(April 18th, 2016)
Just over a week ago, that is on April 9th, I powered up my station and directed the EME array towards the Moon as there was a good Europeen window in effect. As the array was in motion, I noticed an extremely intense increase in the noise level, something in the 10-15 dB range. I was extremely surprised by the intensity of it and quite curious to find out where these massive bursts of noise were coming from.
After going back and forth with the array in order to locate the source, I quickly realized that the noise source was nothing else than the Sun! I had never seen the Sun being so "enraged" and noisy.
Clearly, with a 10-15 dB of intensity, the noise generated by the Sun on that day was completely off the chart and far above what was expected. As a precaution measure before drawing any conclusion, I asked W2HRO-Paul to take some measurements in order to validate what I was seeing on my side. Paul quickly came back to me and confirmed that he was measuring the same noise intensity on the East coast. Wow!
There are several articles on the internet which explain how the Sun can be utilized as a "calibrated" noise source in order to evaluate the RX performance of an EME station. The intensity of the Sun noise is apparently a function of the solar flux (sfu) and the frequency of operation. For intance, for a station of my size (18.5 dBi), and with a low solar flux of 62 sfu, the difference in the noise level between the Cold Sky and the Sun should be in the order of 4dB (famous Y-Factor). On that day, the Solar Flux was low at 106 sfu so the noise measured was evidently a way above the expected figure.
A day later, I decided to run a "Sun Pass" experiment. I parked the array at 125AZ/52EL and measured the noise level (every second) as the Sun was making its way through the Sky and passing right in front of the main antenna lobe. The results of the experiment are shown in the graph above to the right. One can see how the noise gradually increased as the Sun was approaching the main antenna lobe and then decreased as expected while it was moving away from it. What was quite remarkable again was the intensity of the noise itself, which averaged about 10dB with many outbursts in the 12-15 dB range!
A week later, the Sun had significantly quieted down so I repeated the "Sun Pass" experiment using the exact same protocol. The results obtained are shown in the plot to the right.
The noise figures obtained were much more in line with the expected value of <4dB in noise increase. The Sun had clealy returned to its "Normal" state. Interestingly, the Solar Flux on that day was at 113 sfu, that is very close to the figure on April 9th. 
W2HRO - Paul confirmed that he was getting similar noise figures on his side so once again, the phenomenon observed was validated by a separate source.
I was able to capture both extreme cases on video and created a montage in order to show how dramatic the phenomenon is (see video to the right).
The first portion of the video (April 9th) shows the difference in noise level between the Cold Sky and the Sun. The second portion of the video (April 16th) shows the same experiment but after the Sun had returned to its "Normal" state.  
 ***WARNING: For Best Video Resolution, it is critical to select 720p HD resolution in the youtube "settings" at bottom right of the player. The 360p default setting won't yield good enough resolution to see the details. It will take several seconds before the High Resolution kicks in, so you will need to restart the video from the beginning when the High Resolution is active and select to view the video in "Full Screen Mode" for best experience...***
First, having the Sun going wild without notice and being capable of spitting out 10-15dB of noise is very dramatic and consequential in the world of EME! The RX performance degradation can be very significant. Imagine the penalty for a station which has an antenna array with signicant side-lobes for instance... No antenna system is perfect anyway, so having a very strong source of noise in the sky is just not a good thing for EME. Practically speaking, this means that the true "Effective Performance Degradation" could be much higher than what is calculated by WSJT.
The obvious questions I am now asking myself: How often does this happen? Why haven't we heard about this before??? Is there a space indicator/metric which can predict when such phenomenon takes place? Is there a full spectrum of possible scenarios between the two extreme cases documented above (I assume yes)? How can the Sun be used as a reliable source to evaluate the RX performance of a station under such circumstances?
Stay tuned!