Search

Multi-spacecraft observations of solar energetic particle (SEP) events not only enable a deeper understanding and development of particle acceleration and transport theories but also provide important constraints for model validation efforts. However, because of computational limitations, a given physics-based SEP model is usually best suited to capture a particular phase of an SEP event, rather than its whole development from onset through decay. For example, magnetohydrodynamic (MHD) models of the heliosphere often incorporate solar transients only at the outer boundary of their so-called coronal domain – usually set at a heliocentric distance of 20–30 R⊙. This means that particle acceleration at coronal mass ejection (CME)-driven shocks is also computed from this boundary onwards, leading to simulated SEP event onsets that can be many hours later than observed, since shock waves can form much lower in the solar corona. In this work, we aim to improve the modelled onset of SEP events by inserting a “fixed source” of particle injection at the outer boundary of the coronal domain of the coupled WSA–Enlil 3D MHD model of the heliosphere. The SEP model that we employ for this effort is Solar Energetic Particle MODel (SEPMOD), a physics-based test-particle code based on a field line tracer and adiabatic invariant conservation. We apply our initial tests and results of SEPMOD’s fixed-source option to the 2021 October 9 SEP event, which was detected at five well-separated locations in the inner heliosphere – Parker Solar Probe, STEREO-A, Solar Orbiter, BepiColombo, and near-Earth spacecraft.

This study investigates the potential impact of Solar Energetic Particles (SEPs) on the V0 layer of the Venus ionosphere. Electron density profiles obtained from radio occultation experiments conducted by the Venus Express (VEX) and Akatsuki missions were utilized for this purpose. Background data from the Analyzer of Space Plasma and EneRgetic Atoms (ASPERA‐4) aboard VEX were used to detect SEP events. Additionally, observations from the Space Environment Monitor (SEM) suite onboard the Geostationary Operational Environmental Satellite (GOES) during alignments of Venus, Earth, and the Sun were also considered. Our findings indicate that while SEPs may contribute to the formation of the V0 layer, they are not the main driving force in the Venusian ionosphere. Plain Language Summary: The Venusian ionosphere shows sporadic enhancements in electron density around 110 km altitude. Despite extensive investigation, the exact cause of its sporadic presence remains elusive. Previous studies have explored various theories, including meteoric influences, gravity waves, and the impact of minor atmospheric constituents such as NO, O2, C, Ar, H2, and H. However, none have conclusively explained its formation mechanism. In this study, we have investigated the potential influence of Solar Energetic Particles (SEPs) on the V0 layer. By analyzing electron density profiles obtained from VEX/Akatsuki missions, hot plasma measurements by ASPERA‐4 onboard VEX, and SEM suite onboard GOES to identify SEP events, we surmise that while SEPs may contribute to the formation of the V0 layer, they are not the primary driving force in the Venusian ionosphere. Key Points: The Impact of solar energetic particle (SEP) events on the formation of V0 layers in the Venusian ionosphere has been exploredEnhanced ionization is seen above the V2 layer peak whenever a V0 layer seen in the Venus ionosphereSEPs and enhanced ionization at 110 km altitudes, however, are not correlated [ABSTRACT FROM AUTHOR]

