Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Recovering the unsigned photospheric magnetic field from Ca II K
  observations   We reassess the relationship between the photospheric magnetic field strength
and the Ca II K intensity for a variety of surface features as a function of
the position on the disc and the solar activity level. This relationship can be
used to recover the unsigned photospheric magnetic field from images recorded
in the core of Ca II K line. We have analysed 131 pairs of high-quality,
full-disc, near-co-temporal observations from SDO/HMI and Rome/PSPT spanning
half a solar cycle. To analytically describe the observationally-determined
relation, we considered three different functions: a power law with an offset,
a logarithmic function, and a power law function of the logarithm of the
magnetic flux density. We used the obtained relations to reconstruct maps of
the line-of-sight component of the unsigned magnetic field (unsigned
magnetograms) from Ca II K observations, which were then compared to the
original magnetograms. We find that both power-law functions represent the data
well, while the logarithmic function is good only for quiet periods. We see no
significant variation over the solar cycle or over the disc in the derived fit
parameters, independently of the function used. We find that errors in the
independent variable, usually not accounted for, introduce attenuation bias. To
address this, we binned the data with respect to the magnetic field strength
and Ca II K contrast separately and derived the relation for the bisector of
the two binned curves. The reconstructed unsigned magnetograms show good
agreement with the original ones. RMS differences are less than 90 G. The
results were unaffected by the stray-light correction of the SDO/HMI and
Rome/PSPT data. Our results imply that Ca~II~K observations, accurately
processed and calibrated, can be used to reconstruct unsigned magnetograms by
using the relations derived in our study.

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The categories of this paper are: stat
2
The categories of this paper are: math
26
The categories of this paper are: bayes-an
14
The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
13
The categories of this paper are: quant-ph
27
The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
10
The categories of this paper are: q-bio
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The categories of this paper are: patt-sol
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The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat