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  • Journal article
    Guo RL, Yao ZH, Sergis N, Wei Y, Xu XJ, Coates AJ, Delamere PA, Roussos E, Arridge CS, Waite JH, Krupp N, Mitche D, Burch J, Dougherty MK, Wan WXet al., 2019,

    Long-standing Small-scale Reconnection Processes at Saturn Revealed by <i>Cassini</i>

    , ASTROPHYSICAL JOURNAL LETTERS, Vol: 884, ISSN: 2041-8205
  • Journal article
    Gregory JM, Andrews T, Ceppi P, Mauritsen T, Webb MJet al., 2019,

    How accurately can the climate sensitivity to CO₂ be estimated from historical climate change?

    , Climate Dynamics, Vol: 54, Pages: 129-157, ISSN: 0930-7575

    The equilibrium climate sensitivity (ECS, in K) to CO2 doubling is a large source of uncertainty in projections of future anthropogenic climate change. Estimates of ECS made from non-equilibrium states or in response to radiative forcings other than 2×CO2 are called “effective climate sensitivity” (EffCS, in K). Taking a “perfect-model” approach, using coupled atmosphere–ocean general circulation model (AOGCM) experiments, we evaluate the accuracy with which CO2 EffCS can be estimated from climate change in the “historical” period (since about 1860). We find that (1) for statistical reasons, unforced variability makes the estimate of historical EffCS both uncertain and biased; it is overestimated by about 10% if the energy balance is applied to the entire historical period, 20% for 30-year periods, and larger factors for interannual variability, (2) systematic uncertainty in historical radiative forcing translates into an uncertainty of ±30to45% (standard deviation) in historical EffCS, (3) the response to the changing relative importance of the forcing agents, principally CO2 and volcanic aerosol, causes historical EffCS to vary over multidecadal timescales by a factor of two. In recent decades it reached its maximum in the AOGCM historical experiment (similar to the multimodel-mean CO2 EffCS of 3.6 K from idealised experiments), but its minimum in the real world (1.6 K for an observational estimate for 1985–2011, similar to the multimodel-mean value for volcanic forcing). The real-world variations mean that historical EffCS underestimates CO2 EffCS by 30% when considering the entire historical period. The difference for recent decades implies that either unforced variability or the response to volcanic forcing causes a much stronger regional pattern of sea surface temperature change in the real world than in AOGCMs. We speculate that this could be explained by a deficiency in simulated coupled atmosphere
  • Journal article
    Hellinger P, Matteini L, Landi S, Franci L, Verdini A, Papini Eet al., 2019,

    Turbulence versus Fire-hose Instabilities: 3D Hybrid Expanding Box Simulations

    , The Astrophysical Journal, Vol: 883, Pages: 178-178
  • Journal article
    Ceppi P, Gregory JM, 2019,

    A refined model for the Earth’s global energy balance

    , Climate Dynamics, Vol: 53, Pages: 4781-4797, ISSN: 0930-7575

    A commonly-used model of the global radiative budget assumes that the radiative response to forcing, R, is proportional to global surface air temperature T, R= λT. Previous studies have highlighted two unresolved issues with this model: first, the feedback parameter λ depends on the forcing agent; second, λ varies with time. Here, we investigate the factors controlling R in two atmosphere–slab ocean climate models subjected to a wide range of abrupt climate forcings. It is found that R scales not only with T, but also with the large-scale tropospheric stability S (defined here as the estimated inversion strength area-averaged over ocean regions equatorward of 50∘). Positive S promotes negative R, mainly through shortwave cloud and lapse-rate changes. A refined model of the global energy balance is proposed that accounts for both temperature and stability effects. This refined model quantitatively explains (1) the dependence of climate feedbacks on forcing agent (or equivalently, differences in forcing efficacy), and (2) the time evolution of feedbacks in coupled climate model experiments. Furthermore, a similar relationship between R and S is found in observations compared with models, lending confidence that the refined energy balance model is applicable to the real world.
  • Journal article
    Wedlund Simon C, Behar E, Nilsson H, Alho M, Kallio E, Gunell H, Bodewits D, Heritier K, Galand M, Beth A, Rubin M, Altwegg K, Volwerk M, Gronoff G, Hoekstra Ret al., 2019,