Spacecraft‐to‐spacecraft radio occultations experiments are being conducted at Mars between Mars Express (MEX) and Trace Gas Orbiter (TGO), the first ever extensive inter‐spacecraft occultations at a planet other than Earth. Here we present results from the first 83 such occultations, conducted between 2 Nov 2020 and 5th of July 2023. Of these, 44 observations have to‐date resulted in the extraction of vertical electron density profiles. These observations are the successful results of a major feasibility study conducted by the European Space Agency to use pre‐existing relay communication equipment for radio science purposes. Mutual radio occultations have numerous advantages over traditional spacecraft‐to‐ground station occultations. In this work, we demonstrate how raw data are transformed into electron density values and validated with models and other instruments. Plain Language Summary: Radio occultation is a measuring technique involving the passage of a signal through an atmosphere, during which we observe how much the signal bends. This bending effect is precisely measured as a frequency shift. Typically, this technique is employed by transmitting a signal from a satellite orbiting a planet to a ground station on Earth. However, in this article, we explore an alternative on this measurement approach known as mutual RO. Here, both the transmitter and receiver of the signal are positioned in orbit around the same foreign planet, creating a unique scenario. Specifically, we utilized two European Space Agency satellites: Mars Express and ExoMars Trace Gas Orbiter, and have conducted a total of 83 experiments. Within this article, we present the methodology, processing and outcomes of select experiments, underlining their reliability through comparisons with models and data from other instruments. Key Points: Many vertical electron density profiles have been successfully acquired via mutual radio occultation (RO) measurementsConducting RO mutually provides many advantages over conventional spacecraft‐Earth occultationsThis is an example of data‐relay equipment being repurposed for Radio Science [ABSTRACT FROM AUTHOR]

For the past few decades, it has been demonstrated that gravity waves (GWs) and neutral winds can drive ionospheric irregularities on Earth. Still, as far as we know, the formation of ionospheric irregularities on Mars due to GWs has not been well studied. In this study, we use data from the NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission to show evidence of an irregularity event in the Martian ionosphere, potentially seeded by the GWs break (GWB). Statistical findings indicate that the observed ratio of GWB‐related irregularity events varies from ∼0.25 to ∼0.47 each year, and the average ratio in 2015–2020 is ∼0.37. We perform a numerical simulation to provide further insight into the processes behind irregularity formation, which employs neutral wind shear as a source of perturbation in the context of the GWB. The simulations yield results fundamentally aligned with the observed characteristics of ionospheric irregularities in the 2018 event by considering the wind shear as the disturbance source. This study provides supplementary insights into the perturbation sources involved in shaping irregularities within the Martian ionosphere and presents valuable information about the coupling between the Martian ionosphere and the lower atmosphere. Plain Language Summary: Many remote and in situ observations of the Martian ionosphere have been made by landers, orbiting or flyby spacecrafts, for example, Viking, Mars Global Surveyor (MGS), Mars Express (MEX), and Mars Atmosphere and Volatile EvolutioN (MAVEN) missions. It gives us a great opportunity to know the basic ionospheric structure and composition of Mars. However, there are still many scientific questions about the Martian ionosphere that have yet to be studied, for example, the structure of the lower ionosphere, small‐scale ionospheric irregularities, and the role of lower atmospheric effects on the ionosphere. Normally, the ionosphere is a stable stratified structure. However, in some situations, the ionosphere becomes unstable, generating perturbations in the electron density, leading to these so‐called "irregularities." In this study, we report that the break of gravity waves potentially accounts for the formation of ionospheric irregularities observed in the Martian ionosphere by NASA's MAVEN spacecraft. The results of our statistical study support the notion that gravity waves are a crucial seeding source in the formation of irregularities. This investigation adds to the current understanding of perturbation sources responsible for irregularity formation in the Martian ionosphere, while also contributing to our knowledge of the coupling between the Martian ionosphere and the lower atmosphere. Key Points: An example of a Martian ionospheric irregularity event potentially seeded by GWB (the break of gravity waves) is presentedStatistical results show that the proportion ratio of GWB‐related irregularities peaked at ∼0.47 in 2018, reached a minimum of ∼0.25 in 2017, and averaged ∼0.37 over the six yearsThe simulation shows that the formation of the irregularities can be fundamentally reproduced using the GWB‐related wind shear as a perturbation source [ABSTRACT FROM AUTHOR]

