Papers

Coronal mass ejections (CMEs) from the Sun can have severe impacts on the Earth environment in the form of geomagnetic storms. These storms pose a risk to the global technological infrastructure, making the prediction of these events imperative. In this paper, we have broadened the statistical verification of the Bz4Cast tool, the first empirically-driven model to forecast solar wind magnetic vectors inside a CME prior to their Earth arrival. Twenty five CME events (between 2012 and 2016) have been tested with the Bz4Cast model, and the skills have been compared to the heuristic approach of NOAA's Space Weather Prediction Center (SWPC) G-scale for 3-day geomagnetic storm forecasts. For a broad range of scores, and within uncertainty, the Bz4Cast architecture provided the same skill as the experienced on-duty forecasters at SWPC. The most prominent difference is that the Bz4Cast architecture provides a slightly higher false alarm ratio than the SWPC 3-day forecast.

The equations of reduced magnetohydrodynamics (RMHD) isolate the Alfvénic energy transfer and turbulence dynamics from MHD in a computationally tractable way. In this study, we derive the equations of RMHD for a low $\beta$ plasma in a dipole coordinate system aligned with the background magnetic field using a multiscale analysis. The Kreiss theorem is used to derive the equilibrium conditions that the background conditions must satisfy on the time scale of MHD turbulence. From these, we find a connection between plasma flow along flux tubes and changes in the Alfvén speed that may drive nonlinear wave effects. The final equations demonstrate an intimate coupling between field aligned currents and plasma vorticity including the effects of nonuniform flux tube area and realistic plasma density profiles. This work has immediate application to the magnetosphere-ionosphere coupling problem in the Earth's magnetosphere as it provides a way to link dynamically evolving field aligned currents from the magnetosphere, especially important during geomagnetic storms and substorms, with the development of magnetohydrodynamic turbulence at low altitudes in a self-consistent manner. This also clarifies the role of Alfvén waves as a transfer mechanism of energy under these inhomogeneous circumstances while remaining simple enough to make predictions. Furthermore, this study makes use of a novel methodology, a computer algebra system, in performing the brunt of algebraic work under complicated coordinate systems. We hope this approach serves as a template for performing reproducible and verifiable multiscale perturbation analysis under arbitrarily complex geometries.

Atmospheric ion escape driven by the solar wind is a key process controlling the long-term loss of the Martian atmosphere. Localized plasma clouds can carry substantial fluxes of planetary ions away from Mars, representing episodes of bulk escape. However, their origin has remained unclear due to the absence of simultaneous upstream measurements. Using joint observations from the MAVEN and Tianwen-1 missions, which provide real-time upstream monitoring, we present direct evidence that these plasma clouds are nonlinear wave packets generated by the Kelvin-Helmholtz instability (KHI). The spatial scale of KH waves is constrained for the first time via two-point measurements. Ion fluxes within plasma clouds are one to two orders of magnitude higher than those in typical steady-state escape channels. Our results indicate that KHI is an important process for solar wind coupling to planetary upper atmospheres and plays a crucial role in shaping atmospheric ion escape for unmagnetized planets.

Irregularities in electron density within the interplanetary medium (IPM) can cause fluctuations in the Doppler frequency of spacecraft radio signals. The amplitude of these fluctuations depends on factors such as the carrier frequency, propagation geometry, and link configuration. However, quantitative characterization of these effects across different frequencies in various occultation experiments is currently limited. We analyze five complementary datasets: two-way S-band observations from Chandrayaan-3 outside the lunar ionosphere, two-way S-band data from Chandrayaan-2 during lunar occultation, one-way S/X band measurements from the Venus Express Radio Science (VeRa)/Akatsuki Radio Science (Akatsuki) under IPM-only conditions, and one-way X-band Akatsuki data during solar occultation. The Chandrayaan-3 and Akatsuki IPM observations isolate IPM effects by excluding contributions from planetary atmospheres, the lunar ionosphere, and, except during solar occultation, the solar corona. Chandrayaan-3 data sample dynamically evolving Earth-Moon geometries and exhibit weak, mHz-level Doppler fluctuations, while Chandrayaan-2 observations provide near-lunar plasma benchmarks with higher amplitudes, during quiet time solar and geomagnetic conditions. Akatsuki and VeRa's IPM-only measurements capture long-path interplanetary effects, whereas Akatsuki solar occultation data reveal strong coronal signatures. Power spectral density analysis indicates Kolmogorov-like turbulence for lunar occultation and solar occultation cases, while IPM-only spectra show low-amplitude fluctuations. These results quantify the IPM contribution to Doppler noise, demonstrate the enhanced plasma sensitivity of two-way coherent links, and provide constraints relevant to turbulence modelling, precision spacecraft tracking, and the interpretation of radio occultation experiments.

