THE HELIUM VALLEY - COMPARISON OF IMPULSIVE SOLAR-FLARE ION ABUNDANCES AND GYRORESONANT ACCELERATION WITH OBLIQUE TURBULENCE IN A HOT MULTIION PLASMA

We present a detailed interpretation of the heavy ion abundance enhanc ements observed in impulsive flare energetic particles, in terms of th e conditions for gyroresonant acceleration by moderately oblique waves in a hot solar coronal plasma. On the basis of a realistic coronal pl asma containing its complete set of minor ions, we analyze first all p arallel wave modes in terms of their dispersion relation, damping time scale, and condition for gyroresonant acceleration of thermal ions, as a function of temperature. We identify the ''Helium Valley,'' the reg ion in the frequency-wavenumber plane of strong wave damping by therma l He-4(+2) ions, as crucial for explaining the observed abundances: an y ions with charge-to-mass ratio in the neighborhood of 0.5 cannot be accelerated preferentially, relative to He-4(+2). Then solving the dis persion relation equation for oblique waves in a hot e-p-He plasma, we discuss this general class of waves in terms of polarization and damp ing timescale. For waves propagating at moderate angles to the magneti c held (theta not approximate to 90 degrees), our calculations indicat e that the first harmonic n = 1 gyroresonance is dominant, and that th e corresponding He valley narrows down for increasing angle theta. Usi ng this analysis, we calculate the limits of the He valley and investi gate the preferential gyroresonant acceleration of heavy ions by moder ately oblique waves (theta not approximate to 90 degrees) in a solar c oronal plasma. Only for nearly perpendicular waves (theta approximate to 90 degrees), are higher order resonances important and regions of w ave damping by interaction with thermal particles vanishingly narrow i n frequency. We estimate the fraction of ions of each element outside the He valley as a function of temperature and compare the resulting e nhanced abundances with the observed enhancements, for the case of a s pectrum of non-quasi-perpendicular waves, as produced by a cascading o f the general turbulence. The results allow us to specify the range of possible temperatures for the source plasma of the accelerated partic les to between similar to 2.4 and similar to 4.5 x 10(6) K, i.e., comp arable to active region (AR), but not to flaring gas, temperatures. Th is points to an acceleration of the ions taking place, either in the A R gas surrounding the flare itself or within the flaring loop but befo re it became heated. Constraints are set on the typical time At over w hich the ions are accelerated preferentially. We find times between si milar to 5 x 10(-4) and similar to 3 x 10(-2) s (for our nominal plasm a with density and field of n(e) = 10(10) cm(-3) and B = 100 G); it co uld be similar to 10 times larger, if the typical conditions in quiesc ent ARs (n(e) similar to 2 x 10(9) cm(-3) and B similar to 200 G) appl y also to the bulk similar to 3 x 10(6) K gas of flaring ARs. We discu ss another physical interpretation of At, if wave cascading is effecti ve. Preliminary calculations have shown that the proposed selective ac celeration 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 n ot dominant.