solar system physics group grande et al, venus, ras 2010 solar wind interactions and ionospheric...

Solar System Physics Group

Grande et al, Venus, RAS 2010

Solar wind interactions and Solar wind interactions and Ionospheric loss mechanisms Ionospheric loss mechanisms

at Venusat Venus

M Grande, A G Wood, I C Whittaker, G Guymer and A BreenM Grande, A G Wood, I C Whittaker, G Guymer and A Breen

S BarabashS BarabashSwedish Institute of Space PhysicsSwedish Institute of Space Physics

T ZhangT ZhangAustrian Academy of SciencesAustrian Academy of Sciences



• 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


Grande et al, Venus, RAS 2010r

(Rv)

x (Rv)

BoundaryBoundary locationslocations identified by algorithm identified by algorithm

Blue points – Blue points – Bow ShockBow Shock

Red points – ICBRed points – ICB



Bow Shock modelBow Shock model

Fitted curve Fitted curve agrees closely agrees closely with previous with previous resultsresults

But Aber curve But Aber curve is the lowest is the lowest altitude at the altitude at the sub-solar pointsub-solar point

Whittaker et al JGR 2010



Sub solar Sub solar points vary points vary linearly linearly with with average average sunspot sunspot numbernumber

Compare solar conditions of different modelsCompare solar conditions of different models

Bow Shock modelBow Shock model

Whittaker et al JGR 2010


Grande et al, Venus, RAS 2010D

iffer

ence

fro

m f

itted

po

sitio

n (R

v)

Dynamic pressure indicator 7 day averaged EUV

r (R

v)

x (Rv)

Boundary locationsBoundary locations

In agreement with Martinecz et al. (2009) the bow shock position does In agreement with Martinecz et al. (2009) the bow shock position does not correlate with either;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 correlationThe same analysis for the ICB also shows no correlation



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 theplasma sheet and in a boundary layer of the induced magnetosphere. The escape rate

ratios are Q(H+1)/Q(O+)=1.9implying 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.



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+



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



1.

2.

4.

3.

Mean electron energy

Log

elec

tron

den

sity 1. Solar wind

2. Magnetosheath

3. Boundary Layer

4. Magnetopause

(Hapgood & Bryant 1990 GRL 17(11))

Transition parametersTransition parameters



• 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 parametersTransition parameters



Ion energies below the ICBIon energies below the ICB

Lowest Lowest energies near energies near

periapsisperiapsis

Low energies Low energies in the tailin the tail

Higher Higher energies energies close to close to

boundary with boundary with shocked solar shocked solar

windwind



Ion energies below the ICBIon energies below the ICB

Showing only Showing only those ions those ions

with energies with energies above the above the

escape escape velocityvelocity



Transport of higher energy ionsTransport of higher energy ions

Photoelectrons caused by Photoelectrons caused by photoionisation of oxygenphotoionisation of oxygen

Observed on dayside and nightside Observed on dayside and nightside (Coates et al., 2008)(Coates et al., 2008)

Inferred that these are transported Inferred that these are transported from day to nightfrom day to night

Suggest that they set up an E fieldSuggest that they set up an E field

Suggest that this can accelerate Suggest that this can accelerate O+O+

This would then be lost to the solar This would then be lost to the solar wind (Coates et al., 2009)wind (Coates et al., 2009)



Day-to-night flow at Day-to-night flow at solar maximumsolar maximum

From Brace et al. (1995) (left) and Miller and Whitten (1991) (right)

Ion TransportIon Transport



Ion TransportIon Transport

At solar minimum ionopause at a lower altitudeAt solar minimum ionopause at a lower altitude

Inhibits transport processInhibits transport process

Remote sensing experiments suggestRemote sensing experiments suggest

that transport strongly reduced / shut off that transport strongly reduced / shut off

(Knudsen et al., 1987)(Knudsen et al., 1987)

Venus Express conducting first in situ measurements at solar miniumumVenus Express conducting first in situ measurements at solar miniumum



One Venus year of observations from 4 August 2008One Venus year of observations from 4 August 2008

Cover all LT sectors, once in each directionCover all LT sectors, once in each direction

Solar flux approximately constant, around 70 sfuSolar flux approximately constant, around 70 sfu

Ion Counts and PositionIon Counts and PositionIntegrated Ion Counts, 4th August 2008 – 17th March 2009



Ion Counts and PositionIon Counts and Position

One Venus year of observations from 4 August 2008One Venus year of observations from 4 August 2008

Cover all LT sectors, once in each directionCover all LT sectors, once in each direction

Solar flux approximately constant, around 70 sfuSolar flux approximately constant, around 70 sfu

Integrated Ion Counts, 4th August 2008 – 17th March 2009



AssymetriesAssymetries

Can have significant counts Can have significant counts nightward of terminatornightward of terminator

Maintenance of nightside ionsMaintenance of nightside ions

Higher peak counts on dusk side Higher peak counts on dusk side

Could be dawn-dusk asymmetry in dayside Could be dawn-dusk asymmetry in dayside ion density (Miller and Knudsen, 1987)ion density (Miller and Knudsen, 1987)



Ion Transport in Noon-Midnight PlaneIon Transport in Noon-Midnight Plane

Energy of peak countsEnergy of peak counts MedianMedian 1st quartile1st quartile 3rd quartile3rd quartile

Noon-Midnight orbits:Noon-Midnight orbits: 15 eV15 eV 13 eV13 eV 20 eV20 eV

Midnight-Noon orbits:Midnight-Noon orbits: 25 eV25 eV 20 eV20 eV 29 eV29 eV

NoonNoonMidnightMidnight NoonNoonMidnightMidnight

Suggests nightward ion flow of ~4 km/sSuggests nightward ion flow of ~4 km/s



Ion Transport in Noon-Midnight PlaneIon Transport in Noon-Midnight Plane

Suggests Suggests antisunward flow of antisunward flow of several km/sseveral km/s

At the highest At the highest altitudes altitudes approaches the approaches the escape velocityescape velocity



Transport of lower energy ionsTransport of lower energy ionsTransterminator Transterminator

ion flow driven by ion flow driven by pressure gradientpressure gradient

Transterminator Transterminator ion flux greater ion flux greater than required to than required to

populate populate nightside nightside

ionosphere ionosphere (Brace et al., (Brace et al.,

1995)1995)

Suggest that Suggest that some of these some of these

ions could be lost ions could be lost to the solar windto the solar wind



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 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 periodSame is also currently seen at Earth with ACEThree stream structure is a feature of current cycleLong term pattern of coronal holesNote ability of STEREO to image associated CIRs

3



Separation of orbits by initial Solar Wind mean energy

Wake differences fast/slow stream

5

Fast Slow

All

Heavy



CIRs Identified by IPS



• 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 25th/26th 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 observations11

Conclusions



• 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



STEREO for Venus – Solar Wind studies


Grande et al, Venus, RAS 2010a: 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



• 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 0.5RV •The ion populations in the inner magnetosphere, including the ionosphere, are enhanced and energised

solar system physics group grande et al, venus, ras 2010 solar wind interactions and ionospheric...

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