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Journal articleZhang YC, Dai L, Rong ZJ, et al., 2020, , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-11, ISSN: 2169-9380
In this study, we reported the large鈥恆mplitude and fast鈥恉amped flapping of the plasma sheet, which co鈥恛ccurred with magnetic reconnection. Data from the Double Star TC鈥1 and Cluster satellites were used to analyze the features of the plasma sheet flapping 1.4 RE earthward of an ongoing magnetic reconnection event. The flapping was rapidly damped, and its amplitude decreased from the magnetohydrodynamics scale to the subion scale in 5 min. The variation in the flapping period (from 224 to 20 s) indicated that the source of the flapping had highly dynamic temporal characteristics. The plasma sheet flapping propagated duskward through a kink鈥恖ike wave with a velocity of 100 km/s, which was in agreement with the group velocity of the ballooning perturbation. A correlation analysis between the magnetic reconnection and plasma sheet flapping indicated that the magnetic reconnection likely facilitated the occurrence of ballooning instability by altering the state of plasma in the downstream plasma sheet. In this regard, the reconnection鈥恑nduced ballooning instability could be a potential mechanism to generate the flapping motion of the plasma sheet.
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Journal articleBrown Z, Koskinen T, Muller-Wodarg I, et al., 2020, , Nature Astronomy, Vol: 4, Pages: 872-879, ISSN: 2397-3366
Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag1,2. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyse these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturn’s thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy3, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70° to 30° latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.
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Journal articleGood SW, Kilpua EKJ, Ala-Lahti M, et al., 2020, , ASTROPHYSICAL JOURNAL LETTERS, Vol: 900, ISSN: 2041-8205
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- Citations: 17
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Journal articleBradley TJ, Cowley SWH, Bunce EJ, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
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- Citations: 16
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Journal articleZhu X, He J, Verscharen D, et al., 2020, , ASTROPHYSICAL JOURNAL LETTERS, Vol: 901, ISSN: 2041-8205
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- Citations: 25
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Journal articleLotekar A, Vasko IY, Mozer FS, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
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- Citations: 44
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Journal articleMozer FS, Agapitov OV, Bale SD, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380
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- Citations: 26
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Journal articleShatwell P, Czaja A, Ferreira D, 2020, , Geophysical Research Letters, Vol: 47, Pages: 1-9, ISSN: 0094-8276
To study the role of the Atlantic meridional overturning circulation (AMOC) in transient climate change, we perform an abrupt CO2鈥恉oubling experiment using a coupled atmosphere鈥恛cean鈥恑ce model with a simple geometry that separates the ocean into small and large basins. The small basin exhibits an overturning circulation akin to the AMOC. Over the simulated 200 years of change, it stores heat at a faster rate than the large basin by 0.6 ± 0.2 W m−2. We argue that this is due to the small basin MOC. However, we find that as the MOC weakens significantly, it has little impact on the small basin's heat storage rate. We suggest this is due to the effects of both compensating warming patterns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant.
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Journal articleBantges R, 2020,
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Journal articleWalker AP, De Kauwe MG, Bastos A, et al., 2020, , New Phytologist, ISSN: 0028-646X
Atmospheric carbon dioxide concentration ([CO2 ]) is increasing, which increases leaf-scale photosynthesis and intrinsic water-use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2 ] increase and thus climate change. However, ecosystem CO2 -responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO2 ]-driven terrestrial carbon sink can appear contradictory. Here we synthesise theory and broad, multi-disciplinary evidence for the effects of increasing [CO2 ] (iCO2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre-industry. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2-responses are high in comparison with experiments and theory. Plant mortality and soil carbon iCO2-responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.
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Journal articleFranci L, Stawarz JE, Papini E, et al., 2020, , The Astrophysical Journal, Vol: 898, ISSN: 0004-637X
Magnetospheric Multiscale (MMS) observations of plasma turbulence generated by a Kelvin–Helmholtz (KH) event at the Earth's magnetopause are compared with a high-resolution two-dimensional (2D) hybrid direct numerical simulation of decaying plasma turbulence driven by large-scale balanced Alfvénic fluctuations. The simulation, set up with four observation-driven physical parameters (ion and electron betas, turbulence strength, and injection scale), exhibits a quantitative agreement on the spectral, intermittency, and cascade-rate properties with in situ observations, despite the different driving mechanisms. Such agreement demonstrates a certain universality of the turbulent cascade from magnetohydrodynamic to sub-ion scales, whose properties are mainly determined by the selected parameters, also indicating that the KH instability-driven turbulence has a quasi-2D nature. The fact that our results are compatible with the validity of the Taylor hypothesis, in the whole range of scales investigated numerically, suggests that the fluctuations at sub-ion scales might have predominantly low frequencies. This would be consistent with a kinetic Alfvén wave-like nature and/or with the presence of quasi-static structures. Finally, the third-order structure function analysis indicates that the cascade rate of the turbulence generated by a KH event at the magnetopause is an order of magnitude larger than in the ambient magnetosheath.
