Magnetosphere–Ionosphere Coupling

The process of magnetosphere-ionosphere coupling involves the transport of vast quantities of energy and momentum between a magnetized planet and its space environment, or magnetosphere. This transport involves extended, global sheets of electrical current, which flows along magnetic field lines. Some of the charged particles, which carry this current rain down onto the planet’s upper atmosphere and excite aurorae–beautiful displays of light close to the magnetic poles, which are an important signature of the physics of the coupling process. The Earth, Jupiter, and Saturn all have magnetospheres, but the detailed physical origin of their auroral emissions differs from planet to planet. The Earth’s aurora is principally driven by the interaction of its magnetosphere with the upstream solar wind—a flow of plasma continually emanating from the Sun. This interaction imposes a particular pattern of flow on the plasma within the magnetosphere, which in turn determines the morphology and intensity of the currents and aurorae. Jupiter, on the other hand, is a giant rapid rotator, whose main auroral oval is thought to arise from the transport of angular momentum between the upper atmosphere and the rotating, disc-like plasma in the magnetosphere. Saturn exhibits auroral behavior consistent with a solar wind–related mechanism, but there is also regular variability in Saturn’s auroral emissions, which is consistent with rotating current systems that transport energy between the magnetospheric plasma and localized vortices of flow in the upper atmosphere/ionosphere.</p

Trapped Particle Motion in Magnetodisk Fields

The spatial and temporal characterization of trapped charged particle trajectories in magnetospheres has been extensively studied in dipole magnetic field structures. Such studies have allowed the calculation of spatial quantities, such as equatorial loss cone size as a function of radial distance, the location of the mirror points along particular field lines (L‐shells) as a function of the particle's equatorial pitch angle, and temporal quantities such as the bounce period and drift period as a function of the radial distance and the particle's pitch angle at the equator. In this study, we present analogous calculations for the disk‐like field structure associated with the giant rotation‐dominated magnetospheres of Jupiter and Saturn as described by the University College London/Achilleos‐Guio‐Arridge (UCL/AGA) magnetodisk model. We discuss the effect of the magnetodisk field on various particle parameters and make a comparison with the analogous motion in a dipole field. The bounce period in a magnetodisk field is in general smaller the larger the equatorial distance and pitch angle, by a factor as large as ∼8 for Jupiter and ∼2.5 for Saturn. Similarly, the drift period is generally smaller, by a factor as large as ∼2.2 for equatorial distances ∼20–24 RJ at Jupiter and ∼1.5 for equatorial distances ∼7–11 RS at Saturn

Automated bow shock and magnetopause boundary detection with Cassini using threshold and deep learning methods

Two algorithms set for automatic detection of bow shock (BS) and magnetopause (MP) boundaries at Saturn using in situ magnetic field and plasma data acquired by the Cassini spacecraft are presented. Traditional threshold-based and modern deep learning algorithms were investigated for the task of boundary detection. Sections of Cassini’s orbits were pre-selected based on empirical BS and MP boundary models, and from outlier detection in magnetic field data using an autoencoder neural network. The threshold method was applied to pre-selected magnetic field and plasma data independently to compute parameters to which a threshold was applied to determine the presence of a boundary. The deep learning method used a type of convolutional neural network (CNN) called ResNet on images of magnetic field time series data and electron energy-time spectrograms to classify the presence of boundaries. 2012 data were held out of the training data to test and compare the algorithms on unseen data. The comparison showed that the CNN method applied to plasma data outperformed the threshold method. A final multiclass CNN classifier trained on plasma data obtained F1 scores of 92.1% ± 1.4% for BS crossings and 84.7% ± 1.9% for MP crossings on a corrected test dataset (from use of a bootstrap method). Reliable automated detection of boundary crossings could enable future spacecraft experiments like the PEP instrument on the upcoming JUICE spacecraft mission to dynamically adapt the best observing mode based on rapid classification of the boundary crossings as soon as it appears, thus yielding higher quality data and improved potential for scientific discovery

Internally driven large-scale changes in the size of Saturn’s magnetosphere

Saturn’s magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the system’s dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10–15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively “plasma-loaded” or “plasma-depleted” states, defined in terms of the internal suprathermal plasma pressure

Magnetosphere-Ionosphere-Thermosphere coupling at Jupiter using a three-dimensional atmospheric general circulation model

