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Journal articleHe J, Tu C, Marsch E, et al., 2015,
PROTON HEATING IN SOLAR WIND COMPRESSIBLE TURBULENCE WITH COLLISIONS BETWEEN COUNTER-PROPAGATING WAVES
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 813, ISSN: 2041-8205- Author Web Link
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- Citations: 40
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Journal articleSteckiewicz M, Mazelle C, Garnier P, et al., 2015,
Altitude dependence of nightside Martian suprathermal electron depletions as revealed by MAVEN observations
, Geophysical Research Letters, Vol: 42, Pages: 8877-8884, ISSN: 0094-8276The MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft is providing new detailed observations of the Martian ionosphere thanks to its unique orbital coverage and instrument suite. During most periapsis passages on the nightside ionosphere suprathermal electron depletions were detected. A simple criterion was implemented to identify the 1742 depletions observed from 16 November 2014 to 28 February 2015. A statistical analysis reveals that the main ion and electron populations within the depletions are surprisingly constant in time and altitude. Absorption by CO2 is the main loss process for suprathermal electrons, and electrons that strongly peaked around 6 eV are resulting from this interaction. The observation of depletions appears however highly dependent on altitude. Depletions are mainly located above strong crustal magnetic sources above 170 km, whereas the depletions observed for the first time below 170 km are globally scattered onto the Martian surface with no particular dependence on crustal fields.
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Journal articleHadid LZ, Sahraoui F, Kiyani KH, et al., 2015,
Nature of the MHD and kinetic scale turbulence in the magnetosheath of Saturn: Cassini observations
, Astrophysical Journal Letters, Vol: 813, ISSN: 2041-8213Low-frequency turbulence in Saturn's magnetosheath is investigated using in situ measurements of the Cassini spacecraft. Focus is put on the magnetic energy spectra computed in the frequency range of ~[10−4, 1]Hz. A set of 42 time intervals in the magnetosheath were analyzed, and three main results that contrast with known features of solar wind turbulence are reported. (1) The magnetic energy spectra showed a ~f−1 scaling at MHD scales followed by an $\sim {f}^{-2.6}$ scaling at sub-ion scales without forming the so-called inertial range. (2) The magnetic compressibility and the cross-correlation between the parallel component of the magnetic field and density fluctuations $C(\delta n,\delta {B}_{| | })$ indicate the dominance of the compressible magnetosonic slow-like modes at MHD scales rather than the Alfvén mode. (3) Higher-order statistics revealed a monofractal (multifractal) behavior of the turbulent flow downstream of a quasi-perpendicular (quasi-parallel) shock at sub-ion scales. Implications of these results on theoretical modeling of space plasma turbulence are discussed.
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Journal articleFuselier SA, Altwegg K, Balsiger H, et al., 2015,
ROSINA/DFMS and IES observations of 67P: Ion-neutral chemistry in the coma of a weakly outgassing comet
, Astronomy & Astrophysics, Vol: 583, ISSN: 1432-0746Context. The Rosetta encounter with comet 67P/Churyumov-Gerasimenko provides a unique opportunity for an in situ, up-closeinvestigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the Sun.Aims. Observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within thethin coma.Methods. Observations from late October through mid-December 2014 show the continuous presence of the solar wind 30 km fromthe comet nucleus. These and other observations indicate that there is no contact surface and the solar wind has direct access tothe nucleus. On several occasions during this time period, the Rosetta/ROSINA/Double Focusing Mass Spectrometer measured thelow-energy ion composition in the coma. Organic volatiles and water group ions and their breakup products (masses 14 through 19),CO+, and CO+2(masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. Secondary ions includeH3O+and HCO+(masses 19 and 29). These secondary ions indicate ion-neutral chemistry in the thin coma of the comet. A relativelysimple model is constructed to account for the low H3O+/H2O+and HCO+/CO+ratios observed in a water dominated coma. Resultsfrom this simple model are compared with results from models that include a more detailed chemical reaction network.Results. At low outgassing rates, predictions from the simple model agree with observations and with results from more complex modelsthat include much more chemistry. At higher outgassing rates, the ion-neutral chemistry is still limited and high HCO+/CO+ratiosare predicted and observed. However, at higher outgassing rates, the model predicts high H3O+/H2O+ratios and the observed ratiosare often low. These low ratios may be the result of the highly heterogeneous nature of the coma, where CO and CO2 number densitiescan exceed that of water.
