LAUSR

In early September 2017, a series of solar flares and coronal mass ejections (CMEs) erupted from the Sun. The Cor2a coronagraph, a unit of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), onboard the Solar Terrestrial Relations Observatory (STEREO)-A spacecraft recorded two Sun–Earth-directed CMEs on 4 September (referred to as CME04) and 6 September (referred to as CME06). A few days later, the Wind spacecraft (\({\approx}\,212.4\) solar radii: \(\mathrm{R}_{\odot}\)) recorded two interplanetary shocks, presumably driven by CME04 and CME06, at \({\approx}\,22\mbox{:}41~\mbox{UT}\) on 06 September 2017 (referred to as Shock06) and at \({\approx}\,22\mbox{:}48~\mbox{UT}\) on 07 September 2017 (referred to as Shock07), respectively. The travel time of the CME04/Shock06 [\(\Delta t_{\text{shock-CME@18R}\odot}\)] and CME06/Shock07 from \(18~\mathrm{R}_{\odot}\) to the Wind spacecraft was 41.52 hours and 32.47 hours, respectively. The propagating speed [\(V_{\mathrm{CME}}\)] of the CME04 and CME06 at \({\approx}\,18~\mathrm{R}_{\odot}\) was determined with SECCHI/Cor2a as \({\approx}\,886~\mbox{km}\,\mbox{s}^{-1}\) and \({\approx}\,1368~\mbox{km}\,\mbox{s}^{-1}\), respectively. Assuming a constant velocity after \(18~\mathrm{R}_{\odot }\), the estimated \(\Delta t_{\text{shock-CME@18R}\odot}\) is 42.45 and 27.5 hours for CME04 and CME06, respectively. This simple estimate of the CME propagation speed provides a satisfactory result for the CME04 event (error \({\approx}\,2.3\%\)) but not for the CME06 event (error \({\approx}\,15.3\%\)). The second event, CME06, was delayed further due to an interaction with the preceding event (CME04). It is suggested that the CME speed estimated near the Sun with coronagraph images can be a good estimator for the interplanetary CME (ICME) transit time when there is no pre-event. A three-dimensional magnetohydrodynamic simulation is performed to address this issue by providing a panoramic view of the entire process not available from the observations. A southward interplanetary magnetic field [\(B_{\mathrm{s}}\)] increased sharply to −31.6 nT on 7 September at Wind, followed by a severe geomagnetic storm (\(\mathrm{Dst} = -124~\mbox{nT}\)). The sharp increase of the IMF [\(B_{\mathrm{s}}\)] was a result of the interaction between Shock07 and the driver of Shock06 (CME04). This study suggests that a severe geomagnetic storm can be caused by the interaction between a MC, with an impinging IP shock from behind, and the Earth’s magnetosphere. The intensity of a geomagnetic storm will likely be stronger for an event associated with ICME–ICME interaction than for a geomagnetic event caused by only a single ICME.