Storm-time VLF emissions caused by the solar wind disturbances : a case study on 8 December 2013

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>This study is made of temporal variations of the daytime VLF emissions (1–6 kHz) occurred during the moderate magnetic storm (Kp = 6) on 8 December 2013. The storm was associated with the Coronal Mass Ejection (CME). VLF emissions were recorded in the frequency band of 0.2–39 kHz during the dark winter at Kannuslehto (KAN, L ~ 5.5) in Northern Finland. The results were compared with simultaneous variations in the solar wind and Interplanetary Magnetic Field (IMF). It was found that intense VLF chorus started after the pressure jump in solar wind (from 7 to 12 nPa) under the positive IMF Bz. The VLF emissions occurred in two separate frequency bands. The lower frequency (below ~2 kHz) band represents the intense long lasting hiss with right-hand polarization, and in the upper frequency band (above ~2 kHz) the left-hand polarized hiss bursts occurred during about 1 hour. The plasmasphere was strongly compressed, and due to that KAN was mapped outside of the plasmapause. We suppose that VLF chorus exited in the magnetosphere by the cyclotron instability of the radiation belt electrons. The low frequency chorus was generated outside of the plasmapause and arrived to KAN along the direction of N-S meridian. The high-frequency band was generated inside of the plasmasphere arrived to KAN almost along the meridian.</p></div>