Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: WHAM Observations of H-Alpha, [S II], and [N II] toward the Orion and
  Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium   A large portion of the Galaxy (l = 123 deg to 164 deg, b = -6 deg to -35
deg), which samples regions of the Local (Orion) spiral arm and the more
distant Perseus arm, has been mapped with the Wisconsin H-Alpha Mapper (WHAM)
in the H-Alpha, [S II] 6716, and [N II] 6583 lines. Several trends noticed in
emission-line investigations of diffuse gas in other galaxies are confirmed in
the Milky Way and extended to much fainter emission. We find that the [S
II]/H-Alpha and [N II]/H-Alpha ratios increase as absolute H-Alpha intensities
decrease. For the more distant Perseus arm emission, the increase in these
ratios is a strong function of Galactic latitude and thus, of height above the
Galactic plane. The [S II]/[N II] ratio is relatively independent of H-Alpha
intensity. Scatter in this ratio appears to be physically significant, and maps
of it suggest regions with similar ratios are spatially correlated. The Perseus
arm [S II]/[N II] ratio is systematically lower than Local emission by 10%-20%.
With [S II]/[N II] fairly constant over a large range of H-Alpha intensities,
the increase of [S II]/H-Alpha and [N II]/H-Alpha with |z| seems to reflect an
increase in temperature. Such an interpretation allows us to estimate the
temperature and ionization conditions in our large sample of observations. We
find that WIM temperatures range from 6,000 K to 9,000 K with temperature
increasing from bright to faint H-Alpha emission (low to high [S II]/H-Alpha
and [N II]/H-Alpha) respectively. Changes in [S II]/[N II] appear to reflect
changes in the local ionization conditions (e.g. the S+/S++ ratio). We also
measure the electron scale height in the Perseus arm to be 1.0+/-0.1 kpc,
confirming earlier, less accurate determinations.

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The categories of this paper are: eess
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The categories of this paper are: econ
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The categories of this paper are: chao-dyn
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The categories of this paper are: math
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The categories of this paper are: nucl-th
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The categories of this paper are: supr-con
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The categories of this paper are: hep-ph
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The categories of this paper are: stat
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The categories of this paper are: plasm-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: q-fin
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The categories of this paper are: gr-qc
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The categories of this paper are: adap-org
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The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
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The categories of this paper are: solv-int