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Journal articleMyhre G, Samset BH, Hodnebrog Ø, et al., 2018,
Sensible heat has significantly affected the global hydrological cycle over the historical period
, Nature Communications, Vol: 9, ISSN: 2041-1723Globally, latent heating associated with a change in precipitation is balanced by changes to atmospheric radiative cooling and sensible heat fluxes. Both components can be altered by climate forcing mechanisms and through climate feedbacks, but the impacts of climate forcing and feedbacks on sensible heat fluxes have received much less attention. Here we show, using a range of climate modelling results, that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since preindustrial time, because the radiative impact of forcings and feedbacks approximately compensate. The model results show a dissimilar influence on sensible heat and precipitation from various drivers of climate change. Due to its strong atmospheric absorption, black carbon is found to influence the sensible heat very differently compared to other aerosols and greenhouse gases. Our results indicate that this is likely caused by differences in the impact on the lower tropospheric stability.
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Journal articleHunt GJ, Provan G, Cowley SWH, et al., 2018,
Saturn's planetary period oscillations during the closest approach of Cassini's ring grazing orbits
, Geophysical Research Letters, Vol: 45, Pages: 4692-4700, ISSN: 0094-8276Saturn's planetary period oscillations (PPOs) are ubiquitous throughout its magnetosphere. We investigate the PPO's azimuthal magnetic field amplitude interior to the field‐aligned currents, during the closest approaches of Cassini's ring‐grazing orbits (October 2016 to April 2017), with periapses at ~2.5 RS. The amplitudes of the northern and southern PPO systems are shown to vary as a function of latitude. The amplitude ratio between the two PPO systems shows that the northern system is dominant by a factor of ~1.3 in the equatorial plane, and it is dominant to ~ −15° latitude in the southern hemisphere. The dayside amplitudes are approximately half of the 2008 nightside amplitudes, which agree with previous local time‐related amplitude observations. Overall, there is clear evidence that the PPOs are present on field lines that map to the outer edge of Saturn's rings, closer to Saturn than previously confirmed.
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Journal articlePhan TD, Eastwood JP, Shay MA, et al., 2018,
Electron magnetic reconnection without ion coupling in Earth's turbulent magnetosheath
, Nature, Vol: 557, Pages: 202-206, ISSN: 0028-0836Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region 1,2 . On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed 3-5 . Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region 6 . In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales 7-11 . However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth's turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.
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- Citations: 284
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Journal articleGryspeerdt ERI, Quaas J, Goren T, et al., 2018,
An automated cirrus classification
, Atmospheric Chemistry and Physics, Vol: 18, Pages: 6157-669, ISSN: 1680-7316Cirrus clouds play an important role in determining the radiation budget of the earth, but many of their propertiesremain uncertain, particularly their response to aerosol variations and to warming. Part of the reason for this uncertainty isthe dependence of cirrus cloud properties on the cloud formation mechanism, which itself is strongly dependent on the localmeteorological conditions.In this work, a classification system (Identification and Classification of Cirrus or IC-CIR) is introduced to identify cirrusclouds by the cloud formation mechanism. Using re-analysis and satellite data, cirrus clouds are separated in four main types:orographic, frontal, convective and synoptic. Through a comparison to convection-permitting model simulations and back-trajectory based analysis, it is shown that these observation-based regimes can provide extra information on the cloud scaleupdraughts and the frequency of occurrence of liquid-origin ice, with the convective regime having higher updraughts and agreater occurrence of liquid-origin ice compared to the synoptic regimes. Despite having different cloud formation mecha-nisms, the radiative properties of the regimes are not distinct, indicating that retrieved cloud properties alone are insufficient tocompletely describe them.This classification is designed to be easily implemented in GCMs, helping improve future model-observation comparisonsand leading to improved parametrisations of cirrus cloud processes
