Geotail observation of electron cyclotron harmonic waves near the dayside magnetopause

We have been studying electron cyclotron harmonic (ECH) waves observed in the magnetosphere with the Plasma Wave Instrument (PWI) onboard the GEOTAIL spacecraft. The dynamic spectra show various types of ECH waves particularly near the dayside magnetopause. Superimposed on weak ECH waves, intense bursts with multiple harmonic bands are observed intermittently and sporadically in time, which is called "Totem Pole" emission (TP emission). The present paper focuses on the relation between the ECH waves and the corresponding electron velocity distribution functions obtained by the GEOTAIL LEP. The electron velocity distributions consist of multiple components including hot electrons at energy of a few keV and warm electrones of about hundred eV. To correct the total density estimated from the velocity distribution, we added a cold Maxwellian below 70 eV. As studied in linear theory, the GEOTAIL LEP observation also shows that the density ratio between the hot and cold electrons determines unstable harmonic modes. Linear dispersion analysis with the observed velocity distributions modified with the cold component indicates that ECH waves are marginally stable for each case. With increased loss cone factors of the hot electrons, however, multiple harmonic modes become unstable. (C) 1999 COSPAR. Published by Elsevier Science Ltd.