X. H. Han

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.

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.