Publications

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

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.

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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

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

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

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

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

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

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Citations: 40

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

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Citations: 11

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

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

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

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Journal article

Fowler CM, Halekas J, Schwartz S, Goodrich KA, Gruesbeck JR, Benna Met al., 2019,

The Modulation of Solar Wind Hydrogen Deposition in the Martian Atmosphere by Foreshock Phenomena

, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 7086-7097, ISSN: 2169-9380

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Citations: 10

Journal article

Shuster JR, Gershman DJ, Chen L-J, Wang S, Bessho N, Dorelli JC, da Silva DE, Giles BL, Paterson WR, Denton RE, Schwartz SJ, Norgred C, Wilder FD, Cassak PA, Swisdak M, Uritsky V, Schiff C, Rager AC, Smith S, Avanov LA, Vinas AFet al., 2019,

MMS Measurements of the Vlasov Equation: Probing the Electron Pressure Divergence Within Thin Current Sheets

, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7862-7872, ISSN: 0094-8276

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Citations: 24

Journal article

Sillmann J, Stjern CW, Myhre G, Samset BH, Hodnebrog O, Andrews T, Boucher O, Faluvegi G, Forster P, Kasoar MR, Kharin VV, Kirkevag A, Lamarque J-F, Olivie DJL, Richardson TB, Shindell D, Takemura T, Voulgarakis A, Zwiers FWet al., 2019,

Extreme wet and dry conditions affected differently by greenhouse gases and aerosols

, npj Climate and Atmospheric Science, Vol: 2, Pages: 1-7, ISSN: 2397-3722

Global warming due to greenhouse gases and atmospheric aerosols alter precipitation rates, but the influence on extreme precipitation by aerosols relative to greenhouse gases is still not well known. Here we use the simulations from the Precipitation Driver and Response Model Intercomparison Project that enable us to compare changes in mean and extreme precipitation due to greenhouse gases with those due to black carbon and sulfate aerosols, using indicators for dry extremes as well as for moderate and very extreme precipitation. Generally, we find that the more extreme a precipitation event is, the more pronounced is its response relative to global mean surface temperature change, both for aerosol and greenhouse gas changes. Black carbon (BC) stands out with distinct behavior and large differences between individual models. Dry days become more frequent with BC-induced warming compared to greenhouse gases, but so does the intensity and frequency of extreme precipitation. An increase in sulfate aerosols cools the surface and thereby the atmosphere, and thus induces a reduction in precipitation with a stronger effect on extreme than on mean precipitation. A better understanding and representation of these processes in models will provide knowledge for developing strategies for both climate change and air pollution mitigation.

Journal article

Stjern CW, Lund MT, Samset BH, Myhre G, Forster PM, Andrews T, Boucher O, Faluvegi G, Flaeschner D, Iversen T, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson T, Sand M, Shawki D, Shindell D, Smith CJ, Takemura T, Voulgarakis Aet al., 2019,

Arctic amplification response to individual climate drivers

, Journal of Geophysical Research: Atmospheres, Vol: 124, Pages: 6698-6717, ISSN: 2169-897X

The Arctic is experiencing rapid climate change in response to changes in greenhouse gases, aerosols, and other climate drivers. Emission changes in general, as well as geographical shifts in emissions and transport pathways of short‐lived climate forcers, make it necessary to understand the influence of each climate driver on the Arctic. In the Precipitation Driver Response Model Intercomparison Project, 10 global climate models perturbed five different climate drivers separately (CO2, CH4, the solar constant, black carbon, and SO4). We show that the annual mean Arctic amplification (defined as the ratio between Arctic and the global mean temperature change) at the surface is similar between climate drivers, ranging from 1.9 (± an intermodel standard deviation of 0.4) for the solar to 2.3 (±0.6) for the SO4 perturbations, with minimum amplification in the summer for all drivers. The vertical and seasonal temperature response patterns indicate that the Arctic is warmed through similar mechanisms for all climate drivers except black carbon. For all drivers, the precipitation change per degree global temperature change is positive in the Arctic, with a seasonality following that of the Arctic amplification. We find indications that SO4 perturbations produce a slightly stronger precipitation response than the other drivers, particularly compared to CO2.

