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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,
Comparative analysis of the vlasiator simulations and MMS observations of multiple X‐line reconnection and flux transfer events
, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-22, ISSN: 2169-9380The Vlasiator hybrid‐Vlasov code was developed to investigate global magnetospheric dynamics at ion‐kinetic scales. Here, we focus on the role of magnetic reconnection in the formation and evolution of the magnetic islands at the low‐latitude magnetopause, under southward interplanetary magnetic field (IMF) conditions. The simulation results indicate that: 1) the magnetic reconnection ion kinetics, including the Earthward‐pointing Larmor electric field on the magnetospheric‐side of an X‐point and anisotropic ion distributions, are well‐captured by Vlasiator, thus enabling the study of reconnection‐driven magnetic island evolution processes, 2) magnetic islands evolve due to continuous reconnection at adjacent X‐points, ‘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‐points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross‐sectional growth rates up to +0.3 RE2/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion‐scale flux transfer events (FTEs) sandwiched between two dominant X‐lines. 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‐scale FTEs may grow Earth‐sized within ~10 minutes, which is comparable to the average transport time for FTEs formed in the subsolar region to the high‐latitude magnetopause. Future simulations shall revisit reconnection‐driven island evolution processes with improved spatial resolutions.
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Journal articleEggington JWB, Eastwood JP, Mejnertsen L, et al., 2020,
Dipole tilt effect on magnetopause reconnection and the steady‐state magnetosphere‐ionosphere system: global MHD simulation
, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-17, ISSN: 2169-9380The Earth’s dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere‐ionosphere 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‐uniform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle‐dependence, we perform MHD simulations of the steady‐state magnetosphere‐ionosphere system under purely southward IMF conditions for tilt angles from 0°‐90°. We identify the location of the magnetic separator in each case, and find that an increasing tilt angle shifts the 3‐D X‐line 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‐I field‐aligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north‐south asymmetry in magnetospheric convection, thus driving more FAC in the northern (sunward‐facing) 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,
Quantitative assessment of fire and vegetation properties in simulations with fire-enabled vegetation models from the Fire Model Intercomparison Project
, Geoscientific Model Development, Vol: 13, Pages: 3299-3318, ISSN: 1991-959XGlobal 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,
Response of surface shortwave cloud radiative effect to greenhouse gases and aerosols and its impact on summer maximum temperature
, Atmospheric Chemistry and Physics, Vol: 20, Pages: 8251-8266, ISSN: 1680-7316Shortwave 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,
The global methane budget 2000-2017
, Earth System Science Data, Vol: 12, Pages: 1561-1623, ISSN: 1866-3508Understanding 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,
Investigating Mercury’s environment with the two-spacecraft BepiColombo mission
, Space Science Reviews, Vol: 216, Pages: 1-78, ISSN: 0038-6308The 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,
Generalized Ohm’s Law Decomposition of the Electric Field in Magnetosheath Turbulence: Magnetospheric Multiscale Observations
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Journal articleBowen TA, Mallet A, Bale SD, et al., 2020,
Constraining Ion-Scale Heating and Spectral Energy Transfer in Observations of Plasma Turbulence
, PHYSICAL REVIEW LETTERS, Vol: 125, ISSN: 0031-9007- Author Web Link
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- Citations: 46
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Journal articleMartin CJ, Ray LC, Constable DA, et al., 2020,
Evaluating the ionospheric mass source for Jupiter's magnetosphere: An ionospheric outflow model for the auroral regions
, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380Ionospheric outflow is the flow of plasma initiated by a loss of equilibrium along a magnetic field line which induces an ambipolar electric field due to the separation of electrons and ions in a gravitational field and other mass dependant sources. We have developed an ionospheric outflow model using the transport equations to determine the number of particles that flow into the outer magnetosphere of Jupiter. The model ranges from 1400 km in altitude above the 1 bar level to 2.5 RJ along the magnetic field line and considers H+ and H3+ as the main ion constituents. Previously, only pressure gradients and gravitational forces were considered in modelling polar wind. However, at Jupiter we need to evaluate the affect of field‐aligned currents present in the auroral regions due to the breakdown of corotation in the magnetosphere, along with the centrifugal force exerted on the particles due to the fast planetary rotation rate. The total number flux from both hemispheres is found to be 1.3‐1.8 x 1028 s‐1 comparable in total number flux to the Io plasma source. The mass flux is lower due to the difference in ion species. This influx of protons from the ionosphere into the inner and middle magnetosphere needs to be included in future assessments of global flux tube dynamics and composition of the magnetosphere system.
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Journal articleCao H, Dougherty MK, Hunt GJ, et al., 2020,
The landscape of Saturn's internal magnetic field from the Cassini Grand Finale
, ICARUS, Vol: 344, ISSN: 0019-1035- Cite
- Citations: 41
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Journal articleDiaz-Aguado MF, Bonnell JW, Bale SD, et al., 2020,
Experimental Investigation of the Secondary and Backscatter Electron Emission from Spacecraft Materials
, JOURNAL OF SPACECRAFT AND ROCKETS, Vol: 57, Pages: 793-808, ISSN: 0022-4650- Author Web Link
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- Citations: 1
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Journal articleHuang SY, Zhang J, Sahraoui F, et al., 2020,
Kinetic Scale Slow Solar Wind Turbulence in the Inner Heliosphere: Coexistence of Kinetic Alfven Waves and Alfven Ion Cyclotron Waves
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 897, ISSN: 2041-8205- Author Web Link
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- Citations: 40
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Journal articleStarkey M, Fuselier SA, Desai MI, et al., 2020,
MMS Observations of Accelerated Interstellar Pickup He<SUP>+</SUP>Ions at an Interplanetary Shock
, ASTROPHYSICAL JOURNAL, Vol: 897, ISSN: 0004-637X- Author Web Link
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- Citations: 2
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Journal articleOliveros JCM, Diaz Castillo SM, Krupar V, et al., 2020,
An In Situ Interplanetary "U-burst": Observation and Results
, ASTROPHYSICAL JOURNAL, Vol: 897, ISSN: 0004-637X- Author Web Link
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- Citations: 1
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Journal articleMagurno D, Cossich W, Maestri T, et al., 2020,
Cirrus cloud identification from airborne far-infrared and mid-infrared spectra
, Remote Sensing, Vol: 12, Pages: 1-19, ISSN: 2072-4292Airborne interferometric data, obtained from the Cirrus Coupled Cloud-Radiation Experiment (CIRCCREX) and from the PiknMix-F field campaign, are used to test the ability of a machine learning cloud identification and classification algorithm (CIC). Data comprise a set of spectral radiances measured by the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) and the Airborne Research Interferometer Evaluation System (ARIES). Co-located measurements of the two sensors allow observations of the upwelling radiance for clear and cloudy conditions across the far- and mid-infrared part of the spectrum. Theoretical sensitivity studies show that the performance of the CIC algorithm improves with cloud altitude. These tests also suggest that, for conditions encompassing those sampled by the flight campaigns, the additional information contained within the far-infrared improves the algorithm’s performance compared to using mid-infrared data only. When the CIC is applied to the airborne radiance measurements, the classification performance of the algorithm is very high. However, in this case, the limited temporal and spatial variability in the measured spectra results in a less obvious advantage being apparent when using both mid- and far-infrared radiances compared to using mid-infrared information only. These results suggest that the CIC algorithm will be a useful addition to existing cloud classification tools but that further analyses of nadir radiance observations spanning the infrared and sampling a wider range of atmospheric and cloud conditions are required to fully probe its capabilities. This will be realised with the launch of the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, ESA’s 9th Earth Explorer.
