Heliophysics Senior Review 2008

The Reuven Ramaty

High Energy Solar Spectroscopic Imager

(RHESSI)

Robert Lin, Principal Investigator

Brian Dennis, Mission Scientist

Manfred Bester, Mission Operations Manager

Bryan Mendez, E/PO Manager

**Table of Contents**

1 Executive Summary [4](#executive-summary)

2 Science [5](#science)

2.1 Accomplishments [6](#accomplishments)

2.1.1 Hard X-rays & Parent Electrons [6](#hard-x-rays-parent-electrons)

2.1.1.1 Photospheric Compton Scattering of X‑rays (Albedo)
[6](#photospheric-compton-scattering-of-xrays-albedo)

2.1.1.2 New Imaging Spectroscopy Methods
[7](#new-imaging-spectroscopy-methods)

2.1.1.3 HXR Emission Along Flare Ribbons
[8](#hxr-emission-along-flare-ribbons)

2.1.1.4 Electron Acceleration During Magnetic Reconnection
[9](#electron-acceleration-during-magnetic-reconnection)

2.1.1.5 Partially Disk-occulted Flares
[9](#partially-disk-occulted-flares)

2.1.1.6 Hard X-ray emissions from the high corona associated with CMEs
[10](#hard-x-ray-emissions-from-the-high-corona-associated-with-cmes)

2.1.1.7 HXR Source Motions Along Loops
[11](#hxr-source-motions-along-loops)

2.1.2 Gamma-Rays [11](#gamma-rays)

2.1.2.1 Gamma-ray-line Imaging [11](#gamma-ray-line-imaging)

2.1.2.2 Coronal Gamma-ray Continuum Sources
[12](#coronal-gamma-ray-continuum-sources)

2.1.2.3 Polarization of Gamma-ray Continuum
[12](#polarization-of-gamma-ray-continuum)

2.1.2.4 Harder Flare-Accelerated Ion Spectra?
[12](#harder-flare-accelerated-ion-spectra)

2.1.2.5 Correlated Electron and Ion Acceleration
[13](#correlated-electron-and-ion-acceleration)

2.1.2.6 Gamma-ray and radio sub‑millimeter observations
[14](#gamma-ray-and-radio-submillimeter-observations)

2.1.3 HXRs and SEPs [14](#hxrs-and-seps)

2.1.3.1 RHESSI & WIND Observations [14](#rhessi-wind-observations)

2.1.3.2 HXR spectral evolution and SEPs
[15](#hxr-spectral-evolution-and-seps)

2.1.4 Microflares [16](#microflares)

2.1.4.1 Microflare Statistics [16](#microflare-statistics)

2.1.4.2 Microflares with RHESSI & Hinode/XRT
[18](#microflares-with-rhessi-hinodexrt)

2.1.5 Quiet-Sun & Axion Observations [18](#quiet-sun-axion-observations)

2.1.6 HXR correlations with CMEs [19](#_Ref115581988)

2.1.7 Solar Oblateness [20](#solar-oblateness)

2.1.8 Terrestrial Gamma-ray Flashes (TGFs)
[20](#terrestrial-gamma-ray-flashes-tgfs)

2.1.9 Astrophysics Results [21](#astrophysics-results)

2.1.9.1 Gamma-Ray Bursts (GRB's) [21](#gamma-ray-bursts-grbs)

2.1.9.2 Accreting X-ray binary A0535+26
[21](#accreting-x-ray-binary-a053526)

2.2 SCIENCE OBJECTIVES for 2008 - 2012
[22](#science-objectives-for-2008---2012)

2.2.1 Flare Rates and Energetic Events
[22](#flare-rates-and-energetic-events)

2.2.2 RHESSI Flare Studies with Hinode and STEREO
[22](#rhessi-flare-studies-with-hinode-and-stereo)

2.2.3 In Situ Particle Observations and HXR Emission
[24](#in-situ-particle-observations-and-hxr-emission)

2.2.4 Microflares [24](#microflares-1)

2.2.5 Quiet-Sun & Axion Observations
[24](#quiet-sun-axion-observations-1)

2.2.6 Gamma-Ray Observations [25](#gamma-ray-observations)

2.2.6.1 Coordination with GLAST [25](#coordination-with-glast)

2.2.7 Solar Limb Data [26](#solar-limb-data)

2.2.8 Terrestrial Gamma-ray Flashes [26](#terrestrial-gamma-ray-flashes)

2.2.9 Astrophysical Objectives [26](#astrophysical-objectives)

2.2.10 Proposed Solar High-Energy Events GI Program
[26](#proposed-solar-high-energy-events-gi-program)

2.3 Potential for Performance during FY-09 to FY-12
[26](#potential-for-performance-during-fy-09-to-fy-12)

2.3.1 Relevance to HP Research Objectives
[26](#relevance-to-hp-research-objectives)

2.3.2 Impact of Scientific Results [27](#impact-of-scientific-results)

2.3.3 Spacecraft and Instrument Health
[27](#spacecraft-and-instrument-health)

2.3.4 Productivity, and Vitality of Science Team
[27](#productivity-and-vitality-of-science-team)

2.3.4.1 Science Nuggets [27](#science-nuggets)

2.3.5 Future Promise [28](#future-promise)

2.3.6 Data Accessibility and Usability
[28](#data-accessibility-and-usability)

2.3.6.1 Browser [28](#browser)

2.3.6.2 Facilities for Multi-wavelength Analysis
[28](#facilities-for-multi-wavelength-analysis)

3 TECHNICAL STATUS [29](#technical-status)

3.1 Observatory [29](#observatory)

3.2 Instruments [29](#instruments)

3.2.1 Spectrometer and Cryocooler [29](#spectrometer-and-cryocooler)

3.2.2 Detector Annealing [30](#detector-annealing)

3.2.3 Imager [31](#imager)

3.2.4 Aspect System [31](#aspect-system)

3.3 Software [31](#software)

3.4 Ground System [32](#ground-system)

3.4.1 Ground Stations [32](#ground-stations)

3.4.2 Mission Operations Center [32](#mission-operations-center)

3.4.2.1 Normal Operations [32](#normal-operations)

3.4.3 Science Operations Center [33](#science-operations-center)

3.4.4 Max Millennium Program [33](#max-millennium-program)

3.4.5 Education and Public Outreach [33](#education-and-public-outreach)

4 Appendices [34](#appendices)

4.1 Budget [34](#budget)

4.2 Acronym List [35](#acronym-list)

# Executive Summary

The RHESSI Small Explorer mission is designed to investigate particle
acceleration and energy release in solar flares through imaging and
spectroscopy of hard X-ray (HXR)/gamma-ray continua emitted by energetic
electrons, and of gamma-ray lines produced by energetic ions. The single
RHESSI instrument provides ground-breaking imaging and spectroscopy
measurements over the broad energy range from soft X-rays (3 keV) to
gamma-rays (17 MeV).

RHESSI was launched in February 2002, and has been operating
successfully ever since. Over 11,000 flares with detectable emission
have been identified above 12 keV, \~950 above 25 keV, and 30 above 300
keV with 18 showing gamma-ray line emission. In addition over 25,000
microflares have been detected above 6 keV. Since the start of the
mission, all the data and the analysis software have been made
immediately available to the scientific community.

RHESSI provides unique observations of high-energy processes close to
the Sun that address the key goals of the Heliophysics Great
Observatory: understanding the fundamental processes of particle
acceleration and energy release in solar eruptions, both flares and
CMEs. The resulting photon emissions and accelerated particles directly
affect our Home in Space, and are especially important for our Journey
Outward.

In the \~2 years since the last Senior Review, analysis of RHESSI
observations has resulted in over 200 publications, and many press
releases, popular articles, and awards. Some of the new results include:

-   The first comprehensive survey of \~25,000 hard X-ray microflares,
    providing their occurrence frequency vs. energy, both thermal and
    non-thermal, and showing that essentially all of them occur in
    active regions, and that they generally show steep non-thermal
    spectra.

-   The best upper limits on the hard X-ray emission of the quiet Sun,
    \>2 orders of magnitude lower than the weakest microflare. These
    upper limits constrain the production of axions (a dark matter
    candidate) in the Sun's core, thus limiting the axion-photon
    coupling constant.

-   The discovery of non-thermal hard X-ray sources very high (up to
    \>0.3 Rs) in the corona (when the associated flare is occulted)
    accompanying all fast (\>\~1000 km/s) coronal mass ejections (CMEs).

-   The first survey of partially disk-occulted flares, showing rapid
    variations of the coronal non-thermal HXR emission (likely
    thin-target emission without significant collisional energy losses),
    often well above the thermal source.

-   The detection of X-ray source motions, first from the flare looptop
    down the legs and then back up, likely related to chromospheric
    evaporation.

-   The discovery of large numbers of relativistic electrons with
    extremely hard spectra in coronal loops above the footpoints in
    large flares.

-   The first measurements (with \~2σ significance) of polarization of
    \~100-350 keV X-rays in large solar flares.

-   The finding of a very close linear correlation between \>0.3 MeV
    gamma-ray continuum and 2.223 MeV neutron-capture line fluences,
    indicating that the acceleration of the parent relativistic
    electrons and \>\~30 MeV protons must be closely related.

-   The development of powerful new model-independent mathematical
    methods to infer the energy spectra of the parent electrons as well
    as information on their angular distribution, from the uniquely
    precise RHESSI X-ray measurements.

-   The development of plasma theory and simulations of the magnetic
    reconnection process in flares that directly accelerate electrons
    with high efficiency, as required by RHESSI and previous
    observations.

-   The discovery of a close correlation between the \>50 keV power law
    spectral indices of energetic electrons in prompt impulsive events
    observed by the Wind 3DP at \~1 AU and of the associated flare HXR
    burst, indicating a common source.

-   The discovery that the excess solar oblateness detected by RHESSI's
    SAS (solar aspect sensor) may be due to low level faculae that cover
    the entire solar disk and not just the active latitudes.

-   The discovery that the electrons up to 30 MeV that produce
    Terrestrial Gamma-Ray Flashes (TGFs) are accelerated at 13-15 km
    altitude, and that some of the electrons enter the magnetosphere.

The numerous results over the RHESSI lifetime have changed our
perspective on solar flares, particularly on high-energy processes in
flares. The most recent RHESSI workshop in June 2007 was devoted to
planning for a \~500-page book, to be published by Space Sciences
Reviews (for easy access). Most of the chapter drafts are finished, and
publication is expected later this year.

The spacecraft and instrument continue to operate nominally. After five
and a half years RHESSI's germanium detectors were showing strong
effects of radiation damage, as predicted, including significant loss of
active volume. The detectors were annealed in November 2007 by heating
up to \>\~90º C for a week, and they have recovered close to their full
sensitive volume and to the high energy resolution they had in 2005.
Further anneals are planned as needed to maintain acceptable detector
performance. Since the mission was designed with no expendables and the
orbit decay has been minimal, RHESSI should be able to operate for years
to come. Thus, RHESSI is ready for the rise from solar minimum to the
expected maximum of the next activity cycle in 2009-2012.

The recent successful launches of Hinode and STEREO bring unique new
observations that are highly complementary to RHESSI for the study of
solar flares, CMEs, and solar energetic particles. In particular, Hinode
provides the previously missing high cadence measurements in broad--band
soft X-rays, diagnostic EUV spectroscopy, and optical/ vector
magnetograph required for flare studies; while STEREO provides stereo
EUV imaging of flares, coronagraph imaging of CMEs, radio burst
tracking, and multi-point in situ measurements of energetic particles
and plasma/fields. The launch of the Solar Dynamics Observatory later
this year will add the full-Sun, high cadence, multi-wavelength imaging
of the Sun. Finally, the launch of GLAST later this year will provide
complementary measurements of ultra-high-energy gamma-rays from solar
flares. Thus, for the first time the elements of the Heliophysics Great
Observatory (HGO) required for comprehensive studies of energy release
and particle acceleration processes in flares and CMEs will be
available.

We propose a Solar High Energy Events GI program to maximize the science
return by supporting studies (not possible under the in-guide funding)
that integrate the remarkably complete suite of new high-energy and
supporting observations of solar explosions, and their energetic
consequences obtained by the HGO in this upcoming rise to solar maximum.

# Science

The past two years have been tremendously productive scientifically for
the RHESSI mission. RHESSI has provided (and continues to provide) an
enormously rich data set. A detailed description of the RHESSI mission,
instrument, and software is given in the first six papers of the Nov.
2002 issue of Solar Physics (vol. 210, p. 3-124). Here is a brief
summary.

RHESSI provides high-resolution imaging and spectroscopy of
bremsstrahlung X-ray/gamma-ray continuum emitted by energetic electrons,
and gamma-ray lines produced by energetic ions. At HXR and gamma-ray
energies, the only viable method of obtaining arcsec-class images within
the SMEX constraints is with Fourier-transform imaging. The RHESSI
instrument has an imager made up of nine Rotating Modulation Collimators
(RMCs), each consisting of a pair of widely separated grids mounted on a
rotating spacecraft, to achieve angular resolution as fine as \~2 arcsec
and imaging up to gamma-ray energies. Behind each RMC is a segmented
germanium detector (GeD) to detect photons from 3 keV to 17 MeV. The
GeDs are cooled to \~90 K by a space-qualified long-life mechanical
cryocooler, to achieve the high spectral resolution (\~1 to 10 keV
FWHM). The GeDs were successfully annealed in November, 2007, to
mitigate radiation damage.

As the spacecraft rotates, the RMCs convert the spatial information from
the source into temporal modulation of the photon counting rates of the
GeDs. Pointing information is provided by the Solar Aspect System (SAS)
and redundant Roll Angle Systems (RASs). An automated shutter system
allows a wide dynamic range (\>10^7^) of flare intensities, from
microflares to the largest X-class flares, to be handled without
instrument saturation, and it allows the sensitivity to be maximized
when low-energy (\<\~20 keV) X-ray fluxes are low. The spin-stabilized
(\~15 rpm) spacecraft is Sun-pointing to within \~0.2º and operates
autonomously, with the energy and time of arrival for every photon
stored in a solid-state memory (sized to handle the largest flare) and
telemetered to the ground.

The RHESSI observations provide powerful diagnostics because: 1) they
combine high spatial resolution, high spectral resolution, and excellent
precision; 2) they span an enormous energy range from soft X-rays (\~3
keV) emitted by hot thermal plasmas, through the hard X-ray/gamma-ray
continuum emitted by accelerated electrons, to gamma-ray lines emitted
by accelerated ions; 3) data for every detected count are brought to the
ground so there is unlimited flexibility in the data analysis; and 4)
the X‑ray and gamma-ray emission processes are quantitatively
well-understood.

Thus, significant effort over the past two years was devoted to
inferring the parent electron distribution in energy, angle, time, and
space from the RHESSI X-ray measurements. The methods developed take
into account the effect of albedo -- scattering of the photons from the
dense photosphere -- on the X-ray spectrum. This also allows information
on the directivity of the electrons to be inferred, and to correct for
it in deriving the parent electron source spectrum. Recently, a new
technique has been developed that utilizes the imaging spectroscopy
measurements to go directly to the parent electron distribution in
energy and space.

The observations appear to be generally consistent with the flare
electron (and ion) acceleration being intimately related to the magnetic
reconnection. For many flares the accelerated tens of keV electrons
contains \>\~10-50% of the total energy released in the flare. Since
reconnection ideally results in bulk flows, a long-standing question is
how can the energy released be converted to energetic electrons. In the
last couple of years this problem has been addressed by leading plasma
theorists and simulation experts, and a possible explanation has been
put forward that arises naturally from the reconnection process, and
that also appears to explain electron acceleration in a reconnection
event observed in situ in the Earth's deep magnetotail.

Another puzzle has been the detection of HXR emission only at a few
footpoints while optical/EUV emissions are seen along the entire ribbons
of the flare loop arcade. Recent detailed analyses of RHESSI
observations with the finest spatial resolution show HXR emission along
the entire loop arcade. It's not yet clear whether the normal lack of
ribbon HXR emission is due to dynamic range limitations of RHESSI's
imaging.

In the past two years there have been systematically searches for
relatively weak coronal HXR emission when the flare is occulted by the
limb. Coronal non-thermal emissions are found to be common -- above
flare loops and behind fast CMEs. Furthermore, at high energy
(\>\~200keV) HXR emissions seem to be stronger in the corona compared
with tens of keV emissions.

Comparisons of RHESSI HXR observations with Wind energetic electron
observations for prompt impulsive SEP events show that the \>50 keV HXR
spectra are closely correlated to the escaping electron spectra,
suggesting a common source for the HXR-producing and escaping electrons.
However, the relationship is not consistent with standard thick or thin
target models.

