Plasma in the near Venus tail: Venus Express observations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 8, , doi:./3ja964, 3 Plasma in the near Venus tail: Venus Express observations E. Dubinin, M. Fraenz, T. L. Zhang, J. Woch, Y. Wei, A. Fedorov, 3 S. Barabash, 4 and R. Lundin 4 Received 7 June 3; revised 8 November 3; accepted November 3; published 9 December 3. [] 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 3R V ) 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. Citation: Dubinin, E., M. Fraenz, T. L. Zhang, J. Woch, Y. Wei, A. Fedorov, S. Barabash, and R. Lundin (3), Plasma in the near Venus tail: Venus Express observations, J. Geophys. Res. Space Physics, 8, , doi:./3ja964.. Introduction [] Venus has no intrinsic magnetic field and its magnetosphere is formed by the direct interaction of solar wind with the ionosphere. The existence of an induced magnetosphere with a magnetic tail was already observed by the first space missions Venera 9 and and in laboratory simulation experiments [Yeroshenko, 979; Dubinin et al., 978, 98; Dolginov et al., 98; Podgornyi et al., 98]. Most information about properties of induced magnetospheres was obtained by the Pioneer Venus Orbiter (PVO) which has explored the Venus space for almost 4 years [Russell et al., 6]. [3] The magnetic flux tubes carried by solar wind and mass loaded by planetary photoions are draped around Venus and stretched antisunward founding the long magnetotail which consists of two lobes of opposite polarity of the magnetic field separated by a plasma sheet [Russell and Vaisberg, 983]. The tail boundary was well defined, at least, up to R v, the most distant space sampled by PVO Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany. Space Research Institute, Graz, Austria. 3 IRAP, CNRS, Toulouse, France. 4 Swedish Institute of Space Physics, Kiruna, Sweden. Corresponding author: E. Dubinin, Max Planck Institute for Solar System Research, Max-Planck-Str.,, D-379 Katlenburg-Lindau, Germany. (dubinin@mps.mpg.de) 3. American Geophysical Union. All Rights Reserved /3/./3JA [Saunders and Russell, 986]. The Venus Express (VEX) mission was launched to Venus in 5 and provide us with a new data set of the characteristics of the space environment at Venus. The locations of the main plasma boundaries, namely bow shock, magnetopause (magnetic pileup boundary), and ionopause (ion composition boundary) at the period of solar minimum were studied and compared to simulations by Zhang et al. [8a, 8b], Martinecz et al. [8, 9], Kallio et al. [8], and Ma et al. [3]. The first comparative analysis of Venus and Mars magnetotails investigated by almost identical plasma packages was performed by Fedorov et al. [8]. Fedorov et al. [8] have observed similar plasma domains in the wakes of both unmagnetized planets. Barabash et al. [7b] have reported the first observations of loss of different sorts of ions from Venus through the tail with a distinct identification of different escape channels. An estimate of the absolute values of escape rates during solar minimum conditions was done by Fedorov et al. []. [4] In this paper we discuss in more detail the main characteristics of the Venusian plasma sheet which were measured at solar minimum at distances X 3R v.this region was almost not sampled during the previous missions. Only two passes were made by Venera through the intermediate tail (.5 to 5R v downstream) in 976, also during solar minimum conditions [Vaisberg et al., 995]. Vaisberg et al. [995] have observed a regular increase of ion energy each time when the spacecraft approached or crossed the current sheet, although the absence of ion mass resolution has imposed serious constraints on the determination of plasma parameters.

