Interplanetary magnetic field enhancements in the solar wind: Statistical properties at 1 AU

A new class of magnetic field signatures in the solar wind has been studied using interplanetary data obtained in the vicinity of Earth. Typical behavior of these signatures is a slow rise of the total magnetic field to a sharp peak and then a gradual and almost symmetric decrease to the background interplanetary magnetic field. These events last from a minimum of 30 min to a maximum of more than 11 hr. The background magnetic field is enhanced from 13 to up to 168%. However, there is no correlation between the amplification of the background field and the duration of each event. A total of 45 events have thus been found in a survey of the magnetometer data of two spacecraft (ISEE-3 and IMP-8), corresponding to a total of more than 6 years of available survey data. The peak magnetic pressure of these events is always less than the solar wind dynamic pressure by a factor of 10 or more. These signatures are similar to those observed at Venus by the Pioneer Venus Orbiter. Based on these observations and the previously discovered signatures at Venus, the cause of these events is postulated to be small outgassing bodies which have passed by the spacecraft. These bodies must outgas enough to produce the field distortion observed and yet they must be small enough to be generally undetected by observers on Earth.