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Indian Journal of Radio & Space Physics

 

 

 

ISSN : 0367-8393

 

CODEN:IJRSAK

VOLUME 32

NUMBER 5

OCTOBER 2003

 

 

CONTENTS

 

Day-to-day variability of the equatorial electrojet strength

251

R P Kane, N B Trivedi, Y Tanaka, Y Hajime & B M Pathan

 

Incoherent Cerenkov radiation as the source of auroral and polar hiss emissions

261

Ram Prakash, Bal Mukund Agrawal & A P Singh

 

Observations of recovery time characteristics in cosmic ray Forbush type decreases

266

Pankaj K Shrivastava & Prashanti Shrivastava

 

Consistency of limiting and bounce averaged diffusion coefficients in pitch angle diffusion and energetic electron precipitation in the inner radiation belt

 

271

Ram Prakash, A P Singh & Bal Mukund Agrawal

 

Characteristics of cirrus clouds observed at a low latitude tropical coastal station, Trivandrum using lidar

278

S Veerabuthiran & M Satyanarayana

 

Measurements of raindrop size distribution over Gadanki during south-west and north-east monsoon

 

286

K Krishna Reddy & Toshiaki Kozu

 

Rain drop size distribution from radar reflectivity measurements

296

P Mali, S K Sarkar & J Das

 

Column density of atmospheric water vapour over Ahmedabad

301

D K Chakrabarty, S Patel, P Vala & K N Iyer

 

Atmospheric aerosols and air pollution

306

N V Raju, B S N Prasad & B Narasimhamurthi

 

Convective boundary layer information revealed by lower atmospheric wind profiler over Gadanki, India

 

312

K Krishna Reddy

 

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 251-260

 

 Day-to-day variability of the equatorial electrojet strength

R P Kane, N B Trivedi, Y Tanaka, Y Hajime and B M Pathan

 

 

The H ranges at the equatorial electrojet locations Ancon, Peru and Trivandrum, India, show substantial periodicities in the 2-20 day range which are absent in solar flux, and hence, should be attributed mainly to planetary wave activity and more so in quiet-sun intervals. However, the results for the two locations differ in details, indicating that these effects may not be global. The variations are seen mostly in the daytime values, indicating their association with the daytime electrojet strength only.

 

Keywords: Equatorial electrojet, Electrojet, Magnetic field component

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 261-265

  

  

Incoherent Cerenkov radiation as the source of auroral and polar hiss emissions

Ram Prakash, Bal Mukund Agrawal & A P Singh

 

 

For known plasma parameters and observed energetic electron spectra, detailed power calculations are carried out under incoherent Cerenkov radiation mechanism for auroral and polar VLF hiss emissions to confirm their generation mechanism. The results indicate a discrepancy of about an order of magnitude between the calculated and observed peak power intensities. The precipitating electrons having energy of the order of a few tens of electron volt radiating incoherently in Cerenkov mode seem to be the fundamental sources of these emissions, which are further amplified probably by electron cyclotron resonance instability mechanism involving energetic electrons up to the observed power levels during their downward propagation from the source region.

 

Keywords: VLF hiss, Cerenkov radiation, Auroral emissions

 

 

 

Indian Journal of Radio & Space Physics

Vol. 33, October 2003, pp. 266-270

 

 

Observations of recovery time characteristics in cosmic ray Forbush type decreases

Pankaj K Shrivastava & Prashanti Shrivastava

 

 

An attempt has been made to select about 33 Forbush type decreases occurring during 1986-1999 at Kiel Neutron Monitor Station. This interval covered the whole period of sunspot solar cycle 22 and ascending part of solar cycle 23. Recovery time characteristics of Forbush decreases are studied. The average recovery time of neutron monitor energies is 4 days, but varies from 2 to 10 days. No significant difference in the average recovery time is observed from two different rigidities. The relative amplitudes of these decreases are examined.

 

Keywords: Comic rays, Forbush decrease, Solar cycle

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 271-277

 

 

 Consistency of limiting and bounce averaged diffusion coefficients in pitch angle diffusion and energetic electron precipitation in the inner radiation belt

Ram Prakash, A P Singh & Bal Mukund Agrawal

 

 

Limiting and bounce averaged diffusion coefficients are calculated at different L-values in the inner radiation zone. The results show that their calculated values decrease with increasing equatorial pitch angles (a). The values of bounce averaged diffusion coefficient at a 90 are found to be nearly 2-3 orders of magnitude lower than those at a 30, whereas limiting diffusion coefficients approach a zero value at a 90. The results agree well with the experimental fact that farther the pitch angles from the loss cone, less efficient is the diffusion of electrons into the loss cone and that the energetic electrons having pitch angles close to 90 are stably trapped in the inner radiation zone.

 

Keywords: Pitch angle diffusion, Electron precipitation, ELF hiss

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 278-285

 

 

 

Characteristics of cirrus clouds observed at a low latitude tropical coastal station, Trivandrum using lidar

S Veerabuthiran & M Satyanarayana

 

 

High altitude cirrus clouds at about 1-2 km below the tropopause were observed using the multiwavelength lidar system located at a tropical coastal station, Trivandrum (833N, 77E). Seasonal variations of these clouds were studied over a period of one year. The clouds were seen almost 50% of the time of lidar operation. An analysis of lidar data has shown frequent occurrence of such clouds near the tropopause. Based on their optical depth, cirrus clouds were classified into two categories, namely thick cirrus (optical depth > 0.03) and thin cirrus (optical depth < 0.03) and their occurrence frequencies were about 64% and 36% of the total occurrences.

