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One of the most intriguing
aspects of the aurora, especially in the age of
low-cost, high-sensitivity cameras, is the vibrant
colors they exhibit. From the striking green to deep
reds and pink/violet colors, they present almost
unnatural colors. This is due to the production of the
optical emission by electron impact excitation of the
atomic and molecular gases. Since the interactions
excite quantum states in the electron orbitals, the
permitted emission wavelengths are discrete and limited.
This has been the backbone of studying the energy
deposition and distribution of the precipitating
electrons using ground-based and some satellite based
studies. With advanced sensitivity and proliferation of
multi-camera arrays in Alaska, Canada and Scandinavia,
as well as a much expanded cadre of citizen science
observers, we are finding that not all emissions are
discrete. The discovery of the STEVE (Strong Thermal
Emission Velocity Enhancement) phenomenon, and
subsequent measurement with an imaging spectrograph
showed that the emissions were broadband, not discrete.
While associated with very high electron temperature,
calculations do not support the idea that it is a
blackbody glow. And more recently a new set of broadband
filtered cameras in Canada have captured several active
auroral events where the color characteristics did not
follow normal red-green-blue ratios. Subsequent
measurements with the same imaging spectrographs again
show a distinct broadband emission overlaying the
typical auroral emissions. I will present these examples
(STEVE and broadband active aurora), and present a
little speculation on the possible mechanism, though the
measurements to date don't allow a definitive answer.
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