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Journal articleBandyopadhyay R, Chasapis A, Chhiber R, et al., 2018,
Solar Wind Turbulence Studies Using MMS Fast Plasma Investigation Data
, ASTROPHYSICAL JOURNAL, Vol: 866, ISSN: 0004-637X- Author Web Link
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- Citations: 44
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Journal articleKhurana KK, Dougherty MK, Provan G, et al., 2018,
Discovery of atmospheric-wind-driven electric currents in Saturn's magnetosphere in the gap between Saturn and its rings
, Geophysical Research Letters, Vol: 45, Pages: 10068-10074, ISSN: 0094-8276Magnetic field observations obtained by the Cassini spacecraft as it traversed regions inside of Saturn's D ring packed a genuine surprise. The azimuthal component of the magnetic field recorded a consistent positive perturbation with a strength of 15–25 nT near closest approach. The closest approaches were near the equatorial plane of Saturn and were distributed narrowly around local noon and brought the spacecraft to within 2,550 km of Saturn's cloud tops. Modeling of this perturbation shows that it is not of internal origin but is produced by external currents that couple the low‐latitude northern ionosphere to the low‐latitude southern ionosphere. The azimuthal perturbations diminish at higher latitudes on field lines that connect to Saturn's icy rings. The sense of the current system suggests that the southern feet of the field lines in the ionosphere leads their northern counterparts. We show that the observed field perturbations are consistent with a field‐aligned current whose strength is ~1 MA/radian, that is, comparable in strength to the planetary‐period‐oscillation‐related current systems observed in the auroral zone. We show that the Lorentz force in the ionosphere extracts momentum from the faster moving low‐latitude zonal belt and delivers it to the northern ionosphere. We further show that the electric current is generated when the two ends of a field line are embedded in zonal flows with differing wind speeds in the low‐latitude thermosphere. The wind‐generated currents dissipate 2 × 1011W of thermal power, similar to the input from the solar extreme ultraviolet flux in this region.
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Journal articleHanna E, Fettweis X, Hall RJ, 2018,
Brief communication: Recent changes in summer Greenland blocking captured by none of the CMIP5 models
, CRYOSPHERE, Vol: 12, Pages: 3287-3292, ISSN: 1994-0416- Author Web Link
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- Citations: 56
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Journal articleGewin V, Keith D, Haigh J, et al., 2018,
Tackling harassment
, NATURE, Vol: 562, Pages: 449-450, ISSN: 0028-0836 -
Journal articleJones G, Agarwal J, Bowles N, et al., 2018,
The proposed Caroline ESA M3 mission to a Main Belt Comet
, Advances in Space Research, Vol: 62, Pages: 1921-1946, ISSN: 0273-1177We 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.
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Journal articleSnodgrass C, Jones GH, Boehnhardt H, et al., 2018,
The Castalia mission to Main Belt Comet 133P/Elst-Pizarro
, Advances in Space Research, Vol: 62, Pages: 1947-1976, ISSN: 0273-1177We 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.
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Journal articleBelmonte MT, Pickering JC, Clear CP, et al., 2018,
The laboratory astrophysics spectroscopy programme at Imperial College London
, Galaxies, Vol: 6, ISSN: 2075-4434Accurate atomic parameters, such as transition probabilities, wavelengths, and energy levels, are indispensable for the analysis of stellar spectra and the obtainment of chemical abundances. However, the quantity and quality of the existing data in many cases lie far from the current needs of astronomers, creating an acute need for laboratory measurements of matching accuracy and completeness to exploit the full potential of the very expensively acquired astrophysical spectra. The Fourier Transform Spectrometer at Imperial College London works in the vacuum ultraviolet-visible region with a resolution of 2,000,000 at 200 nm. We can acquire calibrated spectra of neutral, singly, and doubly ionized species. We collaborate with the National Institute of Standards and Technology (NIST) and the University of Lund to extend our measurements into the infrared region. The aim of this review is to explain the current capabilities of our experiment in an understandable way to bring the astronomy community closer to the field of laboratory astrophysics and encourage further dialogue between our laboratory and all those astronomers who need accurate atomic data. This exchange of ideas will help us to focus our efforts on the most urgently needed data.
