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.

scholarly journals On the seasonal variations of the threshold height for the occurrence of equatorial spread F during solar minimum and maximum years

2007 ◽  
Vol 25 (4) ◽  
pp. 855-861 ◽  
Author(s):  
G. Manju ◽  
C. V. Devasia ◽  
R. Sridharan
Keyword(s):  
Solar Minimum ◽  
Solar Maximum ◽  
Decisive Role ◽  
Meridional Wind ◽  
Equatorial Spread F ◽  
Clear Cut ◽  
Meridional Winds ◽  
Different Seasons ◽  
Spread F ◽  
Bottom Side

Abstract. A study has been carried out on the occurrence of bottom side equatorial spread F (ESF) and its dependence on the polarity and magnitude of the thermospheric meridional wind just prior to ESF occurrence during summer, winter and equinox seasons of solar maximum (2002) and minimum years (1995), using ionosonde data of Trivandrum (8.5° N, 76.5° E, dip=0.5° N) and SHAR (13.7° N, 80.2° E, dip ~5.5° N) in the Indian longitude sector. In this study, we have examined the changes in the threshold height of the base of the F layer for the triggering of ESF, irrespective of the magnitude and polarity of the meridional winds during the above periods. The study indicates that the threshold height above which ESF triggering is entirely controlled only by the collisional R-T instability is least for summer months, with higher values for winter and equinox, during the solar minimum period, whereas for the solar maximum period the threshold height is least for winter, with higher values for summer and equinox. But the range over which the threshold height varies is very narrow (<15 km) for solar minimum in relation to the large range of variation (>50 km) in the solar maximum epoch. Further to this, the study also reveals a clear-cut increase in threshold height with solar activity for all seasons. Clear-cut seasonal variability is also observed in the threshold height, especially for solar maximum. The study quantifies the level of the base of the F layer below which neutral dynamical effects play a decisive role in the triggering of ESF during different seasons and solar epochs.

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 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 Meteor heights during the recent solar minimum

2014 ◽  
Vol 12 ◽  
pp. 161-165 ◽  
Author(s):  
Ch. Jacobi
Keyword(s):  
Solar Cycle ◽  
Solar Minimum ◽  
Middle Atmosphere ◽  
Low Frequency ◽  
Late Summer ◽  
Vhf Radar ◽  
Linear Temperature ◽  
Reflection Height ◽  
Atmosphere Temperature

Abstract. Average meteor heights have been continuously observed using a SKiYMET VHF radar at Collm (51.3° N, 13.0° E) since late summer of 2004. Initially, the daily mean meteor height was about 89.4 km. Since that time, average meteor heights have decreased. This is consistent with earlier results on middle atmosphere temperature change from the literature and from earlier results of low-frequency reflection height changes measured at Kühlungsborn and Collm. During the recent solar minimum 2008/2009 the meteor heights further decreased. Linear fitting of a trend and a solar cycle to the heights reveals a linear decrease of about −56 m year−1 and a solar cycle effect of +450 m per 100 sfu. Assuming that meteor heights, on a long-term average, approximately refer to a level of constant pressure, this decrease can be converted to a mean middle atmosphere linear temperature decrease of −0.23 K year−1 and a solar cycle effect of +1.8 K per 100 sfu during the last decade, which is in the range of observed trends reported in the literature.

scholarly journals Solar cycle dependent characteristics of the equatorial blanketing E<sub><i>s</i></sub> layers and associated irregularities

2006 ◽  
Vol 24 (11) ◽  
pp. 2931-2947 ◽  
Author(s):  
C. V. Devasia ◽  
V. Sreeja ◽  
S. Ravindran
Keyword(s):  
Solar Activity ◽  
Solar Cycle ◽  
Electric Field ◽  
Solar Minimum ◽  
Solar Maximum ◽  
Dominant Role ◽  
Onset Time ◽  
Vhf Radar ◽  
Cycle Dependent ◽  
June July

Abstract. The occurrence of blanketing type Es (Esb) layers and associated E-region irregularities over the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip ~0.5°) during the summer solstitial months of May, June, July and August has been investigated in detail for the period 1986–2000 to bring out the variabilities in their characteristics with the solar cycle changes. The study has been made using the ionosonde and magnetometer data of Trivandrum from 1986–2000 along with the available data from the 54.95 MHz VHF backscatter radar at Trivandrum for the period 1995–2000. The appearance of blanketing Es layers during these months is observed to be mostly in association with the occurrence of afternoon Counter Electrojet (CEJ) events. The physical process leading to the occurrence of a CEJ event is mainly controlled by the nature of the prevailing electro dynamical/neutral dynamical conditions before the event. Hence it is natural that the Esb layer characteristics like the frequency of occurrence, onset time, intensity, nature of gradients in its top and bottom sides etc are also affected by the nature of the background electro dynamical /neutral dynamical processes which in turn are strongly controlled by the solar activity changes. The occurrence of Esb layers during the solstitial months is found to show very strong solar activity dependence with the occurrence frequency being very large during the solar minimum years and very low during solar maximum years. The intensity of the VHF radar backscattered signals from the Esb irregularities is observed to be controlled by the relative roles of the direction and magnitude of the prevailing vertical polarization electric field and the vertical electron density gradient of the prevailing Esb layer depending on the phase of the solar cycle. The gradient of the Esb layer shows a more dominant role in the generation of gradient instabilities during solar minimum periods while it is the electric field that has a more dominant role during solar maximum periods.

scholarly journals The effects of the pre-reversal ExB drift, the EIA asymmetry, and magnetic activity on the equatorial spread F during solar maximum

2005 ◽  
Vol 23 (3) ◽  
pp. 745-751 ◽  
Author(s):  
C.-C. Lee ◽  
J.-Y. Liu ◽  
B. W. Reinisch ◽  
W.-S. Chen ◽  
F.-D. Chu
Keyword(s):  
Solar Maximum ◽  
Magnetic Activity ◽  
The West ◽  
West Side ◽  
Gps Receivers ◽  
Equatorial Spread F ◽  
Equatorial Ionization Anomaly ◽  
Spread F ◽  
A Chain ◽  
June July

Abstract. We use a digisonde at Jicamarca and a chain of GPS receivers on the west side of South America to investigate the effects of the pre-reversal enhancement (PRE) in ExB drift, the asymmetry (Ia) of equatorial ionization anomaly (EIA), and the magnetic activity (Kp) on the generation of equatorial spread F (ESF). Results show that the ESF appears frequently in summer (November, December, January, and February) and equinoctial (March, April, September, and October) months, but rarely in winter (May, June, July, and August) months. The seasonal variation in the ESF is associated with those in the PRE ExB drift and Ia. The larger ExB drift (>20m/s) and smaller |Ia| (<0.3) in summer and equinoctial months provide a preferable condition to development the ESF. Conversely, the smaller ExB drift and larger |Ia| are responsible for the lower ESF occurrence in winter months. Regarding the effects of magnetic activity, the ESF occurrence decreases with increasing Kp in the equinoctial and winter months, but not in the summer months. Furthermore, the larger and smaller ExB drifts are presented under the quiet (Kp<3) and disturbed (Kp≥3) conditions, respectively. These results indicate that the suppression in ESF and the decrease in ExB drifts are mainly caused by the decrease in the eastward electric field.

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