Modelling 1-D quasi-static potential structures at Jupiter

<p>Quasi-static potentials have long thought to be one of the significant drivers of the main ultraviolet emission associated with Jupiter’s auroral oval. The magnetic field lines connecting to the auroral zone extend into Jupiter’s middle magnetosphere, at radii of 20R<sub>J</sub> – 50 R<sub>J</sub>. Such quasi-static potential structures are capable of accelerating charged particles into the planetary ionosphere and generating aurora, with the Juno JEDI instrument observing inverted-V potential structures on the order of megavolts. However, Juno’s observation of quasi-static potentials has not been as ubiquitous as was initially theorised. Juno has observed more frequent instances of bi-directional electron beams on the same field line, indicating the presence of dynamic processes occurring at different altitudes. In addition, this suggests that quasi-static potentials may not be a significant driver for the main UV emission.</p><p> </p><p>In this paper, we present new results from a 1-D Vlasov model of the high-latitude magnetic field lines in the Jovian mid-magnetosphere. Our model is time-dependent and features a non-uniform mesh close to the ionosphere, allowing us to examine the formation of quasi-static potential structures in the upward current region over the course of a simulation. We will also present simulations showing the collapse and reformation of these potential structures, with the collapse showing the propagation of electron beams in both directions along the modelled field line.</p>

Comparison of Terrestrial and Martian TEC at Dawn and Dusk during Solstices

<p class="p1">Planetary ionospheres undergo many changes at dawn and dusk due to both photochemical and transport processes.<span class="Apple-converted-space">  </span>The relative importance of these different processes vary depending on a variety of factors, including the amount of solar radiation, the composition of the thermosphere, and the characteristics of any local magnetic fields.<span class="Apple-converted-space">  </span>This study uses the similarities between the ionospheres on Mars and Earth to examine the behaviour of the ionosphere at dawn and dusk.<span class="Apple-converted-space">  </span>It has notable implications for comparative aeronomy, as a solid understanding of ionospheric processes on planets with and without magnetic fields is important for characterising the environments of solar and exoplanets, as well as atmospheric evolution over long time scales.</p> <p class="p1">The amount of plasma present in the ionosphere was measuring using the total electron content (TEC), and grouped so that both solstices and different phases of the solar cycle could be examined.<span class="Apple-converted-space">  </span>To allow comparisons between the ionospheres of Mars and Earth, which differ greatly in density, the rate of change of TEC as a function of solar zenith angle was used to compare the plasma production and losses in the main layer of each planetary ionosphere.<span class="Apple-converted-space">  </span>Examination of the dawn and dusk TEC slopes shows that, to first order, the Martian slopes are symmetric while those at Earth are not.<span class="Apple-converted-space">  </span>This symmetry is interpreted as an indicator of photochemical equilibrium, and different reasons for deviations from symmetry were explored.<span class="Apple-converted-space">  </span>The presence or absence of a magnetic field played a large role in shaping plasma transport, with photochemical processes in both ionospheres behaving similarly in the absence of a magnetic field.<span class="Apple-converted-space">  </span>At Mars, it was found that transport processes were most important at solar maximum, while at Earth transport processes were most important at solar minimum. </p>

Investigating the effects of the planetary rings on the azimuthal magnetic field at Saturn

<p>Magnetic field observations from the 22 Cassini Grand Finale orbits have shown a mean lagging azimuthal magnetic field configuration on magnetic field lines mapping from Saturn to its main rings in the equatorial plane, with some orbit to orbit variability. A prominent feature is observed in the southern hemisphere on field lines connecting to the B-ring on 70% of the orbits, which is spatially consistent with the location of in-falling dust indicated by the Cosmic Dust Analyser instrument. In our work, we examine the possible connection between the in-falling charged dust and the B-ring magnetic field feature. We also use a simple steady-state model to couple the planetary ionosphere to a weakly conducting ring ionosphere over the main rings, where the model output shows an expected leading field configuration associated with the rings. The discrepancy between the simple theoretical model and the data indicates the presence of additional processes (e.g. departure from Keplerian velocity of the charged ring particles), which will be discussed. We will further discuss the likely connection between the observed lagging field configuration in the middle magnetosphere and in the inner magnetosphere.  </p>