2 [5] PVO sampled the Venus tail either near periapsis (5 5 km) at solar maximum or near apoapsis (5 R v ). There were only scarce plasma observations in the distant plasma sheet of the Venus tail. An indirect assessment of plasma properties was performed by Saunders and Russell [986] assuming a balance between the lobe magnetic field pressure and thermal ion pressure in the plasma sheet. Slavin et al. [984] have observed an asymmetry in fluxes of H + and O + ions near the cross-tail current sheet at X = 8to R v and suggested that the magnetotail of Venus may be resupplied with magnetic flux tubes in an asymmetrical fashion. PVO observations and recent simulations show that asymmetry also exists in the pileup of the IMF and the ionopause height on the dayside [Luhmann et al., 985; Phillips et al., 988; Jarvinen et al., 3]. The magnetic barrier which separates the magnetosheath from the ionosphere is shifted from the subsolar region in the direction of the motional electric field of the solar wind. On the other hand, the ionopause is shifted in the direction opposite to the motional electric field. [6] We show in this paper that plasma in the plasma sheet accelerated by magnetic field stresses carries the bulk of energy providing a dynamic balance between the magnetic field and plasma pressures. Due to the specific variations of these stresses across the current sheet, the energy of accelerated ions displays a bell-shaped distribution. An important feature of the plasma sheet in the near Venus tail is a strong asymmetry controlled by the direction of the motional electric field in the upstream solar wind. This asymmetry is revealed not only in plasma characteristics but also in the strongly asymmetrical supply of plasma sheet by ionospheric plasma.. Instrumentation and Data Analysis [7] VEX was inserted in 6 into a highly elliptical polar orbit around Venus with a 4 h period and pericenter and apocenter of 8 35 km and 66, km, respectively. Plasma and magnetic field measurements on VEX are performed by the ASPERA-4 instrument and the magnetometer MAG, respectively [Barabash et al., 7a; Zhang et al., 6]. [8] The Ion Mass Analyzer (IMA/ASPERA-4) on VEX detects ions in the ev/q 3 kev/q energy range and 44 amu/charge range, including both solar wind and planetary ions with time resolution of 9 s and field of view 9 ı 36 ı. The electron spectrometer (ELS/ASPERA-4 ) provides a -D distribution (6 sectors) of electron fluxes in the energy range of 5 ev kev with a time resolution of 4 s (on some orbits resolution of s was used). Plasma parameters were calculated using the algorithms described by Fraenz et al. [6] which were applied after a background noise reduction made by Fedorov et al. []. [9] We use the magnetic field measurements carried out with 4 s resolution. The magnetic field data are presented either in Venus Solar Orbital (VSO) or in Venus Sun Electric field (VSE) coordinates. In VSE coordinates, the X * axis is antiparallel to the upstream solar wind flow and the Y * axis along the cross-flow magnetic field component of the interplanetary magnetic field (IMF), such that the IMF is in the X * Y * plane and B * y is always positive. Then the DUBININ ET AL.: PLASMA SHEET AT VENUS 765 Z* R v - - Y, R v * median Figure. Magnetic field polarity of the Venus tail in the Y*Z* plane (B x component). motional electric field V sw B IMF is along the Z * axis and points outward from the planet in the Z * >hemisphere. Since the orientation of the tail lobes and current sheet in induced magnetospheres is controlled by the clock angle of the upstream interplanetary magnetic field (IMF), this coordinate system is the most relevant for the description of the Venus magnetotail. [] In this paper we have selected about 6 VEX orbits from August 6 to January with signatures of a current sheet crossing (change of sign in the B x component) and rather steady IMF orientation on the inbound and outbound legs of the orbit (less than 3 ı clock angle variations). [] Figure shows the magnetic field polarity of the Venus tail in the Y * Z * plane using the measurements in the time intervals of 5 min around the current sheet crossing. All field values within the tail ( 3R v X.5R v )are combined. A clear separation between two magnetic lobes is observed, although in the Venus Solar Orbital coordinates, the distribution is random. The magnetic lobes are separated by the plasma sheet filled by the plasma of the planetary origin. This plot justifies our choice of the coordinate system for studying the plasma sheet in the Venus tail. 3. Ion Acceleration [] Plasma in the plasma sheet is accelerated in the tailward direction by the magnetic field tangential stresses [Dubinin et al., 993]. Since this force is the strongest in the center of the tail, the ion energy gradually increases, reaches a maximum, and then again decreases when the spacecraft crosses the current sheet. Figure shows several examples of such an ion behavior. Oxygen ions, which are the major ion species, gain energy up to about of kev near the center of the tail. Since the tangential force (B r)b across o the current sheet varies as *,whereb? and B x are the transverse and the longitudinal magnetic field components, respectively, and y * is the distance to the center of the - B x, nt

