Flares III
Presiding: D Alexander, Rice University; G D Holman, NASA Goddard Space Flight Center
SP41C-01 08:30h
Particle Acceleration and Plasma Heating by Turbulence in Solar Flares
There is mounting theoretical and observational evidence in support of the idea that plasma turbulence plays an important role in solar flares. It is likely that the energy released by magnetic reconnections is channelled mainly into plasma waves or turbulence. The importance of turbulence in accelerating electrons and ions have been recognized for some time now and models based on particle acceleration by plasma waves have been shown to successfully reproduce many spectral and temporal features of the Solar Energetic Particles observed near the Earth and the X- and gamma-ray radiations produced by electrons and ions during the impulsive phase. New high spatial resolution observations by RHESSI have provided further evidence for this model. In this paper we will review briefly the model and describe some of the results based on particle and photon spectra and on the new high resolution images. Here are two examples: 1. We will show how the model can account for the prominent enhancement and distinct spectrum of 3He ions in some impulsive solar flares and discuss its potential to explain the enhancements of ultra-heavy ions observed by ACE and WIND, and the observed correlation between ion enhancements and the iron spectral index. 2. In the gradual decaying phase of many solar flares, RHESSI observations show that the flare plasma is being continuously heated near the looptop region, and that a suppression of conduction is needed to confine the source in the looptop region as observed. Turbulence could play important roles in both processes. The work is supported by NSF grant ATM-0312344, NASA grants NAG5-12111, and NAG5 11918-1 at Stanford.
SP41C-02 08:45h
Stochastic Particle Acceleration in a Self-Consistent Solar Flare Atmosphere
We present results from a unified and self-consistent model of particle acceleration and atmospheric response in impulsive solar flares. In our model, electrons and ions are stochastically energized from thermal to relativistic energies on short timescales by cascading MHD turbulence, which is assumed to have been excited initially in the coronal region of a flare loop during the primary energy release phase. The accelerated particles then propagate to the denser transition region and chromosphere, where they can deposit a large fraction of their energy and drive the formation of a hydrodynamic shock that propagates back into the corona. The density enhancements that accompany this shock in turn modify the particle acceleration processes in the corona by altering (in a spatially-dependent manner) the density and Alfvén speed, and hence the acceleration rates and threshold energies. The two main components of this simulation are the NRL Dynamic Solar Flux Tube Model code and a spatially-dependent quasilinear particle acceleration/wave evolution code. As such, it provides a comprehensive treatment of both macroscopic (chromospheric evaporation) and microscopic (wave-particle interactions) processes. We demonstrate the coupling between acceleration and atmospheric response by presenting simulation results for realistic flare parameters, and show the importance of including the later process in particle acceleration studies. We also show that acceleration by cascading MHD turbulence is able to account for all the major features of flare energetic particles. This work was supported by NASA grant NAG5-12794.
SP41C-03 09:00h
The Low-Energy Cutoff to the Nonthermal Electron Spectrum Determined from RHESSI Solar Flare Observations
Since the spectrum of electrons accelerated during a solar flare is a steeply falling function, most of the energy in these electrons resides with the lower energy particles. Thus, it is most important to determine the low energy end of the spectrum, particularly the low energy cutoff (or flattening from the steep power-law spectrum seen at higher energies) that must surely exist to keep the total energy finite. The determination of such a cutoff to the electron spectrum is decisive for the estimation of the total energy in nonthermal electrons at any given time during a flare. Unfortunately, thermal bremsstrahlung usually dominates the low-energy part of flare X-ray emission so that the flattening of the nonthermal X-ray spectrum that such a cutoff would produce at low energies has never been unequivocally identified. An M1.0 flare observed with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 2002 June 2 had a particularly favorable time history for the detection of a spectral flattening at low X-ray energies. Two impulsive X-ray peaks occurred within 1 minute of the soft X-ray start of the flare. They were noticeable in the time histories over a broad energy range down to RHESSI's 3 keV lower limit. Both peaks showed a spectral flattening at energies below 20 to 30 keV, as expected for a low-energy cutoff to the distribution of nonthermal electrons. The deduced cutoff electron energy varies from ~20 to as high as 38 keV during the 30 seconds that spectral flattening is evident. We will discuss the implications of these results on the flare energetics and on particle acceleration and transport models.
