Spatial variation of AIA coronal Fourier power spectra

Abstract:

We describe a study of the spatial distribution of the properties of the
Fourier power spectrum of time-series of AIA 171Å and 193Å data. The
area studied includes examples of physically different components of
the corona, such as coronal moss, a sunspot, quiet Sun and
fan loop footpoints.

We show that a large fraction of the power spectra are well
modeled by a power spectrum that behaves like a power law f^-n (n>0)
at lower frequencies f, dropping to a constant value at higher
frequencies. We also show that there are areas where the power spectra
are better described by the above power spectrum model, plus a narrow
band oscillatory feature, centered in the 3-5 minute oscillation
range. These narrow-band spectral features are thought to be due to
the propagation of oscillations from lower down in solar atmosphere to
hotter. This allows us to produce maps of large areas of the corona
showing where the propagation from one waveband to another does and
does not occur. This is an important step in understanding wave
propagation in different layers in the corona.

We also show the 171Å and 193Å power spectrum power law indices are
correlated, with 171Å power law indices in the range n = 1.8 to 2.8,
and 193Å power law indices n = 2 to 3.5 approximately. Maps of the
power law index show that different ranges of values of the power law
indices occur in spatially contiguous parts of the corona, indicating
that local spatial structure may play a role in defining the power law
index value. Taken with our previous result from Ireland et al. (2015)
that physically different parts of the corona have different mean
values of the power law index, this new result strongly suggests that
the same mechanism producing the observed power law power spectrum is
operating everywhere across the corona. We discuss the nanoflare
hypothesis as a possible explanation of these observations.