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Journal articleKoskinen TT, Sandel BR, Yelle RV, et al., 2015,
Saturn's variable thermosphere from Cassini/UVIS occultations
, Icarus, Vol: 260, Pages: 174-189, ISSN: 0019-1035We retrieved the density and temperature profiles in Saturn’s thermosphere from 26 stellar occultations observed by the Cassini/UVIS instrument. These results expand upon and complement the previous analysis of 15 Cassini/UVIS solar occultations by Saturn’s upper thermosphere. We find that the exospheric temperatures based on the stellar occultations agree with the solar occultations and range from 380 K to 590 K. These temperatures are also consistent with the recent re-analysis of the Voyager/UVS occultations. The retrieved density profiles support our earlier inference that the shape of the atmosphere at low pressures is consistent with a meridional trend of increasing temperatures with absolute latitude. This implies a high-latitude heat source, such as auroral heating, although the existing circulation models that include auroral heating still underestimate the equatorial temperatures by overestimating the meridional temperature gradient. This suggests either that the circulation models are somehow incomplete or there is some other heat source at low to mid latitudes that is relatively less efficient than high-latitude heating. We also find evidence for the expansion of the exobase by about 500 km between 2006 and 2011 near the equator, followed by possible contraction after 2011. The expansion appears to be caused by significant warming of the lower thermosphere that anti-correlates with solar activity and may be connected to changes in global circulation. Lastly, we note that our density profiles are in good general agreement with the Voyager/UVS data. In particular, the Voyager density profiles are most consistent with the Cassini/UVIS stellar occultations from late 2008 and early 2009 that roughly coincide in season with the Voyager flybys.
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Journal articleEngelhardt IAD, Wahlund J-E, Andrews DJ, et al., 2015,
Plasma regions, charged dust and field-aligned currents near Enceladus
, PLANETARY AND SPACE SCIENCE, Vol: 117, Pages: 453-469, ISSN: 0032-0633 -
Journal articleBeth A, Garnier P, Toublanc D, et al., 2015,
Theory for planetary exospheres: I. Radiation pressure effect on dynamical trajectories
, Icarus, Vol: 266, Pages: 410-422, ISSN: 0019-1035The planetary exospheres are poorly known in their outer parts, since the neutral densities are low comparedwith the instruments detection capabilities. The exospheric models are thus often the main sourceof information at such high altitudes. We present a new way to take into account analytically the additionaleffect of the radiation pressure on planetary exospheres. In a series of papers, we present with anHamiltonian approach the effect of the radiation pressure on dynamical trajectories, density profiles andescaping thermal flux. Our work is a generalisation of the study by Bishop and Chamberlain (Bishop, J.,Chamberlian, J.W. [1989]. Icarus 81, 145–163). In this first paper, we present the complete solutions ofparticles trajectories, which are not conics, under the influence of the solar radiation pressure with someassumptions. This problem is similar to the classical Stark problem (Stark, J. [1914]. Ann. Phys. 348,965–982). This problem was largely tackled in the literature and more specifically, recently by Lantoineand Russell (Lantoine, G., Russell, R.P. [2011]. Celest. Mech. Dynam. Astron. 109, 333–366) and byBiscani and Izzo (Biscani, F., Izzo, D. [2014]. Mon. Not. R. Astron. Soc. 439, 810–822) as we will discussin this paper. We give here the full set of solutions for the motion of a particle (in our case for an atomor a molecule), i.e. the space coordinates and the time solution for bounded and unbounded trajectoriesin terms of Jacobi elliptic functions. We thus provide here the complete set of solutions for this so-callStark effect (Stark, J. [1914]. Ann. Phys. 348, 965–982) in terms of Jacobi elliptic functions (Jacobi, C.G.J. [1829]. Fundamenta nova theoriae functionum ellipticarum. Sumtibus fratrum), which may be usedto model the trajectories of particles in planetary exospheres.
