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  • Journal article
    Dorfman S, Hietala H, Astfalk P, Angelopoulos Vet al., 2017,

    Growth rate measurement of ULF waves in the ion foreshock

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 44, Pages: 2120-2128, ISSN: 0094-8276
  • Journal article
    Volwerk M, Jones GH, Broiles T, Burch J, Carr C, Coates AJ, Cupido E, Delva M, Edberg NJT, Eriksson A, Goetz C, Goldstein R, Henri P, Madanian H, Nilsson H, Richter I, Schwingenschuh K, Wieser GS, Glassmeier K-Het al., 2017,

    Current sheets in comet 67P/Churyumov-Gerasimenko's coma

    , Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 3308-3321, ISSN: 2169-9402

    The Rosetta Plasma Consortium (RPC) data are used to investigate the presence of current sheets in the coma of comet 67P/Churyumov-Gerasimenko. The interaction of the interplanetary magnetic field (IMF) transported by the solar wind toward the outgassing comet consists amongst others of mass loading and field line draping near the nucleus. The draped field lines lead to so-called nested draping because of the constantly changing direction of the IMF. It is shown that the draping pattern is strongly variable over the period of one month. Nested draping results in neighbouring regions with oppositely directed magnetic fields, which are separated by current sheets. Selected events on 5 and 6 June 2015 are studied, which show that there are strong rotations of the magnetic field with associated current sheets that have strengths from several tens up to hundreds of nA/m2. Not all discussed current sheets show the characteristic peak in plasma density at the centre of the sheet, which might be related to the presence of a guide field. There is no evidence for different kinds of plasmas on either side of a current sheet, and no strongly accelerated ions have been observed which could have been an indication of magnetic reconnection in the current sheets.
  • Journal article
    Riley P, Ben-Nun M, Linker JA, Owens MJ, Horbury TSet al., 2017,

    Forecasting the properties of the solar wind using simple pattern recognition

    , SPACE WEATHER-THE INTERNATIONAL JOURNAL OF RESEARCH AND APPLICATIONS, Vol: 15, Pages: 526-540, ISSN: 1542-7390
  • Journal article
    Pulupa M, Bale SD, Bonnell JW, Bowen TA, Carruth N, Goetz K, Gordon D, Harvey PR, Maksimovic M, Martinez-Oliveros JC, Moncuquet M, Saint-Hilaire P, Seitz D, Sundkvist Det al., 2017,

    The solar probe plus radio frequency spectrometer: Measurement requirements, analog design, and digital signal processing

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 2836-2854, ISSN: 2169-9380
  • Journal article
    Hoilijoki S, Ganse U, Pfau-Kempf Y, Cassak PA, Walsh BM, Hietala H, von Alfthan S, Palmroth Met al., 2017,

    Reconnection rates and X line motion at the magnetopause: Global 2D-3V hybrid-Vlasov simulation results

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 2877-2888, ISSN: 2169-9380
  • Journal article
    Hietala H, Artemyev AV, Angelopoulos V, 2017,

    Ion dynamics in magnetotail reconnection in the presence of density asymmetry

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 2010-2023, ISSN: 2169-9380
  • Journal article
    Cahill B, Toumi R, Stenchikov G, Osipov S, Brindley Het al., 2017,

    Evaluation of thermal and dynamic impacts of summer dust aerosols on the Red Sea

    , Journal of Geophysical Research: Oceans, Vol: 122, Pages: 1325-1346, ISSN: 2169-9275

