The time-frequency characteristics of geoelectric storm in Chinese mainland

A total of 83 geoelectric storms (GESs) of Kp≥5 during the period of August of 2014 to August of 2016 are exhaustively analyzed by using the waveform analysis and maximum entropy estimation methods based on the minute-values of geoelectric field from 25 stations along two longitude chains (118°E and 99°E) and two latitude chains (40°N and 34°N) in Chinese mainland, and then the time-frequency variation characteristics of the geoelectric storm (GES) are studied in detail. As a result, the following understandings are obtained: there are two types of GESs, i.e., sudden commencement GES and gradual commencement GES, and they usually continue for one to three days; GESs synchronously occur in a wide area with the same or similar waveforms, suggesting a wide-area characteristic of GES. The response of the GES to the variation of the geomagnetic storm is discussed, and the differences in amplitude variation between different stations and in different directions are also discussed. It is believed that the interaction between the space electromagnetic field caused by the solar wind during the geomagnetic storm and the interaction of the Earth's medium causes a change in the ground current, which results in a strong response to the rapid change in the geoelectric field and the rapid change in the geomagnetic field. The amplitude of GES is related to the underground medium conductivity underneath the geoelectric field stations. The smaller the conductivity is, the lager the amplitude is. In general, the geoelectric storm principally has 1.6 h, 1 h, 15 min and 10 min periodic components according to the size of their power spectrum density, which have a little difference among different GESs.