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Journal articleHausmann U, Czaja A, Marshall J, 2015,
Estimates of air–sea feedbacks on sea surface temperature anomalies in the southern ocean
, Journal of Climate, Vol: 29, Pages: 439-454, ISSN: 1520-0442Sea surface temperature (SST) air–sea feedback strengths and associated decay time scales in the Southern Ocean (SO) are estimated from observations and reanalysis datasets of SST, air–sea heat fluxes, and ocean mixed layer depths. The spatial, seasonal, and scale dependence of the air–sea heat flux feedbacks is mapped in circumpolar bands and implications for SST persistence times are explored. It is found that the damping effect of turbulent heat fluxes dominates over that due to radiative heat fluxes. The turbulent heat flux feedback acts to damp SSTs in all bands and spatial scales and in all seasons, at rates varying between 5 and 25 W m−2 K−1, while the radiative heat flux feedback has a more uniform spatial distribution with a magnitude rarely exceeding 5 W m−2 K−1. In particular, the implied net air–sea feedback (turbulent + radiative) on SST south of the polar front, and in the region of seasonal sea ice, is as weak as 5–10 W m−2 K−1 in the summertime on large spatial scales. Air–sea interaction alone thus allows SST signals induced around Antarctica in the summertime to persist for several seasons. The damping effect of mixed layer entrainment on SST anomalies averages to approximately 20 W m−2 K−1 across the ACC bands in the summer-to-winter entraining season and thereby reduces summertime SST persistence to less than half of that predicted by air–sea interaction alone (i.e., 3–6 months).
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Journal articleBrindley H, Osipov S, Bantges R, et al., 2015,
An assessment of the quality of aerosol retrievals over the Red Sea and evaluation of the climatological cloud-free dust direct radiative effect in the region
, Journal of Geophysical Research: Atmospheres, Vol: 120, Pages: 10862-10878, ISSN: 2169-897XGround-based and satellite observations are used in conjunction with the Rapid Radiative Transfer Model (RRTM) to assess climatological aerosol loading and the associated cloud-free aerosol direct radiative effect (DRE) over the Red Sea. Aerosol optical depth (AOD) retrievals from the Moderate Resolution Imaging Spectroradiometer and Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments are first evaluated via comparison with ship-based observations. Correlations are typically better than 0.9 with very small root-mean-square and bias differences. Calculations of the DRE along the ship cruises using RRTM also show good agreement with colocated estimates from the Geostationary Earth Radiation Budget instrument if the aerosol asymmetry parameter is adjusted to account for the presence of large particles. A monthly climatology of AOD over the Red Sea is then created from 5 years of SEVIRI retrievals. This shows enhanced aerosol loading and a distinct north to south gradient across the basin in the summer relative to the winter months. The climatology is used with RRTM to estimate the DRE at the top and bottom of the atmosphere and the atmospheric absorption due to dust aerosol. These climatological estimates indicate that although longwave effects can reach tens of W m−2, shortwave cooling typically dominates the net radiative effect over the Sea, being particularly pronounced in the summer, reaching 60 W m−2 at the surface. The spatial gradient in summertime AOD is reflected in the radiative effect at the surface and in associated differential heating by aerosol within the atmosphere above the Sea. This asymmetric effect is expected to exert a significant influence on the regional atmospheric and oceanic circulation.
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Journal articleBrindley H, Osipov S, Bantges R, et al., 2015,
An assessment of the quality of aerosol retrievals over the Red Sea and evaluation of the climatological cloud-free dust direct radiative effect in the region
, Journal of Geophysical Research: Atmospheres, Vol: 120, ISSN: 2169-897XGround-based and satellite observations are used in conjunction with the Rapid RadiativeTransfer Model (RRTM) to assess climatological aerosol loading and the associated cloud-free aerosol directradiative effect (DRE) over the Red Sea. Aerosol optical depth (AOD) retrievals from the Moderate ResolutionImaging Spectroradiometer and Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments are firstevaluated via comparison with ship-based observations. Correlations are typically better than 0.9 with verysmall root-mean-square and bias differences. Calculations of the DRE along the ship cruises using RRTM alsoshow good agreement with colocated estimates from the Geostationary Earth Radiation Budget instrumentif the aerosol asymmetry parameter is adjusted to account for the presence of large particles. A monthlyclimatology of AOD over the Red Sea is then created from 5 years of SEVIRI retrievals. This shows enhancedaerosol loading and a distinct north to south gradient across the basin in the summer relative to the wintermonths. The climatology is used with RRTM to estimate the DRE at the top and bottom of the atmosphereand the atmospheric absorption due to dust aerosol. These climatological estimates indicate that althoughlongwave effects can reach tens of W m 2, shortwave cooling typically dominates the net radiativeeffect over the Sea, being particularly pronounced in the summer, reaching 120 W m 2 at the surface.The spatial gradient in summertime AOD is reflected in the radiative effect at the surface and in associateddifferential heating by aerosol within the atmosphere above the Sea. This asymmetric effect is expected toexert a significant influence on the regional atmospheric and oceanic circulation.
