Date:  Wednesday, February 28, 1996
Time:  4:15 PM ­ Refreshments at 4:00
Location:  ERL 126


On the Mysterious Properties of the Polar Summer Mesosphere:  
Forecast for Increasing Cloudiness and Liquid Nitrogen Temperatures or, 
Was Chicken Little Right?

Dr. M. C. Kelley
School of Electrical Engineering,
Cornell University,
Ithaca N. Y. 14853
-- currently at Mission Research Corp. Monterey CA --

Abstract

The Earth's summer polar mesosphere has become the subject of intense study in
recent years.  This is in part due to the fact that the observations of polar
mesospheric clouds seems to be increasing throughout this century.  Called
Noctilucent Clouds due to their visibility in the twilight sky, they are the
highest clouds on the planet. It is  remarkable that they form at all due to
the freeze dried nature of the upper stratosphere but the temperatures are so
low, less than 100K have been observed, that ice can form even at the low water
vapor pressures found at the mesopause. That the temperature can be so low in
full polar sunlight is indeed quite curious and involves a dynamic
refrigeration process. In addition to the optical scattering possible from such
cloud particles they exhibit a remarkably high radar cross-section.  In the
mid-1980's several rocket probes were flown through the cloud region and found
extremely sharp density gradients, too sharp to be explained by normal
ambipolar diffusion estimates.  It is now thought that at least half the
electrical charge in the region is tied up in charged ice particles, a
hypothesis which can explain the radar signatures.  The region thus is one
which can be considered a dusty space plasma well within reach of remote
sensing and space probes.