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Journal articleHeritier K, Galand M, Henri P, et al., 2018,
Plasma source and loss at comet 67P during the Rosetta mission
, Astronomy and Astrophysics, Vol: 618, ISSN: 0004-6361Context.The Rosetta spacecraft provided us with a unique opportunity to study comet 67P/Churyumov-Gerasimenko from a closeperspective and over a two-year time period. Comet 67P is a weakly active comet. It was therefore unexpected to find an active anddynamic ionosphere where the cometary ions were largely dominant over the solar wind ions, even at large heliocentric distances.Aims.Our goal is to understand the different drivers of the cometary ionosphere and assess their variability over time and over thedifferent conditions encountered by the comet during the Rosetta mission.Methods.We used a multi-instrument data-based ionospheric model to compute the total ion number density at the position ofRosetta. In-situ measurements from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the Rosetta PlasmaConsortium (RPC)–Ion and Electron Sensor (IES), together with the RPC–LAngmuir Probe instrument (LAP) were used to computethe local ion total number density. The results are compared to the electron densities measured by RPC–Mutual Impedance Probe(MIP) and RPC–LAP.Results.We were able to disentangle the physical processes responsible for the formation of the cometary ions throughout thetwo-year escort phase and we evaluated their respective magnitudes. The main processes are photo-ionization and electron-impactionization. The latter is a significant source of ionization at large heliocentric distance (>2 au) and was predominant during the lastfour months of the mission. The ionosphere was occasionally subject to singular solar events, temporarily increasing the ambientenergetic electron population. Solar photons were the main ionizer near perihelion at 1.3 au from the Sun, during summer 2015.
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Journal articleDougherty MK, Cao H, Khurana KK, et al., 2018,
Erratum for the Research Article “Saturn’s magnetic field revealed by the Cassini Grand Finale” by M. K. Dougherty, H. Cao, K. K. Khurana, G. J. Hunt, G. Provan, S. Kellock, M. E. Burton, T. A. Burk, E. J. Bunce, S. W. H. Cowley, M. G. Kivelson, C. T. Russell, D. J. Southwood
, Science, Vol: 362, ISSN: 0036-8075 -
Journal articleSourdeval O, Gryspeerdt E, Krämer M, et al., 2018,
Ice crystal number concentration estimates from lidar–radar satellite remote sensing – Part 1: Method and evaluation
, Atmospheric Chemistry and Physics, Vol: 18, Pages: 14327-14350, ISSN: 1680-7316The number concentration of cloud particles is a key quantity for understanding aerosol–cloud interactions and describing clouds in climate and numerical weather prediction models. In contrast with recent advances for liquid clouds, few observational constraints exist regarding the ice crystal number concentration (Ni). This study investigates how combined lidar–radar measurements can be used to provide satellite estimates of Ni, using a methodology that constrains moments of a parameterized particle size distribution (PSD). The operational liDAR–raDAR (DARDAR) product serves as an existing base for this method, which focuses on ice clouds with temperatures Tc < −30°C.Theoretical considerations demonstrate the capability for accurate retrievals of Ni, apart from a possible bias in the concentration in small crystals when Tc≳ − 50°C, due to the assumption of a monomodal PSD shape in the current method. This is verified via a comparison of satellite estimates to coincident in situ measurements, which additionally demonstrates the sufficient sensitivity of lidar–radar observations to Ni. Following these results, satellite estimates of Ni are evaluated in the context of a case study and a preliminary climatological analysis based on 10 years of global data. Despite a lack of other large-scale references, this evaluation shows a reasonable physical consistency in Ni spatial distribution patterns. Notably, increases in Ni are found towards cold temperatures and, more significantly, in the presence of strong updrafts, such as those related to convective or orographic uplifts. Further evaluation and improvement of this method are necessary, although these results already constitute a first encouraging step towards large-scale observational constraints for Ni. Part 2 of this series uses this new dataset to examine the controls on Ni.
