Quasi-periodic Particle Injection into Coronal Loops

Details Quasi-periodic Particle Injection into Coronal Loops Quasi-periodic Particle Injection into Coronal Loops

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...416..857a&link_type=abstract

Astrophysical Journal v.416, p.857

Astronomy and Astrophysics

Astronomy

33

Sun: Corona, Sun: Flares, Sun: Particle Emission, Sun: Radio Radiation, Sun: X-Rays, Gamma Rays

Scientific paper

We present observations of the flare of 1989 June 22, 1445 UT (in active region NOAA 5555), obtained with the Hard X-Ray Burst Spectrometer (HXRBS) on SMM at energies >52 keV, and with the broad-band radio spectrometer PHOENIX at ETH/Zurich in the frequency range of 100-2800 MHz. The radio emission is dominated by a ≲100% polarized decimetric continuum at 400-1400 MHz, peaking at 750 MHz. The decimetric radio flux is highly correlated with the 50-150 keV hard X-ray flux but is delayed by 3.5-5.4 s with respect to the hard X-rays. The HXR emission shows an excess of ≳10 fast (≳100 ms) spikes (according to Poisson statistics). The radio emission exhibits weak fine structure, consisting of ≍45 quasi-periodic pulses with a mean period of 1.6 s. The frequency-time drift pattern of this fine structure is found to be consistent with segments of inverted-U type bursts, suggesting quasi-periodic injection of electron beams into a loop system. The loop system has an average height of 68,000 km and expands with a velocity of 200 km s-1 due to flare heating. Chromospheric evaporation enhances the electron density near the footpoints. The type III- exciting electrons have a mean velocity of υ/c = 0.30±0.10 (22 keV) and propagate along inverted-U burst trajectories with a mean duration of 2.5 s. For those electrons which reach the mirror point near the opposite footpoint of the loop system, we calculate (from the density and loop length) a low energy cutoff of ≥ 8 keV due to collisional deflection, yielding a propagation velocity of v/c = 0.18 and a propagation delay of 5.1±1.0 s, which agrees well with the observed delay of 5.16 s between the cross-correlated HXR and radio flux. The ≥ 8 keV electrons provide free energy for a loss cone instability near the secondary footpoint, which is observed as decimetric continuum polarized in the same sense of circular polarization as the type III bursts. The constraints from the Hα flare position and the magnetic potential field extrapolation indicate that the loss cone emission is produced in the diverging field region above the umbra of the leading sunspot, which has a photospheric field strength of -1600 G.
This flare allows us to deconvolve quasi-periodic particle injection and subsequently triggered coherent radio emission from trapped particles in flare-associated loops. It demonstrates that quasi-periodic modes of particle acceleration, particle dynamics in mirror loops, and the resulting plasma instabilities can be efficiently diagnosed from correlated hard X-ray and radio signatures.

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