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Journal articleGoldstraw EE, Hood AW, Browning PK, et al., 2018,
Comparison of methods for modelling coronal magnetic fields
, Astronomy and Astrophysics, Vol: 610, ISSN: 0004-6361Aims. Four different approximate approaches used to model the stressing of coronal magnetic fields due to an imposed photospheric motion are compared with each other and the results from a full time-dependent magnetohydrodynamic (MHD) code. The assumptions used for each of the approximate methods are tested by considering large photospheric footpoint displacements.Methods. We consider a simple model problem, comparing the full non-linear MHD, determined with the Lare2D numerical code, with four approximate approaches. Two of these, magneto-frictional relaxation and a quasi-1D Grad-Shafranov approach, assume sequences of equilibria, whilst the other two methods, a second-order linearisation of the MHD equations and Reduced MHD, are time dependent.Results. The relaxation method is very accurate compared to full MHD for force-free equilibria for all footpoint displacements, but has significant errors when the plasma β0 is of order unity. The 1D approach gives an extremely accurate description of the equilibria away from the photospheric boundary layers, and agrees well with Lare2D for all parameter values tested. The linearised MHD equations correctly predict the existence of photospheric boundary layers that are present in the full MHD results. As soon as the footpoint displacement becomes a significant fraction of the loop length, the RMHD method fails to model the sequences of equilibria correctly. The full numerical solution is interesting in its own right, and care must be taken for low β0 plasmas if the viscosity is too high.
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Journal articleBowen TA, Badman S, Hellinger P, et al., 2018,
Density Fluctuations in the Solar Wind Driven by Alfven Wave Parametric Decay
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 854, ISSN: 2041-8205- Author Web Link
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- Citations: 25
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Journal articleBeth A, Galand MIF, 2018,
Effects of the convective field on weakly outgassing comets.
, Monthly Notices of the Royal Astronomical Society, Vol: 469, Pages: S824-S841, ISSN: 0035-8711By applying a kinetic approach, we have developed two models in order to assess the influence of one main driver of plasma acceleration, the convective electric field, on the cometary ion distribution at 67P/Churyumov-Gerasimenko (67P/C-G). This electric field is carried by the solar wind and corresponds to the acceleration undergone by cometary ions ultimately picked up. We have quantified its contribution on ion number density and mean velocity profiles, supported by an intercomparison with the recent literature. We found that the ion number density should reflect a departure from the observed ∼1/r law. We discuss reasons for this discrepancy.
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Journal articleBall WT, Alsing J, Mortlock DJ, et al., 2018,
Continuous decline in lower stratospheric ozone offsets ozone layer recovery
, Atmospheric Chemistry and Physics Discussions, Vol: 18, Pages: 1379-1394, ISSN: 1680-7367Ozone forms in the Earth's atmosphere from the photodissociation of molecular oxygen, primarily in the tropical stratosphere. It is then transported to the extratropics by the Brewer–Dobson circulation (BDC), forming a protective "ozone layer" around the globe. Human emissions of halogen-containing ozone-depleting substances (hODSs) led to a decline in stratospheric ozone until they were banned by the Montreal Protocol, and since 1998 ozone in the upper stratosphere is rising again, likely the recovery from halogen-induced losses. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60°S and 60°N outside the polar regions (60–90°). Here we report evidence from multiple satellite measurements that ozone in the lower stratosphere between 60°S and 60°N has indeed continued to decline since 1998. We find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60°S and 60°N. We find that total column ozone between 60°S and 60°N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and thus the causes now urgently need to be established.