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Journal articleGryspeerdt E, Sourdeval E, Quaas J, et al., 2018,
Ice crystal number concentration estimates from lidar–radar satellite remote sensing – Part 2: Controls on the ice crystal number concentration
, Atmospheric Chemistry and Physics, Vol: 18, Pages: 14351-14370, ISSN: 1680-7316The ice crystal number concentration (Ni) is a key property of ice clouds, both radiatively and microphysically. Due to sparse in situ measurements of ice cloud properties, the controls on the Ni have remained difficult to determine. As more advanced treatments of ice clouds are included in global models, it is becoming increasingly necessary to develop strong observational constraints on the processes involved.This work uses the DARDAR-Nice Ni retrieval described in Part 1 to investigate the controls on the Ni at a global scale. The retrieved clouds are separated by type. The effects of temperature, proxies for in-cloud updraft and aerosol concentrations are investigated. Variations in the cloud top Ni (Ni(top)) consistent with both homogeneous and heterogeneous nucleation are observed along with differing relationships between aerosol and Ni(top) depending on the prevailing meteorological situation and aerosol type. Away from the cloud top, the Ni displays a different sensitivity to these controlling factors, providing a possible explanation for the low Ni sensitivity to temperature and ice nucleating particles (INP) observed in previous in situ studies.This satellite dataset provides a new way of investigating the response of cloud properties to meteorological and aerosol controls. The results presented in this work increase our confidence in the retrieved Ni and will form the basis for further study into the processes influencing ice and mixed phase clouds.
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Journal articleNowack PJ, Braesicke P, Haigh J, et al., 2018,
Using machine learning to build temperature-based ozone parameterizations for climate sensitivity simulations
, Environmental Research Letters, Vol: 13, ISSN: 1748-9326A number of studies have demonstrated the importance of ozone in climate change simulations, for example concerning global warming projections and atmospheric dynamics. However, fully interactive atmospheric chemistry schemes needed for calculating changes in ozone are computationally expensive. Climate modelers therefore often use climatological ozone fields, which are typically neither consistent with the actual climate state simulated by each model nor with the specific climate change scenario. This limitation applies in particular to standard modeling experiments such as preindustrial control or abrupt 4xCO2 climate sensitivity simulations. Here we suggest a novel method using a simple linear machine learning regression algorithm to predict ozone distributions for preindustrial and abrupt 4xCO2 simulations. Using the atmospheric temperature field as the only input, the regression reliably predicts three-dimensional ozone distributions at monthly to daily time intervals. In particular, the representation of stratospheric ozone variability is much improved compared with a fixed climatology, which is important for interactions with dynamical phenomena such as the polar vortices and the Quasi-Biennial Oscillation. Our method requires training data covering only a fraction of the usual length of simulations and thus promises to be an important stepping stone towards a range of new computationally efficient methods to consider ozone changes in long climate simulations. We highlight key development steps to further improve and extend the scope of machine learning-based ozone parameterizations.
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Journal articleDougherty MK, Cao H, Khurana KK, et al., 2018,
Saturn's magnetic field revealed by the Cassini Grand Finale
, Science, Vol: 362, Pages: 1-9, ISSN: 0036-8075INTRODUCTIONStarting on 26 April 2017, the Grand Finale phase of the Cassini mission took the spacecraft through the gap between Saturn’s atmosphere and the inner edge of its innermost ring (the D-ring) 22 times, ending with a final plunge into the atmosphere on 15 September 2017. This phase offered an opportunity to investigate Saturn’s internal magnetic field and the electromagnetic environment between the planet and its rings. The internal magnetic field is a diagnostic of interior structure, dynamics, and evolution of the host planet. Rotating convective motion in the highly electrically conducting layer of the planet is thought to maintain the magnetic field through the magnetohydrodynamic (MHD) dynamo process. Saturn’s internal magnetic field is puzzling because of its high symmetry relative to the spin axis, known since the Pioneer 11 flyby. This symmetry prevents an accurate determination of the rotation rate of Saturn’s deep interior and challenges our understanding of the MHD dynamo process because Cowling’s theorem precludes a perfectly axisymmetric magnetic field being maintained through an active dynamo.RATIONALEThe Cassini fluxgate magnetometer was capable of measuring the magnetic field with a time resolution of 32 vectors per s and up to 44,000 nT, which is about twice the peak field strength encountered during the Grand Finale orbits. The combination of star cameras and gyroscopes onboard Cassini provided the attitude determination required to infer the vector components of the magnetic field. External fields from currents in the magnetosphere were modeled explicitly, orbit by orbit.RESULTSSaturn’s magnetic equator, where the magnetic field becomes parallel to the spin axis, is shifted northward from the planetary equator by 2808.5 ± 12 km, confirming the north-south asymmetric nature of Saturn’s magnetic field. After removing the systematic variation with distance from the spin axis, the peak-to-peak
