A. Masters

The magnetic memory of Titan's ionized atmosphere

After 3 years and 31 close flybys of Titan by the Cassini Orbiter, Titan was finally observed in the shocked solar wind, outside of Saturns magnetosphere. These observations revealed that Titans flow-induced magnetosphere was populated by “fossil” fields originating from Saturn, to which the satellite was exposed before its excursion through the magnetopause. In addition, strong magnetic shear observed at the edge of Titans induced magnetosphere suggests that reconnection may have been involved in the replacement of the fossil fields by the interplanetary magnetic field.

MESSENGER observations of Mercury's dayside magnetosphere under extreme solar wind conditions

CLJ and RMW acknowledge support from the Natural Sciences and Engineering Research Council of Canada, and CLJ acknowledges support from MESSENGER Participating Scientist grant NNX11AB84G. The MESSENGER project is supported by the NASA Discovery Program under contracts NASW- 00002 to the Carnegie Institution of Washington and NAS5-97271 to The Johns Hopkins University Applied Physics Laboratory.

An empirical model of Saturn's bow shock: Cassini observations of shock location and shape

We present a new empirical model of Saturns bow shock that utilizes observations from the Cassini spacecraft. Shock crossings are identified in magnetic field and plasma observations made by Cassini between June 2004 and August 2005. The Cassini crossings are then combined with the crossings made during the Saturn flybys of Pioneer 11, Voyager 1, and Voyager 2. Solar wind dynamic pressures for the Cassini crossings are estimated using upstream electron densities determined from Langmuir wave observations made by the Radio and Plasma Wave System. The crossing positions are rotated into aberrated coordinates to correct for the effect of the planets orbital motion. In the case of Saturn this rotation is by similar to 1 degrees. To correct for solar wind dynamic pressure variations, the crossing positions are normalized to the average pressure = 0.048 nPa. The model is then obtained by fitting a conic section to the crossings using a nonlinear least squares technique. To validate the assumptions made in constructing the model, we treat the parameters previously assumed to be constants as variables and fit their values using an optimization routine; this leads to a conic section that is within the positional uncertainty of the model. The spacecraft trajectories are considered, and we conclude that they do not significantly bias the model. The new model is compared to the existing models, and the similarities and differences are discussed. We suggest that the new model gives the most accurate empirical representation of the shape and location of Saturns bow shock.

Transient internally driven aurora at Jupiter discovered by Hisaki and the Hubble Space Telescope

Jupiters auroral emissions reveal energy transport and dissipation through the planets giant magnetosphere. While the main auroral emission is internally driven by planetary rotation in the steady state, transient brightenings are generally thought to be triggered by compression by the external solar wind. Here we present evidence provided by the new Hisaki spacecraft and the Hubble Space Telescope that shows that such brightening of Jupiters aurora can in fact be internally driven. The brightening has an excess power up to similar to 550 GW. Intense emission appears from the polar cap region down to latitudes around Ios footprint aurora, suggesting a rapid energy input into the polar region by the internal plasma circulation process.

The location of magnetic reconnection at Saturn's magnetopause: A comparison with Earth

Data from the Cassini Electron Spectrometer are used to investigate the location of magnetic reconnection at Saturns magnetopause. Heated, streaming electron distributions in the boundary layer on the magnetosheath side of the magnetopause are evidence of reconnection and an open magnetopause. A model for the location of reconnection is used to compare the modeled and observed streaming direction of the heated electron distributions. Magnetic reconnection at Saturns magnetopause is predicted and observed to occur at locations similar to those at Earths magnetopause. Although not conclusive, the results here are consistent with the expected importance of X-line drifts in suppressing low-shear reconnection. Because of different conditions at Saturns magnetopause, this suppression is predicted to be more severe at Saturn than at Earth.

Asymmetric distribution of reconnection jet fronts in the Jovian nightside magnetosphere

Magnetic reconnection plays important roles in mass transport and energy conversion in planetary magnetospheres. It is considered that transient reconnection causes localized auroral arcs or spots in the Jovian magnetosphere, by analogy to the case in the Earths magnetosphere. However, the local structures of transient reconnection events (i.e., magnetospheric plasma parameters) and their spatial distribution have not been extensively investigated for the Jovian magnetosphere. Here we examine plasma velocity and density during strong north-south magnetic field events in the Jovian nightside magnetosphere, which may be associated with tail reconnection. We find prominent reconnection jet fronts predominantly on the dawnside of the nightside magnetosphere, which would be a signature unique to rotation-dominant planetary magnetospheres. The observed plasma structures are consistent with significant field-aligned currents which would generate localized aurora.

