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Toroidal and poloidal magnetic fields at Venus. Venus Express observations
Planetary and Space Science 87 (2013) 19–29
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Toroidal and poloidal magnetic fields at Venus. Venus Express observations E. Dubinin a,n, M. Fraenz a, J. Woch a, T.L. Zhang b, Y. Wei a, A. Fedorov c, S. Barabash d, R. Lundin d a
Max-Planck-Institute for Solar System Research, 37191, Katlenburg-Lindau, Germany Space Research Institute, Austrian Academy of Sciences, A-8042, Graz, Austria c Centre d’Etude Spatiale des Rayonnements, BP-44346, F31028 Toulouse, France d Swedish Institute of Space Physics, Box 812, S-98 128, Kiruna, Sweden b
a r t i c l e i n f o
abstract
Article history: Received 16 February 2012 Received in revised form 23 October 2012 Accepted 12 December 2012 Available online 22 December 2012
Magnetic field and plasma measurements carried out onboard Venus Express during solar minimum conditions suggest the existence of two kinds of magnetic field configuration in the Venusian ionosphere. We interpret these as the manifestation of two different types of generation mechanisms for the induced magnetosphere. A different magnetic field topology (toroidal and poloidal) arises if the induced currents are driven either by the solar wind motional electric field or by the Faraday electric field—a conducting ionosphere sees the magnetic field carried by solar wind as a time-varying field. At the dayside, both driving agents produce a similar draping pattern of the magnetic field. However, different magnetic field signatures inherent to both induction mechanisms appear at lower altitudes in the terminator region. The conditions at low solar EUV flux when the ionosphere of Venus becomes magnetized seem to be favorable to distinguish between two different types of the induced fields. We present cases of both types of the magnetic field topology. The cases when the effects of the Faraday induction become well noticeable are especially interesting since they provide us with an example of solar wind interaction with a tiny induced dipole field immersed into the ionosphere. Another interesting case when poloidal magnetic fields are evidently displayed is observed when the IMF vector is almost aligned with the solar wind velocity. In general case, both mechanisms of induction probably complement each other. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Venus Magnetic field Magnetosphere Ionosphere Venus Express
1. Introduction The idea of a captured or induced planetary magnetosphere was widely discussed after the first mission to the Moon. It was suggested that the induced currents in the lunar interior produce a magnetic field which can deflect the solar wind around the obstacle (see e.g. Gold, 1966; Sonett and Colburn, 1967; Hollweg, 1969; Johnson and Midgley, 1968; Schubert and Schwartz, 1969; Schwartz et al., 1969). Two main driving mechanisms of the electric currents were discussed: (1) Faraday induction (curlE ¼ @B=@t)—currents are induced by temporal changes in the external magnetic field and (2) the motional electric field (V sw BIMF ) of the solar wind plasma in the rest frame of the Moon (V sw and BIMF are the solar wind velocity and interplanetary magnetic field, respectively). In the case of the motional electric field, currents produce a toroidal magnetic field with a classical draping configuration on the dayside (see e.g. Figure 8b in Luhmann, 1992). In the case of the
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0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.12.003
Faraday induction, currents in the conductive body induced by a ‘switch on the external Interplanetary Magnetic Field (IMF)’ produce a dipole magnetic field whose axis is antiparallel to the IMF (see e.g. Figure 5 in Blank and Sill, 1969). However, because of very low electrical conductivity of the Moon, the induced currents occur too small to support these scenarios. The existence of induced magnetospheres was discovered later, namely at Venus and Mars, planets which also have no global magnetic field and solar wind interacts directly with their conducting ionospheres (see e.g. Russell and Vaisberg, 1983). How to distinguish or separate the effects of both mechanisms? An interesting experiment has been performed in the laboratory (Podgornyi et al., 1982). Magnetic field topologies arising at plasma flow interaction with a conductor and with a conductor covered by a insulator have been compared. If the motional electric field drives the current then electrical contact between a moving plasma and a conductor is necessary. If such a contact is inhibited then the motional electric field might be canceled by the polarization field and the only driving electric field which remains is the Faraday field. In this case the flowing plasma interacts with a degenerated induced dipole magnetic field (Podgornyi et al., 1982).
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Fig. 1. Sketch of possible field configurations for the induced currents driven by the motional electric field (a), Faraday electric field (b) and by both mechanisms. (c), in the (Vsw, BIMF) plane.
How to distinguish or separate the effects of both mechanisms at Venus? Fig. 1 shows a sketch of possible field configurations expected for both types of induction. For the currents driven by V sw BIMF electric field the net result is a superposition of the IMF perturbed by the currents at the bow shock and the induced toroidal magnetic field (Fig. 1a) providing us with a draping configuration on the dayside and with wrapping features on the nightside (Luhmann, 1992). The characteristic feature of this configuration might be, although it is not necessary, a change of sign of the cross-flow component of the magnetic field near the terminator. Superposition of the IMF with an induced dipole expected for the case of Faraday induction is shown in Fig. 1b. The characteristic feature of this type of interaction is a wrapping of the field lines around the planet on the nightside with a change of sign in the Bx -component (X-axis is toward the Sun) (Johnson and Midgley, 1968; Podgornyi et al., 1982; Luhmann, 1992). A dipole magnetic field inside the ionosphere is annihilated if the ionosphere is considered as an ideal conductor. A more complicated configuration arises when both mechanisms operate at the same time (see e.g. Fig. 1c). There might be also another interesting test for signatures of an induced dipole. In the case when the IMF is aligned or almost aligned with the solar wind velocity, currents driven by V sw BIMF field become very small and the effects of Faraday induction might be more apparent. Analysis of the first magnetic field measurements at Venus by the Russian spacecraft Venera-9, 10 has shown the occurrence of the toroidal pattern although a change of sign in the Bx -component on the nightside on some orbits suggested a contribution of both mechanisms (Dolginov et al., 1981; Podgornyi et al., 1982). Most information about solar wind interaction with Venus was obtained by the Pioneer Venus Orbiter (PVO) in 1979–1992 (see e.g. Russell et al., 2006). It was observed that at solar maximum a magnetic barrier forms on the dayside of Venus which deflects the solar wind around the ionosphere. This barrier occurs almost impenetrable for the solar wind with the frozen-in IMF, i.e. the ionosphere stays ‘unmagnetized’, except for narrow magnetic flux ropes (Zhang et al., 1991). On the nightside the classical draping configuration with a long magnetic tail is formed (Saunders and Russell, 1986). However, for a range of solar wind overpressure, i.e. when its dynamic pressure exceeds the ionospheric pressure, the dayside ionosphere becomes magnetized (Luhmann et al., 1987). A specific configuration of the magnetic field during such a regime is observed also on the nightside (Luhmann, 1992). Analyzing the data obtained during periods of high dynamic pressure at solar maximum epoch Luhmann (1992) has observed pervasive large amplitude magnetic fields wrapped around the planet with a change of sign in the main transverse component of the magnetic field. Such kind of wrapping was statistically confirmed by Phillips et al. (1986). Luhmann (1992) has interpreted these data in terms of the prevalence of an induced toroidal magnetic field. In this paper we analyze the magnetic field and plasma measurements carried out by VEX in the ionosphere/
magnetosphere mostly looking for signatures of ‘a poloidal induced magnetosphere’. During the PVO epoch, a survey of the ionospheric magnetic fields was done only at solar maximum conditions and such signatures were not found. In contrast, VEX observations during 2006–2010 were made close to minimum solar activity. We present observations which show that the configuration of the induced magnetosphere at Venus at solar minimum conditions sometimes differs from the classical draping model. The magnetic field measurements made in the northern polar ionosphere for the conditions when the Bz component (VSO reference frame) of the IMF dominates reveal features which might be interpreted as a signature of the poloidal magnetic fields induced by the currents driven by the Faraday electric field (curl E ¼ @B=@t). Similar types of signatures are also observed when the IMF is aligned or almost aligned with the solar wind flow.
