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Journal articleSchutgens NAJ, Gryspeerdt E, Weigum N, et al., 2016,
Will a perfect model agree with perfect observations? The impact of spatial sampling
, Atmospheric Chemistry and Physics, Vol: 16, Pages: 6335-6353, ISSN: 1680-7324The spatial resolution of global climate models with interactive aerosol and the observations used to evaluate them is very different. Current models use grid spacings of ∼ 200 km, while satellite observations of aerosol use so-called pixels of ∼ 10 km. Ground site or airborne observations relate to even smaller spatial scales. We study the errors incurred due to different resolutions by aggregating high-resolution simulations (10 km grid spacing) over either the large areas of global model grid boxes ("perfect" model data) or small areas corresponding to the pixels of satellite measurements or the field of view of ground sites ("perfect" observations). Our analysis suggests that instantaneous root-mean-square (RMS) differences of perfect observations from perfect global models can easily amount to 30–160 %, for a range of observables like AOT (aerosol optical thickness), extinction, black carbon mass concentrations, PM2.5, number densities and CCN (cloud condensation nuclei). These differences, due entirely to different spatial sampling of models and observations, are often larger than measurement errors in real observations. Temporal averaging over a month of data reduces these differences more strongly for some observables (e.g. a threefold reduction for AOT), than for others (e.g. a twofold reduction for surface black carbon concentrations), but significant RMS differences remain (10–75 %). Note that this study ignores the issue of temporal sampling of real observations, which is likely to affect our present monthly error estimates. We examine several other strategies (e.g. spatial aggregation of observations, interpolation of model data) for reducing these differences and show their effectiveness. Finally, we examine consequences for the use of flight campaign data in global model evaluation and show that significant biases may be introduced depending on the flight strategy used.
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Journal articleSulaiman AH, Masters A, Dougherty MK, 2016,
Characterization of Saturn's bow shock: magnetic field observations of quasi-perpendicular shocks
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 4425-4434, ISSN: 2169-9380Collisionless shocks vary drastically from terrestrial to astrophysical regimes resulting in radically different characteristics. This poses two complexities. First, separating the influences of these parameters on physical mechanisms such as energy dissipation. Second, correlating observations of shock waves over a wide range of each parameter, enough to span across different regimes. Investigating the latter has been restricted since the majority of studies on shocks at exotic regimes (such as supernova remnants) have been achieved either remotely or via simulations, but rarely by means of in situ observations. Here we present the parameter space of MA bow shock crossings from 2004 to 2014 as observed by the Cassini spacecraft. We find that Saturn's bow shock exhibits characteristics akin to both terrestrial and astrophysical regimes (MA of order 100), which is principally controlled by the upstream magnetic field strength. Moreover, we determined the θBn of each crossing to show that Saturn's (dayside) bow shock is predominantly quasi-perpendicular by virtue of the Parker spiral at 10 AU. Our results suggest a strong dependence on MA in controlling the onset of physical mechanisms in collisionless shocks, particularly nontime stationarity and variability. We anticipate that our comprehensive assessment will yield deeper insight into high MA collisionless shocks and provide a broader scope for understanding the structures and mechanisms of collisionless shocks.
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Journal articleFarrugia CJ, Lavraud B, Torbert RB, et al., 2016,
Magnetospheric Multiscale Mission observations and non-force free modeling of a flux transfer event immersed in a super-Alfvénic flow
, Geophysical Research Letters, Vol: 43, Pages: 6070-6077, ISSN: 0094-8276We analyze plasma, magnetic field, and electric field data for a flux transfer event (FTE) to highlight improvements in our understanding of these transient reconnection signatures resulting from high-resolution data. The ∼20 s long, reverse FTE, which occurred south of the geomagnetic equator near dusk, was immersed in super-Alfvénic flow. The field line twist is illustrated by the behavior of flows parallel/perpendicular to the magnetic field. Four-spacecraft timing and energetic particle pitch angle anisotropies indicate a flux rope (FR) connected to the Northern Hemisphere and moving southeast. The flow forces evidently overcame the magnetic tension. The high-speed flows inside the FR were different from those outside. The external flows were perpendicular to the field as expected for draping of the external field around the FR. Modeling the FR analytically, we adopt a non-force free approach since the current perpendicular to the field is nonzero. It reproduces many features of the observations.