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Journal articleMadanian H, Burch JL, Eriksson AI, et al., 2020, , Planetary and Space Science, Vol: 187, ISSN: 0032-0633
The Rosetta spacecraft detected transient and sporadic diamagnetic regions around comet 67P/Churyumov-Gerasimenko. In this paper we present a statistical analysis of bulk and suprathermal electron dynamics, as well as a case study of suprathermal electron pitch angle distributions (PADs) near a diamagnetic region. Bulk electron densities are correlated with the local neutral density and we find a distinct enhancement in electron densities measured over the southern latitudes of the comet. Flux of suprathermal electrons with energies between tens of eV to a couple of hundred eV decreases each time the spacecraft enters a diamagnetic region. We propose a mechanism in which this reduction can be explained by solar wind electrons that are tied to the magnetic field and after having been transported adiabatically in a decaying magnetic field environment, have limited access to the diamagnetic regions. Our analysis shows that suprathermal electron PADs evolve from an almost isotropic outside the diamagnetic cavity to a field-aligned distribution near the boundary. Electron transport becomes chaotic and non-adiabatic when electron gyroradius becomes comparable to the size of the magnetic field line curvature, which determines the upper energy limit of the flux variation. This study is based on Rosetta observations at around 200 鈥媖m cometocentric distance when the comet was at 1.24 AU from the Sun and during the southern summer cometary season.
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Journal articleFarrell WM, MacDowall RJ, Gruesbeck JR, et al., 2020, , ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 249, ISSN: 0067-0049
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- Citations: 44
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Journal articleHofstadter MD, Fletcher LN, Simon AA, et al., 2020, , Space Science Reviews, Vol: 216, Pages: 1-17, ISSN: 0038-6308
Other papers in this special issue have discussed the diversity of planetary atmospheres and some of the key science questions for giant planet atmospheres to be addressed in the future. There are crucial measurements that can only be made by orbiters of giant planets and probes dropped into their atmospheres. To help the community be more effective developers of missions and users of data products, we summarize how NASA and ESA categorize their planetary space missions, and the restrictions and requirements placed on each category. We then discuss the atmospheric goals to be addressed by currently approved giant-planet missions as well as missions likely to be considered in the next few years, such as a joint NASA/ESA Ice Giant orbiter with atmospheric probe. Our focus is on interplanetary spacecraft, but we acknowledge the crucial role to be played by ground-based and near-Earth telescopes, as well as theoretical and laboratory work.
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Journal articleErgun RE, Ahmadi N, Kromyda L, et al., 2020, , ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X
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- Citations: 38
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Journal articleVasko IY, Kuzichev I, Artemyev A, et al., 2020, , PHYSICS OF PLASMAS, Vol: 27, ISSN: 1070-664X
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- Citations: 25
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Journal articleSimon Wedlund C, Behar E, Nilsson H, et al., 2020, , Astronomy and Astrophysics: a European journal, Vol: 640, Pages: C3-C3, ISSN: 0004-6361
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Journal articleErgun RE, Ahmadi N, Kromyda L, et al., 2020, , ASTROPHYSICAL JOURNAL, Vol: 898, ISSN: 0004-637X
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- Citations: 53
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Journal articleBowen TA, Bale SD, Bonnell JW, et al., 2020, , ASTROPHYSICAL JOURNAL, Vol: 899, ISSN: 0004-637X
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- Citations: 35
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Journal articleWilder FD, Schwartz SJ, Ergun RE, et al., 2020, , GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276
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- Citations: 5
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Journal articleFujita R, Morimoto S, Maksyutov S, et al., 2020, , JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 125, ISSN: 2169-897X
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- Citations: 32
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Journal articleGibbins G, Haigh JD, 2020,
Entropy production rates of the climate
, Journal of the Atmospheric Sciences, ISSN: 0022-4928There is ongoing interest in the global entropy production rate as a climate diagnostic and predictor, but progress has been limited by ambiguities in its definition; different conceptual boundaries of the climate system give rise to different internal production rates. Three viable options are described, estimated and investigated here, two of which -- the material and the total radiative (here `planetary') entropy production rates -- are well-established and a third which has only recently been considered but appears very promising. This new option is labelled the `transfer' entropy production rate and includes all irreversible processes that transfer heat within the climate, radiative and material, but not those involved in the exchange of radiation with space. Estimates in three model climates put the material rate in the range 27-48 mW/m^2K, the transfer rate 67-76mW/m^2K, and the planetary rate 1279-1312 mW/m^2K. The climate-relevance of each rate is probed by calculating their responses to climate changes in a simple radiative-convective model. An increased greenhouse effect causes a significant increase in the material and transfer entropy production rates but has no direct impact on the planetary rate. When the same surface temperature increase is forced by changing the albedo instead, the material and transfer entropy production rates increase less dramatically and the planetary rate also registers an increase. This is pertinent to solar radiation management as it demonstrates the difficulty of reversing greenhouse gas-mediated climate changes by albedo alterations. It is argued that the transfer perspective has particular significance in the climate system and warrants increased prominence.
