Venus Wake Research Articles

Measurement of plasma channels in the Venus wake

Plasma channels have been suggested to account for the observation of ionospheric holes in the Venus nightside ionosphere. They are produced by the erosion of the solar wind on the magnetic polar regions of the Venus ionosphere, and are related to a sharp plasma transition that extends along the flanks of the wake and that is located downstream from the bow shock. The sharp plasma (intermediate) transition is a feature that has been detected with instruments onboard all the main spacecraft that have probed the Venus wake. From the early flyby transit of the Mariner 5 spacecraft to the later orbit crossings of the Venera 10, the Pioneer Venus and more recently the Venus Express there has been accumulated evidence of the presence of that transition by the flanks of the Venus wake. We report here measurements made with the ASPERA-4 instrument of the Venus Express that were conducted in orbit crossings by the midnight plane over a 4 year period (between 2006 and 2009), and that show different positions of the intermediate transition in the Venus wake at locations closer to the sun-Venus axis near solar minimum conditions by 2009. Measurements conducted in this latter time period also indicate that the intermediate transition by the midnight plane derives from the plasma channels and that is replaced by a broad velocity layer in orbits that probed farther away from that plane. It is argued that changes in the position of the intermediate transition with respect to the plasma channels are related to processes produced by the transport of solar wind momentum to the Venus polar ionosphere. The energy spectra of planetary ions measured in VEX orbits that probed by the midnight plane show momentum transport from the solar wind protons. This result is contrary to the conclusions reported by Collinson et al. (2014) in the sense that there is ¨no difference in the peak energy of the ionospheric outflow¨ when measured inside and outside an ionospheric hole.

The Venus–solar wind interaction: Is it purely ionospheric?

The Venus solar wind interaction is often regarded as the prototypical example of an induced magnetosphere. Pioneer Venus Orbiter (PVO) observations during a period of moderate to strong solar EUV fluxes led to a fairly detailed picture in which the currents in the conducting ionosphere produce a nearly impenetrable obstacle to the incident magnetized plasma flow, resulting in a classical draped field magnetosheath region and a comet-like magnetotail. Inspired by the availability of Venus Express (VEX) observations from the north polar region, and their sometimes unexpected behavior, we reanalyzed the observed Venus wake magnetic fields in the altitude range ~150 to ~450km to determine whether some signature of a weak planetary field could have been missed. Our results suggest the presence of a small (few nT) but persistent radial field direction bias in the deep nightside, low to mid-latitude range sampled on PVO. The bias has a hemispheric dependence, with the more positive (outward) fields in the south and the more negative (inward) fields in the north. However the VEX counterpart of these data, obtained just nightward of the north polar terminator, shows no significant bias. This observation raises several questions about our understanding of the fields at the surface of Venus. We investigate whether the PVO radial field bias could be the subtle signature of a weak global dipole with , higher by ~10× than the previously established upper limits. A weak dipole solar wind interaction model produces results in the center of the low altitude wake that compare favorably with the observed field bias seen by PVO; however, the lack of agreement with the higher latitude and VEX observations suggests other explanations need to be considered. For example, effects related to previously observed convection electric field-controlled hemispheric asymmetries provide a possible alternative, as are external fields that diffuse into and through the interior. This work points out the need for better understanding the features introduced by species-dependent plasma processes, and the role of the planet itself, in deciphering weakly magnetized planet interactions.

The impact of a slow interplanetary coronal mass ejection on Venus

AbstractWe present Venus Express observations of the impact of a slow interplanetary coronal mass ejection (ICME), which struck Venus on 23 December 2006, creating unusual quasi steady state upstream conditions for the 2 h close to periapsis: an enhanced (∼ nT) interplanetary magnetic field (IMF), radially aligned with the Sun‐Venus line; and a dense (∼ cm−3) solar wind. Contrary to our current understanding and expectations, the ionosphere became partially demagnetized. We also find evidence for shocked sheathlike solar wind protons and electrons in the wake of Venus, and powerful (≈ nT2/Hz) foreshock whistler mode waves radiating from the bow shock at an unexpectedly low frequency (0.6 Hz). Given the abnormally high density of escaping heavy ions at the magnetopause boundary (295 cm−3, one of the highest of the whole mission) and the enhanced density of escaping heavy ions in the wake, we find that even weak ICMEs with no driving shocks can increase atmospheric loss rates at Venus and suggests that the Bx component of the IMF may be a factor in atmospheric escape rates.

