Khan-Hyuk Kim

Statistical analysis of compressional Pc3–4 pulsations observed by AMPTE CCE at L = 2–3 in the dayside magnetosphere

Few spacecraft observations of Pc3–4 (7–100 mHz) pulsations at L < 3 have been reported, although ground observations of the pulsations in the same L range are routine. In particular, there are no systematic spacecraft observations of compressional pulsations expected at the time of low-latitude ground Pc3–4 pulsations that exhibit a constant frequency over a range of L. These pulsations have been attributed to fast mode waves trapped in the plasmasphere (referred to as plasmaspheric cavity mode waves) or to evanescent waves that exist inward of the turning point for fast mode waves propagating from the outer magnetosphere. We studied the properties of compressional Pc3–4 pulsations at L < 3 using magnetic field data simultaneously acquired by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) and at the Kakioka ground station (L = 1.25) over a 2-year period from 1985 to 1986. From time intervals when the satellite was on the dayside and between L = 2 and L = 3, we identified 63 cases of magnetospheric compressional (bz) Pc3–4 pulsations with or without an accompanying perturbation in the radial (bx) component, depending on the latitude of the satellite. The pulsations were observed primarily when AMPTE CCE was at prenoon hours (magnetic local time (MLT) = 0800–1200), and they were often accompanied by nearly identical magnetic pulsations at Kakioka. In an attempt to construct the spatial mode structure of the compressional pulsations, we have examined the relationship between the bx and bz components and also between the bz (or bx) component and the horizontal H (north–south) component at Kakioka. We find that the phase and amplitude relationship between bx and bz is consistent with waves standing in the meridian plane. The ground-satellite cross phase is clustered near the values expected for radially standing or evanescent waves but also exhibits considerable deviations from the expected values. Although we have demonstrated that compressional Pc3–4 pulsations exist in the low-L magnetosphere and that they give rise to pulsations on the ground, we conclude that further study is required to distinguish between the cavity and evanescent modes.

Global MHD simulation of the geomagnetic sudden commencement on 21 October 1999

[1] Recently, Shinbori et al. (2004) examined the electric field variations associated with geomagnetic sudden commencements (SC) by using data from the Akebono satellite in the inner magnetosphere (L < 5) and reported the following characteristics of the SC-associated electric field variations. (1) The electric field shows a bipolar change. (2) The initial . excursion of the electric field tends to be directed westward. (3) The electric field amplitude does not show a dependence on magnetic local time. By using a global three-dimensional MHD simulation model, we examine how and where such SC-associated electric field variations are established. In our study, we used the SC event that occurred on 21 October 1999, which was caused by a sudden increase in the solar wind dynamic pressure from ∼3 to ∼13 nPa. The solar wind and interplanetary magnetic field conditions observed from the Wind satellite near GSM (x, y, z) ∼ (22, -62, 20) R E are used as the simulation input parameters. The numerical simulation shows that inward flow is first excited near local noon and then flow vortex is generated near the flankside as the solar wind discontinuity is passing over the magnetosphere. Thus, the convection electric field variations change with local time. The vortical structure has a duration of 3-4 min and propagates with a flow speed of ∼75% of the solar wind speed. The electric fields associated with flow vortices show a bipolar structure. We suggest that the flow vortex in our simulation is associated with the main impulse of SC and that the SC-associated electric fields observed at Akebono are due to the convection electric field.

Ground‐satellite coherence analysis of Pc3 pulsations

Although Pc3 magnetic pulsations are commonly observed in the dayside magnetosphere and are believed to propagate into the inner magnetosphere and to the ground, how and where they establish themselves as a regular oscillation are not completely understood. In particular, it is not clear whether the cavity mode plays a significant role in determining the spectral properties of Pc3 pulsations. The mode, in principle, can be identified by multipoint observations of the spatial variation of the amplitude and phase of magnetic field perturbations, with at least one satellite providing evidence of compressional magnetic field oscillation in the magnetosphere. Motivated by these requirements for detection of the cavity mode, we surveyed combined magnetic field data from the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite and the Kakioka ground station. We identified an interval on November 4–5, 1984, during which both the satellite (dipole L = 3–6) and ground station (L = 1.25) were on the morningside (near 8-hour local time) with a small local time separation (within 0.9 hour) and observed Pc3 magnetic pulsations exhibiting a compressional component in space. Unlike previous studies, which used satellites located at L > 6, we observed high coherence in the Pc3 band between the compressional component (bz)in space and the horizontal component (H) on the ground. A cross-phase analysis of the merged bz and H time series indicates that these components oscillated in antiphase over a range of satellite radial distance. We discuss whether the observations can be taken as evidence of a cavity mode oscillation in the magnetosphere.

Effects of ionospheric damping on MHD wave mode structure

We calculate the mode structure of magnetospheric MHD waves on a meridional plane. We have added the effect of ionospheric dissipation to the three-dimensional dipole field MHD model of Lee and Lysak (1999); this model allows a realistic Alfven speed profile for the plasmasphere and realistic boundary conditions at the outer boundaries that vary with respect to local time. Using power spectra and plots of spatial mode structure, we show that the two-dimensional transverse modes on the dipolar meridian are strongly affected by ionospheric damping, but the compressional modes are not. The location of field line resonances spreads wide as the damping increases, but the compressional mode structure remains stable.

