An updated review of polar
mesosphere summer echoes: Observation, theory, and their relation to
noctilucent clouds and subvisible aerosols
Peculiar atmospheric radar echoes from the high-latitude summer
mesosphere have spurred much research in recent years. The radar data
(taken on frequency bands ranging from 2 to 1290 MHz) have been
supplemented by measurements from an increasing arsenal of in situ
(rocket-borne) and remote-sensing (satellites and lidars) instruments.
Theories to explain these so-called polar mesosphere summer echoes
(PMSE) have also proliferated. Although each theory is distinct and
fundamentally different, they all share the feature of being dependent
on the existence of electrically charged aerosols. It is, therefore,
natural to assume that PMSE are intimately linked to the other
fascinating phenomenon of the cold summer mesopause, noctilucent
clouds (NLCs), which are simply ice aerosols that are large enough to
be seen by the naked eye. In this paper we critically examine both
the data collected and the theories proposed, with a special focus on
the relationship between PMSE and NLCs. The picture that emerges is
loosely akin to that of an iceberg, where the visible tip is the small
number of big particles that manifest themselves as NLCs and where the
submerged mass is the large number of smaller, subvisible particles
that create the right condition for PMSE.
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