Venus Express Research Articles

Solar Wind Turbulence and Complexity Probed with Rank-Ordered Multifractal Analysis (ROMA)

The Rank-Ordered Multifractal Analysis (ROMA) is a tool designed to characterize scale (in)variance and multifractality based on rank ordering the fluctuations in “groups” characterized by the same mono-fractal behavior (Hurst exponent). A range-limited structure-function analysis provides the mono-fractal index for each rank-ordered range of fluctuations. We discuss here two examples of multi-scale solar wind turbulence and complexity where ROMA is applied on the following: (a) data collected by Ulysses spacecraft in the fast solar wind, outside the ecliptic, between 25 and 31 January 2007, at roughly 2.5 Astronomical Units (AU) from the Sun, in the Southern heliosphere, at latitudes between −76.5 and −77.3 degrees, and (b) slow solar wind data collected in the ecliptic plane by Venus Express spacecraft, at 0.72 AU, on 28 January 2007. The ROMA spectrum of fast solar wind derived from ULYSSES data shows a scale-dependent structure of fluctuations: (1) at the smallest/kinetic range of scales (800 to 3200 km), persistent fluctuations are dominant, and (2) at the inertial range of scales (104 to 2 × 105 km), anti-persistent fluctuations are dominant, but less clearly developed and possibly indicative for the development of instabilities with cross-over behavior. The ROMA spectrum of the slow solar wind derived from Venus Express data, suggests a different structure of turbulence: (1) fully developed multifractal turbulence across scales between 5 × 104 and 4 × 105 km, with the Hurst index changing from anti-persistent to persistent values for the larger amplitude magnetic fluctuations; (2) at the smallest scales (400 to 6400 km), fluctuations are mainly anti-persistent, and the ROMA spectrum indicates a tendency towards mono-fractal behavior.

Classifying the clouds of Venus using unsupervised machine learning

Because Venus is completely shrouded by clouds, they play an important role in the planet’s atmospheric dynamics. Studying the various morphological features observed on satellite imagery of the Venusian clouds is crucial to understanding not only the dynamic atmospheric processes, but also interactions between the planet’s surface structures and atmosphere. While attempts at manually categorizing and classifying these features have been made many times throughout Venus’ observational history, they have been limited in scope and prone to subjective bias. We therefore present and investigate an automated, objective, and scalable approach for their classification using unsupervised machine learning that can leverage full datasets of past, ongoing, and future missions.To achieve this, we introduce a novel framework to generate nadir observation patches of Venus’ clouds at fixed consistent scales from satellite imagery data of the Venus Express and Akatsuki missions. Such patches are then divided into classes using an unsupervised machine learning approach that consists of encoding the patch images into feature vectors via a convolutional neural network trained on the patch datasets and subsequently clustering the obtained embeddings using hierarchical agglomerative clustering.We find that our approach demonstrates considerable accuracy when tested against a curated benchmark dataset of Earth cloud categories, is able to identify meaningful classes for global-scale (3000km) cloud features on Venus and can detect small-scale (25km) wave patterns. However, at medium scales (∼500km) challenges are encountered, as available resolution and distinctive features start to diminish and blended features complicate the separation of well defined clusters.

Extent of the Magnetotail of Venus From the Solar Orbiter, Parker Solar Probe and BepiColombo Flybys

AbstractWe analyze data from multiple flybys by the Solar Orbiter, BepiColombo, and Parker Solar Probe (PSP) missions to study the interaction between Venus' plasma environment and the solar wind forming the induced magnetosphere. Through examination of magnetic field and plasma density signatures we characterize the spatial extent and dynamics of Venus' magnetotail, focusing mainly on boundary crossings. Notably, we observe significant differences in boundary crossing location and appearance between flybys, highlighting the dynamic nature of Venus' magnetotail. In particular, during Solar Orbiter's third flyby, extreme solar wind conditions led to significant variations in the magnetosheath plasma density and magnetic field properties, but the increased dynamic pressure did not compress the magnetotail. Instead, it is possible that the increased EUV flux at this time rather caused it to expand in size. Key findings also include the identification of several far downstream bow shock (BS), or bow wave, crossings to at least 60 (1 = 6,052 km is the radius of Venus), and the induced magnetospheric boundary to at least 20 . These crossings provide insight into the extent of the induced magnetosphere. Pre‐existing models from Venus Express were only constrained to within 5 of the planet, and we provide modifications to better fit the far‐downstream crossings. The new model BS is now significantly closer to the central tail than previously suggested, by about 10 at 60 downstream.

An investigation into Venusian atmospheric chemistry based on an open-access photochemistry-transport model at 0–112 km

Atmospheric chemistry plays a crucial role in the evolution of climate habitability on Venus. It has been widely explored by chemistry-transport models, but some characteristics are still poorly interpreted. This study is devoted to developing an open-access chemistry-transport model spanning both the middle and lower atmospheres of Venus. It provides a scheme for the structure of the chemistry, especially for the sulfur and oxygen, and investigates the influence of the cloud diffusivity and the SO2 dissolution that are adopted in the clouds. The developed model is based on the VULCAN framework and was updated with the state-of-the-art Venusian atmospheric chemistry. It includes vertical eddy diffusion retrieved recently with the Venus Express observations, and it resolves radiative transfer containing gas absorption and scattering, Mie scattering of the cloud droplets, and absorption of the unknown UV absorber. The obtained abundance profiles of SO, SO2, CO, COS, O, O2, O3, HCl, and NO are in overall agreement with the observations. The results show that the increase in cloud diffusivity has slight effects on the chemical structure. The SO2 mainly dissolves in 50–90 km and evaporates below the clouds. The rapid dissolution-release cycle is responsible for the large upward flux of SO2 at 58 km. At around 70 km, SO has a significant peak that is larger than that of previous studies by an order of magnitude, and S and SO2 also show slight increases. They are attributed to the buffering effects of liquid SO2 in the clouds. O2 is significantly eliminated by SO in this layer. We emphasize the superior regulation of the sulfur cycle on O2 at 70 km and its potential contributions to the long-standing problem of the overestimated O2 abundance.

Do Solar Energetic Particle (SEP) Events Influence the Formation of the V0 Layer in the Venusian Ionosphere?

AbstractThis 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.

3- and 6-degree-of-freedom analysis of the Magellan aerobraking experiment and planar aerobraking at Venus

The first non-Earth aerobraking experiment occurred in 1993 as part of the Magellan satellite's mission to Venus. Magellan reduced its orbit from elliptical to nearly circular over a span of 70 Earth days by leveraging the non-conservative force of aerodynamic drag to reduce its orbital energy via transiting the upper region of Venus' sensible atmosphere. First tested with the Hiten probe in the Earth-Moon system in 1991, the subsequent Magellan experiment helped prove the viability of aerobraking for planetary missions and paved the way for its implementation in Mars missions starting in the late 1990s and the 2014 Venus Express mission. This paper, for the first time in literature, investigates the Magellan aerobraking experiment from the perspective of both 3- and 6-degree-of-freedom analysis. Multiple atmospheric density models are considered, as well as J4 gravitational perturbations in order to understand the complexity and sensitivity of numerous aerobraking maneuvers in the thick Venusian atmosphere. Alternative satellites are also studied to ascertain their capability of maintaining the Magellan aerobraking flight profile. So as to examine a wide range of vehicle mass and drag reference area, the Hubble Space Telescope (HST) and Hiten satellites are modeled and their aerobraking performance analyzed for the hypothetical case of Venus orbital operations. When reconstructing the historical Magellan aerobraking maneuvers, 3DOF and 6DOF analysis yielded minimal trajectory deviation with the Magellan and Hiten models, while more significant deviation was experienced with the larger HST model. Both 3DOF and 6DOF analyses also reveal the sensitivity of the atmospheric density model and indicate a higher fidelity gravity model is necessary for Venusian aerobraking modeling, planning, and analysis.

The Venus Global Ionosphere‐Thermosphere Model (V‐GITM): A Coupled Thermosphere and Ionosphere Formulation

AbstractThis paper introduces the new Venus global ionosphere‐thermosphere model (V‐GITM) which incorporates the terrestrial GITM framework with Venus‐specific parameters, ion‐neutral chemistry, and radiative processes in order to simulate some of the observable features regarding the temperatures, composition, and dynamical structure of the Venus atmosphere from 70 to 170 km. Atmospheric processes are included based upon formulations used in previous Venus GCMs, several augmentations exist, such as improved horizontal and vertical momentum equations and tracking exothermic chemistry. Explicitly solving the momentum equations allows for the exploration of its dynamical effects on the day‐night structure. In addition, V‐GITM's use of exothermic chemistry instead of a strong heating efficiency accounts for the heating due to the solar EUV while producing comparable temperatures to empirical models. V‐GITM neutral temperatures and neutral‐ion densities are compared to upper atmosphere measurements obtained from Pioneer Venus and Venus Express. V‐GITM demonstrates asymmetric horizontal wind velocities through the cloud tops to the middle thermosphere and explains the mechanisms for sustaining the wind structure. In addition, V‐GITM produces reasonable dayside ion densities and shows that the neutral winds can carry the ions to the nightside via an experiment advecting .

Three‐Dimensional Venus Cloud Structure Simulated by a General Circulation Model

AbstractThe clouds have a great impact on Venus's energy budget and climate evolution, but its three‐dimensional structure is still not well understood. Here we incorporate a simple Venus cloud physics scheme into a flexible GCM to investigate the three‐dimensional cloud spatial variability. Our simulations show good agreement with observations in terms of the vertical profiles of clouds and H2SO4 vapor. H2O vapor is overestimated above the clouds due to efficient transport in the cloud region. The cloud top decreases as latitude increases, qualitatively consistent with Venus Express observations. The underlying mechanism is the combination of H2SO4 chemical production and meridional circulation. The mixing ratios of H2SO4 at 50–60 km and H2O vapors in the main cloud deck basically exhibit maxima around the equator, due to the effect of temperature's control on the saturation vapor mixing ratios of the two species. The cloud mass distribution is subject to both H2SO4 chemical production and dynamical transport and shows a pattern that peaks around the equator in the upper cloud while peaks at mid‐high latitudes in the middle cloud. At low latitudes, H2SO4 and H2O vapors, cloud mass loading and acidity show semidiurnal variations at different altitude ranges, which can be validated against future missions. Our model emphasizes the complexity of the Venus climate system and the great need for more observations and simulations to unravel its spatial variability and underlying atmospheric and/or geological processes.

Three-dimensional Venusian ionosphere model

To study the Venusian ionosphere, a 3D ionospheric model is included in the Venus Planetary Climate Model (Venus PCM), which we present here with a number of recent extensions and improvements. Our ionospheric model of Venus consists of 15 charged species (electron and 14 ions), 13 photo-ionization for 8 species, an ion-neutral chemistry (61 reactions) and an ambipolar diffusion scheme for the ion vertical transport. The electron temperature is assumed independent of the solar activity. Simulation results are compared with observation from the Pioneer Venus (PV) and Venus Express (VEX) for high and low/intermediate solar activity respectively. The model shows that ambipolar diffusion dominates photochemical equilibrium above 180 km and above 130 km altitude on the dayside and the nightside respectively. On the dayside, the electron density predicted by Venus PCM and its variation with solar zenith angle (SZA) are in good agreement with PV observations at high solar activity, even if the secondary electron peak is underestimated by the model, due to the absence of (photo-)electron impact ionization process, in Venus PCM. At higher solar activity, the predicted horizontal and vertical variations in electron and ion densities are both significantly underestimated on the nightside, probably due to the currently predicted weak day-to-night transport and the absence of photo-electron impact ionization process which is the main source of ion production on the nightside. On the dayside at low solar activity, the electron density predicted by Venus PCM are over-estimated by a factor 1.25 at the altitude of the main ionospheric peak and by a factor 2–3 at 250 km, compared to VEX observations. We suggest that this difference between high and low solar activity is linked to the overestimation of the neutral density at low solar activity and the independence of electron temperature with solar activity used in Venus PCM, which should have a significant effect on ion chemistry and the scale height of ion species.

One-dimensional Microphysics Model of Venusian Clouds from 40 to 100 km: Impact of the Middle-atmosphere Eddy Transport and SOIR Temperature Profile on the Cloud Structure

We conducted a simulation of H2SO4 vapor, H2O vapor, and H2SO4–H2O liquid aerosols from 40 to 100 km, using a 1D Venus cloud microphysics model based on the one detailed in Imamura & Hashimoto. The cloud distribution obtained is in good agreement with in situ observations by Pioneer Venus and remote-sensing observations from Venus Express (VEx). Case studies were conducted to investigate sensitivities to atmospheric parameters, including eddy diffusion and temperature profiles. We find that efficient eddy transport is important for determining upper haze population and its microphysical properties. Using the recently updated eddy diffusion coefficient profile by Mahieux et al., our model replicates the observed upper haze distribution. The H2O vapor distribution is highly sensitive to the eddy diffusion coefficient in the 60–70 km region. This indicates that updating the eddy diffusion coefficient is crucial for understanding the H2O vapor transport through the cloud layer. The H2SO4 vapor abundance varies by several orders of magnitude above 85 km, depending on the temperature profile. However, its maximum value aligns well with observational upper limits found by Sandor et al., pointing to potential sources other than H2SO4 aerosols in the upper haze layer that contribute to the SO2 inversion layer. The best-fit eddy diffusion profile is determined to be ∼2 m2 s−1 between 60 and 70 km and ∼360 m2 s−1 above 85 km. Furthermore, the observed increase of H2O vapor concentration above 85 km is reproduced by using the temperature profile from the VEx/SOIR instrument.

