B. B. Tang

Properties of Kelvin‐Helmholtz waves at the magnetopause under northward interplanetary magnetic field: Statistical study

We search the plasma and magnetic field data of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes B and C during 2008 and 2009 for observation evidences of the Kelvin-Helmholtz instability (KHI). Fourteen KHI events with rolled-up vortices are identified under the northward interplanetary magnetic field (IMF) at the low-latitude boundary layer (LLBL). We collect another 42 events reported from the observations of the Geotail, Double Star TC-1, and Cluster for a statistical study of the KH wave properties. All the 56 rolled-up KH wave events are quantitatively characterized by the dominant period, phase velocity, and the wavelength. We further explore the relationship between the KH wave period and the solar wind velocity (VSW) and the IMF clock angle. It is found that the KH period tends to be shorter under a higher VSW, and longer with a larger IMF clock angle. The spatial distribution of the KH wavelength shows a longitudinal growth with increasing distance from the subsolar point along the flank magnetopause. The statistical results provide new insights for the development of KH waves and their connection with the interplanetary conditions and deepen our understanding of the KHI at the magnetopause.

The magnetosphere under the radial interplanetary magnetic field: A numerical study

We investigate the magnetosphere under radial interplanetary magnetic fields (IMF) by using global magnetohydrodynamic simulations. The magnetosphere-ionosphere system falls into an unexpected state under this specific IMF orientation when the solar wind electric field vanishes. The most important features that characterize this state include (1) magnetic reconnections can still occur, which take place at the equatorward of the cusp in one hemisphere, the tailward of the cusp in the other hemisphere, and also in the plasma sheet; (2) significant north-south asymmetry exists in both magnetosphere and ionosphere; (3) the polar ionosphere mainly presents a weak two-cell convection pattern, with the polar cap potential valued at approximate to 30 kV; (4) the whole magnetosphere-ionosphere system stays in a very quiet state, and the AL index does not exceed -70 nT; and (5) the Kelvin-Helmholtz instability can still be excited at both flanks of the magnetosphere. These results imply the controlling role of the IMF direction between the solar wind and magnetosphere interactions and improve our understanding of the solar wind-magnetosphere-ionosphere system.

Kinetic evidence of magnetic reconnection due to Kelvin-Helmholtz waves

The Kelvin-Helmholtz (KH) instability at the Earths magnetopause is predominantly excited during northward interplanetary magnetic field (IMF). Magnetic reconnection due to KH waves has been suggested as one of the mechanisms to transfer solar wind plasma into the magnetosphere. We investigate KH waves observed at the magnetopause by the Magnetospheric Multiscale (MMS) mission; in particular, we study the trailing edges of KH waves with Alfvenic ion jets. We observe gradual mixing of magnetospheric and magnetosheath ions at the boundary layer. The magnetospheric electrons with energy up to 80 keV are observed on the magnetosheath side of the jets, which indicates that they escape into the magnetosheath through reconnected magnetic field lines. At the same time, the low-energy (below 100 eV) magnetosheath electrons enter the magnetosphere and are heated in the field-aligned direction at the high-density edge of the jets. Our observations provide unambiguous kinetic evidence for ongoing reconnection due to KH waves.

Modeling geomagnetically induced electric field and currents by combining a global MHD model with a local one-dimensional method

Geomagnetically induced currents (GIC) flowing in long conductor systems on the ground are a well-known space weather hazard. We develop a new approach to simulating GICs by combining a global MHD model with a local one-dimensional method. As an example, we apply this approach to model the GIC at the Pirttikoski 400 kV transformer of the Finnish high-voltage power system during the space weather event of 22-23 September 1999. The modeled results can capture the main observational features, and the model performances is better than two GIC persistence models, which demonstrates this promising new approach in GIC forecasting.

Comparison of equivalent current systems for the substorm event of 8 March 2008 derived from the global PPMLR-MHD model and the KRM algorithm

Geomagnetic perturbation is an important aspect to determine the capability of a 3-D MHD model in predicting space weather. Taking the substorm event of 8 March 2008 as an example, we compare the equivalent current systems (ECS) in the ionosphere derived from the global PPMLR-MHD simulation model and the ground-based magnetic field observations using the KRM inversion algorithm. The evolution of ECS is utilized to give a global view of the temporal and spatial development of the magnetic fields on the ground. The PPMLR-MHD model has generally reproduced the main characters of the large-scale magnetic field variation on the ground. The magnetic latitude and local time distribution of the ECS is in reasonably agreement with the inversion results during the disturbed period. We hopefully consider the ECS to be a promising numerical forecast product of the global geomagnetic variation from an global 3-D MHD model in the future.

