Astrophysics, abstract
astro-ph/0602092

From: Mikhail V. Medvedev [view email]

Date: Sat, 4 Feb 2006 08:41:11 GMT (144kb)

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

Subj-class: Astrophysics; Geophysics; Plasma Physics; Atmospheric and Oceanic Physics; Populations and Evolution

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.

http://arxiv.org/abs/astro-ph/0602092