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Using the 8.5‐year Venus Express measurements, we demonstrate the asymmetric plasma distributions in the Venusian magnetosheath. An escaping plume is formed by pickup oxygen ions in the hemisphere where the motional electric field points outward from Venus, while the velocity of solar wind protons is faster in the opposite hemisphere. The pickup O+ escape rate is estimated to be (3.6 ± 1.4) × 1024 s−1 at solar maximum, which is comparable to the ion loss rate through the magnetotail, and (1.3 ± 0.4) × 1024 s−1 at solar minimum. The increase of O+ fluxes with extreme ultraviolet (EUV) intensity is significant upstream of the bow shock, partially attributed to the increase of exospheric neutral oxygen density. However, the solar wind velocity just has a slight effect on the pickup O+ escape rate in the magnetosheath, while the effect of solar wind density is not observed. Our results suggest the pickup O+ escape rate is mainly controlled by EUV radiation. Plain Language Summary The atmospheric evolution and water escape of Venus might be influenced by the solar wind‐Venus interaction. The atoms outside the induced magnetosphere are ionized by the solar radiation and accelerated to the escape velocity by solar wind electric field. In this way, the oxygen ions are picked up by solar wind and lost from the atmosphere to space. We use the data from Venus Express spacecraft to analyze the distribution of pickup oxygen ions in the vicinity of the planet. The planetary oxygen ions form a strong escaping plume, indicating the pickup process is an efficient escape channel removing the atmospheric particles. With an enhanced solar extreme ultraviolet radiation, the escape rate through this channel would be higher because more ions are produced and then picked up. This indicates an enhanced ion loss billions of years ago since the young Sun is more active, which might be a reason for the disappearance of a presumably‐existed ocean. Key Points The pickup O+ escape rate at Venus increases with solar activity, and it is comparable to the ion loss rate through the magnetotail The solar wind velocity has a slight effect on the pickup O+ escape rate in the magnetosheath The neutral oxygen density upstream of the bow shock might increase by a factor of two from solar minimum to maximum

We investigated, for the first time, the impact of the disappearing solar wind (DSW) event [26–28 December 2022] on the deep nightside ionospheric species using MAVEN data sets. An enhanced plasma density has been observed in the Martian nightside ionosphere during extreme low solar wind density and pressure periods. At a given altitude, the electron density surged by ∼2.5 times, while for ions (NO+, O2+, CO2+, C+, N+, O+, and OH+), it enhanced by > 10 times, respectively, compared to their typical average quiet‐time periods. This investigation suggests that an upward ionospheric expansion likely took place in a direct consequence to the contrasting low dynamic/magnetic pressure and relatively higher nightside ionospheric pressure (by 1–2 orders) causing an increased ionospheric density. Moreover, the day‐to‐night plasma transport may also be a contributing factor to the increased plasma density. Thus, this study offers a new insight about planetary atmosphere/ionosphere during extreme quiescent solar wind periods. Plain Language Summary The evolution of the Mars climate over time depends upon the solar wind‐Mars interactions. The varying activity over the Sun intermittently produce extreme low density solar wind or also called as disappearing Solar Wind (DSW), which can affect the planetary environment in many ways. Beyond Earth, the effect of DSW on other planetary atmospheres is not well studied. In order to understand this aspect, we have explored the behavior of Martian nightside plasma environment (species: e−, NO+, O2+, CO2+, C+, N+, O+, and OH+) during the DSW event. A dense ionosphere is observed during DSW compared to non‐DSW periods. During DSW, the magnitude of peak nightside electron and ions density are increased by ∼2.5 and more than 10 times, respectively compared to their typical average quiet‐time scenario. The higher plasma density could be due to an expansion from the lower to the topside ionosphere, in consequence to the higher ionospheric pressure as compared to the low solar wind pressure. Furthermore, it could also be enhanced by the transport of plasma from dayside to nightside. Hence, this study, for the first instance, guides us to a new understanding of the impact of a rarest solar wind phenomenon on the Martian ionosphere. Key Points An increased plasma density is observed in the nightside ionosphere during the disappearing solar wind periods around Mars The electron and ions abundance surged by a factor of ∼2.5 and >10 respectively, compared to the average quiet‐time periods The contrast between the higher nightside ionospheric pressure and the dynamic/magnetosheath pressure led to increased plasma densities

