**Modeling Solar Flare**

**Hard X-ray Images and Spectra Observed with RHESSI**

# Background

A solar flare is an enormous explosion in the solar atmosphere,
involving sudden bursts of particle acceleration, plasma heating, and
bulk mass motion. In common flares, 10^32^ ergs or more of energy can be
released in a few minutes to a few tens of minutes. Instrumentation on
board a variety of spacecraft over the last two decades has generated a
wealth of data which has significantly improved our understanding of
solar flares (Aschwanden, 2002). But there are still many unsolved
problems, such as how flare energy is stored, what triggers a solar
flare, and how particles are accelerated, and so on.

The most accepted flare energy release process is magnetic reconnection,
but so far there is no direct observational evidence to support this.
The most convincing observation is the hard X-ray source above the flare
loops. The canonical event is the flare of 1992 January 13, first
described by Masuda et al. (1994) and later analyzed by

Alexander & Metcalf (1997). This flare, which is clearly delineated by a
soft X-ray (thermal) loop, shows three compact hard X-ray sources, two
located at the footpoints and a third above the top of the soft X-ray
loop. Recently Petrosian et al. (2002) analyzed 18 X-ray bright limb
flares observed with the Yohkoh solar satellite. Fifteen showed
detectable impulsive loop-top emission. Considering the finite dynamic
range of the instrument, they concluded that loop-top emission is a
common feature of all flares.

Hard X-ray emission from flares is bremsstrahlung from interactions
between energetic electrons and the ambient atmosphere, so hard X-ray
observations of solar flares provide diagnostic information on the
acceleration and subsequent transport of energetic particles in the
solar corona. The launch of the Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) provides an unprecedented opportunity to study solar
flares (Lin et al. 2002). It provides high temporal (down to tens of
milliseconds) and spatial (as fine as 2 arcseconds) hard X-ray and
gamma-ray images (Hurford et al. 2002). It also provides spatially
integrated and spatially resolved spectra with the highest resolution
(as fine as 1 keV) (Smith et al. 2002).

**Purpose**

If loop-top hard X-ray sources are common for solar flares (Petrosian et
al. 2002), then RHESSI would also detect them. In order to check this
assumption, a systematic study of RHESSI observational results is
necessary. Furthermore, the mechanism of loop-top emission is still not
well understood. Only by modeling flares can one understand the
mechanism of particle acceleration and transport in flares. RHESSI
software allows one to simulate flares with models to create images and
spectra. By comparing the simulated results with observations, validity
of the models and of the analysis software can be evaluated.

## 

## Methodology

I will first do a detailed analysis of a few simple events observed by
RHESSI, together with other supplementary data, such as from the Extreme
ultraviolet Imaging Telescope (EIT) and the Michelson Doppler Imager
(MDI), which are both onboard [the SOlar and Heliospheric
Observatory](http://sohowww.nascom.nasa.gov/) (SOHO). Then I will use
selected flare models to simulate these events.

I will start with a single loop model, and then try a few acceleration
and transport models (Holman 1996), which include Coulomb collisions,
pitch-angle diffusion and magnetic mirroring, to obtain electron
spectral distributions along the loop (McTiernan & Petrosian 1990).
Finally, I will obtain X-ray images and spectra from the X-ray
bremsstrahlung produced by electrons with both thermal and nonthermal
distributions (Sui et al. 2002). The model will be applied to study both
limb and disk events, and will handle nonsymmetrical flare loops
observed from any perspective.

Microwave observations provide complementary information to the hard
X-ray data for the same flares. Because both microwave emission and hard
X-ray emission are produced by highly energetic electrons, studying
microwave data and hard X-ray data together will give a more
comprehensive picture of the accelerated particles. My computerized
flare model can also create simulated microwave images and spectra from
the calculated electron gyrosynchrotron emission. If appropriate
microwave data are obtained from Owens Valley Solar Observatory and/or
the Very Large Array (VLA), I plan to use them to compare with my model
predictions.

**Contribution**

By using the best up-to-date hard X-ray images and spectra of solar
flares, my flare modeling will contribute to a better understanding of
loop-top emission in flares. Furthermore, it will be very helpful in
understanding the particle acceleration and transport mechanism.

**Selected Bibliography**

Alexander, D., Metcalf, T. R., 1997, ApJ, 489, 442

Aschwanden, M.J., 2002, '*Particle Acceleration and Kinematics in Solar
Flares. A Synthesis of Recent Observations and Theoretical Concepts'*
Space Sci. Reviews

Holman, G. D., 1996, B.A.A.S., 28, 939

Hurford, G. et al., 2002, Solar Phys., in press

Lin, R.P. et al., 2002, Solar Phys., in press

Masuda, S., Kosugi, T, Hara, H., Tsuneta, S., Ogawara, Y., 1994, Nature,
371, 495

McTiernan, J. M., Petrosian, V., 1990, ApJ, 359, 524

Petrosian, V., Donaghy, T. Q., McTieran, J. M., 2002, ApJ, 569, 459

Smith, D.M. et al., 2002, Solar Phys., in press

Sui, L., Holman G. D., Dennis, B. R., Krucker S., Schwartz R. A.,
Tolbert K., 2002, Solar Phys., in press