Publications

Journal article

Wang S, Rashid T, Thorp H, Toumi Ret al., 2020,

A shortening of the life-cycle of major tropical cyclones

, Geophysical Research Letters, Vol: 47, Pages: 28 Jul 2020-28 Jul 2020, ISSN: 0094-8276

In this study a comprehensive picture of the changing intensity life cycle of major (Category 3 and higher) tropical cyclones (TCs) is presented. Over the past decades, the lifetime maximum intensity has increased, but there has also been a significant decrease in duration of time spent at intensities greater than Category 1. These compensating effects have maintained a stable global mean‐accumulated cyclone energy of individual major TCs. The global mean duration of major TCs has shortened by about 1 day from 1982 to 2018. There has been both faster intensification (Categories 1 to 3) and weakening (Categories 3 to 1) by about 40%. The probabilities of rapid intensification and rapid weakening have both risen in the period 2000–2018 compared to 1982–1999. A statistically significant anticorrelation is found between the lifetime maximum intensity and the following duration of the final weakening. This suggests an element of self‐regulation of TC life cycles.

Journal article

Archer MO, 2020,

Space Sound Effects Short Film Festival: using the film festival model to inspire creative art–science and reach new audiences

, Geoscience Communication, Vol: 3, Pages: 147-166, ISSN: 2569-7110

The ultra-low frequency analogues of sound waves in Earth'smagnetosphere play a crucial role in space weather; however, the publicis largely unaware of this risk to our everyday lives and technology.As a way of potentially reaching new audiences, SSFX (Space Sound Effects) made 8 yearsof satellite wave recordings audible to the human ear with the aimof using it to create art. Partnering with film industry professionals,the standard processes of international film festivals were adoptedby the project in order to challenge independent filmmakers to incorporatethese sounds into short films in creative ways. Seven films coveringa wide array of topics and genres (despite coming from the same sounds)were selected for screening at a special film festival out of 22 submissions.The works have subsequently been shown at numerous established filmfestivals and screenings internationally. These events have attracteddiverse non-science audiences resulting in several unanticipated impacts on them, thereby demonstrating how working with the art world canopen up dialogues with both artists and audiences who would not ordinarily engage with science.

Journal article

Madanian H, Schwartz SJ, Halekas JS, Wilson LBet al., 2020,

Nonstationary Quasiperpendicular Shock and Ion Reflection at Mars

, GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276

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Citations: 6

Journal article

Vasko IY, Wang R, Mozer FS, Bale SD, Artemyev AVet al., 2020,

On the Nature and Origin of Bipolar Electrostatic Structures in the Earth's Bow Shock

, FRONTIERS IN PHYSICS, Vol: 8, ISSN: 2296-424X

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Citations: 19

Journal article

Bandyopadhyay R, Sorriso-Valvo L, Chasapis A, Hellinger P, Matthaeus WH, Verdini A, Landi S, Franci L, Matteini L, Giles BL, Gershman DJ, Moore TE, Pollock CJ, Russell CT, Strangeway RJ, Torbert RB, Burch JLet al., 2020,

In situ observation of hall magnetohydrodynamic cascade in space plasma

, Physical Review Letters, Vol: 124, Pages: 225101 – 1-225101 – 7, ISSN: 0031-9007

We present estimates of the turbulent energy-cascade rate derived from a Hall-magnetohydrodynamic (MHD) third-order law. We compute the contribution from the Hall term and the MHD term to the energy flux. Magnetospheric Multiscale (MMS) data accumulated in the magnetosheath and the solar wind are compared with previously established simulation results. Consistent with the simulations, we find that at large (MHD) scales, the MMS observations exhibit a clear inertial range dominated by the MHD flux. In the subion range, the cascade continues at a diminished level via the Hall term, and the change becomes more pronounced as the plasma beta increases. Additionally, the MHD contribution to interscale energy transfer remains important at smaller scales than previously thought. Possible reasons are offered for this unanticipated result.

