Publications

Journal article

Sergis N, Achilleos N, Guio P, Arridge CS, Sorba AM, Roussos E, Krimigis SM, Paranicas C, Hamilton DC, Krupp N, Mitchell DG, Dougherty MK, Balasis G, Giannakis Oet al., 2018,

Mapping Saturn's nightside plasma sheet using Cassini's proximal orbits

, Geophysical Research Letters, Vol: 45, Pages: 6798-6804, ISSN: 0094-8276

Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5‐day‐period proximal orbits, covering the mid‐latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos‐Guio‐Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long‐term, near‐equatorial measurements.

Journal article

Sulaiman AH, Kurth WS, Hospodarsky GB, Averkamp TF, Persoon AM, Menietti JD, Ye S-Y, Gurnett DA, Pisa D, Farrell WM, Dougherty MKet al., 2018,

Auroral Hiss Emissions During Cassini's Grand Finale: Diverse Electrodynamic Interactions Between Saturn and Its Rings

, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 6782-6789, ISSN: 0094-8276

The Cassini Grand Finale orbits offered a new view of Saturn and its environment owing to multiple highly inclined orbits with unprecedented proximity to the planet during closest approach. The Radio and Plasma Wave Science instrument detected striking signatures of plasma waves in the southern hemisphere. These all propagate in the whistler mode and are classified as (1) a filled funnel‐shaped emission, commonly known as auroral hiss. Here however, our analysis indicates that they are likely associated with currents connected to the rings. (2) First observations of very low frequency saucers directly linked to the planet on field lines also connected to the rings. The latter observations are unique to low altitude orbits, and their presence at the Earth and Saturn alike shows that they are fundamental plasma waves in planetary ionospheres. Our results give an insight, from a unique perspective, into the dynamic and diverse nature of Saturn's environment.

Journal article

Reid J, Hood AW, Parnell CE, Browning PK, Cargill PJet al., 2018,

Coronal energy release by MHD avalanches: continuous driving

, Astronomy and Astrophysics: a European journal, Vol: 615, Pages: 1-10, ISSN: 0004-6361

Previous work has confirmed the concept of a magnetohydrodynamic (MHD) avalanche in pre-stressed threads within a coronal loop. We undertook a series of full, three-dimensional MHD simulations in order to create three threads by twisting the magnetic field through boundary motions until an instability ensues. We find that, following the original instability, one unstable thread can disrupt its neighbours with continued driving. A “bursty” heating profile results, with a series of ongoing energy releases, but no evident steady state. For the first time using full MHD, we show that avalanches are a viable mechanism for the storing and release of magnetic energy in the solar corona, as a result of photospheric motions.

Journal article

Banks JR, Schepanski K, Heinold B, Huenerbein A, Brindley HEet al., 2018,

The influence of dust optical properties on the colour of simulated MSG-SEVIRI Desert Dust infrared imagery

, ATMOSPHERIC CHEMISTRY AND PHYSICS, Vol: 18, Pages: 9681-9703, ISSN: 1680-7316

Satellite imagery of atmospheric mineral dust is sensitive to the optical properties of the dust, governed by the mineral refractive indices, particle size, and particle shape. In infrared channels the imagery is also sensitive to the dust layer height and to the surface and atmospheric environment. Simulations of mineral dust in infrared "Desert Dust" imagery from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) have been performed, using the COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model) dust transport model and the Radiative Transfer for TOVS (RTTOV) program, in order to investigate the sensitivity of the imagery to assumed dust properties. This paper introduces the technique and performs initial validation and comparisons with SEVIRI measurements over North Africa for daytime hours during 6 months covering June and July of 2011–2013. Using T-matrix scattering theory and assuming the dust particles to be spherical or spheroidal, wavelength- and size-dependent dust extinction values are calculated for a number of different dust refractive index databases, along with several values of the particle aspect ratio, denoting the particle shape. The consequences for the infrared extinction values of both the particle shape and the particle orientation are explored: this analysis shows that as the particle asphericity increases, the extinctions increase if the particles are aligned horizontally, and decrease if they are aligned vertically. Randomly oriented spheroidal particles have very similar infrared extinction properties as spherical particles, whereas the horizontally and vertically aligned particles can be considered to be the upper and lower bounds on the extinction values. Inputting these values into COSMO-MUSCAT-RTTOV, it is found that spherical particles do not appear to be sufficient to describe fully the resultant colour of the dust in the infrared imagery. Comparisons

Journal article

Heritier KL, Altwegg K, Berthelier J-J, Beth A, Carr CM, De Keyser J, Eriksson AI, Fuselier SA, Galand M, Gombosi TI, Henri P, Johansson FL, Nilsson H, Rubin M, Wedlund CS, Taylor MGGT, Vigren Eet al., 2018,

On the origin of molecular oxygen in cometary comae

, NATURE COMMUNICATIONS, Vol: 9, ISSN: 2041-1723

Journal article

Futaana Y, Barabash S, Wieser M, Wurz P, Hurley D, Mihaly H, Mall U, Andre N, Ivchenko N, Oberst J, Retherford K, Coates A, Masters A, Wahlund JE, Kallio E, SELMA proposal teamet al., 2018,

SELMA mission: how do airless bodies interact with space environment? The Moon as an accessible laboratory

, Planetary and Space Science, Vol: 156, Pages: 23-40, ISSN: 0032-0633

The Moon is an archetypal atmosphere-less celestial body in the Solar System. For such bodies, the environments are characterized by complex interaction among the space plasma, tenuous neutral gas, dust and the outermost layer of the surface. Here we propose the SELMA mission (Surface, Environment, and Lunar Magnetic Anomalies) to study how airless bodies interact with space environment. SELMA uses a unique combination of remote sensing via ultraviolet and infrared wavelengths, and energetic neutral atom imaging, as well as in situ measurements of exospheric gas, plasma, and dust at the Moon. After observations in a lunar orbit for one year, SELMA will conduct an impact experiment to investigate volatile content in the soil of the permanently shadowed area of the Shackleton crater. SELMA also carries an impact probe to sound the Reiner-Gamma mini-magnetosphere and its interaction with the lunar regolith from the SELMA orbit down to the surface. SELMA was proposed to the European Space Agency as a medium-class mission (M5) in October 2016. Research on the SELMA scientific themes is of importance for fundamental planetary sciences and for our general understanding of how the Solar System works. In addition, SELMA outcomes will contribute to future lunar explorations through qualitative characterization of the lunar environment and, in particular, investigation of the presence of water in the lunar soil, as a valuable resource to harvest from the lunar regolith.

Journal article

Menelaou K, Yau MK, Lai T-K, 2018,

Possible Three-Dimensional Mechanism for Oscillating Wobbles in Tropical Cyclone-Like Vortices with Concentric Eyewalls

, JOURNAL OF THE ATMOSPHERIC SCIENCES, Vol: 75, Pages: 2157-2174, ISSN: 0022-4928

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Journal article

Hanna E, Hall RJ, Cropper TE, Ballinger TJ, Wake L, Mote T, Cappelen Jet al., 2018,

Greenland blocking index daily series 1851-2015: Analysis of changes in extremes and links with North Atlantic and UK climate variability and change

, INTERNATIONAL JOURNAL OF CLIMATOLOGY, Vol: 38, Pages: 3546-3564, ISSN: 0899-8418

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Citations: 50

Journal article

Vasko IY, Mozer FS, Krasnoselskikh VV, Artemyev AV, Agapitov OV, Bale SD, Avanov L, Ergun R, Giles B, Lindqvist P-A, Russell CT, Strangeway R, Torbert Ret al., 2018,

Solitary Waves Across Supercritical Quasi-Perpendicular Shocks

, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 5809-5817, ISSN: 0094-8276

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Citations: 38

Journal article

Kasoar MR, Shawki D, Voulgarakis A, 2018,

Similar spatial patterns of global climate response to aerosols from different regions

, npj Climate and Atmospheric Science, Vol: 12, ISSN: 2397-3722

Anthropogenic aerosol forcing is spatially heterogeneous, mostly localised around industrialised regions like North America, Europe, East and South Asia. Emission reductions in each of these regions will force the climate in different locations, which could have diverse impacts on regional and global climate. Here, we show that removing sulphur dioxide (SO2) emissions from any of these northern-hemisphere regions in a global composition-climate model results in significant warming across the hemisphere, regardless of the emission region. Although the temperature response to these regionally localised forcings varies considerably in magnitude depending on the emission region, it shows a preferred spatial pattern independent of the location of the forcing. Using empirical orthogonal function analysis, we show that the structure of the response is tied to existing modes of internal climate variability in the model. This has implications for assessing impacts of emission reduction policies, and our understanding of how climate responds to heterogeneous forcings.

Journal article

Khatiwala S, Graven H, Payne S, Heimbach Pet al., 2018,

Changes to the air‐sea flux and distribution of radiocarbon in the ocean over the 21st century

, Geophysical Research Letters, Vol: 45, Pages: 5617-5626, ISSN: 0094-8276

We investigate the spatiotemporal evolution of radiocarbon (Δ14C) in the ocean over the 21st century under different scenarios for anthropogenic CO2 emissions and atmospheric CO2 and radiocarbon changes using a 3‐D ocean carbon cycle model. Strong decreases in atmospheric Δ14C in the high‐emission scenario result in strong outgassing of 14C over 2050–2100, causing Δ14C spatial gradients in the surface ocean and vertical gradients between the surface and intermediate waters to reverse sign. Surface Δ14C in the subtropical gyres is lower than Δ14C in Pacific Deep Water and Southern Ocean surface water in 2100. In the low‐emission scenario, ocean Δ14C remains slightly higher than in 1950 and relatively constant over 2050–2100. Over the next 20 years we find decadal changes in Δ14C of −30‰ to +5‰ in the upper 2 km of the ocean, which should be detectable with continued hydrographic surveys. Our simulations can help in planning future observations, and they provide a baseline for investigating natural or anthropogenic changes in ocean circulation using ocean Δ14C observations and models.

Journal article

Tang T, Shindell D, Samset BH, Boucher O, Forster PM, Hodnebrog O, Myhre G, Sillmann J, Voulgarakis A, Andrews T, Faluvegi G, Flaschner D, Iversen T, Kasoar M, Kharin V, Kirkevag A, Lamarque J-F, Olivie D, Richardson T, Stjern CW, Takemura Tet al., 2018,

Dynamical response of Mediterranean precipitation to greenhouse gases and aerosols

, ATMOSPHERIC CHEMISTRY AND PHYSICS, Vol: 18, Pages: 8439-8452, ISSN: 1680-7316

Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and, thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the beginning of the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare the modeled dynamical response of Mediterranean precipitation to individual forcing agents in a set of global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive sea level pressure (SLP) pattern similar to the North Atlantic Oscillation–Arctic Oscillation, characterized by higher SLP at midlatitudes and lower SLP at high latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901–2010, roughly one-third (31±17%) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs, whereas global scattering sulfate aerosols would have negligible impacts. Aerosol–cloud interactions appear to have minimal impacts on Mediterranean precipitation in these models, at least in part because many simulations did not fully include such processes; these merit further study. The findings from this study suggest that future BC and WMGHG emissions may significantly affect regional water resources, agricultural practices, ecosystems and

Journal article

Wang B, Nishimura Y, Hietala H, Lyons L, Angelopoulos V, Plaschke F, Ebihara Y, Weatherwax Aet al., 2018,

Impacts of Magnetosheath High-Speed Jets on the Magnetosphere and Ionosphere Measured by Optical Imaging and Satellite Observations

, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 4879-4894, ISSN: 2169-9380

Journal article

Coustenis A, Atreya S, Castillo-Rogez J, Mueller-Wodarg I, Spilker L, Strazzulla Get al., 2018,

