Solar System Physics Group Grande et al, Venus, RAS 2010 Solar wind interactions and Ionospheric loss mechanisms at Venus M Grande, A G Wood, I C Whittaker, G Guymer and A Breen S Barabash Swedish Institute of Space Physics T Zhang Austrian Academy of Sciences

Solar System Physics Group Grande et al, Venus, RAS 2010 Boundaries and effect of Solar Cycle. Atmospheric loss Flux tubes and transition parameter Trans-terminator flow. CME CIR structure observed by IPS and in situ at Venus 11 Outline

Solar System Physics Group Grande et al, Venus, RAS 2010 r (R v ) x (R v ) Boundary locations identified by algorithm Blue points – Bow Shock Red points – ICB

Solar System Physics Group Grande et al, Venus, RAS 2010 Bow Shock model Fitted curve agrees closely with previous results But Aber curve is the lowest altitude at the sub-solar point Whittaker et al JGR 2010

Solar System Physics Group Grande et al, Venus, RAS 2010 Sub solar points vary linearly with average sunspot number Compare solar conditions of different models Bow Shock model Whittaker et al JGR 2010

Solar System Physics Group Grande et al, Venus, RAS 2010 Difference from fitted position (R v ) Dynamic pressure indicator7 day averaged EUV r (R v ) x (R v ) Boundary locations In agreement with Martinecz et al. (2009) the bow shock position does not correlate with either; the solar wind dynamic pressurethe solar wind dynamic pressure or the solar EUV fluxor the solar EUV flux The same analysis for the ICB also shows no correlation

Solar System Physics Group Grande et al, Venus, RAS 2010 Water Loss at Venus Spatial distribution of the escaping plasma. The measured O + (a), and H + (b) flux distributions in the tail region from 33 orbits were integrated over XVse [20.5, 23.0] and are shown in a YVse–ZVse plane across the tail. The geometrical eclipse of Venus is shown by the thin grey circle. To ensure that no solar- wind protons affect the mass composition measurements inside the IMB, we restrict the area of the analysis to R,1.2RV. Blank circles show measurements with zero flux. The plasma sheet region is identified by red dashed lines and labelled PS, the boundary layer at the IMB is identified by black dashed lines and labelled BL, and the direction of the convection electric field is labelled E. S. Barabash et al Nature Vol 450|29 November 2007 Escaping ions leave Venus through the plasma sheet and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H + 1)/Q(O + )=1.9 implying that the escape of H + and O +, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Solar System Physics Group Grande et al, Venus, RAS 2010 H+He++He+ X-R(signed) plot H+ This cylindrical plot shows that the data fits the bow shock in all three dimensions. The wake can also be seen to flow at an angle towards positive y. This is associated with the 5° angle the solar wind makes with Venus. X-Y plot of O+ The main ion concentration is found over the pole, corresponding to low altitude The planetary wake is the clear escape route for O+

Solar System Physics Group Grande et al, Venus, RAS 2010 Double energy populations occur in conjunction with flux ropes in the Venusian ionosphere with ionosheric oxygen and SW protons on the same flux tube, The small number of cases studied so far do not preclude a chance association

Solar System Physics Group Grande et al, Venus, RAS Mean electron energy Log electron density 1.Solar wind 2. Magnetosheath 3. Boundary Layer 4. Magnetopause (Hapgood & Bryant 1990 GRL 17(11)) Transition parameters

Solar System Physics Group Grande et al, Venus, RAS 2010

Solar System Physics Group Grande et al, Venus, RAS 2010 Traditionally a transition parameter is defined by anti- correlation between electron density and mean electron energy (perpendicular to the electric field) The parameter has been used previously to characterise boundary crossings at Earth based on electron data. (Hapgood & Bryant 1990 GRL 17(11)) Once defined, the transition parameter can be used to reveal ordering in other, independent data sets. We are using it to decide whether flux rope events offer a small scale mechanism for atmospheric loss at Venus r (R v ) Transition parameters

Solar System Physics Group Grande et al, Venus, RAS 2010 Ion energies below the ICB Lowest energies near periapsis Low energies in the tail Higher energies close to boundary with shocked solar wind

Solar System Physics Group Grande et al, Venus, RAS 2010 Ion energies below the ICB Showing only those ions with energies above the escape velocity

Solar System Physics Group Grande et al, Venus, RAS 2010 Transport of higher energy ions Photoelectrons caused by photoionisation of oxygen Observed on dayside and nightside (Coates et al., 2008) Inferred that these are transported from day to night Suggest that they set up an E field Suggest that this can accelerate O+ This would then be lost to the solar wind (Coates et al., 2009)

Solar System Physics Group Grande et al, Venus, RAS 2010 Day-to-night flow at solar maximum From Brace et al. (1995) (left) and Miller and Whitten (1991) (right) Ion Transport