 

Keywords: Clouds characteristics, Lidar, Aerosols

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 286-295

 

 

 

Measurements of raindrop size distribution over Gadanki during south-west and north-east monsoon

K Krishna Reddy and Toshiaki Kozu

 

 

Simultaneous observations of disdrometer and optical rain gauge (at National MST Radar Facility, Gadanki) and their application in rainfall estimation are evaluated. Comparison of rain rate obtained from disdrometer and optical rain gauge in different precipitation events in different seasons show reasonably good agreement between the two instruments. Southern India has two distinct rainfall seasons, namely south-west (S-W) monsoon and north-east (N-E) monsoon periods. Disdrometer data analysis shows a clear seasonal dependence in radar reflectivity factor-rainfall rate (Z-R) relations (i.e. raindrop size distribution characteristics) in S-W monsoon and N-E monsoon periods over Gadanki. It is also found that during S-W monsoon precipitation generally has bigger drops than during N-E monsoon. During the S-W monsoon most of the precipitating cloud systems are associated with mesoscale convection activities. These precipitating systems are short-lived (~1-2 h) with high intensity of rainfall.

 

Keywords: Raindrops, Raindrop size distribution, Rainfall, Radar reflectivity

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 296-300

 

 

Rain drop size distribution from radar reflectivity measurements

P Mali, S K Sarkar and J Das

 

 

Rain drop size distribution (RDSD) is useful for estimation of attenuation of high frequency electromagnetic waves due to rain. In this paper, radar reflectivity (Z) obtained by using an X-band radar operating at ~9.4 GHz has been used for deducing the results on RDSD at different rain intensities in the range 50-150 mm/hr. The present results have been observed to match with the results obtained by other investigators by using distrometers for the same range of rain rates. The equation obtained for RDSD is of general type and it would satisfy RDSD for the other rain rates also, if the radar reflectivity (Z) is calibrated properly with various rain rates. The unique feature of this distribution equation has its direct application in radar reflectivity data and rain rate for RDSD.

 

Keywords: Rain drop size, Rain rate, Radar reflectivity

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 301-305

 

 

Column density of atmospheric water vapour over Ahmedabad

D K Chakrabarty, S Patel, P Vala and K N Iyer

 

 

Water vapour spectrum has a strong absorption feature around 442.7 nm. Using this property column density of water vapour has been determined at Ahmedabad, a low latitude station. Measurements have been made from 1996 to 2000. Slant column density has been found to be ~1024 cm-2. The period stretches from a minimum to maximum solar activity. A decreasing trend of water vapour content with an increase of solar activity level is also seen.

 

Keywords: Water vapour, Column density

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 306-311

 

Atmospheric aerosols and air pollution

 N V Raju and B S N Prasad & B Narasimhamurthy

 

 

Multiwavelength radiometer measurement of aerosol optical depth (AOD) at ten wavelengths in the visible and near IR is an ongoing programme since 1988 at Mysore (12N, 76E), a semi-urban continental area with a population of about 1 million. The analysis of 10-yr data indicates an increase in the spectral AOD which is attributed to the increased air pollution resulting from anthropogenic activities. The pollution related aerosol turbidity parameters a and b are determined graphically from AOD. The size-dependent parameter a shows a decrease initially and later on an increase. The number density parameter b remains almost a constant throughout the data period. Aerosol size spectrum is retrieved by numerically inverting the AODs, and the aerosol characteristics are estimated. These indicate that a depends on the number of accumulation mode particles, and b on the mass loading.

 

Keywords : Aerosol, Optical depth, Air pollution

 

 

 

Indian Journal of Radio & Space Physics

Vol. 32, October 2003, pp. 312-319

 

 Convective boundary layer information revealed by the lower
atmospheric wind profiler over Gadanki, India

K Krishna Reddy

 

 

A technique for determining the height of the convective atmospheric boundary layer (CBL) with lower atmospheric wind profiler (LAWP) at Gadanki is discussed. The results are qualitatively compared with simultaneous radiosonde observations. Gadanki-LAWP provides continuous CBL height measurements with very good time resolution (5 minutes or less), allowing for detailed understanding of growth and fluctuations of CBL. During dry season, on clear days, a thin enhanced echo layer corresponding to the top of planetary boundary layer (PBL) appeared at about 500 m height in the morning and ascended to about 1500 m in the afternoon. Strong upward/downward velocities were observed below the echo layer (or inside PBL), reaching 1.5 km in the afternoon. Two types of strong echo structures were found to appear systematically in the tropical PBL with diurnal variations on clear days during wet season. The first type, the striking appearance of an echo layer ascending from below 300 m (in the morning) to above 3-5 km (in the afternoon), was identified as a diurnal variation of the top of the mixing PBL. Another type is an elevated layer echo appearing at 2-3 km height from almost all the times, which seems to be coincided with humidity gradient. It is found, with a few exceptions, that the drier period had higher boundary layer than the wet period indicating that most of the net solar radiation evaporated the moisture rather than heating the surface, and therefore contributed a little to buoyant force.

 

Keywords: Boundary layer, Turbulence, Atmospheric wind profiler