Frédéric Clette Royal Observatory Belgium
Ringlaan 3
1180 Brussels
not yet available
Polarimetry of the solar coronaF. Clette, J.-R. Gabryl, P. Cugnon Departement Zonnefysica van de Koninklijke Sterrenwacht van België
Ringlaan 3
1180 Brussels
Our purpose
Despite of the developement of space observation techniques, total solar eclipses
still remain the unique occasion to achieve some specific observations of the upper
solar atmosphere. For instance, the experiment prepared by the Solar Physics
team at the ROB will take the opportunity to determine the density of free
electrons which constitute together with protons the main component of the
coronal plasma in the range of 1 to 3 solar radii above the solar limb. Other
observation techniques poorly cover this very important region which is now considered
as the onset place of the solar wind. This solar wind which results from an
expansion motion of the solar plasma is continuously spreading over and interacting
with the whole solar system, Earth included.
Measuring the electron density and its inhomogeneities is one of the key parameter
to modelize the physical process leading to an extreme heating (temperature over
1 million Kelvin) and the coronal plasma acceleration. A by-product of our analysis
will consist in bringing an independant photometric reference to instruments
onboard remote space probes, especially SOHO in which our Solar Physics Department
is involved (EIT experiment).
Experiment basics
In order to separate the light scattered by the coronal free electrons from parasiting
sources (the light scattered by interplanetary dust and Earth atmosphere), our optical
device is equiped with a rotating polarizing filter. Analysing a set of pictures
according to a series of different polarizer orientations allows us to extract the polarized
brightness due to electrons from the other unpolarized light sources. Moreover, the
polarization gives us some insights about the electron density distribution along the
lines of sight, so that we are able to rebuild a large scale 3D electron density
distribution.
A long story
Our 1999 observation campaign is part of a long term study which started with the
observation of 1973 total solar eclipse and which includes no less than 7 total solar
eclipses (1973, 1976, 1980, 1983, 1991, 1994, 1998), of which up to now just the 1976
eclipse failed to give results, due to cloud coverage! The set of all the collected pictures
gives us a quite unique view of the morphological evolution of the corona. The corona
reveals the magnetic fields architecture and continuously evolves along the solar
cycle activity which spans over an approximate period of 11 years.
Equipment of August 11th, 1999 observation.
The basic experiment, almost unchanged since 1973, uses a small photographic refractor
equiped with a polarizer which can be rotated over 3 or 6 indexed positions. We are
going on with it, since it is simple and reliable, and because it allows an easy
comparison with previous eclipse observations as a long-term study requires it.
Two observing teams will be located at two distant sites (North-East of France
and center of Romania) in order to increase the probability to observe the corona
under clear sky conditions and to study any changes in the corona which might occur
during the time range separating eclipse totality at both sites (30 minutes).
Furthemore, a more sophisticated device based on the same polarimetry principle will
be used at both sites to record digital pictures of the corona. Thanks to CCD cameras
(silicium based chips), the instruments will record a hundred of pictures in less than
two minutes (a short totality!), i.e. more than what we successfully got during longer
eclipses of 1991 and 1994. These data are ready-to-use without any intermediate
photographic processing and will allow a more accute and precise analysis of the visible
and near infrared radiation scattered by the corona.
Waarneemproject voor de effecten die de totale zonsverduistering op
11 augustus 1999 heeft op de atmosferische druk en de zwaartekracht.
M. van Ruymbeke, B. Ducarme en A. Somerhausen Koninklijke Sterrenwacht van België
Ringlaan 3
1180 Brussels
not yet available
Waarneming van de kolomdichtheid van de elektronen in de ionosfeer met
het Global Positioning System tijdens de totale zonsverduistering op
11 augustus 1999René Warnant Koninklijke Sterrenwacht van België
Ringlaan 3
1180 Brussels
not yet available
De totale zonsverduistering in het Centrum voor Geofysica in DourbesJean-Claude Jodogne Koninklijk Meteorologisch Instituut
Ringlaan 3
1180 Brussels
not yet available
Contribution of the Department « Aerology » of the Royal Meteorological
Institute to the observation of the solar eclipse on August 11, 1999Alexandre Joukoff Royal Meteorological Institute
Ringlaan 3
1180 Brussels
The solar radiation reaching the Earth's surface is the main parameter
affected during a solar eclipse. The solar radiation has a direct
influence on temperature and the temperature gradients generated during an
eclipse may induce variations in the surface wind field. Therefore, it is
interesting to observe the variations of the various components of the
solar radiation as well as those of the terrestrial radiation in order to
deduce the surface budget and establish correlations with other
meteorological parameters.