Galactic cosmic rays at 0.7 A.U. with Venus Express housekeeping data

We apply a previously developed procedure to characterize galactic cosmic rays (GCRs) at 0.7 A.U. with engineering data coming from the Venus Express mission. The engineering parameters are the Error Detection and Correction EDAC cumulative counters, used for detection and correction of memory errors induced by highly energetic particles. It has already been demonstrated that the slope of this counter measures GCR fluxes using data from Mars Express (1.5 A.U.) and Rosetta (up to 4 A.U.) data. Here, we reproduce these methods using Venus Express EDAC data in order to understand the behavior of GCRs closer to the Sun. We again witness the anti-correlation of EDAC slope with the solar activity and further investigate this procedure. The resulting time-lag between maximum sunspot number and minimum GCRs intensity at Venus is close to one day instead of the expected several months. This work represents one of the first characterization of galactic cosmic rays at small distances to the Sun over a long period of time and further cements the value of using EDAC counters as scientific information.

Structure of a Quasi-parallel Shock Front

The aim of this study is to compare observations of the magnetic field structure of observed quasi-parallel collisionless shock fronts with the results obtained analytically. A two-fluid analytic model of the shock front structure was derived under the assumptions that the shock is stationary and planar. The ion and electron kinetic pressures were assumed to be scalar, and polytropic state equations were used. The results of this analytical approach show that the shock magnetic field has an oscillatory structure. Venus Express (VEX) observations of the Venusian bow shock have been used to validate these theoretical findings. The Venusian bow shock and corresponding foreshock are significantly smaller than those of Earth. Thus, observations of the underlying structure of the quasi-parallel shock at Venus are not masked by the presence of high-amplitude waves and nonlinear structures originating in the foreshock. It is shown that the structure of the shock front, as observed by VEX, has a very strong similarity to the structure obtained analytically.

Statistical Mapping of Magnetic Topology at Venus

AbstractDespite Venus having insignificant intrinsic magnetic fields, the magnetic connectivity between the solar wind and the Venus ionosphere, or magnetic topology, is not as simple as expected and is also important for characterizing the Venus space environment. This study provides a technique combining superthermal electron energy and pitch angle distributions to infer up to 6 subtypes of magnetic topology at Venus. This enables us to determine magnetic topology with automated procedures using the Venus Express (VEx) observations from May 2006 to November 2014. We find that the draped topology (both ends of a field line not connected to the collisional ionosphere) is the dominant topology in the near‐Venus space environment, >70%, except at low altitudes close to the ionosphere. The open (a field line connected to both the solar wind and the collisional ionosphere) and closed (a field line connected only to the collisional ionosphere) topologies make up 20%–30% on average of the magnetotail and up to 50% at low altitudes. This study provides the first characterization of the statistical distributions of different magnetic topologies at Venus.

Bimodal aerosol distribution in Venus' upper haze from joint SPICAV-UV and -IR observations on Venus Express

Spectroscopic solar occultation measurements by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus/Solar Occultation at Infrared instrument (SPICAV/SOIR) onboard the Venus Express orbiter gave new data about upper haze aerosol properties, its vertical distribution and spatial and temporal variations. Early study with three channels of SPICAV/SOIR instrument using a few selected orbits indicated presence of two aerosol modes in particle size distribution (Wilquet et al., 2009). Analysis of aerosol properties from the SPICAV−IR spectrometer for the whole Venus Express data set obtained from May 2006 till November 2014 has proved it for some occultations (Luginin et al., 2016). In this work, we report retrieval of the upper haze (81–100 km) aerosol properties from 101 simultaneous SPICAV−UV and –IR solar occultation sessions acquired between March 2007 and January 2013. A joint analysis of the data from two spectrometers allowed us to characterize the size distribution ∼10 km higher in the atmosphere compared to previous analysis and to detect bimodal distribution in ∼50% of observations previously believed to be unimodal. At altitudes 81–92 km bimodality is observed in >50% of cases. Mode 2 particles are detected up to 98 km and mode 1 up to 100 km. Mean radius equals 0.14 ± 0.03 μm for mode 1 and 0.78 ± 0.18 μm for mode 2. Number density profiles for both modes of particles exponentially decrease with altitude, starting from 50 cm−3 and 0.3 cm−3 at 82 km for mode 1 and mode 2, respectively, and reaching 3 cm−3 at 98 km for mode 1 and 0.03 cm−3 at 94 km for mode 2.

