Venus Tail Research Articles

A Revised Understanding of the Structure of the Venusian Magnetotail From a High‐Altitude Intercept With a Tail Ray by Parker Solar Probe

AbstractOne of the major discoveries of NASA's 1979–1991 Pioneer Venus Orbiter is that the nightside ionosphere becomes filamentary at high altitude, forming comet‐like tail rays. Pioneer Venus Orbiter could not establish how much farther into the wake of Venus tail rays extend, nor understand how they form. Here we present plasma and fields data from the fourth flyby of Venus by NASA's Parker Solar Probe consistent with an intercept with an ionospheric tail ray. The observations unambiguously demonstrate that Venusian Ionotail Rays can extend to at least 7,800 km in altitude. Using the new Parker observations we are able to identify a tail ray encounter within the Venus Express dataset. We thus present a unified picture of the structure of the magnetotail of Venus based on combined Venus Express, Pioneer Venus Orbiter, and Parker observations, and recent hybrid modeling.

The extension of ionospheric holes into the tail of Venus

AbstractIonospheric holes are Cytherian nightside phenomena discovered by the NASA Pioneer Venus Orbiter, featuring localized plasma depletions driven by prominent and unexplained enhancements in the draped interplanetary magnetic field. Observed only during solar maximum, the phenomenon remains unexplained, despite their frequent observation during the first 3 years of the mission and more than 30 years having elapsed since their first description in the literature. We present new observations by the European Space Agency Venus Express showing that ionospheric holes can extend much further into the tail than previously anticipated (1.2 to 2.4 planetary radii) and may be observed throughout the solar cycle and over a wide range of solar wind conditions. We find that ionospheric holes are a manifestation of a deeper underlying phenomenon: tubes of enhanced draped interplanetary magnetic field that emerge in pairs from below the ionosphere and stretch far down the tail. We speculate on two possible explanations for the magnetic fields underlying the phenomena: magnetic pileup due to stagnation of ionospheric flow and internal draping around a metallic core.

Plasma in the near Venus tail: Venus Express observations

Although Venus has no global intrinsic magnetic fields, it possesses a long magnetotail of induced origin. The topology of the tail is determined by the interplanetary magnetic field orientation. We present recent plasma and magnetic field observations in the near Venus tail (X≥−3RV) made by the Venus Express spacecraft. We show that ion acceleration in the Venus plasma sheet is produced by the slingshot effect of the draping magnetic field lines, though some features as differential streaming of different ion species point to the existence of other forces. We explain a bell shape of ion spectrograms while the spacecraft crosses the current sheet. The absence of a balance between the lobe magnetic pressure and thermal pressure of plasma in the plasma sheet indicates a dynamic rather than a static equilibrium in the Venus magnetotail. A strong asymmetry of the plasma sheet is controlled by the direction of the motional electric field in the upstream solar wind. In the hemisphere pointed in the direction of the motional electric field, the j×B force accelerates plasma tailward supplying the plasma sheet, while in the opposite hemisphere, the flow pattern occurs less regularly with smaller speeds but higher number densities.

Hemispheric asymmetry of the magnetic field wrapping pattern in the Venusian magnetotail

We examine statistically the magnetic field in the Venusian magnetotail which is formed by the draping of interplanetary magnetic field lines. Although the near‐planet and distant magnetotail regions have been sampled by the various missions to Venus and the general magnetic features of the distant magnetotail are well established, the near wake region from about 1.3 to 3 Venusian radii downstream of the planet remained unexplored until the Venus Express mission. Here we report the unanticipated finding of a draped field reversal in one hemisphere of the near Venus tail. When ordered by the interplanetary electric field orientation, the magnetic field lines in the hemisphere with inward motional electric field apparently are wrapped more tightly around Venus than in the other hemisphere, thus forming a field reversal region in the this portion of the near tail. A global hybrid simulation produces what we see and provides a three‐dimensional view of the observed hemispherical asymmetry.

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.

