Relationships between pre-sunset electrojet strength, pre-reversal enhancement and equatorial spread-F onset

Abstract. The virtual height of the bottom side F-region (h'F) and equatorial spread-F (ESF) onsets at Chumphon (10.7° N, 99.4° E; 3.3° N magnetic latitude) were compared with the behaviour of equatorial electrojet (EEJ) ground strength at Phuket (8.1° N, 98.3° E; 0.1° N magnetic latitude) during the period from November 2007 to October 2008. Increase in the F-layer height and ESF onsets during the evening hours were well connected with the EEJ ground strength before sunset, namely, both the height increase and ESF onsets were suppressed when the integrated EEJ ground strength for the period from 1 to 2 h prior to sunset was negative. The finding suggests observationally that the pre-sunset E-region dynamo current and/or electric field are related to the F-region dynamics and ESF onsets around sunset.

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Multistation digisonde observations of equatorial spread F in South America

Annales Geophysicae ◽

10.5194/angeo-22-3145-2004 ◽

2004 ◽

Vol 22 (9) ◽

pp. 3145-3153 ◽

Cited By ~ 39

Author(s):

B. W. Reinisch ◽

M. Abdu ◽

I. Batista ◽

G. S. Sales ◽

G. Khmyrov ◽

...

Keyword(s):

South America ◽

Field Line ◽

Magnetic Equator ◽

Simple Correlation ◽

Equatorial Spread F ◽

F Region ◽

Sporadic E Layers ◽

Spread F ◽

E Region ◽

Sporadic E

Abstract. Directional ionogram and F-region drift observations were conducted at seven digisonde stations in South America during the COPEX campaign from October to December 2002. Five stations in Brazil, one in Argentina, and one in Peru, monitored the ionosphere across the continent to study the onset and development of F-region density depletions that cause equatorial spread F (ESF). New ionosonde techniques quantitatively describe the prereversal uplifting of the F layer at the magnetic equator and the eastward motion of the depletions over the stations. Three of the Brazilian stations were located along a field line with a 350-km apex over the equator to investigate the relation of the occurrence of ESF and the presence of sporadic E-layers at the two E-region intersections of the field line. No simple correlation was found.

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Observational evidence for the plausible linkage of Equatorial Electrojet (EEJ) electric field variations with the post sunset F-region electrodynamics

Annales Geophysicae ◽

10.5194/angeo-27-4229-2009 ◽

2009 ◽

Vol 27 (11) ◽

pp. 4229-4238 ◽

Cited By ~ 18

Author(s):

V. Sreeja ◽

C. V. Devasia ◽

◽

Keyword(s):

Observational Study ◽

Electric Field ◽

Solar Maximum ◽

Equatorial Electrojet ◽

Observational Evidence ◽

Related Parameter ◽

F Region ◽

Seasonal Characteristics ◽

Vertical Drift ◽

Spread F

Abstract. The paper is based on a detailed observational study of the Equatorial Spread F (ESF) events on geomagnetically quiet (Ap≤20) days of the solar maximum (2001), moderate (2004) and minimum (2006) years using the ionograms and magnetograms from the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip lat ~0.5° N) in India. The study brings out some interesting aspects of the daytime Equatorial Electrojet (EEJ) related electric field variations and the post sunset F-region electrodynamics governing the nature of seasonal characteristics of the ESF phenomena during these years. The observed results seem to indicate a plausible linkage of daytime EEJ related electric field variations with pre-reversal enhancement which in turn is related to the occurrence of ESF. These electric field variations are shown to be better represented through a parameter, termed as "E", in the context of possible coupling between the E- and F-regions of the ionosphere. The observed similarities in the gross features of the variations in the parameter "E" and the F-region vertical drift (Vz) point towards the potential usage of the EEJ related parameter "E" as an useful index for the assessment of Vz prior to the occurrence of ESF.

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VHF radar observations of the dip equatorial E-region during sunset in the Brazilian sector

Annales Geophysicae ◽

10.5194/angeo-24-1617-2006 ◽

2006 ◽

Vol 24 (6) ◽

pp. 1617-1623 ◽

Cited By ~ 7

Author(s):

C. M. Denardini ◽

M. A. Abdu ◽

E. R. de Paula ◽

C. M. Wrasse ◽

J. H. A. Sobral

Keyword(s):

