structure, with the largest outflow velocities
    occurring along network boundaries and at the intersection of
    network boundaries. This can be considered the first direct
    spectroscopic determination of the source regions of the fast solar
    wind in coronal holes.

-   UVCS has discovered that the high-speed solar wind in coronal holes
    achieves its high velocity - up to 800 km/s - by "surfing" magnetic
    waves in the Sun\'s outer atmosphere.

-   UVCS observations show that solar-maximum equatorial coronal holes
    exhibit 3-5 times lower outflow speeds than solar-minimum polar
    coronal holes. Since wind speeds observed at 1AU are similar in the
    two cases, the bulk of the acceleration must occur outside the
    region observed by UVCS, i.e. above 3 solar radii.

-   CELIAS has doubled the number of elements and isotopes previously
    recorded in the solar wind.

-   During a Venus disk passage, CELIAS observed, for the first time
    from such a large distance, pick-up ions from Venus\' ionosphere,
    finding a much smaller diffusion cone than anticipated.

**Comets**

-   SWAN has measured the total amount of water ice present in comet
    C/1999 S4 (LINEAR) on 25 May 2000 and subsequent times, until all of
    its water ice had been vaporised as the comet fragmented. The amount
    of water vapour released throughout the observing period was
    estimated at 3.3 million tonnes. Combined with an HST estimate of
    the total volume of the fragments on 5 August, a remarkably low
    value for the

density - about 15 kg per cubic meter - was found.

-   UVCS made the first high resolution UV spectroscopic measurements of
    several comets. Measurements of comet C/1996Y1 obtained at 6.8 R~~
    confirmed predictions of models of the cometary bow shock driven by
    mass-loading, as cometary molecules are ionized and swept up in the
    solar wind. From line widths and Doppler shifts, the solar wind
    speed at 6.8 R~~ could be determined (640 km/s). The outgassing
    rate of the comet was estimated at 20 kg/s, implying a net active
    area of the nucleus of only about 6.7 m in diameter and a mass of
    about 120,000 kg.

**Other discoveries and milestones include:**

-   Discovery of a magnetic surface "carpet", subducted and replaced on
    timescales of 1.5-3 days, that may be the source for the energy
    required to heat the corona.

-   The first detection of a flare-induced "sun-quake".

-   First measurement of the acceleration profile of the slow solar wind
    from 3 to 30 solar radii.

-   First measurement of the acceleration profile of the fast solar wind
    from 1.5 through 3.5 solar radii.

-   Detection of 11-25 minute polar plume oscillations indicative of
    compressive waves.

-   The discovery of \"EIT waves\" and their relation to CMEs,
    dramatically improving space weather forecasting reliability.

-   Invaluable statistics on, and spectacular images and movies of,
    coronal mass ejections and other energetic events, elevating space
    weather studies to a new level.

-   First direct observations of a post-CME current sheet.

-   Measurement of ion freeze-in temperatures and its variation with
    element and over time.

-   First detection of active regions on the backside of the Sun through
    their "lighthouse" effect.

-   Discovery of a comet's shadow.

We would like to point out that almost all the discoveries made by
*SOHO* is due not only to the comprehensive complement of instruments,
but also due to the unique vantage point of the spacecraft, providing a
highly stable platform with an uninterrupted and undisturbed view of the
Sun. Another key element in SOHO science is the large number of
collaborations with numerous ground-based observatories as well as other
spacecraft, such as *Ulysses, TRACE*, *NEAR*, *Galileo*, and *Yohkoh.*

**[3. Scientific Rationale for the Proposed Mission
Extension]{.underline}**

A mission extension of four years from April 2003 through March 2007 is
proposed in order to cover an entire 11-year solar cycle, fulfilling the
scientific promise of *SOHO* to answer fundamental questions about the
Sun and heliosphere. Launched on 2 December 1995, *SOHO*'s nominal
science operations phase started on 2 May 1996, close to the last solar
minimum. The next solar minimum is expected to occur around 2007.

The following sections list some of the open questions in solar and
heliospheric physics, forming the scientific rationale of the mission
extension. Many of them will be answered with the help of future *SOHO*
observations.

***Solar Interior***

We have evidence of large scale, organized subsurface flows toward
active regions in MDI data taken during special "continuous contact"
runs of 2 -- 3 months each year from 1997 -- 2001. Each of those samples
represents only two solar rotations, so we do not yet know how these
flows evolve and relate to the growth/decay phases of activity. Do such
flows drive the emergence and decay processes for solar active regions?
It is important to obtain additional samples in the declining phase of
the solar cycle --- without a SOHO extension the next opportunity will
not come until after 2012.