Saturn's low‐latitude boundary layer: 1. Properties and variability

Transport of solar wind plasma into a planetary magnetosphere produces an internal boundary layer adjacent to the magnetopause that contains a mixture of solar wind and magnetospheric plasma. The potential processes responsible for this transport are varied, and their relative importance remains unclear, as well as how their competition varies between the magnetized planets. In this paper we examine the properties and variability of Saturns low-latitude boundary layer (LLBL) using observations made by the Cassini spacecraft; the electron structure of Saturns LLBL is examined in a companion paper. The duration of spacecraft excursions into the LLBL was generally between similar to 3 and similar to 23 min, and the electron environment of Saturns LLBL was intermediate between that of the magnetosheath and that of the magnetosphere, as expected. Estimates of the speed of the magnetopause current layer are of the order of 100 km s (1), and the estimated thickness of the LLBL is of the order of 1 Saturn radius. Our results do not reveal a strong influence of the orientation of the interplanetary magnetic field on the thickness of Saturns LLBL, which is unlike its terrestrial counterpart. There is also no clear dawn-dusk asymmetry in the thickness of Saturns LLBL, which might be expected on the basis of our understanding of the growth of the Kelvin-Helmholtz instability at Saturns magnetopause. We discuss the implications of these findings for the physics of Saturns magnetospheric boundary and what this could mean for our understanding of how boundary layers are formed in planetary magnetospheres.

Hot flow anomalies at Saturn's bow shock

[1] We present evidence for the occurrence of hot flow anomalies (HFAs) at Saturn’s bow shock. A survey of Cassini magnetic field and electron data taken upstream of the dawn flank bow shock is carried out in order to identify Kronian HFAs. Seventeen events are identified that were all associated with energization of the solar wind electrons and satisfied the majority of the conditions for HFA formation. The majority of the events possessed a central cavity of rarefied plasma; however, for two of the events the central cavity was over dense, possibly indicating that these examples were at an early stage of formation. For the event that occurred on 8 November 2004 the calculation of ion moments is possible, revealing an ion temperature increase by a factor of � 800 in the central region of the event that was associated with a significant deflection of the solar wind bulk flow. The spatial extent of the event was � 4.6 Saturn radii in the direction normal to the current sheet underlying the event. Pressure calculations imply that the heated central region was expanding at the time of the encounter. We conclude that the 8 November 2004 event was a spacecraft encounter with an HFA at Saturn’s bow shock and we propose that the other 16 events identified by the survey were also HFA encounters. These observations suggest that HFAs are a solar-system-wide phenomenon.

Electron acceleration to relativistic energies at a strong quasi-parallel shock wave

Data from the Cassini spacecraft identify strong electron acceleration as the solar wind approaches the magnetosphere of Saturn. This so-called bow shock unexpectedly occurs even when the magnetic field is roughly parallel to the shock-surface normal. Knowledge of the magnetic dependence of electron acceleration will aid understanding of supernova remnants.

Electron-Ion Temperature Equilibration in Collisionless Shocks: The Supernova Remnant-Solar Wind Connection

Collisionless shocks are loosely defined as shocks where the transition between pre-and post-shock states happens on a length scale much shorter than the collisional mean free path. In the absence of collision to enforce thermal equilibrium post-shock, electrons and ions need not have the same temperatures. While the acceleration of electrons for injection into shock acceleration processes to produce cosmic rays has received considerable attention, the related problem of the shock heating of quasi-thermal electrons has been relatively neglected.In this paper we review the state of our knowledge of electron heating in astrophysical shocks, mainly associated with supernova remnants (SNRs), shocks in the solar wind associated with the terrestrial and Saturnian bowshocks, and galaxy cluster shocks. The solar wind and SNR samples indicate that the ratio of electron temperature, (Te) to ion temperature (Tp) declining with increasing shock speed or Alfvén Mach number. We discuss the extent to which such behavior can be understood on the basis of waves generated by cosmic rays in a shock precursor, which then subsequently damp by heating electrons, and speculate that a similar explanation may work for both solar wind and SNR shocks.