H.-J. Kwon

Loss of geosynchronous relativistic electrons by EMIC wave scattering under quiet geomagnetic conditions

We have examined relativistic electron flux losses at geosynchronous orbit under quiet geomagnetic conditions. One 3 day period, from 11 to 13 October 2007, was chosen for analysis because geomagnetic conditions were very quiet (3 day average of Kp 2 MeV electron flux at geosynchronous orbit shows typical diurnal variations with a maximum near noon and a minimum near midnight for each day. The flux level of the daily variation significantly decreased from first day to third day for the 3 day period by a factor of >10. The total magnetic field strength (BT) of the daily variation on the third day, however, is comparable to that on the first day. Unlike electron flux decreases, the flux of protons with energies between 0.8 and 4 MeV adiabatically responses to the daily variation of BT. That is, there is no significant decrease of the proton flux when the electron flux decreases. During the interval of quiet geomagnetic conditions, well-defined electromagnetic ion cyclotron (EMIC) waves were detected at geosynchronous spacecraft. Low-altitude polar-orbiting spacecraft observed the precipitation of energetic protons and relativistic electrons in the interval of EMIC waves enhancement. From these observations, we suggest that the EMIC waves at geosynchronous orbit cause pitch angle scattering and relativistic electron losses to the atmosphere under quiet geomagnetic conditions.

Spectral characteristics of steady quiet-time EMIC waves observed at geosynchronous orbit

We have studied the spectral properties of quiet-time electromagnetic ion cyclotron (EMIC) waves following a steady quiet condition, which is defined with Kp values 1 during 12h, using GOES 10, 11, and 12 magnetometer data for solar minimum years 2007-2008. We identified 6584 steady quiet-time EMIC wave samples using a semiautomated procedure. Approximately 82% of the samples were observed in the morning-to-early afternoon sector (0700-1500 magnetic local time) with a maximum occurrence near noon, and their peak frequencies were mostly in the He band. We found that the occurrence rate of steady quiet-time EMIC waves is higher than that of EMIC waves for all or quiet geomagnetic conditions (Dst > 0nT or AE < 100nT) reported in previous studies by a factor of 2 or more. The frequency ratio f(peak) (samples peak frequency)/ fH+ (the local proton gyrofrequency) of the He-band waves (approximate to 0.11-0.16) under steady quiet conditions is lower than that (approximate to 0.14-0.24) in previous studies. These results may be due to the fact that the plasmasphere expanded more frequently to the geosynchronous region under extremely quiet geomagnetic conditions in 2007-2008 than the periods selected in previous studies. The amplitude and frequency of He-band EMIC waves for nonlinear wave growth are examined as changing cold plasma density at geosynchronous orbit. We confirm that the spectral properties of observed EMIC waves are in good agreement with the nonlinear theory.

Statistical analysis of geosynchronous magnetic field perturbations near midnight during sudden commencements

When an interplanetary (IP) shock passes over the Earths 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 (+ΔBT) in the magnetic field strength. (2) In winter, however, ΔBT exhibits a positive (+ΔBT) or negative (–ΔBT) enhancement. (3) In summer, the midnight geosynchronous SC perturbations in the BH component (positive northward) in VDH coordinates are mostly (∼88%, 35 out of 40 events) positive (+ΔBH), while the occurrence rate of the positive perturbation (∼43%) in the Bz component (positive northward) in GSM coordinates is lower than that of the negative perturbation (∼57%). (4) In winter, the negative perturbations in ΔBH (∼61%) and ΔBz (∼74%) are dominant. (5) Both the north-south components (BH and Bz) in spring and fall are scattered around zero. To explain the observations, we suggest that SC-associated cross-tail current (JSC) 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 ΔBH, ΔBz, and ΔBT is due to the seasonal variation of the spacecraft position relative to JSC.

Occurrence of EMIC waves and plasmaspheric plasmas derived from THEMIS observations in the outer magnetosphere: Revisit

We have statistically studied the relationship between electromagnetic ion cyclotron (EMIC) waves and cold plasmaspheric plasma (Nsp) in the L range of 6-12 using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data for 2008-2011. The important observational results are as follows: (1) Under quiet geomagnetic conditions (Kp≤ 1), the maximum occurrence rate of the hydrogen (H) band EMIC waves appears in the early morning sector (0600-0900 MLT) at the outermost region (L = 10-12). (2) Under moderate and disturbed conditions (Kp≥ 2), the H-band occurrence rate is higher in the morning-to-early afternoon sector for L> 10. (3) The high occurrence region of helium (He) band waves for Kp≤ 1 varies from L = 7 to 12 in radial distances along the local time (i.e., at L∼ 7 near noon and at L = 8-12 near late afternoon). (4) The He-band waves for Kp≥ 2 are mainly localized between 1200 and 1800 MLT with a peak around 1500-1600 MLT at L = 8-10. (5) Nsp is much higher for the He-band intervals than for the H-band intervals by a factor of 10 or more. The He-band high occurrence appears at a steep Nsp gradient region. (6) The morning-afternoon asymmetry of the normalized frequency seen both in H-band and He-band is similar to the asymmetric distribution of Nsp along the local time. These observations indicate that the cold plasma density plays a significant role in determining the spectral properties of EMIC waves. We discuss whether a morning-afternoon asymmetry of the EMIC wave properties can be explained by the spatial distribution of cold plasmaspheric plasma.

Statistical Analysis of Low-latitude Pi2 Pulsations Observed at Bohyun Station in Korea

We statistically investigated the properties of low-latitude Pi2 pulsations using Bohyun (BOH, Mlat

SC‐Associated Electric Field Variations in the Magnetosphere and Ionospheric Convective Flows

We examine magnetic and electric field perturbations associated with a sudden commencement (SC), caused by an interplanetary (IP) shock passing over the Earths magnetosphere on 16 February 2013. The SC was identified in the magnetic and electric field data measured at THEMIS-E (THE-E: MLT = 12.4, L = 6.3), Van Allen Probe-A (VAP-A: MLT = 3.2, L = 5.1), and Van Allen Probe-B (VAP-B: MLT = 0.2. L= 4.9) in the magnetosphere. During the SC interval, THE-E observed a dawnward-then-duskward electric (E) field perturbation around noon, while VAP-B observed a duskward E-field perturbation around midnight. VAP-A observed a dawnward-then-duskward E-field perturbation in the postmidnight sector, but the duration and magnitude of the dawnward E-perturbation are much shorter and weaker than that at THE-E. That is, the E-field signature changes with local time during the SC interval. The SuperDARN radar data indicate that the ionospheric plasma motions during the SC are mainly due to the E-field variations observed in space. This indicates that the SC-associated E-field in space plays a significant role in determining the dynamic variations of the ionospheric convection flow. By comparing previous SC MHD simulations and our observations, we suggest that the E-field variations observed at the spacecraft are produced by magnetospheric convection flows due to deformation of the magnetosphere as the IP shock sweeps the magnetopause.

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 [2014] (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