Plasma Environment Of Venus Research Articles

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.

Proton Temperature Anisotropies in the Venus Plasma Environment During Solar Minimum and Maximum

AbstractThe proton population in Venus' plasma environment is characterized during periods of solar minimum and maximum using data from a particle mass‐energy spectrometer. Such characterizations at different levels of solar activity provides physical insight into solar‐cycle‐dependent plasma phenomena around the planet, for example mirror modes in the magnetosheath. Statistical distributions of proton bulk speeds and temperatures are generated using a previously developed method which applies Maxwellian fits to measurements of the protons' velocity distribution function. Spatial maps and probability‐density histograms comparing the proton parameters between the two time periods are presented. The temperatures perpendicular (T⊥) and parallel (T∥) to the background magnetic field are found to be 20%–35% lower during solar maximum. Though the overall distributions of the temperature ratio T⊥/T∥ do not change, the regions with higher anisotropy (T⊥/T∥ > 1) are found farther downstream from the bow shock during solar maximum than minimum. This is consistent with the previously observed growth of mirror modes during solar maximum and their decay during minimum.

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.

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.

The Response of the Venusian Plasma Environment to the Passage of an ICME: Hybrid Simulation Results and Venus Express Observations

AbstractOwing to the heritage of previous missions such as the Pioneer Venus Orbiter and Venus Express, the typical global plasma environment of Venus is relatively well understood. On the other hand, this is not true for more extreme driving conditions such as during passages of interplanetary coronal mass ejections (ICMEs). One of the outstanding questions is how do ICMEs, either the ejecta or sheath portions, impact (1) the Venusian magnetic topology and (2) escape rates of planetary ions? One of the main issues encountered when addressing these problems is the difficulty of inferring global dynamics from single spacecraft obits; this is where the benefits of simulations become apparent. In the present study, we present a detailed case study of an ICME interaction with Venus on 5 November 2011 in which the magnetic barrier reached over 250 nT. We use both Venus Express observations and hybrid simulation runs to study the impact on the field draping pattern and the escape rates of planetary O+ ions. The simulation showed that the magnetic field line draping pattern around Venus during the ICME is similar to that during typical solar wind conditions and that O+ ion escape rates are increased by approximately 30% due to the ICME. Moreover, the atypically large magnetic barrier appears to manifest from a number of factors such as the flux pileup, dayside compression, and the driving time from the ICME ejecta.

Statistical features of the global polarity reversal of the Venusian induced magnetosphere in response to the polarity change in interplanetary magnetic field

AbstractIn this study we present the first statistical analysis on the effects of heliospheric current sheet crossings on the induced magnetosphere of Venus. These events are of particular interest because they lead to the reconfiguration of the induced magnetosphere with opposite polarity. We use a statistical approach based on 117 orbit pairs, and we study the spatial distribution of the heavy ion flux measurements in the plasma environment of Venus. The average and median heavy ion flux measurements are compared before and after the polarity reversal events. The results show that after the events the average and median heavy ion fluxes in the magnetotail are reduced by the factors of 0.75 ± 0.09 and 0.52, respectively. We find that even if a passage of a current sheet is a short time scale event lasting about 10 min, its effect on the near‐Venus plasma environment lasts for a few hours. We conclude that the observations show similarities to the previous comet studies and the polarity reversal of the induced magnetosphere might be accompanied with dayside reconnection and magnetic disconnection of the plasma tail from the planetary ionosphere.

The morphology of the topside ionosphere of Mars under different solar wind conditions: Results of a multi-instrument observing campaign by Mars Express in 2010

Since the internally-generated magnetic field of Mars is weak, strong coupling is expected between the solar wind, planetary magnetosphere, and planetary ionosphere. However, few previous observational studies of this coupling incorporated data that extended from the solar wind to deep into the ionosphere. Here we use solar wind, magnetosphere, and ionosphere data obtained by the Mars Express spacecraft during March/April 2010 to investigate this coupling. We focus on three case studies, each centered on a pair of ionospheric electron density profiles measured by radio occultations, where the two profiles in each pair were obtained from the same location at an interval of only a few days. We find that high dynamic pressures in the solar wind are associated with compression of the magnetosphere, heating of the magnetosheath, reduction in the vertical extent of the ionosphere, and abrupt changes in electron density at the top of the ionosphere. The first three of these associations are analogous to the behavior of the plasma environment of Venus, but the final one is not. These results reinforce the notion that changes in solar forcing influence the behaviors of all of the tightly coupled regions within the Martian plasma environment.

