Modulation of total electron content by ULF Pc5 waves

Abstract

An intriguing effect was found while analyzing the small-scale variations of total electron content (TEC) derived from global positioning system (GPS) signals. We found a response in TEC variations to intense global Pc5 pulsations with periods of a few millihertz covering the corrected geomagnetic latitudes~58°-75°d uring the recovery phase of the strong magnetic storms on 31 October 2003. Earlier studies demonstrated that the GPS-TEC technique is a powerful method to study the propagation pattern of transient disturbances in the ionosphere, generated by seismic or internal gravity waves. This technique has turned out to be sensitive enough to ULF waves as well. During periods with intense Pc5 geomagnetic wave activity, distinct pulsations with the same periodicity were found in the TEC data from high-latitude GPS receiving stations in Scandinavia. Wavelet and cross-spectral analysis showed a high coherence (~0.9) between the periodic geomagnetic and TEC variations. Moreover, the relative amplitude of TEC periodic fluctuations ΔTEC/TEC was about or even larger than the relative amplitude of geomagnetic variations ΔB/B. So far, the effect of TEC modulation by Pc5 waves is not well understood and is still a challenge for the MHD wave theory. Various possible modulation mechanisms have been estimated, but no mechanism has been firmly identified.

Modulation of the ionosphere by Pc5 waves observed simultaneously by GPS/TEC and EISCAT

“…The radar observations showed that Pc5 waves can noticeably modulate the ionospheric plasma: the electric field E, plasma convection velocity V, E-layer electron density N e and the ionosphere conductance Σ, and electron T e and ion T i temperatures in both F-and E-layers (see references in Pilipenko et al 2014b). Recent observations by Pilipenko et al (2014a) and Watson et al (2015) have demonstrated that Pc5 waves are capable to modulate TEC as well.…”

“…This additional plasma heating may shift the ionizationrecombination balance due to the dependence of the recombination coefficient β(T) on temperature and cause plasma density variations (Pilipenko et al 2014a). The periodic ion heating in the bottom ionosphere during October 31, 2003, event by Pc5 wave electric field indeed can be seen in the EISCAT data (Pilipenko et al 2014b).…”

“…Early results, during the era of Faraday technique with geostationary beacons, reported TEC fluctuations related to geomagnetic variations in the Pc3-4 range (30-to 50-s period) (Davies and Hartman 1976;Okuzawa and Davies 1981). The TEC modulation by intense Pc5 pulsations was found by Pilipenko et al (2014a) and Watson et al (2015). Thus, the standard TEC/GPS technique is sufficiently sensitive to detect ULF waves in some cases.…”

“…Additional periodic ionization and consequently TEC variations may occur owing to the ULFmodulated precipitation of energetic electrons into the ionosphere (Watson et al 2015). Other possible mechanisms of TEC modulation by incident MHD waves comprise (a) plasma compression by evanescent fast compressional mode wave arising from the interaction of an Alfven wave with the anisotropic ionosphere (Pilipenko and Fedorov 1995); (b) periodic advection across a lateral gradient of the ionospheric plasma (Waters and Cox 2009); (c) periodic vertical shift and reconfiguration of the plasma vertical profile (Poole and Sutcliffe 1987); (d) frictional heating of ionospheric ions owing to periodic dragging through neutrals, and (e) field-aligned plasma transport by an Alfven wave (Pilipenko et al 2014a). These possible mechanisms of TEC modulation by magnetospheric ULF waves provide main contribution either to the upper ionosphere (F-layer) or to the lower ionosphere (E-layer and bottom of F-layer).…”

“…Here we analyze a unique event when the same global Pc5 waves were detected in the ionosphere by the GPS/ TEC technique (Pilipenko et al 2014a) and EISCAT radar (Pilipenko et al 2014b). We analyze these observations simultaneously which has provided an additional information on the relationship between geomagnetic and ionospheric variations.…”