3 Figure. Variations in the B x component of the magnetic field and energy spectra of oxygen ions at crossings of the current sheet of the Venusian tail. A drop-like picture of ion fluxes is due to a slow ( 3 min) elevation scan by the IMA instrument. Dotted curves show the energy gained by oxygen ions according to the equation (). current sheet, we can easily estimate the velocity V i and energy W i gained by x B dv ix dv? ix i i V ix dt dx * () o Here we neglect a contribution of the magnetic pressure gradient force. Then the energy gained by ions is as x W i * L, () o n i where i = n i m i is the mass density of the ions and L is the characteristic scale of the acceleration. Then a bell shape of the ion spectrograms across the current sheet can be easily explained. [3] Indeed, variations of the B x component of the magnetic field across the current sheet are well described by the hyperbolic tangent of the distance y * to the center of the current sheet B x B xo tanh(y * /ı), whereı is the characteristic halfwidth of the current sheet. Then, the tangential forces (B r)b and the energy W i will vary with distance as the squared hyperbolic secant:w i * L x o n i B? B xo sech ( y* o n i ı )L, yielding a bell-like shaping (Figure 3). ı [4] Let us consider a typical case of the field and ion energy variations across the current sheet on July 8 (see Figure ). The two top panels in Figure 4 depict the variations of the magnetic field (B x, B? ), and the plasma number density (n e ) averaged over 6 s time intervals as functions of the distance y * to the center of the current sheet. The dotted curve on the top panel shows the function B xo tanh(y * (in km)/35 km) with B xo =7nT, which fits reasonably the observations and provides us with estimate of the current sheet width. Figure 4 (third panel) shows 766 the value db x averaged on two data points and the squared dy * hyperbolic secant, sech ( y* ) (the dotted curve). Figure 4 35 (bottom panel) presents the value of the energy gained by the ions evaluated from the equation (). It is worth noting that the spatial variations in the number density and the crossflow component of the magnetic field make the observed profile of the energy wider as compared to that in Figure 3. We used here that L ı that is justified below. The dotted curve in Figure b shows the calculated energy in the time series plot. Similar procedure applied to other events shown in Figure yields the values plotted by dots. A reasonable agreement between the observations and this simple model is observed. It should be noted that although the real width of the current sheet cannot be determined by a single spacecraft, it is not very critical for our analysis since we compare the observed magnetic field variations with the observed plasma variations, both of them suffering possible motions of the plasma sheet. The model does not take also into account a possible shift of the whole bell-like distribution due to the existence of the forces directed to the center W i ~sech (y/d) B x ~tanh(y/d) y/d Figure 3. Hyperbolic tangent and squared hyperbolic secant approximately describe variations of the B x component of the magnetic field and the magnetic tangential stresses across the current sheet.

4 B x, B, nt - B B x 8 Jul 7tanh(Y * /35) Normalized number of samples L/d Figure 5. Histogram of the acceleration length L normalized to the maximum number of samples. cm-3 n e, nt/km *, /dy - db x ev Energy, Y*,km * sech(y/35) sech (Y /35) * that the assumption that the ion acceleration occurs at the distance L comparable to the width of the current sheet is reasonable. [6] Figure 6 shows how the peak energy gained by oxygen ions varies with distance X to the tail. We see that ions are accelerated approximately to the same energy at all distances X sampled by VEX confirming that acceleration occurs in a rather narrow layer. It is worth noting that the absence of the notable ion acceleration tailward of R M was also reported for the Martian tail [Nilsson et al., ]. [7] Besides oxygen ions, there are also other ion species, H + (of planetary origin) and He + ions in the Venus tail. Figure 7 gives an example of energy spectrograms of different sorts of ions across the plasma sheet. We observe a bell shaping of the spectrograms for all ion species while more heavy ions gain higher (lower) energies (velocities). Although the difference in ion velocities is small as compared to the Alfvén speed, a clear differential streaming of ion species is a typical feature for the Venus tail (X 3R v ). Note that a typical electrostatic analyzer without ion mass discrimination mostly used in the previous space missions to Venus would detect these ions as single ions with a broad energy distribution [see, e.g., Vaisberg et al., 995]. [8] In some cases we observe that the ratio of energies of different ion species is not proportional to the ratio of their masses as, for example, in the case shown in Figure 8 [see also Barabash et al., 