Ion populations in the tail of Venus

Plasma measurements in the tails of Venus showed the existence of several ion populations. Measurements performed on Venera and Pioneer Venus spacecraft at different planetocentric distances showed the evolution of the plasma parameters along the tail. Low-energy ion fluxes measured in the tail at close downstream distances, are also observed farther downstream, and show low acceleration from 0.5 R(sub V) to 12 R(sub V). High energy ions (energetic O(+) ions) reported from Pioneer Venus Orbiter (PVO) observations in the tail at 10-12 R(sub V) seem to be the same ion component that was observed as energetic ions at the tail boundary close to the planet on Venera spacecraft. We give evidence that these ions are accelerated in the narrow shear layer near the tail boundary.

A model of the ionospheric tail rays of Venus

Distinctive structural patterns in the "tail rays" that form the outermost reaches of the Venus nightside ionosphere were observed on the Pioneer Venus Orbiter. The measurements further suggested that the tail rays are the vehicle for significant escape of atmospheric oxygen, but the manner in which they fit into the scenario of solar wind scavenging and the physics behind their formation remain unexplained. In this paper a model of tail ray generation and morphology is proposed which is based on knowledge of the magnetic field structure in the low‐altitude magnetosheath and near‐wake, where the tail rays are observed. The model applies specifically to those regions where the plasma pressure in the tail rays is less than the field pressure. It is shown that many of the observed structural patterns can be explained if it is assumed that initially low energy ionospheric O+ ions are picked up by an externally imposed convection electric field from a thin source region around the terminator. Gravity is found to play a significant role in determining the ion trajectories and hence the modeled tail ray structures. The model can produce single, double, triple, or quadruple thin tail rays, all of which are observed. The energies of the particles are also consistent with the available data. The reported tail ray dependence on solar EUV flux and solar wind dynamic pressure naturally fits into the proposed concept. The implication for ion escape is that no special mechanisms are required to explain Venus tail rays. They may be simply interpreted as the low‐altitude, low‐energy manifestation of the standard ion pickup process at a weakly magnetized planet.

Venus ionospheric tail rays: Spatial distributions and interplanetary magnetic field control

Venus tail rays were first observed in the Pioneer Venus Langmuir probe data. These rays appear to be filaments of ionospheric plasma extending far downstream from the nightside ionosphere. The characteristic signatures of tail rays are in many ways similar to those of plasma “clouds” observed above the terminator ionopause. These signatures include a localized electron density enhancement, and occasional magnetic field reversal in the Bx component and/or enhanced electric field noise. Here, selected Pioneer Venus orbits from the interval between orbits 700 and 3500 during the period November 1980 to May 1988 are surveyed for tail ray structures as seen in electron density time series from the Orbiter Langmuir probe. After rotating the upstream transverse magnetic field (interplanetary magnetic field (IMF)) to point in the same direction, the resulting distribution of tail rays is found to be scattered rather than organized around a central “plasma sheet” in the plane perpendicular to the IMF. If tail rays originate from clouds, this lack of organization is consistent with the observed scatter of cloud locations around the planet in the terminator plane. On the other hand, statistical analyses indicate that tail ray formation does not appear to require an extraordinary degree of rotation of the IMF during the pass, unlike the clouds. The steadiness of the IMF during passes containing tail rays is further supported by gas dynamic magnetosheath field modeling. Only one upstream magnetic field vector is needed as input to simulate the magnetometer time series for the magnetosheath during orbits on which tail rays are seen. Thus, although tail rays and clouds exhibit similar plasma and field signatures, there is no conclusive evidence that tail rays evolve from clouds. The results of the present study suggest that tail rays are a normal feature of the steady solar wind interaction with Venus but are not generally associated with a central tail plasma sheet. Instead, a terminator source, or multiple sources, is suggested.

A comparison of induced magnetotails of planetary bodies: Venus, Mars, and Titan

The Pioneer Venus orbiter (PVO), PHOBOS 2, and Voyager 1 spacecraft have together provided observations of three planetary bodies with induced magnetotails: Venus, Mars, and Titan. During the extended mission of PVO, the tail of Venus was probed at an altitude of ∼1.3 planetary radii, which provided a more appropriate basis for comparison with the Mars data (at ∼2.7 planetary radii), and Titan data (∼2.5 planetary radii downstream), than the previously analyzed Venus tail data obtained near PVO apoapsis (∼12 planetary radii). A parallel examination of the magnetic properties of these tails at downstream distances within 3 planetary radii reveals the following similarities and differences. In the cases of Venus and Mars, which are always embedded in the supermagnetosonic solar wind flow, the tail lobe fields are smoothly joined to the draped magnetosheath fields at their outer boundaries, but separated in the center by a distinct, and sometimes narrow, current sheet. The tail of Mars has a cross section that is wider, when scaled by the planet radius, than that at Venus (as found by earlier MARS spacecraft experiments), a lobe field strength that is about the same as that at Venus in spite of the factor of ∼3 smaller interplanetary field at Mars, and a cross‐tail field strength that exceeds that at Venus by ∼1.5 times. The tail of Titan appears similar to the others except that there is no bow shock and little or no draped magnetosheath field signature since the surrounding magnetospheric plasma flow is submagnetosonic (although super‐Alfvenic). The lobe field strengths are about half those at Venus and Mars, while the cross‐tail field is almost negligible. The near‐Titan tail diameter is close to the body diameter. In place of the smooth transition to a draped magnetosheath field at the tail boundaries, as seen at Venus and Mars, the Titan observations show current sheets where the field rotates to its external orientation. It is shown that the Titan wake magnetic signature can be simulated with a model field composed of a cylindrical boundary containing antiparallel axial “tail lobe” fields, surrounded by a field described by streamlines of incompressible flow around a cylinder. Simulation of the magnetic fields observed at Mars and Venus, on the other hand, requires a draped magnetosheath field model with an appropriately oriented comet‐tail‐like model in its interior.

A statistical study of ions and magnetic fields in the Venus magnetotail

Plasma and magnetic field data from 223 orbits in the first nine tail seasons of Pioneer Venus are examined to provide a statistical description of the combined ion and magnetic field properties of the Venus magnetosheath and magnetotail. This description is based solely on instrumental responses, with no a priori assumptions about existing regions or their plasma and field characteristics. Over 22,000 plasma spectra taken from 7 to 12 Rv downtail are categorized as to whether they contain shocked solar wind, pickup ions, or no discernible plasma. The magnetic field characteristics associated with each plasma spectrum category are examined, and the average two‐dimensional cross‐tail ion flow and magnetic field structures of the Venus tail are determined at a resolution of 0.25×0.25 Rv. Plasma flows everywhere tailward, slowing from 450 km/s in the sheath to less than 300 km/s at the tail axis. Weak outward deflections of 50 km/s or less are found within the tail. The magnetotail is found to be highly draped, with a field‐reversing current sheet not more than 0.25 Rv thick. At these distances the tail is filled with shocked solar wind but also contains pickup ions and a plasma component with fluxes that are not detectable by the Pioneer Venus orbiter plasma analyzer. The bulk flow and field configurations imply a number density of 1.2 cm−3 and a temperature of 9×106 °K for a current sheet composed of 90% protons and 10% O+. These conditions are at or below the limit of instrumental detection. Weak magnetic field asymmetries are associated with the plasma dropouts. The undiscernible plasma component in the tail is consistent with a planetary ion population with fluxes below the instrumental detection threshold. A high E/q plasma population previously interpreted as planetary pickup ions (O+) is found asymmetrically both within the tail and in the adjacent sheath. The undiscernible and pickup ion plasmas (plasmas of Venusian origin) are observed from 5 to 15% of the time within the tail region. This rate depends on solar EUV flux, indicating a photoionization source. Thus the Venus tail is filled with plasma that is primarily shocked solar wind at sometimes undetectable fluxes, which coexists with a photoion population that produces asymmetries in the bulk plasma and magnetic field properties.

The magnetotail of Mars: Phobos observations

Magnetic measurements obtained by the Phobos spacecraft in circular orbit at 2.86 Mars radii show a 2‐lobed magnetic tail. The peak solar‐antisolar component of the magnetic field in these lobes is about 14 nT. The polarity of the tail lobes is sensitive to the direction of the interplanetary magnetic field as expected for a magnetic field induced by the solar wind interaction. The ratio of the cross‐tail component of the tail field is over twice that in the solar wind, or about a factor of 4 larger than the ratio observed in the Venus tail at greater distances. Since the ratio is observed to decrease with increasing distance in the Venus tail, we conclude that the Mars and Venus magnetotails are in fact similar. If there is an influence of an intrinsic planetary magnetic field on the magnetotail of Mars that influence is weak.