2. Observations 2.1. Instrumentation Venus Express (VEX) has a highly elliptical polar orbit with a 24 h period and pericenter and apocenter of 180–350 km and 66,000 km, respectively. The magnetic field is measured by the MAG instrument (Zhang et al., 2006) and plasma by the ASPERA-4 package (Barabash et al., 2007). The Ion Mass Analyzer (IMA/ ASPERA-4) on VEX detects ions in the 10 eV/q–30 keV/q energy range and 1–44 amu/charge range, including both solar wind and planetary ions with time resolution of 192 s and field of view 901 3601. The electron spectrometer (ELS/ASPERA-4) provides the 2D distribution (16 sectors) of electron fluxes in the energy range of 5 eV–20 keV with a time resolution of 4 (1) s. In this paper 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 the VSO coordinates, the (XY) plane coincides with the Venus orbital plane, where X is directed to the Sun, Y is opposite to the planet’s orbital velocity, Z is perpendicular to X and Y and positive to ecliptic north. Since the orbital inclination of Venus is small (i¼ 3.341), the orbital plane nearly coincides with the ecliptic plane. In VSE coordinates, the X % -axis is antiparallel to the upstream solar wind flow (and equal to X VSO), and the Y % -axis along the cross-flow magnetic field component of the IMF, such that the IMF is in the (X % ,Y % )-plane and By% is always positive. Then the motional electric field E ¼ V sw BIMF is along the Z % -axis and points out of the planet in the (Z% 40) hemisphere. 2.2. Toroidal magnetic fields Fig. 2 presents a typical case of a classical draping configuration well described in terms of the toroidal magnetic field. From top to bottom are the magnetic field data in the VSO coordinate
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Fig. 3. Examples of MAG observations which are consistent with the sketch for toroidal field, shown in Fig. 1a. The maps are plotted in the (X % Y % )-plane which contains the solar wind velocity vector and the cross-flow component of the IMF.
system, spectrograms of electron, proton and oxygen ion fluxes, respectively, measured by VEX on February 9, 2007 (06:30– 07:45UT). The right panel shows the projections of the magnetic field vectors onto the XY (VSO) plane. The orbit of the s/c was in the terminator plane and the IMF was almost in the ecliptic plane and so approximately in the XY (VSO) plane; its cross-flow component was much larger than the parallel one. Positions of the bow shock (BS) and the induced Magnetospheric Boundary (MB) are marked by the dashed vertical lines. The magnetospheric cavity is almost void of solar wind plasma but filled by cold plasma of ionospheric origin dominated by O þ and H þ ions (see spectrograms of the fluxes of proton and oxygen ions on the third and fourth panels, respectively). The magnetic field permeates the ionosphere; this configuration is called ‘magnetized ionosphere’ and is typical for solar minimum conditions
and/or high solar wind dynamic pressure (Luhmann and Cravens, 1991). The characteristic features of the classical induced magnetosphere with a pile-up of the IMF, formation of the Magnetic pile-up Boundary (MB) and the magnetic field draping around the ionospheric obstacle are clearly observed in this example. The Plasma Sheet (PS), indicated by a drop in the magnetic field strength ( 07 : 12 UT) and separating two lobes of the magnetotail with opposite polarity of the field is also well seen. Note that a draping may occur without wrapping, as in the given case, or with wrapping on the nightside, where the cross-flow component of the magnetic field changes sign near the terminator. Examples of observations by VEX with wrapping signatures are shown in Fig. 3. The data are presented in VSE coordinates which are most suitable to study the solar wind interaction with planets like Mars or Venus having draped magnetospheric
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can be also interpreted as the signature of the induced dipole field. A sketch at the bottom assumes a reversal of the dipole axis, as compared to Fig. 5, in response to a change in the IMF direction. Fig. 7 depicts more examples, now for s/c orbital plane near the terminator plane; data are plotted in the (X % Y % ) plane. The cross-flow component of the IMF is always along þY % . Classical draping configuration assumes Bx 40 (Bx o 0) at Y % o 0 (Y % 4 0), respectively. Indeed, such a draping (see blue field lines in Fig. 7) is observed in the magnetosheath and upper part of the ionosphere. At lower altitudes, which correspond to the orbital segments adjacent to Y % 0, a wrapping with a change of sign in the Bx component is clearly observed (see red field lines in Fig. 7). This also supports a contribution of the poloidal component to the general field pattern. 2.4. Cases with IMF almost aligned with the solar wind flow
Fig. 4. Sketch of the field geometry in the northern polar region sampled by VEX: (a) for the toroidal (IMF lies in the ecliptic plane) and (b) for the poloidal (IMF is perpendicular to the ecliptic plane) field configurations, respectively.
configurations. We observe a reversal of the cross-flow component By% of the field near the terminator that is consistent with the sketch shown in Fig. 1a. 2.3. Poloidal magnetic fields At the dayside the effects of the electric currents driven by the motional electric field and the Faraday electric field are similar and hardly to be distinguished—the field lines are draping around the planet (Fig. 1a,b). On the nightside a distinction between the two different magnetic field topologies can be detected more easily from the variations in the Bx component. Superposition of the IMF with an induced dipole expected for the case of Faraday induction leads to a wrapping of the field lines around the planet on the nightside with a change of sign in the Bx -component near the terminator (Fig. 1b). It is worth noting that the highly elliptical polar orbit of the VEX spacecraft with its pericenter in the north is not very suitable for this analysis since for the nominal IMF lying in the ecliptic plane, a change of sign of the Bx component in the northern polar region due to Faraday currents might be misinterpreted with a change due to a plasma sheet crossing (Fig. 4a). On the other hand, for an IMF with significant component out of the XY (VSO) plane, i.e. when the northward or southward IMF component dominates, this region becomes very appropriate to observe possible effects of the poloidal component (Fig. 4b). Fig. 5 depicts four cases of the measurements made for a northward IMF, and where the s/c orbital plane was near the noon-midnight plane. Draping of the IMF results in a negative Bx (VSO) component in the þZ hemisphere sampled by VEX. A change of sign of the Bx component (shown as red vectors in Fig. 5) in all these cases does not fit to the classical draping pattern and can be attributed to the induced dipole component. A sketch on the bottom assumes the solar wind interaction with a small induced dipole which is consistent with the observations. One notices that the region occupied by a ‘dipole’ magnetic field is rather localized and usually observed near the terminator at low altitudes (hr 500 km). Fig. 6 shows examples for the southward IMF, again for s/c orbital plane near the noon-midnight plane. In this case a pure draping of the IMF results in a positive Bx (VSO) component near the pole (in the solar wind the Bx component is negative while in the inner þZ (VSO) magnetosheath, it becomes positive because of draping). There should be no change in sign of Bx near the closest approach. However, vectors deviating from this general draping pattern are observed and shown in red on the plot. These
The case when the IMF vector is aligned with the solar wind velocity vector is interesting since the motional electric field as a driver for the induced currents ceases or is strongly reduced. In this case, the currents induced by curl E ¼ @B=@t (solar wind carries a spatially varying Interplanetary Magnetic Field (IMF) across the planet) will produce a magnetic field which looks like the field of a degenerated dipole with its axis aligned with the solar wind flow. Fig. 8 (left panel) shows the magnetic field components and the total field strength for the orbit on November 15, 2006. The IMF conditions were rather steady during the crossing of the magnetosphere by VEX. In the solar wind the IMF had a negative Bx -component and the cross-flow component is small (Bx 7, 1 nT, B? 1:4 nT). Inside the magnetosphere, at altitudes hr 1500 km, the Bx -component changed its sign. This is well seen in the panel (b) which shows the magnetic field vectors projected onto the (XY) VSO plane. A change of sign is not consistent with a simple wrapping pattern and can be interpreted as the manifestation of the degenerated dipole-like magnetospheric configuration (Fig. 8c). Fig. 9 shows the magnetic field vectors projected onto (XY) VSO plane for two other cases where the IMF was also almost aligned with the solar wind flow. In the left panel (November 15, 2009) the IMF has a negative Bx -component, while in the right panel (May 11, 2008) Bx is positive. However, a change of sign occurs in both cases at altitudes h r 1000 km. As in the previous example, such a magnetic field configuration can appear due to superposition of the IMF and an induced dipole field. The axis of the dipole is antiparallel to the IMF direction (Fig. 9c, d). Note that there were no plasma measurements for the orbits on November 15, 2006 and November 15, 2009. For the case of May 11, 2008, where the IMF has a positive Bx VSO component, Fig. 10 displays details of the magnetic field and plasma measurements. On the inbound leg of the trajectory, the magnetic field is almost aligned with the X-axis ( 5:21). The angle increases to 28:61 when the spacecraft returned to the solar wind on the outbound leg. There is no signature at all of pile-up of the magnetic field in the inner part of the magnetosheath—the magnetic field value gradually decreases. This is particularly clearly seen on the inbound leg of the orbit. Zhang et al. (2009) have described similar cases as a ‘disappearing magnetosphere’. However, as is seen from the plasma data, the usual magnetospheric cavity void of solar wind plasma but filled with cold planetary plasma composed of O þ and H þ ions, exists. An increase in the field, ‘a magnetic barrier’, is observed just at the border of the magnetospheric cavity and is characterized by the reversal of the field orientation that is consistent with formation of the induced dipole magnetosphere. On the outbound leg, a reversal of the magnetic field vector is also observed.
