1 / 15

Physics 681: Solar Physics and Instrumentation – Lecture 22

Physics 681: Solar Physics and Instrumentation – Lecture 22. Carsten Denker NJIT Physics Department Center for Solar–Terrestrial Research. The Magnetic Force. Lorentz force (non-relativistic Ohm’s law = magnetohydrodynamic approximation)

satin
Télécharger la présentation

Physics 681: Solar Physics and Instrumentation – Lecture 22

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. Physics 681: Solar Physics and Instrumentation – Lecture 22 Carsten Denker NJIT Physics Department Center for Solar–Terrestrial Research

  2. The Magnetic Force • Lorentz force (non-relativistic Ohm’s law = magnetohydrodynamic approximation) • The volume force can be divided into a magnetic pressure gradient and a magnetic tension • Magnetic flux tube applies a lateral pressure to the gas into which it is embedded • Typical pressure 104 Pa can be balanced by B ≈ 0.15 T • In sunspots we see at deeper layer  2  104 Pa  B ≈ 0.3 T • Magnetic tension is the tendency of lines of force to shorten themselves  restoring force to perturbations Center for Solar-Terrestrial Research

  3. Magnetic Flux Tubes • Converging plasma motion is capable of concentrating magnetic flux • Cellular flows (granulation, mesogranulation, supergranulation, and giant cells) • Kinematic approximation (the flow v is given, the Lorentz force is neglected) • 2D, stationary flow consisting of rolls • Magnetic Reynolds number Rm = ul / η = 250 • Boundary conditions: field is vertical at all times at all boundaries • Field lines become deformed  diffusion term in the induction equation is no longer negligible  field line reconnection  magnetic flux is expelled from the interior and accumulated in sheets near the cell edges Center for Solar-Terrestrial Research

  4. Clark and Johnson (1967) Galloway and Weiss (1981) Center for Solar-Terrestrial Research

  5. Steady state: time scale of field decay d 2 / η equals time scale of advection l / u • Final flux after field concentration • Field amplification is rapid l / u (turnover time) • Expulsion of flux is slower 5( l / u ) and depends on Rm • Flux sheets may exist (chain-like crinkles) • Equipartition between kinetic and magnetic energy densities (dynamic regime) • Regions of motion and regions of fields mutually exclude each other • Critical flux • Field BP corresponds to an equilibrium between magnetic and gas pressure Center for Solar-Terrestrial Research

  6. Galloway and Weiss (1981) Center for Solar-Terrestrial Research

  7. Surface density ρ = 3  10-4 kg/m3, velocity of granules u = 2.0 km/s  equipartition field Be = 0.04 T • Observed fields are a factor 3 larger  convective collapse (convective instability in the presence of a magnetic field) • Stable flux tube exist for a minimum field of 0.1 T capable of suppressing the convective instability • The magnetic field is very weak for the major fraction of the solar surface • Locally stronger fields of >0.1 T in flux tubes • Solar magnetic fields are intermittent • Pores are sunspots lacking a penumbra (B ≈ 0.15 T, lifetime ≈ 1 day, size ≈ 5 arcsec) • Magnetic knots (B ≈ 0.1-0.2 T, “line gaps” in spectra, lifetime ≈ 1 hour, size 1-2 arcsec, IR observations, abundant near sunspots, ≈ 10 knots per 100 granules, knots have predominantly the opposite field of sunspots, flux is balanced) • Unresolved fields  filling factor (d ≈ 100-200 km) Center for Solar-Terrestrial Research

  8. http://www.kis.uni-freiburg.de/~steiner/ Center for Solar-Terrestrial Research

  9. Lin and Rimmele (1999) Center for Solar-Terrestrial Research

  10. Wang et al. (1998) Center for Solar-Terrestrial Research

  11. http://nsosp.nso.edu/dst/images/fill1.gif Center for Solar-Terrestrial Research

  12. Center for Solar-Terrestrial Research

  13. Center for Solar-Terrestrial Research

  14. Langhans et al. (2002) Center for Solar-Terrestrial Research

  15. http://dotdb.phys.uu.nl/DOT/Showpiece/movies.html Center for Solar-Terrestrial Research

More Related