7b]. This trend is also seen in Figure 9, which compares the velocities Figure 4. (from top to bottom) Variations in B x, B?, n e, db x /dy *, and the energy W i evaluated from the equation () as functions of the distance y * to the center of the current sheet crossed by VEX on July 8. Dotted curves show the functions of the hyperbolic tangent and squared hyperbolic secant. of the plasma sheet which are often not symmetrical [see, e.g., McComas et al., 986, Figure 5]. [5] To justify the choice of the acceleration length L, we have analyzed 5 VEX orbits crossing the plasma sheet. Since the equation () can be rewritten in the form W i B? B xo L, we can estimate the ratio Ł/ı using the observed o n ı values of W i (peak values), B?, B xo,andn. Figure 5 shows the histogram of the L/ı ratio normalized to the number of samples corresponding to the maximum value. It is seen ev + ), Energy (O X, Rv Figure 6. Maximum energy gained by oxygen ions (E i > ev) in the center of the plasma sheet portion in the Z * > hemisphere as a function of the distance X(R v ) to the tail. 767

5 nt Bx, ev Energy, ev Energy, ev Energy, Dist., R v.9 UT 4 VEX/ASPERA-4/MAG O + He + H DUBININ ET AL.: PLASMA SHEET AT VENUS 8 Jul Figure 7. Variations of the B x component of the magnetic field, energy spectra, and bulk speeds (color curves) of different ion species across the plasma sheet on July 8. of oxygen ions, protons, and He + ions in the center of the plasma sheet. Dotted lines correspond to the mutual relations between the ion speeds proportional to the square roots of ion masses (V(O + )=V(H + )/ and V(O + )=V(He + )/ / ). Such a relation is more clearly exposed for O + and He + ions. A differential streaming of different ion species under action of external forces is probably not surprising because of different ion inertia. In multifluid MHD approach the additional terms related to the velocity difference appear in the momentum equations [see, e.g., Dubinin et al., ]. These additional forces result in the momentum exchange between ion fluids and can bring about a transition of the whole system to a steady equilibrium with a remnant of a differential streaming [see, e.g., Dubinin et al., 5]. Presently, it is not known whether the observed relations between the ion speeds are compatible to a stationary differential flow of multi-ion species or characterize only a transition to a flow with equal velocities. Measurements at larger distances in the tail are necessary to resolve this issue. Note that a similar differential streaming between the main proton population and minor ions in solar wind remains an open problem [see, e.g., Neugebauer, 98; Marsch et al., 98]. 4. Pressure Balance [9] Saunders and Russell [986] using the PVO observations of the magnetic field in the distant tail have pointed km/s Velocity, km/s Velocity, km/s Velocity, ev/cm^-s-sr-ev ev/cm^-s-sr-ev ev/cm^-s-sr-ev 768 out that a pressure balance between the lobe magnetic field pressure and thermal ion pressure in the Venus plasma sheet requires an energy density nkt of about kev cm 3. However, due to absence of adequate plasma observations on PVO, this suggestion was not tested. Figuresa d present the examples of the distribution functions of oxygen ions close to the center of the plasma sheet crossed on the orbits shown in Figure. Dotted curves depict the Maxwellian distributions fitted to the observations. The corresponding values of the bulk and thermal energies of the distributions are also given. It is seen that oxygen ions are streaming as cold beams, and their thermal energy density nkt is about.. kev n cm 3, i.e., significantly smaller than the magnetic field energy B o. [] Figure compares the typical profiles of the magnetic field pressure (P M ) and thermal plasma pressure (ion P th,i and electron P th,e components) evaluated from the plasma moments across the current sheet of the Venus tail for two VEX orbits. We see that the plasma pressure is several times less than the magnetic pressure implying that the plasma sheet subsists not in a static equilibrium. On the other hand, the dynamic pressure of the streaming planetary plasma (P d ) provides us the required pressure balance. This suggests that the electric currents which support the observed magnetic field configuration are mainly the inertia currents rather than the pressure gradient currents. Maps of the distribution of the mean values of the magnetic field and ion thermal pressures plotted in the Y * Z * and X * Y * planes (Figure ) confirm the absence of a static equilibrium. It is interesting to note that Matsumoto et al. [] have also found the absence of the pressure balance in the distant geomagnetic tail sampled by the Geotail spacecraft. Although the origin of the geomagnetic tail is different from that on Venus, the total pressure in the current sheet was about 5% less than the pressure in the lobes in 36% cases. An excess of the magnetic field energy was converted to the plasma kinetic energy. On Venus we observe that the major part of the magnetic field energy is converted to the ion kinetic energy. [] However, in some cases (see, e.g., Figures e and f ), the ion distributions in the plasma sheet contain broad suprathermal tails implying the existence of other important mechanisms of ion energization. For example, ion-ion instabilities can result in anomalous friction between interpenetrating magnetosheath and ionospheric plasmas and significant ion heating with generation of suprathermal tails in the ion distributions [e.g., Dobe et al., 999]. Such an interpenetration of the magnetosheath protons and the ionospheric H + and O + ions is clearly seen on the ion spectrograms shown in Figure 3. Contribution of suprathermal ions and the magnetosheath protons can sometimes provide a balance between the magnetic field and thermal pressures (Figure 3). 