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Fig. 5. Examples of magnetic field observations for northward IMF. Vectors are projected onto the (XZ)-VSO plane. Red arrows indicate the vectors pointing in the þ X-direction which are not consistent with the classical draping model. A possible field pattern for northward IMF with superposed poloidal induced field is shown in the (X,Z)-plane at the bottom. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
2.5. Statistical study Although we presented only two cases (Fig. 3) with wrapping signatures expected for toroidal magnetic fields, such a field topology is rather common. In the following we present results of a statistical study of magnetic field measurements recorded between March 2008 and February 2010 on 110 VEX orbits, at altitudes below 1500 km. Fig. 11 shows a map in the (X % Y % )plane of the cross-flow component By% for two hemispheres, the
E þ hemisphere (Z % 40), in which the motional electric field in the solar wind points outward from the planet, and the E hemisphere (Z % o0) with the electric field pointing towards the planet. A reversal of sign of the By% component is clearly observed mainly in the E hemisphere while a draping without wrapping occurs in the E þ hemisphere, i.e. the field geometry occurs different in the E þ and E hemispheres. A similar difference in the draping pattern in the near tail was reported by Zhang et al., 2010. A change of sign appears close to the
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Fig. 6. Examples of magnetic field observations for southward IMF. Vectors are projected onto the (XZ)-VSO plane. Red arrows indicate the vectors pointed in the Xdirection which are not consistent with the classical draping model. A possible field pattern for southward IMF with superposed poloidal induced field is shown in the (X,Z)-plane at the bottom. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
terminator plane (solar zenith angle, SZA 901) and at altitudes of r 350 km. In Section 2.3 for poloidal induced fields we presented 14 cases with the signatures of ‘anomalous’ draping—a change of sign in the Bx component where it is not expected in the classical draping pattern of the magnetic field. Here we plot the statistical map of the Bx component in the (X % Y % )-plane for 110 VEX orbits (Fig. 12). In the VSE reference frame, where the cross-flow component By% is always positive, classical draping geometry assumes that Bx must be positive for Y % o0 and negative for Y % 4 0. However, a
deviation from this pattern occurs: regions with an anomalous reversal of the Bx component are observed in both half-planes (indicated by white ellipses). Such a behavior is consistent with the case studies shown with a contribution of a poloidal field, as in Fig. 7.
3. Discussion The problem of the origin of the electric field driving induction currents is not new. It was widely discussed in applications to the
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Fig. 7. Examples of magnetic field observations with ‘anomalous’ draping features, for s/c orbits near the terminator plane. Vectors are projected onto the (X % Y % )-VSE plane where the cross-flow component of the IMF is in þ Y % direction. Wrapping with a change of sign of the Bx component (red field lines) is consistent with a poloidal induced magnetic field. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
Moon, Mars and Venus (see e.g. Sonett and Colburn, 1967; Dessler, 1967; Michel, 1971; Cloutier and Daniell, 1973; Johnson and Hanson, 1979; Podgornyi et al., 1982; Luhmann, 1992). The electric field (V sw BIMF ) related to the motion of the magnetized solar wind and ‘seen’ by a conductor in its reference frame, as well as the Faraday electric field (curl E ¼ @B=@t) can drive the electric currents and generate an induced magnetosphere. Although the topology of the magnetic fields for both kinds of currents is rather different, the difference is hardly distinguishable on the dayside of the obstacle where superposition of the IMF, perturbed by the currents at the bow shock, and the induced magnetic field results in a draping configuration. Near the terminator a difference between the field patterns becomes more visible. Toroidal magnetic fields are observed either without or with a wrapping around the planet (see Figs. 1– 3). The latter is easily noticed by a change of sign in the cross-flow component of the magnetic field near the terminator; similar features were observed by PVO (Phillips et al., 1986; Luhmann, 1992). We also observe that on orbits sampling the pericenter region in the E þ hemisphere (defined by the motional electric field) draping occurs without wrapping, while in the E hemisphere it occurs with wrapping; a similar difference was reported for the near tail by Zhang et al. (2010). For the first time, we here present observations which clearly demonstrate the existence of poloidal magnetic fields at Venus. In the VSO reference frame, this type of field can be identified by a change of sign of the Bx component in a region in space where its change is not expected for an ordinary draping model. In the case of an IMF with a significant Bz component (northward or southward) a change of sign in the Bx component due to the
poloidal field, cannot be misinterpreted as a crossing of the current sheet separating the lobes of the induced magnetosphere in the tail. The detection of signatures of both the toroidal and poloidal magnetic fields implies the coexistence of both induction mechanisms. It is often difficult to distinguish between both types of field lines because it depends on the relative strength of the induced currents and might differ from orbit to orbit. An interesting situation occurs when the IMF is nearly aligned with the solar wind velocity vector and the contribution of the motional electric field (nearly) vanishes or is strongly reduced. The data presented here show that also in this case a magnetospheric cavity void of solar wind is formed, i.e. in terms of plasma observations there is no visible difference as compared to the observations for a nominal IMF orientation. From the perspective of the magnetic field configuration, a striking difference appears. The magnetic barrier formed by the piled-up magnetic field lines is often absent (at least near the terminator) and the magnetic field inside the major part of the magnetosphere/ionosphere in the terminator region has a polarity opposite to that in the solar wind. Here, the existence of an induced dipole magnetic field is in reasonable agreement with the observations. The dipole axis occurs parallel or antiparallel to the solar wind flow. Therefore, the central part of the generated magnetosphere is filled with field lines of a polarity opposite to that in the solar wind. It is interesting to compare this to the earlier MHD simulations of the solar wind interaction with a conducting sphere for aligned field-flow conditions (De Zeeuw et al., 1996). In the central-most wake of that model the magnetic field orientation occurred opposite to the one in the upstream flow. A change of sign of the Bx component was even more pronounced in the associated PVO
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Fig. 9. (a, b) Magnetic field vectors projected onto the XY VSO plane for two VEX orbits in the terminator plane when Bx b B? in the solar wind. (c,d) A reversal of the field direction (red vectors in panels (a) and (b) within the magnetosphere/ionosphere) can be explained by induction of a poloidal dipole field with axis antiparallel to the IMF direction.
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measurements made in the near planetary wake for steady, near flow aligned conditions (De Zeeuw et al., 1996). This is consistent
2008 May 11
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Fig. 10. Magnetic field and plasma measurements for May 11, 2008 where the IMF was nearly parallel to solar wind flow. From top to bottom: magnetic field (Bx ,By ,Bz ) in VSO coordinates and the field value (Bt), spectrogram of electron fluxes and solar zenith angle (red line), spectrogram of the proton fluxes and VEX altitude (red line) and spectrogram of oxygen ions. Vertical lines indicate the boundaries of the magnetospheric cavity. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
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with our observations and the proposed field configuration of an induced field-antiparallel dipole. The characteristic feature of solar wind interaction with a flow aligned dipole is the existence of a circular plasma sheet (Podgornyi et al., 1977; Voigt et al., 1983). Therefore, it seems not surprising that for such type of IMF orientation, the spatial distribution of fluxes of the energized oxygen ions occurs rather circularly symmetrical (Masunaga et al., 2011). It is important to note that the analogy between the ionosphere and an ideal conductor is a rather crude approximation, although it seems to be able to explain the main features of the observations. Let us consider the relevant current systems on the dayside of the Venus ionosphere in the MHD approach which is better suitable to describe the ionospheric electrodynamics. In plasmas, like the solar wind or the top-side Venus ionosphere, a pattern of electrical currents is derived from the momentum equations for the ions and electrons. The component of j perpendicular to magnetic field B is composed of the pressure
Y, Rv
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Fig. 11. Map of the cross-flow component By% in the E þ (Z % 40) hemisphere in the VSE reference frame, in which the motional electric field of the solar wind points outward from the planet, and in the E (Z % o 0) hemisphere with the electric field pointing towards the planet. The data are projected onto the (X % Y % )-plane. Red arrow shows the cross-flow component of the IMF.
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E. Dubinin et al. / Planetary and Space Science 87 (2013) 19–29
Ionosphere Psh
Pi
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j
i
b
i
P
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sh
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sh
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Fig. 13. Sketch of the relevant current circuits on the dayside (view from side). (a) For solar maximum conditions the ionosphere is not magnetized. The current in the barrier (jb) providing a pileup of the magnetic field is closing via the shielding current (ji) on the ionopause. A matching of these currents implies a pressure balance between the magnetosheath plasma pressure Psh, the magnetic pressure in the pileup region (Pm) and the ionospheric pressure (Pi). (b) For solar minimum conditions when the ionospheric pressure is less than the solar wind dynamic pressure the current system shifts to lower altitudes where a new equilibrium is reached. (c) In this scenario, a part of the ionospheric currents flow entirely within the resistive ionosphere (red curve) closing in the layer where the conductivity is maximum. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
gradient, inertial and frictional currents dðrVÞ ; þF d B rP þ dt j? ¼ B2
ð1Þ
where r, P, V, Fd are the mass density, the plasma thermal pressure, the bulk speed, and the drag force respectively. The collisional drag force, Fd rvin(V Vn) arises from relative motion between plasma and neutrals. Here vin and Vn are the ion-neutral collision frequency and the bulk speed of neutrals, respectively. For a quasi-static equilibrium, the above expression for the current flowing in the interface between the regions occupied by collisionless plasma and magnetic field, respectively, is reduced to j? ¼ ðB rPÞ=B2 . This situation is realized for solar maximum conditions. The induced currents flowing at the ionopause cancel the field inside the ionosphere and enhance it outside forming a magnetic field barrier. These currents are closed in the outer part of the barrier and in the magnetosheath where they are maintained by the
pressure gradient of the magnetosheath plasma (jb ¼ ðB rPms Þ=B2 ) (Fig. 13a). The force jb B decelerates the incoming solar wind and provides a balance between solar wind dynamic pressure and the magnetic pressure of the pileup magnetic field Pdyn ¼ Pm . The current which flows on the ionopause is maintained by the pressure gradient of the ionospheric plasma ji ¼ ðrPi BÞ=B2 . Correspondingly, a pressure balance between the thermal ionospheric pressure and the pressure of the piled-up magnetic field is established (Pm ¼ Pi ). This pressure balance corresponds to a matching of the currents flowing in the magnetic barrier and on the ionopause. In the regime of solar wind overpressure typical for solar minimum conditions or for the cases with high solar wind pressure during the solar maximum period, the pressure gradient currents ji flowing on the ionopause are not sufficient to balance the currents jb flowing in the magnetic barrier and the adjacent magnetosheath. Such an imbalance of the currents and, correspondingly, of pressures leads to a broadening of the current system to lower altitudes and to a shift of the ionopause to a position where a new pressure balance can be maintained (Fig. 13b). As a result of this entry into the ionosphere, the solar wind is strongly mass-loaded and transfers its momentum to the ionospheric plasma. This implies the generation of additional photo-ion currents (the second term in Eq. (1) (Dubinin et al., 2011). As a result, the ionosphere becomes ‘magnetized’. The toroidal current loops which envelop the sunlit hemispherical ionospheric shell (Fig. 13a, b) produce a toroidal magnetic field around Venus. With a decrease of altitude, the collisions of ions and electrons with neutrals and, correspondingly, the frictional currents (the last term in Eq. (1)) become more and more important. Since the electrical contact of the ionosphere with solar wind plasma shrinks with decreasing altitude, the current closure can occur entirely within the ionosphere where the resistive conductivity reaches a maximum. Such a current circuit (shown in Fig. 13c in red), produces the poloidal magnetic field configuration. We conclude that, although the model used here explains the existence of toroidal and poloidal magnetic fields in the Venus ionosphere a lot of questions concerning the mechanisms of formation of an induced magnetosphere (for example a strong asymmetry between E þ and E hemispheres and giant flux ropes (Zhang et al., 2012) observed at low altitudes) remain open.
Acknowledgments Authors (E.D., M.F., J.W., Y.W.) wish to acknowledge support from DFG for supporting this work by Grants WO 910/3-1, MO 539/17-1 and DLR by Grants 50QM99035, FKZ 50 QM 0801. ED is also strongly indebted to ISSI for the support given to the International team ‘Comparative Studies of Induced Magnetospheres’. References Barabash, S., et al., 2007. The analyzer of space plasma and energetic atoms (ASPERA-4) for the Venus Express mission. Planetary and Space Science 55, 1772, http://dx.doi.org/10.1016/j.pss.2007.01.014. Blank, J.L., Sill, W.R., 1969. The response of the Moon to the time-varying interplanetary magnetic field. Journal of Geophysical Research 74, 736. Cloutier, P.A., Daniell, R.E., 1973. Ionospheric currents induced by solar wind interaction with planetary atmospheres. Planetary and Space Science 21, 463. Dessler, A.J., 1967. Ionizing plasma flux in the Martian upper atmosphere. In: Brandt, J.C., McElroy, M.B. (Eds.), Atmospheres of Mars and Venus. Gordon and Breach, London, pp. 241. De Zeeuw, D.L., et al., 1996. A new axisymmetric MHD model of the interaction of the solar wind with Venus. Journal of Geophysical Research 101, 5447. Dolginov, S.S., Dubinin, E., Yeroshenko, Ye., et al., 1981. About configuration of the magnetic field in the Venus tail. Cosmic Research 19, 624.
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Dubinin, E., Fraenz, M., Fedorov, A., et al., 2011. Ion energization and escape on Mars and Venus. Space Science Reviews 162, 173–211, http://dx.doi.org/ 10.1007/s11214-011-9831-7. Gold, T., 1966. In: Mackin, R.L. Jr., Neugebauer, M. (Eds.), The Solar Wind. Pergamon Press. Hollweg, J.V., 1969. Interaction of solar wind with the Moon and formation of a lunar limb shock. Journal of Geophysical Research 73, 7269. Johnson, F.S., Midgley, I.E., 1968. Notes on the Lunar magnetosphere. Journal of Geophysical Research 73, 1523. Johnson, F.S., Hanson, W.B., 1979. A new concept for the daytime magnetosphere. Geophysical Research Letters 6, 581. Luhmann, J.G., Phillips, J.L., Russell, C.T., 1987. Studies of the configuration of the Venus ionospheric magnetic field. Advances in Space Research 7 (12), 101. Luhmann, J.G., Cravens, T.E., 1991. Magnetic fields in the ionosphere of Venus. Space Science Reviews 55, 201. Luhmann, J.G., 1992. Pervasive large-scale magnetic fields in the Venus nightside ionosphere and their implications. Journal of Geophysical Research 97, 6103. Masunaga, K., et al., 2011. O þ outflow channels around Venus controlled by directions of the IMF: observations of high energy O þ ions around terminator. Journal of Geophysical Research 116, A09326, http://dx.doi.org/10.1029/ 2011JA016705. Michel, F.C., 1971. Solar wind interaction with planetary atmospheres. Reviews of Geophysics and Space Physics 9, 427. Phillips, J.L., Luhmann, J.G., Russell, C.T., 1986. Magnetic field configuration of the Venus magnetosheath. Journal of Geophysical Research 91, 7931. Podgornyi, I., Dubinin, E., Israelevich, P., Potanin, Yu., 1977. Dynamics of plasma in laboratory models of Earth and Uranus magnetospheres. Herold of Russian Academy of Sciences, Physics 41, 1870. Podgornyi, I., Dubinin, E., Israelevich, P., Sonett, C.P., 1982. Comparison of measurements of electromagnetic induction in the magnetosphere of Venus with laboratory simulations. The Moon and Planets 27, 397.
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Russell, C.T., Vaisberg, O., 1983. The interaction of the solar wind with Venus. In: Hunten, D.M. (Ed.), Venus. University of Arizona Press, Tucson. Russell, C.T., Luhmann, J.G., Strangeway, R.J., 2006. The solar wind interaction with Venus through the eyes of the Pioneer Venus Orbiter 54, 1482, http://dx.doi. org/10.1016/j.pss.2006.04.025. Saunders, M.A., Russell, C.T., 1986. Average dimension and magnetic structure of the distant Venus magnetotail. Journal of Geophysical Research 91, 5589. Schubert, G., Schwartz, K., 1969. A theory for the interpretation of Lunar surface magnetometer data. The Moon 1, 106. Schwartz, K., Sonett, C.P., Schubert, G., 1969. Unipolar induction in the Moon and lunar limb shock mechanism. The Moon 1, 7. Sonett, C.P., Colburn, D.S., 1967. Establishment of a Lunar unipolar generator and associated shock and wake by solar wind. Nature 216, 340. Voigt, G.-H., Hill, T.W., Dessler, A.J., 1983. The magnetosphere of Uranus, plasma sources, convection and field configuration. The Astrophysics Journal 266, 390. Zhang, T.L., Luhmann, J.G., Russell, C.T., 1961. The magnetic barrier at Venus. Journal of Geophysical Research 96, 11145, http://dx.doi.org/10.1029/ 91JA00088. Zhang, T.L., et al., 2006. Magnetic field investigation of the Venus plasma environment: expected new results. Planetary and Space Science 54, 1336, http://dx.doi.org/10.1016/j.pss.2006.04.018. Zhang, T.L., Du, J., Ma, Y., et al., 2009. Disappearing induced magnetosphere of Venus. Implications for close-in exoplanets. Geophysics Research Letters 36, L20203, http://dx.doi.org/10.1029/2009GL040515. Zhang, T.L., Baumjohann, W., Du, J., et al., 2010. Hemispheric asymmetry of the magnetic field wrapping pattern in the Venusian magnetotail. Geophysical Research Letters 37, L14202, http://dx.doi.org/10.1029/2010GL044020. Zhang, T.L., Baumjohann, W., Teh, W., et al. , 2012. Giant flux ropes observed in the magnetized ionosphere at Venus. Geophysical Research Letters 39, L23103, http://dx.doi.org/10.1029/2012GL054236.
