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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
<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,
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
<jats:p>Collisionless shocks in space plasma are regions of heating andacceleration of charged particles and dissipation of kinetic energy.These accelerated particles are the source of electromagnetic emissionsfrom supernova remnants and other astrophysical structures. At high Machnumbers, shocks can be inherently nonstationary and exhibit modulatedenergy transfer and recurring plasma compression areas in the form ofreformation. We use data from the Mars Atmosphere and Volatile Evolution(MAVEN) spacecraft to study reformation of the Martian bow shock whichhas a relatively high curvature compared to that at Earth and theupstream solar wind is often mass loaded with a population of pickupions. We show evidence of ion reflection effects in reformation of asupercritical quasi-perpendicular shock.</jats:p>
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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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Journal articleHunt GJ, Bunce EJ, Cao H, et al., 2020,
Saturn's auroral field-aligned currents: observations from the Northern Hemisphere dawn sector during cassini's proximal orbits
, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field‐aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F‐ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LT). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept‐forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO‐independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass‐by‐pass we find that the main upward current at dawn is stronger compared to near‐noon. This results in the current density been ~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO‐independent upward current electron power ~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal orbits may show evidence of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere.
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Journal articleVerniero JL, Larson DE, Livi R, et al., 2020,
Parker Solar Probe Observations of Proton Beams Simultaneous with Ion-scale Waves
, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 248, ISSN: 0067-0049- Author Web Link
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- Citations: 93
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Journal articleStaniland N, Dougherty M, Masters A, et al., 2020,
Determining the nominal thickness and variability of the magnetodisc current sheet at saturn
, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-15, ISSN: 2169-9380The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 R S . The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability.
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Journal articleFuselier, Petrinec, Sawyer, et al., 2020,
Suppression of magnetic reconnection at Saturn’s low-latitude magnetopause
, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-16, ISSN: 2169-9380Observations from the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) are used in an in‐depth investigation of the occurrence and location of reconnection at Saturn's magnetopause. Heated, streaming electrons parallel and/or antiparallel to the magnetic field in the magnetosheath adjacent to the magnetopause indicate that reconnection is occurring somewhere on the boundary. In these instances, the Cassini spacecraft is connected to open magnetic field lines that thread the magnetopause boundary. A survey of 99 crossings with sufficient pitch angle coverage from CAPS/ELS indicates that 65% of the crossings had this evidence of reconnection. Specific crossings from this survey are used to demonstrate that there are times when reconnection at Saturn's low‐latitude magnetopause may be suppressed.
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