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Journal articleKollmann, Cohen, Allen, et al., 2020,
Magnetospheric studies: a requirement for addressing interdisciplinary mysteries in the Ice Giant systems
, Space Science Reviews, Vol: 216, ISSN: 0038-6308Uranus and Neptune are the least-explored planets in our Solar System. This paper summarizesmysteries about these incredibly intriguing planets and their environments spurred by our limitedobservations from Voyager 2 and Earth-based systems. Several of these observations are eitherinconsistent with our current understanding built from exploring other planetary systems, orindicate such unique characteristics of these Ice Giants that they leave us with more questions thananswers. This paper specifically focuses on the value of all aspects of magnetosphericmeasurements, from the radiation belt structure to plasma dynamics to coupling to the solar wind,through a future mission to either of these planets. Such measurements have large interdisciplinaryvalue, as demonstrated by the large number of mysteries discussed in this paper that cover othernon-magnetospheric disciplines, including planetary interiors, atmospheres, rings, and moons.
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Journal articleArcher MO, 2020,
Space Sound Effects Short Film Festival: using the film festival model to inspire creative art–science and reach new audiences
, Geoscience Communication, Vol: 3, Pages: 147-166, ISSN: 2569-7110The ultra-low frequency analogues of sound waves in Earth'smagnetosphere play a crucial role in space weather; however, the publicis largely unaware of this risk to our everyday lives and technology.As a way of potentially reaching new audiences, SSFX (Space Sound Effects) made 8 yearsof satellite wave recordings audible to the human ear with the aimof using it to create art. Partnering with film industry professionals,the standard processes of international film festivals were adoptedby the project in order to challenge independent filmmakers to incorporatethese sounds into short films in creative ways. Seven films coveringa wide array of topics and genres (despite coming from the same sounds)were selected for screening at a special film festival out of 22 submissions.The works have subsequently been shown at numerous established filmfestivals and screenings internationally. These events have attracteddiverse non-science audiences resulting in several unanticipated impacts on them, thereby demonstrating how working with the art world canopen up dialogues with both artists and audiences who would not ordinarily engage with science.
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Journal articleWang S, Rashid T, Thorp H, et al., 2020,
A shortening of the life-cycle of major tropical cyclones
, Geophysical Research Letters, Vol: 47, Pages: 28 Jul 2020-28 Jul 2020, ISSN: 0094-8276In this study a comprehensive picture of the changing intensity life cycle of major (Category 3 and higher) tropical cyclones (TCs) is presented. Over the past decades, the lifetime maximum intensity has increased, but there has also been a significant decrease in duration of time spent at intensities greater than Category 1. These compensating effects have maintained a stable global mean‐accumulated cyclone energy of individual major TCs. The global mean duration of major TCs has shortened by about 1 day from 1982 to 2018. There has been both faster intensification (Categories 1 to 3) and weakening (Categories 3 to 1) by about 40%. The probabilities of rapid intensification and rapid weakening have both risen in the period 2000–2018 compared to 1982–1999. A statistically significant anticorrelation is found between the lifetime maximum intensity and the following duration of the final weakening. This suggests an element of self‐regulation of TC life cycles.
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Journal articleMadanian H, Schwartz SJ, Halekas JS, et al., 2020,
Nonstationary Quasiperpendicular Shock and Ion Reflection at Mars
, GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276- Author Web Link
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- Citations: 6
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Journal articleVasko IY, Wang R, Mozer FS, et al., 2020,
On the Nature and Origin of Bipolar Electrostatic Structures in the Earth's Bow Shock
, FRONTIERS IN PHYSICS, Vol: 8, ISSN: 2296-424X- Author Web Link
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- Citations: 36
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Journal articleBandyopadhyay R, Sorriso-Valvo L, Chasapis A, et al., 2020,
In situ observation of hall magnetohydrodynamic cascade in space plasma
, Physical Review Letters, Vol: 124, Pages: 225101 – 1-225101 – 7, ISSN: 0031-9007We present estimates of the turbulent energy-cascade rate derived from a Hall-magnetohydrodynamic (MHD) third-order law. We compute the contribution from the Hall term and the MHD term to the energy flux. Magnetospheric Multiscale (MMS) data accumulated in the magnetosheath and the solar wind are compared with previously established simulation results. Consistent with the simulations, we find that at large (MHD) scales, the MMS observations exhibit a clear inertial range dominated by the MHD flux. In the subion range, the cascade continues at a diminished level via the Hall term, and the change becomes more pronounced as the plasma beta increases. Additionally, the MHD contribution to interscale energy transfer remains important at smaller scales than previously thought. Possible reasons are offered for this unanticipated result.