Bow shock and magnetopause contributions to the magnetospheric current system: Hints from the Cluster observations

We perform a statistical study of the surface current at the high-latitude magnetopause (HLMP) and the bow shock under different southward interplanetary magnetic field (IMF) conditions, taking advantage of the crossing events of these discontinuities by the Cluster spacecraft. With an enhancement of southward IMF B-Z, the surface current at HLMP reduces, while increasing at the bow shock. Since the amount of the magnetospheric current increases with the increase of southward IMF B-Z, a synthesis analysis based on the Cluster observations suggests the bow shock and HLMP together contribute to the magnetospheric current system, and the bow shock would become an important current generator when the southward IMF B-Z becoming large. This scenario accords with the previous global magnetohydrodynamic (MHD) simulations.

Magnetospheric vortices and their global effect after a solar wind dynamic pressure decrease

Using multipoint data from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites, we report a magnetospheric flow vortex driven by a negative solar wind dynamic pressure pulse. The observed vortex rotated in a direction opposite to that associated with positive solar wind dynamic pressure pulses. The vortex was moving tailward, as confirmed by a global magnetohydrodynamics (MHD) simulation. In addition, the equivalent ionospheric currents (EICs) deduced from ground magnetometer station data reveal that a current vortex in the ionosphere near the foot point of the satellites has a rotation sense consistent with that observed in the magnetosphere. The field-aligned current (FAC) density estimated from three THEMIS satellites is about 0.15nA/m(2), and the total FAC of the vortex is about 1.5-3x10(5)A, on the order of the total FAC in a pseudobreakup, but less than the total FAC in most moderate substorms, 10(6)A. Key Points

Bow shock and magnetopause contributions to the cross-tail current from global MHD simulations

We performed a series of global MHD simulations to study the closure of the cross-tail current in the magnetotail under different solar wind conditions. The cross-tail current closes totally within the magnetopause, forming the classical theta structure when IMF is set to be zero. The situation changes for southward IMF cases: part of the cross-tail current passes through the magnetosheath and closes across the bow shock, forming an overlapped q structure of the cross-tail current, when viewed from the Sun. Quantitative analysis shows that a larger strength of the southward IMF B-Z results in a higher percentage of current closed through the bow shock. Nearly a constant quantity of the cross-tail current comes from the bow shock despite variations in the solar wind speed. An increase in the ionospheric Pedersen conductance leads to an increase in the bow shock contribution but a decrease in the magnetopause contribution to the cross-tail current; therefore, the net cross-tail current is almost independent of the ionospheric conductance. Cross-tail current that closes across the bow shock rather than the magnetopause can be classified as the magnetic reconnection current, providing energy supply for dissipation needed at the magnetic reconnection region in the magnetotail.

Intensification of the Cowling current in the global MHD simulation model

We examine the effects of the ionospheric conductance on the intensification of the westward electrojet current in the ionosphere based on the piecewise parabolic method with a Lagrangian remap (PPMLR) global MHD simulation model. The ionospheric conductance is empirically linked to the plasma pressure in the plasma sheet. The simulation results are consistent with observations: When the Pedersen and Hall conductances are small, the ionospheric current shows a two-cell pattern; when the conductances increase and the ratio Sigma(H)/Sigma(P) >= 2, an intense westward electrojet appears in the midnight sector. This intense westward electrojet is the Cowling current driven by the induced southward electric field due to the blockage of the northward Hall current from closure in the equatorial plasma sheet. The simulation shows the development of the Cowling electrojet is essential to the intensification of the westward electrojet in the ionosphere.

Dayside magnetospheric and ionospheric responses to solar wind pressure increase: Multispacecraft and ground observations

We provide in-situ observations of the transient phenomena in the dayside magnetosphere during the preliminary impulse (PI) and main impulse (MI) event on 30 September 2008. The PI and MI geomagnetic signals are induced by twin traveling convection vortices (TCVs) with opposite polarities in the equivalent ionospheric currents (EICs) due to a sudden increase of the solar wind dynamic pressure. The two PIs associated ionospheric current vortices centered at ~07 magnetic local time(MLT), 67° magnetic latitude (MLAT) in the dawn side and ~14 MLT, 73°MLAT in dusk side, respectively. The dawnside MI current vortex centered at ~68° MLAT and 6 MLT, while the duskside vortex center was traveling poleward from ~67° MLAT to ~75° MLAT at a speed of ~5.6-7.4 km/s around 14 MLT . It is found that both dawn side PI and MI related current vortices were azimuthally seen up to 4 MLT. Following the magnetosphere sudden impulse (SI), clockwise flow vortex with a radial scale larger than 3 Re, associated with positive field-aligned current (FAC) was observed by THEMIS spacecraft in the outer dayside magnetosphere. The flow vortex expanded and traveled tailward in the magnetosphere, also being reproduced with global MHD simulations. Based on both observation and simulation technique, we show that the MI related FACs are correlated with the large scale flow vortex. The PI FACs are partially provided by the mode conversion of fast mode waves into the Alfven waves near the equatorial plane. While, most of it may be generated at a higher latitude region in the magnetosphere.