Abstract
Abstract. In this study we investigate the spectral structure of Pc3–4 pulsations observed at low and midlatitudes. For this purpose, ground-based magnetometer data recorded at the MM100 stations in Europe and at two low latitude stations in South Africa were used. In addition, fluxgate magnetometer data from the CHAMP (CHAllenging Minisatellite Payload) low Earth orbit satellite were used. The results of our analysis suggest that at least three mechanisms contribute to the spectral content of Pc3–4 pulsations typically observed at these latitudes. We confirm that a typical Pc3–4 pulsation contains a field line resonance (FLR) contribution, with latitude dependent frequency, and an upstream wave (UW) contribution, with frequency proportional to the IMF (interplanetary magnetic field) magnitude BIMF. Besides the FLR and UW contributions, the Pc3–4 pulsations consistently contain signals at other frequencies that are independent of latitude and BIMF. We suggest that the most likely explanation for these additional frequency contributions is that they are fast mode resonances (FMRs) related to cavity, waveguide, or virtual modes. Although the above contributions to the pulsation spectral structure have been reported previously, we believe that this is the first time where evidence is presented showing that they are all present simultaneously in both ground-based and satellite data.
Highlights
It is commonly accepted that field line resonances (FLRs) excited by fast mode waves in the plasmasphere are the prime source of Pc3–4 pulsations observed at low and middle latitudes during local daytime (Menk et al, 1994)
It is widely accepted that the source of the fast mode waves responsible for driving the FLRs are upstream waves generated by an ion cyclotron instability driven by ion beams reflected from the bow shock region (Le and Russell, 1994)
A number of investigators have suggested that fast mode resonances (FMRs) due to cavity or waveguide modes in the plasmasphere are the drivers of the FLRs (Allan et al, 1985; Samson et al, 1995; Waters et al, 2000; Menk et al, 2000)
Summary
In this study we use spectral analysis techniques to show that Pc3–4 pulsations observed at low and midlatitude ground stations and at F region satellite altitudes contain signals arising from at least three different sources. Since most of the events used in this study exhibited peaks at the low frequency end of the spectrum, around 20 mHz, we checked these to confirm that the peaks were real This was done by recomputing the spectra with different filter cut-off frequencies and by plotting spectra of unfiltered data on loglog scales; examples of the latter are shown in some of the figures. In this study we utilised certain solar wind parameters such as solar wind speed and IMF magnitude and cone angle For this purpose, we used the one-minute OMNI high resolution solar wind data, which are provided after being time-shifted to the magnetosphere’s bow-shock nose from the original locations of the observations and available on the OMNI website: http://omniweb.gsfc.nasa.gov
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