Finally, the approach to solar minimum in the last few years has been
ideal for systematic studies of microflares and their implications for
coronal heating, and of quiet Sun (no active regions) X-ray emissions.
With the attenuators out RHESSI provides uniquely high sensitivity at
3-20 keV energies.

In addition, RHESSI's Solar Aspect System (SAS) has provided the best
solar optical shape measurements ever obtained, opening up a new area of
research. RHESSI's discovery that TGFs commonly extend up to \>\~20 MeV
has revitalized the study of lightning related high-energy phenomena.
Finally RHESSI provides unique measurements for a variety of
astrophysical high-energy phenomena.

To tap the real power of RHESSI requires the integration of its data
with that of the other HGO missions and with the Gamma-ray Large Area
Space Telescope (GLAST) that will be launched in a few months. To
understand what is special about the flare and CME-related particle
acceleration and energy release regions identified by RHESSI, we need
good magnetic field models based on accurate vector photospheric (and
chromospheric if available) measurements. We need information on the
magnetic topology, waves, shocks, and ambient conditions in the corona
obtained by instruments on Hinode and STEREO, as well as on TRACE, SOHO,
and groundbased observatories; SDO will soon provide additional
capabilities. To understand the Sun's connection to the heliosphere will
require ACE, Wind, STEREO, SOHO, Ulysses and other spacecraft measuring
energetic particles, radio emission, magnetic fields, and the solar
wind. We want to understand what conditions lead to these high-energy
eruptive phenomena. To further the joint analysis of the many events
expected to be observed in common, the RHESSI team is proposing a GI
Program specifically for this purpose. A joint workshop is already
planned for December 2008 in Napa, CA, to discuss the results of the
early cross-platform (RHESSI, Hinode, STEREO) observations. Preliminary
results (see below) already show the promise of such multi-platform
comparisons.

## Accomplishments

To date, there have been over 670 papers and at least 25 PhD theses
published using RHESSI data. They cover a very wide range of topics so
we are only able to highlight some of the areas of current research.

### Hard X-rays & Parent Electrons 

#### Photospheric Compton Scattering of X‑rays (Albedo)

The observed hard X-ray emission from solar flares is a combination of
primary bremsstrahlung photons with a reflected component from dense
layers of solar atmosphere of the downward-directed primary emission.
This backscattered component can be significant, creating new features
and/or distorting or masking features in the primary spectrum, and so
substantially modifying key diagnostics such as the electron spectrum or
electron energy budget. This effect is well-known in solar physics, and
more generally in X-ray astronomy, and it has been widely studied
theoretically (e.g., Tomblin, 1972; Santangelo et al., 1973; Bai and
Ramaty, 1978). But it is only with the remarkably precise RHESSI
measurements that photospheric albedo can be detected and studied in the
X-ray spectrum (Kasparova et al 2005, Kontar et al, 2006) and in X-ray
images (Schmahl and Hurford, 2003). It is straightforward to calculate
the albedo by Monte-Carlo simulations (e.g. Bai and Ramaty, 1978) if the
primary X-ray spectrum is known. However, the primary spectrum is
unknown and unlikely to be an exact power-law, as is sometimes assumed.
Kontar et al. (2006) have developed a Green\'s function approach to the
backscatter spectral deconvolution problem - constructing a Green\'s
matrix including photoelectric absorption - that allows a
spectrum-independent extraction of the primary spectrum from the
spectrum observed by RHESSI. The method is now a fully integrated part
of the RHESSI OSPEX software package.

As a result of the dependence of the albedo component on the
heliocentric angle (the albedo contribution should be stronger for disk
events as compared to limb events), the shape of photon spectra should
vary as a function of their position on the solar disk (Bai and Ramaty,
1978; Kontar et al., 2006). A statistical analysis of RHESSI flares
(Kasparova et al., 2007) demonstrates a clear center-to-limb variation
of photon spectral indices in the 15 - 20 keV energy range and a weaker
dependency in the 20 - 50 keV range. The observed spectral variations
were found to be consistent with the predictions of albedo-induced
spectral index changes.

The albedo spectral "contaminant\" in fact can provide information on
the anisotropy of the flare fast electron distribution, by providing a
view of the hard X-ray flare from behind, like a dentist\'s mirror.
Moreover, the solar albedo "mirror\" is spectrally distorting, resulting
in an albedo "bump\" feature, the strength of which is an indicator of
the degree of downward beaming of the electron distribution. This solar
"mirror\" allows us to obtain information on the directionality of the
primary X-ray source, and therefore on the accelerated electron
distribution, from a single spacecraft. Application of this spectrometry
method to two strong flare X-ray bursts (Kontar and Brown, 2006)
observed by RHESSI shows that the ratio of downward and upward
propagating X-ray emitting electrons is close to one in a wide range of
energies.

In the standard thick-target flare model, electrons are accelerated high
in the corona and travel down the loop field lines to the dense
chromosphere where they produce the observed flare hard X-ray emission.
The isotropization (and energy loss) of the downward electron beam is
due to the collisions only and hence isotropy is only approached when
electrons have lost most of their energy and are therefore no longer
emitting very energetic radiation. The lack of a strong electron
beaming, inferred from RHESSI data for these two intense flare hard
X-ray bursts close to disk centre events (Kontar and Brown, 2006), rules
out the basic thick-target model that has been around for decades,
unless the electrons are effectively isotropized somehow. For example,
X-ray source electrons could be locally and continuously reaccelerated
with near isotropy along the entire length of a magnetic loop. Such a
model would also solve other difficulties of the thick-target model,
such as the problematic high beam density and currents involved.

More recently, in a study of nine flares showing spectral flattening at
low energies, Sui et al. (2007) found that albedo from isotropically
emitted photons could account for this flattening in only three of the
flares, all located near disk center where the contribution from albedo
is greatest. We note here that for some flares, RHESSI can directly
measure the albedo component to provide information on the height of the
HXR source and its directivity.

Kontar et al. (2007) have shown that the quality of the RHESSI spectra
at energies \>300 keV is sufficient to clearly identify the role of
electron-electron bremsstrahlung in the hard X-ray spectrum, and to
modify the parent electron flux distribution accordingly. Application of
their method shows that previously unexplained "breaks" in flare
electron spectra may simply be an artifact of neglecting this important
component of the emission.

Kontar, E.P., Emslie, A.G., Massone, A.M., Piana, M., Brown, J.C., &
Prato, M. 2007, Ap.J., 670, 857.

Sui, L., Holman, G. D., and Dennis, B. R. 2007, ApJ, 670, 862.

#### New Imaging Spectroscopy Methods

One of the main science goals of RHESSI is ***imaging spectroscopy***,
the production of an "image cube" of HXR intensity I versus 2-D position
(*x, y*) and photon energy ε. Knowledge of the cross-section for the
hard X-ray emission process then allows us to transform this into the
real science goal -- a map of the accelerated electron flux spectrum F
versus (*x, y*) and electron energy E. Over the past year or so, major
advances in this transformation have been made.

RHESSI stores imaging information through the temporal modulation of
flux by the nine sets of rotating grid pairs. This information is most
readily translated into visibilities, two-dimensional spatial Fourier
transforms of the image (Hurford et al. 2002, 2008). "Traditional"
imaging spectroscopy then proceeds by "stacking" count-based images at
different ε and examining the hard X-ray intensity *I*(ε) in prominent
features. However, this method has several drawbacks. First, it does not
fully optimize the way in which spatial information is encoded by
RHESSI. This is because the inferred flux in any feature (or spatial
region) is determined by a mix of a finite number of spatial Fourier
components which also extend beyond the feature under scrutiny. Thus,
there is not only a loss of information due to limited spatial frequency
sampling but also contamination due to Fourier sidelobes from
neighboring features. Second, since the counts in different energy
ranges are independent statistical quantities, there is no a priori
guarantee that, in the presence of statistical noise, images will vary
smoothly with photon (or count) energy, as they must in the physical
case of an electron flux that does vary smoothly with energy.

A new imaging spectroscopy technique (Piana et al. 2007) has recently
been developed which overcomes these difficulties and produces the
desired "information cube" F(x, y; E). The method accomplishes the
transformation from count data to electron flux information by
performing a regularized spectral inversion on the count spectrum at
sampled points in the spatial frequency domain, i.e., on the photon (or
count) visibility spectra V(u, v; ε). This algorithm demands smoothness
in the electron spectrum at each spatial frequency point, thus enhancing
patterns that persist over a relatively wide energy band, while
suppressing noise-related signals that are present only over a narrow
range of energies. Well-established image reconstruction techniques
(Hurford et al. 2002) can then be applied to the resulting "electron
flux visibilities" W(u, v; E) to produce the desired "electron flux
images" F(x, y; E). Since the visibilities used to construct these
electron flux images are obtained using a regularized inversion
technique, they, and their corresponding "electron images," will, by
construction, vary smoothly with E. We can then reliably infer
physically plausible electron spectra in prominent spatial features by
"stacking" the recovered electron images.

#### HXR Emission Along Flare Ribbons

One of the best examples of the use of RHESSI's imaging spectroscopy
with the finest grids is shown in fig. 2.1.1-1 for the M8.0 flare on May
13, 2005 (Liu et al. 2007a, b, 2008; Jing et al. 2007). This event was
sufficiently intense and showed significant modulation in the count
rates of all nine detectors to allow fine structure at the 2‑arcsecond
level to be imaged. Remarkably, the 25 -- 50 keV image shows the
emission extending along the full extent of the ribbons and closely
matches the TRACE 1600 Å brightenings. This is in contrast to the more
usual situation earlier in this flare and in many other flares where
just one or two bright HXR footpoints are seen on each EUV or Hα ribbon.
Presumably, in those cases, energetic electrons are precipitating at all
points along the ribbons but only the brightest points show up in the
RHESSI images because of its limited dynamic range of \<50:1. At the
time of the image shown in fig. 2.1.1‑1, the flux of precipitating
electrons was sufficiently uniform along the ribbons to allow their
presence to be revealed at all locations in the HXR image. The 6 -- 12
keV contours show the usual high--temperature (\~20 MK) source extending
along the top of the arcade of loops that produce the ribbons.

![](media/image1.png){width="3.1in" height="3.073611111111111in"}**Fig.
2.1.1-1.** TRACE 1600 Å image (with blue-green-red-yellow color scale)
near the peak of the two-ribbon GOES M8 flare at 16:45:28 UT on 13 May
2005. The overlaid contours show the 6 -- 12 keV (black) and 25 -- 50
keV (purple) sources obtained from one-minute integrations of RHESSI
data about this time. The RHESSI images were reconstructed using the
CLEAN algorithm with modulated count rates equally weighted for all nine
subcollimators, giving a resolution as fine as the 2 arcsecond (FWHM)
capability of the finest grids. The TRACE image has been moved by 2
arcsec in X and 5 arcsec in Y to provide the best match to the RHESSI 25
-- 50 keV sources.

This 13 May 2005 event was associated with an EUV sigmoid eruption, and
the extensive observations have been analyzed in detail by Liu et al.
(2007a, b, 2008) and Jing et al. (2007). They have attempted to explain
why the HXR sources appear first as one or two individual bright
footpoints but later extend relatively uniformly along the full extent
of the ribbons. Early in the event when only one or two point-like HXR
sources were seen on each ribbon, Jing et al. (2007) found a good
spatial correlation between the HXR flux and an energy release rate
consistent with the standard 2D reconnection model. They suggest that
tether-cutting reconnection first occurs between the two elbows in the
middle of the sigmoid, resulting in electron acceleration concentrated
on a few field lines and the HXR sources appearing footpoint-like, as
usual (e.g. Moore et al. 2001). Later on, reconnection takes place in
the current sheet above the magnetic arcade of loops and the accelerated
electrons bombard the chromosphere all along the footpoints of the
arcade, resulting in the ribbon-like HXR emissions. Exploiting RHESSI's
capability of imaging spectroscopy in each pixel, Liu et al. (2008)
further found a spatial anticorrelation relationship between the local
HXR flux and the local spectral index. They suggested this as a spatial
analog of the temporal soft-hard-soft spectral evolution pattern of the
integrated HXR flux, which may contribute to explaining the usual
confined nature of HXR sources compared with extended H/UV ribbons.
They also found an anticorrelation between the HXR spectral index and
the inferred convestive electric field along the ribbon.

Emslie, A. G. et al. 2003, ApJ., 595, L107.

Hurford, G. J., et al. 2002, Sol. Phys., 201, 61

Hurford, G. J. et al. 2008, in preparation.

Jing, J. et al. 2007, ApJ, 664, L127

Liu, C. et al. 2007a, ApJ, 658, L127

Liu, C. et al. 2007b, ApJ, 669, 1372

Liu, C. et al. 2008, ApJ, 672, L69.

Moore et al., ApJ 552, 833, 2001

Piana, M., et al. 2007, ApJ, 665, 846.

Sui, L, et al. 2002, Sol. Phys., 210, 245

#### Electron Acceleration During Magnetic Reconnection

HXR observations show that the accelerated tens-of-keV electrons often
contain \>\~10-50% of the total energy released in the flare, implying
that electron acceleration is intimately related to the energy release
process (Lin & Hudson, 1971), presumably magnetic reconnection. Electron
acceleration to hundreds of keV is also observed inside the reconnection
region in the Earth's deep magnetotail (Oieroset et al, 2002). In the
last few years, very significant progress has been made in the
understanding of collisionless magnetic reconnection, through in situ
space measurements, laboratory studies, and extensive theory and
simulation work. Recently, the long-standing problem - to explain how
energetic electrons are efficiently produced as magnetic fields
reconnect and release energy - has been addressed by Drake et al
(2006a). Particle-in-cell simulations of reconnection show that the
narrow current layers form at the X-line and produce secondary magnetic
islands (Drake et al 2006b) that result in strong electron energization.
While some acceleration comes from parallel electric fields, the primary
energy gain is due to the contraction of the initially squashed magnetic
islands. Electrons circulating rapidly within the islands gain energy
through a Fermi process, reflecting off the ends of the islands as they
move inward at the Alfvén speed. Up to 60% of the released magnetic
energy is transferred to the electrons in the process. The computed
energy spectra depend on the plasma β, and agree well with the
magnetotail observations (Oieroset et al 2002), while the limiting β = 0
case produces very flat spectra similar to those observed in the most
intense flares.

Further work by the same group (Cassak et al 2006) show how a
spontaneous transition from slow Sweet-Parker reconnection (needed to be
able to store magnetic energy) to fast magnetic reconnection can occur
in the solar corona.

Oieroset, M. et al. 2002, Phys. Rev. Lett. 89, 195001.

Lin, R. P., & H. S. Hudson, 1971, Solar Phys., 17, 412.

Drake, J.F., et al.2006a, Nature, 443, 553.

Drake,J.F., et al.2006b, Geophys. Res. Lett. 33,L13105

Cassak, P.a. et al. 2006, Ap.J.644, L145.

#### Partially Disk-occulted Flares

Initial RHESSI results (Krucker & Lin 2008) of 55 partially
disk-occulted flares reveal that 90% show two components in the HXR
range - thermal emission at lower energies and additional emission
extending to higher energies with fast time variations and a soft
spectrum. Krucker & Lin (2008) suggest that, at least in some events,
the rapidly-varying component could be produced in the thin-target
scenario (i.e. faint HXR production without significant collisional
energy losses) by the same population of electrons that later
precipitate and lose their energy by collisions in loop footpoints
(thick target).

A typical example is shown in the left panels of fig. 2.1.1-2, where
thermal emission originates from a simple loop at the western limb.
Above 18 keV, faint nonthermal emission with fast time variations (of
the order of tens of seconds) is seen that comes from a loop \~2000 km
above the thermal loop observed at the same time. However, the
nonthermal loop agrees well in altitude with the thermal flare loop seen
later, at the time of the soft X-ray peak. This is consistent with
simple flare models (from Krucker et al. 2007).

The right panels of fig. 2.1.1-2 show a rare example where the
nonthermal emission originates clearly from above the thermal flare
loops (\~8000 km above in this case), similar to the Masuda flare
(Masuda et al. 1994). However, later in this event, the thermal loops
never reach the altitude of the nonthermal source.

![](media/image2.png){width="3.0972222222222223in"
height="2.2777777777777777in"}**Fig. 2.1.1-2**. Two examples of RHESSI
X-ray observations of partially occulted flares. Top panels: GOES soft
X-ray (red) and RHESSI hard X-ray (blue) time profiles. Bottom panels:
RHESSI imaging results with thermal emission in red and nonthermal
emission in blue.

![](media/image3.png){width="2.0in" height="1.9083333333333334in"}Within
the next few years, we expect that there will be many joint observations
of flares that will be disk-occulted from RHESSI and Hinode/XRT but that
one of the STEREO spacecraft will see on the disk. Thus, RHESSI and
Hinode/XRT observations will provide imaging of thermal coronal
emissions at different temperatures (first results were published by
Krucker et al. 2007), while one of the STEREO spacecraft will show the
full flare geometry including the location of EUV ribbons. This will
place the coronal HXR emission in the context of the flare ribbons.