    The seasonal response of upper ocean processes in the Red Sea to summer-time dust aerosol perturbations is investigated using an uncoupled regional ocean model. We find that the upper limit response is highly sensitive to dust-induced reductions in radiative fluxes. Sea surface cooling of −1°C and −2°C is predicted in the northern and southern regions, respectively. This cooling is associated with a net radiation reduction of −40 W m−2 and −90 W m−2 over the northern and southern regions, respectively. Larger cooling occurs below the mixed layer at 75 m in autumn, −1.2°C (north) and −1.9°C (south). SSTs adjust more rapidly (ca. 30 days) than the subsurface temperatures (seasonal time scales), due to stronger stratification and increased mixed layer stability inhibiting the extent of vertical mixing. The basin average annual heat flux reverses sign and becomes positive, +4.2 W m−2 (as compared to observed estimates −17.3 W m−2) indicating a small gain of heat from the atmosphere. When we consider missing feedbacks from atmospheric processes in our uncoupled experiment, we postulate that the magnitude of cooling and the time scales for adjustment will be much less, and that the annual heat flux will not reverse sign but nevertheless be reduced as a result of dust perturbations. While our study highlights the importance of considering coupled ocean-atmosphere processes on the net surface energy flux in dust perturbation studies, the results of our uncoupled dust experiment still provide an upper limit estimate of the response of the upper ocean to dust-induced radiative forcing perturbations.
  • Journal article
    Wu Y, Han Y, Voulgarakis A, Wang T, Li M, Wang Y, Xie M, Zhuang B, Li Set al., 2017,

    An agricultural biomass burning episode in eastern China: transport, optical properties, and impacts on regional air quality

    , Journal of Geophysical Research: Atmospheres, Vol: 122, Pages: 2304-2324, ISSN: 2169-897X

    Agricultural biomass burning (ABB) has been of particular concern due to its influence on air quality and atmospheric radiation, as it produces large amounts of gaseous and aerosol emissions. This paper presents an integrated observation of a significant ABB episode in Nanjing, China, during early June 2011, using combined ground-based and satellite sensors (Moderate Resolution Imaging Spectroradiometer, Atmospheric Infrared Sounder, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), and Ozone Monitoring Instrument products). The time-height distribution, optical properties, sources and transport of smoke, and its impacts on air quality are investigated. Lidar profiles indicate that the smoke aerosols are confined to the planetary boundary layer (PBL) and have a depolarization ratio of less than 0.08. The aerosol optical depths increase from 0.5 to 3.0 at 500 nm, while the extinction-related Angstrom exponent increases from 1.1 to 1.6 at the wavelength pair of 440–870 nm. The single-scattering albedo becomes lower at 670–1020 nm following the ABB intrusion and particularly shows a decreasing tendency between wavelengths of 440 to 1020 nm. The absorption Angstrom exponent (0.7) is smaller than 1.0, which may indicate the aged smoke particles mixed or coated with the urban aerosols. Surface particular matter PM10 and PM2.5 show a dramatic increase, reaching hourly mean of 800 µg/m3 and 485 µg/m3, respectively, which results in a heavy air pollution event. The stagnant and high-moisture weather provides favorable conditions for the aerosols to accumulate near the surface. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) also illustrate that the large-scale aerosols are primarily present in the PBL and transported to the ocean, but some dense smoke plumes are misclassified as cloud or polluted dust. By comparing with the observations, we found that the
  • Journal article
    Khurana KK, Fatemi S, Lindkvist J, Roussos E, Krupp N, Holmstroem M, Russell CT, Dougherty MKet al., 2017,

    The role of plasma slowdown in the generation of Rhea's Alfven wings

    , Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 1778-1788, ISSN: 2169-9380

    Alfvén wings are known to form when a conducting or mass-loading object slows down a flowing plasma in its vicinity. Alfvén wings are not expected to be generated when an inert moon such as Rhea interacts with Saturn's magnetosphere, where the plasma impacting the moon is absorbed and the magnetic flux passes unimpeded through the moon. However, in two close polar passes of Rhea, Cassini clearly observed magnetic field signatures consistent with Alfvén wings. In addition, observations from a high-inclination flyby (Distance > 100 RRh) of Rhea on 3 June 2010 showed that the Alfvén wings continue to propagate away from Rhea even at this large distance. We have performed three-dimensional hybrid simulations of Rhea's interaction with Saturn's magnetosphere which show that the wake refilling process generates a plasma density gradient directed in the direction of corotating plasma. The resulting plasma pressure gradient exerts a force directed toward Rhea and slows down the plasma streaming into the wake along field lines. As on the same field lines, outside of the wake, the plasma continues to move close to its full speed, this differential motion of plasma bends the magnetic flux tubes, generating Alfvén wings in the wake. The current system excited by the Alfvén wings transfers momentum to the wake plasma extracting it from plasma outside the wake. Our work demonstrates that Alfvén wings can be excited even when a moon does not possess a conducting exosphere.
  • Journal article
    Sergis N, Jackman CM, Thomsen MF, Krimigis SM, Mitchell DG, Hamilton DC, Dougherty MK, Krupp N, Wilson RJet al., 2017,

