IAC Winter School 2013Solar magnetic fields 2Philip Judge 1

Solar magnetic fields 2

Philip Judge, High Altitude Observatory, NCAR

1. Magnetised plasmas

2. How we measure solar (plasmas and) magnetic fields

IAC Winter School 2013Solar magnetic fields 2Philip Judge

What is a plasma?

Universe = dark energy +dark matter +plasma +gas + solid + liquid

• free ions and e- (hot and/or energized), yet quasi-neutral• many interacting particles (Coulomb force, plasma no >> 1)• collective effects• complex nonlinear behavior, especially when magnetized

Plasma:

m=E/c2

IAC Winter School 2013Solar magnetic fields 2Philip Judge 3

Newtonian momentum equations and O(u/c) relativistic transformations

Maxwell

σ huge

Bulk fluid momentum equation

Nonlinear coupling

“This is supposed to be about magnetic fields: Why do we have to talk about plasmas...?”

0

…Where do r, u, j, s, T, p… come from?

IAC Winter School 2013Solar magnetic fields 2Philip Judge 4

Simple plasma, Boltzmann transport: Braginskii 1965

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Plasma b,

equipartition

Bulk equation of motion (e+p)

IAC Winter School 2013Solar magnetic fields 2Philip Judge 6

Currents (npvpe-nev e): Ohm’s law in collisional plasmas

problem

IAC Winter School 2013Solar magnetic fields 2Philip Judge

some plasmas

IAC Winter School 2013Solar magnetic fields 2Philip Judge

why is plasma so prevelant? Range of plasma characteristics

…wow

IAC Winter School 2013Solar magnetic fields 2Philip Judge

Allen 1973 compilation

IAC Winter School 2013Solar magnetic fields 2Philip Judge

plasma characteristics

…wow

IAC Winter School 2013Solar magnetic fields 2Philip Judge

plasmas vs gases

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The Sun as a laboratory for plasmaphysics in space and astrophysical regimes where

Rm >>1, b >>1, b <<1,b=ratio plasma/magnetic pressure

Large Rm regimes exist outside of laboratory plasmas,

-> MHD regime (frozen fields)

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Induction equation and Large magnetic Reynolds number RM

Induction diffusion (non-linear)

timescale L/u L2/h

RM = diffusion/induction times = Lu/h

h 0, Frozen flux condition (Alfven theorem)

IAC Winter School 2013Solar magnetic fields 2Philip Judge 14

RM = ∞ dynamics:

Lagrangian model (no diffusion)

C. de ForestL. Rachmeler

IAC Winter School 2013Solar magnetic fields 2Philip Judge

Gold (1964)

IAC Winter School 2013Solar magnetic fields 2Philip Judge

High and low b in the photosphere, quiet b >1, sunspot b <1,

b = plasma/magnetic pressure

Solar magnetic fields seen at the “surface”are intermittent-

Spots on large scales

tubes, sheets, ribbonson convective scales

IAC Winter School 2013Solar magnetic fields 2Philip Judge

High and low b regimes, photosphere b >1, corona b <<1, b = plasma/magnetic pressure

dense, turbulent solar plasma patches (b >1) with tenuous field-aligned coronal plasma loops (b <<1)

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signatures of solar magnetism

Slides from R. Casini

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SPINOR- a real spectropolarimeter

IAC Winter School 2013Solar magnetic fields 2Philip Judge

The “right stuff”: Navarro 2005a,bInversions of Fe I and Ca II lines in a sunspot, hydrostatic, DST 76cm, SPINOR, sensitivity 1.5x10-4

Current density

Temperature

“bandwidth limited”

IAC Winter School 2013Solar magnetic fields 2Philip Judge

Instrument fringeremoval using2D PCA

Casini, Judge, Schad2012 in press ApJSi I/ He I 1083 nm regionFIRS/DST, noise ~ 3x10-4

Measuring magnetic fields in solarlow beta plasma is hard

IAC Winter School 2013Solar magnetic fields 2Philip Judge

Measuring magnetic fields in solarlow beta plasma is hard

22

Uitenbroek 2011Ca II 854 nm

200-2000G

200-2000G