Excitation of low‐frequency waves by auroral electron beams

Abstract

The electron distribution functions measured by DE 1 at high altitudes during an auroral pass on day 318, 1981, are used to conduct a linear instability analysis of low‐frequency electromagnetic and electrostatic waves near and below the hydrogen gyrofrequency. The free energy sources examined are electron beams in inverted‐V structures and in the central plasma sheet (CPS) region. It is found that inverted‐V electron beams with energies of several keV can excite electromagnetic ion cyclotron (EMIC) waves propagating obliquely with frequencies below the hydrogen gyrofrequency. When an oxygen ion is the dominant component, energetic electron beams can also excite electromagnetic oxygen cyclotron waves over a broad frequency range. We suggest that the oxygen and hydrogen EMIC waves excited by energetic electron beams might explain low‐frequency electric and magnetic noise (<100 Hz) in the auroral zone. Throughout the CPS, DE 1 measured energetic (>1 keV) trapped electrons, upward and downward streaming electrons with a beam energy less than 100 eV, and conic ions at energies less than 300 eV. Integration of the plasma data for several energy and pitch angle ranges shows that the conic ion number flux in the CPS correlates best with the number flux of streaming electrons with energies less than 235 eV, but not with the energetic trapped electron fluxes. Due to the high density (∼6 cm−3), the CPS electron beams can excite hydrogen and oxygen electrostatic ion cyclotron (EIC) waves with large growth rates. Although the CPS beam energy is too low to excite oblique kinetic Alfvén waves or EMIC waves through Landau resonance, the CPS electron beams can excite reactively left‐hand Alfvén waves propagating parallel to the magnetic field at frequencies below the O+ gyrofrequency. Based on the instability analysis, a possible interpretation for the correlation between the streaming electron and conic ion fluxes is that upward and downward streaming electrons throughout the CPS provide the free energy for heating oxygen ions through oxygen EIC waves. However, EMIC waves propagating from the inverted‐V structure into the CPS as Alfvén waves can also heat ions through electromagnetic ion cyclotron resonance. Detailed modeling of the observed conic ion distributions needs to include both oxygen EIC and Alfvén waves to resolve the question of oxygen heating in the CPS.