scholarly journals Identification of active fossil bubbles based on coordinated VHF radar and airglow measurements

2007 ◽  
Vol 25 (10) ◽  
pp. 2099-2102 ◽  
Author(s):  
R. Sekar ◽  
D. Chakrabarty ◽  
S. Sarkhel ◽  
A. K. Patra ◽  
C. V. Devasia ◽  
...  
Keyword(s):  
Vhf Radar ◽  
Equatorial Spread F ◽  
Plasma Irregularity ◽  
Scale Size ◽  
Spread F ◽  
Dip Equator ◽  
Plasma Depletion ◽  
First Time ◽  
Airglow Intensity

Abstract. Plasma irregularity structures associated with an Equatorial Spread-F (ESF) event were recorded by the Indian VHF Radar on 26–27 April 2006 near midnight hours. The plasma structures were found to be isolated without having bottomside structure. They moved predominantly downward and the structures were found to be less turbulent than their post-sunset counterparts. However, the structures were characterized by meter-scale size irregularities. These structures are identified for the first time as plasma depletion structures using simultaneous, collocated measurements of OI 630.0 nm airglow intensity variations. The variation of the base height of ionospheric F layer over dip equator is also presented to buttress the result. Further, these plasma structures are shown to be "active fossil bubbles".

scholarly journals Characterization of VHF radar observations associated with equatorial Spread F by narrow-band optical measurements

2004 ◽  
Vol 22 (9) ◽  
pp. 3129-3136 ◽  
Author(s):  
R. Sekar ◽  
D. Chakrabarty ◽  
R. Narayanan ◽  
S. Sripathi ◽  
A. K. Patra ◽  
...  
Keyword(s):  
Large Scale ◽  
Temporal Variations ◽  
Density Fluctuations ◽  
Optical Measurements ◽  
Small Scale ◽  
Upward Movement ◽  
Vhf Radar ◽  
Equatorial Spread F ◽  
Spread F ◽  
Airglow Intensity

Abstract. The VHF radars have been extensively used to investigate the structures and dynamics of equatorial Spread F (ESF) irregularities. However, unambiguous identification of the nature of the structures in terms of plasma depletion or enhancement requires another technique, as the return echo measured by VHF radar is proportional to the square of the electron density fluctuations. In order to address this issue, co-ordinated radar backscatter and thermospheric airglow intensity measurements were carried out during March 2003 from the MST radar site at Gadanki. Temporal variations of 630.0-nm and 777.4-nm emission intensities reveal small-scale ("micro") and large-scale ("macro") variations during the period of observation. The micro variations are absent on non-ESF nights while the macro variations are present on both ESF and non-ESF nights. In addition to the well-known anti-correlation between the base height of the F-region and the nocturnal variation of thermospheric airglow intensities, the variation of the base height of the F-layer, on occasion, is found to manifest as a bottomside wave-like structure, as seen by VHF radar on an ESF night. The micro variations in the airglow intensities are associated with large-scale irregular plasma structures and found to be in correspondence with the "plume" structures obtained by VHF radar. In addition to the commonly observed depletions with upward movement, the observation unequivocally reveals the presence of plasma enhancements which move downwards. The observation of enhancement in 777.4-nm airglow intensity, which is characterized as plasma enhancement, provides an experimental verification of the earlier prediction based on numerical modeling studies.

scholarly journals Radar Observations of 8.3-m scale equatorial spread F irregularities over Trivandrum

2004 ◽  
Vol 22 (3) ◽  
pp. 911-922 ◽  
Author(s):  
D. Tiwari ◽  
A. K. Patra ◽  
C. V. Devasia ◽  
R. Sridharan ◽  
N. Jyoti ◽  
...  
Keyword(s):  
Spectral Width ◽  
Plasma Waves ◽  
Equatorial Ionosphere ◽  
Doppler Velocity ◽  
Spectral Parameters ◽  
Equatorial Spread F ◽  
Ionosphere Plasma ◽  
Scale Size ◽  
Spread F ◽  
Present Understanding

