Title: Characteristics of the Background Spectrum

Investigator: J. Harvey (National Solar Observatory).

Team Members: TBD, I'd be happy to work with anyone on this. I imagine
              that David Hathaway may propose something related.

SOI Coordinator: TBD

SSSC Programmer: TBD

Abstract:

This investigation will determine the power as a function
of degree and frequency of the background on which p modes
are observed. This background spectrum carries information
about convective processes near the solar surface such
as their characteristic time and spatial scales. It includes
very broad chromospheric resonances.

Data required are one or more calibrated l-nu spectra
after m averaging. An existing analysis program will be
used to isolate the background spectrum.

Investigation plan:

Justification -

SOI will produce a vast amount of information about p modes.
At the same time an unparalleled opportunity will exist to study
other velocity fields near the solar surface. These velocity
fields include those due to granulation, mesogranulation,
supergranulation, active regions, differential rotation, and
meridional flows. More speculative is the possible existence
of giant cells.

The velocity fields at the surface produce an amazingly structured
background spectrum on which p modes are superimposed. While
it is important to know the background spectrum in order to
properly analyze the p-mode spectrum, the background spectrum
is an important diagnostic in its own right. It represents
a statistical description of the uppermost layers of the
convection zone. The spectrum can be transformed into a more
familiar time and space correlation function and compared
with models.

At some point in the future, it will be possible to make
similar statistical spectra (without spatial resolution) of other
stars. Using the sun as a key to interpreting such spectra
will provide insight on how mass motions work on stars in
general.

Specifics -

This investigation is specifically aimed at producing an
accurate description of the background spectrum and its variation
with time. The focus is on those velocity fields which are
meaningfully described in statistical terms. This excludes
very low spatial frequency velocity fields such as differential
rotation and meridional flows, and isolated features such as
sunspots and plages. The low level of activity expected
during the SOI observations makes this an ideal time for such
an investigation.

Observational studies of integrated light background spectra
as a function of frequency have been done by the BISON
(Elsworth et al. 1994) and IAC (Palle et al. 1995) groups.
Hathaway et al. (1991) studied low frequency, imaged data.
Resolved l-nu diagrams have been analyzed by Harvey et al.
(1993) and Tripathy and Hill (1995).  Recent unpublished
results from South Pole 1994-5 have revealed the second
chromospheric mode predicted long ago.

We start with a calibrated l-nu diagram. The next step
is to determine a function that represents the background
part of the spectrum in the presence of p modes. An IRAF
procedure has been developed to do this using South Pole
and GONG spectra. It should work well on SOI spectra.
Once the background spectrum is determined and corrected for
effects of instrumental noise, aliasing, etc., it can be
used to fit a model directly or it can be transformed
into a correlation form and then compared with a model.
The final result is a description of the non-p-mode
velocity spectrum which can be compared with physical
models of convective and overshoot processes.

If spectra could be produced from restricted regions of
the solar disk (e.g. various mu angles), then the statistical
description could separate information about horizontal and
vertical components as well as provide some height dependence
information. If spectra from more than one time interval were
made available, one could investigate possible time
variations. There is some evidence that the vigor of granulation
changes during the solar cycle (Livingston et al. 1995).

References:

Elsworth, Y., Howe, R., Isaak, G.R., McLeod, C.P., Miller, B.A., New,
R., Speake, C.C., Wheeler, S.J., 1994, MNRAS 269, 529

Harvey, J.W., Duvall, T.L.Jr., Jefferies, S.M., Pomerantz, M.A., 1993,
in GONG 1992, 111

Hathaway, D.H., Rhodes, E.J.Jr., Cacciani, A., Korzennik, S.G., 1991,
in Lecture Notes in Physics 388, 163

Livingston, W., Wallace, L., White, O.R., Huang, Y.R., 1995, Nature,
submitted

Palle, P.L., Jimenez, A., Perez Hernandez, F., Regulo, C., Roca
Cortes, T., Sanchez, L., 1995, ApJ 441, 952

Tripathy, S.C., Hill, F., 1995, in GONG 1994, 334