Analysis of the waves associated with the unusual interplanetary shock observed on day 109, 1992

Abstract

On day 109 of 1992 Ulysses observed an extraordinary interplanetary shock at a time of extremely unusual interplanetary magnetic field conditions. The field was closely aligned with the radial direction for nearly a day ahead of the shock, suggesting that it was most likely to be a quasi‐parallel shock. Unlike most interplanetary shocks, this shock was associated with substantial upstream and downstream wave fields, which is consistent with the shock being quasi‐parallel in nature. There were several separate intervals of wave activity in the upstream region, the first occurring on day 108 and the second two intervals occurring on day 109 leading up to the shock. In addition, just ahead of the shock the background field magnitude decreased to a value close to zero, and superimposed on this decrease were a number of field magnitude enhancements. The downstream region was populated by large‐amplitude compressive waves. A combination of spectral and wavelet analysis techniques was used to characterize the different intervals of wave activity. The waves on day 108 and the first interval of wave activity on day 109 are consistent with generation by an ion beam instability, but the waves on day 109 increased in amplitude as the shock was approached, suggesting that these waves were directly produced by the shock.