Space Forum / SETI / February 2006

From this morning's astro-ph:

astro-ph/0602092
Title: Do extragalactic cosmic rays induce cycles in fossil diversity?
Authors: Mikhail V. Medvedev, Adrian L. Melott (University of Kansas)
Comments: 13 pages, 3 figures, submitted to Nature

The idea of cycles in fossil diversity has recently been put on a firm
statistical footing, revealing a 62{plus-minnus}3-million-year cycle
in the number of marine genera. The strong signal requires a periodic
process extending back at least 540 My, which is difficult to explain
by any terrestrial process. While astro- and geophysical phenomena may
be periodic for such a long time, no plausible mechanism has been
found. The fact that the period of the diversity cycle is close to the
64 My period of the vertical oscillation of the Solar system relative
to the galactic disk is suggestive. However, any model involving
cosmogenic processes modulated by the Sun's midplane crossing or its
maximal vertical distance from the galactic plane predicts a
half-period cycle, i.e. about 32 My. Here we propose that the
diversity cycle is caused by the anisotropy of cosmic ray (CR)
production in the galactic halo/wind/termination shock and the
shielding effect of the galactic magnetic fields. CRs influence cloud
formation, can affect climate and harm live organisms directly via
increase of radiation dose. The CR anisotropy is caused by the
galactic north-south asymmetry of the termination shock due to the
interaction with the ``warm-hot intergalactic medium'' as our galaxy
falls toward the Virgo cluster (nearly in the direction of the
galactic north pole) with a velocity of order 200 km/s. Here we
revisit the mechanism of CR propagation in the galactic magnetic
fields and show that the shielding effect is strongly
position-dependent. It varies by a factor of a hundred and reaches a
minimum at the maximum northward displacement of the Sun. Very good
phase agreement between maximum excursions of the Sun toward galactic
north and minima of the fossil diversity cycle further supports our
model.