Publications

Journal article

Palmroth M, Hietala H, Plaschke F, Archer M, Karlsson T, Blanco-Cano X, Sibeck D, Kajdič P, Ganse U, Pfau-Kempf Y, Battarbee M, Turc Let al., 2018,

Magnetosheath jet properties and evolution as determined by a global hybrid-Vlasov simulation

, Annales Geophysicae: atmospheres, hydrospheres and space sciences, Vol: 36, Pages: 1171-1182, ISSN: 0992-7689

Abstract. We use a global hybrid-Vlasov simulation for the magnetosphere, Vlasiator, to investigate magnetosheath high-speed jets. Unlike many other hybrid-kinetic simulations, Vlasiator includes an unscaled geomagnetic dipole, indicating that the simulation spatial and temporal dimensions can be given without scaling. Thus, for the first time, this allows investigating the magnetosheath jet properties and comparing them directly with the observed jets within the Earth's magnetosheath. In the run shown in this paper, the interplanetary magnetic field (IMF) cone angle is 30°, and a foreshock develops upstream of the quasi-parallel magnetosheath. We visually detect a structure with high dynamic pressure propagating from the bow shock towards the magnetopause. The structure is confirmed as a jet using three different criteria, which have been adopted in previous observational studies. We compare these criteria against the simulation results. We find that the magnetosheath jet is an elongated structure extending Earthward of the bow shock by ~ 2.3 RE, while its size perpendicular to the direction of propagation is ~ 0.5 RE. We also investigate the jet evolution, and find that the jet originates due to the interaction of the foreshock Ultra Low Frequency (ULF) waves with the bow shock surface. The simulation shows that magnetosheath jets can develop also under steady IMF, as inferred by observational studies.

Journal article

Schillings A, Nilsson H, Slapak R, Wintoft P, Yamauchi M, Wik M, Dandouras I, Carr CMet al., 2018,

O+ Escape During the Extreme Space Weather Event of 4-10 September 2017

, Space Weather-the International Journal of Research and Applications, Vol: 16, Pages: 1363-1376, ISSN: 1539-4956

We have investigated the consequences of extreme space weather on ion outflow from the polar ionosphere by analyzing the solar storm that occurred early September 2017, causing a severe geomagnetic storm. Several X‐flares and coronal mass ejections were observed between 4 and 10 September. The first shock—likely associated with a coronal mass ejection—hit the Earth late on 6 September, produced a storm sudden commencement, and began the initial phase of the storm. It was followed by a second shock, approximately 24 hr later, that initiated the main phase and simultaneously the Dst index dropped to Dst = −142 nT and Kp index reached Kp = 8. Using COmposition DIstribution Function data on board Cluster satellite 4, we estimated the ionospheric O+ outflow before and after the second shock. We found an enhancement in the polar cap by a factor of 3 for an unusually high ionospheric O+ outflow (mapped to an ionospheric reference altitude) of 1013 m−2 s−1. We suggest that this high ionospheric O+ outflow is due to a preheating of the ionosphere by the multiple X‐flares. Finally, we briefly discuss the space weather consequences on the magnetosphere as a whole and the enhanced O+ outflow in connection with enhanced satellite drag.

Journal article

Masters A, 2018,

A more viscous-like solar wind interaction with all the giant planets

, Geophysical Research Letters, Vol: 45, Pages: 7320-7329, ISSN: 0094-8276

Identifying and quantifying the different drivers of energy ﬂow through a planetarymagnetosphere is crucial for understanding how each planetary system works. The magnetosphere of ourown planet is primarily driven externally by the solar wind through global magnetic reconnection, while aviscous-like interaction with the solar wind involving growth of the Kelvin-Helmholtz (K-H) instability is asecondary effect. Here we consider the solar wind-magnetosphere interaction at all magnetized planets,exploring the implications of diverse solar wind conditions. We show that with increasing distance fromthe Sun the electric ﬁelds arising from reconnection at the magnetopause boundary of a planetarymagnetosphere become weaker, whereas the boundaries become increasingly K-H unstable. Our resultssupport the possibility of a predominantly viscous-like interaction between the solar wind and every oneof the giant planet magnetospheres, as proposed by previous authors and in contrast with the solarwind-magnetosphere interaction at Earth.