    Solar wind charge exchange in cometary atmospheres III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko

    , Astronomy and Astrophysics, Vol: 630, ISSN: 0004-6361

    Solar wind charge-changing reactions are of paramount importance to thephysico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission tocomet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to studycharge-changing processes in situ. To understand the role of these reactions inthe evolution of the solar wind plasma, and interpret the complex in-situmeasurements made by Rosetta, numerical or analytical models are necessary. Weuse an extended analytical formalism describing solar wind charge-changingprocesses at comets along solar wind streamlines. The model is driven by solarwind ion measurements from the Rosetta Plasma Consortium-Ion CompositionAnalyzer (RPC-ICA) and neutral density observations from the RosettaSpectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS),as well as charge-changing cross sections of hydrogen and helium particles in awater gas. A mission-wide overview of charge-changing efficiencies at comet 67Pis presented. Electron capture cross sections dominate and favor the productionof He and H energetic neutral atoms, with fluxes expected to rival those of H+and He2+ ions. Neutral outgassing rates are retrieved from local RPC-ICA fluxmeasurements, and match ROSINA's estimates very well. From the model, we findthat solar wind charge exchange is unable to fully explain the magnitude of thesharp drop of solar wind ion fluxes observed by Rosetta for heliocentricdistances below 2.5 AU. This is likely because the model does not take intoaccount the relative ion dynamics and, to a lesser extent, ignore the formationof bow shock-like structures upstream of the nucleus. This work also shows thatthe ionization by solar EUV radiation and energetic electrons dominates thesource of cometary ions, although solar wind contributions may be significantduring isolated events.
  • Journal article
    Moore L, Melin H, O'Donoghue J, Stallard TS, Moses J, Galand M, Miller S, Schmidt CAet al., 2019,

    Modelling H-3(+) in planetary atmospheres: effects of vertical gradients on observed quantities

    , Philosophical Transactions of the Royal Society A. Mathematical, Physical and Engineering Sciences, Vol: 377, Pages: 1-19, ISSN: 1364-503X

    Since its detection in the aurorae of Jupiter approximately 30 years ago, the H3+ ion has served as an invaluable probe of giant planet upper atmospheres. However, the vast majority of monitoring of planetary H3+ radiation has followed from observations that rely on deriving parameters from column-integrated paths through the emitting layer. Here, we investigate the effects of density and temperature gradients along such paths on the measured H3+ spectrum and its resulting interpretation. In a non-isothermal atmosphere, H3+ column densities retrieved from such observations are found to represent a lower limit, reduced by 20% or more from the true atmospheric value. Global simulations of Uranus' ionosphere reveal that measured H3+ temperature variations are often attributable to well-understood solar zenith angle effects rather than indications of real atmospheric variability. Finally, based on these insights, a preliminary method of deriving vertical temperature structure is demonstrated at Jupiter using model reproductions of electron density and H3+ measurements. The sheer diversity and uncertainty of conditions in planetary atmospheres prohibits this work from providing blanket quantitative correction factors; nonetheless, we illustrate a few simple ways in which the already formidable utility of H3+ observations in understanding planetary atmospheres can be enhanced.This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H3+, H5+ and beyond’.
  • Journal article
    Wedlund CS, Bodewits D, Alho M, Hoekstra R, Behar E, Gronoff G, Gunell H, Nilsson H, Kallio E, Beth Aet al., 2019,

    Solar wind charge exchange in cometary atmospheres I. Charge-changing and ionization cross sections for He and H particles in H2O

    , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
  • Journal article
    Wedlund CS, Behar E, Kallio E, Nilsson H, Alho M, Gunell H, Bodewits D, Beth A, Gronoft G, Hoekstra Ret al., 2019,

    Solar wind charge exchange in cometary atmospheres II. Analytical model

    , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
  • Journal article
    Luspay-Kuti A, Altwegg K, Berthelier JJ, Beth A, Dhooghe F, Fiethe B, Fuselier SA, Gombosi T, Hansen KC, Hassig M, Livadiotis G, Mall U, Mandt KE, Mousis O, Petrinec SM, Rubin M, Trattner KJ, Tzou C-Y, Wurz Pet al., 2019,

    Comparison of neutral outgassing of comet 67P/Churyumov-Gerasimenko inbound and outbound beyond 3 AU from ROSINA/DFMS

    , ASTRONOMY & ASTROPHYSICS, Vol: 630, ISSN: 1432-0746
  • Journal article
    Mandt KE, Eriksson A, Beth A, Galand M, Vigren Eet al., 2019,

    Influence of collisions on ion dynamics in the inner comae of four comets

    , ASTRONOMY & ASTROPHYSICS, Vol: 630, Pages: 1-8, ISSN: 1432-0746

    Context. Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form.Aims. The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft.Methods. We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed.Results. Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.
  • Journal article
    Hoang M, Garnier P, Gourlaouen H, Lasue J, Reme H, Altwegg K, Balsiger H, Beth A, Calmonte U, Fiethe B, Galli A, Gasc S, Jaeckel A, Korth A, Le Roy L, Mall U, Rubin M, Semon T, Tzou C-Y, Waite JH, Wurz Pet al., 2019,

    Two years with comet 67P/Churyumov-Gerasimenko: H2O, CO2, and CO as seen by the ROSINA/RTOF instrument of Rosetta

    , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
  • Journal article
    Myllys M, Henri P, Galand M, Heritier KL, Gilet N, Goldstein R, Eriksson A, Johansson F, Deca Jet al., 2019,

    Plasma properties of suprathermal electrons near comet 67P/Churyumov-Gerasimenko with Rosetta

    , Astronomy and Astrophysics: a European journal, Vol: 630, Pages: 1-14, ISSN: 0004-6361

    Context. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko from 2014 to September 2016. The mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance.Aims. In this study, different electron populations, called warm and hot, observed by the Ion and Electron Sensor (IES) of the Rosetta Plasma Consortium (RPC) are investigated near the comet during the escorting phase of the Rosetta mission.Methods. The estimates for the suprathermal electron densities and temperatures were extracted using IES electron data by fitting a double-kappa function to the measured velocity distributions. The fitting results were validated using observations from other RPC instruments. We give upgraded estimates for the warm and hot population densities compared to values previously shown in literature.Results. The fitted density and temperature estimates for both electron populations seen by IES are expressed as a function of heliocentric distance to study their evolution with the cometary activity. In addition, we studied the dependence between the electron properties and cometocentric distance.Conclusions. We observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. In addition, warm and hot populations show a significant dependence with the heliocentric distance. The populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. Moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. Finally, based on our results we reject the hypothesis that hot electron population seen by IES consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of
  • Journal article
    Pancost R, Rogelj J, Scoones I, Haigh JD, Whitmarsh L, Fankhauser S, Williams Jet al., 2019,

    The pathway toward a net-zero-emissions future

    , One Earth, Vol: 1, Pages: 18-20, ISSN: 2590-3322

    The UK government recently committed to achieving net-zero carbon emissions by 2050. We asked a selection of UK-based experts to reflect upon this commitment, the challenges ahead, and the actions required to make it a reality.
  • Journal article
    Beth A, Galand M, Heritier K, 2019,

    Comparative study of photo-produced ionosphere in the close environment of comets

    , Astronomy & Astrophysics, Vol: 630, ISSN: 0004-6361

    Context. The Giotto and Rosetta missions gave us the unique opportunity of probing the close environment of cometary ionospheres of 1P/Halley (1P) and 67P/Churyumov-Gerasimenko (67P). The plasma conditions encountered at these two comets were very different from each other, which mainly stem from the different heliocentric distances, which drive photoionization rates, and from the outgassing activities, which drive the neutral densities.Aims. We asses the relative contribution of different plasma processes that are ongoing in the inner coma: photoionization, transport, photoabsorption, and electron–ion dissociative recombination. The main goal is to identify which processes are at play to then quantitatively assess the ionospheric density.Methods. We provide a set of analytical formulas to describe the ionospheric number density profile for cometary environments that take into account some of these processes. We discuss the validity of each model in the context of the Rosetta and Giotto missions.Results. We show that transport is the dominant loss process at large cometocentric distances and low outgassing rates. Chemical plasma loss through e−-ion dissociative recombination matters around 67P near perihelion and at 1P during the Giotto flyby: its effects increase as the heliocentric distance decreases, that is, at higher outgassing activity and higher photoionization frequency. Photoabsorption is of importance for outgassing rates higher than 1028 s−1 and only close to the cometary nucleus, well below the location of both spacecraft. Finally, regardless of the processes we considered, the ion number density profile always follows a 1∕r law at large cometocentric distances.
  • Journal article
    Carnielli G, Galand M, Leblanc F, Leclercq L, Modolo R, Beth A, Huybrighs HLF, Jia Xet al., 2019,

    First 3D test particle model of Ganymede's ionosphere

    , Icarus, Vol: 330, Pages: 42-59, ISSN: 0019-1035

    We present the first three-dimensional multi-species ionospheric model for Ganymede, based on a test particle Monte Carlo approach. Inputs include the electromagnetic field configuration around the moon from the magnetospheric models developed by Leclercq et al. (2016) and by Jia et al. (2009), and the number density, bulk velocity and temperature distributions of the neutral exosphere simulated by Leblanc et al. (2017). According to our simulations, O2+ is the most abundant ion species, followed by O+, H2+ and H2O+. For O+ and O2+, the majority of ions produced impact the moon's surface, while for the other species the majority escapes Ganymede's magnetosphere. For all ion species, the escape occurs either in the direction of corotation of the Jovian plasma or through the Alfvén wings.To validate our model, the output of our simulations, performed under the Galileo G2 flyby conditions, are compared to the observations. These include the electron density derived by the plasma wave instrument (PWS), the ion energy spectrogram measured by the plasma analyzer (PLS) and the associated plasma moments (Frank et al., 1997a).On the one hand, the electron density found by our model is consistently underestimated throughout the flyby, being at least one order of magnitude lower compared to observations. We argue that the prime reason for this discrepancy comes from the exospheric density, which may be underestimated. On the other hand, we find a remarkably good agreement between the modeled ion energy spectrogram and that recorded by PLS, providing a validation of the test particle model. Finally, we compare the modeled plasma moments along the G2 flyby with those analyzed by Frank et al. (1997a). The data seems to be more consistent with an ionosphere dominated by O2+ instead of H+ or O+, as suggested previously in the literature. This supports our finding that O2+ is the dominant ion species close to the surface.
  • Journal article
    Kuzichev I, Vasko IY, Soto-Chavez AR, Tong Y, Artemyev A, Bale SD, Spitkovsky Aet al., 2019,

    Nonlinear Evolution of the Whistler Heat Flux Instability

    , ASTROPHYSICAL JOURNAL, Vol: 882, ISSN: 0004-637X
  • Journal article
    Krupar V, Magdalenic J, Eastwood JP, Gopalswamy N, Kruparova O, Szabo A, Nemec Fet al., 2019,

    Statistical survey of coronal mass ejections and interplanetary type II bursts

    , The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 882, Pages: 1-5, ISSN: 0004-637X

    Coronal mass ejections (CMEs) are responsible for most severe space weather events, such as solar energetic particle events and geomagnetic storms at Earth. Type II radio bursts are slow drifting emissions produced by beams of suprathermal electrons accelerated at CME-driven shock waves propagating through the corona and interplanetary medium. Here, we report a statistical study of 153 interplanetary type II radio bursts observed by the two STEREO spacecraft between 2008 March and 2014 August. The shock associated radio emission was compared with CME parameters included in the Heliospheric Cataloguing, Analysis and Techniques Service catalog. We found that faster CMEs are statistically more likely to be associated with the interplanetary type II radio bursts. We correlate frequency drifts of interplanetary type II bursts with white-light observations to localize radio sources with respect to CMEs. Our results suggest that interplanetary type II bursts are more likely to have a source region situated closer to CME flanks than CME leading edge regions.
  • Journal article
    Perrone D, Stansby D, Horbury TS, Matteini Let al., 2019,

    Thermodynamics of pure fast solar wind: radial evolution of the temperature-speed relationship in the inner heliosphere

    , Monthly Notices of the Royal Astronomical Society, Vol: 488, Pages: 2380-2386, ISSN: 0035-8711

    A strong correlation between speed and proton temperature has been observed, across many years, on hourly averaged measurements in the solar wind. Here, we show that this relationship is also observed at a smaller scale on intervals of a few days, within a single stream. Following the radial evolution of a well-defined stream of coronal-hole plasma, we show that the temperature–speed (T–V) relationship evolves with distance, implying that the T–V relationship at 1 au cannot be used as a proxy for that near the Sun. We suggest that this behaviour could be a combination of the anticorrelation between speed and flux-tube expansion factor near the Sun and the effect of a continuous heating experienced by the plasma during the expansion. We also show that the cooling index for the radial evolution of the temperature is a function of the speed. In particular, T⊥ in faster wind, although higher close to the Sun, decreases more quickly with respect to slower wind, suggesting that it has less time to interact with the mechanism(s) able to heat the plasma. Finally, we predict the expected T–V relationship in fast streams closer to the Sun with respect to the Helios observations, which Parker Solar Probe will explore in the near future.
  • Journal article
    Sorba AM, Achilleos NA, Sergis N, Guio P, Arridge CS, Dougherty MKet al., 2019,

    Local Time Variation in the Large-Scale Structure of Saturn's Magnetosphere

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 7425-7441, ISSN: 2169-9380
  • Journal article
    Poh G, Slavin JA, Lu S, Le G, Ozturk DS, Sun W, Zou S, Eastwood JP, Nakamura R, Baumjohann W, Russell CT, Gershman DJ, Giles BL, Pollock CJ, Moore TE, Torbert RB, Burch JLet al., 2019,

    Dissipation of earthward propagating flux rope through re‐reconnection with geomagnetic field: An MMS case study

    , Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 7477-7493, ISSN: 2169-9380

    Three‐dimensional global hybrid simulations and observations have shown that earthward‐moving flux ropes (FRs) can undergo magnetic reconnection (or re‐reconnection) with the near‐Earth dipole field to create dipolarization front (DF)‐like signatures that are immediately preceded by brief intervals of negative BZ. The simultaneous erosion of the southward BZ field at the leading edge of the FR and continuous reconnection of lobe magnetic flux at the X‐line tailward of the FR result in the asymmetric south‐north BZ signature in many earthward‐moving FRs and possibly DFs with negative BZ dips prior to their observation. In this study, we analyzed Magnetospheric MultiScale (MMS) observation of fields and plasma signatures associated with the encounter of an ion diffusion region ahead of an earthward‐moving FR on 3 August 2017. The signatures of this re‐reconnection event were (i) +/− BZ reversal, (ii) −/+ bipolar‐type quadrupolar Hall magnetic fields, (iii) northward super‐Alfvénic electron outflow jet of ~1,000–1,500 km/s, (iv) Hall electric field of ~15 mV/m, (v) intense currents of ~40–100 nA/m2, and (vi) J·E′ ~0.11 nW/m3. Our analysis suggests that the MMS spacecraft encounters the ion and electron diffusion regions but misses the X‐line. Our results are in good agreement with particle‐in‐cell simulations of Lu et al. (2016, https://doi.org/10.1002/2016JA022815). We computed a dimensionless reconnection rate of ~0.09 for this re‐reconnection event and through modeling, estimating that the FR would fully dissipate by −16.58 RE. We demonstrated pertubations in the high‐latitude ionospheric currents at the same time of the dissipation of earthward‐moving FRs using ground‐ and space‐based measurements.
  • Journal article
    Manners H, Masters A, 2019,

