Imperial College London


Faculty of Natural SciencesDepartment of Physics

Senior Lecturer







6M69Huxley BuildingSouth Kensington Campus





Publication Type

106 results found

Zomerdijk-Russell S, Masters A, Korth H, Heyner Det al., 2023, Modelling the time-dependent magnetic fields that BepiColombo will use to probe down into Mercury’s mantle, Geophysical Research Letters, Vol: 50, ISSN: 0094-8276

External solar wind variability causes motion of the magnetopause and changes of this boundary's current structure, and the resulting inductive processes, may be exploited to determine the interior structure of magnetized planets. In preparation for the arrival of the BepiColombo spacecraft at Mercury, we here assess solar wind ram pressure forcing in this planet's environment, through analysis of data acquired by the Helios spacecraft, and the impact on the magnetopause's inducing field. These measurements suggest that BepiColombo will see highly unpredictable solar wind conditions and that the inducing field generated in response to variable solar wind ram pressure is non-uniform across the planet's surface. The inducing magnetic field spectrum, with frequencies in the range of ∼5.5 x10¯⁵ -1.5 x 10¯²Hz, suggests that the transfer functions derived from the two BepiColombo spacecraft could allow us to obtain a profile of conductivity through Mercury's crust and mantle.

Journal article

Masters A, Ioannou C, Rayns N, 2022, Does Uranus’ asymmetric magnetic field produce a relatively weak proton radiation belt?, Geophysical Research Letters, Vol: 49, ISSN: 0094-8276

Since the Voyager 2 flyby in 1986 the radiation belts of Uranus have presented a problem for physicists. The observations indicate the electron radiation belt is far more intense than the proton radiation belt, and while the electron intensities are close to the upper theoretical limit, proton intensities are well below. Here we propose the relatively weak proton radiation belt could be due to Uranus' asymmetric magnetic field. We model test particle motion through the field to show that perturbations arising from asymmetry are greater the larger the particle gyroradius, predominantly affecting urn:x-wiley:00948276:media:grl65197:grl65197-math-0001100-keV protons. For these particles, more rapid changes in maximum distance from the planet during a bounce motion promote trajectory evolution into regions where they could be lost through impact with the rings, impact with the atmosphere, or to the distant magnetosphere and solar wind. We suggest this could explain a relatively weak proton radiation belt at Uranus.

Journal article

Paranicas C, Mauk B, Kollmann P, Clark G, Haggerty D, Westlake J, Liuzzo L, Masters A, Cassidy T, Bagenal F, Bolton Set al., 2022, Energetic charged particle fluxes relevant to Ganymede's polar region, Geophysical Research Letters, Vol: 49, ISSN: 0094-8276

The JEDI instrument made measurements of energetic charged particles near Ganymede during a close encounter with that moon. Here we find ion flux levels are similar close to Ganymede itself but outside its magnetosphere and on near wake and open field lines. But energetic electron flux levels are more than a factor of 2 lower on polar and near-wake field lines than on nearby Jovian field lines at all energies reported here. Flux levels are relevant to the weathering of the surface, particularly processes that affect the distribution of ice, since surface brightness has been linked to the open-closed field line boundary. For this reason, we estimate the sputtering rates expected in the polar regions due to energetic heavy ions. Other rates, such as those related to radiolysis by plasma and particles that can reach the surface, need to be added to complete the picture of charged particle weathering.

Journal article

Mousis O, Bouquet A, Langevin Y, Andre N, Boithias H, Durry G, Faye F, Hartogh P, Helbert J, Iess L, Kempf S, Masters A, Postberg F, Renard J-B, Vernazza P, Vorburger A, Wurz P, Atkinson DH, Barabash S, Berthomier M, Brucato J, Cable M, Carter J, Cazaux S, Coustenis A, Danger G, Dehant V, Fornaro T, Garnier P, Gautier T, Groussin O, Hadid LZ, Ize J-C, Kolmasova I, Lebreton J-P, Le Maistre S, Lellouch E, Lunine JI, Mandt KE, Martins Z, Mimoun D, Nenon Q, Munoz Caro GM, Rannou P, Rauer H, Schmitt-Kopplin P, Schneeberger A, Simons M, Stephan K, Van Hoolst T, Vaverka J, Wieser M, Woerner Let al., 2022, Moonraker: Enceladus Multiple Flyby Mission, PLANETARY SCIENCE JOURNAL, Vol: 3

Journal article

Masters A, Sergis N, Sulaiman A, Palmaerts B, Hunt Get al., 2022, Near-magnetic-field-aligned energetic electrons above Saturn’s dark polar regions, Journal of Geophysical Research: Space Physics, Vol: 127, Pages: 1-14, ISSN: 2169-9380

