~Vanishing twilight days data still very compelling.
February 12, 2015, there are high energy Lows basically cut off by High pressure system covering a wider area of the High Arctic. The nearest High energy systems were 976 and 991 mb in strength, they are not expansive and isolated. The sunlight ray path
on feb 12 went through a much smaller Upper Air dT/dZ rate (difference in temperature/altitude depth) than some preceding events.
Dec 25 santa storm brightness versus February 12 low dT/dZ sunray transect.
The higher energy event on Dec 25 was literally more visible.
With -13.1 degree sun way below horizon, the last High Arctic observation days of ideal High Energy Events were basically free of haze particulates, because during the long night, there was next to no photochemistry at all. December 25 2014 is a class 10 extra bright event which was so strong it likely affected the sea Ice extent numbers later:
Watch closely the biggest drop in recent sea ice extent was preceded by the "santa storm" a literal heat laden from the South Atlantic cyclone which headed almost strait to the North Pole , eventually it merged with other Gulf Stream cyclones and covered a huge extent of the circumpolar world, forming a true sub vortex.
Fast forward to today (feb 12, 2015):
The energy systems are nowhere as intense, where did the brightness at -12.9 degrees below the horizon gone? Although there was one recent event rivaling December 25, it dissipated on the North American West Coast. But the twilight observation days are drawing to a close. The Arctic atmosphere is loaded with photochemically sensitive aerosols, Arctic haze will be setting in, however the haze dissipates during darkness , so the low just returning orange sun is not sufficient to affect twilight, not yet, but soon there will be no weak twilight to observe. WDFeb12,2015