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  • Journal article
    Lorfing CY, Reid HAS, Gómez-Herrero R, Maksimovic M, Nicolaou G, Owen CJ, Rodriguez-Pacheco J, Ryan DF, Trotta D, Verscharen Det al., 2023,

    Solar Electron Beam—Langmuir Wave Interactions and How They Modify Solar Electron Beam Spectra: Solar Orbiter Observations of a Match Made in the Heliosphere

    , Astrophysical Journal, Vol: 959, ISSN: 0004-637X

    Solar Orbiter's four in situ instruments have recorded numerous energetic electron events at heliocentric distances between 0.5 and 1 au. We analyze energetic electron fluxes, spectra, pitch-angle distributions, associated Langmuir waves, and type III solar radio bursts for three events to understand what causes modifications in the electron flux and identify the origin and characteristics of features observed in the electron spectrum. We investigate what electron beam properties and solar wind conditions are associated with Langmuir wave growth and spectral breaks in the electron peak flux as a function of energy. We observe velocity dispersion and quasilinear relaxation in the electron flux caused by the resonant wave-particle interactions in the deca-keV range, at the energies at which we observe breaks in the electron spectrum, cotemporal with the local generation of Langmuir waves. We show, via the evolution of the electron flux at the time of the event, that these interactions are responsible for the spectral signatures observed around 10 and 50 keV, confirming the results of simulations by Kontar and Reid. These signatures are independent of pitch-angle scattering. Our findings highlight the importance of using overlapping FOVs when working with data from different sensors. In this work, we exploit observations from all in situ instruments to address, for the first time, how the energetic electron flux is modified by the beam-plasma interactions and results in specific feature appearing in the local spectrum. Our results, corroborated with numerical simulations, can be extended to a wider range of heliocentric distances.

  • Journal article
    Rasca AP, Farrell WM, Gruesbeck JR, MacDowall RJ, Bale SD, Kasper JCet al., 2023,

    Switchbacks and Associated Magnetic Holes Observed near the Alfvén Critical Surface

    , Astrophysical Journal, Vol: 959, ISSN: 0004-637X

    During recent solar encounters, the Parker Solar Probe (PSP) began its initial dips below the Alfvén critical surface to measure in situ the sub-Alfvénic coronal wind. While the near-Sun super-Alfvénic solar wind is shown to be dominated by impulsive magnetic switchbacks (short magnetic field reversals), these brief encounters with the sub-Alfvénic coronal wind show switchbacks and associated magnetic holes (MHs) to still be present but different in character. In this work, we compare and contrast specific features of the switchbacks, including the change in B r and V r and associated boundary B-field dropouts (MHs) at locations when PSP was both above and below the Alfvén critical surface. We use observations from the PSP perihelion Encounters 8 (E8) and 12 (E12) in the analysis. We first perform a superposed epoch analysis to identify common features in the switchback boundaries, including the formation of the associated ∣B∣ dropouts/MHs in slow and fast flows. We then examine the presence of B-field dropouts/MHs as a function of Alfvén Mach number, M A. From E12, we find that the switchbacks have a systematic reduction in rotation (and reduction in B r deflection) with decreasing M A. Further, the ∣B∣ dropouts/MHs associated with the boundaries were also found to decrease in strength and occurrence with M A (with no or few ∣B∣ dropouts at M A < 0.7). The results suggest that the switchback rotation and boundary-associated MHs are connected, possibly consistent with diamagnetic effects at the boundary that require large rotations to be initiated.

  • Journal article
    Rosenfeld D, Kokhanovsky A, Goren T, Gryspeerdt E, Hasekamp O, Jia H, Lopatin A, Quaas J, Pan Z, Sourdeval Oet al., 2023,

    Frontiers in satellite‐based estimates of cloud‐mediated aerosol forcing

    , Reviews of Geophysics, Vol: 61, ISSN: 8755-1209

    Atmospheric aerosols affect the Earth's climate in many ways, including acting as the seeds on which cloud droplets form. Since a large fraction of these particles is anthropogenic, the clouds' microphysical and radiative characteristics are influenced by human activity on a global scale leading to important climatic effects. The respective change in the energy budget at the top of the atmosphere is defined as the effective radiative forcing due to aerosol-cloud interaction (ERFaci). It is estimated that the ERFaci offsets presently nearly 1/4 of the greenhouse-induced warming, but the uncertainty is within a factor of two. A common method to calculate the ERFaci is by the multiplication of the susceptibility of the cloud radiative effect to changes in aerosols by the anthropogenic change of the aerosol concentration. This has to be done by integrating it over all cloud regimes. Here we review the various methods of the ERFaci estimation. Global measurements require satellites' global coverage. The challenge of quantifying aerosol amounts in cloudy atmospheres are met with the rapid development of novel methodologies reviewed here. The aerosol characteristics can be retrieved from space based on their optical properties, including polarization. The concentrations of the aerosols that serve as cloud drop condensation nuclei can be also estimated from their impact on the satellite-retrieved cloud drop number concentrations. These observations are critical for reducing the uncertainty in the ERFaci calculated from global climate models (GCMs), but further development is required to allow GCMs to properly simulate and benefit these novel observables.

