Modeling Non-Force-Free and Deformed Flux Ropes in Titan’s Ionosphere

Previous work at Titan presented a set of 85 flux ropes detected during Cassini flybys of Titan from 2005 to 2017. In that study a force‐free model was used to determine the radii and axial magnetic field of the flux ropes. In this work we apply non‐force‐free models. The non‐force‐free model shows an improvement in the number of flux ropes that can be fitted with a model, along with improved uncertainties and χ2 values. A number of asymmetries and features in the magnetometer data cannot be reproduced by either model; therefore, we deform the force‐free model to show that small deformations can replicate these features. One such deformation is to use an elliptical cross section, which replicates a plateau in magnetic field strength along with asymmetries on either side of the center of the flux ropes. Additionally, we explore the properties of bending a flux rope, where we find that minimum variance analysis becomes increasingly degenerate with bending, along with a slight bend causing the switching of the axial field direction from intermediate to maximum variance direction. We conclude that the flux ropes at Titan show aspects of developing flux ropes, compared to other planetary bodies, which exhibit more agreement to the force‐free assumptions of mature flux ropes

Energy-banded ions in Saturn's magnetosphere

Using data from the Cassini Plasma Spectrometer ion mass spectrometer, we report the first observation of energy-banded ions at Saturn. Observed near midnight at relatively high magnetic latitudes, the banded ions are dominantly H+, and they occupy the range of energies typically associated with the thermal pickup distribution in the inner magnetosphere (L < 10), but their energies decline monotonically with increasing radial distance (or time or decreasing latitude). Their pitch angle distribution suggests a source at low (or slightly southern) latitudes. The band energies, including their pitch angle dependence, are consistent with a bounce-resonant interaction between thermal H+ ions and the standing wave structure of a field line resonance. There is additional evidence in the pitch angle dependence of the band energies that the particles in each band may have a common time of flight from their most recent interaction with the wave, which may have been at slightly southern latitudes. Thus, while the particles are basically bounce resonant, their energization may be dominated by their most recent encounter with the standing wave

Energy-banded ions in Saturn's magnetosphere

Using data from the Cassini Plasma Spectrometer ion mass spectrometer, we report the first observation of energy-banded ions at Saturn. Observed near midnight at relatively high magnetic latitudes, the banded ions are dominantly H+, and they occupy the range of energies typically associated with the thermal pickup distribution in the inner magnetosphere (L < 10), but their energies decline monotonically with increasing radial distance (or time or decreasing latitude). Their pitch angle distribution suggests a source at low (or slightly southern) latitudes. The band energies, including their pitch angle dependence, are consistent with a bounce-resonant interaction between thermal H+ ions and the standing wave structure of a field line resonance. There is additional evidence in the pitch angle dependence of the band energies that the particles in each band may have a common time of flight from their most recent interaction with the wave, which may have been at slightly southern latitudes. Thus, while the particles are basically bounce resonant, their energization may be dominated by their most recent encounter with the standing wave

Auroral evidence of radial transport at Jupiter during January 2014

We present Jovian auroral observations from the 2014 January Hubble Space Telescope (HST) campaign and investigate the auroral signatures of radial transport in the magnetosphere alongside contemporaneous radio and Hisaki EUV data. HST FUV auroral observations on day 11 show, for the first time, a significantly superrotating polar spot poleward of the main emission on the dawnside. The spot transitions from the polar to main emission region in the presence of a locally broad, bright dawnside main emission feature and two large equatorward emission features. Such a configuration of the main emission region is also unreported to date. We interpret the signatures as part of a sequence of inward radial transport processes. Hot plasma inflows from tail reconnection are thought to flow planetward and could generate the superrotating spot. The main emission feature could be the result of flow shears from prior hot inflows. Equatorward emissions are observed. These are evidence of hot plasma injections in the inner magnetosphere. The images are thought to be part of a prolonged period of reconnection. Radio emissions measured by Wind suggest that hectometric (HOM) and non-Io decametric (DAM) signatures are associated with the sequence of auroral signatures, which implies a global magnetospheric disturbance. The reconnection and injection interval can continue for several hours

Characterization of Jupiter's secondary auroral oval and its response to hot plasma injections

We present Jovian auroral observations from the 2014 January Hubble Space Telescope (HST) campaign and characterize the auroral second oval feature with particular attention to the response to hot plasma injections. The location of the second oval feature lies between the Ganymede and Europa moon footprint contours between 150 and 240° system III longitude, corresponding to a source in the inner magnetosphere between 9 and 13 RJ. At the examined longitudes, this is in the same region of 11–16 RJ known as the pitch angle distribution boundary, beyond which electrons are thought to be scattered into a field-aligned configuration and cause auroral precipitation. The feature is enhanced in both brightness and longitudinal spread 1–3 days after large hot plasma injections. The precipitating electrons have a higher-energy and lower flux than the electrons generating large injection signatures. We suggest that wave-particle interactions are responsible for the scattering of electrons in this region. We also suggest that the plasma injections can act as a temperature anisotropy and particle source to enhance electron scattering into the aurora and the brightness of the second oval feature. Changes to the magnetic field topology around an injection may also generate shear Alfvén waves and therefore accelerate electrons parallel to the magnetic field resulting in precipitation

Observations of Continuous Quasiperiodic Auroral Pulsations on Saturn in High Time-Resolution UV Auroral Imagery

Saturn's aurora represents the ionospheric response to plasma processes occurring in the planet's entire magnetosphere. Short-lived ∼1-hr quasiperiodic high-energy electron injections, frequently observed in in situ particle and radio measurements, should therefore entail an associated flashing auroral signature. This study uses high time-resolution ultraviolet (UV) auroral imagery from the Cassini spacecraft to demonstrate the continuous occurrence of such flashes in Saturn's northern hemisphere and investigate their properties. We find that their recurrence periods of order 1 hr and preferential occurrence near dusk match well with previous observations of electron injections and related auroral hiss features. A large spread in UV auroral emission power, reaching more than 50% of the total auroral power, is observed independent of the flash locations. Based on an event observed both by the Hubble Space Telescope and the Cassini spacecraft, we propose that these auroral flashes are not associated with low-frequency waves and instead directly caused by recurrent small-scale magnetodisc reconnection on closed field lines. We suggest that such reconnection processes accelerate plasma planetward of the reconnection site toward the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent-back to a more dipolar configuration. This manifests as a sawtooth-shaped discontinuity observed in magnetic field data and indicates a release of magnetospheric energy through plasmoid release

Testing the relationship between Saturn's ENA and narrowband radio emissions

Saturn’s kilometric radiation (SKR) and Energetic Neutral Atom (ENA) emissions are important remote diagnostics of the planet’s magnetospheric dynamics, intensifying during periods of global-scale plasma injection, and displaying characteristic planetary periodicity. Global-scale ENA signatures have been associated with narrowband radio emissions around 5 and 20 kHz, particularly at evening local times where plasma injections are expected to have moved inwards through the magnetosphere, triggering interchange instabilities. Narrowband radio emission sources are associated with density gradients at the inner edges of the Enceladus plasma torus that promote wave mode conversion, but any radial distance dependence with the ENA emission is untested. We constrain ENA keograms to distances covering the ‘inner’ and ‘outer’ magnetosphere separately, and quantify the correlation between the ENA intensity with narrowband flux density in the 5 and 20 kHz emission bands. One case study shows a spiral ENA morphology that indicates global-scale plasma injection activity. ‘Bursts’ of narrowband emission coincide with the rotation of ENA enhancements through the dusk-midnight local time sector in the inner magnetosphere, but at earlier times in the outer magnetosphere, consistent with inward flow of the injected plasma as it drifts around the planet. A second case study with similar observing conditions shows clear 5 kHz radio bursts, but very low levels of ENA detections, indicating that the relationship is not always so general in these data. These results contribute towards our developing picture of how global plasma injection events can influence Saturn’s inner magnetosphere, linking together two valuable sources of remotely sensed global emissions, the ENAs and narrowband radio emissions

A complete dataset of equatorial projections of Saturn's energetic neutral atom emissions observed by Cassini-INCA

Observations of energetic neutral atoms (ENAs) are a useful tool for analyzing ion and neutral abundances in planetary magnetospheres. They are created when hot plasma, originating for example from magnetic reconnection sites, charge-exchanges with the ambient neutral population surrounding the planet. The motion of ENAs is not governed by the magnetic field, allowing remote imaging. During the Cassini mission, the Ion Neutral Camera (INCA) of the Magnetosphere Imaging Instrument (MIMI) collected vast amounts of hydrogen and oxygen ENA observations of Saturn's magnetosphere from a variety of different viewing geometries. In order to enable investigations of the morphology and dynamics of Saturn's ring current, it is useful to re-bin and re-project the camera-like views from the spacecraft-based perspective into a common reference frame. We developed an algorithm projecting INCA's ENA observations into a regular grid in Saturn's equatorial plane. With most neutrals and ions being confined into an equatorial rotating disc, this projection is quite accurate in both spatial location and preservation of ENA intensity, provided the spacecraft is located at large enough elevations. Such projections were performed for all INCA ENA data from the Cassini Saturn tour; the data is available for download together with a Python routine flagging contaminated data and returning detailed spacecraft geometry information. The resulting dataset is a good foundation for investigating for example the statistical properties of Saturn's ring current and its complicated dynamics in relation to other remote and in situ observations of, for example, auroral emissions and magnetotail reconnection events

Ground-based observations of Saturn’s auroral ionosphere over three days:trends in H3+ temperature, density and emission with Saturn local time and planetary period oscillation

On 19–21 April 2013, the ground-based 10-m W.M. Keck II telescope was used to simultaneously measure View the MathML sourceH3+ emissions from four regions of Saturn’s auroral ionosphere: (1) the northern noon region of the main auroral oval; (2) the northern midnight main oval; (3) the northern polar cap and (4) the southern noon main oval. The View the MathML sourceH3+ emission from these regions was captured in the form of high resolution spectral images as the planet rotated. The results herein contain twenty-three View the MathML sourceH3+ temperatures, column densities and total emissions located in the aforementioned regions – ninety-two data points in total, spread over timescales of both hours and days. Thermospheric temperatures in the spring-time northern main oval are found to be cooler than their autumn-time southern counterparts by tens of K, consistent with the hypothesis that the total thermospheric heating rate is inversely proportional to magnetic field strength. The main oval View the MathML sourceH3+ density and emission is lower at northern midnight than it is at noon, in agreement with a nearby peak in the electron influx in the post-dawn sector and a minimum flux at midnight. Finally, when arranging the northern main oval View the MathML sourceH3+ parameters as a function of the oscillation period seen in Saturn’s magnetic field – the planetary period oscillation (PPO) phase – we see a large peak in View the MathML sourceH3+ density and emission at ∼115° northern phase, with a full-width at half-maximum (FWHM) of ∼44°. This seems to indicate that the influx of electrons associated with the PPO phase at 90° is responsible at least in part for the behavior of all View the MathML sourceH3+ parameters. A combination of the View the MathML sourceH3+ production and loss timescales and the ±10° uncertainty in the location of a given PPO phase are likely, at least in part, to be responsible for the observed peaks in View the MathML sourceH3+ density and emission occurring at a later time than the peak precipitation expected at 90° PPO phase