[ips-si-alert] Automatic detector missed sudden storm commencement [SEC=UNCLASSIFIED]

[rwc]

The SWS automatic geomagentic sudden impulse storm detector failed to 
detect the sudden storm commencement at around 2308UT on 7 Sep 2017 (Time not official).
This appears to be due to the sudden storm commencement (SSC) not 
having a sustained following elevated "plateau" in increased magnetic 
field values usually associated with compression of the Earth's magnetic 
field following a shock arrival, due to elevated solar wind speed and density. 
Instead the main phase (subsequent drop in values due to the ring current) of the 
storm appeared to start immediately following the impulse.  The detector 
looks for a step function in the magentometer data and so did not detect 
the SSC which was more of a "spike" in shape. This may be related to the 
solar wind denisty rapidly dropping off after solar wind shock arrival, 
reducing solar wind pressure. See: http://www.sws.bom.gov.au/Geophysical/1/1/5
SWS