RECENT OBSERVATIONS AND MODELING OF THE SOLAR WIND: IMPLICATIONS FOR THE SPACE ENVIRONMENT OF THE PL

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• RECENT OBSERVATIONS AND MODELING OF THE SOLAR
  WIND IMPLICATIONS FOR THE SPACE ENVIRONMENT OF
  THE PLANETS LOCATED IN THE VERY INNER HELIOSPHERE
  
• Alexis P. Rouillard

Space Environment Physics Group, University of
Southampton, UK
Special thanks to the HI RAL team and Imperial
College VEX team Mike Lockwood, Jackie Davies,
Chris Davis, Richard Harrison, Tielong Zhang, Bob
Forsyth, Adam Rees, Chris Carr.
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Plan of the talk
Plan of the talk

• Quiet (backgound) solar wind
• STEREO observations of CIRs/Streamers
• Coronal Mass Ejections (CMEs)
• importance for Space Weather
• STEREO observations of CMEs
• Planetary Impacts
• Conclusions

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• Quiet (backgound) heliosphere

The solar wind, consisting of ionised coronal
plasma, flows supersonically and radially outward
from the Sun due to the large pressure difference
between the hot solar corona and the interstellar
medium.
At 1AU, a typical 3 months of solar wind data
looks typically like
Two types of solar wind fast and slow
Solar wind speed (km.s-1)
Density (n/cc)
Dense and tenuous.
Hot and Cold
Temperature (K)
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• Quiet solar wind and the Parker field

The solar wind carries a magnetic field frozen in
the plasma. A field line connec- ting parcels of
plasma emitted by the same region on the solar
surface will be a spiral.
View of the solar equator
Parker Spiral Field
A current sheet usually separates the solar wind
regions of opposite magnetic polarity.
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• Quiet solar wind

Ulysses was the first spacecraft to measure the
plasma properties of the solar wind at high
latitudes.
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• Quiet solar wind

The background solar wind consists of the
corotating geometry of fast and slow flow
dictated by the evolution of coronal holes
McComas et al., 1998
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• Quiet solar wind

• Ulysses found continuous fast solar wind (750
  km/s) at high latitudes at solar minimum in
  agreement with the idea that fast solar wind
  originated in coronal holes. This fast wind was
  associated with large stable polar coronal holes.
  
• Slow solar wind is associated with the streamers
  (dense plasma regions) seen in coronagraph
  images, but its exact source is unclear.
• The large variations in solar wind speed at
  mid-latitudes are induced by corotating fast
  Solar wind speed (extensions of polar holes).

Red Outward Field
Blue Inward Field
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• Quiet solar wind source in coronal holes

Polar Coronal hole extensions force fast wind in
the ecliptic region where usually slow wind
predominates.
Carrington Map produced by Dr E Benevolenskaya
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• Potential Field Source Surface

Potential Field Source Surface can be used to
predict the background solar wind.
Fast Wind
Potential Field Source Surface uses magnetograph
observations to form a 3-D prediction of the
magnetic topology of the coronal magnetic field
near the Sun.
Coronal source surface
Slow Wind
??B 0
The Wang and Sheeley empirical law correlates
flux-tube expansion factors to solar wind speed.
closed flux tube
Photosphere
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• Solar Minimum versus Solar max Heliospheric
• Current Sheet

Stack plot of Carrington rotations from 1983 to
1994, showing the location of the heliospheric
current sheet (HCS) on the source surface at
2.5 Rs.
Hoeksema, Space Sci. Rev., 72, 137, 1995
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• Solar Minimum versus Solar max streamers

Monthly averages of the Sunspot number
Helmet streamers change topology over the solar
cycle.
They broaden, shrink in size and split.
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• Solar wind speed ENLIL predictions

STEREO EUVI 195A
ENLIL is a time-dependent 3D MHD model of the
heliosphere. It solves for plasma mass, momentum
and energy density, and magnetic field (Odstrcil
and Pizzo, 1999)
ENLILs inner radial boundary is located beyond
the sonic point, typically at 21.5 or 30 solar
radii. It can accept boundary condition
information from either the WSA or MAS models.
The outer radial boundary can be adjusted to
include planets or spacecraft of interest.
ENLIL prediction of the radial solar wind speed.
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• Corotating Interaction Regions ENLIL predictions

