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Journal articleSteckiewicz M, Garnier P, André N, et 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: 2169-9402Nightside 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.
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Journal articleBeth A, Altwegg K, Balsiger H, et 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: 0035-8711In 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.
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Journal articleJohnson CD, Hood AW, de Moortel I, et al., 2017,
A New Approach for Modelling Chromospheric Evaporation in Response to Enhanced Coronal Heating: 1 The Method
, Astronomy & Astrophysics, Vol: 597, ISSN: 0004-6361We 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.
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Journal articleVigren E, Altwegg K, Edberg NJT, et 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 chapterBrindley HE, Russell JE, 2017,
Top of atmosphere broadband radiative fluxes from geostationary satellite observations
, Comprehensive Remote Sensing, Pages: 85-113This 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.
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Conference paperVoulgarakis 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: 2194-5217Fire 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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Journal articleLotz SI, Heyns MJ, Cilliers PJ, 2017,
Regression-based forecast model of induced geoelectric field
, SPACE WEATHER-THE INTERNATIONAL JOURNAL OF RESEARCH AND APPLICATIONS, Vol: 15, Pages: 180-191- Author Web Link
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- Citations: 7
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Book chapterGaland M, Unruh Y, 2017,
Modelling the Upper Atmosphere of Gas-Giant Exoplanets Irradiated by Low-Mass Stars Supervisors' Foreword
, MODELLING THE UPPER ATMOSPHERE OF GAS-GIANT EXOPLANETS IRRADIATED BY LOW-MASS STARS, Publisher: SPRINGER-VERLAG BERLIN, Pages: V-VI, ISBN: 978-3-319-63350-3 -
Journal articleKubicka M, Mostl C, Amerstorfer T, et al., 2016,
Prediction of geomagnetic storm strength from inner heliospheric in situ observations
, Astrophysical Journal, Vol: 833, ISSN: 1538-4357Prediction of the effects of coronal mass ejections (CMEs) on Earth strongly depends on knowledge of the interplanetary magnetic field southward component, B z . Predicting the strength and duration of B z inside a CME with sufficient accuracy is currently impossible, forming the so-called B z problem. Here, we provide a proof-of-concept of a new method for predicting the CME arrival time, speed, B z , and resulting disturbance storm time (Dst) index on Earth based only on magnetic field data, measured in situ in the inner heliosphere (<1 au). On 2012 June 12–16, three approximately Earthward-directed and interacting CMEs were observed by the Solar Terrestrial Relations Observatory imagers and Venus Express (VEX) in situ at 0.72 au, 6° away from the Sun–Earth line. The CME kinematics are calculated using the drag-based and WSA–Enlil models, constrained by the arrival time at VEX, resulting in the CME arrival time and speed on Earth. The CME magnetic field strength is scaled with a power law from VEX to Wind. Our investigation shows promising results for the Dst forecast (predicted: −96 and −114 nT (from 2 Dst models); observed: −71 nT), for the arrival speed (predicted: 531 ± 23 km s−1; observed: 488 ± 30 km s−1), and for the timing (6 ± 1 hr after the actual arrival time). The prediction lead time is 21 hr. The method may be applied to vector magnetic field data from a spacecraft at an artificial Lagrange point between the Sun and Earth or to data taken by any spacecraft temporarily crossing the Sun–Earth line.
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Journal articleBarnes WT, Cargill PJ, Bradshaw SJ, 2016,
Inference of heating properties from "hot" non-flaring plasmas in active region cores. II. nanoflare trains
, Astrophysical Journal, Vol: 833, ISSN: 1538-4357Despite its prediction over two decades ago, the detection of faint, high-temperature (\hot") emissiondue to nano are heating in non- aring active region cores has proved challenging. Using an e cienttwo- uid hydrodynamic model, this paper investigates the properties of the emission expected fromrepeating nano ares (a nano are train) of varying frequency as well as the separate heating of electronsand ions. If the emission measure distribution (EM(T)) peaks atT=Tm, we nd that EM(Tm) isindependent of details of the nano are train, and EM(T) above and belowTmre ects di erent aspectsof the heating. BelowTmthe main in uence is the relationship of the waiting time between successivenano ares to the nano are energy. AboveTmpower-law nano are distributions lead to an extensiveplasma population not present in a monoenergetic train. Furthermore, in some cases characteristicfeatures are present in EM(T). Such details may be detectable given adequate spectral resolution anda good knowledge of the relevant atomic physics. In the absence of such resolution we propose somemetrics that can be used to infer the presence of \hot" plasma.
