Abstract
The objective of this study is (1) to determine the magnetic cloud (MC) structure associated with the 17 March 2015 storm and (2) to gain an insight into how the storm developed responding to the solar wind conditions. First, we search MC geometries which can explain the observed solar wind magnetic fields by fitting to both cylindrical and toroidal flux rope models. Then, we examine how the resultant MC geometries can be connected to the solar source region to find out the most plausible model for the observed MC. We conclude that the observations are most consistently explained by a toroidal flux rope with the torus plane nearly parallel to the ecliptic plane. It is emphasized that the observations are characterized by the peculiar spacecraft crossing through the MC, in that the magnetic fields to be observed are southward throughout the passage. For understanding of the storm development, we first estimate the injection rate of the storm ring current from the observed Dst variation. Then, we derive an expression to calculate the estimated injection rate from the observed solar wind variations. The point of the method is to evaluate the injection rate by the convolution of the dawn-to-dusk electric field in the solar wind and a response function. By using the optimum response function thus determined, we obtain a modeled Dst variation from the solar wind data, which is in good agreement with the observed Dst variation. The agreement supports the validity of our method to derive an expression for the ring current injection rate as a function of the solar wind variation.
Highlights
The 17 March 2015 geomagnetic storm is the largest in Solar Cycle 24 so far with minimum Dst of −223 nT and has stimulated many research activities (Kamide and Kusano 2015; Kataoka et al 2015; Liu et al 2015; Cho et al 2016; Wang et al 2016)
The foregoing studies (Gopalswamy et al 2015; Kataoka et al 2015; Liu et al 2015; Cho et al 2016; Wang et al 2016) agree that a halo coronal mass ejection (CME) associated with a C9.1 flare which started at 01:15 UT on 15 March in AR 12297 (S22W25) was the solar source event of the magnetic cloud (MC)
We studied the 17 March 2015 storm focusing on the MC structure associated with the storm and on the storm development as geomagnetic response to the solar wind conditions
Summary
The 17 March 2015 geomagnetic storm is the largest in Solar Cycle 24 so far with minimum Dst of −223 nT and has stimulated many research activities (Kamide and Kusano 2015; Kataoka et al 2015; Liu et al 2015; Cho et al 2016; Wang et al 2016). The large geomagnetic storm was somewhat surprising, and space weather agencies worldwide failed to predict that such a severe storm would occur (Kamide and Kusano 2015; Kataoka et al 2015, Wang et al 2016). While some particular ideas have been proposed such. One reason is the complexity in the solar wind data due to superposed disturbances which prevents easy determination of boundaries for this particular
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