## SCIENCE OBJECTIVES for 2007 - 8 and 2009 - 10

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height="3.2104166666666667in"}

Figure 2.2‑1. Predicted rates of X-ray flares for the next solar cycle
based on the rates from the three previous cycles as measured with the
indicated instruments. The measured rates have been normalized to the
rate of GOES-class C3 and higher by dividing by the indicated factors
for each instrument.

Solar minimum is predicted for 2006-7, with a significant upturn in
activity expected for 2008 (Figure 2.2-1). 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. In 2006, STEREO and SOLAR-B will be launched to join the
S^3^C Great Observatory. 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 SOLAR-B, and the 3-D imaging in
EUV and white light coronagraphy of STEREO for studying flares and CME
initiation (section 2.2.2). 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 (section
2.2.3). Solar minimum will also enable RHESSI to make deep searches for
microflares (2.2.4) and quiet-Sun nonthermal emissions (2.2.5).

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 the S^3^C Great Observatory in the last solar maximum
(2.2.7). As always RHESSI will contribute unique measurements for study
of Terrestrial Gamma-ray Flashes (2.2.5) and cosmic gamma-ray bursts and
other high energy astrophysical phenomena (2.2.6).

In 2009-10, solar activity will be steeply rising towards its peak, and
RHESSI's high energy measurements will be essential to the S^3^C Great
Observatory.

### Flare Rates and Energetic Events

Since the choice of which RHESSI science objectives will be pursued
depends on the level of solar activity, we first present our best
estimates of predicted flare rates through the next solar maximum. From
the rate of \>25 keV HXR flares for the last three solar cycles shown in
figure 2.2-1, we use as a prediction for the next cycle the average rate
(solid red curve) for each month of the cycle assuming a 10.5-year
period. For the solar minimum period (2006-7), we expect \~0.2 HXR
flares per day, for a total of \~75 above 25 keV or \~500 above 12 keV,
assuming a 50% duty cycle. The activity is likely to be more intense for
short periods with long intervals of virtually no activity at all -- a
so-called spotless Sun. Activity is expected to pick up rapidly in 2008
with the flare rate more than doubling in that year as we rise toward
the next solar maximum. During the solar minimum period from January
1986 - January 1988, the SMM Gamma-Ray Spectrometer (GRS) observed 5
gamma-ray flares with detectable flux above 300 keV, with four coming
from a single active region. We should detect a similar number during
the two-year minimum period. Several large SEP events should be detected
as well.

The flare rate does not predict the occurrence of some major phenomena
very well, however. Huge SEP events are well known to occur away from
solar maximum. In the past minimum, for example, the X‑class flare of
July 1996 resulted in the first detection of a flare-generated
helioseismic wave. In the recent late-phase activity of 2003 and 2005,
further examples have been found. RHESSI HXR and gamma-ray line imaging
of such a flare could determine whether high energy particles are the
energy (and momentum) source for the wave, which has unique potential
for sounding sub-surface active-region structure that may have
implications for event prediction.

Finally, we note that the July 1996 region (and its flares and CMEs)
resulted in many important studies simply because this "test particle"
activity dropped into a greatly simplified solar-minimum coronal
structure.

### RHESSI Flare Studies with SOLAR-B, STEREO, and GOES N

Comparing STEREO and SOLAR B observations with the RHESSI data set will
greatly enhance the scientific return for all three missions. SOLAR-B,
to be launched in August 2006, 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. SOLAR-B/SOT will provide the
first stable time-series observations of the solar vector magnetic
field, at the highest possible angular resolution. Together with
SOLAR-B/XRT soft X-ray imaging and SOLAR-B/EIS EUV imaging and
spectroscopy, this will certainly 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 SOLAR-B observations thus
will provide our best and most objective determination of the trigger
conditions.

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, SOLAR‑B/XRT
and RHESSI will provide thermal flare energy estimates, SOLAR‑B/EIS will
give flow measurements, and vector-magnetograms by SOLAR‑B/SOT will be
needed to for accurate modeling of the coronal field. This will allow
the spatial and temporal evolution during a flare to be modeled and
different models to be tested. 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 show that about 10% of all flares are partially occulted.
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.

The Gamma Ray Large Area Space Telescope (GLAST) (late 2007 launch)
provides a unique opportunity to make simultaneous nuclear-line and
very-high energy (several GeV) gamma-ray observations, extending RHESSI
measurements of the accelerated-particle spectrum to \>1 GeV. 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 (Section 2.1.3.2). 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.

Very soon, perhaps as early as the end of 2005, a new GOES spacecraft
will be launched (GOES-N). This spacecraft will carry the next
generation of the Solar X-ray Imager (SXI), with much better spatial
resolution that the SXI currently in operation, roughly like the
Yohkoh/SXT half resolution images. After launch there will be a checkout
period of several months during which the much improved SXI instrument
will be imaging the X-ray Sun. Eventually, the GOES spacecraft will be
turned off until needed, but the months of overlapping coverage with
RHESSI will be invaluable for joint studies. Topics of interest include
cross-calibrations of the instruments and complementary observations of
solar activity. It was most unfortunate that the Yohkoh spacecraft
failed just before the launch of RHESSI. With the launch of the new SXI
instrument, much of the joint science planned with SXT can be recovered.