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Journal articleBall WT, Alsing J, Mortlock DJ, et al., 2018,
Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery
, Atmospheric Chemistry and Physics, Vol: 18, Pages: 1379-1394, ISSN: 1680-7316Ozone forms in the Earth's atmosphere from the photodissociation of molecular oxygen, primarily in the tropical stratosphere. It is then transported to the extratropics by the Brewer–Dobson circulation (BDC), forming a protective "ozone layer" around the globe. Human emissions of halogen-containing ozone-depleting substances (hODSs) led to a decline in stratospheric ozone until they were banned by the Montreal Protocol, and since 1998 ozone in the upper stratosphere is rising again, likely the recovery from halogen-induced losses. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60° S and 60° N outside the polar regions (60–90°). Here we report evidence from multiple satellite measurements that ozone in the lower stratosphere between 60° S and 60° N has indeed continued to decline since 1998. We find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60° S and 60° N. We find that total column ozone between 60° S and 60° N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and thus the causes now urgently need to be established.
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Journal articleAkhavan-Tafti M, Slavin JA, Le G, et al., 2018,
MMS examination of FTEs at the earth's subsolar magnetopause
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 1224-1241, ISSN: 2169-9380Determining the magnetic field structure, electric currents, and plasma distributions within flux transfer event (FTE)-type flux ropes is critical to the understanding of their origin, evolution, and dynamics. Here the Magnetospheric Multiscale mission's high-resolution magnetic field and plasma measurements are used to identify FTEs in the vicinity of the subsolar magnetopause. The constant-α flux rope model is used to identify quasi-force free flux ropes and to infer the size, the core magnetic field strength, the magnetic flux content, and the spacecraft trajectories through these structures. Our statistical analysis determines a mean diameter of 1,700 ± 400 km (~30 ± 9 d i ) and an average magnetic flux content of 100 ± 30 kWb for the quasi-force free FTEs at the Earth's subsolar magnetopause which are smaller than values reported by Cluster at high latitudes. These observed nonlinear size and magnetic flux content distributions of FTEs appear consistent with the plasmoid instability theory, which relies on the merging of neighboring, small-scale FTEs to generate larger structures. The ratio of the perpendicular to parallel components of current density, R J , indicates that our FTEs are magnetically force-free, defined as R J < 1, in their core regions ( < 0.6 R flux rope ). Plasma density is shown to be larger in smaller, newly formed FTEs and dropping with increasing FTE size. It is also shown that parallel ion velocity dominates inside FTEs with largest plasma density. Field-aligned flow facilitates the evacuation of plasma inside newly formed FTEs, while their core magnetic field strengthens with increasing FTE size.
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Journal articleGershman DJ, F-Vinas A, Dorelli JC, et al., 2018,
Energy partitioning constraints at kinetic scales in low-beta turbulence
, PHYSICS OF PLASMAS, Vol: 25, ISSN: 1070-664X -
Journal articleCeppi P, Zappa G, Shepherd TG, et al., 2018,
Fast and slow components of the extratropical atmospheric circulation response to CO2 forcing
, Journal of Climate, Vol: 31, Pages: 1091-1105, ISSN: 0894-8755Poleward shifts of the extratropical atmospheric circulation are a common response to CO2 forcing in global climate models (GCMs), but little is known about the time dependence of this response. Here it is shown that in coupled climate models, the long-term evolution of sea surface temperatures (SSTs) induces two distinct time scales of circulation response to steplike CO2 forcing. In most GCMs from phase 5 of the Coupled Model Intercomparison Project as well as in the multimodel mean, all of the poleward shift of the midlatitude jets and Hadley cell edge occurs in a fast response within 5–10 years of the forcing, during which less than half of the expected equilibrium warming is realized. Compared with this fast response, the slow response over subsequent decades to centuries features stronger polar amplification (especially in the Antarctic), enhanced warming in the Southern Ocean, an El Niño–like pattern of tropical Pacific warming, and weaker land–sea contrast. Atmosphere-only GCM experiments demonstrate that the SST evolution drives the difference between the fast and slow circulation responses, although the direct radiative effect of CO2 also contributes to the fast response. It is further shown that the fast and slow responses determine the long-term evolution of the circulation response to warming in the representative concentration pathway 4.5 (RCP4.5) scenario. The results imply that shifts in midlatitude circulation generally scale with the radiative forcing, rather than with global-mean temperature change. A corollary is that time slices taken from a transient simulation at a given level of warming will considerably overestimate the extratropical circulation response in a stabilized climate.