Saturn's main auroral emissions define two oval-shaped regions, one encircling each magnetic pole. The regions at higher latitudes are generally “dark” (i.e., devoid of auroras), and are magnetically connected to the distant planetary magnetosphere where there is a much-debated interaction with the solar wind. Electric currents flow into the atmosphere along the magnetic field within these polar regions. Establishing whether polar magnetic flux is “open” or “closed” is key for diagnosing how the solar wind interaction works. Because energetic electrons moving almost parallel or anti-parallel to the magnetic field shed light on the field topology, we survey Cassini energetic particle data for rare instances when the spacecraft was able to measure these parts of the distribution in the polar field environment close to the planet. Over the entire mission we find 16 intervals when measurements at ∼0urn:x-wiley:21699380:media:jgra57498:jgra57498-math-0001 and ∼180urn:x-wiley:21699380:media:jgra57498:jgra57498-math-0002 pitch angles were made simultaneously without sunlight contamination. Across all the events, above-background field-aligned fluxes were measured intermittently by the >15 keV electron channels, extending up to ∼300 keV when present. Uni-directional anti-planetward fluxes were observed during 10 of the events, and bi-directional fluxes were observed during 6 of the events. We suggest the uni-directional anti-planetward fluxes indicate the presence of field-aligned beams, and that the bi-directional fluxes indicate regions of locally closed magnetic field. These results either mean the solar wind interaction is predominantly via global magnetic reconnection but is more complex than initially proposed, or that the interaction is instead predominantly “viscous-like” at Saturn.

Journal article

Sulaiman A, Mauk B, Szalay J, Allegrini F, Clark G, Gladstone R, Kotsiaros, Kurth W, Bagenal F, Bonfond B, Connerney J, Ebert R, Elliott S, Gershman D, Hospodarsky G, Hue V, Lysak R, Masters A, Santolik, Saur J, Bolton Set al., 2022, Jupiter’s low-altitude auroral zones: Fields, particles, plasma waves, and density depletions, Journal of Geophysical Research: Space Physics, Vol: 127, ISSN: 2169-9380

The Juno spacecraft's polar orbits have enabled direct sampling of Jupiter's low-altitude auroral field lines. While various data sets have identified unique features over Jupiter's main aurora, they are yet to be analyzed altogether to determine how they can be reconciled and fit into the bigger picture of Jupiter's auroral generation mechanisms. Jupiter's main aurora has been classified into distinct “zones”, based on repeatable signatures found in energetic electron and proton spectra. We combine fields, particles, and plasma wave data sets to analyze Zone-I and Zone-II, which are suggested to carry upward and downward field-aligned currents, respectively. We find Zone-I to have well-defined boundaries across all data sets. H+ and/or H3+ cyclotron waves are commonly observed in Zone-I in the presence of energetic upward H+ beams and downward energetic electron beams. Zone-II, on the other hand, does not have a clear poleward boundary with the polar cap, and its signatures are more sporadic. Large-amplitude solitary waves, which are reminiscent of those ubiquitous in Earth's downward current region, are a key feature of Zone-II. Alfvénic fluctuations are most prominent in the diffuse aurora and are repeatedly found to diminish in Zone-I and Zone-II, likely due to dissipation, at higher altitudes, to energize auroral electrons. Finally, we identify significant electron density depletions, by up to 2 orders of magnitude, in Zone-I, and discuss their important implications for the development of parallel potentials, Alfvénic dissipation, and radio wave generation.

Journal article

Agiwal O, Masters A, Hunt G, Dougherty Met al., 2022, The contribution of planetary period oscillations towards circulation and mass loss in Saturn’s magnetosphere, Journal of Geophysical Research: Space Physics, Vol: 127, Pages: 1-17, ISSN: 2169-9380

Magnetic reconnection is a process during which magnetic energy is released as kinetic energy. It is considered a crucial driver of energy transport and mass loss within Saturn's magnetosphere. On long-term timescales, is thought to be predominantly driven by the rapid rotation of equatorially mass-loaded flux tubes (i.e., the Vasyliunas cycle), but there is some non-negligible driving from the solar wind as well (i.e., the Dungey cycle). In this study, we investigate an atmospheric driven phenomenon that modulates Saturn's magnetosphere every ∼10.6–10.8 hr, known as planetary period oscillations (PPOs), as an additional driver of magnetic reconnection at Saturn. Using an empirical model of PPO dynamics and Cassini magnetic field and plasma measurements, we find that PPO-driven magnetic reconnection is likely to occur in Saturn's magnetosphere, however, the occurrence of the phenomenon depends on temporally variable characteristics of the PPO systems and spatial asymmetries within Saturn's equatorial magnetosphere. Thus, it is not expected to be an on-going process. On year-long timescales, we find that PPOs are expected to be on par with the Dungey Cycle in driving circulation within Saturn's magnetosphere. However, on ∼1–2 weeks-long timescales, under specific conditions where PPO-driven reconnection is expected to be active, this phenomenon can become more significant than the Vasyliunas cycle, and thus dominate circulation within Saturn's magnetosphere. On year-long timescales, this process is estimated to remove upwards of ∼20% of the mass loaded into the magnetosphere by Enceladus.

