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Journal articleStawarz JE, Pouquet A, 2015,
Small-scale behavior of Hall magnetohydrodynamic turbulence
, PHYSICAL REVIEW E, Vol: 92, ISSN: 2470-0045- Author Web Link
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- Citations: 10
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Journal articleRegoli LH, Roussos E, Feyerabend M, et al., 2015,
Access of energetic particles to Titan's exobase: A study of Cassini's T9 flyby
, Planetary and Space Science, Vol: 130, Pages: 40-53, ISSN: 1873-5088We study how the local electromagnetic disturbances introduced by Titan affect the ionization rates of the atmosphere. For this, we model the precipitation of energetic particles, specifically hydrogen and oxygen ions with energies between 1 keV and 1 MeV, into Titan׳s exobase for the specific magnetospheric configuration of the T9 flyby. For the study, a particle tracing software package is used which consists of an integration of the single particle Lorentz force equation using a 4th order Runge–Kutta numerical method. For the electromagnetic disturbances, the output of the A.I.K.E.F. hybrid code (kinetic ions, fluid electrons) is used, allowing the possibility of analyzing the disturbances and asymmetries in the access of energetic particles originated by their large gyroradii. By combining these methods, 2D maps showing the access of each set of particles were produced. We show that the access of different particles is largely dominated by their gyroradii, with the complexity of the maps increasing with decreasing gyroradius, due to the larger effect that local disturbances introduced by the presence of the moon have in the trajectory of the particles with lower energies. We also show that for particles with gyroradii much larger than the moon׳s radius, simpler descriptions of the electromagnetic environment can reproduce similar results to those obtained when using the full hybrid simulation description, with simple north–south fields being sufficient to reproduce the hybrid code results for O+ ions with energies larger than 10 keV but not enough to reproduce those for H+ ions at any of the energies covered in the present study. Finally, by combining the maps created with upstream plasma flow measurements by the MIMI/CHEMS instrument, we are able to estimate normalized fluxes arriving at different selected positions of the moon׳s exobase. We then use these fluxes to calculate energy deposition and non-dissociative N2 ionization rates for precipitati
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Journal articleRussell AJB, Yeates AR, Eastwood JP, 2015,
Magnetic reconnection now and in the future
, Astronomy and Geophysics, Vol: 56, ISSN: 1366-8781 -
Journal articleTinetti G, Drossart P, Eccleston P, et al., 2015,
The EChO science case
, Experimental Astronomy, Vol: 40, Pages: 329-391, ISSN: 1572-9508The discovery of almost two thousand exoplanets has revealed an unexpectedlydiverse planet population. We see gas giants in few-day orbits, whole multi-planet systemswithin the orbit of Mercury, and new populations of planets with masses between that of theEarth and Neptune—all unknown in the Solar System. Observations to date have shown thatour Solar System is certainly not representative of the general population of planets in ourMilky Way. The key science questions that urgently need addressing are therefore: What areexoplanets made of? Why are planets as they are? How do planetary systems work and whatcauses the exceptional diversity observed as compared to the Solar System? The EChO(Exoplanet Characterisation Observatory) space mission was conceived to take up thechallenge to explain this diversity in terms of formation, evolution, internal structure andplanet and atmospheric composition. This requires in-depth spectroscopic knowledge of theatmospheres of a large and well-defined planet sample for which precise physical, chemicaland dynamical information can be obtained. In order to fulfil this ambitious scientificprogram, EChO was designed as a dedicated survey mission for transit and eclipsespectroscopy capable of observing a large, diverse and well-defined planet sample withinits 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signalfrom the star and planet are differentiated using knowledge of the planetary ephemerides,allows us to measure atmospheric signals from the planet at levels of at least 10−4 relative tothe star. This can only be achieved in conjunction with a carefully designed stable payloadand satellite platform. It is also necessary to provide broad instantaneous wavelengthcoverage to detect as many molecular species as possible, to probe the thermal structureof the planetary atmospheres and to correct for the contaminating effects of the stellarphotosphere. This requires wavelength coverage of at l
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Journal articleRussell AJB, Yeates AR, Eastwood JP, 2015,
Magnetic reconnection now and in the future
, ASTRONOMY & GEOPHYSICS, Vol: 56, Pages: 18-23, ISSN: 1366-8781 -
Journal articleArridge CS, Eastwood J, Jackman CM, et al., 2015,
Cassini in situ observations of long duration magnetic reconnection in Saturn’s magnetotail
, Nature Physics, Vol: 12, Pages: 268-271, ISSN: 1745-2481Magnetic reconnection is a fundamental process in solar system and astrophysical plasmas, through which stored magnetic energy associated with current sheets is converted into thermal, kinetic and wave energy1, 2, 3, 4. Magnetic reconnection is also thought to be a key process involved in shedding internally produced plasma from the giant magnetospheres at Jupiter and Saturn through topological reconfiguration of the magnetic field5, 6. The region where magnetic fields reconnect is known as the diffusion region and in this letter we report on the first encounter of the Cassini spacecraft with a diffusion region in Saturn’s magnetotail. The data also show evidence of magnetic reconnection over a period of 19 h revealing that reconnection can, in fact, act for prolonged intervals in a rapidly rotating magnetosphere. We show that reconnection can be a significant pathway for internal plasma loss at Saturn6. This counters the view of reconnection as a transient method of internal plasma loss at Saturn5, 7. These results, although directly relating to the magnetosphere of Saturn, have applications in the understanding of other rapidly rotating magnetospheres, including that of Jupiter and other astrophysical bodies.