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Journal article

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

Enceladus auroral hiss emissions during Cassini's grand finale

, Geophysical Research Letters, Vol: 45, Pages: 7347-7353, ISSN: 0094-8276

Cassini's Radio and Plasma Wave Science (RPWS) instrument detected intense auroral hiss emissions during one of its perikrone passes of the Grand Finale orbits. The emissions were detected when Cassini traversed a flux tube connected to Enceladus' orbit (L‐shell = 4) and at a time when both the spacecraft and the icy moon were in similar longitudes. Previous observations of auroral hiss related to Enceladus were made only during close flybys and here we present the first observation of such emissions close to Saturn. Further, ray‐tracing analysis shows the source location at a latitude of 63°, in excellent agreement with earlier UVIS observations of Enceladus' auroral footprint by Pryor et al. (2011, https://doi.org/10.1038/nature09928). The detection has been afforded exclusively by the Grand Finale phase, which enabled sampling of Enceladus' high‐latitude flux tube near Saturn. This result provides new insight into the spatial extent of the electrodynamic interaction between Saturn and Enceladus.

Journal article

Ryan E, Wild O, Voulgarakis A, Lee Let al., 2018,

Fast sensitivity analysis methods for computationally expensive models with multi-dimensional output

, GEOSCIENTIFIC MODEL DEVELOPMENT, Vol: 11, Pages: 3131-3146, ISSN: 1991-959X

Global sensitivity analysis (GSA) is a powerful approach in identifying which inputs or parameters most affect a model's output. This determines which inputs to include when performing model calibration or uncertainty analysis. GSA allows quantification of the sensitivity index (SI) of a particular input – the percentage of the total variability in the output attributed to the changes in that input – by averaging over the other inputs rather than fixing them at specific values. Traditional methods of computing the SIs using the Sobol and extended Fourier Amplitude Sensitivity Test (eFAST) methods involve running a model thousands of times, but this may not be feasible for computationally expensive Earth system models. GSA methods that use a statistical emulator in place of the expensive model are popular, as they require far fewer model runs. We performed an eight-input GSA, using the Sobol and eFAST methods, on two computationally expensive atmospheric chemical transport models using emulators that were trained with 80 runs of the models. We considered two methods to further reduce the computational cost of GSA: (1) a dimension reduction approach and (2) an emulator-free approach. When the output of a model is multi-dimensional, it is common practice to build a separate emulator for each dimension of the output space. Here, we used principal component analysis (PCA) to reduce the output dimension, built an emulator for each of the transformed outputs, and then computed SIs of the reconstructed output using the Sobol method. We considered the global distribution of the annual column mean lifetime of atmospheric methane, which requires ∼ 2000 emulators without PCA but only 5–40 emulators with PCA. We also applied an emulator-free method using a generalised additive model (GAM) to estimate the SIs using only the training runs. Compared to the emulator-only methods, the emulator–PCA and GAM methods accurately estimated the SIs

Journal article

Guo RL, Yao ZH, Wei Y, Ray LC, Rae IJ, Arridge CS, Coates AJ, Delamere PA, Sergis N, Kollmann P, Grodent D, Dunn WR, Waite JH, Burch JL, Pu ZY, Palmaerts B, Dougherty MKet al., 2018,