Preface to the special issue of PSS on "Surfaces, atmospheres and magnetospheres of the outer planets, their satellites and ring systems: Part XII"

, PLANETARY AND SPACE SCIENCE, Vol: 155, Pages: 1-1, ISSN: 0032-0633

Journal article

Breuillard H, Matteini L, Argall MR, Sahraoui F, Andriopoulou M, Le Contel O, Retino A, Mirioni L, Huang SY, Gershman DJ, Ergun RE, Wilder FD, Goodrich KA, Ahmadi N, Yordanova E, Vaivads A, Turner DL, Khotyaintsev YV, Graham DB, Lindqvist P-A, Chasapis A, Burch JL, Torbert RB, Russell CT, Magnes W, Strangeway RJ, Plaschke F, Moore TE, Giles BL, Paterson WR, Pollock CJ, Lavraud B, Fuselier SA, Cohen IJet al., 2018,

New Insights into the Nature of Turbulence in the Earth's Magnetosheath Using Magnetospheric MultiScale Mission Data

, ASTROPHYSICAL JOURNAL, Vol: 859, ISSN: 0004-637X

The Earth's magnetosheath, which is characterized by highly turbulent fluctuations, is usually divided into two regions of different properties as a function of the angle between the interplanetary magnetic field and the shock normal. In this study, we make use of high-time resolution instruments on board the Magnetospheric MultiScale spacecraft to determine and compare the properties of subsolar magnetosheath turbulence in both regions, i.e., downstream of the quasi-parallel and quasi-perpendicular bow shocks. In particular, we take advantage of the unprecedented temporal resolution of the Fast Plasma Investigation instrument to show the density fluctuations down to sub-ion scales for the first time. We show that the nature of turbulence is highly compressible down to electron scales, particularly in the quasi-parallel magnetosheath. In this region, the magnetic turbulence also shows an inertial (Kolmogorov-like) range, indicating that the fluctuations are not formed locally, in contrast with the quasi-perpendicular magnetosheath. We also show that the electromagnetic turbulence is dominated by electric fluctuations at sub-ion scales (f > 1 Hz) and that magnetic and electric spectra steepen at the largest-electron scale. The latter indicates a change in the nature of turbulence at electron scales. Finally, we show that the electric fluctuations around the electron gyrofrequency are mostly parallel in the quasi-perpendicular magnetosheath, where intense whistlers are observed. This result suggests that energy dissipation, plasma heating, and acceleration might be driven by intense electrostatic parallel structures/waves, which can be linked to whistler waves.

Journal article

Weiss Z, Steers EBM, Pickering JC, 2018,

Transition rate diagrams and excitation of titanium in a glow discharge in argon and neon

, SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY, Vol: 144, Pages: 20-28, ISSN: 0584-8547

Journal article

Chen L-J, Wang S, Wilson LB, Schwartz S, Bessho N, Moore T, Gershman D, Giles B, Malaspina D, Wilder FD, Ergun RE, Hesse M, Lai H, Russell C, Strangeway R, Torbert RB, F-Vinas A, Burch J, Lee S, Pollock C, Dorelli J, Paterson W, Ahmadi N, Goodrich K, Lavraud B, Le Contel O, Khotyaintsev YV, Lindqvist P-A, Boardsen S, Wei H, Le A, Avanov Let al., 2018,

Electron Bulk Acceleration and Thermalization at Earth's Quasiperpendicular Bow Shock.

, Phys Rev Lett, Vol: 120

Electron heating at Earth's quasiperpendicular bow shock has been surmised to be due to the combined effects of a quasistatic electric potential and scattering through wave-particle interaction. Here we report the observation of electron distribution functions indicating a new electron heating process occurring at the leading edge of the shock front. Incident solar wind electrons are accelerated parallel to the magnetic field toward downstream, reaching an electron-ion relative drift speed exceeding the electron thermal speed. The bulk acceleration is associated with an electric field pulse embedded in a whistler-mode wave. The high electron-ion relative drift is relaxed primarily through a nonlinear current-driven instability. The relaxed distributions contain a beam traveling toward the shock as a remnant of the accelerated electrons. Similar distribution functions prevail throughout the shock transition layer, suggesting that the observed acceleration and thermalization is essential to the cross-shock electron heating.

