Reply to comment by U. Villante and M. Piersanti on “Statistical analysis of geosynchronous magnetic field perturbations near midnight during sudden commencements”

We thank Villante and Piersanti [2015 hereinafter referred to as VP] for their comments on our recent paper Park et al. [2014, hereinafter referred to as P14]. The main comment in VP is that geosynchronous magnetic field perturbations near midnight during sudden commencement (SC) can be explained by different combinations of the hinging point distance and current sheet thickness. We basically agree with VP. However, we have question as to whether all SC-associated magnetic field perturbations observed at geosynchronous orbit near midnight can be explained by empirical models developed by Tsyganenko [2002a, 2002b, referred to as T01] in VP and developed by Tsyganenko and Sitnov [2005, referred to as TS05] in P14. It should be noted that T01 and TS05 are not time-dependent models. This indicates that the magnetic field strength and magnetic field vectors of strongly disturbed magnetic field perturbations, which is caused by the passage of interplanetary shock over the Earth's magnetosphere, during a typical SC interval less than ∼15 min cannot be easily estimated from both T01 and TS05 models. The models do not provide the distance of the hinging point from Earth and current sheet thickness in the magnetotail as a function of the solar wind dynamic pressure during the SC interval. In particular, the location where the SC-associated current establishes has not been provided from the models. This is a reason that we do not make a quantitative examination whether the SC observation at geosynchronous orbit is consistent with the model field variation (see Figures 7 and 8 in P14). Using the hinging point distance of 7.38 RE and current sheet thickness of 2.05 RE in T01 model, VP shows that the model predictions describe the observations quite well for the SC event occurred at 09:10 UT on 8 June 2000. In P14, we focus on the difference between onset times of the positive enhancement of BH in VDH coordinates and Bz in GSM coordinates, as plotted in Figure 1 which is the same as Figure 3 in P14, to explain where the initial SC-associated cross-tail current is enhanced, and suggest that the SC-associated cross-tail current is enhanced near geosynchronous orbit (see Figure 12 in P14). We have a question whether the onset time difference between the BH and Bz components can be expected from the model in VP because the authors uses only the magnetic field components in GSM coordinates. Figure 2 shows the TS05 model magnetic field perturbations in ΔBz caused by solar wind dynamic pressure (PSW) change from PSW = 2 to PSW>2 at midnight geosynchronous orbit as a function of PSW with the same parameters for the hinging point distance (RH = 9RE and 7.38 RE) and current sheet thickness (D0 = 2.4RE and 2.05 RE) as those in VP. We consider only the inner tail current in TS05 model (see the text in P14), which is more relatively recent model than T01. If all currents are considered in TS05, ΔBz is negative for all PSW (data not shown here). As shown in Figure 2, there is no significant difference between case 1 (RH = 9RE and D0 = 2.4RE) and case 2 (RH=7.38RE and D0 = 2.05RE) in terms of the polarity of ΔBz for PSW change. In Figure 2, the location of the inner tail current density is fixed at Xsy = 6 (see the text in P14 for detailed descriptions). As suggested and discussed in P14 (see Figures 7–9 in P14), however, the radial distribution of the inner tail current density is a main factor to control the change of ΔBz polarity at midnight geosynchronous orbit in TS05. This work was supported by BK21+ through the National Research Foundation (NRF) funded by the Ministry of Education of Korea. Work of K.-H. Kim was supported by the Basic Science Research Program through NRF funded by NRF-2013R1A1A2A10004414 and also supported by NRF-2012M1A3A3A02033285. Larry Kepko thanks Atsuki Shinbori for his assistance in evaluating this paper.