Determination of the eddy diffusion in the Venusian clouds from VeRa sulfuric acid observations

Context. The vertical eddy diffusion coefficient (Kzz) characterizing the efficiency of vertical atmospheric mixing is essential for 1D planetary atmospheric modeling, but poorly constrained in the Venusian clouds, where our ability to observe tracer gases is limited. The Venusian clouds are mainly composed of H2SO4, which has significant mass cycles in this region. A critical process herein is that the H2SO4 vapor abundance in the middle and lower clouds of Venus is regulated by both condensation and eddy diffusion processes. Aims. This study is devoted to proposing a novel approach to estimating the Venusian cloud Kzz, examining the variability of the cloud Kzz in both equatorial and polar regions, and evaluating the derived Kzz through the implementation of a 1D photochemical model. Methods. The H2SO4 vapor data used in this study were obtained from observations conducted by Venus Express. A novel approach that relies on the premise that both eddy diffusion and condensation regulate the abundance of H2SO4 vapor was then applied to estimate the Venusian cloud Kzz. The global mean Kzz and its latitudinal variation were discussed. A 1D photochemistry-diffusion model was applied to evaluate the estimations. Results. Our calculations indicate that the global mean Kzz reaches 5 × 108 cm2 s−1 in the lower clouds, which is an order of magnitude larger than several observation-based estimations and model results. It rapidly decreases as the altitude increases above 54 km. Equatorial Kzz is three times as large as polar Kzz at 48 km, while polar Kzz reaches its peak below 46.5 km, where equatorial Kzz rapidly decreases as the altitude decreases. Conclusions. We provide an estimate of the Venusian cloud Kzz based on H2SO4 vapor observations. Significant latitudinal variations exist in the Venusian cloud Kzz.

Exploring sensitivity: Unveiling the impact of input parameters on Venus ionosphere [formula omitted] layer characteristics

This study investigates the impact of model input parameters on the characteristic features of the dayside V2 layer in the Venus ionosphere using an in-house developed one-dimensional photochemical model (1D-PCM) and observations from Venus Express radio science experiments (VeRa). 1D-PCM is simulated for different combinations of neutral density models (VTS3, a global empirical model of the Venus Thermosphere, and VenusGRAM, the Venus Global Reference Atmospheric Model), solar flux models (Solar 2000 (S2K), and observations by SEE (Solar EUV Experiment) onboard the NASA’s TIMED, the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics, satellite). The hmV2 (height of the V2 layer plasma density) is found to be better reproduced in VTS3-SEE and VTS3-S2K pairs of combination, indicating that VTS3 is a more representative model for Venusian neutral density. The nmV2 (peak plasma density of the V2 layer) remains consistently higher when using SEE data for solar fluxes, but the model deviates significantly for Solar Zenith Angles (SZA) between 65∘ and 85∘, possibly due to differences in neutral density at high SZA where the temperature is lower than predicted by the neutral density model. Quantitative analysis reveals that errors in the 1D-PCM, with deviations of 1 to 2 km in hmV2 and 5%–10% difference in nmV2 between the model and observations, could be due to uncertainties in the neutral density and solar flux models. Comparison with the Transplanet model suggests that accounting for input uncertainties can improve the model’s ability to reproduce observations. This study provides insights into the influence of input parameters on the Venus ionosphere and highlights the importance of considering uncertainties in modeling studies.

Statistical distribution of mirror-mode-like structures in the magnetosheaths of unmagnetized planets – Part 2: Venus as observed by the Venus Express spacecraft