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.

Global expansion of the dayside magnetopause for long-duration radial IMF events: Statistical study on GOES observations

For decades, unusual locations of the magnetopause under radial interplanetary magnetic field (IMF) have been reported in many studies using in situ satellite observations. These studies have shown that the magnetopause is expanded either over all magnetic local times (MLTs) on the dayside (global expansion) or just near the noon (bullet-like expansion) during radial IMF conditions. With limited observations near the magnetopause, the type of the magnetopause expansion is still undetermined. In this study, 19 years of dayside geosynchronous magnetic field data obtained from Geostationary Operational Environmental Satellites (GOES) are statistically processed to infer the shape of the dayside magnetopause under radial IMF conditions. The MLT distributions of geosynchronous magnetic fields for radial IMF are compared with those for northward IMF with the correction of solar wind dynamic pressure distributions between two different IMF conditions. After removal of the solar wind dynamic pressure effect, we have found that the geosynchronous magnetic field for radial IMF is smaller than that for northward IMF over almost all MLTs on the dayside regardless of magnetic latitudes and seasons. The difference between geosynchronous magnetic fields for the northward and radial IMFs supports the suggestion that the dayside magnetopause is globally expanded during the radial IMF conditions.

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.

Electron Debye scale Kelvin-Helmholtz instability: Electrostatic particle-in-cell simulations

In this paper, we investigated the electron Debye scale Kelvin-Helmholtz (KH) instability using two-dimensional electrostatic particle-in-cell simulations. We introduced a velocity shear layer with a thickness comparable to the electron Debye length and examined the generation of the KH instability. The KH instability occurs in a similar manner as observed in the KH instabilities in fluid or ion scales producing surface waves and rolled-up vortices. The strength and growth rate of the electron Debye scale KH instability is affected by the structure of the velocity shear layer. The strength depends on the magnitude of the velocity and the growth rate on the velocity gradient of the shear layer. However, the development of the electron Debye scale KH instability is mainly determined by the electric field generated by charge separation. Significant mixing of electrons occurs across the shear layer, and a fraction of electrons can penetrate deeply into the opposite side fairly far from the vortices across the shear layer.

Detailed study of the Mare Crisium northern magnetic anomaly

Low-altitude Lunar Prospector Magnetometer (LP-MAG) data for Mare Crisium show two magnetic anomalies near its inner northern and southern edges. Because these features are located inside a basin, they were likely formed by slow cooling of the basins melt, or the partially melted mantle, instead of by any impact-related shock magnetization process. Therefore, they are important for assessing the nature of the ancient dynamo field that produced them. In this study we confine our attention to the simpler northern anomaly (CNA) and use low altitude (∼ 22 km) LP data to model its source body as a dipole and magnetized disks of different radii. We infer that the source is likely located ∼20-30 km from the surface and horizontally localized within a small region (<1∘ or <∼30 km radius). The surface field intensity calculated from the best-fit dipole is in good agreement with that obtained from LP Electron Reflectometer (LP-ER) data. Our magnetization directions are substantially different from two previous studies, largely due to using lower altitude data to perform our inversions. We also find a surprising sensitivity to small changes in source body latitude (∼1∘). The magnetic paleopoles implied by our best-fit models are distant from previous estimates by up to ∼50∘ of great circle arc, and are substantially distant from the Moons present rotation axis. Our results demonstrate how multiple altitude datasets must be used when estimating paleopoles and other properties of even the simplest of the Moons magnetic anomalies.

Dependence of Electromagnetic Ion Cyclotron Wave Occurrence on North‐South Orientation of Interplanetary Magnetic Field: THEMIS Observations

We investigate the L-MLT (i.e., L-shall versus magnetic local time) distributions of electromagnetic ion cyclotron (EMIC) waves, using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data from 2009 to 2014, under prolonged (more than four hours) intervals of northward and southward interplanetary magnetic fields (IMFs). H-band EMIC waves show high occurrence in the dawn-to-postnoon sector (0400-1600 MLT) at L > ~7 for northward IMFs. On the other hand, for southward IMFs H-band EMIC waves occur in the morning-to-afternoon sector (0700-1500 MLT), and no strong wave activity is found in the dawn sector (0400-0700 MLT). H-band EMIC waves are frequently observed in the region where the ambient temperature anisotropy for the energetic ions (~1-25 keV) is high. He-band EMIC waves show high occurrence in the noon-to-dusk sector (1100-2000 MLT) for both IMF BZ conditions. In the dusk sector, however, the high occurrence region is shifted into the inner L-shell for southward IMFs (L < 8) in contrast to that observed for northward IMFs (L < 10). He-band EMIC waves are shown to be generated in the region where the total plasma density is high (especially near the plasmapause). The occurrence rates of EMIC waves are lower overall for southward IMFs than for northward IMFs for both H-band and He-band waves. We suggest that an increase of heavy ion (i.e., He+ and O+) concentration is likely associated with low occurrence rates in both frequency bands for southward IMFs.