[Lowfer] SIDs

[Garry]

I've been monitoring VLF trnasmitter signal strengths in the daytime for 
awhile, looking for sudden ionospheric disturbances (SIDs). Using 
Spectrum Lab software signal strength - versus - time graphs have been 
created each day for four VLF stations: (1) NPM, Hawaii, 21.4 kHz; (2) 
NAA, Maine, 24.0 kHz; (3) NLK, Washington, 24.8 kHz; and (4) NML4, ND, 
25.2 kHz. GOES x-ray flux spikes in time indicate such events (see 
https://www.swpc.noaa.gov/products/goes-x-ray-flux) but so far here the 
impact on VLF signal strength has generally only been a few dB. But 
today around 1550Z a very large event occurred, followed around 1722Z by 
a typical event.

73, Garry, K3SIW, EN52ta, Elgin, IL
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