That sound wave speed is enhanced beneath magnetic fields is now well
established for sunspots, plage, and even pores. Similarly, it has been
shown that there is an ordered flow beneath large sunspots. In the
future we hope to be able to use such diagnostics to derive space
weather predictive information.

How does the rotational shear at the tachocline vary? This shear layer
at the base of the convection zone shows variations near the equator
with a period of about 1.3 years. This variation, as well as its
implications, is not yet understood. It has withstood a number of tests
of reality and the result remains intact. We anticipate the opportunity
of seeing what happens in the declining phase of the solar cycle.

How does the poleward flow vary with depth, latitude, and phase of the
solar cycle? The existence of a poleward flow of 10-20 m/s at the
surface has been known for two cycles but only with MDI observations has
it been well characterized and observed to extend into at least the
first 10% of the convection zone. There is at best a suggestion in the
noisy, groundbased data that the poleward flow extends only to lower
latitudes in the years nearer to maximum. This possible variation with
the cycle is a topic of current investigation with *SOHO*.

***The extended solar cycle***

There are several solar parameters that vary with a cycle time longer
than 11 years. There are at least three aspects to the extended cycle
that can be examined with continued MDI observations into the declining
and minimum phases of the cycle.

-   There is evidence that the toroidal component of the field can be
    detected in MDI magnetograms. We can watch the emergence of high
    latitude new cycle fields, measure their toroidal component, and
    detect the interactions of the next cycle's fields with those of the
    current cycle.

-   MDI magnetograms, obtained every 96 minutes, can be used to study
    the emergence of ephemeral regions at high latitude. We can thus
    study, in greater detail than possible from the ground, the
    evolution of the new cycle emerging regions in order to compare them
    to ephemeral regions of the present cycle: are there significant
    differences? This study will also lead to better understanding of
    the connections between the current and the next cycle.

-   We can continue to study the zonal flows in the convection zone.
    Historical data predicts that we will shortly be able to detect the
    shear zone which will later be associated with the centerline of
    next-cycle activity. By watching this process unfold within the
    convection zone we should gain insight into the operation of the
    cycle dynamo, or at least further constrain models.

***Acceleration of the solar wind***

Polar coronal holes are enlarged by episodic surges of
alternating-polarity magnetic flux bundles moving poleward from
mid-latitudes. A solar-cycle-long *SOHO* mission would give us the
opportunity to understand the full life cycle of polar coronal holes,
the source of much of the solar wind flow for a significant part of the
solar cycle.

UVCS measurements of density and ion kinetic temperature perpendicular
to the magnetic field show an apparent anticorrelation. Is this because
higher density regions are collision-dominated, or is there a density
dependence of ion cyclotron wave generation and dissipation?
Spectroscopic observations at the solar cycle phase when polar holes and
the dipole streamer belt are growing rapidly will provide new insights.

The declining phase of a solar cycle is also the period most likely to
feature low-latitude coronal holes. In the high-speed streams from those
holes, we can use *in situ* instruments on *SOHO* and ACE to
characterize coronal electron temperatures, non-Maxwellian distribution,
and ion differential speeds, and *SOHO* spectrographs (SUMER, CDS, and
UVCS) to measure plasma properties for some of the ions available to the
*in situ* instruments. Models consistent with both *in situ* and remote
sensing measurements should constrain, for example, flow speeds for
different ions of the same species in the corona. That, in turn, should
allow us to determine how heating and acceleration processes depend on
ion charge, without having to consider ion mass dependencies.

The heliospheric magnetic field and its origin, the solar magnetic
field, will be "relaxing" over the next few years to their simpler,
solar minimum configurations. Do coronal streamer properties change
smoothly or abruptly during this period? Do the same physical processes
heat streamers and accelerate the slow solar wind? If not, can we
distinguish which processes are responsible for each?

***Coronal Mass Ejections and Space Weather***

Data from the last six solar cycles show that the distribution of events
disturbing the geomagnetic field is broader, and peaks later, than
traditional measures of solar activity (*e.g.* sunspot "number"). The
time lag from "solar maximum" is typically 2 -- 3 years. Why is this so?
Is it because new-cycle magnetic flux, migrating from high solar
latitudes, leads to solar events with different properties than earlier
in the cycle? Is it because CMEs have a less complex solar magnetic
field from which to escape, because the number of active regions is low?
Or is it because, as a result of these solar changes, the interplanetary
field is more amenable to the propagation of fast shocks? Or is the
well-known persistence of large flares and fast CME's throughout the
decline from solar maximum a reflection of interactions with the
low-latitude coronal holes?

