High Energy Physics with author PLB is supported by a NATO travel grant (CRG 9100072). B. Wiebel-Sooth and H. Meyer are supported by the BMBF, FRG, under contract number . PLB would like to thank Drs. G. Battistoni, T. Gaisser, P. Lipari, D. Müller, R. Protheroe, M. Shapiro, and T. Stanev for many hours of intense discussion of the topics raised in this paper.
© European Southern Observatory (ESO) 1998
Online publication: January 8, 1998