Pioneer Venus Orbiter magnetic field and plasma observations in the Venus magnetotail

This study uses Pioneer Venus orbiter (PVO) magnetometer and plasma analyzer measurements to investigate the draped‐field tail of Venus with an emphasis on determining the magnetic field and plasma conditions within the various tail regions and their dependence upon interplanetary magnetic field (IMF) orientation. For this purpose PVO orbits during which the spacecraft's high inclination trajectory took it through the central magnetotail were identified. The criteria used to select these orbits were the existence of relatively complete magnetic field and plasma data during crossings of the magnetotail which lasted more than 3 hours. Examination of observations from four PVO tail seasons in 1981–1983 produced 12 orbits meeting these requirements. Analysis of the observations taken during those orbits indicates that the distribution of plasma within the magnetotail is highly asymmetric and controlled by the orientation of the IMF. In the plasma sheet and adjacent lobe regions downstream of the Venus hemisphere over which the solar wind motional electric field, Esw = −Vsw × Bsw, is directed away from the planet, PVO observed increasing fluxes of H+ and O+ as the spacecraft moves away from the tail axis toward the outer boundary of the tail. The greatest concentration of these O+ ions was found in the vicinity of the cross‐tail current layer downstream of this Venus hemisphere. No O+ ions were observed outside of the magnetotail based upon the magnetic field data and the definitions adopted in this study. However, only slow, <310 km/s, antisolar directed O+ is detectable by the PVO plasma analyzer and this factor may be the reason for the frequent observation of O+ in the cross‐tail current and the absence of measurable O+ outside of the magnetotail where the bulk flow speeds are significantly slower than in the distant ionosheath. Downstream of the Venus hemisphere over which the solar wind motional electric field is directed in toward the planet, PVO does not usually observe significant fluxes of E/Q = 0–8 kV ions, except sometimes directly adjacent to the outer boundary of the tail. The outer boundary of the Venus magnetotail downstream of this hemisphere is typically a well‐defined magnetopauselike current layer. The magnetotail‐ionosheath interface downstream of Venus hemisphere over which the solar wind motional electric field is outward, however, is very broad and resembles a slow mode expansion fan with field strength slowly decreasing and plasma density gradually increasing as PVO moves outward and enters the ionosheath. These results are interpreted as being due to the more efficient pick‐up of newly ionized atmospheric neutrals over the Venus hemisphere where the initial gyromotion takes the newly created ions away from the dense, lower atmosphere where they might be lost due to scattering (Cloutier et al., 1974). The implications of these findings for the formation and maintenance of the Venus magnetotail are discussed.

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.

The ionotail of Venus: Its configuration and evidence for ion escape

The Pioneer Venus Orbiter measurements of the plasma and magnetic environment of the near tail of Venus show that the ionosphere becomes increasingly filamentary with increasing altitude, apparently forming cometlike tail rays that extend several thousand kilometers behind the planet. We call this region the ionotail of Venus. The tail rays are envisioned as plumes of high‐beta plasma of ionospheric origin that are surrounded by regions of low‐density, low‐beta plasma. The ionotail appears to be in quasi‐equilibrium, with the plasma pressure in the rays approximately balanced by the magnetic pressure of the region surrounding the rays. The magnetic field in this region is approximately sunaligned as we assume are the tail rays. Magnetic field reversals observed in the tail ray boundaries suggest the presence of strong current sheets there. Unlike the lower ionosphere whose major ion is thermal O+, a detailed study of tail ray plasma between 2000 and 2500 km altitude shows that the major ions are superthermal O+, with energies in the range of 9‐16 eV. The electrons are much cooler, with energies of about 1 eV. A minor, more energetic ion component, having energies exceeding 40 eV is also observed within the tail rays and occasionally between the rays as well. These Pioneer Venus Orbiter measurements reveal an ionotail that is highly dynamic, a region in which solar wind induced magnetic fields configure the ionospheric structures and accelerate the ions beyond the planetary escape velocity. We estimate a total planetary O+ escape rate of 5 × 1025 ions/s, and we infer an H+ escape rate of about half that value, about a factor of 2 below the hydrogen escape rate due to H+ charge exchange with the hydrogen exosphere of Venus.

