[1] Effects of solar eclipses on the propagation characteristics of worldwide VLF sferics from lightning activity require more investigation. An attempt was made on the occasion of two solar eclipses during 22nd July, 2009 and 15th January, 2010 to study the effects of the two eclipses on the propagation characteristics of VLF sferics in the Earth-ionosphere waveguide. Identical experimental setups were used to study the VLF sferics during the two eclipse events. The spectral character of VLF sferics propagating inside the waveguide is studied at a fixed receiver location (23.75°N, 91.25°E) at six discrete frequencies in between 3 and 20 kHz. During both the eclipse events, it is observed that VLF sferics at all the six discrete frequencies is increased from the mean normal average ambient level. The increment peaks around 10–12 kHz with an overall increment of 6.4 dB with respect to its ambient level. The VLF spectral character of enhancement of sferics show similar characters in two eclipses. The percentage decrease in electron density using standard modeling equations is found to be 90% at the height of 71 km for both the eclipses, supporting linear variation of electron density with solar radiation at the D-region of the ionosphere. The results are explained qualitatively on the basis of a decrease in electron density at the lower ionosphere modifying the reflection coefficient which affected the propagation of VLF sferics in Earth-ionosphere waveguide during eclipsed condition.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JA017498

Spectral character of VLF sferics propagating inside the Earth-ionosphere waveguide during two recent solar eclipses

Electromagnetic waves from lightning activity, commonly known as atmospherics or sferics serve as an effective tool for studying the lower ionosphere as well as thunderstorm activity. It is also useful for locating lightning strokes regionally and globally. In this paper, we present the analysis of the Integrated Field Intensity of Sferics (IFIS) at six discrete VLF frequencies for 30 lightning-associated overhead thunderstorms in Tripura, within the period from August 2009 to October 2010. An ingeniously developed well calibrated GPS locked software VLF receiver, located at the Department of Physics, Tripura University (23.5°N, 91.25°E), is used for the present study. Two distinct types of variations of IFIS, (i) single peak and (ii) dual peak are found characterizing each thunderstorm and their occurrence show nearly inverse character. The spectral character of IFIS rise rate, fall rate and rate of enhancement for each type is studied searching for suitable frequencies in the VLF range to forecast a thunderstorm. It is concluded that VLF sferics from 3–10 kHz are the most effective in terms nowcasting an incoming thunderstorm well before 3–4 hours of its peak occurrence, when there may not be any visual indication of the thunderstorm.

/pdf/characteristics-of-severe-thunderstorms-studied-with-the-aid-2nw2qihxkw.pdf

Characteristics of severe thunderstorms studied with the aid of VLF atmospherics over North–East India

The electrical effects due to monsoon clouds in conjunction with the VLF atmospherics data have been extensively analyzed. The cloud distribution and rainfall pattern during the SW monsoon period are examined. The diurnal curves of rainfall show that the maximum rain generally occurs in the afternoon hours between 13 to 18 IST. The coefficient of variation (CV) of monsoon rainfall plotted against rainfall amount reveals that CV decreases with increasing rainfall amounts upto about 40 inches. The differences in the mean dry bulb temperature as well as mean relative humidity values at the standard levels between strong and weak monsoon are studied. The monthly median of the hourly average together with the respective upper and lower decile values of atmospherics have been considered. Also, the frequency dependence of afternoon maximum (or late afternoon minimum) to morning minimum in the sferics level is taken into account to determine the seasonal variation. During monsoon months the sferics level with higher cloud amount (⩾4 okta) increases considerably but the width of the sferics is reduced. The results are interpreted by considering the activity of the sources involved at such times.

Some distinct effects of tropical monsoon clouds as derived from atmospherics

Radar observations of two pre-monsoon tornadoes as experienced over the Gangetic West Bengal and the integrated field intensity of atmospherics (IFIA) at 30 kHz recorded over the tropical station, Calcutta, have been analysed. The records of IFIA exhibit a pronounced variation in the noise level showing first a sudden enhancement and then a distinct fall which is repetative in nature in both the cases. The characteristics of the variations are analysed and discussed. The synoptic situations in connection with both the tornadoes have also been critically studied. The results obtained are explained by considering the existing knowledge of electrical activity.