Journal article

Carnielli G, Galand M, Leblanc F, Modolo R, Beth A, Jia Xet al., 2020,

Constraining Ganymede's neutral and plasma environments through simulations of its ionosphere and Galileo observations

, Icarus, Vol: 343, Pages: 1-11, ISSN: 0019-1035

Ganymede's neutral and plasma environments are poorly constrained by observations. Carnielli et al. (2019) developed the first 3D ionospheric model aimed at understanding the dynamics of the present ion species and at quantifying the presence of each component in the moon's magnetosphere. The model outputs were compared with Galileo measurements of the ion energy flux, ion bulk velocity and electron number density made during the G2 flyby. A good agreement was found in terms of ion energy distribution and bulk velocity, but not in terms of electron number density. In this work, we present some improvements to our model Carnielli et al. (2019) and quantitatively address the possible sources of the discrepancy found in the electron number density between the Galileo observations and our ionospheric model. We have improved the ion model by developing a collision scheme to simulate the charge-exchange interaction between the exosphere and the ionosphere. We have simulated the energetic component of the O$_2$ population, which is missing in the exospheric model of Leblanc et al. (2017) and added it to the original distribution, hence improving its description at high altitudes. These improvements are found to be insufficient to explain the discrepancy in the electron number density. We provide arguments that the input O$_2$ exosphere is underestimated and that the plasma production acts asymmetrically between the Jovian and anti-Jovian hemispheres. In particular, we estimate that the O$_2$ column density should be greater than $10^{15}$~cm$^{-2}$, i.e., higher than previously derived upper limits (and a factor 10 higher than the values from Leblanc et al. (2017)), and that the ionization frequency from electron impact must be higher in the anti-Jovian hemisphere for the G2 flyby conditions.

Journal article

Yang L, Wang L, Zhao L, Tao J, Li G, Wimmer-Schweingruber RF, He J, Tian H, Bale SDet al., 2020,

Quiet-time Solar Wind Suprathermal Electrons of Different Solar Origins

, ASTROPHYSICAL JOURNAL LETTERS, Vol: 896, ISSN: 2041-8205

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Citations: 2

Journal article

Dimmock AP, Hietala H, Zou Y, 2020,

Compiling magnetosheath statistical data sets under specific solar wind conditions: lessons learnt from the dayside kinetic southward IMF GEM challenge

, Earth and Space Science, Vol: 7, Pages: 1-13, ISSN: 2333-5084

The Geospace Environmental Modelling (GEM) community offers a framework for collaborations between modelers, observers, and theoreticians in the form of regular challenges. In many cases, these challenges involve model‐data comparisons to provide wider context to observations or validate model results. To perform meaningful comparisons, a statistical approach is often adopted, which requires the extraction of a large number of measurements from a specific region. However, in complex regions such as the magnetosheath, compiling these data can be difficult. Here, we provide the statistical context of compiling statistical data for the southward IMF GEM challenge initiated by the “Dayside Kinetic Processes in Global Solar Wind‐Magnetosphere Interaction” focus group. It is shown that matching very specific upstream conditions can severely impact the statistical data if limits are imposed on several solar wind parameters. We suggest that future studies that wish to compare simulations and/or single events to statistical data should carefully consider at an early stage the availability of data in context with the upstream criteria. We also demonstrate the importance of how specific IMF conditions are defined, the chosen spacecraft, the region of interest, and how regions are identified automatically. The lessons learnt in this study are of wide context to many future studies as well as GEM challenges. The results also highlight the issue where a global statistical perspective has to be balanced with its relevance to more‐extreme, less‐frequent individual events, which is typically the case in the field of space weather.

Journal article

Curtis PE, Ceppi P, Zappa G, 2020,

Role of the mean state for the Southern Hemispheric Jet Stream response to CO₂ forcing in CMIP6 models

, Environmental Research Letters, Vol: 15, Pages: 1-7, ISSN: 1748-9326

Global climate models indicate that the Southern Hemispheric (SH) jet stream shifts poleward in response to CO2 forcing, but the magnitude of this shift remains highly uncertain. Here we analyse the SH jet stream response to 4×CO2 forcing in Coupled Model Intercomparison Project phase 6 (CMIP6) simulations, and find a substantially muted jet shift during winter compared with CMIP5. We suggest this muted response results from a more poleward mean jet position, consistent with a strongly reduced bias in jet position relative to the reanalysis during 1980--2004. The improved mean jet position cannot be explained by changes in the simulated sea surface temperatures. Instead, we find indications that increased horizontal grid resolution in CMIP6 relative to CMIP5 has contributed to the higher mean jet latitude, and thus to the reduced jet shift under CO2 forcing. These results imply that CMIP6 models can provide more realistic projections of SH climate change.

