Abstract
This study reports the temporal and spatial distributions of the extremely/very low frequency (ELF/VLF) wave activities and the energetic electron fluxes in the ionosphere during an intense storm (geomagnetic activity index Dst of approximately −174 nT) that occurred on 26 August 2018, based on the observations by a set of detectors onboard the China Seismo-Electromagnetic Satellite (CSES). A good correlation of the ionospheric ELF/VLF wave activities with energetic electron precipitations during the various storm evolution phases was revealed. The strongest ELF/VLF emissions at a broad frequency band extending up to 20 kHz occurred from the near-end main phase to the early recovery phase of the storm, while the wave activities mainly appeared at the frequency range below 6 kHz during other phases. Variations in the precipitating fluxes were also spotted in correspondence with changing geomagnetic activity, with the max values primarily appearing outside of the plasmapause during active conditions. The energetic electrons at energies below 1.5 MeV got strong enhancements during the whole storm time on both the day and night side. Examinations of the half-orbit data showed that under the quiet condition, the CSES was able to depict the outer/inner radiation belt as well as the slot region well, whereas under disturbed conditions, such regions became less sharply defined. The regions poleward from geomagnetic latitudes over 50° were found to host the most robust electron precipitation regardless of the quiet or active conditions, and in the equatorward regions below 30°, flux enhancements were mainly observed during storm time and only occasionally in quiet time. The nightside ionosphere also showed remarkable temporal variability along with the storm evolution process but with relatively weaker wave activities and similar level of fluxes enhancement compared to the ones in the dayside ionosphere. The ELF/VLF whistler-mode waves recorded by the CSES mainly included structure-less VLF waves, structured VLF quasi-periodic emissions, and structure-less ELF hiss waves. A wave vector analysis showed that during storm time, these ELF/VLF whistler-mode waves obliquely propagated, mostly likely from the radiation belt toward the Earth direction. We suggest that energetic electrons in the high latitude ionosphere are most likely transported from the outer radiation belt as a consequence of their interactions with ELF/VLF waves.
Highlights
Within the solar-terrestrial system, the ionospheric layers can mirror both Earthward and Sunward disturbances at various scales
Zhima et al [10] firstly found the evidence of whistler-mode chorus penetrating the plasmapause and entering into the low altitude ionosphere, and Zhang et al [26] firstly reported that chorus waves in the magnetosphere accelerated energetic electrons (1–3 MeV) in the ionosphere from the China Seismo-Electromagnetic Satellite (CSES) observations
Another important extremely/very low frequency (ELF/very low frequency (VLF)) whistler-mode wave often appearing in the ionosphere is the quasi-periodic (QP) wave at frequencies from several hundred Hz to ~4 kHz with varying periodic modulations of wave intensity over time scales from several seconds to a few minutes; they are often observed by low Earth orbit (LEO) satellites and ground stations [13,15]
Summary
Within the solar-terrestrial system, the ionospheric layers can mirror both Earthward and Sunward disturbances at various scales. ELF hiss waves are structure-less and incoherent electromagnetic waves that preferentially appear at a broad frequency range from several hundred Hz to 3 kHz, playing vital roles in either the loss of energetic electrons or the formation of radiation belt slot region [22,27,28] Another important ELF/VLF whistler-mode wave often appearing in the ionosphere is the quasi-periodic (QP) wave at frequencies from several hundred Hz to ~4 kHz with varying periodic modulations of wave intensity over time scales from several seconds to a few minutes; they are often observed by low Earth orbit (LEO) satellites and ground stations [13,15]. Zhima et al [20] reported the well-pronounced rising-tone structure QP waves and simultaneous energetic electron precipitations (from ~400 keV to 1 MeV) in the high-latitude ionosphere firstly based on the CSES’s observations
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