科研之友
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摘要
In this paper, a new ionospheric index pair-the single-station index (Js) and the planetary index (Jp), which describe ionospheric disturbances-is constructed using the spectral-whitening method. Observations covering more than 30 years and obtained from 21 ionosonde stations widely distributed across middle-and low-latitude areas are used in the study. Variations in the Js index at all stations generally follow the variation of the geomagnetic Dst index during periods of strong to moderate geomagnetic activity. Nevertheless, the inertial response of Js to weak geomagnetic activity results in a weak correlation between Js and Dst. The extrema of Js and Dst during the same geomagnetic activity period have a much higher correlation coefficient, since they avoid the delaying effect of the ionospheric response to geomagnetic activity. To smooth out local differences among ionospheric disturbances, the Jp index was defined as the means of all Js indices and so describes ionospheric disturbances at the planetary scale. The Jp index is significantly correlated to the Dst index, and the correlation coefficient between the extrema of Jp and Dst is up to -0.8. This correlation remains robust when the data set length varies, and data from approximately 10 stations are sufficient to derive a reasonable estimate of Jp. The quantitative relationship between Jp and Dst can be used to reproduce Jp by adopting Dst as the precursor, and this raises the possibility of predicting planetary ionospheric disturbances based on geomagnetic activity.
