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  • Journal article
    Zelinka MD, Myers TA, Qin Y, Chao L-W, Klein SA, Po-Chedley S, Ma P-L, Wall CJ, Ceppi P, Gettelman Aet al., 2026,

    Recent cloud trends and extremes reaffirm established bounds on cloud feedback and aerosol-cloud interactions

    , Communications Earth & Environment
  • Journal article
    Wang S, Yang P, Brindley HE, Huang X, L'Ecuyer TSet al., 2026,

    Enhanced Full Spectral Temperature-Dependent Refractive Index of Liquid Water From Supercooled to Ambient Conditions

    , Geophysical Research Letters, Vol: 53, ISSN: 0094-8276

    A new compilation of the complex refractive index of liquid water is presented, spanning temperatures from (Formula presented.) (near homogeneous freezing) to (Formula presented.) K and wavelengths from (Formula presented.) μm to 10 m. The real part of the refractive index is derived using the Kramers–Kronig relation, where the imaginary part is constrained by measurements reported in literature and validated through the f-sum rule. The result reveals a significant temperature dependence, especially at wavelengths beyond the near-infrared. Sensitivity analyses in the infrared split-window and microwave spectral regime demonstrate substantial differences in bulk optical properties between supercooled and ambient conditions. These findings manifest the importance of accounting for temperature-dependent refractive indices in optical radiative transfer and simulations.
  • Journal article
    Ceppi P, Wilson Kemsley S, Andersen H, Andrews T, Kramer RJ, Nowack P, Wall CJ, Zelinka MDet al., 2026,

    Emerging low-cloud feedback and adjustment in global satellite observations

    , Atmospheric Chemistry and Physics (ACP), Vol: 26, Pages: 4153-4171, ISSN: 1680-7316

    From mid-2003 to mid-2024, a global decrease in low-cloud amount enhanced the absorption ofsolar radiation by 0.22±0.07Wm−2 per decade (±1σ range), accelerating the energy imbalance trend duringthat period (0.44Wm−2 per decade). Through controlling factor analysis, here we show that the low-cloudtrend is due to a combination of cloud feedback and adjustments to greenhouse gases and aerosols (respectively 0.09±0.02, 0.05±0.03, and 0.03±0.03Wm−2 per decade), which jointly account for 74% of the trend. The contribution of natural climate variability is weak but uncertain (0.01±0.08Wm−2 per decade), owing to apoorly constrained trend in boundary-layer inversion strength. Importantly, the observed low-cloud radiativetrend lies well within the range of values simulated by contemporary global climate models under conditionsclose to present day. Any systematic model error in the representation of present-day global energy imbalancetrends is thus likely to originate in processes unrelated to low clouds.
  • Journal article
    Nair V, Gryspeerdt E, Arola A, Lipponen A, Virtanen Tet al., 2026,

    Observing the role of wind-driven processes in the evolution of warm marine cloud properties

    , Atmospheric Chemistry and Physics, Vol: 26, Pages: 4049-4066, ISSN: 1680-7316

    The cloud droplet effective radius is a key variable when evaluating the interactions between aerosols and clouds. The activation of fine-sized sea salt from the ocean results in the formation of more but smaller cloud droplets (reducing the effective radius) in marine stratocumulus. Coarse sea spray aerosols are generated for high surface wind speeds and act as giant cloud condensation nuclei, which activate to form larger droplets. This increases the effective radius and initiates precipitation. These high wind speeds also lead to enhanced moisture fluxes from the ocean surface. Although the opposing impacts of wind-driven fine and coarse marine sea spray aerosols have been documented, their observations have been limited to instantaneous satellite images. In this work, a novel framework is introduced that uses short-timescale observations of the temporal evolution of clouds to identify, isolate, and extract the process fingerprints of marine sea-salt and surface fluxes on stratocumulus cloud properties. This method shows that changes in droplet size previously attributed to aerosol are actually due to increases in evaporation from the ocean surface due to high surface wind speeds. However, when this is accounted for, a clear impact of giant cloud condensation nuclei is observed, reducing cloud droplet number concentrations by initiating precipitation in polluted clouds. By isolating the causal aerosol impact on clouds from confounding factors, this method provides a pathway to improved constraints on the human forcing of the climate, whilst also demonstrating how marine aerosols limit the effectiveness of anthropogenic aerosol perturbations.
  • Journal article
    Snodgrass C, Epifani EM, Tubiana C, Sanchez JP, Biver N, Inno L, Knight MM, Lacerda P, De Keyser J, Donaldson A, Edberg NJT, Galand M, Guilbert-Lepoutre A, Henri P, Kasahara S, Kawakita H, Kokotanekova R, Kueppers M, Micheli M, Pajusalu M, Rubin M, Sakatani N, Yoshioka K, Della Corte V, Eriksson AI, Fulle M, Holt C, Lara L, Rotundi A, Jehin Eet al., 2026,

    Considerations on the process of target selection for the Comet Interceptor mission

    , Icarus, Vol: 447, ISSN: 0019-1035

    Comet Interceptor is an ESA science mission with payload contributions from ESA Member States and with an international participation by JAXA. It is the first mission that is being designed, built, and potentially launched before its target is known. This approach will enable the spacecraft to perform the first mission to a Long Period Comet from the Oort Cloud, as these comets have fleeting visits to the inner Solar System lasting only months to years from first discovery, too short for the usual process of mission development to be followed. In this paper we describe a number of factors that need to be considered in selecting a target for the mission, including scientific, orbital, spacecraft and instrument constraints, and discussion of different prioritisation strategies. We find that, in the case where we have a choice of targets, our decisions will mostly be driven by orbital information, which we will have relatively early on, with information on the activity level of the comet an important but secondary consideration. As cometary activity levels are notoriously hard to predict based on early observations alone, this prioritisation / decision approach based more on orbits gives us confidence that a good comet that is compatible with the spacecraft constraints will be selectable with sufficient warning time to allow the mission to intercept it.
  • Conference paper
    Gryspeerdt E, Driver OGA, Marjani S, Maruhashi J, Neely III RR, Rhodes L, Stettler MEJ, Tippett A, Walden CJ, Walker Det al., 2026,

    Aircraft as a natural experiment on ice clouds

    <jats:p>Aerosol impacts on ice clouds remain a highly uncertain component of the effective radiative forcing from aerosol-cloud interactions, with models simulating a wide range of responses. Developing observational constraints for these aerosol-cloud effects is challenging. The low aerosol concentrations involved hinder their direct observation and the meteorological conditions that affect cloud properties (such as temperature and updraught speed) also impact ice crystal number, limiting its use for inferring information about aerosol. Variations in meteorological conditions can also impact cloud and aerosol properties together, obscuring the causal impact of aerosol on cloud.Similar to the use of ship emitted aerosol and the resulting 'shiptrack' cloud perturbation to understand aerosol-cloud interactions in liquid clouds, here we use aircraft to understand the response of ice clouds to aerosol perturbations.  Aircraft release water, aerosol and heat into the atmosphere as they fly, creating contrails in clear sky if conditions are suitable and perturbing existing clouds they fly through. The perturbation sizes vary with aircraft type, allowing a more detailed assessment of cloud responses.Using a range of satellite data and ground-based radar observations, we composite contrails and aircraft impacts on existing clouds under a variety of conditions from a range of different aircraft types. We see that contrails formed from different aircraft types have varying lifetimes, consistent with an aerosol effect that increases cloud lifetime. Impacts on existing clouds vary significantly with time since the perturbation and meteorological conditions, highlighting the importance of the background cloud conditions. We also demonstrate how non-aerosol effects can be isolated and removed, to better constrain the impact of aerosols and aircraft on ice clouds and climate. </jats:p>
  • Conference paper
    Maruhashi J, Marjani S, Driver O, Itcovitz J, Gryspeerdt E, Stettler Met al., 2026,

    Success and failure of contrail models: a flight-by-flight investigation using satellite observations

    <jats:p>A realistic quantification of aviation’s net global climate impact depends on how well models represent aviation-induced aerosols (e.g., soot and sulfate) and their dual role: contributing to net warming through the formation of persistent ice clouds (contrails) and contributing to cooling by altering the microphysical properties of existing liquid clouds. Here, we focus on the warming pathway. Persistent contrails are estimated to produce warming over a year comparable to the warming from aviation CO₂ accumulated over several decades [1] and may account for ~2% of the total anthropogenic surface temperature increase since pre-industrial times [2]. Given their importance, contrails must be modelled both accurately and efficiently to support operational mitigation and to track aviation’s climate impact.The Contrail Cirrus Prediction (CoCiP) tool is a widely used Lagrangian model that predicts contrail formation and evolution on a flight-by-flight basis. CoCiP is integrated into the Non-CO₂ Aviation Effects Tracking System (NEATS), which supports compliance with recent European reporting requirements for non-CO₂ aviation effects. Despite its broad adoption, CoCiP has been shown to underestimate lifetime-integrated optical depth relative to higher-fidelity models [3], motivating further evaluation against observations.We analyze ~500 flights from 2025 that flew through the UK and surrounding region (approximately 48°N-63°N, 20°W-4°E) that have been contrail-matched using detections from the Earth Cloud Aerosol and Radiation Explorer (EarthCARE) mission. For each flight, we run CoCiP and compare its output at the advected waypoint closest to the satellite-detected contrail at the detection time. We find that CoCiP fails to predict a contrail for roughly half of the cases. For ~20% of flights, contrail formation is not expected based on the Schmidt–Appleman criterion, which depends on both atmospheric and aircraft character
  • Journal article
    Recchiuti D, Franci L, Matteini L, Papini E, Battiston R, Piersanti Met al., 2026,

    Evolution of ion distribution functions in ionospheric plasmas perturbed by Alfvén waves

    , Journal of Plasma Physics, Vol: 92, ISSN: 0022-3778

    This study investigates ion kinetic effects during the parametric decay instability (PDI) of parallel-propagating Alfvén waves under plasma conditions characteristic of the Earth’s ionosphere. By using a series of hybrid particle-in-cell simulations, we examine the evolution of ion velocity distribution functions (VDFs) in ultra-low-beta plasmas. Our numerical campaign systematically explores the dependence on key parameters (plasma beta, pump-wave amplitude and polarisation, and ion composition). To emphasise the role of kinetic effects, we choose to trigger the PDI with a dispersive mother wave with wavelength comparable to the ion characteristic inertial length. Our results reveal pronounced non-thermal VDF modifications, including parallel heating and the formation of secondary ion beams, linked to the nonlinear evolution of parametric decay instability. By varying the plasma beta and the pump-wave amplitude, we identify a critical regime where rapid and complete broadening of the velocity distribution function is observed, triggering bidirectional ion acceleration. Notably, simulations modelling realistic ionospheric conditions demonstrate that even low-amplitude Alfvénic perturbations can induce significant VDF spreading and ion beam generation, with hydrogen ions exhibiting stronger effects than oxygen. These non-thermal microscopic processes offer a plausible mechanism for particle precipitation in space weather events. This work represents the first comprehensive study with hybrid simulations of PDI-driven ion kinetics in ultra-low-beta plasmas, providing quantitative estimates for the time delay between electromagnetic wave impact and ion VDF modification, and new insights into wave–particle interactions that may contribute to ion acceleration, precipitation processes and space plasma dynamics.
  • Journal article
    Schwartz SJ, Trattner KJ, Raptis S, Ergun RE, Wilson LB, Gomez RG, Cohen IJ, Gershman DJ, Goodrich KA, Kucharek H, Vo Tet al., 2026,

