Extended Gamma-Ray Emission of Solar Flares
G. Rank, V. Schönfelder and J. M. Ryan
An exciting discovery during the last solar cycle was the existence of extended gamma-ray emission of solar flares. COMPTEL measured extended emission of the flares on 9, 11 and 15 June 1991 in the 2.223 MeV neutron capture line for several hours after the impulsive phase (1). The emission originates mainly from accelerated protons with energies of 10 to 100 MeV. The long-term time profiles for all three flares are shown in the figure below. Our results can be compared to data from other instruments, including measurements of gamma-rays at higher energies, hard X-rays and microwaves.
The following main characteristics of the extended phase have been found:
- The long-term emission can be described by a two-fold exponential decay (see figure). The distinct break in the light curve suggests either the existence of two separate proton populations or a dramatic change in the acceleration/emission environment.
Two models have been proposed to explain the extended emission: the magnetic trapping of protons that have been accelerated during the impulsive phase (2), and the continuous acceleration of protons for many hours (3). By comparing the time profiles of spectral features which probe different energy regimes in the parent proton spectrum, we can show that the accelerated proton spectrum stays constant during the extended phase. This rules out the trapping scenario which predicts energy-dependent energy loss processes in the magnetic trap.
- The emission process during the extended phase appears to be different from the impulsive phase (e.g. the accelerated proton spectrum is harder; nucleonic processes dominate over electronic contributions). The most promising approach (3) utilizes a stochastic acceleration mechanism in large coronal loops with a high level of magnetic turbulence to accelerate protons up to the required energies of several hundred MeV.
- As can be seen in the figure, all three flares observed in June 1991 reveal a remarkably similar time profile during the extended phase. This behaviour indicates long-term stability of large (coronal size) magnetic structures in the solar atmosphere for several days.
1. Rank, G., 1997, 25th ICRC, SH 1.3.2
2. Ramaty R. & Mandzhavidze N., 1994, AIP Proc., 294, 26
3. Ryan, J. M. & Lee, M. A., 1991, Ap.J., 368, 316
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