Som Sharma

Long term ionospheric trends over Ahmedabad

Ionospheric data over Ahmedabad (23°N, 73°E) during 1955–96 have been examined for the long term changes arising due to the predicted cooling of the mesosphere and thermosphere because of the increased concentration of trace gases resulting from anthropogenic causes. The long term changes in the critical frequencies of the ionospheric layers and the height of the maximum ionization as characterized by hPF2, the virtual height at 0.834 of foF2, have been obtained after removing the seasonal and solar cycle variations. While the trend of a small decrease in foE (0.15 MHz in 40 years) may not be considered significant, there is a clear indication of a decrease in foF2 (1.6 MHz for midday, 1.0 MHz for midnight). A small increase is noted in foF1 (0.3 MHz). There is a decrease in hPF2 (16 km for midday and 10 km for midnight). The lowering of F2 layer peak will affect radio propagation.

Ionospheric measurements during the total solar eclipse of 11 August 1999

A number of radio experiments were conducted at Ahmedabad (23°N, 73°E) with the aim of studying the ionospheric effects of the total solar eclipse of 11 August 1999. Rapid radio soundings from the ionosonde were made on the eclipse day and on control days. A riometer was operating at 30 MHz, and field strength measurements along the three oblique incidence paths of Colombo-Ahmedabad (11905 kHz), Bombay-Ahmedabad (558 kHz) and Rajkot-Ahmedabad (810 kHz) were also made. A reduction of about 20% was observed in the minimum frequency of reflection from the ionosonde (fmin), which indicates a reduction in D-region ionization. The critical frequency of the E-layer was not measurable beyond 1600 h IST on eclipse day due to the strong blanketing sporadic-E, but there is a 20% decrease in the critical frequency of the F1-layer. Although there was no change in the minimum virtual height of the F-layer on eclipse day, there appears to have been a decrease in the height of maximum ionization (hpF2) during the eclipse, indicating a reduction in the thickness of the F-layer. The signal strength of the Colombo-Ahmedabad path shows an initial rapid increase with the start of the eclipse (indication of a decrease in ionization in the D- and lower E-regions), but subsequently decreases until the maximum of the eclipse (excessive deviative absorption because of the wave penetrating to the E-region). The field strength measurements of the Bombay-Ahmedabad path show a large fading after sunset as the sky wave also appeared. On eclipse day the fading started about an hour earlier. Riometer recordings also show a higher signal during eclipse day, which again indicates an eclipse-associated decrease in ionization.

Spread-F at anomaly crest regions in the Indian and American longitudes

Occurrence features of spread-F over Ahmedabad, situated near the anomaly crest region in Indian zone are studied from the quarter hourly ionograms for the years 1983–1995. The occurrences of range and frequency types of spread-F over Ahmedabad are seasonal and solar cycle dependent. Frequency type of spread-F is most frequent during June-solstices of low sunspot years. The range type of spread-F is frequent during equinoxes and December-solstices (with lower occurrence) of high sunspot years. The occurrence features of range spread at Ahmedabad are compared with the occurrence features of range spread over Cachoeira Paulista, a station near the anomaly crest region in Brazil in the American sector. The occurrence of range spread-F over Cachoeira Paulista shows strong seasonal dependence with a distinct maximum during local summer (December solstices) for both low and high sunspot years. The occurrence of range spread over Cachoeira Paulista is much higher than over Ahmedabad.

Long-term changes in ionospheric parameters over Ahmedabad

Ionospheric data over Ahmedabad obtained from regular radio soundings made during past four decades are analyzed to study long term changes. Midday values of the critical frequencies of E, F1 and F2 layers show a small positive trend with changes of less than 0.1 MHz for E and F1 layers and about 0.2 MHz for F2 layer in of 40 years. Increase in the peak altitude of F2 layer is also noticed. However the increases are within the limits of accuracies involved in estimation.

Ionospheric effects of the total solar eclipse of 24 October 1995 over Ahmedabad

Abstract Ionospheric effects of the total solar eclipse of 24 October 1995 over Ahmedabad, with maximum obscuration of 83 % at 0800 hr (75° EMT), are studied based on radio soundings using KEL (KEL Aerospace Pvt. Ltd.) digital ionosonde. Comparison with the data recorded on control day, 23 October 1995, shows reduction of 35 %, 45 % and 28 % in the critical frequencies of E, F1, and F2 regions respectively. True height analysis of the quarter hourly ionograms shows maximum eclipse associated decrease at 200 km.