Radar observations of tornadoes and the field intensity of atmospherics. Radarbeobachtungen von Tornados und die Feldstärke der Atmospherics

The influence of thunderstorms on the atmospheric radio noise field strength (ARNFS) over Kalyani (West Bengal) at 27 kHz has been analyzed over a two-year period. The results show that the median noise level decreases during the winter and reaches a maximum during the monsoon months. A comparison of the seasonal occurrence of thunderstorm-days with ARNFS indicates that both of them have similar trend over time. However, the trend is more pronounced during daytime, indicating relationship between solar activity, thunderstorms and ARNFS. In this paper, we have attempted to investigate how tropical thunderstorm and lightning activity might be responsible for the solar modulation of atmospheric electrification. The influence of ionizing radiation on the electrical global circuit of the atmosphere is also critically analyzed.

Response of thunderstorm and lightning activity to solar modulation of atmospheric electrification

Thunderstorm associated atmospherics during the monsoon, premonsoon and postmonsoon seasons over the tropical station Calcutta show some typical characteristics. In all the cases gradual rise of atmospherics (GRA) and steady recovery of atmospherics (SRA) have been found. In between the above two changes, there is the sudden enhancement of atmospherics (SEA) which is some times composed of two components SEA1 and SEA2. All the above facts of the integrated field intensity of atmospherics (IFIA) observed at 10 and 20 kHz have been critically examined here in relation to monsoon, premonsoon and postmonsoon thunderstorms. The results exhibit that the precipitation is associated with the steady recovery of IFIA during the monsoon and postmonsoon thunderstorms while that during premonsoon thunderstorms is associated with the second sudden enhancement. These anomalies have been interpreted in terms of charge separation within thunderclouds. Moreover, the difference of the respective amplitudes at 10 and 20 kHz of the different variations of IFIA have also been critically focussed.

Characteristics of VLF atmospherics during tropical thunderstorms

On the seasonal variations of VLF atmospherics and rainfall over Calcutta

Simultaneous observations of the field strength of atmospheric radio noise at three harmonically related frequencies in the VLF band for a period of two years from October 1976 to September 1978 have been analyzed to explore the frequency spectrum of atmospherics The results have been shown for different months and seasons of the year and interpreted in terms of source activity and propagation effect

Very low frequency atmospheric radio noise field intensities

The field intensity of VLF-atmospherics recorded over the tropical station, Calcutta (22°34′N, 88°24′E) occasionally exhibit a characteristic fall in association with the ground rainfall during monsoon months. The sudden fall is followed by a pause and a subsequent recorvery thus forming a clipped saw-tooth like appearance. The occurrences and magnitudes of the fall are predominant at 20 kHz in the VLF-band. They mostly occur around local noon and midnight hours.

On some characteristics of tropical atmospherics due to monsoon clouds. Über Charakteristiken tropischer Atmospherics aus Monsunwolken

The records of VLF atmospherics over Calcutta during the torrential rainfall due to violent monsoon depression in 1978, exhibit distinct long period fadings both at day and night. Interesting results obtained from an analysis of the round-the-clock atmospherics data and associated meteorological parameters, are reported in this paper. A tentative conclusion is reached suggesting an origin of the fading from atmospheric gravity waves generated in the centre of activity of the monsoon depression.

A. B. Bhattacharya

Papers

Characteristics of VLF atmospherics during tropical thunderstorms

On the seasonal variations of VLF atmospherics and rainfall over Calcutta

Very low frequency atmospheric radio noise field intensities

On some characteristics of tropical atmospherics due to monsoon clouds. Über Charakteristiken tropischer Atmospherics aus Monsunwolken

Fading of VLF atmospherics associated with torrential rain due to monsoon depression