SOLAR ECLIPSE CORONAL IMAGING SYSTEM (SECIS)

The Solar Eclipse Coronal Imaging System (SECIS) is an experiment designed to look for very fast changes possibly occurring in the solar corona, the extensive and very hot (temperatures of about 1 million Kelvin or 1 MK) region of the solar atmosphere that is visible as a white, highly structured glow around the moon's disk during a total eclipse of the sun. SECIS using a pair of very fast CCD cameras that image portions of the solar corona with frame rates of up to 70 frames a second. One of the objects is to see if the heating mechanism for the corona, which is still in doubt after more than 50 years of study, is due to the dissipation of waves associated with the magnetic field that is a strong feature of the solar atmosphere. The waves in question are magnetic in nature, or more precisely `magnetohydrodynamic' or MHD, and would originate from the sun's interior. An alternative hypothesis for coronal heating, which has some observational support, is that numerous very small releases of energy -- dubbed `nanoflares' -- occur all over the solar corona producing plasma (ionized gas) heated to temperatures of about a million K. There are many supporters for both heating ideas; it is even possible that the real heating mechanism, at least in some parts of the corona, is a combination of nanoflare and MHD wave heating.

There are theoretical reasons for believing that, if MHD wave heating is predominant, the periods of the waves are very short, maybe only a few seconds. So repeated imaging of the corona, say with a spacecraft working in the X-ray or ultraviolet regions of the spectrum, would have to be extremely fast so that oscillations or modulations in the light intensity resulting from MHD waves could be seen. In practice, imaging even relatively small portions of the solar corona with spacecraft like Yohkoh (the Japanese solar X-ray flare mission) or SOHO (the ESA/NASA solar mission) takes approximately 2 or more minutes because of telemetry limitations. There is then a need for faster imaging if short-period MHD waves are to be searched for. The CCD cameras used by the SECIS system are linked to an adapted PC which is able to capture the data streams from each camera and store in a hard-disk device. The CCD cameras, manufactured by EEV (Chelmsford), have a 512 x 512 pixel format and have a 12-bit digital output. The computer (hardware and software supplied by Carr--Crouch Computer Company in Maidenhead) is able to store up to about 8 Gigabytes of data for later analysis.

Here is the SECIS set-up as used in the 1999 total solar eclipse that was visible along a track right across Europe (including south-west England) and southern Asia. In this image, the telescope (a 200mm Schmidt--Cassegrain) collects light from the eclipsed sun, directs it to a collimator with a beam splitter. The beam of light reflected by the beam splitter goes to one of the cameras without any filter in place, while the transmitted beam is directed to a narrow-band filter with central wavelength at 530.3nm -- this is the wavelength of a strong emission line produced by the hot coronal plasma due to highly ionized iron atoms and is known by solar physicists as the `green' line. After the filter, the beam goes to a second camera.

The whole equipment during the 1999 eclipse was mounted on a sturdy trestle that in turn was supported on wooden blocks sunk into the ground. Our Polish colleagues from the University of Wroclaw provided a high-precision tracking mirror, or heliostat, with a 250mm flat mirror that rotates with half the earth's rotation period, resulting in a steady beam of light in one particular direction. This picture shows the Wroclaw heliostat just before the 1999 eclipse and without the mirror in place.

The instrument was taken to Zambia, at the Physics Dept., University of Zambia (Lusaka), for the June 2001 eclipse. We got even better results than in the 1999 eclipse. The weather conditions were perfect and our equipment worked flawlessly.

The cameras are now at Bialkow Observatory, University of Wroclaw, Poland, and are being used by Dr Pawel Rudawy and his student Krzysztof Radziszewski in conjunction with the MSDP spectrograph to study high-time-cadence observations of H-alpha flares. These are being compared with light curves from the RHESSI instrument. Several flares were observed in 2003 and 2004, and preliminary results were presented at COSPAR, Paris (2004).

The following are SECIS-related publications which I have personal involvement with -- any others without my name I do not have involvement with:

Phillips, K. J. H. et al. 2000, SECIS: The Solar Eclipse Coronal Imaging System', Solar Phys., 193, 259-271

Dwivedi, B., and Phillips, K. J. H. 2001, The Paradox of the Sun's Hot Corona', Scientific American, 284, 40-47.

Williams, D. R., Mathioudakis, M., Gallagher, P. T., Phillips, K. J. H., et al. 2002, An Observational Study of a Magneto-Acoustic Wave in the Solar Corona', MNRAS, 336, 747.

Rudawy, P., Phillips, K. J. H., Gallagher, P. T., et al. 2004, Search for 1--10 Hz Modulations in Coronal Emission with SECIS during the 1999 August 11 Total Solar Eclipse', A and A, 416, 1179-1186.

Radziszewsi, K., Rudawy, P., Phillips, K. J. H., and Dennis, B. R. 2004, High Time Resolution of Solar Flare H-alpha Emission', Adv. Space Res., to be published.

 

Page created by Ken Phillips (Kenneth.J.Phillips.1@gsfc.nasa.gov).

Last updated November 8, 2004