Solar cycle dependence of ion cyclotron wave frequencies

Abstract

Electromagnetic ion cyclotron (EMIC) waves have been studied for decades, though remain a fundamentally important topic in heliospheric physics. The connection of EMIC waves to the scattering of energetic particles from Earth's radiation belts is one of many topics that motivate the need for a deeper understanding of characteristics and occurrence distributions of the waves. In this study, we show that EMIC wave frequencies, as observed at Halley Station in Antarctica from 2008 through 2012, increase by approximately 60% from a minimum in 2009 to the end of 2012. Assuming that these waves are excited in the vicinity of the plasmapause, the change in Kp in going from solar minimum to near solar maximum would drive increased plasmapause erosion, potentially shifting the generation region of the EMIC to lower L and resulting in the higher frequencies. A numerical estimate of the change in plasmapause location, however, implies that it is not enough to account for the shift in EMIC frequencies that are observed at Halley Station. Another possible explanation for the frequency shift, however, is that the relative density of heavier ions in the magnetosphere (that would be associated with increased solar activity) could account for the change in frequencies. In terms of effects on radiation belt dynamics, the shift to higher frequencies tends to mean that these waves will interact with less energetic electrons, although the details involved in this process are complex and depend on the specific plasma and gyrofrequencies of all populations, including electrons. In addition, the change in location of the generation region to lower L shells means that the waves will have access to higher number fluxes of resonant electrons. Finally, we show that a sunlit ionosphere can inhibit ground observations of EMIC waves with frequencies higher than ∼0.5 Hz and note that the effect likely has resulted in an underestimate of the solar-cycle-driven frequency changes described here.

Generation of EMIC Waves and Effects on Particle Precipitation During a Solar Wind Pressure Intensification With Bz>0

“…Electromagnetic ion cyclotron (EMIC) waves are generated near the geomagnetic equator by anisotropic (T ⊥ > T ‖ ) energetic (∼10–100 kev) proton distributions (Cornwall et al, ; Horne & Thorne, ; Kozyra et al, ; Remya et al, ). The waves have frequencies that depend on the background ion populations, including effects from heavy ions, thus leading to a solar cycle dependence of wave frequencies (Lessard et al, ), following variations in heavy ion densities that follow this trend.…”

Measuring the seeds of ion outflow: Auroral sounding rocket observations of low‐altitude ion heating and circulation

“…Generally speaking, it has been thought that transverse heating by WPI occurs at a minimum altitude of approximately 500 km on the nightside [ Whalen et al , ; Yau et al , ; Arnoldy et al , ]. Many investigations have shown that the outflow of oxygen into the magnetosphere has a profound effect on magnetospheric dynamics [ Kronberg et al , , and references therein], such as the generation of magnetosphere sawtooth oscillations by ionospheric O + outflow [ Brambles et al , ] and changes in wave frequencies in ion cyclotron waves [ Lessard et al , ].…”

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

“…Kangas et al (1998) have provided a review of the occurrence of Pc1 waves and its link with the geomagnetic storm, solar cycle, Sun's and Earth's rotation and orbital motion. A recent study carried out using EMIC observations at Halley station in Antarctica, indicates that the frequency of EMIC wave is also influenced by the solar activity (Lessard et al 2015). There is observational evidence that supports the solar activity influence on the EMIC wave occurrence.…”