    Radial and local time structure of the Saturnian ring current, revealed by Cassini

    , Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 1803-1815, ISSN: 2169-9380

    We analyze particle and magnetic field data obtained between July 2004 and December 2013 in the equatorial magnetosphere of Saturn, by the Cassini spacecraft. The radial and local time distribution of the total (thermal and suprathermal) particle pressure and total plasma beta (ratio of particle to magnetic pressure) over radial distances from 5 to 16 Saturn radii (RS = 60,258 km) is presented. The average azimuthal current density Jϕ and its separate components (inertial, pressure gradient, and anisotropy) are computed as a function of radial distance and local time and presented as equatorial maps. We explore the relative contribution of different physical mechanisms that drive the ring current at Saturn. Results show that (a) the particle pressure is controlled by thermal plasma inside of ~8 RS and by the hot ions beyond ~12 RS, exhibiting strong local time asymmetry with higher pressures measured at the dusk and night sectors; (b) the plasma beta increases with radial distance and remains >1 beyond 8–10 RS for all local times; (c) the ring current is asymmetric in local time and forms a maximum region between ~7 and ~13 RS, with values up to 100–115 pA/m2; and (d) the ring current is inertial everywhere inside of 7 RS, exhibits a mixed nature between 7 and 11 RS and is pressure gradient driven beyond 11 RS, with the exception of the noon sector where the mixed nature persists. In the dawn sector, it appears strongly pressure gradient driven for a wider range of radial distance, consistent with fast return flow of hot, tenuous magnetospheric plasma following tail reconnection.
  • Journal article
    Badia A, Jorba O, Voulgarakis A, Dabdub D, Perez Garcia-Pando C, Hilboll A, Goncalves M, Janjic Zet al., 2017,

    Description and evaluation of the Multiscale Online Nonhydrostatic AtmospheRe CHemistry model (NMMB-MONARCH) version 1.0: gas-phase chemistry at global scale

    , Geoscientific Model Development, Vol: 10, Pages: 609-638, ISSN: 1991-959X

    This paper presents a comprehensive description and benchmark evaluation of the tropospheric gas-phase chemistry component of the Multiscale Online Nonhydrostatic AtmospheRe CHemistry model (NMMB-MONARCH), formerly known as NMMB/BSC-CTM, that can be run on both regional and global domains. Here, we provide an extensive evaluation of a global annual cycle simulation using a variety of background surface stations (EMEP, WDCGG and CASTNET), ozonesondes (WOUDC, CMD and SHADOZ), aircraft data (MOZAIC and several campaigns), and satellite observations (SCIAMACHY and MOPITT). We also include an extensive discussion of our results in comparison to other state-of-the-art models. We note that in this study, we omitted aerosol processes and some natural emissions (lightning and volcano emissions).The model shows a realistic oxidative capacity across the globe. The seasonal cycle for CO is fairly well represented at different locations (correlations around 0.3–0.7 in surface concentrations), although concentrations are underestimated in spring and winter in the Northern Hemisphere, and are overestimated throughout the year at 800 and 500 hPa in the Southern Hemisphere.Nitrogen species are well represented in almost all locations, particularly NO2 in Europe (root mean square error – RMSE – below 5 ppb). The modeled vertical distributions of NOx and HNO3 are in excellent agreement with the observed values and the spatial and seasonal trends of tropospheric NO2 columns correspond well to observations from SCIAMACHY, capturing the highly polluted areas and the biomass burning cycle throughout the year. Over Asia, the model underestimates NOx from March to August, probably due to an underestimation of NOx emissions in the region. Overall, the comparison of the modeled CO and NO2 with MOPITT and SCIAMACHY observations emphasizes the need for more accurate emission rates from anthropogenic and biomass burning sources (i.e., specification of temporal variability).The
  • Journal article
    Sundberg T, Burgess D, Scholer M, Masters A, Sulaiman AHet al., 2017,

