Geomagnetopause surface fluctuations observed by Voyager 1

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 A/sub i/, wavelength lambda/sub i/, and speed V/sub i/. 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/sub -500/+3800 km, and the wavelengths were of the order of 47,000/sub -12,000//sup +30,000/ km. The wave speed was approximately 340/sub -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