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Journal articleWolf G, Brayshaw DJ, Klingaman NP, et al., 2018,
Quasi-stationary waves and their impact on European weather and extreme events
, Quarterly Journal of the Royal Meteorological Society, Vol: 144, Pages: 2431-2448, ISSN: 0035-9009Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society. Large-scale, quasi-stationary atmospheric waves (QSWs) have long been known to be associated with weather extremes such as the European heatwave in 2003. There is much debate in the scientific literature as to whether QSW activity may increase under a changing climate, providing a strong motivation for developing a better understanding of the behaviour and drivers of QSWs. This paper presents the first steps in this regard: the development of a robust objective method for a simple identification and characterization of these waves. A clear connection between QSWs and European weather and extreme events is confirmed for all seasons, indicating that blocking anti-cyclones are often part of a broader-scale wave pattern. Investigation of the QSW climatology in the Northern Hemisphere reveals that wave activity is typically strongest in midlatitudes, particularly at the exit of the Atlantic and Pacific storm track, with weaker intensities in summer. In general, the structure of individual QSW events tends to follow the climatological pattern, except in winter where the strongest and most persistent QSWs are typically shifted polewards, indicating a distinct evolution of the “strongest” QSW events. Modes of interannual variability are calculated to better understand their importance and connection to European temperatures and to identify relevant QSW patterns. This analysis highlights that European winter temperatures are strongly associated with the meridional location of QSW activity whereas high European summer temperatures are associated with increases in the overall intensity of midlatitude QSW activity. QSWs are shown to be strongly connected to commonly used indices to describe the large-scale atmospheric circulation (NAO, AO, Niño 3.4, PNA) but offer a more direct link to understanding their impact on
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Journal articleMoore L, Cravens TE, Mueller-Wodarg I, et al., 2018,
Models of Saturn's equatorial ionosphere based on in situ data from Cassini's grand finale
, Geophysical Research Letters, Vol: 45, Pages: 9398-9407, ISSN: 0094-8276We present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low‐altitude ionosphere, with a mean ion mass of 11 Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H3+ variability and an overabundance of modeled H3+ potentially hint at missing physical processes in the model, including a loss process that affects H3+ but not H+.
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Journal articleManners 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: 0094-8276Energy 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.
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Journal articleBowen TA, Mallet A, Bonnell JW, et al., 2018,
Impact of Residual Energy on Solar Wind Turbulent Spectra
, ASTROPHYSICAL JOURNAL, Vol: 865, ISSN: 0004-637X- Author Web Link
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- Citations: 17
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Journal articleStawarz JE, Eastwood JP, Genestreti KJ, et al., 2018,
Intense electric fields and electron‐scale substructure within magnetotail flux ropes as revealed by the Magnetospheric Multiscale mission
, Geophysical Research Letters, Vol: 45, Pages: 8783-8792, ISSN: 0094-8276Three flux ropes associated with near‐Earth magnetotail reconnection are analyzed using Magnetospheric Multiscale observations. The flux ropes are Earthward propagating with sizes from ∼3 to 11 ion inertial lengths. Significantly different axial orientations are observed, suggesting spatiotemporal variability in the reconnection and/or flux rope dynamics. An electron‐scale vortex, associated with one of the most intense electric fields (E) in the event, is observed within one of the flux ropes. This E is predominantly perpendicular to the magnetic field (B); the electron vortex is frozen‐in with E × B drifting electrons carrying perpendicular current and causing a small‐scale magnetic enhancement. The vortex is ∼16 electron gyroradii in size perpendicular to B and potentially elongated parallel to B. The need to decouple the frozen‐in vortical motion from the surrounding plasma implies a parallel E at the structure's ends. The formation of frozen‐in electron vortices within reconnection‐generated flux ropes may have implications for particle acceleration.
