Results
- Showing results for:
- Reset all filters
Search results
-
Journal articleWedlund Simon C, Behar E, Nilsson H, et al., 2019, , 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 articleMoore L, Melin H, O'Donoghue J, et al., 2019, , 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 articleHoang M, Garnier P, Gourlaouen H, et al., 2019, , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
-
Journal articleBeth A, Galand M, Heritier K, 2019, , 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鈭時 law at large cometocentric distances.
-
Journal articlePancost R, Rogelj J, Scoones I, et al., 2019, , 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 articleMyllys M, Henri P, Galand M, et al., 2019, , 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 articleMandt KE, Eriksson A, Beth A, et al., 2019, , 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 articleLuspay-Kuti A, Altwegg K, Berthelier JJ, et al., 2019, , ASTRONOMY & ASTROPHYSICS, Vol: 630, ISSN: 1432-0746
-
Journal articleWedlund CS, Bodewits D, Alho M, et al., 2019, , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
-
Journal articleWedlund CS, Behar E, Kallio E, et al., 2019, , Astronomy and Astrophysics: a European journal, Vol: 630, ISSN: 0004-6361
-
Journal articleCarnielli G, Galand M, Leblanc F, et al., 2019, , 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 articleKuzichev I, Vasko IY, Soto-Chavez AR, et al., 2019, , ASTROPHYSICAL JOURNAL, Vol: 882, ISSN: 0004-637X
- Cite
- Citations: 40
-
Journal articleKrupar V, Magdalenic J, Eastwood JP, et al., 2019, , 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 articlePerrone D, Stansby D, Horbury TS, et al., 2019, , 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 articleSorba AM, Achilleos NA, Sergis N, et al., 2019, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 7425-7441, ISSN: 2169-9380
- Cite
- Citations: 11
-
Journal articlePoh G, Slavin JA, Lu S, et al., 2019, , Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 7477-7493, ISSN: 2169-9380
Three鈥恉imensional global hybrid simulations and observations have shown that earthward鈥恗oving flux ropes (FRs) can undergo magnetic reconnection (or re鈥恟econnection) with the near鈥怑arth dipole field to create dipolarization front (DF)鈥恖ike 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鈥恖ine tailward of the FR result in the asymmetric south鈥恘orth BZ signature in many earthward鈥恗oving 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鈥恗oving FR on 3 August 2017. The signatures of this re鈥恟econnection event were (i) +/− BZ reversal, (ii) −/+ bipolar鈥恡ype quadrupolar Hall magnetic fields, (iii) northward super鈥怉lfvénic 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鈥恖ine. Our results are in good agreement with particle鈥恑n鈥恈ell simulations of Lu et al. (2016, https://doi.org/10.1002/2016JA022815). We computed a dimensionless reconnection rate of ~0.09 for this re鈥恟econnection event and through modeling, estimating that the FR would fully dissipate by −16.58 RE. We demonstrated pertubations in the high鈥恖atitude ionospheric currents at the same time of the dissipation of earthward鈥恗oving FRs using ground鈥 and space鈥恇ased measurements.
-
Journal articleManners H, Masters A, 2019, , Geophysical Research Letters, Vol: 46, Pages: 9344-9351, ISSN: 0094-8276
Quasi鈥恜eriodic pulsations in the ultra鈥恖ow鈥恌requency 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鈥恠itu 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鈥恏armonic ultra鈥恖ow鈥恌requency event in Jupiters equatorial plasma sheet. The harmonic periods lie in the 4鈥22鈥恗in range, and the nodal structure is confined to the plasma sheet. Polarization analysis reveals several elliptically鈥恜olarized 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鈥恏armonic waves therefore potentially explain the full range of periods in quasi鈥恜eriodic pulsations in Jupiters magnetosphere.
-
Journal articleHunt G, Cowley S, Provan G, et al., 2019, , 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 articleVigren E, Edberg NJT, Eriksson A, et al., 2019, , 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 articleDeca J, Henri P, Divin A, et al., 2019, , 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 articleFadanelli S, Lavraud B, Califano F, et al., 2019, , 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 articleFowler CM, Halekas J, Schwartz S, et al., 2019, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 7086-7097, ISSN: 2169-9380
- Cite
- Citations: 10
-
Journal articleShuster JR, Gershman DJ, Chen L-J, et al., 2019, , GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7862-7872, ISSN: 0094-8276
- Cite
- Citations: 24
-
Journal articleSillmann J, Stjern CW, Myhre G, et al., 2019, , 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 articleStjern CW, Lund MT, Samset BH, et al., 2019, , 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鈥恖ived 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 articleSulaiman AH, Farrell WM, Ye S-Y, et al., 2019, , GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7166-7172, ISSN: 0094-8276
-
Journal articleAkhavanTafti M, Slavin JA, Eastwood JP, et al., 2019, , 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鈥恜oint measurements. The FTEs are initially modeled as quasi鈥恌orce鈥恌ree flux ropes in order to infer their geometry and the spacecraft trajectory relative to their central axis. The MMS observations reveal a radially鈥恑nward 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 articleLavergne A, Graven H, De Kauwe MG, et al., 2019, , Global Change Biology, Vol: 25, Pages: 2242-2257, ISSN: 1354-1013
Plant water鈥恥se 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鈥恡erm observation鈥恇ased estimates of WUE that will better inform the representation of WUE in vegetation models.
-
Journal articleShi J, Zhang Z, Torkar K, et al., 2019, , 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鈥恆ligned 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鈥恠outh 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鈥恗agnetosphere interaction processes, such as quasi鈥恦iscous 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鈥恆ligned current at low altitude. This requires further studies, however.
-
Journal articleWilson LB, Chen L-J, Wang S, et al., 2019, , ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 243, ISSN: 0067-0049
- Cite
- Citations: 65
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.