Imperial College London


Faculty of Natural SciencesDepartment of Physics

Senior Lecturer







Blackett LaboratorySouth Kensington Campus





Publication Type

95 results found

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, 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, 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 (FLR) 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

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, 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

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

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

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

Simon AA, Fletcher LN, Arridge C, Atkinson D, Coustenis A, Ferri F, Hofstadter M, Masters A, Mousis O, Reh K, Turrini D, Witasse Oet al., 2020, A review of the in situ probe designs from recent ice giant mission concept studies, Space Science Reviews, Vol: 216, Pages: 1-13, ISSN: 0038-6308

For the Ice Giants, atmospheric entry probes provide critical measurements not attainable via remote observations. Including the 2013–2022 NASA Planetary Decadal Survey, there have been at least five comprehensive atmospheric probe engineering design studies performed in recent years by NASA and ESA. International science definition teams have assessed the science requirements, and each recommended similar measurements and payloads to meet science goals with current instrument technology. The probe system concept has matured and converged on general design parameters that indicate the probe would include a 1-meter class aeroshell and have a mass around 350 to 400-kg. Probe battery sizes vary, depending on the duration of a post-release coast phase, and assumptions about heaters and instrument power needs. The various mission concepts demonstrate the need for advanced power and thermal protection system development. The many completed studies show an Ice Giant mission with an in situ probe is feasible and would be welcomed by the international science community.

Journal article

Hofstadter M, Simon A, Atreya S, Banfield D, Fortney Jonathan, Hayes A, Hedman M, Hospodarsky G, Mandt K, Masters A, Showalter M, Soderlund K, Turrini D, Turtle E, Reh K, Elliot J, Arora N, Petropoulos A, Ice giant mission study teamet al., 2019, Uranus and Neptune missions: a study in advance of the next Planetary Science Decadal Survey, Planetary and Space Science, Vol: 177, ISSN: 0032-0633

The ice giant planets, Uranus and Neptune, represent an important and unexplored class of planets. Mostof our detailed information about themcomes from fleeting looks by the Voyager 2 spacecraftin the 1980s.Voyager,and ground-based work since then, found that these planets, their satellites, rings, and magnetospheres, challenge our understanding of the formation and evolution of planetarysystems. We also now knowthat Uranus-Neptune size planetsare common around other stars. These are some of the reasons ice giant exploration was a high priority in NASA’smost recent Planetary Science Decadal Survey. In preparation for the next Decadal Survey,NASA, with ESA participation,conducted a broad study of possible ice giant missions in the 2024 –2037 timeframe. This paper summarizes the key resultsof the study,and addressesquestionsthat have been raised by the science communityand in a recent NASA review. Foremost amongstthese are questions about the science objectives, the science payload, and the importance of an atmospheric probe. Theconclusions ofthe NASA/ESA study remain valid. In particular, it is a high priority to sendan orbiterand atmospheric probeto at least one of the ice giants, with instrumentationto studyall components of an ice giant system.Uranus and Neptune are found to be equally compelling as science targets. The two planets are not equivalent, however, and each systemhas thingsto teach us the other cannot. An additional mission study is needed to refine plans for future exploration of these worlds.

Journal article

Manners H, Masters A, 2019, First evidence for multiple‐harmonic standing Alfvén waves in Jupiter's equatorial plasma sheet, Geophysical Research Letters, Vol: 46, Pages: 9344-9351, ISSN: 0094-8276

Quasi‐periodic pulsations in the ultra‐low‐frequency band are ubiquitously observed in the jovian magnetosphere, but their source and distribution have until now been a mystery. Standing Alfvén waves on magnetic field lines have been proposed to explain these pulsations and their large range in observed periods. However, in‐situ evidence in support of this mechanism has been scarce. Here we use magnetometer data from the Galileo spacecraft to report first evidence of a multiple‐harmonic ultra‐low‐frequency event in Jupiters equatorial plasma sheet. The harmonic periods lie in the 4‐22‐min range, and the nodal structure is confined to the plasma sheet. Polarization analysis reveals several elliptically‐polarized odd harmonics, and no presence of even harmonics. The harmonic periods, their polarization, and the confinement of the wave to the plasma sheet, are strong evidence supporting the standing Alfvén wave model. Multiple‐harmonic waves therefore potentially explain the full range of periods in quasi‐periodic pulsations in Jupiters magnetosphere.

