Imperial College London

DrAdamMasters

Faculty of Natural SciencesDepartment of Physics

Senior Lecturer
 
 
 
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Contact

 

a.masters

 
 
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Location

 

Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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87 results found

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, ISSN: 0148-0227

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

Blanc, Prieto-Ballesteros, Andre, Gomez-Elvira, Jones, Sterken, Desprats, Gurvits, Khurana, Balmino, Blöcker, Broquet, Bunce, Cavel, Choblet, Collins, Coradini, Cooper, Dirkx, Fontaine, GARNIER, Gaudin, Hartogh, Hussmann, Genova, Jaeggi, Kempf, Krupp, Lara, Lasue, Lainey, Leblanc, Lebreton, Longobardo, Lorenz, Martins, Martins, Marty, Masters A, Mimoun, Palomba, Parro, Regnier, Saur, Schutte, Sittler, Spohn, Srama, Stephan, Szego, Tosi, Vance, Wagner, Van Hoolst, Wahlund, Westall, Volwerk, Wurzet al., Joint Europa Mission (JEM) a multi-scale study of Europa to characterize its habitability and search for extant life, Planetary and Space Science, 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

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

Davies E, Masters A, Dougherty M, Hansen K, Coates A, Hunt Get al., 2017, Swept Forward Magnetic Field Variability in High-Latitude Regions of Saturn's Magnetosphere, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 12328-12337, ISSN: 2169-9380

Swept forward field is the term given to configurations of magnetic field wherein the field lines deviate from the meridional planes of a planet in the direction of its rotation. Evidence is presented for swept-forward field configurations on Cassini orbits around Saturn from the first half of 2008. These orbits were selected on the basis of high inclination, spatial proximity, and temporal proximity, allowing for the observation of swept-forward field and resolution of dynamic effects using data from the Cassini magnetometer. Nine orbits are surveyed; all show evidence of swept-forward field, with typical sweep angle found to be 23°. Evidence is found for transient events that lead to temporary dramatic increases in sweep-forward angle. The Michigan Solar Wind Model is employed to investigate temporal correlation between the arrivals of solar wind shocks at Saturn with these transient events, with two shown to include instances corresponding with solar wind shock arrivals. Measurements of equatorial electron number density from anode 5 of the Cassini Plasma Spectrometer instrument are investigated for evidence of magnetospheric compression, corresponding with predicted shock arrivals. Potential mechanisms for the transfer of momentum from the solar wind to the magnetosphere are discussed.

Journal article

Masters A, 2017, Model-based assessments of magnetic reconnection and Kelvin-Helmholtz instability at Jupiter’s magnetopause, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 11154-11174, ISSN: 2169-9380

The interaction between the solar wind and Jupiter's magnetic field confines the planetary field to the largest magnetosphere in the Solar System. However, the full picture of when and where key processes operate at the magnetopause boundary of the system remains unclear. This is essential for testing understanding with observations and for determining the relative importance of different drivers of Jovian magnetospheric dynamics. Here we present a global analytical model of Jovian magnetopause conditions under steady state, which forms the basis of boundary process assessments. Sites of magnetic reconnection at Jupiter's magnetopause are expected to be in regions of sufficiently high magnetic shear across the boundary, controlled by the orientation of the interplanetary magnetic field. Reconnection rates are also most sensitive to changes in the highly variable IMF, followed by changes in the solar wind plasma mass density. The largest plasma flow shear across the boundary is in the equatorial dawn region, producing a region that is typically unstable to growth of the Kelvin-Helmholtz (K-H) instability. Compared to magnetopause reconnection site locations, this K-H-unstable region at dawn is less sensitive to changing conditions. Motion of K-H boundary perturbations typically includes dawn-to-dusk motion across the subsolar region. Model-predicted reconnection voltages are typically hundreds of kV but rely on steady solar wind conditions on a time scale that is longer than typical at Jupiter's orbit. How the reconnection voltage compares to the voltage applied due to the “viscous-like” interaction involving K-H instability remains unclear.

