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Physics Physics Space Physics

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Tuesday 8 September

Space Physics · Cross-listed from astro-ph.SR

On modelling the 2017 galactic cosmic ray depression

O.P.M. Aslam, D. MacTaggart, M.S. Potgieter et al. (+2 more)

In the second half of 2017, as Solar Cycle 24 approached its minimum, a pronounced and rigidity-dependent depression in the galactic cosmic ray (GCR) proton flux was observed using AMS-02. This event was driven by a sequence of strong coronal mass ejections (CMEs). In this work, we model the modulation of GCR protons during this period using a steady-state Parker transport framework with parameters constrained by AMS-02 observations. Treating the event as a purely long-term modulation interval yields excellent agreement with the observed proton intensities, but implies an inferred rigidity dependence of particle mean free paths that is inconsistent with basic turbulence theory, corresponding to an effective inversion of the scattering regime. We argue that this behaviour arises from the ability of highly flexible transport parameterizations to reproduce integrated modulation signatures through correlated adjustments of diffusion coefficients during transient events. To address this inconsistency, we relax the commonly adopted assumption of a time-independent ratio of perpendicular to parallel diffusion coefficients and introduce a physically motivated reduction of this ratio during the CME-dominated interval. This modification preserves the quality of the spectral fits whilst restoring the expected rigidity ordering of mean free paths. Our results indicate that allowing the perpendicular-to-parallel diffusion ratio to vary in time provides an effective means of constraining transport solutions during the 2017 depression and may help to resolve similar inconsistencies reported in other GCR modulation studies.
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All authors O.P.M. Aslam, D. MacTaggart, M.S. Potgieter, M.D. Ngobeni, I.I. Ramokgaba

Monday 7 September

Space Physics · Cross-listed from astro-ph.SR

Amplification of active region equatorial dipole

Ismo Tähtinen

The effect of active region parameters on the evolution of solar axial dipole is well understood. However, their effect on the solar equatorial dipole has not been studied in detail. Understanding the development of the equatorial dipole is important as it drives the interplanetary magnetic field on intracyclic timescales (~ 1yr). We study how the latitude, tilt angle, and the polarity separation affect the evolution of solar equatorial dipole. We use dipole flux transport (DFT), a matrix implementation of surface flux transport (SFT) model, to simulate the dipole evolution of synthetic bipolar magnetic regions (BMRs) with a wide range of tilt angles and latitudes. We also study the evolution of active regions in HMI SHARP database and their associated BMRs. We quantify the dipole evolution by means of the maximum and mean amplification and the growth and decay times of the equatorial dipole. Tilt angle controls the amplification of the equatorial dipole, with larger tilt angles leading to stronger equatorial dipole. Latitude controls the timescale at which the equatorial dipole grows and decays. Low-latitude regions produce the longest living equatorial dipole. We find that only 31% of anti-Joy regions experience amplification of equatorial dipole, compared with 96 of regularly tilted active regions. Latitudes and tilt angles of active regions can have considerable effect on the solar equatorial dipole. Systematic changes in their properties could increase/decrease the strength of the equatorial dipole relative to the solar activity, which could lead to similar relative change in geomagnetic activity. We suggest that large low-latitude coronal holes and relatively high geomagnetic activity in the declining phase of solar cycle 20, could be related to emergence of unusually low-latitude active regions that produced strong and persistent equatorial dipole.
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Space Physics · Cross-listed from astro-ph.IM

Optical Flashes from Beam-Driven Light Sails with the Roman, Rubin and Euclid Observatories

James Guillochon and Abraham Loeb

The primary challenge of rocket propulsion is the burden of accelerating the spacecraft's own fuel. Light sails leave the propellant at home, with the achievable speed set by the sail area, the thermal tolerance of its material, and the power of the driving array. In GL15 we showed that leakage from a microwave array driving such a sail between habitable worlds produces Jansky-level radio transients lasting tens of seconds at $100$~pc. We take that leak to optical and near-infrared, where Fresnel matching would shrink the aperture to $\sim\!20$–$100$~m at $1\,\mu{\rm m}$, but the intensity on that aperture is $2\times10^{8}\,{\rm W\,m^{-2}}$, five orders above published directed-energy loadings. Spreading $1.5$~TW at $\sim20\,{\rm kW\,m^{-2}}$ is a $\sim10$~km phased array, pushing the emitters; Lubin's $10^{3}\,{\rm W\,m^{-2}}$ loading is a $40$~km array, comfortable but more expensive. A tenth-wave delay on $40$~m tiles sized so one tile still covers the sail leaks $0.054$ of the power ($80$~GW) into a halo. The typical Galactic detection is a single $\sim30$~s peak at $m_{\rm AB}\simeq19.4$ in F146 at $8$~kpc; a pair $87$~s apart, about one in five, is the confirmation test. Roman's Galactic Bulge Time Domain Survey reaches $109$~kpc at $8\sigma$ on a faint host, so a pointed flash of that halo is visible from anywhere in the Galaxy if the beam points at us. For $10$–$40$~km optical beamers held to $\sim\lambda/10$ at $v_{\rm max}\gtrsim250\,{\rm km\,s^{-1}}$, $N_{\rm det}=1$ wants $\Gamma_{6{\rm h}}\simeq16$ and a null search limits $\Gamma_{6{\rm h}}\lesssim49$, a factor-of-three window in launch rate. The real threshold is set by PSF-coincident stellar and instrumental transients. If intensity-limited optical beamers are commonly employed in our galaxy, this activity could be revealed by Roman, Rubin and Euclid at no additional observing cost.
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Space Physics · Cross-listed from physics.optics

Mechanical quality factor estimation from a nonlinear optical cavity response

Zohran Ali, Davide Tomasella, Daniel Allepuz Requena et al. (+2 more)