Probability of occurrence of planetary ionosphere storms associated with the magnetosphere disturbance storm time events

The ionospheric W index allows to distinguish state of the ionosphere and plasmasphere from quiet conditions (W = 0 or ±1) to intense storm (W = ±4) ranging the plasma density enhancements (positive phase) or plasma density depletions (negative phase) regarding the quiet ionosphere. The global W index maps are produced for a period 1999–2014 from Global Ionospheric Maps of Total Electron Content, GIM-TEC, designed by Jet Propulson Laboratory, converted from geographic frame (−87.5:2.5:87.5° in latitude, −180:5:180° in longitude) to geomagnetic frame (−85:5:85° in magnetic latitude, −180:5:180° in magnetic longitude). The probability of occurrence of planetary ionosphere storm during the magnetic disturbance storm time, Dst, event is evaluated with the superposed epoch analysis for 77 intense storms (Dst ≤ −100 nT) and 230 moderate storms (−100 < Dst ≤ −50 nT) with start time, t0, defined at Dst storm main phase onset. It is found that the intensity of negative storm, iW-, exceeds the intensity of positive storm, iW+, by 1.5–2 times. An empirical formula of iW+ and iW- in terms of peak Dst is deduced exhibiting an opposite trends of relation of intensity of ionosphere-plasmasphere storm with regard to intensity of Dst storm.

Significance of Dungey-cycle flows in Jupiter's and Saturn's magnetospheres, and their identification on closed equatorial field lines

We consider the contribution of the solar wind-driven Dungey-cycle to flux transport in Jupiter's and Saturn's magnetospheres, the associated voltages being based on estimates of the magnetopause reconnection rates recently derived from observations of the interplanetary medium in the vicinity of the corresponding planetary orbits. At Jupiter, the reconnection voltages are estimated to be ~150 kV during several-day weak-field rarefaction regions, increasing to ~1 MV during few-day strong-field compression regions. The corresponding values at Saturn are ~25 kV for rarefaction regions, increasing to ~150 kV for compressions. These values are compared with the voltages associated with the flows driven by planetary rotation. Estimates of the rotational flux transport in the "middle" and "outer" magnetosphere regions are shown to yield voltages of several MV and several hundred kV at Jupiter and Saturn respectively, thus being of the same order as the estimated peak Dungey-cycle voltages. We conclude that under such circumstances the Dungey-cycle "return" flow will make a significant contribution to the flux transport in the outer magnetospheric regions. The "return" Dungey-cycle flows are then expected to form layers which are a few planetary radii wide inside the dawn and morning magnetopause. In the absence of significant cross-field plasma diffusion, these layers will be characterized by the presence of hot light ions originating from either the planetary ionosphere or the solar wind, while the inner layers associated with the Vasyliunas-cycle and middle magnetosphere transport will be dominated by hot heavy ions originating from internal moon/ring plasma sources. The temperature of these ions is estimated to be of the order of a few keV at Saturn and a few tens of keV at Jupiter, in both layers.

Physics of the Ion Composition Boundary: a comparative 3-D hybrid simulation study of Mars and Titan

The plasma environments of Mars and Titan have been studied by means of a 3-D hybrid simulation code, treating the electrons as a massless, charge-neutralizing fluid, whereas ion dynamics are covered by a kinetic approach. As neither Mars nor Titan possesses a significant intrinsic magnetic field, the upstream plasma flow interacts directly with the planetary ionosphere. The characteristic features of the interaction region are determined as a function of the alfvénic, sonic and magnetosonic Mach number of the impinging plasma. For the Martian interaction with the solar wind as well as for the case of Titan being located outside Saturn's magnetosphere in times of high solar wind dynamic pressure, all three Mach numbers are larger than 1. In such a scenario, the interaction gives rise to a so-called Ion Composition Boundary, separating the ionospheric plasma from the ambient flow and being highly asymmetric with respect to the direction of the convective electric field. The formation of these features is explained by analyzing the Lorentz forces acting on ionospheric and ambient plasma particles. Titan's plasma environment is highly variable and allows various different combinations of the three Mach numbers. Therefore, the Ion Composition Boundary may vanish under certain circumstances. The relevant physical mechanism is illustrated as a function of the Mach numbers in the upstream plasma flow.