Venus Magnetosheath Research Articles

Propagation of nonlinear ion-acoustic fluctuations in the mantle of Venus

Motivated by the observations of ion-acoustic fluctuations with the Parker Solar Probe (PSP) and earlier by the Pioneer Venus Orbiter (PVO) in the Venusian magnetosheath, we investigate the nature of ion-acoustic solitary and double-layer (DL) structures in the mantle. We employed a hydrodynamic description along with reductive perturbation theory to derive the nonlinear Zakharov—Kuznetsov equation that elucidates the dynamics of three-dimensional ion-acoustic wave packets. Using the spacecraft measurements of the plasma configuration at Venus, we carried out a parametric analysis of these structures, including the influence of the magnetic field strength and the relative densities and temperatures, considering two cases: quasi-parallel and oblique propagation. Moreover, we determined the structural characteristics of these waves, where oblique (quasi-parallel) solitary waves have a potential of 0.4 V (0.4 V) and a maximum electric field amplitude Em ~ 0.024 mV m−1 (8 m V m−1) across spatial and temporal widths of ~40–80 km (~140–200 m) and 0.4 s (1.6 ms). These waves produce low-frequency electrostatic activity in the frequency range of 1.6–10 Hz (630–3160 Hz). Quasi-parallel DLs have potential drops of (6.5–13) V and Em ~ (0.16–0.35) mV m−1 with a width and duration of (100–120) m and ~1 ms, and a frequency range of ~630–3980 Hz. These outcomes can explain the detected electrostatic fluctuations above the ionosphere via PVO in the frequency channels of 730 Hz and 5.4 kHz. Furthermore, the DL features estimated in this work are in line with the recent PSP measurements of the DLs propagating in the magnetosheath of Venus.

Electrostatic Solitary Waves in the Venusian Ionosphere Pervaded by the Solar Wind: A Theoretical Perspective

Electrostatic solitary waves (ESWs) in the Venusian ionosphere that are impinged by the solar wind are investigated using a homogeneous, collisionless, and magnetized multicomponent plasma consisting of Venusian H+ and O+ ions, Maxwellian Venusian electrons and streaming solar wind protons, and suprathermal electrons following κ − distribution. The model supports the propagation of positive potential slow O+ and H+ ion-acoustic solitons. The evolution and properties of the solitons occurring in two sectors, viz., dawn-dusk and noon-midnight sector of the Venus ionosphere at an altitude of (200–2000) km, are studied. The theoretical model predicts positive potential solitons with amplitude ∼(0.067–56) mV, width ∼(1.7–53.21) m, and velocity ∼(1.48–8.33) km s−1. The bipolar soliton electric field has amplitude ∼(0.03–27.67) mV m−1 with time duration ∼(0.34–22) ms. These bipolar electric field pulses when Fourier transformed to the frequency domain occur as a broadband electrostatic noise, with frequency varying in the range of ∼9.78 Hz–8.77 kHz. Our results can explain the observed electrostatic waves in the frequency range of 100 Hz–5.4 kHz in the Venus ionosphere by the Pioneer Venus Orbiter mission. The model can also be relevant in explaining the recent observation of ESWs in the Venus magnetosheath by the Solar Orbiter during its first gravity assist maneuver of Venus.

Solar Control of the Pickup Ion Plume in the Dayside Magnetosheath of Venus

AbstractUsing 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.

Kinetic‐Scale Turbulence in the Venusian Magnetosheath

AbstractWhile not specifically designed as a planetary mission, NASA's Parker Solar Probe (PSP) mission uses a series of Venus gravity assists (VGAs) in order to reduce its perihelion distance. These orbital maneuvers provide the opportunity for direct measurements of the Venus plasma environment at high cadence. We present first observations of kinetic scale turbulence in the Venus magnetosheath from the first two VGAs. In VGA1, PSP observed a quasi‐parallel shock, β ∼ 1 magnetosheath plasma, and a kinetic range scaling of k−2.9. VGA2 was characterized by a quasi‐perpendicular shock with β ∼ 10, and a steep k−3.4 spectral scaling. Temperature anisotropy measurements from VGA2 suggest an active mirror mode instability. Significant coherent waves are present in both encounters at sub‐ion and electron scales. Using conditioning techniques to exclude these electromagnetic wave events suggests the presence of developed sub‐ion kinetic turbulence in both magnetosheath encounters.

