Characteristics of Pc5 activity at high latitudes stations in Antarctica

Highlights

• Pc5 pulsations observed at high latitude ground stations located in Antarctica in 2017.

• Latitudinal dependence of diurnal seasonal variations of Pc5 activities.

• The comparison of Pc5 and solar wind speed.

• Difference generation mechanism of Pc5 waves at auroral latitudes and polar-cap region.

Abstract

We examined wave activities in the Pc5 frequency band (~2–7 mHz) using the magnetic field data from five Antarctic stations, which are AGO3 (72.5° S Altitude-Adjusted Corrected Geomagnetic latitude), South Pole (SPA, 74.6° S), McMurdo (MCM) and Jang Bogo Station (JBS, 80° S), and Dome C (DMC, 89.1° S), during 2017. Pc5 waves at AGO3 and SPA show characteristics associated with Kelvin-Helmohltz instability on the magnetopause and substorm activities, under closed field lines conditions. The local time and seasonal dependence of Pc5 wave activities at polar cap stations (MCM, JBS, and DMC) are significantly different from those at AGO3 and SPA. These indicate that the generation mechanism of Pc5 activities in the open field line region at polar cap is different from that in the closed field lines. We suggest that polar-cap Pc5 is generated by ionospheric current variations produced by solar dynamo between solar wind plasma and geomagnetic field.

Introduction

Ultralow frequency (ULF) waves in the Pc5 frequency range (~2–7 mHz) are commonly observed at high-latitude regions (L > 6) on the ground (Jacobs et al., 1964). The variation of Pc5 polarization and amplitude with latitude and local time (Samson et al., 1971) is associated with field line resonance (FLR) (Chen and Hasegawa, 1974; Southwood, 1974). The theory of FLR describes the coupling of an evanescent fast mode wave, which is generated on the magnetopause through Kelvin-Helmohltz instability (KHI), to internal transverse modes (standing Alfvén waves) on selected L shells. Since the solar wind velocity controls the KHI on the magnetopause, increase in Pc5 power is well correlated with increase in the solar wind velocity (e.g., Engebretson et al., 1998).

Many previous studies reported that the occurrence rate and intensity of Pc5 waves are higher in the dawn sector than in the dusk sector in space (e.g., Anderson et al., 1990; Nosé et al., 1995; Takahashi et al., 2015). Recently, Takahashi et al. (2016) reported that the dawn-dusk asymmetry of Pc5 waves does not originate from a dawn-dusk asymmetry of KHI on the magnetopause but from weaker FLR in the dusk sector than in the dawn sector. Thus, FLR is an important factor to understand the spatial variations of the intensity of Pc5 waves.

Geomagnetic pulsations in Pc5 band can be also be excited by changes in magnetopause current due to solar wind dynamic pressure variations (e.g., Kepko et al., 2002; Kim et al., 2002; Takahashi and Ukhorskiy, 2007). Therefore, wave amplitude depends on the change in solar wind dynamic pressures. Since the magnetospheric compression is due to magnetopause displacement caused by solar wind pressure variations and is strongest at the frontside magnetopause owing to strong magnetopause current variations in the subsolar region, the intensity of the compressional Pc5 waves has a peak near magnetic local noon (Takahashi and Ukhorskiy, 2007). These compressional waves driven by solar wind pressure variations can resonantly couple to standing Alfvén waves.

Magnetopause surface waves are one of possible mechanism to generate broadband irregular pulsations at cusp latitude. Pilipenko et al. (2018) compared long-period pulsations in Pc 5–6 band and optical measurements of open-closed field line boundary (OCB). They found that the ULF waves maximized at lower latitudes than optical OCB latitude.

Recently, De Lauretis et al. (2016) reported that Pc5 waves observed at a polar cap station in Antarctica around local magnetic noon are associated with ionospheric currents carried by Alfvén waves through the direct coupling (i.e., FLR) of the compressional waves generated by oscillations of solar wind dynamic pressure at lower latitudes. Francia et al. (2005, 2009) reported that Pc5 power observed at a polar cap station has a peak power near noon and suggested that such daytime Pc5 pulsations are related to FLRs occurring at lower latitudes. Although many previous studies showed the occurrence of several possible sources and types of ULF activity in the polar cap, mechanisms of their excitation have not been firmly established yet (Pilipenko and Engebretson, 2002; Engebretson et al., 2006; Pilipenko et al., 2015).

In this study, we statistically examine Pc5 activities observed at stations located in Antarctica. We observed that the local time dependence of Pc5 power at polar cap stations is different from that at the stations in the closed field lines, which is consistent with previous studies (e.g., Francia et al., 2005, 2009). We suggest that the generation mechanism of polar-cap Pc5 activities is not FLR, which is considered in the previous studies, and that polar-cap Pc5 activities are associated with the variation of solar wind velocity.