    The dynamics of very high Alfvén Mach number shocks in space plasmas

    , Astrophysical Journal Letters, Vol: 836, ISSN: 2041-8213

    Astrophysical shocks, such as planetary bow shocks or supernova remnant shocks, are often in the high or very-high Mach number regime, and the structure of such shocks is crucial for understanding particle acceleration and plasma heating, as well inherently interesting. Recent magnetic field observations at Saturn's bow shock, for Alfvén Mach numbers greater than about 25, have provided evidence for periodic non-stationarity, although the details of the ion- and electron-scale processes remain unclear due to limited plasma data. High-resolution, multi-spacecraft data are available for the terrestrial bow shock, but here the very high Mach number regime is only attained on extremely rare occasions. Here we present magnetic field and particle data from three such quasi-perpendicular shock crossings observed by the four-spacecraft Cluster mission. Although both ion reflection and the shock profile are modulated at the upstream ion gyroperiod timescale, the dominant wave growth in the foot takes place at sub-proton length scales and is consistent with being driven by the ion Weibel instability. The observed large-scale behavior depends strongly on cross-scale coupling between ion and electron processes, with ion reflection never fully suppressed, and this suggests a model of the shock dynamics that is in conflict with previous models of non-stationarity. Thus, the observations offer insight into the conditions prevalent in many inaccessible astrophysical environments, and provide important constraints for acceleration processes at such shocks.
  • Journal article
    Czaja A, Vanniere B, Dacre H, 2017,

    A "cold path" for Gulf Stream - troposphere connection

    , Journal of Climate, Vol: 30, Pages: 1363-1379, ISSN: 1520-0442

    The mechanism by which the Gulf Stream sea surface temperature (SST)front anchors a band of precipitation on its warm edge is still a matter of debateand little is known about how synoptic activity contributes to the meanstate. In the present study, the influence of the SST front on precipitationis investigated during the course of a single extratropical cyclone using a regionalconfiguration of the Met Office Unified Model. The comparison of acontrol run with a simulation in which SST gradients were smoothed broughtthe following conclusions: a band of precipitation is reproduced for a singleextratropical cyclone and the response to the SST gradient is dominated bya change of convective precipitation in the cold sector of the storm. Severalclimatological features described by previous studies, such as surface windconvergence on the warm edge or a meridional circulation cell across the SSTfront, are also reproduced at synoptic time scales in the cold sector. Based onthese results, a simple boundary layer model is proposed to explain the convectiveand dynamical response to the SST gradient in the cold sector. In thismodel, cold and dry air parcels acquire more buoyancy over a sharp SST gradientand become more convectively unstable. The convection sets a pressureanomaly over the entire depth of the boundary layer which drives wind convergence.This case study offers a new pathway by which the SST gradientcan anchor a climatological band of precipitation.
  • Journal article
    Gingell IL, Sorriso-Valvo L, Burgess D, de Vita G, Matteini Let al., 2017,

    Three dimensional simulations of sheared current sheets: transition to turbulence?

    , Journal of Plasma Physics, Vol: 83, ISSN: 1469-7807

    Systems of multiple current sheets arise in various situations in natural plasmas, such asat the heliospheric current sheet in the solar wind and in the outer heliosphere in theheliosheath. Previous three-dimensional simulations have shown that such systems candevelop turbulent-like fluctuations resulting from forward and inverse cascade in wavevector space. We present a study of the transition to turbulenceof such multiple currentsheet systems, including the effects of adding a magnetic guide field and velocity shearsacross the current sheets. Three-dimensional hybrid simulationsare performed of systemswith eight narrow current sheets in a triply-periodic geometry. We carry out a numberof different analyses of the evolution of the fluctuations as the initially highly orderedstate relaxes to one which resembles turbulence. Despite the evidence of forward andinverse cascade in the fluctuation power spectra, we find that none of the simulated caseshave evidence of intermittency after the initial period of fast reconnection associatedwith the ion tearing instability at the current sheets. Cancellation analysis confirms thatthe simulations have not evolved to a state which can be identified as fully developedturbulence. The addition of velocity shears across the current sheets slows the evolutionin the properties of the fluctuations, but by the end of the simulation they are broadlysimilar. However, if the simulation is constrained to be two-dimensional, differences arefound, indicating that fully three-dimensional simulations are important when studyingthe evolution of an ordered equilibrium towards a turbulent-like state.
  • Journal article
    Chen P, Wang T, Lu X, Yu Y, Kasoar M, Xie M, Zhuang Bet al., 2017,