Rotationally driven magnetic reconnection in Saturn's dayside

, Nature Astronomy, Vol: 2, Pages: 640-645, ISSN: 2397-3366

Magnetic reconnection is a key process that explosively accelerates charged particles, generating phenomena such as nebular flares1, solar flares2 and stunning aurorae3. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisc, where thin-current-sheet conditions are conducive to reconnection4. The dayside magnetodisc is usually considered thicker than the nightside due to the compression of solar wind, and is therefore not an ideal environment for reconnection. In contrast, a recent statistical study of magnetic flux circulation strongly suggests that magnetic reconnection must occur throughout Saturn’s dayside magnetosphere5. Additionally, the source of energetic plasma can be present in the noon sector of giant planetary magnetospheres6. However, so far, dayside magnetic reconnection has only been identified at the magnetopause. Here, we report direct evidence of near-noon reconnection within Saturn’s magnetodisc using measurements from the Cassini spacecraft. The measured energetic electrons and ions (ranging from tens to hundreds of keV) and the estimated energy flux of ~2.6 mW m–2 within the reconnection region are sufficient to power aurorae. We suggest that dayside magnetodisc reconnection can explain bursty phenomena in the dayside magnetospheres of giant planets, which can potentially advance our understanding of quasi-periodic injections of relativistic electrons6 and auroral pulsations7.

Journal article

Wang S, Toumi R, 2018,

Reduced sensitivity of tropical cyclone intensity and size to sea surface temperature in a radiative-convective equilibrium environment

, Advances in Atmospheric Sciences, Vol: 35, Pages: 981-993, ISSN: 1861-9533

It has been challenging to project the tropical cyclone (TC) intensity, structure and destructive potential changes in a warming climate. Here, we compare the sensitivities of TC intensity, size and destructive potential to sea surface warming with and without a pre-storm atmospheric adjustment to an idealized state of Radiative-Convective Equilibrium (RCE). Without RCE, we find large responses of TC intensity, size and destructive potential to sea surface temperature (SST) changes, which is in line with some previous studies. However, in an environment under RCE, the TC size is almost insensitive to SST changes, and the sensitivity of intensity is also much reduced to 3% °C−1–4% °C−1. Without the pre-storm RCE adjustment, the mean destructive potential measured by the integrated power dissipation increases by about 25% °C−1 during the mature stage. However, in an environment under RCE, the sensitivity of destructive potential to sea surface warming does not change significantly. Further analyses show that the reduced response of TC intensity and size to sea surface warming under RCE can be explained by the reduced thermodynamic disequilibrium between the air boundary layer and the sea surface due to the RCE adjustment. When conducting regional-scale sea surface warming experiments for TC case studies, without any RCE adjustment the TC response is likely to be unrealistically exaggerated. The TC intensity–temperature sensitivity under RCE is very similar to those found in coupled climate model simulations. This suggests global mean intensity projections under climate change can be understood in terms of a thermodynamic response to temperature with only a minor contribution from any changes in large-scale dynamics.

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Journal article

Graven H, Zazzeri G, Acuña Yeomans E, 2018,

Global and regional emissions of radiocarbon from nuclear power plants from 1972 to 2016

, Radiocarbon, Vol: 60, Pages: 1067-1081, ISSN: 0033-8222

CH4 and CO2 emissions from geologic sources, which are devoid of radiocarbon (14C), dilute the atmospheric 14C/C ratio. Observations of 14C/C can be used to estimate fossil fuel-derived CH4 and CO2. However, the atmospheric 14C/C ratio is perturbed by emissions of 14C from nuclear power plants (NPPs) and fuel reprocessing sites, which may affect such 14C/C-based estimation if they are not correctly quantified. We calculate NPP 14C emissions for CO2 and CH4 from 1972–2016 using standard emission factors (14C emitted per unit of power produced) and analyze trends in global and regional emissions. We use available observations of 14C emissions and power generation in Europe to assess emission factors for different reactor types, as well as potential differences related to the age or manufacturer of the NPPs. Globally, nuclear 14C emissions increase until 2005 and then decrease, mostly because of the closure of gas-cooled reactors in the United Kindom and the shutdown of light water reactors after the Fukushima nuclear accident in March 2011. Observed emission factors in Europe show strong variability, spanning values from 0.003 to 2.521 TBq/GWa for PWR and from 0.007 to 1.732 TBq/GWa for BWR reactors, suggesting more information and more sophisticated models are needed to improve estimates of 14C emissions.

