Y. Wei

Day-to-night transport in the Martian ionosphere: Implications from total electron content measurements

The nightside Martian ionosphere is thought to be contributed by day-to-night transport and electron precipitation, of which the former has not been well studied. In this work, we evaluate the role of day-to-night transport based on the total electron content (TEC) measurements made by the Mars Advanced Radar for Subsurface and Ionospheric Sounding on board Mars Express. This is accomplished by an examination of the variation of nightside TEC in the time domain rather than the traditional solar zenith angle domain. Our analyses here, being constrained to the Northern Hemisphere where the effects of crustal magnetic fields can be neglected, reveal that day-to-night transport serves as the dominant source for the nightside Martian ionosphere from terminator crossing up to time in darkness of ≈5.3 × 103 s, beyond which it is surpassed by electron precipitation. The observations are compared with predictions from a simplified time-dependent ionosphere model. We conclude that the solid body rotation of Mars is insufficient to account for the observed depletion of nightside TEC but the data could be reasonably reproduced by a zonal electron flow velocity of ≈1.9 km s−1.

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.

Plasma Sheet Pressure Variations in the Near‐Earth Magnetotail During Substorm Growth Phase: THEMIS Observations

We investigate the plasma sheet pressure variations in the near-Earth magnetotail (radius distance, R, from 7.5xa0RE to 12xa0RE and magnetic local time, MLT, from 18:00 to 06:00) during substorm growth phase with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. It is found that, during the substorm growth phase, about 39.4% (76/193) of the selected events display a phenomenon of equatorial plasma pressure (Peq) decrease. The occurrence rates of Peq decrease cases are higher in the dawn (04:00 to 06:00) and dusk (18:00 to 20:00) flanks (> 50%) than in the midnight region (20:00 to 04:00, xa0−16%). The mean value of Peq increase percentages at the end of substorm growth phase is the highest (~ 40%) in the premidnight MLT bin (22:00 to 00:00) and is almost unchanged in the dawn and dusk flanks. Further investigations show that 13.0% of the events have more than 10% of Peq decrease at the end of substorm growth phase comparing to the value before the growth phase, and ~ 28.0% of the events have small changes (< 10%), and ~ 59.0% events have a more than 10% increase. This study also reveals the importance of electron pressure (Pe) in the variation of Peq in the substorm growth phase. The Pe variations often account for more than 50% of the Peq changes, and the ratios of Pe to ion pressure often display large variations (~xa050%). Among the investigated events, during the growth phase, an enhanced equatorial plasma convection flow is observed, which diverges in the midnight tail region and propagates azimuthally toward the dayside magnetosphere with velocity of ~ 20xa0km/s. It is proposed that the Peq decreases in the near-Earth plasma sheet during the substorm growth phase may be due to the transport of closed magnetic flux toward the dayside magnetosphere driven by dayside magnetopause reconnection. Both solar wind and ionospheric conductivity effects may influence the distributions of occurrence rates for Peq decrease events and the Peq increase percentages in the investigated region.

Cold Ion Outflow Modulated by the Solar Wind Energy Input and Tilt of the Geomagnetic Dipole

The solar wind energy input into the Earths magnetosphere-ionosphere system drives ionospheric outflow, which plays an important role in both the magnetospheric dynamics and evolution of the atmos ...