Solar System Physics Group Grande et al, Venus, RAS 2010 Ion Transport At solar minimum ionopause at a lower altitude Inhibits transport process Remote sensing experiments suggest that transport strongly reduced / shut off (Knudsen et al., 1987) Venus Express conducting first in situ measurements at solar miniumum

Solar System Physics Group Grande et al, Venus, RAS 2010 One Venus year of observations from 4 August 2008 Cover all LT sectors, once in each direction Solar flux approximately constant, around 70 sfu Ion Counts and Position Integrated Ion Counts, 4 th August 2008 – 17 th March 2009

Solar System Physics Group Grande et al, Venus, RAS 2010 Ion Counts and Position One Venus year of observations from 4 August 2008 Cover all LT sectors, once in each direction Solar flux approximately constant, around 70 sfu Integrated Ion Counts, 4 th August 2008 – 17 th March 2009

Solar System Physics Group Grande et al, Venus, RAS 2010Assymetries Can have significant counts nightward of terminator Maintenance of nightside ions Higher peak counts on dusk side Could be dawn-dusk asymmetry in dayside ion density (Miller and Knudsen, 1987)

Solar System Physics Group Grande et al, Venus, RAS 2010 Ion Transport in Noon-Midnight Plane Energy of peak countsMedian1st quartile3rd quartile Noon-Midnight orbits:15 eV13 eV20 eV Midnight-Noon orbits:25 eV20 eV29 eV Noon Midnight Noon Midnight Suggests nightward ion flow of ~4 km/s

Solar System Physics Group Grande et al, Venus, RAS 2010 Ion Transport in Noon-Midnight Plane Suggests antisunward flow of several km/s At the highest altitudes approaches the escape velocity

Solar System Physics Group Grande et al, Venus, RAS 2010 Transport of lower energy ions Transterminator ion flow driven by pressure gradient Transterminator ion flux greater than required to populate nightside ionosphere (Brace et al., 1995) Suggest that some of these ions could be lost to the solar wind

Solar System Physics Group Grande et al, Venus, RAS 2010 An image taken by the inner camera HI-1A on STEREO A more than a day after the CME launch. The front and core of the CME are visible in this image and labeled A and B respectively. The bright body on the left hand-side of the figure is Mercury. A. P. Rouillard et al 2008

Solar System Physics Group Grande et al, Venus, RAS 2010 Solar Wind periodicity CIR Identified with Slow/Fast stream CIR structures in Solar Wind near solar minimum Note repeat of structure when repeated on 28 day period Same is also currently seen at Earth with ACE Three stream structure is a feature of current cycle Long term pattern of coronal holes Note ability of STEREO to image associated CIRs 3

Solar System Physics Group Grande et al, Venus, RAS 2010 Separation of orbits by initial Solar Wind mean energy Wake differences fast/slow stream 5 Fast Slow All Heavy

Solar System Physics Group Grande et al, Venus, RAS 2010

Solar System Physics Group Grande et al, Venus, RAS 2010 CIRs Identified by IPS

Solar System Physics Group Grande et al, Venus, RAS 2010 The ion composition plots match the bow shock model well and show tail-ward cold O+ escape. Trans terminator flow identified as possible ion loss mechanism The CME arriving on the 25 th /26 th May produces a relaxation and compression of the bow shock and unusual energization patterns in the inner magnetosphere. The solar wind ion densities show periodicities related to CIR structure These correlate with IPS observations 11 Conclusions

Solar System Physics Group Grande et al, Venus, RAS 2010

Solar System Physics Group Grande et al, Venus, RAS 2010 Note repeat of structure when repeated on 28 day period Same is also currently seen at Earth with ACE Three stream structure is a feature of current cycle Long term pattern of coronal holes Note ability of STEREO to image associated CIRs

Solar System Physics Group Grande et al, Venus, RAS 2010 STEREO for Venus – Solar Wind studies

Solar System Physics Group Grande et al, Venus, RAS 2010 a: the radial component of the magnetic field in VSO coordinates measured by the Venus Express magnetometer before the arrival of the first front (blue), during the pas sage of front A (green) (and the associated flux rope) and after the passage of front A (red). b, The magnitude of the magnetic field vector before the passage of front A. c, The magnitude of the magnetic field vector during the passage of front A. d: The magnitude of the magnetic field vector after the passage of front A. e: the radial distance between VEX and Venus. The time of inbound and outbound crossing of the bow shock are shown by vertical lines for each of the passages shown in the above panels. A. P. Rouillard et al 2008

Solar System Physics Group Grande et al, Venus, RAS 2010 The event caused the bow shock to relax outward before strongly compressing it inwards then relaxing back to close to its original position for the outbound crossing. The location of the bow shock as determined by the VEX magnetometer was confirmed by ASPERA4. The range of variation of the shock distance seen during both inbound and outbound passes is roughly 0.5R V The ion populations in the inner magnetosphere, including the ionosphere, are enhanced and energised