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Session 18 - Flares I.
Oral session, Tuesday, July 01
Ballroom A, Chair: Mona Hagyard
We measure energy-dependent time delays of \approx20-200 keV hard X-ray (HXR) emission from 78 flares simultaneously observed with the Compton Gamma Ray Observatory (CGRO) and Yohkoh. Fast time structures (\lesssim1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons. For the smooth HXR flux we find systematic time delays in the range of \tau_S=t_50 keV-t_200 keV\approx -(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 fit a model of the collisional deflection time t^Defl(E) \propto E^3/2 n_e^-1 to the observed HXR delays to infer electron densities n_e^Trap in the trap. Independently we determine the electron density n_e^SXR in flare loops from soft X-ray (SXR) peak emission measures EM=\int n_e^2 dh, using loop width (w) measurements to estimate the column depth dh \approx w. Comparing the two independent density measurements in HXR and SXR we find a mean ratio of q_e=n_e^Trap/n_e^SXR \approx 1, with a relatively small scatter by a factor of \approx 2. It is likely that the SXR-bright flare loops have generally a higher density than the trapping regions (when q_e<1), but are also subject to filling factors less than unity (when q_e>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 \le 200 keV.
Program listing for Tuesday