Statistical Analysis of Solar Events Associated with Storm Sudden Commencements over One Year of Solar Maximum During Cycle 23: Propagation from the Sun to the Earth and Effects

Abstract

Taking the 32 storm sudden commencements (SSCs) listed by the International Service of Geomagnetic Indices (ISGI) of the Observatory de l’Ebre during 2002 (solar activity maximum in Cycle 23) as a starting point, we performed a multi-criterion analysis based on observations (propagation time, velocity comparisons, sense of the magnetic field rotation, radio waves) to associate them with solar sources, identified their effects in the interplanetary medium, and looked at the response of the terrestrial ionized and neutral environment. We find that 28 SSCs can be related to 44 coronal mass ejections (CMEs), 15 with a unique CME and 13 with a series of multiple CMEs, among which 19 (68%) involved halo CMEs. Twelve of the 19 fastest CMEs with speeds greater than 1000 km s⁻¹ are halo CMEs. For the 44 CMEs, including 21 halo CMEs, the corresponding X-ray flare classes are: 3 X-class, 19 M-class, and 22 C-class flares. The probability for an SSC to occur is 75% if the CME is a halo CME. Among the 500, or even more, front-side, non-halo CMEs recorded in 2002, only 23 could be the source of an SSC, i.e. 5%. The complex interactions between two (or more) CMEs and the modification of their trajectories have been examined using joint white-light and multiple-wavelength radio observations. The detection of long-lasting type IV bursts observed at metric–hectometric wavelengths is a very useful criterion for the CME–SSC events association. The events associated with the most depressed Dst values are also associated with type IV radio bursts. The four SSCs associated with a single shock at L1 correspond to four radio events exhibiting characteristics different from type IV radio bursts. The solar-wind structures at L1 after the 32 SSCs are 12 magnetic clouds (MCs), 6 interplanetary coronal mass ejections (ICMEs) without an MC structure, 4 miscellaneous structures, which cannot unambiguously be classified as ICMEs, 5 corotating or stream interaction regions (CIRs/SIRs), one CIR caused two SSCs, and 4 shock events; note than one CIR caused two SSCs. The 11 MCs listed in 3 or more MC catalogs covering the year 2002 are associated with SSCs. For the three most intense geomagnetic storms (based on Dst minima) related to MCs, we note two sudden increases of the Dst, at the arrival of the sheath and the arrival of the MC itself. In terms of geoeffectiveness, the relation between the CME speed and the magnetic-storm intensity, as characterized using the Dst magnetic index, is very complex, but generally CMEs with velocities at the Sun larger than 1000 km s⁻¹ have larger probabilities to trigger moderate or intense storms. The most geoeffective events are MCs, since 92% of them trigger moderate or intense storms, followed by ICMEs (33%). At best, CIRs/SIRs only cause weak storms. We show that these geoeffective events (ICMEs or MCs) trigger an increased and combined auroral kilometric radiation (AKR) and non-thermal continuum (NTC) wave activity in the magnetosphere, an enhanced convection in the ionosphere, and a stronger response in the thermosphere. However, this trend does not appear clearly in the coupling functions, which exhibit relatively weak correlations between the solar-wind energy input and the amplitude of various geomagnetic indices, whereas the role of the southward component of the solar-wind magnetic field is confirmed. Some saturation appears for Dst values $< -100$ nT on the integrated values of the polar and auroral indices.

Appendix: Event Timing and Solar-Wind Observations

We look into solar-wind data a few hours before each SSC in order to find the cause of the compression. Taking into account the propagation time, we are able to find an increase in the density and/or velocity that causes the SSC. We refer to these jumps as discontinuities of solar-wind perturbations. The discontinuity times at L1 are gathered in Table 14 (ACE data), column $t_{ \mathrm{d}}$; they usually happen from 30 to 60 minutes prior to the SSC. Then we identify later on if the perturbations following these shocks are ICMEs or other perturbation. $t_{\mathrm{s}}$ and $t_{\mathrm{e}}$ when different from $t_{\mathrm{d}}$ indicate the start and end times of the perturbation. The $[t_{\mathrm{s}},t_{ \mathrm{e}}]$ interval marks the sheath of the solar-wind perturbation, when observed.

