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Journal articleBercic L, Larson D, Whittlesey P, et al., 2020,
Coronal electron temperature inferred from the strahl electrons in the inner heliosphere: parker solar probe and helios observations
, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 892, Pages: 1-14, ISSN: 0004-637XThe shape of the electron velocity distribution function plays an important role in the dynamics of the solar wind acceleration. Electrons are normally modeled with three components, the core, the halo, and the strahl. We investigate how well the fast strahl electrons in the inner heliosphere preserve the information about the coronal electron temperature at their origin. We analyzed the data obtained by two missions, Helios, spanning the distances between 65 and 215 R S, and Parker Solar Probe (PSP), reaching down to 35 R S during its first two orbits around the Sun. The electron strahl was characterized with two parameters: pitch-angle width (PAW) and the strahl parallel temperature (T s∥). PSP observations confirm the already reported dependence of strahl PAW on core parallel plasma beta (${\beta }_{\mathrm{ec}\parallel }$). Most of the strahl measured by PSP appear narrow with PAW reaching down to 30°. The portion of the strahl velocity distribution function aligned with the magnetic field is for the measured energy range well described by a Maxwellian distribution function. T s∥ was found to be anticorrelated with the solar wind velocity and independent of radial distance. These observations imply that T s∥ carries the information about the coronal electron temperature. The obtained values are in agreement with coronal temperatures measured using spectroscopy, and the inferred solar wind source regions during the first orbit of PSP agree with the predictions using a PFSS model.
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Journal articleNair V, Heus T, Van Reeuwijk M, 2020,
Dynamics of subsiding shells in actively growing clouds with vertical updrafts
, Journal of the Atmospheric Sciences, Vol: 77, Pages: 1353-1369, ISSN: 0022-4928The dynamics of a subsiding shell at the edges of actively growing shallow cumulus clouds with updrafts is analyzed using direct numerical simulation. The actively growing clouds have a fixed in-cloud buoyancy and velocity. Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell which grows with time. A self-similar regime is observed for first and second order moments when normalized with respective maximum values. Internal scales derived from integral properties of the flow problem are identified. Self-similarity analysis conducted by normalizing using these scales reveal that contrary to classical self similar flows, the turbulent kinetic energy budget terms and velocity moments scale according to the buoyancy and not with the mean velocity. The shell thickness is observed to increase linearly with time. The buoyancy scale remains time-invariant and is set by the initial cloud-environment thermodynamics. The shell accelerates ballistically with a magnitude set by the saturation value of the buoyancy of the cloud-environment mixture. In this regime, the shell is buoyancy driven and independent of the in-cloud velocity. Relations are obtained for predicting the shell thickness and minimum velocities by linking the internal scales with external flow parameters. The values thus calculated are consistent with the thickness and velocities observed in typical shallow cumulus clouds. The entrainment coefficient is a function of the initial state of the cloud and the environment, and is shown to be of the same order of magnitude as fractional entrainment rates calculated for large scale models.
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Journal articleAdhikari S, Shay MA, Parashar TN, et al., 2020,
Reconnection from a turbulence perspective
, Physics of Plasmas, Vol: 27, Pages: 1-10, ISSN: 1070-664XThe spectral properties associated with laminar, anti-parallel reconnection are examined using a 2.5D kinetic particle in cell simulation. Both the reconnection rate and the energy spectrum exhibit three distinct phases: an initiation phase where the reconnection rate grows, a quasi-steady phase, and a declining phase where both the reconnection rate and the energy spectrum decrease. During the steady phase, the energy spectrum exhibits approximately a double power law behavior, with a slope near −5/3 at wave numbers smaller than the inverse ion inertial length and a slope steeper than −8/3 for larger wave numbers up to the inverse electron inertial length. This behavior is consistent with a Kolmogorov energy cascade and implies that laminar reconnection may fundamentally be an energy cascade process. Consistent with this idea is the fact that the reconnection rate exhibits a rough correlation with the energy spectrum at wave numbers near the inverse ion inertial length. The 2D spectrum is strongly anisotropic with most energy associated with the wave vector direction normal to the current sheet. Reconnection acts to isotropize the energy spectrum, reducing the Shebalin angle from an initial value of 70° to about 48° (nearly isotropic) by the end of the simulation. The distribution of energy over length scales is further analyzed by dividing the domain into spatial subregions and employing structure functions.
