Hanying Wei

Proton cyclotron waves at Mars: Exosphere structure and evidence for a fast neutral disk

[1] Mars is an unmagnetized planet whose hydrogen exosphere extends into the solar wind, creating proton cyclotron waves. Mars Global Surveyor data reveal the occurrence of waves to be extensive and often intermittent at large distance, indicating that the exosphere is time varying or non-spherical. When the region of wave occurrence is examined in a magnetic-electric coordinate system, a strong asymmetry in the occurrence of these waves is seen in the direction of the interplanetary electric field. The extensive, yet asymmetric and intermittent, occurrence of waves can be understood if, after protons are first picked up near Mars, the ions are neutralized by charge exchange and transported across field lines to distant regions, allowing the pickup process to extend far from Mars on only one side of the planet. Thus the exosphere of Mars appears to extend in a disk of fast hydrogen atoms both downstream and to the side of Mars in the direction of the interplanetary electric field.

Induced magnetosphere and its outer boundary at Venus

The induced magnetosphere at Venus consists of regions near the planet and its wake for which the magnetic pressure dominates all other pressure contributions. Initial Venus Express measurements indicate a well-defined outer boundary, the magnetopause, of the induced magnetosphere. This magnetopause acts as an obstacle to deflect the solar wind. Across this boundary, the magnetic field exhibits abrupt directional changes and pronounced draping. In this paper, we examine the structure of the magnetopause using Venus Express magnetic measurements. We find that the magnetopause is a directional discontinuity resembling either a tangential or a rotational discontinuity depending on the interplanetary magnetic field orientation.

Cold ionospheric plasma in Titan's magnetotail

The interaction between Titan and the corotating Saturnian plasma forms an induced magnetosphere with an elongated Alfven-wing-style magnetotail. On 26 December 2005, the Cassini spacecraft flew th ...

Generation of ion cyclotron waves in the corona and solar wind

To examine the generation and nonlinear evolution of ion cyclotron waves in the corona and solar wind, we perform electromagnetic simulations using a wide range of plasma conditions and ion velocity distribution functions. The source of the instability is temperature anisotropy of ions with temperature perpendicular to the magnetic field larger than parallel. For velocity distribution we use Maxwellian, bi-Maxwellian, and Fermi-accelerated functions with perpendicular temperature larger than parallel with the aim to understand the extent to which the details of the distribution function impact the general properties and the nonlinear evolution of the instability. The results show that in a proton-electron plasma, ion cyclotron waves are generated over a wide range of temperature anisotropies and plasma beta. Also, the general properties of the instability and the nonlinear evolution of the waves are not sensitive to the details of the velocity distribution functions. Allowing for the presence of minor ion species we show that these ions by themselves can drive the instability and generate waves with frequencies below the gyrofrequency of the minor ions. In the event that protons also have temperature anisotropy, waves on the proton branch are also generated. Results using bi-Maxwellian or Fermi-accelerated velocity distribution functions show similar properties for the instability and the nonlinear evolution of the waves. However, differences are found when allowing for relative drifts between the protons and minor ions in that when using Fermi-accelerated distribution functions oblique ion cyclotron waves are generated that are not observed in simulations using bi-Maxwellian distribution function.

Force Balance at the Magnetopause Determined with MMS: Application to Flux Transfer Events

The Magnetospheric Multiscale mission (MMS) consists of four identical spacecraft forming a closely separated (≤10 km) and nearly regular tetrahedron. This configuration enables the decoupling of spatial and temporal variations and allows the calculation of the spatial gradients of plasma and electromagnetic field quantities. We make full use of the well cross-calibrated MMS magnetometer and fast plasma instruments measurements to calculate both the magnetic and plasma forces in flux transfer events (FTEs), and evaluate the relative contributions of different forces to the magnetopause momentum variation. This analysis demonstrates that some but not all FTEs, consistent with previous studies, are indeed force-free structures in which the magnetic pressure force balances the magnetic curvature force. Furthermore, we contrast these events with FTE events that have non-force-free signatures.

Characterizing the low-altitude magnetic belt at Venus: Complementary observations from the Pioneer Venus Orbiter and Venus Express

Using Venus Express, Zhang et al. (2012b) identified strong magnetic field enhancements at low altitudes over the north polar region of Venus as giant flux ropes. Strong fields at low altitudes were also observed during the Pioneer Venus Orbiter mission, but at low latitudes near the subsolar and midnight regions. We examine the possibility that the Venus Express observations are not giant flux ropes, but part of a low-altitude magnetic belt that builds up in the subsolar region, passes over the terminator, and extends to the nightside. Our analysis indicates the magnetic belt is dominantly horizontal over the dayside and gains a radial component nightward. The peak magnetic field strength of the belt and the altitude at which it peaks also varies around the planet, with the lowest altitude and strongest field strength in the subsolar region, consistent with the idea of the belt forming on the dayside. Zhang et al. (2012b) also noted the fields in the polar region had a bias in the +By direction in Venus Solar Orbital coordinates. The multifluid magnetohydrodynamic simulation we present shows an asymmetry in the plasma flow from the subsolar region to the poles due to the oxygen ion and proton mass ratio. This causes the magnetic field to preferentially accumulate in the north for a +By interplanetary magnetic field direction, providing an explanation for this bias.