    Energy Partition at a Collisionless Supercritical Quasi-Parallel Shock

    , Journal of Geophysical Research Space Physics, Vol: 131, ISSN: 2169-9380

    Shocks in collisionless astrophysical plasmas redistribute some of the incident flow energy into both thermal and non-thermal energy. Quantifying the partition of that energy amongst various particle species or their sub-populations, and electromagnetic energy, represents a fundamental goal of shock physics. It embodies the role of the equation of state for the system. Here we apply a framework to assess all the incident and downstream energy fluxes at a crossing of Earth's bow shock for which the upstream magnetic field was roughly aligned with the shock normal direction. Such quasi-parallel shocks are known to be non-steady and to produce significant populations of suprathermal particles. We quantify the evolution of all the important carriers of energy flux through the shock region. We sub-divide the proton population into thermal, suprathermal, and energetic components in order to investigate the shock's efficiency in energizing the nonthermal particles. While the largest energy fluxes are found in the incident proton ram energy and downstream proton thermal enthalpy fluxes, a significant suprathermal population pervades the regions both up- and downstream. We also evaluate the energy fluxes attributable to fluctuations in the fluid and field parameters.
  • Journal article
    Wivell L, Dougherty MK, Masters A, 2026,

    The dynamic inducing magnetic field signal at Triton

    , Geophysical Research Letters, Vol: 53, ISSN: 0094-8276

    Triton, Neptune's largest moon, is suspected of harboring a subsurface ocean. Detecting sub-surface oceans requires measuring the ocean's induced magnetic field, typically exploiting common frequencies at which the field environment of the moon changes, in this case Triton's path through Neptune's magnetosphere. Triton's orbit around Neptune, whilst nearly completely circular, is highly inclined, with an inclination of approximately 157° to Neptune's equator. This results in a rotation signal strength and frequency which has significant orbital dependence, unlike at other ocean worlds. The work conducted in this study highlights how the inducing signal at Triton is dynamic in both power and frequency caused by Triton's high inclination. This means that there is a richer set of frequency signals than previously thought, that may be useful for induction studies. This work quantifies the effect that Triton's inclination has on the inducing signal, and rules out other potential sources of signal disruption.
  • Journal article
    Jia H, Quaas J, Kroese W, van Diedenhoven B, Gryspeerdt E, Böhm C, Block K, Hasekamp Oet al., 2026,

    Optimal choice of proxy for cloud condensation nuclei reduces uncertainty in aerosol-cloud-climate forcing

    , Science Advances, Vol: 12, ISSN: 2375-2548

    Aerosol-cloud interactions (ACI) remain the largest uncertainty in anthropogenic climate forcings. Observation-based estimates of instantaneous radiative forcing from ACI (RFaci; the Twomey effect) rely on the choice of aerosol quantities as proxies for cloud condensation nuclei (CCN) concentrations, which differ in their ability to represent cloud-base CCN and data accuracy. Using diverse observations and aerosol-climate models, we evaluate the utility of different proxies with two independent approaches. Both approaches reveal that surface CCN exhibits the smallest bias in predicting RFaci (+5%), followed by aerosol index, surface sulfate and column CCN with similar biases of +25%, while aerosol optical depth and column sulfate show the largest biases (−60% and +92%). Constraining RFaci with the optimal proxy reduces uncertainty from 66 to 43%, yielding a less negative RFaci (−1.0 W m−2) than the unconstrained case (−1.2 W m−2). Our findings highlight the crucial role of proxy constraint in reconciling and improving RFaci estimates.
  • Journal article
    Bianco JS, Tenerani A, Gonzalez C, Matteini L, Klein KGet al., 2026,

    Evolution of an Alfvén Wave–driven Proton Beam in the Expanding Solar Wind

    , The Astrophysical Journal, Vol: 998, Pages: 194-194, ISSN: 0004-637X

    <jats:title>Abstract</jats:title> <jats:p>We investigate the self-consistent formation and long-term evolution of proton beams in the expanding solar wind using an ensemble of one-dimensional hybrid expanding box simulations. Initial conditions are chosen to represent a range of plasma states observed by the Helios spacecraft at 0.3 au, including an amplitude-modulated Alfvén wave that nonlinearly drives a proton beam aligned with the magnetic field. We compare simulation results with solar wind data out to 1.5 au and show that our model reproduces key observed features of proton beams on average, such as the radial evolution of the drift and the relative core-to-beam density ratio. These findings support the theory that the observed evolution of the proton beam drift in the solar wind is determined by kinetic instabilities. More broadly, our results indicate that the interplay between nonlinear Alfvén wave dynamics, expansion effects, and kinetic instabilities plays a fundamental role in solar wind dynamics, with implications for interpreting solar wind heating rate estimates.</jats:p>
  • Journal article
    Wilson Kemsley S, Nowack P, Ceppi P, 2026,

    Recent Cloud Controlling Factor Analyses Indicate Higher Climate Sensitivity

    , Geophysical Research Letters, Vol: 53, ISSN: 0094-8276

    Cloud feedback is a dominant source of uncertainty in climate model estimates of equilibrium climate sensitivity (ECS). Cloud controlling factor analysis can observationally constrain cloud feedback. For the first time, we use separate rather than unified frameworks to assess high- and low-cloud feedbacks and constrain the net cloud feedback and subsequently, the ECS. We find a robustly positive cloud feedback (i.e., a negative feedback is (Formula presented.) % probable), indicating that clouds amplify global warming. We assess the individual and combined impacts of our cloud feedback constraints on ECS using three approaches. Two indicate an upward ECS shift with reduced uncertainty, preserving ECS–feedback correlations but using cloud feedback as a single line of evidence. The third, a Bayesian framework combining multiple lines of evidence, also suggests a higher ECS but with a smaller increase and broader confidence range.
  • Journal article
    Mosselmans S, Elizabeth Brindley H, Cox C, Gryspeerdt E, Murray J, Panditharatne S, Warwick L, Schuettemeyer D, Oetjen H, Carlsen T, Foth A, David Ret al., 2026,

    Exploring clear‐sky longwave radiative closure in the arctic: a downwelling case study

    , Atmospheric Science Letters, Vol: 27, ISSN: 1530-261X

    The far infrared region plays a critical role in the Earth's greenhouse effect. Measurements across this range are limited and previous studies have found it challenging to achieve radiative closure between different instruments across the mid-far infrared. This study compares high-resolution downwelling spectra measured from the surface in the Arctic to radiative transfer simulations run with atmospheric profiles from a radiosonde, microwave radiometer, and the 5th ECMWF atmospheric reanalysis. The radiosonde-driven simulation generates the closest agreement to the radiance observations. The HATPRO-driven simulation matches within its uncertainty across the entire range. However, it has a larger uncertainty envelope than the other sources. There is a bias in all three simulations associated with underestimated humidity. Our results reiterate the sensitivity of the far infrared to the humidity profile. This suggests that the wider deployment of spectrometers capable of measuring across this region, in parallel with efforts to improve knowledge of far infrared water vapour spectroscopy, could markedly improve our ability to routinely characterise polar environments.
  • Journal article
    Hadid LZ, Chust T, Wahlund JE, Morooka MW, Roussos E, Witasse O, Rabia J, Pisa D, Kim K, Edberg NJT, Rymer AM, Lamy L, Kotsiaros S, Aizawa S, Jeandet A, Modolo R, André N, Canu P, Bowers CF, Jia X, Coates AJ, Jones GH, Parsec-Wallis A, Agiwal O, Holmberg MKG, Nénon Q, Cao H, Kurth WS, Dougherty MKet al., 2026,

    Evidence of an Extended Alfvén Wing System at Enceladus: Cassini's Multi-Instrument Observations

    , Journal of Geophysical Research Space Physics, Vol: 131, ISSN: 2169-9380

    We report in situ evidence for Enceladus' Alfvén wing system and its coupling with Saturn's ionosphere, based on multi-instrument observations from the Cassini spacecraft. Analysis of 36 events, including 13 from non-flyby paths, confirms the existence of a Main Alfvén Wing (MAW) current system generated at Enceladus, and associated Reflected Alfvén Wings (RAWs) occurring both at Saturn's ionosphere and on the density gradient of Enceladus' plasma torus, extending longitudinally to at least (Formula presented.) ((Formula presented.) 2,000 moon radii) downstream of the moon. Additionally, the observations reveal the systematic existence of a filamentation process of these large-scale Alfvénic perturbations (MAW and RAWs) during their propagation at any distance from their source. These findings demonstrate a more extensive electrodynamic coupling than previously reported for Enceladus and more generally for any moon-magnetosphere interaction. Moreover, the observation of energetic electron depletions and water-group ion signatures at longitudes even further from the moon supports the interpretation of an extended and persistent interaction region. These results highlight Enceladus' role in shaping Saturn's magnetospheric environment and underscore the importance of future missions to exhaustively analyze this type of complex interaction between a moon and a planet.
  • Journal article
    Wivell L, Dougherty MK, Masters A, 2026,

    Inductive response of Enceladus' ice shell and potentially stratified ocean

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

    Saturn's moon Enceladus harbors a global subsurface ocean beneath its icy crust. Understanding the structure and composition of this ocean and ice is critical to assessing its potential habitability. Modern electromagnetic (EM) sounding techniques, which measure a celestial body's induced response to external electromagnetic fields, offer a powerful tool for probing internal structures. These techniques are well-established for Earth and the Moon, modeled for Europa, and here evaluated for Enceladus. By modeling higher frequency range (1 mHz−1 kHz), which sound to shallower depths than lower frequencies, this study shows that induction can provide a constraint on ice composition. The induced response also gives insight into other ice-shell properties, including potential water layers, as well as different stratified ocean conditions. The findings of this study highlight the potential for future missions to use EM sounding to constrain properties of the ice-shell, including composition, as well as identifying potential ocean stratification.
  • Journal article
    Fargette N, Eastwood JP, Phan TD, Matteini L, Franci Let al., 2026,