Contents lists available at ScienceDirect
Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
Toroidal and poloidal magnetic fields at Venus. Venus Express observations E. Dubinin a,n, M. Fraenz a, J. Woch a, T.L. Zhang b, Y. Wei a, A. Fedorov c, S. Barabash d, R. Lundin d a
Max-Planck-Institute for Solar System Research, 37191, Katlenburg-Lindau, Germany Space Research Institute, Austrian Academy of Sciences, A-8042, Graz, Austria c Centre d’Etude Spatiale des Rayonnements, BP-44346, F31028 Toulouse, France d Swedish Institute of Space Physics, Box 812, S-98 128, Kiruna, Sweden b
a r t i c l e i n f o
abstract
Article history: Received 16 February 2012 Received in revised form 23 October 2012 Accepted 12 December 2012 Available online 22 December 2012
Magnetic field and plasma measurements carried out onboard Venus Express during solar minimum conditions suggest the existence of two kinds of magnetic field configuration in the Venusian ionosphere. We interpret these as the manifestation of two different types of generation mechanisms for the induced magnetosphere. A different magnetic field topology (toroidal and poloidal) arises if the induced currents are driven either by the solar wind motional electric field or by the Faraday electric field—a conducting ionosphere sees the magnetic field carried by solar wind as a time-varying field. At the dayside, both driving agents produce a similar draping pattern of the magnetic field. However, different magnetic field signatures inherent to both induction mechanisms appear at lower altitudes in the terminator region. The conditions at low solar EUV flux when the ionosphere of Venus becomes magnetized seem to be favorable to distinguish between two different types of the induced fields. We present cases of both types of the magnetic field topology. The cases when the effects of the Faraday induction become well noticeable are especially interesting since they provide us with an example of solar wind interaction with a tiny induced dipole field immersed into the ionosphere. Another interesting case when poloidal magnetic fields are evidently displayed is observed when the IMF vector is almost aligned with the solar wind velocity. In general case, both mechanisms of induction probably complement each other. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Venus Magnetic field Magnetosphere Ionosphere Venus Express
1. Introduction The idea of a captured or induced planetary magnetosphere was widely discussed after the first mission to the Moon. It was suggested that the induced currents in the lunar interior produce a magnetic field which can deflect the solar wind around the obstacle (see e.g. Gold, 1966; Sonett and Colburn, 1967; Hollweg, 1969; Johnson and Midgley, 1968; Schubert and Schwartz, 1969; Schwartz et al., 1969). Two main driving mechanisms of the electric currents were discussed: (1) Faraday induction (curlE ¼ @B=@t)—currents are induced by temporal changes in the external magnetic field and (2) the motional electric field (V sw BIMF ) of the solar wind plasma in the rest frame of the Moon (V sw and BIMF are the solar wind velocity and interplanetary magnetic field, respectively). In the case of the motional electric field, currents produce a toroidal magnetic field with a classical draping configuration on the dayside (see e.g. Figure 8b in Luhmann, 1992). In the case of the
n
Corresponding author. Tel.: þ49 55 56979129. E-mail address: [email protected] (E. Dubinin).
0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.12.003
Faraday induction, currents in the conductive body induced by a ‘switch on the external Interplanetary Magnetic Field (IMF)’ produce a dipole magnetic field whose axis is antiparallel to the IMF (see e.g. Figure 5 in Blank and Sill, 1969). However, because of very low electrical conductivity of the Moon, the induced currents occur too small to support these scenarios. The existence of induced magnetospheres was discovered later, namely at Venus and Mars, planets which also have no global magnetic field and solar wind interacts directly with their conducting ionospheres (see e.g. Russell and Vaisberg, 1983). How to distinguish or separate the effects of both mechanisms? An interesting experiment has been performed in the laboratory (Podgornyi et al., 1982). Magnetic field topologies arising at plasma flow interaction with a conductor and with a conductor covered by a insulator have been compared. If the motional electric field drives the current then electrical contact between a moving plasma and a conductor is necessary. If such a contact is inhibited then the motional electric field might be canceled by the polarization field and the only driving electric field which remains is the Faraday field. In this case the flowing plasma interacts with a degenerated induced dipole magnetic field (Podgornyi et al., 1982).
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Fig. 1. Sketch of possible field configurations for the induced currents driven by the motional electric field (a), Faraday electric field (b) and by both mechanisms. (c), in the (Vsw, BIMF) plane.
How to distinguish or separate the effects of both mechanisms at Venus? Fig. 1 shows a sketch of possible field configurations expected for both types of induction. For the currents driven by V sw BIMF electric field the net result is a superposition of the IMF perturbed by the currents at the bow shock and the induced toroidal magnetic field (Fig. 1a) providing us with a draping configuration on the dayside and with wrapping features on the nightside (Luhmann, 1992). The characteristic feature of this configuration might be, although it is not necessary, a change of sign of the cross-flow component of the magnetic field near the terminator. Superposition of the IMF with an induced dipole expected for the case of Faraday induction is shown in Fig. 1b. The characteristic feature of this type of interaction is a wrapping of the field lines around the planet on the nightside with a change of sign in the Bx -component (X-axis is toward the Sun) (Johnson and Midgley, 1968; Podgornyi et al., 1982; Luhmann, 1992). A dipole magnetic field inside the ionosphere is annihilated if the ionosphere is considered as an ideal conductor. A more complicated configuration arises when both mechanisms operate at the same time (see e.g. Fig. 1c). There might be also another interesting test for signatures of an induced dipole. In the case when the IMF is aligned or almost aligned with the solar wind velocity, currents driven by V sw BIMF field become very small and the effects of Faraday induction might be more apparent. Analysis of the first magnetic field measurements at Venus by the Russian spacecraft Venera-9, 10 has shown the occurrence of the toroidal pattern although a change of sign in the Bx -component on the nightside on some orbits suggested a contribution of both mechanisms (Dolginov et al., 1981; Podgornyi et al., 1982). Most information about solar wind interaction with Venus was obtained by the Pioneer Venus Orbiter (PVO) in 1979–1992 (see e.g. Russell et al., 2006). It was observed that at solar maximum a magnetic barrier forms on the dayside of Venus which deflects the solar wind around the ionosphere. This barrier occurs almost impenetrable for the solar wind with the frozen-in IMF, i.e. the ionosphere stays ‘unmagnetized’, except for narrow magnetic flux ropes (Zhang et al., 1991). On the nightside the classical draping configuration with a long magnetic tail is formed (Saunders and Russell, 1986). However, for a range of solar wind overpressure, i.e. when its dynamic pressure exceeds the ionospheric pressure, the dayside ionosphere becomes magnetized (Luhmann et al., 1987). A specific configuration of the magnetic field during such a regime is observed also on the nightside (Luhmann, 1992). Analyzing the data obtained during periods of high dynamic pressure at solar maximum epoch Luhmann (1992) has observed pervasive large amplitude magnetic fields wrapped around the planet with a change of sign in the main transverse component of the magnetic field. Such kind of wrapping was statistically confirmed by Phillips et al. (1986). Luhmann (1992) has interpreted these data in terms of the prevalence of an induced toroidal magnetic field. In this paper we analyze the magnetic field and plasma measurements carried out by VEX in the ionosphere/
magnetosphere mostly looking for signatures of ‘a poloidal induced magnetosphere’. During the PVO epoch, a survey of the ionospheric magnetic fields was done only at solar maximum conditions and such signatures were not found. In contrast, VEX observations during 2006–2010 were made close to minimum solar activity. We present observations which show that the configuration of the induced magnetosphere at Venus at solar minimum conditions sometimes differs from the classical draping model. The magnetic field measurements made in the northern polar ionosphere for the conditions when the Bz component (VSO reference frame) of the IMF dominates reveal features which might be interpreted as a signature of the poloidal magnetic fields induced by the currents driven by the Faraday electric field (curl E ¼ @B=@t). Similar types of signatures are also observed when the IMF is aligned or almost aligned with the solar wind flow.
2. Observations 2.1. Instrumentation Venus Express (VEX) has a highly elliptical polar orbit with a 24 h period and pericenter and apocenter of 180–350 km and 66,000 km, respectively. The magnetic field is measured by the MAG instrument (Zhang et al., 2006) and plasma by the ASPERA-4 package (Barabash et al., 2007). The Ion Mass Analyzer (IMA/ ASPERA-4) on VEX detects ions in the 10 eV/q–30 keV/q energy range and 1–44 amu/charge range, including both solar wind and planetary ions with time resolution of 192 s and field of view 901 3601. The electron spectrometer (ELS/ASPERA-4) provides the 2D distribution (16 sectors) of electron fluxes in the energy range of 5 eV–20 keV with a time resolution of 4 (1) s. In this paper 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 the VSO coordinates, the (XY) plane coincides with the Venus orbital plane, where X is directed to the Sun, Y is opposite to the planet’s orbital velocity, Z is perpendicular to X and Y and positive to ecliptic north. Since the orbital inclination of Venus is small (i¼ 3.341), the orbital plane nearly coincides with the ecliptic plane. In VSE coordinates, the X % -axis is antiparallel to the upstream solar wind flow (and equal to X VSO), and the Y % -axis along the cross-flow magnetic field component of the IMF, such that the IMF is in the (X % ,Y % )-plane and By% is always positive. Then the motional electric field E ¼ V sw BIMF is along the Z % -axis and points out of the planet in the (Z% 40) hemisphere. 2.2. Toroidal magnetic fields Fig. 2 presents a typical case of a classical draping configuration well described in terms of the toroidal magnetic field. From top to bottom are the magnetic field data in the VSO coordinate
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X, Rv Fig. 2. Magnetic field and plasma measurements by MAG and ASPERA-4 on February 9, 2007. (Left): Magnetic field components (Bx ,By ,Bz ) in the VSO coordinate system and the total field value (Bt), spectrogram of electron fluxes and solar zenith angle (SZA, red line), spectrogram of proton fluxes and VEX altitude, spectrogram of oxygen ion fluxes. Locations of the bow shock (BS), the Magnetospheric Boundary (MB) and the Plasma Sheet (PS) are shown by dashed lines. (Right): Map of the magnetic field vectors projected onto the XY VSO plane along the VEX orbit. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.) -2
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Fig. 3. Examples of MAG observations which are consistent with the sketch for toroidal field, shown in Fig. 1a. The maps are plotted in the (X % Y % )-plane which contains the solar wind velocity vector and the cross-flow component of the IMF.
