A relatively modest value of the initial rotation of the iron core , a period of \sim 6 – 31 s , will give a very rapidly rotating protoneutron star and hence strong differential rotation with respect to the infalling matter .
Under these conditions , a seed field is expected to be amplified by the MRI and to grow exponentially .
Exponential growth of the field on the time scale \Omega ^ { -1 } by the magnetorotational instability ( MRI ) will dominate the linear growth process of field line “ wrapping ” with the same characteristic time .
The shear is strongest at the boundary of the newly formed protoneutron star .
Modest initial rotation velocities of the iron core result in sub–Keplerian rotation and a sub–equipartition magnetic field that nevertheless produce substantial MHD luminosity and hoop stresses : saturation fields of order 10 ^ { 15 } – 10 ^ { 16 } G develop \sim 300 msec after bounce with an associated MHD luminosity of \sim 10 ^ { 49 } – 10 ^ { 53 } erg s ^ { -1 } .
Bi-polar flows driven by this MHD power can affect or even cause the explosions associated with core-collapse supernovae .
If the initial rotation is too slow , then there will not be enough rotational energy to power the supernova despite the high luminosities .
The MRI should be active and may qualitatively alter the flow if a black hole forms directly or after a fall-back delay .