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Journal articleStaniland 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: 0094-8276We 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.
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Journal articleTurner DL, Wilson LB, Liu TZ, et al., 2018,
Autogenous and efficient acceleration of energetic ions upstream of Earth's bow shock
, NATURE, Vol: 561, Pages: 206-+, ISSN: 0028-0836- Author Web Link
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- Citations: 43
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Journal articlePalmroth M, Hietala H, Plaschke F, et al., 2018,
Magnetosheath jet properties and evolution as determined by a global hybrid-Vlasov simulation
, Annales Geophysicae: atmospheres, hydrospheres and space sciences, Vol: 36, Pages: 1171-1182, ISSN: 0992-7689Abstract. We use a global hybrid-Vlasov simulation for the magnetosphere, Vlasiator, to investigate magnetosheath high-speed jets. Unlike many other hybrid-kinetic simulations, Vlasiator includes an unscaled geomagnetic dipole, indicating that the simulation spatial and temporal dimensions can be given without scaling. Thus, for the first time, this allows investigating the magnetosheath jet properties and comparing them directly with the observed jets within the Earth's magnetosheath. In the run shown in this paper, the interplanetary magnetic field (IMF) cone angle is 30°, and a foreshock develops upstream of the quasi-parallel magnetosheath. We visually detect a structure with high dynamic pressure propagating from the bow shock towards the magnetopause. The structure is confirmed as a jet using three different criteria, which have been adopted in previous observational studies. We compare these criteria against the simulation results. We find that the magnetosheath jet is an elongated structure extending Earthward of the bow shock by ~ 2.3 RE, while its size perpendicular to the direction of propagation is ~ 0.5 RE. We also investigate the jet evolution, and find that the jet originates due to the interaction of the foreshock Ultra Low Frequency (ULF) waves with the bow shock surface. The simulation shows that magnetosheath jets can develop also under steady IMF, as inferred by observational studies.
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Journal articleSchillings A, Nilsson H, Slapak R, et al., 2018,
O+ Escape During the Extreme Space Weather Event of 4-10 September 2017
, Space Weather-the International Journal of Research and Applications, Vol: 16, Pages: 1363-1376, ISSN: 1539-4956We have investigated the consequences of extreme space weather on ion outflow from the polar ionosphere by analyzing the solar storm that occurred early September 2017, causing a severe geomagnetic storm. Several X‐flares and coronal mass ejections were observed between 4 and 10 September. The first shock—likely associated with a coronal mass ejection—hit the Earth late on 6 September, produced a storm sudden commencement, and began the initial phase of the storm. It was followed by a second shock, approximately 24 hr later, that initiated the main phase and simultaneously the Dst index dropped to Dst = −142 nT and Kp index reached Kp = 8. Using COmposition DIstribution Function data on board Cluster satellite 4, we estimated the ionospheric O+ outflow before and after the second shock. We found an enhancement in the polar cap by a factor of 3 for an unusually high ionospheric O+ outflow (mapped to an ionospheric reference altitude) of 1013 m−2 s−1. We suggest that this high ionospheric O+ outflow is due to a preheating of the ionosphere by the multiple X‐flares. Finally, we briefly discuss the space weather consequences on the magnetosphere as a whole and the enhanced O+ outflow in connection with enhanced satellite drag.
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Journal articleSulaiman AH, Kurth WS, Hospodarsky GB, et al., 2018,
Enceladus auroral hiss emissions during Cassini's grand finale
, Geophysical Research Letters, Vol: 45, Pages: 7347-7353, ISSN: 0094-8276Cassini's Radio and Plasma Wave Science (RPWS) instrument detected intense auroral hiss emissions during one of its perikrone passes of the Grand Finale orbits. The emissions were detected when Cassini traversed a flux tube connected to Enceladus' orbit (L‐shell = 4) and at a time when both the spacecraft and the icy moon were in similar longitudes. Previous observations of auroral hiss related to Enceladus were made only during close flybys and here we present the first observation of such emissions close to Saturn. Further, ray‐tracing analysis shows the source location at a latitude of 63°, in excellent agreement with earlier UVIS observations of Enceladus' auroral footprint by Pryor et al. (2011, https://doi.org/10.1038/nature09928). The detection has been afforded exclusively by the Grand Finale phase, which enabled sampling of Enceladus' high‐latitude flux tube near Saturn. This result provides new insight into the spatial extent of the electrodynamic interaction between Saturn and Enceladus.