Intrinsic damping rate and quality factor ($Q$) are key parameters in optomechanical systems. However, in high-Q devices, their measurement is often challenging because probe-induced dynamical backaction (DBA) and the need to resolve and accurately track the mechanical resonance limit the performance of state-of-the-art techniques. Here, we introduce an all-optical ringdown method in the unresolved-sideband regime that exploits the nonlinear, time-averaged optical response induced by strong mechanical motion. We derive an analytical model of the time-averaged optical cavity response probed by a weak frequency-scanning laser. During the evolution of the mechanical oscillator, the excited motion modulates the Lorentzian lineshape into a double-horned profile whose peak separation enables an accurate estimation of the modulation amplitude. Tracking this response during the mechanical ringdown allows the intrinsic damping rate to be determined without resolving the mechanical oscillation. We apply the method to a density-modulated phononic crystal membrane placed inside a high-finesse cavity and obtain mechanical quality factors consistent with independent measurements on the same platform. Our method requires a minimal optical setup, while offering an in situ diagnostic and reduced impact from probe-induced DBA and robustness against measurement noise. Finally, the model can describe other resonant systems dispersively coupled to coherent frequency modulations, providing new insights and measurement strategies that expand beyond the optomechanical platform.
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All authors Zohran Ali, Davide Tomasella, Daniel Allepuz Requena, Alexander Huck, Ulrik Lund Andersen

Sunday 6 September

Space Physics

AETHER-P3 Nowcast v1.0: Model Description, Training-Data Construction, and Validation Technical report prepared in support of CCMC onboarding

Ruochen Wang and Xiaoli Bai

AETHER-P\textsuperscript{3} Nowcast v1.0 is a machine-learning-based global thermospheric neutral-density model developed for low-Earth-orbit applications and prepared for onboarding to NASA's Community Coordinated Modeling Center (CCMC). The model provides pointwise neutral-density estimates together with predictive uncertainty using a deep evidential regression framework driven by causal solar, solar-wind, geomagnetic, spatial, temporal, and empirical-model inputs. This report documents the released model configuration, training-data construction, software traceability, output products, validation strategy, benchmark performance, and known limitations. The training archive combines accelerometer- and mission-derived density observations from CHAMP, GRACE-A, GOCE, Swarm-C, and GRACE-FO spanning 2000–2023. More than 40 million eligible 30-s observations are available, but the archive is strongly dominated by consecutive quiet-time measurements. To preserve coverage of physically important regimes, the final 1.67-million-sample training set is constructed using deterministic regime-aware sampling that progressively subsamples quiet conditions while retaining all available extreme-condition observations. Validation uses temporally disjoint chronological blocks with exclusion guards to reduce information leakage. Evaluation across quiet, moderate, and extreme conditions shows competitive performance relative to HASDM, JB2008, NRLMSISE-00, and available WAM-IPE cases, while also identifying limitations associated with sparse training coverage, mission-dependent density products, and condition-dependent uncertainty calibration. The report provides a reproducible technical description of the AETHER-P\textsuperscript{3} Nowcast v1.0 research release and its current CCMC onboarding configuration.
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Space Physics · Cross-listed from astro-ph.IM

Mechanisms of Electrostatic Charge Formation and Retention in Lunar Regolith

Minhyeok Kim, Hyun Jung Kim, Sang H. Choi et al. (+1 more)

As the Artemis program advances toward the lunar south pole and permanently shadowed regions (PSRs), understanding lunar charging is increasingly important for protecting astronauts, robotic systems, instruments, and infrastructure. Persistent darkness, cryogenic temperatures, low regolith conductivity, and long charge-relaxation times may allow energetic-particle-induced charge to accumulate beneath the surface. This Technical Memorandum addresses a key unresolved question: how is electrostatic charge generated, separated, retained, and accumulated within lunar regolith? Existing models predict that solar energetic particles and galactic cosmic rays may produce subsurface electric fields approaching dielectric breakdown thresholds, but the microscopic connection between incident particles and macroscopic volumetric charge sources remains unclear. Energy deposition alone does not determine retained charge. Incident particles and their secondary particles may stop, implant, backscatter, transmit, recombine, become trapped, or escape. This memorandum therefore defines the required microscopic input as the average signed retained-charge distribution per unit depth and incident particle, resolved by particle species and energy. Combined with incident flux and energy spectra, this response provides a depth-dependent volumetric charge-source rate that can be coupled with charge continuity, conduction, dielectric relaxation, and Poisson's equation. Key uncertainties include the effects of mineralogy, grain and pore geometry, temperature, and pre-existing potential on charge retention. Particle-resolved modeling and cryogenic high-vacuum irradiation experiments are needed to constrain these processes and assess subsurface electric fields, dielectric breakdown, dust transport, contamination, and charge-mitigation requirements for sustained lunar polar exploration.
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All authors Minhyeok Kim, Hyun Jung Kim, Sang H. Choi, Ashley Daeun Jung

Saturday 5 September

Space Physics · Cross-listed from physics.pop-ph

Project Setu: 3D Multi-Physics Design and Scaled Structural Analysis for a Relativistic Lightsail Architecture

Prashant Suresh Kamble

Deep-space exploration beyond the solar system requires eliminating chemical propellant mass penalties to achieve relativistic flight velocities (0.166c at 180 s, reaching the mission target of 0.20c at 227 s). This study presents a 3D multi-physics numerical framework for a 4.0-meter circular lightsail propelled by a 100 GW ground laser array, coupling 3D Maxwell FDTD wave optics, non-linear membrane mechanics, and Stefan-Boltzmann thermal radiation in ANSYS Mechanical APDL and Ansys Lumerical. A four-level grid convergence study establishes numerical independence with an ASME GCI_21 of 0.13%, resolving peak membrane stresses of 530.88 MPa with a 3.77x safety factor against stoichiometric Si3N4 tensile failure. With optical absorption constrained to 10 ppm (A = 1.0 x 10^-5), the steady-state core temperature stabilizes at 923.02 K (0.44% deviation from radiation theory), maintaining a +1,247 K margin below sublimation, while fundamental drumhead modal resonance (7.92 Hz) provides a 7.92x safety buffer against laser jitter. The electrodynamic radiation pressure formulation is cross-verified against published flight telemetry from JAXA IKAROS and NASA LightSail 2 within 0.12% and 2.13%, confirming classical momentum transfer modeling across solar and beamed propulsion regimes.
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Space Physics · Cross-listed from eess.SP

Wideband HF Skywave Propagation: A Review with Link Modeling Corroborated by Propagation Data

Eric Weber, Ted Nowak, Joseph Berg et al. (+1 more)

