Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Principal component analysis of Birkeland currents determined by the
  Active Magnetosphere and Planetary Electrodynamics Response Experiment   Principal component analysis is performed on Birkeland or field-aligned
current (FAC) measurements from the Active Magnetosphere and Planetary
Electrodynamics Response Experiment. Principal component analysis (PCA)
identifies the patterns in the FACs that respond coherently to different
aspects of geomagnetic activity. The regions 1 and 2 current system is shown to
be the most reproducible feature of the currents, followed by cusp currents
associated with magnetic tension forces on newly reconnected field lines. The
cusp currents are strongly modulated by season, indicating that their strength
is regulated by the ionospheric conductance at the foot of the field lines. PCA
does not identify a pattern that is clearly characteristic of a substorm
current wedge. Rather, a superposed epoch analysis of the currents associated
with substorms demonstrates that there is not a single mode of response, but a
complicated and subtle mixture of different patterns.

1
The categories of this paper are: cs
31
The categories of this paper are: ao-sci
18
The categories of this paper are: econ
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
29
The categories of this paper are: funct-an
15
The categories of this paper are: nucl-ex
4
The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
25
The categories of this paper are: patt-sol
37
The categories of this paper are: supr-con