  • Journal article
    Hwang K-J, Nakamura R, Eastwood JP, Fuselier SA, Hasegawa H, Nakamura T, Lavraud B, Dokgo K, Turner DL, Ergun RE, Reiff PHet al., 2023,

    Cross-scale processes of magnetic reconnection

    , Space Science Reviews, Vol: 219, ISSN: 0038-6308

    Various physical processes in association with magnetic reconnection occur over multiple scales from the microscopic to macroscopic scale lengths. This paper reviews multi-scale and cross-scale aspects of magnetic reconnection revealed in the near-Earth space beyond the general global-scale features and magnetospheric circulation organized by the Dungey Cycle. Significant and novel advancements recently reported, in particular, since the launch of the Magnetospheric Multi-scale mission (MMS), are highlighted being categorized into different locations with different magnetic topologies. These potentially paradigm-shifting findings include shock and foreshock transient driven reconnection, magnetosheath turbulent reconnection, flow shear driven reconnection, multiple X-line structures generated in the dayside/flankside/nightside magnetospheric current sheets, development and evolution of reconnection-driven structures such as flux transfer events, flux ropes, and dipolarization fronts, and their interactions with ambient plasmas. The paper emphasizes key aspects of kinetic processes leading to multi-scale structures and bringing large-scale impacts of magnetic reconnection as discovered in the geospace environment. These key features can be relevant and applicable to understanding other heliospheric and astrophysical systems.

  • Journal article
    Hartinger MD, Elsden T, Archer MO, Takahashi K, Wright AN, Artemyev A, Zhang X, Angelopoulos Vet al., 2023,

    Properties of Magnetohydrodynamic normal modes in the Earth's magnetosphere

    , JGR: Space Physics, Vol: 128, ISSN: 2169-9402

    The Earth's magnetosphere supports a variety of Magnetohydrodynamic (MHD) normal modes with Ultra Low Frequencies (ULF) including standing Alfvén waves and cavity/waveguide modes. Their amplitudes and frequencies depend in part on the properties of the magnetosphere (size of cavity, wave speed distribution). In this work, we use ∼13 years of Time History of Events and Macroscale Interactions during Substorms satellite magnetic field observations, combined with linearized MHD numerical simulations, to examine the properties of MHD normal modes in the region L > 5 and for frequencies <80 mHz. We identify persistent normal mode structure in observed dawn sector power spectra with frequency-dependent wave power peaks like those obtained from simulation ensemble averages, where the simulations assume different radial Alfvén speed profiles and magnetopause locations. We further show with both observations and simulations how frequency-dependent wave power peaks at L > 5 depend on both the magnetopause location and the location of peaks in the radial Alfvén speed profile. Finally, we discuss how these results might be used to better model radiation belt electron dynamics related to ULF waves.

  • Journal article
    Klein KG, Spence H, Alexandrova O, Argall M, Arzamasskiy L, Bookbinder J, Broeren T, Caprioli D, Case A, Chandran B, Chen L-J, Dors I, Eastwood J, Forsyth C, Galvin A, Genot V, Halekas J, Hesse M, Hine B, Horbury T, Jian L, Kasper J, Kretzschmar M, Kunz M, Lavraud B, Le Contel O, Mallet A, Maruca B, Matthaeus W, Niehof J, OBrien H, Owen C, Retinò A, Reynolds C, Roberts O, Schekochihin A, Skoug R, Smith C, Smith S, Steinberg J, Stevens M, Szabo A, TenBarge J, Torbert R, Vasquez B, Verscharen D, Whittlesey P, Wickizer B, Zank G, Zweibel Eet al., 2023,

    HelioSwarm: a multipoint, multiscale mission to characterize turbulence

    , Space Science Reviews, Vol: 219, ISSN: 0038-6308

    HelioSwarm (HS) is a NASA Medium-Class Explorer mission of the Heliophysics Division designed to explore the dynamic three-dimensional mechanisms controlling the physics of plasma turbulence, a ubiquitous process occurring in the heliosphere and in plasmas throughout the universe. This will be accomplished by making simultaneous measurements at nine spacecraft with separations spanning magnetohydrodynamic and sub-ion spatial scales in a variety of near-Earth plasmas. In this paper, we describe the scientific background for the HS investigation, the mission goals and objectives, the observatory reference trajectory and instrumentation implementation before the start of Phase B. Through multipoint, multiscale measurements, HS promises to reveal how energy is transferred across scales and boundaries in plasmas throughout the universe.