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Journal articleNakamura R, Varsani A, Genestreti KJ, et al., 2018,
Multiscale Currents Observed by MMS in the Flow Braking Region
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 1260-1278, ISSN: 2169-9380- Author Web Link
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- Citations: 33
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Journal articleTang T, Shindell D, Samset BH, et al., 2018,
Mediterranean Precipitation Response to Greenhouse Gases andAerosols
<jats:p>Abstract. 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 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 Mediterranean precipitation responses to individual forcing agents in a set of state-of-the-art 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 North Atlantic Oscillation (NAO)/Arctic Oscillation (AO)-like sea level pressure (SLP) pattern, characterized by higher SLP at mid-latitudes 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 have negligible impacts. The results from this study suggest that future BC emissions may significantly affect regional water resources, agricultural practices, ecosystems, and the economy in the Mediterranean region. </jats:p>
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Journal articleFranci L, Landi S, Verdini A, et al., 2018,
Solar Wind Turbulent Cascade from MHD to Sub-ion Scales: Large-size 3D Hybrid Particle-in-cell Simulations
, ASTROPHYSICAL JOURNAL, Vol: 853, ISSN: 0004-637X -
Journal articleYang L, Wang L, Li G, et al., 2018,
The Strongest Acceleration of >40 keV Electrons by ICME-driven Shocks at 1 au
, ASTROPHYSICAL JOURNAL, Vol: 853, ISSN: 0004-637X- Author Web Link
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- Citations: 9
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Journal articleBroll JM, Fuselier SA, Trattner KJ, et al., 2018,
MMS Observation of Shock-Reflected He<SUP>++</SUP> at Earth's Quasi-Perpendicular Bow Shock
, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 49-55, ISSN: 0094-8276- Author Web Link
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- Citations: 7
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Journal articleBruneau N, Toumi R, Wang S, 2018,
Impact of wave whitecapping on land falling tropical cyclones
, Scientific Reports, Vol: 8, ISSN: 2045-2322Predicting tropical cyclone structure and evolution remains challenging. Particularly, the surface wave interactions with thecontinental shelf and their impact on tropical cyclones have received very little attention. Through a series of state-of-the-arthigh-resolution, fully-coupled ocean-wave and atmosphere-ocean-wave experiments, we show here, for the first time, thatin presence of continental shelf waves can cause substantial cooling of the sea surface. Through whitecapping there is atransfer of momentum from the surface which drives deeper vertical mixing. It is the waves and not just the wind which becomethe major driver of stratified coastal ocean ahead-of-cyclone cooling. In the fully-coupled atmosphere-ocean-wave model anegative feedback is found. The maximum wind speed is weaker and the damaging footprint area of hurricane-force winds isreduced by up to 50% due to the strong wave induced ocean cooling ahead. Including wave-ocean coupling is important toimprove land falling tropical cyclone intensity predictions for the highly populated and vulnerable coasts.
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Conference paperZabori B, Hirn A, Eastwood J, et al., 2018,
Space radiation and magnetic field environment specification for the Radcube space weather related CubeSat mission
, ISSN: 0074-1795To study space weather environment in space, as a first step, it is necessary to develop and establish an advanced, real-time monitoring system. Such a monitoring system may be able to provide scientific data on space radiation (electron and proton spectra, flux of heavier ions) and the status of the magnetosphere in order to gain the possibility for a reliable forecast capability. The expansion of the CubeSat/SmallSat industry will make it possible in the near future to launch orbital constellations with relevant, miniaturised instrumentation in order to study the space weather environment in near real-time. Thus the development of RADCUBE, a 3U CubeSat demonstration mission lead by a Hungarian company, called C3S LLC, for space weather monitoring purpose, has begun within the European Space Agency (ESA) CubeSat programme. As part of the development a new, combined, space weather monitoring instrument package (called RadMag) has been initiated at the Centre for Energy Research, Hungarian Academy of Sciences in the framework of ESA General Support Technology Programme (GSTP) in collaboration with Imperial College London and Astronika. The RadMag measurement capabilities were defined by reconstructing the expected space radiation and magnetic field environments for different orbit scenarios. The space radiation environment was analyzed considering the following parameters: flux of Galactic Cosmic Rays, trapped protons and electrons, solar particle events, corresponding Linear Energy Transfer (LET) spectra and Total Ionizing Dose (TID) levels. The expected magnetic field environment was modeled with the IGRF2015 + Tsyganenko-96 model both for quiet and stormy conditions. This paper addresses the results of these radiation and magnetic field environment reconstruction and calculations for the different possible orbital parameters of the RADCUBE mission in order to characterise the expected performance of the RadMag instrument during the RADCUBE mission. An overview of
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Conference paperMurray JE, Pickering JC, Brindley H, et al., 2018,
Spectrally resolved radiative observations of the Earth in the Far-Infrared using the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS)
The Far-Infrared contributes up to 50% of the radiative emission from Earth to space, however, FIR observations are lacking. Satellite instruments are proposed or set for deployment, we discuss the practicalities of supporting these missions with TAFTS.
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Conference paperMurray JE, Brindley HE, Fox S, et al., 2018,
Far-Infrared emissivity of ice and snow: Resolving the paucity of observational data
Far-Infrared up-welling radiance measurements over Greenland have yielded the first estimates of the emissivity of ice and snow in this spectral region. I describe the complexity of undertaking such work and what the future holds.
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Journal articleSamset BH, Myhre G, Forster PM, et al., 2018,
Weak hydrological sensitivity to temperature change over land, independent of climate forcing
, npj Climate and Atmospheric Science, Vol: 1, ISSN: 2397-3722We present the global and regional hydrological sensitivity (HS) to surface temperature changes, for perturbations to CO2, CH4, sulfate and black carbon concentrations, and solar irradiance. Based on results from ten climate models, we show how modeled global mean precipitation increases by 2–3% per kelvin of global mean surface warming, independent of driver, when the effects of rapid adjustments are removed. Previously reported differences in response between drivers are therefore mainly ascribable to rapid atmospheric adjustment processes. All models show a sharp contrast in behavior over land and over ocean, with a strong surface temperature-driven (slow) ocean HS of 3–5%/K, while the slow land HS is only 0–2%/K. Separating the response into convective and large-scale cloud processes, we find larger inter-model differences, in particular over land regions. Large-scale precipitation changes are most relevant at high latitudes, while the equatorial HS is dominated by convective precipitation changes. Black carbon stands out as the driver with the largest inter-model slow HS variability, and also the strongest contrast between a weak land and strong sea response. We identify a particular need for model investigations and observational constraints on convective precipitation in the Arctic, and large-scale precipitation around the Equator.
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Journal articleTaylor SA, Coates AJ, Jones GH, et al., 2018,
Modeling, Analysis, and Interpretation of Photoelectron Energy Spectra at Enceladus Observed by Cassini
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 287-296, ISSN: 2169-9380 -