Masuda et al., Nature, 371, 495, 1994.

Krucker, S.; Lin, R. P., ApJ, 673, 1181, 2008.

#### Hard X-ray emissions from the high corona associated with CMEs

For flares occurring 20 or more degrees behind the solar limb, not only
the HXR footpoints but also the main thermal and nonthermal HXR
emissions from the corona are occulted, and flare-related emissions from
the high corona (\>200 arcsec above the flare site) can be seen. Krucker
et al. (2007a) found evidence that all fast (\>1500 km s^-1^) farside
CMEs that originated from flares occulted by 20^o^ to 45^o^ may show
related HXR emissions from the high corona.

RHESSI provides the first high-resolution imaging spectroscopy of HXR
emission from such sources (Fig. 2.1.1‑3 & 4). The first RHESSI
observations analyzed show large sources (\>200 arcsec) that expand and
move outwards. The speed of the hard X-ray source is lower than the
speed of the CME front, but similar to the speed of the trailing
filament. This suggests that energetic electrons are being advected
outwards in magnetic flux tubes related to the CME.

![](media/image4.png){width="3.1in" height="3.267361111111111in"}**Fig.
2.1.1-3.** Time profile of the October 27, 2002 event that was seen
on-disk from Mars, while for Earth-orbiting spacecraft the flare site
was occulted by at least 200 arcsec (0.2 solar radius). From top to
bottom the panels show, (1) GRS X-ray and gamma-ray time profiles seen
from Mars (entire flare is seen), (2) GOES soft X-ray flux seen from
Earth, (3) RHESSI 15-25 keV time profile seen from Earth, (4) RHESSI HXR
spectrogram.

**Fig. 2.1.1-4.** RHESSI X-ray imaging in the thermal range (3-7 keV,
red contours) and nonthermal range (10-30 keV, blue contours) reveal
large sources (\>200 arcsec) seen just above the limb. The RHESSI CLEAN
algorithm was used to reconstruct these images, and the contour levels
are at 15, 30, 45, 60, 75, and 90% of the maximum. The thermal emission
on the disk around ‑800/200 arcsec is from AR10717 and was already
present before the flare started.

Multi-spacecraft observations had already shown that such HXR emissions
from the high corona could occur during the impulsive phase of the
flare, simultaneously with the HXR footpoint emissions. The time
profiles of the high coronal events all look similar and show a
relatively simple time evolution, with a fast rise and a slower
exponential decay. The exponential decay is surprisingly constant,
lasting sometimes several minutes without significant deviation, and the
photon spectrum exhibits progressive spectral hardening. This might
indicate that collisional losses dominate without further acceleration.
The relatively simple spectral shape and temporal evolution of these
events simplifies spectral inversion and the direct determination of the
spectrum of the accelerated electrons.

#### HXR Source Motions Along Loops

In addition to looptop and above-the-looptop sources and their motions,
RHESSI has observed X-ray source motions along the legs of flare loops
(Sui et al. 2006, Liu et al. 2006). Sui et al. observed a source region
move downward from the top of a flare loop to the footpoints and then
back up to the looptop during the duration of a flare. They argue that
this motion is partially explained by the soft-hard-soft evolution of
the accelerated electron distribution and note that this X-ray source
motion can be used to deduce the evolution of the plasma density in the
legs of the flare loop. Liu et al. observed upward X-ray source motion
in a flare loop and used this motion to deduce the evolution of
chromospheric evaporation in the loop.

We expect a dramatic increase in our understanding of coronal hard X-ray
sources with the advent of Hinode and STEREO. With STEREO observations,
we will often be able to tie down a limb-flare location unambiguously.
There also may be improved knowledge of the magnetic structure. This
will greatly enhance our understanding of the event geometry. Hinode
will also be making coronal observations with its high-resolution soft
X-ray telescope and with its EUV imaging spectrograph. For CME-related
events, we will have the opportunity to locate the coronal hard X-ray
sources (with their information about the acceleration and the energy
release) in the evolving magnetic structure of the eruption for the
first time.

Asai, A., Nakajima, H., Shimojo, M., White, S. M., Hudson, H. S., and
Lin, R. P. 2006, PASJ, 58, L1

Lin, R.P. and 12 coauthors, ApJL, 595, L69 (2003a).

Lin, R.P. Krucker, S., Holman, G. D., Sui, L., Hurford, G. J., and
Schwartz, R. A., Proc. 28^th^ Int. Cosmic Ray Conf., p. 3207 (2003b).

Liu, W., Liu, S.; Jiang, Yan W.; Petrosian, V. 2006, ApJ, 649, 1124.

[Masuda, S.](http://adsabs.harvard.edu/cgi-bin/author_form?author=Masuda,+S&fullauthor=Masuda, S.&charset=ISO-8859-1&db_key=AST),
[Kosugi, T.](http://adsabs.harvard.edu/cgi-bin/author_form?author=Kosugi,+T&fullauthor=Kosugi, T.&charset=ISO-8859-1&db_key=AST),
[Hara, H.](http://adsabs.harvard.edu/cgi-bin/author_form?author=Hara,+H&fullauthor=Hara, H.&charset=ISO-8859-1&db_key=AST),
[Tsuneta, S.](http://adsabs.harvard.edu/cgi-bin/author_form?author=Tsuneta,+S&fullauthor=Tsuneta, S.&charset=ISO-8859-1&db_key=AST),
[Ogawara, Y.](http://adsabs.harvard.edu/cgi-bin/author_form?author=Ogawara,+Y&fullauthor=Ogawara, Y.&charset=ISO-8859-1&db_key=AST),
Nature, 371, 495 (1994).

Qiu, J., Lee, J., and Gary, D. E., ApJ 603, 335 (2004).

Sui, L., Holman, G. D., and Dennis, B. R. 2006, ApJ, 645, L157.

Veronig, A.M., and Brown, J.C., ApJ. 603, L117 (2004).

White, S.M., Krucker, Sam, Shibasaki, K., Yokoyama, T., Shimojo, M., and
Kundu, M.R., ApJL, 595, L111 (2003).

### Gamma-Rays

Gamma rays are produced when electrons and ions above several hundred
keV interact with the solar atmosphere. The gamma-ray spectrum observed
from 200 keV to 8.5 MeV by RHESSI during the 20 January 2005 flare is
plotted in Figure 2.1.2-3.

#### Gamma-ray-line Imaging

Gamma-ray imaging provides the only available observational technique
for studying the spatial distribution of accelerated ions near the Sun.
RHESSI is the only instrument available for acquiring such images. The
first gamma-ray image of a solar flare was obtained using the intense
neutron-capture line at 2.2 MeV in the X4.8 flare of 2002 July 23
(Hurford et al., 2003). Since that event, four more flares have been
imaged in the neutron-capture line. For all of these events, most if not
all of the gamma-ray line emission came from compact sources in the
flare region.

![](media/image5.png){width="3.0708333333333333in"
height="3.0166666666666666in"}

**Figure 2.1.2-1.** Imaging of the 2.223 MeV neutron-capture line (blue
contours) and the HXR electron bremsstrahlung (red contours) of the
flare on October 28, 2003. The underlying image is from TRACE at 195 Å.
The HXR and γ-ray line double footpoint sources are separated by
\~10,000km. Note the apparent loop-top source in the hard X-ray
contours.

The, 2003 October 28 flare showed a double source at 2.2 MeV straddling
the EUV arcade (Figure 2.1.2-1, Hurford et al, 2006), similar to the HXR
footpoints. This is as expected if the ion acceleration is related to
magnetic reconnection, and thus strongly suggests that the parent ions
are accelerated in the flare process and not by CME shocks. The
gamma-ray line sources are displaced by 10-15 thousand km from the HXR
sources for the 2002 July 23 and 2003 Oct 28 flares. At present there is
no convincing explanation for this separation.

Hurford, G. J., Schwartz, R. A., Krucker, S., Lin, R. P., Smith, D. M.,
and Vilmer, N., 2003, ApJ, 595, L77.

Hurford, G. J., et al. 2006, ApJ. 644, L93.

#### Coronal Gamma-ray Continuum Sources

RHESSI provides imaging above 100 keV for the first time, so we now have
direct diagnostics of non-thermal electron bremsstrahlung of
relativistic flare electrons. Imaging at these high energies is limited
by counting statistics, and only the largest flares have high enough
counting rates for imaging. So far, only the three events with best
counting statistics have been analyzed for coronal emission at the
highest energies (Krucker et al., 2008). These reveal, besides footpoint
sources, the existence of coronal gamma-ray sources with very hard
spectra. For the usual power-law fit to the spectrum, we find indices
between 1.5 and 2, suggesting that \>1 MeV electrons confined in the
corona produce these emissions (Figure 2.1.2-2).

#### Polarization of Gamma-ray Continuum

Boggs et al (2006) and Suarez-Garcia et al. (2006) have reported on the
first measurements by RHESSI of the polarization of \~100 --350 keV
solar flare gamma-ray continuum produced by near-relativistic electrons.
Such polarization would be expected if the parent electrons are beamed,
and the direction of polarization is related to the direction of the
beaming. The polarization is measured by looking for photons that
Compton scatter from one RHESSI detector to another. Since every energy
loss is recorded with its time to a microsecond, it is straightforward
to search for coincident energy losses in two detectors and determine
the direction between the detectors. Systematic errors are reduced since
RHESSI rotates rapidly (4s period). The six X-class and one M-class
flare range from 0 to \~50% polarization, highly intriguing but only a
2σ result.

Boggs et al. 2006, Ap.J., 638,1129-1139.

Suarez-Garcia et al. 2006, Solar Phys.,239,149.

#### Harder Flare-Accelerated Ion Spectra?

During Solar Cycles 21 and 22, gamma-ray observations made by
spectrometers on SMM, Yohkoh, and CGRO indicated that flare-accelerated
ions could be represented as power laws with indices ranging from \~3 to
\~5. In contrast, four of the best-studied RHESSI flares appeared to
have indices ranging from \~2.2 to 3.5 based on early spectral studies.
The hardness of the RHESSI flare spectra was inferred from diagnostic
line ratios, e.g. Ne/O line ratios, and from the relative weakness of
these lines compared to the continuum.

![](media/image6.png){width="3.091666666666667in"
height="2.873611111111111in"} **Figure 2.1.2-2.** **(Top left)** Time
profiles in GOES soft X-rays and RHESSI gamma-rays for the flare of
January 20, 2005. The red line is an exponential fit to the 250-450 keV
count rate with a decay time of 257 ± 2s. The vertical lines mark the
time intervals of the two images shown below. **(Bottom)** Imaging
during the gamma-ray peak **(left)** and the decay **(right).** .Both
figures show a TRACE 1600Å image taken at 06:45:11 UT overplotted with
12-15 keV (red) and 250-500 keV (blue) contours. During the peak, the
gamma-ray emission is localized at the footpoints, while later an
additional coronal source becomes visible. **(Top Right)** Imaging
spectroscopy results averaged between 06:46:44 and 06:55:01 UT: The
spectrum of the coronal source and the combined footpoint sources are
given in magenta and blue, respectively. The power-law fits give a much
harder spectrum for the coronal source (gamma = 1.5 ± 0.2) than for the
footpoints (gamma = 2.9 ± 0.1). For comparison, the spatially integrated
spectrum (gray) and the sum of the coronal and footpoint emission
(black) are also shown.

One of the hardest spectra for solar energetic particles (SEPs) ever
seen in space was measured during the 2005 January 20 solar flare. The
RHESSI gamma-ray spectrum shown in fig. 2.1.2‑3 is dominated by the
511-keV annihilation line and the 2.223 MeV neutron-capture line, and by
strong bremsstrahlung and nuclear continua. The nuclear de-excitation
lines are not prominent. The strong annihilation line was in part due to
\>20 MeV solar photons interacting in the material surrounding the
detectors. The time histories of the different components of the
gamma-ray spectrum are plotted in Fig. 2.1.2‑4.

![](media/image7.png){width="3.1in"
height="2.232638888888889in"}**Figure 2.1.2-3**. RHESSI gamma-ray
spectrum for the 2005 January 20 gamma-ray line flare showing the
different contributions to the count rate spectrum.

![](media/image8.jpeg){width="3.33125in"
height="2.5944444444444446in"}**Figure 2.1.2‑4**. Time history of
various gamma-ray components measured by RHESSI during the 2005 January
20 solar flare.

The bremsstrahlung component rises and peaks before the nuclear
de-excitation line emission, consistent with what is expected for
particle transport in a magnetic loop. The neutron-capture line peaks
later due to the slowing down and capture of the neutrons deep in the
solar atmosphere. The annihilation line peaks even later due its
dominant origin from radioactive decays (a fraction of the line flux
comes from positrons arising in pion decays).

A comparison of the various line intensities suggests that the spectrum
of accelerated particles is consistent with a power law with an index of
\~3. The bottom panel shows the time history of the count rate of pulses
that triggered the upper-level-discriminator set at 20 MeV. It rises
much more sharply and lasts longer than any of the lower-energy
components. As we show in a later section, this high-energy emission
(\>20 MeV) appears to have been detected into the next RHESSI orbit. We
attribute it to the decay of charged and neutral pions produced by \>300
MeV protons (this is confirmed by Coronas SONG measurements). The
spectrum of accelerated particles producing the pion-decay radiation
appears to be a distinctly different component having a significantly
harder spectrum than the normal impulsive component. Some of the
conclusions reached in this study may change once the flare is
reanalyzed using an improved gamma-ray detector response function. We
return to this discussion of SEPs in Section 2.1.3.

#### Correlated Electron and Ion Acceleration

We have found a remarkably tight linear correlation (*r*^2^ = 0.92)
between the 2.223 MeV neutron-capture line fluence and \>300 keV
bremsstrahlung fluence (Fig, 2.1.2-5) extending over more than three
orders of magnitude, taking all RHESSI flares with complete coverage and
correcting for limb darkening for the 2.223 MeV line (Shih et al. 2008).

![](media/image9.png){width="2.0in"
height="1.9041666666666666in"}**Figure 2.1.2-5** Correlation plot
between neutron-capture line fluence and the \>300 keV bremsstrahlung
fluence for the flares that satisfy the selection criteria (RHESSI in
solid diamonds, SMM in hollow circles). The dotted line indicates the
best-fit line in linear space that passes through the origin giving a
line/continuum ratio of 0.067.

These emissions are produced by \>\~30 MeV accelerated ions and \>300
keV accelerated electrons, respectively. The correlation fits to a
straight line passing through the origin, with a ratio of the fluences
of 0.067. The individual ratios of 14 of the 16 flares lie within a
factor of 2 away from the average, taking into account the ±1s errors
bars. The correlation also agrees with SMM/GRS data when making the same
selection cuts on flares. This proportionality implies that the
processes in flares that accelerate electrons above 300 keV and ions
above 30 MeV are closely related, and that the relative acceleration of
these two populations is independent of flare size.

It appears that solar flares accelerate ions to \>30 MeV whenever they
accelerate \>300 keV electrons down to RHESSI's detection limit. RHESSI
has observed flares with \>300 keV bremsstrahlung emission that lack
neutron-capture line emission, but these flares are invariably near the
solar limb or over the solar limb, and thus any neutron-capture line
emission would have been severely attenuated by the intervening
atmosphere. Thus, there does not appear to be a distinct class of
electron-dominated flares.

It is important to note that there are many other flares observed by
RHESSI with comparably large GOES classes that have both insignificant
\>300 keV bremsstrahlung and insignificant gamma-ray emission. This
difference could indicate that electrons must be accelerated to some
minimum energy before ions are accelerated to high energies.
Alternatively, flares with soft energy spectra may accelerate many ions,
but not to sufficient energies to produce de-excitation lines or the
neutron-capture line.

Shih, A. Y., et al., 2008, in preparation.

#### Gamma-ray and radio sub‑millimeter observations

**Figure 2.1.2-6.** Imaging in radio, UV, and gamma-rays during the
October 28, 2003 flare (from Trottet et al. 2008). The TRACE 1600Å image
is shown with 210 GHz centroid positions over-plotted as red crosses.
The average position and source size of the radio emission is
represented by a thick red cross. The dashed circle gives a rough size
of the field-of-view of the radio imaging with radio sources outside
this circle not represented by the derived source position and size. The
thick magenta circles give the flare-averaged 2.2 MeV gamma-ray
footpoint locations (from Hurford et al. 2006) with the circle radius
representing the 1s![](media/image10.png){width="3.1in"
height="1.8256944444444445in"} uncertainty in the source location.