Journal article

Horbury TS, Matteini L, Stansby D, 2018,

Short, large-amplitude speed enhancements in the near-Sun fast solar wind

, Monthly Notices of the Royal Astronomical Society, Vol: 478, Pages: 1980-1986, ISSN: 0035-8711

We report the presence of intermittent, short discrete enhancements in plasma speed in the near-Sun high speed solar wind. Lasting tens of seconds to minutes in spacecraft measurements at 0.3 AU, speeds inside these enhancements can reach 1000 km/s, corresponding to a kinetic energy up to twice that of the bulk high speed solar wind. These events, which occur around 5% of the time, are Alfvenic in nature with large magnetic field deflections and are the same temperature as the surrounding plasma, in contrast to the bulk fast wind which has a well-established positive speed-temperature correlation. The origin of these speed enhancements is unclear but they may be signatures of discrete jets associated with transient events in the chromosphere or corona. Such large short velocity changes represent a measurement and analysis challenge for the upcoming Parker Solar Probe and Solar Orbiter missions.

Journal article

Gingell IL, Schwartz SJ, Gershman DJ, Paterson WR, Desai RT, Giles BL, Pollock CJ, Avanov LAet al., 2018,

Production of negative hydrogen ions within MMS Fast Plasma Investigation due to solar wind bombardment

, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 6161-6170, ISSN: 2169-9380

The particle data delivered by Fast Plasma Investigation (FPI) instrument aboard NASA's Magnetospheric Multiscale (MMS) mission allows for exceptionally high-resolution examination of the electron and ion phase space in the near-Earth plasma environment. It is necessary to identify populations which originate from instrumental effects. Using FPI's Dual Electron Spectrometers (DES) we isolate a high energy (~keV) beam, present while the spacecraft are in the solar wind, which exhibits an azimuthal drift with period associated with the spacecraft spin. We show that this population is consistent with negative hydrogen ions H- generated by a double charge exchange interaction between the incident solar wind H+ ions and the metallic surfaces within the instrument. This interaction is likely to occur at the deflector plates close to the instrument aperture. The H- density is shown to be approximately 0.2-0.4% of the solar wind ion density, and the energy of the negative ion population is shown to be 70% of the incident solar wind energy. These negative ions may introduce errors in electron velocity moments on the order of 0.2-0.4% of the solar wind velocity, and significantly higher errors in the electron temperature.

Journal article

Verscharen D, Klein KG, Chandran BDG, Stevens ML, Salem CS, Bale SDet al., 2018,

ALPS: the Arbitrary Linear Plasma Solver

, JOURNAL OF PLASMA PHYSICS, Vol: 84, ISSN: 0022-3778

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Citations: 37

Journal article

Plaschke F, Hietala H, Archer M, Blanco-Cano X, Kajdic P, Karlsson T, Lee SH, Omidi N, Palmroth M, Roytershteyn V, Schmid D, Sergeev V, Sibeck Det al., 2018,

Jets downstream of collisionless shocks

, Space Science Reviews, Vol: 214, ISSN: 0038-6308

The magnetosheath flow may take the form of large amplitude, yet spatially localized, transient increases in dynamic pressure, known as “magnetosheath jets” or “plasmoids” among other denominations. Here, we describe the present state of knowledge with respect to such jets, which are a very common phenomenon downstream of the quasi-parallel bow shock. We discuss their properties as determined by satellite observations (based on both case and statistical studies), their occurrence, their relation to solar wind and foreshock conditions, and their interaction with and impact on the magnetosphere. As carriers of plasma and corresponding momentum, energy, and magnetic flux, jets bear some similarities to bursty bulk flows, which they are compared to. Based on our knowledge of jets in the near Earth environment, we discuss the expectations for jets occurring in other planetary and astrophysical environments. We conclude with an outlook, in which a number of open questions are posed and future challenges in jet research are discussed.

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.

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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

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: 63

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.

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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.

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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

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.

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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.

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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.

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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

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

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

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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