Generation of Magnetic Fields in Gravitationally

Forming Structures in the Early Universe

Russell M. Kulsrud

Princeton Plasma Physics Laboratory

OUTLINE

• PROPERTIES OF OUR GALACTIC MAGNETIC FIELD

• STANDARD — DISC DYNAMO THEORY

• PROTOGALACTIC ORIGIN

OUR GALACTIC MAGNETIC FIELD

• The field is toroidal.

• Its strength is 2 microgauss.

• It varies in radius possibly changing every spiral arm or about one kiloparsec.

• It does not change in the direction perpendicular to the disc.

Figure 1: The galactic magnetic field

THE ALPHA-OMEGA DISC DYNAMO THEORY

• THE THEORY CONSERVES FLUX, SO TO WORK IT HAS TO EXPEL NEGATIVE FLUX

• THIS IS VERY DIFFICULT BECAUSE THE GALACTIC DISC IS IN A DEEP POTENTIAL WELL AND THE FLUX MUST CARRY MATTER OUT WITH IT

Figure 2: The escape of negative flux in the Alpha-Omega dynamo.

OTHER ORIGINS

• The Galaxy was originally a plasma in the form of a protogalaxy » 100kpc with a mass of 1011 solar masses.

• It resulted from a gravitational collapse and smaller parts also suffered gravitational collapse creating many small shocks.

Shocks Creating Turbulence

These shocks produced vorticity, turbulence, weak magnetic fields (Biermann) and heating that led to the support of the protogalaxy against gravity.

k

The Magnetic Differential Equation

The Solution

B0=6£10-6 gauss

lnB/B0

lnk/k0

CONCLUSION

• Magnetic Fields may have already been generated before our galaxy was fully formed due to plasma processes in the preceding protogalaxy.

• This may be a common phenomenon.