The onset of the impulsive component of the radio sub-millimeter
emission during the giant flare of October 28, 2003 is simultaneous with
the start of high-energy (\>200 MeV/nucleon) proton acceleration and the
production of pions. The 210 GHz source size is compact (\<10 arcsec)
and its location is co-spatial with the 2.2 MeV footpoint emission seen
by RHESSI (fig. 2.1.2-6). The close correlation in time and space of the
sub-millimeter emission with the production of pions suggests that
synchrotron emission of positrons produced in charged-pion decay might
be responsible for the observed compact radio source. However,
order-of-magnitude approximations suggest that the derived number of
positrons from charged-pion decay is probably too small. As no spectral
information around 210 GHz is available, the emission mechanism of the
210 GHz emission is unclear. A second example of correlated gamma-ray
and submm emission is given by Silva et al. (2007) for the 2 Nov. 2003
flare.

Hurford at al., 2006, ApJ 644, L93.

Silva et al. 2007, Solar Phys. 245, 311.

Trottet et al. 2008, in preparation.

### HXRs and SEPs

#### RHESSI & WIND Observations

Comparison of Wind measurements of energetic electrons in solar
impulsive events with the electrons producing the flare X-ray emission
at the Sun observed by RHESSI provides unique information on the
energetic electron acceleration and escape processes. For prompt events
(where the injection of the electrons at the Sun, inferred from their
velocity dispersion, is simultaneous with the X-ray burst) we find that
the power-law exponents of the spectra of the electrons at 1 AU and of
the X-ray producing electrons are closely correlated, suggesting a
common source (Figure 2.1.3‑1, Krucker et al., 2007).

However, a simple model, where the electrons observed at 1 AU escape
directly from the acceleration source while the downward going
accelerated electrons produce the X‑rays while losing all their energy
to collisions in the dense lower solar atmosphere, does not fit. Some
additional acceleration of the downward going electrons may be
occurring; if magnetic reconnection does the initial acceleration,
possibly the downward going electrons are further accelerated by Fermi
and betatron processes as the newly closed field line becomes more
dipolar. For several of these events, RHESSI\'s X-ray imaging together
with TRACE imaging of lower temperature plasma, show accompanying jets
consistent with reconnection of an open field line with a closed loop
(Figure 2.1.3-2).

The spectrum of protons inferred from RHESSI\'s gamma-ray measurements
was found to be very similar to the spectrum of SEP protons observed at
1AU by Wind, ACE, GOES and SAMPEX, for magnetically well-connected
flares such as 2 November 2003 and 20 January 2005, suggesting that in
those cases some SEPs may indeed come from the flare.

Recent studies show that fast (\>1000 km/s) CMEs are accompanied by
nonthermal X-ray emission (indicating energetic electrons) detected by
RHESSI very high in the corona, but behind the front of the CME.
Comparison with energetic electrons accompanying the ICME when it passes
Wind will help to resolve the origin of these energetic particles.

#### HXR spectral evolution and SEPs

In a 1995 study, Alan Kiplinger found that flares with HXR spectra that
evolved from soft to hard to harder (SHH) are closely associated with
high-energy solar proton events observed in interplanetary space. While
the physical association between the progressively hardening X-ray
spectrum and the particles is not understood at present, the Kiplinger
study obtained a 96% success rate in predicting large proton events. If
one saw the SHH pattern in HXR spectral evolution, a proton event was
almost sure to happen. This association strongly suggests a physical
connection between the X-ray-producing electrons in the flare (on the
closed flare loops that create the magnetic mirror geometry) and the
escaping energetic protons on open field lines. This correlation is
puzzling, especially since it is generally believed that an
interplanetary shock front, remote from the flare itself, is the main
proton accelerator.

RHESSI images show that 20 to 100 keV X-rays originate from footpoints
during times with SHH behavior (Figure 2.1.3-3). This was previously
noted by Qiu et al. 2004, using Yohkoh/HXT observations of a flare with
similar progressive hardening. Hence, if coronal trapping is responsible
for the gradual hardening, the hard X-ray emission is not produced by
electrons in the corona, but by electrons leaving the trap and
precipitating to the footpoints. Therefore, the trapping time in the
corona must be shorter than the collisional loss time, at least at
energies below \~100 keV. At higher energies (\>200 keV), efficient
trapping is observed, and, during the decay phase of the HXR burst,
coronal emission can even dominate the footpoint emission.

R. Saldanha, S. Krucker, R.P. Lin, ApJ, 673, 1169, 2008

Qiu, J., Lee, J., and Gary, D.E., ApJ, 603, 335, 2004.

![](media/image11.png){width="5.946527777777778in"
height="2.015972222222222in"}

**Figure 2.1.3-1.** **Left:** Example of an in-situ electron spectrum
and a hard X-ray photon spectrum of a prompt impulsive electron event
(April 14, 2002). The top curve shows the in-situ electron peak flux
spectrum with the pre-event background spectrum subtracted. A broken
power-law fit to the data is shown with spectral indices of -1.5 and
-3.9, respectively below and above a break around \~40 keV. The X-ray
photon spectrum during the peak of the HXR event is shown below where a
thermal (red) and a broken power law are fitted to the data. The thermal
component with a temperature of \~14 MK dominates below 12 keV, while at
higher energies the spectrum is well represented by a broken power law
with spectral indices of 2.8 and 3.4 respectively below and above a
break energy of \~48 keV. The gray curves give the pre-event and
instrumental background. **Center and Right**: Correlation plot of the
photon spectral index γ and electron spectral index δ above \~50 keV for
16 prompt events (middle) and 16 delayed events (right). A positive
correlation is found for the prompt events, but no clear dependence is
seen for delayed events.

![](media/image12.png){width="5.995833333333334in"
height="2.9868055555555557in"}**Figure 2.1.3-2.** Example of
multi-wavelength imaging. **Left:** RHESSI contours at 6-12 keV (red,
thermal emission) and 20-50 keV (blue, nonthermal emission) are overlaid
on a TRACE 195Å EUV image (dark corresponds to enhanced emission).
Around 700/-245 arcsec, the X-ray emission suggests a loop with two
nonthermal footpoints.. The strongest nonthermal source, however, is
slightly to the south east (683/-257 arcsec) and shows surprisingly a
lower intensity thermal source. TRACE observations show an EUV jet at
720/-270 arcsec escaping from the flare region. **Right:** Simple
magnetic reconnection models (\'interchange reconnection\') show similar
source structure. The red box marks the reconnection site from where
downward moving electrons produce the HXR sources and upward moving
electrons escape into interplanetary space.

![](media/image13.png){width="3.058333333333333in"
height="2.597916666666667in"}

**Figure 2.1.3-3.** HXR spectral evolution and imaging of the January
19, 2005 X-class flare (Saldanha et al 2008). **Top**. GOES soft X-ray
flux (red, low-energy channel) and RHESSI 50-100 keV HXR (blue) count
rate. Temporal evolution of the power-law index in the energy range
between 50 and 100 keV. **Bottom**. HXR imaging results of the January
19, 2005 flare taken during the first HXR peaks (08:11:40 to 08:13:41
UT, left) with SHS behavior and later during the progressively hardening
phase (08:43:10 to 08:45:40 UT, right). Both figures show HXR contours
in the thermal (red contours at 20, 40, 60, and 80% of the peak) and
nonthermal (blue contours at 7.5, 15, 30, 50, 70, and 90% of the peak)
energy ranges superposed on TRACE 1600 Å images taken at 08:25:30 UT
(the first image available for this flare) and 08:45:03UT. For both
spectral behaviors, soft-hard-soft (SHS) and soft-hard-harder (SHH),
non-thermal HXR emissions in the 50-100 keV range are observed from
footpoints.

### Microflares

#### Microflare Statistics

One proposal for heating the solar atmosphere is via very small flares
(microflares, like ordinary flares but weaker, or Parker's "nanoflares")
occurring quasi-continuously over the solar disk. The viability of this
mechanism depends on the integrated energy of these events and on their
ubiquity, particularly in the absence of sunspots. Their integrated
energy depends on two factors: their frequency and the low-energy cutoff
to the steep nonthermal electron spectrum, which RHESSI has shown
dominates the energetics of the weakest events (Krucker et al., 2002).
RHESSI has high sensitivity (to detect the smallest events), spectral
resolution and coverage to characterize nonthermal and thermal spectra,
and imaging to associate individual microflares with active regions.

RHESSI results (Benz and Grigis 2002, Krucker et al. 2002, Qui et al.,
Liu et al. 2004, Battaglia et al. 2005, Hannah et al. 2008a) have shown
that microflares have relatively steep power-law (viz. nonthermal)
spectra extending down to below 10 keV in some cases. This has extended
the low-energy cutoff well below that available with previous
instruments, a result which has significantly raised estimates of their
integrated energy.

Christe et al. (2008) and Hannah et al (2008b) describe a sample of
\~25,000 events observed over RHESSI's first five years. Figure 2.1.4-1
shows this sample distributed in approximate heliographic coordinates.
There is a strong preference for the microflares in this figure to occur
in active regions. They also found that small flares are not necessarily
spatially small, with no correlation between the observed thermal loop
size and flare magnitude. This study characterized the energy content of
these small flares, and presented the first thermal energy (shown in
Figure 2.1.4-2) and non-thermal power distributions determined with
RHESSI. These observations confirm the strong similarity between
microflares and "real" flares that are orders of magnitude more
energetic. All events with sufficient counts for imaging are found to be
located in active regions. They are too intermittent in both space and
time to "explain" the heating of the general corona but their total
energy is roughly comparable to that needed to support the general
corona or the non-flaring active-region loop structures.

![](media/image14.png){width="3.0625in" height="3.0055555555555555in"}

**Figure 2.1.4-1.** Locations of \~25,000 microflares observed by RHESSI
during its first five years of operation from 2002 through to 2007.

The limb events are particularly interesting. A glance at fig. 2.1.4-1
shows that many locations lie just above the limb. (Note that RHESSI's
high angular resolution is precise at the sub-arc-second level for
centroiding, even for weak events.) Thus, a study analogous to that done
by Matsushita et al. (1992) for the much inferior HXT observations needs
urgently to be carried out. This and related studies can now be done in
the context of the STEREO data, which will greatly help in understanding
the geometry (even in 2D, i.e., just for footpoint identification) of
limb events seen by RHESSI.

![](media/image15.png){width="3.0833333333333335in"
height="2.4472222222222224in"}

Figure 2.1.4-2 The first RHESSI thermal energy distribution of
microflares (Hannah et al. 2008). The peak thermal energy for the RHESSI
microflares is shown in comparison to the thermal energy distributions
found for soft X-ray active region transient brightenings and EUV
nanoflares with EIT and TRACE.

Battaglia, M.; Grigis, P. C.; Benz, A. O., A&A. 439, 737, 2005.

Christe S., Hannah I.G., Krucker S. Lin R.P, McTiernan J, ApJ, 2008, in
press.

Hannah I.G., Christe S., Krucker S. Hurford G.J, Hudson H.S, Lin R.P,
ApJ 2008a, in press

Hannah I.G., Krucker S., Hudson H.S., Christe S., Lin R.P., A&A 2008b,
in press.

Hudson, H. S., Hannah, I. G., DeLuca, E. E., and Weber, M., in Proc. 1st
*Hinode* Sci. mtg (2008).

Krucker, S., Christe, S., Lin, R.P., Hurford, G.J., and Schwartz, R.A.;
Solar Phys. 210, 445-456, 2002.

Liu, C.; Qiu, J.; Gary, D. E.; Krucker, S.; Wang, H., ApJ 604, 442,
2004.

Matsushita, K., Masuda, S., Kosugi, T., Inda, M., and Yaji, K., Solar
Phys. 44, L89 (1992).

Neupert, W., Ann. Rev. Aston. Astophys. 7, 121-148, 1969.

Qiu, J., Liu, C., Gary, D. E., Nita, G. M., Wang, H., ApJ, 612, 530,
2004.

#### Microflares with RHESSI & Hinode/XRT

Individual microflares have been analyzed using both RHESSI and
Hinode/XRT. An example is shown in fig. 2.1.4-3. RHESSI provides
information regarding the energy content of the accelerated electrons
and where this energy is deposited. The high sensitivity and spatial
resolution of Hinode/XRT can then be used to observe the dynamic
response to this deposited energy. RHESSI is also available to image and
provide physical characteristics (through spectroscopy) of the hotter
component of the thermal plasma response. This has been done in detail
for a single microflare, also using TRACE EUV data in addition to RHESSI
and Hinode/XRT, to observe the cooler EUV response (Hannah 2008).

Hannah I.G., Krucker S., Hudson H.S., Christe S., Lin R.P., A&A 2008b,
in press.

![](media/image16.png){width="3.071527777777778in"
height="3.051388888888889in"}

**Figure 2.1.4-3** X-ray image from both RHESSI (coloured contours) and
Hinode/XRT (background image) (Hannah et al. 2008b). RHESSI shows where
the accelerated electrons are depositing their energy (blue contours)
and Hinode/XRT the dynamical response of the heated plasma as it fills
the post-flare loops.

### Quiet-Sun & Axion Observations

RHESSI's design is optimized for flare observations but we have been
able to push RHESSI's capabilities to observe fainter solar emission,
which we plan to continue through 2008 and 2009. The fan-beam modulation
technique (Hannah et al. 2007a) was developed to allow the faint solar
disk to be distinguished from instrumental and terrestrial background.
It requires pointing RHESSI slightly off the solar disk (between 0.4 and
1˚), thus chopping the solar signal as the spacecraft rotates at \~15
rpm. This off-pointing process has been successfully conducted several
times since 2005 during periods of quiet Sun when there have been no
active regions/sunspots on the disk. Initial analysis of these
observations has produced upper limits to the quiet-Sun X-ray flux
between 3 to 200 keV and showed that the 3-6 keV emission from the quiet
Sun is at least two orders of magnitude smaller than the smallest
microflares observed with RHESSI (Hannah et al. 2007b). This is shown in
Figure 2.1.5-1 as well as the fact that the non-flaring sun emission
appears to correlate to the softer GOES X‑ray emission in a similar
manner to microflares, suggesting that there might be an unresolved
ensemble of microflares being responsible for this non-active region
emission.

![](media/image17.png){width="3.09375in" height="2.577777777777778in"}

**Figure 2.1.5-1**. The use of the Fan beam modulation technique (Hannah
et al. 2007a) allows RHESSI to observe the non-flaring Sun, finding that
the quiet emission is over two orders of magnitude fainter than that in
small microflares and that it correlates with the softer GOES observed
X-ray emission in a similar way to the microflares (Hannah et al.
2007b).

The limits of the quiet-Sun emission found with RHESSI have been used to
put limits on the coupling constant of the theoretical axion particle
(Hannah 2007b). These particles have been postulated to solve the
strong-CP problem in quantum chromodynamics and have a cosmological
significance as a dark-matter candidate. Solar axions should be produced
copiously in the Sun's core via nuclear reactions, with the average
energy calculated to be 4.2 keV (Van Bibber et al. 1989) and with a
narrow ^57^Fe line near 14 keV (Moriyama 1995). These axions can
directly convert to X-ray photons of the same energy in the corona
through interactions with magnetic fields perpendicular to their
velocity vector. The limits found with RHESSI are comparable to the
limits on the axion-photon coupling constant found with the CERN Axion
Solar Telescope (CAST) (Andrianonje et al. 2007) for low mass axions.

Andrianonje et al., J. Cosmology and Astroparticle Phys. JCAP, 04 2007.

Hannah I.G., Hurford G.J, Hudson H.S., Lin R.P., Rev Sci Ins 78, 024501
2007a.

Hannah I.G., Hurford G.J, Hudson H.S., Lin R.P, van Bibber K, ApJL 659
L77 2007b.

1.  []{#_Ref115581988 .anchor}HXR correlations with CMEs

Given the geoeffectiveness of CMEs, the effort to understand and
eventually to predict these powerful solar events has become a major
activity of the HD program. As we know, CMEs sometimes occur without
flares and vice-versa, but the largest flares tend to be associated with
fast and powerful CMEs.

Most models of these events include an intimate magnetic connection
between the initiation and early acceleration of the CME and the energy
release in the associated flare. Dramatic support for this connection
has come from the comparison of the acceleration rate of two CMEs and
the HXR light curve seen with RHESSI - see fig. 2.1.6-1 from Temmer et
al. (2008). They studied two fast halo CMEs where early signatures of
the CME impulsive acceleration phase could be identified in on-disk SXI
and TRACE images with high time cadence, and where the impulsive phase
of the associated flare was fully covered by RHESSI. Since the flare HXR
flux is directly related to the number and energy distribution of
accelerated electrons which contain a large fraction of the total flare
energy, this data set enabled them to study the relationship between the
large-scale CME acceleration and the energy release in the associated
flare.