Journal article

Graven H, Fischer ML, Lueker T, Jeong S, Guilderson TP, Keeling RF, Bambha R, Brophy K, Callahan W, Cui X, Frankenberg C, Gurney K, LaFranchi BW, Lehman SJ, Michelson H, Miller JB, Newman S, Paplawsky W, Parazoo NC, Sloop C, Walker SJet al., 2018,

Assessing fossil fuel CO₂ emissions in California using atmospheric observations and models

, Environmental Research Letters, Vol: 13, ISSN: 1748-9326

Analysis systems incorporating atmospheric observations could provide a powerful tool for validating fossil fuel CO2 (ffCO2) emissions reported for individual regions, provided that fossil fuel sources can be separated from other CO2 sources or sinks and atmospheric transport can be accurately accounted for. We quantified ffCO2 by measuring radiocarbon (14C) in CO2, an accurate fossil-carbon tracer, at nine observation sites in California for three months in 2014–15. There is strong agreement between the measurements and ffCO2 simulated using a high-resolution atmospheric model and a spatiotemporally-resolved fossil fuel flux estimate. Inverse estimates of total in-state ffCO2 emissions are consistent with the California Air Resources Board's reported ffCO2 emissions, providing tentative validation of California's reported ffCO2 emissions in 2014–15. Continuing this prototype analysis system could provide critical independent evaluation of reported ffCO2 emissions and emissions reductions in California, and the system could be expanded to other, more data-poor regions.

Journal article

Dougherty MK, Spilker LJ, 2018,

Review of Saturn's icy moons following the Cassini mission

, Reports on Progress in Physics, Vol: 81, ISSN: 0034-4885

We review our knowledge of the icy moons of Saturn prior to the Cassini orbital mission, describe the discoveries made by the instrumentation onboard the Cassini spacecraft.

Journal article

Liu L, Shawki D, Voulgarakis A, Kasoar M, Samset B, Myhre G, Forster P, Hodnebrog Ø, Sillmann J, Aalbergsjø S, Boucher O, Faluvegi G, Iversen T, Kirkevag A, Lamarque J-F, Olivie D, Richardson T, Shindell D, Takemura Tet al., 2018,

A PDRMIP multi-model study on the impacts of regional aerosol forcings on global and regional precipitation

, Journal of Climate, Vol: 31, Pages: 4429-4447, ISSN: 0894-8755

Atmospheric aerosols such as sulfate and black carbon (BC) generate inhomogeneous radiative forcing and can affect precipitation in distinct ways compared to greenhouse gases (GHGs). Their regional effects on the atmospheric energy budget and circulation can be important for understanding and predicting global and regional precipitation changes, which act on top of the background GHG-induced hydrological changes. Under the framework of the Precipitation Driver Response Model Inter-comparison Project (PDRMIP), multiple models were used for the first time to simulate the influence of regional (Asian and European) sulfate and BC forcing on global and regional precipitation. The results show that, as in the case of global aerosol forcing, the global fast precipitation response to regional aerosol forcing scales with global atmospheric absorption, and the slow precipitation response scales with global surface temperature response. Asian sulphate aerosols appear to be a stronger driver of global temperature and precipitation change compared to European aerosols, but when the responses are normalised by unit radiative forcing or by aerosol burden change, the picture reverses, with European aerosols being more efficient in driving global change. The global apparent hydrological sensitivities of these regional forcing experiments are again consistent with those for corresponding global aerosol forcings found in the literature. However, the regional responses and regional apparent hydrological sensitivities do not align with the corresponding global values. Through a holistic approach involving analysis of the energy budget combined with exploring changes in atmospheric dynamics, we provide a framework for explaining the global and regional precipitation responses to regional aerosol forcing.

Journal article

Wilson LB, Stevens ML, Kasper JC, Klein KG, Maruca BA, Bale SD, Bowen TA, Pulupa MP, Salem CSet al., 2018,

The Statistical Properties of Solar Wind Temperature Parameters Near 1 au

, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 236, ISSN: 0067-0049

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Citations: 79

Journal article

Eastwood J, Mistry R, Phan TD, Schwartz SJ, Ergun RE, Drake JF, Oieroset M, Stawarz JE, Goldman MV, Haggerty C, Shay MA, Burch JL, Gershman DJ, Giles BL, LIndqvist PA, Torbert RB, Strangeway RJ, Russell CTet al., 2018,

Guide field reconnection: exhaust structure and heating

, Geophysical Research Letters, Vol: 45, Pages: 4569-4577, ISSN: 0094-8276

Magnetospheric Multiscale (MMS) observations are used to probe the structure and temperature profile of a guide field reconnection exhaust ~100 ion inertial lengths downstream from the X‐line in the Earth's magnetosheath. Asymmetric Hall electric and magnetic field signatures were detected, together with a density cavity confined near one edge of the exhaust and containing electron flow toward the X‐line. Electron holes were also detected both on the cavity edge and at the Hall magnetic field reversal. Predominantly parallel ion and electron heating was observed in the main exhaust but within the cavity, electron cooling and enhanced parallel ion heating was found. This is explained in terms of the parallel electric field, which inhibits electron mixing within the cavity on newly reconnected field lines, but accelerates ions. Consequently, guide field reconnection causes inhomogeneous changes in ion and electron temperature across the exhaust.

Journal article

Brophy K, Graven H, Manning AJ, White E, Arnold T, Fischer ML, Jeong S, Cui X, Rigby Met al., 2018,

Characterizing uncertainties in atmospheric inversions of fossil fuel CO2 emissions in California

, Atmospheric Chemistry and Physics Discussions, Pages: 1-44, ISSN: 1680-7367

Atmospheric inverse modelling has become an increasingly useful tool for evaluating emissions of greenhouse gases including methane, nitrous oxide, and synthetic gases such as hydrofluorocarbons (HFCs). Atmospheric inversions for emissions of CO2 from fossil fuel combustion (ffCO2) are currently being developed. The aim of this paper is to investigate potential errors and uncertainties related to the spatial and temporal prior representation of emissions and modelled atmospheric transport for the inversion of ffCO2 emissions in the US state of California. We perform simulation experiments based on a network of ground-based observations of CO2 concentration and radiocarbon in CO2 (a tracer of ffCO2), combining prior (bottom-up) emission models and transport models currently used in many atmospheric studies. The potential effect of errors in the spatial and temporal distribution of prior emission estimates is investigated in experiments by using perturbed versions of the emission estimates used to create the pseudo-data. The potential effect of transport error was investigated by using three different atmospheric transport models for the prior and pseudo-data simulations. We find that the magnitude of biases in posterior total state emissions arising from errors in the spatial and temporal distribution in prior emissions in these experiments are 1 %–15 % of posterior total state emissions and are generally smaller than the 2σ uncertainty in posterior emissions. Transport error in these experiments introduces biases of −10 % to +6 % into posterior total state emissions. Our results indicate that uncertainties in posterior total state ffCO2 estimates arising from the choice of prior emissions or atmospheric transport model are on the order of 15 % or less for the ground-based network in California we consider. We highlight the need for temporal variations to be included in prior emissions and for continuing efforts to

Journal article

Hunt GJ, Provan G, Bunce EJ, Cowley SWH, Dougherty MK, Southwood DJet al., 2018,

Field-aligned currents in Saturn’s magnetosphere: Observations from the F-ring orbits

, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 3806-3821, ISSN: 2169-9402