SP41C-04 09:15h
Quasiperiodic Electron Acceleration in the 15 June 2003 Solar Flare
We report a comprehensive analysis of strong quasiperiodic radio- and X-ray pulsations observed from the X1.3-class flare which occurred at S06E78 on 23:42-23:50 UT, 15 June 2003. Because of the favorable time of the flare, it was jointly observed by the Owens Valley Solar Array (OVSA) and the Nobeyama Solar Radio Observatory, which allowed us the advantage of combining high spectral, temporal, and spatial resolution radio observations. In addition, the part of this event displaying the strongest pulsations was also observed in hard X-rays with RHESSI. We study the frequency dependence of quantitative measures of the pulsations, including Fourier spectra, phase differences, modulation depth, as well as the degree of (radio) polarization. We compare these measures with the expectations of competing radio pulsation models, i.e., MHD loop oscillations and quasiperiodic electron injection. Although the Fourier spectra display a few significant peaks at each observing frequency, we found that none of them can be explained by the MHD-oscillations. In contrast, the model of quasiperiodic particle acceleration/injection is capable of explaining all quantitative measures observed for this event in a natural way. We discuss implications of these findings for electron acceleration and transport in solar flares.
SP41C-05 09:30h
Distinguishing between thermal and non-thermal electron populations in solar flares using RHESSI
Solar flare plasmas contain thermal and non-thermal electron populations, both of which emit characteristic X-ray bremsstrahlung spectra. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes X-rays above 3~keV with a spectral resolution of ~1~keV FWHM. RHESSI spectra typically show a thermal continuum dominant below ~10~keV and a non-thermal continuum dominant above ~30~keV. Distinguishing between the two electron populations is crucial in determining the total energy contained in non-thermal electrons. High-temperature plasmas also emit characteristic line spectra, including emission in the Fe and Fe/Ni "line complexes" at ~6.7 and ~8~keV. Recent predictions and subsequent empirical confirmation have shown that the fluxes and equivalent widths of these complexes are strongly temperature-dependent (Phillips 2004, Caspi et al. 2004). We use the Fe and Fe/Ni line complexes to obtain constraints on the thermal electrons for a variety of large (GOES class M and X) solar flares, with emphasis on the X4.8 event on 23 July 2002, to determine the time-varying characteristics of the thermal and non-thermal electron populations. We estimate and compare the energy contained in these electrons, and discuss the implications for acceleration and heating of electrons in solar flares.
http://sprg.ssl.berkeley.edu/~cepheid/agu_spd2005
SP41C-06 09:45h
Impulsive Phase Hard X-Ray Source Motions Observed by RHESSI
High temporal, spatial, and spectral resolution observations by RHESSI have provided some interesting and surprising results. In this paper we present results of our analyses of the spatial evolution during the impulsive phase, in particular the motions of the looptop (LT) and footpoint (FP) sources of flares with a simple loop structure. For the first time, on November 13, 2003 RHESSI observed HXR emissions from the legs of a flaring loop, in contrast to the commonly observed LT and FP emissions. In the 9-30 keV energy range, emissions from the FPs initially appear to extend toward the legs, then rise up, and eventually merge at the LT. This motion happens first at lower energies and proceeds to higher energies, indicating a gradual density increase in the flaring loop, induced possibly by chromospheric evaporation. Spectral analyses are carried out to determine the nature of these sources. We discuss the implications of these results on the particle acceleration, plasma heating, and chromospheric evaporation processes. The work is supported by NASA grants NAG5-12111, NAG5 11918-1, and NSF grant ATM-0312344 at Stanford University.