The high-frequency (HF) band, ranging from 3 to 30 MHz, is widely used in a number of applications such as beyond-line-of-sight (BLOS) communications and over-the-horizon radar systems. The long-range propagation characteristics of the HF band rely on the Earth's ionosphere as a refracting medium to establish BLOS wireless links. However, traditional HF communications links often use channels with narrow bandwidths (typically 3 kHz), which limits the amount of data that can be transmitted. In this perspective article, we discuss the challenges of establishing wideband HF communications links with emphasis on the ionosphere, noise characteristics, antenna performance, and available bandwidth. Solar radiation drives ionospheric layering, where fluctuating electron densities determine usable frequencies that change with time of day, seasons, and latitude. Since background noise at HF decreases with increasing frequency, operating at the upper HF limit is advantageous. We present a noise prediction model following ITU Recommendation P.372-17 and highlight the role of high-directivity antennas such as Log-Periodic Dipole Arrays, in optimizing the signal-to-noise ratio of the link. Using ray tracing techniques, we model several representative wideband HF links that employ near-vertical-incidence skywave (NVIS) or long-range skywave modes over distances ranging from approximately 390 to 5,200 km. We examine the performance of these links for bandwidths up to 1 MHz and present their day and night ionograms, with transmitter and receiver locations spanning near-equatorial to high northern latitudes. The modeled maximum usable frequencies are corroborated against crowdsourced FT8 reception data and direct ionosonde measurements over the same period for links for which propagation data was available.
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All authors Eric Weber, Ted Nowak, Joseph Berg, Nader Behdad

Thursday 3 September

Space Physics · Cross-listed from astro-ph.EP

Detection of hydrocarbons in Titan using high-resolution cross-correlation spectroscopy

Maria Coelho, Rafael Rianço-Silva, Diogo Gonçalves et al. (+2 more)

High-resolution cross-correlation spectroscopy (HRCCS) is a powerful technique for detecting molecules whose individual spectral lines are too weak to be identified directly, but its sensitivity is limited by the availability and quality of high-resolution opacity data. Many molecules of atmospheric and astrobiological interest lack complete line lists, restricting traditional template-based searches. In this work, we use Titan as a controlled testbed to develop and validate a new cross-section-based methodology for HRCCS template construction. We analyse K-band CRIRES+ observations of Titan (1.99 - 2.48 {\mu}m), and compute cross-correlation functions using both line-by-line and cross-section-based templates. Our analysis recovers known hydrocarbons such as methane (CH4) and acetylene (C2H2), and yields the first HRCCS detection of ethane (C2H6) with a significance of SNRpeak = 5.17 {\pm} 0.07. The ethane detection was made possible exclusively through cross-section-based templates, as no high-resolution line list currently exists for this molecule. These results demonstrate that cross-section-based template construction is a practical and powerful strategy for extending HRCCS to molecules that currently lack reliable line lists, and establish Titan as a benchmark for calibrating molecular detection techniques that can be applied to both solar system and exoplanet atmospheres. Future applications of this approach to other ground-based high-resolution spectrographs, as well as to JWST's highest-resolution modes and next-generation facilities such as the ELT, could significantly expand the inventory of molecules detectable in planetary atmospheres.
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All authors Maria Coelho, Rafael Rianço-Silva, Diogo Gonçalves, Pedro Machado, Zita Martins

Space Physics · Cross-listed from astro-ph.SR

The Role of Preceding CMEs and SIRs in Enhancing Shock Acceleration of Electrons

Xiaomin Chen, Seve Nyberg, Rami Vainio et al. (+3 more)

The role of large-scale pre-existing interplanetary structures, including preceding coronal mass ejections (CMEs) and stream interaction regions (SIRs), in shaping the shock acceleration environment for energetic electrons remains not fully understood. In this study, we investigate nine interplanetary shocks observed by the Solar Terrestrial Relations Observatory (STEREO) that are associated with significant MeV electron enhancements, as such enhancements are rarely observed at interplanetary shocks. We combine remote-sensing observations, drag-based modeling, and in-situ measurements to analyze the shock propagation through pre-existing interplanetary structures. Eight of the nine events are associated with a preceding slow or intermediate-speed CME, while six shocks are in-situ observed propagating within preceding ICMEs, indicating that large-scale upstream trapping structures are a common feature of these events. Further analysis identifies three distinct scenarios associated with enhanced electron acceleration: shocks propagating through preceding ICMEs, shock-SIR interactions, and direct injection of flare-accelerated electrons into SIRs. As a representative shock-in-ICME event, the 2012 January 29 low-$\beta$, quasi-perpendicular shock ($\theta_{Bn}\sim87^\circ$) is further investigated using observations together with one-dimensional Monte Carlo test-particle simulations of a shock propagating into a large-scale upstream magnetic loop. The simulation suggests that the upstream loop prolongs electron residence near the shock and substantially enhances acceleration efficiency. These results demonstrate that large-scale interplanetary structures can precondition the upstream magnetic environment, providing favorable conditions for prolonged electron residence and efficient shock acceleration.
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All authors Xiaomin Chen, Seve Nyberg, Rami Vainio, Immanuel Christopher Jebaraj, Chuan Li, Nina Dresing

Space Physics · Cross-listed from astro-ph.SR

Bifurcations in the Heliosphere: Global-scale Coronal Pseudostreamer Evolution and the Creation of Secondary Heliospheric Current Sheets

Lizet S. Casillas, Benjamin J. Lynch, Victor Réville et al. (+2 more)

The heliospheric current sheet (HCS) is the boundary between open magnetic fields of opposite polarity in the solar wind that connect back to their respective open flux regions in the corona. The HCS is often described as a disc-like sheet warped by the combined effects of the inclination of the magnetic equator on the Sun, solar rotation, and solar wind expansion. The HCS is an extension of the helmet streamer belt-the closed-flux region resulting from the global dipole component of the observed photospheric magnetic field. In the absence of a strong dipole, a dominant quadrupole can result in two separate HCSs. Here we report on a set of idealized, axisymmetric solar wind simulations designed to study the transition of the single-HCS corona and inner heliosphere into a multiple-HCS configuration, run with the WindPredict-AW PLUTO MHD code. We systematically vary the relative strengths of the dipole/quadrupole contributions to the global field and show this results in the formation of a large-scale, closed-flux pseudostreamer that interacts with the ambient solar wind. We find that when the dipole and quadrupole magnitudes are approximately equal, our model corona can oscillate between a one- and two-HCS configuration on the timescale of days-to-weeks. We characterize the plasma dynamics of the transition of a global-scale pseudostreamer into a localized, secondary helmet-streamer belt with its own HCS surrounding a newly-opened, opposite-polarity coronal hole. We discuss the implications of our results for solar cycle evolution and the dynamic coupling of the highly structured corona and inner heliosphere.
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All authors Lizet S. Casillas, Benjamin J. Lynch, Victor Réville, Olga A. Panasenco, Marco Velli