The average magnetic field draping and consistent plasma properties of the Venus magnetotail

A new technique has been developed to determine the average structure of the Venus magnetotail (in the range from −8 Rv to −12 Rv) from the Pioneer Venus magnetometer observations. The spacecraft position with respect to the cross‐tail current sheet is determined from an observed relationship between the field‐draping angle and the magnitude of the field referenced to its value in the nearby magnetosheath. This allows us statistically to remove the effects of tail flapping and variability of draping for the first time and thus to map the average field configuration in the Venus tail. From this average configuration we calculate the cross‐tail current density distribution and J × B forces. Continuity of the tangential electric field is utilized to determine the average variations of the X‐directed velocity which is shown to vary from −250 km/s at −8 Rv to −470 km/s at −12 Rv. From the calculated J × B forces, plasma velocity, and MHD momentum equation the approximate plasma acceleration, density, and temperature in the Venus tail are determined. The derived ion density is approximately ∼0.07 p+/cm³ (0.005 O+/cm³) in the lobes and ∼0.9 p+/cm³ (0.06 O+/cm³) in the current sheet, while the derived approximate average plasma temperature for the tail is ∼6×106 K for a hydrogen plasma or ∼9×107 K for an oxygen plasma.

The Martian magnetotail

There is evidence of a strong influence of an atmospheric (cometary) interaction on the Martian tail formation: small total magnetic flux in the tail, the existence of plasma flow of apparently planetary origin, interplanetary magnetic field control of magnetic field orientation in the tail and other evidence. At the same time the large radius of the Martian magnetotail (about 2 planetary radii) can be considered as a strong evidence for the existence of a planetary magnetic field. Plasma and magnetic field properties in the Martian tail are in many respects similar to the ones observed in the tail of Venus. The limited amount of near-Mars measurements leads to some reservations in coming to definite conclusions. A combined magnetosphere of Mars is suggested that consists of two polar-tied magnetic tubes connected to the tail and an equatorial Venus-type interaction region in-between.

The venus magnetotail

The magnetic structure of the Venus magnetotail has been measured using 9 seasons of Pioneer Venus measurements in the distant wake region. The magnetotail region is about 4 Venus radii wide and about 3.2 Venus radii high at 12 Venus radii behind the planet. The tail contains about 2.3 Megawebers of flux, much of which closes across the tail. The J X B force associated with this cross tail component accelerates the tail plasma down the tail causing an ion loss rate of about 10 26 ions per second. The Venus tail does respond to solar activity and contains significantly less magnetic flux at solar minimum.

Mass-loading and the formation of the Venus tail

Despite its lack of an intrinsic magnetic field Venus has a well defined magnetotail, containing about 3 megawebers of magnetic flux in a tail about 4 R V across with perhaps a slightly elliptical cross section. This tail arises through the mass-loading of magnetic flux tubes passing by the planet. Mass-loading can occur due to charge exchange and photo-ionization as well as from the diffusion of magnetic field into the ionosphere. Various evidence exists for the mass-loading process, including the direct observation of the picked up ions with both the Venera and Pioneer Venus plasma analyzers.

A comparative study of distant magnetotail structure at Venus and Earth

Pioneer Venus and ISEE‐3 observations are used to conduct a comparative investigation of distant magnetotail structure at Venus and Earth. Both magnetotails are found to be well ordered out to the apoapses of PVO and ISEE‐3 which are at 11‐12 Rv and 220‐240 Re. The estimated lengths of the Venus and Earth magnetotails are 100‐150 Rv and 600‐6000 Re. Despite their very different interactions with the solar wind, the magnetotails of these two planets possess many similarities. The Venus tail appears to have distinct mantle, lobe, plasma sheet boundary layer, and plasma sheet regions much like its terrestrial analogue. It is further shown that the plasma sheet magnetic field distributions in the disant tails of Venus and Earth are consistent with disconnection from the planet. In particular, the 〈BZ〉 = −0.25 nT observed by ISEE‐3 X = −210 to −225 Re is sufficiently negative on the basis of flux conservation for the average location of the tail neutral line to always earthward of X = −210 Re. Finally the lobe field magnitudes are approximately those expected on the basis of average solar wind conditions, while the mean plasma sheet beta values inferred from pressure balance arguments are 8.5 and 3.8 for Venus and Earth, respectively.