Hemispheric asymmetries of the Venus plasma environment

We study the Venus‐solar wind interaction and the hemispheric asymmetries of the Venus plasma environment in the global HYB‐Venus hybrid simulation. We concentrate especially on the role of the flow‐aligned interplanetary magnetic field (IMF) component (i.e., the Parker spiral angle or the IMF cone angle) and analyze the dawn‐dusk and Esw asymmetries between four magnetic quadrants around Venus. Using the simulation model, we study two upstream condition cases in detail: the perpendicular IMF to the solar wind flow case and the nominal Parker spiral case (dominant flow‐aligned IMF component). Several differences and similarities were found in these two simulation runs. Common features of the Venus plasma environment between the two cases include asymmetric magnetic barrier and tail lobes and asymmetric planetary ion escape in the direction of the solar wind convection electric field. Further, protons of planetary origin and of solar wind origin were found to follow similar velocity patterns in the Venus plasma wake in both cases. The differences when the IMF flow‐aligned component is dominating compared to the perpendicular IMF case, the so‐called (magnetic) dawn‐dusk asymmetries, include the parallel bow shock and the foreshock region, the asymmetric magnetic barrier, the asymmetric tail current system, and the asymmetric central tail current sheet. Further, the escaping planetary H+ and O+ ion fluxes are concentrated more on the hemisphere of the parallel bow shock. When interpreting in situ plasma and magnetic observations from Venus, the features of at least these two basic IMF configurations should be considered.

The Plasma Environment of Venus, Mars, and Titan, an Introduction

In December 2009 the EUROPLANET Research Infrastructure, a project funded by the European Community under the Capacities programme of the Seventh Framework Programme (grant agreement number 228319) and the International Space Science Institute, organized a workshop in Bern to overview and summarize our current knowledge on the plasma environments of Venus, Mars, and Titan. This was the first workshop organized in the series of four planned in the networking activity NA-2 of Europlanet RI. Emphasis was on new results, since many excellent reviews were published recently. The comparison of the plasma environment of these three solar system objects was a primary focus of the workshop, which also aimed at defining directions for future research, through further use of existing data or development of new space missions. In this introductory paper, we summarize the main results of this workshop, via an overview of the different chapters it contains. Unlike the Earth, our neighbours Venus and Mars have no global magnetic fields, although Mars has small-scale crustal fields; and Titan, to our best knowledge, is also a nonmagnetic body. These three objects are the only non-magnetic solar system bodies having a dense atmosphere. Venus and Mars have the supersonic and super-Alfvenic solar wind as their plasma environment. Titan, in contrast, is located inside Saturn’s hot, rapidly rotating, subsonic, multicomponent magnetospheric flow (although it occasionally makes excursions into the magnetosheath) and its environment is highly variable in time. Accordingly,

Ion flow and momentum transfer in the Venus plasma environment

An analysis of ion data from 390 Venus Express, VEX, orbits demonstrates that the flow of solar wind- and ionospheric ions near Venus is characterized by a marked asymmetry. The flow asymmetry of solar wind H + and ionospheric O + points steadily in the opposite direction to the planet’s orbital motion, and is most pronounced near the Pole and in the tail/nightside region. The flow asymmetry is consistent with aberration forcing, here defined as lateral forcing induced by the planet’s orbital motion. In addition to solar wind forcing by the radial solar wind expansion, Venus is also subject a lateral/aberration forcing induced by the planet’s orbital motion transverse to the solar wind flow. The ionospheric response to lateral solar wind forcing is analyzed from altitude profiles of the ion density, ion velocity and ion mass-flux. The close connection between decreasing solar wind H + mass-flux and increasing ionospheric O + mass-flux, is suggestive of a direct/local solar wind energy and momentum transfer to ionospheric plasma. The bulk O + ion flow is accelerated to velocities less than 10 km/s inside the dayside/flank Ionopause, and up to 6000 km in the tail. Consequently, the bulk O + outflow does not escape, but remains near Venus as a fast (km/s) O + zonal wind in the Venus polar and nightside upper ionosphere. Furthermore, the total O + mass-flux in the Venus induced magnetosphere, increases steadily downward to a maximum of 2 × 10 −14 kg/(m 2 s) at ≈400 km altitude, suggesting a downward transport of energy and momentum. The O +, and total mass-flux, decay rapidly below 400 km. With no other plasma mass-flux as replacement, we argue that the reduction of ion mass-flux is caused by ion-neutral drag, a transfer of ion energy and momentum to neutrals, implying that the O + plasma wind is converted to a neutral (thermosphere) wind at Venus. Incidentally, such a neutral wind would go in the same direction as the Venus atmosphere superrotation.