Wednesday 2 September

Space Physics

Direct Four-Spacecraft Measurement of Reconnection Exhaust Thickness in the Solar Wind

Mani K Chettri, Rupak Mukherjee and Hemam D. Singh

The thickness of a reconnecting solar wind current sheet is normally inferred from a single spacecraft as the product of the boundary-normal speed and the crossing duration. Multipoint measurements have tested this method for generic solar wind current sheets and constrained the geometry of reconnection exhausts, but a direct comparison with single-spacecraft thickness estimates for the same reconnection-associated crossings is still lacking. We analyze four current-sheet crossings observed by Magnetospheric Multiscale in the pristine solar wind on 2017 November 10 the two boundaries of a confirmed reconnection exhaust and two nearby current sheets. The tetrahedron separation was $\approx 16$ km. Cross-correlation of the magnetic-field ramps gives millisecond-level lag uncertainties, boundary normals to $4^\circ-8^\circ$ and speeds to $5\%-8\%$, while single-threshold timing at these separations is noise dominated. The exhaust edges are 29-31 ion inertial lengths ($d_i$) thick. The two edge normals differ by $22^\circ$ (68% interval [$19^\circ,26^\circ$]), resolving a nonparallel exhaust geometry. At the exhaust edges, the measured boundary speeds agree with three standard single-spacecraft estimates to 4%-15% and remain within about one Alfvén speed of the plasma motion. Estimating the normal from one spacecraft is the larger source of error, though the thickness is still recovered to within tens of percent when a degenerate minimum-variance solution is rejected. At the actively reconnecting sheet, the boundary speed exceeds the local plasma speed along the normal by $29\% \pm 10\%$.
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Space Physics · Cross-listed from astro-ph.SR

Electron acceleration by turbulent reconnection in solar flares

Zining Ren, Xin Cheng, Yulei Wang et al. (+1 more)

Solar flares can release magnetic energy explosively in the corona and produce high-energy particles on short timescales. However, how and where these particles are accelerated remains an open question. Here, we investigate the acceleration and transport of electrons during self-developed three-dimensional turbulent reconnection of solar flares by solving Parker's transport equation in the framework of high-resolution MHD simulations. We find that thermal electrons at both the flare current sheet and loop top are rapidly accelerated up to ~90keV, with energy spectra exhibiting a power-law feature. Although the population of accelerated electrons at the flare loop top is larger than that at the current sheet, their spectral indices are similar, close to the values usually observed. More importantly, the acceleration is achieved by turbulence-driven compression structures of various scales rather than the supposed termination shock, particularly at the flare loop top. A portion of compression structures even forms shocks. These results highlight the critical role of turbulent reconnection in accelerating electrons, thereby shedding new light on the acceleration and transport of particles in other high-energy phenomena.
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All authors Zining Ren, Xin Cheng, Yulei Wang, Mingde Ding

Tuesday 1 September

Space Physics · Cross-listed from physics.plasm-ph

A Magnetothermodynamic Theory of Energy Transport In Collisionless Plasmas

Dominic Payne

Presented here is an exploratory conceptual framework with the aim of describing electromagnetic and thermal energy transport and transfer in collisionless plasmas in a coherent way, starting from an arrow-of-time perspective. That is, arguments about the transport of energy are fundamentally based on the idea that systems tend to fill the phase space available to them. First, we describe electric fields, magnetic fields, and plasma particles as interacting subsystems that are thermodynamically linked in specific ways. Second, we discuss the importance of the stochastic electric field in the transport of plasma particles down energy density gradients until the energy flux contributions from so-called electrothermodynamic (ETD) interactions become sufficiently uniform. Third, the logic of ETD transport is extended to a magnetized system invoking 2 types of magnetothermodyamic (MTD) transport: 1) transport of electromagnetic energy through stochastic Poynting flux and 2) plasma energy transport through stochastic drift energy fluxes associated with local ExB drift motion. The magnitudes of the ETD and MTD mechanisms are tied to local characteristic scales of the electric field fluctuations and critical scales are derived beyond which ETD mechanisms dominate MTD mechanisms. Finally, we discuss the interplay between electrothermodynamic and magnetothermodynamic equilibria (ETE and MTE), dynamics in the small critical scale (MTD) limit, and the potential relevance of this type of description to the problem of reconnection onset and scale coupling.
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Monday 31 August

Space Physics · Cross-listed from astro-ph.SR

Direct Measurement of Polar Coronal Hole-like Solar Wind in its Acceleration Phase

Samuel T. Badman, Yeimy J. Rivera, Michael L. Stevens et al. (+38 more)

The early evolution of fast polar coronal hole (PCH) solar wind remains largely unconstrained by in situ measurements. In March 2025, Parker Solar Probe (Parker) at its closest approach of 9.86 Solar Radii ($R_\odot$) measured outflow from a large equatorial coronal hole (ECH) which was also measured at 1\,au and at intermediate distances by Solar Orbiter (also near its perihelion). At 1\,au the stream properties are consistent with PCH properties established by Ulysses. The stream was measured by Parker substantially below the Alfvén surface, with proton temperatures in excess of 2\,MK and a speed at $\sim$10\,$R_\odot$ which was only $\sim$60% of its asymptotic value. The Solar Orbiter data indicates that the acceleration is largely complete by 60~$R_{\odot}$. Spherically-polarized fluctuations in the stream are observed to develop from near-transverse and small-angle at Parker to full reversal “switchbacks” at Solar Orbiter. Comparison of the implied acceleration profile to historical doppler-dimming measurements suggests that the stream's low coronal acceleration is similar to that of PCH flows. Consistent with previous work, this acceleration requires significantly more energy than can be provided by the observed thermal pressure gradients, with a significant contribution likely from the abundant Alfvénic fluctuation energy observed at Parker. These observations provide unique constraints on models of the radial evolution of the fastest solar wind, and indicate that these wind streams experience gradual, steady acceleration over their first few tens of solar radii of evolution.
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All authors Samuel T. Badman, Yeimy J. Rivera, Michael L. Stevens, Mari Paz Miralles, Steven R. Cranmer, Andrea Verdini, Lynn B. Wilson III, Xiangyu Wu, Georgios Nicolaou, Jean-Baptiste Dakeyo, Kai Jaffarove, Tamar Ervin, Dominic S. Payne, Michael Terres, Kristopher G. Klein, Jasper S. Halekas, Sujan Prasad Gautam, William H. Matthaeus, Riddhi Bandyopadhyay, Srijan Bharati Das, Tatiana Niembro, Christopher J. Owen, Rungployphan Kieokaew, Ali Rahmati, Orlando M. Romeo, Federico Fraschetti, Marco Velli, Lorenzo Matteini, Kristoff W. Paulson, Lidiya Ahmed, Stuart D. Bale, Phyllis Whittlesey, Marc Pulupa, Nour E. Raouafi, Timothy S. Horbury, Jean Morris, Helen O'Brien, Roberto Livi, Davin E. Larson, Adam J. Finley, Philippe Louarn

Space Physics · Cross-listed from astro-ph.IM

Feasibility of Capillary-Driven Orbital Liquid Mirror Telescopes

Janoah Dietrich and Álvaro Romero-Calvo

Space-based liquid mirror telescopes (LMTs) have been recently proposed to overcome the scaling limits of traditional rigid mirrors. In these new space architectures, capillary forces would shape a reflective liquid into a spherical optical surface in microgravity. However, the equilibrium, stability, and dynamic behavior of such liquid interfaces under orbital perturbations remain largely uncharacterized. This study investigates the feasibility of space LMTs by combining axisymmetric capillary with thin-film models. Solar radiation pressure and self-gravitation are found to be the dominant perturbations in Sun–Earth L2 orbits, causing equilibrium deformations that exceed the $\lambda/20$ optical threshold at apertures above 0.86 m for visible ($\lambda = 380$ nm) and 1.58 m for near-infrared observatories ($\lambda = 2.5$ $\mu$m). Micrometeoroid impacts self-heal within seconds to minutes, settling after filling requires years to decades at large apertures, and propulsive maneuvers breach optical limits within seconds to hours. These results highlight the need for interface control in large-scale systems such as NASA's 50 m Fluidic Telescope (FLUTE). Thermocapillary actuation supports apertures of 2.3/8.0 m (VIS/NIR) at 100 $\mu$K and 10.6/37.7 m at 10 $\mu$K. Simultaneously, parasitic Marangoni flows impose sub-mK uniformity for NIR and 1–100 $\mu$K for visible operation on any capillary LMT. Departing the capillary regime by imposing a surface-normal settling acceleration, ranging from 39 mm s$^{-2}$ (NIR,5 m) to 17 m s$^{-2}$ (VIS,50 m), is the most promising route to tens of meters in aperture. This approach enables a passive implementation effectively suppressing perturbation induced interface distortions and thermal Marangoni flows. Keywords: Liquid mirror telescope, Optics, Space
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Sunday 30 August

Space Physics · Cross-listed from astro-ph.SR

Ion-Scale Waves Regulated by Plasma Beta and Cross Helicity: Observational Evidence for the Helicity Barrier in Imbalanced Solar Wind Turbulence

G. Q. Zhao, R. Meyrand, H. Q. Feng et al. (+3 more)

Ion-scale waves are believed to play an important role in heating of the solar corona and wind, though their generation mechanism remains unclear. Based on Parker Solar Probe observations, this Letter investigates the occurrence of ion-scale waves in the near-Sun solar wind with heliocentric distances between 0.1 and 0.2 au. Results show that the occurrence rate of left-handed polarized waves significantly depends on the plasma beta ($\beta$) and cross helicity ($\sigma_c$). The occurrence rate rapidly increases with decreasing $\beta$ and increasing $\sigma_c$. Overall, the occurrence rate exceeds 45$\%$ when $\beta<0.2$ and $\sigma_c > 0.7$ are satisfied. These observations are consistent with the direct predictions of the helicity barrier theory that suggests the generation of ion cyclotron waves via imbalanced magnetized turbulence.
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All authors G. Q. Zhao, R. Meyrand, H. Q. Feng, H. F. Yang, L. Xiang, Z. Wang

Space Physics · Cross-listed from astro-ph.SR

Origin of small-scale evaporation flows deep in the chromosphere during a solar flare

L. P. Chitta, H. N. Smitha, F. A. Iglesias et al. (+29 more)

Flares are caused by an abrupt release of magnetic energy in the solar atmosphere. Plasma heated to well over 10 MK filling the post-flare corona originates from a rapid heating and ablation of the cooler chromospheric material. This chromospheric evaporation is thought to be facilitated primarily by nonthermal electrons impinging on to the lower atmosphere. Questions on when and where in the chromosphere these upflows originate, however, are not fully resolved. Here we report on unprecedented high-resolution observations of an M-class flare recorded by the Sunrise Ultraviolet Spectropolarimeter and Imager on board the balloon-borne SUNRISE observatory, that reveal highly structured upflows on spatial scales of ~100 km originating deep in the chromosphere. The flows even precede the onset of nonthermal electrons by about 10 minutes and last through the impulsive phase of the flare. Our observations shed new light on the lower atmospheric heating and mass circulation in flares that are challenging to reconcile with the standard solar flare model.
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All authors L. P. Chitta, H. N. Smitha, F. A. Iglesias, T. L. Riethmüller, A. Feller, W. Chen, A. Lagg, A. Gandorfer, J. Hölken, S. K. Solanki, J. C. del Toro Iniesta, Y. Katsukawa, P. Bernasconi, T. Berkefeld, A. Álvarez-Herrero, M. Kubo, D. Orozco Suárez, B. Grauf, M. Carpenter, A. Bell, Valentín Martínez Pillet, F. J. Bailén, J. Blanco Rodríguez, J. Sebastián Castellanos Durán, E. Harnes, R. T. Ishikawa, Y. Kawabata, T. Matsumoto, T. Oba, A. L. Siu-Tapia, H. Strecker, D. Vukadinović

