X. C. Shen

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.

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

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.

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.

Dayside Magnetospheric and Ionospheric Responses to a Foreshock Transient on 25 June 2008: 2.2‐D Evolution Based on Dayside Auroral Imaging

The foreshock region involves localized and transient structures such as foreshock cavities and hot flow anomalies due to solar wind-bow shock interactions, and foreshock transients have been shown to lead to magnetospheric and ionospheric responses. In this paper, the interaction between a foreshock transient and the magnetosphere-ionosphere system is investigated using dayside aurora imagers revealing structures and propagation in greater detail than previously possible. A foreshock transient was detected by THEMIS-B and C during 1535-1545 UT on June 25, 2008. THEMIS-A, D and E observed magnetopause compression, cold plasma enhancement and ULF waves in the dayside magnetosphere. The all-sky imager (ASI) at South Pole observed that both diffuse and discrete aurora brightened locally soon after the appearance of this foreshock transient. The diffuse aurora brightening, which corresponded to a region a few Re size in GSM-Y in the equatorial plane, propagated duskward with an average speed of ~100 km/s. Soon after the diffuse aurora brightened, discrete aurora also brightened and extended duskward, which was consistent with the motion of the foreshock transient as it swept through the magnetosheath while impacting the magnetopause. Equivalent horizontal currents measured by magnetometers revealed a pair of field-aligned currents (FACs) moving duskward consistent with motion of the discrete aurora patterns. We conclude that the high-resolution and two-dimensional observation of auroral responses by ground-based ASI can help to estimate the evolution and propagation of upstream foreshock transients and their substantial impacts on the magnetosphere-ionosphere coupling system, including magnetospheric compression and currents in the ionosphere.

Observations of Kelvin‐Helmholtz Waves in the Earth's Magnetotail Near the Lunar Orbit

Kelvin‐Helmholtz waves (KHWs), which have been widely observed at the magnetopause in the region near the Earth, play an essential role in the transport of solar wind plasma and energy into the mag ...

Initial responses of magnetospheric plasma flows to the dynamic pressure enhancements

Using the solar wind data obtained from OMNI web, we find 1778 dynamic pressure enhancement events during Mar 2007 to Dec 2012. Then we check the responses of Magnetospheric plasma flows to these events and find 64 responses show clear sudden impulse (SI) signatures from THEMIS observations. We superpose the initial responses of the flows in one plot to find two flow vortices exist at two sides of the near earth magnetotail respectively, which was not observed in-situ before. There are some ULF wave observations after initial responses.

Characteristics of dayside magnetospheric flows during solar wind dynamic pressure pulse

The response scenario of dayside magnetospheric plasma flows to the sudden increase of solar wind dynamic pressures was studied with 5 events from THEMIS observation. In all events we observed bipolar Vx(-/+) signatures as a response. The inferred flow vortices presented to be rotating clockwise and counterclockwise in the morning and afternoon side, respectively, as see from the north. Moreover, the pre and after noon flow vortices move toward -y and +y direction. The in situ observation of dayside plasmasheet flow vortices may provide a new possible mechanism for the solar wind dynamic impulse associated dayside ionospheric convection vortices.