[P-EM28_29AM2] Magnetosphere-Ionosphere Coupling

[PEM28-01] MLT dependence in the response of ionospheric electric fields at mid-low latitude during geomagnetic sudden commencement

*Naoko TAKAHASHI¹, Yasumasa KASABA¹, Atsuki SHINBORI², Yukitoshi NISHIMURA³, Takashi KIKUCHI⁴, Tsutomu NAGATSUMA⁵ (1.Graduate School of Science, Tohoku University, 2.Research Institute for Sustainable Humanosphere (RISH), Kyoto University, 3.University of California, Los Angeles, 4.Solar-Terrestrial Environment Laboratory, Nagoya University, 5.National Institute of Information and Communications Technology)

The geomagnetic sudden commencement (SC) is one of the geomagnetic disturbance phenomena triggered by an enhancement of the magnetopause current associated with the compression of the magnetosphere due to solar wind disturbances [e.g., Araki, 1994]. Detailed evolution and propagating processes of the electromagnetic field associated with SCs are observed three-dimensionally in the entire geospace. Unlike magnetic storms and substorms which involve complex plasma physical processes, SCs can be identified as distinct magnetic variations that sharply change on a global scale. However, the characteristics of SCs have been extensively investigated mainly by means of the magnetic field variations obtained by ground-based observations, which could be affected by conductivities when deducing electric fields. Thus, investigating the electric field variations is needed to understand the transport of electromagnetic energy (Poynting fluxes, E×B/μ) associated with SCs. In this study, we examined two critical subjects about the ionospheric electric field associated with SCs using the in-situ electric field data.The in-situ ionospheric electric field was derived from the drift velocity observed by the Ionospheric Plasma and Electrodynamics Instrument (IPEI) onboard ROCSAT-1, which orbited at an ionospheric altitude (about 600 km), with magnetic field from the IGRF-10 model. We also used the geomagnetic field data from ground stations at the subauroral region, mid and low latitudes, and dip equator with a high time resolution of 1 second.The first subject is the transmission time of the ionospheric electric field from the subauroral region to the dip equator. We found the simultaneous SC onset between the ionospheric electric field by the ROCSAT-1 observations and geomagnetic fields by ground-based observations, and the time delay in the peak amplitudes of the preliminary impulse (PI) and main impulse (MI) occur irrespective of the magnetic local time (MLT). In statistical analyses, we showed that peak signatures of the ionospheric electric field at the low latitude appeared simultaneously with that of the geomagnetic field at the subauroral region. We also found that the peak signature at the equatorial region was observed with the time delay, and its value is about 20-40 seconds in the PI peak and 80-140 seconds in the MI peak. The instantaneous onset can be explained by means of the TM₀ mode waves propagating at the speed of light in the Earth-ionosphere waveguide, while the time delay in the peaks is interpreted as the difference of the time constant L/R of an equivalent circuit. From these results, we demonstrated the transmission of the electric field from the subauroral region and the common energy transport process for both the PI and MI.The second subject is the global structure of the ionospheric field. Ground-based observations are limited to mid and low latitudes, and provide only the horizontal component (E_phi) of the electric field. Thus, it is difficult to estimate the global electric field variation, especially at the terminator sector where SC signatures tend to appear in the radial component (E_r) of the electric field. We found the MLT dependence of the SC amplitude both the PI and MI signatures in the E_r and E_phi electric fields. In addition, the dayside characteristics of the PI signature extended to the evening terminator sector (18-21 h MLT) with an enhancement around 20 h MLT. This tendency is consistent with previous results obtained by the ground-based observations and model calculations. We consider that enhancements associated with SCs are influenced by the non-uniform ionospheric conductivity.In the present study, we revealed the global instant response of the ionospheric electric field during SCs based on the in-situ ionospheric electric field observations. Our results can serve as a basis for understanding energy transmission paths du