Saturday 29 August

Space Physics · Cross-listed from astro-ph.EP

Dynamical Mass Demography of Single S-type Planets Shaped by Stellar Companions

Xiumin Huang and Jianghui Ji

More than 700 S-type planets have been identified from Gaia and TESS observations, including over 150 transiting targets, enabling the precise dynamical orbits and planetary mass characterization in binary environments. Here we present the first systematic N-body dynamical mass determination for 40 single S-type (Satellite-type orbit) planets in close binaries using an improved Markov Chain Monte Carlo (MCMC) framework with Gaussian priors to model stellar companion perturbations. {For binary systems with $a_\mathrm{B}<100$ au, companion-induced perturbations can bias the RV semi-amplitude and consequently the inferred planetary masses. The impact is strongest for short-period sub-Jovian planets, whose masses decrease by up to 20% when transitioning from Keplerian to N-body modeling, while massive Jovian planets with $M_{\mathrm{Jup}} < M_{\mathrm{p}} < 5$ $M_{\mathrm{Jup}}$ show only a 7.2% reduction.} Applying the NAFF stability criterion to TOI-4633, $i_{\mathrm{mut}} = 70^{\circ}$ is the upper limit for the fast rejection of unstable orbital solutions. Our results reveal that intermediate-mass planets exhibit both positive or negative biases in the inferred masses and influence the observed population of the sub-Jovian desert.
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Friday 28 August

Space Physics · Cross-listed from physics.optics

High-performance silicon-metal laser welding resisting extreme conditions

Jiawei Yin, Markus Blothe, Hagen P. Kohl et al. (+3 more)

Reliable material joining is essential for countless industrial applications. While femtosecond laser welding provides a route beyond conventional bonding methods, demonstrations of silicon-metal joints are rare due to nonlinear propagation effects and have so far been limited to shear joining strengths of a few MPa. Here, we demonstrate high-strength silicon-Kovar laser welding using sub-nanosecond pulses. By optimizing the focal position, the welding pattern, the laser polarization, and the metal roughness, remarkable shear joining strengths up to 15.8 MPa are achieved. The silicon-metal joints withstand temperatures of up to 500 {\deg}C and are hermetically sealed. Together with the remarkable strength values, the resistance to harsh environments underpins the applicability of silicon-metal welding in various fields including aerospace, nuclear science, and metallurgy.
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All authors Jiawei Yin, Markus Blothe, Hagen P. Kohl, Christina Schütze, Stefan Nolte, Maxime Chambonneau

Space Physics · Cross-listed from astro-ph.SR

A high-frequency type II radio burst associated with an X2.3 class flare

Divya Paliwal, Anshu Kumari, Vishwa Vijay Singh et al. (+3 more)

Radio observations provide a powerful diagnostic of the solar corona, enabling investigations of dynamic phenomena associated with solar flares, coronal mass ejections (CMEs), and shock waves. We present a multiwavelength analysis of a rare high-frequency type II radio burst (starting frequency of $\sim$750 MHz and frequency drift rate of $\sim$0.5 MHz s$^{-1}$) associated with the X2.3-class solar flare that occurred on 6 November 2024. A propagating EUV disturbance was observed shortly after the flare onset in the SDO/AIA field of view, while radio spectrographs recorded the type II burst between 13:46 and 13:56 UT over a frequency range of $\sim$750 to 45 MHz. Radio imaging observations from the Nançay Radioheliograph (NRH) show that the radio sources propagate southward during the event. X-ray spectroscopy from HEL1OS onboard Aditya-L1 and imaging observations from STIX onboard Solar Orbiter reveal signatures of efficient non-thermal electron acceleration associated with the flare. An NLFFF extrapolation identifies a pre-eruptive magnetic flux rope in the source region, while white-light coronagraph observations obtained during the event show no detectable large-scale CME. The speed of the erupting flux rope, derived from stereoscopic EUV observations, is consistent, within measurement uncertainties, with the shock speed inferred from the radio dynamic spectrum. Together, these observations suggest that compact flux rope eruptions, even in the absence of a detectable white-light CME, can generate low-coronal shocks capable of producing high-frequency type II radio emission.
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All authors Divya Paliwal, Anshu Kumari, Vishwa Vijay Singh, Dinesh Mishra, Pritam Das, Nadiya K

Space Physics

Solar Magnetic Configuration Control over Radiation Belt Electrons

Ahmad Lalti, Jonathan Rae, Clare Watt et al. (+2 more)

Earth is surrounded by two highly dynamic concentric belts of particle radiation. The outer radiation belt exhibits coherent variability at solar-cycle, seasonal, and 27-day (Carrington) timescales. While the solar-cycle and Carrington variations have been attributed to the recurrence of coronal hole solar wind, the seasonal variation has long been explained by local geometric effects that modulate the solar wind-magnetosphere coupling. Here, we challenge this paradigm by showing that the periodic recurrence of coronal hole solar wind likewise drives the seasonal variation. We further demonstrate that the Alfvénic nature of this solar wind is responsible for the observed electron flux enhancement in the outer radiation belt. These findings provide a unifying framework linking solar magnetic topology, solar wind properties, and magnetospheric dynamics across multiple timescales at Earth and beyond.
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All authors Ahmad Lalti, Jonathan Rae, Clare Watt, Stephanie Yardley, Savvas Raptis

Thursday 27 August

Space Physics · Cross-listed from astro-ph.HE

Magnetospheric birefringence breaks the rotating-vector model and depolarises X-ray pulsars

Alexander A. Mushtukov, Sergey S. Tsygankov, Silvia Zane et al. (+4 more)

Accreting X-ray pulsars have long been expected to show large linear polarisation below the cyclotron resonance. In the standard picture, this follows from two facts: radiative opacities in the strongly magnetised neutron star environment depend strongly on polarisation mode, and the emission comes from compact polar regions. The emerging radiation should therefore be locally polarised and add almost coherently, with a polarisation angle tracing the projected magnetic field as in the rotating-vector model. Recent X-ray polarimetric observations, instead, show modest polarisation fractions and position angle swings that often depart from this simple picture. Here we show that propagation through the magnetosphere of an accreting neutron star can naturally produce both effects. Using general-relativistic ray tracing and Stokes-parameter transport, we find that photons emitted from compact polar regions acquire ray-dependent polarisation angle offsets in two ways. First, nearly field-aligned trajectories can cross local non-adiabatic pockets inside the QED birefringent region. Second, photons that intersect the plasma-loaded accretion flow accumulate additional phase shifts between plasma normal modes. Both effects broaden the distribution of polarisation angles across the ray bundle and reduce the hotspot-integrated linear polarisation, while also producing systematic deviations from rotating-vector-model expectations. Magnetospheric propagation should therefore be treated as part of the polarimetric transfer problem in accreting X-ray pulsars, and may help explain the observed low polarisation fractions and anomalous angle swings.
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All authors Alexander A. Mushtukov, Sergey S. Tsygankov, Silvia Zane, Nabil Brice, Ruth M. E. Kelly, Roberto Taverna, Roberto Turolla