We investigate the azimuthal magnetic field signatures associated with high‐latitude field‐aligned currents observed during Cassini's F‐ring orbits (October 2016–April 2017). The overall ionospheric meridional current profiles in the northern and southern hemispheres, that is, the regions poleward and equatorward of the field‐aligned currents, differ most from the 2008 observations. We discuss these differences in terms of the seasonal change between data sets and local time (LT) differences, as the 2008 data cover the nightside while the F‐ring data cover the post‐dawn and dusk sectors in the northern and southern hemispheres, respectively. The F‐ring field‐aligned currents typically have a similar four current sheet structure to those in 2008. We investigate the properties of the current sheets and show that the field‐aligned currents in a hemisphere are modulated by that hemisphere's “planetary period oscillation” (PPO) systems. We separate the PPO‐independent and PPO‐related currents in both hemispheres using their opposite symmetry. The average PPO‐independent currents peak at ~1.5 MA/rad just equatorward of the open closed field line boundary, similar to the 2008 observations. However, the PPO‐related currents in both hemispheres are reduced by ~50% to ~0.4 MA/rad. This may be evidence of reduced PPO amplitudes, similar to the previously observed weaker equatorial oscillations at similar dayside LTs. We do not detect the PPO current systems' interhemispheric component, likely a result of the weaker PPO‐related currents and their closure within the magnetosphere. We also do not detect previously proposed lower latitude discrete field‐aligned currents that act to “turn off” the PPOs.

Journal article

Fujita R, Morimoto S, Umezawa T, Ishijima K, Patra PK, Worthy DEJ, Goto D, Aoki S, Nakazawa Tet al., 2018,

Temporal Variations of the Mole Fraction, Carbon, and Hydrogen Isotope Ratios of Atmospheric Methane in the Hudson Bay Lowlands, Canada

, Journal of Geophysical Research: Atmospheres, Vol: 123, Pages: 4695-4711, ISSN: 2169-897X

©2018. American Geophysical Union. All Rights Reserved. We have conducted simultaneous measurements of the mole fraction and carbon and hydrogen isotope ratios (δ13C and δD) of atmospheric methane (CH4) at Churchill (58°44′N, 93°49′W) in the northern part of the Hudson Bay Lowlands (HBL), Canada, since 2007. Compared with the measurements at an Arctic baseline monitoring station, Ny-Ålesund, Svalbard (78°55′N, 11°56′E), CH4 mole fraction is generally higher and δ13C and δD are lower at Churchill due to regional biogenic CH4 emissions. Clear seasonal cycles in the CH4 mole fraction, δ13C, and δD are observable at Churchill, and their seasonal phases in summer are earlier by approximately 2 weeks than those at Ny-Ålesund. Using the one-box model analysis, the phase difference is ascribed to the different seasonal influence of CH4 emissions from boreal wetlands on the two sites. Short-term CH4 variations are also observed at Churchill throughout the year. The analysis of the observed isotopic signatures of atmospheric CH4 confirmed that the short-term CH4 variations are mainly produced by biogenic CH4 released from the HBL wetlands in summer and by fossil fuel CH4 transported over the Arctic in winter. Forward simulations of an atmospheric chemistry-transport model, with wetland CH4 fluxes prescribed by a process-based model, show unrealistically high CH4 mole fractions at Churchill in summer, suggesting that CH4 emissions assigned to the HBL wetlands are overestimated. Our best estimate of the HBL CH4 emissions is 2.7 ± 0.3 Tg CH4 yr−1 as an average of 2007–2013, consistent with recent estimations by inverse modeling studies.

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Journal article

Hanna E, Fettweis X, Hall RJ, 2018,

Recent changes in summer Greenland blocking captured by none ofthe CMIP5 models

<jats:p>Abstract. Recent studies note a signiﬁcant increase in high-pressure blocking over the Greenland region (Greenland Blocking Index, GBI) in summer since the 1990s. Such a general circulation change, indicated by a negative trend in the North Atlantic Oscillation (NAO) index, is generally highlighted as a major driver of recent surface melt records observed on the Greenland Ice Sheet (GrIS). Here we compare reanalysis-based GBI records with those from the Coupled Model Intercomparison Project 5 (CMIP5) suite of global climate models over 1950–2100. We ﬁnd that the recent summer GBI increase lies well outside the range of modelled past reconstructions (Historical scenario) and future GBI projections (RCP4.5 and RCP8.5). The models consistently project a future decrease in GBI (linked to an increase in NAO), which highlights a likely key deﬁciency of current climate models if the recently-observed circulation changes continue to persist. Given well-established connections between atmospheric pressure over the Greenland region and air temperature and precipitation extremes downstream, e.g. over Northwest Europe, this brings into question the accuracy of simulated North Atlantic jet stream changes and resulting climatological anomalies over densely populated regions of northern Europe as well as of future projections of GrIS mass balance produced using global and regional climate models. </jats:p>

Journal article

Hanna E, Fettweis X, Hall RJ, 2018,

Supplementary material to &quot;Recent changes in summer Greenland blocking captured by none ofthe CMIP5 models&quot;

Journal article

Myhre G, Samset BH, Hodnebrog Ø, Andrews T, Boucher O, Faluvegi G, Fläschner D, Forster PM, Kasoar M, Kharin V, Kirkevåg A, Lamarque J-F, Olivié D, Richardson TB, Shawki D, Shindell D, Shine KP, Stjern CW, Takemura T, Voulgarakis Aet al., 2018,

Sensible heat has significantly affected the global hydrological cycle over the historical period

, Nature Communications, Vol: 9, ISSN: 2041-1723

Globally, latent heating associated with a change in precipitation is balanced by changes to atmospheric radiative cooling and sensible heat fluxes. Both components can be altered by climate forcing mechanisms and through climate feedbacks, but the impacts of climate forcing and feedbacks on sensible heat fluxes have received much less attention. Here we show, using a range of climate modelling results, that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since preindustrial time, because the radiative impact of forcings and feedbacks approximately compensate. The model results show a dissimilar influence on sensible heat and precipitation from various drivers of climate change. Due to its strong atmospheric absorption, black carbon is found to influence the sensible heat very differently compared to other aerosols and greenhouse gases. Our results indicate that this is likely caused by differences in the impact on the lower tropospheric stability.

Journal article

Hunt GJ, Provan G, Cowley SWH, Dougherty MK, Southwood DJet al., 2018,

Saturn's planetary period oscillations during the closest approach of Cassini's ring grazing orbits

, Geophysical Research Letters, Vol: 45, Pages: 4692-4700, ISSN: 0094-8276

Saturn's planetary period oscillations (PPOs) are ubiquitous throughout its magnetosphere. We investigate the PPO's azimuthal magnetic field amplitude interior to the field‐aligned currents, during the closest approaches of Cassini's ring‐grazing orbits (October 2016 to April 2017), with periapses at ~2.5 RS. The amplitudes of the northern and southern PPO systems are shown to vary as a function of latitude. The amplitude ratio between the two PPO systems shows that the northern system is dominant by a factor of ~1.3 in the equatorial plane, and it is dominant to ~ −15° latitude in the southern hemisphere. The dayside amplitudes are approximately half of the 2008 nightside amplitudes, which agree with previous local time‐related amplitude observations. Overall, there is clear evidence that the PPOs are present on field lines that map to the outer edge of Saturn's rings, closer to Saturn than previously confirmed.

Journal article

Phan TD, Eastwood JP, Shay MA, Drake JF, Sonnerup BUO, Fujimoto M, Cassak PA, Øieroset M, Burch JL, Torbert RB, Rager AC, Dorelli JC, Gershman DJ, Pollock C, Pyakurel PS, Haggerty CC, Khotyaintsev Y, Lavraud B, Saito Y, Oka M, Ergun RE, Retino A, Le Contel O, Argall MR, Giles BL, Moore TE, Wilder FD, Strangeway RJ, Russell CT, Lindqvist PA, Magnes Wet al., 2018,

Electron magnetic reconnection without ion coupling in Earth's turbulent magnetosheath

, Nature, Vol: 557, Pages: 202-206, ISSN: 0028-0836

Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region 1,2 . On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed 3-5 . Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region 6 . In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales 7-11 . However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth's turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.

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