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 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 Parker’s solar wind solution (1958) v = cS at r = rC Velocity 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 left Sun 5 days ago Plasma leaving Sun now Magnetic field forms a spiral structure Plasma flows radially along black arrows. 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. Slow Fast Fast Fast Fast Sun Slow Current Sheet Slow Slow Fast Fast 5AU (15 days) 3AU (9 days) 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 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)