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When the cone angle of the interplanetary magnetic field (IMF) becomes small, induced magnetospheres of unmagnetized planets degenerate. Using hybrid simulations, we study ionospheric ion escape in a 4° cone angle case and compare it with the nominal 55° cone angle (Parker spiral) case. The total escape rate is 1.7×1⁢025 s−1, nearly an order of magnitude higher than the nominal rate of 2.2×1⁢024 s−1. The escape probability is four times higher. The unique feature of the degenerate induced magnetosphere is the upstream escape driven by the ambipolar electric field, contributing 42% to the total escape, a channel absent in the nominal case. Additionally, 52% of escape occurs through a cross-flow plume, formed by the drift of ionospheric ions in the weak convective field and IMF. This channel is dominant, exhibiting an intensity seven times greater than that of the plume driven by the convective electric field in the nominal case.

Unmagnetized bodies with sufficiently dense ionospheres, such as Mars, form induced magnetospheres when interacting with the solar wind carrying the frozen-in interplanetary magnetic field (IMF). Mars Express equipped with the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) operating for 20 years over two solar cycles made fundamental contributions to our understanding of how the induced magnetosphere of Mars works. ASPERA-3 established the ion escape rate from 2 × 1024 s−1 to 4 × 1024 s−1 depending on the phase of the solar cycle. The measured empirical dependences of the escape rate on the solar wind dynamical pressure and UV fluxes allowed to determine the total atmospheric pressure lost over the past 4 billion years to be on the order of 10 mbars, i.e. small, contrary to long-standing expectations of a strong ion escape process. Comparing the measured escape rates from Mars with Venus and the Earth resulted in formulating the paradigm-shifting statement that the intrinsic magnetic field increases the escape rates and does not protect planetary atmospheres. Due to the long longevity of the mission, ASPERA-3 captured a number of extreme solar weather events and the unique encounter of Mars with the comet Siding-Spring. ASPERA-3 conducted the first-ever energetic neutral atom imaging of an induced magnetosphere, revealing the global periodic variability of the system, the significant precipitation of ENAs originating in the solar wind and magnetosheath, and the enhancement of ENA emissions from the Martian magnetic anomalies. ASPERA-3 conducted studies of the particles responsible for the discrete Martian aurora and characterized the precipitation of solar wind protons and alpha particles onto the atmosphere. The latter turned out to be a significant contribution to the helium balance on Mars. ASPERA-3 made several important findings outside its main science objectives among those are detection of the tentative signatures of backscattered ions from the Phobos surface, investigations for the first time of radar accelerated ions and electrons in non-magnetized environments, and measurements of heliospheric ENAs. Despite the significant progress following outstanding Mars Express results in the field of the Mars – solar wind interaction there is a broad spectrum of unsolved problems and unanswered questions to be addressed by future mission. The most fundamental one is the ionosphere – magnetosphere interactions.

Jovian magnetospheric plasma irradiates the surface of Ganymede and is postulated to be the primary agent that changes the surface brightness of Ganymede, leading to asymmetries between polar and equatorial regions as well as between the trailing and leading hemispheres. As impinging ions sputter surface constituents as neutrals, ion precipitation patterns can be remotely imaged using the Energetic Neutral Atoms (ENA) measurement technique. Here we calculate the expected sputtered ENA flux from the surface of Ganymede to help interpret future observations by ENA instruments, particularly the Jovian Neutrals Analyzer (JNA) onboard the JUpiter ICy moon Explorer (JUICE) spacecraft. We use sputtering models developed based on laboratory experiments to calculate sputtered fluxes of H2O, O2, and H2. The input ion population used in this study is the result of test particle simulations using electric and magnetic fields from a hybrid simulation of Ganymede's environment. This population includes a thermal component (H+ and O+ from 10 eV to 10 keV) and an energetic component (H+, O++, and S+++ from 10 keV to 10 MeV). We find a global ENA sputtering rate from Ganymede of 1.42 × 1027 s−1, with contributions from H2, O2, and H2O of 34%, 17%, and 49% respectively. We also calculate the energy distribution of sputtered Energetic Neutral Atoms (ENAs), give an estimate of a typical JNA count rate at Ganymede, and investigate latitudinal variations of sputtered fluxes along a simulated orbit track of the JUICE spacecraft. Our results demonstrate the capability of the JNA sensor to remotely map ion precipitation at Ganymede.