    Source apportionment of size-fractionated particles during the 2013 Asian Youth Games and the 2014 Youth Olympic Games in Nanjing, China

    , SCIENCE OF THE TOTAL ENVIRONMENT, Vol: 579, Pages: 860-870, ISSN: 0048-9697
  • Journal article
    Eastwood J, Biffis E, Hapgood MA, Green L, Bisi MM, Bentley RD, Wicks R, McKinnell LA, Gibbs M, Burnett Cet al., 2017,

    The economic impact of space weather: where do we stand?

    , Risk Analysis, Vol: 37, Pages: 206-218, ISSN: 0272-4332

    Space weather describes the way in which the Sun, and conditions in space more generally, impact human activity and technology both in space and on the ground. It is now well understood that space weather represents a significant threat to infrastructure resilience, and is a source of risk that is wide‐ranging in its impact and the pathways by which this impact may occur. Although space weather is growing rapidly as a field, work rigorously assessing the overall economic cost of space weather appears to be in its infancy. Here, we provide an initial literature review to gather and assess the quality of any published assessments of space weather impacts and socioeconomic studies. Generally speaking, there is a good volume of scientific peer‐reviewed literature detailing the likelihood and statistics of different types of space weather phenomena. These phenomena all typically exhibit “power‐law” behavior in their severity. The literature on documented impacts is not as extensive, with many case studies, but few statistical studies. The literature on the economic impacts of space weather is rather sparse and not as well developed when compared to the other sections, most probably due to the somewhat limited data that are available from end‐users. The major risk is attached to power distribution systems and there is disagreement as to the severity of the technological footprint. This strongly controls the economic impact. Consequently, urgent work is required to better quantify the risk of future space weather events.
  • Journal article
    Han D-S, Hietala H, Chen X-C, Nishimura Y, Lyons LR, Liu J-J, Hu H-Q, Yang H-Get al., 2017,

    Observational properties of dayside throat aurora and implications on the possible generation mechanisms

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 1853-1870, ISSN: 2169-9380
  • Journal article
    Liu F, Fiencke C, Guo J, Rieth R, Dong R, Pfeiffer E-Met al., 2017,

    Performance evaluation and optimization of field-scale bioscrubbers for intensive pig house exhaust air treatment in northern Germany

    , Science of The Total Environment, Vol: 579, Pages: 694-701, ISSN: 0048-9697
  • Journal article
    Artemyev AV, Angelopoulos V, Hietala H, Runov A, Shinohara Iet al., 2017,

    Ion density and temperature profiles along (X-GSM) and across (Z(GSM)) the magnetotail as observed by THEMIS, Geotail, and ARTEMIS

    , JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 1590-1599, ISSN: 2169-9380
  • Journal article
    Heyn I, Block K, Mülmenstädt J, Gryspeerdt ERI, Kühne P, Salzmann M, Quaas Jet al., 2017,

    Assessment of simulated aerosol effective radiative forcings in the terrestrial spectrum

