During the propagation of interplanetary coronal mass ejections (ICMEs), evolution of the ICME-driven shock along with interactions with other solar wind structures, planetary bodies, and general changes to their morphology can alter particle acceleration efficiency and transport effects at their associated shocks. While the underlying mechanisms for these processes have been studied, the connection between the radial evolution of the ICME-driven shock during propagation and resulting gradual Solar Energetic Particle (SEP)and Energetic Storm Particle (ESP) intensities, composition, and acceleration has yet to be fully understood. The current distributed array of spacecraft at varying heliocentric distances provides a welcome opportunity to statistically analyze the radial dependency of particle populations and acceleration mechanisms present at ICME-driven shocks. We compile a database of 39 multipoint ICME events from 2016-2023, which are observed in situ by at least two of the following spacecraft: Parker Solar Probe (PSP), Solar Orbiter, ACE, Wind, and STEREO-A. Using the magnetic field, plasma, and ion compositional data provided by these spacecraft, we derive both local shock and ESP spectral shape parameters. By comparing the changes in these parameters at different stages of ICME propagation, we analyze the connection between the evolution of the local shock conditions and the spectral shape. We find evidence to suggest a consistent increase in shock acceleration efficiency with heliocentric distance while the parent ICME is within 0.7 au, followed by a reduction in shock efficiency at further distances.

In-situ measurements from the Sun-Earth Lagrangian L1 point typically provide a 20-minute to 1-hour advanced warning of incoming interplanetary (IP) shocks, magnetic clouds before impact at the nose of Earth's magnetopause. Sub-L1 monitors may provide measurements sunward of the L1 point to improve the lead times for such transients to several hours, and various mission architecture have been proposed for more than 25 years. Because CMEs and shocks do not propagate exactly radially, the location of such a monitor with respect to the Sun-Earth line is a key parameter to take into account when designing such missions. Here, we highlight some recent results and measurements of CMEs that show that small angular separations may result in drastic differences in the CME properties measured by two spacecraft, and examples showing that CME evolution over a few hours may differ significantly from the average evolution as obtained from statistical studies over several decades. We highlight how a pathfinder mission is required to better understand the variation of properties within CMEs on moderate scales and the evolution of CMEs over a few hours. Such an improved knowledge will then allow for a dedicated fleet of operational monitors that will improve the lead time of space weather forecasting without a loss of accuracy