Conference paperKrupp N, Roussos E, Mitchell DG, et al., 2018,
Charged particle measurements in the Saturnian magnetosphere during the Cassini era 2004-2017
, Symposium Celebrating Prof. Wing-Huen Ip's 70th Birthday - Serendipities in the Solar System and Beyond, Publisher: ASTRONOMICAL SOC PACIFIC, Pages: 163-175, ISSN: 1050-3390 -
Conference paperSergis N, Bunce EJ, Carbary JF, et al., 2018,
The Ring Current of Saturn
, AGU Chapman Conference on Currents in Geospace and Beyond, Publisher: AMER GEOPHYSICAL UNION, Pages: 139-154, ISSN: 0065-8448- Author Web Link
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- Citations: 5
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Conference paperFranci L, Hellinger P, Guarrasi M, et al., 2018,
Three-dimensional simulations of solar wind turbulence with the hybrid code CAMELIA
, 12th International Conference on Numerical Modeling of Space Plasma Flows (ASTRONUM), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588- Author Web Link
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- Citations: 23
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Journal articleMejnertsen L, Eastwood J, Hietala H, et al., 2017,
Global MHD simulations of the Earth's bow shock shape and motion under variable solar wind conditions
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 259-271, ISSN: 2169-9380Empirical models of the Earth's bow shock are often used to place in situ measurements in context and to understand the global behavior of the foreshock/bow shock system. They are derived statistically from spacecraft bow shock crossings and typically treat the shock surface as a conic section parameterized according to a uniform solar wind ram pressure, although more complex models exist. Here a global magnetohydrodynamic simulation is used to analyze the variability of the Earth's bow shock under real solar wind conditions. The shape and location of the bow shock is found as a function of time, and this is used to calculate the shock velocity over the shock surface. The results are compared to existing empirical models. Good agreement is found in the variability of the subsolar shock location. However, empirical models fail to reproduce the two-dimensional shape of the shock in the simulation. This is because significant solar wind variability occurs on timescales less than the transit time of a single solar wind phase front over the curved shock surface. Empirical models must therefore be used with care when interpreting spacecraft data, especially when observations are made far from the Sun-Earth line. Further analysis reveals a bias to higher shock speeds when measured by virtual spacecraft. This is attributed to the fact that the spacecraft only observes the shock when it is in motion. This must be accounted for when studying bow shock motion and variability with spacecraft data.
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Journal articleDavies E, Masters A, Dougherty M, et al., 2017,
Swept Forward Magnetic Field Variability in High-Latitude Regions of Saturn's Magnetosphere
, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 12328-12337, ISSN: 2169-9380Swept forward field is the term given to configurations of magnetic field wherein the field lines deviate from the meridional planes of a planet in the direction of its rotation. Evidence is presented for swept-forward field configurations on Cassini orbits around Saturn from the first half of 2008. These orbits were selected on the basis of high inclination, spatial proximity, and temporal proximity, allowing for the observation of swept-forward field and resolution of dynamic effects using data from the Cassini magnetometer. Nine orbits are surveyed; all show evidence of swept-forward field, with typical sweep angle found to be 23°. Evidence is found for transient events that lead to temporary dramatic increases in sweep-forward angle. The Michigan Solar Wind Model is employed to investigate temporal correlation between the arrivals of solar wind shocks at Saturn with these transient events, with two shown to include instances corresponding with solar wind shock arrivals. Measurements of equatorial electron number density from anode 5 of the Cassini Plasma Spectrometer instrument are investigated for evidence of magnetospheric compression, corresponding with predicted shock arrivals. Potential mechanisms for the transfer of momentum from the solar wind to the magnetosphere are discussed.
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Journal articleMackie A, Palmer PI, Brindley H, 2017,
Characterizing energy budget variability at a Sahelian site: a test of NWP model behaviour
