L. H. Chai

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.

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.

Compressibility of Mercury's dayside magnetosphere

The Mercury is experiencing significant variations of solar wind forcing along its large eccentric orbit. With 12 Mercury years of data from Mercury Surface, Space ENvironment, GEochemistry, and Ranging, we demonstrate that Mercurys distance from the Sun has a great effect on the size of the dayside magnetosphere that is much larger than the temporal variations. The mean solar wind standoff distance was found to be about 0.27 Mercury radii (RM) closer to the Mercury at perihelion than at aphelion. At perihelion the subsolar magnetopause can be compressed below 1.2 RM of ~2.5% of the time. The relationship between the average magnetopause standoff distance and heliocentric distance suggests that on average the effects of the erosion process appears to counter balance those of induction in Mercurys interior at perihelion. However, at aphelion, where solar wind pressure is lower and Alfvenic Mach number is higher, the effects of induction appear dominant.

Discrepancy between ionopause and photoelectron boundary determined from Mars Express measurements

The Martian ionosphere directly interacts with the solar wind due to lack of a significant intrinsic magnetic field, and an interface is formed in between. The interface is usually recognized by two kinds of indicators: the ionopause identified from ionospheric density profiles and the photoelectron boundary (PEB) determined from the electron energy spectrum at higher energies. However, the difference between them remains unclear. We have determined the locations of crossings of the ionopause and PEB from Mars Express observations during 2005–2013 and found that the average position of the PEB appears to be ~200 km higher than that of the ionopause, which corresponds to 103 cm–3 in the electron density profile. The discrepancy can be explained by cross-field transport of photoelectrons.

Solar zenith angle-dependent asymmetries in Venusian bow shock location revealed by Venus Express

It has been long known that the Venusian bow shock (BS) location is asymmetric from the observations of the long-lived Pioneer Venus Orbiter mission. The Venus Express (VEX) mission crossed BS near perpendicularly not only in the terminator region but also in the near-subsolar and tail regions. Taking the advantage of VEX orbit geometry, we examined a large data set of BS crossings observed during the long-lasting solar minimum between solar cycles 23 and 24 and found that the Venusian BS asymmetries exhibit dependence of solar zenith angle. In the terminator and tail regions, both the magnetic pole-equator and north-south asymmetries are observed in Venusian BS location, which is similar to the Pioneer Venus Orbiter (PVO) observation near terminator. However, in the near-subsolar region, only the magnetic north-south is observed; i.e., the BS shape is indented inward over magnetic south pole and bulged outward over magnetic north pole. The absence of the magnetic pole-equator asymmetry in the near-subsolar region suggests that the magnetic pole-equator asymmetry is mainly caused by the asymmetric wave propagation rather than the ion pickup process. The evident magnetic north-south asymmetry in solar minimum, which is not observed by PVO, suggests that even during the low long-lasting solar minimum, the ion pickup process is very important in Venusian space environment.

Increasing exposure of geosynchronous orbit in solar wind due to decay of Earth's dipole field

The Earths dipole moment has been decaying over the past 1.5 centuries. The magnetosphere thus has been shrinking and the chance of geosynchronous magnetopause crossings has been increasing. We quantitatively evaluate the increasing exposure of geosynchronous orbit in the solar wind caused by the decay of dipole moment and the variation of solar wind condition and study the possible situation if such decay persists for several more centuries. The results show that the average subsolar magnetopause distance would move earthward by ~0.3 RE per century, assuming the linear decreasing of the Earths dipole moment at present rate. The minimum solar wind dynamic pressure required for geosynchronous magnetopause crossings will decrease by ~4 nPa (2 nPa) in the next 100 years under northward (southward) interplanetary magnetic field. Under normal solar wind conditions, the noon region of the geosynchronous orbit will be exposed to the solar wind in the next few centuries. These results suggest that the secular variations of geomagnetic field are of paramount importance for our understanding of space climate.

Solitary kinetic Alfven waves in adiabatic process

Solitary kinetic Alfven waves (SKAWs) have been an important subject in the field of space plasma physics because of their nonzero parallel electrical field and density fluctuations. Under different thermodynamic processes, SKAWs, within the limit of small amplitude, are studied analytically and numerically using the Sagdeev potential method. The results show that the width of the solitary structures is larger and the amplitude of the density humps is smaller under constant entropy than those under constant temperature with other relevant parameters being the same. The perturbed electromagnetic fields Ex, By, and Ez are also studied further.

The Magnetic Field Structure of Mercury's Magnetotail

In this study, we use the magnetic field data measured by MESSENGER from 2011 to 2015 to investigate the average magnetic field morphology of Mercury’s magnetotail in the down tail 0~ 3 RM (RM = 2440 km, Mercurys radius). It is found that Mercury has a terrestrial-like magnetotail, the magnetic field structure beyond ~1.5 RM down tail is stretched significantly with the typical flaring lobe field ~50 nT. A tail current sheet separating the antiparallel field lines of lobes is present on the equatorial plane. The magnetotail width in north-south direction is ~5 RM, while the transverse width is ~ 4 RM. Thus, magnetotail is elongated along north-south direction. At current sheet center, the normal component of magnetic field (10~20 nT) is much larger than the cross-tail component. The magnetic field profile over current sheet can be well fitted by the Harris sheet model. The fitting shows that the curvature radius of field lines at sheet center usually reaches a minimum around the midnight (100 ~200 km) with stronger current density (40~50 nA/m2). While the curvature radius increases towards both flanks (400~600 km) with the decreased current density (~20 nA/m2). The typical half-thickness of current sheet around the midnight is about ~0.25 RM (~600 km), and the inner edge of current sheet is located at the down tail ~ 1.5 RM. Our results about the tail field structure does not show the evident dawn-dusk asymmetry as that found in the Earth’s magnetotail. Possible reasons are provided and discussed.

Is the flow‐aligned component of IMF really able to impact the magnetic field structure of Venusian magnetotail?

An earlier statistical survey suggested that the flow-aligned component of upstream interplanetary magnetic field (IMF) may play an important role in controlling the lobe-asymmetries of the Venusian magnetotail. The tail current sheet would be displaced and the magnetic field configuration would show asymmetries with respect to the current sheet. The asymmetries are expected to be more evident when the flow-aligned component becomes dominant. Here, with carefully selected cases as well as a statistical study based on Venus Express observations in the near Venus tail, we show that the lobe-asymmetries of the magnetic field as well as the displacement of the current sheet are common characteristics of the Venusian magnetotail. However, the asymmetries and the displacement of the current sheet do not show a significant dependence on the flow-aligned component of the IMF. Our results suggest that the flow-aligned component of IMF cannot penetrate into the near magnetotail to impact the magnetic field structure.