The correlation length of ULF waves around Venus: VEX observations

A study of the correlation length of ultra-low frequency (ULF) waves around Venus was developed using electron density and magnetic field data obtained from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) and magnetometer (MAG), respectively, on board of the mission Venus Express (VEX). The analysis was conducted using the whole interval of the mission (2006–2014). The correlation scales have been calculated by the correlation length parameter that is a characteristic distance over which fluctuations in a variable are correlated. We limited the study to the frequency range from 8 to 50 mHz because previous studies have shown that ULF waves produced in the foreshock have the highest power in this range. In this study the correlation length was calculated by an exponential fit employed to the auto-correlation curve. The auto-correlation function was calculated lagged by a time between 0 and 60 s and sliding a window with 120s across the data. This analysis has been also extended to correlation length determinations in spatial scale. In order to obtain the correlation length in a spatial domain, the temporal correlation length must be multiplied by the solar wind velocity. Here, the ASPERA-4/IMA (Ion Mass Analyzer) velocity data was used. It was found that the dominant correlation length in temporal scale varies from 9 to 14 s in electron density and between 7.5 and 11 s in the magnetic field. In spatial scale, correlation length varies between 2.8×103 and 5×103 km in electron density data, and between 1.7×103-4X103km for the total magnetic field data value in this frequency window. Fluctuations in the magnetosheath and in the MPB may be correlated with fluctuations at the ionosphere, since correlation lengths in those regions are larger than the size of these regions, indicating that local resonant effect of wave trains at the ionopause may enhance the atmospheric ion escape at Venus. Our results also show that pickup heavy ions can interact with discontinuities in the magnetosheath of Venus and can destroy ULF wave trains during periods of low solar wind pressure. The results obtained here are compared with a similar previous analysis in the Mars environment.

Proton Temperature Anisotropies in the Plasma Environment of Venus

AbstractVelocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T⊥) and parallel (T‖) to the background magnetic field. We calculate average values of T⊥ and T‖ for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T⊥/T‖ > 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near‐subsolar magnetosheath (T⊥/T‖≈3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere.

Mirror mode waves in Venus's magnetosheath: solar minimum vs. solar maximum

Abstract. 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.

Statistical investigation on the power-law behavior of magnetic fluctuations in the Venusian magnetosheath

The present work aims to develop a better understanding of wave and turbulence processes in the planetary magnetosheath region. We study the plasma conditions (temperature, flow velocity, and magnetic field), the low-frequency wave properties, and the energy spectra for magnetic field fluctuations in the Venus magnetosheath. We use the magnetic field data of 101 magnetosheath flank crossings from the Venus Express magnetometer experiment in the years 2006 and 2008. The statistical investigation of the plasma conditions shows that the mean magnetic field amplitude is about 10 nT, the average proton temperature of the order of MK, and the super-Alfvenic, subsonic bulk plasma flow. Below 0.07 Hz, the angle of propagation is about 80° for the most of the cases, and it varies from 10° to 90° above the frequency 0.07 Hz. The compressibility shows similar distribution at low (below 0.07 Hz) and high frequencies (above 0.07 Hz). The energy spectra in the spacecraft frequency frame reveal the power-law behaviors which give physical insight on the energy transfer from larger to smaller scales due to wave–wave interaction. A spectral break (sudden change in slope) is observed at 0.25 Hz, above which the spectral curve becomes steeper with spectral indices between −4 and −1.5 (close to the Kolmogorov slope, −5/3). The low-frequency part (below 0.07 Hz) having a spectral index close to −1 indicates the energy cascades due to mirror mode waves, and the steepen spectra at high frequencies (above 0.07 Hz) with spectral indices between −4 and −0.5 are interpreted as the energy accumulation due to mirror mode and proton cyclotron waves.

Magnetic fluctuations and turbulence in the Venus magnetosheath and wake

Recent research has shown that distinct physical regions in the Venusian induced magnetosphere are recognizable from the variations of strength and of wave/fluctuation activity of the magnetic field. In this paper the statistical properties of magnetic fluctuations are investigated in the Venusian magnetosheath, terminator, and wake regions. The latter two regions were not visited by previous missions. We found 1/f fluctuations in the magnetosheath, large‐scale structures near the terminator and more developed turbulence further downstream in the wake. Location independent short‐tailed non‐Gaussian statistics was observed.