Correction to “Plasma environment of Venus: Comparison of Venus Express ASPERA‐4 measurements with 3‐D hybrid simulations”

[1] In the paper “Plasma environment of Venus: Comparison of Venus Express ASPERA-4 measurements with 3-D hybrid simulations” by C. Martinecz et al. (Journal of Geophysical Research, 114, E00B30, doi:10.1029/2008JE003174, 2009), 9-11–12 were numbered incorrectly. The correct numbered figures and their captions are shown here.

Plasma environment of Venus: Comparison of Venus Express ASPERA‐4 measurements with 3‐D hybrid simulations

We use data of the ASPERA‐4 ion and electron spectrometers onboard Venus Express to determine the locations and shapes of the plasma boundaries (bow shock, ion composition boundary, and mantle) at Venus. We also investigate the variation of the terminator bow shock position as a function of the solar wind dynamic pressure and solar EUV flux. We compare the results with a 3‐D hybrid simulation. In the hybrid model, ions are treated as individual particles moving in self‐consistently generated electromagnetic fields and electrons are modeled as a massless charge neutralizing fluid. The planetary heavy ion plasma is generated by an oxygen ionosphere and exosphere adapted to a profile, which depends on the solar zenith angle (Chapman layer). A comparison between observations and simulations indicates that the hybrid model is able to produce an adequate picture of the global plasma environment at Venus. The positions of the plasma boundaries are well reproduced by the model but a significant disagreement appears in the absolute values of the considered parameters.

Magnetic field investigation of the Venus plasma environment: Expected new results from Venus Express

The Venus Express mission is scheduled for launch in 2005. Among many other instruments, it carries a magnetometer to investigate the Venus plasma environment. Although Venus has no intrinsic magnetic moment, magnetic field measurements are essential in studying the solar wind interaction with Venus. Our current understanding of the solar wind interaction with Venus is mainly from the long lasting Pioneer Venus Orbiter (PVO) observations. In this paper, we briefly describe the magnetic field experiment of the Venus Express mission. We compare Venus Express mission with PVO mission with respect to the solar wind interaction with Venus. Then we discuss what we will achieve with the upcoming Venus Express mission.

Steady‐state plasma transition in the Venus ionosheath

The results of an extended analysis of the plasma and electric field data of the Pioneer Venus Orbiter (PVO) are presented. We report the persistent presence of a plasma transition embedded in the flanks of the Venus ionosheath between the bow shock and the ionopause. This transition is identified by the repeated presence of characteristic bursts in the 30 kHz channel of the electric field detector of the PVO. The observed electric field signals coincide with the onset of different plasma conditions in the inner ionosheath where more rarified plasma fluxes are measured. The repeated identification of this intermediate ionosheath transition in the PVO data indicates that it is present as a steady state feature of the Venus plasma environment. The distribution of PVO orbits in which the transition is observed suggests that it is more favourably detected in the vicinity of and downstream from the terminator.

The Pioneer Venus Orbiter Plasma Analyzer Experiment

The plasma analyzer experiment on the Pioneer Venus Orbiter was designed to determine the basic characteristics of the plasma environment of Venus and the nature of the solar wind interaction at Venus. The plasma analyzer experiment is an electrostatic energy-per-unit charge (E/Q) spectrometer which measures ions and electrons. There is a curved plate electrostatic analyzer system with multiple collectors. The experiment obtains the three dimensional plasma distribution function. Some of the scientific objectives of the instrument are briefly discussed, the general characteristics of the experiment are summarized, and some of the analyses based on the data are presented.