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Journal articleWalker SN, Balikhin MA, Shklyar DR, et al., 2015,
Experimental determination of the dispersion relation of magnetosonic waves
, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 9632-9650, ISSN: 2169-9402Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave-particle interactions depend upon the characteristic properties of the wave mode. Hence determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilises multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory.
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Journal articleProvan G, Tao C, Cowley SWH, et al., 2015,
Planetary period oscillations in Saturn's magnetosphere: Examining the relationship between abrupt changes in behavior and solar wind-induced magnetospheric compressions and expansions
, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 9524-9544, ISSN: 2169-9402We examine planetary period oscillations (PPOs) observed in Saturn's magnetospheric magnetic field data from the time of Saturn's equinox in 2009. In particular, we focus on the time period commencing February 2011, when the oscillations started to display sudden and unexpected changes in behavior at ~100–200 day intervals. These were characterized by large simultaneous changes in the amplitude of the northern and southern PPO systems, together with small changes in period and jumps in phase. Nine significant abrupt changes have been observed in the postequinox interval to date, commencing as the Sun started to emerge from a long extended solar minimum. We perform a statistical study to determine whether these modulations in PPO behavior were associated with changes in the solar and/or upstream solar wind conditions. We report that the upstream solar wind conditions show elevated values of solar wind dynamic pressure and density around the time of PPO behavioral transitions, as opposed to before and after these times. We suggest that abrupt changes in PPO behavior may be related to significant changes in the size of the Saturnian magnetosphere in response to varying solar wind conditions.
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Journal articleField RD, Luo M, Kim D, et al., 2015,
Sensitivity of simulated tropospheric CO to subgrid physics parameterization: a case study of Indonesian biomass burning emissions in 2006
, Journal of Geophysical Research: Atmospheres, Vol: 120, Pages: 11743-11759, ISSN: 2169-8996Recent cumulus and turbulence parameterization changes to the NASA GISS ModelE2 have improved representation of the Madden-Julian Oscillation and low cloud distribution, but their effect on composition-related quantities is not known. In this study, we simulate the vertical transport of carbon monoxide (CO) from uncontrolled biomass burning in Indonesia in late 2006, during which uniquely high CO was detected in the upper troposphere. Two configurations of ModelE2, one without the changes (AR5) and one with the changes (AR5′), are used for an ensemble simulation of the transport of CO from the biomass burning. The simulation results are evaluated against new CO profiles retrieved jointly from the Aura Tropospheric Emission Spectrometer and the Microwave Limb Sounder. Modeled upper tropospheric CO using the AR5 physics was unrealistically high. The AR5′ physics suppress deep convection that reaches near the tropopause, reducing vertical transport of CO to the upper troposphere and bringing the model into better agreement with satellite CO. In this regard, the most important changes were related to the strength of entrainment of environmental air into the convective column, the strength of re-evaporation above cloud base, and a negative plume buoyancy threshold based on density temperature. This study illustrates how individual, noncomposition model changes can lead to significantly different modeled composition, which in this case improved agreement with satellite retrievals. This study also illuminates the potential usefulness of CO satellite observations in constraining unobservable processes in general circulation models.
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Journal articleTurner DL, O'Brien TP, Fennell JF, et al., 2015,
The effects of geomagnetic storms on electrons in Earth's radiation belts
, GEOPHYSICAL RESEARCH LETTERS, Vol: 42, Pages: 9176-9184, ISSN: 0094-8276- Author Web Link
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- Citations: 66
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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: 36
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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, 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 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 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: 6
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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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