    , Geophysical Research Letters, Vol: 44, Pages: 1001-1007, ISSN: 1944-8007

    In its fifth assessment report (AR5), the Intergovernmental Panel on Climate Change provides a best estimate of the effective radiative forcing (ERF) due to anthropogenic aerosol at −0.9 W m−2. This value is considerably weaker than the estimate of −1.2 W m−2 in AR4. A part of the difference can be explained by an offset of +0.2 W m−2 which AR5 added to all published estimates that only considered the solar spectrum, in order to account for adjustments in the terrestrial spectrum. We find that, in the CMIP5 multimodel median, the ERF in the terrestrial spectrum is small, unless microphysical effects on ice- and mixed-phase clouds are parameterized. In the latter case it is large but accompanied by a very strong ERF in the solar spectrum. The total adjustments can be separated into microphysical adjustments (aerosol “effects”) and thermodynamic adjustments. Using a kernel technique, we quantify the latter and find that the rapid thermodynamic adjustments of water vapor and temperature profiles are small. Observation-based constraints on these model results are urgently needed.
  • Journal article
    Sorba AM, Achilleos NA, Guio P, Arridge CS, Pilkington NM, Masters A, Sergis N, Coates AJ, Dougherty MKet al., 2017,

    Modeling the compressibility of Saturn's magnetosphere in response to internal and external influences

    , Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 1572-1589, ISSN: 2169-9402

    The location of a planetary magnetopause is principally determined by the balance between solar wind dynamic pressure DP and magnetic and plasma pressures inside the magnetopause boundary. Previous empirical studies assumed that Saturn's magnetopause standoff distance varies as math formula and measured a constant compressibility parameter α corresponding to behavior intermediate between a vacuum dipole appropriate for Earth (α≈6) and a more easily compressible case appropriate for Jupiter (α≈4). In this study we employ a 2-D force balance model of Saturn's magnetosphere to investigate magnetospheric compressibility in response to changes in DP and global hot plasma content. For hot plasma levels compatible with Saturn observations, we model the magnetosphere at a range of standoff distances and estimate the corresponding DP values by assuming pressure balance across the magnetopause boundary. We find that for “average” hot plasma levels, our estimates of α are not constant with DP but vary from ∼4.8 for high DP conditions, when the magnetosphere is compressed (≤25 RS), to ∼3.5 for low DP conditions. This corresponds to the magnetosphere becoming more easily compressible as it expands. We find that the global hot plasma content influences magnetospheric compressibility even at fixed DP, with α estimates ranging from ∼5.4 to ∼3.3 across the range of our parameterized hot plasma content. We suggest that this behavior is predominantly driven by reconfiguration of the magnetospheric magnetic field into a more disk-like structure under such conditions. In a broader context, the compressibility of the magnetopause reveals information about global stress balance in the magnetosphere.
  • Journal article
    Fu HS, Vaivads A, Khotyaintsev YV, André M, Cao JB, Olshevsky V, Eastwood JP, Retinò Aet al., 2017,

    Intermittent energy dissipation by turbulent reconnection

    , Geophysical Research Letters, Vol: 44, Pages: 37-43, ISSN: 1944-8007

    Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub-ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines.
  • Journal article
    Sulaiman AH, Gurnett DA, Halekas JS, Yates JN, Kurth WS, Dougherty MKet al., 2017,

    Whistler mode waves upstream of Saturn

    , Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 227-234, ISSN: 2169-9380

    Whistler mode waves are generated within and can propagate upstream of collisionless shocks. They are known to play a role in electron thermodynamics/acceleration and, under certain conditions, are markedly observed as wave trains preceding the shock ramp. In this paper, we take advantage of Cassini's presence at ~10 AU to explore the importance of whistler mode waves in a parameter regime typically characterized by higher Mach number (median of ~14) shocks, as well as a significantly different interplanetary magnetic field structure, compared to near Earth. We identify electromagnetic precursors preceding a small subset of bow shock crossings with properties which are consistent with whistler mode waves. We find these monochromatic, low-frequency, and circularly polarized waves to have a typical frequency range of 0.2–0.4 Hz in the spacecraft frame. This is due to the lower ion and electron cyclotron frequencies near Saturn, between which whistler waves can develop. The waves are also observed as predominantly right handed in the spacecraft frame, the opposite sense to what is typically observed near Earth. This is attributed to the weaker Doppler shift, owing to the large angle between the solar wind velocity and magnetic field vectors at 10 AU. Our results on the low occurrence of whistler waves upstream of Saturn also underpin the predominantly supercritical bow shock of Saturn.
  • Journal article
    Archer MO, Hartinger MD, Walsh BM, Angelopoulos Vet al., 2017,