Journal article

Tilquin H, Eastwood JP, Phan TD, 2020,

Solar wind reconnection exhausts in the inner heliosphere observed by helios and detected via machine learning

, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 895, Pages: 1-10, ISSN: 0004-637X

Reconnecting current sheets in the solar wind play an important role in the dynamics of the heliosphere and offer an opportunity to study magnetic reconnection exhausts under a wide variety of inflow and magnetic shear conditions. However, progress in understanding reconnection can be frustrated by the difficulty of finding events in long time-series data. Here we describe a new method to detect magnetic reconnection events in the solar wind based on machine learning, and apply it to Helios data in the inner heliosphere. The method searches for known solar wind reconnection exhaust features, and parameters in the algorithm are optimized to maximize the Matthews Correlation Coefficient using a training set of events and non-events. Applied to the whole Helios data set, the trained algorithm generated a candidate set of events that were subsequently verified by hand, resulting in a database of 88 events. This approach offers a significant reduction in construction time for event databases compared to purely manual approaches. The database contains events covering a range of heliospheric distances from ~0.3 to ~1 au, and a wide variety of magnetic shear angles, but is limited by the relatively coarse time resolution of the Helios data. Analysis of these events suggests that proton heating by reconnection in the inner heliosphere depends on the available magnetic energy in a manner consistent with observations in other regimes such as at the Earth's magnetopause, suggesting this may be a universal feature of reconnection.

Journal article

Madanian H, Schwartz SJ, Halekas JS, Wilson LBet al., 2020,

Nonstationary Quasi-perpendicular Shock and Ion Reflection at Mars

Journal article

Lester JG, Lovenduski NS, Graven HD, Long MC, Lindsay Ket al., 2020,

Internal variability dominates over externally forced ocean circulation changes seen through CFCs

, Geophysical Research Letters, Vol: 47, Pages: 1-10, ISSN: 0094-8276

Observations of oceanic transient tracers have indicated that the circulation in the Southern Ocean has changed in recent decades, potentially driven by changes in external climate forcing. Here, we use the CESM Large Ensemble to analyze changes in two oceanic tracers that are affected by ocean circulation: the partial pressure of chlorofluorocarbon‐12 (pCFC12) and the idealized model tracer Ideal Age (IAGE) over the 1991 to 2005 period. The small ensemble mean change in IAGE suggests that there has been very little externally forced change in Southern Ocean circulation over this period, in contrast to strong internal variability. Further, our analysis implies that real‐world observations of changes in pCFC12 may not be a robust way to characterize externally driven changes in Southern Ocean circulation because of the large internal variability in pCFC12 changes exhibited by the individual ensemble members.

Journal article

Shebanits O, Hadid LZ, Cao H, Morooka MW, Hunt G, Dougherty MK, Wahlund J-E, Waite Jr JH, Mueller-Wodarg Iet al., 2020,

Saturn’s near-equatorial ionospheric conductivities from in situ measurements

, Scientific Reports, Vol: 10, ISSN: 2045-2322

Cassini’s Grand Finale orbits provided for the first time in-situ measurements of Saturn’s topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10−5–10−4 S/m at (or close to) the ionospheric peak, a factor 10–100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300–800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.

Journal article

Lavraud B, Fargette N, Réville V, Szabo A, Huang J, Rouillard AP, Viall N, Phan TD, Kasper JC, Bale SD, Berthomier M, Bonnell JW, Case AW, Dudok de Wit T, Eastwood JP, Génot V, Goetz K, Griton LS, Halekas JS, Harvey P, Kieokaew R, Klein KG, Korreck KE, Kouloumvakos A, Larson DE, Lavarra M, Livi R, Louarn P, MacDowall RJ, Maksimovic M, Malaspina D, Nieves-Chinchilla T, Pinto RF, Poirier N, Pulupa M, Raouafi NE, Stevens ML, Toledo-Redondo S, Whittlesey PLet al., 2020,

The heliospheric current sheet and plasma sheet during Parker Solar Probe’s first orbit