Space Physics · Cross-listed from eess.SY

Horizon-Dependent Tube MPC for Elliptical-Orbit Rendezvous Under Mass Uncertainty

Omer Burak Iskender and Keck-Voon Ling

A spacecraft closing on a target from three hundred kilometres to contact flies one guidance law across five orders of magnitude of range, and a controller that is provably safe at close range can lose that guarantee completely at long range while continuing to fly as though nothing were wrong. This paper derives the range at which the guarantee lapses and uses it as a design rule. The bound compares the prediction model's own linearisation error against the disturbance set the controller was built to reject, and needs only the sampling period, the orbit and that disturbance bound, so it can be evaluated before any simulation. On a Mars Sample Return approach it disqualifies the homing phase, where most of the propellant is spent, and clears the other two. Re-posing the disqualified phase in relative orbital elements restores the guarantee; re-posing a phase the rule already clears, in a frame two orders of magnitude more accurate, changes propellant by under a tenth of one per cent, and it is that second prediction that makes the rule falsifiable rather than descriptive. The constraint tightening also ties the prediction horizon to feasibility, so the horizon search limit becomes a mission parameter rather than a solver setting. Against a reimplementation of a published benchmark that reproduces its propellant to within one per cent, over five hundred dispersed Monte Carlo transfers per case on matched seeds, the controller saves 29% of the propellant on a circular target orbit and 40% on an eccentric one, docking inside the 0.20 m capture requirement on essentially every draw at a median miss near 5 cm. Two results run the other way: the saving is bought with time of flight and computation, and it comes from what the guarantee demanded of the terminal condition rather than from better disturbance rejection. Recursive feasibility and asymptotic stability are not claimed.
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2608.27659 · 27 Aug · 19 pages, 12 figures, 8 tables. Accepted for presentation at the 77th International Astronautical Congress (IAC 2026), Antalya, Turkiye, 5-9 October 2026; paper IAC-26,C1,4,9,x108305. Accepted version (authors' preprint), not the version of record. Typeset in a plain article class rather than the IAF conference template

Abstract PDF $BNCH ×1

Space Physics · Cross-listed from astro-ph.EP

Narrow-Shell Stochasticity in Source-Sink Models of the Low Earth Orbit Environment

Jaewon Choi, Souvik Dhara and Harsha Honnappa

Deterministic source-sink models are widely used to assess the long-term evolution, capacity, and sustainability of the low Earth orbit (LEO) environment. These models propagate shell-averaged populations through ordinary differential equations (ODEs), relying on individual collision, disposal, and decay events to average out within sufficiently large altitude shells. As constellation traffic is increasingly organized into kilometer- and sub-kilometer-scale shells, this averaging assumption becomes strained. We formulate the multi-shell, multi-species LEO environment as a Markov jump process and, from this formulation, recover the conventional source-sink ODE as a large-volume limit and derive a stochastic differential equation (SDE) approximation whose fluctuations scale as V^{-1/2} in the per-shell volume V. The diffusion approximation is validated against an exact discrete-event simulation of the underlying jump process. Sweeping V at fixed spatial density over the 450-800 km band, we find that the two descriptions agree at shell volumes comparable to those used in established source-sink models but diverge as shells narrow. At the finest shell volume considered, the mean debris population reaches roughly 4.5 times the ODE prediction, with several realizations undergoing runaway growth absent from the ODE trajectory. This departure is driven by nonlinear collision terms, through which population variance and covariance raise expected collision rates, generating further debris and reinforcing the collision-debris feedback. Narrow-shell stochasticity thus both widens the outcome distribution and shifts the expected trajectory. Because the stochastic model shares its parameterization with the deterministic one, it provides a scale-consistent extension of existing source-sink models for evaluating shell configuration, collision risk, and long-term LEO sustainability.
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Space Physics

Statistical Models of Ionospheric Variability and Irregularities in the Topside Ionosphere Based on the Swarm Satellite Data

Daria Kotova, Alan Wood, Eelco Doornbos et al. (+9 more)

The ionosphere is a highly complex plasma containing electron density structures with a wide range of spatial scales. Coupling of the ionosphere with the Earth's magnetosphere and the solar wind, as well as to the neutral atmosphere, makes the ionosphere highly dynamic and highly dependent on the driving processes. Thus, modelling the ionosphere and capturing its full dynamic range considering all spatiotemporal scales is challenging. Swarm is the European Space Agency's (ESA) first constellation mission for Earth Observation, comprising multiple satellites in low Earth orbit. During the Swarm-VIP-Dynamic project, a suite of statistical models has been developed using observations from Swarm and proxies for heliogeophysical processes. The statistical modelling technique of Generalised Linear Modelling was used to create models for both the electron density and the variability of the plasma structures at horizontal spatial scales between 7.5 km and 100 km. Separate models were created for low, middle, auroral and polar latitudes. The models make predictions based on explanatory variables, which act as proxies for the underlying physical processes. The performance of the models of the electron density approached the theoretical best values for some of the goodness-of-fit statistics. This suggests that the modelling method is appropriate for the task undertaken. The models of ionospheric variability at larger spatial scales (about 100 km) also perform well, however the model performance decreases at smaller spatial scales. This suggests that there are physical processes missing from the models. Possible candidates are instability processes or driving forces of the ionosphere by wave activity from below, neither of which are captured by the models.
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All authors Daria Kotova, Alan Wood, Eelco Doornbos, Jaroslav Urbář, Luca Spogli, Yaqi Jin, Lucilla Alfonsi, Gareth Dorrian, Mainul Hoque, Kasper van Dam, Elisabetta Iorfida, Wojciech Miloch

Space Physics · Cross-listed from astro-ph.EP

Study on the Venusian Atmospheric Thermal Structure: A Comparative Analysis between VEX/Akatsuki and Venus-GRAM/VCD

Soumyaneal Banerjee, R. K. Choudhary, Keshav R. Tripathi et al. (+5 more)

