Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Stationary waves and slowly moving features in the night upper clouds of
  Venus   At the cloud top level of Venus (65-70 km altitude) the atmosphere rotates 60
times faster than the underlying surface, a phenomenon known as superrotation.
Whereas on Venus's dayside the cloud top motions are well determined and Venus
general circulation models predict a mean zonal flow at the upper clouds
similar on both day and nightside, the nightside circulation remains poorly
studied except for the polar region. Here we report global measurements of the
nightside circulation at the upper cloud level. We tracked individual features
in thermal emission images at 3.8 and 5.0 $\mathrm{\mu m}$ obtained between
2006 and 2008 by the Visible and Infrared Thermal Imaging Spectrometer
(VIRTIS-M) onboard Venus Express and in 2015 by ground-based measurements with
the Medium-Resolution 0.8-5.5 Micron Spectrograph and Imager (SpeX) at the
National Aeronautics and Space Administration Infrared Telescope Facility
(NASA/IRTF). The zonal motions range from -110 to -60 m s$^{-1}$, consistent
with those found for the dayside but with larger dispersion. Slow motions (-50
to -20 m s$^{-1}$) were also found and remain unexplained. In addition,
abundant stationary wave patterns with zonal speeds from -10 to +10 m s$^{-1}$
dominate the night upper clouds and concentrate over the regions of higher
surface elevation.

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The categories of this paper are: gr-qc
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The categories of this paper are: q-fin
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The categories of this paper are: cmp-lg
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The categories of this paper are: math
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The categories of this paper are: cs
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The categories of this paper are: q-bio
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The categories of this paper are: funct-an
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The categories of this paper are: hep-ex
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The categories of this paper are: econ
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The categories of this paper are: alg-geom
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The categories of this paper are: supr-con
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The categories of this paper are: solv-int
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The categories of this paper are: atom-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: quant-ph
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The categories of this paper are: nucl-th