Solar wind‐driven plasma fluxes from the Venus ionosphere

AbstractMeasurements conducted with the ASPERA‐4 instrument and the magnetometer of the Venus Express spacecraft show that the kinetic pressure of planetary O+ ion fluxes measured in the Venus wake can be significantly larger than the local magnetic pressure, and as a result, those ions are not solely being driven by magnetic forces but also by the kinetic energy of the solar wind. Beams of planetary O+ ions with those properties have been detected in several orbits of the Venus Express through the Venus wake as the spacecraft traverses by the noon‐midnight plane along its near‐polar trajectory. The momentum flux of the O+ ions leads to superalfvenic flow conditions. It is suggested that such O+ ion beams are produced in the vicinity of the magnetic polar regions of the Venus ionosphere where the solar wind erodes the local plasma leading to plasma channels that extend downstream from those regions. The distribution of the number of cases where superalfvenic and subalfvenic conditions are measured along the Venus Express trajectory leads to dominant values when the total kinetic plasma pressure (including that of the solar wind protons) and the magnetic pressure are comparable, thus suggesting a possible equipartition of energy between the plasma and the magnetic field.

Widely different characteristics of oxygen and hydrogen ion escape from Venus

We study the solar wind induced escape of O+ and H+ ions from Venus' atmosphere in the HYB‐Venus hybrid simulation. Most of the previous Venus global plasma modelling studies have concentrated only on the O+ escape. According to the hybrid simulation, planetary O+ and H+ ions behave very differently from each other in the Venusian induced magnetosphere. Both species are asymmetrically distributed in the direction of the interplanetary electric field and in the dawn‐dusk plane. The H+ flow can be understood by E × B drift motion but finite Larmor radius (FLR) effects are essential to the behavior of O+ ions. These differences result in different H+/O+ escape ratios globally and in the plasma wake. Further, the energy ratio of the escaping planetary ions was found to be consistent with the observations made in the near Venus wake by the ASPERA‐4 instrument onboard the Venus Express spacecraft.

Solar wind erosion of the Venus polar ionosphere

The geometry of the Venus nightside ionosphere is reexamined on the basis of plasma and magnetic field data obtained with the Pioneer Venus Orbiter (PVO) and the Mariner 5 spacecraft. There is evidence indicating that the shocked solar wind that streams over the magnetic polar regions of the Venus ionosphere experiences a sharp expansion and also a strong deceleration to speeds much smaller than those present in the outer ionosheath. It is argued that the deficiency of solar wind momentum that is inferred from the PVO and Mariner 5 measurements along the flanks of the Venus ionosheath may be responsible for the erosion of the polar upper ionosphere in plasma channels that extend downstream from the magnetic polar regions. The erosion of the Venus ionosphere is mostly produced within those channels and represents the main source of material delivered to the Venus wake. The configuration that is proposed accounts for the PVO observation of plasma clouds and ionospheric holes in the nightside hemisphere and leads to a mass loss smaller than that expected from a global erosion. The formation and evolution of the ionospheric polar channels should also be important in the solar wind interaction and erosion of the Mars ionosphere.

Rarefied plasma fluxes within the polar expansion fan of the Venus ionosheath

Abstract Measurements obtained with the PVO plasma instrument show strong changes in the particle flux intensity at the intermediate transition of the Venus ionosheath. In particular, the strong plasma fluxes present in the outer ionosheath sharply decrease to low values across this transition and are not observed in the inner ionosheath. These conditions are most frequently seen near the regions where the draped interplanetary magnetic field lines slip over the planet to enter the wake (magnetic polar regions) and imply the presence of an expansion fan that extends in the downstream direction of the solar wind. Low magnetic field intensities are also observed within the expansion fan with a sharp change at the intermediate transition. Similar characteristics can be obtained from data obtained during the Mariner 5 flyby across the Venus wake. In this case there is evidence of a strong decrease of the solar wind flow speed across the intermediate transition downstream from the terminator. The magnetic field data obtained in that pass further indicates that this event was detected as the spacecraft moved behind a magnetic polar region and that as a result the solar wind becomes strongly decelerated over those regions. Thus, in addition to having low densities and magnetic field intensities the plasma within the expansion fan decelerates and exhibits weak particle fluxes.

