Comparative Examination of Plasmoid Ejection at Mercury, Earth, Jupiter, and Saturn

The onset of magnetic reconnection in the near-tail of Earth, long known to herald the fast magnetospheric convection that leads to geomagnetic storms and substorms, is very closely associated with the formation and down-tail ejection of magnetic loops or flux ropes called plasmoids. Plasmoids form as a result of the fragmentation of preexisting cross-tail current sheet as a result of magnetic reconnection. Depending upon the number, location, and intensity of the individual reconnection X-lines and how they evolve, some of these loop-like or helical magnetic structures may also be carried sunward. At the inner edge of the tail they are expected to "re-reconnect' with the planetary magnetic field and dissipate. Plasmoid ejection has now been observed in the magnetotails of Mercury, Earth, Jupiter, and Saturn. These magnetic field and charged particle measurements have been taken by the MESSENGER, Voyager, Galileo, Cassini, and numerous Earth missions. Here we present a comparative examination of the structure and dynamics of plasmoids observed in the magnetotails of these 5 planets. The results are used to learn more about how these magnetic structures form and to assess similarities and differences in the nature of magnetotail reconnection at these planets

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

Machine Learning Applications to Kronian Magnetospheric Reconnection Classification

The products of magnetic reconnection in Saturn’s magnetotail are identified in magnetometer observations primarily through characteristic deviations in the north–south component of the magnetic field. These magnetic deflections are caused by traveling plasma structures created during reconnection rapidly passing over the observing spacecraft. Identification of these signatures have long been performed by eye, and more recently through semi-automated methods, however these methods are often limited through a required human verification step. Here, we present a fully automated, supervised learning, feed forward neural network model to identify evidence of reconnection in the Kronian magnetosphere with the three magnetic field components observed by the Cassini spacecraft in Kronocentric radial–theta–phi coordinates as input. This model is constructed from a catalog of reconnection events which covers three years of observations with a total of 2093 classified events, categorized into plasmoids, traveling compression regions and dipolarizations. This neural network model is capable of rapidly identifying reconnection events in large time-span Cassini datasets, tested against the full year 2010 with a high level of accuracy (87%), true skill score (0.76), and Heidke skill score (0.73). From this model, a full cataloging and examination of magnetic reconnection events in the Kronian magnetosphere across Cassini's near Saturn lifetime is now possible

Cassini observations of ionospheric plasma in Saturn's magnetotail lobes

Studies of Saturn's magnetosphere with the Cassini mission have established the importance of Enceladus as the dominant mass source for Saturn's magnetosphere. It is well known that the ionosphere is an important mass source at Earth during periods of intense geomagnetic activity but lesser attention has been dedicated to study the ionospheric mass source at Saturn. In this paper we describe a case study of data from Saturn's magnetotail, when Cassini was located at ∼2200 hours Saturn local time at 36 RS from Saturn. During several entries into the magnetotail lobe, tailward-flowing cold electrons and a cold ion beam were observed directly adjacent to the plasma sheet and extending deeper into the lobe. The electrons and ions appear to be dispersed, dropping to lower energies with time. The composition of both the plasma sheet and lobe ions show very low fluxes (sometimes zero within measurement error) of water group ions. The magnetic field has a swept-forward configuration which is atypical for this region and the total magnetic field strength larger than expected at this distance from the planet. Ultraviolet auroral observations show a dawn brightening and upstream heliospheric models suggest that the magnetosphere is being compressed by a region of high solar wind ram pressure. We interpret this event as the observation of ionospheric outflow in Saturn's magnetotail. We estimate a number flux between 2.95±0.43×109 1.43±0.21×1010 cm-2 s-1, one or about two orders magnitude larger than suggested by steady state MHD models, with a mass source between 1.4×102 and 1.1×103 kg/s. After considering several configurations for the active atmospheric regions, we consider as most probable the main auroral oval, with associated mass source between 49.7±13.4 and 239.8±64.8 kg/s for an average auroral oval, and 10±4 and 49±23 kg/s for the specific auroral oval morphology found during this event. It is not clear how much of this mass is trapped within the magnetosphere and how much is lost to the solar wind

Tailward propagation of magnetic energy density variations With respect to substorm onset times

During geomagnetic substorms, around 1015 J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail ( urn:x-wiley:jgra:media:jgra54303:jgra54303-math-0001) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail ( urn:x-wiley:jgra:media:jgra54303:jgra54303-math-0002). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time

Increases in plasma sheet temperature with solar wind driving during substorm growth phases