    First evidence for multiple‐harmonic standing Alfvén waves in Jupiter's equatorial plasma sheet

    , Geophysical Research Letters, Vol: 46, Pages: 9344-9351, ISSN: 0094-8276

    Quasi‐periodic pulsations in the ultra‐low‐frequency band are ubiquitously observed in the jovian magnetosphere, but their source and distribution have until now been a mystery. Standing Alfvén waves on magnetic field lines have been proposed to explain these pulsations and their large range in observed periods. However, in‐situ evidence in support of this mechanism has been scarce. Here we use magnetometer data from the Galileo spacecraft to report first evidence of a multiple‐harmonic ultra‐low‐frequency event in Jupiters equatorial plasma sheet. The harmonic periods lie in the 4‐22‐min range, and the nodal structure is confined to the plasma sheet. Polarization analysis reveals several elliptically‐polarized odd harmonics, and no presence of even harmonics. The harmonic periods, their polarization, and the confinement of the wave to the plasma sheet, are strong evidence supporting the standing Alfvén wave model. Multiple‐harmonic waves therefore potentially explain the full range of periods in quasi‐periodic pulsations in Jupiters magnetosphere.
  • Journal article
    Hunt G, Cowley S, Provan G, Cao H, Bunce E, Dougherty M, Southwood Det al., 2019,

    Currents associated with Saturn's intra-D ring azimuthal field perturbations

    , Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5675-5691, ISSN: 2169-9380

    During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn’s D ring and the denser layers of the planet’s atmosphere, revealing the presence of an unanticipated low-latitude current system via the associated azimuthal perturbation field peaking typically at ~10-30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field-aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field-aligned current densities above the ionosphere are typically ~5-10 nA m-2 , more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field-aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north-south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller-scale structures, possibly due to rapid temporal or local time variations.
  • Journal article
    Vigren E, Edberg NJT, Eriksson A, Galand M, Henri P, Johansson FL, Odelstad E, Rubin M, Vallieres Xet al., 2019,

    The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March

    , The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 881, ISSN: 0004-637X

    A comet ionospheric model assuming the plasma moves radially outward with the same bulk speed as the neutral gas and not being subject to severe reduction through dissociative recombination has previously been tested in a series of case studies associated with the Rosetta mission at comet 67P/Churyumov–Gerasimenko. It has been found that at low activity and within several tens of kilometers from the nucleus such models (which originally were developed for such conditions) generally work well in reproducing observed electron number densities, in particular when plasma production through both photoionization and electron-impact ionization is taken into account. Near perihelion, case studies have, on the contrary, shown that applying similar assumptions overestimates the observed electron number densities at the location of Rosetta. Here we compare Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure sensor-driven model results with Rosetta Plasma Consortium/Mutual Impedance Probe-derived electron number densities for an extended time period (2015 November through 2016 March) during the postperihelion phase with southern summer/spring. We observe a gradual transition from a state when the model grossly overestimates (by more than a factor of 10) the observations to being in reasonable agreement during 2016 March.
  • Journal article
    Vuorinen L, Hietala H, Plaschke F, 2019,