A tight synchronization between the CME acceleration and the flare HXR
bursts was found in both cases; onsets as well as peaks occurred
simultaneously within about 5 minutes (i.e. within the observational
limitations). These findings show a close relationship between the two
phenomena, consistent with magnetic reconnection occurring in a current
sheet behind the CME, as envisaged in the standard flare/CME picture. On
the one hand, magnetic reconnection adds poloidal flux to the CME
sustaining the Lorentz force which drives the CME acceleration. On the
other hand, the higher the acceleration of the CME, the larger the space
that is evacuated per unit time in the coronal region behind it.
External magnetic pressure then drives a stronger mass inflow into the
reconnection region, which, in turn, causes stronger magnetic
reconnection in the current sheet beneath, i.e., stronger flare energy
release.

![](media/image18.png){width="3.1in" height="4.247916666666667in"}**Fig.
2.1.6-1**. Event of 2005 January 17. From top to bottom, distance-time
profile, d(t), velocity v(t), and acceleration a(t) of the CME as
observed by SXI and LASCO-C2 & C3. In the bottom panel, the red line
shows the RHESSI 50--100 keV HXR count-rate light curve of the
associated flare plotted on a log scale extending from 10 to 10^4^
counts s^‑1^ detector^-1^.

In the 2002 April 15 flare, the coronal X-ray source changed its motion
within about one minute from being approximately stationary to moving
outward at about 300 km s^-1^. This speed was found to be consistent
with that of an associated CME observed later with LASCO. Several weak
X-ray sources were later observed along the line of motion of the
original coronal source, indicating the continued presence of a current
sheet below the CME and the presence of instabilities within the current
sheet (Sui et al. 2005). For the homologous flare on 2002 April 16,
*SOHO* SUMER spectra combined with RHESSI data have provided evidence
for the presence of upward and downward reconnection jets in the
above-the-loop-top region (Wang et al. 2007).

Wang, T. et al., ApJ 661L, 207, 2007

Temmer, M. et al., ApJ 673, 95, 2008

### Solar Oblateness 

The RHESSI system for precise aspect determination includes a Solar
Aspect System (SAS) with a simple design but remarkable properties
(Fivian et al. 2005). Essentially it follows the idea of Dicke's "Solar
oblateness telescope" (Dicke and Goldenberg, 1967) in having sensors on
a rotating telescope continually scanning the limb of the Sun. The data
are very numerous: about 100 measures per second, each with a
statistical error measured at a few *mas* (milli arc sec). Because of
RHESSI's \~15 rpm rotation, these data are distributed around the entire
limb and are thus an excellent tool for measuring the shape of the Sun
and its time variations. We regard this as a new independent window on
the structure and dynamics of the solar interior, independent of
helioseismology and of the neutrino observations.

The SAS data have recently produced their first result, a measure of the
solar oblateness with statistical errors of about 0.1 *mas*
(corresponding to less than 100 m in height). At this level of precision
many systematic terms need to be considered, and the data reduction
scheme -- though simple in principle -- is quite complex. Accordingly we
have only thus far reduced a preliminary 3-month data set as described
below. In particular, faculae (and sunspots) produce obvious signatures,
as seen in Figure2.1.9-1.

![](media/image19.wmf){width="3.0833333333333335in"
height="1.1354166666666667in"}**Figure 2.1.9-1**. Gray-scale
representation of RHESSI radius observations for a two-week interval in
2004, at moderate solar activity. The color bar is calibrated in arcsec.
Sunspots are radius deficits, hence dark features; faculae are radius
excesses, hence bright features. The rotationally-induced oblateness can
be seen as blurred excesses at the W and E limbs (90° and 270°).

Fig. 2.1.9-2 shows a limb profile (the Y-axis of Fig. 2.1.9-1 for
selected data), illustrating the great precision of the observations.
The novel finding is that there is a large apparent oblateness
significantly in excess of that expected from rotational flattening.
This is present even for the data selected to omit all apparent faculae,
based on synoptic charts such as that of Fig. 2.1.9-1. Fig. 2.1.9-2
(right) shows the four limb quadrants averaged together; the lowest line
shows Dicke's (1970) estimate of the rotational flattening.

To make the first oblateness determination, Fivian et al. (2008) use
SOHO/EIT observations as a sensitive proxy for faculae, finding a good
correlation with RHESSI radius as expected from Figure 2.1.9-1. Using
this correlation to identify clean data produces a strikingly different
result from that seen in Figure 2.1.9‑2: the measured oblateness is now
consistent, within errors, with that expected from surface rotation (and
helioseismology). This result shows the existence of a general level of
facular brightening extended across the disk, evident in EUV
observations and also in the RHESSI radius determinations. We believe
this to be the "active network" that Leighton-type models of solar
surface magnetism describe as magnetic flux diffusing away from decayed
sunspot groups. RHESSI has sufficient resolution to avoid these regions
and thus to be able to measure the "true" oblateness, i.e. that created
by rotation.

![](media/image20.wmf){width="3.089583333333333in"
height="1.7465277777777777in"}**Figure 2.1.9-2.** Limb profiles for data
selected to avoid faculae. The right-hand plot folds all four quadrants
together. The dashed line shows the full data set with its obvious
active-region faculae at 70-80˚ co-latitude. The gold line at the bottom
of the plot shows Dicke's estimate of rigid-body rotational flattening.
RHESSI detects a broad excess oblateness.

Dicke, R. H., and Goldenberg, H. M.: Phys. Rev. Lett. 18, 313, 1967.

Dicke, R. H., 1970, ApJ 159, 1.

Fivian, M.D., Hudson, H.S., and Lin, R.P., 2005, in *Proc. 11th European
Solar Physics Meeting*, ESA SP-600, 41.

Fivian, M.D., Hudson, H. S., Lin, R. P., and Zahid, H. J., *Nature*
(submitted 2008).

### Terrestrial Gamma-ray Flashes (TGFs)

Beginning in 2005 (Smith et al. 2005), RHESSI unexpectedly revitalized
the study of Terrestrial Gamma-Ray Flashes (TGFs), and indeed the entire
field of high-energy processes in thunderstorms, by detecting over 100
of these events per year. This is greater than the entire previous
dataset (75 events) from the Compton Gamma-ray Observatory, the only
mission to detect this phenomenon previously (Fishman et al. 1994). All
that was known about TGFs prior to 2005 was that they are millisecond
bursts of gamma-rays extending up to \~20 MeV seen above thunderstorms.
From analyzing the RHESSI data, we now know the following about TGFs:

-   The gamma-ray spectrum shows that they are the result of a process
    called Relativistic Runaway that accelerates electrons to \>\~30 MeV
    in \<\~1ms (Dwyer and Smith 2005; Carlson et al. 2007);

-   Nearly all TGFs can be associated with individual lightning strikes
    (Cummer et al. 2005; Inan et al. 2006);

-   The type of associated lightning is primarily intra-cloud (Stanley
    et al. 2006; Williams et al. 2006);

-   They are primarily produced at altitudes centered around 13-15 km
    (Dwyer and Smith 2005);

-   They also produce beams of electrons up to 30 MeV that sometimes
    enter the magnetosphere and can be directly seen from orbit (Dwyer
    et al. 2008; Smith et al. 2008).

Carlson, B. E. et al. 2007, GRL, 34, CiteID L08809

Cummer, S. A. et al. 2005, GRL, 32, CiteID L08811

Dwyer, J. R. and Smith, D. M. 2005, GRL, 32, CiteID L22804

Dwyer, J. R. et al. 2008, accepted to GRL

Inan, U.S. et al. 2006, GRL, 33, CiteID L18802

Fishman, G. J. et al. 1994, Science, 264, 1313

Smith, D. M. 2004, Proc. 5th INTEGRAL Workshop, p. 45

Smith, D. M. et al. 2005, Science, 307, 1085

Smith, D. M. et al. 2008, in preparation for JGR

Williams, E. et al. 2006, JGRD 111, CiteID D16209

### Astrophysics Results

#### Gamma-Ray Bursts (GRB's)

Thanks to its broad energy range and wide field of view, RHESSI has made
valuable contributions to the study of astrophysical Gamma Ray Bursts
(GRBs). These energetic transients have peak energies ranging from tens
to hundreds of keV. Determining this peak energy via spectral fitting
helps constrain overall burst energetics and may provide information
about its intrinsic luminosity via a variety of empirical correlations.
The Swift GRB satellite which localizes many bursts is sensitive only to
150 keV, so RHESSI has played a key role in extending spectral coverage
of GRBs into the MeV range.

Through July 2007, RHESSI has observed \~420 GRBs, though the majority
do not have the localization required for further spectral analysis.
Spectral parameters were reported to the community via the GCN Circulars
for eight GRBs beginning in 2006. Detailed studies of GRBs 061126 and
070125 were reported by Perley et al. (2007) and Bellm et al. (2007a).
Wigger et al. (2008) found a statistically significant high-energy
excess in the RHESSI data of GRB 021206, providing support for a
specific model of the still poorly-understood GRB emission mechanism.
Bellm et al. (2007b) performed joint spectral fits on 26 bursts observed
by RHESSI and Swift-BAT.

RHESSI\'s excellent timing resolution has also made it possible to probe
the Quantum Gravity energy scale in the range 10^16^-10^19^ GeV, up to
the Planck mass scale, place limits on quantum gravity effects,
manifested as a modification in the electromagnetic radiation dispersion
relation, specifically, a (small) energy dependence of the velocity of
light. This is seen as energy-dependent delays in GRB pulses. To
approach the Planck mass scale, we must measure arrival time differences
e.g. of on the order of 0.5 ms from soft to \~10 MeV photons from an
event at a redshift of a few. Boggs et al (2004) used GRB 021206 to
derive one of the most stringent limits on the quantum gravity energy
scale to date. There is a systematic search for millisecond flares in
RHESSI GRBs with \>1 MeV photons. This work to date is summarized in
Wunderer et al (2007).

In December 27, 2004 the known soft gamma-ray repeater SGR 1806-20
underwent a cataclysmic giant flare, the brightest gamma-ray event
observed in over 30 years of monitoring the high-energy sky. A series of
letters were published in Nature, led by a full article on the RHESSI
analysis and results (Hurley et al., 2005). Recently, the comprehensive
analysis of the RHESSI (and Wind) observations of this giant flare was
published by Boggs et al. (2007).

RHESSI GCN Circulars: 5418, 5685, 5725, 5838, 5867, 6025, 6135, 6399

Bellm, E., et al. 2007a, arXiv:0710.4590

Bellm, E., et al. 2007b, arXiv:0801.2417, to appear in the proceedings
of Gamma Ray Bursts 2007

Boggs, S., C. Wunderer, K. Hurley, and W. Coburn 2004, ApJ 611, L77

Boggs, S.E. et al. 2005, ApJ., 611, L77-L80.

Boggs, S., A. Zoglauer, E. Bellm, K. Hurley, R. P. Lin, D. M. Smith, C.
Wigger, W. Hajdas 2007, ApJ 661, 458

Hurley et al., 2005, Nature, 434, 109.

Perley, D., et al. 2008, ApJ 672, 449

Wigger, C., et al. 2008, ApJ 675, 553

Wunderer, C., E. Bellm, S. Boggs, and K. Hurley 2007, AIPC 921, 512 O

#### Accreting X-ray binary A0535+26

In May 2005, the accreting X-ray binary A0535+26 underwent a rare (the
last outburst was in 1994) and remarkable outburst reaching a brightness
several times that of the Crab Nebula. At the time, it was tens of
degrees from the Sun, and Swift was only able to observe it briefly
because of the solar pointing constraints, missing most of the outburst.
This also prevented any other instrument but RHESSI from observing it.
RHESSI was able to take advantage of its off-pointing capabilities to
follow this source for several weeks during this outburst. Boggs & Smith
(2006) reported on a search in the RHESSI data for a red-shifted
neutron-capture (2.223 MeV) line, and obtained the first upper limits
for an accreting neutron star source.

##  SCIENCE OBJECTIVES for 2008 - 2012

### Flare Rates and Energetic Events 

We are now very near the minimum in solar activity as indicated in
Figure 2.2-1 by the flare rates recorded by different instruments over
the last three cycles. In 2009-10, solar activity will be steeply rising
again towards the next solar maximum, and RHESSI's high-energy
measurements will be essential to the HGO.

Large flares, fast CMEs, and SEP events still occur but are less
frequent near solar minimum. Such events are important because of
simpler conditions in the coronal and solar wind. STEREO and Hinode were
launched in 2006 to join the HGO and the Solar Dynamics Observatory
(SDO) will be launched in 2008. RHESSI is the ideal high-energy
complement to the high-resolution optical imager and vector
magnetograph, soft X-ray imaging, and EUV imaging spectroscopy of
Hinode, and the 3-D imaging in EUV and white-light coronagraphy of
STEREO for studying flares and CME initiation. STEREO's highly sensitive
energetic electron and ion measurements, separated in longitude, are
perfect for comparisons with RHESSI's sensitive measurements of
energetic particles at the Sun near solar minimum when the background is
low. Solar minimum will also enable RHESSI to make deep searches for
microflares and quiet-Sun nonthermal emissions.

As pointed out earlier, much work remains to be done to develop methods
and software to mine the information in the rich RHESSI data set. In
addition we propose a GI program to integrate the remarkable new
observations of solar eruptions and their energetic consequences
obtained by HGO in the last solar maximum. As always RHESSI will
contribute unique measurements for study of Terrestrial Gamma-ray
Flashes and cosmic gamma-ray bursts and other high energy astrophysical
phenomena.

![](media/image21.png){width="3.1in"
height="3.1930555555555555in"}**Figure 2.2‑1**. Predicted rates of X-ray
flares for the next solar cycle (blue curve) based on the average of the
rates from the three previous cycles as measured with other X-ray
instruments (ISEE‑3, SMM/HXRBS, CGRO/BATSE, and Yohkoh/HXT (Aschwanden,
private communication). The black histogram shows the rate of GOES \>C3
flares, and the measured rates for the other two instruments (Yohkoh/HXT
in red and RHESSI in orange) have been normalized to that rate by
dividing by the indicated factors.

### RHESSI Flare Studies with Hinode and STEREO

Comparing STEREO and Hinode observations with the RHESSI data set will
greatly enhance the scientific return for all three missions. Hinode
carries a high-resolution optical telescope (SOT), an EUV imaging
spectrograph (EIS), and a soft X-ray telescope (XRT), perfect
complements to RHESSI's high-energy observations for studies of solar
activity. Hinode/SOT will provide the first stable time-series
observations of the solar vector magnetic field, at the highest possible
angular resolution. Together with Hinode/XRT soft X‑ray imaging and
Hinode/EIS EUV imaging and spectroscopy, this will give the best
information to date regarding the pre-event physical conditions in the
solar corona. In some manner not yet understood, these conditions change
gradually to a "trigger point", at which time an instability or loss of
equilibrium occurs (e.g., Forbes 2000). Arguably the best signs of the
initiation are the non-thermal effects, as observed by RHESSI, even in
the weakest microflares. The combination of RHESSI and Hinode
observations thus will provide our best and most objective determination
of the trigger conditions.

![](media/image22.png){width="5.947916666666667in" height="4.09375in"}

.

**Figure 2.2.2-1.** Example of RHESSI-STEREO observations of a partially
disk-occulted flare from Earth view seen on December 31, 2007. The
cartoon at the bottom right gives the different view angles (to scale).
The X-ray time profiles from GOES and RHESSI are shown to the left
revealing intense non-thermal emissions above 20 keV. Imaging results
are presented in the three figures shown at the top. STEREO/EUVI shows
two flare ribbons in north-south direction. The RHESSI source originates
most likely from above the loop arcade connecting the EUV ribbons. About
10 minutes later, STEREO A shows the escaping Coronal Mass Ejection
(preliminary results).

RHESSI can identify the coronal regions, both pre-flare and during the
impulsive phase, that appear to be the sites of flare energy release,
magnetic reconnection, and electron (and ion) acceleration. XRT will
provide the high-cadence high-resolution imaging of these
high-temperature regions (not presently available). RHESSI footpoint
motions (Krucker et al., 2003) give a clear indication of the
reconnection process in the evolution of the coronal magnetic field. EIS
imaging spectroscopy can provide the key measurements of flows and
turbulence in these regions to clarify the energy release and particle
acceleration processes.

STEREO/EUVI will provide 3‑dimensional observations of flare loops seen
in EUV. Although RHESSI HXR observations only provide 2‑dimensional
information, HXR footpoints occur at high density in the chromosphere.
Hence, the HXR footpoints can be relatively easily compared with the
3‑dimensional geometry obtained from STEREO. Furthermore, Hinode/XRT and
RHESSI will provide thermal flare energy estimates, Hinode/EIS will give
flow measurements. Hinode vector magnetograms are the first from space,
and have high resolution. The development of more accurate coronal field
models is a major Hinode objective. The results of this work will allow
the field evolution during a flare to be modeled so that different flare
models can be compared. RHESSI will provide both thermal and non-thermal
electron diagnostics.

