Wenlong Liu

Determination of the substorm initiation region from a major conjunction interval of THEMIS satellites

[1] We investigate in detail the time history of substorm disturbances in the magnetotail observed during a major tail conjunction of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites on 29 January 2008, 0700―0900 UT. During this interval, all THEMIS satellites were closely aligned along the tail axis near midnight and were bracketed in local time by GOES 11 and 12. The radial distance covered ranges from the geosynchronous altitude to ∼30 R E in the tail. This interval consists of three activations detected by the THEMIS satellites with good ground all-sky-camera observations of auroral activity. The first activation is a small substorm with spatially limited disturbance in the tail. The onset arc was equatorward of an undisturbed arc. The second activation is a moderate size substorm with the onset arc also being equatorward of an undisturbed arc. The third activation is an intensification of the substorm with its onset indicated by the second activation. The active auroral arc for this intensification was near the poleward boundary of the auroral oval. Analysis of these observations indicates that the first activation is a small substorm initiated in the near-Earth plasma sheet and does not involve magnetic reconnection of open magnetic field lines. Magnetic reconnection on closed field lines can be ruled out for this substorm because it cannot generate the observed high-speed plasma flow. The second and third activations are part of a moderate size substorm initiated also in the near-Earth plasma sheet, with a subsequent substorm intensification involving activity initiated tailward of ∼30 R E . Overall, the time history of substorm activity for these two substorms is consistent with the near-Earth initiation model.

Whistler‐mode waves inside flux pileup region: Structured or unstructured?

During reconnection, a flux pileup region (FPR) is formed behind a dipolarization front in an outflow jet. Inside the FPR, the magnetic field magnitude and Bz component increase and the whistler-mode waves are observed frequently. As the FPR convects toward the Earth during substorms, it is obstructed by the dipolar geomagnetic field to form a near-Earth FPR. Unlike the structureless emissions inside the tail FPR, we find that the whistler-mode waves inside the near-Earth FPR can exhibit a discrete structure similar to chorus. Both upper band and lower band chorus are observed, with the upper band having a larger propagation angle (and smaller wave amplitude) than the lower band. Most chorus elements we observed are “rising-tone” type, but some are “falling-tone” type. We notice that the rising-tone chorus can evolve into falling-tone chorus within <3 s. One of the factors that may explain why the waves are unstructured inside the tail FPR but become discrete inside the near-Earth FPR is the spatial inhomogeneity of magnetic field: we find that such inhomogeneity is small inside the near-Earth FPR but large inside the tail FPR.

Three‐dimensional lunar wake reconstructed from ARTEMIS data

Data from the two-spacecraft Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun mission to the Moon have been exploited to characterize the lunar wake with unprecedented fidelity. The differences between measurements made by a spacecraft in the solar wind very near the Moon and concurrent measurements made by a second spacecraft in the near lunar wake are small but systematic. They enabled us to establish the perturbations of plasma density, temperature, thermal, magnetic and total pressure, field, and flow downstream of the Moon to distances of 12 lunar radii (R-M). The wake disturbances are initiated immediately behind the Moon by the diamagnetic currents at the lunar terminator. Rarefaction waves propagate outward at fast MHD wave velocities. Beyond similar to 6.5 R-M, all plasma and field parameters are poorly structured which suggests the presence of instabilities excited by counter-streaming particles. Inward flowing plasma accelerated through pressure gradient force and ambipolar electric field compresses the magnetic field and leads to continuous increase in magnitude of magnetic perturbations. Besides the downstream distance, the field perturbation magnitude is also a function of the solar wind ion beta and the angle between the solar wind and the interplanetary magnetic field (IMF). Both ion and electron temperatures increase as a consequence of an energy dispersion effect, whose explanation requires fully kinetic models. Downstream of the Moon, the IMF field lines are observed to bulge toward the Moon, which is unexpected and may be caused by a plasma pressure gradient force or/and the pickup of heavy charged dust grains behind the Moon.

Solar wind pressure pulse‐driven magnetospheric vortices and their global consequences

We report the in situ observation of a plasma vortex induced by a solar wind dynamic pressure enhancement in the nightside plasma sheet using multipoint measurements from Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. The vortex has a scale of 5–10 Re and propagates several Re downtail, expanding while propagating. The features of the vortex are consistent with the prediction of the Sibeck (1990) model, and the vortex can penetrate deep (~8 Re) in the dawn-dusk direction and couple to field line oscillations. Global magnetohydrodynamics simulations are carried out, and it is found that the simulation and observations are consistent with each other. Data from THEMIS ground magnetometer stations indicate a poleward propagating vortex in the ionosphere, with a rotational sense consistent with the existence of the vortex observed in the magnetotail.

Dynamic plasmapause model based on THEMIS measurements

This paper presents a dynamic plasmapause location model established based on 5 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) measurements from 2009 to 2013. In total, 5878 plasmapause crossing events are identified, sufficiently covering all 24 magnetic local time (MLT) sectors. Based on this plasmapause crossing database, we investigate the correlations between plasmapause locations with solar wind parameters and geomagnetic indices. Input parameters for the best fits are obtained for different MLT sectors, and finally, we choose five input parameters to build a plasmapause location model, including 5 min-averaged SYM-H, AL, and AU indices as well as hourly-averaged AE and Kp indices. two out-of-sample comparisons on the evolution of the plasmapause is shown during two magnetic storms, demonstrating good agreement between model results and observations. Two major advantages are achieved by this model. First, this model provides plasmapause locations at 24 MLT sectors, still providing good consistency with observations. Second, this model is able to reproduce dynamic variations of the plasmapause on timescales as short as 5 min.

