MIST

By Gregory J. Hunt, Department of Physics, Imperial College London, UK.

In a magnetized planetary system, large-scale electrical currents that flow along the magnetic field lines are fundamental in the transfer of angular momentum through the coupling of the magnetosphere and ionosphere [e.g., Cowley, 2000]. In the case of Saturn, two such types of these current systems have been deduced from Cassini magnetometer data and studied in detail [e.g. Bunce et al., 2008; Talboys et al., 2009a; Talboys et al., 2009b; Southwood & Kivelson, 2009; Talboys et al., 2011; Hunt et al., 2014, 2015, 2016; Bradley et al., 2018]. The first type is an axisymmetric, quasi-static field-aligned current system, which is associated with the transfer of angular momentum from the planet to Saturn’s outer magnetospheric plasma. The second type is associated with the planetary period oscillation (PPO) phenomenon at Saturn [e.g., Carbary & Mitchell, 2013]. Specifically, there are two rotating field-aligned current systems with oppositely directed currents on either side of the pole. One is associated with the northern hemisphere and the other with the southern hemisphere. These two rotating current systems result in the near 10.7-hour oscillations observed throughout the Saturnian system [e.g., Southwood & Kivelson, 2007; Andrews et al., 2010; Southwood & Cowley, 2014].

Hunt et al. [2018a] performed a statistical survey for both the northern and southern hemisphere auroral field-aligned current regions from a set of orbits prior to Cassini’s Grand Finale, known as the F-ring orbits. This analysis showed in each hemisphere there was the quasi-static and that hemisphere’s PPO field aligned current systems. Interestingly, the PPO current systems’ strengths had decreased by approximately 50% when compared to previous results [Hunt et al., 2014, 2015]. This reduction is in agreement with a decrease in the PPO amplitudes as determined by Hunt et al. [2018b]. The general form and strengths of the overall current profiles for both hemispheres are shown in the figure below. Other differences were observed in the azimuthal field poleward and equatorward of the field-aligned current region. These imply possible seasonal and local time effects on the overall field-aligned current structure and azimuthal field topology.

For more information, see our paper below:

Hunt, G. J., Provan, G., Bunce, E. J., Cowley, S. W. H., Dougherty, M. K., & Southwood, D. J. (2018a). Field‐aligned currents in Saturn's magnetosphere: Observations from the F‐ring orbits. Journal of Geophysical Research: Space Physics, 123, 3806–3821. https://doi.org/10.1029/2017JA025067

Figure: Overall current profiles versus northern (a) and southern (b) ionospheric colatitudes. Coloured profiles are the F-ring orbit data, with color code shown at the top of the figure. A mean profile is shown by the joined filled circles. (c, d) Comparison between the F-ring orbit mean profiles from (a) and (b) and the 2008 mean profile (joined crosses) for the northern and southern hemisphere, respectively. The error bars are the standard deviation of the F-ring means. Grey shaded regions are standard deviation of the 2008 means. Black squares show colatitude bins where Welch’s T test shows the 2008 and F-ring averages are significantly different. The open-closed field line boundary (OCB) is shown by the vertical dashed lines.

By Catherine Diéval, Department of Physics, Lancaster University, UK.

The dayside Mars ionosphere is thought to be reasonably well understood (see e.g. a review by Withers, 2009). The top of the ionosphere is influenced, among various factors, by localized crustal magnetic fields (e.g. Acuña et al., 1999), solar EUV and solar wind input, in the absence of a global magnetic moment. However a peculiar ionospheric feature is still the subject of ongoing research: non-horizontal electron density structures are regularly observed in localized areas with strong and near vertical crustal magnetic fields, in the topside ionospheric levels remotely sampled by the MARSIS radar (Picardi et al., 2004) onboard the Mars Express orbiter (e.g. Andrews et al. 2014; Diéval et al., 2015; Duru et al., 2006; Gurnett et al., 2005). These structures are detectable via oblique echoes returned to the radar after it sends a radio wave pulse through the ionosphere.The reflectors often appear at higher apparent altitude than the surrounding ionosphere, and so are nicknamed "bulges".

Previous studies also used radar returns uncorrected for signal dispersion. Actually, the group velocity of the radio waves varies with the refractive index of the plasma layers encountered, until reflection occurs. The apparent ranges of the received echoes are calculated using the time delays of the echoes and assuming the speed of light in vacuum. However this leads to overestimating the ranges,so interpretations on the shape of the structures based on these are uncertain.

Our work (Diéval et al., 2018), is a statistical study using timeseries of electron density profiles (electron density function of altitude, corrected for signal dispersion) to study the shape of 48 structures, in their full frequency (thus altitude) range, during the period that Mars Express passes over them.

Figure 1 shows that at any frequency, the most frequent shape is the bulge, dwarfing three other types of detected shapes: dips, downhill slopes and uphill slopes. All these shapes are inclined, thus able to reflect oblique echoes. Interestingly, bulges were reproduced in simulation results of Matta et al. (2015).

For more information, see the paper below:

Diéval, C., Kopf, A. J., & Wild, J. A. (2018). Shapes of magnetically controlled electron density structures in the dayside Martian ionosphere. Journal of Geophysical Research: Space Physics, 123, 3919–3942. https://doi.org/10.1002/2017JA025140

Figure 1: Distribution of the four simplest shapes of structures as a function of frequency, for the 48 events, displayed as colored symbols: bulges (red dots), dips (black diamonds), uphill slopes (blue ‘x’), downhill slopes (green ‘+’). Data points at frequency levels within in the sensitivity gaps are not displayed.

