NOPR Collection:

Analysis of Solar Optical Flares in Relation to Their Peak Intensities

1973-06-01T00:00:00Z

Title: Analysis of Solar Optical Flares in Relation to Their Peak Intensities Authors: Gupta, M. K. Das; Mitra, R. K.; Sarkar, S. K. Abstract: Peak intensities of 5200 optical flares reported during the period January 1968 to December 1971 were examined in relation to their (i) occurrences, (ii) durations, (Hi) ratios' of rise time to duration, and (iv) classifications, both area-wise and intensity-wise. important results obtained are (i) flares as reported by the different observatories fall under two major groups, one group confined within the percentage intensity increase range 40-100% and the other within the range 140-300%, (ii) intensity-wise flare classification in vogue does not show any direct relationship with the present system of quantitative intensity increase evaluation and (iii) majority of the flares are 'impulsive' in nature. Page(s): 158-160

Long Period Fading of hf & lf Signals during Meteorological Activities in Relation to Internal Atmospheric Gravity Waves

1973-06-01T00:00:00Z

Title: Long Period Fading of hf & lf Signals during Meteorological Activities in Relation to Internal Atmospheric Gravity Waves Authors: Sen, A. K.; Trehan, S. K.; Saha, B. Abstract: Long period fading with quasi-periods of the order of 1 hr, was observed at Calcutta (lat., 22ᵒ33 N; long., 88ᵒ24E) simultaneously in the field strength of hf signals, during local meteorological activities. The fading usually starts suddenly and the effects observed at hf and if are closely associated. The results obtained from an analysis of the observations in 1972 during the period June-December; when meteorological disturbances were prevalent in and around Calcutta, are presented. It is suggested that the fading might be due to internal atmospheric gravity waves of meteorological origin affecting the ionosphere. It appears that the closely associated hf and if fading effects are produced by the gravity waves bringing about disturbances in a common region of the ionosphere responsible for the absorption of the signal and the reflection of the if signal, respectively. Page(s): 156-157

Anomaly in the If Propagation Characteristics during Meteor Shower Activities

1973-06-01T00:00:00Z

Title: Anomaly in the If Propagation Characteristics during Meteor Shower Activities Authors: Sen, A. K.; Saha, B. Abstract: An anomalous behaviour of the If signal strength in the form of a gradual decrease followed by a sudden increase has been observed frequently during major meteor showers. The anomaly occurred both during day and night. Some of the interesting results obtained from an analysis of the observations made in Calculta (lat., 22°34'N; long., 88°24' E) for a major meteor shower of the year 1972-73 are reported in this paper. The anomaly is interpreted in terms of the ionization produced by a gradual diffusion of an ablazed meteor at the ionospheric heights involved. Page(s): 153-155

Night-time Changes in Field Strength of 11.8 MHz Radio Signals over Coldmbo-Ahmedabad Transmission Path

1973-06-01T00:00:00Z

Title: Night-time Changes in Field Strength of 11.8 MHz Radio Signals over Coldmbo-Ahmedabad Transmission Path Authors: Chhipa, G. M.; Patel, B. M. Abstract: The results of a study of night-time records of the field strength of radio signals on 11.8 MHz transmitted from Colombo (6.9^ᵒN, 79.9^ᵒE, I=5^ᵒS) and obtained at Ahmedabad (23^ᵒN, 72.6^ᵒE, I=34^ᵒN) are reported. It is shown that the maximum strength is observed an hour after sunset in summer and almost at sunset in winter. This maximum is most pronounced in equinoctial months. The records show a fast fading of small amplitude on quiet day’s characteristic of spread-F. On magnetically disturbed days, the field strength under-goes large fluctuations with reduced fading rate and absence of flutter-type fading. On some occasions of high magnetic activity, signals become very weak. Such changes are believed to be caused by large-scale movements in the F2-layer and the accompanying changes in the electron density during the magnetic storm. Several instances of temporary sharp increases of field strengths over duration of about 10 min have been recorded in the post-sunset to pre-midnight interval. Most of these are found to occur after solar radio bursts with a lag of 12-17 min. The observed effect of increased field strength may be due to a transient disturbance produced in the F2-layer due to the burst. Page(s): 148-152