Using measurements from the Mars Atmosphere and Volatile EvolutioN mission, we investigate the densities of H+ (nH+${n}_{{\\mathrm{H}}^{+}}$ ), O+ (nO+${n}_{{\\mathrm{O}}^{+}}$ ), and O2+ (no2+${n}_{{\\mathrm{o}}_{2}^{+}}$ ), respectively, in the Martian magnetotail current sheet. We find that the current sheet when it is closer to the terminator than 0.75 Mars radii is mostly dominated by heavy ions ((nO++no2+${n}_{{\\mathrm{O}}^{+}}+{n}_{{\\mathrm{o}}_{2}^{+}}$ )>2 nH+${n}_{{\\mathrm{H}}^{+}}$ ), regardless of the variation of the upstream solar wind, but that it is sometimes dominated by H+ (nH+${n}_{{\\mathrm{H}}^{+}}$>2(nO++no2+${n}_{{\\mathrm{O}}^{+}}+{n}_{{\\mathrm{o}}_{2}^{+}}$ )) at downstream distances exceeding 0.75 Mars radii. The occurrence rate of the dominant H+ weakly increases (and that of the heavy ions decreases) with solar wind density and dynamic pressure. Our results suggest that solar wind protons could enter the Martian tail and may become the dominant ion species in the current sheet, particularly when the solar wind density or dynamic pressure is high. Plain Language Summary The current sheet of the Martian magnetotail is a major channel for the escape of planetary ions. The ion composition in the current sheet is essential to our understanding of this escape, as well as the magnetotail plasma dynamics. Our current knowledge, however, is poor. Based on the measurements of the ion density of different species in the current sheet from the Mars Atmosphere and Volatile EvolutioN spacecraft, we report that the current sheets we have surveyed are dominated by either the heavy ions from the planet or H+ (mostly) from the solar wind. We find that the downstream distance and the variation of the upstream solar wind are the two key factors that account for which ion species dominates in the tail current sheet. Key Points Current sheets are mostly dominated by heavy ions but are sometimes dominated by H+ at the downstream distance exceeding 0.75 Mars radii The occurrence rate of current sheets with dominant H+ (heavy ions) weakly increases (decreases) with solar wind density and dynamic pressure Our results suggest that the dominant H+ in the current sheet could originate from solar wind

A series of coronal mass ejections (CMEs) from the Sun, interacted with one another and formed a complex interplanetary‐CME (ICME) that impinged Earth's magnetosphere on 10 May 2024 and caused the strongest geomagnetic storm of the past two decades. We present a unique pulse‐like enhanced solar wind density structure associated with a giant Kelvin‐Helmholtz (KH) wave vortex formed within the ICME due to compression of sheath region by the surrounding ejecta. When impinged on the magnetosphere, it caused enhanced eastward and westward currents, leading to strong positive and negative geomagnetic field impulses at low and auroral latitudes, respectively. The negative disturbances extended to mid‐latitudes around the midnight sector. Interestingly, the westward current pulse (negative disturbance) penetrated deep into low‐latitudes (up to 24.49°N and 28.92°S) in both hemispheres, but exclusively on the dawn side. The mechanisms responsible for such an intensification and low‐latitude penetration of westward current pulse are discussed.

We report a citizen science‐motivated study on the cause of an unusually bright red aurora as witnessed from Hokkaido, Japan during a magnetic storm on 1 December 2023. The auroral brightness of 5 kR is unusual for the Dst index peak of only −107 nT. In spite of the moderate storm amplitude, the extremely high solar wind density of >50/cc and dynamic pressure of >25 nPa caused the aurora oval extension to 53 magnetic latitudes (L = 2.8). We discuss that the drift loss of the ring current particles across the small‐size magnetopause is important, and Hokkaido was at the right position to see the direct effect of the large particle injection of the storm‐time substorm. Plain Language Summary Citizen scientists identified an unusually bright red aurora from Hokkaido, Japan during a not‐so‐unusual magnetic storm on 1 December 2023. The large dynamic pressure, driven by large density of >50/cc, contributed to a small magnetopause and the effects observed at such low latitude. The hypothesis of this study is that the loss of ring current particles across the small‐size magnetopause played an important role. Also, we discuss that Hokkaido was at the right position to see the direct effect of storm‐time substorm. Key Points Unusually bright red aurora was witnessed by citizen scientists from Hokkaido, Japan on 1 December 2023 The magnetic storm amplitude was not unusually large, but the solar wind density was high (50/cc) Dynamic pressure and asymmetric evolution of the ring current are important to understand the cause of red‐aurora magnetic storm events

We re-examined the contributions of the sheath region and the magnetic cloud to the main phase of the major geomagnetic storm on May 15, 1997. Our analysis revealed that the maximum values of the southward component of the interplanetary magnetic field (IMF) and the solar wind electric field were both located within the magnetic cloud. However, the sheath region’s contribution to the main phase of the storm was significantly greater than that of the magnetic cloud, indicating that this storm was predominantly driven by the sheath region. Interestingly, we found that the time integral of the solar wind electric field in the magnetic cloud was greater than that in the sheath region, which suggests that storm intensity is not solely determined by the time integral of the solar wind electric field. The only advantage of the sheath region over the magnetic cloud was its density, which was substantially higher in the sheath region than in the magnetic cloud. This finding underscores that solar wind density is a crucial parameter influencing the evolution of the ring current, in addition to the southward IMF and the solar wind speed.

