<DOC>
<DOCNO>
296
</DOCNO>
<TITLE>
notes on waves through gases at pressures small compared
with the magnetic pressure, with applications to upper
atmosphere aerodynamics .
</TITLE>
<AUTHOR>
lighthill,m.j.
</AUTHOR>
<BIBLIO>
j. fluid mech. 9, 1960, 465.
</BIBLIO>
<TEXT>
most treatments of magnetohydrodynamic
waves have confined physical interpretation
to cases when the alfven velocity a
is small compared with the sound
velocity a .  here we consider the 'low-beta
situation', in which a is much
larger than a .  then, except for two modes with
wave velocity a the only possible
waves are longitudinal ones, propagated
unidirectionally along lines of magnetic
force with velocity a .  these can be
interpreted as sound waves, confined to
effectively rigid magnetic tubes of force .
hall-current effects do not alter these
conclusions (in contrast to the high-beta
situation), and finite conductivity
introduces only small dissipation .
  an application is made to the flow pattern
around a body moving through the
f layer of the ionosphere, where, although
neutral particles have a very large
mean free path, charged particles interact
electrostatically and, it is argued, may
be regarded as forming a continuous fluid
whose movement is independent of
that of the neutral particles .  a body moving
at satellite speed or below would
then excite the above-mentioned unidirectional
sound waves, but no waves at
much faster alfven velocity .  these considerations
suggest that its movement
would be accompanied by a v-shaped pattern of
electron density (figure 2),
which might be in part responsible for some
anomalous radar echoes that have
been reported .
</TEXT>
</DOC>