Venus tail ray observation near Earth

In June, 1996, Venus passed through a very close inferior conjunction with the Sun. At that time the CTOF detector of the CELIAS mass spectrometer experiment on the SOHO spacecraft near Earth's L1 Lagrangian point was measuring heavy ions in the solar wind ∼4.5 × 107 km downstream of Venus. Close to the time predicted by simple geometric arguments for passage of SOHO through the Venus wake, CTOF made three encounters with unusual fluxes of O+ and C+ ions. Their energy distributions resembled those of tail rays originating in the Venus ionosphere or ionopause region rather than of ions produced in the corona of neutral atoms that surrounds the planet. The C+ abundance was ≈ 10% of O+. The observed O+ speed was very close to the simultaneous solar wind speed and the O+ temperature was a cool 5600 K/amu. The flux densities for the three events were (2.4–4.4) × 10³ cm−2s−1.

Polar expansion fan in the Venus ionosheath

Magnetic field data of the Pioneer Venus Orbiter (PVO) are used to examine the configuration of the intermediate transition around the Venus wake. The intermediate transition is a plasma boundary characterized by a sudden decrease of the magnetic field intensity to low values in the inner ionosheath and a strong rotation of the magnetic field orientation to a direction closer to the Sun‐Venus axis. Measurements show that the intermediate transition is not seen across the entire Venus wake but in an expansion fan that is formed around the solar wind‐induced magnetic polar regions of the Venus ionosheath. The magnetic field geometry within the expansion fan is produced by ionosheath magnetic field lines that are draped around those regions and that are then converted by the shocked solar wind. Examples are shown where the intermediate transition is seen at positions different from those of ionospheric plasma clouds. Across both features the magnetic field may also exhibit a different rotation including changes in the polarity of its component along the Sun‐Venus axis. A schematic of the magnetic field geometry within and in the vicinity of the expansion fan is presented. It is argued that the expansion fan flares away from the Venus tail and that the local orientation of the magnetic field is different from that observed around the Venus dayside ionosphere.

Role of the Mode Theory on Lightning Generated Waves Propagating in Venus-Ionosphere Waveguide.

The wave mode either electromagnetic or electrostatic of the OEFD measured signals is unknown and needs to be ascertained by indirect means. Considering these waves to be generated by global Venus lightning, the wave attenuation along the Venus-ionosphere wave-guide has been computed. The effective wave attenuation versus frequency shows a “knee” characterizing minimum attenuation 6 kHz. This “knee” is consistent with the amplitude spectral peak of OEFD signals around this frequency. This feature of the OEFD signals implies that the Venus lightning may have global occurrence. The lightning generated signals may travel considerable distance within the Venus-ionosphere waveguide. The encounter of these signals with plasma “holes” or ion “troughs” in the nightside of Venus wake region results in the leakage of the signals out of the shielding ionosphere. These signals are occasionally recorded by the OEFD. The occurrence rate statistics shows that the leakage of 5.4 kHz signals through the plasma “holes” in the vicinity of PVO periapsis are most effective. The maximum occurrence rate of higher frequency signals change in altitude slightly from one PVO season to another.

The Venus transterminator ion flux at solar maximum

The median Venus transterminator ion flux nv(h) derived from Pioneer Venus retarding potential analyzer measurements of the ion velocity and ion density during a period of solar maximum is presented as a function of altitude h from 150 km to the solar maximum median ionopause altitude of 800 km. This flux is integrated with altitude to provide the integrated total ion flux S(h) crossing the terminator plane as a function of altitude. S(h) is nearly a linear function of h, reflecting the nearly constant value of nv with altitude. The value of S at 800 km altitude, called F, is 5×1026 s−1, a value equal to the total nightside hemispheric molecular ion recombination rate L calculated previously. The loss of ions down the wake of Venus is smaller than F or L by at least a factor of 5. Thus the equality of F and L is evidence that the dominant Venus nightside ionization source at solar maximum is transterminator ion transport.