, Letters of the Astrophysical Journal, Vol: 894, Pages: 1-8, ISSN: 2041-8205

We present heliospheric current sheet (HCS) and plasma sheet (HPS) observations during Parker Solar Probe's (PSP) first orbit around the Sun. We focus on the eight intervals that display a true sector boundary (TSB; based on suprathermal electron pitch angle distributions) with one or several associated current sheets. The analysis shows that (1) the main density enhancements in the vicinity of the TSB and HCS are typically associated with electron strahl dropouts, implying magnetic disconnection from the Sun, (2) the density enhancements are just about twice that in the surrounding regions, suggesting mixing of plasmas from each side of the HCS, (3) the velocity changes at the main boundaries are either correlated or anticorrelated with magnetic field changes, consistent with magnetic reconnection, (4) there often exists a layer of disconnected magnetic field just outside the high-density regions, in agreement with a reconnected topology, (5) while a few cases consist of short-lived density and velocity changes, compatible with short-duration reconnection exhausts, most events are much longer and show the presence of flux ropes interleaved with higher-β regions. These findings are consistent with the transient release of density blobs and flux ropes through sequential magnetic reconnection at the tip of the helmet streamer. The data also demonstrate that, at least during PSP's first orbit, the only structure that may be defined as the HPS is the density structure that results from magnetic reconnection, and its byproducts, likely released near the tip of the helmet streamer.

Journal article

Ala-Lahti M, Ruohotie J, Good SW, Kilpua E, Lugaz Net al., 2020,

Spatial coherence of interplanetary coronal mass ejection sheaths at 1 AU

Journal article

Hunt GJ, Bunce EJ, Cao H, Cowley SWH, Dougherty MK, Provan G, Southwood DJet al., 2020,

Saturn's auroral field-aligned currents: observations from the Northern Hemisphere dawn sector during cassini's proximal orbits

, Journal of Geophysical Research: Space Physics, Vol: 125, ISSN: 2169-9380

We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field‐aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F‐ring orbits prior to the Proximal orbits. First, we show that the position of the main auroral upward current is displaced poleward between the two local times (LT). This is consistent with the statistical position of the ultraviolet auroral oval for the same time interval. Second, we show the overall average ionospheric meridional current profile differs significantly on the equatorward boundary of the upward current with a swept‐forward configuration with respect to planetary rotation present at dawn. We separate the planetary period oscillation (PPO) currents from the PPO‐independent currents and show their positional relationship is maintained as the latitude of the current shifts in LT implying an intrinsic link between the two systems. Focusing on the individual upward current sheets pass‐by‐pass we find that the main upward current at dawn is stronger compared to near‐noon. This results in the current density been ~1.4 times higher in the dawn sector. We determine a proxy for the precipitating electron power and show that the dawn PPO‐independent upward current electron power ~1.9 times higher than at noon. These new observations of the dawn auroral region from the Proximal orbits may show evidence of an additional upward current at dawn likely associated with strong flows in the outer magnetosphere.

Journal article

Fuselier, Petrinec, Sawyer, Mukherjee, Masters Aet al., 2020,

Suppression of magnetic reconnection at Saturn’s low-latitude magnetopause

, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-16, ISSN: 2169-9380

Observations from the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) are used in an in‐depth investigation of the occurrence and location of reconnection at Saturn's magnetopause. Heated, streaming electrons parallel and/or antiparallel to the magnetic field in the magnetosheath adjacent to the magnetopause indicate that reconnection is occurring somewhere on the boundary. In these instances, the Cassini spacecraft is connected to open magnetic field lines that thread the magnetopause boundary. A survey of 99 crossings with sufficient pitch angle coverage from CAPS/ELS indicates that 65% of the crossings had this evidence of reconnection. Specific crossings from this survey are used to demonstrate that there are times when reconnection at Saturn's low‐latitude magnetopause may be suppressed.

Journal article

Staniland N, Dougherty M, Masters A, Bunce Eet al., 2020,

Determining the nominal thickness and variability of the magnetodisc current sheet at saturn

, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-15, ISSN: 2169-9380

The thickness and variability of the Saturnian magnetodisc current sheet is investigated using the Cassini magnetometer data set. Cassini performed 66 fast, steep crossings of the equatorial current sheet where a clear signature in the magnetic field data allowed for a direct determination of its thickness and the offset of its center. The average, or nominal, current sheet half‐thickness is 1.3 R S , where R S is the equatorial radius of Saturn, equal to 60,268 km. This is thinner than previously calculated, but both spatial and temporal dependencies are identified. The current sheet is thicker and more variable by a factor ∼2 on the nightside compared to the dayside, ranging from 0.5–3 R S . The current sheet is on average 50% thicker in the nightside quasi‐dipolar region (≤15 R S ) compared to the dayside. These results are consistent with the presence of a noon‐midnight electric field at Saturn that produces a hotter plasma population on the nightside compared to the dayside. It is also shown that the current sheet becomes significantly thinner in the outer region of the nightside, while staying approximately constant with radial distance on the dayside, reflecting the dayside compression of the magnetosphere by the solar wind. Some of the variability is well characterized by the planetary period oscillations (PPOs). However, we also find evidence for non‐PPO drivers of variability.