Since 2021, a new surge in discrete aurora detections at Mars has been observed by the Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM) Hope Orbiter as EMUS started to regularly obtain synoptic auroral images with a high sensitivity. Here we report on a fortuitous conjunction between EMM and Mars Express (MEX) using far ultraviolet (FUV) imaging of discrete aurora by EMM EMUS, in situ measurements of suprathermal electrons by the MEX Analyzer of Space Plasma and Energetic Atoms Electron Spectrometer (ELS), and topside radar sounding of the nightside ionosphere by the MEX Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). In this event, EMM EMUS imaged a clear discrete aurora signature around moderately strong crustal magnetic fields on the nightside near the dusk terminator, 11 min before which MEX MARSIS measured a prominent local enhancement of the peak electron density in the nightside ionosphere and MEX ELS observed an in situ enhancement of suprathermal electrons at the corresponding location. A remarkable geographic agreement is found between the enhancements of the aurora, ionosphere, and suprathermal electrons, suggesting that the enhanced ionization and auroral emission are caused concurrently by precipitating suprathermal electrons. Subsequent images indicate that the discrete aurora slightly changed its shape in 15 min and mostly disappeared in a few hours. The MEX MARSIS measurements of the auroral ionosphere display overlapping ionospheric and surface echoes indicative of horizontal gradients of the peak electron density. Analysis of the overlapping echoes implies that the auroral ionosphere and electron precipitation could be highly structured with horizontal spatial scales on the order of several tens of km. MEX MARSIS also observed a non-auroral ionospheric enhancement with a wider spatial extent than the local auroral enhancement, suggesting alternative sources of the enhanced nightside ionosphere such as plasma transport. The comparison between the ionospheric structures measured by MEX MARSIS, suprathermal electron flux measured by MEX ELS, and discrete auroral emission imaged by EMM EMUS underscores the complexity of the auroral and non-auroral nightside ionospheres. This motivates further investigations of their sources, transport, and connections to the magnetotail dynamics of Mars. [ABSTRACT FROM AUTHOR]

The electron density of the Martian ionosphere is modulated by solar wind forcing and crustal magnetic fields. Sounding observations from the orbital Shallow Radar (SHARAD) map the ionospheric total electron content (TEC) at a spatial resolution of ∼35 km. Averaging over a 250‐km diameter window from data collected weeks to years apart yields the first map of long‐term stable dayside martian TEC features. An extensive region of suppressed TEC in the southern highlands correlates with strong radial magnetic fields, but in other areas no simple correlation is observed. The TEC maps do follow the outlines of exposed Noachian crust and patches of magnetization in Tharsis not reset by volcanic activity. SHARAD TEC mapping may capture magnetic field strength at an intermediate height between the surface and the altitudes of orbital measurements underlying spherical harmonic models. Existing and future data will allow SHARAD TEC mapping to ∼100 km spatial resolution. Plain Language Summary: The upper atmosphere of Mars hosts a layer of charged particles that form the ionosphere. The vertical distribution of electrons is affected by the stream of solar wind and by magnetic fields imprinted on rocks that formed about 4 b.y. ago when the interior of Mars was hot enough to sustain a dynamo like that of present‐day Earth. We use a novel technique based on radar echoes from the surface observed by the Shallow Radar (SHARAD) on the NASA Mars Reconnaissance Orbiter to measure the total electron content (TEC) of the atmosphere over the past 15 years. Averaging many observations at a location reduces the solar wind effect and leaves a map of persistent features of the ionosphere. Long‐term stable TEC features correlate with the most ancient crust, but also occur in areas covered by younger lava flows where volcanic heating has not totally erased the magnetic field of the rocks. This method provides a new way of mapping regional geologic and magnetic field correlations that complement results from orbiter and lander magnetometer instruments. Key Points: MRO extended mission Shallow Radar measurements map stable Total Electron Content (TEC) patterns in the dayside ionosphere of MarsTEC maps align with regional geology and partially match crustal magnetic field models based on orbital dataTEC mapping offers a new and complementary method for delineating magnetized crust [ABSTRACT FROM AUTHOR]