Alexandre Wennmacher

Titan's near magnetotail from magnetic field and electron plasma observations and modeling: Cassini flybys TA, TB, and T3

[1] The first close Titan encounters TA, TB, and T3 of the Cassini mission at almost the same Saturnian local time � 1030 and in the same spatial region downstream of Titan have enabled us to study the formation of the tail of its induced magnetosphere. The study is based on magnetic field and electron plasma observations as well as threedimensional modeling. Our most important findings are the following: (1) No crossings of a bow shock of Titan were observed, and all encounters occurred at high plasma b > 1 for

The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals

We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiters time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0 ◦ ± 1.3 ◦ . Our model calculations predict a significantly stronger oscillation by 5.8 ◦ ± 1.3 ◦ without ocean compared to 2.2 ◦ ± 1.3 ◦ if an ocean is present. Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymedes dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields.

Variability of Titan's induced magnetotail: Cassini magnetometer observations

We study the structure and variability of Titans magnetotail by analyzing Cassini magnetic field observations from all tail crossings between 2005 and 2013. Titans magnetotail is strongly affected by fluctuations in the ambient magnetospheric field conditions. Therefore, even Titan flybys with nearly identical trajectories may reveal a completely different location and strength of the perturbations in the moons tail. Short-scale variations of the ambient magnetospheric field may cause a “fragmentation” of Titans magnetic lobes, as also seen in Cassini Plasma Spectrometer ion data. By transforming the magnetic field perturbations detected during all available tail crossings to the Draping Coordinate System, we identified the following general characteristics of Titans plasma interaction: (1) Perpendicular to the background magnetic field and the corotation direction, Titans magnetotail is confined to a narrow region with a diameter of about 5 Titan radii. Thus, Titans tail exhibits a rather “flat” structure reminiscent of a delta wing. (2) The plasma incident upon Titan does not possess a significant velocity component along the Saturn-Titan line. (3) The nonzero component of the background field along the corotation direction generates an asymmetry of Titans magnetotail which clearly manifests in Cassini magnetometer data.

The UV Spectrum of the Ultracool Dwarf LSR J1835+3259 Observed with the Hubble Space Telescope

An interesting question about ultracool dwarfs recently raised in the literature is whether their emission is purely internally driven or partially powered by external processes similar to planetary aurora known from the solar system. In this work we present Hubble Space Telescope observations of the energy fluxes of the M8.5 ultracool dwarf LSR J1835+3259 throughout the UV. The obtained spectra reveal that the object is generally UV-fainter compared to other earlier-type dwarfs. We detect the \ion{Mg}{2} doublet at 2800 \A and constrain an average flux throughout the Near-UV. In the Far-UV without Lyman alpha, the ultracool dwarf is extremely faint with an energy output at least a factor of 1000 smaller as expected from auroral emission physically similar to that on Jupiter. We also detect the red wing of the Lyman alpha emission. Our overall finding is that the observed UV spectrum of LSR J1835+3259 resembles the spectrum of mid/late-type M-dwarf stars relatively well, but it is distinct from a spectrum expected from Jupiter-like auroral processes.

Interplanetary Field Enhancements: Observations from 0.3 AU to 1 AU

Interplanetary Field Enhancements are rare but very distinct increases in the magnetic field strength, reaching a nearly symmetric cusp‐shaped peak. These increases are usually accompanied by a thin central current sheet. Their associations with the perihelion passages of the asteroid 2201 Oljato and with the comet De Vico have led to the hypothesis that these events are associated with the interaction of the solar wind with dust particles. In this paper we examine observations of these events with the Helios 1/2 spacecraft that flew as close to the Sun as 0.29 AU. These events are observed over the entire range of distances studied by Helios 1 and 2. The ponderomotive force exerted by the magnetic field strength decreases with radial distance roughly inversely proportional to the distance squared.