Neutrino mass spectrum with νμ→νs oscillations of atmospheric neutrinos

We consider the "standard" spectrum of the active neutrinos (characterized by strong mass hierarchy and small mixing) with additional sterile neutrino, nu(s). The sterile neutrino mixes strongly with the muon neutrino, so that nu(mu) <--> nu(s) oscillations solve the atmospheric neutrino problem. We show that the parametric enhancement of the nu(mu) <--> nu(s) oscillations occurs for the high energy atmospheric neutrinos which cross the core of the Earth. This can be relevant for the anomaly observed by the MACRO experiment. Solar neutrinos are converted both to nu(mu) and nu(s). The heaviest neutrino (approximate to nu(tau)) may compose the hot dark matter of the Universe. The phenomenology of this scenario is elaborated and crucial experimental signatures are identified. We also discuss properties of the underlying neutrino mass matrix. (C) 1998 Published by Elsevier Science B.V.