By Andrew Smith, Department of Physics and Astronomy, University of Southampton, UK.

Magnetic reconnection in a planet's magnetotail allows the stretched field to snap back towards the planet, carrying with it a bundle of plasma.  This is known as a dipolarization front, which often manifest in spacecraft data as rapid rotations of the magnetic field accompanied by a change in the local plasma character.  Dipolarization fronts have been observed at Earth, Mercury, Jupiter and Saturn and are thought to be linked to bright auroral displays.

We performed a large automated survey of Cassini data, identifying 28 intervals when the spacecraft was in the path of dipolarization fronts sweeping towards Saturn.  The changes in plasma properties were investigated, along with the supra-thermal composition.  A large dawn-dusk asymmetry was present in the observations, with 79% of the events located post-midnight.  Figure 1 shows the change in plasma characteristics from that preceding the front (a) to within the dipolarizing material (b).  All of the identified events showed an increase in the electron temperature and a coupled reduction in the electron density.  Figures 1c and (d) show the relative change in temperature and density respectively.  Overall, the temperature was found to increase by factors between 4 and 12, while the density dropped by factors of 3-10.  The variable plasma properties are thought to be linked to a variable reconnection location, particularly post-midnight.

Figure 1: Panels (a) and (b) show the electron density plotted aainst the electron temperature for before (a) and after (b) the dipolarization front.  These panels are plotted on the same axes scale for direct comparison.  The gray lines indicate how the events move in density-temperature space.  Panels (c) and (d) show the electron temperature and density (respectively) before the front plotted against the electron temperature and density after the passage of the front.  The points and error bars provided are the mean and standard error of the mean respectively.  The diagonal black dashed line shows the location of $y = x$: where the points would lie if there was no change following the passage of the front.  The red dashed lines indicate least squares linear fits to the data; the details of the fit parameters are provided on the panels.  The color bar for all four panels indicates the radial distance at which the spacecraft encountered the event.

For more information, see the paper below:

Smith, A. W., Jackman, C. M., Thomsen, M. F., Sergis, N., Mitchell, D. G., & Roussos, E. (2018). Dipolarization fronts with associated energized electrons in Saturn's magnetotail. Journal of Geophysical Research: Space Physics, 123, 2714–2735. https://doi.org/10.1002/2017JA024904

By Jennifer Carter, Department of Physics and Astronomy, University of Leicester, UK

Under northward interplanetary magnetic field conditions, when the IMF Bz > 0 nT, non-filamentary auroral emissions may be seen within the dayside polar cap and separate from the main auroral oval. These emissions are associated with lobe reconnection occurring at the high-latitude magnetopause on open field lines. Two mechanisms have been proposed to explain these emissions. The first involves the precipitation of magnetosheath plasma at the footprint of the high-latitude reconnection site, resulting in a “cusp spot”. This cusp spot has been shown to move in response to the east-west (B_Y) orientation of the solar wind. The second mechanism associates the auroral emissions known as High-Latitude Detached Arcs (HiLDAs) with upward field-aligned currents inside the polar cap. Under northward IMF, twin-cell field-aligned currents (NBZ system) can be found inside of the main region 1-region 2 field aligned current system. Under the influence of positive IMF B_Y, the upward NBZ cell expands across the noon sector in the Northern Hemisphere, whereas under negative B_Y, the downward cell will enlarge. The reverse scenario occurs in the Southern Hemisphere for either B_Ydirection.

Previous observations of HiLDAs have been limited to the Northern Hemisphere for a small data set, and previous authors have linked this phenomenon to season, as the HiLDAs have only been detected during the summer. We used concurrent auroral observations from Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imager (SSUSI) experiment, and FAC distributions constructed from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), from the Iridium telecommunication satellite constellation, to perform a large statistical study of HiLDAs under varying IMF for both hemispheres. We observe a patch of auroral emission that is co-located with the upward NBZ FAC in the dayside polar cap in both the Northern and Southern Hemispheres under northward IMF conditions.

We observe the HiLDA emission to move in response to changes in the IMF B_Ycomponent (e.g. Figure 1), whereby the HiLDAs are seen to move into the polar cap under positive B_Y, or be pushed up against, and therefore indiscernible from, the main auroral oval under negative B_Y(Northern Hemisphere case). We also support the hypothesis that these emissions are only detectable in the summer hemisphere, indicating a dependence on ionospheric conductivity via photoionisation in the predominantly sunlit hemisphere.

For more information, see the paper below:

Carter, J. A., Milan, S. E., Fogg, A. R., Paxton, L. J., & Anderson, B. J. (2018). The association of high‐latitude dayside aurora with NBZ field‐aligned currents. Journal of Geophysical Research: Space Physics, 123. https://doi.org/10.1029/2017JA025082

Figure 1: Northern Hemisphere summer auroral emissions in the Lyman-Birge-Hopfield long band with overlaid field-aligned current contours, for the Northern (N, row a) and Southern (S, row b) Hemispheres. Clock angles are given in the left-hand column. Interplanetary magnetic field magnitudes are between 5 and 10 nT. Field-aligned current contours are overlaid for upward (red) and downward (turquoise) currents, at absolute magnitudes of 0.1 (solid line), 0.3 (dashed line), and 0.5 (dotted line) μA/m2.