The two‐tier structure of comet Giacobini‐Zinner's magnetic tail

The structure of comet Giacobini Zinner's magnetic tail is interpreted in terms of a flow geometry similar to that proposed for the Venus wake. It is suggested that the two‐step shape of the magnetic field profile obtained from the ICE measurements across each lobe of the magnetic tail of that comet is due to the different entry of interplanetary magnetic fluxes into that region. The low (∼30 γ) intensity outer wings of the magnetic field profile result from magnetic fluxes convected by the mass‐loaded shocked solar wind that enters the tail from the polar boundary of the comet's magnetic cavity. The high (∼60 γ) intensity inner parts of the lobes result, on the other hand, from the compression that this flow produces on interplanetary magnetic field lines that have slipped past the polar boundary of the cavity after draping around the comet. The latter magnetic fluxes are not convected by the local plasma but forced to accumulate around the central plasma tail that extends behind the comet.

Results of the first statistical study of Pioneer Venus Orbiter plasma observations in the distant Venus tail: Evidence for a hemispheric asymmetry in the pickup of ionospheric ions

Pioneer Venus Orbiter plasma and magnetometer observations from the first nine tail seasons of crossings of the Venus wake are used to study ion pickup in the far wake of an unmagnetized object embedded in the solar wind. This first statistical study treats all of the plasma spectra containing pickup ions in the vicinity of the Venus tail. We find a hemispheric asymmetry in the pickup of ionospheric ions, with approximately four times more O+ events observed in the “northern” magnetic hemisphere (where Z″ > O), i.e., the induced electric field points outward, (away from the ionopause boundary) than in the “southern” (Z″ < O) magnetic hemisphere. Out of a total of 167 large O+ events, 125, or 75%, occurred in the northern hemisphere when position is calculated in terms of Venus radii and 129 or 77% occurred in the “northern” hemisphere when position is expressed in gyroradii. This hemispheric asymmetry in ion pickup is consistent with the prediction of the Cloutier et al. (1974) mass loading model for Venusian ions above the ionopause boundary.

Average dimension and magnetic structure of the distant Venus magnetotail

We report the first major statistical investigation of the far wake of an unmagnetized object embedded in the solar wind. The investigation is based on Pioneer Venus Orbiter magnetometer data from 70 crossings of the Venus wake at altitudes between 5 and 11 Venus radii during reasonably steady IMF conditions. We find that Venus has a well‐developed tail, flaring with altitude and possibly broader in the direction parallel to the IMF cross‐flow component. Tail lobe field polarities and the direction of the cross‐tail field are consistent with tail accretion from the solar wind. Average values for the cross‐tail field (2 nT) and the distant tail flux (3 MWb) indicate that most distant tail field lines close across the center of the tail and are not rooted in the Venus ionosphere. We illustrate our findings in a three‐dimensional schematic.

Magnetic field in the wake of Venus and the formation of ionospheric holes

Magnetic field structures are analyzed for both the ionospheric hole region and the magnetosheath‐ionosphere interaction region of the nightside of Venus, in search of possible coupling between these two regimes. A magnetic coordinate system based on the directions of the solar wind and the interplanetary magnetic field (IMF) is found to order the data reasonably well, allowing consistent superposition of observational data from individual passes of the Pioneer Venus orbiter. The main findings are (1) ionospheric holes form in a zone of ±45° magnetic latitude covering possibly the entire width of the Venus wake in the longitudinal direction; (2) the magnetic field within the holes is strong and ordered, and its direction is determined by the IMF direction, being consistent with the field line draping concept; (3) the magnetic field direction in the holes, particularly at low altitudes near the terminator, shows evidence for the existence of strong plasma flow toward the antisolar meridian in the direction parallel to the magnetic equatorial plane; and (4) the average magnetic field distribution in the nightside magnetosheath also evidences strong plasma flow toward the antisolar meridian. Together these results strongly indicate that the magnetosheath plasma flow in the wake region plays an important role in forming the ionospheric holes, through deformation of the nightside ionopause. These results are combined in a model of the three‐dimensional magnetic field structure around the ionosphere of Venus.