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Journal articleYang L, Wang L, Zhao L, et al., 2020,
Quiet-time Solar Wind Suprathermal Electrons of Different Solar Origins
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 896, ISSN: 2041-8205- Author Web Link
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- Citations: 4
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Journal articleCarnielli G, Galand M, Leblanc F, et al., 2020,
Constraining Ganymede's neutral and plasma environments through simulations of its ionosphere and Galileo observations
, Icarus, Vol: 343, Pages: 1-11, ISSN: 0019-1035Ganymede's neutral and plasma environments are poorly constrained by observations. Carnielli et al. (2019) developed the first 3D ionospheric model aimed at understanding the dynamics of the present ion species and at quantifying the presence of each component in the moon's magnetosphere. The model outputs were compared with Galileo measurements of the ion energy flux, ion bulk velocity and electron number density made during the G2 flyby. A good agreement was found in terms of ion energy distribution and bulk velocity, but not in terms of electron number density. In this work, we present some improvements to our model Carnielli et al. (2019) and quantitatively address the possible sources of the discrepancy found in the electron number density between the Galileo observations and our ionospheric model. We have improved the ion model by developing a collision scheme to simulate the charge-exchange interaction between the exosphere and the ionosphere. We have simulated the energetic component of the O$_2$ population, which is missing in the exospheric model of Leblanc et al. (2017) and added it to the original distribution, hence improving its description at high altitudes. These improvements are found to be insufficient to explain the discrepancy in the electron number density. We provide arguments that the input O$_2$ exosphere is underestimated and that the plasma production acts asymmetrically between the Jovian and anti-Jovian hemispheres. In particular, we estimate that the O$_2$ column density should be greater than $10^{15}$~cm$^{-2}$, i.e., higher than previously derived upper limits (and a factor 10 higher than the values from Leblanc et al. (2017)), and that the ionization frequency from electron impact must be higher in the anti-Jovian hemisphere for the G2 flyby conditions.
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Journal articleCurtis PE, Ceppi P, Zappa G, 2020,
Role of the mean state for the Southern Hemispheric Jet Stream response to CO₂ forcing in CMIP6 models
, Environmental Research Letters, Vol: 15, Pages: 1-7, ISSN: 1748-9326Global climate models indicate that the Southern Hemispheric (SH) jet stream shifts poleward in response to CO2 forcing, but the magnitude of this shift remains highly uncertain. Here we analyse the SH jet stream response to 4×CO2 forcing in Coupled Model Intercomparison Project phase 6 (CMIP6) simulations, and find a substantially muted jet shift during winter compared with CMIP5. We suggest this muted response results from a more poleward mean jet position, consistent with a strongly reduced bias in jet position relative to the reanalysis during 1980--2004. The improved mean jet position cannot be explained by changes in the simulated sea surface temperatures. Instead, we find indications that increased horizontal grid resolution in CMIP6 relative to CMIP5 has contributed to the higher mean jet latitude, and thus to the reduced jet shift under CO2 forcing. These results imply that CMIP6 models can provide more realistic projections of SH climate change.
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Journal articleTilquin H, Eastwood JP, Phan TD, 2020,
Solar wind reconnection exhausts in the inner heliosphere observed by helios and detected via machine learning
, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 895, Pages: 1-10, ISSN: 0004-637XReconnecting current sheets in the solar wind play an important role in the dynamics of the heliosphere and offer an opportunity to study magnetic reconnection exhausts under a wide variety of inflow and magnetic shear conditions. However, progress in understanding reconnection can be frustrated by the difficulty of finding events in long time-series data. Here we describe a new method to detect magnetic reconnection events in the solar wind based on machine learning, and apply it to Helios data in the inner heliosphere. The method searches for known solar wind reconnection exhaust features, and parameters in the algorithm are optimized to maximize the Matthews Correlation Coefficient using a training set of events and non-events. Applied to the whole Helios data set, the trained algorithm generated a candidate set of events that were subsequently verified by hand, resulting in a database of 88 events. This approach offers a significant reduction in construction time for event databases compared to purely manual approaches. The database contains events covering a range of heliospheric distances from ~0.3 to ~1 au, and a wide variety of magnetic shear angles, but is limited by the relatively coarse time resolution of the Helios data. Analysis of these events suggests that proton heating by reconnection in the inner heliosphere depends on the available magnetic energy in a manner consistent with observations in other regimes such as at the Earth's magnetopause, suggesting this may be a universal feature of reconnection.
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Journal articleLester JG, Lovenduski NS, Graven HD, et al., 2020,
Internal variability dominates over externally forced ocean circulation changes seen through CFCs
, Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276Observations of oceanic transient tracers have indicated that the circulation in the Southern Ocean has changed in recent decades, potentially driven by changes in external climate forcing. Here, we use the CESM Large Ensemble to analyze changes in two oceanic tracers that are affected by ocean circulation: the partial pressure of chlorofluorocarbon‐12 (pCFC12) and the idealized model tracer Ideal Age (IAGE) over the 1991 to 2005 period. The small ensemble mean change in IAGE suggests that there has been very little externally forced change in Southern Ocean circulation over this period, in contrast to strong internal variability. Further, our analysis implies that real‐world observations of changes in pCFC12 may not be a robust way to characterize externally driven changes in Southern Ocean circulation because of the large internal variability in pCFC12 changes exhibited by the individual ensemble members.
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Journal articleShebanits O, Hadid LZ, Cao H, et al., 2020,
Saturn’s near-equatorial ionospheric conductivities from in situ measurements
, Scientific Reports, Vol: 10, ISSN: 2045-2322Cassini’s Grand Finale orbits provided for the first time in-situ measurements of Saturn’s topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10−5–10−4 S/m at (or close to) the ionospheric peak, a factor 10–100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300–800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.
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Journal articleMadanian H, Schwartz SJ, Halekas JS, et al., 2020,
Nonstationary Quasi-perpendicular Shock and Ion Reflection at Mars
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Journal articleLavraud B, Fargette N, Réville V, et al., 2020,
The heliospheric current sheet and plasma sheet during Parker Solar Probe’s first orbit
, Letters of the Astrophysical Journal, Vol: 894, Pages: 1-8, ISSN: 2041-8205We present heliospheric current sheet (HCS) and plasma sheet (HPS) observations during Parker Solar Probe's (PSP) first orbit around the Sun. We focus on the eight intervals that display a true sector boundary (TSB; based on suprathermal electron pitch angle distributions) with one or several associated current sheets. The analysis shows that (1) the main density enhancements in the vicinity of the TSB and HCS are typically associated with electron strahl dropouts, implying magnetic disconnection from the Sun, (2) the density enhancements are just about twice that in the surrounding regions, suggesting mixing of plasmas from each side of the HCS, (3) the velocity changes at the main boundaries are either correlated or anticorrelated with magnetic field changes, consistent with magnetic reconnection, (4) there often exists a layer of disconnected magnetic field just outside the high-density regions, in agreement with a reconnected topology, (5) while a few cases consist of short-lived density and velocity changes, compatible with short-duration reconnection exhausts, most events are much longer and show the presence of flux ropes interleaved with higher-β regions. These findings are consistent with the transient release of density blobs and flux ropes through sequential magnetic reconnection at the tip of the helmet streamer. The data also demonstrate that, at least during PSP's first orbit, the only structure that may be defined as the HPS is the density structure that results from magnetic reconnection, and its byproducts, likely released near the tip of the helmet streamer.
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