1 / 15

Lecture 18: The solar wind

Lecture 18: The solar wind. Topics to be covered: Solar wind Inteplanetary magnetic field. The solar wind. Biermann (1951) noticed that many comets showed excess ionization and abrupt changes in the outflow of material in their tails - is this due to a solar wind?

eitan
Download Presentation

Lecture 18: The solar wind

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 18: The solar wind • Topics to be covered: • Solar wind • Inteplanetary magnetic field PY4A01 - Solar System Science

  2. The solar wind • Biermann (1951) noticed that many comets showed excess ionization and abrupt changes in the outflow of material in their tails - is this due to a solar wind? • Assumed comet orbit perpendicular to line-of-sight (vperp) and tail at angle  => tan = vperp/vr • From observations, tan  ~ 0.074 • But vperp is a projection of vorbit => vperp = vorbitsin ~ 33 km s-1 • From 600 comets, vr ~ 450 km s-1. • See Uni. New Hampshire course (Physics 954) for further details: http://www-ssg.sr.unh.edu/Physics954/Syllabus.html PY4A01 - Solar System Science

  3. The solar wind • STEREO satellite image sequences of comet tail buffeting and disconnection. PY4A01 - Solar System Science

  4. Parker’s solar wind • Parker (1958) assumed that the outflow from the Sun is steady, spherically symmetric and isothermal. • As PSun>>PISM => must drive a flow. • Chapman (1957) considered corona to be in hydrostatic equibrium: • If first term >> than second => produces an outflow: • This is the equation for a steadily expanding solar/stellar wind. Eqn. 1 Eqn. 2 PY4A01 - Solar System Science

  5. Parker’s solar wind (cont.) • As, or • Called the momentum equation. • Eqn. 3 describes acceleration (1st term) of the gas due to a pressure gradient (2nd term) and gravity (3rd term). Need Eqn. 3 in terms of v. • Assuming a perfect gas, P = R  T /  (R is gas constant;  is mean atomic weight),the 2nd term of Eqn. 3 is: Eqn. 3 Isothermal wind => dT/dr 0 Eqn. 4 PY4A01 - Solar System Science

  6. Parker’s solar wind (cont.) Eqn. 5 • Now, the mass loss rate is assumed to be constant, so the Equation of Mass Conservation is: • Differentiating, • Substituting Eqn. 6 into Eqn. 4, and into the 2ndterm of Eqn. 3, we get • A critical point occurs when dv/dr  0i.e., when • Setting Eqn. 6 PY4A01 - Solar System Science

  7. Parker’s solar wind (cont.) • Rearranging => • Gives the momentum equation in terms of the flow velocity. • If r = rc, dv/dr -> 0 or v = vc, and if v = vc, dv/dr -> ∞ or r = rc. • An acceptable solution is when r = rcand v = vc(critical point). • A solution to Eqn. 7 can be found by direct integration: where C is a constant of integration. Leads to five solutions depending on C. Eqn. 7 Eqn. 8 Parker’s “Solar Wind Solutions” PY4A01 - Solar System Science

  8. Parker’s solutions • Solution I and II are double valued. Solution II also doesn’t connect to the solar surface. • Solution III is too large (supersonic) close to the Sun - not observed. • Solution IV is called the solar breeze solution. • Solution V is the solar wind solution (confirmed in 1960 by Mariner II). It passes through the critical point at r = rcand v = vc. v/vc Critical point r/rc PY4A01 - Solar System Science

  9. Parker’s solutions (cont.) • Look at Solutions IV and V in more detail. • Solution IV: For large r, v 0and Eqn. 8 reduces to: • Therefore, r2v  rc2vc = const or • From Eqn. 5: • From Ideal Gas Law: P∞ = R  ∞ T /  => P∞ = const • The solar breeze solution results in high density and pressure at large r =>unphysical solution. PY4A01 - Solar System Science

  10. Parker’s solutions (cont.) • Solution V: From the figure, v >> vcfor large r. Eqn. 8 can be written: • The density is then: =>   0 as r  ∞. • As plasma is isothermal (i.e., T = const.), Ideal Gas Law => P  0 as r ∞. • This solution eventually matches interstellar gas properties => physically realistic model. • Solution V is called the solar wind solution. PY4A01 - Solar System Science

  11. Observed solar wind • Fast solar wind (v~700 km s-1) comes from coronal holes. • Slow solar wind (v<500 km s-1) comes from closed magnetic field areas. PY4A01 - Solar System Science

  12. Interplanetary magnetic field Bor v  • Solar rotation drags magnetic field into an Archimedian spiral (r = a). • Predicted by Eugene Parker => Parker Spiral: r - r0 = -(v/)( - 0) • Winding angle: • Inclined at ~45º at 1 AU ~90º by 10 AU. Br or vr B r (r0, 0)  PY4A01 - Solar System Science

  13. Alfven radius • Close to the Sun, the solar wind is too weak to modify structure of magnetic field: • Solar magnetic field therefore forces the solar wind to co-rotate with the Sun. • When the solar wind becomes super-Alfvenic • This typically occurs at ~50 Rsun (0.25 AU). • Transition between regimes occurs at the Alfven radius (rA), where • Assuming the Sun’s field to be a dipole, PY4A01 - Solar System Science

  14. The Parker spiral • http://beauty.nascom.nasa.gov/~ptg/mars/movies/ PY4A01 - Solar System Science

  15. Heliosphere PY4A01 - Solar System Science

More Related