Journal article

Cochrane CJ, Vance SD, Nordheim TA, Styczinski M, Masters A, Regoli LHet al., 2021, In search of subsurface oceans within the Uranian moons, Journal of Geophysical Research: Planets, Vol: 126, ISSN: 2169-9097

The Galileo mission to Jupiter discovered magnetic signatures associated with hidden sub-surface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter’s strong time-varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager-2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager-2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with sub-surface oceans. Future missions to the ice giants may therefore be capable of discovering sub-surface oceans, thereby adding to the family of known “ocean worlds” in our solar system. Here, we assess magnetic induction as a technique for investigating sub-surface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity, and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single-pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.

Journal article

Lai, Jia, Russell, Jia X, Masters A, Dougherty M, Cuiet al., 2021, Magnetic flux circulation in the Saturnian magnetosphere as constrained by Cassini observations in the inner magnetosphere, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-9, ISSN: 2169-9380

In steady state, magnetic flux conservation must be maintained in Saturn’s magnetosphere. The Enceladus plumes add mass to magnetic flux tubes in the inner magnetosphere, and centrifugal force pulls the mass-loaded flux tubes outward. Those flux tubes are carried outward to the magnetotail where they deposit their mass and return to the mass loading region. It may take days for the magnetic flux to be carried outward to the tail, but the return of the nearly empty flux tubes can last only several hours, with speeds of inward motion around 200 km/s. Using time sequences of Cassini particle count rate, the difference in curvature drift and gradient drift is accounted for to determine the return speed, age, and starting dipole L-shell of return flux tubes. Determination of this flux-return process improves our understanding of the magnetic flux circulation at Saturn and provides insight into how other giant planets remove the mass added by their moons.

Journal article

Palmerio, Nieves-Chinchilla, Kilpua, Barnes D, Zhukov, Jian, Witasse, Provan, Tao, Lamy, Bradley, Mays, Mostl, Roussos, Futaana, Masters A, Sanchez-Canoet al., 2021, Magnetic structure and propagation of two interacting CMEs from the Sun to Saturn, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-28, ISSN: 2169-9380

One of the grand challenges in heliophysics is the characterization of coronal mass ejection (CME) magnetic structure and evolution from eruption at the Sun through heliospheric propagation. At present, the main difficulties are related to the lack of direct measurements of the coronal magnetic fields and the lack of 3D in-situ measurements of the CME body in interplanetary space. Nevertheless, the evolution of a CME magnetic structure can be followed using a combination of multi-point remote-sensing observations and multi-spacecraft in-situ measurements as well as modeling. Accordingly, we present in this work the analysis of two CMEs that erupted from the Sun on April 28, 2012. We follow their eruption and early evolution using remote-sensing data, finding indications of CME–CME interaction, and then analyze their interplanetary counterpart(s) using in-situ measurements at Venus, Earth, and Saturn. We observe a seemingly single flux rope at all locations, but find possible signatures of interaction at Earth, where high-cadence plasma data are available. Reconstructions of the in-situ flux ropes provide almost identical results at Venus and Earth but show greater discrepancies at Saturn, suggesting that the CME was highly distorted and/or that further interaction with nearby solar wind structures took place before 10 AU. This work highlights the difficulties in connecting structures from the Sun to the outer heliosphere and demonstrates the importance of multi-spacecraft studies to achieve a deeper understanding of the magnetic configuration of CMEs.

Journal article

Rymer AM, Runyon KD, Clyde B, Nunez JI, Nikoukar R, Soderlund KM, Sayanagi K, Hofstadter M, Quick LC, Stern SA, Becker T, Hedman M, Cohen I, Crary F, Fortney JJ, Vertesi J, Hansen C, de Pater I, Paty C, Spilker T, Stallard T, Hospodarsky GB, Smith HT, Wakeford H, Moran SE, Annex A, Schenk P, Ozimek M, Arrieta J, McNutt Jr RL, Masters A, Simon AA, Ensor S, Apland CT, Bruzzi J, Patthoff DA, Scott C, Campo C, Krupiarz C, Cochrane CJ, Gantz C, Rodriguez D, Gallagher D, Hurley D, Crowley D, Abel E, Provornikova E, Turtle EP, Clark G, Wilkes J, Hunt J, Roberts JH, Rehm J, Murray K, Wolfarth L, Fletcher LN, Spilker L, Martin ES, Parisi M, Norkus M, Izenberg N, Stough R, Vervack Jr RJ, Mandt K, Stevenson KB, Kijewski S, Cheng W, Feldman JD, Allen G, Prabhu D, Dutta S, Young C, Williams Jet al., 2021, Neptune Odyssey: A Flagship Concept for the Exploration of the Neptune-Triton System, PLANETARY SCIENCE JOURNAL, Vol: 2