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Journal articleChen CHK, Matteini L, Burgess D, et al., 2015,
Erratum: magnetic field rotations in the solar wind at kinetic scales
, Monthly Notices of the Royal Astronomical Society: Letters, Vol: 455, Pages: L51-L51, ISSN: 1745-3933 -
Journal articlePalmroth M, Archer M, Vainio R, et al., 2015,
ULF foreshock under radial IMF: THEMIS observations and global kinetic simulation Vlasiator results compared
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 120, Pages: 8782-8798, ISSN: 2169-9380 -
Journal articleVigren E, Galand M, Eriksson AI, et al., 2015,
ON THE ELECTRON-TO-NEUTRAL NUMBER DENSITY RATIO IN THE COMA OF COMET 67P/CHURYUMOV-GERASIMENKO: GUIDING EXPRESSION AND SOURCES FOR DEVIATIONS
, ASTROPHYSICAL JOURNAL, Vol: 812, ISSN: 0004-637X -
Journal articleParfitt R, Czaja A, 2015,
On the contribution of synoptic transients to the mean atmospheric state in the Gulf Stream region
, Quarterly Journal of the Royal Meteorological Society, Vol: 142, Pages: 1554-1561, ISSN: 1477-870XA new decomposition of the time mean sea level pressure, precipitation, meridional velocity (v) and pressure vertical velocity (ω) is applied to ERA-Interim reanalysis data over the North Atlantic ocean for the December-February 1979–2011 time period. The decomposition suggests that the atmosphere over the Gulf Stream is dominated by a continuous series of synoptic systems, or baroclinic waves, propagating across the region. The time mean value of precipitation, meridional velocity and ω (the latter being taken as a proxy for upward and downward motion) is accordingly set by the propagating waves. The result is particularly striking for ω (v) considering that ascent and descent (poleward and equatorward flow) could reasonably be expected to cancel out in such a series of waves.These results shed a new light on analyses of the storm track heat budget in which the residual between diabatic heating and “transient” eddy heat fluxes (singled out through band pass time filtering or spatial Fourier analysis) is interpreted as a Rossby wave source. This interpretation is questioned because, as a consequence of the filtering used, these studies prevent any direct contribution of the “transients” to the time mean ω or meridional velocity, attributing entirely both fields to the circulation associated with the thermally forced Rossby wave. The fact that “transients” directly contribute to the observed time mean ω over the Gulf Stream might also explain the discrepancy between the observed and predicted response of the vertical motion field to heating in midlatitudes.
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Journal articleFranci L, Landi S, Matteini L, et al., 2015,
High-resolution hybrid simulations of kinetic plasma turbulence at proton scales
, Astrophysical Journal, Vol: 812, ISSN: 1538-4357We investigate properties of plasma turbulence from magnetohydrodynamic (MHD) to sub-ion scales by means oftwo-dimensional, high-resolution hybrid particle-in-cell simulations. We impose an initial ambient magneticfield perpendicular to the simulation box, and we add a spectrum of large-scale magnetic and kineticfluctuations with energy equipartition and vanishing correlation. Once the turbulence is fully developed, weobserve an MHD inertial range, where the spectra of the perpendicular magnetic field and the perpendicular protonbulk velocity fluctuations exhibit power-law scaling with spectral indices of -5 3 and -3 2, respectively. Thisbehavior is extended over a full decade in wavevectors and is very stable in time. A transition is observed aroundproton scales. At sub-ion scales, both spectra steepen, with the former still following a power law with a spectralindex of ~-3. A-2.8 slope is observed in the density and parallel magnetic fluctuations, highlighting the presenceof compressive effects at kinetic scales. The spectrum of the perpendicular electric fluctuations follows that of theproton bulk velocity at MHD scales, and flattens at small scales. All these features, which we carefully testedagainst variations of many parameters, are in good agreement with solar wind observations. The turbulent cascadeleads to on overall proton energization with similar heating rates in the parallel and perpendicular directions. Whilethe parallel proton heating is found to be independent on the resistivity, the number of particles per cell, and theresolution employed, the perpendicular proton temperature strongly depends on these parameters.
