Widely spaced wave‐particle observations during GEOTAIL and Wind magnetic conjunctions in the Earth's ion foreshock with near‐radial interplanetary magnetic field

Abstract

Several events have been identified of an ion foreshock extending up to 250 RE upstream of the Earth. These events occur mostly during periods of slowly drifting radial interplanetary magnetic field (IMF) when the 1‐min average values of the strengths of the IMF and the solar wind (SW) speeds are mostly steady. For their analysis an analytical solution to the problem of the closest approach of an IMF line to two spacecraft is given. We used this method to find intervals of magnetic conjunction between the bow shock and the upstream regions at GEOTAIL and Wind. This solution is obtained by determining the minimum angle θ (as a function of time) between the mean direction of the IMF (measured at Wind) and the vector‐difference (rWI‐r) of the locations of Wind and the point (attached on the field line) which went earlier by GEOTAIL. Here we take into account the mean drift of the flux lines with the SW, by assuming that the spacecraft were located in the same heliospheric magnetic domain. We have tested this method against a set of selected cases which show a steady presence of the ion foreshock close to the bow shock (GEOTAIL) and its sporadic presence far upstream (Wind). We have found our method to be accurate within a few Earth radii (RE). We have identified an outstanding candidate for the bow shock, GEOTAIL, and Wind sequential magnetic conjunction, which occurred on June 11, 1995. Additionally, this diagnostic technique has been applied to nine more intervals of simultaneous occurrences of intensity enhancements of broadband ultralow‐frequency (ULF) waves, and fluctuating fluxes of scattered energetic ions (40–140 keV). Very broad ion foreshock regions (> 40 RE) are commonly observed during the subset of events characterized by a high‐speed SW. The observed frequencies of the ULF waves are basically enhanced transversal modes in the range from ∼ 1/10 to 2/3 of proton cyclotron frequency, fcp. Fluctuations in the energetic ion fluxes were also observed in this frequency range for all the cases. Therefore we argue that the nature of the coupling between ULF waves and energetic ions is similar both in the near as well as far upstream regions of the Earth's bow shock.