Bangalore: The Indian space agency is launching five rockets on Friday to study the effects of the millennium's longest annular solar eclipse in the southern part of the country, an official said.

"Five rockets are being launched on Friday between 1 p.m. and 3 p.m. to investigate the effects of the solar eclipse in the lower and middle levels of the atmosphere," Indian Space Research Organisation (ISRO) Project Director P. Ratnakar Rao told the sources from Thiruvananthapuram on Friday.

The Rohini (RH) sounding rockets will be launched from the Thumba Equatorial Rocket Launch Station (TERLS) at the Vikram Sarabhai Space Centre (VSSC) of ISRO in the Kerala capital.

Of the five rockets, three will be in the RH 300MK-II series and two in RH 200 series carrying scientific instruments to measure the effects of the eclipse in the earth's atmosphere and ionosphere.

"The effects are measured by the instruments during the rockets' flight path after they zoom into the sky and their data is relayed to our space centre before they plunge into the Arabian Sea or the Indian Ocean," Rao said.

The space agency will also launch one larger Rohini rocket (RH 560 MK II) between 1 p.m. and 2 p.m. from its spaceport at Sriharikota, about 80 km from northeast of Chennai to a peak altitude of 548 km for collecting additional data.

The state-run space agency had launched three rockets on Thursday around the same time to peak altitudes of 70 km and 116 km for collecting data during a normal day for comparison with the data gathered during the eclipse.

"We will compare the data obtained on normal days with data during and immediately after the eclipse to study the difference," Rao noted.

An annular eclipse occurs when the moon covers the centre of the sun, but not its edges. This leaves a ring (or annulus) of the sun, which will be visible around its edges, with the moon darkening its centre.

"We will launch an RH 300 rocket on Saturday between 1 p.m. and 2 p.m. from TERLS for collecting data on the after-effects of the eclipse in the atmosphere. We will get more insights into the effect of this phenomenon on the earth," Rao said.

The occurrence of eclipse will result in a sudden cut-off of solar radiation. This affects the atmospheric structure and dynamics.

"There will be a large reduction in ionization and temperature. At around 1:14 p.m. IST, the eclipse will pass close to TERLS with 91 percent obscuration and the edges will touch at the Sriharikota spaceport, with an obscuration of 85 percent," Rao pointed out.

Though the centre line just misses the main land, the path, being 323 km wide, will offer a unique opportunity to investigate the effects of the fast varying solar flux on the photochemistry and electrodynamics of the different atmospheric regions, especially the equatorial mesopause and ionosphere-thermosphere regions.

The uniqueness of this eclipse is that it occurs around noontime, when the incoming solar radiation is at its maximum, the sun being at its zenith.

The obscuration of sun during this eclipse is exceptionally long, about 11 minutes and eight seconds. The maximum obscuration occurs during noon hours (13.15 IST). As a consequence, it provides an opportunity to study, perhaps for the first time, the solar eclipse-induced effects in the noontime equatorial region.

"These experiments will coordinate modern ground-based eclipse observations with in situ space measurements. Interpretation of eclipse data together with space data will give new insights to the earlier eclipse observations," Rao added. IANS