Geomagnetopause surface fluctuations observed by Voyager 1

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Abstract
As Voyager 1 moved out of the dawnside of the earth's magnetosphere on September 5, 1977 (at position (−2.6, −16.5, 1.1) earth radii in GSE), it crossed the magnetopause apparently seven times, despite the high spacecraft speed of 11 km/s. Normals to the magnetopause and their associated error cones were well determined for each of the crossings through the use of a minimum variance analysis of the internal magnetic field. The oscillating nature of the ecliptic plane component of these normals indicates that most of the multiple crossings were due to a wavelike surface disturbance moving tailward along the magnetopause. We modeled the wave, which was aperiodic, as a sequence of sine waves with amplitude Ai, wavelength λi, and speed Vi. These quantities were determined for two pairs of intervals from the measured slopes, occurrence times, and relative positions of six magnetopause crossings. The average amplitude was A =2100−500+3800 km, and the wavelengths were of the order of 47,000−12,000+30,000km. The wave speed was approximately 340−95+210 km/s, and typical periods were in the neighborhood of 170±60 s. The magnetopause thickness was estimated to lie in the range 300–700 km with higher values possible. The estimated amplitude of these waves was obviously small compared to their wavelengths; this conclusion is independent of any bulk normal motion of the magnetopause that might have been present.

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