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Journal articleMasters A, 2018,
A more viscous-like solar wind interaction with all the giant planets
, Geophysical Research Letters, Vol: 45, Pages: 7320-7329, ISSN: 0094-8276Identifying 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.
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Journal articleRyan E, Wild O, Voulgarakis A, et al., 2018,
Fast sensitivity analysis methods for computationally expensive models with multi-dimensional output
, GEOSCIENTIFIC MODEL DEVELOPMENT, Vol: 11, Pages: 3131-3146, ISSN: 1991-959XGlobal sensitivity analysis (GSA) is a powerful approach in identifying which inputs or parameters most affect a model's output. This determines which inputs to include when performing model calibration or uncertainty analysis. GSA allows quantification of the sensitivity index (SI) of a particular input – the percentage of the total variability in the output attributed to the changes in that input – by averaging over the other inputs rather than fixing them at specific values. Traditional methods of computing the SIs using the Sobol and extended Fourier Amplitude Sensitivity Test (eFAST) methods involve running a model thousands of times, but this may not be feasible for computationally expensive Earth system models. GSA methods that use a statistical emulator in place of the expensive model are popular, as they require far fewer model runs. We performed an eight-input GSA, using the Sobol and eFAST methods, on two computationally expensive atmospheric chemical transport models using emulators that were trained with 80 runs of the models. We considered two methods to further reduce the computational cost of GSA: (1) a dimension reduction approach and (2) an emulator-free approach. When the output of a model is multi-dimensional, it is common practice to build a separate emulator for each dimension of the output space. Here, we used principal component analysis (PCA) to reduce the output dimension, built an emulator for each of the transformed outputs, and then computed SIs of the reconstructed output using the Sobol method. We considered the global distribution of the annual column mean lifetime of atmospheric methane, which requires ∼ 2000 emulators without PCA but only 5–40 emulators with PCA. We also applied an emulator-free method using a generalised additive model (GAM) to estimate the SIs using only the training runs. Compared to the emulator-only methods, the emulator–PCA and GAM methods accurately estimated the SIs
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Journal articleWang B, Nishimura Y, Hietala H, et al., 2018,
Dayside Magnetospheric and Ionospheric Responses to a Foreshock Transient on 25 June 2008: 2. 2-D Evolution Based on Dayside Auroral Imaging
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 6347-6359, ISSN: 2169-9380 -
Journal articleGuo RL, Yao ZH, Wei Y, et al., 2018,
Rotationally driven magnetic reconnection in Saturn's dayside
, Nature Astronomy, Vol: 2, Pages: 640-645, ISSN: 2397-3366Magnetic reconnection is a key process that explosively accelerates charged particles, generating phenomena such as nebular flares1, solar flares2 and stunning aurorae3. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisc, where thin-current-sheet conditions are conducive to reconnection4. The dayside magnetodisc is usually considered thicker than the nightside due to the compression of solar wind, and is therefore not an ideal environment for reconnection. In contrast, a recent statistical study of magnetic flux circulation strongly suggests that magnetic reconnection must occur throughout Saturn’s dayside magnetosphere5. Additionally, the source of energetic plasma can be present in the noon sector of giant planetary magnetospheres6. However, so far, dayside magnetic reconnection has only been identified at the magnetopause. Here, we report direct evidence of near-noon reconnection within Saturn’s magnetodisc using measurements from the Cassini spacecraft. The measured energetic electrons and ions (ranging from tens to hundreds of keV) and the estimated energy flux of ~2.6 mW m–2 within the reconnection region are sufficient to power aurorae. We suggest that dayside magnetodisc reconnection can explain bursty phenomena in the dayside magnetospheres of giant planets, which can potentially advance our understanding of quasi-periodic injections of relativistic electrons6 and auroral pulsations7.
