A Novel Determination of the Foreshock ULF Boundary: Statistical Approach

The location and spatial extent of the region populated by the foreshock waves depend on the IMF orientation. We performed a systematic statistical study of wave activity in the frequency range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mrow><mml:mn>0.03</mml:mn> <mml:mo>-</mml:mo> <mml:mn>0.15</mml:mn></mml:mrow> </mml:mrow> </mml:math> Hz observed during an initial phase of the THEMIS mission. Wave activity is quantified by standard deviations of the IMF magnitude and its components over 10-min intervals. We apply the foreshock coordinate system defined as the angle between the bow shock normal and upstream magnetic field vectors and the distance from the spacecraft to bow shock along the magnetic field line. We have found that the Ultra-low Frequency (ULF) foreshock boundary (a) is well defined in these coordinates, (b) it tends to shift outward with an increasing solar wind bulk speed, and (c) with an increasing Mach number. However, the change of the fluctuation level in the foreshock is not uniform because the increasing solar wind bulk speed enhances the fluctuation level mainly in a close proximity of the bow shock whereas the increasing Mach number leads to an intensification of fluctuation levels at the foreshock boundary.