We report on the Balloon-borne Investigation of Temperature and Speed of Electrons in the corona (BITSE) mission launched recently to observe the solar corona from ≈ 3  Rs to 15 Rs at four wavelengths (393.5, 405.0, 398.7, and 423.4 nm). The BITSE instrument is an externally occulted single stage coronagraph developed at NASA’s Goddard Space Flight Center in collaboration with the Korea Astronomy and Space Science Institute (KASI). BITSE used a polarization camera that provided polarization and total brightness images of size 1024 × 1024 pixels. The Wallops Arc Second Pointer (WASP) system developed at NASA’s Wallops Flight Facility (WFF) was used for Sun pointing. The coronagraph and WASP were mounted on a gondola provided by WFF and launched from the Fort Sumner, New Mexico station of Columbia Scientific Balloon Facility (CSBF) on September 18, 2019. BITSE obtained 17,060 coronal images at a float altitude of ≈ 128,000 feet ( ≈ 39  km) over a period of ≈ 4  hrs. BITSE flight software was based on NASA’s core Flight System, which was designed to help develop flight quality software. We used EVTM (Ethernet Via Telemetry) to download science data during operations; all images were stored on board using flash storage. At the end of the mission, all data were recovered and analyzed. Preliminary analysis shows that BITSE imaged the solar minimum corona with the equatorial streamers on the east and west limbs. The narrow streamers observed by BITSE are in good agreement with the geometric properties obtained by the Solar and Heliospheric Observatory (SOHO) coronagraphs in the overlapping physical domain. In spite of the small signal-to-noise ratio ( ≈ 14 ) we were able to obtain the temperature and flow speed of the western steamer. In the heliocentric distance range 4 – 7 Rs on the western streamer, we obtained a temperature of ≈ 1.0 ± 0.3  MK and a flow speed of ≈ 260  km s −1 with a large uncertainty interval.

The observational rate of mirror mode waves in Venus's magnetosheath for solar maximum conditions is studied and compared with previous results for solar minimum conditions. It is found that the number of mirror mode events is approximately 14 % higher for solar maximum than for solar minimum. A possible cause is the increase in solar UV radiation, ionizing more neutrals from Venus's exosphere and the outward displacement of the bow shock during solar maximum. Also, the solar wind properties (speed, density) differ for solar minimum and maximum. The maximum observational rate, however, over Venus's magnetosheath remains almost the same, with only differences in the distribution along the flow line. This may be caused by the interplay of a decreasing solar wind density and a slightly higher solar wind velocity for this solar maximum. The distribution of strengths of the mirror mode waves is shown to be exponentially falling off, with (almost) the same coefficient for solar maximum and minimum. The plasma conditions in Venus's magnetosheath are different for solar minimum as compared to solar maximum. For solar minimum, mirror mode waves are created directly behind where the bow shock will decay, whereas for solar maximum all created mirror modes can grow.

Coronal mass ejections (CMEs) cause various disturbances of the space environment; therefore, forecasting their arrival time is very important. However, forecasting accuracy is hindered by limited CME observations in interplanetary space. This study investigates the accuracy of CME arrival times at the Earth forecasted by three-dimensional (3D) magnetohydrodynamic (MHD) simulations based on interplanetary scintillation (IPS) observations. In this system, CMEs are approximated as spheromaks with various initial speeds. Ten MHD simulations with different CME initial speed are tested, and the density distributions derived from each simulation run are compared with IPS data observed by the Institute for Space-Earth Environmental Research (ISEE), Nagoya University. The CME arrival time of the simulation run that most closely agrees with the IPS data is selected as the forecasted time. We then validated the accuracy of this forecast using 12 halo CME events. The average absolute arrival-time error of the IPS-based MHD forecast is approximately 5.0 h, which is one of the most accurate predictions that ever been validated, whereas that of MHD simulations without IPS data, in which the initial CME speed is derived from white-light coronagraph images, is approximately 6.7 h. This suggests that the assimilation of IPS data into MHD simulations can improve the accuracy of CME arrival-time forecasts. The average predicted arrival times are earlier than the actual arrival times. These early predictions may be due to overestimation of the magnetic field included in the spheromak and/or underestimation of the drag force from the background solar wind, the latter of which could be related to underestimation of CME size or background solar wind density.

Imaging techniques provide essential information in astronomical and space physics studies. The Soft X-ray Imager (SXI) will obtain images of the Earth’s magnetosphere via the solar wind charge exchange process in a global view. However, it is a challenge to reconstruct its 3-D structures from the observed 2-D image(s). In this paper, a recently proposed method, Tangent Fitting Approach (TFA), is validated to reconstruct the large-scale magnetopause from a single X-ray image obtained by instrument simulation. It is revealed that the large-scale magnetopause under a medium solar wind number density can be well reconstructed, although the locations of maximum X-ray photon counts are scattered in the image due to instrumental effects and diffusive sky background. Higher solar wind number density leads to stronger signals and further leads to better reconstruction results. For lower solar wind density, the X-ray maximum photon counts may not be identified from the SXI simulations, preprocessing of the images shall be considered before applying TFA. Furthermore, the subsolar magnetopause can be well derived when the satellite is on the dayside orbits.