system, spectrograms of electron, proton and oxygen ion fluxes, respectively, measured by VEX on February 9, 2007 (06:30– 07:45UT). The right panel shows the projections of the magnetic field vectors onto the XY (VSO) plane. The orbit of the s/c was in the terminator plane and the IMF was almost in the ecliptic plane and so approximately in the XY (VSO) plane; its cross-flow component was much larger than the parallel one. Positions of the bow shock (BS) and the induced Magnetospheric Boundary (MB) are marked by the dashed vertical lines. The magnetospheric cavity is almost void of solar wind plasma but filled by cold plasma of ionospheric origin dominated by O þ and H þ ions (see spectrograms of the fluxes of proton and oxygen ions on the third and fourth panels, respectively). The magnetic field permeates the ionosphere; this configuration is called ‘magnetized ionosphere’ and is typical for solar minimum conditions
and/or high solar wind dynamic pressure (Luhmann and Cravens, 1991). The characteristic features of the classical induced magnetosphere with a pile-up of the IMF, formation of the Magnetic pile-up Boundary (MB) and the magnetic field draping around the ionospheric obstacle are clearly observed in this example. The Plasma Sheet (PS), indicated by a drop in the magnetic field strength ( 07 : 12 UT) and separating two lobes of the magnetotail with opposite polarity of the field is also well seen. Note that a draping may occur without wrapping, as in the given case, or with wrapping on the nightside, where the cross-flow component of the magnetic field changes sign near the terminator. Examples of observations by VEX with wrapping signatures are shown in Fig. 3. The data are presented in VSE coordinates which are most suitable to study the solar wind interaction with planets like Mars or Venus having draped magnetospheric
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IMF in ecliptic plane
VEX
IMF perpendicular to ecliptic plane
VEX
can be also interpreted as the signature of the induced dipole field. A sketch at the bottom assumes a reversal of the dipole axis, as compared to Fig. 5, in response to a change in the IMF direction. Fig. 7 depicts more examples, now for s/c orbital plane near the terminator plane; data are plotted in the (X % Y % ) plane. The cross-flow component of the IMF is always along þY % . Classical draping configuration assumes Bx 40 (Bx o 0) at Y % o 0 (Y % 4 0), respectively. Indeed, such a draping (see blue field lines in Fig. 7) is observed in the magnetosheath and upper part of the ionosphere. At lower altitudes, which correspond to the orbital segments adjacent to Y % 0, a wrapping with a change of sign in the Bx component is clearly observed (see red field lines in Fig. 7). This also supports a contribution of the poloidal component to the general field pattern. 2.4. Cases with IMF almost aligned with the solar wind flow
Fig. 4. Sketch of the field geometry in the northern polar region sampled by VEX: (a) for the toroidal (IMF lies in the ecliptic plane) and (b) for the poloidal (IMF is perpendicular to the ecliptic plane) field configurations, respectively.
configurations. We observe a reversal of the cross-flow component By% of the field near the terminator that is consistent with the sketch shown in Fig. 1a. 2.3. Poloidal magnetic fields At the dayside the effects of the electric currents driven by the motional electric field and the Faraday electric field are similar and hardly to be distinguished—the field lines are draping around the planet (Fig. 1a,b). On the nightside a distinction between the two different magnetic field topologies can be detected more easily from the variations in the Bx component. Superposition of the IMF with an induced dipole expected for the case of Faraday induction leads to a wrapping of the field lines around the planet on the nightside with a change of sign in the Bx -component near the terminator (Fig. 1b). It is worth noting that the highly elliptical polar orbit of the VEX spacecraft with its pericenter in the north is not very suitable for this analysis since for the nominal IMF lying in the ecliptic plane, a change of sign of the Bx component in the northern polar region due to Faraday currents might be misinterpreted with a change due to a plasma sheet crossing (Fig. 4a). On the other hand, for an IMF with significant component out of the XY (VSO) plane, i.e. when the northward or southward IMF component dominates, this region becomes very appropriate to observe possible effects of the poloidal component (Fig. 4b). Fig. 5 depicts four cases of the measurements made for a northward IMF, and where the s/c orbital plane was near the noon-midnight plane. Draping of the IMF results in a negative Bx (VSO) component in the þZ hemisphere sampled by VEX. A change of sign of the Bx component (shown as red vectors in Fig. 5) in all these cases does not fit to the classical draping pattern and can be attributed to the induced dipole component. A sketch on the bottom assumes the solar wind interaction with a small induced dipole which is consistent with the observations. One notices that the region occupied by a ‘dipole’ magnetic field is rather localized and usually observed near the terminator at low altitudes (hr 500 km). Fig. 6 shows examples for the southward IMF, again for s/c orbital plane near the noon-midnight plane. In this case a pure draping of the IMF results in a positive Bx (VSO) component near the pole (in the solar wind the Bx component is negative while in the inner þZ (VSO) magnetosheath, it becomes positive because of draping). There should be no change in sign of Bx near the closest approach. However, vectors deviating from this general draping pattern are observed and shown in red on the plot. These
The case when the IMF vector is aligned with the solar wind velocity vector is interesting since the motional electric field as a driver for the induced currents ceases or is strongly reduced. In this case, the currents induced by curl E ¼ @B=@t (solar wind carries a spatially varying Interplanetary Magnetic Field (IMF) across the planet) will produce a magnetic field which looks like the field of a degenerated dipole with its axis aligned with the solar wind flow. Fig. 8 (left panel) shows the magnetic field components and the total field strength for the orbit on November 15, 2006. The IMF conditions were rather steady during the crossing of the magnetosphere by VEX. In the solar wind the IMF had a negative Bx -component and the cross-flow component is small (Bx 7, 1 nT, B? 1:4 nT). Inside the magnetosphere, at altitudes hr 1500 km, the Bx -component changed its sign. This is well seen in the panel (b) which shows the magnetic field vectors projected onto the (XY) VSO plane. A change of sign is not consistent with a simple wrapping pattern and can be interpreted as the manifestation of the degenerated dipole-like magnetospheric configuration (Fig. 8c). Fig. 9 shows the magnetic field vectors projected onto (XY) VSO plane for two other cases where the IMF was also almost aligned with the solar wind flow. In the left panel (November 15, 2009) the IMF has a negative Bx -component, while in the right panel (May 11, 2008) Bx is positive. However, a change of sign occurs in both cases at altitudes h r 1000 km. As in the previous example, such a magnetic field configuration can appear due to superposition of the IMF and an induced dipole field. The axis of the dipole is antiparallel to the IMF direction (Fig. 9c, d). Note that there were no plasma measurements for the orbits on November 15, 2006 and November 15, 2009. For the case of May 11, 2008, where the IMF has a positive Bx VSO component, Fig. 10 displays details of the magnetic field and plasma measurements. On the inbound leg of the trajectory, the magnetic field is almost aligned with the X-axis ( 5:21). The angle increases to 28:61 when the spacecraft returned to the solar wind on the outbound leg. There is no signature at all of pile-up of the magnetic field in the inner part of the magnetosheath—the magnetic field value gradually decreases. This is particularly clearly seen on the inbound leg of the orbit. Zhang et al. (2009) have described similar cases as a ‘disappearing magnetosphere’. However, as is seen from the plasma data, the usual magnetospheric cavity void of solar wind plasma but filled with cold planetary plasma composed of O þ and H þ ions, exists. An increase in the field, ‘a magnetic barrier’, is observed just at the border of the magnetospheric cavity and is characterized by the reversal of the field orientation that is consistent with formation of the induced dipole magnetosphere. On the outbound leg, a reversal of the magnetic field vector is also observed.
