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Indian Journal of Radio & Space Physics

 

ISSN : 0367-8393)

 

CODEN:IJRSAK  

VOLUME 32

NUMBER 4

AUGUST 2003

 

CONTENTS

 

Generation of field-aligned currents by VLF waves in the magnetosphere

189

V H Kulkarni

 

Comparison of space-based microwave polarization difference index and normalized difference vegetation index for crop growth monitoring

 

193

R P Singh & V K Dadhwal

 

Diagnostic study on vertical structure of tropical precipitating cloud systems using lower atmospheric wind profiler

 

198

K Krishna Reddy

 

A study of the forced Lorenz model of relevance to monsoon predictability

209

Ashok Kumar Mittal, Suneet Dwivedi & Avinash Chandra Pandey 

 

Drop size distribution of rainfall of different intensities

217

V Sasi Kumar, S Sampath & P V S S K Vinayak

 

Diurnal and seasonal variations of point discharge current during thunderstorms at a tropical inland station

 

221

S S Kandalgaonkar, M I R Tinmaker, M K Kulkarni & Asha Nath

 

Aerosol optical depth variation observed using sun-photometer over Indore

229

Pawan Gupta, Harish Gadhavi & A Jayaraman

 

Harmful electromagnetic environment near transmission tower

238

P P Pathak, Vijay Kumar & R P Vats 

 

 


 

Indian Journal of Radio & Space Physics

Vol. 32, August 2003, pp. 189-192

 

 

 

Generation of field-aligned currents by VLF waves in the magnetosphere

V H Kulkarni

 

 

This paper shows that VLF waves can be a source for the high energy electrons of tens of keV, carrying the small scale magnetic field-aligned Birkeland currents of sizes < 50 km in the magnetosphere. The dispersion relation for whistler mode waves in a magnetized plasma consisting of cold, warm and hot electrons and a beam of electrons carrying the current is examined. The VLF growth rates in the presence of field-aligned currents are found to be weaker and the current carrying electrons extract energy from the VLF waves. Along with the dependence of field-aligned currents and VLF waves on the magnetic activity, both these parameters also depend on each other directly.

 

Key words: VLF Waves, Field-aligned current, Whistler, Plasma

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, August 2003, pp. 193-197

 

 

Comparison of space-based microwave polarization difference index and normalized difference vegetation index for crop growth monitoring

R P Singh & V K Dadhwal

 

 

This paper reports the passive microwave radiometry application for regional assessment of crop growth in India. Brightness temperature (TB) of various vegetation classes observed at 37 GHz using special sensor microwave imager (SSMI) data, during Kharif season in 1999 in India are analyzed. Vertical as well as horizontal polarization brightness temperature is used to derive microwave polarization difference index (MPDI) for different crops/vegetation classes at various phenological stages. This MPDI index is related with the ten-day maximum value of composite of normalized difference vegetation index (NDVI) data of NOAA-AVHRR sensor. A non-linear inverse relationship has been developed between the MPDI and NDVI of different crops over various regions. It has been observed that the surface wetness/soil moisture in rice growing region influenced the MPDI-NDVI relationship.

 

Key words: Microwave, Radiometry, Crop growth, Remote sensing

 

  

 

Indian Journal of Radio & Space Physics

Vol. 32, August 2003, pp. 198-208

 

 

 

Diagnostic study on vertical structure of tropical precipitating cloud
systems using lower atmospheric wind profiler

K Krishna Reddy

 

 

Temporal variations of vertical structure of precipitating clouds in tropical India have been studied based on the observations of 1.357 GHz lower atmospheric wind profiler (LAWP) at Gadanki. In the presence of precipitating clouds, backscattering from hydrometeors is dominant and the vertically pointing LAWP measures the backscattered energy and the vertical velocities of the hydrometeors directly overhead. These observations yield time-altitude cross-sections of equivalent reflectivity, Doppler velocity and spectral width that illustrate the evolution of precipitating cloud systems. The vertical structure of these three parameters has been used to classify the precipitating cloud systems into three types: convective, transition and stratiform. Diurnal and seasonal variations of the occurrence of precipitating cloud systems show that the precipitation primarily occurs in the afternoon and also the convective and transition clouds are most frequent in the south-west monsoon, while the occurrence of stratiform clouds is predominant in the north-east monsoon. Gadanki-LAWP provides means for the development of climatology of tropical precipitating cloud systems. Such climatology is needed to specify diabatic heating rates in large-scale numerical weather prediction and climate models.

 

Key words: Tropical clouds, Clouds, Rain, Wind profiler

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, August 2003, pp. 209-216

 

 

A study of the forced Lorenz model of relevance to monsoon predictability

 Ashok Kumar Mittal, Suneet Dwivedi & Avinash Chandra Pandey

 

 

The forced Lorenz model introduced as a paradigmatic model for monsoon prediction is explored in greater detail. In particular, a study is made of the bifurcation structure of the forced Lorenz model as the forcing is varied. Shift in the probability distribution function between the two branches of the Lorenz attractor as a function of the forcing is also studied.

