The Solar Wind. The Ulysses orbit from launch through 1995.

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

Create the solar wind adding one complicating 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)

Complexity in the Solar Wind

Non-rotating, unmagnetised star

Vel

ocit

y

speed = sound speed

Distance from starr = rC

Parker’s solar wind solution (1958)v = cS at r = rC

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.

Coronal expansion with pure dipole magnetic field added (MHD Solution)

Fast wind

Slow windCurrent 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

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 7 days ago

Plasma leaving Sun now

Plasma left Sun 3 days ago

Plasma left Sun 5 days ago

Add rotation: Create spiral magnetic field structure

Plasma flows radially along black arrows.

Magnetic field forms a spiral structure

Solar wind in sun’s equatorial plane

Coronal Magnetic Field Structure – Solar Minimum

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.

Coronal magnetic field at 5 RSUN obtained by extrapolating photospheric field – yellow line is current sheet where radial component changes sign.

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

Current Sheet

Fast

Fast

Fast

Fast

Slow

Slow

Slow

Slow

Fast

Fast

5AU (15 days) 3AU (9 days) 1AU (3 days)

Sun

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.

Interaction of fast and slow solar wind streamsEquatorial 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.

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)

Solar Wind -- Summary