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Journal articleAla-Lahti M, Ruohotie J, Good S, et al., 2020,
Spatial coherence of interplanetary coronal mass ejection-driven sheaths at 1 AU
<jats:p> &lt;p&gt;&lt;span&gt;We report on the longitudinal coherence of sheath regions driven by interplanetary coronal mass ejections (ICMEs). ICME sheaths are significant drivers of geomagnetic activity at the Earth, with a considerable fraction of ICME-driven storms being either entirely or primarily induced by the sheath. Similarly to Lugaz et al. (2018; doi:10.3847/2041-8213/aad9f4&lt;/span&gt;&lt;span&gt;), we have analyzed two-point magnetic field measurements made by the ACE and &lt;em&gt;Wind &lt;/em&gt;spacecraft in 29 ICME sheaths to estimate the coherence scale lengths, defined as the spatial scale at which correlation between measurements falls to zero, of the field magnitude and components. Scale lengths for the sheath are found to be mostly smaller than the corresponding values in the ICME driver, an expected result given that ICME sheaths are characterized by highly fluctuating, variable magnetic fields, in contrast to the often more coherent ejecta. A relatively large scale length for the magnetic field component in the GSE &lt;em&gt;y&lt;/em&gt;-direction was found. We discuss how magnetic field line draping around the ejecta and the alignment of pre-existing magnetic structures by the preceding shock may explain the observed results. In addition, we consider the existence of longitudinally extended and possibly geoeffective magnetic field fluctuations within ICME sheaths, the full understanding of which requires further multi-spacecraft analysis.&lt;/span&gt;&lt;/p&gt; </jats:p>
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Journal articleTrotta D, Franci L, Burgess D, et al., 2020,
The role of turbulence strength on the acceleration of transrelativistic electrons
<jats:p> &lt;p&gt;Energetic particles are widely observed in many astrophysical systems, but the physical mechanisms responsible for their acceleration are not yet fully understood. We address the interaction of suprathermal, transrelativistic electrons with plasma turbulence at ion and sub-ion scales using a combination of hybrid particle-in-cell and test particle simulations. First, we present results of simulations with different turbulence amplitude. Two different mechanisms for electron energisation are identified: one is consistent with the picture of stochastic acceleration in turbulence, yielding to moderate electron energisation, while the other one involves electron trapping in turbulent structures, resulting in an efficient and fast electron energisation. The latter is observed to be active only for certain combinations of turbulence amplitude and electron initial energy. Furthermore, varying the injection scale, we explore the importance of the size of turbulent magnetic structures and of the nonlinear time associated to their dynamical evolution on electron acceleration. These results have important implications for electron acceleration in a wide range of space and astrophysical systems.&lt;/p&gt; </jats:p>
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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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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 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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Journal articleDavies E, Forsyth R, Good S, 2020,
Using In-Situ Juno Observations to Understand the Evolution of Interplanetary Coronal Mass Ejections Within 1 AU and Beyond
<jats:p> &lt;p&gt;Understanding the evolution of interplanetary coronal mass ejections (ICMEs) as they propagate through the heliosphere is essential in forecasting space weather severity. Much of our knowledge of ICMEs has been gained using in-situ measurements from single spacecraft, although the increasing number of missions in the inner heliosphere has led to an increase in multi-spacecraft studies improving our understanding of the global structure of ICMEs. Whilst most such recent studies have focused on the inner heliosphere within 1 AU, Juno cruise phase data provides a new opportunity to study ICME evolution over greater distances. We present analysis of ICMEs observed in-situ both by Juno and at least one other spacecraft within 1 AU to investigate their evolution as they propagate through the heliosphere. Investigation of the sheath region and timing considerations between spacecraft allows for the general shape of the shock front to be reconstructed. Combining in-situ observations and results of flux rope fitting techniques determines the global picture of the ICME as it propagates. However, effects on in-situ observations due to radial evolution and due to the longitudinal separation between multi-spacecraft remain hard to separate. We note the importance of the interplanetary environment in which the ICME propagates and the need for caution in radial alignment studies. &amp;#160;&lt;/p&gt; </jats:p>
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Journal articleKilpua E, Good S, Palmerio E, et al., 2020,
Multi-spacecraft Observations of interacting CME flux ropes
<jats:p> &lt;p&gt;Interactions between coronal mass ejections (CMEs) in interplanetary space are a highly important aspect for understanding their physical dynamics and evolution as well as their space weather consequences. Here we present an analysis of three CMEs that erupted from the Sun on June 12-14, 2012 using almost radially aligned spacecraft at Venus and Earth, complemented by heliospheric imaging and modelling with EUHFORIA. These multi-spacecraft observations were critical for interpreting the event correctly, in particular regarding the last two CMEs in the series (June 13 and June 14). At the orbit of Venus these CMEs were mostly separate with the June 14 CME just about to reach the previous CME. A significant interaction occurred before the CMEs reached the Earth. The shock of the June 14 CME had propagated through the June 13 CME and the two CMEs had coalesced into a single large flux rope structure before they reached the Earth. This merged flux rope had one of the largest magnetic field magnitudes observed in the near-Earth solar wind during Solar Cycle 24. We discuss also the general importance of multi-spacecraft observations and modelling using them in analyzing solar eruptions.&lt;/p&gt; </jats:p>
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Journal articleGood S, Ala-Lahti M, Palmerio E, et al., 2020,
Radial evolution of magnetic field fluctuations in an ICME sheath
<jats:p> &lt;p&gt;The sheaths of compressed solar wind that precede interplanetary coronal mass ejections (ICMEs) commonly display large-amplitude magnetic field fluctuations. As ICMEs propagate radially from the Sun, the properties of these fluctuations may evolve significantly. We present a case study of an ICME sheath observed by a pair of radially aligned spacecraft at around 0.5 and 1 AU from the Sun. Radial changes in fluctuation amplitude, compressibility, inertial-range spectral slope, permutation entropy, Jensen-Shannon complexity, and planar structuring are characterised.&amp;#160; We discuss the extent to which the observed evolution in the fluctuations is similar to that of solar wind emanating from steady sources at quiet times, how the evolution may be influenced by evolving local factors such as leading-edge shock orientation, and how the perturbed heliospheric environment associated with ICME propagation may impact the evolution more generally.&lt;/p&gt; </jats:p>
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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 articlePlaschke F, Jernej M, Hietala H, et al., 2020,
On the alignment of velocity and magnetic fields within magnetosheath jets
, Annales Geophysicae: atmospheres, hydrospheres and space sciences, Vol: 38, Pages: 287-296, ISSN: 0992-7689Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Based on Magnetospheric Multiscale (MMS) jet observations and corresponding superposed epoch analyses of the angles ϕ between the velocity and magnetic fields, we can confirm that this suggestion is correct. However, while the alignment is more significant for faster than for slower jets, and for jets observed close to the bow shock, the overall effect is small: typically, reductions in ϕ of around 10∘ are observed at jet core regions, where the jets' velocities are largest. Furthermore, time series of ϕ pertaining to individual jets significantly deviate from the superposed epoch analysis results. They usually exhibit large variations over the entire range of ϕ: 0 to 90∘. This variability is commonly somewhat larger within jets than outside them, masking the systematic decrease in ϕ at core regions of individual jets.
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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 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: 40
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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: 4
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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 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 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 articleNakamura TKM, Stawarz JE, Hasegawa H, et al., 2020,
Effects of Fluctuating Magnetic Field on the Growth of the Kelvin-Helmholtz Instability at the Earth's Magnetopause
, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380- Author Web Link
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- Citations: 19
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Journal articleHietala H, Dimmock AP, Zou Y, et al., 2020,
The challenges and rewards of running a geospace environment modeling challenge
, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-5, ISSN: 2169-9380Geospace Environment Modeling (GEM) is a community‐driven, National Science Foundation‐sponsored research program investigating the physics of the Earth's magnetosphere and its coupling to the solar wind and the atmosphere. This commentary provides an introduction to a Special Issue collating recent studies related to a GEM Challenge on kinetic plasma processes in the dayside magnetosphere during southward interplanetary magnetic field conditions. We also recount our experiences of organizing such a collaborative activity, where modelers and observers compare their results, that is, of the human side of bringing researchers together. We give suggestions on planning, managing, funding, and documenting these activities, which provide valuable opportunities to advance the field.
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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 articleNemecek Z, Durovcova T, Safrankova J, et al., 2020,
What is the solar wind frame of reference?
, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 889, Pages: 1-14, ISSN: 0004-637XVarious solar wind ion species move with different speeds and theoretical considerations as well as limited observations in a region close to the Sun show that heavy solar wind ions tend to flow faster than protons, at least in less-aged fast solar wind streams. The solar wind flow carries the frozen-in interplanetary magnetic field (IMF) and this situation evokes three related questions: (i) what is the proper solar wind speed, (ii) is this speed equal to the speed of the dominant component, whatever that may be, and (iii) what is the speed of the magnetic field? We show that simple theoretical considerations based on the MHD approximation as well as on the dynamics of charged particles in electric and magnetic fields suggest that the IMF velocity of motion (de Hoffmann–Teller (HT) velocity) would be deliberated as the velocity appropriate for solar wind studies. Our analysis based on the Wind, Helios, ACE, and SOHO observations of differential streaming of solar wind populations shows that their energy is conserved in the HT frame. On the other hand, the noise and temporal resolution of the data do not allow us to decide whether the total momentum is also conserved in this frame.
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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 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 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 -
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 articleLeske RA, Christian ER, Cohen CMS, et al., 2020,
Observations of the 2019 April 4 Solar Energetic Particle Event at theParker Solar Probe
, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 246, ISSN: 0067-0049
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