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Proposal ID Benz_22

Date of Proposal



ETH Zurich/Switzerland

Institute of Astronomy
8092 Zurich


+41-1-632 42 23

+41-1-632 12 05

The present proposal has been described in JOP46 and refers to
observations taken at the Very Large Array from

1996, July 12, 13:47:30 - 21:46:15 UT
1996, July 27, 14:48:15 - 22:47:00 UT

The four radio frequencies used have sampled the upper
chromosphere and transition region in short snapshots
(2 minute time resolution) and long-duration single-frequency
scans (10s time resolution). The origin of radio emission is
mostly thermal bremsstrahlung from the upper chromosphere,
but non-thermal contributions cannot be excluded.

In our previous VLA observations of Feb-20-95 we have seen a
correlation between radio observations at 1.3, 2.0, and 3.6 cm
and soft X-ray enhancements in YOHKOH data (Benz et al., 1996).
The radio emission was observed to be very variable. Some
brightenings in the network of the quiet Sun correlated with soft
X-ray peaks (Krucker et al, 1996). Now we want to investigate
their temperature and emission measure at the resolution of EIT
and determine their energy.

We have also obserevrd at Kitt Peak with the Zurich Imaging
Stokes Polarimeter ZIMPOL and have recorded the full Stokes
vector at the 1.5 m McMath-Pierce telescope at Kitt Peak with
four CCD cameras in parallel in various spectral lines to
determine the magnetic field vector at various heights in the
photosphere and the lower chromosphere.

We will later also request the simultaneously observed SUMER and
CDS data to investigate the energy input into the transition
region and lower corona.

Barbara Thompson


Quiet Sun


Radio/EIT correlation of microflares in the network of the quiet Sun

update added on 1998/03/09 at 13:19:57
First results (with some minor mistakes) were presented at the
SoHO workshop in Oslo (Krucker/Benz/Delaboudiniere 1997,
in: The Corona and Solar Wind Near Minimum Activity, ESA SP-404,
p. 465. We are ready to submit a paper about the temporal
variations observed by EIT in the high-temperature EUV lines
in the next weeks. The abstract will be the following:

Heating Events in the Quiet Solar Corona

Sensitive observations of the quiet Sun observed by EIT on the SoHO
satellite in high-temperature EUV line emission are presented. The
thermal radiation of the quiet corona is detected to impulsively increase
more than can be attributed to noise in about 44\% of all pixels of the
image within the observing time of 42 minutes. The more prominent
enhancements are identified with previously reported X-ray network
flares (Krucker, Benz \ Acton 1997) above the magnetic network of
the quiet chromosphere. In coronal EUV lines they are amenable to
detailed analysis suggesting that the brightenings are caused by
additional plasma injected from below and heated to slightly higher
temperature than the preexisting corona. Statistical investigations
are consistent with the hypothesis that the weaker EUV enhancements
originate from the same parent population. The power spectrum of the
emission measure time variations is approximately a power law with an
exponent of 1.79$\pm 0.08$ for the brightest pixels and 1.69$\pm 0.08$
for the average and the faintest pixels. The power input derived from the
impulsive brightenings is linearly proportional to the radiative loss in
the observed part of the corona. The absolute amount of impulsive input
is model dependent. It cannot be excluded that it can satisfy the total
requirement for heating. These observations give convincing evidence that
a significant fraction of the heating in quiet coronal regions is impulsive.

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