Abstract. In this series of papers, we present statistical maps of mirror-mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on extreme ultraviolet (EUV) solar flux levels (high and low). The detection of these structures is done through magnetic-field-only criteria, and ambiguous determinations are checked further. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution, facilitating comparisons between planetary and cometary environments. Applied to the magnetometer data of the Venus Express (VEX) spacecraft from May 2006 to November 2014, we detect structures closely resembling MMs lasting in total more than 93 000 s, corresponding to about 0.6 % of VEX's total time spent in Venus's plasma environment. We calculate MM-like occurrences normalized to the spacecraft's residence time during the course of the mission. Detection probabilities are about 10 % at most for any given controlling parameter. In general, MM-like structures appear in two main regions: one behind the shock and the other close to the induced magnetospheric boundary, as expected from theory. For solar maximum, the active region behind the bow shock is further inside the magnetosheath, near the solar minimum bow shock location. The ratios of the observations during solar minimum and maximum are slightly dependent on the depth ΔB/B of the structures; deeper structures are more prevalent at solar maximum. A dependence on solar EUV (F10.7) flux is also present, where at higher F10.7 flux the events occur at higher values than the daily-average value of the flux. The main dependence of the MM-like structures is on the condition of the bow shock: for quasi-perpendicular conditions, the MM occurrence rate is higher than for quasi-parallel conditions. However, when the shock becomes “too perpendicular” the chance of observing MM-like structures reduces again. Combining the plasma data from the Ion Mass Analyser (IMA on board Venus Express) with the magnetometer data shows that the instability criterion for MMs is reduced in the two main regions where the structures are measured, whereas it is still enhanced in the region between these two regions, implying that the generation of MMs is transferring energy from the particles to the field. With the addition of the Electron Spectrometer (ELS on board Venus Express) data, it is possible to show that there is an anti-phase between the magnetic field strength and the density for the MM-like structures. This study is Part 2 of a series of papers on the magnetosheaths of Mars and Venus.

Shocklets and Short Large Amplitude Magnetic Structures (SLAMS) in the High Mach Foreshock of Venus

AbstractShocklets and short large‐amplitude magnetic structures (SLAMS) are steepened magnetic fluctuations commonly found in Earth's upstream foreshock. Here we present Venus Express observations from the 26th of February 2009 establishing their existence in the steady‐state foreshock of Venus, building on a past study which found SLAMS during a substantial disturbance of the induced magnetosphere. The Venusian structures were comparable to those reported near Earth. The 2 Shocklets had magnetic compression ratios of 1.23 and 1.34 with linear polarization in the spacecraft frame. The 3 SLAMS had ratios between 3.22 and 4.03, two of which with elliptical polarization in the spacecraft frame. Statistical analysis suggests SLAMS coincide with unusually high solar wind Alfvén mach‐number at Venus (12.5, this event). Thus, while we establish Shocklets and SLAMS can form in the stable Venusian foreshock, they may be rarer than at Earth. We estimate a lower limit of their occurrence rate of ≳14%.

A Survey of Strong Electric Potential Drops in the Ionosphere of Venus

AbstractEvery planet or moon with an ionosphere is thought to generate a weak electrical potential which helps ions overcome gravity and escape to space. A pilot study at Venus by Collinson et al. (2016, https://doi.org/10.1002/2016GL068327) indicated a planetary potential an order of magnitude stronger than expected. Here we present a statistical study of the electrical potential drop in the ionosphere of Venus, which was found to be an average of 7.04 ± 2.19 V. However, these strong potentials measured by Venus Express are likely atypical and extreme outliers associated with a transient phenomenon in the Venusian ionosphere. We posit they are associated with transient and sporadic density cavities in the ionosphere and may be the result of sporadic electrostatic double layer formation in the dayside ionosphere.

The magnetic field clock angle departure in the Venusian magnetosheath and its response to IMF rotation

We investigate the characteristics of interplanetary magnetic field (IMF) draping in the Venusian magnetosheath using both Venus Express (VEX) observations and magnetohydrodynamics simulations. The distributions of magnetosheath field clock angle illustrate the nearly symmetric morphology of draped magnetic field with respect to the solar wind electric field, and the departure of the IMF clock angle is larger at closer distances. Based on VEX data, the sheath field clock angle departures are found to be <45 degrees for 90% of the instances under steady IMF and this parameter can respond almost immediately to the unsteady IMF. We suggest the magnetosheath field just slips around the planet without significant pileup or bending. Our time-dependent simulations indicate that the response time of sheath field to IMF variation is not more than 1 min and it depends on the involved regions of magnetosheath: the timescale in the inner part of magnetosheath adjacent to the induced magnetosphere is longer than that in the outer part. We find this timescale is controlled by the convection velocity in the magnetosheath, emphasizing the magnetohydrodynamic characteristics of the behavior of the sheath field. The finite magnetosheath field clock angle departure and its quick response to IMF variation suggest that the magnetic field clock angle measured within the Venusian magnetosheath can be used as a reasonable proxy for the upstream IMF clock angle.