Journal article

Sulaiman AH, Farrell WM, Ye S-Y, Kurth WS, Gurnett DA, Hospodarsky GB, Menietti JD, Pisa D, Hunt GJ, Agiwal O, Dougherty MKet al., 2019,

A Persistent, Large-Scale, and Ordered Electrodynamic Connection Between Saturn and Its Main Rings

, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7166-7172, ISSN: 0094-8276

Journal article

AkhavanTafti M, Slavin JA, Eastwood JP, Cassak PA, Gershman DJet al., 2019,

MMS multi‐point analysis of FTE evolution: physical characteristics and dynamics

, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5376-5395, ISSN: 2169-9380

Previous studies have indicated that flux transfer events (FTEs) grow as they convect away from the reconnection site along the magnetopause. This increase in FTE diameter may occur via adiabatic expansion in response to decreasing external pressure away from the subsolar region or due to a continuous supply of magnetic flux and plasma to the FTEs' outer layers by magnetic reconnection. Here we investigate an ensemble of 55 FTEs at the subsolar magnetopause using Magnetospheric Multiscale (MMS) multi‐point measurements. The FTEs are initially modeled as quasi‐force‐free flux ropes in order to infer their geometry and the spacecraft trajectory relative to their central axis. The MMS observations reveal a radially‐inward net force at the outer layers of FTEs which can accelerate plasmas and fields toward the FTE's core region. Inside the FTEs, near the central axis, plasma density is found to decrease as the axial net force increases. It is interpreted that the axial net force accelerates plasmas along the axis in the region of compressing field lines. Statistical analysis of the MMS observations of the 55 FTEs indicates that plasma pressure, Pth, decreases with increasing FTE diameter, λ, as Pth,obsv ∝ λ−0.24. Assuming that all 55 FTEs started out with similar diameters, this rate of plasma pressure decrease with increasing FTE diameter is at least an order of magnitude slower than the theoretical rate for adiabatic expansion (i.e., Pth,adiab. ∝ λ−3.3), suggesting the presence of efficient plasma heating mechanisms, such as magnetic reconnection, to facilitate FTE growth.

Journal article

Lavergne A, Graven H, De Kauwe MG, Keenan FT, Medlyn BE, Prentice Iet al., 2019,

Observed and modelled historical trends in the water use efficiency of plants and ecosystems

, Global Change Biology, Vol: 25, Pages: 2242-2257, ISSN: 1354-1013

Plant water‐use efficiency (WUE, the carbon gained through photosynthesis per unit of water lost through transpiration) is a tracer of the plant physiological controls on the exchange of water and carbon dioxide between terrestrial ecosystems and the atmosphere. At the leaf level, rising CO2 concentrations tend to increase carbon uptake (in the absence of other limitations) and to reduce stomatal conductance, both effects leading to an increase in leaf WUE. At the ecosystem level, indirect effects (e.g. increased leaf area index, soil water savings) may amplify or dampen the direct effect of CO2. Thus, the extent to which changes in leaf WUE translate to changes at the ecosystem scale remains unclear. The differences in the magnitude of increase in leaf versus ecosystem WUE as reported by several studies are much larger than would be expected with current understanding of tree physiology and scaling, indicating unresolved issues. Moreover, current vegetation models produce inconsistent and often unrealistic magnitudes and patterns of variability in leaf and ecosystem WUE, calling for a better assessment of the underlying approaches. Here, we review the causes of variations in observed and modelled historical trends in WUE over the continuum of scales from leaf to ecosystem, including methodological issues, with the aim of elucidating the reasons for discrepancies observed within and across spatial scales. We emphasize that even though physiological responses to changing environmental drivers should be interpreted differently depending on the observational scale, there are large uncertainties in each data set which are often underestimated. Assumptions made by the vegetation models about the main processes influencing WUE strongly impact the modelled historical trends. We provide recommendations for improving long‐term observation‐based estimates of WUE that will better inform the representation of WUE in vegetation models.

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Journal article

Shi J, Zhang Z, Torkar K, Cheng Z, Escoubet P, Farzakeley A, Dunlop M, Carr Cet al., 2019,

South‐North Hemispheric Asymmetry of the FAE Distribution Around the Cusp Region: Cluster Observation

, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5342-5352, ISSN: 2169-9380

Cluster data from late July to early October were used to study the distribution of field‐aligned electron (FAE) events around the two cusps. An FAE event was defined as electron parallel flux >3 × 108 (cm2 s)−1. The total number of FAE events around the two cusps was basically identical, but downward FAE events prevailed in the south and upward FAE events in the north. In the southern cusp, the peak of the FAE events distribution versus altitude was about 1.3 RE higher and the peak of the FAE events distribution versus invariant latitude (ILAT) was about 4° ILAT lower. Only the downward FAEs around the southern cusp had a second ILAT peak, which was located about 11° higher than the main peak. The normalized number of FAEs showed nearly the same features as the unnormalized number of the FAEs events. These results indicated a north‐south asymmetry of the FAE distribution around the two cusps. Some causes for the asymmetry are discussed, the main ones being the asymmetry of the magnetospheric configuration resulting from geomagnetic dipolar tilt and solar wind flows, the interplanetary magnetic field asymmetry related to the magnetosphere, and the difference of ionospheric conductivity in the two hemispheres. Various solar wind‐magnetosphere interaction processes, such as quasi‐viscous interaction and reconnection, are responsible for the asymmetry, too. The second distribution peak (at higher ILAT) of the downward FAE events around the southern cusp corresponded to high solar wind speed and may be associated with the northward interplanetary magnetic field Bz field‐aligned current at low altitude. This requires further studies, however.

Journal article

Wilson LB, Chen L-J, Wang S, Schwartz SJ, Turner DL, Stevens ML, Kasper JC, Osmane A, Caprioli D, Bale SD, Pulupa MP, Salem CS, Goodrich KAet al., 2019,

Electron Energy Partition across Interplanetary Shocks. I. Methodology and Data Product

, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 243, ISSN: 0067-0049

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Citations: 65

Journal article

Good SW, Kilpua EKJ, LaMoury AT, Forsyth RJ, Eastwood JP, Möstl Cet al., 2019,

Self‐similarity of ICME flux ropes: Observations by radially aligned spacecraft in the inner Heliosphere

, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 4960-4982, ISSN: 2169-9380

Interplanetary coronal mass ejections (ICMEs) are a significant feature of the heliospheric environment and the primary cause of adverse space weather at the Earth. ICME propagation and the evolution of ICME magnetic field structure during propagation are still not fully understood. We analyze the magnetic field structures of 18 ICME magnetic flux ropes observed by radially aligned spacecraft in the inner heliosphere. Similarity in the underlying flux rope structures is determined through the application of a simple technique that maps the magnetic field profile from one spacecraft to the other. In many cases, the flux ropes show very strong underlying similarities at the different spacecraft. The mapping technique reveals similarities that are not readily apparent in the unmapped data and is a useful tool when determining whether magnetic field time series observed at different spacecraft are associated with the same ICME. Lundquist fitting has been applied to the flux ropes, and the rope orientations have been determined; macroscale differences in the profiles at the aligned spacecraft may be ascribed to differences in flux rope orientation. Assuming that the same region of the ICME was observed by the aligned spacecraft in each case, the fitting indicates some weak tendency for the rope axes to reduce in inclination relative to the solar equatorial plane and to align with the solar east‐west direction with heliocentric distance.Plain Language SummaryCoronal mass ejections (CMEs) are large eruptions of magnetic field and plasma from the Sun. When they arrive at the Earth, these eruptions can cause significant damage to ground and orbital infrastructure; forecasting this “space weather” impact of CMEs at the Earth remains a difficult task. The impact of individual CMEs is largely dependent on their magnetic field configurations, and an important aspect of space weather forecasting is understanding how CME field configuration changes with distance from t

Journal article

Kronberg EA, Grigorenko EE, Malykhin A, Kozak L, Petrenko B, Vogt M, Roussos E, Kollmann P, Jackman C, Kasahara S, Malova KV, Tao C, Radioti A, Masters Aet al., 2019,

Acceleration of ions in Jovian plasmoids: does turbulence play a role?

, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5056-5069, ISSN: 2169-9380

The dissipation processes which transform electromagnetic energy into kinetic particle energy in space plasmas are still not fully understood. Of particular interest is the distribution of the dissipated energy among different species of charged particles. The Jovian magnetosphere is a unique laboratory to study this question because outflowing ions from the moon Io create a high diversity in ion species. In this work, we use multispecies ion observations and magnetic field measurements by the Galileo spacecraft. We limit our study to observations of plasmoids in the Jovian magnetotail, because there is strong ion acceleration in these structures. Our model predicts that electromagnetic turbulence in plasmoids plays an essential role in the acceleration of oxygen, sulfur, and hydrogen ions. The observations show a decrease of the oxygen and sulfur energy spectral index γ at ∼30 to ∼400 keV/nuc with the wave power indicating an energy transfer from electromagnetic waves to particles, in agreement with the model. The wave power threshold for effective acceleration is of the order of 10 nT2Hz−1, as in terrestrial plasmoids. However, this is not observed for hydrogen ions, implying that processes other than wave‐particle interaction are more important for the acceleration of these ions or that the time and energy resolution of the observations is too coarse. The results are expected to be confirmed by improved plasma measurements by the Juno spacecraft.

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