Z. J. Rong

Statistical survey on the magnetic structure in magnetotail current sheets

On the basis of the multipoint magnetic observations of Cluster in the region 15-19 R-E downtail, the magnetic field structure in magnetotail current sheet (CS) center is statistically surveyed. It is found that the B-y component (in GSM coordinates) is distributed mainly within vertical bar B-y vertical bar < 5nT, while the B-z component is mostly positive and distributes mainly within 1 similar to 10 nT. The plane of the magnetic field lines (MFLs) is mostly vertical to the equatorial plane, with the radius of curvature (Rc) of the MFLs being directed earthward and the binormal (perpendicular to the curvature and magnetic field direction) being directed azimuthally westward. The curvature radius of MFLs reaches a minimum, R-c,R-min, at the CS center and is larger than the corresponding local half thickness of the neutral sheet, h. Statistically, it is found that the overall surface of the CS, with the normal pointing basically along the south-north direction, can be approximated to be a plane parallel to equatorial plane, although the local CS may be flapping and is frequently tilted to the equatorial plane. The tilted CS (normal inclined to the equatorial plane) is apt to be observed near both flanks and is mainly associated with the slippage of magnetic flux tubes. It is statistically verified that the minimum curvature radius, R-c,R-min, half thickness of neutral sheet, h, and the slipping angle of MFLs, delta, in the CS satisfies h = R-c,R-min cos delta. The current density, with a mean strength of 4-8 nA/m(2), basically flows azimuthally and tangentially to the surface of the CS, from dawn side to the dusk side. There is an obvious dawn-dusk asymmetry of CS, however. For magnetic local times (MLT) similar to 21:00-similar to 01:00, the CS is relatively thinner; the minimum curvature radius of MFLs, R-c,R-min (0.6-1 R-E) and the half-thickness of neutral sheet, h (0.2-0.4 R-E), are relatively smaller, and B-z (3-5 nT) and the minimum magnetic field, B-min (5-7 nT), are weaker. It is also found that negative B-z has a higher probability of occurrence and the cross-tail current density j(Y) is dominant (2-4 nA/m(2)) in comparison to those values near both flanks. This implies that magnetic activity, e. g., magnetic reconnection and current disruption, could be triggered more frequently in CS with similar to 21:00-similar to 01:00 MLT. Accordingly, if mapped to the region in the auroral ionosphere, it is expected that substorm onset would be optically observed with higher probability for similar to 21:00-similar to 01:00 MLT, which is well in agreement with statistical observations of auroral substorm onset.

Flattened current sheet and its evolution in substorms

In this research, the properties of a tail current sheet, which has a flattened geometry, and its evolution during substorm activity have been investigated. The geometrical configuration of the magnetic field and the spatial distribution of the current density in a flattened current sheet have been revealed with certainty for the first time. It is found that such a flattened current sheet has sufficiently strong B-y (GSM) within its neutral sheet that the magnetic field lines (MFLs) in the neutral sheet are lie almost in the GSM equatorial plane and that the normal directions are generally northward. Detailed analyses show that, the magnetic field lines are spiral-like, not plane curves, which are left-handed or right-handed spirals for B-y > 0 or B-y < 0. This magnetic rotation occurs predominantly in the neutral sheet. The flattened current sheet may be very thin, and the thickness of the neutral sheet is much less than the minimum radius of the curvature of the MFLs in the current sheet. The analysis also suggests that the neutral sheet current is field-aligned and lies mainly duskward. The curvature current makes little contribution to the total current in the flattened current sheet. The main current carriers in the neutral sheet of the flattened current sheet are electrons. A statistical survey shows that there is one positive correlation between B-y in the flattened current sheet and IMF B-y and penetration efficiency is 0.67. Flattened current sheets may occur in both quiet and disturbed periods and may appear at all phases of the substorms. During the growth phase of a substorm event, the neutral sheet of the flattened current sheet is shown to become progressively thinner, while the associated current density is increasing gradually. It is found that the northern turning of the IMF has triggered the explosive growth phase at the end of the growth phase, which lasts several minutes. At the explosive growth phase, the flattened current sheet becomes much thinner and the current density in the neutral sheet then increases considerably and reaches a value larger than 0.017 mu Am-2. Just after the onset of the substorm, the current density in the neutral sheet drops abruptly and varies turbulently.

Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause

Based on high-resolution measurements from NASAs Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earths magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20 eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials).

Profile of strong magnetic field By component in magnetotail current sheets

The strong magnetic field B-y component (in GSM coordinates) has been increasingly noticed to play an important role in the dynamics of tail current sheet (CS). The distribution profile of strong B-y components in the tail CS (i.e., those with guide field), however, is not well known. In the present work, by using the simultaneous multipoint observations of Cluster satellites, the profile of a strong B-y component in tail current sheets is explored, through detailed case studies, as well as in a statistical study. It is discovered that around the midnight meridian, the strength of the strong B-y component, i.e., |B-y|, is basically enhanced at the center of the CS relative to that in the CS boundaries and lobes and forms a north-south symmetric distribution about the center of CS. Generally, however, for strong guide field cases in the non-midnight meridian, the profile of B-y strength basically becomes north-south asymmetric, the strength of the B-y component in the northern side of the CS is found to be either stronger or weaker than that in the counterpart southern side. By considering the modulation of the tail flaring magnetic field with magnetic local time, we propose an interpretation to account for the variation of the B-y-profile, which is supported by the statistical survey. These results offer an observation basis for further studies.

The flapping motion of the Venusian magnetotail: Venus Express observations

With a newly developed technique and magnetic field measurements obtained by the magnetometer on Venus Express, we study the flapping motion of the Venusian magnetotail. We find that the flapping motion generally comprises contributions both from a nonpropagating steady flapping and a propagating kink-like flapping. The flapping motion tilts the current sheet normal significantly in the plane perpendicular to the Venus-Sun line. The kink-like flapping waves traveling along solar wind electric field or its antidirection can be found in either magnetotail hemisphere where solar wind electric field pointing toward/away. The traveling behaviors suggest that the locations of the triggers for kink-like flappings are near the boundaries between magnetotail current sheet and magnetosheath, not near the central region of magnetotail as is for the Earths magnetotail.

Morphology of magnetic field in near‐Venus magnetotail: Venus express observations

Knowledge of the magnetic field morphology in the near-Venus wake is essential to the studies of magnetotail dynamics and the planetary plasma escape. In this study we use the magnetic field measurements made by Venus Express during the period of April 2006 to December 2012 to investigate the global magnetic field morphology in the near-Venus magnetotail (0–3 Venusian radii, RV, down tail) in the frame of solar wind electric field coordinates. The hemisphere with electric field pointing toward/away is indicated as ±E hemisphere. It has been reported that the cross-tail field component has a hemispheric asymmetry in the Venusian magnetotail. We report here that this asymmetry should have been formed at the terminator and would transport tailward. In addition, we find that the draped magnetic field lines near both hemispheric flanks are directed equatorward in the region 0–1.5 RV down tail as it looks like “sinking” into Venus umbra. We estimate the thickness of the magnetotail current sheet and the current density at the sheet center. We find that the average half thickness of central current sheet near +E hemispheric flank (~460 km) is almost twice as thick as that near magnetic equatorial plane (~200 km), but the corresponding current densities at the sheet center are comparable (~6.0 nA/m2). As a result, the larger cross-tail field component found near the +E hemispheric flank suggests a stronger tailward j × B force, i.e., the more efficient tailward acceleration of plasma in this region, showing the agreement with previous observations of heavy ion outflow from Venus. In contrast, the average magnetic field structure near −E hemispheric flank is irregular, which suggests that dynamic activities, such as magnetic reconnection and magnetic field turbulence, preferentially appear there.