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Journal articleSaunois M, Bousquet P, Poulter B, et al., 2016,
The global methane budget 2000-2012
, Earth System Science Data, Vol: 8, Pages: 697-751, ISSN: 1866-3516The global methane (CH4) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH4 over the past decade. Emissions and concentrations of CH4 are continuing to increase, making CH4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular (∼ biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories and data-driven approaches (including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations). For the 2003–2012 decade, global methane emissions are estimated by top-down inversions at 558 Tg CH4 yr−1, range 540–568. About 60 % of global emissions are anthropogenic (range 50–65 %). Since 2010, the bottom-up global emission inventories have been closer to methane emissions in the most carbon-intensive Representative Concentrations Pathway (RCP8.5) and higher
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Journal articleFranci L, Landi S, Matteini L, et al., 2016,
Plasma beta dependence of the ion-scale spectral break of solar wind turbulence: high-resolution 2D hybrid simulations
, Astrophysical Journal, Vol: 833, ISSN: 0004-637XWe investigate properties of the ion-scale spectral break of solar wind turbulence by means of two-dimensional high-resolution hybrid particle-in-cell simulations. We impose an initial ambient magnetic field perpendicular to the simulation box and add a spectrum of in-plane, large-scale, magnetic and kinetic fluctuations. We perform a set of simulations with different values of the plasma β, distributed over three orders of magnitude, from 0.01 to 10. In all cases, once turbulence is fully developed, we observe a power-law spectrum of the fluctuating magnetic field on large scales (in the inertial range) with a spectral index close to −5/3, while in the sub-ion range we observe another power-law spectrum with a spectral index systematically varying with β (from around −3.6 for small values to around −2.9 for large ones). The two ranges are separated by a spectral break around ion scales. The length scale at which this transition occurs is found to be proportional to the ion inertial length, d i , for β Lt 1 and to the ion gyroradius, ${\rho }_{i}={d}_{i}\sqrt{\beta }$, for β Gt 1, i.e., to the larger between the two scales in both the extreme regimes. For intermediate cases, i.e., β ~ 1, a combination of the two scales is involved. We infer an empiric relation for the dependency of the spectral break on β that provides a good fit over the whole range of values. We compare our results with in situ observations in the solar wind and suggest possible explanations for such a behavior.
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Journal articleFrancis DBK, Flamant C, Chaboureau J-P, et al., 2016,
Dust emission and transport over Iraq associated with the summer Shamal winds
, AEOLIAN RESEARCH, Vol: 24, Pages: 15-31, ISSN: 1875-9637 -
Journal articleGoetz C, Koenders C, Hansen KC, et al., 2016,
Structure and evolution of the diamagnetic cavity at comet 67P/Churyumov-Gerasimenko
, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S459-S467, ISSN: 0035-8711The long duration of the Rosetta mission allows us to study the evolution of the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko in detail. From 2015 April to 2016 February 665 intervals could be identified where Rosetta was located in a zero-magnetic-field region. We study the temporal and spatial distribution of this cavity and its boundary and conclude that the cavity properties depend on the long-term trend of the outgassing rate, but do not respond to transient events at the spacecraft location, such as outbursts or high neutral densities. Using an empirical model of the outgassing rate, we find a functional relationship between the outgassing rate and the distance of the cavity to the nucleus. There is also no indication that this unexpectedly large distance is related to unusual solar wind conditions. Because the deduced shape of the cavity boundary is roughly elliptical on small scales and the distances of the boundary from the nucleus are much larger than expected we conclude that the events observed by Rosetta are due to a moving instability of the cavity boundary itself.
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Journal articleKasper JC, Abiad R, Austin G, et al., 2016,
Solar Wind Electrons Alphas and Protons (SWEAP) Investigation: Design of the Solar Wind and Coronal Plasma Instrument Suite for Solar Probe Plus
, SPACE SCIENCE REVIEWS, Vol: 204, Pages: 131-186, ISSN: 0038-6308- Author Web Link
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- Citations: 383
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Journal articleFox NJ, Velli MC, Bale SD, et al., 2016,
The Solar Probe Plus Mission: Humanity's First Visit to Our Star
, SPACE SCIENCE REVIEWS, Vol: 204, Pages: 7-48, ISSN: 0038-6308- Author Web Link
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- Citations: 733
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Journal articleMcComas DJ, Alexander N, Angold N, et al., 2016,
Integrated Science Investigation of the Sun (ISIS): Design of the Energetic Particle Investigation
, SPACE SCIENCE REVIEWS, Vol: 204, Pages: 187-256, ISSN: 0038-6308- Author Web Link
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- Citations: 122
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Journal articleStawarz JE, Eriksson S, Wilder FD, et al., 2016,
Observations of turbulence in a Kelvin-Helmholtz event on September 8, 2015 by the Magnetospheric Multiscale Mission
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 11021-11034, ISSN: 2169-9380Spatial and high-time-resolution properties of the velocities,magnetic eld, and 3D electric eld within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale Mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a rst look at the properties of turbulence in the KHI. For the rst time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra ex-hibit power law behavior with changes in slope near the ion gyrofrequency and lower-hybrid frequency. The work provides the rst observational evi-dence for turbulent intermittency and anisotropy consistent with quasi-two-dimensional turbulence in association with the KHI. The behavior of kinetic scale intermittency is found to have di erences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics inthe KHI.
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Journal articleO'Shea SJ, Choularton TW, Lloyd G, et al., 2016,
Airborne observations of the microphysical structure of two contrasting cirrus clouds
, Journal of Geophysical Research: Atmospheres, Vol: 121, Pages: 13510-13536, ISSN: 2169-8996We present detailed airborne in situ measurements of cloud microphysics in two midlatitude cirrus clouds, collected as part of the Cirrus Coupled Cloud-Radiation Experiment. A new habit recognition algorithm for sorting cloud particle images using a neural network is introduced. Both flights observed clouds that were related to frontal systems, but one was actively developing while the other dissipated as it was sampled. The two clouds showed distinct differences in particle number, habit, and size. However, a number of common features were observed in the 2-D stereo data set, including a distinct bimodal size distribution within the higher-temperature regions of the clouds. This may result from a combination of local heterogeneous nucleation and large particles sedimenting from aloft. Both clouds had small ice crystals (<100 µm) present at all levels However, this small ice mode is not present in observations from a holographic probe. This raises the possibility that the small ice observed by optical array probes may at least be in part an instrument artifact due to the counting of out-of-focus large particles as small ice. The concentrations of ice crystals were a factor ~10 higher in the actively growing cloud with the stronger updrafts, with a mean concentration of 261 L−1 compared to 29 L−1 in the decaying case. Particles larger than 700 µm were largely absent from the decaying cirrus case. A comparison with ice-nucleating particle parameterizations suggests that for the developing case the ice concentrations at the lowest temperatures are best explained by homogenous nucleation.
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Journal articleGaland M, Héritier KL, Odelstad E, et al., 2016,
Ionospheric plasma of comet 67P probed by Rosetta at 3 AU from the Sun
, Monthly Notices of the Royal Astronomical Society, Vol: 462, Pages: S331-S351, ISSN: 1365-2966We propose to identify the main sources of ionization of the plasma in the coma of comet 67P/Churyumov–Gerasimenko at different locations in the coma and to quantify their relative importance, for the first time, for close cometocentric distances (<20 km) and large heliocentric distances (>3 au). The ionospheric model proposed is used as an organizing element of a multi-instrument data set from the Rosetta Plasma Consortium (RPC) plasma and particle sensors, from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis and from the Microwave Instrument on the Rosetta Orbiter, all on board the ESA/Rosetta spacecraft. The calculated ionospheric density driven by Rosetta observations is compared to the RPC-Langmuir Probe and RPC-Mutual Impedance Probe electron density. The main cometary plasma sources identified are photoionization of solar extreme ultraviolet (EUV) radiation and energetic electron-impact ionization. Over the northern, summer hemisphere, the solar EUV radiation is found to drive the electron density – with occasional periods when energetic electrons are also significant. Over the southern, winter hemisphere, photoionization alone cannot explain the observed electron density, which reaches sometimes higher values than over the summer hemisphere; electron-impact ionization has to be taken into account. The bulk of the electron population is warm with temperature of the order of 7–10 eV. For increased neutral densities, we show evidence of partial energy degradation of the hot electron energy tail and cooling of the full electron population
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Journal articleJeong S, Newman S, Zhang J, et al., 2016,
Estimating methane emissions in California's urban and rural regions using multitower observations
, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 121, Pages: 13031-13049, ISSN: 2169-897X -
Journal articlePfau-Kempf Y, Hietala H, Milan SE, et al., 2016,
Evidence for transient, local ion foreshocks caused by dayside magnetopause reconnection
, Annales Geophysicae, Vol: 34, Pages: 943-959, ISSN: 0992-7689We present a scenario resulting in time-dependent behaviour of the bow shock and transient, local ion reflection under unchanging solar wind conditions. Dayside magnetopause reconnection produces flux transfer events driving fast-mode wave fronts in the magnetosheath. These fronts push out the bow shock surface due to their increased downstream pressure. The resulting bow shock deformations lead to a configuration favourable to localized ion reflection and thus the formation of transient, travelling foreshock-like field-aligned ion beams. This is identified in two-dimensional global magnetospheric hybrid-Vlasov simulations of the Earth's magnetosphere performed using the Vlasiator model (http://vlasiator.fmi.fi). We also present observational data showing the occurrence of dayside reconnection and flux transfer events at the same time as Geotail observations of transient foreshock-like field-aligned ion beams. The spacecraft is located well upstream of the foreshock edge and the bow shock, during a steady southward interplanetary magnetic field and in the absence of any solar wind or interplanetary magnetic field perturbations. This indicates the formation of such localized ion foreshocks.
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Journal articleThomas RT, Prentice IC, Graven H, et al., 2016,
Increased light-use efficiency in northern terrestrial ecosystems indicated by CO2 and greening observations
, Geophysical Research Letters, Vol: 43, Pages: 11339-11349, ISSN: 1944-8007Observations show an increasing amplitude in the seasonal cycle of CO2 (ASC) north of 45°N of 56 ± 9.8% over the last 50 years and an increase in vegetation greenness of 7.5–15% in high northern latitudes since the 1980s. However, the causes of these changes remain uncertain. Historical simulations from terrestrial biosphere models in the Multiscale Synthesis and Terrestrial Model Intercomparison Project are compared to the ASC and greenness observations, using the TM3 atmospheric transport model to translate surface fluxes into CO2 concentrations. We find that the modeled change in ASC is too small but the mean greening trend is generally captured. Modeled increases in greenness are primarily driven by warming, whereas ASC changes are primarily driven by increasing CO2. We suggest that increases in ecosystem-scale light use efficiency (LUE) have contributed to the observed ASC increase but are underestimated by current models. We highlight potential mechanisms that could increase modeled LUE.
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Journal articleGraven HD, 2016,
The carbon cycle in a changing climate
, Physics Today, Vol: 69, Pages: 48-54, ISSN: 0031-9228 -
Journal articleChakravorty S, Gnanaseelan C, Pillai PA, 2016,
Combined influence of remote and local SST forcing on Indian Summer Monsoon Rainfall variability
, Climate Dynamics, Vol: 47, Pages: 2817-2831, ISSN: 0930-7575 -
Journal articleMistry R, Eastwood JP, Haggerty CC, et al., 2016,
Observations of Hall reconnection physics far downstream of the X-line
, Physical Review Letters, Vol: 117, ISSN: 1079-7114Observations made using the Wind spacecraft of Hall magnetic fields in solar wind reconnection exhausts are presented. These observations are consistent with the generation of Hall fields by a narrow ion inertial scale current layer near the separatrix, which is confirmed with an appropriately scaled particle-in-cell simulation that shows excellent agreement with observations. The Hall fields are observed thousands of ion inertial lengths downstream from the reconnection X line, indicating that narrow regions of kinetic dynamics can persist extremely far downstream.
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Journal articleWang S, Toumi R, 2016,
On the relationship between hurricane cost and the integrated wind profile
, Environmental Research Letters, Vol: 11, ISSN: 1748-9326It is challenging to identify metrics that best capture hurricane destructive potential and costs. Although it has been found that the sea surface temperature and vertical wind shear can both make considerable changes to the hurricane destructive potential metrics, it is still unknown which plays a more important role. Here we present a new method to reconstruct the historical wind structure of hurricanes that allows us, for the first time, to calculate the correlation of damage with integrated power dissipation and integrated kinetic energy of all hurricanes at landfall since 1988. We find that those metrics, which include the horizontal wind structure, rather than just maximum intensity, are much better correlated with the hurricane cost. The vertical wind shear over the main development region of hurricanes plays a more dominant role than the sea surface temperature in controlling these metrics and therefore also ultimately the cost of hurricanes.
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Journal articleWang S, Toumi R, 2016,
On the relationship between hurricane cost and the integrated wind profile
, Environmental Research Letters, Vol: 11, ISSN: 1748-9326It is challenging to identify metrics that best capture hurricane destructive potential and costs. Although it has been found that the sea surface temperature and vertical wind shear can both make considerable changes to the hurricane destructive potential metrics, it is still unknown which plays a more important role. Here we present a new method to reconstruct the historical wind structure of hurricanes that allows us, for the first time, to calculate the correlation of damage with integrated power dissipation and integrated kinetic energy of all hurricanes at landfall since 1988. We find that those metrics, which include the horizontal wind structure, rather than just maximum intensity, are much better correlated with the hurricane cost. The vertical wind shear over the main development region of hurricanes plays a more dominant role than the sea surface temperature in controlling these metrics and therefore also ultimately the cost of hurricanes.
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Journal articleProvan G, Cowley SWH, Lamy L, et al., 2016,
Planetary period oscillations in Saturn's magnetosphere: coalescence and reversal of northern and southern periods in late northern spring
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 9829-9862, ISSN: 2169-9402We investigate planetary period oscillations (PPOs) in Saturn's magnetosphere using Cassini magnetic field and Saturn kilometric radiation (SKR) data over the interval from late 2012 to the end of 2015, beginning ~3 years after vernal equinox and ending ~1.5 years before northern solstice. Previous studies have shown that the northern and southern PPO periods converged across equinox from southern summer values ~10.8 h for the southern system and ~10.6 h for the northern system and near coalesced ~1 year after equinox, before separating again with the southern period ~10.69 h remaining longer than the northern ~10.64 h. We show that these conditions ended in mid-2013 when the two periods coalesced at ~10.66 h and remained so until mid-2014, increasing together to longer periods ~10.70 h. During coalescence the two systems were locked near magnetic antiphase with SKR modulations in phase, a condition in which the effects of the generating rotating twin vortex flows in the two ionospheres reinforce each other via hemisphere-to-hemisphere coupling. The magnetic-SKR relative phasing indicates the dominance of postdawn SKR sources in both hemispheres, as was generally the case during the study interval. In mid-2014 the two periods separated again, the northern increasing to ~10.78 h by the end of 2015, similar to the southern period during southern summer, while the southern period remained fixed near ~10.70 h, well above the northern period during southern summer. Despite this difference, this behavior resulted in the first enduring reversal of the two periods, northern longer than southern, during the Cassini era.
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Journal articleYates JN, Southwood DJ, Dougherty MK, et al., 2016,
Saturn's quasiperiodic magnetohydrodynamic waves
, Geophysical Research Letters, Vol: 43, Pages: 102-111, ISSN: 1944-8007Quasi-periodic ∼1-hour fluctuations have been recently reported by numerous instruments on-board the Cassini spacecraft. The interpretation of the sources of these fluctuations has remained elusive to date. Here we provide an explanation for the origin of these fluctuations using magnetometer observations. We find that magnetic field fluctuations at high northern latitudes are Alfvénic, with small amplitudes (∼0.4 nT), and are concentrated in wave-packets similar to those observed in Kleindienst et al. [2009]. The wave-packets recur periodically at the northern magnetic oscillation period. We use a magnetospheric box model to provide an interpretation of the wave periods. Our model results suggest that the observed magnetic fluctuations are second harmonic Alfvén waves standing between the northern and southern ionospheres in Saturn’s outer magnetosphere
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