5. Asymmetry of Plasma Sheet [] It is generally suggested that the Venusian plasma sheet is supplied by plasma once the draped magnetic field lines are slipping around the poles from the dayside and drag the ionospheric plasma to the tail by their tangential stresses. However, the observations show a distinct

6 4 VEX/ASPERA-4 :7:33 O + 6 Aug 9 E O+ = 4 ev E = 74 ev He + E H + =4 ev E O + / E He + / E H + = 3.78 /.9 Energy, ev 3 He + H + Counts s^3/km^6 O + H + He Mass channels Energy, ev Figure 8. (a) Energy-mass matrix of ion fluxes measured by VEX in the center of the Venus plasma sheet. (b) Bulk energies of different ion species are not proportional to the ratios of the ion masses. asymmetry in supply of the plasma sheet. Such an asymmetry is seen between the hemisphere pointed in the direction of the motional electric field (E + hemisphere) and the hemisphere pointed in the opposite direction (E hemisphere). [3] Figure 4 depicts the projections of the velocity vectors of oxygen ions and protons of planetary origin onto the X * Z * plane. In the E + hemisphere, the jb force accelerates plasma tailward filling the plasma sheet. In the E hemisphere, the flow pattern occurs less regularly with smaller speeds. A similar flow asymmetry of the planetary ions into the Venus wake is observed in the recent hybrid simulations [Jarvinen et al., 3]. The pattern of ion velocities in the E + hemisphere shows that ions in the near Venus tail are streaming in the direction opposite to the motional electric field, i.e., move as a fluid under action of the j B forces rather than test particles experienced by the action of the V sw B IMF electric field. Note also the existence of another ion population (pickup ions), not observed during the selected time intervals, whose motion is mostly determined by the motional electric field [see, e.g., McEnulty et al., ; Jarvinen et al., 3]. [4] Asymmetry is exposed not only in a flow pattern but also in plasma characteristics. Figure 5 compares maps of distributions of the velocities and densities of oxygen ions in the Y * Z * and X * Z * planes. It is seen that the density in the E hemisphere is significantly higher than that in the E + hemisphere, while the ion velocity there is much lower. [5] The observed strong asymmetry is controlled by the direction of the motional electric field in the upstream solar wind and probably takes its origin already on the dayside. This asymmetry is attributed to the action of the motional electric field. While oxygen ions which are not magnetized are extracted and accelerated by the electric field, the electrons drift in the crossed electric and magnetic fields feeding the Hall current that enhances the magnetic field [Luhmann km/s Velocity, O km/s Velocity, O 3 3 H + Velocity/, km/s 3 3 He+ Velocity/sqrt(), km/s Figure 9. Scatter plots of velocities of oxygen ions and velocities of protons and He + ions in the center of the plasma sheet, respectively. Dotted lines correspond to the mutual relations between the ion speeds proportional to square roots of ion masses (V(O + )=V(H + )/ and V(O + )=V(He + )/ / ). 769

7 Figure. (a d) Examples of distribution functions of oxygen ions close to the center of the current sheet of the Venus tail for the events shown in Figure. Dotted curves show the Maxwellian distributions fitted to the observations. The corresponding values for the bulk and thermal ion energies are given. (e and f) The anomalous events of a dense plasma with broad tails of the suprathermal ions. 3 7 Dec 6 Sep 6 PRESSURE, npa Bx, nt Dist, Rv. UT P M PRESSURE, npa Bx, nt Dist, Rv UT. 5 P M Figure. Examples of pressure variations across the current sheet of the Venus tail showing the absence of the pressure balance. 763