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Journal articleMatteini L, Hellinger P, Schwartz SJ, et al., 2015,
FIRE HOSE INSTABILITY DRIVEN BY ALPHA PARTICLE TEMPERATURE ANISOTROPY
, Astrophysical Journal, Vol: 812, ISSN: 1538-4357We investigate properties of a solar wind-like plasma, including a secondary alpha particle population exhibiting aparallel temperature anisotropy with respect to the background magnetic field, using linear and quasi-linearpredictions and by means of one-dimensional hybrid simulations. We show that anisotropic alpha particles candrive a parallel fire hose instability analogous to that generated by protons, but that, remarkably, can also betriggered when the parallel plasma beta of alpha particles is below unity. The wave activity generated by the alphaanisotropy affects the evolution of the more abundant protons, leading to their anisotropic heating. When both ionspecies have sufficient parallel anisotropies, both of them can drive the instability, and we observe the generationof two distinct peaks in the spectra of the fluctuations, with longer wavelengths associated to alphas and shorterones to protons. If a non-zero relative drift is present, the unstable modes propagate preferentially in the directionof the drift associated with the unstable species. The generated waves scatter particles and reduce their temperatureanisotropy to a marginally stable state, and, moreover, they significantly reduce the relative drift between the twoion populations. The coexistence of modes excited by both species leads to saturation of the plasma in distinctregions of the beta/anisotropy parameter space for protons and alpha particles, in good agreement with in situ solarwind observations. Our results confirm that fire hose instabilities are likely at work in the solar wind and limit theanisotropy of different ion species in the plasma.
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Journal articleSouthwood D, 2015,
James Wynne Dungey 1923-2015 OBITUARY
, Astronomy & Geophysics, Vol: 56, Pages: 8-8, ISSN: 1468-4004 -
Journal articleBeskin VS, Balogh A, Falanga M, et al., 2015,
Magnetic Fields at Largest Universal Strengths: Overview
, SPACE SCIENCE REVIEWS, Vol: 191, Pages: 1-12, ISSN: 0038-6308- Author Web Link
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- Citations: 8
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Journal articleHellinger P, Matteini L, Landi S, et al., 2015,
Plasma turbulence and kinetic instabilities at ion scales in the expanding solar wind
, Astrophysical Journal Letters, Vol: 811, ISSN: 2041-8213The relationship between a decaying strong turbulence and kinetic instabilities in a slowly expanding plasma isinvestigated using two-dimensional (2D) hybrid expanding box simulations. We impose an initial ambientmagnetic field perpendicular to the simulation box, and we start with a spectrum of large-scale, linearly polarized,random-phase Alfvénic fluctuations that have energy equipartition between kinetic and magnetic fluctuations andvanishing correlation between the two fields. A turbulent cascade rapidly develops; magnetic field fluctuationsexhibit a power-law spectrum at large scales and a steeper spectrum at ion scales. The turbulent cascade leads to anoverall anisotropic proton heating, protons are heated in the perpendicular direction, and, initially, also in theparallel direction. The imposed expansion leads to generation of a large parallel proton temperature anisotropywhich is at later stages partly reduced by turbulence. The turbulent heating is not sufficient to overcome theexpansion-driven perpendicular cooling and the system eventually drives the oblique firehose instability in a formof localized nonlinear wave packets which efficiently reduce the parallel temperature anisotropy. This workdemonstrates that kinetic instabilities may coexist with strong plasma turbulence even in a constrained 2D regime.
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Journal articleHietala H, Drake JF, Phan TD, et al., 2015,
Ion temperature anisotropy across a magnetotail reconnection jet
, Geophysical Research Letters, Vol: 42, Pages: 7239-7247, ISSN: 1944-8007A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by anti-parallel reconnection in conjunction with Particle-In-Cell simulations, showing spatial variations in the anisotropy across the outflow far (> 100di) downstream of the X-line. A consistent pattern is found in both the spacecraft data and the simulations: Whilst the total temperature across the exhaust is rather constant, near the boundaries Ti,|| dominates. The plasma is well-above the firehose threshold within patchy spatial regions at |BX| ∈ [0.1, 0.5]B0, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the mid-plane (|BX|0.1 B0), Ti,⊥ > Ti,|| and ions undergo Speiser-like motion despite the large distance from the X-line.
