ABSTRACT
Jae-Ok Leeljoking@kasi.re.kr
Korea Astronomy and Space Science Institute
| Title : | Effects of geometries and substructures of ICMEs on geomagnetic storms |
| Abstract | |
| To better understand of storm generation by an ICME, we consider the geometries (impact location of flux rope at the Earth) and substructures (sheath and magnetic cloud (MC)) of the ICME. We apply the toroidal magnetic flux rope fitting model to 59 CDAW ICME-storm pairs to identify their substructures and geometric trajectory, and select 25 MC-associated and 5 sheath-associated storm events. We investigate the relationship between the storm strength indicated by Dst index and the solar wind conditions relating southward magnetic field [Bs], such as the minimum value [Bs_min], duration time [T_Bs], solar wind speed when the Bs is minimum [Vsw], convectional electric field [Ey], which is induced by [Vsw × Bs_min], and integrated Ey during T_Bs [Ey × T_Bs]. As the results, we find that all solar wind parameters have relatively high correlation coefficients (CC) with the storm strength (CC ∼ 0.8), except T_Bs (CC < 0.2). And the sheath-storm events show better correlations than MC-storm events for all parameters; for Ey , CC = 0.97 in sheath and CC = 0.77 in MC. Moreover, the slope of a linear regression line for sheath-storm events is about two times steeper than that of the MC-storm events in the relationship between storm strength (Dst index) and Ey × T_Bs, implying that the sheaths can produce stronger storms than MCs at a given solar wind condition. Regarding the geometric encounter of ICME, we find that 73 % (11/15) of storms and 100 % (4/4) of intense storms (Dst min ≤ -100 nT) occur in the regions at negative Py (relative position of the Earth trajectory from the ICME axis in the Y component in GSE coordinate) when the eastern flanks of ICMEs encounter the Earth, while 60 % (6/10) of storms and 83 % (5/6) of intense storms occur in positive Py regions when the western flanks hit the Earth. Our results demonstrate that the strength of a geomagnetic storm is strongly affected by not only the solar wind conditions (Bs_min, Ey, and Ey × T_Bs) and substructures (sheaths and MCs) but also ICME-Earth impact geometries (Py on ICME-Earth trajectory). |
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