Journal article

Masters A, Sulaiman A, Stawarz L, Reville B, Sergis N, Fujimoto M, Burgess D, Coates A, Dougherty Met al., 2017, An in situ Comparison of Electron Acceleration at Collisionless Shocks under Differing Upstream Magnetic Field Orientations, Astrophysical Journal, Vol: 843, ISSN: 1538-4357

A leading explanation for the origin of Galactic cosmic rays is acceleration at high-Mach number shock waves in the collisionless plasma surrounding young supernova remnants. Evidence for this is provided by multi-wavelength non-thermal emission thought to be associated with ultrarelativistic electrons at these shocks. However, the dependence of the electron acceleration process on the orientation of the upstream magnetic field with respect to the local normal to the shock front (quasi-parallel/quasi-perpendicular) is debated. Cassini spacecraft observations at Saturn's bow shock have revealed examples of electron acceleration under quasi-perpendicular conditions, and the first in situ evidence of electron acceleration at a quasi-parallel shock. Here we use Cassini data to make the first comparison between energy spectra of locally accelerated electrons under these differing upstream magnetic field regimes. We present data taken during a quasi-perpendicular shock crossing on 2008 March 8 and during a quasi-parallel shock crossing on 2007 February 3, highlighting that both were associated with electron acceleration to at least MeV energies. The magnetic signature of the quasi-perpendicular crossing has a relatively sharp upstream–downstream transition, and energetic electrons were detected close to the transition and immediately downstream. The magnetic transition at the quasi-parallel crossing is less clear, energetic electrons were encountered upstream and downstream, and the electron energy spectrum is harder above ~100 keV. We discuss whether the acceleration is consistent with diffusive shock acceleration theory in each case, and suggest that the quasi-parallel spectral break is due to an energy-dependent interaction between the electrons and short, large-amplitude magnetic structures.

Journal article

Sundberg T, Burgess D, Scholer M, Masters A, Sulaiman AHet al., 2017, The dynamics of very high Alfvén Mach number shocks in space plasmas, Astrophysical Journal Letters, Vol: 836, ISSN: 2041-8213

Astrophysical shocks, such as planetary bow shocks or supernova remnant shocks, are often in the high or very-high Mach number regime, and the structure of such shocks is crucial for understanding particle acceleration and plasma heating, as well inherently interesting. Recent magnetic field observations at Saturn's bow shock, for Alfvén Mach numbers greater than about 25, have provided evidence for periodic non-stationarity, although the details of the ion- and electron-scale processes remain unclear due to limited plasma data. High-resolution, multi-spacecraft data are available for the terrestrial bow shock, but here the very high Mach number regime is only attained on extremely rare occasions. Here we present magnetic field and particle data from three such quasi-perpendicular shock crossings observed by the four-spacecraft Cluster mission. Although both ion reflection and the shock profile are modulated at the upstream ion gyroperiod timescale, the dominant wave growth in the foot takes place at sub-proton length scales and is consistent with being driven by the ion Weibel instability. The observed large-scale behavior depends strongly on cross-scale coupling between ion and electron processes, with ion reflection never fully suppressed, and this suggests a model of the shock dynamics that is in conflict with previous models of non-stationarity. Thus, the observations offer insight into the conditions prevalent in many inaccessible astrophysical environments, and provide important constraints for acceleration processes at such shocks.

Journal article

Sorba AM, Achilleos NA, Guio P, Arridge CS, Pilkington NM, Masters A, Sergis N, Coates AJ, Dougherty MKet al., 2017, Modeling the compressibility of Saturn's magnetosphere in response to internal and external influences, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 1572-1589, ISSN: 2169-9402

The location of a planetary magnetopause is principally determined by the balance between solar wind dynamic pressure DP and magnetic and plasma pressures inside the magnetopause boundary. Previous empirical studies assumed that Saturn's magnetopause standoff distance varies as math formula and measured a constant compressibility parameter α corresponding to behavior intermediate between a vacuum dipole appropriate for Earth (α≈6) and a more easily compressible case appropriate for Jupiter (α≈4). In this study we employ a 2-D force balance model of Saturn's magnetosphere to investigate magnetospheric compressibility in response to changes in DP and global hot plasma content. For hot plasma levels compatible with Saturn observations, we model the magnetosphere at a range of standoff distances and estimate the corresponding DP values by assuming pressure balance across the magnetopause boundary. We find that for “average” hot plasma levels, our estimates of α are not constant with DP but vary from ∼4.8 for high DP conditions, when the magnetosphere is compressed (≤25 RS), to ∼3.5 for low DP conditions. This corresponds to the magnetosphere becoming more easily compressible as it expands. We find that the global hot plasma content influences magnetospheric compressibility even at fixed DP, with α estimates ranging from ∼5.4 to ∼3.3 across the range of our parameterized hot plasma content. We suggest that this behavior is predominantly driven by reconfiguration of the magnetospheric magnetic field into a more disk-like structure under such conditions. In a broader context, the compressibility of the magnetopause reveals information about global stress balance in the magnetosphere.

