Occurrence and characteristics of nighttime ULF waves at low latitude: The results of a comprehensive analysis

Abstract

The occurrence and characteristics of ULF events (f ≈ 10–100 mHz) detected during the night at low latitude (L'Aquila, Italy, λ ≈ 36.3°), during quiet and moderately perturbed magnetospheric conditions, have been examined by means of a long‐term analysis between 1996 and 2012. Clearly defined events (≈8000 on each component) are typically more energetic in H than in D and basically consist of penetrating upstream waves, resonances of local field lines, and Pi2 waves. The global event occurrence shows a strong asymmetry about midnight, with a much higher wave activity before dawn than after dusk: it mostly comes from the intense penetration of upstream waves through the dawn flank of the magnetopause. D events are more frequent in summer and H events more frequent in winter, suggesting a different influence of the ionospheric modification of the downgoing signals. Between f ≈ 30 and 45 mHz, the reversal of the dominant polarization across midnight reveals tailward propagation of penetrating waves. Below f ≈ 25 mHz, intermingled with continuous Pc3 and Pc4 waves, a large fraction of events exhibit Pi2 characteristics: the dominant left‐handed polarization and the switch of the tilt angle of the major axis of the polarization ellipses are consistent with the pattern expected for waves related to the substorm current wedge. A relevant percentage of the power spectra shows a second enhancement above f ≈ 55 mHz, revealing resonance of local field lines also during the night.

Nighttime Pc3 pulsations: MM100 and MAGDAS observations

“…Our study confirms the conclusion of Villante and Tiberi (2016) on the key role of foreshock waves in formation of night Pc3 energy spectrum. A good correspondence between predicted and observed Pc3 wave central frequencies is found for Pc3n D population, and to some extent for Pc3n L groups as well.…”

“…With these two tests, we have found that night Pc3s of day origin, selected only with amplitude and coherence criterion, demonstrate features typical for dayside Pc3s and nighttime Pc3 pulsations of intermediate and high frequencies studied in Villante and Tiberi (2016) at L = 1.6. On the contrary, the V and B distributions for the intervals of local night Pc3s agree with the mean annual distributions of these parameters.…”

“…A comprehensive statistical analysis of nighttime Pc3 waves was performed with the data of L' Aquila observatory (L = 1.6) (Villante and Tiberi 2016). The authors found that nighttime Pc3s had lower amplitudes than those at dayside, but their principal features, such as the IMF control of the central frequency, were the same.…”

“…The frequency of upstream waves is determined by the magnitude B IMF of the interplanetary magnetic field (IMF) (Guglielmi and Troitskaya 1974) due to their ion-cyclotron origin. Following Villante and Tiberi (2016), we use the f − B IMF dependence in its simplest form, suggested in Troitskaya (1988) However, the relationship (1) has a statistical character and holds for values averaged over a long (e.g., hourly) time interval. On the wave event basis, this…”

Inherent Length Scales of Periodic Mesoscale Density Structures in the Solar Wind Over Two Solar Cycles

“…The apparent frequency of a PDS as it flows past Earth or an in situ spacecraft is related to the radial length scale of the structure as f pds = V sw / L pds , where f pds is simply the inverse of the Δ T between each density enhancement. Numerous event studies have observed direct links between the periodicities in solar wind density and periodicities in radar (Fenrich & Waters, 2008; Stephenson & Walker, 2002), ionospheric (Dyrud et al., 2008), and ground magnetometer (Villante et al., 2007; Villante & Tiberi, 2016) observations, at frequencies from ∼4 mHz down to ∼0.2 mHz. Viall, Kepko, et al.…”