Faraday Rotation: Unique Measurements of Magnetic

Fields in the Outer CoronaJustin C. Kasper (UM), Ofer Cohen (SAO), Steven Spangler (Iowa), Gaetan Le Chat

(Meudon)

Summary• Faraday Rotation is a unique way to remotely measure magnetic fields

few-10 solar radii from Sun• These observations are probably best suited for comparison with models

– Model validation– CME magnetic field topology forecasting

• Initial comparisons are promising– Diagnostics of the solar corona from comparison between faraday rotation

measurements and magnetohydrodynamic simulations, G. Le Chat; J.C. Kasper; O. Cohen; S.R. Spangler, Astrophysical Journal., 2014, 789.

• Plans for the future

There are many ways to validate and in the inner heliosphere

Observations BATS-R-US Simulations

Is there a similar way to validate ?• Remotely measure magnetic fields

– In the quiet corona and solar wind– In coronal mass ejections– Validate models, nowcast CME fields, …

Close to the Sun

Zeeman effect in the photosphere

Interplanetary space

In situ measurements with a magnetometer

What about in between?

Low frequency radio emission• Diagnose plasma

– Faraday Rotation = – RM = Rotation Measure

• Use modern low frequency radio arras– Larger means sensitive to smaller – Many (thousands) removes ambiguity– Wide FOV permits ionospheric correction– Why not lower: Ionospheric cutoff at 2-10 MHz

Helios and steady state corona

Volland et al. (1977); Bird et al. (1980); Pätzold et al (1987);

Helios and CMEs

Bird et al., 1985Volland et al. (1977); Bird et al. (1980); Pätzold et al (1987);

Imaging heliospheric fields

Mancuso and Spangler, 2000

Monitor galaxies instead of spacecraft!

Recent example• Revisit some existing observations and compare them with a model• Previous interpretations of Faraday Rotation used toy models of the density and magnetic

field in the inner heliosphere• Here, for the first time, we will use an MHD simulation (BATS-R-US from Michigan) driven

by photospheric measurements• We use linear polarization observations made with the NRAO Very Large Array at

frequencies of 1465 and 1665 MHz of 34 polarized radio sources occulted by the solar corona between 5 and 14 solar radii.

– May 1997 (Mancuso & Spangler 2000) , corresponding to Carrington rotation numbers 1922, 1923– March 2005 and April 2005 (Ingleby et al. 2007), or CR2027 and CR2028.

• Compare simulations and observations over 8-14 Rs from Sun and determine how well simulation reproduces magnetic field at those distances

Process• Start with precise time each observation was made and the location of

each target galaxy• Calculate lines of sight from Earth to each galaxy through the inner

heliosphere in Carrington coordinate system (matched to synoptic photospheric maps) (IDL->TECPLOT->IDL)

• Download appropriate photospheric map for Carrington rotation corresponding to the observations and use as input to BATS-R-US code

• Run simulation of entire corona out to at least 0.25 AU• Calculate Rotation Measure along each line of sight• Compare with VLA observations

White Light Maps

Rotation Measure Maps

May 1997

Zoomed in• Except for a couple

outliers very good overall agreement within error bars

• Only if correct Carrington Rotation is used to drive model

• Outer corona magnetic field changes significantly in one month even in solar minimum

Outliers due to streamer belt• Dashed line indicates the

outlier from this Carrington Rotation

• The outliers always pass close to the streamer belt and heliospheric current sheet

• MHD model less reliable at HCS

Now look at solar maximum• Trend line is off

from observations

• Try rerunning calculations with density and field adjusted by a constant value

B*n reduced by 35%

Realistic CME eruption T+24 hrs

Simulation courtesy Chip Manchester (UM)

Lots of antennas, big computers

1 @ $1MNarrow field of view

Slow to slew

2000 @ $20Full sky field of view

128 tiles

16 dipoles per tile

Electronic delay lines

Steer in microseconds

MWA Core

LOFAR Core

Conclusions• Routine Faraday Rotation observations of the corona may soon be possible

– Model validation? (Streamers, solar cycle, HCS)– CME magnetic field topology!

• Can we incorporate FR LOS calculations into BATS-R-US simulations, or at least existing BAT-R-US analysis tools?– RA, DEC, time -> HGI steps, LOS parameters

• Opportunity to partner with MWA and LOFAR teams (NSF proposal?)SOHO Coronagraph Simulated light Simulated Faraday rotation