Abstract. In this paper, we present observations of equatorial spread F (ESF) irregularities made using a newly installed 18MHz radar located at Trivandrum. We characterize the morphology and the spectral parameters of the 8.3-m ESF irregularities which are found to be remarkably different from that observed so extensively at the 3-m scale size. We also present statistical results of the irregularities in the form of percentage occurrence of the echoes and spectral parameters (SNR, Doppler velocity, Spectral width). The Doppler spectra are narrower, less structured and less variable in time as compared to those observed for 3-m scale size. We have never observed the ESF irregularity velocities to be supersonic here unlike those at Jicamarca, and the velocities are found to be within ±200ms–1. The spectral widths are found to be less than 150ms–1. Hence, the velocities and spectral width both are smaller than those reported for 3-m scale size. The velocities and spectral widths are further found to be much smaller than those of the American sector. These observations are compared with those reported elsewhere and discussed in the light of present understanding on the ESF irregularities at different wavelengths. Key words. Ionoshphere (equatorial ionosphere, plasma waves and instabilities; ionospheric irregularities)

scholarly journals Time and Altitude spread F echoes distribution over the Christmas Island VHF radar

2022 ◽  
Author(s):  
Ricardo Yvan de La Cruz Cueva ◽  
Eurico Rodrigues de Paula ◽  
Acácio Cunha Neto
Keyword(s):  
Solar Minimum ◽  
Solar Maximum ◽  
Equatorial Region ◽  
Peak Time ◽  
Vhf Radar ◽  
Equatorial Spread F ◽  
Plasma Drift ◽  
Christmas Island ◽  
Spread F

Abstract. The goal of this work is to study the time and altitude echoes characteristics under different solar and seasonality conditions using the VHF radar RTI images. The occurrence of equatorial spread F depends on the existence of conditions that can seed the Raileight-Taylor instability, and these conditions can change with solar flux, seasonality, longitude distributions, and day-to-day variability. So, the equatorial spread F is observed as its time and altitude occurrence. The VHF radar of Christmas Island (2.0° N, 157.4° W, 2.9° N dip latitude) has been operational in the equatorial region for some time now, allowing long-term observations. The occurrence of echoes during solar minimum conditions are observed all throughout the night since the post reversal westward electric field is weaker than the solar maximum and the possibilities for the vertical plasma drift to become positive are larger. On other hand, echoes during solar maximum will be controlled by dynamics near the time of the Pre-reversal Peak (PRE). Our results indicate peak time occurrence of echoes along this period shows a well-defined pattern, with echoes being distributed as closer to local sunset during solar maximum and around/closer midnight during solar minimum conditions, meanwhile, the peak altitude occurrence of echoes shows a slightly regular pattern with higher altitude occurrences during solar maxima and lower altitudes during solar minimum conditions.

Equatorial Spread-F and Magnetic Activity

Nature ◽  
1958 ◽  
Vol 181 (4625) ◽  
pp. 1724-1725 ◽  
Author(s):  
A. J. LYON ◽  
N. J. SKINNER ◽  
R. W. WRIGHT
Keyword(s):  
Magnetic Activity ◽  
Equatorial Spread F ◽  
Spread F

scholarly journals Seasonal dependence of the "forecast parameter" based on the EIA characteristics for the prediction of Equatorial Spread F (ESF)

2008 ◽  
Vol 26 (7) ◽  
pp. 1751-1757 ◽  
Author(s):  
S. V. Thampi ◽  
S. Ravindran ◽  
T. K. Pant ◽  
C. V. Devasia ◽  
R. Sridharan
Keyword(s):  
Seasonal Variability ◽  
Seasonal Pattern ◽  
Onset Time ◽  
Critical Value ◽  
Latitudinal Gradients ◽  
Electron Content ◽  
Total Electron ◽  
Equatorial Spread F ◽  
Spread F ◽  
Definite Role

Abstract. In an earlier study, Thampi et al. (2006) have shown that the strength and asymmetry of Equatorial Ionization Anomaly (EIA), obtained well ahead of the onset time of Equatorial Spread F (ESF) have a definite role on the subsequent ESF activity, and a new "forecast parameter" has been identified for the prediction of ESF. This paper presents the observations of EIA strength and asymmetry from the Indian longitudes during the period from August 2005–March 2007. These observations are made using the line of sight Total Electron Content (TEC) measured by a ground-based beacon receiver located at Trivandrum (8.5° N, 77° E, 0.5° N dip lat) in India. It is seen that the seasonal variability of EIA strength and asymmetry are manifested in the latitudinal gradients obtained using the relative TEC measurements. As a consequence, the "forecast parameter" also displays a definite seasonal pattern. The seasonal variability of the EIA strength and asymmetry, and the "forecast parameter" are discussed in the present paper and a critical value for has been identified for each month/season. The likely "skill factor" of the new parameter is assessed using the data for a total of 122 days, and it is seen that when the estimated value of the "forecast parameter" exceeds the critical value, the ESF is seen to occur on more than 95% of cases.

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