Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: What is the appropriate coordinate system for magnetometer data when
  analyzing ionospheric currents?   In this paper we investigate which coordinate representation is most
appropriate when analyzing ground magnetometer data in terms of ionospheric
currents, in particular the westward electrojet. The $\textit{AL}$ and the
recently introduced $\textit{SML}$ index are frequently used as monitors of the
westward electrojet. Both indices are based on ground magnetometers at auroral
latitudes. From these magnetometers, the largest perturbation in the southward
direction is selected as the $\textit{ AL/SML }$ index at 1 min cadence. The
southward component is defined as antiparallel to the orientation of the
horizontal part of the Earths' main field, $\textbf{B}_{0, \textit{H}}$. The
implicit assumption when using these indices as a monitor of the westward
electrojet is that the electrojet flows perpendicular to $\textbf{B}_{0,
\textit{H}}$. However, $\textbf{B}_{0, \textit{H}}$ is, in general, not
perpendicular to the westward direction in coordinate systems that take
nondipole terms of the Earth's magnetic field into account, such as apex and
the Altitude Adjusted Corrected Geomagnetic coordinate systems. In this paper
we derive a new $\textit{SML}$ index, based on apex coordinates. We find that
the new index has less variation with longitude and universal time (UT),
compared to the traditionally defined $\textit{SML}$. We argue that when
analyzing ionospheric currents using magnetometers, it is appropriate to
convert the components to a corrected geomagnetic system. This is most
important when considering longitudinal or UT variations, or when data from a
limited region are used.

20
The categories of this paper are: acc-phys
4
The categories of this paper are: cond-mat
17
The categories of this paper are: q-fin
27
The categories of this paper are: comp-gas
10
The categories of this paper are: q-bio
18
The categories of this paper are: econ
29
The categories of this paper are: funct-an
0
The categories of this paper are: hep-ph
8
The categories of this paper are: hep-ex
28
The categories of this paper are: alg-geom
14
The categories of this paper are: math-ph
1
The categories of this paper are: cs
2
The categories of this paper are: math
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph