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Journal articleKilpua EKJ, Fontaine D, Good S, et al., 2020,
Magnetic field fluctuation properties of coronal mass ejection-driven sheath regions in the near-Earth solar wind
<jats:p>Abstract. In this work, we investigate the magnetic field fluctuations in three coronal mass ejection (CME)-driven sheath regions at 1 AU with their speeds ranging from slow to fast. The data set we use consists primarily of high resolution (0.092 s) magnetic field measurements from the Wind spacecraft. We analyse magnetic field fluctuation amplitudes and fluctuation amplitudes normalised to the mean magnetic field, compressibility, and spectral properties of fluctuations. We also analyse intermittency using various approaches: we apply the partial variance of increments (PVI) method, investigate probability distribution functions of fluctuations, including their skewness and kurtosis, and perform a structure function analysis. Our analysis is conducted separately for three different subregions in the sheath and in the solar wind ahead of it, each 1 hr in duration. We find that, for all cases, the transition from the solar wind ahead to the sheath generates new fluctuations and the intermittency and compressibility increase, while the region closest to the ejecta leading edge resembled the solar wind ahead. The spectral indices exhibit large variability in different parts of the sheath, but are typically steeper than Kolmogorov's in the inertial range. The structure function analysis produced generally much better fit with the extended p-model (Kraichnan's form) than with the standard version, implying that turbulence is not fully developed in CME sheaths near Earth's orbit. The p-values obtained (p~0.8–0.9) also suggest relatively high intermittency. At the smallest timescales investigated, the spectral indices indicate relatively shallow slopes (between −2 and −2.5), suggesting that in CME-driven sheaths at 1 AU the energy cascade from larger to smaller scales could still be ongoing through the ion scale. Regarding many properties (e.g., spectral indices and compressibility) turbulent properties in sheath
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Journal articleNordheim TA, Wellbrock A, Jones GH, et al., 2020,
Detection of Negative Pickup Ions at Saturn's Moon Dione
, GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276 -
Journal articleWilson LB, Chen L-J, Wang S, et al., 2020,
Electron Energy Partition across Interplanetary Shocks. III. Analysis
, ASTROPHYSICAL JOURNAL, Vol: 893, ISSN: 0004-637X- Author Web Link
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- Citations: 22
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Journal articleWild O, Voulgarakis A, O'Connor F, et al., 2020,
Global sensitivity analysis of chemistry-climate model budgets of tropospheric ozone and OH: exploring model diversity
, Atmospheric Chemistry and Physics, Vol: 20, Pages: 4047-4058, ISSN: 1680-7316Projections of future atmospheric composition change and its impacts on air quality and climate depend heavily on chemistry–climate models that allow us to investigate the effects of changing emissions and meteorology. These models are imperfect as they rely on our understanding of the chemical, physical and dynamical processes governing atmospheric composition, on the approximations needed to represent these numerically, and on the limitations of the observations required to constrain them. Model intercomparison studies show substantial diversity in results that reflect underlying uncertainties, but little progress has been made in explaining the causes of this or in identifying the weaknesses in process understanding or representation that could lead to improved models and to better scientific understanding. Global sensitivity analysis provides a valuable method of identifying and quantifying the main causes of diversity in current models. For the first time, we apply Gaussian process emulation with three independent global chemistry-transport models to quantify the sensitivity of ozone and hydroxyl radicals (OH) to important climate-relevant variables, poorly characterised processes and uncertain emissions. We show a clear sensitivity of tropospheric ozone to atmospheric humidity and precursor emissions which is similar for the models, but find large differences between models for methane lifetime, highlighting substantial differences in the sensitivity of OH to primary and secondary production. This approach allows us to identify key areas where model improvements are required while providing valuable new insight into the processes driving tropospheric composition change.
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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: 21
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Journal articleWeiss Z, Pickering JC, 2020,
Charge transfer from doubly charged ions of transition elements in a neon glow discharge: evidence based on emission spectra
, Plasma Sources Science and Technology, Vol: 29, Pages: 1-12, ISSN: 0963-0252An extensive study of Mn II, Fe II, Ti II, Cr II and Cu II emission spectra from a Grimm-type glow discharge in neon was performed, using the formalism of transition rate (TR) diagrams. In this method, radiative depopulation rates of individual excited levels of a species under study are established based on the emission spectrum, prospective contributions from radiative decay of higher excited levels (cascade excitation) are subtracted and the resulting net depopulation rates are plotted as function of energy of the levels involved. A peak at a particular energy in such a diagram reflects a collisional process in operation, selectively populating levels in a narrow interval around that energy. By comparing net TR diagrams of ionic spectra of the elements listed above, a common pattern was found indicating that singly charged ions of these elements are created, in addition to other mechanisms, by charge transfer between doubly charged ions of the element under study and metastable neutral neon atoms. This mechanism appears to be significant and needs to be taken into account in collisional–radiative models describing excitation and ionization of some elements in neon glow discharges.
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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 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 articleGood SW, Ala-Lahti M, Palmerio E, et al., 2020,
Radial Evolution of Magnetic Field Fluctuations in an Interplanetary Coronal Mass Ejection Sheath
, ASTROPHYSICAL JOURNAL, Vol: 893, ISSN: 0004-637X- Author Web Link
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- Citations: 15
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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: 37
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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 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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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 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 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 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 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 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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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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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 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 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 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 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 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 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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