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Journal articleMcCoy DT, Hartmann DL, Zelinka MD, et al., 2015,
Mixed-phase cloud physics and Southern Ocean cloud feedback in climate models
, Journal of Geophysical Research: Atmospheres, Vol: 120, Pages: 9539-9554, ISSN: 2169-897XIncreasing optical depth poleward of 45° is a robust response to warming in global climate models. Much of this cloud optical depth increase has been hypothesized to be due to transitions from ice‐dominated to liquid‐dominated mixed‐phase cloud. In this study, the importance of liquid‐ice partitioning for the optical depth feedback is quantified for 19 Coupled Model Intercomparison Project Phase 5 models. All models show a monotonic partitioning of ice and liquid as a function of temperature, but the temperature at which ice and liquid are equally mixed (the glaciation temperature) varies by as much as 40 K across models. Models that have a higher glaciation temperature are found to have a smaller climatological liquid water path (LWP) and condensed water path and experience a larger increase in LWP as the climate warms. The ice‐liquid partitioning curve of each model may be used to calculate the response of LWP to warming. It is found that the repartitioning between ice and liquid in a warming climate contributes at least 20% to 80% of the increase in LWP as the climate warms, depending on model. Intermodel differences in the climatological partitioning between ice and liquid are estimated to contribute at least 20% to the intermodel spread in the high‐latitude LWP response in the mixed‐phase region poleward of 45°S. It is hypothesized that a more thorough evaluation and constraint of global climate model mixed‐phase cloud parameterizations and validation of the total condensate and ice‐liquid apportionment against observations will yield a substantial reduction in model uncertainty in the high‐latitude cloud response to warming.
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Journal articleHunt GJ, Cowley SWH, Provan G, et al., 2015,
Field-aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations
, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 7552-7584, ISSN: 2169-9402We investigate the magnetic perturbations associated with field-aligned currents observed on 34 Cassini passes over the premidnight northern auroral region during 2008. These are found to be significantly modulated not only by the northern planetary-period oscillation (PPO) system, similar to the southern currents by the southern PPO system found previously, but also by the southern PPO system as well, thus providing the first clear evidence of PPO-related interhemispheric current flow. The principal field-aligned currents of the two PPO systems are found to be co-located in northern ionospheric colatitude, together with the currents of the PPO-independent (subcorotation) system, located between the vicinity of the open-closed field boundary and field lines mapping to ~9 Saturn radius (Rs) in the equatorial plane. All three systems are of comparable magnitude, ~3 MA in each PPO half-cycle. Smaller PPO-related field-aligned currents of opposite polarity also flow in the interior region, mapping between ~6 and ~9 Rs in the equatorial plane, carrying a current of ~ ±2 MA per half-cycle, which significantly reduce the oscillation amplitudes in the interior region. Within this interior region the amplitudes of the northern and southern oscillations are found to fall continuously with distance along the field lines from the corresponding hemisphere, thus showing the presence of cross-field currents, with the southern oscillations being dominant in the south, and modestly lower in amplitude than the northern oscillations in the north. As in previous studies, no oscillations related to the opposite hemisphere are found on open field lines in either hemisphere.
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Journal articleYang L, Wang L, Li G, et al., 2015,
THE ANGULAR DISTRIBUTION OF SOLAR WIND SUPERHALO ELECTRONS AT QUIET TIMES
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 811, ISSN: 2041-8205- Author Web Link
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- Citations: 9
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Journal articleSulaiman AH, Masters A, Dougherty MK, et al., 2015,
Quasiperpendicular high Mach number shocks
, Physical Review Letters, Vol: 115, ISSN: 1079-7114Shock waves exist throughout the Universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasiperpendicular shocks across 2 orders of magnitude in Alfvén Mach number (MA) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted time scale of ∼0.3τc, where τc is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and MA and focus on the magnetic structure of such shocks to further show that for the same MA, a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.
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Journal articlePilkington NM, Achilleos N, Arridge CS, et al., 2015,
Internally driven large-scale changes in the size of Saturn's magnetosphere
, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 7289-7306, ISSN: 2169-9402Saturn's magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the system's dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10–15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively “plasma-loaded” or “plasma-depleted” states, defined in terms of the internal suprathermal plasma pressure.
