Statistical analysis of geosynchronous magnetic field perturbations near midnight during sudden commencements. Issue 6 (24th June 2014)

• Record Type: Journal Article
• Authors: Park, J.‐S.; Kim, K.‐H.; Kwon, H.‐J.; Lee, E.; Lee, D.‐H.; Jin, H.; Hwang, J.
• Is Part Of: Journal of geophysical research. Volume 119:Issue 6(2014:Jun.)
• Page Start: 4668
• Page End: 4680
• Publication Date: 2014-06-24
• Subjects: sudden commencement -- interplanetary shock; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2013JA019380
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: When an interplanetary (IP) shock passes over the Earth's magnetosphere, the geosynchronous magnetic field strength near noon is always enhanced except for the magnetopause crossing events. Near midnight, however, it increases or decreases. This indicates that the nightside magnetosphere is not always compressed by a sudden increase in the solar wind dynamic pressure. To understand midnight geosynchronous magnetic field responses to IP shocks, we statistically examined geosynchronous magnetic field perturbations, corresponding to 120 sudden commencements (SCs), observed when geosynchronous spacecraft were near midnight between 2200 and 0200 magnetic local times. Out of the 120 SCs, 107 SCs were identified by one geosynchronous spacecraft, and 13 SCs were identified by two geosynchronous spacecraft. Thus, 133 events were used in our statistical study. We observed 23 events in spring, 40 events in summer, 32 events in fall, and 38 events in winter, respectively. A statistical study of the midnight geosynchronous SC perturbations reveals the following characteristics. (1) In summer, all events show a positive enhancement (+Δ B T ) in the magnetic field strength. (2) In winter, however, Δ B T exhibits a positive (+Δ B T ) or negative (–Δ B T ) enhancement. (3) In summer, the midnight geosynchronous SC perturbations in the B H component (positive northward) in VDH coordinates are mostly (∼88%, 35 out of 40 events) positive (+Δ B H ), while the occurrence rate of the positive perturbation (∼43%) in the B z component (positive northward) in GSM coordinates is lower than that of the negative perturbation (∼57%). (4) In winter, the negative perturbations in Δ B H (∼61%) and Δ B z (∼74%) are dominant. (5) Both the north‐south components ( B H and B z ) in spring and fall are scattered around zero. To explain the observations, we suggest that SC‐associated cross‐tail current ( J SC ) has a peak intensity around geosynchronous orbit and thus is a main controlling factor of midnight geosynchronous magnetic field perturbations during SCs. Specifically, we suggest that the seasonal variation of the sign of Δ B H, Δ B z, and Δ B T is due to the seasonal variation of the spacecraft position relative to J SC . Key Points: Midnight SC‐associated B field perturbations Seasonal variations of SC‐associated geosynchronous B perturbations SC‐associated cross‐tail current near geosynchronous orbit