  • Journal article
    Eglinton TI, Graven HD, Raymond PA, Trumbore SEet al., 2023,

    A special issue preface: radiocarbon in the Anthropocene

    , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 381, ISSN: 1364-503X

    The Anthropocene is defined by marked acceleration in human-induced perturbations to the Earth system. Anthropogenic emissions of CO2 and other greenhouse gases to the atmosphere and attendant changes to the global carbon cycle are among the most profound and pervasive of these perturbations. Determining the magnitude, nature and pace of these carbon cycle changes is crucial for understanding the future climate that ecosystems and humanity will experience and need to respond to. This special issue illustrates the value of radiocarbon as a tool to shed important light on the nature, magnitude and pace of carbon cycle change. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

  • Journal article
    Eglinton TI, Graven HD, Raymond PA, Trumbore SE, Aluwihare L, Bard E, Basu S, Friedlingstein P, Hammer S, Lester J, Sanderman J, Schuur EAG, Sierra CA, Synal H-A, Turnbull JC, Wacker Let al., 2023,

    Making the case for an International Decade of Radiocarbon

    , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 381, ISSN: 1364-503X

    Radiocarbon (14C) is a critical tool for understanding the global carbon cycle. During the Anthropocene, two new processes influenced 14C in atmospheric, land and ocean carbon reservoirs. First, 14C-free carbon derived from fossil fuel burning has diluted 14C, at rates that have accelerated with time. Second, 'bomb' 14C produced by atmospheric nuclear weapon tests in the mid-twentieth century provided a global isotope tracer that is used to constrain rates of air-sea gas exchange, carbon turnover, large-scale atmospheric and ocean transport, and other key C cycle processes. As we write, the 14C/12C ratio of atmospheric CO2 is dropping below pre-industrial levels, and the rate of decline in the future will depend on global fossil fuel use and net exchange of bomb 14C between the atmosphere, ocean and land. This milestone coincides with a rapid increase in 14C measurement capacity worldwide. Leveraging future 14C measurements to understand processes and test models requires coordinated international effort-a 'decade of radiocarbon' with multiple goals: (i) filling observational gaps using archives, (ii) building and sustaining observation networks to increase measurement density across carbon reservoirs, (iii) developing databases, synthesis and modelling tools and (iv) establishing metrics for identifying and verifying changes in carbon sources and sinks. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

  • Journal article
    Guilbert-Lepoutre A, Benseguane S, Martinien L, Lasue J, Besse S, Grieger B, Beth Aet al., 2023,

    Pits on Jupiter-family Comets and the Age of Cometary Surfaces

    , Planetary Science Journal, Vol: 4

    Large and deep depressions, also known as pits, are observed at the surface of all Jupiter-family comets (JFCs) imaged by spacecraft missions. They offer the opportunity to glimpse the subsurface characteristics of comet nuclei and study the complex interplay between surface structures and cometary activity. This work investigates the evolution of pits at the surface of 81P/Wild 2, 9P/Tempel 1, and 103P/Hartley 2, in continuation of the work by Benseguane et al. on 67P/Churyumov-Gerasimenko. Pits are selected across the surface of each nucleus, and high-resolution shape models are used to compute the energy they receive. A thermal evolution model is applied to constrain how cometary activity sustained under current illumination conditions could modify them. Similar to what was found for 67P, we show that erosion resulting from water-driven activity is primarily controlled by seasonal patterns that are unique to each comet as a consequence of their shape and rotational properties. However, progressive erosion sustained after multiple perihelion passages is not able to carve any of the observed pits. Instead, cometary activity tends to erase sharp morphological features; they become wider and shallower over time. Our results reinforce the evolutionary sequence evidenced from independent measurables to transform “young” cometary surfaces, with sharp surface topography prone to outbursts, into “old” cometary surfaces. Finally, we suggest that the mechanism at the origin of the pits on JFCs should be able to carve these structures in a region of the solar system where water ice does not sublimate; the Centaur phase thus appears critical to understand JFC surface properties.