    Fluid and kinetic properties of the near-sun heliospheric current sheet

    , The Astrophysical Journal, Vol: 997, ISSN: 0004-637X

    The heliospheric current sheet (HCS) is an important large-scale structure of the heliosphere, and, for the first time, the Parker Solar Probe (PSP) mission enables us to study its properties statistically, close to the Sun. We visually identify the 39 HCS crossings measured by PSP below 50 R⊙ during encounters 6–21, and investigate the occurrence and properties of magnetic reconnection, the behavior of the spectral properties of the turbulent energy cascade, and the occurrence of kinetic instabilities at the HCS. We find that 82% of the HCS crossings present signatures of reconnection jets, showing that the HCS is continuously reconnecting close to the Sun. The proportion of inward and outward jets depends on heliocentric distance, and the main HCS reconnection X-line has a higher probability of being located close to the Alfvén surface. We also observe a radial asymmetry in jet acceleration, where inward jets do not reach the local Alfvén speed, contrary to outward jets. We find that turbulence levels are enhanced in the ion kinetic range, consistent with the triggering of an inverse cascade by magnetic reconnection. Finally, we highlight the ubiquity of magnetic hole trains in the high-β environment of the HCS, showing that the mirror mode instability plays a key role in regulating the ion temperature anisotropy in HCS reconnection. Our findings shed new light on the properties of magnetic reconnection in the high-β plasma environment of the HCS, its interplay with the turbulent cascade, and the role of the mirror mode instability.
  • Journal article
    Clear CP, Uylings P, Raassen T, 2026,

    Calculated oscillator strengths and transition probabilities of singly ionised nickel (Ni II)

    , Astronomy and Astrophysics (A & A), Vol: 706, ISSN: 0004-6361

    Aims. This work reports calculated transition probabilities for spectral lines of singly ionised nickel (Ni II) incorporating newly determined experimental energy levels, addressing critical gaps in atomic data required for astrophysical spectroscopy and plasma diagnostics.Methods. Transition probabilities of Ni II were calculated using the semi-empirical orthogonal operator method for both odd and even energy levels. Calculated eigenvalues were fine-tuned to experimental energy levels, determined using Fourier transform spectroscopy, further increasing the accuracy of these calculated transition probabilities.Results. In total, transition probabilities have been calculated for nearly 118 000 electric dipole transitions between 361 even and 735 odd levels. The resulting transition probabilities show strong agreement with existing experimental and semi-empirical data, while offering improved consistency and coverage across a wide range of line strengths. The calculated transitions span the far-infrared to the vacuum ultraviolet spectral regions, providing extensive coverage for astrophysical applications. This dataset significantly enhances the calculated atomic data available for Ni II and represents a critical contribution to the advancement of our understanding of astrophysical phenomena through improved spectroscopic analysis.
  • Journal article
    Im U, Samset BH, Nenes A, Thomas JL, Kokkola H, Dubovik O, Amiridis V, Arola A, Bellouin N, Benedetti A, Bilde M, Blichner S, Decesari S, Ekman AML, GarcíaPando CP, Gross S, Gryspeerdt E, Hasekamp O, Kahn RA, Laakso A, Lohmann U, Marelle L, Massling AH, Myhre CL, Pöhlker M, Quaas J, Raatikainen T, Riipinen I, Schmale J, Seifert P, Skov H, Smith C, Sporre MK, Stier P, Storelvmo T, Tsigaridis K, van Diedenhoven B, Virtanen A, Wandinger U, Wilcox LJ, Zieger Pet al., 2026,

    Aerosol‐cloud interactions: overcoming a barrier to projecting near‐term climate evolution and risk

    , AGU Advances, Vol: 7, ISSN: 2576-604X

    Aerosol-cloud interactions (ACI) are a major source of uncertainty in climate science, critically affecting our ability to project near-term climate evolution and assess societal risks. These interactions influence effective radiative forcing, cloud dynamics, and precipitation patterns, yet remain insufficiently constrained due to limitations in observations, modeling, and process understanding. This uncertainty hampers robust policy advice across multiple domains—from estimating remaining carbon budgets and climate sensitivity, to anticipating regional extreme events and evaluating climate interventions such as solar radiation modification. In many cases, the influence of ACI is either underappreciated or excluded from decision-making frameworks due to its complexity and lack of quantification. This perspective outlines a path forward to overcome these barriers by leveraging emerging opportunities in satellite remote sensing, ground-based and airborne observations, high-resolution climate modeling, and machine learning. We identify key areas where rapid progress is feasible, including improved retrievals of cloud microphysical properties, better representation of natural aerosols in a warming world, and enhanced integration of observational and modeling communities. Even as anthropogenic aerosol and its impacts on clouds is reducing owing to emissions controls, addressing ACI uncertainties remains essential for refining climate projections, supporting effective mitigation and adaptation strategies, and delivering actionable science to policymakers in a rapidly changing climate system.
  • Journal article
    Tsilimigkras A, Lazaridis M, Voulgarakis A, Varotsos KV, Karali A, Giannakopoulos C, Stamou A, Koutroulis Aet al., 2026,

    Climate projections for Greece: Defining a regional sub-ensemble from the CMIP6 landscape