Journal article

Kronberg EA, Grigorenko EE, Malykhin A, Kozak L, Petrenko B, Vogt M, Roussos E, Kollmann P, Jackman C, Kasahara S, Malova KV, Tao C, Radioti A, Masters Aet al., 2019, Acceleration of ions in Jovian plasmoids: does turbulence play a role?, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5056-5069, ISSN: 2169-9380

The dissipation processes which transform electromagnetic energy into kinetic particle energy in space plasmas are still not fully understood. Of particular interest is the distribution of the dissipated energy among different species of charged particles. The Jovian magnetosphere is a unique laboratory to study this question because outflowing ions from the moon Io create a high diversity in ion species. In this work, we use multispecies ion observations and magnetic field measurements by the Galileo spacecraft. We limit our study to observations of plasmoids in the Jovian magnetotail, because there is strong ion acceleration in these structures. Our model predicts that electromagnetic turbulence in plasmoids plays an essential role in the acceleration of oxygen, sulfur, and hydrogen ions. The observations show a decrease of the oxygen and sulfur energy spectral index γ at ∼30 to ∼400 keV/nuc with the wave power indicating an energy transfer from electromagnetic waves to particles, in agreement with the model. The wave power threshold for effective acceleration is of the order of 10 nT2Hz−1, as in terrestrial plasmoids. However, this is not observed for hydrogen ions, implying that processes other than wave‐particle interaction are more important for the acceleration of these ions or that the time and energy resolution of the observations is too coarse. The results are expected to be confirmed by improved plasma measurements by the Juno spacecraft.

Journal article

Snodgrass C, Jones GH, Boehnhardt H, Gibbings A, Homeister M, Andre N, Beck P, Bentley MS, Bertini I, Bowles N, Capria MT, Carr C, Ceriotti M, Coates AJ, Della Corte V, Donaldson Hanna KL, Fitzsimmons A, Gutiérrez PJ, Hainaut OR, Herique A, Hilchenbach M, Hsieh HH, Jehin E, Karatekin O, Kofman W, Lara LM, Laudan K, Licandro J, Lowry SC, Marzari F, Masters A, Meech KJ, Moreno F, Morse A, Orosei R, Pack A, Plettemeier D, Prialnik D, Rotundi A, Rubin M, Sánchez JP, Sheridan S, Trieloff M, Winterboer Aet al., 2018, The Castalia mission to Main Belt Comet 133P/Elst-Pizarro, Advances in Space Research, Vol: 62, Pages: 1947-1976, ISSN: 0273-1177

We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these.

Journal article

Jones G, Agarwal J, Bowles N, Burchell M, Coates A, Fitzsimmons A, Graps A, Hsieh H, Lisse C, Lowry S, Masters A, Snodgrass C, Tubiana Cet al., 2018, The proposed Caroline ESA M3 mission to a Main Belt Comet, Advances in Space Research, Vol: 62, Pages: 1921-1946, ISSN: 0273-1177

We describe Caroline, a mission proposal submitted to the European Space Agency in 2010 in response to the Cosmic Visions M3 call for medium-sized missions. Caroline would have travelled to a Main Belt Comet (MBC), characterizing the object during a flyby, and capturing dust from its tenuous coma for return to Earth. MBCs are suspected to be transition objects straddling the traditional boundary between volatile–poor rocky asteroids and volatile–rich comets. The weak cometary activity exhibited by these objects indicates the presence of water ice, and may represent the primary type of object that delivered water to the early Earth. The Caroline mission would have employed aerogel as a medium for the capture of dust grains, as successfully used by the NASA Stardust mission to Comet 81P/Wild 2. We describe the proposed mission design, primary elements of the spacecraft, and provide an overview of the science instruments and their measurement goals. Caroline was ultimately not selected by the European Space Agency during the M3 call; we briefly reflect on the pros and cons of the mission as proposed, and how current and future mission MBC mission proposals such as Castalia could best be approached.

