Effect of solar and magnetic activity on VHF scintillations near the equatorial anomaly crest

Abstract

Abstract. The VHF amplitude scintillation recorded during the period January 1991 to December 1993 in the declining phase of a solar cycle and April 1998 to December 1999 in the ascending phase of the next solar cycle at Varanasi (geogr. lat.=25.3 • , long. =83.0 • , dip=37 • N) have been analyzed to study the behavior of ionospheric irregularities during active solar periods and magnetic storms. It is shown that irregularities occur at arbitrary times and may last for <30 min. A rise in solar activity increases scintillations during winter (November-February) and near equinoxes (March-April; September-October), whereas it depresses the scintillations during the summer (May-July). In general, the role of magnetic activity is to suppress scintillations in the pre-midnight period and to increase it in the post-midnight period during equinox and winter seasons, whilst during summer months the effect is reversed. The pre-midnight scintillation is sometimes observed when the main phase of D st corresponds to the pre-midnight period. The annual variation shows suppression of scintillations on disturbed days, both during premidnight and post-midnight period, which becomes more effective during years of high solar activity. It is observed that for magnetic storms for which the recovery phase starts postmidnight, the probability of occurrence of irregularities is enhanced during this time. If the magnetic storm occurred during daytime, then the probability of occurrence of scintillations during the night hours is decreased. The penetration of magnetospheric electric fields to the magnetic equator affects the evolution of low-latitude irregularities. A delayed disturbance dynamo electric field also affects the development of irregularities.

Effect of magnetic activity on plasma bubbles over equatorial and low-latitude regions in East Asia

“…However, the dependence of the first occurrence of low latitude plasma bubbles on local time is at its maximum at 23:00 LT, and most plasma bubble events last for <45 min. It indicates that the duration of plasma bubbles becomes greater as one approaches the equator (Chandra et al, 1993;Singh et al, 2004). In addition, a time delay of about 3 h exists between the first occurrence time of equatorial and low-latitude plasma bubbles.…”

Occurrence and zonal drifts of small-scale ionospheric irregularities over an equatorial station during solar maximum – Magnetic quiet and disturbed conditions

“…Then, the storm presented a slow recovery phase that took more than 68 h. Previous studies have shown occurrence of postmidnight scintillation activity when the maximum of the main phase of the storms begins at midnight to dawn local time sector. According to Singh et al (2004) the chance of occurrence increases if the D st index is below À100 nT. This may be expected to be the case here for the storm of 5 November, in which SYM-H index is well below À300 nT.…”

“…According to Aarons (1991) the ring current (as seen by the D st index) may be a controlling factor in the generation of F-region irregularities. Recent studies have followed the Aarons' method to study geomagnetic storm effects on the generation or inhibition of F-layer irregularities, such as Kumar and Gwal (2000), Biktash (2004), Singh et al (2004) and Shang et al (2008). The advantage in the use of the SYM-H index is its 1-min time resolution which provides more precisely the time occurrence of the SCs, the maximum negative values of the horizontal component of geomagnetic field and the initial-to-main and recovery phase duration of the storms.…”

Characteristics of low latitude ionospheric E-region irregularities linked with daytime VHF scintillations measured from Varanasi

“…The scintillation at Varanasi predominantly is observed in the pre-midnight periods in small patches with duration < 30 minutes (Singh and Singh 1997;Singh et al 2004). VHF scintillation data recorded during daytime for the period January 1991 to December 1993 in the declining phase of solar cycle and April 1998 to December 1999 in the ascending phase of the next solar cycle as well as recent year of January 2008 to December 2008 have been analyzed in this study.…”

“…The scintillation index in dB has been scaled manually every 15 minutes by measuring peak-to-peak P max -P min excursion in dB and using a calibration chart and conversion chart (Whitney et al 1969), where 1 dB = 12% scintillation index, P max is the power amplitude of the third peak-to-peak down from the maximum excursion and P min is the power amplitude of the third level up from the minimum excursion. The similar method of analysis was adopted by All India Coordinated Program of Ionospheric and Thermospheric Studies (AICPITS) of Department of Science, Government of India Kumar et al 2000;Singh et al 2004Singh et al , 2006Vijayakumar et al 2007). Typical examples of daytime scintillations recorded at low latitude station Varanasi are shown in figure 1.…”