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Journal articlePlaschke F, Hietala H, 2018,
Plasma flow patterns in and around magnetosheath jets
, ANNALES GEOPHYSICAE, Vol: 36, Pages: 695-703, ISSN: 0992-7689 -
Journal articleProvan G, Cowley SWH, Bradley TJ, et al., 2018,
Planetary period oscillations in Saturn's magnetosphere: Cassini magnetic field observations over the northern summer solstice interval
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 3859-3899, ISSN: 2169-9380We determine properties of Saturn's planetary period oscillations from Cassini magnetic measurements over the ~2‐year interval from September 2015 to end of mission in September 2017, spanning Saturn northern summer solstice in May 2017. Phases of the northern system oscillations are derived over the whole interval, while those of the southern system are not discerned in initial equatorial data due to too low amplitude relative to the northern, but are determined once southern polar data become available from inclined orbits beginning May 2016. Planetary period oscillation periods are shown to be almost constant over these intervals at ~10.79 hr for the northern system and ~10.68 hr for the southern, essentially unchanged from values previously determined after the periods reversed in 2014. High cadence phase and amplitude data obtained from the short‐period Cassini orbits during the mission's last 10 months newly reveal the presence of dual modulated oscillations varying at the beat period of the two systems (~42 days) on nightside polar field lines in the vicinity (likely either side) of the open‐closed field boundary. The modulations differ from those observed previously in the equatorial region, indicative of a reversal in sign of the radial component oscillations, but not of the colatitudinal component oscillations. Brief discussion is given of a possible theoretical scenario. While weak equatorial beat modulations indicate a north/south amplitude ratio >5 early in the study interval, polar and equatorial region modulations suggest a ratio ~1.4 during the later interval, indicating a significant recovery of the southern system.
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Journal articleBradley TJ, Cowley SWH, Provan G, et al., 2018,
Field-aligned currents in Saturn's nightside magnetosphere: subcorotation and planetary period oscillation components during northern spring
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 3602-3636, ISSN: 2169-9380We newly analyze Cassini magnetic field data from the 2012/2013 Saturn northern spring interval of highly inclined orbits and compare them with similar data from late southern summer in 2008, thus providing unique information on the seasonality of the currents that couple momentum between Saturn's ionosphere and magnetosphere. Inferred meridional ionospheric currents in both cases consist of a steady component related to plasma subcorotation, together with the rotating current systems of the northern and southern planetary period oscillations (PPOs). Subcorotation currents during the two intervals show opposite north‐south polar region asymmetries, with strong equatorward currents flowing in the summer hemispheres but only weak currents flowing to within a few degrees of the open‐closed boundary (OCB) in the winter hemispheres, inferred due to weak polar ionospheric conductivities. Currents peak at ~1 MA rad−1 in both hemispheres just equatorward of the open‐closed boundary, associated with total downward polar currents ~6 MA, then fall across the narrow auroral upward current region to small values at subauroral latitudes. PPO‐related currents have a similar form in both summer and winter with principal upward and downward field‐aligned currents peaking at ~1.25 MA rad−1 being essentially collocated with the auroral upward current and approximately equal in strength. Though northern and southern PPO currents were approximately equal during both intervals, the currents in both hemispheres were dual modulated by both systems during 2012/2013, with approximately half the main current closing in the opposite ionosphere and half cross field in the magnetosphere, while only the northern hemisphere currents were similarly dual modulated in 2008.
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Journal articleHarrison RA, Davies JA, Barnes D, et al., 2018,
CMEs in the Heliosphere: I. A Statistical Analysis of the Observational Properties of CMEs Detected in the Heliosphere from 2007 to 2017 by STEREO/HI-1
, Solar Physics, Vol: 293, ISSN: 0038-0938We present a statistical analysis of coronal mass ejections (CMEs) imaged by the Heliospheric Imager (HI) instruments on board NASA’s twin-spacecraft STEREO mission between April 2007 and August 2017 for STEREO-A and between April 2007 and September 2014 for STEREO-B. The analysis exploits a catalogue that was generated within the FP7 HELCATS project. Here, we focus on the observational characteristics of CMEs imaged in the heliosphere by the inner (HI-1) cameras, while following papers will present analyses of CME propagation through the entire HI fields of view. More specifically, in this paper we present distributions of the basic observational parameters – namely occurrence frequency, central position angle (PA) and PA span – derived from nearly 2000 detections of CMEs in the heliosphere by HI-1 on STEREO-A or STEREO-B from the minimum between Solar Cycles 23 and 24 to the maximum of Cycle 24; STEREO-A analysis includes a further 158 CME detections from the descending phase of Cycle 24, by which time communication with STEREO-B had been lost. We compare heliospheric CME characteristics with properties of CMEs observed at coronal altitudes, and with sunspot number. As expected, heliospheric CME rates correlate with sunspot number, and are not inconsistent with coronal rates once instrumental factors/differences in cataloguing philosophy are considered. As well as being more abundant, heliospheric CMEs, like their coronal counterparts, tend to be wider during solar maximum. Our results confirm previous coronagraph analyses suggesting that CME launch sites do not simply migrate to higher latitudes with increasing solar activity. At solar minimum, CMEs tend to be launched from equatorial latitudes, while at maximum, CMEs appear to be launched over a much wider latitude range; this has implications for understanding the CME/solar source association. Our analysis provides some supporting evidence for the systematic dragging of CMEs to lower latitude
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Journal articleSmith CW, Vasquez BJ, Coburn JT, et al., 2018,
Correlation Scales of the Turbulent Cascade at 1 au
, ASTROPHYSICAL JOURNAL, Vol: 858, ISSN: 0004-637X -
Journal articleLyons JR, Herde H, Stark G, et al., 2018,
VUV pressure-broadening in sulfur dioxide
, JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER, Vol: 210, Pages: 156-164, ISSN: 0022-4073 -
Book chapterMoore L, Galand M, Kliore AJ, et al., 2018,
Saturn's Ionosphere
, Saturn in the 21st Century, Editors: Baines, Publisher: Cambridge University PressThis chapter summarizes our current understanding of the ionosphere ofSaturn. We give an overview of Saturn ionospheric science from the Voyager erato the present, with a focus on the wealth of new data and discoveries enabledby Cassini, including a massive increase in the number of electron densityaltitude profiles. We discuss recent ground-based detection of the effect of"ring rain" on Saturn's ionosphere, and present possible model interpretationsof the observations. Finally, we outline current model-data discrepancies andindicate how future observations can help in advancing our understanding of thevarious controlling physical and chemical processes.
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Journal articleStansby D, Horbury T, 2018,
Number density structures in the inner heliosphere
, Astronomy and Astrophysics, Vol: 613, ISSN: 0004-6361Aims.The origins and generation mechanisms of the slow solar wind are still unclear. Part of the slow solar wind is populated by“number density structures”, discrete patches of increased number density that are frozen in to and move with the bulk solar wind. Inthis paper we aimed to provide the first in-situ statistical study of number density structures in the inner heliosphere.Methods.We reprocessed in-situ ion distribution functions measured by Helios in the inner heliosphere to provide a new reliable setof proton plasma moments for the entire mission. From this new data set we looked for number density structures measured within0.5 AU of the Sun and studied their properties.Results.We identified 140 discrete areas of enhanced number density. The structures occurred exclusively in the slow solar wind andspanned a wide range of length scales from 50 Mm to 2000 Mm, which includes smaller scales than have been previously observed.They were also consistently denser and hotter that the surrounding plasma, but had lower magnetic field strengths, and thereforeremained in pressure balance.Conclusions.Our observations show that these structures are present in the slow solar wind at a wide range of scales, some of whichare too small to be detected by remote sensing instruments. These structures are rare, accounting for only 1% of the slow solar windmeasured by Helios, and are not a significant contribution to the mass flux of the solar wind.
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Journal articleKarlsson T, Plaschke F, Hietala H, et al., 2018,
Investigating the anatomy of magnetosheath jets - MMS observations
, ANNALES GEOPHYSICAE, Vol: 36, Pages: 655-677, ISSN: 0992-7689We use Magnetosphere Multiscale (MMS) mission data to investigate a small number of magnetosheath jets, which are localized and transient increases in dynamic pressure, typically due to a combined increase in plasma velocity and density. For two approximately hour-long intervals in November, 2015 we found six jets, which are of two distinct types. (a) Two of the jets are associated with the magnetic field discontinuities at the boundary between the quasi-parallel and quasi-perpendicular magnetosheath. Straddling the boundary, the leading part of these jets contains an ion population similar to the quasi-parallel magnetosheath, while the trailing part contains ion populations similar to the quasi-perpendicular magnetosheath. Both populations are, however, cooler than the surrounding ion populations. These two jets also have clear increases in plasma density and magnetic field strength, correlated with a velocity increase. (b) Three of the jets are found embedded within the quasi-parallel magnetosheath. They contain ion populations similar to the surrounding quasi-parallel magnetosheath, but with a lower temperature. Out of these three jets, two have a simple structure. For these two jets, the increases in density and magnetic field strength are correlated with the dynamic pressure increases. The other jet has a more complicated structure, and no clear correlations between density, magnetic field strength and dynamic pressure. This jet has likely interacted with the magnetosphere, and contains ions similar to the jets inside the quasi-parallel magnetosheath, but shows signs of adiabatic heating. All jets are associated with emissions of whistler, lower hybrid, and broadband electrostatic waves, as well as approximately 10 s period electromagnetic waves with a compressional component. The latter have a Poynting flux of up to 40 µW m−2 and may be energetically important for the evolution of the jets, depending on the wave excitation mechanism. Only one of th
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Journal articleHellinger P, Verdini A, Landi S, et al., 2018,
von Karman-Howarth Equation for Hall Magnetohydrodynamics: Hybrid Simulations
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 857, ISSN: 2041-8205A dynamical vectorial equation for homogeneous incompressible Hall-magnetohydrodynamic (MHD) turbulence together with the exact scaling law for third-order correlation tensors, analogous to that for the incompressible MHD, is rederived and applied to the results of two-dimensional hybrid simulations of plasma turbulence. At large (MHD) scales the simulations exhibit a clear inertial range where the MHD dynamic law is valid. In the sub-ion range the cascade continues via the Hall term, but the dynamic law derived in the framework of incompressible Hall-MHD equations is obtained only in a low plasma beta simulation. For a higher beta plasma the cascade rate decreases in the sub-ion range and the change becomes more pronounced as the plasma beta increases. This break in the cascade flux can be ascribed to nonthermal (kinetic) features or to others terms in the dynamical equation that are not included in the Hall-MHD incompressible approximation.
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Journal articleBruneau N, Toumi R, Wang S, 2018,
Publisher correction: Impact of wave whitecapping on land falling tropical cyclones
, Scientific Reports, Vol: 8, ISSN: 2045-2322 -
Journal articleKrupar V, Maksimovic M, Kontar EP, et al., 2018,
Interplanetary Type III Bursts and Electron Density Fluctuations in the Solar Wind
, ASTROPHYSICAL JOURNAL, Vol: 857, ISSN: 0004-637XType III bursts are generated by fast electron beams originated from magnetic reconnection sites of solar flares. As propagation of radio waves in the interplanetary medium is strongly affected by random electron density fluctuations, type III bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements. Here, we performed a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial RElations Observatory mission. We investigated their time–frequency profiles in order to retrieve decay times as a function of frequency. Next, we performed Monte Carlo simulations to study the role of scattering due to random electron density fluctuations on time–frequency profiles of radio emissions generated in the interplanetary medium. For simplification, we assumed the presence of isotropic electron density fluctuations described by a power law with the Kolmogorov spectral index. Decay times obtained from observations and simulations were compared. We found that the characteristic exponential decay profile of type III bursts can be explained by the scattering of the fundamental component between the source and the observer despite restrictive assumptions included in the Monte Carlo simulation algorithm. Our results suggest that relative electron density fluctuations $\langle \delta {n}_{{\rm{e}}}\rangle /{n}_{{\rm{e}}}$ in the solar wind are 0.06–0.07 over wide range of heliospheric distances.
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Journal articleCarbary JF, Mitchell DG, Kollmann P, et al., 2018,
Energetic electron pitch angle distributions during the Cassini final orbits
, Geophysical Research Letters, Vol: 45, Pages: 2911-2917, ISSN: 0094-8276Pitch angle distributions (PADs) of very energetic electrons (110–365 keV) are examined during the ring‐grazing and proximal orbits of the Cassini spacecraft, from day 320 2016 (15 November) to day 257 2017 (14 September). These repeating orbits allowed a statistical evaluation of the PADs within the magnetopause on the nightside of Saturn. Along L‐shells (i.e., equatorial crossing distances of magnetic field lines) near and outside that of Titan and north of the equator, the electron fluxes were unidirectionally field‐aligned going away from Saturn. Along L‐shells inside Titan's and south of the equator, the electrons had bidirectional or pancake (trapping) PADs. This behavior suggests that the field lines within Titan's L‐shell are generally closed, while those outside of that L‐shell are generally open. This result strictly applies only to the nightside local times sampled during the final Cassini orbits, but one may infer a similar behavior at other times.
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Journal articleGristey JJ, Chiu JC, Gurney RJ, et al., 2018,
Insights into the diurnal cycle of global Earth outgoing radiation using a numerical weather prediction model
, ATMOSPHERIC CHEMISTRY AND PHYSICS, Vol: 18, Pages: 5129-5145, ISSN: 1680-7316A globally complete, high temporal resolution and multiple-variable approach is employed to analyse the diurnal cycle of Earth's outgoing energy flows. This is made possible via the use of Met Office model output for September 2010 that is assessed alongside regional satellite observations throughout. Principal component analysis applied to the long-wave component of modelled outgoing radiation reveals dominant diurnal patterns related to land surface heating and convective cloud development, respectively explaining 68.5 and 16.0 % of the variance at the global scale. The total variance explained by these first two patterns is markedly less than previous regional estimates from observations, and this analysis suggests that around half of the difference relates to the lack of global coverage in the observations. The first pattern is strongly and simultaneously coupled to the land surface temperature diurnal variations. The second pattern is strongly coupled to the cloud water content and height diurnal variations, but lags the cloud variations by several hours. We suggest that the mechanism controlling the delay is a moistening of the upper troposphere due to the evaporation of anvil cloud. The short-wave component of modelled outgoing radiation, analysed in terms of albedo, exhibits a very dominant pattern explaining 88.4 % of the variance that is related to the angle of incoming solar radiation, and a second pattern explaining 6.7 % of the variance that is related to compensating effects from convective cloud development and marine stratocumulus cloud dissipation. Similar patterns are found in regional satellite observations, but with slightly different timings due to known model biases. The first pattern is controlled by changes in surface and cloud albedo, and Rayleigh and aerosol scattering. The second pattern is strongly coupled to the diurnal variations in both cloud water content and height in convective regions but only cloud water content in marine stratocu
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Journal articleCheng ZW, Shi JK, Zhang JC, et al., 2018,
Influence of the IMF cone angle on invariant latitudes of polar region footprints of FACs in the magnetotail: cluster observation
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 2588-2597, ISSN: 2169-9380The influence of the interplanetary magnetic field (IMF) cone angle θ (the angle between the IMF direction and the Sun-Earth line) on the invariant latitudes of the footprints of the field-aligned currents (FACs) in the magnetotail has been investigated. We performed a statistical study of 542 FAC cases observed by the four Cluster spacecraft in the Northern Hemisphere. The results show that there are almost no FACs when the IMF cone angle is less than 10°, and there are indications of the FACs in the plasma sheet boundary layers being weak under the radial IMF conditions. The footprints of the large FAC ( > 10 nA/m 2 ) cases are within invariant latitudes < 71° and mainly within IMF cone angles θ > 60°, which implies that the footprints of the large FACs mainly expand equatorward with large IMF cone angle. The equatorward boundary of the FAC footprints in the polar region decreases with increasing IMF cone angle (and has a better correlation for northward IMF), which shows that the IMF cone angle plays an important controlling role in FAC distributions in the magnetosphere-ionosphere coupling system. There is almost no correlation between the poleward boundary and the IMF cone angle for both northward and southward IMF. This is because the poleward boundary movement is limited by an enhanced lobe magnetic flux. This is the first time a correlation between FAC footprints in the polar region and IMF cone angles has been determined.
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Journal articleWang S, Toumi R, 2018,
A historical analysis of the mature stage of tropical cyclones
, International Journal of Climatology, Vol: 38, Pages: 2490-2505, ISSN: 0899-8418The characteristics of tropical cyclone intensity and size during the mature stage are presented. Rooted in the classic description by Herbert Riehl, the mature stage is identified as the period from the time of lifetime maximum intensity to the time of lifetime maximum size. This study is the first to analyse the global climatology of the mature stage of tropical cyclones in detail. Three basic features at the mature stage are observed: the reduction of intensity, the outward expansion of the eyewall, and the increase of tangential wind in the outer primary circulation. Globally, about a quarter of tropical cyclones undergo the mature stage. High intensity at the end of the immature stage favours the likelihood of the occurrence of the mature stage. The intensity reduction during the mature stage is considerable with nearly three-quarters of cyclones decreasing by more than 10%, which makes the conventional ‘steady-state’ presumption questionable. The increase in the radius of damaging-force wind is typically about 50 km, while the decrease in maximum wind speed is typically 20% at the mature stage. However, the average integrated kinetic energy and hence destructive potential increases substantially by about 70%. This is consistent with our finding that most of the highly damaging landfalling hurricanes undergo a mature stage. Intensity downgrades during the mature stage may be misinterpreted as they are mostly not accompanied by an overall danger reduction.
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Journal articleHall RJ, Hanna E, 2018,
North Atlantic circulation indices: links with summer and winter UK temperature and precipitation and implications for seasonal forecasting
, INTERNATIONAL JOURNAL OF CLIMATOLOGY, Vol: 38, Pages: E660-E677, ISSN: 0899-8418- Author Web Link
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- Citations: 42
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Journal articleErgun RE, Goodrich KA, Wilder FD, et al., 2018,
Magnetic Reconnection, Turbulence, and Particle Acceleration: Observations in the Earth's Magnetotail
, Geophysical Research Letters, Vol: 45, Pages: 3338-3347, ISSN: 0094-8276We report observations of turbulent dissipation and particle acceleration from large-amplitude electric fields (E) associated with strong magnetic field (B) fluctuations in the Earth's plasma sheet. The turbulence occurs in a region of depleted density with anti-earthward flows followed by earthward flows suggesting ongoing magnetic reconnection. In the turbulent region, ions and electrons have a significant increase in energy, occasionally > 100 keV, and strong variation. There are numerous occurrences of |E| > 100 mV/m including occurrences of large potentials ( > 1 kV) parallel to B and occurrences with extraordinarily large J · E (J is current density). In this event, we find that the perpendicular contribution of J · E with frequencies near or below the ion cyclotron frequency (f ci ) provide the majority net positive J · E. Large-amplitude parallel E events with frequencies above f ci to several times the lower hybrid frequency provide significant dissipation and can result in energetic electron acceleration.
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Journal articleRichardson TB, Forster PM, Andrews T, et al., 2018,
Carbon Dioxide Physiological Forcing Dominates Projected Eastern Amazonian Drying
, Geophysical Research Letters, Vol: 45, Pages: 2815-2825, ISSN: 0094-8276Future projections of east Amazonian precipitation indicate drying, but they are uncertain and poorly understood. In this study we analyze the Amazonian precipitation response to individual atmospheric forcings using a number of global climate models. Black carbon is found to drive reduced precipitation over the Amazon due to temperature-driven circulation changes, but the magnitude is uncertain. CO 2 drives reductions in precipitation concentrated in the east, mainly due to a robustly negative, but highly variable in magnitude, fast response. We find that the physiological effect of CO 2 on plant stomata is the dominant driver of the fast response due to reduced latent heating and also contributes to the large model spread. Using a simple model, we show that CO 2 physiological effects dominate future multimodel mean precipitation projections over the Amazon. However, in individual models temperature-driven changes can be large, but due to little agreement, they largely cancel out in the model mean.
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Journal articleMendillo M, Trovato J, Moore L, et al., 2018,
Comparative ionospheres: terrestrial and giant planets
, Icarus, Vol: 303, Pages: 34-46, ISSN: 0019-1035The study of planetary ionospheres within our solar system offers a variety of settings to probe mechanisms of photo-ionization, chemical loss, and plasma transport. Ionospheres are a minor component of upper atmospheres, and thus their mix of ions observed depends on the neutral gas composition of their parent atmospheres. The same solar irradiance (x-rays and extreme-ultra-violet vs. wavelength) impinges upon each of these atmospheres, with solar flux magnitudes changed only by the inverse square of distance from the Sun. If all planets had the same neutral atmosphere—with ionospheres governed by photochemical equilibrium (production = loss)—their peak electron densities would decrease as the inverse of distance from the Sun, and any changes in solar output would exhibit coherent effects throughout the solar system.Here we examine the outer planet with the most observations of its ionosphere (Saturn) and compare its patterns of electron density with those at Earth under the same-day solar conditions. We show that, while the average magnitudes of the major layers of molecular ions at Earth and Saturn are approximately in accord with distance effects, only minor correlations exist between solar effects and day-to-day electron densities. This is in marked contrast to the strong correlations found between the ionospheres of Earth and Mars. Moreover, the variability observed for Saturn's ionosphere (maximum electron density and total electron content) is much larger than found at Earth and Mars. With solar irradiance changes far too small to cause such effects, we use model results to explore the roles of other agents. We find that water sources from Enceladus at low latitudes, and ‘ring rain’ at middle latitudes, contribute substantially to variability via water ion chemistry. Thermospheric winds and electrodynamics generated at auroral latitudes are suggested causes of high latitude ionospheric variability, but remain inconclusive due to the l
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Journal articleMunoz V, Dominguez M, Alejandro Valdivia J, et al., 2018,
Evolution of fractality in space plasmas of interest to geomagnetic activity
, NONLINEAR PROCESSES IN GEOPHYSICS, Vol: 25, Pages: 207-216, ISSN: 1023-5809- Author Web Link
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- Citations: 9
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Journal articleGood SW, Forsyth RJ, Eastwood JP, et al., 2018,
Correlation of ICME magnetic fields at radially aligned spacecraft
, Solar Physics, Vol: 293, ISSN: 0038-0938The magnetic field structures of two interplanetary coronal mass ejections (ICMEs), each observed by a pair of spacecraft close to radial alignment, have been analysed. The ICMEs were observed in situ by MESSENGER and STEREO-B in November 2010 and November 2011, while the spacecraft were separated by more than 0.6 AU in heliocentric distance, less than 4° in heliographic longitude, and less than 7° in heliographic latitude. Both ICMEs took approximately two days to travel between the spacecraft. The ICME magnetic field profiles observed at MESSENGER have been mapped to the heliocentric distance of STEREO-B and compared directly to the profiles observed by STEREO-B. Figures that result from this mapping allow for easy qualitative assessment of similarity in the profiles. Macroscale features in the profiles that varied on timescales of one hour, and which corresponded to the underlying flux rope structure of the ICMEs, were well correlated in the solar east–west and north–south directed components, with Pearson’s correlation coefficients of approximately 0.85 and 0.95, respectively; microscale features with timescales of one minute were uncorrelated. Overall correlation values in the profiles of one ICME were increased when an apparent change in the flux rope axis direction between the observing spacecraft was taken into account. The high degree of similarity seen in the magnetic field profiles may be interpreted in two ways. If the spacecraft sampled the same region of each ICME (i.e. if the spacecraft angular separations are neglected), the similarity indicates that there was little evolution in the underlying structure of the sampled region during propagation. Alternatively, if the spacecraft observed different, nearby regions within the ICMEs, it indicates that there was spatial homogeneity across those different regions. The field structure similarity observed in these ICMEs points to the value of placing in situ space weather monitors w
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Journal articleGenestreti KJ, Varsani A, Burch JL, et al., 2018,
MMS observation of asymmetric reconnection supported by 3-D electron pressure divergence
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 1806-1821, ISSN: 2169-9380We identify the electron diffusion region (EDR) of a guide field dayside reconnection site encountered by the Magnetospheric Multiscale (MMS) mission and estimate the terms in generalized Ohm's law that controlled energy conversion near the X-point. MMS crossed the moderate-shear (∼130°) magnetopause southward of the exact X-point. MMS likely entered the magnetopause far from the X-point, outside the EDR, as the size of the reconnection layer was less than but comparable to the magnetosheath proton gyroradius, and also as anisotropic gyrotropic "outflow" crescent electron distributions were observed. MMS then approached the X-point, where all four spacecraft simultaneously observed signatures of the EDR, for example, an intense out-of-plane electron current, moderate electron agyrotropy, intense electron anisotropy, nonideal electric fields, and nonideal energy conversion. We find that the electric field associated with the nonideal energy conversion is (a) well described by the sum of the electron inertial and pressure divergence terms in generalized Ohms law though (b) the pressure divergence term dominates the inertial term by roughly a factor of 5:1, (c) both the gyrotropic and agyrotropic pressure forces contribute to energy conversion at the X-point, and (d) both out-of-the-reconnection-plane gradients (∂/∂M) and in-plane (∂/∂L,N) in the pressure tensor contribute to energy conversion near the X-point. This indicates that this EDR had some electron-scale structure in the out-of-plane direction during the time when (and at the location where) the reconnection site was observed.
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Journal articleKacem I, Jacquey C, Génot V, et al., 2018,
Magnetic reconnection at a thin current sheet separating two interlaced flux tubes at the Earth's magnetopause
, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 1779-1793, ISSN: 2169-9380The occurrence of spatially and temporally variable reconnection at the Earth's magnetopause leads to the complex interaction of magnetic fields from the magnetosphere and magnetosheath. Flux transfer events (FTEs) constitute one such type of interaction. Their main characteristics are (1) an enhanced core magnetic field magnitude and (2) a bipolar magnetic field signature in the component normal to the magnetopause, reminiscent of a large-scale helicoidal flux tube magnetic configuration. However, other geometrical configurations which do not fit this classical picture have also been observed. Using high-resolution measurements from the Magnetospheric Multiscale mission, we investigate an event in the vicinity of the Earth's magnetopause on 7 November 2015. Despite signatures that, at first glance, appear consistent with a classic FTE, based on detailed geometrical and dynamical analyses as well as on topological signatures revealed by suprathermal electron properties, we demonstrate that this event is not consistent with a single, homogenous helicoidal structure. Our analysis rather suggests that it consists of the interaction of two separate sets of magnetic field lines with different connectivities. This complex three-dimensional interaction constructively conspires to produce signatures partially consistent with that of an FTE. We also show that, at the interface between the two sets of field lines, where the observed magnetic pileup occurs, a thin and strong current sheet forms with a large ion jet, which may be consistent with magnetic flux dissipation through magnetic reconnection in the interaction region.
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Journal articleLiang M, Czaja A, Graversen R, et al., 2018,
Poleward energy transport: is the standard definition physically relevant at all time scales?
, Climate Dynamics, Vol: 50, Pages: 1785-1797, ISSN: 0930-7575Poleward energy transport in the atmosphere and oceans constitutes an important branch of the global energy budget, and its role in the climate system has been the subject of many studies. In the atmosphere, the transport is affected by “eddies” and large scale meridional cells, both with zero net mass transport across latitude circles, but also partly by processes associated with a net transport of mass across latitude circles. The latter must cease to operate in steady state, but they may be significant when time variability of the heat budget is considered. Indeed, examination of reanalysis data on short (daily to monthly) timescales shows that mass variations on these timescales result in surprisingly large fluctuations (in excess of 1015 W = 1 PW) in the poleward heat transport. These fluctuations are referred to as “extensive”, for they primarily alter the mass integrated energy of the region considered, but not its averaged value. It is suggested that extensive fluctuations mask more meaningful climate signals present in the heat transport variability on monthly and interannual timescales, and a new formulation is proposed to isolate the latter. This new formulation is applied successfully to reanalysis data and climate model simulations.
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Journal articleKiehas SA, Runov A, Angelopolos V, et al., 2018,
Magnetotail Fast Flow Occurrence Rate and Dawn-Dusk Asymmetry at <i>X</i><sub>GSM</sub> ∼-60 <i>R<sub>E</sub></i>
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 1767-1778, ISSN: 2169-9380- Author Web Link
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