Table 14 Observations at L1 for each SSC in 2002. Events are classified in five categories: ICME with (MC) or without (ICME) magnetic clouds, corotating and stream interaction regions (CIR/SIR), shocks (Shock), and miscellaneous (Misc.) when we cannot decide. $t_{ \mathrm{d}}$ is the time of the discontinuity that later on causes the SSC. For each MC and ICME, $t_{\mathrm{s}}$ and $t_{\mathrm{e}}$ are the start and end times (rounded up) of the ICME. Velocities are given: before the discontinuity ($V_{\mathrm{up}}$), at the discontinuity ($V_{ \mathrm{d}}$), at $t_{\mathrm{s}}$ ($V_{s}$) and at $t_{\mathrm{e}}$ ($V _{\mathrm{e}}$). $B^{*}_{\mathrm{z<0}}$ is a normalized value of the negative part of the IMF z-component (see Section 4.2). We provide reference to catalogs that list the events. The references to these catalogs have been numbered as 1, 2, 3, 4, 5, 6, 7, and 8, and the numbers correspond to Huttunen et al. (2005), Jian et al. (2006a), Jian et al. (2006b), Lepping et al. (2006), Li et al. (2011), Richardson and Cane (2010), Zhang et al. (2007), and Gopalswamy et al. (2010b), respectively.

To look at the geoeffectiveness of the solar-wind structures, we use OMNIWEB data to transport data from L1 to the bow-shock nose. Although the bow-shock crossing can modify SW properties, these data do not contain the exact cause of the magnetosphere compression. Nevertheless one can reasonably believe that an increase of either the SW density or the velocity upstream of the bow shock is still present downstream, even with a different intensity.

In the interplanetary medium, ICMEs are generally first identified by a shock (also called leading shock) in velocity, density, and magnetic field, then followed by a highly fluctuating region, the so-called ICME sheath. MCs appear in the central part of ICMEs with a magnetic structure that is well defined and resembles a flux rope with a slowly rotating magnetic field and low temperatures. We thus consider the following dataset at L1 (cf. also Figure 1): the date, density, temperature, and bulk flow velocity of the solar wind, the solar-wind pressure, and the IMF intensity and orientation. The orientation is defined by two angles ($\theta_{\mathrm{IMF}}$ and $\phi_{\mathrm{IMF}}$). $\theta_{ \mathrm{IMF}}$ is defined by the deviation of the IMF from the $(X,Y)_{\mathrm{GSM}}$ plane ($\theta_{\mathrm{IMF}}=90^{\circ}$ is aligned with $Z_{\mathrm{GSM}}$), $\phi_{\mathrm{IMF}}$ is the azimuth in the $(X,Y)_{\mathrm{GSM}}$ plane ($\phi_{\mathrm{IMF}}=0^{\circ}$ is parallel to the Earth–Sun axis),

By coupling these measurements with several lists describing the properties of the different solar-wind processes, it is possible to identify the cause of the terrestrial magnetosphere response.

At L1 we sort the SSC-related events into five categories: MC, ICME, Misc., Shock, and CIR/SIR (see Section 2.1). Table 15 gathers ICME and MC properties observed at L1. We compare parameter values in the sheath to the values in the cloud itself. Similar parameters are presented for the Misc. and Shocks in Table 16, and for CIR/SIR events in Table 17. In the latter case, we compare properties before and after the stream interface (SI). Many results presented in Section 4.2 are based on these tables.

Table 15 Solar-wind properties in the sheath and in the ICME (MC and non-MC) itself. Each ICME is identified using its negative peak value $B_{\mathrm{pz}}$. For the sheath and the ICME itself, we list the following: total (sheath or ICME) duration, duration between structure start time and time of the minimum values of Dst, Dst minimum value inside the structure, mean $\beta$, mean $M_{A}$, normalized and integrated $B_{\mathrm{z<0}}^{*}$ value until the time of min(Dst) (n/a means no negative z-component of the IMF during the period for which $B_{\mathrm{z<0}}^{*}$ is calculated), field rotation (e.g. SEN stands for south–east–north and indicates the sequence of the field direction, small letters indicate a small component) and chirality (in the case of MC). We considered the OMNI dataset propagated to the bow-shock nose location.

Table 16 Solar-wind properties in the Misc. structures and the Shocks. We list the following: normalized and integrated $B_{\mathrm{z<0}} ^{*}$ parameter, event duration, duration between SSC and min(Dst), min(Dst) value, negative peak value $B_{\mathrm{pz}}$, mean $\beta$, mean $M_{A}$, mean $P_{\mathrm{SW}}$, and maximum jump in solar-wind velocity ration inside the Misc. structures or across the shock for the Shocks. We considered the OMNI dataset propagated to the bow-shock nose location.

Table 17 Solar-wind properties in the CIR/SIR structures. Each event is presented with its normalized and integrated $B_{\mathrm{z}<0}^{*}$ value. We list the following before and after the SI: event duration (from SSC to SI, and from SI to the end), duration between SSC and the time of min(Dst), min(Dst), negative peak value $B_{\mathrm{pz}}$, mean $\beta$, mean $M_{A}$. The last two columns show the maximum $P_{\mathrm{SW}}$ reached and the maximum jump in solar-wind velocity ratio across the SI. Note that the time between SSC and min(Dst) can be negative before the SI. This is due to the fact that the CIR/SIR start time in the literature can be recorded as being before the SSC time. We considered the OMNI dataset propagated to the location of the bow-shock nose.