A comprehensive analysis of the occurrence and characteristics of midperiod ULF waves at low latitude

Abstract

AbstractA long‐term analysis (1985–2012) allowed to investigate the occurrence and characteristics of clearly defined ULF events (f ≈ 10–100 mHz) detected during daytime at low‐latitude (L'Aquila, Italy; L ≈ 1.6) during quiet and moderately perturbed magnetospheric conditions. Selected events (≈30,000 on each component) typically have much greater amplitude on the H component with respect to D. They often reveal two prominent spectral peaks, one in band A (f < ≈ 45 mHz), mostly related to the transmission of upstream waves, and the other one in band B (f > ≈ 55 mHz), including resonances of local field lines and higher‐frequency upstream waves. The occurrence of resonant phenomena on the D component is also clearly evidenced. The solar wind speed is confirmed as the key element for the manifestation of events; by contrast, their appearance is inhibited during extremely low solar wind densities. The events mostly manifest between dawn and early afternoon, with highest occurrence at ≈08:00–10:00 LT, as might be expected for the usual distribution of the interplanetary magnetic field orientation determining more or less favorable conditions for a foreshock region on the morning flank of the bow shock. No evidence is found for a favorite occurrence for low cone angles (θXB < ≈ 10°); rather, they preferentially manifest for θXB ≈ 25°–40°. The polarization pattern, much more definite in the afternoon, is consistent with the expected antisunward propagation; in the morning sector, it also suggests the possible occurrence of sunward propagating modes, mostly at f < ≈ 22 mHz. The tilt angle of the major axis of the polarization ellipses during daytime hours is oriented in the NW/SE quadrant and experiences remarkable changes at sunrise and sunset; it also shows a seasonal modulation with larger angles in the winter.