During the substorm growth phase, magnetic reconnection extracts ~10^15 J from the solar wind through magnetic reconnection at the magnetopause, which is then stored in the magnetotail lobes. Plasma sheet pressure then increases to balance magnetic flux density increases in the lobes. We examine plasma sheet pressure, density and temperature during substorm growth phases using nine years of Cluster data (>316,000 data points). We show that plasma sheet pressure and temperature are higher during growth phases with higher solar wind driving whereas the density is approximately constant. We also show a weak correlation between plasma sheet temperature before onset and the minimum SuperMAG SML auroral index in the subsequent substorm. We discuss how energization of the plasma sheet before onset may result from thermodynamically adiabatic processes; how hotter plasma sheets may result in magnetotail instabilities and how this relates to the onset and size of the subsequent substorm expansion phase

Classification of Cassini’s Orbit Regions as Magnetosphere, Magnetosheath, and Solar Wind via Machine Learning

Several machine learning algorithms and feature subsets from a variety of particle and magnetic field instruments on-board the Cassini spacecraft were explored for their utility in classifying orbit segments as magnetosphere, magnetosheath or solar wind. Using a list of manually detected magnetopause and bow shock crossings from mission scientists, random forest (RF), support vector machine (SVM), logistic regression (LR) and recurrent neural network long short-term memory (RNN LSTM) classification algorithms were trained and tested. A detailed error analysis revealed a RNN LSTM model provided the best overall performance with a 93.1% accuracy on the unseen test set and MCC score of 0.88 when utilizing 60 min of magnetometer data (|B|, Bθ, Bϕ and BR) to predict the region at the final time step. RF models using a combination of magnetometer and particle data, spanning H+, He+, He++ and electrons at a single time step, provided a nearly equivalent performance with a test set accuracy of 91.4% and MCC score of 0.84. Derived boundary crossings from each model’s region predictions revealed that the RNN model was able to successfully detect 82.1% of labeled magnetopause crossings and 91.2% of labeled bow shock crossings, while the RF model using magnetometer and particle data detected 82.4 and 74.3%, respectively

X-rays Studies of the Solar System

X-ray observatories contribute fundamental advances in Solar System studies by probing Sun-object interactions, developing planet and satellite surface composition maps, probing global magnetospheric dynamics, and tracking astrochemical reactions. Despite these crucial results, the technological limitations of current X-ray instruments hinder the overall scope and impact for broader scientific application of X-ray observations both now and in the coming decade. Implementation of modern advances in X-ray optics will provide improvements in effective area, spatial resolution, and spectral resolution for future instruments. These improvements will usher in a truly transformative era of Solar System science through the study of X-ray emission.Comment: White paper submitted to Astro2020, the Astronomy and Astrophysics Decadal Surve

Jupiter's X-ray Emission 2007 Part 2:Comparisons with UV and Radio Emissions and In-Situ Solar Wind Measurements

We compare Chandra and XMM‐Newton X‐ray observations of Jupiter during 2007 with a rich multi‐instrument dataset including: upstream in‐situ solar wind measurements from the New Horizons spacecraft, radio emissions from the Nançay Decametric Array and Wind/Waves, and UV observations from the Hubble Space Telescope. New Horizons data revealed two corotating interaction regions (CIRs) impacted Jupiter during these observations. Non‐Io decametric bursts and UV emissions brightened together and varied in phase with the CIRs. We characterise 3 types of X‐ray aurorae: hard X‐ray bremsstrahlung main emission, pulsed/flared soft X‐ray emissions and a newly identified dim flickering (varying on short‐timescales, but quasi‐continuously present) aurora. For most observations, the X‐ray aurorae were dominated by pulsed/flaring emissions, with ion spectral lines that were best fit by Iogenic plasma. However, the brightest X‐ray aurora was coincident with a magnetosphere expansion. For this observation, the aurorae were produced by both flickering emission and erratic pulses/flares. Auroral spectral models for this observation required the addition of solar wind ions to attain good fits, suggesting solar wind entry into the outer magnetosphere or directly into the pole for this particularly bright observation. X‐ray bremsstrahlung from high energy electrons was only bright for one observation, which was during a forward shock. This bremsstrahlung was spatially coincident with bright UV main emission (power> 1TW) and X‐ray ion spectral line dusk emission, suggesting closening of upward and downward current systems during the shock. Otherwise, the bremsstrahlung was dim and UV main emission power was also lower(<700 GW), suggesting their power scaled together