VLA Expansion Project Science Working Group:
The Magnetic Universe
(24 November 1998)

Summary by M. Rupen.

Chaired by Frazer Owen
Also present:
  Tim Bastian, Alan Bridle, Chris Carilli, Mark Claussen, Ketan Desai,
  Vivek Dhawan, Jean Eilek, Chris Fassnacht, Dale Frail, Ken Kellerman,
  Amy Mioduszewski, Rick Perley, Michael Rupen, Greg Taylor, Joan Wrobel


1- How to detect magnetic fields at radio wavelengths
  a. We can derive three things: existence, orientation, strength

  b. The methods
    - Synchrotron and non-thermal radiation
    - Rotation measures
    - Anisotropic scattering
    - Zeeman splitting
    - Maser polarization
    - Gyrosynchrotron emission
    - Dust orientation at 7mm: use high-resolution to track dust alignment
        Follow-up: Dale Frail
    - Cyclotron lines (different frequencies correpsond to different plasma
        layers --> 3D maps)
    - Razin-Tsytovich

  c. Background sources as probes 
    * We get lots of background sources:
      at 20cm, about 127 suitable sources per 30amin beam, vs. perhaps
      5-10 w/ current VLA
    * Expansion Project offers
                     SENSITIVITY, for detectability
                     BANDWIDTH and SPECTRAL RESOLUTION, to track the pol'n 
                       angle
                     POLARIMETRY at the same time, vs. now have to toss 
                       channels

  d. Zeeman splitting
    * Not clear we win big. 
    * Would be nice to do Zeeman on ULIRGs, perhaps with A+.
    * CHECK WITH TROLAND: are you ever limited by the instrument?
        How about if we could give you time-stable D terms?

2- The Sun ***
  Follow-up: Tim Bastian
  Mostly covered by the 1995 workshop
  * Three-dimensional tomography: use broad frequency coverage (up to 15 GHz)
    to probe cyclotron emission at different depths
    - can't do flares, since we can't change frequencies rapidly enough
  * Map the heliosphsere using scattering and rotation measures (see solar
    science working group)


3- Planets
  Follow-up: Bryan Butler
  see planetary science working group


4- Stars ***
  Covered in the solar/stellar working group, under
  "origins of stellar activity"


5- Pulsars
  * Single-pulse polarimetry to get the geometry of the magnetic fields...
    but GBT will do this


6- Stellar masers
  * OH is too scattered for VLBA --> A+ mapping might be good
  * sensitivity will help for 1720 MHz shock masers
  Nothing stunning -- don't emphasize this


7- SNRs
  * Masers in shocks
  * RM, Faraday rotation probes

8- Galactic Center ***

9- Galactic molecular clouds ***
   - detect mangnetic fields through Faraday rotation and statistical
     depol'n

10- Pol'n of galactic background (as WSRT/de Bruyn)

12- Galaxies
  * galaxy halos: 20 rads/m^2
  * Spiral galaxies
  * Map normal rather than special galaxies
  * perhaps can also get to higher z???
  * look for emission from "boring" galaxies -- e.g. dE -- are there mag.
      fields?

13- Galaxy clusters ***
  * Use background sources to probe the RM across a cluster
    - many current RM probes are cluster members themselves, making it
      difficult to disentangle RM from galaxy and from cluster
    - typical RM= 0.8 Bparallel/microG n_e/cm^-3 l/pc
      --> 10-20 rad/m^2
    - Indications of extremely high RM
      : Halpha; 3C295; cooling flows; etc.
    --> figure out pressure support due to mag. field and how that affects
        DM derivations
    --> high-z magnetized clusters (.ps file)
    --> where does magnetic field come from in cosmological objects?
  * Cluster halos
    - properties of halos vs. frequency: E Config. at high freq.
    - fainter halos
    - Uniform vs. filamented mag. fields --> sensitivity is paramount!
      What happens on Mpc to kpc scales? 
      --> L or P band

14- Integalactic fields
  * Faraday rotation vs. z
    - can be done now, but you always win with more sources:
      Bridle calls this a problem in pattern recognition, given the
      spatial-frequency behavior of the Faraday rotation
    - Art Wolfe claims excess Faraday rotation in damped Ly alpha absorbers
  * Relationships along adjacent lines-of-sight: this goes as N^2, so you
      win by a lot if you add more sources
    - "Let confusion be your friend!"
    - Find 3 quasars within 1arcmin of each other --> large-scale magnetized
      structures in the early universe

15- High-redshift sources
  * pol'n maps currently limited by resolution and sensitivity
  * known source are all < 10asec; we want A+ at 1.4 GHz
  * "We have a long history of misinterpreting pol'n at low resolution"

16- Lenses: RM of different images
  * currently sensitivity limited
  * CLASS lenses look spiral, not elliptical -- why do they have high RM?


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Last modified 08 December 1999

mrupen@nrao.edu