I was curious to see how SID numbers increase over the solar cycle period. So plotted total SID numbers from jan 2008 to march 2012 using AAVSO SID database. What I get is here in the attahment:
If you pull all the AAVSO SID submissions from the NGDC ftp site:
and graph the duration (from start to end) of the SIDs going back to 1958 this is what it looks like. You can see the last 3 solar cycles in there, I think.
To my knowledge the data stored at NGDC does have contributions to the SID database from other professional observatories all over the world, although, most seem to be amateur stations: ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/Sudden_Ionospheric_Disturbances/sid_stations
Since most of NOAA's interest was from 1957 (after Sputnik) on it is interesting to read about how it started: ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/Sudden_Ionospheric_Disturbances/1README.TXT
I think the NGDC data make more sense when you look all the way back to 1958. In this graph attached, you can see how the solar cycles show up from 1960 - 1990 , then it seems a decision was made to compare the SID events to the GOES satellite: http://en.wikipedia.org/wiki/GOES_8
This seems to have made a big difference in 'variability' of the SID event data. The AAVSO still compares all SID submissions to the GOES satellites (GOES 15 now).
Is this a good idea?