The largest CMEs are in general the most geoeffective (when
earthward-directed), as well as the fastest. Does size, as determined by
LASCO and EIT, correlate with thermal content, as UVCS should be able to
determine? What is the temperature structure of CMEs? Will the latitude
of origin of CMEs evolve gradually back to the dipole streamer belt, or
will it shift abruptly from its current, broad distribution in latitude?

CMEs are only geoeffective when they contain or drive several hours of
southward B~z~ when they arrive at Earth. It has been shown that field
extrapolations based on the photospheric magnetic field underlying the
areas where CMEs erupt yield good estimates of the B~z~. Recent
developments the in processing of MDI magnetograms has led to the
concept of a "synoptic frame" in which synoptic, whole-Sun Carrington
grids of magnetic field are supplemented by rapidly updated magnetic
information for the visible disk. Initial analyses of these data are
promising and may lead to better estimates of the sign and strength of
the B~z~ component of the field carried by CMEs. Tools to use these data
are presently being developed. Combined with future SOHO observations
they should significantly improve space weather forecasting
capabilities.

Is there a relationship between flare energetics and the post-CME
current sheet? Is there a spatial relationship between the volume in
which impulsive phase flare particle acceleration takes place, and that
in which post-CME heating (presumably due to reconnection) occurs? One
of several areas in which *SOHO* can support the upcoming NASA *HESSI*
mission is to take advantage of the opportunity to characterize both the
primary energy release plasma (with *HESSI*) and the plasma heated,
presumably by reconnection, in what has been considered the "gradual"
phase of flares but is really the early phase of CMEs.

## 

## Why keep SOHO going? 

While we have made considerable progress in the understanding of the
Sun, we have still not realized the full potential of *SOHO* to explore
the diverse parameter space offered by the solar cycle:

We have learned many new things about the solar interior, but still do
not know whether there are gravity-driven oscillation modes in the deep
interior of our star. The chances of a detection is directly related to
the length of the observation time series.

We do not yet understand the time evolution of zonal flows, or whether
the convergence zones for active region flows drive the latitudinal
distribution of the regions over the cycle.

We have seen the effects of coronal mass ejections on the lowest layers
of the corona, studied the Doppler motions and heating characteristics
of CME ejecta, and detected inflows in the corona, but we still do not
know what causes CMEs to erupt, nor how they are accelerated, nor
whether the variation in the rate is the same from cycle to cycle.

We have made great progress in understanding the origin and acceleration
of solar wind plasma in polar coronal holes, and in extending that
understanding to low-latitude holes, but we still need to understand the
origins of the slower, low-latitude wind and how the spatial
distribution of the wind varies over the solar cycle.

We are entering the phase of maximum geoeffectiveness in the solar
cycle, and as we learned inadvertently during the mission interruption,
there is a recognition that *SOHO*'s space weather capabilities are
international resources that should continue uninterrupted. Until at
least the launch of the *STEREO* mission in 2005/6, no similar
capabilities are planned.

The combination of *SOHO*, *Yohkoh*, and *TRACE* is likewise unique in
characterizing solar activity, and when joined by *HESSI*, the
scientific community will have the once-in-a-lifetime ability to cover
*all temporal scales of solar activity from milliseconds to a solar
cycle*.

Coupling this grand alliance of sunward-gazing spacecraft with the
fine-tuned, Earth-centered observations of *Cluster*, *SOHO* can make
large and vital contributions, unrivalled in quality and scope, to a
long chain of observations. We may be able to follow solar activity from
its subsurface stages through the emergence of surface active regions,
whether on the near or far side of the Sun, to the brilliance of flares
and violent onset of CMEs, racing outward, passing *SOHO* and other
spacecraft before crashing into Earth's protective cocoon of magnetic
flux; in short, following solar activity from cradle to grave.

Finally, due to its influence on the terrestrial atmosphere, solar
variability may constitute an important driver of climate changes.
Variations in the total solar irradiance will directly influence the
amount of energy absorbed by land and oceans. Furthermore, the EUV
spectrum at wavelengths below 1200 Å is the dominant source of energy
for heating and ionization in the terrestrial upper atmosphere at
altitudes above 90 km. Changes in the amount of EUV radiation will
therefore change the chemistry, dynamics and temperature of the Earth\'s
atmosphere. Good knowledge of the solar spectral irradiance is thus of
critical importance for understanding climate variability, and to
disentangle natural variations from human made climate changes.