Context: Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. The solar wind was observed for much of this time, but was significantly deflected and to some extent slowed down by the interaction with the coma.

Aims: We use the different changes in the speed of H⁺ and He²⁺ when they interact with the coma to estimate the upstream speed of the solar wind. The different changes in the speed are due to the different mass per charge of the particles, while the electric force per charge due to the interaction is the same. A major assumption is that the speeds of H⁺ and He²⁺ were the same in the upstream region. This is investigated.

Methods: We derived a method for reconstructing the upstream solar wind from H⁺ and He²⁺ observations. The method is based on the assumption that the interaction of the comet with the solar wind can be described by an electric potential that is the same for both H⁺ and He²⁺. This is compared to estimates from the Tao model and to OMNI and Mars Express data that we propagated to the observation point.

Results: The reconstruction agrees well with the Tao model for most of the observations, in particular for the statistical distribution of the solar wind speed. The electrostatic potential relative to the upstream solar wind is derived and shows values from a few dozen volts at large heliocentric distances to about 1 kV during solar events and close to perihelion. The reconstructed values of the solar wind for periods of high electrostatic potential also agree well with propagated observations and model results.

Conclusions: The reconstructed upstream solar wind speed during the Rosetta mission agrees well with the Tao model. The Tao model captures some slowing down of high-speed streams as compared to observations at Earth or Mars. At low solar wind speeds, below 400 km s^-1, the agreement is better between our reconstruction and Mars observations than with the Tao model. The magnitude of the reconstructed electrostatic potential is a good measure of the slowing-down of the solar wind at the observation point.

The present‐day Venusian atmosphere is dry, yet, in its earlier history a significant amount of water evidently existed. One important water loss process comes from the energy and momentum transfer from the solar wind to the atmospheric particles. Here, we used measurements from the Ion Mass Analyzer onboard Venus Express to derive a relation between the power in the upstream solar wind and the power leaving the atmosphere through oxygen ion escape in the Venusian magnetotail. We find that on average 0.01% of the available power is transferred, and that the percentage decreases as the available energy increases. For Mars the trend is similar, but the efficiency is higher. At Earth, the ion escape does not behave similarly, as the ion escape only increases after a threshold in the available energy is reached. These results indicate that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind.

We investigate the heavy ion density and velocity in the Venusian upper ionosphere near the North Pole, using the Ion Mass Analyzer, a part of the Analyzer of Space Plasmas and Energetic Atoms 4, together with the magnetic field instruments on Venus Express. The measurements were made during June-July 2014, covering the aerobraking campaign with lowered altitude measurements (similar to 130 km). The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400-km altitude. A clear trend of dusk-to-dawn heavy ion flow across the polar ionosphere was found, with speeds of similar to 2-10 km/s. In addition, the flow has a significant downward radial velocity component. The flow pattern does not depend on the interplanetary magnetic field directions nor the ionospheric magnetization states. Instead, we suggest a thermal pressure gradient between the equatorial and polar terminator regions, induced by the decrease in density between the regions, as the dominant mechanism driving the ion flow. Plain Language Summary We have calculated the ion density and velocities in the Venusian polar ionosphere using measurements from the Ion Mass Analyzer on board the Venus Express spacecraft. During June-July 2014 the periapsis was lowered to similar to 130 km, which allowed for measurements down to low altitudes of the ionosphere near the North Pole. The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400 km, which is similar to what was found near the equatorial region by the Pioneer Venus mission. In addition, there is a clear trend of dusk-to-dawn flow, along the terminator, for the heavy ions. This is surprising, as a general flow from day-to-night is expected for the Venusian ionosphere due to the long nights and significant heating of the dayside upper atmosphere. The interplanetary magnetic field direction does not appear to affect the ion flow pattern. Instead, we propose a thermal pressure gradient as the dominant accelerating mechanism, induced by the decrease in density from the equator toward the pole.