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Journal articleAkhavanTafti M, Palmroth M, Slavin JA, et al., 2020, , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-22, ISSN: 2169-9380
The Vlasiator hybrid鈥怴lasov code was developed to investigate global magnetospheric dynamics at ion鈥恔inetic scales. Here, we focus on the role of magnetic reconnection in the formation and evolution of the magnetic islands at the low鈥恖atitude magnetopause, under southward interplanetary magnetic field (IMF) conditions. The simulation results indicate that: 1) the magnetic reconnection ion kinetics, including the Earthward鈥恜ointing Larmor electric field on the magnetospheric鈥恠ide of an X鈥恜oint and anisotropic ion distributions, are well鈥恈aptured by Vlasiator, thus enabling the study of reconnection鈥恉riven magnetic island evolution processes, 2) magnetic islands evolve due to continuous reconnection at adjacent X鈥恜oints, ‘coalescence’ which refers to the merging of neighboring islands to create a larger island, ‘erosion’ during which an island loses magnetic flux due to reconnection, and ‘division’ which involves the splitting of an island into smaller islands, and 3) continuous reconnection at adjacent X鈥恜oints is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross鈥恠ectional growth rates up to +0.3 RE2/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion鈥恠cale flux transfer events (FTEs) sandwiched between two dominant X鈥恖ines. The MMS measurements similarly reveal: 1) anisotropic ion populations, and 2) normalized reconnection rate ~0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion鈥恠cale FTEs may grow Earth鈥恠ized within ~10 minutes, which is comparable to the average transport time for FTEs formed in the subsolar region to the high鈥恖atitude magnetopause. Future simulations shall revisit reconnection鈥恉riven island evolution processes with improved spatial resolutions.
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Journal articleEggington JWB, Eastwood JP, Mejnertsen L, et al., 2020, , Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-17, ISSN: 2169-9380
The Earth’s dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere鈥恑onosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying IMF orientation and non鈥恥niform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle鈥恉ependence, we perform MHD simulations of the steady鈥恠tate magnetosphere鈥恑onosphere system under purely southward IMF conditions for tilt angles from 0°鈥90°. We identify the location of the magnetic separator in each case, and find that an increasing tilt angle shifts the 3鈥怐 X鈥恖ine southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50° tilt angle, characteristic of regular FTE generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90°. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region鈥怚 field鈥恆ligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north鈥恠outh asymmetry in magnetospheric convection, thus driving more FAC in the northern (sunward鈥恌acing) hemisphere for large tilt angles than in the south independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.
