EvidenceofSolarWind:Comets

Solar wind ripping off the tail of a comet

SlowSolarWind

Slow Solar Wind (dotted redlines)

• 200-600 km/s• Very Low Density: about 5-

10 particles (electrons andprotons) per cm3 at 1AU

• Highly variable, canchange by factor of 2 in 24hrs

• Lower latitudes, thusaffects the planets morethan fast wind

• Associated with closedfield lines Extreme UV from SOHO Spacecraft

SlowSolarWind

• Slow solar windoriginates fromclosed magneticfield lines at base ofhelmet streamersthat are confined tolow latitudes

FastSolarWind

Fast Solar Wind (solid redlines)

• ~ 750 km/s• Very Low Density: about

half as dense as the slowsolar wind

• Associated with open fieldlines (coronal holes)

Extreme UV from SOHO Spacecraft

Coronal Holes

• A coronal hole is a region of very low plasma density in thecorona that shows up as a dark region in X-ray images.

• The correspond to regions ofthe solar magnetic field thatextend radially away from thephotosphere (no looping).

• The low plasma density isdue to the radial nature of thefield, plasma escapes bymoving along the magneticfield.• The result is a directed beamof very high velocity plasma.

ParkerSpiral• Sun rotates (~25 day

period at equator)• Solar wind flows out

radially with magneticfield “frozen” in it

• Field lines tied to thesun, so rotation resultsin a spiral magneticfield

• Resultant field is like alawn sprinkler

• Eugene Parkertheorized this in the1950’s

ParkerSpiralandthePlanets

• At increasing distancesfrom the sun, themagnetic field gets moreand more twisted

• 450 at Earth• 900 at Jupiter’s orbit

(~10 AU)

Wheredoesthesolarwindstop?

• Other stars have winds too, called stellarwinds

• Hence, universe is not really a vacuum.• Tenuous magnetized plasma (and a little dust)

known as the local interstellar medium (LISM)fills interstellar space.

• But “near” to our sun, the solar winddominates over the LISM.

• This bubble where the solar wind dominates isknown as the heliosphere

Sun’s galactic rotation period = 250 million years

Spiral arm rotation period = 50 million years

Theboundarybetweenthesolarwindandinterstellarmedium

1. Bow shock: regionwhere the LISM beginsto slow

2. Termination shock:region where the solarwind begins to slow

3. Heliopause: regionwhere the solar windand LISM are inbalance

The two magnetized plasmas “bounce” off of each other in the outersolar system. At the boundary:

HeliopauseiswellpastorbitofPluto

Doweknowexactlywheretheheliopauseislocated?

• Voyager I and II spacecrafts• Launched in 1977• First close-up images of

Jupiter, Saturn, Uranus,Neptune

• Presently about 104 and 84AU from earth (or 13/11 lighthours)

Doweknowexactlywheretheheliopauseislocated?

• Voyager I passed thetermination shock in 2004,maybe.

• Voyager II passed thetermination shock five timesin 2007.

• The boundaries are notstatic, but depend on solaractivity (solar wind, CMEs…)

• Scientists are waiting forthem cross the heliopause