  • Journal article
    Mozer FS, Agapitov O, Bale SD, Livi R, Romeo O, Sauer K, Vasko IY, Verniero Jet al., 2023,

    Density Enhancement Streams in The Solar Wind

    , Astrophysical Journal Letters, Vol: 957, ISSN: 2041-8205

    This Letter describes a new phenomenon on the Parker Solar Probe of recurring plasma density enhancements that have Δn/n ∼ 10% and that occur at a repetition rate of ∼5 Hz. They were observed sporadically for about 5 hr between 14 and 15 solar radii on Parker Solar Probe orbit 12 and they were also seen in the same radial range on both the inbound and outbound orbits 11. Their apparently steady-state existence suggests that their pressure gradient was balanced by the electric field. The X-component of the electric field component produced from this requirement is in good agreement with that measured. This provides strong evidence for the measurement accuracy of the density fluctuations and the X- and Y-components of the electric field (the Z-component was not measured). The electrostatic density waves were accompanied by an electromagnetic low-frequency wave, which occurred with the electrostatic harmonics. The amplitudes of these electrostatic and electromagnetic waves at ≥1 Hz were greater than the amplitude of the Alfvénic turbulence in their vicinity so they can be important for the heating, scattering, and acceleration of the plasma. The existence of this pair of waves is consistent with the observed plasma distributions and is explained as an oscilliton due to the nonlinear coupling between the kinetic Alfvén wave and the ion cyclotron mode, which belongs with the minor population of alpha particles.

  • Journal article
    Bizien N, Dudok de Wit T, Froment C, Velli M, Case AW, Bale SD, Kasper J, Whittlesey P, MacDowall R, Larson Det al., 2023,

    Are Switchback Boundaries Observed by Parker Solar Probe Closed?

    , Astrophysical Journal, Vol: 958, ISSN: 0004-637X

    Switchbacks are sudden and large deflections in the magnetic field that Parker Solar Probe frequently observes in the inner heliosphere. Their ubiquitous occurrence has prompted numerous studies to determine their nature and origin. Our goal is to describe the boundary of these switchbacks using a series of events detected during the spacecraft’s first encounter with the Sun. Using FIELDS and SWEAP data, we investigate different methods for determining the boundary normal. The observed boundaries are arc-polarized structures with a rotation that is always contained in a plane. Classical minimum variance analysis gives misleading results and overestimates the number of rotational discontinuities. We propose a robust geometric method to identify the nature of these discontinuities, which involves determining whether or not the plane that contains them also includes the origin ( B = 0). Most boundaries appear to have the same characteristics as tangential discontinuities in the context of switchbacks, with little evidence for having rotational discontinuities. We find no effect of the size of the Parker spiral deviation. Furthermore, the thickness of the boundary is within MHD scales. We conclude that most of the switchback boundaries observed by Parker Solar Probe are likely to be closed, in contrast to previous studies. Our results suggest that their erosion may be much slower than expected.

  • Journal article
    Afanasiev A, Vainio R, Trotta D, Nyberg S, Talebpour Sheshvan N, Hietala H, Dresing Net al., 2023,

    Self-consistent modeling of the energetic storm particle event of November 10, 2012

    , Astronomy and Astrophysics, Vol: 679, ISSN: 0004-6361

    Context. It is thought that solar energetic ions associated with coronal and interplanetary shock waves are accelerated to high energies by the diffusive shock acceleration mechanism. For this mechanism to be efficient, intense magnetic turbulence is needed in the vicinity of the shock. The enhanced turbulence upstream of the shock can be produced self-consistently by the accelerated particles themselves via streaming instability. Comparisons of quasi-linear-theory-based particle acceleration models that include this process with observations have not been fully successful so far, which has motivated the development of acceleration models of a different nature. Aims. Our aim is to test how well our self-consistent quasi-linear SOLar Particle Acceleration in Coronal Shocks (SOLPACS) simulation code, developed earlier to simulate proton acceleration in coronal shocks, models the particle foreshock region. Methods. We applied SOLPACS to model the energetic storm particle (ESP) event observed by the STEREO A spacecraft on November 10, 2012. Results. All but one main input parameter of SOLPACS are fixed by the in situ plasma measurements from the spacecraft. By comparing a simulated proton energy spectrum at the shock with the observed one, we were able to fix the last simulation input parameter related to the efficiency of particle injection to the acceleration process. A subsequent comparison of simulated proton time-intensity profiles in a number of energy channels with the observed ones shows a very good correspondence throughout the upstream region. Conclusions. Our results strongly support the quasi-linear description of the foreshock region.

  • Journal article
    Dunn C, Bowen TA, Mallet A, Badman ST, Bale SDet al., 2023,

    Effect of Spherical Polarization on the Magnetic Spectrum of the Solar Wind

    , Astrophysical Journal, Vol: 958, ISSN: 0004-637X

    Magnetic fluctuations in the solar wind are often observed to maintain constant magnitude of the magnetic field in a manner consistent with spherically polarized large-amplitude Alfvén waves. We investigate the effect of spherical polarization on the magnetic spectral index through a statistical survey of magnetic fluctuations observed by Parker Solar Probe between 20 R ⊙ and 200 R ⊙. We find that deviations from spherical polarization, i.e., changes in ∣ B ∣ (compressive fluctuations) and one-dimensional discontinuities, have a dramatic effect on the scaling behavior of the turbulent fluctuations. We show that shallow k −3/2 spectra are only observed for three-dimensional structures of constant magnetic field strength, which we identify as large-amplitude Alfvén waves. The presence of compressive fluctuations coincides with a steepening of the spectrum up to k −5/3. Steeper power-law scalings approaching k −2 are observed when the fluctuations are dominated by discontinuities. Near-Sun fluctuations are found to be the most spherically polarized, suggesting that this spherical state is fundamental to the generation of the solar wind. With increasing distance from the Sun, fluctuations are found to become less three-dimensional and more compressive, which may indicate the breakdown of the Alfvénic equilibrium state.