Equatorial spread F: Statistical comparison between ionosonde and scintillation observations and longitude dependence

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Space Weather Effects of Coronal Mass Ejection

This paper describes the space weather effects of a major CME which was accompanied by extremely violent events on the Sun. The signatures of the event in the interplanetary medium (IPM) sensed by Ooty Radio Telescope, the solar observations by LASCO coronagraph onboard SOHO, GOES X-ray measurements, satellite measurements of the interplanetary parameters, GPS based ionospheric measurements, the geomagnetic storm parameter Dst and ground based ionosonde data are used in the study to understand the space weather effects in the different regions of the solar-terrestrial environment. The effects of this event are compared and possible explanations attempted.

Interplanetary and terrestrial observations of an Earth‐directed coronal mass ejection

In this article we report interplanetary scintillation observations at 103 and 327 MHz of an Earth-directed coronal mass ejection (CME) which occurred near the center of the solar disk at 0435 UT on May 12, 1997. The disturbance was found to have plasma density ∼4 times more than that of the ambient plasma at a distance of ∼ 0.5 AU from the Sun. The most peculiar aspect of this CME is that it appears that the disturbance moved slightly slower than the ambient medium. Solar and Heliospheric Observatory (SOHO) and interplanetary scintillation (IPS) estimates of solar wind are quite different; it appears that the difference could be due to the projection effect of the SOHO image.Though the disturbance was not very severe, its impact on Earths environment produced a geomagnetic storm. This event was associated with a two-ribbon flare.The ionospheric effects of soft X rays from this solar flare were observed by a digital ionosonde at Ahmedabad in the form of excess ionization (∼1200 el cm−3) in the D region of the ionosphere.

Fog/low clouds detection over the Delhi Earth Station using the Ceilometer and the INSAT-3D/3DR satellite data

ABSTRACT Fog is a meteorological phenomenon in which cloud has its base near to the surface which disrupts public health and transportation due to decrease in visibility. In the present study, a collective method has been implemented for detection of fog/low clouds over the Delhi Earth Station during fog season for the years 2016 and 2017. A detailed comparison has been performed with the use of ground-based cloud-base height measurements from the Ceilometer and the Indian National Satellites (INSAT-3D/3DR)-retrieved fog product. Fog/low clouds have been effectively detected by both the Ceilometer and the INSAT-3D/3DR during single-layer cloud in the range of 0–200 m in most of the cases. Since the INSAT-3D/3DR fog product is derived under clear sky condition, fog detection during multilayer clouds is a challenging task. Results indicate that the Ceilometer can capture fog/low clouds effectively even in presence of single- and multilayer clouds. Remote-sensing technology can provide better opportunity by providing complete temporal and spatial coverage of fog. The present technique which incorporates both ground-based Ceilometer and space-based INSAT-3D/3DR measurements is observed to be more promising than other conventional methods for an improved detection and monitoring of fog/low clouds.

Evaluation of clouds simulated by a weather model over western India

ABSTRACT Cloud is one of the most crucial parameters, which influences the predictive ability of the weather models. This study is first of its kind to assess the efficiency of the numerical weather prediction (NWP) models to simulate the cloud types over Indian region. Two broadly used NWP models named as European Centre for Medium Range Weather Forecast (ECMWF) Reanalysis Interim (ERAI) and Weather Research and Forecasting (WRF) model are chosen to compare analyses and forecasts of cloud types along with cloud base height (CBH) measured from ceilometer over Ahmedabad (23.03°N, 72.5°E, 55 m amsl), India. Three months of the Indian summer monsoon period (July to September, 2014) are used for initial verifications. Results show that both the ERAI and WRF models are able to capture the cloud cover correctly over this region. This investigation has shown that the WRF model shows mostly mid-level cloud as low-level cloud, which needs further modification in WRF physics to improve the skills of cloud forecast. Further investigations of temporal variations of clouds and rainfall from in situ observations reveal that low-level multi-layer clouds provide favourable environment for rainfall.