IntroductionMars’ lack of a global magnetic field led to low expectations for auroral phenomena on the planet, but MAVEN observations showed auroral activity to be frequent, diverse in nature, and often global in scope. Subsequent observations by the Emirates Mars Mission [1] further expanded the breadth of observable types of aurora, and it is likely that the variety will increase further as observations expand. Figure 1 below shows three fundamentally different types of aurora on Mars. Ironically, Mars’ lack of a global field is actually responsible for most of the activity, which leads to a new perspective for non-magnetized objects in our solar system and beyond.Making Sense of Diverse Auroral ActivityEach of type of Mars aurora is a tracer for a different important process involving the interaction between solar influences and the near-Mars magnetic and charged particle environment. Originally, types of aurora were named for their resemblance to terrestrial or other types, with later forms named after their morphology, geographical location, precipitating particles, etc. The nomenclature became cumbersome, confusing, and not insightful to those not working in the field. At the same time, modeling showed that all types could fit into three categories based on the precipitating particles and their origins: solar energetic particles (SEPs), suprathermal electrons, and solar wind protons (see Figure 1). These distinctions usefully group the phenomena into clusters of phenomena observed and modelled similarly, and also allow for new varieties to be added.Figure 1. Three types of aurora on Mars, as observed by the Imaging UltraViolet Spectrograph (IUVS) on MAVEN (left and right images) and the Emirates Ultraviolet Spectrometer (EMUS) on the Emirates Mars Mission (center image). Each is diagnostic of a specific interaction between solar or internal influences and Mars’ magnetic and plasma environment. SEP Aurora on the Rise to Solar Max This presentation will focus on SEP aurora, the type of aurora most dependent on solar activity. MAVEN/IUVS discovered that the entire visible nightside of Mars can be engulfed in auroral emissions [Figure 1, left, and reference 2]. The phenomenon can also be studied in limb scan mode, which revealed that solar energetic particles can penetrate down to ~60 km altitude. Contemporaneous MAVEN/SEP observations of electrons up to 200 keV confirmed the correlation with solar activity. Some diffuse aurora events have been observed to last for days during extended solar events. Both SEP protons and electrons can be responsible [3]. SEP aurora events are currently occurring at the highest frequency of MAVEN’s 10 year mission. The phenomenon was discovered in 2014 during the declining phase of the solar cycle, and only two additional major events occurred in the following 7 years. Since August 2022, IUVS has observed 8 major events, half of which have occurred since February 2024 (Figure 2). (Additional SEP activity did occur throughout this period, but observations at those other times were not possible.) All auroral events are closely correlated with the arrival of SEP particles as measured by MAVEN’s SEP instrument. Figure 2. MAVEN/IUVS image of SEP aurora (also known as diffuse aurora) on 18 March 2024 during a space weather event. Both the bright limb and on-disk emission are attributed to auroral emission. The near-UV image of Mars’ south polar region in southern spring was obtained contemporaneously with the auroral portion of the image by alternating detector gain during nadir scanning. The substantial dataset of recent events will be ideal for comparative studies to determine (1) whether protons or electrons are responsible, and which energy ranges matter most; (2) how Mars’ hybrid magnetosphere responds to the SEP flux, and whether the auroral brightness is modulated by local structures or crustal magnetic fields. In additional to auroral science goals, our efforts aim to quantify how aurora can serve as a proxy for space weather hazard for human exploration of Mars. This leads to the ironic situation where the aurora may be so impressive it’s necessary to seek shelter against the radiation. Reaching Solar Max does not imply that auroral activity will soon decrease. In fact, the type of solar activity giving rise to SEP events may increase in the declining phase, with more burst of SEP aurora. MAVEN’s potential mission extension will be able to test this hypothesis. The Case for Visible Nightside Imaging from Orbit. Visible wavelength emissions are predicted to accompany UV emissions from SEP aurora, based on atomic and molecular physics with known branching ratios [4]. Oxygen green line emission is expected at 557 nm, as frequently seen at Earth. Brightnesses are expected to be detectable with visible wavelength cameras capable of long exposures. No orbital instruments on existing spacecraft have yet made detections, probably due to sensitivity limits in short exposures. Future Mars missions with instruments designed for nightside imaging offer tremendous low-cost potential for breakthrough observations in auroral science. Visible filter imaging of the aurora with conventional technology is likely to be orders of magnitude more sensitive than the complex and expensive slit-scanning spectral imagers in orbit at Mars today. In addition to the [OI] green line emission, SEP aurora should also cause emission near the blue end of the visible range, emanating from the FDB bands of CO2+. The recent discovery of visible-wavelength nightglow [5] offers another compelling target for nightside imaging. The M-MATISSE mission, currently in a competitive Phase A Study for an ESA M-class mission, carries such a camera [6] . Even if selected, missions beyond should also carry low-cost visible imagers optimized for the orbit, observational capabilities and science goals of the mission. References: [1] Lillis et al. (2022), GRL, doi: 10.1029/2022GL099820; [2] Schneider, et al. (2018). GRL, doi: 10.1029/ 2018GL077772. [3] Nakamura et al. (2020), JGR, doi: 10.1029/2021JA029914; [4] Gérard et al., (2015) JGR. 120, 6749–6765 doi: 10.1002/ 2015JA021150; [5] Gerard et al., Nat. Astr., 10.1038/s41550-023-02104-8; [6] Sanchez-Cano et al. (2023): doi 10.3389/fspas.2022.1101945

The evolution of coronal mass ejections (CMEs) as they travel away from the Sun is one of the major issues in heliophysics and space weather. After erupting, CMEs propagate outwards through the background solar wind flow, which in turn may significantly affect CME evolution by means of e.g. acceleration, deflection, and/or rotation. In order to determine to which extent the ambient wind can alter the speed, trajectory, and orientation of a CME, we run a series of 3D magnetohydrodynamics simulations (using the coupled solar–heliospheric WSA–Enlil model) to conduct a multi-vantage point study of the radial and longitudinal evolution of CME structures as they propagate up to Earth’s (1 AU) and Mars’ (1.5 AU) orbits. We explore a broad range of input CME parameters (initial radial speed, angular width) and ambient solar wind conditions (slow versus fast wind) to investigate the different evolutionary behaviours of CMEs and their driven shocks and sheath regions. To study the radial and longitudinal evolution for the modelled CME ejecta and shock events, we examine the resulting magnetic field and plasma time series at different heliocentric distances (0.5 AU, 1 AU, and 1.5 AU) and heliolongitudes (in 30° increments). This work will help establish a set of expected CME behaviours at Earth’s and Mars’ radial distances, which can be used for analysing real CME events.