Variation of Magnetic Field (By, Bz) Polarity and Statistical Analysis of Solar Wind Parameters during the Magnetic Storm Period

Abstract

It is generally believed that the occurrence of a magnetic storm depends upon the solar wind conditions, particularly the southward interplanetary magnetic field (IMF) component. To understand the relationship between solar wind parameters and magnetic storms, variations in magnetic field polarity and solar wind parameters during magnetic storms are examined. A total of 156 storms during the period of 1997~2003 are used. According to the interplanetary driver, magnetic storms are divided into three types, which are coronal mass ejection (CME)-driven storms, co-rotating interaction region (CIR)-driven storms, and complicated type storms. Complicated types were not included in this study. For this purpose, the manner in which the direction change of IMF <TEX>$B_y$</TEX> and <TEX>$B_z$</TEX> components (in geocentric solar magnetospheric coordinate system coordinate) during the main phase is related with the development of the storm is examined. The time-integrated solar wind parameters are compared with the time-integrated disturbance storm time (Dst) index during the main phase of each magnetic storm. The time lag with the storm size is also investigated. Some results are worth noting: CME-driven storms, under steady conditions of <TEX>$B_z$</TEX> < 0, represent more than half of the storms in number. That is, it is found that the average number of storms for negative sign of IMF <TEX>$B_z$</TEX> (T1~T4) is high, at 56.4%, 53.0%, and 63.7% in each storm category, respectively. However, for the CIR-driven storms, the percentage of moderate storms is only 29.2%, while the number of intense storms is more than half (60.0%) under the <TEX>$B_z$</TEX> < 0 condition. It is found that the correlation is highest between the time-integrated IMF <TEX>$B_z$</TEX> and the time-integrated Dst index for the CME-driven storms. On the other hand, for the CIR-driven storms, a high correlation is found, with the correlation coefficient being 0.93, between time-integrated Dst index and time-integrated solar wind speed, while a low correlation, 0.51, is found between timeintegrated <TEX>$B_z$</TEX> and time-integrated Dst index. The relationship between storm size and time lag in terms of hours from <TEX>$B_z$</TEX> minimum to Dst minimum values is investigated. For the CME-driven storms, time lag of 26% of moderate storms is one hour, whereas time lag of 33% of moderate storms is two hours for the CIR-driven storms. The average values of solar wind parameters for the CME and CIR-driven storms are also examined. The average values of <TEX>${\mid}Dst_{min}{\mid}$</TEX> and <TEX>${\mid}B_{zmin}{\mid}$</TEX> for the CME-driven storms are higher than those of CIR-driven storms, while the average value of temperature is lower.