Abstract
A long-duration upstream ultralow frequency (ULF) wave event was detected on 15 February 2009 by Cluster satellites, close to the bow shock nose. A clear wave activity was identified when the interplanetary magnetic field orientation was favorable to the local generation. We examined the wave properties in both the solar wind and the spacecraft frame during a selected time interval and found that foreshock waves were essentially Alfven waves propagating at a small angle with respect to the interplanetary magnetic field. A comparison of Cluster observations with those on the ground, in the polar cap and at low-latitude stations, confirms the results of previous studies, indicating that upstream waves can reach different ground regions along different paths.
Highlights
The propagation to the ground of mid-frequency (10 to 100 mHz) ultralow frequency (ULF) waves generated upstream the Earth's bow shock is a much debated subject
Russell et al (1971) discussed that the foreshock waves are left-hand polarized in the spacecraft frame, while they are intrinsically right-hand polarized, resulting from the reversal of phase speed direction; in addition, the wave frequency in the spacecraft frame is modified by the Doppler shift: ωsc = ωr + k∙VSW, where ωsc, ωr, k, and Vsw represent the angular frequency in the spacecraft frame and in the solar wind (SW) frame, the wave number vector, and the SW flow velocity, respectively (Narita et al 2004)
Since the ULF signals observed at ground could propagate from a not a priori identified region, we looked for a possible source region on the basis of formula (1), in which the wave frequency depends on θnB; for each B, we varied θnB between 0° and 45° to evaluate the highest and lowest frequency in correspondence to the boundaries of the favorable generation region (Regi et al 2014)
Summary
The propagation to the ground of mid-frequency (10 to 100 mHz) ultralow frequency (ULF) waves generated upstream the Earth's bow shock is a much debated subject Such waves are the result of a wave-particle interaction between already-existing waves and solar wind (SW) protons reflected off the bow shock, and mostly occur in the region of quasi-parallel bow shock, i.e., where the interplanetary magnetic field (IMF) makes a small angle θnB (
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