  In previous publications we have shown that long-ignored approximations in
standard model calculations could have significant implications for very low
mass particles such as neutrinos and photons. In particular we showed that, in
a dilute plasma such as that in the solar corona, a significant decay
probability of $\nu' \rightarrow \nu + \gamma$ will be possible as a
consequence of the terms ignored in making the approximations. Here the $\nu'$
and $\nu$ are high and low mass eigenstates of the neutrino. In this paper, we
investigate the effect in the vicinity of an expanding supernovae remnant such
as SN1987A. We show that, in the dilute plasma external to the remnant, such
decays are possible and significant. We describe a calculation of effects of
such decays on the anti-neutrinos from SN1987A. The calculated anti-neutrino
energy against arrival time agrees reasonably well with that observed, assuming
that the expansion velocity of the remnant is $\approx 0.2 c$ and that the
plasma density is high within the expanding remnant.
