Low‐frequency waves and associated energetic ions downstream of Saturn

Abstract

Coherent, low‐frequency waves were observed occasionally in the interplanetary magnetic field (IMF) by Voyager 2 during 5 months in 1981–1982 following its encounter with Saturn. These waves had periods ranging from 1.4 to 2.8 hours in the spacecraft reference frame (frequencies near 10−4 Hz) and were detected at distances ranging from 150 to 2800 RS (9.0×106 to 1.7×108 km) from Saturn. The waves were characterized by large‐amplitude fluctuations in both magnitude and direction of B, an ellipticity >0.5 on average, and in most cases have been interpreted to have right‐hand polarization in the plasma frame. Although there was at best only a weak signature of these waves in the low‐energy (<6 keV) plasma, large enhancements were observed in the count rate of energetic (28–80 keV) ions in association with wave occurrences out to a distance of ∼1300 RS (7.8×107 km) from Saturn. The distribution of the enhanced ions was highly anisotropic, with the highest count rates measured in the 45° sector most nearly facing Saturn. The observation of ion enhancements was found to depend on the latitude angle of the IMF. It is concluded that the waves were excited by energetic plasma ions propagating away from Saturn. They are similar to waves produced upstream of earth by ions reflected from the bow shock or escaping from the magnetosphere. Because of the unique combination of IMF orientation at Saturn's orbit and the Voyager 2 postencounter trajectory, these observations represent the most remote detection of such planet‐associated (foreshock) waves. It is shown that viable mechanisms for exciting such waves under the given conditions include two beam‐driven resonant cyclotron instabilities as well as the nonresonant firehose instability. The absence of any strong signature of these waves in the thermal plasma suggests that the magnetic field fluctuations may arise from the firehose instability.