Flares IV
Presiding: D Alexander, Rice University; G D Holman, NASA Goddard Space Flight Center
SP42A-01 10:30h
Energetics and Temporal Variability in RHESSI Gamma-Ray Flares
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) has detected nuclear gamma-ray line emission from at least eleven solar flares over the past three years. These gamma-ray lines are produced when flare-accelerated ions collide with the ambient solar medium. In this paper, we concentrate on the energy content and time profiles of accelerated ions and electrons. Ion spectral information is derived from suitable line ratios. We also compare the fluxes and timing of gamma-ray emission and high-energy electron bremsstrahlung emission. Finally, for the larger flares, we investigate the temporal variability of the nuclear de-excitation lines as well.
SP42A-02 10:45h
Accelerated and Ambient Abundances in RHESSI Gamma-Ray Flares
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) has detected nuclear gamma-ray line emission from at least eleven solar flares over the past three years. These gamma-ray lines are produced when flare-accelerated ions collide with the ambient solar medium. In this paper, we use gamma-ray line ratios and Doppler profiles to constrain the relative fluxes of accelerated protons, alphas, and heavier nuclei in the brighter RHESSI gamma-ray flares. We also study the relative fluxes of narrow lines to compare our conclusions about ambient solar abundances in the interaction region to earlier work from the Solar Maximum Mission Gamma-Ray Spectrometer. The work at the University of California was supported by NASA contract NAS 5-98033.
SP42A-03 11:00h
RHESSI Positron-Electron Annihilation Line Observations
We discuss our analysis of the electron-positron annihilation line and continuum from flares observed by RHESSI. RHESSI observed what appears to be a thermally broadened line (~6.5 keV FWHM) at 511 keV over the first 10 min of the 2003 October 28 flare that was consistent with emission from a plasma >105 K. The line narrowed to a width of ~1 keV over the next 2 min. The narrow width suggests emission from a <104 K region that is at least 20% ionized. This suggests a rapid variation in the solar atmosphere at densities <1014 H cm-3. In contrast the shape of the annihilation line from the first ten minutes of the 2003 November 2 flare is consistent with emission from a quiet solar atmosphere at ~5200 K; however, the flux in the 3 γ continuum may be inconsistent with this location. RHESSI has recently observed gamma-ray emission from flares erupting from AR0720 on 2005 January 15, 17, 19, and 20. Our preliminary analysis of the January 20 flare reveals a gamma spectrum with a hard continuum, intense lines at 511 keV and 2223 keV (neutron capture), and relatively weak nuclear de-excitation lines. This gamma-ray spectrum is consistent with a very-hard accelerated particle spectrum that produces most of the positrons from decay of positive pi mesons produced near the photosphere. The annihilation line has a width of ~6.5 keV that is again suggestive of thermal broadening. This work has been supported by NASA DPR NNG04ED181 and by ONR.
http://heseweb.nrl.navy.mil/gamma/solar/share.html
SP42A-04 11:15h
Microflare Statistics and Frequency Distribution
RHESSI is uniquely suited to observe solar microflares due to its unique sensitivity in the 3-15 keV energy range (up to ~100 times better than previous solar instruments). As such, it provides new information on these low level transients. Initial results (Krucker et al 2002, Benz & Grigis 2002) suggest that microflares are different from larger flares. They are more often associated with steep nonthermal spectra (power law index -5 to -7). In this study, we present microflare statistics from times of low activity. A list of microflares was created by applying the standard RHESSI flare-finding algorithm to the lower 6-12 keV energy range (~10,000 events). Imaging was used in order to obtain positions of solar events and reject non-solar events. These solar events were then each spectrally analyzed. We present microflare statistics, including active region productivity, and the microflare frequency distribution. This work was supported by NASA contract NAS5-98033.
SP42A-05 11:30h
Size Distribution of Flares: Is it Universal?
Size distributions of solar flares, measured in appropriate variables, often exhibit power-law forms with negative indices. From this, an avalanche model has been proposed. This model assumes that the size distribution is universal; that is, it does not easily change from under one condition to another. In my previous study, I showed that the size distribution varies with the phase of the 152-day periodicity. In the peak phase, the size distribution was flatter. I also reported that the size distribution was flatter in the declining phase of cycle 21 than in the maximum phase. In this paper, I am going to study of a size distribution, using GOES soft X-ray observations of solar flares. The size distribution is flatter for flares produced by super-active regions than for other flares. The size distribution is also flatter for flares produced in 2003 and 2004, during which unusually high flare activity was observed. Such variability of size distributions does not support the avalanche model.
SP42A-06 11:45h
From Raw Data to Flare Predictions: A Fully Automated Technique
With the large volume of solar data which already exists, and expected in the near future with SDO, automated techniques are becoming increasingly vital. We present a fully automated active region extraction routine based on boundary extraction and region growing techniques applied to full disc MDI longitudinal magnetograms. Once extracted, any number of image processing techniques can be applied to the data leading to the possibility of automated classification. We discuss a large scale (9 years of MDI data, ~10,000 active region images) fractal survey of this data. This quantifies the meaning of magnetic complexity, relating lower threshold fractal dimension to the onset of large flares.