An extended *SOHO* mission, covering an entire solar cycle, will be a
key element in the quest to better understand our variable star and its
influence on Earth.

# 

# **Annex A: Spacecraft Status and Trend Analysis**

## General

After six years in space, the spacecraft is healthy and performs
entirely within specifications. All  the equipment is running on the
main branch - no recourse to the redundant units has been required.
Although all three gyros were lost after the successful recovery of the
spacecraft in September 1998, the gyroless software performs flawlessly.

## Power Subsystem

The solar array has degraded by 13% in the six years since launch -- at
about half of the forecasted rate (4% per year). The present power
budget allows 30% degradation before any conservation measures need to
be devised.

## Data Handling Subsystem

All the telecommand and telemetry functions operate nominally. The Solid
State Recorder works well, with no degradation, as does the tape
Recorder, which undergoes periodic maintenance as the backup telemetry
storage.  The Onboard Time drift is two orders of magnitude better than
specifications. The High Gain Antenna Pointing Mechanism shows no
indication of degradation.

## AOCS Subsystem

Attitude and orbit control is performing flawlessly, with the reaction
wheels taking on the tasks normally performed by gyros when not using
the star tracker for roll control. The new "star swap" feature has
allowed the spacecraft to remain in normal mode throughout all of the
recent severe proton events. There are no signs of degradation.

## 

## Propulsion Subsystem

The propulsion subsystem is nominal. The average hydrazine consumption
is on the order of 2 kg/year, with a current reserve of 122 kg.

## Thermal Subsystem

All spacecraft temperatures are nominal. As predicted, temperatures on
the +X panel increase slowly due to aging of the materials, and +X
heaters have been trimmed down. Nothing indicates that the thermal
subsystem cannot support the mission beyond 2007.

## Conclusion

The spacecraft is healthy using no redundant subsystems. Fuel reserves
and the solar power budget have comfortable margins given the rate of
consumption and degradation. Nothing indicates that the spacecraft
cannot support an extension through March 2007.

**[Annex B: Instrument Status]{.underline}**

# **GOLF**

-   Operating nominally

-   Progressive, expected fall-off in throughput not worrying, and will
    not prevent GOLF from observing an entire solar cycle

-   Continuous observations in red wing of Na D line since *SOHO*
    recovery

```{=html}
<!-- -->
```
-   Noise in red and blue wings similar

-   Avoid risk of operation to move to blue wing

```{=html}
<!-- -->
```
-   Reserve: redundant data channel validated but unused

# 

# **VIRGO**

-   All instruments within VIRGO are fully operational and work fine and
    there is no reason to believe that the instruments would not operate
    for at least another 5 years.

-   The degradation of sensitivity is well within specifications and
    much less than expected from other missions; the main reason being
    the high degree of cleanliness of instruments and spacecraft.

# **MDI**

-   MDI is working very well, having made about 50 million images on
    board, delivering about 8 million to the ground after onboard
    computations.

-   There is an expected degradation in total light throughput due to
    changes in the front window transmission. The reduction in
    transmission to March 2001 was about 10%, or about 2.3% per year.

-   The reduction in transmission causes a slow increase of the front
    window temperature (\~ 2ºC per year), which in turn causes a slow
    drift of the best focus position.

-   The drift in best focus position has moved the nominal focus setting
    back almost to the design point. Shortly after launch it was at the
    limit of the adjustment range.

-   The drift in central wavelength of the Michelson interferometer has
    nearly stopped.

-   There is no detected change in the CCD flat field except for
    variations with focus change.

-   There is a reduction in shutter exposure time uniformity from one
    part in 12000 to one part in 4000. This corresponds to jitter of 40
    μsec for a 165 ms exposure. The pre-launch specification was 50 μs.
    So, while still within the pre-launch requirement we will watch for
    changes. There might be a low-impact change to the observing
    sequence that could reduce shutter usage by about 30%. This will be
    evaluated if there are further changes. The change occurred in March
    2000.

-   In summary, with the possible exception of the recently detected
    shutter jitter, there is no known limit to MDI\'s useful life within
    the *SOHO* solar array life expectancy.

# 

# **SUMER**

-   The detectors are near their calibrated lifetime. In order to
    preserve detector lifetime and reduce the total counts collected by
    the detectors as much as possible, SUMER has been and will be
    operated in a "campaign mode", where the instrument is switched on
    only during certain selected periods.

