Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Galactic Halo UV Field, Magellanic Stream and High Velocity Clouds   Significant numbers of high-velocity HI clouds (HVCs) have now been detected
in H-alpha, with a subset seen in low ionization lines (e.g. [NII]). It was
originally hoped that the observed H-alpha strength would provide a distance
constraint to individual clouds. This idea requires that a useful fraction
(f_esc > 1%) of ionizing photons escape the Galaxy, and that the halo ionizing
field is relatively smooth, as we discuss. Most HVCs which are known to be
close to the Sun are H-alpha; the brightest clouds also show enhanced [NII]
emission, in contrast to the Magellanic Stream where the low ionization
emission lines are always weak compared to H-alpha. But an acute complication
for H-alpha distances is the apparent H-alpha brightness of the Magellanic
Stream along several sight lines. To account for this, we present three
possible configurations for the Magellanic Stream and propose a follow-up
experiment. If we normalize the distances to local HVCs, some HVCs appear to be
scattered throughout the Galactic halo on scales of tens of kiloparsecs.

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The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
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The categories of this paper are: bayes-an
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The categories of this paper are: hep-lat
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The categories of this paper are: acc-phys
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The categories of this paper are: nlin
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The categories of this paper are: q-bio
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The categories of this paper are: q-alg
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The categories of this paper are: solv-int
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The categories of this paper are: q-fin
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The categories of this paper are: alg-geom
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The categories of this paper are: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: nucl-ex
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The categories of this paper are: mtrl-th