  • Journal article
    Kruparova O, Krupar V, Szabo A, Pulupa M, Bale SDet al., 2023,

    Quasi-thermal Noise Spectroscopy Analysis of Parker Solar Probe Data: Improved Electron Density Model for Solar Wind

    , Astrophysical Journal, Vol: 957, ISSN: 0004-637X

    We present a comprehensive analysis of electron density measurements in the solar wind using quasi-thermal noise (QTN) spectroscopy applied to data from the first 15 encounters of the Parker Solar Probe mission (2018 November-2023 March). Our methodology involves the identification of the plasma line frequency and the calculation of plasma density based on in situ measurements. By analyzing over 2.1 million data points, we derive a power-law model for electron density as a function of radial distance from the Sun in the range of 13 to 50 R ☉: n e(r) = (343,466 ± 19921) × r (−1.87±0.11). This model provides improved estimates for localizing interplanetary solar radio bursts. Moreover, obtained electron densities can be used for calibrating particle instruments on board the Parker Solar Probe. We discuss its limitations and potential for further refinement with additional Parker Solar Probe encounters.

  • Journal article
    Stephenson P, Beth A, Deca J, Galand M, Goetz C, Henri P, Heritier K, Lewis Z, Moeslinger A, Nilsson H, Rubin Met al., 2023,

    The source of electrons at comet 67P

    , Monthly Notices of the Royal Astronomical Society, Vol: 525, Pages: 5041-5065, ISSN: 0035-8711

    We examine the origin of electrons in a weakly outgassing comet, using Rosetta mission data and a 3D collisional model of electrons at a comet. We have calculated a new data set of electron-impact ionization (EII) frequency throughout the Rosetta escort phase, with measurements of the Rosetta Plasma Consortium’s Ion and Electron Sensor (RPC/IES). The EII frequency is evaluated in 15-min intervals and compared to other Rosetta data sets. EII is the dominant source of electrons at 67P away from perihelion and is highly variable (by up to three orders of magnitude). Around perihelion, EII is much less variable and less efficient than photoionization at Rosetta. Several drivers of the EII frequency are identified, including magnetic field strength and the outgassing rate. Energetic electrons are correlated to the Rosetta-upstream solar wind potential difference, confirming that the ionizing electrons are solar wind electrons accelerated by an ambipolar field. The collisional test particle model incorporates a spherically symmetric, pure water coma and all the relevant electron-neutral collision processes. Electric and magnetic fields are stationary model inputs, and are computed using a fully kinetic, collision-less Particle-in-Cell simulation. Collisional electrons are modelled at outgassing rates of Q = 1026 s−1 and Q = 1.5 × 1027 s−1. Secondary electrons are the dominant population within a weakly outgassing comet. These are produced by collisions of solar wind electrons with the neutral coma. The implications of large ion flow speed estimates at Rosetta, away from perihelion, are discussed in relation to multi-instrument studies and the new results of the EII frequency obtained in this study.

  • Journal article
    Palmerio E, Maharana A, Lynch BJ, Scolini C, Good SW, Pomoell J, Isavnin A, Kilpua EKJet al., 2023,

    Modeling a Coronal Mass Ejection from an Extended Filament Channel. II. Interplanetary Propagation to 1 au

    , Astrophysical Journal, Vol: 958, ISSN: 0004-637X

    We present observations and modeling results of the propagation and impact at Earth of a high-latitude, extended filament channel eruption that commenced on 2015 July 9. The coronal mass ejection (CME) that resulted from the filament eruption was associated with a moderate disturbance at Earth. This event could be classified as a so-called “problem storm” because it lacked the usual solar signatures that are characteristic of large, energetic, Earth-directed CMEs that often result in significant geoeffective impacts. We use solar observations to constrain the initial parameters and therefore to model the propagation of the 2015 July 9 eruption from the solar corona up to Earth using 3D magnetohydrodynamic heliospheric simulations with three different configurations of the modeled CME. We find the best match between observed and modeled arrival at Earth for the simulation run that features a toroidal flux rope structure of the CME ejecta, but caution that different approaches may be more or less useful depending on the CME-observer geometry when evaluating the space weather impact of eruptions that are extreme in terms of their large size and high degree of asymmetry. We discuss our results in the context of both advancing our understanding of the physics of CME evolution and future improvements to space weather forecasting.

