Research PaperThe relationship between geomagnetic indices and the interplanetary medium parameters in magnetic storm main phases during CIR and ICME events
Introduction
It is known that during magnetic storm the large-scale magnetospheric current systems are enhanced, auroral electrojets shift to lower latitudes, and intense substorm disturbances occur. Simultaneously we can observe a significant decrease in the horizontal magnetic field component at low latitudes and an increase of geomagnetic indices (Akasofu and Chapman, 1961; Nishida, 1978).
Geomagnetic indices calculated by the ground-based observations data allow us to describe the development dynamics and intensity of magnetic disturbances during periods of magnetic storms. The low-latitude Dst index is used to evaluate the ring current intensity during magnetic storms and to measure the geoeffectiveness of interplanetary disturbances (Sugiura, 1964; Burton et al., 1975). The high-latitude AE index and the mid-latitude Kp index characterize the auroral current intensity during magnetic storms and are indicators of the substorm activity (Davis and Sugiura, 1966).
The correlation between the geomagnetic indices, variations in the parameters of the interplanetary magnetic field (IMF) and solar wind during periods of magnetic storms are widely represented in papers (e.g. Gonzalez et al., 1994; Grafe et al., 1997; Grafe and Feldstein, 2000; Kane, 2010), and references therein). In the papers (Gonzalez et al., 1994; Kane, 2010) it was shown that the southward IMF Bz component, whose efficiency is connected with the influence of solar wind electric field (Esw = V × Bz), is the main reason for generation of magnetic disturbances. The AE (Kp) indices correlate with the Dst index during the magnetic storm (Grafe et al., 1997; Grafe and Feldstein, 2000, and references therein).
In early researches, Akasofu and Chapman (1961) and McIlwain (1974) believed that the sequence of intense substorm disturbances lead to the development of magnetic storm. According to the opinion of supporters of this hypothesis, during a substorm there occurred an injection of energetic particles from the plasma sheet into the ring current. Amplification of ring current causes a decrease of Earth's magnetic field, and, correspondingly, a decrease of Dst index. The first doubts in the correctness of this hypothesis appeared after the paper by Burton et al. (1975), in which it was shown that the decrease of Dst index is obviously connected with the southward IMF Bz. The time of response of the Dst to the southward IMF Bz is much less than the characteristic time of development of the substorm. Further, it was proved in papers (Iyemori, 1994; Iyemori and Rao, 1996). However, the development of ring current is not associated with substorms. According to (Iyemori and Rao, 1996; Sharma et al., 2003), the main cause of ring current amplification is the enhancement of magnetospheric convection during periods of the long-term southward IMF Bz component whose efficiency is attributed to the SW electric field effect (Gonzalez et al., 1994; Kane, 2005). Therefore, during the periods of magnetic storm the geomagnetic indices correlate between each other. However, the relation of these indices has more complex nature (Consolini and De Michelis, 2011; De Michelis et al., 2015; Alberti et al., 2017).
It is known that the development of magnetic storms and substorms depends on the type of SW (see, for example, papers by Borovsky and Denton (2006); Yermolaev et al. (2010)). There exist the following types of SW: interplanetary manifestations of coronal mass ejections (ICME) including magnetic clouds (MC) and Ejecta (Ej), corotating interaction regions (CIR), and also the sheath regions before ICMEs (ShMC, ShEj). Each type of SW has a certain set of SW and IMF parameters. For example, in CIR and Sheath events, the plasma has increased values of density, temperature, and thermal pressure prevails over the magnetic one (β > 1). In ICME events, the magnetic pressure prevails over the thermal pressure and has a magnetic field structure in the form of flux rope. MC differs from Ejecta by a higher (>10 nT) and more regular magnetic field. In the CIR events, in contrary to the ICME, there are high SW velocities and small values of the southward IMF Bz. The difference in the SW and IMF parameters in the SW flow types are manifested in variations of the AE, Kp and Dst indices.
The analysis between SW parameters for different types of SW and indices of geomagnetic activity is shown in the papers (Plotnikov and Barkova, 2007; Yermolaev et al, 2010, 2012; Guo et al., 2011; Cramer et al., 2013), in which the magnitude of minimum value of Dst index increases with the growth of SW electric field for all types of SW, but for events of ICME (MC + Ejecta) the value of |Dstmin| will not increase at high values of Esw. The AE index, unlike Dst, during the magnetic storm main phase does not depend on Esw for almost all SW types except MC. There is a nonlinear dependence of AE on Esw in MC. The relationship between Kp and Esw is characterized by a linear empirical dependence for CIR and a non-monotonic dependence for MC (Plotnikov and Barkova, 2007; Yermolaev et al., 2012). However, in papers by Boroyev (2016), Boroyev and Vasiliev (2018) it is shown that for CIR events, unlike ICME events, the value of AE increases with the growth of SW electric field (Esw). The relation of Kp index with the SW electric field is more expressed for the ICME events (r = 0.81), than for the CIR events (r = 0.5). A distinction of results, apparently, is caused by a different approach in calculating of AE and Kp indices during the main phase of magnetic storm.
The aim of this paper is to continue studies of auroral activity during periods of magnetic storms induced by different SW types. This study was started in (Boroyev and Vasiliev, 2018) for the periods of main phases of magnetic storms according to the data for 1979–2000. The second time is to check or refute the results obtained previously by Boroyev and Vasiliev (2018) using more events (magnetic storms) and to exclude the possible dependence of results on the number of events.
Section snippets
Data and methods
In this paper we estimate the geomagnetic activity using the AE, Kp, and Dst indices (https://omniweb.gsfc.nasa.gov/form/dx1.html). The values of mid-latitude geomagnetic activity index (Kp) obtained from the OMNIWeb Data Explorer website are given as Kp × 10.146 CIR and ICME induced magnetic storms (MC + Ejecta) with Dstmin ≤ −50 nT for the 1979 to 2017 period have been selected. Other SW types that induce a magnetic storm were not considered in this paper. A magnetic storm is considered to be
Results
The dependence of average value of the AE index (AЕaver) on the rate of storm development (|ΔDst׀/ΔT) for magnetic storms induced by the CIR and ICME events is shown in Fig. 1 (a, c). The squares denote individual magnetic storms, straight lines are linear approximations. The dependence of average value of the mid-latitude Kp index (Kpaver) on the rate of storm development is presented in Fig. 1 (b, d) for different types of the SW. In Table 1 the equations of linear regressions between the AЕ
Discussion
In (Boroyev and Vasiliev, 2018), the relationship of the mid-latitude Kp index both with the rate of storm development and SW electric field during the main phase of magnetic storms induced by the CIR and ICME events for the 1979 to 2000 period was studied. It was shown that for the CIR and ICME events, the average value of Kp index (Kpaver) increased with the growth of rate of storm development (׀ΔDst׀/ΔT). The highest coefficient between Kpaver & ׀ΔDst׀/ΔT was observed for the CIR events
Conclusion
The results of analysis allow us to make the following conclusions:
- 1.
The increase of the set of data (events) confirm the results obtained in (Boroyev and Vasiliev, 2018). This fact supports the assumption that the results obtained in the previous (Boroyev and Vasiliev, 2018) and in the present work do not depend on the number of events but are related to the nature of phenomena.
- 2.
It is shown that for the CIR and ICME events, with the growth of development rate of the magnetic storm, the increase
Acknowledgments
This work is supported by Project II.16.2.1 (registration number AAAA-A17-117021450059-3). The work done under the project SCOSTEP program « Variability of the Sun and Its Terrestrial Impact (VarSITI)».
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