    , Theoretical and Applied Climatology, Vol: 157, ISSN: 0177-798X

    Most climate change impact studies, regardless of scope, traditionally rely on a predefined set of climate model simulations without thoroughly examining representativeness, model skill, and diversity. This approach risks overlooking regional nuances and limits the utility of projections for tailored adaptation strategies. In the Mediterranean—and particularly Greece, where climate risks are high—addressing these limitations is essential for reliable, actionable projections. The CMIP6 ensemble is extensive, but its size and internal variability pose challenges for regional use, leaving users to navigate an “ensemble of opportunity” with interdependent models and diverse historical and future behaviors. Here we evaluate 35 CMIP6 models over Greece against bias-adjusted GSWP3-W5E5 observations, assessing both annual and seasonal historical performance with multiple diagnostics (correlation, standard deviation, CRMSE, bias, RMSE) and summarizing skill via a composite Historical Performance Score (HPS): the harmonic mean of Taylor Skill Score (pattern fidelity) and a variability-aware bias score that penalizes systematic offsets relative to observed interannual variability. Future responses are analyzed for 2081–2100 (high-emission Shared Socioeconomic Pathway SSP5-8.5) using a quadrant framework based on temperature change (tas) and late-century precipitation (pr); changes in maximum temperature (tasmax) are also incorporated to characterize the amplification of hot conditions. By integrating model performance and ensemble spread, the methodology refines model selection to balance historical credibility with diversity of future outcomes, enabling compact regional sub-ensembles that capture the range from moderate to severe warming and from drier to wetter states. Results show that historical skill does not necessarily translate into capturing future extremes in warming or drying, but a carefully chosen sub-ensemble can maximize the range o
  • Journal article
    Di Natale G, Brindley H, Murray J, Warwick L, Panditharatne S, Yang P, David RO, Carlsen T, Vâjâiac SN, Ghemulet S, Bantges R, Foth A, Flügge M, Lyngra R, Oetjen H, Schuettemeyer D, Palchetti L, Murray Jet al., 2026,

    Achieving consistency between in-situ and remotely sensed optical and microphysical properties of Arctic cirrus: the impact of far-infrared radiances

    , Atmospheric Chemistry and Physics (ACP), Vol: 26, Pages: 1373-1394, ISSN: 1680-7316

    This paper explores whether it is possible to achieve consistency between ground-based infrared radiance measurements made in the presence of cirrus, co-located in-situ aircraft measurements of the cirrus microphysics, and ancillary ground-based remote sensing. Specifically we use spectrally resolved radiances covering the range 400–1500 cm−1, in-situ measurements of cirrus particle sizes and habits, backscatter ceilometer observations of cloud vertical structure and microwave inferred temperature and humidity profiles to investigate whether we can obtain consistency between the derived cloud properties and atmospheric state from these independent sources of data. The primary focus of this study is on the sensitivity of the retrieved cloud particle size to the assumed crystal habit. Excellent consistency between the retrieved cloud parameters is achieved both with the ceilometer derived optical depth and the size distribution measured by the aircraft by assuming the crystal habit to be comprised of bullet rosettes. The averaged values of the effective diameter and optical depth obtained from radiometric measurements are (26.5 ± 1.8) µm and (0.12 ± 0.01) in comparison with the values derived from in-situ and ceilometer measurements equal to (31.5 ± 5.0) µm and (0.13 ± 0.01), respectively. Furthermore, we show that the radiance information contained within the far-infrared (wavenumbers < 650 cm−1) spectrum is critical to achieving this level of agreement with the in-situ aircraft observations. The results emphasize why it is vital to expand the current limited database of measurements encompassing the far-infrared spectrum, particularly in the presence of cirrus, to explore whether this finding holds over a wider range of conditions.
  • Journal article
    Badman ST, Fargette N, Matteini L, Agapitov OV, Akhavan-Tafti M, Bale SD, BharatiDas S, Bizien N, Bowen TA, DudokdeWit T, Froment C, Horbury T, Huang J, Jagarlamudi VK, Larosa A, Madjarska MS, Panasenco O, Pariat E, Raouafi NE, Rouillard AP, Ruffolo D, Sioulas N, Soni SL, Sorriso-Valvo L, Suen GHH, Velli M, Verniero Jet al., 2026,

    Properties of magnetic switchbacks in the near-sun solar wind

    , Space Science Reviews, Vol: 222, ISSN: 0038-6308

    Magnetic switchbacks are fluctuations in the solar wind in which the interplanetary magnetic field sharply deflects away from its background direction so as to create folds in magnetic field lines while remaining of roughly constant magnitude. The magnetic field and velocity fluctuations are extremely well correlated in a way corresponding to Alfvénic fluctuations propagating away from the Sun. For a background field which is nearly radial this causes an outwardly propagating jet to form. Switchbacks and their characteristic velocity jets have recently been observed to be nearly ubiquitous by Parker Solar Probe with in situ measurements in the inner heliosphere within 0.3 AU. Their prevalence, substantial energy content, and potentially fundamental role in the dynamics of the outer corona and solar wind motivate the significant research efforts into their understanding. Here we review the in situ measurements of these structures (primarily by Parker Solar Probe). We discuss how they are identified and measured, and present an overview of the primary observational properties of these structures, both in terms of individual switchbacks and their collective arrangement into “patches”. We identify both properties for which there is a strong consensus and those that have limited or qualified support and require further investigation. We identify and collate several open questions and recommendations for future studies.
  • Journal article
    Smith C, Kasoar M, Perkins O, Millington JDA, Mistry Jet al., 2026,

    Small-scale livelihood and cultural fire: global spatiotemporal characteristics, and gaps in data

