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Venus has intrigued planetary scientists for decades because of its huge contrasts to Earth, in spite of its nickname of “Earth’s Twin”. Its invisible upper atmosphere and space environment are also part of the larger story of Venus and its evolution. In 60s to 70s, several missions (Venera and Mariner series) explored Venus-solar wind interaction regions. They identified the basic structure of the near-Venus space environment, for example, existence of the bow shock, magnetotail, ionosphere, as well as the lack of the intrinsic magnetic field. A huge leap in knowledge about the solar wind interaction with Venus was made possible by the 14-year long mission, Pioneer Venus Orbiter (PVO), launched in 1978. More recently, ESA’s probe, Venus Express (VEX), was inserted into orbit in 2006, operated for 8 years. Owing to its different orbit from that of PVO, VEX made unique measurements in the polar and terminator regions, and probed the near-Venus tail for the first time. The near-tail hosts dynamic processes that lead to plasma energization. These processes in turn lead to the loss of ionospheric ions to space, slowly eroding the Venusian atmosphere. VEX carried an ion spectrometer with a moderate mass-separation capability and the observed ratio of the escaping hydrogen and oxygen ions in the wake indicates the stoichiometric loss of water from Venus. The structure and dynamics of the induced magnetosphere depends on the prevailing solar wind conditions. VEX studied the response of the magnetospheric system on different time scales. A plethora of waves was identified by the magnetometer on VEX; some of them were not previously observed by PVO. Proton cyclotron waves were seen far upstream of the bow shock, mirror mode waves were observed in magnetosheath and whistler mode waves, possibly generated by lightning discharges were frequently seen. VEX also encouraged renewed numerical modeling efforts, including fluid-type of models and particle-fluid hybrid type of models, describing the plasma interaction on scales ranging from ion gyro radius to the entire induced magnetosphere. In this review article, we review what has been found from space physics measurements around Venus (from the solar wind down to the ionopause), with a particular emphasis on updated results since the Venus Express mission. We conclude the article by a short discussion on the remaining open scientific questions and the future of this field.

Space weather refers to dynamic conditions on the Sun and in the space environment of the Earth, which are often driven by solar eruptions and their subsequent interplanetary disturbances. It has been unclear how an extreme space weather storm forms and how severe it can be. Here we report and investigate an extreme event with multi-point remote-sensing and in situ observations. The formation of the extreme storm showed striking novel features. We suggest that the in-transit interaction between two closely launched coronal mass ejections resulted in the extreme enhancement of the ejecta magnetic field observed near 1 AU at STEREO A. The fast transit to STEREO A (in only 18.6 h), or the unusually weak deceleration of the event, was caused by the preconditioning of the upstream solar wind by an earlier solar eruption. These results provide a new view crucial to solar physics and space weather as to how an extreme space weather event can arise from a combination of solar eruptions. Coronal mass ejections are large expulsions of plasma from the solar corona into space, and are drivers of major space weather effects. Here, the authors report observations of two successive ejections, whose interaction led to extremely enhanced magnetic fields and high solar wind speeds near 1 AU.

Venus, lacking an intrinsic global dipole magnetic field, serves as a textbook example of an induced magnetosphere, formed by interplanetary magnetic fields (IMF) enveloping the planet. Yet, various aspects of its magnetospheric dynamics and planetary ion outflows are complex and not well understood. Here we analyze plasma and magnetic field data acquired during the fourth Venus flyby of the Parker Solar Probe (PSP) mission and show evidence for closed topology in the nightside and downstream portion of the Venus magnetosphere (i.e., the magnetotail). The formation of the closed topology involves magnetic reconnection—a process rarely observed at non-magnetized planets. In addition, our study provides an evidence linking the cold Venusian ion flow in the magnetotail directly to magnetic connectivity to the ionosphere, akin to observations at Mars. These findings not only help the understanding of the complex ion flow patterns at Venus but also suggest that magnetic topology is one piece of key information for resolving ion escape mechanisms and thus the atmospheric evolution across various planetary environments and exoplanets. Magnetic reconnection dynamics in Venus’ magnetosphere are not well-known due to limited observations. Here, the authors show direct evidence for closed magnetic topology in Venus’ magnetotail and a link between the cold ion flow in the magnetotail and its direct magnetic connectivity to the ionosphere.

The MAVEN Solar Energetic Particle (SEP) instrument is designed to measure the energetic charged particle input to the Martian atmosphere. SEP consists of two sensors mounted on corners of the spacecraft deck, each utilizing a dual, double-ended solid-state detector telescope architecture to separately measure fluxes of electrons from 20 to 1000 keV and ions from 20–6000 keV, in four orthogonal look directions, each with a field of view of 42 ∘ by 31 ∘ . SEP, along with the rest of the MAVEN instrument suite, allows the effects of high energy solar particle events on Mars’ upper atmospheric structure, temperatures, dynamics and atmospheric escape rates, to be quantified and understood. Given that solar activity was likely substantially higher in the early solar system, understanding the relationship between energetic particle input and atmospheric loss today will enable more confident estimates of total atmospheric loss over Mars’ history.