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Journal articleMalaspina DM, Szalay JR, Pokorny P, et al., 2020,
In Situ Observations of Interplanetary Dust Variability in the Inner Heliosphere
, ASTROPHYSICAL JOURNAL, Vol: 892, ISSN: 0004-637X- Author Web Link
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- Citations: 30
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Journal articleKrasnoselskikh V, Larosa A, Agapitov O, et al., 2020,
Localized Magnetic-field Structures and Their Boundaries in the Near-Sun Solar Wind from Parker Solar Probe Measurements
, ASTROPHYSICAL JOURNAL, Vol: 893, ISSN: 0004-637X- Author Web Link
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- Citations: 53
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Journal articleJohnston CD, Cargill PJ, Hood AW, et al., 2020,
Modelling the solar transition region using an adaptive conduction method
, Astronomy & Astrophysics, Vol: 635, Pages: A168-A168, ISSN: 0004-6361Modelling the solar Transition Region with the use of an Adaptive Conduction (TRAC) method permits fast and accurate numerical solutions of the field-aligned hydrodynamic equations, capturing the enthalpy exchange between the corona and transition region, when the corona undergoes impulsive heating. The TRAC method eliminates the need for highly resolved numerical grids in the transition region and the commensurate very short time steps that are required for numerical stability. When employed with coarse spatial resolutions, typically achieved in multi-dimensional magnetohydrodynamic codes, the errors at peak density are less than 5% and the computation time is three orders of magnitude faster than fully resolved field-aligned models. This paper presents further examples that demonstrate the versatility and robustness of the method over a range of heating events, including impulsive and quasi-steady footpoint heating. A detailed analytical assessment of the TRAC method is also presented, showing that the approach works through all phases of an impulsive heating event because (i) the total radiative losses and (ii) the total heating when integrated over the transition region are both preserved at all temperatures under the broadening modifications of the method. The results from the numerical simulations complement this conclusion.
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Journal articleBruneau N, Wang S, Toumi R, 2020,
Long memory impact of ocean mesoscale temperature anomalies on tropical cyclone size
, Geophysical Research Letters, Vol: 47, ISSN: 0094-8276Mesoscale ocean temperature anomalies modify a tropical cyclone (TC). Through a modeling study we show that, while the maximum wind speed is rapidly restored after the TC passes a warm‐ or cold‐ (eddy size) sea surface temperature (SST) anomaly, the storm size changes are more significant and persistent. The radius of gale force winds and integrated kinetic energy (IKE) can change by more than 10% per degree and this endures several days after crossing an SST anomaly. These properties have a long memory of the impact from the ocean fluxes and depend on the integrated history of SST exposure. They are found to be directly proportional to the storm total precipitation. Accurate continuous forecast of the SST along the track may therefore be of central importance to improving predictions of size and IKE, while instantaneous local SST near the TC core is more important for the forecast of maximum wind speed.
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Journal articleKaweeyanun N, Masters A, Jia X, 2020,
Favorable conditions for magnetic reconnection at ganymede’s upstream magnetopause
, Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276Ganymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady‐state conditions at Ganymede's magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede's closed magnetic field encounters Jupiter's ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter's rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer mission.
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Journal articleFargette N, Lavraud B, Øieroset M, et al., 2020,
On the ubiquity of magnetic reconnection inside flux transfer event‐like structures at the earth's magnetopause
, Geophysical Research Letters, Vol: 47, Pages: 1-9, ISSN: 0094-8276Flux transfer events (FTEs) are transient phenomena frequently observed at the Earth's magnetopause. Their usual interpretation is a flux rope moving away from the reconnection region. However, the Magnetospheric Multiscale Mission revealed that magnetic reconnection sometimes occurs inside these structures, questioning their flux rope configuration. Here we investigate 229 FTE‐type structures and find reconnection signatures inside 19% of them. We analyze their large‐scale magnetic topology using electron heat flux and find that it is significantly different across the FTE reconnecting current sheets, demonstrating that they are constituted of two magnetically disconnected structures. We also find that the interplanetary magnetic field (IMF) associated with reconnecting FTEs presents a strong By component. We discuss several formation mechanisms to explain these observations. In particular, the maximum magnetic shear model predicts that for large IMF By, two spatially distinct X lines coexist at the magnetopause. They can generate separate magnetic flux tubes that may become interlaced.
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Conference paperGryspeerdt E, Smith T, O'Keefe E, et al., 2020,
Impact of ship emission controls recorded by cloud properties
<jats:p> &lt;p&gt;The impact of aerosols on cloud properties is one of the largest uncertainties in the anthropogenic forcing of the climate system. As large, isolated sources of aerosol, ships provide the ideal opportunity to investigate aerosol-cloud interactions. However, their use for quantifying the aerosol impact on clouds has been limited by a lack on information on the aerosol perturbation generated by the ship.&lt;/p&gt;&lt;p&gt;In this work, satellite cloud observations are combined with ship emissions estimated from transponder data. Using over 17,000 shiptracks during the implementation of emission controls, the central role of sulphate aerosol in controlling shiptrack properties is demonstrated. Meteorological factors are shown to have a significant impact on shiptrack formation, particularly cloud-top relative humidity. Accounting for this meteorological variation, this work also demonstrates the potential for satellite retrievals of ship sulphate emissions, providing a pathway to the use of cloud observations for monitoring air pollution.&lt;/p&gt; </jats:p>
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Journal articleFujita R, Graven H, 2020,
Impact of atmospheric radiocarbon and stable isotope measurements on understanding the global CH4 budget over 1850&#8211;2015
<jats:p> &lt;p&gt;Measurements of stable isotope ratios of atmospheric CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;(&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;D-CH&lt;sub&gt;4&lt;/sub&gt;) have been utilized to evaluate contributions of individual CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;sources and sinks to global atmospheric CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;budget. However, given the uncertainty of both the source isotope signatures and kinetic isotope effects, recent estimates of the global atmospheric CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;budget using stable isotope observations are still inconclusive&lt;span&gt;.&amp;#160;&lt;/span&gt;Radiocarbon measurements (&amp;#916;&lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;) could provide &lt;span&gt;stronger&amp;#160;&lt;/span&gt;additional&amp;#160;&lt;span&gt;constraint&amp;#160;&lt;/span&gt;&lt;span&gt;on&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;fossil-fuel&amp;#160;&lt;/span&gt;CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;&lt;span&gt;sources &lt;/span&gt;(i.e.,&lt;sup&gt;14&lt;/sup&gt;C-free)&lt;span&gt;, but the uncertainty of &lt;sup&gt;14&lt;/sup&gt;CH&lt;sub&gt;4&lt;/sub&g
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Conference paperSourdeval O, Gryspeerdt E, Mülmenstädt J, et al., 2020,
Satellite-based estimate of the climate forcing due to aerosol - ice cloud interactions
<jats:p> &lt;p&gt;Substantial efforts have been led over the last decades to improve our understanding of the interactions between clouds and anthropogenic aerosols (aci). The effective radiative forcing associated with these interactions (ERFaci), which combines the radiative forcing (i.e. Twomey effect) and cloud adjustments, still constitutes a large part of our current uncertainties on climate predictions.&lt;/p&gt;&lt;p&gt;Important progress has been made in the assessment of ERFaci for liquid clouds, partly due to advances in the joint use of satellite and modelling data to tackle this problem. More particularly, the retrieval of the droplet number concentration from satellite remote sensing - a property closely related to droplet nucleation processes - has been extremely helpful to better quantify ERFaci. However, similar estimations for ice clouds have for long suffered from a lack of observational constraint on the ice crystal number concentration (N&lt;sub&gt;i&lt;/sub&gt;), a challenging task due to the high complexity of the physical processes associated with the nucleation and growth of ice crystals. However, a novel long-term global dataset of N&lt;sub&gt;i&lt;/sub&gt; from active satellite measurements has recently (DARDAR-Nice) opened the door to new observation-based estimates of RFaci for ice clouds.&lt;/p&gt;&lt;p&gt;This study investigates aerosol - ice clouds interactions using N&lt;sub&gt;i&lt;/sub&gt; profiles from the DARDAR-Nice product together with collocated aerosol information from the Copernicus Atmospheric Monitoring Service (CAMS) reanalyses. A multitude of cloud regimes, subdivided into seasonal and regional bins, are considered in order to disentangle meteorological effects from the aci signature. First results of joint-histograms between N&am
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Journal articleNowack P, Runge J, Eyring V, et al., 2020,
Causal networks for climate model evaluation and constrained projections
, Nature Communications, Vol: 11, ISSN: 2041-1723Global climate models are central tools for understanding past and future climate change. The assessment of model skill, in turn, can benefit from modern data science approaches. Here we apply causal discovery algorithms to sea level pressure data from a large set of climate model simulations and, as a proxy for observations, meteorological reanalyses. We demonstrate how the resulting causal networks (fingerprints) offer an objective pathway for process-oriented model evaluation. Models with fingerprints closer to observations better reproduce important precipitation patterns over highly populated areas such as the Indian subcontinent, Africa, East Asia, Europe and North America. We further identify expected model interdependencies due to shared development backgrounds. Finally, our network metrics provide stronger relationships for constraining precipitation projections under climate change as compared to traditional evaluation metrics for storm tracks or precipitation itself. Such emergent relationships highlight the potential of causal networks to constrain longstanding uncertainties in climate change projections.
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Journal articleHajra R, Henri P, Vallières X, et al., 2020,
Ionospheric total electron content of comet 67P/Churyumov-Gerasimenko
, Astronomy & Astrophysics, Vol: 635, Pages: A51-A51, ISSN: 0004-6361We study the evolution of a cometary ionosphere, using approximately two years of plasma measurements by the Mutual Impedance Probe on board the Rosetta spacecraft monitoring comet 67P/Churyumov-Gerasimenko (67P) during August 2014–September 2016. The in situ plasma density measurements are utilized to estimate the altitude-integrated electron number density or cometary ionospheric total electron content (TEC) of 67P based on the assumption of radially expanding plasma. The TEC is shown to increase with decreasing heliocentric distance (rh) of the comet, reaching a peak value of ~(133 ± 84) × 109 cm−2 averaged around perihelion (rh < 1.5 au). At large heliocentric distances (rh > 2.5 au), the TEC decreases by ~2 orders of magnitude. For the same heliocentric distance, TEC values are found to be significantly larger during the post-perihelion periods compared to the pre-perihelion TEC values. This “ionospheric hysteresis effect” is more prominent in the southern hemisphere of the comet and at large heliocentric distances. A significant hemispheric asymmetry is observed during perihelion with approximately two times larger TEC values in the northern hemisphere compared to the southern hemisphere. The asymmetry is reversed and stronger during post-perihelion (rh > 1.5 au) periods with approximately three times larger TEC values in the southern hemisphere compared to the northern hemisphere. Hemispheric asymmetry was less prominent during the pre-perihelion intervals. The correlation of the cometary TEC with the incident solar ionizing fluxes is maximum around and slightly after perihelion (1.5 au < rh < 2 au), while it significantly decreases at larger heliocentric distances (rh > 2.5 au) where the photo-ionization contribution to the TEC variability decreases. The results are discussed based on cometary ionospheric production and loss processes.
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Journal articleZappa G, Ceppi P, Shepherd TG, 2020,
Time-evolving sea-surface warming patterns modulate the climate change response of subtropical precipitation over land
, Proceedings of the National Academy of Sciences of the United States of America, Vol: 117, Pages: 4539-4545, ISSN: 0027-8424Greenhouse gas (GHG) emissions affect precipitation worldwide. The response is commonly described by two timescales linked to different processes: a rapid adjustment to radiative forcing, followed by a slower response to surface warming. However, additional timescales exist in the surface-warming response, tied to the time evolution of the sea-surface-temperature (SST) response. Here, we show that in climate model projections, the rapid adjustment and surface mean warming are insufficient to explain the time evolution of the hydro-climate response in three key Mediterranean-like areas—namely, California, Chile, and the Mediterranean. The time evolution of those responses critically depends on distinct shifts in the regional atmospheric circulation associated with the existence of distinct fast and slow SST warming patterns. As a result, Mediterranean and Chilean drying are in quasiequilibrium with GHG concentrations, meaning that the drying will not continue after GHG concentrations are stabilized, whereas California wetting will largely emerge only after GHG concentrations are stabilized. The rapid adjustment contributes to a reduction in precipitation, but has a limited impact on the balance between precipitation and evaporation. In these Mediterranean-like regions, future hydro-climate–related impacts will be substantially modulated by the time evolution of the pattern of SST warming that is realized in the real world.
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Journal articleAgapitov OV, de Wit TD, Mozer FS, et al., 2020,
Sunward-propagating Whistler Waves Collocated with Localized Magnetic Field Holes in the Solar Wind: <i>Parker Solar Probe</i> Observations at 35.7 <i>R<sub>⊙</sub></i> Radii
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 891, ISSN: 2041-8205- Author Web Link
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- Citations: 51
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Journal articleHanson ELM, Agapitov OV, Vasko IY, et al., 2020,
Shock Drift Acceleration of Ions in an Interplanetary Shock Observed by <i>MMS</i>
, ASTROPHYSICAL JOURNAL LETTERS, Vol: 891, ISSN: 2041-8205- Author Web Link
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- Citations: 10
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Journal articleAgiwal O, Hunt GJ, Dougherty MK, et al., 2020,
Modeling the Temporal Variability in Saturn's Magnetotail Current Sheet From the Cassini F-ring Orbits
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380 -
Journal articleDAmicis R, Matteini L, Bruno R, et al., 2020,
Large amplitude fluctuations in the alfvénic solar wind
, Solar Physics, Vol: 295, Pages: 1-12, ISSN: 0038-0938Large amplitude fluctuations, often with characteristics reminiscent of large amplitude Alfvén waves propagating away from the Sun, are ubiquitous in the solar wind. Such features are most frequently found within fast solar wind streams and most often at solar minimum. The fluctuations found in slow solar wind streams usually have a smaller relative amplitude, are less Alfvénic in character and present more variability. However, intervals of slow wind displaying Alfvénic correlations have been recently identified in different solar cycle phases. In the present paper we report Alfvénic slow solar wind streams seen during the maximum of solar activity that are characterized not only by a very high correlation between velocity and magnetic field fluctuations (as required by outwardly propagating Alfvén modes) – comparable to that seen in fast wind streams – but also by higher amplitude relative fluctuations comparable to those seen in fast wind. Our results suggest that the Alfvénic slow wind has a different origin from the slow wind found near the boundary of coronal holes, where the amplitude of the Alfvénic fluctuations decreases together with decreasing the wind speed.
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Journal articleBellouin N, Quaas J, Gryspeerdt E, et al., 2020,
Bounding global aerosol radiative forcing of climate change
, Reviews of Geophysics, Vol: 58, Pages: 1-45, ISSN: 8755-1209Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the im balance in the Earth’s radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable and arguable lines of evidence, including modelling approaches, theoretical considerations, and obser vations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol61 driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of −1.60 to −0.65 W m−2, or −2.0 to −0.4 W m−2 with a 90% like lihood. Those intervals are of similar width to the last Intergovernmental Panel on Cli mate Change assessment but shifted towards more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds.
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Journal articleLavergne A, Voelker S, Csank A, et al., 2020,
Historical changes in the stomatal limitation of photosynthesis: empirical support for an optimality principle
, New Phytologist, Vol: 225, Pages: 2484-2497, ISSN: 0028-646XThe ratio of leaf‐internal (ci) to ambient (ca) partial pressure of CO2, defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation models uncertainties for predicting terrestrial carbon uptake and water use.We test a theory based on the least‐cost optimality hypothesis for modelling historical changes in χ over the 1951‐2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely‐dated tree‐ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure.The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the inter‐site variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model.Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant‐available soil water and other site‐specific characteristics might improve the predictions.
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Journal articleHaaland S, Paschmann G, Øieroset M, et al., 2020,
Characteristics of the flank magnetopause: MMS results
, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-13, ISSN: 2169-9380We have used a large number of magnetopause crossings by the Magnetospheric Multi Spacecraft (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1000 km near the terminator. Current densities varies in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn‐dusk asymmetry in many of the macroscopic parameters; The dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity.
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Journal articleSimon AA, Fletcher LN, Arridge C, et al., 2020,
A review of the in situ probe designs from recent ice giant mission concept studies
, Space Science Reviews, Vol: 216, Pages: 1-13, ISSN: 0038-6308For the Ice Giants, atmospheric entry probes provide critical measurements not attainable via remote observations. Including the 2013–2022 NASA Planetary Decadal Survey, there have been at least five comprehensive atmospheric probe engineering design studies performed in recent years by NASA and ESA. International science definition teams have assessed the science requirements, and each recommended similar measurements and payloads to meet science goals with current instrument technology. The probe system concept has matured and converged on general design parameters that indicate the probe would include a 1-meter class aeroshell and have a mass around 350 to 400-kg. Probe battery sizes vary, depending on the duration of a post-release coast phase, and assumptions about heaters and instrument power needs. The various mission concepts demonstrate the need for advanced power and thermal protection system development. The many completed studies show an Ice Giant mission with an in situ probe is feasible and would be welcomed by the international science community.
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Journal articlePhan TD, Bale SD, Eastwood JP, et al., 2020,
Parker solar probe In situ observations of magnetic reconnection exhausts during encounter 1
, The Astrophysical Journal Supplement, Vol: 246, Pages: 34-34, ISSN: 0067-0049Magnetic reconnection in current sheets converts magnetic energy into particle energy. The process may play an important role in the acceleration and heating of the solar wind close to the Sun. Observations from Parker Solar Probe (PSP) provide a new opportunity to study this problem, as it measures the solar wind at unprecedented close distances to the Sun. During the first orbit, PSP encountered a large number of current sheets in the solar wind through perihelion at 35.7 solar radii. We performed a comprehensive survey of these current sheets and found evidence for 21 reconnection exhausts. These exhausts were observed in heliospheric current sheets, coronal mass ejections, and regular solar wind. However, we find that the majority of current sheets encountered around perihelion, where the magnetic field was strongest and plasma β was lowest, were Alfvénic structures associated with bursty radial jets, and these current sheets did not appear to be undergoing local reconnection. We examined conditions around current sheets to address why some current sheets reconnected while others did not. A key difference appears to be the degree of plasma velocity shear across the current sheets: the median velocity shear for the 21 reconnection exhausts was 24% of the Alfvén velocity shear, whereas the median shear across 43 Alfvénic current sheets examined was 71% of the Alfvén velocity shear. This finding could suggest that large, albeit sub-Alfvénic, velocity shears suppress reconnection. An alternative interpretation is that the Alfvénic current sheets are isolated rotational discontinuities that do not undergo local reconnection.
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Journal articleHorbury T, Woolley T, Laker R, et al., 2020,
Sharp Alfvenic impulses in the near-Sun solar wind
, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 246, Pages: 1-8, ISSN: 0004-637XMeasurements of the near-Sun solar wind by Parker Solar Probe have revealed the presence of largenumbers of discrete Alfv ́enic impulses with an anti-Sunward sense of propagation. These are similarto those previously observed near 1 AU, in high speed streams over the Sun’s poles and at 60 solarradii. At 35 solar radii, however, they are typically shorter and sharper than seen elsewhere. Inaddition, these spikes occur in “patches” and there are also clear periods within the same stream whenthey do not occur; the timescale of these patches might be related to the rate at which the spacecraftmagnetic footpoint tracks across the coronal hole from which the plasma originated. While the velocityfluctuations associated with these spikes are typically under 100 km/s, due to the rather low Alfv ́enspeeds in the streams observed by the spacecraft to date, these are still associated with large angulardeflections of the magnetic field - and these deflections are not isotropic. These deflections do notappear to be related to the recently reported large scale, pro-rotation solar wind flow. Estimates ofthe size and shape of the spikes reveal high aspect ratio flow-aligned structures with a transverse scaleof≈104km. These events might be signatures of near-Sun impulsive reconnection events.
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Journal articleMartinovic MM, Klein KG, Kasper JC, et al., 2020,
The Enhancement of Proton Stochastic Heating in the Near-Sun Solar Wind
, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 246, ISSN: 0067-0049 -
Journal articleTenerani A, Velli M, Matteini L, et al., 2020,
Magnetic field kinks and folds in the solar wind
, Astrophysical Journal Supplement Series, Vol: 246, Pages: 1-7, ISSN: 0067-0049Parker Solar Probe (PSP) observations during its first encounter at 35.7 R ⊙ have shown the presence of magnetic field lines that are strongly perturbed to the point that they produce local inversions of the radial magnetic field, known as switchbacks. Their counterparts in the solar wind velocity field are local enhancements in the radial speed, or jets, displaying (in all components) the velocity–magnetic field correlation typical of large amplitude Alfvén waves propagating away from the Sun. Switchbacks and radial jets have previously been observed over a wide range of heliocentric distances by Helios, Wind, and Ulysses, although they were prevalent in significantly faster streams than seen at PSP. Here we study via numerical magnetohydrodynamics simulations the evolution of such large amplitude Alfvénic fluctuations by including, in agreement with observations, both a radial magnetic field inversion and an initially constant total magnetic pressure. Despite the extremely large excursion of magnetic and velocity fields, switchbacks are seen to persist for up to hundreds of Alfvén crossing times before eventually decaying due to the parametric decay instability. Our results suggest that such switchback/jet configurations might indeed originate in the lower corona and survive out to PSP distances, provided the background solar wind is sufficiently calm, in the sense of not being pervaded by strong density fluctuations or other gradients, such as stream or magnetic field shears, that might destabilize or destroy them over shorter timescales.
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Journal articleSzalay JR, Pokorny P, Bale SD, et al., 2020,
The Near-Sun Dust Environment: Initial Observations fromParker Solar Probe
, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 246, ISSN: 0067-0049 -
Journal articleChen CHK, Bale SD, Bonnell JW, et al., 2020,
The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere
, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 246, ISSN: 0067-0049 -
Journal articleGiacalone J, Mitchell DG, Allen RC, et al., 2020,
Solar Energetic Particles Produced by a Slow Coronal Mass Ejection at similar to 0.25 au
, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 246, ISSN: 0067-0049
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