Plasma waves in the distant magnetotail

In this study we identify the principal types of plasma waves which occur in the distant magnetotail and we investigate the relationship of these waves to simultaneous plasma and magnetic field measurements made on the same spacecraft. The observations used in this study are from the Imp 8 spacecraft, which passes through the magnetotail at radial distances ranging from about 23.1 to 46.3 R_E. Three principaltypes of plasma waves are detected by Imp 8 in the distant magnetotail: broad band electrostatic noise, whistler mode magnetic noise bursts, and electrostatic electron cyclotron waves. The electrostatic noise is a broad band emission which occurs in the frequency range from about 10 Hz to a few kilohertz and is the most intense and frequently occurring type of plasma wave detected in the distant magnetotail. This noise is found in regions with large gradients in the magnetic field near the outer boundaries of the plasma sheet and in regions with large plasma flow speeds, 10³ km s^-1, directed either toward or away from the earth. The whistler mode magnetic bursts observed by Imp 8 consist of nearly monochromatic tones which last from a few seconds to a few tens of seconds. These noise bursts occur in the same region as the broad band electrostatic noise, although much less frequently, and are thought to be associated with regions carrying substantial field-aligned currents. Electrostatic electron cyclotron waves are seldom detected by Imp 8 in the the distant magnetotail. Although these waves occur very infrequently, they may be of considerable importance, since they have been observed in regions near the neutral sheet when the plasma is extremely hot.