Journal article

Manners H, Masters A, Yates J, 2018, Standing Alfvén waves in Jupiter’s magnetosphere as a source of ∼10-60 minute quasi-periodic pulsations, Geophysical Research Letters, Vol: 45, Pages: 8746-8754, ISSN: 0094-8276

Energy transport inside the giant magnetosphere at Jupiter is poorly understood. Since the Pioneer era, mysterious quasiperiodic (QP) pulsations have been reported. Early publications successfully modeled case studies of ∼60‐min (rest‐frame) pulsations as standing Alfvén waves. Since then, the range of periods has increased to ∼10–60 min, spanning multiple data sets. More work is required to assess whether a common QP modulation mechanism is capable of explaining the full range of wave periods. Here we have modeled standing Alfvén waves to compute the natural periods of the Jovian magnetosphere, for varying plasma sheet thicknesses, field line lengths, and Alfvén speeds. We show that variability in the plasma sheet produces eigenperiods that are consistent with all the reported observations. At least the first half‐dozen harmonics (excluding the fundamental) may contribute but are indistinguishable in our analysis. We suggest that all QP pulsations reported at Jupiter may be explained by standing Alfvén waves.

Journal article

Staniland N, Dougherty M, Masters A, 2018, Quantifying the stress of the Saturnian magnetosphere during the Cassini era, Geophysical Research Letters, Vol: 45, Pages: 8704-8711, ISSN: 0094-8276

We quantify the magnetospheric stress state of Saturn, revealing the nature of the planetary environment and its current systems. The complete magnetic field data set collected by the Cassini spacecraft is used to track the global behavior of the Saturnian magnetosphere during the Cassini era. Variations in the magnetodisc current model parameter μoIo determine when the system is stretched, compressed, or near the ground state. Of the 111 orbits that pass through our chosen region, 69 are well described by the model, indicating a steady state current sheet during this interval. While the stress state displays a dependence on local time, it is also shown to vary temporally. We conclude that the Saturnian magnetosphere remained in a quiet state for a significant period of the Cassini orbital mission at Saturn, with occasional large‐scale deviations observed.

Journal article

Masters A, 2018, A more viscous-like solar wind interaction with all the giant planets, Geophysical Research Letters, Vol: 45, Pages: 7320-7329, ISSN: 0094-8276

Identifying and quantifying the different drivers of energy flow through a planetarymagnetosphere is crucial for understanding how each planetary system works. The magnetosphere of ourown planet is primarily driven externally by the solar wind through global magnetic reconnection, while aviscous-like interaction with the solar wind involving growth of the Kelvin-Helmholtz (K-H) instability is asecondary effect. Here we consider the solar wind-magnetosphere interaction at all magnetized planets,exploring the implications of diverse solar wind conditions. We show that with increasing distance fromthe Sun the electric fields arising from reconnection at the magnetopause boundary of a planetarymagnetosphere become weaker, whereas the boundaries become increasingly K-H unstable. Our resultssupport the possibility of a predominantly viscous-like interaction between the solar wind and every oneof the giant planet magnetospheres, as proposed by previous authors and in contrast with the solarwind-magnetosphere interaction at Earth.

Journal article

Futaana Y, Barabash S, Wieser M, Wurz P, Hurley D, Mihaly H, Mall U, Andre N, Ivchenko N, Oberst J, Retherford K, Coates A, Masters A, Wahlund JE, Kallio E, SELMA proposal teamet al., 2018, SELMA mission: how do airless bodies interact with space environment? The Moon as an accessible laboratory, Planetary and Space Science, Vol: 156, Pages: 23-40, ISSN: 0032-0633

The Moon is an archetypal atmosphere-less celestial body in the Solar System. For such bodies, the environments are characterized by complex interaction among the space plasma, tenuous neutral gas, dust and the outermost layer of the surface. Here we propose the SELMA mission (Surface, Environment, and Lunar Magnetic Anomalies) to study how airless bodies interact with space environment. SELMA uses a unique combination of remote sensing via ultraviolet and infrared wavelengths, and energetic neutral atom imaging, as well as in situ measurements of exospheric gas, plasma, and dust at the Moon. After observations in a lunar orbit for one year, SELMA will conduct an impact experiment to investigate volatile content in the soil of the permanently shadowed area of the Shackleton crater. SELMA also carries an impact probe to sound the Reiner-Gamma mini-magnetosphere and its interaction with the lunar regolith from the SELMA orbit down to the surface. SELMA was proposed to the European Space Agency as a medium-class mission (M5) in October 2016. Research on the SELMA scientific themes is of importance for fundamental planetary sciences and for our general understanding of how the Solar System works. In addition, SELMA outcomes will contribute to future lunar explorations through qualitative characterization of the lunar environment and, in particular, investigation of the presence of water in the lunar soil, as a valuable resource to harvest from the lunar regolith.

Journal article

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