Mathematics – Logic
Scientific paper
May 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aps..may..g201k&link_type=abstract
American Physical Society, APS/AAPT Joint Meeting, May 2-5, 1996, abstract #G2.01
Mathematics
Logic
Scientific paper
It is proposed that the origin of our galactic magnetic field occurred during the protogalactic formation phase of our galaxy. It is assumed that prior to the formation there was no cosmic field at all. It is shown that as the protogalaxy formed the thermoelectric currents in cosmic plasma increased the magnetic field from zero by the Biermann battery mechanism up to a value of order 10-20 gauss. From numerical simulations, it is found that there there is very strong Kolmogoroff turbulence present in the protogalaxy. This turbulence acts on the magnetic field resulting from the Biermann battery and amplifies it at a rate γ = (k_max/k_min )^2/3 × 10-16 sec-1 where k_min and k_max are the minimum and maximum wave numbers for the turbulence. The value of k_min is found to be of order 1 megaparsec-1 , but the value of k_max lies below the grid resolution of the numerical simulation and must be determined by the physics of the cosmic plasma on small scales. During a Hubble time there is plenty of time to amplify the magnetic field from 10-20 gauss to a value that would serve as a seed field for the galactic field. The question that arises is will this field be coherent on large scales or will all the energy be concentrated in small scales. This question is addressed in this talk. the important consideration is that the cosmic plasma at this stage is very hot and has a very low density. As a result, the mean free path is extremely long of order a sizable fraction of the entire size of the protogalaxy. Therefore, it is necessary to treat the effect of the turbulent motions of the cosmic magnetic field by a semicollionless theory on scales shorter than the mean free path. It turns out that as long as the ion gyroradius is small the magnetic field controls the motion of ions through the magnetic mirror effect. this is true even if the magnetic energy is tiny compared to the thermal or kinetic energy of the plasma. As a result of this process the magnetic energy is prevented from developing on scales shorter than the mean free path. This will be demonstrated in the talk. The consequences to be drawn from these results are: The magnetic energy can built up to a large value by the protogalactic turbulence. (2) As long as the mean free path stays long the generated magnetic field will be coherent on sufficiently large scales to provide a primordial field for the galaxy. However, eventually the plasma cools and the mean free path decreases It is important that the magnetic field becomes strong enough before this happens that it can inhibit the buildup of small scale turbulence. Finally, it has to be realized that the entire galactic plasma passes through this protogalactic phase so that whatever field emerges from it must be present as the initial field for the galaxy. If the result turns out to be that the field is very incoherent with lots of energy on small scales then this is the initial field for the galaxy. On the other hand if the field manages to remain coherent as it becomes strong, then the above theory provides a logical origin for a primordial magnetic field. If this is the case then for the galaxy and one need not worry about the problems involved with galactic dynamos is seeking the origin of galactic magnetic fields.
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