    Jets in the magnetosheath: IMF control of where they occur

    , ANNALES GEOPHYSICAE, Vol: 37, Pages: 689-697, ISSN: 0992-7689
  • Journal article
    Fadanelli S, Lavraud B, Califano F, Jacquey C, Vernisse Y, Kacem I, Penou E, Gershman D, Dorelli J, Pollock C, Giles B, Avanov L, Burch J, Chandler M, Coffey V, Eastwood J, Ergun R, Farrugia C, Fuselier S, Genot V, Grigorenko E, Hasegawa H, Khotyaintsev Y, Le Contel O, Marchaudon A, Moore T, Nakamura R, Paterson W, Phan T, Rager A, Russell C, Saito Y, Sauvaud J-A, Schiff C, Smith S, Toledo Redondo S, Torbert R, Wang S, Yokota Set al., 2019,

    Four-spacecraft measurements of the shape and dimensionality of magnetic structures in the near-Earth plasma environment

    , Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 6850-6868, ISSN: 2169-9380

    We present a new method for determining the main relevant features of the local magnetic field configuration, based entirely on the knowledge of the magnetic field gradient using four- spacecraft measurements. The method, named “Magnetic Configuration Analysis” (MCA), estimates the spatial scales on which the magnetic field varies locally. While it directly derives from the well-known Magnetic Directional Derivative (MDD) and Magnetic Rotational Analysis (MRA) procedures (Shi et al., 2005, doi:10.1029/2005GL022454; Shen et al., 2007, doi:10.1029/2005JA011584), MCA was specifically designed to address the actual magnetic field geometry. By applying MCA to multi-spacecraft data from the MMS satellites, we perform both case and statistical analyses of local magnetic field shape and dimensionality at very high cadence and small scales. We apply this technique to different near-Earth environments and define a classification scheme for the type of configuration observed. While our case studies allow us to benchmark the method with those used in past works, our statistical analysis unveils the typical shape of magnetic configurations and their statistical distributions. We show that small-scale magnetic configurations are generally elongated, displaying forms of cigar and blade shapes, but occasionally being planar in shape like thin pancakes (mostly inside current sheets). Magnetic configurations, however, rarely show isotropy in their magnetic variance. The planar nature of magnetic configurations and, most importantly, their scale lengths strongly depend on the plasma β parameter. Finally, the most invariant direction is statistically aligned with the electric current, reminiscent of the importance of electromagnetic forces in shaping the local magnetic configuration
  • Journal article
    Deca J, Henri P, Divin A, Eriksson A, Galand M, Beth A, Ostaszewski K, Horányi Met al., 2019,

    Building a weakly outgassing comet from a generalized Ohm’s law

    , Physical Review Letters, Vol: 123, Pages: 055101-1-055101-7, ISSN: 0031-9007

    When a weakly outgassing comet is sufficiently close to the Sun, the formation of an ionized coma results in solar wind mass loading and magnetic field draping around its nucleus. Using a 3D fully kinetic approach, we distill the components of a generalized Ohm’s law and the effective electron equation of state directly from the self-consistently simulated electron dynamics and identify the driving physics in the various regions of the cometary plasma environment. Using the example of space plasmas, in particular multispecies cometary plasmas, we show how the description for the complex kinetic electron dynamics can be simplified through a simple effective closure, and identify where an isotropic single-electron fluid Ohm’s law approximation can be used, and where it fails.
  • Journal article
    Kilpua EKJ, Fontaine D, Moissard C, Ala-Lahti M, Palmerio E, Yordanova E, Good SW, Kalliokoski MMH, Lumme E, Osmane A, Palmroth M, Turc Let al., 2019,

    Solar Wind Properties and Geospace Impact of Coronal Mass Ejection-Driven Sheath Regions: Variation and Driver Dependence

    , SPACE WEATHER-THE INTERNATIONAL JOURNAL OF RESEARCH AND APPLICATIONS, Vol: 17, Pages: 1257-1280
  • Journal article
    Fowler CM, Halekas J, Schwartz S, Goodrich KA, Gruesbeck JR, Benna Met al., 2019,

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