    , PLoS ONE, Vol: 21, ISSN: 1932-6203

    Human fire use is a key activity and process in many landscapes and ecosystems around the world, varying spatiotemporally depending on social, economic, and ecological factors. Recently, initiatives have begun to synthesise data on global fire use from across multiple disciplines and disparate sources into coherent databases. Here, we draw on information from one of these databases, the Livelihood Fire Database, which collates data on fire use practices worldwide from case studies in the literature. We examine data from 345 case study locations spanning 69 countries regarding return interval, area burned, and seasonality of anthropogenic fires set to meet small-scale rural livelihood objectives and/or for cultural reasons. We distinguish patterns in the spatiotemporal nature of fires associated with different fire-use purposes, such as clearing vegetation for agriculture, maintaining pasture for livestock, promoting certain plant species for gathering, or driving game when hunting. For many fire uses, especially those related to hunting, gathering, human wellbeing, and social signalling, there are very limited quantitative data available, but it is possible to draw qualitative insights from case studies. Case studies demonstrate that environmental and social conditions drive variation in fire use for the same purpose, reiterating that assumptions of uniform drivers of anthropogenic fire may be misleading. Nonetheless where quantitative data are available, we find some correspondence between the spatiotemporal nature of fires and fire-use purpose, suggesting that distinguishing between different fire-use purposes may be useful to understand and to better model their likely timing, size, and frequency relative to climate and other drivers. We recommend examples where the diagnosis of these broad relationships between fire-use purpose and fire properties could enable improved representation of anthropogenic fire in global land surface models, and aid interpretation of
  • Journal article
    Dai AZ, Gregory J, Ceppi P, 2026,

    Understanding the Climate Response to Different Vertical Patterns of Radiative Forcing

    , Geophysical Research Letters, Vol: 53, ISSN: 0094-8276

    The dependence of climate response on the vertical structure of radiative forcing is studied using a set of idealized experiments, with horizontally uniform and vertically confined forcings. We find for a given effective forcing magnitude, higher-altitude forcing causes a smaller global warming, owing to more negative cloud feedback. We present novel evidence relating this altitude dependence to sea-surface temperature patterns and tropospheric static stability. The imposed instantaneous forcings are horizontally uniform, but higher-altitude forcings more effectively suppress convection in the tropical warm pool, producing a more positive effective (adjusted) surface forcing in that region. This gives rise, during the subsequent climate change, to greater warming contrast between the warm pool and rest of the globe, and hence to increase in low cloud amount. Our results show that to achieve accurate climate projections under anthropogenic forcings, it is important to correctly represent the vertical structures of the applied radiative forcing.
  • Journal article
    Eastwood JP, Phan TD, Drake JF, Shay MA, Øieroset M, Fargette N, Waters CL, Lewis HC, Badman ST, Stevens ML, Halekas J, Bale SDet al., 2026,

    Magnetic reconnection energy fluxes in the near-sun heliospheric current sheet as observed by parker solar probe

    , The Astrophysical Journal, Vol: 996, ISSN: 0004-637X

    The Heliospheric Current Sheet (HCS) is a fundamental feature of the heliosphere, playing a key role organizing the magnetic structure of the solar wind. In contrast to observations previously made through the majority of the heliosphere, Parker Solar Probe has recently revealed that the HCS is typically reconnecting in the inner heliosphere. This provides a new opportunity to study reconnection dynamics in large-scale current sheets and assess how this is different from smaller systems such as Earth’s magnetosphere. We use Parker data to explore HCS reconnection energy partition in two case studies from Encounter 07 and 08. In both cases, we find that in the exhaust, the proton enthalpy flux density is largest, with significant contributions from the proton kinetic energy flux density and electron enthalpy flux density. In contrast, the exhaust Poynting flux density is small in both events. The size and stability of the HCS allows for a control volume analysis to be performed, thus allowing us to estimate changes in energy flux during reconnection. This analysis shows that energy is primarily transferred from the magnetic field to the protons, manifested as the kinetic energy of the exhaust and proton heating. Although the exhaust electron enthalpy flux density is significant, the incoming and outgoing electron enthalpy fluxes are found to be similar, and there is minimal electron heating. The small contribution of the Poynting flux in the outflow may be an important feature of HCS reconnection, with implications for reconnection in large-scale solar and astrophysical current sheets more generally.
  • Journal article
    Hartinger MD, Archer MO, Masongsong E, Shi X, Alexander R, Vidugiris E, Holland A, Harold J, Laca J, Blum LW, Coyle S, Candey RM, Collins K, Angelopoulos V, Costello T, Williams Let al., 2026,

    Inverted radial Alfvén continua: first results from “heliophysics audified: resonances in plasmas”

    , Frontiers in Astronomy and Space Sciences, Vol: 13

    Ultra Low Frequency (ULF) waves with periods of (Formula presented.) 10–1,000 s can lead to space weather impacts such as induced electrical currents in power grids, thus it is important to understand the factors controlling wave dynamics. This is challenging, however, as waves (1) are affected by multiple factors simultaneously, (2) are non-stationary which in some cases precludes use of identification methods that assume stationarity, (3) can occur in superposition with each other making them difficult to separate and identify. Past studies have addressed these challenges through combined audiovisual analysis tools to identify complex but recurring patterns in ULF wave activity that eluded standard visual inspection and automated detection algorithms, as well as through crowd-sourced wave identification. The “Heliophysics Audified: Resonances in Plasmas” NASA citizen science project follows these studies by deploying a Graphical User Interface (GUI) for crowd-sourced ULF wave identification to a large online audience before and during the Heliophysics Big Year (HBY). In this study, we discuss the initial development, beta testing, and deployment of the GUI in April 2023. We further discuss the key initial scientific findings of the HARP project, in particular the discovery by volunteers of anomalous standing Alfvén wave activity with frequency increasing with distance from the Earth. Finally, we discuss participant impacts and lessons learned, as well broader impacts beyond the scope of the original project such as collaborations with museums and musicians. We place these results in context with previous work and discuss implications for future studies.
  • Journal article
    Giacalone J, Trotta D, Mitchell DG, Cohen CMS, Fraschetti F, Bale SDet al., 2026,

