Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Solar-cycle variation of sound speed near the solar surface   We present evidence that the sound-speed variation with solar activity has a
two-layer configuration, similar to the one observed below an active region,
which consists of a negative layer near the solar surface and a positive one in
the layer immediately below the first one. Frequency differences between the
activity minimum and maximum of solar cycle 23, obtained applying global
helioseismology to the Michelson Doppler Imager (MDI) on board SOHO, is used to
determine the sound-speed variation from below the base of the convection zone
to a few Mm below the solar surface. We find that the sound speed at solar
maximum is smaller than at solar minimum at the limit of our determination (5.5
Mm). The min-to-max difference decreases in absolute values until ~7 Mm. At
larger depths, the sound speed at solar maximum is larger than at solar minimum
and their difference increases with depth until ~10 Mm. At this depth, the
relative difference is less than half of the value observed at the lowest depth
determination. At deeper layers, it slowly decreases with depth until there is
no difference between maximum and minimum activity.

26
The categories of this paper are: bayes-an
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an
33
The categories of this paper are: plasm-ph
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th
12
The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys