T. R. Sun

Case study of nightside magnetospheric magnetic field response to interplanetary shocks

[1] Observations show that the geosynchronous magnetic field in midnight sector sometimes decreases when an interplanetary (IP) fast forward shock (FFS) passes Earth, even though the magnetosphere is always compressed. We perform case studies of the response observed by the GOES spacecraft at geosynchronous orbit near midnight to two IP shocks passing Earth. One shock produces a decrease in B z (a negative response) and the other an increase in B z (a positive response). A global 3D MHD code is run to reproduce the responses at geosynchronous orbit, and to further provide information on the initiation and development of B z variations in the entire magnetosphere. The model reveals that when a FFS sweeps over the magnetosphere, there exist mainly two regions, a positive response region caused by the compressive effect of the shock and a negative response region which is probably associated with the temporary enhancement of earthward convection in the nightside magnetosphere. The spacecraft may observe an increase or decrease of the magnetic field depending on which region it is in. The numerical results reproduce the main characters of the geosynchronous magnetic field response to IP shocks for these two typical cases.

Nightside geosynchronous magnetic field response to interplanetary shocks: Model results

Inspired by the fact that spacecraft at geosynchronous orbit may observe an increase or decrease in the magnetic field in the midnight sector caused by interplanetary fast forward shocks (FFS), we perform global MHD simulations of the nightside magnetospheric magnetic field response to interplanetary (IP) shocks. The model reveals that when a FFS sweeps over the magnetosphere, there exist mainly two regions: a positive response region caused by the compressive effect of the shock and a negative response region which is probably associated with the temporary enhancement of earthward convection in the nightside magnetosphere. IP shocks with larger upstream dynamic pressures have a higher probability of producing a decrease in B(z) that can be observed in the midnight sector at geosynchronous orbit, and other solar wind parameters such as the interplanetary magnetic field (IMF) B(z) and IP shock speed do not seem to increase this probability. Nevertheless, the southward IMF B(z) leads to a stronger and larger negative response region, and a higher IP shock speed results in stronger negative and positive response regions. Finally, a statistical survey of nightside geosynchronous B(z) response to IP shocks between 1998 and 2005 is conducted to examine these model predictions.

Effects of the interplanetary magnetic field on the twisting of the magnetotail: Global MHD results

We used the global magnetohydrodynamic (MHD) simulation to investigate effects of the interplanetary magnetic field (IMF) on the twisting of the magnetotail. It is shown that the cross section of the magnetotail is elongated along a certain direction close to the IMF orientation. The elongated direction twists with the IMF orientation, magnitude, and the distance away from Earth, and the quantitative relationship has been given. In addition, the current sheet has a similar twisting behavior as the magnetotail magnetopause, with a smaller twisting angle. Our simulated results fall within the range that people have deduced from observations.

The chain response of the magnetospheric and ground magnetic field to interplanetary shocks

In response to interplanetary (IP) shocks, magnetic field may decrease/increase (negative/positive response) in nightside magnetosphere, while at high latitudes on the ground it has two-phase bipolar variations: preliminary impulse and main impulse (MI). Using global MHD simulations, we investigate the linkage between the MI phase variation on the ground and the magnetospheric negative response to an IP shock. It is revealed that although the two phenomena occur at largely separated locations, they are physically related and form a response chain. The velocity disturbances near the flanks of the magnetopause cause the magnetic field to decrease, resulting in a dynamo which thus powers the transient field-aligned currents (FACs). These FACs further generate a pair of ionospheric current vortex, leading to MI variations on the ground. Therefore, we report here the intrinsic physically related chain response of the magnetospheric and ground magnetic field to IP shocks, and thus link the magnetospheric sudden impulse (SI) and ground SI together.

Different Bz response regions in the nightside magnetosphere after the arrival of an interplanetary shock: Multipoint observations compared with MHD simulations

We present different magnetic field changes in the nightside magnetosphere in response to the interplanetary (IP) shock on 17 December 2007, using multiple spacecraft observations and global MHD simulations. The coexistence of two distinct B-z response regions in the nightside magnetosphere in a single event has been observationally identified for the first time. From the inner magnetosphere to the tail, they are the positive response (B-z increase) and the negative response (B-z decrease). This scenario reasonably agrees with the MHD model prediction. Moreover, the analysis of the response delay time shows that, for the three satellites which observed the negative responses of B-z, the one closest to Earth was the last to respond. This phenomenon can also be understood based on the model prediction that the negative response region develops toward Earth after its formation. In addition, the temporarily enhanced earthward flows in the negative response region, which were suggested to be responsible for the formation of this region by previous model studies, were also supported by the observation. At last, a global view of the B-z response processes in the nightside magnetosphere is presented based on MHD simulations.

Risk assessment of the extreme interplanetary shock of 23 July 2012 on low‐latitude power networks

Geomagnetic sudden commencements (SCs), characterized by a rapid enhancement in the rate of change of the geomagnetic field perturbation (dB/dt), are considered to be an important source of large geomagnetically induced currents (GICs) in middle- and low-latitude power grids. In this study, the extreme interplanetary shock of 23 July 2012 is simulated under the assumption that it had hit the Earth with the result indicating the shock-caused SC would be 123nT. Based on statistics, the occurrence frequency of SCs with amplitudes larger than the simulated one is estimated to be approximately 0.2% during the past 147 years on the Earth. During this extreme event, the simulation indicates that dB/dt, which is usually used as a proxy for GICs, at a dayside low-latitude substation would exceed 100nT/min; this is very large for low-latitude regions. We then assess the GIC threat level based on the simulated geomagnetic perturbations by using the method proposed by Marshall et al. (2011). The results indicate that the risk remains at low level for the low-latitude power network on a global perspective. However, the GIC risk may reach moderate or even high levels for some equatorial power networks due to the influence of the equatorial electrojet. Results of this study feature substantial implications for risk management, planning, and design of low-latitude electric power networks.

X-ray imaging of Kelvin-Helmholtz waves at the magnetopause

This paper simulates the Kelvin-Helmholtz wave (KHW)-induced X-ray emissions at the low-latitude magnetopause based on a global MHD code. A method is proposed to extract the KHW information from the X-ray intensity measured by a hypothetical X-ray telescope onboard a satellite assumed with a low Earth orbit. Specifically, the X-ray intensity at high latitude is subtracted from the intensity map as a background to highlight the role of KHW. Using this method, global features of KHW such as the vortex velocity, perturbation degree, spatial distribution, and temporal evolution could be evaluated from the X-ray intensity map. The validity of this method during intervals of solar wind disturbances is also verified. According to the simulation results, X-ray imaging of KHW is suggested as a promising observation technique to essentially see the large-scale configuration and evolution of KHW for the first time.

Shock waves standing in the middle‐ and high‐latitude magnetosheath from global MHD simulations

Standing shock waves (SSWs) are found to exist in the middle-and high-latitude magnetosheath through the global magnetohydrodynamic simulations. There are two (or one) SSWs for constant northward (or southward) interplanetary magnetic field (IMF); they extend into the magnetosheath region and further interact with the bow shock. Because of the extension of SSWs into the interplanetary space, especially when IMF turns southward, an indented bow shock emerges in front of the magnetosphere. The SSWs are excited by the indentations of the magnetopause in the supermagnetosonic solar wind flows in the magnetosheath; for northward IMF, one of the indentations is located in the cusp region and the other corresponds to the neutral point in the tailward of the cusp; for southward IMF, the indentation simply locates in the cusp region. We examine the Rankine-Hugoniot relations across the shock fronts and find the numerical model results are consistent with theoretical predictions.

Temporal and spatial variations in sand and dust storm events in East Asia from 2007 to 2016: Relationships with surface conditions and climate change

We analyzed the frequency and intensity of sand and dust storms (SDSs) in East Asia from 2007 to 2016 using observational data from ground stations, numerical modeling, and vegetation indices obtained from both satellite and reanalysis data. The relationships of SDSs with surface conditions and the synoptic circulation pattern were also analyzed. The statistical analyses demonstrated that the number and intensity of SDS events recorded in spring during 2007 to 2016 showed a decreasing trend. The total number of spring SDSs decreased from at least ten events per year before 2011 to less than ten events per year after 2011. The overall average annual variation of the surface dust concentration in the main dust source regions decreased 33.24μg/m3 (-1.75%) annually. The variation in the temperatures near and below the ground surface and the amount of precipitation and soil moisture all favored an improvement in vegetation coverage, which reduced the intensity and frequency of SDSs. The strong winds accompanying the influx of cold air from high latitudes showed a decreasing trend, leading to a decrease in the number of SDSs and playing a key role in the decadal decrease of SDSs. The decrease in the intensity of the polar vortex during study period was closely related to the decrease in the intensity and frequency of SDSs.

ENERGY DISSIPATION PROCESSES IN SOLAR WIND TURBULENCE

Turbulence is a chaotic flow regime filled by irregular flows. The dissipation of turbulence is a fundamental problem in the realm of physics. Theoretically, dissipation cannot be ultimately achieved without collisions, and so how turbulent kinetic energy is dissipated in the nearly collisionless solar wind is a challenging problem. Wave particle interactions and magnetic reconnection are two possible dissipation mechanisms, but which mechanism dominates is still a controversial topic. Here we analyze the dissipation region scaling around a solar wind magnetic reconnection region. We find that the magnetic reconnection region shows a unique multifractal scaling in the dissipation range, while the ambient solar wind turbulence reveals a monofractal dissipation process for most of the time. These results provide the first observational evidences for the intermittent multifractal dissipation region scaling around a magnetic reconnection site, and they also have significant implications for the fundamental energy dissipation process.