, Atmospheric Chemistry and Physics, Vol: 17, Pages: 15095-15119, ISSN: 1680-7316We use observations of surface and top-of-theatmosphere(TOA) broadband radiation fluxes determinedfrom the Atmospheric Radiation Measurement programmemobile facility, the Geostationary Earth Radiation Budget(GERB) and Spinning Enhanced Visible and Infrared Imager(SEVIRI) instruments and a range of meteorologicalvariables at a site in the Sahel to test the ability of theECMWF Integrated Forecasting System cycle 43r1 to describeenergy budget variability. The model has daily averagebiases of −12 and 18 W m−2for outgoing longwaveand reflected shortwave TOA radiation fluxes, respectively.At the surface, the daily average bias is 12(13) W m−2for the longwave downwelling (upwelling) radiation fluxand −21(−13) W m−2for the shortwave downwelling (upwelling)radiation flux. Using multivariate linear models ofobservation–model differences, we attribute radiation fluxdiscrepancies to physical processes, and link surface andTOA fluxes. We find that model biases in surface radiationfluxes are mainly due to a low bias in ice water path (IWP),poor description of surface albedo and model–observationdifferences in surface temperature. We also attribute observeddiscrepancies in the radiation fluxes, particularly duringthe dry season, to the misrepresentation of aerosol fieldsin the model from use of a climatology instead of a dynamicapproach. At the TOA, the low IWP impacts the amount ofreflected shortwave radiation while biases in outgoing longwaveradiation are additionally coupled to discrepancies inthe surface upwelling longwave flux and atmospheric humidity
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Journal articleCeppi P, Gregory JM, 2017,
Relationship of tropospheric stability to climate sensitivity and Earth's observed radiation budget
, Proceedings of the National Academy of Sciences of the United States of America, Vol: 114, Pages: 13126-13131, ISSN: 0027-8424Climate feedbacks generally become smaller in magnitude over time under CO2 forcing in coupled climate models, leading to an increase in the effective climate sensitivity, the estimated global-mean surface warming in steady state for doubled CO2. Here, we show that the evolution of climate feedbacks in models is consistent with the effect of a change in tropospheric stability, as has recently been hypothesized, and the latter is itself driven by the evolution of the pattern of sea-surface temperature response. The change in climate feedback is mainly associated with a decrease in marine tropical low cloud (a more positive shortwave cloud feedback) and with a less negative lapse-rate feedback, as expected from a decrease in stability. Smaller changes in surface albedo and humidity feedbacks also contribute to the overall change in feedback, but are unexplained by stability. The spatial pattern of feedback changes closely matches the pattern of stability changes, with the largest increase in feedback occurring in the tropical East Pacific. Relationships qualitatively similar to those in the models among sea-surface temperature pattern, stability, and radiative budget are also found in observations on interannual time scales. Our results suggest that constraining the future evolution of sea-surface temperature patterns and tropospheric stability will be necessary for constraining climate sensitivity.
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Journal articleChadney JM, Koskinen TT, Galand M, et al., 2017,
Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b
, Astronomy and Astrophysics, Vol: 608, ISSN: 0004-6361Stellar flares are a frequent occurrence on young low-mass stars around whichmany detected exoplanets orbit. Flares are energetic, impulsive events, andtheir impact on exoplanetary atmospheres needs to be taken into account wheninterpreting transit observations. We have developed a model to describe theupper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. Themodel simulates thermal escape from the upper atmospheres of close-in EGPs.Ionisation by solar radiation and electron impact is included and photochemicaland diffusive transport processes are simulated. This model is used to studythe effect of stellar flares from the solar-like G star HD209458 and the youngK star HD189733 on their respective planets. A hypothetical HD209458b-likeplanet orbiting the active M star AU Mic is also simulated. We find that theneutral upper atmosphere of EGPs is not significantly affected by typicalflares. Therefore, stellar flares alone would not cause large enough changes inplanetary mass loss to explain the variations in HD189733b transit depth seenin previous studies, although we show that it may be possible that an extremestellar proton event could result in the required mass loss. Our simulations dohowever reveal an enhancement in electron number density in the ionosphere ofthese planets, the peak of which is located in the layer where stellar X-raysare absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flarelevels and enhanced electron densities last from about 3 to 10 hours after theonset of the flare. The strength of the flare and the width of its spectralenergy distribution affect the range of altitudes that see enhancements inionisation. A large broadband continuum component in the XUV portion of theflaring spectrum in very young flare stars, such as AU Mic, results in a broadrange of altitudes affected in planets orbiting this star.
