Evolution and propagation of electric fields during magnetic impulses based on multi­-point observations

Thursday, October 1, 2015
Naoko Takahashi1, Yasumasa Kasaba1, Atsuki Shinbori2, Toshi Nishimura3, Takashi Kikuchi2,4, Tomoaki Hori4, Yusuke Ebihara2, Tsutomu Nagatsuma5 and Nozomu Nishitani4, (1)Tohoku University, Sendai, Japan, (2)Kyoto University, Kyoto, Japan, (3)University of California Los Angeles, Los Angeles, CA, United States, (4)Nagoya University, Solar-Terrestrial Environment Laboratory, Nagoya, Japan, (5)NICT National Institute of Information and Communications Technology, Tokyo, Japan
Abstract:
We focus on the evolution and propagation of the electric field associated with sudden commencements (SCs) by in situ multi-point observations.

First, we investigate the time response and local time dependence of in situ ionospheric electric field using by low-earth-orbit satellite, ROCSAT-1. We find that the ionospheric electric field variation associated with SCs instantaneously responds to the preliminary impulse (PI) signature on the ground regardless of observed local time. It supports the global instant transmission of electric field in the ionosphere from the polar region. In contrast, the peak time detected in the ionospheric electric field is earlier than that of the equatorial geomagnetic field (~20 s before in the PI phase). Based on the ground–ionosphere waveguide model, this time lag can be attributed to the latitudinal difference of ionospheric conductivity. Moreover, the westward electric field in the ionosphere, which produces the preliminary reverse impulse of the geomagnetic field on the dayside feature, appears at 18-22 h LT where the ionospheric conductivity, beyond the duskside terminator (18 h LT) is still lower than on the dayside. The result of an MHD simulation for an ideal SC shows that the electric potential distribution is asymmetric with respect to the noon–midnight meridian. This produces the local time distribution of ionospheric electric fields similar to our observed result. Both can be explained by the divergence of the Hall current under non-uniform ionospheric conductivity.

Now, as the next step, we investigate the evolution of the magnetospheric electric field using multiple in situ observations, such as THEMIS, Van Allen Probes, associated with ionospheric electric field observed by SuperDARN and several magnetometer data sets. We find the time delay of the onsets between dayside and nightside magnetospheric electric fields. It can be explained by a fast mode wave propagation in the equatorial plane of the magnetosphere. However, in some events we find peculiar signature, i.e., the simultaneous response between dayside and nightside electromagnetic fields. We will present the direction of the magnetospheric compression using in conjunction with the other in situ data, and consider some ideas for this explanation.

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