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Changes in ionosphere observed during solar eclipse

Last Updated: Tuesday, August 4, 2009 - 12:17

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

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

"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

First Published: Tuesday, August 4, 2009 - 12:17
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