Geomagnetically induced current (GIC) is known to be closely related to the rate of change of local horizontal magnetic field (dBx/dt); and their spectra can give better insight into the relationship. We study the spectral characteristics of GIC measured in Finland and dBx/dt measured 30 km away during the 17 March 2013 intense geomagnetic storm (SymHMin = -132 nT). Two bursts of large GIC (up to 32A) and dBx/dt occurred at ~ 16 UT and 18 UT during the storm main phase, though their values were generally small. For the first time, the Cross Wavelet Transform (XWT) and Wavelet Coherence (WTC) techniques are used to investigate the correlation and phase relationship of GIC and dBx/dt in time-frequency domain. Their WTC correlation is strong (over 0.9) over the entire storm period, indicating dBx/dt is the main factor causing GIC and dBx/dt leading GIC. Their XWT spectra show two enclosed periods (8–42 min and 2–42 min) in the high energy region corresponding to the two bursts of activity in GIC and dBx/dt. Moreover, we use continuous wavelet transform (CWT) and discrete wavelet transform (DWT) to analyze the spectral characteristics of GIC and dBx/dt. It is found that the CWT and DWT spectra of the two are very similar, especially in the low frequency characteristics, without continuous periodicity. Wavelet coefficients become large when GIC and dBx/dT are large; and the third-order coefficient, which corresponds to low-frequency part, best reflects the disturbance of GIC and dBx/dt.
An onboard Langmuir probe instrument equipped with a nonlinear micro-current acquisition device has been developed and is described in this paper. Langmuir probes are commonly used sensors for ionospheric plasma diagnosis, but the plasma density varies significantly with the altitude, latitude, and day/night; therefore, the currents collected by Langmuir probes have an extensive dynamic range and tiny amplitude. We developed a novel Langmuir probe instrument with a nonlinear micro-current acquisition device that can acquire micro-current signals with high precision and has an extended dynamic range. The instrument has a low mass (about 950 g), a power consumption of about 700 mW, and meets the payload requirements of micro-nano satellites. The instrument is equipped with high-speed analog-to-digital converters (ADCs) and a third-order low-pass filter to respectively achieve submeter spatial resolution and signal-tonoise ratio (SNR) measurement of higher than 120 dB. A data inversion algorithm based on polynomial fitting is used to accurately trace back the collected current signals to achieve highprecision diagnosis of plasma in the density range from 10 8 to 10 13 m −3 . The instrument has multiprobe acquisition capability, can run in two modes, and is suitable for different detection platforms. Various experiments were carried out in the Space Plasma Simulation Chamber to evaluate the performance of the instrument. The results demonstrate that the developed Langmuir probe instrument can accurately diagnose plasma in a large density range and meet the requirements for ionospheric plasma diagnosis.
<p>Both simulations and observations had shown that step function-like increase/decrease of solar wind dynamic pressure pulse would excite flow vortex pairs in the dawn and dusk high latitude ionosphere simultaneously. However, some plasma structures, hot flow anomaly, sheath jets etc. existing in the solar wind or magnetosheath are often accompanied with spike-like changes of the dynamic pressure. Whether they can drive the ionospheric vortices or not is still unclear. In this work we report a traveling convection vortex like (TCV-like) event that was induced by a positive-negative pulse pair of dynamic pressure(&#9651;p/p~1) accompanying a large scale (~9min) magnetic hole in the solar wind. It is found that following the magnetic hole, two traveling convection vortices first in anticlockwise then in clockwise rotation were detected by geomagnetic stations located along the 10:30MLT meridian. Meanwhile, another pair of ionospheric vortices azimuthally seen up to 3 MLT first in clockwise then in anticlockwise rotation were appeared in the afternoon sector (~14MLT) centered at ~75MLAT with a trend of poleward moving. The duskside vortices were also confirmed by SuperDARN radar data. The processes following magnetosphere struck by a positive-negative pulse pair were simulated and it found that two pairs of flow vortices in the dawn and dusk magnetosphere may provide the field-aligned currents(FACs) required for the flow/current vortices observed in ionosphere. This work provides a way to understand how the momentum and energy injects to the ionosphere under spike-like dynamic pressures imposing on the magnetosphere.</p>
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