Homepage of Lars Næsheim
I am a Ph.D. student working on various problems associated with the
upper polar mesosphere. Specifically, I am looking at noctilucent
clouds, the polar mesospheric summer radio echo, and mesospheric dust.
Noctilucent clouds (NLC) are very tenuous clouds appearing immediately
after sunset in mid-latitude regions. Their tenuous nature only allows
them to be directly observed when they are illuminated from below, thus
their name and the time of their appearance.
The polar mesospheric summer echo (PMSE) is a phenomenon observed at more
or less the same
conditions as NLC, with echoes observed from the 50 MHz range right up
to 1.3 GHz. No generally accepted theory for explaining these echoes
has been constructed.
Some of the questions I am pondering are:
- Why were NLC's not observed before the 1880's, and why does their
frequency seem to be increasing?
- What relationships exist between NLC, PMSE and mesospheric dust?
All three can be observed independent of each others, but they often show
up together, sometimes very nicely correlated as I was able to
demonstrate in my master's thesis.
- How can sufficient electric charge gradients be created in the
mesopause to create PMSE? And can existing observational data be used to
affirm or disprove any of the proposed hypotheses?
- In what ways are NLC, PMSE and mesospheric dust linked to the
current changes in the upper and middle atmosphere, specifically changes
in the temperature and average height of the F-layer.
I will first and foremost examine observational data, but hopefully
be able to contribute something to elaborate on existing theories.
Hopefully, I will have answers to some of the questions above which I can
defend at my dissertation in 2003...
You can download my cand.scient.-thesis as a PDF-file here(16.2 Mb)
Lars I. Næsheim,
Tromsø, 22. September 2000