E. Dubinin et al. / Planetary and Space Science 87 (2013) 19–29
2009 Jan 18
VEX
2009 Jun 12
VEX nT
10 nT
08:28 08:38
01:28 08:18
08:48
01:18 08:08
08:58
01:38
01:08 01:48
Z, Rv
Z, Rv
23
00:58
09:08
X,Rv
X,Rv
2009 Jun 17
VEX
2009 May 21
VEX nT
nT
01:48
01:18
01:58
01:38
01:28
01:08 01:38
Z, Rv
Z, Rv
02:08
X, Rv
00:58 01:48
X, Rv
Fig. 5. Examples of magnetic field observations for northward IMF. Vectors are projected onto the (XZ)-VSO plane. Red arrows indicate the vectors pointing in the þ X-direction which are not consistent with the classical draping model. A possible field pattern for northward IMF with superposed poloidal induced field is shown in the (X,Z)-plane at the bottom. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
2.5. Statistical study Although we presented only two cases (Fig. 3) with wrapping signatures expected for toroidal magnetic fields, such a field topology is rather common. In the following we present results of a statistical study of magnetic field measurements recorded between March 2008 and February 2010 on 110 VEX orbits, at altitudes below 1500 km. Fig. 11 shows a map in the (X % Y % )plane of the cross-flow component By% for two hemispheres, the
E þ hemisphere (Z % 40), in which the motional electric field in the solar wind points outward from the planet, and the E hemisphere (Z % o0) with the electric field pointing towards the planet. A reversal of sign of the By% component is clearly observed mainly in the E hemisphere while a draping without wrapping occurs in the E þ hemisphere, i.e. the field geometry occurs different in the E þ and E hemispheres. A similar difference in the draping pattern in the near tail was reported by Zhang et al., 2010. A change of sign appears close to the
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E. Dubinin et al. / Planetary and Space Science 87 (2013) 19–29
2008 Mar 23
VEX
2009 Mar 5
VEX
nT
10 nT
04:03
05:58
04:13
05:48
06:08
03:53
05:38
Z, Rv
Z, Rv
06:18 03:43
05:28
06:28
X, Rv
X, Rv
2008 Mar 4
VEX
2009 Jan 30
VEX
nT
5 nT
04:28 04:18
08:18 04:38
08:28
08:08 07:58
04:08
Z, Rv
Z, Rv
04:48 03:58
08:38
X, Rv
X, Rv
Fig. 6. Examples of magnetic field observations for southward IMF. Vectors are projected onto the (XZ)-VSO plane. Red arrows indicate the vectors pointed in the Xdirection which are not consistent with the classical draping model. A possible field pattern for southward IMF with superposed poloidal induced field is shown in the (X,Z)-plane at the bottom. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
terminator plane (solar zenith angle, SZA 901) and at altitudes of r 350 km. In Section 2.3 for poloidal induced fields we presented 14 cases with the signatures of ‘anomalous’ draping—a change of sign in the Bx component where it is not expected in the classical draping pattern of the magnetic field. Here we plot the statistical map of the Bx component in the (X % Y % )-plane for 110 VEX orbits (Fig. 12). In the VSE reference frame, where the cross-flow component By% is always positive, classical draping geometry assumes that Bx must be positive for Y % o0 and negative for Y % 4 0. However, a
deviation from this pattern occurs: regions with an anomalous reversal of the Bx component are observed in both half-planes (indicated by white ellipses). Such a behavior is consistent with the case studies shown with a contribution of a poloidal field, as in Fig. 7.
3. Discussion The problem of the origin of the electric field driving induction currents is not new. It was widely discussed in applications to the
E. Dubinin et al. / Planetary and Space Science 87 (2013) 19–29
Sept 3, 2008
-2
Sept 5, 2008
-2
10 nT
10 nT
25
Sept 13, 2008
-2 10 nT
07:08
06:58 06:58
-1
07:18
-1
06:48
-1 07:08
06:38
0
Y,* Rv
Y,* Rv
Y,* Rv
06:48
0
0 06:58
06:38 06:28
1
1
06:48
1
06:28
06:38
06:18 06:28 06:08
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2
2 -1
0
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2
X,* Rv
10 nT
* Y,Rv
Y,* Rv
06:38
-2
Feb 11, 2007
-2 10 nT
04:28
-1
06:48
-1
0 X,* Rv
04:38
06:58
0
1
2
March 29, 2009
-2
10 nT
-1
-2
X,* Rv May 26, 2007
-2
-1
0
-1
Y,* Rv
2
04:18
0
06:58
07:08
0
04:08 06:28
1
1 03:58
06:18
2
2 2
1
0
07:18
1
-1
-2
2 2
1
X,* Rv
0 X,* Rv
-1
-2
2
1
0
-1
-2
X,* Rv
Fig. 7. Examples of magnetic field observations with ‘anomalous’ draping features, for s/c orbits near the terminator plane. Vectors are projected onto the (X % Y % )-VSE plane where the cross-flow component of the IMF is in þ Y % direction. Wrapping with a change of sign of the Bx component (red field lines) is consistent with a poloidal induced magnetic field. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
Moon, Mars and Venus (see e.g. Sonett and Colburn, 1967; Dessler, 1967; Michel, 1971; Cloutier and Daniell, 1973; Johnson and Hanson, 1979; Podgornyi et al., 1982; Luhmann, 1992). The electric field (V sw BIMF ) related to the motion of the magnetized solar wind and ‘seen’ by a conductor in its reference frame, as well as the Faraday electric field (curl E ¼ @B=@t) can drive the electric currents and generate an induced magnetosphere. Although the topology of the magnetic fields for both kinds of currents is rather different, the difference is hardly distinguishable on the dayside of the obstacle where superposition of the IMF, perturbed by the currents at the bow shock, and the induced magnetic field results in a draping configuration. Near the terminator a difference between the field patterns becomes more visible. Toroidal magnetic fields are observed either without or with a wrapping around the planet (see Figs. 1– 3). The latter is easily noticed by a change of sign in the cross-flow component of the magnetic field near the terminator; similar features were observed by PVO (Phillips et al., 1986; Luhmann, 1992). We also observe that on orbits sampling the pericenter region in the E þ hemisphere (defined by the motional electric field) draping occurs without wrapping, while in the E hemisphere it occurs with wrapping; a similar difference was reported for the near tail by Zhang et al. (2010). For the first time, we here present observations which clearly demonstrate the existence of poloidal magnetic fields at Venus. In the VSO reference frame, this type of field can be identified by a change of sign of the Bx component in a region in space where its change is not expected for an ordinary draping model. In the case of an IMF with a significant Bz component (northward or southward) a change of sign in the Bx component due to the
poloidal field, cannot be misinterpreted as a crossing of the current sheet separating the lobes of the induced magnetosphere in the tail. The detection of signatures of both the toroidal and poloidal magnetic fields implies the coexistence of both induction mechanisms. It is often difficult to distinguish between both types of field lines because it depends on the relative strength of the induced currents and might differ from orbit to orbit. An interesting situation occurs when the IMF is nearly aligned with the solar wind velocity vector and the contribution of the motional electric field (nearly) vanishes or is strongly reduced. The data presented here show that also in this case a magnetospheric cavity void of solar wind is formed, i.e. in terms of plasma observations there is no visible difference as compared to the observations for a nominal IMF orientation. From the perspective of the magnetic field configuration, a striking difference appears. The magnetic barrier formed by the piled-up magnetic field lines is often absent (at least near the terminator) and the magnetic field inside the major part of the magnetosphere/ionosphere in the terminator region has a polarity opposite to that in the solar wind. Here, the existence of an induced dipole magnetic field is in reasonable agreement with the observations. The dipole axis occurs parallel or antiparallel to the solar wind flow. Therefore, the central part of the generated magnetosphere is filled with field lines of a polarity opposite to that in the solar wind. It is interesting to compare this to the earlier MHD simulations of the solar wind interaction with a conducting sphere for aligned field-flow conditions (De Zeeuw et al., 1996). In the central-most wake of that model the magnetic field orientation occurred opposite to the one in the upstream flow. A change of sign of the Bx component was even more pronounced in the associated PVO
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E. Dubinin et al. / Planetary and Space Science 87 (2013) 19–29
Nov 15, 2006
B t , nT
MB
2006 Nov 15
40 30 20 10 0 -10 -20 40
06:38
Bx 06:28
By
Bz
30
Y, Rv
B xyz , nT
BS
Bt
06:18
20 06:08
10
0 SZA, deg 46.2 Alt, km 8770.9 UT 0540
50.9 3532.2 0600
99.3 328.1 0620
05:58
134 3932.5 0640
05:48 05:38
X, Rv VEX
Fig. 8. (a) The magnetic field components (Bx ,By ,Bz ) in VSO coordinates and the total field strength Bt on November 15, 2006 where the IMF was nearly parallel to Vsw. (b) Projection of the magnetic field vectors onto (XY)-VSO plane. Red vectors indicate a reversal of the magnetic field within the magnetosphere/ionosphere. (c) Sketch illustrating the superposition of the draped IMF (blue lines) and poloidal dipole field lines (red lines) in the (X,Y)-plane. The dipole axis is antiparallel to the IMF direction. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
Nov15, 2009; 04:00-05:30 UT
May 11, 2008; 02:40-04:00 UT
02:38 02:48 02:58
05:28 05:18 05:08
03:08
04:48
03:18
Y, Rv
Y, Rv
04:58
03:28
04:38
5 nT
03:38
04:28 04:18 04:08 03:58
03:48 03:58 5 nT
X, Rv
X, Rv
Vsw
BIMF
Vsw
BIMF
Fig. 9. (a, b) Magnetic field vectors projected onto the XY VSO plane for two VEX orbits in the terminator plane when Bx b B? in the solar wind. (c,d) A reversal of the field direction (red vectors in panels (a) and (b) within the magnetosphere/ionosphere) can be explained by induction of a poloidal dipole field with axis antiparallel to the IMF direction.