Spatial gradients from irregular, multiple-point spacecraft configurations

We present a generalized multipoint analysis of physical quantities, such as magnetic field and plasma flow, based on spatial gradient properties, where the multipoint data may be taken by irregular (distorted) configurations of any number of spacecraft. The methodology is modified from a previous, fully 3-D gradient analysis technique, designed to apply strictly to 4-point measurements and to be stable for regular spacecraft configurations. Here, we adapt the method to be tolerant against distorted configurations and to return a partial result when fewer spacecraft measurements are available. We apply the method to a variety of important physical quantities, such as the electric current density and the vorticity of plasma flows based on Cluster and THEMIS multiple-point measurements. The method may also have valuable applications on the coming Swarm mission.

Time delay of interplanetary magnetic field penetration into Earth's magnetotail

Many previous studies have demonstrated that the interplanetary magnetic field (IMF) can control the magnetospheric dynamics. Immediate magnetospheric responses to the external IMF have been assumed for a long time. The specific processes by which IMF penetrates into magnetosphere, however, are actually unclear. Solving this issue will help to accurately interpret the time sequence of magnetospheric activities (e.g., substorm and tail plasmoids) exerted by IMF. With two carefully selected cases, we found that the penetration of IMF into magnetotail is actually delayed by 1-1.5 h, which significantly lags behind the magnetotail response to the solar wind dynamic pressure. The delayed time appears to vary with different auroral convection intensity, which may suggest that IMF penetration in the magnetotail is controlled considerably by the dayside reconnection. Several unfavorable cases demonstrate that the penetration lag time is more clearly identified when storm/substorm activities are not involved.

Mercury's three‐dimensional asymmetric magnetopause

Mercurys magnetopause is unique in the solar system due to its relatively small size and its close proximity to the Sun. Based on 3 years of MErcury Surface, Space ENvironment, GEochemistry, and Ranging orbital Magnetometer and the Fast Imaging Plasma Spectrometer data, the mean magnetopause location was determined for a total of 5694 passes. We fit these magnetopause locations to a three-dimensional nonaxially symmetric magnetopause which includes an indentation for the cusp region that has been successfully applied to the Earth. Our model predicts that Mercurys magnetopause is highly indented surrounding the cusp with central depth ~0.64 RM and large dayside extension. The dayside polar magnetopause dimension is, thus, smaller than the equatorial magnetopause dimension. Cross sections of the dayside magnetopause in planes perpendicular to the Mercury-Sun line are prolate and elongated along the dawn-dusk direction. In contrast, the magnetopause downstream of the terminator plane is larger in the north-south than the east-west directions by a ratio of 2.6 RM to 2.2 RM at a distance of 1.5 RM downstream of Mercury. Due to the northward offset of the internal dipole, the model predicts that solar wind has direct access to the surface of Mercury at middle magnetic latitudes in the southern hemisphere. During extremely high solar wind pressure conditions, the northern hemisphere middle magnetic latitudes may also be subject to direct solar wind impact.

Direct calculation of the ring current distribution and magnetic structure seen by Cluster during geomagnetic storms

Magnetic disturbances caused by the Earths ring current, particularly during storm time activity, have a dominant effect on the geomagnetic field. Strong currents and large kinetic and magnetic energies can change considerably local field geometry and depress the ground geomagnetic field. The multispacecraft magnetic measurements of Cluster allow extensive in situ coverage of the ring current. We select 48 storm time Cluster crossing events to investigate the variation of the local current density distribution and magnetic configuration of the ring current. We find direct evidence for the existence of an inner, eastward flowing current in addition to the dominant westward current, in the ring plane. The radius of curvature of the magnetic field lines (MFLs) is found to be increasingly reduced at all local times during increasing storm activity, changing the resulting ring current magnetic geometry considerably, where the MFL configuration and the azimuthal current density distribution are asymmetric with the local time. During similar storm activity the radius of curvature of the local MFLs, R-c, is smallest on the nightside to duskside, medium on the dawnside, and largest on the dayside. This change in geometry may have significant influence on the spatial distribution of the particles with various energies in the plasmasphere, ring current, and radiation belts.