Proton cyclotron waves at Mars and Venus

Proton cyclotron frequency waves are present upstream of the Mars bow shock and have been interpreted as due to the ionization of exospheric hydrogen and the subsequent acceleration of the proton by the solar wind electric field. These waves are left handed polarized on the spacecraft frame and propagate at a small angle to the magnetic field, presumably growing from the free energy of newly accelerated protons. A second consequence of this acceleration is the loss of the planet’s hydrogen exosphere. Proton cyclotron waves have not previously been reported for Venus. Herein, we report the occurrence of such waves in the Venus magnetosheath. However, we do not find clear examples of such waves in the solar wind upstream of the Venus shock. At Mars, the waves are found up to at least 12 planetary radii to the side of the planet. A process that could produce such an extended pattern of newly created ionization and resultant waves is a multi-step one; ionization near the planets, acceleration of ions by the solar wind electric field, reneutralization of ions allowing them to travel across field lines, and a second ionization far from the planet producing the waves.

Neutral atom imaging of solar wind interaction with the Earth and Venus

Observations from the Low‐Energy Neutral Atom (LENA) imager on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) mission have emerged as a promising new tool for studying the solar wind interaction with the terrestrial magnetosphere. Strong LENA emissions are seen from the magnetosheath during magnetic storms, especially during high solar wind dynamic pressure when the magnetopause is strongly compressed and the magnetosheath penetrates deeply into the exosphere. Venus, unlike the Earth, has no intrinsic magnetic field, so the solar wind penetrates deeply and interacts directly with its upper atmosphere. Energy transfer processes enhance atomic escape and thus play a potentially important role in the evolution of the atmosphere. We have performed simulations of the solar wind interaction with both Earth and Venus and compared the results to LENA observations at the Earth. Low‐energy neutral atom emissions from Venus are calculated based on the global MHD model of Tanaka. We found the simulated energetic neutral atom (ENA) emissions from Venus magnetosheath are comparable or greater than for the Earth. The Venus ionopause is clearly seen in the modeled oxygen ENA images. This simulation work demonstrates the feasibility of remotely sensing the Venusian solar wind interaction and resultant atmospheric escape using fast neutral atom imaging.

Interchange instability of the Venusian ionopause

Within the magnetohydrodynamic (MHD) approach, the interchange instability is studied for the subsolar magnetosheath of Venus. The instability analysis considers the profiles of magnetic field and plasma parameters between the bow shock and the ionopause which are obtained from the numerical MHD solution of the solar wind flow around the ionosphere. With the Fourier transformations, the linearized MHD equations are reduced to a second-order differential equation for the total pressure perturbation as a function of the normal distance from the ionopause. This equation is integrated numerically, and the interchange instability growth rate is obtained as a function of the wave number. The instability growth time is found to be smaller than the time scale of magnetic barrier formation.

MHD effects in the venus magnetosheath including mass loading

We use the “magnetic string” approach to integrate the dissipationless MHD equations for a 3-D flow around planetary obstacles, and apply it to some aspects of the flow in the magnetosheath of Venus. Our method has both analytical and numerical components, and is particularly suited to study the structure of the magnetic barrier (depletion layer). Including pick-up processes, we study the contribution to the structure of the Venus magnetosheath made by the pick-up ions. As one main conclusion, we show that the used method leads to a stand-off bow shock position which in good agreement with observations.

MHD effects of the solar wind flow around planets

Abstract. The study of the interaction of the solar wind with magnetized and unmagnetized planets forms a central topic of space research. Focussing on planetary magnetosheaths, we review some major developments in this field. Magnetosheath structures depend crucially on the orientation of the interplanetary magnetic field, the solar wind Alfvén Mach number, the shape of the obstacle (axisymmetric/non-axisymmetric, etc.), the boundary conditions at the magnetopause (low/high magnetic shear), and the degree of thermal anisotropy of the plasma. We illustrate the cases of Earth, Jupiter and Venus. The terrestrial magnetosphere is axisymmetric and has been probed in-situ by many spacecraft. Jupiter's magnetosphere is highly non-axisymmetric. Furthermore, we study magnetohydrodynamic effects in the Venus magnetosheath.

MHD effects in the Venus magnetosheath

Abstract We study features of flow around Venus using a numerical algorithm to integrate the dissipationless magnetohydrodynamic (MHD) equations, which includes from the start magnetic forces on the flow. We show first that, contrary to previous estimates based on gas dynamics, for a reasonable Alfven Mach number the MHD-modelled subsolar magnetosheath is nearly as thick as that obtained from observations. Concentrating next on the region off the subsolar line we present model results for the variation of all plasma and field parameters along paths normal to the ionopause for different values of the solar zenith angle.

