Download AS414-SolarWind

The Solar Wind
The Ulysses orbit from launch through 1995
The Ulysses orbit showing second solar polar pass
in 2000/2001 near solar max.
Note:
Red trace indicates
magnetic field directed
away from Sun,
Blue trace indicates
magnetic field directed
towards Sun
Complexity in the Solar Wind
Create the solar wind adding one complicating
feature at a time
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Non-rotating, unmagnetised star (Parker Solution)
Add a dipole field (heliospheric current sheet)
Add solar rotation (spiral magnetic field)
Add dipole tilt (undulating current sheet)
Add flow structure (corotating interaction regions)
Add solar transients (blast waves & interplanetary
coronal mass ejections ICMEs – next lecture)
Non-rotating, unmagnetised star
Velocity
Parker’s solar wind solution (1958)
v = cS at r = rC
speed = sound speed
Parker’s solution used simple
hydrodynamics – no magnetic
field effects – and showed that
a supersonic stellar wind is the
only allowed solution if the
pressure far from the star is
small.
r = rC
Distance from star
Coronal expansion with pure dipole
magnetic field added (MHD Solution)
Fast wind
Slow wind
Current Sheet
Helmet Streamer
(Not Flowing)
White light coronagraphs
taken near solar maximum
(above) and solar minimum
(right) showing helmet
streamers -- Inset: Sketch
of solar coronal magnetic
field inferred from image.
Heliospheric current sheet near solar minimum
Streamer belt
(closed loops)
surrounds Sun’s
equator.
Undulating current sheet divides
field lines of opposite polarity
coming from coronal holes in
northern and southern polar regions.
Coronal holes and streamer belt seen
in EUV image
Dark areas are low density
coronal holes from which
corona easily escapes
(open field lines)
Bright areas are the
streamer belt, dense
plasma held close to Sun
by closed field lines in
helmet streamers
Add rotation: Create spiral magnetic field structure
Solar Wind Plasma originating from same point on rotating Sun
at 1 day intervals. Plasma flows radially along black arrows. But
frozen-in-flow drags field line into a spiral structure.
Plasma left Sun 3 days ago
Plasma leaving
Sun now
Plasma left Sun 5 days ago
Magnetic field forms
a spiral structure
Plasma left Sun 7 days ago
Solar wind in sun’s equatorial plane
Coronal Magnetic Field Structure – Solar Minimum
Coronal magnetic field at 5
RSUN obtained by
extrapolating photospheric
field – yellow line is current
sheet where radial
component changes sign.
Photospheric magnetogram
obtained from observation
(Zeeman splitting of
spectral line)
Solar Wind velocity
predicted from expansion of
flux tubes from photosphere
to corona – note slow wind
near current sheet.
Heliospheric current sheet near solar minimum
Streamer belt
(closed loops)
surrounds Sun’s
equator.
Undulating current sheet divides
field lines of opposite polarity
coming from coronal holes in
northern and southern polar regions.
Interaction of fast and slow solar wind streams
North-south cut through solar wind showing undulating current sheet. Slow
flow near current sheet is preceded and followed by fast flow which creates
regions of rarefaction and compression.
Fast
Fast
Slow
5AU (15 days)
3AU (9 days)
Sun
Slow
1AU (3 days)
Interaction of fast and slow solar wind streams
Equatorial view of interacting fast and slow streams. Compression and
rarefaction shocks form when the speed difference between the fast and slow
streams exceeds the sound speed in the solar wind (about 50 km/s). These
are called corotating shocks as they rotate with the sun every 27 days.
Sun
Current sheet and flow structure near
solar minimum and solar maximum
Current sheet and slow flow
confined to equatorial region
Current sheet and slow flow
extend to high latitudes
Where does the solar wind end ? – the interaction of
the solar wind and the interstellar medium.
Plasma
temperature and
flow lines
Plasma density
In December 2004
Voyager 1 passed
through the termination
shock at a distance of
approx 100 AU.
Solar Wind -- Summary
Create the solar wind adding one feature at a time






Non-rotating, unmagnetised star (Parker Solution)
Add a dipole field (heliospheric current sheet)
Add solar rotation (Spiral magnetic field)
Add dipole tilt (undulating current sheet)
Add flow structure (corotating interaction regions)
Add solar transients (blast waves & interplanetary
coronal mass ejections ICMEs – next lecture)