Occurrence characteristics of electromagnetic ion cyclotron waves at sub-auroral Antarctic station Maitri during solar cycle 24

Aditi Upadhyay,Ashwini Kumar Sinha,Amar Kakad,Yoshiharu Omura,Bharati Kakad

doi:10.1186/s40623-020-01157-7

Abstract

We present a statistical study of electromagnetic ion cyclotron (EMIC) waves observed at Antarctic station (geographic 70.7{^circ },hbox {S}, 11.8{^circ },hbox {E}, L=5) on quiet and disturbed days during 2011–2017. The data span a fairly good period of both ascending and descending phases of the solar cycle 24, which has witnessed extremely low activity. We noted EMIC wave occurrence by examining wave power in different frequency ranges in the spectrogram. EMIC wave occurrence during ascending and descending phases of solar cycle 24, its local time, seasonal dependence and durations have been examined. There are total 2367 days for which data are available. Overall, EMIC waves are observed for 3166.5 h (approx 5.57% of total duration) which has contributions from 1263 days. We find a significantly higher EMIC wave occurrence during the descending phase (approx 6.83%) as compared to the ascending phase (approx 4.08%) of the solar cycle, which implies nearly a twofold increase in EMIC wave occurrence. This feature is attributed to the higher solar wind dynamic pressure during descending phase of solar activity. There is no evident difference in the percentage occurrence of EMIC waves on magnetically disturbed and quiet days. On ground, EMIC waves show marginally higher occurrence during winter as compared to summer. This seasonal tendency is attributed to lower electron densities and conductivities in the ionosphere, which can affect the propagation of EMIC waves through ionospheric ducts. In local time, the probability distribution function of EMIC wave occurrence shows enhancement during 11.7–20.7 LT (i.e., afternoon–dusk sector). Daily durations of EMIC waves are in the range of 5–1015 min and it is noted that the longer duration (240–1015 min) events are prevalent on quiet days and are mostly seen during the descending phase of solar cycle.

Highlights

The electromagnetic ion cyclotron (EMIC) waves are generated by the cyclotron-resonant instability of anisotropic medium-energy ring current ions (10–100 keV) (Cornwall 1965; Kennel and Petschek 1966)
Out of 2557 days, the data were available for 2367 days, of which 1964 were quiet and 403 were disturbed days
The peak of solar cycle 24 was in April 2014

Summary

Introduction

The electromagnetic ion cyclotron (EMIC) waves are generated by the cyclotron-resonant instability of anisotropic medium-energy ring current ions (10–100 keV) (Cornwall 1965; Kennel and Petschek 1966). The EMIC waves can undergo anomalous cyclotron resonance with relativistic electrons (Lorentzen et al 2000; Summers and Thorne 2003) and are known to play a substantial role in the loss mechanism of MeV electrons in the radiation belt (Bortnik et al 2006; Usanova et al 2014; Shprits et al 2017) They interact with ring current ions and cause them to precipitate into the mid-latitude ionosphere, resulting in the isolated proton auroras (Nomura et al 2012; Søraas et al 2013; Ozaki et al 2018). Studies of the EMIC waves are important for better understanding of their overall role in the particle dynamics in the Earth’s magnetosphere–ionosphere–atmosphere coupled system

Results

Discussion

Conclusion