JOP 13: JOINT CDS/MDI/SUMER PROGRAMME
Temporal Variability in the Solar Transition Region
Per Maltby & Olav Kjeldseth-Moe, ITA, University of Oslo, Richard
A. Harrison, RAL, Klaus Wilhelm, MPI-Ae, & Philip H. Scherrer,
Stanford University.
Scientific Justification:
The quiet Sun is not quiet, particularly not in
the transition region. A pervasive temporal variability has been observed,
but the reported results differ widely. Apart from explosive events, micro-flares
and temporal changes within 40 -- 60s in bright features in active regions
(Porter et al. 1984), rapid changes have also been reported for the quiet
Sun. Cheng (1991) found from HRTS time series observations that temporal
fluctuations in chromospheric and transition region lines occur in all locations
associated with or without strong magnetic fields. He reported changes ranging
from 10 to 20 percent to a factor of 3 in a time period of 30 sec. Porter
et al. (1987), on the other hand, found from SMM observations of the C IV
resonance line 1548 A that some bright sites (individual 3" pixels)
are short lived, lasting less than 2 minute, while others last for an hour.
Based on a study of spectroheliograms from the Harvard experiment on Skylab
(with time resolution of 5.5 min and spatial resolution of 5"x 5")
Rabin and Dowdy (1992) found that the C III line at 977 A showed significant
variation in 93 percent of the spatial elements. From a total of 6 lines
the amplitude of the fractional variability was highest (32 percent) for
the O IV line at 554 A.
In order to obtain a more quantitative and systematic treatment of the temporal
variability in the quiet Sun, simultaneous observations in different transition
region lines with high spatial, spectral and temporal resolution are required.
The data reduction should include a search for solar oscillations as well
as determinations of the fraction of the spatial elements that show significant
variation above noise in transition region line intensity, line shift and
line width.
It seems very likely that the transition region variability depends on the
magnetic field. Accordingly, since the supergranular network may be followed
from the photosphere into the transition region, nearly simultaneous observations
of the photospheric magnetic field observations with high spatial resolution
will give valuable information. The study may be extended to include several
transition region lines and various degree of solar activity.
References:
Cheng, C.C. 1991 in Mechanisms of Chromospheric and Coronal Heating, eds.
P. Ulmschneider, E.R. Priest, & R. Rosner (Springer), 77
Porter, J.G., Moore, R.L., Reichmann, E.J., Engvold, O., & Harvey, K.L.
1987, ApJ 323, 380
Porter, J.G., Toomre, J., & Gebbie, K.B. 1984, ApJ 283, 879
Rabin, D., & Dowdy, J.F. 1992, ApJ 398, 665
No additional text was submitted for this proposal.