Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...476..403s&link_type=abstract
Astrophysical Journal v.476, p.403
Astronomy and Astrophysics
Astronomy
15
Acceleration Of Particles, Plasmas, Sun: Abundances, Sun: Corona, Sun: Flares, Sun: Magnetic Fields, Sun: Particle Emission
Scientific paper
We present a detailed interpretation of the heavy ion abundance enhancements observed in impulsive flare energetic particles, in terms of the conditions for gyroresonant acceleration by moderately oblique waves in a hot solar coronal plasma. On the basis of a realistic coronal plasma containing its complete set of minor ions, we analyze first all parallel wave modes in terms of their dispersion relation, damping timescale, and condition for gyroresonant acceleration of thermal ions, as a function of temperature. We identify the "Helium Valley," the region in the frequency-wavenumber plane of strong wave damping by thermal 4He+2 ions, as crucial for explaining the observed abundances: any ions with charge-to-mass ratio in the neighborhood of 0.5 cannot be accelerated preferentially, relative to 4He+2. Then solving the dispersion relation equation for oblique waves in a hot e-p-He plasma, we discuss this general class of waves in terms of polarization and damping timescale. For waves propagating at moderate angles to the magnetic field ( theta ≉ 90 deg), our calculations indicate that the first harmonic n = 1 gyroresonance is dominant, and that the corresponding He valley narrows down for increasing angle theta . Using this analysis, we calculate the limits of the He valley and investigate the preferential gyroresonant acceleration of heavy ions by moderately oblique waves ( theta ≉ 90 deg) in a solar coronal plasma. Only for nearly perpendicular waves ( theta ~ 90 deg), are higher order resonances important and regions of wave damping by interaction with thermal particles vanishingly narrow in frequency. We estimate the fraction of ions of each element outside the He valley as a function of temperature and compare the resulting enhanced abundances with the observed enhancements, for the case of a spectrum of non--quasi-perpendicular waves, as produced by a cascading of the general turbulence. The results allow us to specify the range of possible temperatures for the source plasma of the accelerated particles to between ~2.4 and ~4.5 x 106 K, i.e., comparable to active region (AR), but not to flaring gas, temperatures. This points to an acceleration of the ions taking place, either in the AR gas surrounding the flare itself or within the flaring loop but before it became heated. Constraints are set on the typical time Delta t over which the ions are accelerated preferentially. We find times between ~5 x 10-4 and ~3 x 10-2 s (for our nominal plasma with density and field of ne = 1010 cm-3 and B = 100 G); it could be ~10 times larger, if the typical conditions in quiescent ARs (ne ~ 2 x 109 cm-3 and B ~ 200 G) apply also to the bulk ~3 x 106 K gas of flaring ARs. We discuss another physical interpretation of Delta t, if wave cascading is effective. Preliminary calculations have shown that the proposed selective acceleration mechanism can be applied in underdense ( omega p/ Omega e < 1) as well as in overdense plasmas (our nominal case), provided that quasi-perpendicular waves (generated, e.g., by an electron beam) are not dominant.
Meyer Jean-Paul
Reames Donald. V.
Steinacker Adriane
Steinacker Juergen
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