B/Bp
Polar view of the solar equator.
A simulation using ENLIL of the magnetic field
compressed in a solar maximum CIR
Corotating interaction regions occur where fast
wind catches up slow wind, where a stream
interface forms.
Hundhausen, 1973 Pizzo, 1978
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Plan of the talk
Plan of the talk

• Quiet (backgound) and Active heliosphere
• STEREO observations of CIRs/Streamers
• Coronal Mass Ejections (CMEs)
• importance for Space Weather
• STEREO observations of CMEs
• Planetary Impacts
• Conclusions

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HI- Basic Design and operation

The Heliospheric Imager (HI) is a wide-angle
imaging system for the detection of coronal mass
ejection (CME) events in interplanetary space
and, in particular, of events directed towards
the Earth.
The system is composed of four cameras, two inner
and two outer cameras.
The HI instrument on each STEREO spacecraft
comprises two wide-field imagers, HI-1 and HI-2,
which observe in white light with a band-pass of
630-730 nm and 400-1000 nm, respectively.
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HI- Basic Design and operation

• Geometrical Requirements
• To view the Sun-Earth line with unbroken coverage
  from corona to Earth orbit
• Opening angle of 45 degrees governed by average
  CME width over equator
• Brightness Levels
• Needed to achieve rejection to lt 3x10-13 lt
  10-14 B/Bo to detect CME signal
• Have to contend with contributions from the
  F-corona, planets, stars, the Earth and Moon

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Thomson Scattering
Sphere of Maximum Thomson Scattering
View from North Pole
Hi1A
E
A
E
V
Hi2A
Front-side events at intermediate longitudes
exhibit nearly constant levels of brightness over
a wide range of heliocentric distances.
See Vourlidas and Howard, ApJ, 2006
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Typical HI background-subtracted image
CME observed by HI-1A
A monthly Minimum Is subtracted to remove the
F-corona.
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Typical HI difference image
Coronal Mass Ejection
The solar wind mass-flow visualised in the Hi
cameras is not only CME associated material but
also consists of
Plasma blobs ejected by the streamer belt.
Height versus time plots
Are there CIR fronts?
Plasma blobs from the streamer belt.
Comet 2P-Encke
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Apparent Acceleration
Assuming the CME propagates radially out and at a
constant speed, the elongation variation with
time will not be a straight line
a(o)
ß
Sun
A
Ho1AU
Sheeely et al., JGR (1999)
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The fitting technique
rms of difference between observed and predicted
?
Each track in elongation versus time plots can be
fitted to obtain speed and direction
Event direction, ? (deg.) ?
best fit V 320 km s-1 ? 48 (?E 6) (VE
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2? level
Event speed, V (km s-1) ?
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HI-2A
HI-1A
Apparent Acceleration.
HI-2A
HI-1A
Elongation versus time plot a CME
reaches elongations gt 70 degrees.
Best fit a 48 degrees, V320km.s-1
Davies et al., 2008
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Interesting recurrent features appeared in the
J-plots
Tracks converge on this maps
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Elongation variations corotating source
Elongation variation expected for two plasma
parcels moving along different longitudes
Elongation angle
ß
Sun
A
Time
Ho1AU
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CIR tracks converge in A and diverge in B
Tracks converge in A
Tracks diverge in B
Rouillard et al., submitted JGR, 2008
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CME observation in a difference image of Hi1A
We extract latitude cuts at constant elongation
from the Sun.
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Streamer belt warp
km.s-1
(Courtesy of Yi-Ming Wang)
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Streamer belt warp
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Imaging the Streamer Belt
Constant elongation of 5 degrees.
Strong (localised) increases in brightness are
observed inside the streamer belt regions in HI.
These dense parcels are separated by 0.5 days.
HI-1A
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Corotating Interaction Region formation in HI
STEREO A J-plot
Height (o)
STEREO A (5o)
Latitude (o)
Using HI-1A Difference Images We can extract
- Height versus time plots - Latitude versus
time plots
MHD (density)
Latitude (o)
Rouillard et al., Geophys. Res. Let., 2007
Carrington Longitude (o)
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Streamer Belt in HI-1A
STEREO A J-plot
These characteristic shapes form systematically
at the warps of the streamer belt.
Height (o)
STEREO A (5o)
Latitude (o)
Isolated Coronal Hole
MHD (density)
Latitude (o)
Rouillard et al., Geophys. Res. Let., 2007
Carrington Longitude (o)
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Movie (HI-1A)
HI-1A
HI-1A
HI-1A
This density enhancement formed in the cameras
CIR!
HI-2A
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STEREO EUVI 195A
HI-1A
HI-1A
HI-1A
HI-2A
ENLIL prediction of the radial solar wind speed.
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Reconnection at the tip of a single current sheet
LASCO C3 images showing a density inhomogeneity
moving radially outward along an equatorial
streamer stalk on July 28, 1997. (top)
Instantaneous, background subtracted image
recorded at 0845 UT the centroid of the blob is
located at r 9.5 RS. (bottom) Difference of
images taken at 0845 and 0740 UT, where white
(black) indicates an increase (decrease) in the
local intensity during the elapsed interval.
Wang et al., 2000
Observation Sheeley et al., 1999 Wang et al.,
2000
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Reconnection at the tip of a single current sheet
Theory Dahlburg and Karpen, 1997 Van Aalst, M.K.
et al., ApJ., 1999 Wang et al., ApJ., 1998, 2000
Observation Sheeley et al., 1999 Wang et al.,
2000
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Another streamer belt warp

