[evlatests] Frequency-time variable polarized lunar reflection

[Rick Perley]

Eric has written a useful pair of tasks to permit looking at array 
visibility data in a very useful way.

The task 'DFTIM' makes a 'waterfall' image (time vs. frequency) for the 
vector-summed visibility data for a given position in the image.

The task 'DFTPL' makes a time plot of the vector summed visibility data 
for a given image position.

The former proves to be very useful in looking into these curious 
reflected terrestial RFI from the moon.

The attached plots show the 'I', 'Q', 'U', and 'V' spectra vs time 
('waterfall plot') for the central spot on the moon. Frequency increases 
to the right, time advances upwards.  These show only the first three 
scans (each ~8 minutes long).  The large black 'gaps' in time are when I 
was observing DA240 and 3C147. The narrow vertical black gaps are 
channels flagged due to RFI.  I have shown only the data from 384 to 448 
MHz.

The values shown are effectively the brightness of the central spot of 
the lunar emission.  For reference, the disk brightness is about 1 
Jy/beam, and the total flux from the lunar disk (not counting the 
reflection) is about 60 -- 70 Jy.

The 'I' and 'V' images are of greatest interest.  Both show a strong, RH 
circularly polarized signal (positive) which slowly decreases in 
frequency over the time span shown, declining from 394.25 MHz at the 
beginning to 389.25 MHz at the end of the third scan.  (It disappears 
after this).  The brightness exceeds 100 Jy/beam on occasion.  The 
frequency width of this signal is ~0.5 MHz.  Although not easily visible 
in the 'V' image, there is a very clear pulsation in the signal of about 
4 second period.  (The time sampling is 2 seconds).  This signal is 
linearly polarized as well, but at a much lower level.  Both 'Q' and 'U' 
change in time -- this may be due to Faraday rotation effects.

There is a weaker, broader, negative (LCP) reflected signal as well, 
which increases in frequency from ~430 to ~433 MHz over the same time 
span (1 hour).  This signal is broader -- about 1 MHz. It also 'pulses' 
at the same rate as the stronger RCP signal. This signal is seen in 
later scans, but gradually fades away.

The much stronger LCP signals that I reported on yesterday are all seen 
at later times, but do not show this curious changing frequency.

Any ideas of what is going on here?

Rick
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