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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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Journal articleGraven H, Zazzeri G, Acuña Yeomans E, 2018,
Global and regional emissions of radiocarbon from nuclear power plants from 1972 to 2016
, Radiocarbon, Vol: 60, Pages: 1067-1081, ISSN: 0033-8222CH4 and CO2 emissions from geologic sources, which are devoid of radiocarbon (14C), dilute the atmospheric 14C/C ratio. Observations of 14C/C can be used to estimate fossil fuel-derived CH4 and CO2. However, the atmospheric 14C/C ratio is perturbed by emissions of 14C from nuclear power plants (NPPs) and fuel reprocessing sites, which may affect such 14C/C-based estimation if they are not correctly quantified. We calculate NPP 14C emissions for CO2 and CH4 from 1972–2016 using standard emission factors (14C emitted per unit of power produced) and analyze trends in global and regional emissions. We use available observations of 14C emissions and power generation in Europe to assess emission factors for different reactor types, as well as potential differences related to the age or manufacturer of the NPPs. Globally, nuclear 14C emissions increase until 2005 and then decrease, mostly because of the closure of gas-cooled reactors in the United Kindom and the shutdown of light water reactors after the Fukushima nuclear accident in March 2011. Observed emission factors in Europe show strong variability, spanning values from 0.003 to 2.521 TBq/GWa for PWR and from 0.007 to 1.732 TBq/GWa for BWR reactors, suggesting more information and more sophisticated models are needed to improve estimates of 14C emissions.
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Journal articleVerscharen D, Klein KG, Chandran BDG, et al., 2018,
ALPS: the Arbitrary Linear Plasma Solver
, JOURNAL OF PLASMA PHYSICS, Vol: 84, ISSN: 0022-3778- Author Web Link
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- Citations: 14
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Journal articleHorbury TS, Matteini L, Stansby D, 2018,
Short, large-amplitude speed enhancements in the near-Sun fast solar wind
, Monthly Notices of the Royal Astronomical Society, Vol: 478, Pages: 1980-1986, ISSN: 0035-8711We report the presence of intermittent, short discrete enhancements in plasma speed in the near-Sun high speed solar wind. Lasting tens of seconds to minutes in spacecraft measurements at 0.3 AU, speeds inside these enhancements can reach 1000 km/s, corresponding to a kinetic energy up to twice that of the bulk high speed solar wind. These events, which occur around 5% of the time, are Alfvenic in nature with large magnetic field deflections and are the same temperature as the surrounding plasma, in contrast to the bulk fast wind which has a well-established positive speed-temperature correlation. The origin of these speed enhancements is unclear but they may be signatures of discrete jets associated with transient events in the chromosphere or corona. Such large short velocity changes represent a measurement and analysis challenge for the upcoming Parker Solar Probe and Solar Orbiter missions.
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Journal articleSergis N, Achilleos N, Guio P, et al., 2018,
Mapping Saturn's nightside plasma sheet using Cassini's proximal orbits
, Geophysical Research Letters, Vol: 45, Pages: 6798-6804, ISSN: 0094-8276Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5‐day‐period proximal orbits, covering the mid‐latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos‐Guio‐Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long‐term, near‐equatorial measurements.
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Journal articleSulaiman AH, Kurth WS, Hospodarsky GB, et al., 2018,
Auroral Hiss Emissions During Cassini's Grand Finale: Diverse Electrodynamic Interactions Between Saturn and Its Rings
, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 6782-6789, ISSN: 0094-8276The Cassini Grand Finale orbits offered a new view of Saturn and its environment owing to multiple highly inclined orbits with unprecedented proximity to the planet during closest approach. The Radio and Plasma Wave Science instrument detected striking signatures of plasma waves in the southern hemisphere. These all propagate in the whistler mode and are classified as (1) a filled funnel‐shaped emission, commonly known as auroral hiss. Here however, our analysis indicates that they are likely associated with currents connected to the rings. (2) First observations of very low frequency saucers directly linked to the planet on field lines also connected to the rings. The latter observations are unique to low altitude orbits, and their presence at the Earth and Saturn alike shows that they are fundamental plasma waves in planetary ionospheres. Our results give an insight, from a unique perspective, into the dynamic and diverse nature of Saturn's environment.
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Journal articleReid J, Hood AW, Parnell CE, et al., 2018,
Coronal energy release by MHD avalanches: continuous driving
, Astronomy and Astrophysics: a European journal, Vol: 615, Pages: 1-10, ISSN: 0004-6361Previous work has confirmed the concept of a magnetohydrodynamic (MHD) avalanche in pre-stressed threads within a coronal loop. We undertook a series of full, three-dimensional MHD simulations in order to create three threads by twisting the magnetic field through boundary motions until an instability ensues. We find that, following the original instability, one unstable thread can disrupt its neighbours with continued driving. A “bursty” heating profile results, with a series of ongoing energy releases, but no evident steady state. For the first time using full MHD, we show that avalanches are a viable mechanism for the storing and release of magnetic energy in the solar corona, as a result of photospheric motions.
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Journal articleBanks JR, Schepanski K, Heinold B, et al., 2018,
The influence of dust optical properties on the colour of simulated MSG-SEVIRI Desert Dust infrared imagery
, ATMOSPHERIC CHEMISTRY AND PHYSICS, Vol: 18, Pages: 9681-9703, ISSN: 1680-7316Satellite imagery of atmospheric mineral dust is sensitive to the optical properties of the dust, governed by the mineral refractive indices, particle size, and particle shape. In infrared channels the imagery is also sensitive to the dust layer height and to the surface and atmospheric environment. Simulations of mineral dust in infrared "Desert Dust" imagery from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) have been performed, using the COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model) dust transport model and the Radiative Transfer for TOVS (RTTOV) program, in order to investigate the sensitivity of the imagery to assumed dust properties. This paper introduces the technique and performs initial validation and comparisons with SEVIRI measurements over North Africa for daytime hours during 6 months covering June and July of 2011–2013. Using T-matrix scattering theory and assuming the dust particles to be spherical or spheroidal, wavelength- and size-dependent dust extinction values are calculated for a number of different dust refractive index databases, along with several values of the particle aspect ratio, denoting the particle shape. The consequences for the infrared extinction values of both the particle shape and the particle orientation are explored: this analysis shows that as the particle asphericity increases, the extinctions increase if the particles are aligned horizontally, and decrease if they are aligned vertically. Randomly oriented spheroidal particles have very similar infrared extinction properties as spherical particles, whereas the horizontally and vertically aligned particles can be considered to be the upper and lower bounds on the extinction values. Inputting these values into COSMO-MUSCAT-RTTOV, it is found that spherical particles do not appear to be sufficient to describe fully the resultant colour of the dust in the infrared imagery. Comparisons
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Journal articleHeritier KL, Altwegg K, Berthelier J-J, et al., 2018,
On the origin of molecular oxygen in cometary comae
, NATURE COMMUNICATIONS, Vol: 9, ISSN: 2041-1723 -
Journal articleFutaana Y, Barabash S, Wieser M, et 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: 0032-0633The 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.
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Journal articleMenelaou K, Yau MK, Lai T-K, 2018,
Possible Three-Dimensional Mechanism for Oscillating Wobbles in Tropical Cyclone-Like Vortices with Concentric Eyewalls
, JOURNAL OF THE ATMOSPHERIC SCIENCES, Vol: 75, Pages: 2157-2174, ISSN: 0022-4928
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