• Quiet (backgound) solar wind
• STEREO observations of CIRs/Streamers
• Coronal Mass Ejections (CMEs)
• STEREO observations of CMEs
• Planetary Impacts
• Conclusions

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Coronal Mass Ejections
Coronal Mass Ejections (CMEs) are large-scale
eruptions of solar material from the Suns lower
atmosphere into the solar corona and are one of
the most spectacular of solar phenomena1. During
a CME, the Sun releases 1012 kg to 1013 kg of
matter moving at speeds ranging from less than
100 to over 1500 kms-1 (Hudson et al., 2005 St
Cyr et al., 2005)
A typical CME has a three part structure
consisting of a cavity of low electron density, a
dense core (the prominence, which appears as a
bright region on coronagraph images) embedded in
this cavity, and a bright leading edge. It should
be noted, however, that many CMEs are missing one
of these elements, or even all three (Hundhausen,
1988).
The first detection of a CME was made on December
14, 1971 by R. Tousey (1973) using the 7th
Orbiting Solar Observatory (OSO-7)
STEREO was designed to observe CMEs up to
distances gt 1AU
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Another streamer belt warp

• Quiet (backgound) solar wind
• STEREO observations of CIRs/Streamers
• Coronal Mass Ejections (CMEs)
• STEREO observations of CMEs
• Planetary Impacts
• Conclusions

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HI can be used to study shocks and streamer
deformation
CME observed with HI-1A
Difference image ( red (positive change in
brigthness)- Blue (negative change) ).
Many density fronts can be seen
Photospheric Material of high density
Regions where fast wind catches up slow
wind (Odstrcil an Pizzo, 2001)
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HI can be used to study shocks and streamer
deformation
HI will be used in the long term to study shock
front formation and predict energetic particle
acceleration.

We have shown that we can extract speeds and
directions, can we predict impacts at planets?
Venus was located in the HI-1A/2A cameras in
March-July and was ideal to test out methods..
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Another streamer belt warp

• Quiet (backgound) solar wind
• STEREO observations of CIRs/Streamers
• Coronal Mass Ejections (CMEs)
• STEREO observations of CMEs
• Planetary Impacts
• Conclusions

