LONG TIME-SCALE MAGNETODYNAMIC NOISE IN GEOMAGNETIC TAIL

Hourly ranges of the magnetic field in the Earth's magnetotail have been determined from 5.46-min field averages measured by the IMP 1 satellite. The amplitude of fluctuations parallel to the local average field B decreases with increasing distance from the neutral sheet. The amplitude of fluctuations perpendicular to B decreases with increasing distance from the Earth. Fluctuations in two directions perpendicular to B, (parallel and perpendicular to the neutral sheet), are coupled, but there is almost no evidence of a correlation between fluctuations parallel to B and fluctuations perpendicular to B. The tail field fluctuations observed in the vicinity of the Earth are well correlated with the geomagnetic activity measured by the Kp-index. It is suggested that two independent types of noise exist in the geomagnetic tail. The first type corresponds to the disturbances propagating across the tail, perpendicular to B, and the second one corresponds to disturbances propagating from the vicinity of the Earth along the tail field-lines. © 1969.