Journal article

Yates JN, Southwood DJ, Dougherty MK, Sulaiman AH, Masters A, Cowley SWH, Kivelson MG, Chen CHK, Provan G, Mitchell DG, Hospodarsky GB, Achilleos N, Sorba AM, Coates AJet al., 2016, Saturn's quasiperiodic magnetohydrodynamic waves, Geophysical Research Letters, Vol: 43, Pages: 102-111, ISSN: 1944-8007

Quasi-periodic ∼1-hour fluctuations have been recently reported by numerous instruments on-board the Cassini spacecraft. The interpretation of the sources of these fluctuations has remained elusive to date. Here we provide an explanation for the origin of these fluctuations using magnetometer observations. We find that magnetic field fluctuations at high northern latitudes are Alfvénic, with small amplitudes (∼0.4 nT), and are concentrated in wave-packets similar to those observed in Kleindienst et al. [2009]. The wave-packets recur periodically at the northern magnetic oscillation period. We use a magnetospheric box model to provide an interpretation of the wave periods. Our model results suggest that the observed magnetic fluctuations are second harmonic Alfvén waves standing between the northern and southern ionospheres in Saturn’s outer magnetosphere

Journal article

Mejnertsen L, Eastwood JP, Chittenden J, Masters Aet al., 2016, Global MHD Simulations of Neptune's Magnetosphere, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 7497-7513, ISSN: 2169-9380

A global magnetohydrodynamic (MHD) simulation has been performed in order to investigate the outer boundaries of Neptune's magnetosphere at the time of Voyager 2's flyby in 1989 and to better understand the dynamics of magnetospheres formed by highly inclined planetary dipoles. Using the MHD code Gorgon, we have implemented a precessing dipole to mimic Neptune's tilted magnetic field and rotation axes. By using the solar wind parameters measured by Voyager 2, the simulation is verified by finding good agreement with Voyager 2 magnetometer observations. Overall, there is a large-scale reconfiguration of magnetic topology and plasma distribution. During the “pole-on” magnetospheric configuration, there only exists one tail current sheet, contained between a rarefied lobe region which extends outward from the dayside cusp, and a lobe region attached to the nightside cusp. It is found that the tail current always closes to the magnetopause current system, rather than closing in on itself, as suggested by other models. The bow shock position and shape is found to be dependent on Neptune's daily rotation, with maximum standoff being during the pole-on case. Reconnection is found on the magnetopause but is highly modulated by the interplanetary magnetic field (IMF) and time of day, turning “off” and “on” when the magnetic shear between the IMF and planetary fields is large enough. The simulation shows that the most likely location for reconnection to occur during Voyager 2's flyby was far from the spacecraft trajectory, which may explain the relative lack of associated signatures in the observations.

Journal article

Masters A, Sulaiman AH, Sergis N, Stawarz L, Fujimoto M, Coates AJ, Dougherty MKet al., 2016, Suprathermal electrons at Saturn’s bow shock, Astrophysical Journal, Vol: 826, ISSN: 1538-4357

The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernovaRemnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks.However, the Cassini spacecraft has shown that the shock standingin the solar wind sunward of Saturn (Saturn’s bow shock) can occasionally reach this high-Mach number astrophysical regime.In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magnetic conditions. Here we present the full picture of suprathermal electrons at Saturn’s bow shock revealed by Cassini. The downstream thermal electron distribution is resolved in all data taken by the low-energy electron detector (CAPS-ELS, <28 keV)during shock crossings, but the higher energy channels were at(or close to) background. The high-energy electron detector (MIMI-LEMMS, >18 keV) measured a suprathermal electron signatureat 31 of 508 crossings, where typically only the lowest energy channels (<100 keV) were above background. We show that these results are consistent with theory in which the “injection” of thermal electrons into an acceleration process involves interaction with whistler waves at the shock front, and becomes possible for all upstream magnetic field orientations at high Mach numbers like those of the strong shocks around young SNRs. A future dedicated study will analyse the rare crossings with evidence for relativisticelectrons (up to ~1 MeV).

