Changes in ionosphere observed during solar eclipse

Changes in ionosphere observed during solar eclipse Mumbai: Significant changes in the lowest part of earth-ionosphere have been observed by the scientists from the Indian Institute of Geomagnetism (IIG) who conducted experiments on Very Low Frequency (VLF) radiowaves for the first time during total solar eclipse (TSE) on July 22.

The scientists monitored very low frequency (VLF) radiowaves travelling long distances through the earth-ionosphere wave guide and found increase in VLF signal strength during TSE at around 50 to 60 kms above the earth.

IIG Scientists campaigned to record at Allahabad, Varanasi, and Nainital, VLF radio wave signals transmitted from Japan, Australia and a place near Tirunelveli in Tamil Nadu, using special receivers, its director Dr Archana Bhattacharyya said.

"The VLF signals that were monitored had frequencies around 20 kHz, and travel long distances through the earth-ionosphere wave guide, undergoing reflections from the lowest part of the ionosphere, the D region," Bhattacharyya said.

VLF radio wave signals are usually used for communication with submarines immersed in sea which is electrically conducting and hence would attenuate higher frequency radiowaves.

These three stations -- Allahabad, Varanasi and Nainital -- are also part of a global network of VLF recording stations with AWESOME (Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education) receivers, a collaboration started by Stanford University, USA during the International Heliophysical Year.

IIG also found that during the TSE, there was no production of ionisation in the ionosphere and the ions and electrons in the lowest part of the ionosphere recombined at a rapid rate causing the electron density in the 'D' region of the ionosphere to decrease significantly, she said.

"The scientists found that the VLF signal amplitude was very much enhanced because there was no production of ionisation during TSE and at the same time the existing electrons and ions recombined rapidly," Bhattacharyya said.

For the signal from Japan to Allahabad and Varanasi, the propagation paths were largely along the solar eclipse totality path. At these two stations large increases in the strength of the signal were observed around the time of the TSE as compared to a control day, said Bhattacharyya.

This is attributed to the significant decrease in the electron density in the 'D' region of the ionosphere and the simultaneous increase in the width of the earth-ionosphere wave guide during the TSE.

This is only preliminary analysis and further modelling is being done with the data collected during the TSE, she said.

"Observations of ultra-low frequency (ULF) variations in the magnetic field carried out by the IIG scientists at Bhagalpur using an induction coil magnetometer during the TSE showed a decrease in the amplitude of the so-called Schumann resonance," she said.

Schumann resonance is a global resonance at a frequency of about 8Hz caused by lightning discharges in the electromagnetic resonant cavity formed by the surface of the earth and earth's ionosphere, Bhattacharya added.

All these observations need further analysis and modelling, she said.

This is the first time Indian scientists have conducted these types of experiments during this rare celestial event, she said adding the team of IIG scientists who made the observations of the effects of total solar eclipse on ULF and VLF electromagnetic waves are AK Sinha at Bhagalpur, Rajesh Singh, B Veenadhari, and P Vohat at Allahabad, and AK Maurya at Nainital.

Simultaneously, scientists from Banaras Hindu University in Varanasi and ARIES in Nainital also took part in the VLF radio wave observations, she said.

Bureau Report