Journal article

Zomerdijk-Russell S, Masters A, Heyner D, 2021, Variability of the interplanetary magnetic field as a driver of electromagnetic induction in Mercury’s interior, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-15, ISSN: 2169-9380

Mercury’s magnetosphere is a unique and dynamic system, primarily due to the proximity of the planet to the Sun and its small size. Interactions between solar wind and embedded Interplanetary Magnetic Field (IMF) and the dayside Hermean magnetosphere drive an electric current on the system’s magnetopause boundary. So far, electromagnetic induction due to magnetopause motion in response to changing external pressure has been used to constrain Mercury’s iron core size. Here we assess the impact a changing IMF direction has on the Hermean magnetopause currents, and the resulting inducing magnetic field. Observations made by MESSENGER during dayside magnetopause boundary crossings in the first ‘hot season’, are used to demonstrate the importance of the IMF direction to Mercury’s magnetopause currents. Our 16 boundary crossings show that introduction of external IMFs change the magnetopause current direction by 10° to 100°, compared to the case where only the internal planetary field is considered. Analytical modelling was used to fill in the bigger picture and suggests for an east-west reversal of the IMF, typical of the heliospheric current 3 sheet sweeping over Mercury’s magnetosphere, the inducing field at Mercury’s surface caused by the resulting magnetopause current dynamics is on the order of 30% of the global planetary field. These results suggest that IMF variability alone has an appreciable effect on Mercury’s magnetopause current and generates a significant inducing magnetic field around the planet. The arrival of the BepiColombo mission will allow this response to be further explored as a method of probing Mercury’s interior.

Journal article

Kaweeyanun N, Masters A, Jia X, 2021, Analytical assessment of Kelvin-Helmholtz instability growth at Ganymede's upstream magnetopause, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-14, ISSN: 2169-9380

Ganymede is the only Solar System moon that generates a permanent magnetic field. Dynamics within the Ganymedean magnetosphere is thought to be driven by energy-transfer interactions on its upstream magnetopause. Previously in Kaweeyanun et al. (2020), we created a steady-state analytical model of Ganymede's magnetopause and predicted global-scale magnetic reconnection to occur frequently throughout the surface. This paper subsequently provides the first assessment of Kelvin-Helmholtz (K-H) instability growth on the magnetopause. Using the same analytical model, we find that linear K-H waves are expected on both Ganymedean magnetopause flanks. Once formed, the waves propagate downstream at roughly half the speed of the external Jovian plasma flow. The Ganymedean K-H instability growth is asymmetric between magnetopause flanks due to the finite Larmor radius effect arising from large gyroradii of Jovian plasma ions. A small but notable enhancement is expected on the sub-Jovian flank according to the physical understanding of bulk plasma and local ion flows alongside comparisons to the well-observed magnetopause of Mercury. Further evaluation shows that nonlinear K-H vortices should be strongly suppressed by concurring global-scale magnetic reconnection at Ganymede. Reconnection is therefore the dominant cross-magnetopause energy-transfer mechanism and driver of global-scale plasma convection within Ganymede's magnetosphere.

Journal article

Masters A, Dunn W, Stallard T, Manners H, Stawarz Jet al., 2021, Magnetic reconnection near the planet as a possible driver of Jupiter's mysterious polar auroras, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-10, ISSN: 2169-9380

Auroral emissions have been extensively observed at the Earth, Jupiter, and Saturn. These planets all have appreciable atmospheres and strong magnetic fields, and their auroras predominantly originate from a region encircling each magnetic pole. However, Jupiter’s auroras poleward of these “main” emissions are brighter and more dynamic, and the drivers responsible for much of these mysterious polar auroras have eluded identification to date. We propose that part of the solution may stem from Jupiter’s stronger magnetic field. We model large-scale Alfvénic perturbations propagating through the polar magnetosphere toward Jupiter, showing that the resulting <0.1° deflections of the magnetic field closest to the planet could trigger magnetic reconnection as near as ∼0.2 Jupiter radii above the cloud tops. At Earth and Saturn this physics should be negligible, but reconnection electric field strengths above Jupiter’s poles can approach ∼1 V m−1, typical of the solar corona. We suggest this near-planet reconnection could generate beams of high-energy electrons capable of explaining some of Jupiter’s polar auroras.

Journal article

Fletcher LN, Helled R, Roussos E, Jones G, Charnoz S, André N, Andrews D, Bannister M, Bunce E, Cavalié T, Ferri F, Fortney J, Grassi D, Griton L, Hartogh P, Hueso R, Kaspi Y, Lamy L, Masters A, Melin H, Moses J, Mousis O, Nettleman N, Plainaki C, Schmidt J, Simon A, Tobie G, Tortora P, Tosi F, Turrini Det al., 2021, Ice giant system exploration within ESA’s Voyage 2050, Experimental Astronomy, ISSN: 0922-6435

Of all the myriad environments in our Solar System, the least explored are the distant Ice Giants Uranus and Neptune, and their diverse satellite and ring systems. These ‘intermediate-sized’ worlds are the last remaining class of Solar System planet to be characterised by a dedicated robotic mission, and may shape the paradigm for the most common outcome of planetary formation throughout our galaxy. In response to the 2019 European Space Agency call for scientific themes in the 2030s and 2040s (known as Voyage 2050), we advocated that an international partnership mission to explore an Ice Giant should be a cornerstone of ESA’s science planning in the coming decade, targeting launch opportunities in the early 2030s. This article summarises the inter-disciplinary science opportunities presented in that White Paper [1], and briefly describes developments since 2019.

