|
|
|
A COSMIC BATTERY RECONSIDERED Authors: G.S.
Bisnovatyi-Kogan, R.V.L. Lovelace, & V.A. Belinski We
revisit the problem of magnetic field generation in
accretion flows onto black holes owing to the excess radiation
force on electrons. This excess force may arise from the
Poynting-Robertson effect. Instead of a recent claim of
the generation of dynamically important magnetic fields,
we establish the validity of earlier results from 1977
that show that only small magnetic fields are generated.
The radiative force causes the magnetic field to initially
grow linearly with time. However, this linear growth holds
for only a restricted time interval that is of the order
of the accretion time of the matter. The large magnetic
fields recently found result from the fact that the linear
growth is unrestricted. A model of the Poynting-Robertson
magnetic field generation close to the horizon of a
Schwarzschild black hole is solved exactly using general
relativity, and the field is also found to be dynamically
insignificant. These weak magnetic fields may however be important
as seed fields for dynamos.
|