8 Figure. Maps of the distribution of the mean values of the magnetic field and ion (O + and H + ) thermal pressures plotted in the Y * Z * and X * Y * planes. Red (black) shaded bins have pressures higher (less) or equal to the maximum (minimum) value of the color bar. et al., 985] and, correspondingly, the tension forces in the E + hemisphere. On the other hand, the transverse momentum acquired by pickup oxygen ions is balanced by the momentum of the ionospheric plasma expanding in the Z * direction [Phillips et al., 988]. This leads to increase in the density and, correspondingly, decrease in the velocity of the ionospheric plasma transported from the dayside to the nightside in the E hemisphere. [6] Another important factor that influences the dynamics of ions in the near Venus tail is related to a difference in the draping pattern in both hemispheres [Zhang et al., ]. The observations and hybrid simulations show that, in the hemisphere with inward directed electric field (E hemisphere), the magnetic field lines are wrapped more tightly around Venus than in the opposite E + hemisphere. Such a reversal of the cross-flow component (B y *) of the magnetic field produces inward directed field stresses that can modify a plasma transport to the tail in the E hemisphere. [7] Figure 6 shows the V x component of the velocity of different ion species in both hemispheres. Symbols + imposed onto the bins accentuate the regions where the V x component is positive (sunward). We observe a frequent reversal of the V x component in the E hemisphere. Such a behavior is consistent with a wrapping of the magnetic field lines around the planet in this hemisphere [Zhang et al., ] and a change of sign in the magnetic field stresses. The draping topology observed in the E hemisphere might be also favorable for the reconnection accompanied by the appearance of ion fluxes in the sunward direction [Zhang et al., ; Dubinin et al., ]. Both processes probably Figure 3. Examples of pressure variations across the current sheet of the Venus tail showing the pressure balance. Spectrograms of the protons indicate interpenetration between the magnetosheath and ionospheric plasmas supplying the plasma sheet. 763

9 O + km/s H km/s Z ICA,Rv Z ICA,Rv X ICA,Rv -3 - X ICA,Rv -3 Figure 4. Projections of the velocity vectors of oxygen and hydrogen ions onto the X * Z * plane in the near Venus tail. A strong asymmetry between the E + and E hemispheres is seen. median median Z *,Rv Z *,Rv - - Y *, Rv - BS VENUS MB median O xygen Velocity, km/s Oxygen Velocity, km/ s - - Y *, Rv - BS VENUS MB.. median... O xygendensity, cm-3 OxygenDensity, cm X *, Rv - -3 X *, Rv. Figure 5. Maps of the velocity and density of oxygen ions plotted as projections onto the Y * Z * and X * Z * planes in the near Venus tail. Red- (black-)shaded bins have values higher (less) or equal to the maximum (minimum) value of the color bar. Plasma which fills the tail in the E + hemisphere is less dense as compared to that in the E hemisphere and flows with a higher speed. 763

10 Figure 6. Maps of the V x component of the velocity of different ion species onto the X * Z * plane in the near Venus tail. Symbols + show a reversal of the flow toward Venus. operate, and evaluation of their roles in a reversal of plasma flows and the observed asymmetry requires a further study. 6. Conclusions [8] We have studied the characteristics of the plasma sheet on Venus in the near planet (X 3R v ) magnetotail. The measurements were done by the ASPERA-4 and MAG instruments onboard Venus Express at the period close to solar minimum. The plasma sheet on Venus is formed by a drag of the ionospheric plasma, consisting mainly of O +,H +, and He + ions, by the tangential stresses of the magnetic field lines slipping around the planet and forming the magnetotail. We have shown that such a process of plasma supply is very asymmetrical. This asymmetry 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, plasma, pushed by the j B force of the stretched field lines, fill the plasma sheet according to the above scenario. However, in the opposite hemisphere the flow pattern is irregular, ion speeds are significantly less but ion densities are higher. A difference in the magnetic field draping between both hemispheres and the preferential reconnection in the E hemisphere might be responsible for the observed asymmetry. [9] Ion acceleration in the hemisphere, pointed in the direction of the motional electric field, occurs in a relatively narrow layer with a width comparable to the plasma sheet width. A bell shape of ion energy spectrograms measured while VEX crosses the current sheet of the magnetotail is explained by the specific variations of the stress forces across the current sheet. The values of the energy gained by ions are also consistent with the slingshot mechanism of acceleration. [3] Heavier ions are accelerated to higher energies but to slower velocities. In the near Venus tail there is a trend that the ratio of energies of ion species is approximately proportional to the square root of their masses. Presently, it is not clear whether such a relationship corresponds to the stationary differential flow of multi-ion species (the difference in the velocities is smaller than the characteristic Alfvén speed) or whether we see only a transient state to the flow with equal ion velocities [3] The thermal pressure of plasma in the plasma sheet in most cases is not sufficient to balance the magnetic pressure in the lobes, while the dynamic pressure of the accelerated plasma provides the required pressure balance. On some orbits, where the plasma density is higher than usual and there are broad suprathermal tails on the ion distributions, a pressure balance is achieved. [3] Acknowledgments. 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