Electron Trapping Times and Trap Densities in Solar Flare Loops Measured with Compton and YOHKOH

Details Electron Trapping Times and Trap Densities in Solar Flare Loops Measured with Compton and YOHKOH Electron Trapping Times and Trap Densities in Solar Flare Loops Measured with Compton and YOHKOH

Oct 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...487..936a&link_type=abstract

Astrophysical Journal v.487, p.936

Astronomy and Astrophysics

Astronomy

47

Acceleration Of Particles, Sun: Flares, Sun: X-Rays, Gamma Rays

Scientific paper

We measure energy-dependent time delays of ~20--200 keV hard X-ray (HXR) emission from 78 flares observed simultaneously with the Compton Gamma Ray Observatory and Yohkoh. Fast time structures (<~1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons (Aschwanden et al.). For the smooth HXR flux, we find systematic time delays in the range of tau S = t50 keV--t200 keV ~ -(1 ... 10) s, with a sign opposite to TOF delays, i.e., the high-energy HXRs lag the low-energy HXRs. We interpret these time delays of the smooth HXR flux in terms of electron trapping, and we fitted a model of the collisional deflection time t^{{Defl}}(E)~E^{3/2}n^{-1}e to the observed HXR delays in order to infer electron densities n^{{Trap}}e in the trap. Independently, we determine the electron density n^{{SXR}}e in flare loops from soft X-ray (SXR) peak emission measures {EM}=\int \nolimits n2e dh , using loop width (w) measurements to estimate the column depth dh ~ w. Comparing the two independent density measurements in HXR and SXR, we find a mean ratio of qe=n^{{Trap}}e/n^{{SXR}}e~1 , with a relatively small scatter by a factor of ~2. Generally, it is likely that the SXR-bright flare loops have a higher density than the trapping regions (when qe < 1), but they also are subject to filling factors less than unity (when qe > 1). Our measurements provide comprehensive evidence that electron trapping in solar flares is governed in the weak-diffusion limit, i.e., that the trapping time corresponds to the collisional deflection time, while pitch-angle scattering by resonant waves seems not to be dominant in the 20--200 keV energy range. The measurements do not support a second-step acceleration scenario for energies <=200 keV.

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