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SW 101: Lecture 3: the Solar Wind SW 101: Lecture 3: The Solar Wind The solar wind last week – data from the ACE spacecraft The IMF last week – data from the ACE spacecraft ULYSSES data summary – 1992-1998 The Ulysses orbit from launch through 1995 The Ulysses orbit showing second solar polar pass in 2000/2001 near solar max. Ulysses kept going….. Ulysses turned off Ulysses turned off Three Ulysses Orbits: solar min – solar max – solar min Note: Red trace indicates magnetic field directed away from the Sun, Blue trace indicates magnetic field directed towards the Sun SW101 Lecture 3: Solar Wind 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 -- tomorrow) Non-rotating, unmagnetised star Parker’s solar wind solution (1958) v = vs at r = rc Velocity Velocity speed = sound speed Parker’s solution used simple hydrodynamics – no magnetic field effects – and showed that a supersonic stellar wind r = rc is the only allowed solution if Distance from star the pressure far from the star is small. 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 Undulating current sheet divides (closed loops) field lines of opposite polarity surrounds Sun’s coming from coronal holes in equator. 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 Stop and Think Which of the following statements is not true? 1. Most stars have a stellar wind. 2. The solar wind flows out in a spiral pattern. 3. If the solar wind were slower, the spiral pattern would be wound tighter. 4. The heliospheric current sheet is required for there to be an interplanetary magnetic field (IMF) 5. Most of the solar wind originates within coronal holes. 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 Undulating current sheet divides (closed loops) field lines of opposite polarity surrounds Sun’s coming from coronal holes in equator. 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 Sun Slow Slow Slow 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 Stop and Think Corotating interaction regions are the result of: 1. The spiral structure of the interplanetary magnetic field. 2. Regions of the solar wind having different speeds. 3. The heliospheric current sheet. 4. The Sun’s rotation 5. The combination of 2 & 4 Current sheet and flow structure near solar minimum and solar maximum Current sheet and slow flow Current sheet and slow flow confined to equatorial region extend to high latitudes Reexamine the solar wind last week – data from the ACE spacecraft Where does the solar wind end ? – the interaction of the solar wind and the interstellar medium. Plasma temperature and flow lines In December 2004 Voyager 1 passed through the termination shock at a distance of 94 AU. Plasma density Voyager 2 followed in August 2007 at a distance of 84 AU. Interstellar Boundary Explorer (IBEX) Energetic Neutral Atom Images of the interaction between the Local Interstellar Medium and the Heliosphere McComas et al., Science, 2009 The Explanation Parker’s (1961) sketches of the LISM/ Heliosphere interaction for dominant (A) dynamic pressure and (B) magnetic pressure Visualization of the heliopause showing BLISM and direction of motion McComas et al., Science, 2009 Local ISM/Heliosphere interaction with a strong BLISM Opher et al., ApJ, 2006 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 -- tomorrow) .
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