T. Xiao

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.

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

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.

Solar Wind Plasma Entry Observed by Cluster in the High-Latitude Magnetospheric Lobes

Using the Cluster data during the period from January to April between 2001 and 2006, we find an observation of solar wind entry due to magnetic reconnection occurred in the terrestrial high-latitude magnetospheric lobes, tailward of the cusps under northward Interplanetary Magnetic Field (IMF). Occurrence rate of solar wind entry events in this study is of the same order as that for the Cluster orbital interval from August to October between the years of 2002 and 2004 as reported by Shi et al [2013]. In this paper, we further study the role of the IMF Bx and By components in the control of solar wind plasmas entry based on the investigations of different magnetic dipole tilt variations between our database and Shi et al. [2013]. This study shows that the asymmetry distribution of solar wind entry events in the northern and southern lobes could be caused by the variation of magnetic dipole tilt, which could influence the locations of the reconnection site on the high latitude lobe magnetopause. On the other hand, IMF Bx can also affect the solar wind plasma entry rate, which is also consistent with previous results. Therefore, we conclude that the “north-south asymmetry” of solar wind entry events in the lobes could be the combined result of magnetic dipole tilt and IMF Bx. In addition, the IMF By component can influence the entry events in conjunction with the variation of IMF Bx component, which is in line with the Parker Spiral of the IMF.

Cluster observations of magnetic holes near the interplanetary current sheets at 1 AU

Linear Magnetic holes (LMHs) are magnetic field decreasing structures with little change in the field direction. Xiao et al. have studied the geometrical shape and occurrence rate of LMHs in the solar wind at 1 AU from the year 2001 to 2004 by using the magnetic field and plasma data provided by Cluster C1 [1]. In this work, we will investigate LMHs near the region of interplanetary current sheet. It is found that about 60% of LMHs accompanied with the small scale current sheets within 10min. This result may help us to study the formation mechanism of the LMHs in the solar wind.

Cluster-C1 observations of non-train magnetic decreases in the solar wind at 1 AU

By using the magnetic field and plasma data of Cluster-C1 from 2001 to 2009, we have investigated the magnetic field and plasma properties around the linear and nonlinear non-train magnetic decreases (MDs) in the solar wind. Compared with the nonlinear non-train MDs, linear non-train MDs occurred in regions with relative low magnetic field strengths and large densities. It is found that the occurrence rate of linear (nonlinear) non-train MDs decreases (increases first, and then decreases) with magnetic field strength increases, and have no significant change (increases first, and then decreases) when the density increased. In addition, the occurrence rates of linear non-train MDs are basically the same from 2001 to 2009 (nearly one solar cycle), however, the occurrence rates of nonlinear non-train MDs have two peaks in 2003 and 2008 respectively.

The magnetotail current sheet movement detected by Cluster

Using Cluster four spacecraft data, we have made a statistical research on the motion properties of the magnetotail current sheets of the year 2004 and 2005 with various methods. We find that most of the current sheets are propagating from the midnight tail region to dawn and dusk flanks which is consistent with previous studies. Most of the current sheets are propagating with the velocities smaller than 100 km/s, and the exceptions account for 6.5 % of all events. We have also found that ‘Enter/Retreat’ current sheets more favorably occur during the southward interplanetary magnetic field (IMF).

Plasma transport processes at the high latitude magnetosphere observed by cluster

Cluster four spacecraft data is used to study the spatial-temporal characteristics in/near the high and mid-altitude cusp under northward IMF. In our previous work a transition layer equatorward of the cusp was observed, which is suggested to be the entry layer during northward IMF. From event study and a further survey tailward of the Cusp, some transport processes of sheath-like ions in the magnetosphere are studied. Possible (dual) lobe reconnection model in the northward IMF condition are applied to explain these observations.