STEREO observation will provide different view angles of partially
occulted flares seen by RHESSI. For one of the two STEREO spacecraft,
the flare will not be occulted, thus providing information about the
magnetic structure. This will greatly enhance our understanding of the
event geometry. EUV ribbons will give us the likely location of the
occulted HXR footpoints. The CME geometry seen with STEREO coronagraphs
COR-1/2 can be compared with the HXR coronal source location. RHESSI
observations confirm that about 10% of all flares are usefully occulted,
i.e. their bright footpoints obscured from view. However, relatively
large flares are needed to get good counting statistics so it is
expected that only a few good candidates will occur until the flare rate
increases leading up to the next solar maximum. One early example of
this situation has already occurred and the preliminary results are
shown in fig. 2.2.2-1.

Forbes, T., JGR 105, 23,253 (2000).

Krucker, S., Hurford, G.J., and Lin, R.P., ApJ 595, L103 (2003).

### *In Situ* Particle Observations and HXR Emission

For the large flares/SEP events that occur in 2007‑8, RHESSI's gamma-ray
line measurements of the spectrum of energetic protons and its imaging
of the ion source at the Sun can be compared to the ACE, Wind, and
STEREO multi-point SEP spectral measurements spread in longitude, while
Ulysses will be measuring SEPs when crossing the ecliptic plane at \~1.5
AU. These comparisons could determine whether solar flares directly
contribute to SEPs at 1 AU when the flare is magnetically well
connected, as suggested by the 2 Nov 03 and 20 Jan 05 events (Section
2.1.4).

Most of the impulsive electron events detected at 1 AU during solar
minimum will be seen only below \~15 keV. The excellent sensitivity of
RHESSI in the 3-15 keV energy range will give us by far the best chance
ever (with two orders of magnitude better sensitivity) to see related
HXRs from these events, possibly even the thin-target emission from the
escaping electron beams that produce type III radio bursts. Solar
minimum conditions with little thermal emission from the corona are
optimal for these observations. STEREO will provide 3-dimensional event
geometry of the solar source region of the escaping electrons and
STEREO/WAVES will track escaping electrons by their radio emission.
Although the RHESSI observations are only 2-dimensional images, the
relatively simple source structure in X-rays (footpoints) will make it
possible to put the X-ray emission in the 3-dimensional geometry.

The STEREO IMPACT suite SupraThermal Electron (STE) and Solar Electron
Proton Telscopes (SEPT) instruments will measure impulsive electron
events from \~2 to 300 keV. The two STEREO spacecraft will provide these
*in situ* measurements at widely separated solar longitudes. By tracking
the electrons from the solar source HXR emission imaged by RHESSI,
through the interplanetary medium through the Type III radio emission,
to the *in situ* detection by the STEREO spacecraft we can trace and
study the magnetic connectivity from the Sun to the Earth.

### Microflares

Combined microflare observation of RHESSI, STEREO, and Hinode will allow
detailed studies of single microflare events to see if microflares are
indeed just smaller version of large flares or whether they are a
separate type of event. Hinode/EIS will provide observations of plasma
flows, while Hinode/SOT, XRT, and EUVI will provide 3‑dimensional
magnetic structure and estimates of flare volumes. RHESSI observations
will be an essential part of these studies since it is the only
instrument that can provide higher temperature (\>20 MK) diagnostics and
give quantitative measurements of the energy content in non-thermal
electrons. The energetics of small-scale energy release in the solar
atmosphere (microflares, "blinkers", flaring bright points, and many
other manifestations) may have a strong relationship to the coronal
heating problem.

Individual microflares have begun to be analyzed using both RHESSI and
Hinode/XRT (Section.2.1.4..2). The next stage is to investigate
statistically the properties of microflares observed with RHESSI and
Hinode/XRT. This might also help understand the results of the large
RHESSI microflare statistics study (Christe et al. 2008, Hannah et al.
2008).

### Quiet-Sun & Axion Observations

RHESSI's recent anneal has lowered the detector background compared to
previous offpointing periods and so we hope to improve these quiet-Sun
limits further with future offpointings through 2008 and 2009. In
addition, we can make use of the softer X-ray observations from
Hinode/XRT, whose sensitivity and spatial resolution allow for the
detection of bright points across the quiet solar disk. We will
investigate if there is a hard X-ray counterpart to these soft X-ray
bright points.

We plan to search for the sharp ^57^Fe line feature in the axion
spectrum (Moriyama 1995), which will be easier post-anneal with RHESSI's
recovered spatial resolution and suppressed background. We can also
exploit the predicted spatial and spectral properties for solar axions
to improve on the limits previously found with RHESSI.

Andrianonje et al., J. Cosmology and Astroparticle Phys. JCAP, 04 2007.

Christe S., Hannah I.G., Krucker S. Lin R.P, McTiernan J, ApJ 2008 in
press.

Hannah I.G., Hurford G.J, Hudson H.S., Lin R.P., Rev Sci Ins 78, 024501
2007a.

Hannah I.G., Hurford G.J, Hudson H.S., Lin R.P, van Bibber K, ApJL 659
L77 2007b.

Hannah I.G., Christe S. , Krucker S. Hurford G.J, Hudson H.S, Lin R.P,
ApJ 2008 in press.

Moriyama S., Phys. Rev. Lett., 75, 3222, 1995.

### Gamma-Ray Observations

Future work to improve the reconstructed gamma-ray images will be
directed along two lines. First, improved analysis techniques based on
***[visibilities]{.underline}*** will improve the statistical
significance of the results. Second, improved background- and
continuum-subtraction techniques (also visibility-based) represent a
promising avenue to permit imaging in the 511 keV line. The significance
of this is that RHESSI can image such lower energy gamma-ray line
photons with significantly better angular resolution as the finer and
thinner grids give usable modulation at this energy.

Of course, as solar maximum approaches, more gamma-ray flares will be
detected, allowing more systematic studies.

#### Coordination with GLAST

As early as May 2008, RHESSI will be joined in orbit by the Gamma-ray
Large Area Space Telescope (GLAST), providing a unique opportunity to
make simultaneous nuclear-line and high-energy (up to several GeV)
gamma-ray observations. GLAST will also detect neutral pions produced in
the same reactions as the positive pions that yield positrons
responsible for the annihilation line observed by RHESSI. The
high-energy protons can penetrate below the photosphere and may be
responsible for the helioseismic waves and recent near‑IR observations.
Joint high-sensitivity RHESSI and GLAST measurements also may detect
gamma-rays from delayed CME shock-accelerated particles that may impact
the solar atmosphere after the impulsive phase of a flare. Delayed
annihilation and neutron-capture line emissions have been detected by
RHESSI following the impulsive HXR phase of the 10 September 2005 flare.

![](media/image23.jpeg){width="3.1in"
height="2.2597222222222224in"}**Figure 2.2.6-1.** Comparison of nuclear
line and pion decay emissions for two flare spectra.

Figure 2.2.6-1 shows how joint RHESSI and GLAST gamma-ray spectral
observations can be used to determine the spectrum of flare-accelerated
particles. The nuclear-line spectra and the continuum produced by the
decay of neutral and charged pions are plotted for two different
power-law spectra of flare-accelerated particles. The nuclear-line
spectrum dominates over the high-energy continuum for particle with
indices steeper than about 3; for harder particle spectra, the continuum
dominates. The photon spectrum at energies \>1 GeV directly reflects the
flare proton spectrum. Joint RHESSI and GLAST observations will allow
the pion-decay gamma-ray continuum to be distinguished from the primary
electron bremsstrahlung.

High-energy radiation from the 1991 June 11 flare was observed by EGRET
and COMPTEL on CGRO for several hours suggesting continued acceleration
and/or long-term trapping. *RHESSI* observations of \>17 MeV emission
indicate that high-energy interactions also occurred in the solar
atmosphere for at least two hours following the impulsive phase of the
2005 January 20 flare. The *RHESSI* image of this flare plotted in
Figure 2.2.6‑3 reveals two hard X-ray footpoints (contours) lying in two
ribbons revealed by UV images made by *TRACE.* The centroid of radiation
from the 2.223 MeV neutron capture line is identified by the thick white
circle about ten arcsec to the E/SE of the peak in hard X-rays in the
northern footpoint. For comparison we also show the \~30 arcsec position
uncertainty of the LAT for \>500 MeV pion-decay photons. Thus, the
combined RHESSI/GLAST observations will be able to determine whether
high-energy protons also were contained within the closed magnetic loops
ending in the footpoints.

![](media/image24.jpeg){width="2.0in"
height="2.0909722222222222in"}**Fig. 2.2.6-3**. Red circle shows
expected accuracy of LAT location of a 500‑MeV source compared with
RHESSI 250 -- 500 keV contours (in red) and centroid location of the
2.2 MeV emission (white circle) superimposed on a TRACE 1600 Å image
(green and black) of the 2005 January 20 flare. The RHESSI beam size at
2.2 MeV is shown as the blue circle.

### Solar Limb Data

The RHESSI SAS data extend now for six full years but we hope eventually
for the full orbital lifetime of the satellite. They offer a unique
resource for a variety of investigations (Table 2.2.7-1), probably the
most important (and hardest) being the detection of g‑modes. There is
some hope that this is possible since the limb is elevated above the
photosphere resulting in larger amplitudes. Note that there are g‑modes
with periods of hours, and so the RHESSI window function (eclipses in
low-Earth orbit with a 95-min. period) will not interfere with an
analysis of data that spans many years. In general, the longer the data
run, the better the signal-to-noise ratio. This unique data set is
unlikely to be reproduced and may justify the operation of RHESSI even
if its primary mission came to an end.

**Table 2.2.7-1**: Phenomena accessible with precise radius measurements

  ------------------------------ ----------------------------------------
  **Magnetic Phenomena**         **Hydrodynamic Phenomena**

  sunspots                       p-modes

  Wilson depression              g-modes

  faculae                        r-modes ?

  active network                 granulation

  Flares                         other convective motions

  prominences                    oblateness

  coronal holes ?                oblateness time variations ?

  Dynamo signatures ?            Higher order shape terms

                                 Gravitational moments J2, J4

                                 Global temperature variation ?

                                 Planetary tides
  ------------------------------ ----------------------------------------

### Terrestrial Gamma-ray Flashes 

In 2005, over ten collaborative research projects began which correlate
RHESSI TGF data to radio atmospherics from lightning, optical emission
from sprites and elves seen with ISUAL on FORMOSAT-2, relativistic
electrons seen with SAMPEX, and even infra-sound from sprites. RHESSI
TGF times and locations are now made publicly available in near
realtime. RHESSI is the only observatory currently detecting TGFs and
releasing TGF data to the community, so it is critical to the progress
of this young field over the next several years.

### Astrophysical Objectives

The cosmic gamma-ray burst and other transient studies will continue to
exploit the unique timing, spectral resolution, and broad energy
coverage of RHESSI. In addition, we will continue to image the Crab
Nebula and work on improving the aspect solution (the key to obtaining
groundbreaking maps) for those solar off-pointings. The inner features
of the Crab Nebula are extremely dynamic, showing significant variations
from season to season in the optical (HST) and soft X‑ray (Chandra)
bands (Hester, J. J. et al. 2002). Since the highest energy emission
might be expected to be dominated by freshly shock-accelerated
particles, this variation could be greatest in RHESSI\'s hard X‑ray
band. We therefore plan to revisit the Crab for about one week annually
at the Sun\'s closest approach each June.

Hester, J. J. et al. 2002, ApJ 577, L49

### Proposed Solar High-Energy Events GI Program

The new HGO missions -- Hinode, STEREO, and SDO later this year -- as
well as GLAST, provide a near ideal and remarkably complete set of
complementary measurements to RHESSI for the study of particle
acceleration in solar flares, CMEs and SEPs. These are topics of
particular relevance to NASA\'s Vision for Space Exploration and the
Living With a Star initiative. Each of these missions contributes
fundamental new insights, but a global integration and analysis of these
observations will provide the real pay-off in our understanding of these
phenomena. Since the in-guide funding levels of the individual missions
are inadequate to cover such integrated studies, we propose a Guest
Investigator (GI) program in the coming years to stimulate rapid
advancement of our understanding of high-energy solar events - the
buildup, initiation, evolution, and consequences of particle
acceleration in explosive and eruptive events.

## Potential for Performance during FY-09 to FY-12

### Relevance to HP Research Objectives

RHESSI is directly relevant to achieving NASA's strategic goal for
Heliophysics to "Understand the Sun and its effects on the Earth and the
solar system." It addresses the SMD Science Question - "How and why does
the Sun vary?" and the following Research Objectives:

-   Understand the fundamental physical processes of the space
    environment from the Sun to Earth.

-   Develop the capability to predict the extreme and dynamic conditions
    in space in order to maximize the safety and productivity of human
    and robotic explorers.

### Impact of Scientific Results

RHESSI scientific results have been presented at innumerable scientific
meetings and published in over 670 papers listed at the following web
site:

<http://www.lmsal.com/~aschwand/publications/rhessi.html>

Many of them are available electronically in the E‑Print archive at the
Max Millennium web site ‑

<http://solar.physics.montana.edu/max_millennium/>

Over 30 PhD and masters degrees that include significant analysis of
RHESSI observations have been awarded to date or are in preparation --

<http://www.lmsal.com/~aschwand/publications/rhessi_phd.html>

### Spacecraft and Instrument Health

See Section 3.

### Productivity, and Vitality of Science Team

Various awards have been given to members of the RHESSI team in
recognition of their contribution to the success of this mission. The
PI, Bob Lin, was elected to the National Academy of Science and the
American Academy of Arts and Sciences. The PI and CoI, Hugh Hudson,
received the AAS/SPD Hale prize in 2005, and 2008, respectively. CoI,
Gordon Holman received the 2007 SPD Popular Writing Award for a
Scientific American article entitled "The Mysterious Origins of Solar
Flares" that included RHESSI results.

The most newsworthy results have been broadcast to the general public
through over ten press releases from NASA, UC Berkeley, and UC Santa
Cruz covering such topics as magnetic reconnection powering solar
flares, the connection between flares and CMEs, the first gamma-ray
image of a solar flare, particle acceleration in the large flare on Jan.
20, 2005, the brightest gamma-ray burst ever recorded on Dec. 27, 2004,
and terrestrial gamma-ray flashes.

RHESSI personnel have initiated, organized, and attended many scientific
meetings and special sessions. In the last three years, the series of
international RHESSI workshops continued with events in Paris, France
(June 2006), and Santa Cruz, CA (June 2007). Each workshop was attended
by some 50-60 scientists. "Topical" RHESSI workshops also took place in
Paris, France (April 2006) and Berkeley (2007). RHESSI team members
played key roles in several collaborative conferences, such as the one
entitled "Solar & Space Physics and the Vision for Space Exploration"
held in Wintergreen, VA, in October 2005. Another such joint meeting is
being planned on "Anticipating Solar Cycle 24" in Napa, CA, in December
2008. Several international teams of scientists have met in Bern,
Switzerland, under the sponsorship of the International Space Science
Institute, to explore issues related to RHESSI's hard X-ray spectroscopy
and imaging spectroscopy, for example.

Scientists who have attended the RHESSI workshops are preparing a series
of papers for a complete \~500-page volume of Space Science Reviews
(also to be published as a book) detailing the current state of
understanding of all aspects of high-energy solar flares. The chapter
titles (with the lead authors in brackets) are as follows:

-   An Observational Overview of Flares (Fletcher),

-   Interpretation of Thermal and Nonthermal Electron Distributions
    (Holman),

-   Ion Acceleration and Interaction (Vilmer),

-   Correlative Radio and Hard X-Ray Observations (White),

-   A Statistical Study of Flare Parameters (Hannah),

-   Deducing Electron Properties from Hard X-Ray Observations (Kontar),

-   Theoretical Implications (Vlahos).

The papers will be ready for joint publication in the summer of 2008.

RHESSI team members have organized special sessions on high-energy solar
physics topics at every meeting of the AAS/SPD since launch and at
COSPAR General Assemblies. The latest is being organized by CoI, Gordon
Holman, at the Montreal COSPAR meeting in July, 2008. In addition, the
RHESSI team has held a series of workshops to educate the scientific
community on how to use the RHESSI data analysis software in order to
obtain the best possible images and spectra.

Members of the RHESSI team helped to organize and served as faculty
members for the AAS/SPD Summer School on high-energy solar physics held
in Durham, NH, in June 2006. This 10-day summer school was jointly
funded by NASA and NSF and was attended by 50 students and postdocs.