Journal article

Cheng IK, Achilleos N, Masters A, Lewis GR, Kane M, Guio Pet al., 2021, Electron bulk heating at Saturn's magnetopause, Journal of Geophysical Research: Space Physics, Vol: 126, ISSN: 2169-9380

Magnetic reconnection at the magnetopause (MP) energizes ambient plasma via the release of magnetic energy and produces an “open” magnetosphere allowing solar wind particles to directly enter the system. At Saturn, the nature of MP reconnection remains unclear. The current study examines electron bulk heating at MP crossings, in order to probe the relationship between observed and predicted reconnection heating proposed by Phan et al. (2013, under open and closed MP, and how this may pertain to the position of the crossings in the Δβ‐magnetic shear parameter space. The electron heating for 70 MP crossings made by the Cassini spacecraft from April 2005 to July 2007 was found using 1d and 3d moment methods. Minimum variance analysis was used on the magnetic field data to help indicate whether the MP is open or closed. We found better agreement between observed and predicted heating for events suggestive of locally “open” MP. For events suggestive of locally “closed” MP, we observed a cluster of points consistent with no electron heating, but also numerous cases with significant heating. Examining the events in the Δβ‐magnetic shear parameter space, we find 83% of events without evidence of energization were situated in the “reconnection suppressed” regime, whilst between 43% to 68% of events with energization lie in the “reconnection possible” regime depending on the threshold used. The discrepancies could be explained by a combination of spatial and temporal variability which makes it possible to observe heated electrons with different conditions from the putative reconnection site.

Journal article

Heyner, Auster, Fornacon, Carr C, Richter, Mieth, Kolhey, Exner, Motschmann, Baumjohann, Matsuoka, Magnes, Berghofer, Fischer, Plaschke, Nakamura, Narita, Delta, Volwerk, Balogh A, Dougherty M, Horbury T, Langlais, Mandea, Masters A, Oliveira, Sanchez-Cano, Slavin, Vennerstrøm, Vogt, Wicht, Glassmeieret al., 2021, The BepiColombo Planetary Magnetometer MPO-MAG: what can we Learn from the Hermean magnetic field?, Space Science Reviews, Vol: 217, ISSN: 0038-6308

The magnetometer instrument MPO-MAG on-board the Mercury Planetary Orbiter (MPO) of the BepiColombo mission en-route to Mercury is introduced, with its instrument design, its calibration and scientific targets. The instrument is comprised of two tri-axial fluxgate magnetometers mounted on a 2.9 m boom and are 0.8 m apart. They monitor the magnetic field with up to 128 Hz in a ±2048 nT range. The MPO will be injected into an initial 480×1500 km polar orbit (2.3 h orbital period). At Mercury, we will map the planetary magnetic field and determine the dynamo generated field and constrain the secular variation. In this paper, we also discuss the effect of the instrument calibration on the ability to improve the knowledge on the internal field. Furthermore, the study of induced magnetic fields and field-aligned currents will help to constrain the interior structure in concert with other geophysical instruments. The orbit is also well-suited to study dynamical phenomena at the Hermean magnetopause and magnetospheric cusps. Together with its sister instrument Mio-MGF on-board the second satellite of the BepiColombo mission, the magnetometers at Mercury will study the reaction of the highly dynamic magnetosphere to changes in the solar wind. In the extreme case, the solar wind might even collapse the entire dayside magnetosphere. During cruise, MPO-MAG will contribute to studies of solar wind turbulence and transient phenomena.

Journal article

Staniland NR, Dougherty MK, Masters A, Achilleos Net al., 2021, The cushion region and dayside magnetodisc structure at Saturn, Geophysical Research Letters, Vol: 48, Pages: 1-9, ISSN: 0094-8276

A sustained quasi‐dipolar magnetic field between the current sheet outer edge and the magnetopause, known as a cushion region, has previously been observed at Jupiter, but not yet at Saturn. Using the complete Cassini magnetometer data, the first evidence of a cushion region forming at Saturn is shown. Only five examples of a sustained cushion are found, revealing this phenomenon to be rare. Four of the cushion regions are identified at dusk and one pre‐noon. It is suggested that greater heating of plasma post‐noon coupled with the expansion of the field through the afternoon sector makes the disc more unstable in this region. These results highlight a key difference between the Saturn and Jupiter systems.