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Journal articleSukhodolov T, Rozanov E, Ball WT, et al., 2016,
Evaluation of simulated photolysis rates and their response to solar irradiance variability
, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 121, Pages: 6066-6084, ISSN: 2169-897XThe state of the stratospheric ozone layer and the temperature structure of the atmosphere are largely controlled by the solar spectral irradiance (SSI) through its influence on heating and photolysis rates. This study focuses on the uncertainties in the photolysis rate response to solar irradiance variability related to the choice of SSI data set and to the performance of the photolysis codes used in global chemistry-climate models. To estimate the impact of SSI uncertainties, we compared several photolysis rates calculated with the radiative transfer model libRadtran, using SSI calculated with two models and observed during the Solar Radiation and Climate Experiment (SORCE) satellite mission. The importance of the calculated differences in the photolysis rate response for ozone and temperature changes has been estimated using 1-D a radiative-convective-photochemical model. We demonstrate that the main photolysis reactions, responsible for the solar signal in the stratosphere, are highly sensitive to the spectral distribution of SSI variations. Accordingly, the ozone changes and related ozone-temperature feedback are shown to depend substantially on the SSI data set being used, which highlights the necessity of obtaining accurate SSI variations. To evaluate the performance of photolysis codes, we compared the results of eight, widely used, photolysis codes against two reference schemes. We show that, in most cases, absolute values of the photolysis rates and their response to applied SSI changes agree within 30%. However, larger errors may appear in specific atmospheric regions because of differences, for instance, in the treatment of Rayleigh scattering, quantum yields, or absorption cross sections.
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Journal articleCargill PJ, DeMoortel I, Kiddie G, 2016,
Coronal density structure and its role in wave damping in loops
, Astrophysical Journal, Vol: 823, ISSN: 1538-4357It has long been established that gradients in the Alfvén speed, and in particular the plasmadensity, are an essential part of the damping of waves in the magnetically closed solar coronaby mechanisms such as resonant absorption or phase mixing. While models of wave dampingoften assume a fixed density gradient, in this paper the self-consistency of such calculationsis assessed by examining the temporal evolution of the coronal density. It is shownconceptually that for some coronal structures, density gradients can evolve in a way that thewave damping processes are inhibited. For the case of phase mixing we argue that: (a) waveheating cannot sustain the assumed density structure and (b) inclusion of feedback of theheating on the density gradient can lead to a highly structured density, although on longtimescales. In addition, transport coefficients well in excess of classical are required tomaintain the observed coronal density. Hence, the heating of closed coronal structures byglobal oscillations may face problems arising from the assumption of a fixed density gradientand the rapid damping of oscillations may have to be accompanied by a separate (non-wavebased) heating mechanism to sustain the required density structuring.
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Journal articleErgun RE, Holmes JC, Goodrich KA, et al., 2016,
Magnetospheric Multiscale observations of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the magnetopause
, Geophysical Research Letters, Vol: 43, Pages: 5626-5634, ISSN: 0094-8276We report observations from the Magnetospheric Multiscale satellites of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the Earth's magnetopause. The observed waves have parallel electric fields (E||) with amplitudes on the order of 100 mV/m and display nonlinear characteristics that suggest a possible net E||. These waves are observed within the ion diffusion region and adjacent to (within several electron skin depths) the electron diffusion region. They are in or near the magnetosphere side current layer. Simulation results support that the strong electrostatic linear and nonlinear wave activities appear to be driven by a two stream instability, which is a consequence of mixing cold (<10 eV) plasma in the magnetosphere with warm (~100 eV) plasma from the magnetosheath on a freshly reconnected magnetic field line. The frequent observation of these waves suggests that cold plasma is often present near the magnetopause.
