Cusp-latitude Pc3 spectra: band-limited and power-law components

This work attempts to fill a gap in comparative studies of upstream-generated Pc3–4 waves and broad band ULF noise observed at cusp latitudes. We performed a statistical analysis of the spectral properties of three years of cusp-latitude ground magnetometer data, finding that the average daytime Pc3–4 spectra are characterized by two principal components: an upstream-related band-limited enhancement (‘signal’) and a power-law background (‘noise’) with S(f ) α f⁻⁴. Based on this information we developed an algorithm allowing for the deconvolution of these two components in the spectral domain. The frequency of the signal enhancement increases linearly with IMF magnitude as f [mHz] ≃ 4.4 | BIMF | [nT], and its power maximizes around IMF cone angles θxB ≃ 20 and 160° and at 10:30–11:00 MLT. Both spectral components exhibit similar semiannual variations with equinoctial maxima. The back-ground noise power grows with increasing southward Bz and remains nearly constant for northward Bz. Its diurnal variation resembles that of Pc5 field-line resonance power, with a maximum near 09:00 MLT. Both the band-limited signal and broad band noise components show power-law growth with solar wind velocity α V⁵‧⁷¹sw and α V⁴‧¹²sw, respectively. Thus, the effective signal-to-noise ratio increases with in-creasing Vsw. The observations suggest that the noise generation is associated with reconnection processes.