Journal article

Liou K, Paranicas C, Vines S, Kollmann P, Allen R, Clark G, Mitchell D, Jackman C, Masters A, Achilleos N, Roussos E, Krupp Net al., 2021, Dawn-dusk asymmetry in energetic (>20 keV) particles adjacent to Saturn's magnetopause, Journal of Geophysical Research: Space Physics, Vol: 126, ISSN: 2169-9380

Energetic particles (>∼25 keV) have been observed routinely in the terrestrial magnetosheath, but have not been well studied at the magnetosheaths of the outer planets. Here we analyze energetic electrons and ions (mostly protons) in the vicinity (±1 RS) of Saturn's magnetopause, using particle data acquired with the low‐energy magnetosphere measurements system, one of the three sensors of the magnetosphere imaging instrument on board the Cassini spacecraft, during a period of ∼14 years (2004–2017). It is found that energetic particles, especially ions, are also common in Saturn's magnetosheath. A clear inward (toward Saturn) gradient in the electron differential flux is identified, suggestive of magnetospheric sources. Such an inward gradient does not appear in some of the ion channels. We conclude that Saturn's magnetopause acts as a porous barrier for energetic electrons and, to a lesser extent, for energetic ions. A dawn‐dusk asymmetry in the gradient of particle flux across the magnetopause is also identified, with a gradual decrease at the dawn and a sharp decrease at the dusk magnetopause. It is also found that magnetic reconnection enhanced flux levels just outside of the magnetopause, with evidence suggesting that these particles are from magnetospheric sources. These findings strongly suggest that Saturn's magnetosphere is most likely the main source of energetic particles in Saturn's magnetosheath and magnetosphere leakage is an important process responsible for the presence of the energetic particles in Saturn's magnetosheath.

Journal article

Fletcher LN, Simon AA, Hofstadter MD, Arridge CS, Cohen IJ, Masters A, Mandt K, Coustenis Aet al., 2020, Ice giant system exploration in the 2020s: an introduction, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 378, Pages: 1-11, ISSN: 1364-503X

The international planetary science community met in London in January 2020, united in the goal of realizing the first dedicated robotic mission to the distant ice giants, Uranus and Neptune, as the only major class of solar system planet yet to be comprehensively explored. Ice-giant-sized worlds appear to be a common outcome of the planet formation process, and pose unique and extreme tests to our understanding of exotic water-rich planetary interiors, dynamic and frigid atmospheres, complex magnetospheric configurations, geologically-rich icy satellites (both natural and captured), and delicate planetary rings. This article introduces a special issue on ice giant system exploration at the start of the 2020s. We review the scientific potential and existing mission design concepts for an ambitious international partnership for exploring Uranus and/or Neptune in the coming decades.

Journal article

Staniland NR, Dougherty MK, Masters A, Achilleos Net al., 2020, The cushion region and dayside magnetodisc structure at Saturn, Publisher: ESSOAr

A sustained dipolar magnetic field between the current sheet outer edge and the magnetopause, known as a cushion region, has yet to be observed at Saturn. Whilst some signatures of reconnection occurring in the dayside magnetodisc have been identified, the presence of this large-scale structure has not been seen. Using the complete Cassini magnetometer data, the first evidence of a cushion region forming at Saturn is shown. Only five potential examples of a sustained cushion are found, revealing this phenomenon to be rare. This feature more commonly occurs at dusk compared to dawn, where it is found at Jupiter. It is suggested that due to greater heating and expansion of the field through the afternoon sector the disc is more unstable in this region. We show that magnetodisc breakdown is more likely to occur within the magnetosphere of Jupiter compared to Saturn.

Working paper

Blanc M, Prieto-Ballesteros O, Andre N, Gomez-Elvira J, Jones G, Sterken V, Desprats W, Gurvits L, Khurana K, Balmino G, Blocker A, Broquet R, Bunce E, Cavel C, Choblet G, Colins G, Coradini M, Cooper J, Dirkx D, Fontaine D, Garnier P, Gaudin D, Hartogh P, Hussmann H, Genova A, Iess L, Jaggi A, Kempf S, Krupp N, Lara L, Lasue J, Lainey V, Leblanc F, Lebreton J-P, Longobardo A, Lorenz R, Martins P, Martins Z, Marty J-C, Masters A, Mimoun D, Palumba E, Parro V, Regnier P, Saur J, Schutte A, Sittler EC, Spohn T, Srama R, Stephan K, Szego K, Tosi F, Vance S, Wagner R, Van Hoolst T, Volwerk M, Wahlund J-E, Westall F, Wurz Pet al., 2020, Joint Europa Mission (JEM) a multi-scale study of Europa to characterize its habitability and search for extant life, Planetary and Space Science, Vol: 193, ISSN: 0032-0633

