Space Forum / Astronomy / September 2006

t> Are there any windows of observing intergalactic and extragalactic
t> sources below 15MHz above the atmosphere or at the earth's surface?

As you're probably aware, below 30 MHz, the Earth's ionosphere makes
ground-based observations increasingly difficult.  However,
ground-based measurements have been made as low as about 2 MHz.  The
lowest frequency measurements have to be made under special
conditions.

I once put together a list of all of the astronomical measurements
below 30 MHz that I could find (or maybe it was just the "high
resolution" ones).  At these frequencies, making high resolution
observations is difficult, with the exception of measurements of
Jupiter which can be quite strong.  As I recall, most of the
non-Jovian work at these frequencies has been simply surveys, which
indicates that the Galactic background remains strong.  I don't recall
to what extent any discrete sources have been identified.

However, as one goes to lower and lower frequencies, the Galactic
plane begins to *dim*.  This dimming is because of free-free
absorption by ionized gas in the Milky Way.  At a sufficiently low
frequency, we wouldn't even be able to see out of the Galaxy.

As an example, you might want to look at Cane & Whitham, <URL:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1977MNRAS.179...21C

t> I am particularly interested if there is a window below the first
t> schumann resonance of 7.8hz.

*Hertz*?  Nope.  The interplanetary medium has a plasma frequency of
somewhere around 30 kHz.  Thus, waves below this frequency simply
don't propagate.