**2. Vortex structures**

Ample information on the speed and density values of the solar wind and planetary ions is their energy spectra obtained with the ASPERA instrument onboard the VEX spacecraft. A suitable example is provided by the data of the Sept. 26–2009 orbit reproduced in **Figure 2** and that probed by the near vicinity of the midnight plane (small Y-values shown at the bottom of the figure). The energy spectra of the O+ ion component (second panel) exhibit variations that indicate the presence of appreciable planetary O+ ion fluxes between 02:05 UT and 02:30 UT and that lead to enhanced values of their density and speed (third and fifth panels). At the same time the magnetic field intensity exhibits decreased values within that time interval with an oscillating response of its components (seventh panel). The later indicates the possible presence of a vortex structure within the region where enhanced values of the density and speed of the O+ ion fluxes are clearly distinguished. An important property of the observed values is that the kinetic energy density of such fluxes (mnv2 ~ 10 10−10 ergs cm−3) is comparable to the magnetic energy density (B2 /8π ~ 4 10−10 ergs cm−3) measured in the vicinity of that region where B ≈ 10 nT. As a result a near pressure balance condition between values measured outside is suggested within a region with evidence of a vortex structure. It is to be noted that these variations occurred far downstream from the planet (by X < −1.70 RV with this latter parameter being the Venus radius) and thus are unrelated to the crossing of the VEX

**65**

by both vectors).

*and speed values (third and fifth panels).*

**Figure 2.**

*Solar Cycle Variations in the Position of Vortex Structures in the Venus Wake*

spacecraft near the Venus ionosphere where X ≈ 0 (X, Y, and Z represent Cartesian coordinates indicating, respectively, the Venus-sun direction, the direction opposite to the motion of Venus around the sun, and the axis transverse to the plane formed

*Energy spectra of the H+ and O+ ions (upper panels) measured during the sept 26–2009 VEX orbit in the Venus wake by the midnight plane (small Y-values below the figure). In measurements between 02:05 UT and 02:30 UT there are oscillations in the magnetic field components (bottom panel) indicating a vortex structure. In that time interval there are also decreased values of the magnetic field intensity with enhanced O+ density* 

Similar conditions have also been identified in other VEX orbits that were traced by the vicinity of the midnight plane and that exhibit as well a near pressure balance condition between the region where enhanced density and speed values of the planetary ions are measured with the magnetic energy density of the magnetic fluxes encountered in their vicinity [10]. At the same time there is no evidence of a sudden and strong reversal in the direction of the Bx magnetic field component as it would be expected across a plasma sheet embedded by the middle of the wake. On the contrary the enhanced values of the density of the O+ ion population measured by the vortex structure indicates that there is not a local plasma expansion in that region.

Different from those properties it is necessary to examine changes of the vortex structures in their position along the nearly 8 years of observations conducted with the VEX spacecraft (between 2006 and 2013). A quantitative analysis of their location was made when VEX entered and exited those structures that was derived from the energy spectra of the planetary ions in the 20 VEX orbits listed in **Table 1**. A comparative view of the distribution of the vortex structures on the XZ plane of the solar wind velocity direction is presented in **Figure 3** to show the position of the VEX entry and exit crossings in orbits that probed near the midnight plane. Two sets with 4 orbits corresponding to measurements made in 2006 and in 2009 indicate a different displacement of the vortex structures in the Z-direction. There is a general preference of those features to occur closer to Venus in the 2009 measurements since their passage across the Z = 0 axis is by X = −1.7 RV in that set while it reaches X = −2.2 RV in the 2006 measurements.

**3. Distribution of vortex structures in the Venus wake**

*DOI: http://dx.doi.org/10.5772/intechopen.96710*

*Solar Cycle Variations in the Position of Vortex Structures in the Venus Wake DOI: http://dx.doi.org/10.5772/intechopen.96710*

#### **Figure 2.**

*Solar System Planets and Exoplanets*

pattern of the solar wind direction in cylindrical coordinates along the Venus wake is added in the right side panel of **Figure 1** with indications of a sunward directed flow return in the central wake. Comparable variations in the plasma velocity vector have also been recently reported from the VEX measurements [6]. Different from that motion there has also been information derived from the PVO and VEX measurements on an east–west displacement of planetary ion fluxes and that leads to the overall deflection of the trans-terminator ionospheric flow toward the dawn-hemisphere as it moves into the night-side ([7], see Fig. 15; [4], see Fig. 7b). A deflection in that direction can be accounted for in terms of the fluid dynamic Magnus force produced by the joint contribution of the unidirectional solar wind

*(left panel) velocity vectors of H+* ≈ *1–300 eV ions measured with the VEX spacecraft in the Venus wake projected on the YZ plane transverse to the solar wind direction. Data are averaged in 1000 x 1000 km columns at X <* −*1.5 RV (adapted from Figure 4 of [5]). (right panel) average direction of solar wind ion velocity vectors across the Venus near wake collected from many VEX orbits and projected in cylindrical* 

Ample information on the speed and density values of the solar wind and planetary ions is their energy spectra obtained with the ASPERA instrument onboard the VEX spacecraft. A suitable example is provided by the data of the Sept. 26–2009 orbit reproduced in **Figure 2** and that probed by the near vicinity of the midnight plane (small Y-values shown at the bottom of the figure). The energy spectra of the O+ ion component (second panel) exhibit variations that indicate the presence of appreciable planetary O+ ion fluxes between 02:05 UT and 02:30 UT and that lead to enhanced values of their density and speed (third and fifth panels). At the same time the magnetic field intensity exhibits decreased values within that time interval with an oscillating response of its components (seventh panel). The later indicates the possible presence of a vortex structure within the region where enhanced values of the density and speed of the O+ ion fluxes are clearly distinguished. An important property of the observed values is that the kinetic energy density of such fluxes

~ 10 10−10 ergs cm−3) is comparable to the magnetic energy density (B2

10−10 ergs cm−3) measured in the vicinity of that region where B ≈ 10 nT. As a result a near pressure balance condition between values measured outside is suggested within a region with evidence of a vortex structure. It is to be noted that these variations occurred far downstream from the planet (by X < −1.70 RV with this latter parameter being the Venus radius) and thus are unrelated to the crossing of the VEX

/8π ~ 4

velocity and the rotation of the Venus atmosphere/ionosphere [8, 9].

**64**

(mnv2

**2. Vortex structures**

**Figure 1.**

*coordinates [15].*

*Energy spectra of the H+ and O+ ions (upper panels) measured during the sept 26–2009 VEX orbit in the Venus wake by the midnight plane (small Y-values below the figure). In measurements between 02:05 UT and 02:30 UT there are oscillations in the magnetic field components (bottom panel) indicating a vortex structure. In that time interval there are also decreased values of the magnetic field intensity with enhanced O+ density and speed values (third and fifth panels).*

spacecraft near the Venus ionosphere where X ≈ 0 (X, Y, and Z represent Cartesian coordinates indicating, respectively, the Venus-sun direction, the direction opposite to the motion of Venus around the sun, and the axis transverse to the plane formed by both vectors).

Similar conditions have also been identified in other VEX orbits that were traced by the vicinity of the midnight plane and that exhibit as well a near pressure balance condition between the region where enhanced density and speed values of the planetary ions are measured with the magnetic energy density of the magnetic fluxes encountered in their vicinity [10]. At the same time there is no evidence of a sudden and strong reversal in the direction of the Bx magnetic field component as it would be expected across a plasma sheet embedded by the middle of the wake. On the contrary the enhanced values of the density of the O+ ion population measured by the vortex structure indicates that there is not a local plasma expansion in that region.