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Journal articleHantson S, Kelley DI, Arneth A, et al., 2020, , Geoscientific Model Development, Vol: 13, Pages: 3299-3318, ISSN: 1991-959X
Global fire-vegetation models are widely used to assess impacts of environmental change on fire regimes and the carbon cycle and to infer relationships between climate, land use and fire. However, differences in model structure and parameterizations, in both the vegetation and fire components of these models, could influence overall model performance, and to date there has been limited evaluation of how well different models represent various aspects of fire regimes. The Fire Model Intercomparison Project (FireMIP) is coordinating the evaluation of state-of-the-art global fire models, in order to improve projections of fire characteristics and fire impacts on ecosystems and human societies in the context of global environmental change. Here we perform a systematic evaluation of historical simulations made by nine FireMIP models to quantify their ability to reproduce a range of fire and vegetation benchmarks. The FireMIP models simulate a wide range in global annual total burnt area (39–536 Mha) and global annual fire carbon emission (0.91–4.75 Pg C yr−1) for modern conditions (2002–2012), but most of the range in burnt area is within observational uncertainty (345–468 Mha). Benchmarking scores indicate that seven out of nine FireMIP models are able to represent the spatial pattern in burnt area. The models also reproduce the seasonality in burnt area reasonably well but struggle to simulate fire season length and are largely unable to represent interannual variations in burnt area. However, models that represent cropland fires see improved simulation of fire seasonality in the Northern Hemisphere. The three FireMIP models which explicitly simulate individual fires are able to reproduce the spatial pattern in number of fires, but fire sizes are too small in key regions, and this results in an underestimation of burnt area. The correct representation of spatial and seasonal patterns in vegetation appears
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Journal articleTang T, Shindell D, Zhang Y, et al., 2020, , Atmospheric Chemistry and Physics, Vol: 20, Pages: 8251-8266, ISSN: 1680-7316
Shortwave cloud radiative effects (SWCREs), defined as the difference of the shortwave radiative flux between all-sky and clear-sky conditions at the surface, have been reported to play an important role in influencing the Earth's energy budget and temperature extremes. In this study, we employed a set of global climate models to examine the SWCRE responses to CO2, black carbon (BC) aerosols, and sulfate aerosols in boreal summer over the Northern Hemisphere. We found that CO2 causes positive SWCRE changes over most of the NH, and BC causes similar positive responses over North America, Europe, and eastern China but negative SWCRE over India and tropical Africa. When normalized by effective radiative forcing, the SWCRE from BC is roughly 3–5 times larger than that from CO2. SWCRE change is mainly due to cloud cover changes resulting from changes in relative humidity (RH) and, to a lesser extent, changes in cloud liquid water, circulation, dynamics, and stability. The SWCRE response to sulfate aerosols, however, is negligible compared to that for CO2 and BC because part of the radiation scattered by clouds under all-sky conditions will also be scattered by aerosols under clear-sky conditions. Using a multilinear regression model, it is found that mean daily maximum temperature (Tmax) increases by 0.15 and 0.13 K per watt per square meter (W m−2) increase in local SWCRE under the CO2 and BC experiment, respectively. When domain-averaged, the contribution of SWCRE change to summer mean Tmax changes was 10 %–30 % under CO2 forcing and 30 %–50 % under BC forcing, varying by region, which can have important implications for extreme climatic events and socioeconomic activities.
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Journal articleSaunois M, Stavert AR, Poulter B, et al., 2020, , Earth System Science Data, Vol: 12, Pages: 1561-1623, ISSN: 1866-3508
Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations).For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or
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Journal articleMilillo, Fujimoto, Murakami, et al., 2020, , Space Science Reviews, Vol: 216, Pages: 1-78, ISSN: 0038-6308
The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury’s environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors.
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Journal articleStawarz JE, Matteini L, Parashar TN, et al., 2020,
<jats:p>Decomposing the electric field (E) into the contributions fromgeneralized Ohm’s law provides key insight into both nonlinear anddissipative dynamics across the full range of scales within a plasma.Using high-resolution, multi-spacecraft measurements of three intervalsin Earth’s magnetosheath from the Magnetospheric Multiscale mission, theinfluence of the magnetohydrodynamic, Hall, electron pressure, andelectron inertia terms from Ohm’s law, as well as the impact of a finiteelectron mass, on the turbulent spectrum are examined observationallyfor the first time. The magnetohydrodynamic, Hall, and electron pressureterms are the dominant contributions to over the accessible lengthscales, which extend to scales smaller than the electron inertial lengthat the greatest extent, with the Hall and electron pressure termsdominating at sub-ion scales. The strength of the non-ideal electronpressure contribution is stronger than expected from linear kineticAlfvén waves and a partial anti-alignment with the Hall electric fieldis present, linked to the relative importance of electron diamagneticcurrents in the turbulence. The relative contribution of linear andnonlinear electric fields scale with the turbulent fluctuationamplitude, with nonlinear contributions playing the dominant role inshaping for the intervals examined in this study. Overall, the sum ofthe Ohm’s law terms and measured agree to within ~20%across the observable scales. These results both confirm generalexpectations about the behavior of in turbulent plasmas and highlightfeatures that should be explored further theoretically.</jats:p>
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Journal articleBowen TA, Mallet A, Bale SD, et al., 2020, , PHYSICAL REVIEW LETTERS, Vol: 125, ISSN: 0031-9007
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- Citations: 46
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