PARAMETRIC STUDY OF ELECTROMAGNETIC ION-CYCLOTRON INSTABILITY IN THE EARTHS MAGNETOSPHERE

The growth rate of field-aligned electromagnetic ion cyclotron (EMIC) waves in the terrestrial magnetosphere is investigated using an anisot ropic kappa particle distribution to model the energetic ring current ions. Under such conditions, the wave dispersion relationship can be e xpressed in terms of the recently introduced modified plasma dispersio n function. This function is analogous to the plasma dispersion functi on defined by Fried and Conte which has previously been used extensive ly to investigate wave instability in a hot Maxwellian plasma. Calcula tions for kappa distributions under magnetospheric conditions indicate that the previous results, obtained with a Maxwellian distribution, t end to overestimate the peak convective growth rate of L mode waves. N onetheless, the dominant spectral properties of EMIC waves, as reporte d from AMPTE observations, can be understood in terms of cyclotron res onant instability using realistic magnetospheric parameters. Significa nt convective amplification is mainly confined to the outer (L greater than or equal to 5) magnetosphere. In the afternoon sector, where pla sma densities can exceed 10(7) m(-3), intense wave growth is possible in two bands, one above and one below the helium gyrofrequency Omega(H e+). Conversely, on the nightside or for early morning conditions, low er ambient plasma density only allows instability above Omega(He+). Hi gh concentration of thermal helium can strongly suppress instability i n the band above Omega(He+), but it has little effect on wave growth i n the band below Omega(He+).