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RHESSI and STEREO observations of a partially disk-occulted flare

This study focuses on a C8-class solar flare on December 31, 2007, where RHESSI and STEREO observations provided insights into the electron acceleration region. The flare was partially disk-occulted, leading to direct observations of the above-the-loop-top sources. Examining the electron distribution, the study found non-thermal power-law behavior from 10 keV to the relativistic range. Parameters such as density, magnetic field strength, and energy density were measured, indicating the significant role of accelerated electrons in the flare dynamics. Various models and conclusions related to the acceleration process and plasma behavior were discussed based on the observational data.

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RHESSI and STEREO observations of a partially disk-occulted flare

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  1. RHESSI and STEREO observations of a partially disk-occulted flare C8 class flare on December 31, 2007

  2. RHESSI and STEREO observations of a partially disk-occulted flare Direct Observations of the Electron Acceleration Region of a Solar Flare C8 class flare on December 31, 2007 Säm Krucker, H.S. Hudson, J.-P. Wuelser, S. White, R.P. Lin

  3. Above-the-loop-top HXR sources Ambient (thermal) density is low n ~ 109 cm-3 or even smaller number of HXR producing electrons (instantaneous) NHXR ~ n-1 Masuda flare: NHXR > 5* Nthermal NHXR << Nthermal NHXR > Nthermal ambient f f accelerated Masuda et al. 1994, 2000 v v

  4. Above-the-loop-top HXR sources Ambient (thermal) density is low n ~ 109 cm-3 or even smaller number of HXR producing electrons (instantaneous) NHXR ~ n-1 Masuda flare: NHXR > 5* Nthermal NHXR << Nthermal NHXR > Nthermal ambient f f accelerated Masuda et al. 1994, 2000 v v

  5. NHXR > Nthermal means ~ • almost all energy is in accelerated electrons (<Eacc>~20 keV vs Ethermal~0.2 keV) • collisional heating is fast (~ 5 keV/s) • ALL electrons are accelerated flare pre-flare ambient f f accelerated v v

  6. RHESSI and STEREO observations of a partially disk-occulted flare C8 class flare on December 31, 2007

  7. Partially disk-occulted flare of 2007Dec31 HXR peak (impulsive phase) SXR peak microwave limb is higher up, 17 GHz source is co-spatial.

  8. STEREO view shows flare ribbons and post flare loops above-the-loop-top source could be along the flare arcade impulsive phase 1 hour later (post flare loops)

  9. Flare geometry 22”

  10. Flare geometry 22”

  11. Flare geometry 22”

  12. HXR and microwave spectra Distribution of accelerated electrons are non-thermal; power-law from ~10 keV up to relativistic range (few MeV) a ~ 1.8  d ~ 3.4 f~3 GHz  B=30-50 G

  13. Summary: measured parameters ~ 2.109 cm-3 ~ 8.1026 cm3 ~ 30-50 G ~ 0.01 ~ 2.109 cm-3 ~ 3.4 ~ 1 pre-flare density volume magnetic field strength B pre-flare b (T=2 MK) acc. electron density power law distribution with d from ~15 keV up to a few MeV b during HXR burst • energy density of the accelerated electrons is comparable to that of the magnetic field ions?

  14. Discussion of models turbulence (e.g. Liu et al. 2007) Contracting islands (Drake et al. 2006) time evolution given by acceleration and release process.

  15. Conclusions • above-the-loop-top source itself is the acceleration region • ALL electrons are accelerated (non-thermal distribution) • plasma beta b ~ 1

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