Aspects of MHD flow about Venus

We describe the “magnetic string” approach to integrating the dissipationless magnetohydrodynamic (MHD) equations for flow around planetary obstacles and apply it to some aspects of the flow in the magnetosheath of Venus. Our method has both analytical and numerical components and is particularly suited to study the structure of the magnetic barrier (depletion layer). We do not include ion pickup processes and thus discuss only the contribution to the structure of the Venus magnetosheath made by the flow of the shocked solar wind. We work with an interplanetary magnetic field which is directed orthogonal to the solar wind bulk velocity. Magnetic forces on the flow are strongly dependent on the Alfvén Mach number upstream of the bow shock, and one aim of this work is to study the dependence of field and flow quantities in the Venus magnetosheath on this parameter, thus allowing further future comparisons with data under a variety of interplanetary conditions. A second aim is to compare our MHD model results to a synopsis of observations made by the Pioneer Venus Orbiter. As one main conclusion, we show that this method leads, in principle, to a standoff bow shock position in good agreement with observations. We find, namely, that for a low but reasonable Alfvén Mach number, our MHD‐modeled magnetosheath is only ∼ 3.6% thinner in the Sun‐Venus direction than that given by observations. Our method is complementary to three‐dimensional, global MHD simulations of the solar wind‐Venus interaction and offers versatility to modeling other aspects of the complicated interaction of the solar wind with Venus.

The inner magnetosheath of Venus: An analogue for Earth?

The unmagnetized planets provide examples of solar wind interactions that are free from the complications associated with magnetopause reconnection and with sensitive obstacle response to incident solar wind pressure changes. Using the Venus magnetosheath as a testbed, we search for evidence of standing slow mode “transitions” in the inner subsolar region as reported for Earth by Song et al. [1990a; 1992a; b]. Although the system at Venus is much smaller in scale, the Pioneer Venus Orbiter magnetometer data indicate that for perpendicular interplanetary magnetic field conditions the general behavior of the plasma in the magnetosheath is as expected from the simple depletion layer model. In examples of magnetic field measurements chosen for the apparently steady interplanetary conditions during the spacecraft pass, there is no clear evidence for a slow mode structure near the ionopause as might be expected on the basis of the Song et al. study. The implication is that some aspect of the Venus magnetosheath (such as its small size or the presence of local planetary ion production) makes it physically different from Earth's, that the conditions of the magnetosheath during Song's study differed significantly from those in the Venus study, or that the observations of Song et al. do not represent a steady state.

The protonopause — An ion composition boundary in the magnetosheath of comets, Venus and Mars

Abstract The interaction of the solar wind with heavy ions continuously produced by ionization from a background neutral gas cloud is studied in the framework of a collisionless 2D bi-ion fluid model in which the motion of both ion species is coupled exclusively by electromagnetic forces. It is shown that a magnetosheath plasma boundary is formed which has signatures of a bi-ion double layer separating the plasma mixture of protons and heavies from the purely heavy ion plasma. At the self generated potential jump the solar wind protons are stopped and deflected whereas the heavy ions have enough energy to penetrate it. This discontinuity, called here protonopause, is formed in a region where the heavy ion density becomes comparable to the solar wind proton density. We suggest that the protonopause reflects main features of the hitherto unexplained plasma boundaries at comets (cometopause, pile-up boundary), Venus (rarefaction wave, intermediate transition) and Mars (planetopause, mass-loading boundary).

On the spatial range of validity of the gas dynamic model in the magnetosheath of Venus

In the past the global solar wind interaction with Venus has been treated principally with gas dynamic models. While this gas dynamic treatment has proven successful in modeling some of the global characteristics of the interaction, this model does not include the magnetic barrier in a self‐consistent manner. This magnetic barrier is formed in the inner magnetosheath where it transfers solar wind momentum flux to the obstacle via magnetic pressure. In this study we examine the extent to which the gas dynamic fluid approximation describes the magnetic field in the dayside Venus magnetosheath by comparing with two gas dynamic models, one which matches the observed ionopause location and one which matches the bow shock location. We find that each model predicts the field profile reasonably well in the vicinity of the matched bow shock or ionopause but neither model provides an adequate model over the entire range from the ionopause to the bow shock.

The magnetosheath and magnetotail of Venus

We describe the observational history and assess the current understanding of the magnetosheath and magnetotail of Venus, stressing recent developments. We make recommendations for research that can be done using existing observations, as well as desirable trajectory and instrumentation characteristics for future spacecraft missions.