  • Journal article
    Sishtla CP, Pomoell J, Vainio R, Kilpua E, Good Set al., 2023,

    Modelling the interaction of Alfvénic fluctuations with coronal mass ejections in the low solar corona

    , Astronomy and Astrophysics, Vol: 679, ISSN: 0004-6361

    Context. Alfvénic fluctuations of various scales are ubiquitous in the corona; their non-linear interactions and eventual turbulent cascade result in an important heating mechanism that accelerates the solar wind. These fluctuations may be processed by large-scale, transient, and coherent heliospheric structures such as coronal mass ejections (CMEs). In this study we investigate the interactions between Alfvénic solar wind fluctuations and CMEs using magnetohydrodynamic (MHD) simulations. Aims. We study the transmission of upstream solar wind fluctuations into the CME leading to the formation of CME sheath fluctuations. Additionally, we investigate the influence of the fluctuation frequencies on the extent of the CME sheath. Methods. We used an ideal MHD model with an adiabatic equation of state. An Alfvén pump wave is injected into the quiet solar wind by perturbing the transverse magnetic field and velocity components, and a CME is injected by inserting a flux-rope modelled as a magnetic island into the quasi-steady solar wind. Results. The upstream Alfvén waves experience a decrease in wavelength and change in the wave vector direction due to the non-radial topology of the CME shock front. The CME sheath inhibits the transmission of long-wavelength fluctuations due to the presence of non-radial flows in this region. The frequency of the solar wind fluctuations also affects the steepening of MHD fast waves causing the CME shock propagation speed to vary with the solar wind fluctuation frequencies.

  • Journal article
    Trotta D, Horbury TS, Lario D, Vainio R, Dresing N, Dimmock A, Giacalone J, Hietala H, Wimmer-Schweingruber RF, Berger L, Yang Let al., 2023,

    Irregular Proton Injection to High Energies at Interplanetary Shocks

    , The Astrophysical Journal Letters, Vol: 957, Pages: L13-L13, ISSN: 2041-8205

    <jats:title>Abstract</jats:title> <jats:p>How thermal particles are accelerated to suprathermal energies is an unsolved issue, crucial for many astrophysical systems. We report novel observations of irregular, dispersive enhancements of the suprathermal particle population upstream of a high-Mach-number interplanetary shock. We interpret the observed behavior as irregular “injections” of suprathermal particles resulting from shock front irregularities. Our findings, directly compared to self-consistent simulation results, provide important insights for the study of remote astrophysical systems where shock structuring is often neglected.</jats:p>

  • Journal article
    Zomerdijk-Russell S, Masters A, Sun WJ, Fear RC, Slavin JAet al., 2023,

    Does reconnection only occur at points of maximum shear on Mercury’s dayside magnetopause?

    , JGR: Space Physics, Vol: 128, ISSN: 2169-9402

    MESSENGER observations of large numbers of flux transfer events (FTEs) during dayside crossings of Mercury's magnetopause have shown that the highly dynamic Hermean magnetosphere is strongly driven by frequent and intense magnetic reconnection. Since FTEs are products of reconnection, study of them can reveal information about whether reconnection sites favor points of maximum shear on the magnetopause. Here, we analyze 201 FTEs formed under relatively stable upstream solar wind conditions as observed by MESSENGER during inbound magnetopause crossings. By modeling paths of these FTEs along the magnetopause, we determine the conditions and locations of the reconnection sites at which these FTEs were likely formed. The majority of these FTE formation paths were found to intersect with high-magnetic shear regions, defined as shear angles above 135°. Seven FTEs were found where the maximum shear angle possible between the reconnecting magnetic field lines was less than 80° and three of these had shear angles less than 70°, supporting the idea that very low-shear reconnection could be occurring on Mercury's dayside magnetopause under this global-scale picture of magnetic reconnection. Additionally, for the FTEs formed under these low-shear reconnection conditions, tracing a dominant X-line connecting points of maximum shear along the magnetopause that passes through a region of very low-shear may be difficult to justify, implying reconnection could be occurring anywhere along Mercury's magnetopause and may not be confined to points of maximum shear.

  • Book chapter
    Cargill P, 2023,

    Sydney Chapman

    , Oxford's Sedleian Professors of Natural Philosophy, Editors: Hollings, McCartney, Publisher: Oxford University Press, Pages: 157-176, ISBN: 9780192843210

    Sydney Chapman FRS was the Sedleian professor between 1946 and 1953. He was also one of the outstanding geophysicists of the twentieth century. His achievements cover a wide range of topics and include pioneering theory of transport in non-uniform gases, the first proposal of the nature of the ozone layer, and a comprehensive early theory of the nature of geomagnetic storms. The last of these forms the foundation of present-day studies of the Earth’s magnetosphere, and the impact of solar activity on society, referred to as space weather.