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Thomson Scattering
Sphere of Maximum Thomson Scattering
View from North Pole
Hi1A
E
A
E
V
Hi2A
Front-side events at intermediate longitudes
exhibit nearly constant levels of brightness over
a wide range of heliocentric distances.
See Vourlidas and Howard, ApJ, 2006
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Venus Express Magnetometer
Venus Express is an ESA Mission and is a follow
on from the Mars Express mission
It has onboard a magnetometer which is
continuously recording the ambient magnetic
field even in the solar wind (Institut für
Weltraumforschung, PI Dr. Tielong Zhang).
Analyser of Space Plasma and Energetic particles.
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Venus Express Orbit
Normally there is no solar wind plasma data
taken by VEX in the solar wind however MAG is
switched on all the time
24 hour orbit
Bow Shock outbound
Photo-ions
Twenty-four hour orbit
Sunward
Magnetic Barrier
Eclipse
Bow Shock inbound
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Streamer belt warps at Venus
Normally no solar wind plasma measurements coming
from VEX but MAG does.
Hi1A
Front emerges
Hi1A
A clear latitudinal excursion of the plasma sheet
is observed during the 23-25 April 2007.
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Stereo A - Venus - Sun 75 o
Fitting technique gives
Elongation of Venus
Not aligned with Venus
Vr 260 18 kms-1
ß 90 5o
If Venus was at 90o to STEREO A
Time of arrival if Venus was on the Limb
But the structure is corotating! So
Venus is at 90-7515o off the plane of the
sky hence we need to correct arrival time by
1527./360 1.12 days.
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Streamer belt warps at Venus
Normally no solar wind plasma measurements coming
from VEX but MAG does.
Toward Sector
Away Sector
B
No CIR nor Sector Boundary crossing observed on
the 29th (one day before predicted arrival).
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Streamer belt warps at Venus
Normally no solar wind plasma measurements coming
from VEX but MAG does.
Away Sector (South polar coronal hole)
Toward Sector
Away Sector
B
Arrival of the sector boundary/CIR system is
predicted correctly
Current Sheet crossing (Sector boundary)
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Two CME fronts emerged on the Eastern hemisphere
on 21/22 May 2007.
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Prediction of arrival time.
A
B
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Two CME fronts emerged on the Eastern hemisphere
on 21/22 May 2007.
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Conclusions
STEREO can be used to study streamer belt
dynamics far in the heliosphere in great details
as well as coronal hole flows.
CIR normally form where the streamer belt
undergoes latitudinal excursions We observe
fronts forming in the image which do not have
clear CME take-offs Associated with them.
A CIR arrival time was predicted at Venus based
on considering streamer belt warps and J-plots.
We correctly predicted a CME-impact at Venus
first observed planetary impact!
We are in the process of creating a list all
planetary impacting CIR/CME events seen in the
cameras.
We would like to thank Dr Yi-Ming Wang of NRL for
the PFSS data, CCMC for model data.
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Isolated Coronal holes
Solar Maximum
Isolated coronal holes occur during large
activity
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• (C)IRs MHD simulations

Solar Max
Solar Min
Ecliptic

Ecliptic
Latitudinal Cut
Latitudinal Cut
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• (C)IRs semi-analytical approach

The ENLIL code takes one day to calculate the one
rotation worth of solar wind predictions on
large computers.

We wrote a semi-analytical code which
approximates the MHD properties of the solar
wind
Start with a Cauchy integral (Walens equation)
which has the unusual operation of
diferentiation of the position vector with
respect to the coordinates at the original
position.
Approximates the effect of plasma and magnetic
pressure at the stream interface (plasma and
magnetic pressure are of similar magnitude at
1AU.)
(Rouillard, PhD Thesis, 2006)
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• (C)IRs Semi-analytical approach

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• (C)IRs Semi-analytical approach

At solar minimum the polar coronal hole are
largest and emit high speed solar wind.
The latitudinal extent of the fast wind of the
northern polar coronal hole is, over the entire
interval of available predictions, greater than
the extent of the southern polar coronal hole
(Rouillard et al, to be submitted to GRL, 2008).
There are Consequences for geomagnetic activity
(Finch, PhD thesis, 2008).
We are currently investigating the consequence of
a north/south asymmetry in the solar wind speed
for the post-termination shock flows at
distancesgt80AU.
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• (C)IRs semi-analytical approach

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SOHO SMEI STEREO Solar Orbiter