The Aerology department of the RMI will not organise special missions to
perform radiometric observations in the totality zone ; however, a
pyranometer will be used by Marc Vandiepenbeeck to measure the solar global
radiation (direct + diffuse) at a site situated in the totality band.
On the other side, the routine measuring programme will be reinforced in
order to obtain variation curves of the various components of the solar
radiation as well as of the terrestrial and atmospheric radiation during
the eclipse using a high time resolution. So, we shall measure in Uccle
the direct, diffuse and global components of the solar radiation, the short
wave and long wave components of the radiation budget, the spectral
distribution of the solar radiation between 300 and 1000 nm, the UV
radiation between 295 and 385 nm (UVA + UVB) and the light intensity
(visible part of the solar spectrum).
In stations equipped with an automatic data acquisition system (Raversijde,
Melle, Saint-Hubert and Dourbes), we shall measure solar global radiation
and the radiation budget components.
Measurements of other meteorological parameters will also be available in
Uccle, Melle and Dourbes in order to establish correlations, namely with
the temperature.
Observations from space may also bring valuable information.
So, we shall be able to compute the radiation budget along the shadow track
and in the penumbra area, using METEOSAT images. The calculation method
has been developed at the RMI and is presently applied to METEOSAT images,
as preparation to the exploitation of the Earth radiation budget
measurements of GERB (Geostationary Earth Radiation Budget) instrument to
be flown on board of the METEOSAT Second Generation (MSG) satellites, the
first one is scheduled for launch at the end of year 2000.
Eclipse of August 11th, 1999: meteorological observations
during the phenomenon.Marc Vandiepenbeeck Royal Meteorological Institute
Ringlaan 3
1180 Brussels
During eclipses of July 11th, 1991 in San José del Cabo (Mexico),
November 3rd, 1994 in Putre (Chile) and February 26th, 1998 in
Curaçao (Dutch West Indies), one carried out measurements of
various weather parameters. During the first two eclipses quoted above,
one measured the temperature, the relative humidity, the solar radiation
and the atmospheric pressure. During the eclipse observed in Curaçao,
one observed the same parameters and also the speed and the direction
of the wind.
During the eclipse of this year, one plans to carry out measurements
of the same parameters, namely the temperature, the relative humidity,
the solar radiation and the atmospheric pressure with the same instruments
used during the eclipses of July 11th, 1991 and November 3rd, 1994.
A new station measuring the temperature, the relative humidity, the
pressure, the speed and the direction of the wind will be also used.
The precise localisation of the site is not determined yet, but measurements
will be done in the area of Epernay.
Measurements will begin during the morning of August 10th, 1999
and they will stop in the afternoon of August 12th. In the first system
of data acquisition, one will program two periods of measurement with
steps of different times. The second system will make it possible to compare
the temperature measurements, the relative humidity and the atmospheric
pressure with the first system and to have measurements of the wind.
Indeed, in Curaçao, one noticed a strong influence of the eclipse
on the direction of the wind. One thus will be able to confirm or
deny the influence of the eclipse on the direction of the wind.
This effect had already been had a presentiment of Mr. E.A. Bernard at
the time of the eclipse observed in Mexico.
Bernard E.A., Vandiepenbeeck M. et Verhas P., "Météorologie
de l'éclipse totale de soleil du 11 juillet à San
José del Cabo (Mexique)"Ciel et Terre, vol 108, pp3-10, 1992.
Vandiepenbeeck M., "Éclipse totale du 3 novembre 1994
à Putre (Chili) - observations météorologiques ", Ciel
et Terre, vol 112, pp 71-76, 1996
Vandiepenbeeck M. et Verhas P., "Observations météorologiques
lors de l'éclipse du 26 février 1998 à
Curaçao ", Ciel et Terre, vol 115, pp71-74, 1999
The Belgian Institute for Space Aeronomy (IASB-BIRA) will observe the
modifications of the atmospheric composition and radiation related to the
eclipse by means of its ground-based instruments. First, an automatic
station will perform spectral observations of the total, diffuse and direct
components of the solar radiation between 280 and 600 nm. During the
eclipse, the data acquisition rate will be increased up to a frequency of
one per second. It will be then possible to determine with a high temporal
resolution the kinetics of the variation of solar radiation as a function
of the portion of the masked solar disc.
Secondly, IASB-BIRA disposes also of a mobile atmospheric composition
measurement station based on the differential absorption optical
spectroscopy (DOAS). This station could be deployed in Uccle or under the
tract of the eclipse in order to observe changes in atmospheric
composition. The targeted constituents are short lifetime molecules as
nitrogen dioxide and trioxide, these molecules are influenced by
photo-chemical equilibrium and could be perturbed by the few minutes of the
eclipse.
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