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 while crossing the ecliptic plane at
\~1.5 AU distance. 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) instrument will
provide \~50 times greater sensitivity for measurements of impulsive
electron events from \~2 to 30 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 SOLAR B will
allow detailed studies of single microflare events to see if microflares
are indeed just smaller version of large flares or whether they possibly
are a separate type of event. So far RHESSI microflares are observed to
originate from active regions. SOLAR‑B/EIS will provide observations of
plasma flows, while SOLAR B/SOT and STEREO/EUVI will provide \"3D\"
estimates of flare volumes giving better energy estimates. SOLAR‑B/XRT
is sensitive in the RHESSI microflare temperature range (\~5-10 MK)
providing independent energy estimates. RHESSI observations will be an
essential part of these studies since it is the only instrument that can
provide HXR diagnostics and give quantitative measurements of the energy
content in non-thermal electrons and hotter (\>15 MK) thermal plasmas.
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. SOLAR‑B provides unparalleled diagnostic power for such
transients, ideally complemented by RHESSI's sensitivity to higher
temperature plasma and to non-thermal electron acceleration effects.

### X-ray flux from the quiet Sun

Although several interesting mechanisms predict HXR emission from the
quiet Sun, prior to RHESSI no instrument had sufficient sensitivity to
explore it. One of the early pleasant surprises from RHESSI was the
finding that the non-flaring Sun was usually a source of hard X-ray
emission (Krucker et al., 2002). RHESSI imaging immediately identified
the sources with active regions. This suggested that some continuous
X-ray emission might be associated with strong sunspot-associated
magnetic fields. However the question remains as to whether there is
continuous quiet-Sun hard X-ray emission that is not associated with
active regions, but with the diffuse hot corona.

Microwave observations (e.g. Krucker et al. 1997) have shown the
quasi-continuous presence of small 'network flares'. While observations
of individual events are problematic, their integrated emission should
be detectable. Such emission would have significant implications for
coronal heating. There are several other potential mechanisms for
continuous quiet-Sun hard X-ray emission: Parker's nanoflares, double
layers, or non-thermal wave heating to temperatures beyond those
observable with SXI or Yohkoh/SXT. More exotic mechanisms such as albedo
emission secondary to the galactic cosmic rays (Seckel et al., 1991) or
even "axions" (Zioutas et al., 2004) are also possibilities. The RHESSI
capability of defining the non-thermal spectrum and spatial
characteristics of such emission will provide definitive diagnostics.

With its shutters open, RHESSI has much greater sensitivity than any
previous instrument in the 5-15 keV range. To apply this great
sensitivity to the quiet Sun, it is essential to eliminate background
contributions. Furthermore, since widely-distributed, weak hard X-ray
sources do not lend themselves to efficient imaging with rotating
modulation collimators, it is necessary to use a technique that
correctly sums the individual sources. We will use two techniques for
determining the presence and spectrum of quiet Sun emission*,
irrespective of its spatial distribution*. (In each case, conventional
RMC imaging can then be used to infer its spatial properties.) Each
technique is 2 to 3 orders of magnitude more sensitive than the previous
upper limits (Neupert, 1969 and references therein).

The first approach relies on statistical analysis of changes in observed
count rates as RHESSI enters or leaves eclipse. This is relatively
straightforward although background *trends* need to be carefully
handled. The second technique requires RHESSI to offpoint from the Sun
by about 0.4 to 0.8 degrees. In this case, the 1-degree field of view of
the individual grids in 6 of the 9 subcollimators modulates the
integrated solar emission. (In effect, it chops the background signal at
0.5 Hz with a predetermined phase.) The remaining 3 grids act as
controls in that no modulation is expected because of their larger
fields of view. These procedures have been tested with a limited data
set on a few very quiet days. Since the results of such quiet-Sun
observations are expected to be statistically-limited, we require
observations on a large number of days in solar minimum, the only time
that a spotless Sun is likely to be observed.

Krucker, S., Benz A. O., Bastian, T. S., Acton, L. W., ApJ 488, 499-505,
1997.

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

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

Seckel, D., Stanev, T., and Gaisser, T., K.: ApJ 382, 652, 1991.

Zioutas, K., et al.: ApJ 607, 575, 2004.

### 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

#### Gamma-Ray Bursts

There is a strong possibility that GRBs can be used as high-redshift
standard candles (like type Ia supernova) to determine a cosmic distance
scale. However, this can only be done if the full spectrum and fluence
of each GRB is measured. This requires extending the spectrum to well
above the Swift maximum energy of 150 keV. Currently, the only
instruments that are able to make such high-energy spectral measurements
are HETE-2 and RHESSI. HETE-2 observations are due to end at the end of
September, 2005. Thus, until GLAST is launched in 2007 with a BATSE-like
instrument that can detect the burst to GeV energies, RHESSI will be
alone in its ability to make these critical high-energy spectral
measurements.

#### Imaging the Crab Nebula

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 in June.

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

### Proposed Solar Eruptive Events GI Program

In the recent solar maximum, an enormous range of remarkable new
observations of solar explosions and eruptions and their energetic
consequences has become available from ACE, RHESSI, SoHO, TRACE, Wind,
and other missions of the S^3^C Great Observatory. The buildup,
initiation, evolution, and consequences of explosive and eruptive events
are topics of particular relevance to NASA\'s future in this era of the
Vision for Space Exploration and the Living With a Star initiative. In
FY 2007 and 2008, STEREO and SOLAR-B will add more qualitatively new
observations. 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, the science teams of many
of the S^3^C Great Observatory missions (including, alphabetically, ACE,
RHESSI, SOHO, SOLAR-B, STEREO, TRACE, and Wind) propose that NASA
allocate **\$1.5 million/year** in Guest Investigator (GI) funds in the
coming years to stimulate rapid advancement of our understanding of the
buildup, initiation, evolution, and consequences of explosive and
eruptive events. A multi-mission GI program would stimulate analysis of
data from joint observing campaigns as well as stimulate coordination
between instrument and analysis teams. Multi-disciplinary science
workshops, focused on specific aspects of these tasks, will provide
additional stimulus to discover new processes, phenomena, and
correlations in archival and newly-obtained data.