H. Zhang

Mechanism of substorm current wedge formation: THEMIS observations

[1]xa0This paper presents THEMIS measurements of two substorm events to show how the substorm current wedge (SCW) is generated. In the late growth phase when an earthward flow burst in the near-Earth magnetotail brakes and is diverted azimuthally, pressure gradients in the X- and Y-directions are observed to increase in the pileup and diverting regions of the flow. The enhanced pressure gradient in the Y-direction is dawnward (duskward) on the dawnside (duskside) where a clockwise (counter-clockwise) vortex forms. This dawn-dusk pressure gradient drives downward (upward) field-aligned current (FAC) on the dawnside (duskside) of the flow, which, when combined with the FACs generated by the clockwise (counter-clockwise) vortex, forms the SCW. Substorm auroral onset occurs when the vortices appear, Near-Earth dipolarization onset is observed by the THEMIS spacecraft (probes) when a rapid jump in the Y-component of pressure gradient is detected. The total FACs from the vortex and the azimuthal pressure gradient are found to be comparable to the DP-1 current in a typical substorm.

Solar wind entry into the high-latitude terrestrial magnetosphere during geomagnetically quiet times

An understanding of the transport of solar wind plasma into and throughout the terrestrial magnetosphere is crucial to space science and space weather. For non-active periods, there is little agreement on where and how plasma entry into the magnetosphere might occur. Moreover, behaviour in the high-latitude region behind the magnetospheric cusps, for example, the lobes, is poorly understood, partly because of lack of coverage by previous space missions. Here, using Cluster multi-spacecraft data, we report an unexpected discovery of regions of solar wind entry into the Earths high-latitude magnetosphere tailward of the cusps. From statistical observational facts and simulation analysis we suggest that these regions are most likely produced by magnetic reconnection at the high-latitude magnetopause, although other processes, such as impulsive penetration, may not be ruled out entirely. We find that the degree of entry can be significant for solar wind transport into the magnetosphere during such quiet times.

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.

Net terrestrial CO2 exchange over China during 2001–2010 estimated with an ensemble data assimilation system for atmospheric CO2

In this paper we present an estimate of net ecosystem CO2 exchange over China for the years 2001-2010 using the CarbonTracker Data Assimilation System for CO2 (CTDAS). Additional Chinese and Asian CO2 observations are used in CTDAS to improve our estimate. We found that the combined terrestrial ecosystems in China absorbed about -0.33 Pg C yr(-1) during 2001-2010. The uncertainty on Chinese terrestrial carbon exchange estimates as derived from a set of sensitivity experiments suggests a range of -0.29 to -0.64 Pg C yr(-1). This total Chinese terrestrial CO2 sink is attributed to the three major biomes (forests, croplands, and grass/shrublands) with estimated CO2 fluxes of -0.12 Pg C yr(-1) (range from -0.09 to -0.19 Pg C yr(-1)), -0.12 Pg C yr(-1) (range from -0.09 to -0.26 Pg C yr(-1)), and -0.09 Pg C yr(-1) (range from -0.09 to -0.17 Pg C yr(-1)), respectively. The peak-to-peak amplitude of interannual variability of the Chinese terrestrial ecosystem carbon flux is 0.21 Pg C yr(-1) (similar to 64% of mean annual average), with the smallest CO2 sink (-0.19 Pg C yr(-1)) in 2003 and the largest CO2 sink (-0.40 Pg C yr(-1)) in 2007. We stress that our estimate of terrestrial ecosystem CO2 uptake based on inverse modeling strongly depends on a limited number of atmospheric CO2 observations used. More observations in China specifically and in Asia in general are needed to improve the accuracy of terrestrial carbon budgeting for this region.

PROPERTIES OF THE RESISTIVE INSTABILITY IN DOUBLE CURRENT SHEET SYSTEMS WITH STRONG SHEAR FLOWS

Abstract The resistive instability in double current sheet (DCS) systems with shear flows are explored by using a magnetohydrodynamic simulation method. The results showed that the linear growth rate of the antisymmetric and symmetric magnetic reconnection modes depend on the relative vicinity of the two current sheets. When the shear flows are strong and have velocities near or larger than the local Alfven speed, the growth of the antisymmetric mode resistive instability in DCS systems is further increased. It is also found that the growth rate of the resistive instability decreases as the plasma beta increases.

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.

Alfvén wings in the lunar wake: The role of pressure gradients

Strongly conducting or magnetized obstacles in a flowing plasma generate structures called Alfven wings, which mediate momentum transfer between the obstacle and the plasma. Nonconducting obstacles such as airless planetary bodies can generate such structures, which, however, have so far been seen only in sub-Alfvenic regime. A novel statistical analysis of simultaneous measurements made by two ARTEMIS satellites, one in the solar wind upstream of the Moon and one in the downstream wake, and comparison of the data with results of a three-dimensional hybrid model of the interaction reveal that the perturbed plasma downstream of the Moon generates Alfven wings in super-Alfvenic solar wind. In the wake region, magnetic field lines bulge toward the Moon and the plasma flows are significantly perturbed. We use the simulation to show that some of the observed bends of the field result from field-aligned currents. The perturbations in the wake thus arise from a combination of compressional and Alfvenic perturbations. Because of the super-Alfvenic background flow of the solar wind, the two Alfven wings fold back to form a small intersection angle. The currents that form the Alfven wing in the wake are driven by both plasma flow deceleration and a gradient of plasma pressure, positive down the wake from the region just downstream of the Moon. Such Alfven wing structures, caused by pressure gradients in the wake and the resulting plasma slowdown, should exist downstream of any nonconducting body in a super-Alfvenic plasma flow.

Relativistic electron flux dropouts in the outer radiation belt associated with corotating interaction regions

Understanding how the relativistic electron fluxes drop out in the outer radiation belt under different conditions is of great importance. To investigate which mechanisms may affect the dropouts under different solar wind conditions, 1.5–6.0xa0MeV electron flux dropout events associated with 223 corotating interaction regions (CIRs) from 1994 to 2003 are studied using the observations of Solar, Anomalous, Magnetospheric Particle Explorer satellite. According to the superposed epoch analysis, it is found that high solar wind dynamic pressure with the peak median value of about 7xa0nPa is corresponding to the dropout of the median of the radiation belt content (RBC) index to 20% of the level before stream interface arrival, whereas low dynamic pressure with the peak median value of about 3xa0nPa is related to the dropout of the median of RBC index to 40% of the level before stream interface arrival. Furthermore, the influences of Russell-McPherron effect with respect to interplanetary magnetic field orientation on dropouts are considered. It is pointed out that under positive Russell-McPherron effect (+RM effect) condition, the median of RBC index can drop to 23% of the level before stream interface arrival, while for negative Russell-McPherron effect (−RM effect) events, the median of RBC index only drops to 37% of the level before stream interface arrival. From the evolution of phase space density profiles, the effect of +RM on dropouts can be through nonadiabatic loss.

The design of high energy electron detector in FY2 satellite

High energy electron particle detector on FY2 satellite has the capabilities to observe ≥0.2MeV electrons. Based on the high energy particle detector on FY2D satellite, this detector assumes some new design to detect particles more accurately, and it has higher reliability. This paper gives some information about the high energy electron particle detector on FY2.

The design of high energy particle detector in satellites

High energy particle detector is one of the common payloads in Chinese spacecraft. Its purpose is to detect the flux of the high energy particle in space environment. This paper gives some information about the high energy particle detector.