MAGNETIC-FIELD GENERATION IN FIRST-ORDER PHASE-TRANSITION BUBBLE COLLISIONS

We consider the formation of a ring-like magnetic field in collisions of bubbles of broken phase in an Abelian Higgs model. Particular atten tion is paid to multiple collisions. The small collision velocity limi t, appropriate to the electroweak phase transition, is discussed. Assu ming the relevant features of the Abelian Higgs model are appropriate also to the electroweak case, we argue that after the completion of th e electroweak phase transition, when averaged over nucleation center d istances, there exists a mean magnetic field B similar or equal to 2.0 x 10(20) G with a coherence length 9.1 x 10(3) GeV-1 (for m(H) = 68 G eV). Because of the ring-like nature of BI the volume average behaves as B similar to 1/L. Taking into account the turbulent enhancement of the field by inverse cascade, we estimate that colliding electroweak b ubbles would give rise to a mean field B-rms similar or equal to 10(-2 1) G at 10 Mpc comoving scale today.