Y. S. Ge

Dipolarization fronts as earthward propagating flux ropes: A three‐dimensional global hybrid simulation

Dipolarization fronts (DFs) as earthward propagating flux ropes (FRs) in the Earths magnetotail are presented and investigated with a three-dimensional (3-D) global hybrid simulation for the first time. In the simulation, several small-scale earthward propagating FRs are found to be formed by multiple X line reconnection in the near tail. During their earthward propagation, the magnetic field Bz of the FRs becomes highly asymmetric due to the imbalance of the reconnection rates between the multiple X lines. At the later stage, when the FRs approach the near-Earth dipole-like region, the antireconnection between the southward/negative Bz of the FRs and the northward geomagnetic field leads to the erosion of the southward magnetic flux of the FRs, which further aggravates the Bz asymmetry. Eventually, the FRs merge into the near-Earth region through the antireconnection. These earthward propagating FRs can fully reproduce the observational features of the DFs, e.g., a sharp enhancement of Bz preceded by a smaller amplitude Bz dip, an earthward flow enhancement, the presence of the electric field components in the normal and dawn-dusk directions, and ion energization. Our results show that the earthward propagating FRs can be used to explain the DFs observed in the magnetotail. The thickness of the DFs is on the order of several ion inertial lengths, and the electric field normal to the front is found to be dominated by the Hall physics. During the earthward propagation from the near-tail to the near-Earth region, the speed of the FR/DFs increases from ~150 km/s to ~1000 km/s. The FR/DFs can be tilted in the GSM (x, y) plane with respect to the y (dawn-dusk) axis and only extend several Earth radii in this direction. Moreover, the structure and evolution of the FRs/DFs are nonuniform in the dawn-dusk direction, which indicates that the DFs are essentially 3-D.

A statistical analysis of Pi2‐band waves in the plasma sheet and their relation to magnetospheric drivers

We use the Cluster data from 2001 to 2009 to investigate the occurrence of Pi2-band waves in the plasma sheet. To study the generation mechanisms of these waves, we examine the association between Pi2-band waves and dynamic processes in the plasma sheet (fast flows and substorm activity) and the direction of the solar wind velocity. For a total of 80 large-amplitude Pi2-band waves in the plasma sheet, we find that Cluster records fast flows during 62 events, 11 waves without fast flows occur during substorm time, 3 events occur when the solar wind velocity significantly changes its direction, and 4 events are not associated with any of the above activities. Most of the observed Pi2-band waves are predominantly compressional, while 2 events are transverse. Based on this statistical study, we suggest that fast flows maybe the main driver of Pi2-band waves/oscillations in the plasma sheet, especially considering that most of these waves are compressional. The relatively small number of other events indicates that other mechanisms also play a role in creating Pi2-band waves/oscillations in the plasma sheet but are relatively rare. In all wave events of this study, the plasma pressure and magnetic pressure vary in antiphase, suggesting that these waves have the slow-mode feature.

Auroral Signatures of Ballooning Mode Near Substorm Onset: Open Geospace General Circulation Model Simulations

Auroral Phenomen Other Planets Geophysical Mon

Spatial distribution of magnetic fluctuation power with period 40 to 600 s in the magnetosphere observed by THEMIS

Ultralow frequency (ULF) fluctuations are ubiquitous in the magnetosphere and have significant influence on the energetic particle transport. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data to give the spatial distribution of the Pi2/Pc4 and Pc5 band magnetic fluctuation amplitude near the magnetic equator in the magnetosphere. Statistical results can be summarized as follows: (1) strong ULF fluctuations are common in the magnetotail plasma sheet; the amplitude of all three components of magnetic fluctuations decreases with decreasing radial distance; (2) during periods of high AE index, fluctuations can propagate toward the Earth as far as the data cutoff in the nightside of the magnetosphere, and the amplitude of magnetic fluctuations is clearly stronger near the dusk sector of the synchronous orbit than that near the dawn sector, suggesting that the substorm particle injection has significant contribution to these fluctuations; (3) intense compressional Pc5 band magnetic fluctuations are a persistent feature near two flanks of the magnetosphere. Clear peaks of the compressional Pi2/Pc4 band magnetic fluctuation power near two flanks can be found during periods of fast solar wind, while the power of compressional Pi2/Pc4 band fluctuations is weak when the solar wind is slow. (4) Solar wind dynamic pressure and its variations can globally affect the ULF fluctuation power in the magnetosphere. Magnetic fluctuations near the noonside can penetrate from the magnetopause to the synchronous orbit or inner when solar wind pressure variations are large.

Modeling the interaction between the solar wind and Saturn's magnetosphere by the AMR-CESE-MHD method

In this paper, the space-time conservation element and solution element (CESE) method in general curvilinear coordinates is successfully applied to the three-dimensional magnetohydrodynamic (MHD) simulations of the interaction between the solar wind and Saturns magnetosphere on a six-component grid system. As a new numerical model modified for the study of the interaction between the solar wind and Saturns magnetosphere, we obtain the large-scale configurations of Saturns magnetosphere under the steady solar wind with due southward interplanetary magnetic field (IMF) conditions. The numerical results clearly indicate that the global structure of Saturns magnetosphere is strongly affected by the rotation of Saturn as well as by the solar wind. The subsolar standoff distances of the magnetopause and the bow shock in our model are consistent with those predicted by the data-based empirical models. Our MHD results also show that a plasmoid forms in the magnetotail under the effect of the fast planetary rotation. However, somewhat differently from the previous models, we find that there are two flow vortices generated on the duskside under due southward IMF at Saturn. On the duskside, the clockwise one closer to the planet is excited by the velocity shear between the rotational flows and the sunward flows, while the anticlockwise one is generated from the velocity shear between the tailward flows along the magnetopause and the sunward flows.