Journal article

Sulaiman AH, Masters A, Dougherty MK, 2016, Characterization of Saturn's bow shock: magnetic field observations of quasi-perpendicular shocks, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 4425-4434, ISSN: 2169-9380

Collisionless shocks vary drastically from terrestrial to astrophysical regimes resulting in radically different characteristics. This poses two complexities. First, separating the influences of these parameters on physical mechanisms such as energy dissipation. Second, correlating observations of shock waves over a wide range of each parameter, enough to span across different regimes. Investigating the latter has been restricted since the majority of studies on shocks at exotic regimes (such as supernova remnants) have been achieved either remotely or via simulations, but rarely by means of in situ observations. Here we present the parameter space of MA bow shock crossings from 2004 to 2014 as observed by the Cassini spacecraft. We find that Saturn's bow shock exhibits characteristics akin to both terrestrial and astrophysical regimes (MA of order 100), which is principally controlled by the upstream magnetic field strength. Moreover, we determined the θBn of each crossing to show that Saturn's (dayside) bow shock is predominantly quasi-perpendicular by virtue of the Parker spiral at 10 AU. Our results suggest a strong dependence on MA in controlling the onset of physical mechanisms in collisionless shocks, particularly nontime stationarity and variability. We anticipate that our comprehensive assessment will yield deeper insight into high MA collisionless shocks and provide a broader scope for understanding the structures and mechanisms of collisionless shocks.

Journal article

Kimura T, Kraft RP, Elsner RF, Branduardi-Raymont G, Gladstone GR, Tao C, Yoshioka K, Murakami G, Yamazaki A, Tsuchiya F, Vogt MF, Masters A, Hasegawa H, Badman SV, Roediger E, Ezoe Y, Dunn WR, Yoshikawa I, Fujimoto M, Murray SSet al., 2016, Jupiter's X-ray and EUV auroras monitored by Chandra, XMM-Newton, and Hisaki satellite, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 2308-2320, ISSN: 2169-9402

Jupiter's X-ray auroral emission in the polar cap region results from particles which have undergone strong field-aligned acceleration into the ionosphere. The origin of precipitating ions and electrons and the time variability in the X-ray emission are essential to uncover the driving mechanism for the high-energy acceleration. The magnetospheric location of the source field line where the X-ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long-term monitoring of the X-rays and contemporaneous solar wind variability has not been carried out. In April 2014, the first long-term multiwavelength monitoring of Jupiter's X-ray and EUV auroral emissions was made by the Chandra X-ray Observatory, XMM-Newton, and Hisaki satellite. We find that the X-ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X-ray auroral region was found to be open to the interplanetary space. The other half of the X-ray auroral source region is magnetically connected with the prenoon to postdusk sector in the outermost region of the magnetosphere, where the Kelvin-Helmholtz (KH) instability, magnetopause reconnection, and quasiperiodic particle injection potentially take place. We speculate that the high-energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators.

Journal article

Roussos E, Krupp N, Mitchell DG, Paranicas C, Krimigis SM, Andriopoulou M, Palmaerts B, Kurth WS, Badman SV, Masters A, Dougherty MKet al., 2016, Quasi-periodic injections of relativistic electrons in Saturn’s outer magnetosphere, Icarus, Vol: 263, Pages: 101-116, ISSN: 1090-2643

Quasi-periodic, short-period injections of relativistic electrons have been observed in both Jupiter’s and Saturn’s magnetospheres, but understanding their origin or significance has been challenging, primarily due to the limited number of in-situ observations of such events by past flyby missions. Here we present the first survey of such injections in an outer planetary magnetosphere using almost nine years of energetic charged particle and magnetic field measurements at Saturn. We focus on events with a characteristic period of about 60–70 min (QP60, where QP stands for quasi-periodic). We find that the majority of QP60, which are very common in the outer magnetosphere, map outside Titan’s orbit. QP60 are also observed over a very wide range of local times and latitudes. A local time asymmetry in their distribution is the most striking feature, with QP60 at dusk being between 5 and 25 times more frequent than at dawn. Field-line tracing and pitch angle distributions suggest that most events at dusk reside on closed field lines. They are distributed either near the magnetopause, or, in the case of the post-dusk (or pre-midnight) sector, up to about 30 RS inside it, along an area extending parallel to the dawn–dusk direction. QP60 at dawn map either on open field lines and/or near the magnetopause. Both the asymmetries and varying mapping characteristics as a function of local time indicate that generation of QP60 cannot be assigned to a single process. The locations of QP60 seem to trace sites that reconnection is expected to take place. In that respect, the subset of events observed post-dusk and deep inside the magnetopause may be directly or indirectly linked to the Vasyliunas reconnection cycle, while magnetopause reconnection/Kelvin–Helmholtz (KH) instability could be invoked to explain all other events at the duskside. Using similar arguments, injections at the dawnside magnetosphere may result from solar-wind induced storm

Journal article

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