Plasma waves associated with diffuse auroral electrons at mid-altitudes

Using simultaneous observations from the DE-1 High-Altitude Plasma Instrument and the Plasma Wave Instrument, a study is made of 28 auroral-zone crossings, equally divided among dayside and nightside cases covering a range of Kp values from 1- to 8. In the diffuse auroral region, electrostatic emissions of frequencies up to a few kHz are associated with low-energy field-aligned electron beams. Of 18 cases (11 nightside; 7 dayside) examined at high resolution in the particle data, 15 (9 nightside; 6 dayside) exhibited field-aligned beams with electrostatic waves. In most cases, the beams were upward-directed, but occasionally they traveled both up and down the field line. The electrostatic emissions are interpreted as electron-acoustic waves excited by the field-aligned beams.