- Just picked this up from NASA - note the part about LF navigation
systems. Maybe this explains why the hum has been quiet for a while?:
"Scientists are currently tracking a very large flare that occurred on
the Sun around 1:40 pm EDT (17:40 UT). The current estimate of the size
of the explosion is X-17; that would place the flare as the fifth
largest ever observed.
While the blast was not aimed at Earth, the event created a complete
blackout of high frequency communications in North and South America.
According to the NOAA Space Environment Center, communications used by
emergency services along the Gulf Coast may have experienced problems
due to this flare. Low frequency navigation systems may also have
experienced a period of significant degradation. Further, they report
that agencies impacted by space weather storms may experience
disruptions over the next two weeks. These include spacecraft
operators, electric power systems, high frequency communications, and
low-frequency navigations systems."