Impact-generated crater rays are well-documented on the Moon, with most appearing as high-albedo streaks extending radially from a crater's center. On Mars, however, crater rays are significantly rarer and discernible only through thermal imaging due to their lower thermal inertia compared to surrounding terrain. This study presents the first comparative analysis between the lengths of Martian and lunar crater rays, including lunar albedo rays and cold spots, which are ray-like thermal anomalies associated with many of the youngest lunar craters. Our findings indicate that both Martian crater rays and lunar cold spots extend significantly farther than lunar albedo rays, with lengths an order of magnitude greater for craters of equivalent diameter. Furthermore, we propose a connection between the formation mechanisms of Martian crater rays and lunar cold spots based on their thermal properties. By integrating thermal rays into existing ejecta models, we refine the understanding of crater-ray formation and suggest that Martian crater rays and lunar cold spots may share a similar formation mechanism via secondary cratering processes. Advancing knowledge of these features has implications for impact dynamics and surface evolution across planetary bodies.

Understanding the spatial coherence of solar wind plasma and magnetic field properties is essential for interpreting multi-spacecraft observations and for characterizing the large-scale structure of heliospheric transients. In this study, we quantify the spatial correlation of six key solar wind parameters - interplanetary magnetic field components, bulk flow speed, proton number density, and the alpha-to-proton abundance ratio - using simultaneous measurements from the ACE and Wind spacecraft as a function of their instantaneous separation distance. The analysis is performed separately for intervals of background solar wind, Interplanetary Coronal Mass Ejections (ICMEs), and Stream Interaction Regions (SIRs). The decay of the Pearson correlation coefficient with distance is modeled using an exponential function to infer characteristic de-correlation length scales. We find that the bulk solar wind speed is the most spatially coherent parameter in all regimes, while plasma composition exhibits the weakest coherence. Magnetic field coherence shows strong dependence on solar wind structure: ICMEs display near-unity correlations and the largest magnetic coherence scales, consistent with organized, flux-rope-like configurations, whereas SIRs exhibit reduced coherence - particularly in the north - south magnetic field component - reflecting compressed and turbulent plasma. The background solar wind exhibits intermediate behavior, with large-scale coherence in bulk plasma properties but shorter coherence lengths in magnetic fluctuations. These results provide a quantitative framework for distinguishing solar wind structures based on their spatial coherence properties and have important implications for multi-point solar wind studies and space weather applications.

Early detection of major solar flares is critical for defense operations due to their potential to disturb radar and radio systems. Typically, soft X-ray flux is used to monitor and classify solar flares, but since this flux has to be measured in space, it means that its availability itself is dependent on space weather conditions. For this reason, in this paper, we investigated the feasibility of using ground radio observations to monitor major (M5+ class) solar flares. We made use of datasets from the GOES-16 satellite and the Radio Solar Telescope Network in the time range between March 2023 and March 2025. An elastic net regularized logistic regression model was trained on this data, optimized through a grid search and with incorporated class weighting for class imbalance. It was found that especially higher frequencies (8800 MHz) had a reasonable ability in monitoring and predicting major flares (precision and recall for flare events are 53% and 65%, respectively - implying that roughly one third of flares were not detected - with signals appearing, on average, 3 to 4 minutes before the M5 threshold is exceeded). Radio measurements at super high frequencies can thus serve as an alternative method to monitor major solar flaring activity.