Ion cyclotron waves at Mars: Occurrence and wave properties

Ion cyclotron waves (ICWs) are generated during the interaction between the solar wind and the Martian exosphere in a process called ion pickup. Mars Global Surveyor (MGS) detected waves near the proton gyrofrequency, indicating pickup of the exospheric hydrogen. To analyze these waves, we first improve the zero levels of the MGS magnetic field data taken during the first aerobreaking phase and then perform a statistical study of the ICWs observed from just outside the Martian bow shock to over 14 Mars radii away. These ICW events typically last for 5 to 30 min but can occasionally last for hours. The wave power decreases slowly with distance on both the upstream and downstream sides. From the variation of wave properties with the strength of the background field, we find that there are likely still remaining offsets in at least some the data sets even after applying our calibration technique. Thus, we use the events with a strong background field to examine the wave properties that depend on an accurate determination of the field direction and strength. We find the pickup angle associated with the largest occurrence rate of ICWs to be around 45°, but neither the wave amplitude, nor wave frequency, nor wave duration appear to vary with pickup angle. Finally, we find the waves with background field strength greater than 4 nT occur on both the positive and negative electric field sides of Mars but have a larger occurrence rate on the side of Mars in the positive electric field direction (which is defined as the direction of the cross product of the magnetic field vector and solar wind flow vector).

Generation and propagation of ion cyclotron waves in nonuniform magnetic field: Application to the corona and solar wind

With the objective to understand the generation, propagation, and nonlinear evolution of ion cyclotron waves (ICWs) in the corona and solar wind, we use electromagnetic hybrid (kinetic ions and fluid electrons) simulations with a nonuniform magnetic field. ICWs are generated by the temperature anisotropy of O5+ ions as minority species in a proton-electron plasma with uniform density. A number of magnetic field models are used including radial and spiral with field strength decreasing linearly or with the square of the radial distance. O5+ ions with perpendicular temperature larger than parallel are initially placed in the high-magnetic field regions. These ions are found to expand outward along the magnetic field. Associated with this expansion, ion cyclotron waves propagating along the magnetic field are also seen to expand outward. These waves are generated at frequencies below the local gyrofrequency of O5+ ions propagating parallel and antiparallel to the magnetic field. Through analysis of the simulation results we demonstrate that wave generation and absorption take place at all radial distances. Comparing the simulation results to observations of ICWs in the solar wind shows some of the observed wave characteristics may be explained by the mechanism discussed in this paper.

Ion cyclotron waves at Titan

During the interaction of Titans thick atmosphere with the ambient plasma, it was expected that ion cyclotron waves would be generated by the free energy of the highly anisotropic velocity distribution of the freshly ionized atmospheric particles created in the interaction. However, ion cyclotron waves are rarely observed near Titan, due to the long growth times of waves associated with the major ion species from Titans ionosphere, such as CH4+ and N2+. In the over 100 Titan flybys obtained by Cassini to date, there are only two wave events, for just a few minutes during T63 flyby and for tens of minutes during T98 flyby. These waves occur near the gyrofrequencies of proton and singly ionized molecular hydrogen. They are left-handed, elliptically polarized, and propagate nearly parallel to the field lines. Hybrid simulations are performed to understand the wave growth under various conditions in the Titan environment. The simulations using the plasma and field conditions during T63 show that pickup protons with densities ranging from 0.01 cm−3 to 0.02 cm−3 and singly ionized molecular hydrogens with densities ranging from 0.015 cm−3 to 0.25 cm−3 can drive ion cyclotron waves with amplitudes of ~0.02 nT and of ~0.04 nT within appropriate growth times at Titan, respectively. Since the T98 waves were seen farther upstream than the T63 waves, it is possible that the instability was stronger and grew faster on T98 than T63.

Transport of magnetic flux and mass in Saturn's inner magnetosphere

It is well accepted that cold plasma sourced by Enceladus is ultimately lost to the solar wind, while the magnetic flux convecting outward with the plasma must return to the inner magnetosphere. However, whether the interchange or reconnection, or a combination of the two processes is the dominant mechanism in returning the magnetic flux is still under debate. Initial Cassini observations have shown that the magnetic flux returns in the form of flux tubes in the inner magnetosphere. Here we investigate those events with 10 year Cassini magnetometer data and confirm that their magnetic signatures are determined by the background plasma environments: inside (outside) the plasma disk, the returning magnetic field is enhanced (depressed) in strength. The distribution, temporal variation, shape, and transportation rate of the flux tubes are also characterized. The flux tubes break into smaller ones as they convect in. The shape of their cross section is closer to circular than fingerlike as produced in the simulations based on the interchange mechanism. In addition, no sudden changes in any flux tube properties can be found at the “boundary” which has been claimed to separate the reconnection and interchange-dominant regions. On the other hand, reasonable cold plasma loss rate and outflow velocity can be obtained if the transport rate of the magnetic flux matches the reconnection rate, which supports reconnection alone as the dominant mechanism in unloading the cold plasma from the inner magnetosphere and returning the magnetic flux from the tail.