Hanying Wei

Titan has a thick atmosphere, the top of which is ionized and interacts with its plasma environment, i.e. usually the Saturnian magnetospheric plasma, but occasionally the magnetosheath or even the solar wind plasma. When the upstream plasma flow encounters Titan, the plasma slows down and diverts around Titan, and the magnetic field slowly diffuses into Titan’s ionosphere and induces currents in the ionosphere. The resulting magnetic field pattern is that, in the upstream of Titan, field lines drape around it, and in the downstream, the field lines stretch tail-like. Gradually these external fields penetrate into the lower atmosphere and the interior of Titan, and induce currents in any conductive layer if a conductive layer does indeed exist in Titan’s interior. This internally induced current acts to exclude the penetrated field. Both the internally induced field and the externally induced ionospheric field have strength and orientation variable with certain time scales because they are responses to the penetrated external field. The Cassini observations are a sum of fields from both internal and external sources. In this paper, we review the low altitude observations from all Cassini Titan flybys and examine the different behaviors of the external and internal fields, which ultimately provide an upper limit to Titan’s internal field leading to indications for Titan’s interior.

Although electromagnetic ion cyclotron waves (ICWs) have been observed in the solar wind by multiple missions at heliocentric distances from 0.3 to 1 AU, there are still open questions on the generation mechanisms for these waves. Detailed analysis of the plasma distribution is needed to examine whether these waves are possibly generated locally.In the solar wind, there are mainly three types of ion-driven instabilities responsible for parallel-propagating ICWs: ion cyclotron instabilities driven by ion component with temperature anisotropies greater than 1, parallel firehose instabilities driven by ion temperature anisotropies smaller than 1, and ion/ion magnetosonic instabilities driven by the relative drift between two ion components. In the solar wind frame, the waves due to ion cyclotron instability have left-handed polarization, while the waves due to firehose and ion/ion magnetosonic instabilities have right-handed polarization. Depending on the wave propagation parallel or anti-parallel to the magnetic field, the wave frequencies in the spacecraft frame are Doppler shifted higher or lower even with reversed handness. With the plasma data from Magnetospheric Multiscale (MMS) mission, we can examine the possible unstable mode with dispersion analysis and check if the prediction agrees with the observed wave mode. If the plasma measurements of the local solar wind do not support the wave growth, the waves could be possibly generated remotely close to the Sun and propagate away from the source region and are also carried outward by the solar wind flow. If these waves are generated remotely closer to the Sun, the wave properties at different heliocentric distances would help us better understand their sources.The MMS spacecraft spends long periods of its orbit in the “pristine” solar wind starting end of 2017. From the 2017 December data we find over a hundred events and 42 of them last longer than 10 minutes which are called ICW storm events, and the longest event captured lasted over 2 hours. Although only about 17 of them have the plasma data available, we can perform case studies on these events first to investigate the wave properties and possible plasma instabilities, which will help us investigate the wave generation mechanisms due to local or remote sources.

During the interaction of Titan's 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 Titan's 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.

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).

Due to the lack of an intrinsic magnetic field at Venus, the Venus ionosphere acts as the obstacle to the incoming solar wind flow. Induced currents flowing on the ionopause resist the penetration of the solar wind magnetic fields into the ionosphere. The Venus Express magnetometer occasionally observes plasma waves near the ionopause. These waves propagate quasi‐perpendicularly to the background magnetic field, which are distinctively different from the lightning‐generated whistler waves. Both case and statistical studies are performed on the Venus Express magnetometer data in 32 Hz and 128 Hz resolutions for these quasi‐perpendicular propagating waves. These waves are found to be strongly associated with current sheets near the ionopause, either occurring at the center of a sharp current, or occurring at the edge of a current. The earlier Pioneer Venus Orbiter electric field observations also detected plasma waves associated with field‐aligned currents near the ionopause region, and the wave studies in this paper are possibly the magnetic counterparts of those earlier observations. These waves could be generated from the free energy of currents at the ionopause and could lead to dissipation of those currents.

Measurements from the Cassini spacecraft reveal a largely magnetized ionosphere of Titan whose low‐altitude fields could be from the interior of Titan or from penetrating external fields. We use the Cassini magnetic field measurements obtained during 25 Titan flybys to examine the upper limit of Titan's internal magnetic moment. The radial component of the magnetic field near the Titan surface (from 950 to 1100 km) is used to calculate the permanent dipole moment. The calculated upper limit of the permanent dipole moment is 0.78 nT × R_Ti³, that is, for the z, y, and x directions (0.46, 0.55, 0.29, respectively) nT × R_Ti³, with an error of ∼0.5 nT in each component. This value is not significantly different from zero, but it is also a factor of 5 lower than the previous upper limit from Voyager observations. This small (possibly zero) magnetic moment of Titan indicates that the interior of the moon does not support a magnetic dynamo, in agreement with the results of recent gravity study which suggest an incompletely differentiated interior. The induced moment of Titan is examined to test the existence of a subsurface ocean in Titan. The data obtained thus far are not sufficient to provide definitive evidence of the existence and depth of a subsurface ocean. This situation will be improved with the addition of measurements after Saturn's equinox when the external inducing field changes significantly.

To understand the possible large scale influence of Titan on its plasma environment, we study the magnetic fields and plasma measurements, both when Cassini flies close to Titan and when Cassini crosses the moon's orbit far from it. Using 98 Cassini passes from 06/2004 to 12/2008, we examine the plasma environments at the orbit of Titan with the moon present and absent. In particular, the presence of Titan appears to affect the magnetopause location. Near noon, the Saturn magnetopause is more frequently inside of Titan's orbit with the moon absent than with it present. Titan's presence near noon appears to locally enhance the total pressure and reduce the magnetosphere compressibility, possibly by mass‐loading. Near local midnight, the stretching and sweepback angles for cases with Titan present and absent suggest that the moon enhances the tail reconnection rate, in agreement with previous studies of the moon's influence on the Saturnian magnetosphere.

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 through Titan's magnetotail, providing the first distant tail observation, over 5 Titan radii downstream. We examine measurements observed by the magnetometer and Langmuir probe during this pass. We use the direction of the magnetic field along the trajectory to identify the source regions of plasma reaching the spacecraft. Cold plasma, with a density of about 10 cm⁻³, is found magnetically connected to the ionosphere. Titan's ionosphere appears to be escaping along field lines down the tail, leading to particle loss from the atmosphere.