    The Source of Energetic Storm Particles for the 2023 March 13 Event Observed by Parker Solar Probe

    , Astrophysical Journal, Vol: 996, ISSN: 0004-637X

    On 2023 March 13, Parker Solar Probe was located about 0.23 au from the Sun when it was crossed by a very fast interplanetary shock. The intensities of ∼0.08–10 MeV energetic protons were markedly enhanced at the shock crossing. At energies from ∼0.1 to 1 MeV, the intensity at the shock was 3–4 orders of magnitude larger than it was ∼4 hr prior. In this study, we investigate the possible source of particles that led to this enhancement. Pre-existing high-energy particles that were present during the few-hour period prior to the shock could possibly serve as a source, assuming they are accelerated to even higher energies by the shock. We apply a “seeds test”—which determines the expected enhancement at the shock—to test this. We find that none of the pre-existing particle populations identified can account for the observed enhancement at the shock. The solar wind itself provides a very abundant source. To test whether shock-heated thermal solar wind could account for the source, we perform a series of self-consistent hybrid plasma simulations of this event and make direct comparisons with the observed particle spectra and magnetic field. We conclude that shock-heated solar wind protons are the source of energetic particles in this event. The direct comparisons of our simulations and observations also suggest that the shock was moving about 1350 km s<sup>−1</sup> and had an Alfvén Mach number of about 4, both of which are considerably lower than previously estimated.
  • Journal article
    Camacho H, Rotermund KM, Slosar A, Bale SD, Barker DW, Burns J, Bye CH, Jones JD, Fahs A, Goetz K, Herrmann S, Hibbard JJ, Jeong O, Klein-Wolt M, Koopmans LVE, Krajewski J, Li Z, Louis C, Maksimović M, McLean R, Monsalve RA, Oconnor P, Parsons A, Piat M, Pulupa M, Pund R, Rapetti D, Saliwanchik B, Speedie G, Stefanov N, Sundkvist D, Suzuki A, Vedantham HK, Yousuf F, Zarka Pet al., 2026,

    LINEAR MAP-MAKING WITH LUSEE-NIGHT

    , Open Journal of Astrophysics, Vol: 9

    LuSEE-Night is a pathfinder radio telescope on the lunar far side employing four 3-m monopole antennas arranged as two horizontal cross pseudo-dipoles on a rotational stage and sensitive to the radio sky in the 1-50 MHz frequency band. LuSEE-Night measures the corresponding 16 correlation products as a function of frequency. While each antenna combination measures radiation coming from a large area of the sky, their aggregate information as a function of phase in the lunar cycle and rotational stage position can be deconvolved into a low-resolution map of the sky. We study this deconvolution using linear map-making based on the Wiener filter algorithm. We illustrate how systematic effects can be effectively marginalised over as contributions to the noise covariance and demonstrate this technique on beam knowledge uncertainty and gain fluctuations. With reasonable assumptions about instrument performance, we show that LuSEE-Night should be able to map the sub-50 MHz sky at a ∼5-degree resolution.
  • Conference paper
    Alterman BL, Livi SA, Lepri ST, Raines JM, Dewey RM, Trotta D, Spitzer SA, Bert CM, Kistler LM, Galvin A, Nicolaou G, Horbury TS, Rivera YJ, Owen CJ, Verscharen D, Coburn J, Lewis G, D'Amicis RA, Carpenter DT, Shimoun EP, Gershkovich I, Ploof K, Eddy TJ, Fedorov A, Lario D, Kucharek H, Ogasawara K, Allegrini F, Ellis L, Louran P, Stubbs Tet al., 2026,

    High Time Resolution Heavy Ion Sensor Observations across a Collisionless Shock

    , Pages: 216-223, ISSN: 1743-9213

    Charged particle acceleration and heating in the heliosphere requires detailed observa-tions of the velocity distribution functions (VDFs) of these particles across a range of mass and charge states. Such particle energization is often observed concurrent with transient phenom-ena. Proper characterization of such energization processes and their impact on charged particles requires careful consideration of instrument observation timing and the timing at which a tran-sient phenomenon is observed. Solar Orbiter's Heavy Ion Sensor (HIS) observes the composition of the solar wind along with suprathermal and pickup ions. As such, it provides the observations necessary to resolve the mass and charge states appropriate for properly characterizing charged particle energization. In this work, we describe the current status of data aggregation at vari-able time intervals and how that enables proper characterization of charged particle energization concurrent with transient phenomena.
  • Journal article
    Berland GD, Hill ME, Kouloumvakos A, Mitchell DG, McNutt Jr RL, Roelof EC, Cohen CMS, Wiedenbeck ME, Mccomas DJ, Christian ER, Schwadron NA, Khoo LY, Cuesta ME, Muro GD, Xu ZG, Pak S, Farooki HA, Stevens ML, Bale SDet al., 2026,

    Parker Solar Probe Observations of Suprathermal and Energetic Particles during Orbits 18 and 19

    , ASTROPHYSICAL JOURNAL, Vol: 996, ISSN: 0004-637X

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