Faraday Rotation: Unique Measurements of Magnetic Fields in the Outer Corona Justin 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

ObservationsBATS-R-US Simulations

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

Helios and steady state corona Volland et al. (1977); Bird et al. (1980); Pätzold et al (1987);

Helios and CMEs Bird et al., 1985 Volland 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 $1M Narrow field of view Slow to slew $20 Full 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 CoronagraphSimulated light Simulated Faraday rotation