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Proposal ID Reardon_66

Date of Proposal



Osservatorio Astronomico di Capodimonte

via Moiariello 16
Napoli, 80131, Italy





We propose a study using the EIT to measure the frequency spectrum of
intensity oscialltions on coronal loops in the frequency range of 0.02
to 0.002 Hz. These intensity oscillations could be produced by several
different oscillatory processes, such as slow- and fast-mode waves which
would produce strong density fluctuations in coronal loops. These
two wave modes and their damping rates have been theoretically studied
recently by Porter, Klimchuk, and Sturrock (1994a, 1994b) and they find
that such waves, with sufficiently high frequencies, could damp quickly
enough to be important in the balance of coronal heating inputs and
losses. It is not known, however, how slow- or fast-mode waves could
propagate from the photosphere through the steep magnetic and density
gradients in the transition region, but there have been few
observational or theoretical tests of the possible generation of such
modes in the transition region or corona itself. Intermediate mode waves
are incompressible and one would not expect density oscillations.
However, damping mechanisms for such modes, such as resonant absorption,
could produce density fluctuations as a second order effect.

The necessary observations to measure the coronal frequency spectrum are
a continuous sequence of observations taken in a coronal emission line.
The EIT is perhaps uniquely suited to this task, as it is outside of the
earth's atmosphere, reducing spurious seeing-induced fluctuations, but
is not subject to the usual day-night cycle of near-earth-orbiting
spacecraft. With the EIT, we can expect 200 DN/sec from a moderately
bright active region observed through the 171 or 195 Angstrom filters.
This will produce an image with a photon shot noise of 1% in 15 seconds.
With an overhead of five seconds between images, this results in a 20
second cadence, corresponding to a minimum detectable period of around
60 seconds or more. For the simplest model of the resonant periods of a
coronal loop (P=2*L/[m*Valf]), this corresponds to oscillations of a
loop of length ~100 Mm, or a radius of 40-50 arcseconds. The other
periods of interest are those in the range around the five-minute
oscillations since this is where the photospheric energy spectrum peaks.
For these periods, a continuous time series lasting one hour, preferably
several times longer, is necessary.

For a single 32x32 pixel subfield, at 7 bits per pixel, these
observations would produce data at a rate of 500 bps, well within the
EIT's normal average rate. For a 64x64 subfield, the data rate
approaches 2 Kbps, still well below the combined EIT and LASCO telemetry
rate of 5.2 Kbps. Because of the need to resolve the coronal loops in
order to avoid destructive interference of out-of-phase oscillations on
adjoining loops, the data should be taken at the full resolution of 2.6
arcseconds per pixel. The largest source of noise in the observations
after photon shot noise might be produced by errors in the flat-field
calibration images and the spurious oscillations produced by the motion
of the image in the field of view due to image drift and spacecraft
jitter. If possible, these observations might best be done as soon as
possible after a bakeout when the instrument response is highest.

Clette, F., 1996, private communication
Delaboudiniere, J.-P., et al, 1995, Solar Physics, v. 162, pp. 291
Porter, L.J., Klimchuk, J.A., & Sturrock, P.A., 1994a, Astrophys. J., v435, p482
Porter, L.J., Klimchuk, J.A., & Sturrock, P.A., 1994a, Astrophys. J., v435, p502

Frederic Clette


Alfven Waves


Frequency Spectrum of Coronal Intensity Oscillations

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