  • Journal article
    Horner G, Gryspeerdt E, 2023,

    The evolution of deep convective systems and their associated cirrus outflows

    , Atmospheric Chemistry and Physics, Vol: 23, Pages: 14239-14253, ISSN: 1680-7316

    Tropical deep convective clouds, particularly their large cirrus outflows, play an important role in modulating the energy balance of the Earth’s atmosphere. Whilst the cores of these deep convective clouds have a significant shortwave (SW) cooling effect, they dissipate quickly. Conversely, the thin cirrus that flow from these cores can persist for days after the core has dissipated, reaching hundreds of kilometers in extent. These thin cirrus have a potential for large warming in the tropics. Understanding the evolution of these clouds and how they change in response to anthropogenic emissions is therefore important to understand past and future climate change.This work uses a novel approach to investigate the evolution of tropical convective clouds by introducing the concept of ‘Time Since Convection’ (TSC). This is used to build a composite picture of the lifecycle of deep convection, from anvil cirrus to thin detrained cirrus. Cloud properties are a strong function of time since convection, showing decreases in the optical thickness, cloud top height, and cloud fraction over time. After an initial dissipation of the convective core, changes in thin cirrus cloud amount were seen beyond 200 hours from convection.Finally, in the initial stages of convection there was a large net negative cloud radiative effect (CRE). However, once the convective core had dissipated after 6–12 hours, the sign of the CRE flipped, and there was a sustained net warming CRE beyond 120 hours from the convective event. Changes are present in the cloud properties long after the main convective activities have dissipated, signalling the need to continue further analysis at longer time scales than previously studied.

  • Journal article
    Salvi P, Gregory JM, Ceppi P, 2023,

    Time‐evolving radiative feedbacks in the historical period

    , Journal of Geophysical Research: Atmospheres, Vol: 128, ISSN: 2169-8996

    We investigate the time-dependence of radiative feedback in the historical period (since the late 19th century), by analyzing experiments using coupled atmosphere–ocean climate models with historical greenhouse gas, anthropogenic aerosol, and natural forcings, each separately. We find that radiative feedback depends on forcing agent, primarily through the effect of cloud on shortwave radiation, because the various forcings cause different changes in global-mean tropospheric stability per degree of global-mean temperature change. The large time-variation of historical feedback driven by observed sea surface temperature change alone, with no forcing agents, is also consistent with tropospheric stability change, and differs from the similarly large and significant historical time-variation of feedback that is simulated in response to all forcing agents together. We show that the latter results from the varying relative sizes of individual forcings. We highlight that volcanic forcing is especially important for understanding the time-variation, because it stimulates particularly strong feedbacks that tend to reduce effective climate sensitivity. We relate this to stability changes due to enhanced surface temperature response in the Indo-Pacific warm pool.

  • Journal article
    Rosu I-A, Grillakis M, Papadopoulos A, Agop M, Voulgarakis Aet al., 2024,

    Fractal and Spectral Analysis of Recent Wildfire Scars in Greece

    , FIRE TECHNOLOGY, ISSN: 0015-2684
  • Journal article
    Blasl KA, Nakamura TKM, Nakamura R, Settino A, Hasegawa H, Voeroes Z, Hosner M, Schmid D, Volwerk M, Roberts OW, Panov E, Liu Y-H, Plaschke F, Stawarz JE, Holmes JCet al., 2023,

    Electron-Scale Reconnecting Current Sheet Formed Within the Lower-Hybrid Wave-Active Region of Kelvin-Helmholtz Waves

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 50, ISSN: 0094-8276
  • Journal article
    George H, Malaspina DM, Goodrich K, Ma Y, Ramstad R, Conner D, Bale SD, Curry Set al., 2023,

    Non-Lightning-Generated Whistler Waves in Near-Venus Space

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 50, ISSN: 0094-8276
  • Journal article
    Shi P, Scime EE, Barbhuiya MH, Cassak PA, Adhikari S, Swisdak M, Stawarz JEet al., 2023,

    Using Direct Laboratory Measurements of Electron Temperature Anisotropy to Identify the Heating Mechanism in Electron-Only Guide Field Magnetic Reconnection.

    , Phys Rev Lett, Vol: 131

    Anisotropic electron heating during electron-only magnetic reconnection with a large guide magnetic field is directly measured in a laboratory plasma through in situ measurements of electron velocity distribution functions. Electron heating preferentially parallel to the magnetic field is localized to one separatrix, and anisotropies of 1.5 are measured. The mechanism for electron energization is identified as the parallel reconnection electric field because of the anisotropic nature of the heating and spatial localization. These characteristics are reproduced in a 2D particle-in-cell simulation and are also consistent with numerous magnetosheath observations. A measured increase in the perpendicular temperature along both separatrices is not reproduced by our 2D simulations. This work has implications for energy partition studies in magnetosheath and laboratory reconnection.

