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

Abstract

Abstract. 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 ) ∝ 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|B IMF |[nT], and its power maximizes around IMF cone angles θ xB 20 and 160 • and at 10:30-11:00 MLT. Both spectral components exhibit similar semiannual variations with equinoctial maxima. The background noise power grows with increasing southward B z and remains nearly constant for northward B z . 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 ∝ V 5.71 sw and ∝ V 4.12 sw , respectively. Thus, the effective signal-to-noise ratio increases with increasing V sw . The observations suggest that the noise generation is associated with reconnection processes.

A new type of Doppler velocity fluctuations in HF ground scatter from the polar cap

“…Out of 121 daily magnetic records, 23 were excluded due to disturbed conditions. In contrast to the radar data, magnetic spectra show a single near-noon peak, in agreement with findings by Villante et al [2000] and Ponomarenko et al [2002] at polar cap and cusp latitudes, respectively.…”

“…The power spectrum lacks the typical bandlimited component at f '10-50 mHz [Ponomarenko et al, 2002[Ponomarenko et al, , 2005 and instead shows a lower frequency plateau followed by a power law decrease at f 10 mHz;…”

“…Figure 3e shows respective magnetometer spectra, which exhibit a typically steep power law shape across the most of frequency range with a Pc3-4 enhancement between 0.01 and 0.1 Hz at 7 and 12 MLT (blue and black) emphasized by dashed lines / f -3 [e.g., Ponomarenko et al, 2002]. Importantly, this Pc3-4 enhancement is undetectable in the respective radar spectra.…”

Pc3-4 ULF waves observed by the SuperDARN TIGER radar

“…Engebretson et al, 1986;Takahashi and Anderson, 1992;Ponomarenko et al, 2002). On the ground, maximum Pc3-4 power occurs around auroral/cusp latitudes (Fraser-Smith, 1982;Bol'shakova and Troitskaya, 1984).…”

“…Ground-based statistical studies (e.g. Ponomarenko et al, 2002) have revealed that at high latitudes Pc3-4 pulsations comprise two principal components: a band-limited enhancement ("signal") usually centered around 20-30 mHz; and a power-law background ("noise") with power spectral density ∝f −p , where p 3−4. The signal spectrum probably relates to the upstream source (Ponomarenko et al, 2002).…”