Europa is the closest and probably the most promising target to search for extant life in the Solar System, based oncomplementary evidence that it may fulfil the key criteria for habitability: the Galileo discovery of a sub-surface ocean;the many indications that the ice shell is active and may be partly permeable to transfer of chemical species,biomolecules and elementary forms of life; the identification of candidate thermal and chemical energy sourcesnecessary to drive a metabolic activity near the ocean floor. In this article we are proposing that ESA collaborates withNASA to design and fly jointly an ambitious and exciting planetary mission, which we call the Joint Europa Mission(JEM), to reach two objectives: perform a full characterization of Europa’s habitability with the capabilities of a Europaorbiter, and search for bio-signatures in the environment of Europa (surface, subsurface and exosphere) by thecombination of an orbiter and a lander. JEM can build on the advanced understanding of this system which themissions preceding JEM will provide: Juno, JUICE and Europa Clipper, and on the Europa lander concept currentlydesigned by NASA (Maize, report to OPAG, 2019). We propose the following overarching goals for our proposed JointEuropa Mission (JEM): Understand Europa as a complex system responding to Jupiter system forcing, characterisethe habitability of its potential biosphere, and search for life at its surface and in its sub-surface and exosphere. Weaddress these goals by a combination of five Priority Scientific Objectives, each with focused measurement objectivesproviding detailed constraints on the science payloads and on the platforms used by the mission. The JEM observationstrategy will combine three types of scientific measurement sequences: measurements on a high-latitude, low-altitudeEuropan orbit; in-situ measurements to be performed at the surface, using a soft lander; and measurements during thefinal descent to Europa’s surface. T

Journal article

Fletcher, Helled, Roussos, Jones, Charnoz, Andre, Andrews, Bannister, Bunce, Cavalié, Ferri, Fortney, Grassi, Griton, Hartogh, Hueso, Kaspi, Lamy, Masters A, Melin, Moses, Mousis, Nettelmann, Plainaki, Schmidt, Simon, Tobie, Tortora, Tosi, Turriniet al., 2020, Ice giant systems: the scientific potential of orbital missions to Uranus and Neptune, Planetary and Space Science, Vol: 191, ISSN: 0032-0633

Uranus and Neptune, and their diverse satellite and ring systems, represent the least explored environments of our Solar System, and yet may provide the archetype for the most common outcome of planetary formation throughout our galaxy. Ice Giants will be the last remaining class of Solar System planet to have a dedicated orbital explorer, and international efforts are under way to realise such an ambitious mission in the coming decades. In 2019, the European Space Agency released a call for scientific themes for its strategic science planning process for the 2030s and 2040s, known as Voyage 2050. We used this opportunity to review our present-day knowledge of the Uranus and Neptune systems, producing a revised and updated set of scientific questions and motivations for their exploration. This review article describes how such a mission could explore their origins, ice-rich interiors, dynamic atmospheres, unique magnetospheres, and myriad icy satellites, to address questions at the heart of modern planetary science. These two worlds are superb examples of how planets with shared origins can exhibit remarkably different evolutionary paths: Neptune as the archetype for Ice Giants, whereas Uranus may be atypical. Exploring Uranus' natural satellites and Neptune's captured moon Triton could reveal how Ocean Worlds form and remain active, redefining the extent of the habitable zone in our Solar System. For these reasons and more, we advocate that an Ice Giant System explorer should become a strategic cornerstone mission within ESA's Voyage 2050 programme, in partnership with international collaborators, and targeting launch opportunities in the early 2030s.

Journal article

Manners HA, Masters A, 2020, The global distribution of ultra-low-frequency waves in Jupiter's magnetosphere, Journal of Geophysical Research, Vol: 125, ISSN: 0148-0227

Jupiter's giant magnetosphere is a complex system seldom in a configuration approximating steady state, and a clear picture of its governing dynamics remains elusive. Crucial to understanding how the magnetosphere behaves on a large scale are disturbances to the system on length‐scales comparable to the cavity, which are communicated by magnetohydrodynamic waves in the ultra‐low‐frequency band (<1 mHz). In this study we used magnetometer data from multiple spacecraft to perform the first global heritage survey of these waves in the magnetosphere. To map the equatorial region, we relied on the large local‐time coverage provided by the Galileo spacecraft. Flyby encounters performed by Voyager 1 & 2, Pioneer 10 & 11 and Ulysses provided local‐time coverage of the dawn sector. We found several hundred events where significant wave power was present, with periods spanning ~5‐60 min. The majority of events consisted of multiple superposed discrete periods. Periods at ~15, ~30 and ~40 min dominated the event‐averaged spectrum, consistent with the spectra of quasi‐periodic pulsations often reported in the literature. Most events were clustered in the outer magnetosphere close to the magnetopause at noon and dusk, suggesting that an external driving mechanism may dominate. The most energetic events occurred close to the planet, though more sporadically, indicating an accumulation of wave energy in the inner magnetosphere or infrequent impulsive drivers in the region. Our findings suggest that dynamics of the system at large scales is modulated by this diverse population of waves, which permeate the magnetosphere through several cavities and waveguides.