    Magnetospheric and solar wind dependences of coupled fast-mode resonances outside the plasmasphere

    , Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 212-226, ISSN: 2169-9380

    We investigate the magnetospheric and solar wind factors that control the occurrence probabilities, locations, and frequencies of standing Alfvén waves excited via coupled fast-mode resonances (cFMRs) in the outer magnetosphere's dawn and dusk sectors. The variation of these cFMR properties with the observed magnetospheric plasma density profiles and inputs to the semiempirically modeled magnetic field from the numerical cFMR calculations of Archer et al. (2015) are studied. The probability of cFMR occurrence increases with distance between the magnetopause and the Alfvén speed's local maximum. The latter's location depends on magnetospheric activity: during high activity it is situated slightly outside the plasmapause, whereas at low activity it is found at much larger radial distances. The frequencies of cFMR are proportional to the Alfvén speed near the magnetopause, which is affected by both density and magnetic field variations. The location of the excited resonance, however, depends on the relative steepness of the Alfvén speed radial profile. The steeper this is, the closer the resonance is to the outer boundary and vice versa. The variation of the density profiles with solar wind conditions and activity is also shown.
  • Journal article
    Steckiewicz M, Garnier P, André N, Mitchell DL, Andersson L, Penou E, Beth A, Fedorov A, Sauvaud J-A, Mazelle C, Brain DA, Espley JR, McFadden J, Halekas JS, Larson DE, Lillis RJ, Luhmann JG, Soobiah Y, Jakosky BMet al., 2017,

    Comparative study of the Martian suprathermal electron depletions based on Mars Global Surveyor, Mars Express and Mars Atmosphere and Volatile EvolutioN missions observations

    , Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 857-873, ISSN: 2169-9402

    Nightside suprathermal electron depletions have been observed at Mars by three spacecraft to date: Mars Global Surveyor, Mars Express, and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. This spatial and temporal diversity of measurements allows us to propose here a comprehensive view of the Martian electron depletions through the first multispacecraft study of the phenomenon. We have analyzed data recorded by the three spacecraft from 1999 to 2015 in order to better understand the distribution of the electron depletions and their creation mechanisms. Three simple criteria adapted to each mission have been implemented to identify more than 134,500 electron depletions observed between 125 and 900 km altitude. The geographical distribution maps of the electron depletions detected by the three spacecraft confirm the strong link existing between electron depletions and crustal magnetic field at altitudes greater than ~170 km. At these altitudes, the distribution of electron depletions is strongly different in the two hemispheres, with a far greater chance to observe an electron depletion in the Southern Hemisphere, where the strongest crustal magnetic sources are located. However, the unique MAVEN observations reveal that below a transition region near 160–170 km altitude the distribution of electron depletions is the same in both hemispheres, with no particular dependence on crustal magnetic fields. This result supports the suggestion made by previous studies that these low-altitudes events are produced through electron absorption by atmospheric CO2.
  • Journal article
    Beth A, Altwegg K, Balsiger H, Berthelier J-J, Calmonte U, Combi MR, De Keyser J, Dhooghe F, Fiethe B, Fuselier SA, Galand M, Gasc S, Gombosi TI, Hansen KC, Hassig M, Heritier KL, Kopp E, Le Roy L, Mandt KE, Peroy S, Rubin M, Semon T, Tzou C-Y, Vigren Eet al., 2017,

    First in situ detection of the cometary ammonium ion NH4+ (protonated ammonia NH3) in the coma of 67P/C-G near perihelion

    , Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S562-S572, ISSN: 0035-8711

    In this paper, we report the first in situ detection of the ammonium ion NH+44+ at 67P/Churyumov–Gerasimenko (67P/C-G) in a cometary coma, using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA)/Double Focusing Mass Spectrometer (DFMS). Unlike neutral and ion spectrometers onboard previous cometary missions, the ROSINA/DFMS spectrometer, when operated in ion mode, offers the capability to distinguish NH+44+ from H2O+ in a cometary coma. We present here the ion data analysis of mass-to-charge ratios 18 and 19 at high spectral resolution and compare the results with an ionospheric model to put these results into context. The model confirms that the ammonium ion NH+44+ is one of the most abundant ion species, as predicted, in the coma near perihelion.
  • Journal article
    Johnson CD, Hood AW, de Moortel I, Cargill PJet al., 2017,