Journal article

Mackie A, Wild M, Brindley H, Folini D, Palmer Pet al., 2020,

Observed and CMIP5-Simulated Radiative Flux Variability Over West Africa

, EARTH AND SPACE SCIENCE, Vol: 7

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Citations: 5

Journal article

Trotta D, Franci L, Burgess D, Hellinger Pet al., 2020,

Fast Acceleration of Transrelativistic Electrons in Astrophysical Turbulence

, ASTROPHYSICAL JOURNAL, Vol: 894, ISSN: 0004-637X

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Citations: 13

Journal article

Bowen TA, Bale SD, Bonnell JW, de Wit TD, Goetz K, Goodrich K, Gruesbeck J, Harvey PR, Jannet G, Koval A, MacDowall RJ, Malaspina DM, Pulupa M, Revillet C, Sheppard D, Szabo Aet al., 2020,

A Merged Search-Coil and Fluxgate Magnetometer Data Product for Parker Solar Probe FIELDS

, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 125, ISSN: 2169-9380

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Citations: 27

Journal article

Verniero JL, Larson DE, Livi R, Rahmati A, McManus MD, Pyakurel PS, Klein KG, Bowen TA, Bonnell JW, Alterman BL, Whittlesey PL, Malaspina DM, Bale SD, Kasper JC, Case AW, Goetz K, Harvey PR, Korreck KE, MacDowall RJ, Pulupa M, Stevens ML, de Wit TDet al., 2020,

Parker Solar Probe Observations of Proton Beams Simultaneous with Ion-scale Waves

, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 248, ISSN: 0067-0049

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Citations: 51

Journal article

Nordheim TA, Wellbrock A, Jones GH, Desai RT, Coates AJ, Teolis BD, Jasinski JMet al., 2020,

Detection of Negative Pickup Ions at Saturn's Moon Dione

, GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276

Journal article

Kilpua EKJ, Fontaine D, Good S, Ala-Lahti M, Osmane A, Palmerio E, Yordanova E, Moissard C, Hadid LZ, Janvier Met 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

Journal article

Wilson LB, Chen L-J, Wang S, Schwartz SJ, Turner DL, Stevens ML, Kasper JC, Osmane A, Caprioli D, Bale SD, Pulupa MP, Salem CS, Goodrich KAet al., 2020,

Electron Energy Partition across Interplanetary Shocks. III. Analysis

, ASTROPHYSICAL JOURNAL, Vol: 893, ISSN: 0004-637X

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Citations: 22

Journal article

Wild O, Voulgarakis A, O'Connor F, Lamarque J-F, Ryan EM, Lee Let 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: 1680-7316

Projections 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.

Journal article

Malaspina DM, Szalay JR, Pokorny P, Page B, Bale SD, Bonnell JW, de Wit TD, Goetz K, Goodrich K, Harvey PR, MacDowall RJ, Pulupa Met al., 2020,

In Situ Observations of Interplanetary Dust Variability in the Inner Heliosphere

, ASTROPHYSICAL JOURNAL, Vol: 892, ISSN: 0004-637X

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Citations: 21

Journal article

Weiss 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: 0963-0252

An 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.

Journal article

Adhikari S, Shay MA, Parashar TN, Pyakurel PS, Matthaeus WH, Godzieba D, Stawarz JE, Eastwood JP, Dahlin JTet al., 2020,

Reconnection from a turbulence perspective

, Physics of Plasmas, Vol: 27, Pages: 1-10, ISSN: 1070-664X

The 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.

Journal article

Bercic L, Larson D, Whittlesey P, Maksimovic M, Badman ST, Landi S, Matteini L, Bale SD, Bonnell JW, Case AW, de Wit TD, Goetz K, Harvey PR, Kasper JC, Korreck KE, Livi R, MacDowall RJ, Malaspina DM, Pulupa M, Stevens MLet 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: 0004-637X

The 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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