#### Science Nuggets

Beginning in March 2005, we began putting a biweekly series of RHESSI
"science nuggets" on the following web site:

<http://sprg.ssl.berkeley.edu/~tohban/nuggets/>

Each nugget is a brief report that is intended to introduce various
aspects of the scientific material to a technically competent audience.
The Web format allows us to include images, movies, and links. At the
time of writing (February, 2008) there are 68 nuggets online with over
50 different authors. The Webmaster is Steven Christe; CoI Hugh Hudson
serves as general editor and default writer in case of schedule gaps.
The Nuggets are announced in community newsletters that go to an
audience of perhaps 500. Email feedback shows a readership that is both
within and outside the solar physics community.

### Future Promise

RHESSI is still fully operational and shows tremendous promise for the
coming years of increasing solar activity and joint observations with
the newer solar missions now in orbit or to be launched in the next
year. RHESSI continues to be the only mission capable of solar HXR
imaging spectroscopy and is likely to retain that unique position until
the launch of the Spectrometer Telescope for Imaging X-rays (STIX) on
the Solar Orbiter in 2015. RHESSI's gamma-ray imaging spectroscopy
capabilities are not likely to be duplicated or improved on for many
years. Thus, it is critical that RHESSI be fully supported for as long
as possible to provide the high-energy coverage of the flares that will
be seen in increasing numbers with the new and advanced instrumentation
on Hinode, STEREO, SDO, and GLAST.

With the successful anneal and the recovery of useable energy resolution
and sensitive volume in all of the germanium detectors, RHESSI is ready
to make continued observations of X-rays and gamma-rays for at least two
more years. Additional anneals will be necessary as the effects of
radiation damage build up again. Ground tests with a spare germanium
detector indicate that at least four anneals of the same duration and
temperature as the one just completed can be safely carried out without
endangering the detector segmentation. The cryocooler, while showing
signs of degrading performance, is still maintaining the detectors at
the acceptable temperature of 90 -- 95 K with an input power of 75 watts
and negligible microphonics.

### Data Accessibility and Usability

Full details of RHESSI data accessibility and usability are given in the
accompanying Mission Archive Plan. The data archive contains the full
Level‑0 telemetry data, the RHESSI flare list, and a number of
'quicklook products' including prepared lightcurves, spectra, and
images, and summaries of housekeeping data for the entire mission. The
archive, currently \~4.5 TB, resides on three servers, at SSL, Goddard,
and at ETH in Zurich, Switzerland. It is online, available, and easily
accessible by anyone with an Internet connection. The complete RHESSI
software package necessary for the analysis of all RHESSI data is also
freely available online as part of the Solar Software (SSW) tree. It is
mostly written in the Interactive Data Language (IDL licensed from ITT
Visual Data Solutions) and is fully compatible with both UNIX and
Windows operating systems.. It contains all procedures necessary to read
and unpack the FITS data files, prepare and plot light curves,
reconstruct images, and accumulate, display, and analyze spectra. For
the expert analyst, the procedures can all be invoked from the IDL
command line, but a more user-friendly graphical user interface is also
available capable of carrying out all the basic analysis tasks.

There is extensive documentation online at the RHESSI web site --

<http://hesperia.gsfc.nasa.gov/rhessi/>

that describes the mission, the instrument, the science objectives, data
analysis techniques, the software, and the data archive. In addition,
there are support personnel at all three sites to guide scientists in
using the software and interpreting the results.

#### Browser

To allow for quick inspection of the RHESSI data, there are many
automatic routines to produce plots such as quicklook lightcurves and
images and detector monitor rates. Navigating this wealth of
automatically generated plots is accomplished using a web-based data
browser, simply called Browser, found at:

<http://sprg.ssl.berkeley.edu/~tohban/browser/>

Originally written by graduate student Albert Shih in 2003 and
significantly upgraded in 2005, Browser is a powerful tool that allows
the user to load a variety of plots relevant for a specified time, and
then easily step back and forth through time as desired. Browser also
provides the capability of storing the current view as a URL for ease of
sharing among researchers. Finally, Browser dynamically generates links
to non-RHESSI web-based databases (such as magnetograms) corresponding
to the specified time so that the user can investigate the context of
the RHESSI data.

#### Facilities for Multi-wavelength Analysis

To achieve the full scientific potential of RHESSI data and to support
the convenient integration of RHESSI data into studies initiated in
other wavelength regimes, it is essential to have convenient access both
to the data from other instruments and to the relevant software
resources. Two steps have been taken to ensure this. First, the RHESSI
software package includes flexible tools for accessing and integrating
multiple data sets (for example by overlaying images). Coalignment is
simplified since the absolute positions of RHESSI images are inherently
determined to arcsecond accuracy.

Second, we have established a comprehensive RHESSI Synoptic Data Archive
at the GSFC Solar Data Analysis Center (SDAC) that allows ready access
to a broad range of observations for all the RHESSI flares. The archive
contains image, spectrum, and lightcurve data from a wide range of
sources stored in standard formats for direct comparison with RHESSI
data. The sources include SOHO (EIT and MDI), TRACE, Big Bear Solar
Observatory, Kanzelhohe, and Kiepenheuer Hα full-disk images; GOES-12
SXI full-Sun images and GOES 6-12 two-channel light curves with 3 s time
resolution; and Phoenix, Owens Valley Solar Array, Nobeyama, and Nancay
radio images and spectra.

We are continuously improving the Synoptic archive as follows:

-   Increasing the cadence and time coverage of complementary data to
    encompass pre- and post-flare phases of GOES and RHESSI-observed
    events.

-   Facilitating access to data from new missions as they become
    available after launch. We have partnered with the STEREO and Hinode
    science teams to integrate their data analysis tools into the RHESSI
    analysis software to permit joint analysis of multi-wavelength
    observations. We will extend this effort to include next generation
    data from the Solar Dynamics Observatory (SDO)

-   Evolving from a centralized synoptic data archive to a distributed
    system that is incorporated into the Virtual Solar Observatory
    (VSO). We have teamed with the SDAC to develop an IDL client
    interface that directly searches and retrieves VSO datasets. By
    incorporating this interface into the RHESSI analysis software, we
    are augmenting the range of different solar observations accessible
    for joint analysis with RHESSI, while simultaneously extending the
    data analysis capability of the VSO.

# TECHNICAL STATUS

## Observatory 

RHESSI was launched on February 5, 2002, into a circular orbit with an
altitude of 600 km and an inclination of 38°. The observatory continues
to function very well after more than 6 years of operations in its
present 575 x 554 km orbit. All of its subsystems are fully operational.
The solar array power output has declined by only 1.1% since launch, the
battery voltage has been stable for the past 2 years, the absolute
battery pressure has been stable for the past 3 years, and the
differential pressure has been relatively stable for the past 1.5 years.
The average spacecraft temperature has increased by only \~1° C since
launch. The S-band transceiver still generates a stable output power of
4.9 W with clean BPSK modulation at 4.0 Mbps, and the receiver shows no
signs of deterioration in performance. The attitude control system is
stable and the command and data handling subsystem shows no signs of
degradation.

The CPU experienced six resets since launch. Three of these resets
occurred as a result of the very large solar flare in October of 2003,
two were caused by watchdog timeouts during ATS loading, and one had an
unknown cause. The IDPU never reset autonomously, but was reset once by
ground command. On two occasions ATS loads with faulty checksums were
erroneously accepted by the spacecraft. However, these ATS loads were
subsequently reloaded and verified flawlessly prior to their on-board
execution.

Overall, there is no reason to believe that the spacecraft will not
continue to function very well over the next three years. The solar
arrays have shown minimal degradation and are projected to be capable of
providing adequate power for the next three years and beyond. The
battery showed some degradation early in the mission, but has been
stable for the past two years and is projected to provide adequate power
storage for the next several years. The telecommunications subsystem has
been stable since launch, and all other subsystems and sensors are
functioning nominally.

## Instruments

### Spectrometer and Cryocooler

The RHESSI germanium spectrometer and its Sunpower cryocooler continue
to work well. Prior to annealing the detectors in Nov. 2007, all nine
RHESSI detectors were segmented and functioning. The detectors had
suffered radiation damage from trapped and cosmic ray protons at very
close to the rate predicted before launch. This only affects the
measurements of narrow gamma-ray lines, and has no effect on the
performance at soft or hard X-ray energies, to which the vast majority
of flares are restricted. At gamma-ray energies, the result was a loss
of energy resolution (to approximately 8.1 keV FWHM at 2.2 MeV versus
4.4 keV early in the mission) and a loss of active detector volume
(about 15%, with larger changes in some detectors than others).

Three of the detectors, (#2, 5, and 6) have occasionally demonstrated
transient noise and current spikes so that their operating voltages have
been gradually lowered. (Radiation damage may be responsible for this,
but it allows the detectors to remain segmented and operable at lower
voltages than would have been possible at launch).

RHESSI\'s cooler has been powered continuously since the first week of
the mission, logging more than 50,000 hours of failure-free operation.
It has proven to be relatively easy to adjust the amplitude and phase of
the compensating balancer attached to the cooler to reduce the vibration
level so that microphonics do not degrade the detector energy
resolution.

The cooler has been used twice to cool down the spectrometer detectors
and cold plate, from room temperature on the initial cool down, and from
100º C after the anneal cycle in Nov. 2007. It was operated continuously
in its standard constant-amplitude mode at \~65 watts from the first
week of the mission until the anneal process began. For reasons that are
still not completely understood, the average detector temperature has
risen from 75K after the initial cool down to approximately 90K just
prior to the anneal. (The temperature of the detectors fluctuates \~5 K
as the fraction of time in sunlight varies with the orbital precession.)
Since the anneal, the cryocooler has been operated at \~75 watts to
maintain the coldplate temperature at between 90 and 95 K.

![](media/image25.png){width="3.1in" height="1.9145833333333333in"}Most
of the rise in temperature occurred over the first two years of
operation, stabilizing when the spectrometer\'s insulating vacuum volume
was vented to space on Feb. 5, 2004. By implication, there was probably
condensable contamination in the spectrometer vacuum volume causing
degradation of its thermal performance. Opening the vent probably
stopped further accumulation of condensables, but did not release
anything already deposited on the spectrometer\'s interior thermal
surfaces.

The cryocooler was kept powered on at \~50 W during the anneal process
to keep its own components cool and to prevent damage from excess heat.
However, it was operated during that time at an input power that was
higher than its design capability at the actual temperatures reached,
and it appears that it may have suffered some degradation in performance
as a result. For subsequent anneals, the cryocooler will be operated at
a lower power setting when the cold-tip temperature is elevated, more
closely matching the profile used successfully during initial cool down
after launch. The cryocooler performance has stabilized since the
anneal, and the coldplate temperature is being maintained at between 90
and 95 K after increasing the input power to 75 watts.

### Detector Annealing

The germanium detectors were annealed for the first time in November
2007 to reduce the effects of radiation damage caused by energetic
protons and neutrons interacting in the germanium crystals. This
involved heating the detectors to a temperature of \~90° C for 7 days.
This also vents to space accumulated water ice and other volatiles in
the cryostat.

Radiation damage causes two problems: trapping of holes as charges
liberated by a gamma-ray drift through the crystal, and creation of
charge sites in the crystal that compete with the externally applied
electric field. Hole-trapping causes loss of energy resolution due to
incomplete charge collection. Charge sites, once present in sufficient
number, can actually make part of the detector volume passive by
canceling or reversing the applied field. Both of these problems had
become severe by 2007.

![](media/image26.png){width="3.1in"
height="1.9208333333333334in"}**Figure 3.2.2-1.** Background gamma-ray
line at 511 keV as seen at the start of the mission in early 2002
(black), in mid 2006 (blue), in mid 2007 (shortly before the anneal in
green), and at the present time (January 2008 in red).

**Fig. 3.2.2-2**. RHESSI rear segment response to background lines
before anneal (green) and after anneal (red). Black shows the
performance at the start of the mission in 2002, and blue shows the
performance in mid-2006. The full volume and efficiency of the detectors
has been recovered, if not the full resolution.

When the detectors were cooled back down to their operating temperature,
all but one of the detectors (#2) showed considerable improvement in
performance. Approximately half the accumulated radiation damage was
effectively removed. All front segments have nearly their full effective
area again, and most rear segments are now capable of observing nuclear
lines effectively. Neither was the case before the anneal. This will
allow high-resolution X-ray and gamma-ray observations of solar flares
to be continued. Detector #2, which has been anomalous since launch, is
currently operational only as a front segment, with reduced resolution
and effective area; this was also the case through much of the
pre-anneal mission.

Fig. 3.2.2-1 shows the RHESSI rear segment response to the background
511 keV line just before and after the anneal. Figure 3.2.2‑2 shows
similar results for lines at higher energies (the sodium background line
at 1275 keV builds up slowly over the mission so it is not a good point
of comparison for line sensitivity). Although the energy resolution is
worse than it was in 2002, the *efficiency* for collecting lines is
completely recovered. Since most solar lines are naturally broadened,
the most important part of the recovery has been achieved. Even the
current resolution is many times better than previous missions, such as
SMM/GRS, that used scintillators instead of germanium.

By mid-2007, many front segments had lost effective area due to
radiation damage, particularly detector 5. Analysis of the X-ray
count-rate spectra for three C-class flares detected in 2005, 2007
-pre-anneal, and 2007 post-anneal shows that most of this loss has been
recovered by the anneal in all detectors but #2 and 7 that have always
had degraded energy resolution and a higher threshold since launch.

We expect to execute at least one more anneal cycle, probably to be
performed around late 2009, as solar maximum approaches and the
detectors reach a similar level of damage as just before the recent
anneal. The amount of annealing we can do is limited by the drift of
implanted lithium ions at the inner bore of the detectors. The
segmentation of the RHESSI GeDs is caused by a gap in the inner
electrical contact, which is made conductive by the lithium. At high
temperatures, the lithium becomes mobile in the Ge crystal and can drift
so as to close the gap. This would desegment the detector and cause
several problems for its performance, but it could still be operated as
an unsegmented detector. We have repeatedly annealed a spare detector in
the laboratory, and it began to desegment after approximately one month
at 95^o ^C. We are therefore confident that a second anneal of 1 to 2
weeks will be possible without desegmenting the flight detectors.

### Imager

The RHESSI imager subsystem consists of nine rotating modulation
collimators, each of which has a pair of widely separated grids. A
metering structure maintains the relative twist alignment of the nine
grid pairs. The imaging subsystem is inherently stable and has shown no
evidence of change in grid alignment. Improved analysis techniques are
refining the calibration of the grid parameters and locations (now known
to submicron accuracy), the results of which are built into the software
package and are applicable to all data acquired since launch.

### Aspect System

There are three parts to the RHESSI aspect system. The absolute pitch
and yaw angles relative to Sun center are provided by the Solar Aspect
System (SAS), with sub-arcsecond accuracy. The spacecraft roll angle is
provided by two redundant side-looking star scanners, a CCD-based Roll
Angle System (RAS) and a Photomultiplier-Tube-based Roll Angle System
(PMTRAS).

The SAS consists of a set of 3 identical lens/sensor subsystems each of
which focuses a narrow-bandwidth image of the solar disk onto a linear
CCD array. No anomalies in its operation have been observed. During the
first few months of the mission, the sensitivity of the SAS decreased,
but it has now stabilized at \~60% of the original value, a level which
provides an order of magnitude of sensitivity margin with no compromise
in accuracy. The SAS also continues to provide accurate radius
measurements (to 50 mas) for the study of global solar applications as
discussed in section 2.1.7.

The PMT-RAS continues to provide the roll aspect knowledge upon which
most of the solar imaging is based. Its response has remained stable
since launch, easily meeting its 1 arcminute roll-angle requirement with
a large margin in sensitivity.

The CCD-based RAS has also proven to be stable and able to meet the
1 arcminute roll-angle requirement. CCD-RAS data are used to fill
occasional gaps in the PMT-RAS coverage. In addition, by measuring the
polar angle as well as the roll angle, the CCD-RAS can provide full
aspect information with 10 arcsecond accuracy for non-solar targets such
as the Crab nebula and the pulsar A0535+26.

## Software

One of the unique characteristics of RHESSI is that the telemetered data
contain detailed information on each detected photon. This provides
unprecedented flexibility in that decisions and tradeoffs associated
with time resolution, energy resolution, imaging resolution and field of
view, etc, can be made *ex post facto*, enabling the analyst to make
these decisions in the manner best suited to the scientific objectives
and to the characteristics of the event under study. The RHESSI software
fully supports this flexibility. Another advantage of this
photon-oriented database is that ongoing improvements to the software
and instrument calibration can be fully applied retroactively to all the
data since the beginning of the mission.

The RHESSI software package is well-developed, robust, and available via
SolarSoft. It can be used with the IDL command line, a user-friendly
graphical user interface, or scripts for automating user-specific tasks.
The software generates images, and spatially-integrated or
feature-specific spectra and light curves and provides full support for
comparisons with data products from other missions. Recent developments
have included the fully-integrated generation and analysis of image
cubes as a function of time and/or energy to better support imaging
spectroscopy. Many important improvements have been made to the spectral
analysis capabilities, and software has been developed to calculate
visibilities and to carry out visibility-based analyses. The gamma-ray
detector response is also being modified to better account for Compton
scattered photons in the detectors.

