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

A 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 ( θ XB  < ≈ 10°); rather, they preferentially manifest for θ XB  ≈ 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.