E. Dubinin et al. / Planetary and Space Science 87 (2013) 19–29
measurements made in the near planetary wake for steady, near flow aligned conditions (De Zeeuw et al., 1996). This is consistent
2008 May 11
VEX
MB
20
MB Bx Bz
0
By
108
Electrons
SZA
102
107
106
10 104
-2
+
H
103
ALT
B
108
102
O+
*
108 10
3
20
0
0
1
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*
107
102
10 Alt., km SZA UT
-1
107 109
10 104
40 Alt. < 1500 km
109
Bx, nT
103
106 4658.1 102.4 0300
785.4 81.3 0330
8052.7 77.9 0400
Fig. 10. Magnetic field and plasma measurements for May 11, 2008 where the IMF was nearly parallel to solar wind flow. From top to bottom: magnetic field (Bx ,By ,Bz ) in VSO coordinates and the field value (Bt), spectrogram of electron fluxes and solar zenith angle (red line), spectrogram of the proton fluxes and VEX altitude (red line) and spectrogram of oxygen ions. Vertical lines indicate the boundaries of the magnetospheric cavity. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
-40
2 1
2
X,* Rv Fig. 12. Map of the component Bx% in the (X % Y % )-plane. Regions with an ‘anomalous’ draping are indicated by white ellipses.
+
-
E hemisphere -2
-1
20
0
0
1
2 2
1
0
X,* Rv
-1
-2
40
Alt. < 1500km Z * <0
B
20
0
0
-20
1
-20
-40
2
Y,* Rv
-1
By,* nT
Y,*Rv
B
E hemisphere -2
40
Alt. < 1500km Z* > 0
-2
-1
0
By,* nT
Energy, e V
Energy, e V Alt, km
Energy, e V SZA, deg
-10
with our observations and the proposed field configuration of an induced field-antiparallel dipole. The characteristic feature of solar wind interaction with a flow aligned dipole is the existence of a circular plasma sheet (Podgornyi et al., 1977; Voigt et al., 1983). Therefore, it seems not surprising that for such type of IMF orientation, the spatial distribution of fluxes of the energized oxygen ions occurs rather circularly symmetrical (Masunaga et al., 2011). It is important to note that the analogy between the ionosphere and an ideal conductor is a rather crude approximation, although it seems to be able to explain the main features of the observations. Let us consider the relevant current systems on the dayside of the Venus ionosphere in the MHD approach which is better suitable to describe the ionospheric electrodynamics. In plasmas, like the solar wind or the top-side Venus ionosphere, a pattern of electrical currents is derived from the momentum equations for the ions and electrons. The component of j perpendicular to magnetic field B is composed of the pressure
Y, Rv
B, nT
Bt 10
27
-40 2
1
0
-1
-2
X,* Rv
Fig. 11. Map of the cross-flow component By% in the E þ (Z % 40) hemisphere in the VSE reference frame, in which the motional electric field of the solar wind points outward from the planet, and in the E (Z % o 0) hemisphere with the electric field pointing towards the planet. The data are projected onto the (X % Y % )-plane. Red arrow shows the cross-flow component of the IMF.
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E. Dubinin et al. / Planetary and Space Science 87 (2013) 19–29
Ionosphere Psh
Pi
j
j
i
b
i
P
Pi
sh
Ionosphere Psh
B
V
j
b
E
ji
Pi
P
sh
I
Ionosphere
Fig. 13. Sketch of the relevant current circuits on the dayside (view from side). (a) For solar maximum conditions the ionosphere is not magnetized. The current in the barrier (jb) providing a pileup of the magnetic field is closing via the shielding current (ji) on the ionopause. A matching of these currents implies a pressure balance between the magnetosheath plasma pressure Psh, the magnetic pressure in the pileup region (Pm) and the ionospheric pressure (Pi). (b) For solar minimum conditions when the ionospheric pressure is less than the solar wind dynamic pressure the current system shifts to lower altitudes where a new equilibrium is reached. (c) In this scenario, a part of the ionospheric currents flow entirely within the resistive ionosphere (red curve) closing in the layer where the conductivity is maximum. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
gradient, inertial and frictional currents dðrVÞ ; þF d B rP þ dt j? ¼ B2
ð1Þ
where r, P, V, Fd are the mass density, the plasma thermal pressure, the bulk speed, and the drag force respectively. The collisional drag force, Fd rvin(V Vn) arises from relative motion between plasma and neutrals. Here vin and Vn are the ion-neutral collision frequency and the bulk speed of neutrals, respectively. For a quasi-static equilibrium, the above expression for the current flowing in the interface between the regions occupied by collisionless plasma and magnetic field, respectively, is reduced to j? ¼ ðB rPÞ=B2 . This situation is realized for solar maximum conditions. The induced currents flowing at the ionopause cancel the field inside the ionosphere and enhance it outside forming a magnetic field barrier. These currents are closed in the outer part of the barrier and in the magnetosheath where they are maintained by the
pressure gradient of the magnetosheath plasma (jb ¼ ðB rPms Þ=B2 ) (Fig. 13a). The force jb B decelerates the incoming solar wind and provides a balance between solar wind dynamic pressure and the magnetic pressure of the pileup magnetic field Pdyn ¼ Pm . The current which flows on the ionopause is maintained by the pressure gradient of the ionospheric plasma ji ¼ ðrPi BÞ=B2 . Correspondingly, a pressure balance between the thermal ionospheric pressure and the pressure of the piled-up magnetic field is established (Pm ¼ Pi ). This pressure balance corresponds to a matching of the currents flowing in the magnetic barrier and on the ionopause. In the regime of solar wind overpressure typical for solar minimum conditions or for the cases with high solar wind pressure during the solar maximum period, the pressure gradient currents ji flowing on the ionopause are not sufficient to balance the currents jb flowing in the magnetic barrier and the adjacent magnetosheath. Such an imbalance of the currents and, correspondingly, of pressures leads to a broadening of the current system to lower altitudes and to a shift of the ionopause to a position where a new pressure balance can be maintained (Fig. 13b). As a result of this entry into the ionosphere, the solar wind is strongly mass-loaded and transfers its momentum to the ionospheric plasma. This implies the generation of additional photo-ion currents (the second term in Eq. (1) (Dubinin et al., 2011). As a result, the ionosphere becomes ‘magnetized’. The toroidal current loops which envelop the sunlit hemispherical ionospheric shell (Fig. 13a, b) produce a toroidal magnetic field around Venus. With a decrease of altitude, the collisions of ions and electrons with neutrals and, correspondingly, the frictional currents (the last term in Eq. (1)) become more and more important. Since the electrical contact of the ionosphere with solar wind plasma shrinks with decreasing altitude, the current closure can occur entirely within the ionosphere where the resistive conductivity reaches a maximum. Such a current circuit (shown in Fig. 13c in red), produces the poloidal magnetic field configuration. We conclude that, although the model used here explains the existence of toroidal and poloidal magnetic fields in the Venus ionosphere a lot of questions concerning the mechanisms of formation of an induced magnetosphere (for example a strong asymmetry between E þ and E hemispheres and giant flux ropes (Zhang et al., 2012) observed at low altitudes) remain open.
Acknowledgments Authors (E.D., M.F., J.W., Y.W.) wish to acknowledge support from DFG for supporting this work by Grants WO 910/3-1, MO 539/17-1 and DLR by Grants 50QM99035, FKZ 50 QM 0801. ED is also strongly indebted to ISSI for the support given to the International team ‘Comparative Studies of Induced Magnetospheres’. References Barabash, S., et al., 2007. The analyzer of space plasma and energetic atoms (ASPERA-4) for the Venus Express mission. Planetary and Space Science 55, 1772, http://dx.doi.org/10.1016/j.pss.2007.01.014. Blank, J.L., Sill, W.R., 1969. The response of the Moon to the time-varying interplanetary magnetic field. Journal of Geophysical Research 74, 736. Cloutier, P.A., Daniell, R.E., 1973. Ionospheric currents induced by solar wind interaction with planetary atmospheres. Planetary and Space Science 21, 463. Dessler, A.J., 1967. Ionizing plasma flux in the Martian upper atmosphere. In: Brandt, J.C., McElroy, M.B. (Eds.), Atmospheres of Mars and Venus. Gordon and Breach, London, pp. 241. De Zeeuw, D.L., et al., 1996. A new axisymmetric MHD model of the interaction of the solar wind with Venus. Journal of Geophysical Research 101, 5447. Dolginov, S.S., Dubinin, E., Yeroshenko, Ye., et al., 1981. About configuration of the magnetic field in the Venus tail. Cosmic Research 19, 624.
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