Visibilities are calibrated, 'instrument-independent,' Fourier
components of the source image. Mathematically, they correspond to the
output of a single baseline of a radio interferometer. For RHESSI, they
represent an intermediate data product directly obtained from the
observed time-modulated light curves. Visibility measurements have
well-defined statistical errors. Their value lies in the fact that they
depend linearly both on the data and on the source. Consequently, they
can be flexibly combined for comparison between source models and
observations. The basic visibility software implemented to date has
enabled the accurate determination of hard X‑ray source sizes (useful
for example to convert thermal emission measurements to electron
densities), and the evaluation of albedo; it was the computational tool
of choice in obtaining several of the recent results discussed in
section 2. Refinements in the calculation of visibilities and in their
use for other applications (such as improved spectral line imaging,
albedo analysis, and subsecond time resolution studies) are priority
goals of the ongoing software development effort.

## Ground System

In 1999, a multi-mission operations facility was established at SSL to
support the RHESSI and FAST missions. The joint facility, which now also
supports CHIPS and THEMIS operations, includes the Mission Operations
Center (MOC), the Science Operations Center (SOC), the Flight Dynamics
Center (FDC), and the Berkeley Ground Station (BGS). A high degree of
integration and automation combined with flexible system architecture
provides a reliable and cost effective state-of-the-art environment to
perform all functions required to operate multiple spacecraft
simultaneously.

### Ground Stations

The primary ground station for the RHESSI mission is BGS. Backup support
is provided by the Wallops Island (WGS), Santiago (AGO), Merritt Island
(MILA), and the DLR ground station at Weilheim, Germany. BGS has
supported more than 12,000 RHESSI passes since launch - on average 5-6
passes per day. WGS, AGO and MILA support typically 4 passes per day
combined. The average daily data volume is 13.5 Gbits with an overall
telemetry recovery efficiency of \~99%.

The BGS consists of an 11-m parabolic reflector mounted on a pedestal
with a three-axis drive system, and an S-band RF system. Since the
azimuth and elevation gear boxes were rebuilt after a failure in 2002,
BGS operations have been very reliable, supporting more than 21,000
passes for RHESSI, FAST, CHIPS and THEMIS combined.

### Mission Operations Center

The RHESSI Mission Operations Center is part of a secure, shared
state-of-the-art facility with a network of workstations for flight
dynamics, spacecraft command and control, mission planning, command load
generation, and data trending. The facility, typically staffed during
normal working hours only, performs automated pass supports, spacecraft
and instrument state-of-health checks, and generation of all ephemeris
and planning products in a lights-out mode.

#### Normal Operations

RHESSI normal operations comprise mission planning functions, command
load generation, real-time pass supports, spacecraft state-of-health
monitoring, data trending, instrument configuration, and science data
recovery and archiving. Generation of all ephemeris and mission planning
products is based on two-line element sets that are downloaded from the
Space-Track.org web site, quality checked and archived locally in a
fully automated mode. Spacecraft ATS loads are built by the FOT and
uploaded to the spacecraft multiple times per week. Every six weeks, the
spacecraft is re-spun from about 14.5 rpm to its nominal spin rate of 15
rpm. The FOT works closely with project scientists to determine the
optimal instrument configuration consistent with the current level of
solar activity.

During off-hours, FOT members carry two-way pagers to receive
notifications regarding any spacecraft or ground system anomalies. A
number of web-based tools allow FOT members, subsystem engineers, and
instrument scientists to monitor spacecraft and instrument performance
remotely.

Since RHESSI is operated from a multi-mission facility, its resources
are shared with other missions. This approach provides redundancy and
reduces overall operations costs. The flight control team now operates
RHESSI, FAST, CHIPS, and THEMIS for a total of 8 spacecraft.

### Science Operations Center

The RHESSI SOC, located at SSL, consists of three RAID servers, with
\~6.1 Tbytes of data capacity, and six processors. The SOC receives
RHESSI data after every ground-station contact and automatically
processes them to create the Level-0 data files. As of late February
2008 there are approximately 54,500 Level-0 data files, containing
4 Tbytes of data, for an average of approximately 1.8 Gbyte of data per
day.

Other automated procedures generate quick-look data containing the
RHESSI observing summary, flare list, and quick-look light curves,
images, and spectra. Long-term trend plots of SOH data are updated daily
in the SOC. All of these data products are available through the RHESSI
Data Center website at Goddard -

<http://hesperia.gsfc.nasa.gov/rhessidatacenter/>

### Max Millennium Program

The RHESSI PI team created and supports the Max Millennium program for
several functions that are key to the successful acquisition of
observationally complete datasets and the full scientific exploitation
of those observations. These functions are accessible at

<http://solar.physics.montana.edu/max_millennium/>

They include the following:

1.  Joint observing plans between RHESSI and other instrumentation with
    specific scientific goals.

2.  Daily observing targets generated each day by one of four
    experienced solar observers (the Max Millennium Chief Observers)

3.  RHESSI data analysis projects submitted by active scientists
    interested in collaborations.

4.  RHESSI software and hardware updates.

5.  The Solar Physics E-Print Archive. A recent study indicates that
    papers in this archive are cited about twice as often as average.

6.  Majordomo email for items of interest going to 353 self-subscribed
    participants.

### Education and Public Outreach

RHESSI E/PO will continue its partnership with the programs of the
Center for Science Education and the Space Sciences Laboratory (CSE@SSL)
to provide professional development opportunities and classroom support
for teachers in Northern California and at National Conferences. We will
also collaborate with CSE@SSL programs that reach out to informal
education venues and community groups, such as museums during the annual
Sun-Earth Day events and *Traditions of the Sun* community events.
RHESSI has produced an informal *Exploring Magnetism in Space Science*
kit for these kinds of venues.

We will partner with THEMIS and AIM E/PO to train and support a cohort
of Heliophysics Educator Ambassadors. These teachers will be selected
from teachers in the AIM E/PO and THEMIS GEONS programs and will receive
information about Heliophysics missions, training on quality classroom
materials developed by NASA and best practices in teacher professional
development. The teachers will act as Heliophysics Ambassadors with
teachers in their local regions across the country. In addition, RHESSI
E/PO will partner with other missions in the Heliophysics division to
collect all the best classroom resources they have developed and
organize them into a "curriculum" that can be used for the Heliophysics
Educator Ambassador program. RHESSI will continue to disseminate its
electronic E/PO resources via its website and CD-ROMs produced in
partnership with THEMIS, FAST, and STEREO-IMPACT E/PO.

At Goddard, we will continue to combine resources with other HD missions
with a presence here including SOHO, Cluster, Wind, STEREO, and Hinode,
and the upcoming SDO mission. These efforts will be coordinated by the
new hire in the Heliophysics Science Division whose duties will include
leading and coordinating the Education and Public Outreach activities of
the division.

# Appendices

## Budget

The RHESSI budget is presented in the template required for the Senior
Review and is broken down into the following five separate tables.

**I. FY08 - FY013 NASA Guideline with full-cost breakout**

The numbers in this table are the NASA guidelines provided by Chuck
Holmes as the total budget available for RHESSI MO&DA for each of five
fiscal years from FY08 through FY12.

**II. FY08 - FY13 \'5-way\' Functional Breakdown**

The numbers by fiscal year in this table are the breakdown by function
of the guideline numbers given in table I.

2.a Space Communications Services

> The RHESSI share of voice and data tail circuits connecting UCB to the
> NASA network.  The circuit is supported also by FAST and THEMIS.
> Numbers provided by Pat Crouse.

2.b Mission Services

> Predictions of Berkeley MO costs provided by Pat Crouse and agreed to
> by Berkeley.

2.c Other Mission Operations

> Cost of GSFC Operating Spacecraft Project Office management support
> for a fraction of a Mission Director. Numbers provided by Pat Crouse.

3\. Science Center Functions

> Estimated costs of operating Science Center, verifying instrument
> performance and updating calibrations, handling flight data,
> maintaining analysis software, etc. The Science Center Functions costs
> were reduced below the inflation increased values in FY10 through FY12
> to ensure that the Science Data Analysis estimate did not become
> negative.

4\. Science Data Analysis

> Estimated costs for science data analysis. The numbers were calculated
> as the remainder after subtracting all other costs from the NASA
> guideline for each year.

5\. E/PO

> Calculated as 1% of the NASA Guideline for each year.

**III. FY08 - FY13 Instrument team breakdown**

1\. UCB Data Analysis & EPO

2\. GSFC Data Analysis & EPO

These two items were each calculated as 50% of the funds allocated to
Science Center Functions and Science Data Analysis and EPO in Table II.

UCB Mission Operations. These are the same numbers as in Table II, row
2b.

GSFC Project Management. The same numbers as in Table II, row 2c.

**IV. FY08 - FY13 \'5-way\' Breakdown for in-Kind contributions**

These are services to RHESSI that are paid by other funding sources.
These costs are allocated to RHESSI but are not supported with Project
funds. This table was provided by Pat Crouse.

2.a Space Communications Services.

> Contains the costs to use the Ground Network and connect to the
> mission voice and data network.

2.c Other Mission Operations.

> Costs allocated by the SSMO Project for all the elements associated
> with Project Management.

**V. FY08 - FY13 \'5-way\' Breakdown for Optimal Budget:**

The optimal budget is the same as that shown in Table II for FY08 but
with the costs for Science Center Functions (row 3) and Science Data
Analysis (row 4) increased to adjust for inflation in subsequent years.
This allows for continued funding at the present level for Science Data
Analysis through FY12. This is highly desirable since solar activity
will increase during these years and RHESSI observations will be in
demand for joint analysis with data from the advanced instrumentation on
the new solar missions recently launched - Hinode and STEREO - and those
to be launched within the next year - SDO and GLAST.

## Acronym List

> AAS American Astronomical Society
>
> ACS Attitude Control System
>
> AESP Aerospace Education Services Program
>
> AGO Santiago ground station
>
> AGU American Geophysical Union
>
> AIM Aeronomy of Ice in the Mesosphere
>
> AISES American Indian Science and Engineer Society
>
> ATS Absolute Time Sequence
>
> AU Astronomical Unit
>
> BATSE Burst and Transient Source Experiment on CGRO
>
> BGO Bismuth Germinate used as a scintillation detector
>
> BGS Berkeley Ground Station
>
> BPSK Binary Phase Shift Keying
>
> CAST CERN Axion Solar Telescope\
> CCD Charge Coupled Device
>
> CD-ROM Compact Disk -- Read Only Memory
>
> CHIPS Cosmic Hot Interstellar Plasma Spectrometer
>
> CGRO Compton Gamma Ray Observatory
>
> CME Coronal Mass Ejection
>
> COMPTEL Compton Telescope on CGRO
>
> COR-1 Inner Coronograph of STEREO SECCHI instrument
>
> COR-2 Outer Coronograph of STEREO SECCHI instrument
>
> COSPAR COmmittee on SPAce Research
>
> CSE@SSL Center for Science Education at Space Sciences Laboratory
>
> CSTA California Science Teachers Association
>
> DLR Weilheim ground station
>
> EGRET Energetic Gamma Ray Experiment Telescope on CGRO
>
> ERC Educator Resource Center
>
> EEPROM Electrically Erasable Programmable Read-Only Memory
>
> EIT Extreme Ultraviolet Imaging Telescope on SOHO
>
> EIS EUV imaging spectrograph on Hinode
>
> E/PO Education and Public Outreach
>
> ETH Eidgenossische Technische Hochschule, Zurich, Switzerland
>
> EUV Extreme Ultra-Violet
>
> EVA Extra-Vehicular Activity
>
> FAST Fast Auroral SnapshoT
>
> FDC Flight Dynamics Center
>
> FIP First Ionization Potential
>
> FITS Flexible Image Transport System
>
> FORMOSAT-2 Earth Observation Satellite
>
> FOT Flight Operations Team
>
> FoV Field of View
>
> FTE Full time Equivalent
>
> FWHM Full Width at Half Maximum
>
> GCN Gamma-ray Burst Coordination Network
>
> GDS Ground Data System
>
> Ge Germanium
>
> GeD Germanium Detector
>
> GEMS Great Explorations in Math and Science
>
> GEONS Geomagnetic Event Observation Network by Students
>
> GLAST Gamma-ray Large Area Space Telescope
>
> GRB Gamma Ray Burst
>
> GRS Gamma Ray Spectrometer
>
> GSFC Goddard Space Flight Center
>
> G/T Antenna gain to noise temperature ratio
>
> HD Heliophysics Division
>
> HEAD High Energy Astrophysics Division
>
> HGO Heliophysics Great Observatory
>
> Hinode Japanese solar satellite - formerly Solar-B
>
> HST Hubble Space Telescope
>
> HXR Hard X-ray
>
> HXT Hard X-ray Telescope on Yohkoh
>
> IDL Interactive Data Language
>
> IDPU Instrument Data Processing Unit
>
> IGES Institute for Global Environmental Strategies
>
> IMPACT In-situ Measurements of Particles and CME Transients on STEREO
>
> IPM Interplanetary Medium
>
> ISUAL Imager of Sprites and Upper Atmospheric Lightnings on FORMOSAT-2
>
> ITOS Integrated Test and Operations System
>
> KSC Kennedy Space Center
>
> LASCO The Large Angle and Spectrometric Coronagraph on SOHO
>
> LAT Large Area Telescope on GLAST
>
> LHCP Left-Handed Circular Polarization
>
> LHS Lawrence Hall of Science
>
> LSEP Large SEP event
>
> mas milli-arcsecond
>
> MDI Michelson Doppler Imager on SOHO
>
> MILA Merritt Island ground station
>
> MOC Mission Operations Center
>
> MOU Memorandum of Understanding
>
> MPS Mission Planning System
>
> NaI Sodium Iodide used as a scintillation particle detector
>
> NASA National Aeronautics and Space Administration
>
> NSTA National Science Teacher Association\
> NuSTAR Nuclear Spectroscopic Telescope Array
>
> OCA Orbital Carrier Aircraft
>
> OIG Orbital Information Group
>
> PD Professional Development
>
> PMTRAS Photomultiplier Roll Angle System
>
> PSLA Project Service Level Agreement
>
> RAD6000 Radiation-hardened single board computer used on spacecraft
>
> RAS Roll Angle System
>
> RF Radio Frequency
>
> RHCP Right-Handed Circular Polarization
>
> RHESSI Reuven **R**amaty **H**igh **E**nergy **S**olar
> **S**pectroscopic **I**mager
>
> SAA South Atlantic Anomaly
>
> SACNAS Society for Advancement of Chicanos and Native Americans in
> Science
>
> SAMPEX Solar Anomalous and Magnetospheric Particle Explorer
>
> SAS Solar Aspect System
>
> SatTrack Satellite Tracking software
>
> SDAC Solar Data Analysis Center
>
> SDO Solar Dynamics Observatory
>
> SECCHI Sun Earth Connection Coronal and Heliospheric Investigation on
> STEREO
>
> SECEF Sun Earth Connection Education Forum
>
> SEGway Science Education Gateway
>
> SEP Solar Energetic Particles
>
> SEPT Solar Electron Proton Telescope on STEREO / IMPACT
>
> SERS Spacecraft Emergency Response System
>
> SHH Soft, Hard, Harder -- spectral evolution
>
> SHS Soft, Hard, Soft -- spectral evolution\
> SMEX NASA\'s Small Explorer Program
>
> SMM Solar Maximum Mission
>
> SOC Science Operations Center
>
> SOH State-Of-Health
>
> SOHO Solar and Heliophysics Observatory
>
> SOT Solar Optical Telescope on Hinode
>
> SPD Solar Physics Division
>
> SSL Space Sciences Laboratory
>
> STE Suprathermal Electron Telescope on STEREO / IMPACT
>
> STEREO Solar TErestrial RElations ObservatorySolar TErrestrial
> RElations Observatory
>
> STIX Spectrometer Telescope for Imaging X-rays\
> SSW SolarSoftware
>
> STEM Science, Technology, Engineering, and Math
>
> SXI Soft X-ray Imager
>
> TGF Terrestrial Gamma-ray Flash
>
> THEMIS Time History of Events and Macroscale Interactions during
> Substorms
>
> TRACE Transition Region and Coronal Explorer
>
> UCB University of California at Berkeley
>
> UT Universal Time
>
> VSO Virtual Solar Observatory\
> VxWorks A real-time operating system
>
> WGS Wallops Ground Station
>
> WISE Wide Field Infrared Survey Explorer
>
> XRT X-Ray Telescope on Hinode
>
> Yohkoh Japanese Solar Observatory Satellite