Loss of the early Martian atmosphere is often thought to have occurred due to an effective transfer of the solar wind energy through the Martian induced magnetic barrier to the ionosphere. We have quantified the coupling efficiency by comparing the power of the heavy ion outflow with the available power supplied by the upstream solar wind. Constraining upstream solar wind density n_sw, velocity v_sw, and EUV intensity I_EUV/photoionizing flux F_XUV in varying intervals reveals a decrease in coupling efficiency, k,with solar wind dynamic pressure as k ∝ p_dyn^{−0.74±0.13} and with F_XUV as k ∝ F_XUV^{−2.28±0.30}. Despite the decreasein coupling efficiency, higher F_XUV enhances the cold ion outflow, increasing the total ion escape rate as Q(F_XUV) = 10¹⁰(0.82 ± 0.05)F_XUV. The discrepancy between coupling and escape suggests that ion escapefrom Mars is primarily production limited in the modern era, though decreased coupling may lead to an energy-limited solar wind interaction under early Sun conditions.

An extremely strong Coronal Mass Ejection (CME) impacted Mars on 12 July 2011, while theMars Express spacecraft was present inside the nightside ionosphere. Estimated solar wind density andspeed during the event are 39 particles cm⁻³ and 730 km/s, corresponding to nominal solar wind fluxat Mars when the solar system was ∼1.1 Ga old. Comparing with expected average atmospheric heavy ionfluxes under similar XUV conditions, the CME impact is found to have no significant effect on the escaperate 3.3 × 10²⁴ s⁻¹, with an upper limit at 10²⁵ s⁻¹ if the observed tail contraction is not taken into account.On the subsequent orbit, 7 h later after magnetosphere response, fluxes were only 2.4% of average. As such,even under primordial solar wind conditions we are unable to find support for a strong solar wind-driven ion escape, rather the main effect appears to be acceleration of the escaping ions by ×10–×20 typicalcharacteristic energy.

The long operational life (2003-present) of Mars Express (MEX) has allowed the spacecraft tomake plasma measurements in the Martian environment over a wide range of upstream conditions. Wehave analyzed ∼7000 MEX orbits, covering three orders of magnitude in solar wind dynamic pressure, withdata from the on board Analyzer of Space Plasmas and Energetic Particles (ASPERA-3) package, mappingthe locations where MEX crosses the main plasma boundaries, induced magnetosphere boundary (IMB), ionosphere boundary (IB), and bow shock (BS). A coincidence scheme was employed, where data fromthe Ion Mass Analyzer (IMA) and the Electron Spectrometer (ELS) had to agree for a positive boundaryidentification, which resulted in crossings from 1083 orbit segments that were used to create dynamictwo-parameter (solar wind density, n_sw, and velocity v_sw) dependent global dynamic models for the IMB, IB,and BS. The modeled response is found to be individual to each boundary. The IMB scales mainly dependenton solar wind dynamic pressure and EUV intensity. The BS location closely follows the location of the IMB atthe subsolar point, though under extremely low nsw and vsw the BS assumes a more oblique shape. The IBclosely follows the IMB on the dayside and changes its nightside morphology with different trends for nswand vsw. We also investigate the influence of extreme ultraviolet (EUV) radiation on the IMB and BS, findingthat increased EUV intensity expands both boundaries.

Eight years (2007–2015) of ion flux measurements from Mars Express are used to statisticallyinvestigate the influence of the Martian magnetic crustal fields on the atmospheric ion escape rate.We combine all Analyzer of Space Plasmas and Energetic Atoms/Ion Mass Analyzer (ASPERA-3/IMA)measurements taken during nominal upstream solar wind and solar extreme ultraviolet conditions tocompute global average ion distribution functions, individually for the north/south hemispheres and forvarying solar zenith angles (SZAs) of the strongest crustal magnetic field. Escape rates are subsequentlycalculated from each of the average distribution functions. The maximum escape rate (4.2 ± 1.2) × 10²⁴ s⁻¹ is found for SZA = 60^∘–80^∘, while the minimum escape rate (1.7±0.6)×10²⁴ s⁻¹ is found for SZA = 28^∘–60^∘,showing that the dayside orientation of the crustal fields significantly affects the global escape rate (p=97%). However, averaged over time, independent of SZA, we find no statistically significant difference inthe escape rates from the two hemispheres (escape from southern hemisphere 46% ± 18% of global rate).