We investigate the thermal structure of the Venusian middle atmosphere between 45 and 80 km using radio occultation (RO) measurements from Venus Express and Akatsuki spanning 2006-2024. Retrieved temperature profiles are compared with climatological predictions from the Venus-GRAM and the Venus Climate Database (VCD). Systematic deviations exceeding 10 K are observed between the RO temperatures and model climatologies, particularly at high latitudes in both hemispheres. The long-term dataset further reveals a possible decadal-scale temporal variability in temperatures across the low-to-mid latitude regions. This variability becomes less coherent at higher altitudes. A combination of post-stratification and bootstrap analysis on the temperature anomalies indicates that the observed temporal variability at low-to-mid and polar latitudes is not readily explained by the latitudinal sampling bias quantified using the adopted analysis, while the trends at mid-to-high latitudes are affected by sparse and uneven RO sounding. Sensitivity tests using modified cloud albedo inputs in VCD simulations show that adjusting cloud radiative forcing partially reconciles the discrepancies, especially at the lower altitudes, below 60 km. However, above the cloud top, at the lower latitudes, the divergence from the observations increases significantly, thereby failing to provide a general reconciliation between VCD and RO. These results highlight the need for updated empirical climatologies incorporating recent RO measurements and for improved physical parameterizations in Venus general circulation models to better capture the global variability in the planet's middle atmosphere.
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All authors Soumyaneal Banerjee, R. K. Choudhary, Keshav R. Tripathi, K. M. Ambili, J. Oschlisniok, Silvia Tellmann, T. Imamura, H. Ando

Wednesday 26 August

Space Physics

A quiet STEVE disturbs navigation satellites' signals in the Antarctic

Daria Kotova, Luca Spogli, Yaqi Jin et al. (+1 more)

Strong Thermal Emission Velocity Enhancement (STEVE) is a narrow optical phenomenon that occurs equatorward of the auroral oval and is associated with intense subauroral plasma flows and thermospheric heating. Although these conditions can produce plasma irregularities, direct evidence of STEVE effects on radio wave propagation during geomagnetically quiet conditions has so far not been observed. Here we report for the first time a STEVE event observed over Antarctica during quiet geomagnetic conditions and show that it produced measurable fluctuations in the Global Navigation Satellite System (GNSS) signals. Using coordinated optical observations and high-resolution (50 Hz) GNSS scintillation measurements from two Antarctic stations, we identify enhanced phase and amplitude scintillation coincident with intersections between GNSS signal paths and the STEVE arc. The observations indicate the presence of plasma irregularities and suggest substantial temporal variations in the apparent altitude of the optical structure (vertical motion within the 130-270 km range). We find that even a relatively weak STEVE event can affect the GNSS signal propagation in the absence of major geomagnetic disturbances, extending previous studies that associated such effects primarily with storms and intense auroral activity. This finding expands the understanding of STEVE's geophysical impact and highlights potential vulnerabilities in satellite-based navigation systems during seemingly benign space weather conditions. The fact that this STEVE event occurred just off the Antarctic coastline, where scientific expeditions and seagoing vessels rely heavily on precise positioning indicates that STEVE-related plasma structuring should be considered in assessments of the GNSS performance under otherwise quiet geomagnetic conditions in subauroral regions.
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All authors Daria Kotova, Luca Spogli, Yaqi Jin, Wojciech Miloch

Space Physics

Empirical Relationship for Geomagnetically Induced Currents (GIC) and Solar Wind Conditions

Dean Thomas, Lucy A. Wilkerson, Robert S. Weigel et al. (+2 more)

Geomagnetically induced currents (GICs) during extreme space weather events represent a critical hazard to modern infrastructure, including electrical power grids, telecommunication systems, pipelines, and railways. This study presents empirical models designed to provide rapid, site-specific estimates of the maximum GIC, $|\text{GIC}|_{\text{max}}$, expected during a storm. Utilizing data from 67 sites across nine major geomagnetic storms ($K_p \ge 8$) occurring between 2020 and 2025, we performed multivariate linear and quantile regressions incorporating solar wind parameters and local scaling factors. The solar wind parameters address storm intensity and are the minimum interplanetary magnetic field $z$-component, $(B_{z}^{\tiny{\text{IMF}}})_{\text{min}}$, and the maximum magnitude of the solar wind velocity $x$-component, $(V_{x})_{\text{max}}$. The scaling factors account for local differences due to geomagnetic latitude ($\alpha$) and ground conductivity ($\beta$). A 6-parameter model was identified via best subset selection and the corrected Akaike Information Criterion (AICc) as the optimal predictor for mean $|\text{GIC}|_{\text{max}}$, yielding a Pearson correlation of $r = 0.73$ and a coefficient of determination of $R^2 = 0.54$. To examine peak GICs during storms, a 7-parameter quantile regression model was developed to estimate the upper 80% bound of $|\text{GIC}|_{\text{max}}$. Statistical analysis of 39 severe storms since 1995 shows that the $|\text{GIC}|_{\text{max}}$ upper bounds are consistent with a lognormal distribution. These relationships offer a computationally efficient alternative to complex physics-based simulations, enabling space weather practitioners to estimate site-specific GICs in high-level analyses.
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All authors Dean Thomas, Lucy A. Wilkerson, Robert S. Weigel, John Haman, Edward J. Oughton

Tuesday 25 August

Space Physics · Cross-listed from astro-ph.IM

Capra: Scalable HEALPix-Native Intensity Reconstruction for High-Resolution IMAP Analyses

Nikola Bukowiecka, Daniel B. Reisenfeld and Maciej Bzowski

We present Capra, a HEALPix-native pipeline for reconstructing all-sky energetic neutral atom (ENA) intensity maps from the NASA Interstellar Boundary Explorer (IBEX) and Interstellar Mapping and Acceleration (IMAP) missions' event-counting data. Capra produces two products: (1) a baseline boresight-assigned map, and (2) an optional smoothed map on the HEALPix grid. This produces smooth, physically interpretable maps with consistent resolution changes and reference-frame transformations. The smoothed Capra maps recover the large-scale ENA morphology while reducing sampling-driven mottling and strip-like artifacts. Relative comparison with the Theseus reconstruction shows broadly consistent large-scale structure, with the discrepancies reflecting differences in reconstruction methods, rather than major morphological discrepancies. We verify invariance (preservation) of count numbers, rates, and intensities across tessellations under coordinate transformations, and demonstrate numerical convergence up to HEALPix resolution Nside = 64 for a representative dataset. We show that thread-based parallelism performance exhibits approximately linear strong scaling, with bounded memory usage, enabling efficient high-resolution reconstruction and making Capra well-suited for high-resolution IMAP reconstruction.
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