Journal article

Hofstadter MD, Fletcher LN, Simon AA, Masters A, Turrini D, Arridge CSet al., 2020, Future missions to the giant planets that can advance atmospheric science objectives, Space Science Reviews, Vol: 216, Pages: 1-17, ISSN: 0038-6308

Other papers in this special issue have discussed the diversity of planetary atmospheres and some of the key science questions for giant planet atmospheres to be addressed in the future. There are crucial measurements that can only be made by orbiters of giant planets and probes dropped into their atmospheres. To help the community be more effective developers of missions and users of data products, we summarize how NASA and ESA categorize their planetary space missions, and the restrictions and requirements placed on each category. We then discuss the atmospheric goals to be addressed by currently approved giant-planet missions as well as missions likely to be considered in the next few years, such as a joint NASA/ESA Ice Giant orbiter with atmospheric probe. Our focus is on interplanetary spacecraft, but we acknowledge the crucial role to be played by ground-based and near-Earth telescopes, as well as theoretical and laboratory work.

Journal article

Milillo, Fujimoto, Murakami, Benkhoff, Zender, Aizawa, Dósa, Griton, Heyner, Ho, Imber, Jia, Karlsson, Killen, Laurenza, Lindsay, McKenna-Lawlor, Mura, Raines, Rothery, André, Baumjohann, Berezhnoy, Bourdin, Bunce, Califano, Deca, de la Fuente, Dong, Grava, Fatemi, Henri, Ivanovski, Jackson, James, Kallio, Kasaba, Kilpua, Kobayashi, Langlais, Leblanc, Lhotka, Mangano, Martindale, Massetti, Masters A, Morooka, Narita, Oliveira, Odstrcil, Orsini, Pelizzo, Plainaki, Plaschke, Sahraoui, Seki, Slavin, Vainio, Wurz, Barabash, Carr C, Delcourt, Glassmeier, Grande, Hirahara, Huovelin, Korablev, Kojima, Lichtenegger, Livi, Matsuoka, Moissl, Moncuquet, Muinonen, Quèmerais, Saito, Yagitani, Yoshikawa, Wahlundet al., 2020, Investigating Mercury’s environment with the two-spacecraft BepiColombo mission, Space Science Reviews, Vol: 216, Pages: 1-78, ISSN: 0038-6308

The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury’s environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors.

Journal article

Kollmann, Cohen, Allen, Clark, Roussos, Vines, Dietrich, Wicht, de Pater, Runyon, Cartwright, Masters A, Brain, Hibbits, Mauk, Gkioulidou, Rymer, McNutt, Hue, Stanley, Brandtet al., 2020, Magnetospheric studies: a requirement for addressing interdisciplinary mysteries in the Ice Giant systems, Space Science Reviews, Vol: 216, ISSN: 0038-6308

Uranus and Neptune are the least-explored planets in our Solar System. This paper summarizesmysteries about these incredibly intriguing planets and their environments spurred by our limitedobservations from Voyager 2 and Earth-based systems. Several of these observations are eitherinconsistent with our current understanding built from exploring other planetary systems, orindicate such unique characteristics of these Ice Giants that they leave us with more questions thananswers. This paper specifically focuses on the value of all aspects of magnetosphericmeasurements, from the radiation belt structure to plasma dynamics to coupling to the solar wind,through a future mission to either of these planets. Such measurements have large interdisciplinaryvalue, as demonstrated by the large number of mysteries discussed in this paper that cover othernon-magnetospheric disciplines, including planetary interiors, atmospheres, rings, and moons.

Journal article

Staniland N, Dougherty M, Masters A, Bunce Eet al., 2020, Determining the nominal thickness and variability of the magnetodisc current sheet at saturn, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-15, ISSN: 2169-9380

The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 R S . The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability.

Journal article

Fuselier, Petrinec, Sawyer, Mukherjee, Masters Aet al., 2020, Suppression of magnetic reconnection at Saturn’s low-latitude magnetopause, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-16, ISSN: 2169-9380

Observations from the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) are used in an in‐depth investigation of the occurrence and location of reconnection at Saturn's magnetopause. Heated, streaming electrons parallel and/or antiparallel to the magnetic field in the magnetosheath adjacent to the magnetopause indicate that reconnection is occurring somewhere on the boundary. In these instances, the Cassini spacecraft is connected to open magnetic field lines that thread the magnetopause boundary. A survey of 99 crossings with sufficient pitch angle coverage from CAPS/ELS indicates that 65% of the crossings had this evidence of reconnection. Specific crossings from this survey are used to demonstrate that there are times when reconnection at Saturn's low‐latitude magnetopause may be suppressed.

Journal article

Kaweeyanun N, Masters A, Jia X, 2020, Favorable conditions for magnetic reconnection at ganymede’s upstream magnetopause, Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276

Ganymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady‐state conditions at Ganymede's magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede's closed magnetic field encounters Jupiter's ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter's rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer mission.

Journal article

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