Published: 
Nov 2, 2006

EDIT THIS

Temporal variations in gravity field
during solar eclipse on 24 October 1995

D.C. Mishra, M.B.S. Rao, National Geophysical Research Institute, From Current Science, 72 (11) 1997 (783)

"[A one hour feature of the gravimeter record] of 10-12 microGal [10-8 cm/s2]..can neither be classified under short period variations due to tidal effect or drift of the gravimeter nor under high frequency noise which have special patterns. Therefore, this variation is highly significant as it occurs with the onset of the solar eclipse. ..to understand its actual nature and mechanism, more planned experiments of this kind should be carried out during solar eclipses throughout the world whenever such opportunities are available."

Solar eclipses provide an unique opportunity for the study of celestial phenomena such as different parts of the sun like cornoa and chromosphere, its atmosphere and their interaction with the earth and its atmosphere. The solar eclipse on 24 October 1995 starting from sunrise at Iran and ending at sunset at the Pacific Ocean provide a 46 km wide strip for approximately 1800 km in India from Nem Ka Thana (Western Rajasthan) to Diamond Harbour (West Bengal) where the total solar eclipse was observed for some time between 7:22 am to 10:30 am (1). This solar eclipse was unique due to several scientific experiments which provided several interesting results (2).

During this period of solar eclipse, we happened to be at Dhoraji (22 deg, 44’, 70 deg. 27’ Saurashtra) in connection with the gravity survey for oil exploration in that region. This region falls in approximately 80% of the total eclipse and we recorded temporal variation in the gravity field at this place continuously for approximately 12 h before and after the eclipse. The variation in the gravity field is recorded using a Lacoste-Romberg gravimeter of 0.01 microgal accuracy. The temporal varionation in the gravity field recorded at a station can be broadly classified as: (1) Very large period (100-10,000 years) variations related to the mantle processes, sea level changes, glacial rebound and ice processes. (2) Large period (10-100 years) variations due to core-mantle interaction, plate boundary deformation, etc. (3) Medium period (days to years) variations due to earthquakes, volcanoes, etc. (4) Short period (hours to days) variations caused by drift of the gravimeter, ocean tides. (5) Shorter period (hours) variations due to the sudden changes in the atmosphere such as pressure and temperature. (6) Shortest period (seconds to minutes) high frequency noises which are sharp and sudden.

As mentioned above, these variations are largely characterized by their wavelength which is used to classify them in different groups. In the 12-hour record which we made, we can neither record not identify the variations classified under groups (1), (2) and (3). The fourth group of variations show a cycle of 24 h and is represented by smooth cyclic changes as given in Figure 1 which represents the tidal variation on 24 October 1995 at Dhoraji. It is the group (5) kind of variations which can be recorded in our gravimeter and are of present interest. The variations under group (6) are easily identifiable as they are sharp changes due to sudden jerk or some motion in the vicinity. Some experiments of recording gravitational field during solar eclipses (3,4) were conducted previously to find out the tidal effects during that period and gravitational shielding (Majoranna effect).

g1.gif

Figure 1. Tidal effect on 24 October 1995 at Dhoraji (Saurashtra). Normal example of type (4) variation in gravity, e.g. short period (hours to days) variations caused by drift of the gravimeter, ocean tides; as opposed to type (5) in Figure 2, e.g. shorter period (hours) variations due to the sudden changes in the atmosphere such as pressure and temperature. Note the NASA/Marshall experiment is not subject to barometric or thermal effects owing to its active temperature control (<0.3 C), internal vacuum, and barometric compensation in case of buoyancy effects or external vacuum leak. D.C. Mishra, M.B.S. Rao, National Geophysical Research Institute, From Current Science, 72 (11) 1997 (783)

The recorded variation of the gravity field and the tidal (Figure 1) corrected field on 24 October 1995 for a period from 5:00 am to 11:00 am are shown in Figure 2.

g3.gif

Figure 2. Observed (top line uncorrected) and tidal corrected gravity field at Dhoraji on 24 October 1995. The vertical lines indicate the onset and departure of solar coverage between approximately 6:30 and 7:00 am local. The horizontal axis is time in hours (5-11 am), and the gravity scale of the vertical axis is each tick mark (as 2x10^-8 cm/s^2). D.C. Mishra, M.B.S. Rao, National Geophysical Research Institute, From Current Science, 72 (11) 1997 (783).

Both the graphs show a smooth variation due to short period features like tidal and the drift of the gravimeter over which a shorter period feature of 10-12 microgal (10^-8 cm/s^2) between 6:30 and 7:30 am is superimposed. This variation can neither be classified under short period variations due to tidal effect or drift of the gravimeter under high frequency noise which have special patterns. Therefore this variation is highly significant as it occurs with the onset of solar eclipse. It may represent sudden changes in the earth’s atmosphere with the onset of the eclipse. Exact nature of the changes in atmosphere is difficult to visualize. However, it could be due to decrease in the different kinds of radiation such as gamma rays, X-rays, and radio intensity which are found to decrease with the onset of the eclipse or simply due to changes in the atmospheric pressure. Therefore to understand its exact nature and mechanism, more planned experiments of this kind should be carried out during solar eclipses throughout the world whenever such opportunities are available.

1. Bhattarcharya, J.C. Current Science, 1995, 69, 486-488
2. Sapra, B.K., Mayya, Y.S., Sawant, V.D., and Nambi, K.S.V. Curr. Sci. 1997, 72, 321-325
3. Tomaschek, T. Nature, 1955, 4465, 937-939
4. Slichter, L.B. Caputo, M. and Hagger, C.L. J. Geophys. Res., 1965, 70, 1541-1551.


flash!
Join our growing list of subscribers - sign up for our express news delivery and you will receive a mail message every time we post a new story!!!
More
nasanewst.gif
Headlines

For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Curator: Bryan Walls
NASA Official: M. Frank Rose