Jupiter's upper atmosphere is ∼700 K hotter than predicted based on solar extreme ultraviolet heating alone. The reason for this still remains a mystery and is known as the “energy crisis.” It is thought that the interaction between Jupiter and its dynamic magnetosphere plays a vital role in heating its atmosphere to the observed temperatures. Here, we present a new model of Jupiter's magnetosphere‐ionosphere‐thermosphere‐coupled system where we couple a three‐dimensional atmospheric general circulation model to an axisymmetric magnetosphere model. We find that the model temperatures are on average ∼60 K, with a maximum of ∼200 K, hotter than the model's two‐dimensional predecessor making our high‐latitude temperatures comparable to the lower limit of observations. Stronger meridional winds now transport more heat from the auroral region to the equator increasing the equatorial temperatures. However, despite this increase, the modeled equatorial temperatures are still hundreds of kelvins colder than observed. We use this model as an intermediate step toward a three‐dimensional atmospheric model coupled to a realistic magnetosphere model with zonal and radial variation

Local time variation in the large-scale structure of Saturn's magnetosphere

The large‐scale structure of Saturn's magnetosphere is determined by internal and external factors, including the rapid planetary rotation rate, significant internal hot and cold plasma sources, and varying solar wind pressure. Under certain conditions the dayside magnetospheric magnetic field changes from a dipolar to more disk‐like structure, due to global force balance being approximately maintained during the reconfiguration. However, it is still not fully understood which factors dominantly influence this behavior, and in particular how it varies with local time. We explore this in detail using a 2‐D force‐balance model of Saturn's magnetodisk to describe the magnetosphere at different local time sectors. For model inputs, we use recent observational results that suggest a significant local time asymmetry in the pressure of the hot (>3 keV) plasma population, and magnetopause location. We make calculations under different solar wind conditions, in order to investigate how these local time asymmetries influence magnetospheric structure for different system sizes. We find significant day/night asymmetries in the model magnetic field, consistent with recent empirical studies based on Cassini magnetometer observations. We also find dawn‐dusk asymmetries in equatorial current sheet thickness, with the varying hot plasma content and magnetodisk radius having comparable influence on overall structure, depending on external conditions. We also find significant variations in magnetic mapping between the ionosphere and equatorial disk, and ring current intensity, with substantial enhancements in the night and dusk sectors. These results have consequences for interpreting many magnetospheric phenomena that vary with local time, such as reconnection events and auroral observations

Tracking counterpart signatures in Saturn's auroras and ENA imagery during large-scale plasma injection events

Saturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large‐scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures—separated by ~4 hr UT—form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout ~9 hr observation. Mapping of the ionospheric equatorward motion post‐dawn indicates a factor ~5 reduction of the magnetospheric source region's radial speed at a distance of ~14‐20 RS, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field‐aligned currents linked with the southern planetary period oscillation system, or the re‐energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning

No sodium in the vapour plumes of Enceladus

The discovery of water vapour and ice particles erupting from Saturn's moon Enceladus fuelled speculation that an internal ocean was the source. Alternatively, the source might be ice warmed, melted or crushed by tectonic motions. Sodium chloride (that is, salt) is expected to be present in a long-lived ocean in contact with a rocky core. Here we report a ground-based spectroscopic search for atomic sodium near Enceladus that places an upper limit on the mixing ratio in the vapour plumes orders of magnitude below the expected ocean salinity. The low sodium content of escaping vapour, together with the small fraction of salt-bearing particles, argues against a situation in which a near-surface geyser is fuelled by a salty ocean through cracks in the crust. The lack of observable sodium in the vapour is consistent with a wide variety of alternative eruption sources, including a deep ocean6, a freshwater reservoir, or ice. The existing data may be insufficient to distinguish between these hypotheses

Enceladus and Titan: Emerging Worlds of the Solar System (ESA Voyage 2050 White Paper)

Some of the major discoveries of the recent Cassini-Huygens mission have put Titan and Enceladus firmly on the Solar System map. The mission has revolutionised our view of Solar System satellites, arguably matching their scientific importance with that of their planet. While Cassini-Huygens has made big surprises in revealing Titan's organically rich environment and Enceladus' cryovolcanism, the mission's success naturally leads us to further probe these findings. We advocate the acknowledgement of Titan and Enceladus science as highly relevant to ESA's long-term roadmap, as logical follow-on to Cassini-Huygens. In this white paper, we will outline important science questions regarding these satellites and identify the pertinent science themes we recommend ESA cover during the Voyage 2050 planning cycle. Addressing these science themes would make major advancements to the present knowledge we have about the Solar System, its formation, evolution and likelihood that other habitable environments exist outside the Earth's biosphere