-   The east-west drive of the telescope mirror - working in
    high-current mode - caused no difficulties in any of the pointing
    operations, but cannot be used for raster scans anymore.

# **CDS**

-   CDS is nominal and expected to operate at full scientific capability
    for many more years.

-   The detector systems show no signs of degradation above what is
    expected. The CDS NIS detectors see progressive burn-in. This is a
    degradation of sensitivity at the locations of the bright emission
    lines. This is expected and is approaching an asymptotic level for
    the brighter lines. It gives no cause for concern. The burn-in is
    carefully monitored and is readily corrected.

-   The NIS calibration is well understood for data taken prior to the
    loss of *SOHO* in 1998. A post-recovery change in sensitivity and
    its long-term variation is currently being quantified by comparing
    with the SEM, SERTS and quiet Sun data.

-   All voltage levels and currents remain stable and well within
    tolerances.

-   All critical instrument temperatures remain stable. Some temperature
    monitors close to thermal blankets show increases due to expected
    deterioration of the blankets; this causes no major concerns.

-   All Mechanisms continue to operate satisfactorily.

-   The Ground system is fully functional and was recently upgraded.

# **EIT**

-   EIT is nominal and expected to operate at full scientific capability
    for many more years.

-   Instrument throughput is decreasing

    -   Two components: absorption of EUV by a CCD surface contaminant
        (no longer a problem since unintentional bakeout during the
        *SOHO* "vacation"), reduction in charge collection efficiency
        (CCE) in the CCD due to EUV-induced damage (continues)

    -   CCE loss can be tracked accurately with calibration lamp images

    -   Degradation now well understood and modelled

    -   Present exposure times range from 12 s (195 Å) to 2 min (284 Å):
        lots of latitude left

    -   S/N still very high

## UVCS

-   UVCS is expected to continue performing at full scientific
    capability for many more years.

-   O VI Detector:

    -   No significant decrease in efficiency in O VI portion of this
        detector.

    -   Ly-α portion showed 5 to 10% loss of efficiency on *parts* of
        detector at a high voltage of 200 EU.

    -   Voltage increase from 200 to 205 EU in January 2000 restored
        full efficiency

    -   Max of 250 EU allows nine additional increases of 5 EU.

    -   Voltage increase of 5 EU every 1.2 years is expected to keep the
        efficiency loss at \< 5%.

```{=html}
<!-- -->
```
-   Ly-α Detector:

    -   Has been turned off since November 1998 because it draws about
        50% of the maximum current and has regions of elevated
        background.

    -   This behavior is not related to the mission interruption.

    -   Still operational and treated as a back-up for Ly-α
        observations.

# 

# **LASCO**

-   The LASCO/C2 and C3 coronagraphs continue to operate with no
    degradation.

-   Photometric sensitivity is unchanged since launch.

-   The Fabry Pérot interferometer in the LASCO/C1 coronagraph did not
    survive the extreme cold the instrument experienced (-80ºC) during
    the mission interruption.

# **CELIAS**

-   MTOF/PM, STOF/HSTOF, SEM nominal

-   CTOF impaired since 1996 October (HV power supply hardware failure)

# **COSTEP**

-   EPHIN nominal

-   LION: front detectors noisy since launch

    -   Partially operational for higher flux at high solar activity

    -   Separation of electrons and protons possible during solar SEP
        events

## ERNE

-   Operating nominally.

-   Persistent ESU data request error cause still under investigation,
    but software patched to reduce data loss to \< 4 minutes per anomaly

```{=html}
<!-- -->
```
-   Increasing temperatures due to thermal insulation degradation a
    cause for concern, but recent X-panel heater changes have improved
    the situation.

# **SWAN**

-   Two detectors (+Z, -Z) cover whole sky

-   Periscope motors nominal

-   +Z sensor degraded by mission interruption in 1998, but --Z sensor
    unaffected

-   UV contamination of +Z sensor has decreased by more than a factor of
    two since 1998

-   H absorption cells (one per sensor head) decreasing in absorptivity
    (+Z: 15% decrement, -Z: 75%); calibration continues.

In summary, there are no known limitations which should prevent *SOHO*
from completing an entire solar cycle with almost all of its complement
of instruments operating at full scientific capability.

[^1]: ESA/SPC(2000)32, rev.2, Section 2

[^2]: ESA/SPC(2000)32, rev.2, Section 4.6

[^3]: http://soho.estec.esa.nl/publications/