IMF dependence of energetic oxygen and hydrogen ion distributions in the near‐Earth magnetosphere

Energetic ion distributions in the near-Earth plasma sheet can provide important information for understanding the entry of ions into the magnetosphere and their transportation, acceleration, and losses in the near-Earth region. In this study, 11 years of energetic proton and oxygen observations (> similar to 274 keV) from Cluster/Research with Adaptive Particle Imaging Detectors were used to statistically study the energetic ion distributions in the near-Earth region. The dawn-dusk asymmetries of the distributions in three different regions (dayside magnetosphere, near-Earth nightside plasma sheet, and tail plasma sheet) are examined in Northern and Southern Hemispheres. The results show that the energetic ion distributions are influenced by the dawn-dusk interplanetary magnetic field (IMF) direction. The enhancement of ion intensity largely correlates with the location of the magnetic reconnection at the magnetopause. The results imply that substorm-related acceleration processes in the magnetotail are not the only source of energetic ions in the dayside and the near-Earth magnetosphere. Energetic ions delivered through reconnection at the magnetopause significantly affect the energetic ion population in the magnetosphere. We also believe that the influence of the dawn-dusk IMF direction should not be neglected in models of the particle population in the magnetosphere.

Radiation belt electron scattering by whistler‐mode chorus in the Jovian magnetosphere: Importance of ambient and wave parameters

Whistler‐mode chorus waves are regarded as an important acceleration mechanism contributing to the formation of relativistic and ultra‐relativistic electrons in the Jovian radiation belts. Quantitative determination of the chorus wave driven electron scattering effect in the Jovian magnetosphere requires detailed information of both ambient magnetic field and plasma density and wave spectral property, which however cannot be always readily acquired from observations of existed missions to Jupiter. We therefore perform a comprehensive analysis of the sensitivity of chorus induced electron scattering rates to ambient magnetospheric and wave parameters in the Jovian radiation belts to elaborate to which extent the diffusion coefficients depend on a number of key input parameters. It is found that quasi‐linear electron scattering rates by chorus can be strongly affected by the ambient magnetic field intensity, the wave latitudinal coverage, and the peak frequency and bandwidth of the wave spectral distribution in the Jovian magnetosphere, while they only rely slightly on the background plasma density profile and the peak wave normal angle, especially when the wave emissions are confined at lower latitudes. Given the chorus wave amplitude, chorus induced electron scattering rates strongly depend on Jovian L‐shell to exhibit a tendency approximately proportional to LJ3. Our comprehensive analysis explicitly demonstrates the importance of reliable information of both the ambient magnetospheric state and wave distribution property to understanding the dynamic electron evolution in the Jovian radiation belts and therefore has implications for future mission planning to explore the extreme particle radiation environment of Jupiter and its satellites.

The distribution of oscillation frequency of magnetic field and plasma parameters in BBFs: THEMIS statistics

Bursty Bulk Flows (BBFs) have been correlated with Pi2 pulsations and damping oscillations of plasma velocity in many investigations. But the oscillation time scales in BBFs is still an open question. The purpose of this study is to statistically study the oscillated frequency distribution of magnetic field and plasma parameters inside BBFs. The data are obtained by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes during the period of 2008 to 2011. For 424 selected BBF events, we use the wavelet spectrum analysis to select the main, second and third period components of the magnetic field and plasma parameters according to their largest, second and third-largest values of the wavelet power spectral density, respectively. The power spectra show repeated information in the form of multiple peaks or oscillations. The quasi-Gaussian distribution is a good model for the occurrences of the main and secondary periods. The most probable main periods of the magnetic field and plasma parameters are between 143 s and 160 s, which located in the frequency band of Pi2 and Pi3 pulsations. The second and third period ranges from 63 s to 70 s and from 34 s to 37 s, respectively. Main periods of these parameters change little with the radial distance. We conclude that periods of these parameters are formed at the beginning of BBF history. Although the distribution model cannot give the dynamic processes, it identifies the intrinsic frequencies in oscillations of magnetic field and ion velocity inside of BBFs.

Evolution of Kelvin-Helmholtz instability at Venus in the presence of the parallel magnetic field

Two-dimensional MHD simulation was performed to study the evolution of Kelvin-Helmholtz (KH) instability on Venusian ionopause in response to the strong sheared velocity flow in presence of the in-plane magnetic field parallel to the direction of the flow. The Key result from our simulations is that both of the density increase and the parallel magnetic component on the boundary layer play a role of stabilizing the instability. In the high density ratio cases, the value of final total magnetic energy in the quasi-steady status is much more than that of the initial status, which is quite distinct from that with low density increase. The nonlinear development of case with high density increase and uniform magnetic field is of interest that a single magnetic island forms before the instability saturation. In the non-linear development phase, a new magnetic island arises associated with magnetic reconnection occurring inside the narrow high rolled up density region, combining the pre-existing magnetic island together to form a quasi-steady two island pattern.

Study of particle velocity distribution of plasmas in magnetic reconnection with hybrid simulation

Abstract Based on a 2.5 dimensional hybrid code, we have studied the acceleration of particles during the process of magnetic reconnection. It has been found. that a kind of selective acceleration and heating of ions occurred during the magnetic reconnection, which can cause a few particles to reach comparatively high velocities. In particular, the velocity distribution of particles deviates from the initial Maxwellian distribution to a shell or quasi-shell distribution shape. In addition, the evolution of the distributions in different regions present different characteristics, which may correspond to distinct acceleration mechanisms.