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Journal articlePilkington NM, Achilleos N, Arridge CS, et al., 2015,
Asymmetries observed in Saturn's magnetopause geometry
, Geophysical Research Letters, Vol: 42, Pages: 6890-6898, ISSN: 1944-8007For over 10 years, the Cassini spacecraft has patrolled Saturn's magnetosphere and observed its magnetopause boundary over a wide range of prevailing solar wind and interior plasma conditions. We now have data that enable us to resolve a significant dawn-dusk asymmetry and find that the magnetosphere extends farther from the planet on the dawnside of the planet by 7 ± 1%. In addition, an opposing dawn-dusk asymmetry in the suprathermal plasma pressure adjacent to the magnetopause has been observed. This probably acts to reduce the size asymmetry and may explain the discrepancy between the degree of asymmetry found here and a similar asymmetry found by Kivelson and Jia (2014) using MHD simulations. Finally, these observations sample a wide range of season, allowing the “intrinsic” polar flattening (14 ± 1%) caused by the magnetodisc to be separated from the seasonally induced north-south asymmetry in the magnetopause shape found theoretically (5 ± 1% when the planet's magnetic dipole is tilted away from the Sun by 10–17°).
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Journal articleBeth A, Garnier P, Toublanc D, et al., 2015,
Theory for planetary exospheres: II. Radiation pressure effect on exospheric density profiles
, Icarus, Vol: 266, Pages: 423-432, ISSN: 1090-2643The planetary exospheres are poorly known in their outer parts, since the neutral densities are low compared with the instruments detection capabilities. The exospheric models are thus often the main source of information at such high altitudes. We present a new way to take into account analytically the additional effect of the radiation pressure on planetary exospheres. In a series of papers, we present with an Hamiltonian approach the effect of the radiation pressure on dynamical trajectories, density profiles and escaping thermal flux. Our work is a generalization of the study by Bishop and Chamberlain (1989). In this second part of our work, we present here the density profiles of atomic Hydrogen in planetary exospheres subject to the radiation pressure. We first provide the altitude profiles of ballistic particles (the dominant exospheric population in most cases), which exhibit strong asymmetries that explain the known geotail phenomenon at Earth. The radiation pressure strongly enhances the densities compared with the pure gravity case (i.e. the Chamberlain profiles), in particular at noon and midnight. We finally show the existence of an exopause that appears naturally as the external limit for bounded particles, above which all particles are escaping.
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Book chapterHaigh JD, Matthes K, Hanslmeier A, 2015,
The Impact of Solar Variability on Climate.
, Earth’s climate response to a changing Sun, Editors: Lilensten, Dudok de Wit, Matthes, ISBN: 978-2-7598-1733-7 -
Book chapterEastwood J, 2015,
Observing Magnetic Reconnection: The Influence of Jim Dungey
, Magnetospheric Plasma Physics: The Impact of Jim Dungey’s Research, Editors: Southwood, Cowley, Mitton, Publisher: Springer, Pages: 181-197, ISBN: 9783319183589This book makes good background reading for much of modern magnetospheric physics.
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Journal articleYan GQ, Mozer FS, Phan T, et al., 2015,
Quasi-continuous reconnection accompanied by FTEs during IMF Bz≈0 nT observed by Double Star TC-1 at the dawnside magnetopause
, Advances in Space Research, Vol: 58, Pages: 208-217, ISSN: 0273-1177During a one-hour interval of interplanetary magnetic field (IMF) Bz≈0 nT, the equatorialspacecraft Double Star TC-1 encountered the dawn flank magnetopause many times at the magnetic localtime (MLT) of about 08:00 and the latitude of about -27°. During each encounter, reconnection jets wereobserved with their velocities up to more than 500 km/s, significantly higher than the background flow inthe magnetosheath. The fast flows match the theoretical prediction of Alfvénic acceleration well. Themedium temperature and density of ions in the boundary layer indicate the open magnetic field topologyinside this layer. The mainly southward and tailward flows of the plasma jets alongside with the negativeslopes of the Walén test indicate that the spacecraft was located south of the reconnection site, consistentwith both anti-parallel and component reconnection models. The accelerated flows were observed lastingfor about one hour, with some modulations by the oscillations of the magnetopause, but no reversals inthe direction of Vz were found during the interval. The significantly enhanced flows in the boundary layercompared to the adjacent magnetosheath indicate that the reconnection was quasi-continuously active atthe magnetopause northward of the spacecraft under such IMF conditions. At the same time, the bipolarsignatures in BN with enhancements of the magnetic field indicate the occurrence of the Flux TransferEvents (FTEs). The observed reconnection was quasi-continuous, whereas the simultaneouslyaccompanied FTEs were time-dependent under the IMF Bz≈0 nT. For this event, however, it is notpossible to identify whether the reconnection was anti-parallel or component because the TC-1 was faraway from the reconnection site.
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