 

Key words: Lorenz model, Rain, Monsoon predictability

 

  

  

Indian Journal of Radio & Space Physics

Vol. 32, August 2003, pp. 217-220

 

 

 

Drop size distribution of rainfall of different intensities

V Sasi Kumar, S Sampath & P V S S K Vinayak

 

 

Drop size distribution was measured at Thiruvananthapuram, Kerala State, India, during April, June and July 2001 using a Joss-Waldvogel type disdrometer. The instrument gives the number of drops in 20 size classes ranging from 0.3 mm to > 5 mm. Each month's data were sorted, based on rainfall intensity, into periods of rainfall intensity < 1 mm/hr, 1-3 mm/hr, 3-10 mm/hr, 10-30 mm/hr and > 30 mm/hr. The average number of drops in each size class was computed for each set of data. Exponential and  distribution functions were then fitted to each data set. The results showed that, in June and July, the data approximated an exponential distribution for low rainfall intensities, but became closer to the  distribution at higher intensities. However, in April all data fitted the  distribution, irrespective of rainfall intensity.

 

Key words: Drop size distribution, Rainfall, Rainfall intensities

 

 

Indian Journal of Radio & Space Physics 

Vol. 32, August 2003, pp. 221-228

 

 

 

Diurnal and seasonal variations of point discharge current during thunderstorms
at a tropical inland station

 

S S Kandalgaonkar, M I R Tinmaker, M K Kulkarni & Asha Nath

 

 

The data of point discharge current (PDC) measurements during a total of 65 thunderstorms at Pune (18o32¢N, 73o51¢E) are analyzed to study the PDC local diurnal variation and some issues related with the current. The analysis shows that about 83% of the total quarter hours occupied by PDC are localized between 1400 hrs LT and 2400 hrs LT and the remaining 17% are sparsely distributed over midnight to early hours in the morning. The net result of this study revealed that PDC is the dominant agent for the negative charging of the earth’s surface and should be identified as an active element participating in global electrical circuit. The phase relationship between the positive and negative current during the diurnal period suggests that the active period of current of one polarity has a preferred time of occurrence over the other and by the late night hours the positive charge showed its sustained occurrence over the negative one. The seasonal relationship between storm averaged PDC and monthly mean maximum surface air temperature has also been examined. The comparison of seasonal average amplitude of PDC has suggested a strong positive correlation with the monthly mean maximum surface air temperature. The average amplitude of PDC during thunderstorm in premonsoon season is found to be about three times higher than those during the monsoon season. An examination of association between duration and amplitude variation of PDC showed that storms that are closer to the station within 4 km have longer duration and stronger current amplitude than when they are more than 4 km away.

 

Key words: Point discharge current, Thunderstroms, Atmospheric electricity

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, August 2003, pp. 229-237

 

 

 

Aerosol optical depth variation observed using sun-photometer over Indore

Pawan Gupta,  Harish Gadhavi & A Jayaraman

 

 

Columnar aerosol optical depth measurements have been made over Indore (22.7o N, 75.9o E) during May 2001-March 2002 at four spectral bands centred around 400, 499, 668 and 858 nm using a hand-held sun-photometer. The aerosol optical depth (AOD) shows seasonal variation with high values in summer and low values in winter. The summer-increase is found to be due to the high wind speed producing larger amount of wind-derived dust particles. As the summer monsoon sets in at the end of June there is an appreciable decrease in the AOD values. The AOD values decrease further in winter and the decrease is more at higher wavelengths indicating that there is a general reduction in the number of bigger particles. Also during winter months the wind direction changed to southerly and south-easterly which brings air that is more rural to the measurement site. The amplitude of the observed high AOD values in summer and low during winter, is higher for longer wavelengths which shows that the coarse particles contribute more to the observed variation as compared to sub-micron particles. Comparison of the AOD values over Indore with that of Trivandrum and Visakhapatnam shows that the Indore values are comparable to that of Visakhapatnam, but much higher than the Trivandrum values indicating the influence of industrialization and higher population.

 

Key words: Aerosols, Aerosol optical depth, Sun-photometer

 

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, August 2003, pp. 238-241

 

 

Harmful electromagnetic environment near transmission tower

P P Pathak,  Vijay Kumar and R P Vats

 

 

The penetration of RF and microwave electromagnetic fields into human body and its possible consequences are studied. Earlier observations are theoretically verified which show that the radiation from high power transmission tower is more harmful to the people living nearby as compared to that from the low power transmission tower. Absorption of electric component of electromagnetic radiation falling on the body, which increases with frequency, may result into temperature rise in the body. However, the magnetic induction, being very weak in comparison to the earth’s magnetic field, is expected to have no effect on the human body.

 

Key words: Electromagnetic field, Transmission tower, Human body, Radiation