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Journal articleWang S, Toumi R, 2018,
Reduced sensitivity of tropical cyclone intensity and size to sea surface temperature in a radiative-convective equilibrium environment
, Advances in Atmospheric Sciences, Vol: 35, Pages: 981-993, ISSN: 1861-9533It has been challenging to project the tropical cyclone (TC) intensity, structure and destructive potential changes in a warming climate. Here, we compare the sensitivities of TC intensity, size and destructive potential to sea surface warming with and without a pre-storm atmospheric adjustment to an idealized state of Radiative-Convective Equilibrium (RCE). Without RCE, we find large responses of TC intensity, size and destructive potential to sea surface temperature (SST) changes, which is in line with some previous studies. However, in an environment under RCE, the TC size is almost insensitive to SST changes, and the sensitivity of intensity is also much reduced to 3% °C−1–4% °C−1. Without the pre-storm RCE adjustment, the mean destructive potential measured by the integrated power dissipation increases by about 25% °C−1 during the mature stage. However, in an environment under RCE, the sensitivity of destructive potential to sea surface warming does not change significantly. Further analyses show that the reduced response of TC intensity and size to sea surface warming under RCE can be explained by the reduced thermodynamic disequilibrium between the air boundary layer and the sea surface due to the RCE adjustment. When conducting regional-scale sea surface warming experiments for TC case studies, without any RCE adjustment the TC response is likely to be unrealistically exaggerated. The TC intensity–temperature sensitivity under RCE is very similar to those found in coupled climate model simulations. This suggests global mean intensity projections under climate change can be understood in terms of a thermodynamic response to temperature with only a minor contribution from any changes in large-scale dynamics.
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Journal articlePlaschke F, Hietala H, Archer M, et al., 2018,
Jets downstream of collisionless shocks
, Space Science Reviews, Vol: 214, ISSN: 0038-6308The magnetosheath flow may take the form of large amplitude, yet spatially localized, transient increases in dynamic pressure, known as “magnetosheath jets” or “plasmoids” among other denominations. Here, we describe the present state of knowledge with respect to such jets, which are a very common phenomenon downstream of the quasi-parallel bow shock. We discuss their properties as determined by satellite observations (based on both case and statistical studies), their occurrence, their relation to solar wind and foreshock conditions, and their interaction with and impact on the magnetosphere. As carriers of plasma and corresponding momentum, energy, and magnetic flux, jets bear some similarities to bursty bulk flows, which they are compared to. Based on our knowledge of jets in the near Earth environment, we discuss the expectations for jets occurring in other planetary and astrophysical environments. We conclude with an outlook, in which a number of open questions are posed and future challenges in jet research are discussed.
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Journal articleGingell IL, Schwartz SJ, Gershman DJ, et al., 2018,
Production of negative hydrogen ions within MMS Fast Plasma Investigation due to solar wind bombardment
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 6161-6170, ISSN: 2169-9380The particle data delivered by Fast Plasma Investigation (FPI) instrument aboard NASA's Magnetospheric Multiscale (MMS) mission allows for exceptionally high-resolution examination of the electron and ion phase space in the near-Earth plasma environment. It is necessary to identify populations which originate from instrumental effects. Using FPI's Dual Electron Spectrometers (DES) we isolate a high energy (~keV) beam, present while the spacecraft are in the solar wind, which exhibits an azimuthal drift with period associated with the spacecraft spin. We show that this population is consistent with negative hydrogen ions H- generated by a double charge exchange interaction between the incident solar wind H+ ions and the metallic surfaces within the instrument. This interaction is likely to occur at the deflector plates close to the instrument aperture. The H- density is shown to be approximately 0.2-0.4% of the solar wind ion density, and the energy of the negative ion population is shown to be 70% of the incident solar wind energy. These negative ions may introduce errors in electron velocity moments on the order of 0.2-0.4% of the solar wind velocity, and significantly higher errors in the electron temperature.
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