Identifying magnetic reconnection events using the FOTE method

A magnetic reconnection event detected by Cluster is analyzed using three methods: Single-spacecraft Inference based on Flow-reversal Sequence (SIFS), Multispacecraft Inference based on Timing a Structure (MITS), and the First-Order Taylor Expansion (FOTE). Using the SIFS method, we find that the reconnection structure is an X line; while using the MITS and FOTE methods, we find it is a magnetic island (O line). We compare the efficiency and accuracy of these three methods and find that the most efficient and accurate approach to identify a reconnection event is FOTE. In both the guide and nonguide field reconnection regimes, the FOTE method is equally applicable. This study for the first time demonstrates the capability of FOTE in identifying magnetic reconnection events; it would be useful to the forthcoming Magnetospheric Multiscale (MMS) mission.

Simultaneous field‐aligned currents at Swarm and Cluster satellites

We show for the first time, with direct, multispacecraft calculations of electric current density, and other methods, matched signatures of field-aligned currents (FACs) sampled simultaneously near the ionosphere at low (similar to 500km altitude) orbit and in the magnetosphere at medium (similar to 2.5 R-E altitude) orbits using a particular Swarm and Cluster conjunction. The Cluster signatures are interpreted and ordered through joint mapping of the ground/magnetospheric footprints and estimation of the auroral zone boundaries (taken as indication of the boundaries of Region 1 and Region 2 currents). We find clear evidence of both small-scale and large-scale FACs and clear matching of the behavior and structure of the large-scale currents at both Cluster and Swarm. The methodology is made possible through the joint operations of Cluster and Swarm, which contain, in the first several months of Swarm operations, a number of close three-spacecraft configurations.

Storm time evolution of ELF/VLF waves observed by DEMETER satellite

In this paper, using the data of Sun-synchronous satellite (Detection of Electro-Magnetic Waves Transmitted from Earthquake Regions) DEMETER, we investigated the storm time variations of ELF/VLF waves during the intense coronal mass ejections (CME)-driven storms from 2005 to 2009. The results show that there is a good correlation between the enhancement of ELF/VLF waves and the CME events. Immidately following the enhanced wave activity driven by CMEs at the initial phase, the wave intensity decreases temporarily at the beginning of storm main phase. The strongest waves predominantly occur from the late main phase to early recovery phase. The ELF waves below 3 kHz are significantly intensified during the whole storm time, while the high-frequency waves above 3 kHz seem strengthened predominantly during the late main and early recovery phase. The ELF waves below 3 kHz can exist in a wide L shell range, with the intensity peaking at L ~ 3 and 4. High-frequency waves at f > 9 kHz exist mostly outside the plasmapause. The stronger ELF/VLF waves on the dayside can last longer time than those on the nightside.

Polytropic index of central plasma sheet ions based on MHD Bernoulli integral

This paper uses the data of Cluster from 2001 to 2009 to study the polytropic processes of central plasma sheet (CPS) ions. We first adopt the approach of MHD Bernoulli integral (MBI) to identify homogeneous streamflow tubes (quasi-invariant MBI regions) and then calculate the polytropic index of ions for those streamflow tubes whose outward electromagnetic energy ratios δ  < 0.05. The central plasma sheet is actually a complicated system, which comprises many streamflow tubes with different polytropic relations and the transition layers in between. The polytropic indexes of the CPS ions range from 0.1 to 1.8 and have a quasi-Gaussian distribution. The median polytropic index is 0.93 for AE < 200 nT and 0.91 for AE ≥ 200 nT. Thus, there is no obvious difference between the polytropic indexes of the quiet time and the substorm time CPS ions, which suggests that the thinning and thickening processes of plasma sheet during substorm times do not change obviously the polytropic relation of the CPS ions. The statistical analysis using different δ (δ < 0.05, 0.025, and 0.01) shows that the outward emission of electromagnetic energy is an effective cooling mechanism and can make the polytropic index to decrease and shift toward isobaric. It is inferred that the CPS ions as a whole much likely behave in a way between isobaric and isothermal.

Propagation of small size magnetic holes in the magnetospheric plasma sheet

Magnetic holes (MHs), characteristic structures where the magnetic field magnitude decreases significantly, have been frequently observed in space plasmas. Particularly, small size magnetic holes (SSMHs) which the scale is less than or close to the proton gyroradius are recently detected in the magnetospheric plasma sheet. In this study of Cluster observations, by the timing method, the minimum directional difference (MDD) method, and the spatiotemporal difference (STD) method, we obtain the propagation velocity of SSMHs in the plasma flow frame. Furthermore, based on electron magnetohydrodynamics (EMHD) theory we calculate the velocity, width, and depth of the electron solitary wave and compare it to SSMH observations. The result shows a good accord between the theory and the observation.