Geomagnetic Effects of Corotating Interaction Regions

Abstract

We present an analysis of the geoeffectiveness of corotating interaction regions (CIRs), employing the data recorded from 25 January to 5 May 2005 and throughout 2008. These two intervals in the declining phase of Solar Cycle 23 are characterised by a particularly low number of interplanetary coronal mass ejections (ICMEs). We study in detail how four geomagnetic-activity parameters (the Dst, Ap, and AE indices, as well as the Dst time derivative, $\mathrm{dDst}/\mathrm{d}t$ ) are related to three CIR-related solar wind parameters (flow speed, $V$ , magnetic field, $B$ , and the convective electric field based on the southward Geocentric solar magnetospheric (GSM) magnetic field component, $\mathit{VB}_{s}$ ) on a three-hour time resolution. In addition, we quantify statistical relationships between the mentioned geomagnetic indices. It is found that Dst is correlated best to $V$ , with a correlation coefficient of $\mathrm{cc}\approx0.6$ , whereas there is no correlation between $\mathrm{dDst}/\mathrm{d}t$ and $V$ . The Ap and AE indices attain peaks about half a day before the maximum of $V$ , with correlation coefficients ranging from $\mathrm{cc}\approx0.6$ to $\mathrm{cc}\approx0.7$ , depending on the sample used. The best correlations of Ap and AE are found with $\mathit{VB}_{s}$ with a delay of 3 h, being characterised by $\mathrm{cc}\gtrsim 0.6$ . The Dst derivative $\mathrm{dDst}/\mathrm{d}t$ is also correlated with $\mathit{VB}_{s}$ , but the correlation is significantly weaker $\mathrm{cc}\approx 0.4$ – 0.5, with a delay of 0 – 3 h, depending on the employed sample. Such low values of correlation coefficients indicate that there are other significant effects that influence the relationship between the considered parameters. The correlation of all studied geomagnetic parameters with $B$ are characterised by considerably lower correlation coefficients, ranging from $\mathrm{cc}=0.3$ in the case of $\mathrm{dDst}/\mathrm{d}t$ up to $\mathrm{cc}=0.56$ in the case of Ap. It is also shown that peak values of geomagnetic indices depend on the duration of the CIR-related structures. The Dst is closely correlated with Ap and AE ( $\mathrm{cc}=0.7$ ), Dst being delayed for about 3 h. On the other hand, $\mathrm{dDst}/\mathrm{d}t$ peaks simultaneously with Ap and AE, with correlation coefficients of 0.48 and 0.56, respectively. The highest correlation ( $\mathrm{cc}=0.81$ ) is found for the relationship between Ap and AE.