Plasma and electric field measurements of the PVO in the Venus ionosheath

A study of the plasma and electric field measurements of the PVO in the Venus ionosheath near the terminator region is presented. A drastic decrease of the particle flux intensity measured with the plasma instrument is encountered in the inner ionosheath before the inbound crossing of the ionopause. The outer boundary of the region of weak plasma fluxes is consistent with the presence of a rarefaction wave as reported previously from the Mariner 5, Venera and PVO plasma measurements in the Venus wake. Near that boundary there are, in addition, appreciable changes in the electric field signals detected with the PVO electric field detector. These observations suggest the existence of local plasma wave activity associated with turbulent flow conditions in that region.

Wave‐particle interactions in the Venus wake and tail

We present the first analysis of Pioneer Venus Orbiter plasma, electric field, and magnetic field observations in the Venus tail. We have studied the first season of Pioneer Venus Orbiter tail passage (approximately June 1979) in order to determine the main plasma and field configuration in this region at this time and to ascertain some of the basic physical processes. Our analysis shows that the boundary of the Venusian tail is often characterized by changing plasma distributions and enhanced plasma wave activity. We use summary plasma probe parameters to argue that the waves are Doppler‐shifted ion acoustic oscillations. In the magnetotail region there is generally an exclusion of high‐density plasma, but when plasma is detected, the distributions often appear to be non‐Maxwellian at the highest time resolution, and these distorted distribution functions are generally accompanied by enhanced plasma wave signals and magnetic field reversals indicative of electric currents. On the basis of our analyses of the high‐resolution plasma and wave observations for orbit 189, we identify a different time from that previously defined on the basis of the magnetic field data for the spacecraft entrance into the magnetotail. The wave activity in the Venus tail appears similar to the broadband noise identified in the earth's tail, but at Venus the levels are usually higher.

Plasma distribution and magnetic field orientation in the Venus near wake: Solar wind control of the nightside ionopause

A comparative study of the position of the Venus nightside ionopause and of the orientation of the magnetic field in the near Venus wake is presented. It is shown that when the ionopause is detected deep within the umbra the magnetic field vector above that boundary shows consistently a convergent orientation toward the sun‐Venus axis. When the ionopause is encountered outside the umbra, the magnetic field may assume directions not necessarily indicative of closure. The overall configuration of the nightside ionopause seems, in addition, to be strongly controlled by the kinetic energy density of the solar wind. At high values of this quantity the ionopause tends to occur more often at low heights.

Pioneer Venus observations of plasma and field structure in the near wake of Venus

Ionospheric plasma density depletions or ‘holes’ are observed by the Pioneer Venus orbiter in association with radial magnetic fields in the near wake of Venus. This report presents examples of the collected observations of these unexpected features of the Venus nightside ionosphere obtained by the Langmuir probe, magnetometer, ion mass spectrometer, retarding potential analyzer, plasma analyzer, and electric field experiments. The connection between plasma density depletions and temperature changes, changes in ion composition, plasma wave emissions, and magnetic fields with a substantial radial component is illustrated. Mechanisms that may be responsible for the formation and maintenance of holes are suggested.

The distant interplanetary wake of Venus: Plasma observations from Pioneer Venus

In June 1979 the Pioneer Venus orbiter made its first series of passes through the distant solar wind wake of Venus at distances of 8–12 Rv behind the planet. During this period the plasma analyzer aboard the spacecraft detected disturbed magnetosheath plasma that intermittently disappeared and reappeared, suggesting a tattered, filamentary cavity trailing behind the planet. The magnetosheath dropouts almost always occurred inside the region of ‘magnetotail’ observed by Russell et al. Sporadic bursts of energetic ions (E/q ≳ 4 kV) are detected inside and, occasionally, outside the magnetotail; all such bursts are consistent with identification of the ion as O+ of planetary origin moving at the local magnetosheath flow speed. The morphology of the plasma dropouts and of the O+ bursts is analyzed in detail. The cavity appears to contract at times of high solar wind dynamic pressure. The intensity of the O+ component is highly variable, and appears not to be strongly correlated with solar wind dynamic pressure. The most intense bursts correspond to a flux ≲107 ions cm−2 s−1. This maximum flux, if steady and filling a cylinder 1 Rv in radius would correspond to a mass loss rate of ≲1025 ions s−1; the intermittency and variability of the flux suggest that the true mean loss rate is very much lower. The kinetic temperature of the O+ component is estimated as 105–106 K in order of magnitude.