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Journal articleKrupar V, Eastwood JP, Kruparova O, et al., 2016,
An analysis of interplanetary solar radio emissions associated with a coronal mass ejection
, Astrophysical Journal Letters, Vol: 823, ISSN: 2041-8205Coronal mass ejections (CMEs) are large-scale eruptions of magnetized plasma that may cause severe geomagnetic storms if Earth directed. Here, we report a rare instance with comprehensive in situ and remote sensing observations of a CME combining white-light, radio, and plasma measurements from four different vantage points. For the first time, we have successfully applied a radio direction-finding technique to an interplanetary type II burst detected by two identical widely separated radio receivers. The derived locations of the type II and type III bursts are in general agreement with the white-light CME reconstruction. We find that the radio emission arises from the flanks of the CME and are most likely associated with the CME-driven shock. Our work demonstrates the complementarity between radio triangulation and 3D reconstruction techniques for space weather applications.
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Journal articleHausmann U, Czaja A, Marshall J, 2016,
Mechanisms controlling the SST air-sea heat flux feedback and its dependence on spatial scale
, CLIMATE DYNAMICS, Vol: 48, Pages: 1297-1307, ISSN: 0930-7575 -
Journal articleBurgess D, Gingell PW, Matteini L, 2016,
Multiple current sheet systems in the outer heliosphere: energy release and turbulence
, Astrophysical Journal, Vol: 822, ISSN: 1538-4357In the outer heliosphere, beyond the solar wind termination shock, it isexpected that the warped heliospheric current sheet forms a region of closelypacked,multiple, thin current sheets. Such a system may be subject to theion-kinetic tearing instability, and hence generate magnetic islands and hot populationsof ions associated with magnetic reconnection. Reconnection processesin this environment have important implications for local particle transport, andfor particle acceleration at reconnection sites and in turbulence. We study thiscomplex environment by means of three-dimensional hybrid simulations over longtime scales, in order to capture the evolution from linear growth of the tearinginstability to a fully developed turbulent state at late times. The final state developsfrom the highly ordered initial state via both forward and inverse cascades.Component and spectral anisotropy in the magnetic fluctuations is present whena guide field is included. The inclusion of a population of new-born interstellarpickup protons does not strongly affect these results. Finally, we conclude thatreconnection between multiple current sheets can act as an important sourceof turbulence in the outer heliosphere, with implications for energetic particleacceleration and propagation.
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Journal articleMyriokefalitakis S, Daskalakis N, Fanourgakis GS, et al., 2016,
Ozone and carbon monoxide budgets over the Eastern Mediterranean
, Science of the Total Environment, Vol: 563-564, Pages: 40-52, ISSN: 0048-9697The importance of the long-range transport (LRT) on O3 and CO budgets over the Eastern Mediterranean has been investigated using the state-of-the-art 3-dimensional global chemistry-transport model TM4-ECPL. A 3-D budget analysis has been performed separating the Eastern from the Western basins and the boundary layer (BL) from the free troposphere (FT). The FT of the Eastern Mediterranean is shown to be a strong receptor of polluted air masses from the Western Mediterranean, and the most important source of polluted air masses for the Eastern Mediterranean BL, with about 40% of O3 and of CO in the BL to be transported from the FT aloft. Regional anthropogenic sources are found to have relatively small impact on regional air quality in the area, contributing by about 8% and 18% to surface levels of O3 and CO, respectively. Projections using anthropogenic emissions for the year 2050 but neglecting climate change calculate a surface O3 decrease of about 11% together with a surface CO increase of roughly 10% in the Eastern Mediterranean.
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Journal articleField RD, Luo M, Fromm M, et al., 2016,
Simulating the Black Saturday 2009 smoke plume with an interactive composition-climate model: sensitivity to emissions amount, timing, and injection height
, Journal of Geophysical Research: Atmospheres, Vol: 121, Pages: 4296-4316, ISSN: 2169-8996We simulated the high-altitude smoke plume from the early February 2009 Black Saturday bushfires in southeastern Australia using the NASA Goddard Institute for Space Studies ModelE2. To the best of our knowledge, this is the first single-plume analysis of biomass burning emissions injected directly into the upper troposphere/lower stratosphere (UTLS) using a full-complexity composition-climate model. We compared simulated carbon monoxide (CO) to a new Aura Tropospheric Emission Spectrometer/Microwave Limb Sounder joint CO retrieval, focusing on the plume's initial transport eastward, anticyclonic circulation to the north of New Zealand, westward transport in the lower stratospheric easterlies, and arrival over Africa at the end of February. Our goal was to determine the sensitivity of the simulated plume to prescribed injection height, emissions amount, and emissions timing from different sources for a full-complexity model when compared to Aura. The most realistic plumes were obtained using injection heights in the UTLS, including one drawn from ground-based radar data. A 6 h emissions pulse or emissions tied to independent estimates of hourly fire behavior produced a more realistic plume in the lower stratosphere compared to the same emissions amount being released evenly over 12 or 24 h. Simulated CO in the plume was highly sensitive to the differences between emissions amounts estimated from the Global Fire Emissions Database and from detailed, ground-based estimates of fire growth. The emissions amount determined not only the CO concentration of the plume but also the proportion of the plume that entered the stratosphere. We speculate that this is due to either or both nonlinear CO loss with a weakened OH sink or plume self-lofting driven by shortwave absorption of the coemitted aerosols.
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Journal articleGryspeerdt E, Quaas J, Bellouin N, 2016,
Constraining the aerosol influence on cloud fraction
, Journal of Geophysical Research: Atmospheres, Vol: 121, Pages: 3566-3583, ISSN: 2169-897XAerosol-cloud interactions have the potential to modify many different cloud properties. There is significant uncertainty in the strength of these aerosol-cloud interactions in analyses of observational data, partly due to the difficulty in separating aerosol effects on clouds from correlations generated by local meteorology. The relationship between aerosol and cloud fraction (CF) is particularly important to determine, due to the strong correlation of CF to other cloud properties and its large impact on radiation. It has also been one of the hardest to quantify from satellites due to the strong meteorological covariations involved. This work presents a new method to analyze the relationship between aerosol optical depth (AOD) and CF. By including information about the cloud droplet number concentration (CDNC), the impact of the meteorological covariations is significantly reduced. This method shows that much of the AOD-CF correlation is explained by relationships other than that mediated by CDNC. By accounting for these, the strength of the global mean AOD-CF relationship is reduced by around 80%. This suggests that the majority of the AOD-CF relationship is due to meteorological covariations, especially in the shallow cumulus regime. Requiring CDNC to mediate the AOD-CF relationship implies an effective anthropogenic radiative forcing from an aerosol influence on liquid CF of −0.48 W m−2 (−0.1 to −0.64 W m−2), although some uncertainty remains due to possible biases in the CDNC retrievals in broken cloud scenes.
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Journal articleMüller-Wodarg ICF, Bruinsma S, Marty J-C, et al., 2016,
In situ observations of waves in Venus’s polar lower thermosphere with Venus Express aerobraking
, Nature Physics, Vol: 12, Pages: 767-771, ISSN: 1745-2481Waves are ubiquitous phenomena found in oceans and atmospheres alike. From the earliest formal studies of waves in the Earth’s atmosphere to more recent studies on other planets, waves have been shown to play a key role in shaping atmospheric bulk structure, dynamics and variability1, 2, 3, 4. Yet, waves are difficult to characterize as they ideally require in situ measurements of atmospheric properties that are difficult to obtain away from Earth. Thus, we have incomplete knowledge of atmospheric waves on planets other than our own, and we are thereby limited in our ability to understand and predict planetary atmospheres. Here we report the first ever in situ observations of atmospheric waves in Venus’s thermosphere (130–140 km) at high latitudes (71.5°–79.0°). These measurements were made by the Venus Express Atmospheric Drag Experiment (VExADE)5 during aerobraking from 24 June to 11 July 2014. As the spacecraft flew through Venus’s atmosphere, deceleration by atmospheric drag was sufficient to obtain from accelerometer readings a total of 18 vertical density profiles. We infer an average temperature of T = 114 ± 23 K and find horizontal wave-like density perturbations and mean temperatures being modulated at a quasi-5-day period.
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Journal articleArridge CS, Jasinski JM, Achilleos N, et al., 2016,
Cassini observations of Saturn's southern polar cusp
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 3006-3030, ISSN: 2169-9402The magnetospheric cusps are important sites of the coupling of a magnetosphere with the solar wind. The combination of both ground- and space-based observations at Earth has enabled considerable progress to be made in understanding the terrestrial cusp and its role in the coupling of the magnetosphere to the solar wind via the polar magnetosphere. Voyager 2 fully explored Neptune's cusp in 1989, but highly inclined orbits of the Cassini spacecraft at Saturn present the most recent opportunity to repeatedly study the polar magnetosphere of a rapidly rotating planet. In this paper we discuss observations made by Cassini during two passes through Saturn's southern polar magnetosphere. Our main findings are that (i) Cassini directly encounters the southern polar cusp with evidence for the entry of magnetosheath plasma into the cusp via magnetopause reconnection, (ii) magnetopause reconnection and entry of plasma into the cusp can occur over a range of solar wind conditions, and (iii) double cusp morphologies are consistent with the position of the cusp oscillating in phase with Saturn's global magnetospheric periodicities.
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Journal articleLai HR, Russell CT, Jia YD, et al., 2016,
Transport of magnetic flux and mass in Saturn's inner magnetosphere
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 3050-3057, ISSN: 2169-9402It is well accepted that cold plasma sourced by Enceladus is ultimately lost to the solar wind, while the magnetic flux convecting outward with the plasma must return to the inner magnetosphere. However, whether the interchange or reconnection, or a combination of the two processes is the dominant mechanism in returning the magnetic flux is still under debate. Initial Cassini observations have shown that the magnetic flux returns in the form of flux tubes in the inner magnetosphere. Here we investigate those events with 10 year Cassini magnetometer data and confirm that their magnetic signatures are determined by the background plasma environments: inside (outside) the plasma disk, the returning magnetic field is enhanced (depressed) in strength. The distribution, temporal variation, shape, and transportation rate of the flux tubes are also characterized. The flux tubes break into smaller ones as they convect in. The shape of their cross section is closer to circular than fingerlike as produced in the simulations based on the interchange mechanism. In addition, no sudden changes in any flux tube properties can be found at the “boundary” which has been claimed to separate the reconnection and interchange-dominant regions. On the other hand, reasonable cold plasma loss rate and outflow velocity can be obtained if the transport rate of the magnetic flux matches the reconnection rate, which supports reconnection alone as the dominant mechanism in unloading the cold plasma from the inner magnetosphere and returning the magnetic flux from the tail.
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Journal articleMallet A, Schekochihin AA, Chandran BDG, et al., 2016,
Measures of three-dimensional anisotropy and intermittency in strong Alfvénic turbulence
, Monthly Notices of the Royal Astronomical Society, Vol: 459, Pages: 2130-2139, ISSN: 1365-2966We measure the local anisotropy of numerically simulated strong Alfvénic turbulence with respect to two local, physically relevant directions: along the local mean magnetic field and along the local direction of one of the fluctuating Elsasser fields. We find significant scaling anisotropy with respect to both these directions: the fluctuations are ‘ribbon-like’ – statistically, they are elongated along both the mean magnetic field and the fluctuating field. The latter form of anisotropy is due to scale-dependent alignment of the fluctuating fields. The intermittent scalings of the nth-order conditional structure functions in the direction perpendicular to both the local mean field and the fluctuations agree well with the theory of Chandran, Schekochihin & Mallet, while the parallel scalings are consistent with those implied by the critical-balance conjecture. We quantify the relationship between the perpendicular scalings and those in the fluctuation and parallel directions, and find that the scaling exponent of the perpendicular anisotropy (i.e. of the aspect ratio of the Alfvénic structures in the plane perpendicular to the mean magnetic field) depends on the amplitude of the fluctuations. This is shown to be equivalent to the anticorrelation of fluctuation amplitude and alignment at each scale. The dependence of the anisotropy on amplitude is shown to be more significant for the anisotropy between the perpendicular and fluctuation-direction scales than it is between the perpendicular and parallel scales.
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Journal articleLavraud B, Zhang YC, Vernisse Y, et al., 2016,
Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause
, Geophysical Research Letters, Vol: 43, Pages: 3042-3050, ISSN: 1944-8007Based on high-resolution measurements from NASA's Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earth's magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials).
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Journal articleGibbon JD, Gupta A, Krstulovic G, et al., 2016,
Depletion of nonlinearity in magnetohydrodynamic turbulence: insights from analysis and simulations
, Physical Review E, Vol: 93, ISSN: 1539-3755 -
Journal articlePlaschke F, Hietala H, Angelopoulos V, et al., 2016,
Geoeffective jets impacting the magnetopause are very common
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 121, Pages: 3240-3253, ISSN: 2169-9380- Author Web Link
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- Citations: 50
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Journal articleBale SD, Goetz K, Harvey PR, et al., 2016,
The FIELDS Instrument Suite for Solar Probe Plus
, Space Science Reviews, Vol: 204, Pages: 49-82, ISSN: 0038-6308NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurementsof the solar corona and the birthplace of the solar wind. The FIELDS instrument suiteon SPP will make direct measurements of electric and magnetic fields, the properties ofin situ plasma waves, electron density and temperature profiles, and interplanetary radioemissions, amongst other things. Here, we describe the scientific objectives targeted by theSPP/FIELDS instrument, the instrument design itself, and the instrument concept of operationsand planned data products.
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Journal articleSmillie DG, Pickering JC, Nave G, et al., 2016,
The spectrum and term analysis of Co iii measured using Fourier transforms and grating spectroscopy
, Astrophysical Journal Supplement Series, Vol: 223, ISSN: 1538-4365The spectrum of Co iii has been recorded in the region 1562–2564 Å (64,000 cm−1–39,000 cm−1) by Fourier transform (FT) spectroscopy, and in the region 1317–2500 Å (164,000 cm−1–40,000 cm−1) using a 10.7 m grating spectrograph with phosphor image plate detectors. The spectrum was excited in a cobalt–neon Penning discharge lamp. We classified 514 Co iii lines measured using FT spectroscopy, the strongest having wavenumber uncertainties approaching 0.004 cm−1 (approximately 0.2 mÅ at 2000 Å, or 1 part in 107), and 240 lines measured with grating spectroscopy with uncertainties between 5 and 10 mÅ. The wavelength calibration of 790 lines of Raassen & Ortí Ortin and 87 lines from Shenstone has been revised and combined with our measurements to optimize the values of all but one of the 288 previously reported energy levels. Order of magnitude reductions in uncertainty for almost two-thirds of the 3d64s and almost half of the 3d64p revised energy levels are obtained. Ritz wavelengths have been calculated for an additional 100 forbidden lines. Eigenvector percentage compositions for the energy levels and predicted oscillator strengths have been calculated using the Cowan code.
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Journal articleKimura T, Kraft RP, Elsner RF, et al., 2016,
Jupiter's X-ray and EUV auroras monitored by Chandra, XMM-Newton, and Hisaki satellite
, Journal of Geophysical Research: Space Physics, Vol: 121, Pages: 2308-2320, ISSN: 2169-9402Jupiter's X-ray auroral emission in the polar cap region results from particles which have undergone strong field-aligned acceleration into the ionosphere. The origin of precipitating ions and electrons and the time variability in the X-ray emission are essential to uncover the driving mechanism for the high-energy acceleration. The magnetospheric location of the source field line where the X-ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long-term monitoring of the X-rays and contemporaneous solar wind variability has not been carried out. In April 2014, the first long-term multiwavelength monitoring of Jupiter's X-ray and EUV auroral emissions was made by the Chandra X-ray Observatory, XMM-Newton, and Hisaki satellite. We find that the X-ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X-ray auroral region was found to be open to the interplanetary space. The other half of the X-ray auroral source region is magnetically connected with the prenoon to postdusk sector in the outermost region of the magnetosphere, where the Kelvin-Helmholtz (KH) instability, magnetopause reconnection, and quasiperiodic particle injection potentially take place. We speculate that the high-energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators.
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Journal articleTao J, Wang L, Zong Q, et al., 2016,
QUIET-TIME SUPRATHERMAL (∼0.1-1.5 keV) ELECTRONS IN THE SOLAR WIND
, ASTROPHYSICAL JOURNAL, Vol: 820, ISSN: 0004-637X- Author Web Link
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- Citations: 25
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Journal articleMarsham JH, Parker DJ, Todd MC, et al., 2016,
The contrasting roles of water and dust in controlling daily variations in radiative heating of the summertime Saharan heat low
, Atmospheric Chemistry and Physics, Vol: 16, Pages: 3563-3575, ISSN: 1680-7324The summertime Sahara heat low (SHL) is a key component of the West African monsoon (WAM) system. Considerable uncertainty remains over the relative roles of water vapour and dust aerosols in controlling the radiation budget over the Sahara and therefore our ability to explain variability and trends in the SHL, and in turn, the WAM. Here, new observations from Fennec supersite-1 in the central Sahara during June 2011 and June 2012, together with satellite retrievals from GERB, are used to quantify how total column water vapour (TCWV) and dust aerosols (from aerosol optical depth, AOD) control day-to-day variations in energy balance in both observations and ECWMF reanalyses (ERA-I). The data show that the earth-atmosphere system is radiatively heated in June 2011 and 2012. Although the empirical analysis of observational data cannot completely disentangle the roles of water vapour, clouds and dust, the analysis demonstrates that TCWV provides a far stronger control on TOA net radiation, and so the net heating of the earth-atmosphere system, than AOD does. In contrast, variations in dust provide a much stronger control on surface heating, but the decreased surface heating associated with dust is largely compensated by increased atmospheric heating, and so dust control on net TOA radiation is weak. Dust and TCWV are both important for direct atmospheric heating. ERA-I, which assimilated radiosondes from the Fennec campaign, captures the control of TOA net flux by TCWV, with a positive correlation (r = 0.6) between observed and modelled TOA net radiation, despite the use of a monthly dust climatology in ERA-I that cannot capture the daily variations in dustiness. Variations in surface net radiation, and so the vertical profile of radiative heating, are not captured in ERA-I, since it does not capture variations in dust. Results show that ventilation of the SHL by cool moist air leads to a radiative warming, stabilising the SHL with respect to such perturbations. It is k
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Book chapterMoore L, Stallard T, Galand MIF, 2016,
Upper atmospheres of the giant planets
, Heliophysics: Active Stars, their Astrospheres, and Impacts on Planetary Environments, Editors: Schrijver, Bagenal, Sojka, Publisher: Cambridge University Press, Pages: 175-200, ISBN: 9781107090477 -
Journal articleSamset BH, Myhre G, Forster PM, et al., 2016,
Fast and slow precipitation responses to individual climate forcers: a PDRMIP multimodel study
, Geophysical Research Letters, Vol: 43, Pages: 2782-2791, ISSN: 1944-8007Precipitation is expected to respond differently to various drivers of anthropogenic climate change. We present the first results from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where nine global climate models have perturbed CO2, CH4, black carbon, sulfate, and solar insolation. We divide the resulting changes to global mean and regional precipitation into fast responses that scale with changes in atmospheric absorption and slow responses scaling with surface temperature change. While the overall features are broadly similar between models, we find significant regional intermodel variability, especially over land. Black carbon stands out as a component that may cause significant model diversity in predicted precipitation change. Processes linked to atmospheric absorption are less consistently modeled than those linked to top-of-atmosphere radiative forcing. We identify a number of land regions where the model ensemble consistently predicts that fast precipitation responses to climate perturbations dominate over the slow, temperature-driven responses.
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Journal articleParfitt R, czaja A, Minobe S, et al., 2016,
The atmospheric frontal response to SST perturbations in the Gulf Stream region
, Geophysical Research Letters, Vol: 43, Pages: 2299-2306, ISSN: 1944-8007The link between sea surface temperature (SST) gradients and atmospheric fronts is explored in a general circulation model across the Gulf Stream (GS) region from December to February 1981–2000. Two model experiments are analyzed, one with a realistic control SST distribution and one with a spatially smoothed SST distribution. The analysis shows a noticeable change in regional atmospheric frontal frequency between the two experiments (up to 30%), with the distribution of change exhibiting a clear imprint of the GS SST front. Further analysis of the surface sensible heat flux gradient across cold fronts reveals the pattern of change to be mediated by a thermal interaction between the oceanic and atmospheric fronts (“thermal damping and strengthening”). These results not only emphasize the significance of the GS SST gradient for storm development in the North Atlantic but also highlight the importance of resolution in assessing the role of frontal air-sea interaction in midlatitude climate variability.
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Journal articleGoetz C, Koenders C, Richter I, et al., 2016,
First detection of a diamagnetic cavity at comet 67P/Churyumov-Gerasimenko
, Astronomy & Astrophysics, Vol: 588, ISSN: 1432-0746Context. The Rosetta magnetometer RPC-MAG has been exploring the plasma environment of comet 67P/Churyumov-Gerasimenko since August 2014. The first months were dominated by low-frequency waves which evolved into more complex features. However, at the end of July 2015, close to perihelion, the magnetometer detected a region that did not contain any magnetic field at all. Aims. These signatures match the appearance of a diamagnetic cavity as was observed at comet 1P/Halley in 1986. The cavity here is more extended than previously predicted by models and features unusual magnetic field configurations, which need to be explained. Methods. The onboard magnetometer data were analyzed in detail and used to estimate the outgassing rate. A minimum variance analysis was used to determine boundary normals. Results. Our analysis of the data acquired by the Rosetta Plasma Consortium instrumentation confirms the existence of a diamagnetic cavity. The size is larger than predicted by simulations, however. One possible explanation are instabilities that are propagating along the cavity boundary and possibly a low magnetic pressure in the solar wind. This conclusion is supported by a change in sign of the Sun-pointing component of the magnetic field. Evidence also indicates that the cavity boundary is moving with variable velocities ranging from 230-500 m/s.
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Journal articlePhan TD, Shay MA, Eastwood JP, et al., 2016,
Establishing the Context for Reconnection Diffusion Region Encounters and Strategies for the Capture and Transmission of Diffusion Region Burst Data by MMS
, Space Science Reviews, Vol: 199, Pages: 631-650, ISSN: 0038-6308© 2015, The Author(s). This paper describes the efforts of our Inter-Disciplinary Scientist (IDS) team to (a) establish the large-scale context for reconnection diffusion region encounters by MMS at the magnetopause and in the magnetotail, including the distinction between X-line and O-line encounters, that would help the identification of diffusion regions in spacecraft data, and (b) devise possible strategies that can be used by MMS to capture and transmit burst data associated with diffusion region candidates. At the magnetopause we suggest the strategy of transmitting burst data from all magnetopause crossings so that no magnetopause reconnection diffusion regions encountered by the spacecraft will be missed. The strategy is made possible by the MMS mass memory and downlink budget. In the magnetotail, it is estimated that MMS will be able to transmit burst data for all diffusion regions, all reconnection jet fronts (a.k.a. dipolarization fronts) and separatrix encounters, but less than 50 % of reconnection exhausts encountered by the spacecraft. We also discuss automated burst trigger schemes that could capture various reconnection-related phenomena. The identification of candidate diffusion region encounters by the burst trigger schemes will be verified and improved by a Scientist-In-The-Loop (SITL). With the knowledge of the properties of the region surrounding the diffusion region and the combination of automated burst triggers and further optimization by the SITL, MMS should be able to capture most diffusion regions it encounters.
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Journal articleBall WT, Haigh JD, Rozanov EV, et al., 2016,
High solar cycle spectral variations inconsistent with stratospheric ozone observations
, Nature Geoscience, Vol: 9, Pages: 206-209, ISSN: 1752-0894
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