  • Conference paper
    Zhang Z, Desai R, Shebanits O, Miyake Y, Usui Het al., 2023,

    Cassini's floating potential in Titan's ionosphere: 3-D particle-in-cell simulations

    , URSI GASS 2023, Publisher: IEEE, Pages: 1-4

    Accurate determination of Cassini’s spacecraft potential in Titan’s ionosphere is important for interpreting measurements by its low energy plasma instruments. Estimates of the floating potential varied significantly, however, between the various different plasma instruments. In this study we utilize 3-D particle-in-cell simulations to understand the key features of Cassini’s plasma interaction in Titan’s ionosphere. The spacecraft is observed to charge to negative potentials for all scenarios considered, and close agreement is found between the current onto the simulated Langmuir Probe and that observed in Titan’s ionosphere. These simulations are therefore shown to provide a viable technique for modeling spacecraft interacting with Titan’s dusty ionosphere.

  • Journal article
    Ala-Lahti M, Pulkkinen TI, Ruohotie J, Akhavan-Tafti M, Good SW, Kilpua EKJet al., 2023,

    Multipoint Observations of the Dynamics at an ICME Sheath-Ejecta Boundary

    , ASTROPHYSICAL JOURNAL, Vol: 956, ISSN: 0004-637X
  • Journal article
    Good SW, Rantala OK, Jylhä AM, Chen CHK, Möstl C, Kilpua EKJet al., 2023,

    Turbulence Properties of Interplanetary Coronal Mass Ejections in the Inner Heliosphere: Dependence on Proton Beta and Flux Rope Structure

    , Astrophysical Journal Letters, Vol: 956, ISSN: 2041-8205

    Interplanetary coronal mass ejections (ICMEs) have low proton beta across a broad range of heliocentric distances and a magnetic flux rope structure at large scales, making them a unique environment for studying solar wind fluctuations. Power spectra of magnetic field fluctuations in 28 ICMEs observed between 0.25 and 0.95 au by Solar Orbiter and Parker Solar Probe have been examined. At large scales, the spectra were dominated by power contained in the flux ropes. Subtraction of the background flux rope fields increased the mean spectral index from −5/3 to −3/2 at kd i ≤ 10−3. Rope subtraction also revealed shorter correlation lengths in the magnetic field. The spectral index was typically near −5/3 in the inertial range at all radial distances regardless of rope subtraction and steepened to values consistently below −3 with transition to kinetic scales. The high-frequency break point terminating the inertial range evolved approximately linearly with radial distance and was closer in scale to the proton inertial length than the proton gyroscale, as expected for plasma at low proton beta. Magnetic compressibility at inertial scales did not show any significant correlation with radial distance, in contrast to the solar wind generally. In ICMEs, the distinctive spectral properties at injection scales appear mostly determined by the global flux rope structure while transition-kinetic properties are more influenced by the low proton beta; the intervening inertial range appears independent of both ICME features, indicative of a system-independent scaling of the turbulence.

  • Journal article
    Trotta D, Pezzi O, Burgess D, Preisser L, Blanco-Cano X, Kajdic P, Hietala H, Horbury TS, Vainio R, Dresing N, Retinò A, Marcucci MF, Sorriso-Valvo L, Servidio S, Valentini Fet al., 2023,

    Three-dimensional modelling of the shock-turbulence interaction

    , Monthly Notices of the Royal Astronomical Society, Vol: 525, Pages: 1856-1866, ISSN: 0035-8711

    The complex interaction between shocks and plasma turbulence is extremely important to address crucial features of energy conversion in a broad range of astrophysical systems. We study the interaction between a supercritical, perpendicular shock and pre-existing, fully developed plasma turbulence, employing a novel combination of magnetohydrodynamic and small-scale, hybrid-kinetic simulations where a shock is propagating through a turbulent medium. The variability of the shock front in the unperturbed case and for two levels of upstream fluctuations is addressed. We find that the behaviour of shock ripples, i.e. shock surface fluctuations with short (a few ion skin depths, di) wavelengths, is modified by the presence of pre-existing turbulence, which also induces strong corrugations of the shock front at larger scales. We link this complex behaviour of the shock front and the shock downstream structuring with the proton temperature anisotropies produced in the shock-turbulence system. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of novel cross-scale, multispacecraft measurements in resolving shock front irregularities at different scales. These results are relevant for a broad range of astrophysical systems characterized by the presence of shock waves interacting with plasma turbulence.

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