tokens: are very important to understand the ICME’s morphology (Jones et al. 2002 ), propagation and acceleration of energetic particles (Reames 1999 ; Sanderson et al. 2000 ; Manchester et al. 2005 ), and cosmic ray modulation, i.e. Forbush decrease (Raghav et al. 2014 ; Bhaskar, Subramanian Vichare 2016b ; Bhaskar et al. 2016a ; Raghav et al. 2017 ; Shaikh, Raghav Bhaskar 2017 ; Shaikh et al. 2018 ). In fact, the role of these fine structures during geomagnetic storms has also been studied independently (Kataoka et al. 2015 ; Palmerio et al. 2016 ; Raghav et al. 2018 ). Recently, we have reported a unique feature of the ICME sheath, i.e. the coexistence of PMS and Alfvénic fluctuations. Here, we study their influence on the topology of a geomagnetic storm profile in detail. 2 OBSERVATION The studied ICME crossed the Wind spacecraft on 2000 October 12. The temporal variation of in situ plasma parameters during the ICME transit is shown in Fig. 1 . We have used 1 min resolution in situ interplanetary data from the OMNI database available at https://cdaweb.sci.gsfc.nasa.gov/index.html/ . The ICME substructures, i.e. shock–sheath and MC, are shown in different colour bands (see Fig. 1 ). The onset of the shock front is identified as sudden sharp enhancement in IMF B mag , solar wind speed ( V p ), proton temperature ( T ) and proton density ( N p ) (see the first vertical black dashed line). The shock front is followed by high N p , T, V p , and plasma beta (β) and large fluctuations in the IMF vector (i.e. B x, y, z ) region, which corresponds to the ICME shock–sheath (Shaikh et al. 2017 ; Kilpua et al. 2017a ). The estimated thickness of the ICME shock–sheath is about 0.21 au, which is calculated by multiplying the average solar wind velocity (∼450 km s −1 ) by total time duration (∼19.5 h). Information about the shock at 1 au observed by the Wind spacecraft is available at https://www.cfa.harvard.edu/shocks/wi_data/00179/wi_00179.html . Their analysis suggests that it is a fast forward (FF) shock with the following average properties: the shock normal , and the angle between the shock normal and the magnetic field θ Bn = 71.5°. This implies that the shock is a quasi-perpendicular shock. After the passage of the sheath, we observe a decrease in fluctuations in B mag and B comp , a slow variation in θ and φ, and a slow steady trend in V p and low β, indicating the transit of MC (Zurbuchen Richardson 2006 ). The catalogue available at http://space.ustc.edu.cn/dreams/wind_icmes/ corroborates the identified boundary of MC. Figure 1. ICME crossed the Earth during 2000 October 12–14. The figure has seven sub-panels. The first and second panels show the temporal variation of IMF B mag and B vec (i.e. B x , B y and B z ). The third panel represents the azimuth (φ) and elevation (θ) angles of the IMF vector. The fourth panel expresses the temporal variation of the geomagnetic index, i.e. SYM-H and the A L index. The fifth and sixth panels show solar wind
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