Electromagnetic waves with frequencies near the local proton gyrofrequency: ISEE‐3 1 AU observations

Abstract

Low Frequency (LF) electromagnetic waves with periods near the local proton gyrofrequency have been detected in interplanetary space by the magnetometer onboard ISEE‐3. Transverse peak‐to‐peak amplitudes as large as have been noted with compressional components (Δ |B|/ |B|) typically ≤ 0.1. Generally, the waves have even smaller amplitudes, or are not detectable within the solar wind turbulence. The waves are elliptically/linearly polarized and are often, but not always, found to propagate nearly along . Both right‐ and left‐hand polarizations in the spacecraft‐frame have been detected. The waves are observed during all orientations of the interplanetary magnetic field, with the Parker spiral orientation being the most common case. Because the waves are detected at and near the local proton cyclotron frequency, the generation mechanism must almost certainly be solar wind pickup of freshly created hydrogen ions. Possible sources for the hydrogen are the Earth's atmosphere, coronal mass ejections from the Sun, comets and interstellar neutral atoms. At this time it is not obvious which potential source is the correct one. Statistical tests employing over one year of ISEE‐3 data will be done in the near future to eliminate/confirm some of these possibilities.