The Emirates Mars Mission (EMM) – Hope Probe – was developed to understand Mars atmospheric circulation, dynamics, and processes through characterization of the Mars atmosphere layers and its interconnections enabled by a unique high-altitude (19,970 km periapse and 42,650 km apoapse) low inclination orbit that will offer an unprecedented local and seasonal time coverage over most of the planet. EMM has three scientific objectives to (A) characterize the state of the Martian lower atmosphere on global scales and its geographic, diurnal and seasonal variability, (B) correlate rates of thermal and photochemical atmospheric escape with conditions in the collisional Martian atmosphere, and (C) characterize the spatial structure and variability of key constituents in the Martian exosphere. The EMM data products include a variety of spectral and imaging data from three scientific instruments measuring Mars at visible, ultraviolet, and infrared wavelengths and contemporaneously and globally sampled on both diurnal and seasonal timescale. Here, we describe our strategies for addressing each objective with these data in addition to the complementary science data, tools, and physical models that will facilitate our understanding. The results will also fill a unique role by providing diagnostics of the physical processes driving atmospheric structure and dynamics, the connections between the lower and upper atmospheres, and the influences of these on atmospheric escape.

The Cassini Ion Neutral Mass Spectrometer (INMS) has obtained the first in situ composition measurements of the neutral densities of molecular nitrogen, methane, molecular hydrogen, argon, and a host of stable carbon-nitrile compounds in Titan's upper atmosphere. INMS in situ mass spectrometry has also provided evidence for atmospheric waves in the upper atmosphere and the first direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atmosphere. The bulk composition and thermal structure of the moon's upper atmosphere do not appear to have changed considerably since the Voyager 1 flyby.

The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.

This study quantifies several factors controlling the probability of a pickup oxygen ion to escape from the Mars upper atmosphere. It is commonly presumed that ions with sufficient kinetic energy are able to escape to space. To test the validity of this simple assumption, we examined results from our Monte Carlo model, which monitors the motion of billions of test particles due to gravity and the Lorentz force through the electromagnetic fields of a magnetohydrodynamic model solution. It is shown that the electromagnetic fields are the dominant factor, surpassing the deceleration of gravity, in controlling ion transport and thus determine whether particles ultimately escape Mars or return to the planet. The particle kinetic energy and the local time of the crustal fields are also important factors greatly influencing the escape probability. In a simulation case in which the strongest crustal fields face the Sun at nominal solar minimum conditions, on average, only 45% of isotropically distributed newborn particles at ∼400 km altitude are able to escape, even with a sufficiently high initial energy of ∼10 eV. Furthermore, there is a distinct hemispheric asymmetry in the escape probability distribution, as defined by the upstream convection electric field direction (Esw). In the above case, the particles produced in the −Esw hemisphere have a much smaller chance to escape, on average, about 17%. These findings imply that one has to be careful when using satellite periapsis measurements to estimate atmospheric loss, where ion densities are high but escape chances may be very low.

We investigate the effects of solar energetic particle (SEP) events on the Martian ionosphere using observations from the Mars Global Surveyor (MGS) Electron Reflectometer (ER) and Radio Science (RS) experiments. Although MGS/ER is not designed to measure solar storm particles, it detects SEPs as increased instrument background. Using this proxy for SEP fluxes near Mars, we compare electron density profiles obtained from the RS experiment during periods of high and low SEP activity. Six case studies show no clear evidence for an increase in the ionospheric electron density between 200 and 100 km altitudes. However, 4 of the 6 events show a small increase in electron density below 100 km altitude during SEP events, suggesting that high‐energy (10–20 keV) electrons may cause ionization in the lower ionosphere. We also observe an ∼25% decrease in the ionospheric electron density between ∼100 and ∼120 km altitude for the two strongest events, suggesting that SEPs trigger a process that increases electron loss in this altitude range of the lower ionosphere. However, we cannot be confident from only two events that this effect is caused directly or indirectly by increased SEP fluxes. A statistical study confirms the case study results, but not over all solar zenith angles. Additionally, we observe depletions in the topside ionospheric electron density at some solar zenith angles, which can be explained by compression of the ionosphere by the passing CME. Key Points The first study to investigate only SEP effects at Mars Four of six SEP events show a small increase in electron density below 100 km We also observe a ~25% decrease in the electron density between ~100 and 120 km

The atmosphere of Mars, lacking a global magnetic field, is exposed to the precipitation of solar energetic particles (SEPs), resulting in impact ionization and the production of secondary electrons, some of which may escape the atmosphere. In this study, we examine upward traveling fluxes of superthermal electrons between ∼100 and 650 eV, measured by the Mars Global Surveyor Magnetometer/Electron Reflectometer at 400 km altitude during nine of the largest and clearest SEP events of the last solar maximum from November 2000 until the “Halloween” storms of late 2003. We subtract the contribution from backscattered low‐energy precipitating electrons and find that, for the highest and most rarely observed SEP fluxes, we detect a statistically significant flux of SEP‐produced superthermal electrons escaping the Martian atmosphere. The measured fluxes are found to be in broad agreement with a calculation of expected upward electron fluxes resulting from ionization of neutrals by energetic proton impact. Peak SEP ionization rates on the nightside from the Halloween storms are found to be comparable to (although lower than) typical dayside photoionization rates and at least 3 orders of magnitude higher than average nightside electron impact ionization rates. Further advances in our knowledge of SEP effects on the Martian ionosphere await data from the Radiation Assessment Detector (RAD) instrument on the Mars Science Laboratory rover in 2012 and the MAVEN orbiter in 2014. Key Points Solar energetic particle events cause atmospheric ionization Largest SEP events produce detectable superthermal electrons First extraterrestrial detection of SEP‐produced ionospheric electrons