    A New Approach for Modelling Chromospheric Evaporation in Response to Enhanced Coronal Heating: 1 The Method

    , Astronomy & Astrophysics, Vol: 597, ISSN: 0004-6361

    We present a new computational approach that addresses the difficulty of obtaining the correct interaction betweenthe solar corona and the transition region in response to rapid heating events. In the coupled corona/transition region/chromospheresystem, an enhanced downward conductive flux results in an upflow (chromospheric evaporation).However, obtaining the correct upflow generally requires high spatial resolution in order to resolve the transition region.With an unresolved transition region, artificially low coronal densities are obtained because the downward heatflux “jumps” across the unresolved region to the chromosphere, underestimating the upflows. Here, we treat the lowertransition region as a discontinuity that responds to changing coronal conditions through the imposition of a jumpcondition that is derived from an integrated form of energy conservation. To illustrate and benchmark this approachagainst a fully resolved one-dimensional model, we present field-aligned simulations of coronal loops in response to arange of impulsive (spatially uniform) heating events. We show that our approach leads to a significant improvement inthe coronal density evolution than just when using coarse spatial resolutions insufficient to resolve the lower transitionregion. Our approach compensates for the “jumping” of the heat flux by imposing a velocity correction that ensuresthat the energy from the heat flux goes into driving the transition region dynamics, rather than being lost throughradiation. Hence, it is possible to obtain improved coronal densities. The advantages of using this approach in bothone-dimensional hydrodynamic and three-dimensional magnetohydrodynamic simulations are discussed.
  • Journal article
    Vigren E, Altwegg K, Edberg NJT, Eriksson AI, Galand M, Henri P, Johansson F, Odelstad E, Tzou C-Y, Vallieres Xet al., 2017,

    Erratum: “Model–observation comparisons of electron number densities in the coma of 67P/Churyumov-Gerasimenko during 2015 January” (2016, AJ, 152, 59)

    , Astronomical Journal, Vol: 153, Pages: 50-50, ISSN: 0004-6256
  • Book chapter
    Brindley HE, Russell JE, 2017,

    Top of atmosphere broadband radiative fluxes from geostationary satellite observations

    , Comprehensive Remote Sensing, Pages: 85-113

    This chapter provides a description of the Geostationary Earth Radiation Budget (GERB) experiment, placed in the context of the need for high temporal resolution observations of the Earth’s reflected shortwave and outgoing longwave radiation. The GERB instrument design, calibration and data products are outlined in detail, scientific insights gained from the project thus far are highlighted and future directions discussed.
  • Conference paper
    Voulgarakis A, Field R, Fromm M, 2017,

    Fire impacts on high-altitude atmospheric com-position

    , 13th International Conference on Meteorology, Climatology and Atmospheric Physics (COMECAP), Publisher: Springer International Publishing, Pages: 1231-1237, ISSN: 2194-5217

    Fire emissions can strongly impact atmospheric abundances of trace gases and aerosols, in ways that vary strongly in time and space. There is emerging understanding that fires do not only influence areas in the lower troposphere, where the land-surface is in contact with the atmosphere, but can also have significant effects on the upper troposphere and even the stratosphere. Here, I will present example results from our ongoing global modelling studies investigating such effects. First, an overview of recent results will be presented, i.e. from (a) a case study on how high-altitude injections can influence stratospheric composition, and (b) a study that demonstrated how satellite observations can be used to understand the transport of fire pollution into the upper troposphere/lower stratosphere (UTLS), and how such measurements can be used to evaluate convective processes in composition-climate models. Subsequently, the role of typical low-injection fires in driving the interannual variability of UTLS composition will be discussed based on results from recent global model experiments, with a focus on impacts on CO and ozone. The findings show a major role of fire emissions in driving UTLS CO and a minor role in driving UTLS ozone interannual variability.

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