Abstract. Nitric oxide (NO) is produced by solar photolysis and auroral activity in the
upper mesosphere and lower thermosphere region and can, via transport
processes, eventually impact the ozone layer in the stratosphere. This work
uses measurements of NO taken between 2004 and 2016 by the Odin sub-millimeter radiometer (SMR) to build an empirical model that links the
prevailing solar and auroral conditions with the measured number density of
NO. The measurement data are averaged daily and sorted into altitude and
magnetic latitude bins. For each bin, a multivariate linear fit with five
inputs, the planetary K index, solar declination, and the F10.7 cm flux, as
well as two newly devised indices that take the planetary K index and the solar
declination as inputs in order to take NO created on previous days into
account, constitutes the link between environmental conditions and measured
NO. This results in a new empirical model, SANOMA, which only requires the
three indices to estimate NO between 85 and 115 km and between
80∘ S and 80∘ N in magnetic latitude. Furthermore, this work
compares the NO calculated with SANOMA and an older model, NOEM, with
measurements of the original SMR dataset, as well as measurements from four
other instruments: ACE, MIPAS, SCIAMACHY, and SOFIE. The results suggest that
SANOMA can capture roughly 31 %–70 % of the variance of the measured
datasets near the magnetic poles, and between 16 % and 73 % near the
magnetic equator. The corresponding values for NOEM are 12 %–38 %
and 7 %–40 %, indicating that SANOMA captures more of the variance
of the measured datasets than NOEM. The simulated NO for these regions was on
average 20 % larger for SANOMA, and 78 % larger for NOEM, than the
measured NO. Two main reasons for SANOMA outperforming NOEM are identified.
Firstly, the input data (Odin SMR NO) for SANOMA span over 12 years, while
the input data for NOEM from the Student Nitric Oxide Experiment (SNOE) only
cover 1998–2000. Additionally, some of the improvement can be accredited to
the introduction of the two new indices, since they include information of
auroral activity on prior days that can significantly enhance the number
density of NO in the MLT during winter in the absence of sunlight. As a next
step, SANOMA could be used as input in chemical models, as a priori
information for the retrieval of NO from measurements, or as a tool to
compare Odin SMR NO with other instruments. SANOMA and accompanying scripts
are available on http://odin.rss.chalmers.se (last access: 15 September 2018).