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Journal articleGraven H, Allison CE, Etheridge DM, et al., 2017,
Compiled records of carbon isotopes in atmospheric CO2 for historical simulations in CMIP6
, Geoscientific Model Development, Vol: 10, Pages: 4405-4417, ISSN: 1991-959XThe isotopic composition of carbon (Δ14C and δ13C) in atmospheric CO2 and in oceanic and terrestrial carbon reservoirs is influenced by anthropogenic emissions and by natural carbon exchanges, which can respond to and drive changes in climate. Simulations of 14C and 13C in the ocean and terrestrial components of Earth system models (ESMs) present opportunities for model evaluation and for investigation of carbon cycling, including anthropogenic CO2 emissions and uptake. The use of carbon isotopes in novel evaluation of the ESMs' component ocean and terrestrial biosphere models and in new analyses of historical changes may improve predictions of future changes in the carbon cycle and climate system. We compile existing data to produce records of Δ14C and δ13C in atmospheric CO2 for the historical period 1850–2015. The primary motivation for this compilation is to provide the atmospheric boundary condition for historical simulations in the Coupled Model Intercomparison Project 6 (CMIP6) for models simulating carbon isotopes in the ocean or terrestrial biosphere. The data may also be useful for other carbon cycle modelling activities.
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Journal articleTan X, Bao M, Hartmann DL, et al., 2017,
The role of synoptic waves in the formation and maintenance of the Western Hemisphere circulation pattern
, Journal of Climate, Vol: 30, Pages: 10259-10274, ISSN: 0894-8755Previous studies have demonstrated that the NAO, the leading mode of atmospheric low-frequency variability over the North Atlantic, could be linked to northeast Pacific climate variability via the downstream propagation of synoptic waves. In those studies, the NAO and the northeast Pacific climate variability are considered as two separate modes that explain the variance over the North Atlantic sector and the east Pacific–North American sector, respectively. A newly identified low-frequency atmospheric regime—the Western Hemisphere (WH) circulation pattern—provides a unique example of a mode of variability that accounts for variance over the whole North Atlantic–North American–North Pacific sector. The role of synoptic waves in the formation and maintenance of the WH pattern is investigated using the ECMWF reanalysis datasets. Persistent WH events are characterized by the propagation of quasi-stationary Rossby waves across the North Pacific–North American–North Atlantic regions and by associated storm-track anomalies. The eddy-induced low-frequency height anomalies maintain the anomalous low-frequency ridge over the Gulf of Alaska, which induces more equatorward propagation of synoptic waves on its downstream side. The eddy forcing favors the strengthening of the midlatitude jet and the deepening of the mid-to-high-latitude trough over the North Atlantic, whereas the deepening of the trough over eastern North America mostly arises from the quasi-stationary waves propagating from the North Pacific. A case study for the 2013/14 winter is examined to illustrate the downstream development of synoptic waves. The roles of synoptic waves in the formation and maintenance of the WH pattern and in linking the northeast Pacific ridge anomaly with the NAO are discussed.
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Journal articleRhodin AP, Belmonte MT, Engstrom L, et al., 2017,
Lifetime measurements and oscillator strengths in singly ionized scandium and the solar abundance of scandium
, Monthly Notices of the Royal Astronomical Society, Vol: 472, Pages: 3337-3353, ISSN: 0035-8711The lifetimes of 17 even-parity levels (3d5s, 3d4d, 3d6s and 4p2) in the region57 743–77 837 cm−1 of singly ionized scandium (Sc II) were measured by two-step timeresolvedlaser induced fluorescence spectroscopy. Oscillator strengths of 57 lines from thesehighly excited upper levels were derived using a hollow cathode discharge lamp and a Fouriertransform spectrometer. In addition, Hartree–Fock calculations where both the main relativisticand core-polarization effects were taken into account were carried out for both low- andhigh-excitation levels. There is a good agreement for most of the lines between our calculatedbranching fractions and the measurements of Lawler & Dakin in the region 9000–45 000 cm−1for low excitation levels and with our measurements for high excitation levels in the region23 500–63 100 cm−1. This, in turn, allowed us to combine the calculated branching fractionswith the available experimental lifetimes to determine semi-empirical oscillator strengths fora set of 380 E1 transitions in Sc II. These oscillator strengths include the weak lines that wereused previously to derive the solar abundance of scandium. The solar abundance of scandiumis now estimated to log = 3.04 ± 0.13 using these semi-empirical oscillator strengths toshift the values determined by Scott et al. The new estimated abundance value is in agreementwith the meteoritic value (logmet = 3.05 ± 0.02) of Lodders, Palme & Gail.
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