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Session 26 - Solar Flares II.
Oral session, Monday, June 10
Wisconsin Center,

[26.08] The Scaling Law between Electron Time-of-Flight Distances and Loop Lengths in Solar Flares

M. J. Aschwanden (Univ. Maryland), T. Kosugi (NAO, Japan), H. S. Hudson (Univ. Hawaii), M. J. Wills (Harvard, Cambridge), R. A. Schwartz (Hughes STX, NASA/GSFC)

\def\gapprox\lower.4ex\hbox\;\buildrel >øver\scriptstyle\sim\; \def\lapprox\lower.4ex\hbox\;\buildrel <øver\scriptstyle\sim\; >From the complete dataset of solar flares simultaneously observed with the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode (64 ms) and the Hard X-Ray Telescope (HXT) onboard Yohkoh we were able to determine the electron time-of-flight (TOF) distance and the flare loop geometry in 42 events. The electron TOF distances l' were determined from hard X-ray (HXR) time delays (\approx 10-100 ms) occurring in the 16-channel spectra (at \approx 20-200 keV), produced by the velocity difference of the HXR-producing electrons. The flare loops were mostly identified from double footpoint sources in \gapprox 30 keV HXT images, with radii ranging from r=3000 to r=25,000 km. We find a scaling law between the electron TOF distance l' and the flare loop half length s=r(\pi /2), having a mean ratio (and standard deviation) of l'/s=1.41\pm 0.29. In 5 flares we observe coronal \gapprox 30 keV HXR sources of the Masuda-type in the cusp region above the flare loop, and find that their heights are consistent with the electron TOF distance to the footpoints. These results provide strong evidence that particle acceleration in solar flares occurs in the cusp region above the flare loop and that the coronal HXR sources discovered by Masuda are a signature of the acceleration site, probably controlled by a magnetic reconnection process.

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