Electric Field ◽

Equatorial Electrojet ◽

Magnetic Activity ◽

Maximum Intensity ◽

Vhf Radar ◽

F Region ◽

Vertical Electric Field ◽

Tentative Explanation ◽

E Region ◽

Activity Condition

Abstract. Using the RESCO 50 MHz backscatter radar (2.33° S, 44.2° W, DIP: –0.5), at São Luís, Brazil, we obtained Range Time Intensity (RTI) maps covering the equatorial electrojet heights during daytime and evening. These maps revealed a scattering region at an altitude of about 108 km during the sunset period. The type of 3-m irregularity region we present here has not been reported before in the literature, to our knowledge. It was mainly observed around the Southern Hemisphere summer-solstice period, under quiet magnetic activity condition. The occurrence of this echo region coincides in local time with the maximum intensity of an evening pre-reversal eastward electric field of the ionospheric F-region. A tentative explanation is proposed here in terms of the theory of the divergence of the equatorial electrojet (EEJ) current in the evening ionosphere presented by Haerendel and Eccles (1992), to explain the partial contribution of the divergence to the development of the pre-reversal electric field. The theory predicts an enhanced zonal electric field and hence a vertical electric field below 300 km as a consequence of the EEJ divergence in the evening. The experimental results of the enhanced echoes from the higher heights of the EEJ region seem to provide evidence that the divergence of the EEJ current can indeed be the driver of the observed scattering region.

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Evidence for a large scale electric field gradient at the onset of equatorial spread-F

Journal of Atmospheric and Terrestrial Physics ◽

10.1016/0021-9169(84)90120-x ◽

1984 ◽

Vol 46 (4) ◽

pp. 355-362 ◽

Cited By ~ 21

Author(s):

R Raghavarao ◽

J.N Desai ◽

B.G Anandarao ◽

R Narayanan ◽

R Sekar ◽

...

Keyword(s):

Electric Field ◽

Electric Field Gradient ◽

Field Gradient ◽

Large Scale ◽

Equatorial Spread F ◽

Spread F

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Radar and satellite investigations of equatorial evening vertical drifts and spread F

Annales Geophysicae ◽

10.5194/angeo-33-1403-2015 ◽

2015 ◽

Vol 33 (11) ◽

pp. 1403-1412 ◽

Cited By ~ 10

Author(s):

J. M. Smith ◽

F. S. Rodrigues ◽

E. R. de Paula

Keyword(s):

Local Time ◽

Empirical Model ◽

Sufficient Condition ◽

Satellite Measurements ◽

Equatorial Spread F ◽

Annual Variability ◽

F Region ◽

Plasma Drifts ◽

Spread F ◽

Coherent Backscatter

Abstract. We analyzed pre-midnight equatorial F region observations made by the 30 MHz coherent backscatter radar of São Luis, Brazil between August 2010 and February 2012. These measurements were processed, and used to create monthly maps of the echo occurrence as a function of local time and height. The maps show the inter-annual variability associated with equatorial spread F (ESF) occurrence in the Brazilian longitude sector. We also constructed monthly curves of the evening vertical drifts, for the Brazilian sector, using measurements by the ion velocity meter (IVM) onboard the C/NOFS satellite. The IVM evening drifts show a good overall agreement with the Scherliess and Fejer (1999) empirical model. Measured and model drifts show the development of the pre-reversal enhancement (PRE) of the vertical plasma drifts during ESF season. Using joint radar and satellite measurements, we found that evening (18:00–18:30 LT) mean non-negative drifts provide a necessary but not sufficient condition for the occurrence of topside ESF echoes. Evening downward (negative) drifts preceded the absence of topside ESF irregularities.

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Investigation on the role of E-F region coupling processes on the generation of nighttime MSTIDs: Case studies over northern Germany

10.5194/egusphere-egu21-12929 ◽

2021 ◽

Author(s):

Mani Sivakandan ◽

Jorge L Chau ◽

Carlos Martinis ◽

Yuichi Otsuka ◽

Jens Mielich ◽

...

Keyword(s):

Gps Tec ◽

F Region ◽

Perkins Instability ◽

Sporadic E Layers ◽

Spread F ◽

E Region ◽

Sporadic E ◽

Low Latitude Ionosphere ◽

Atmospheric Gravity

Northwest to southeast phase fronts with southwestward moving features are commonly observed in the nighttime midlatitude ionosphere during the solstice months at low solar activity. These features are identified as nighttime MSTIDs (medium scale traveling ionospheric disturbances). Initially, they were considered to be a manifestation of neutral atmospheric gravity waves. Later on, investigations showed that the nighttime MSTIDs are electrified in nature and mostly confined to the mid and low latitude ionosphere. Although the overall characteristics of the nighttime MSTIDs are mostly well understood, the causative mechanisms are not well known. Perkins instability mechanism was believed to be the cause of nighttime MSTIDs, however, the growth rate of the instability is too small to explain the perturbations observed. Recently, model simulations and observational studies suggest that coupling between sporadic-E layers and other type of E-region instabilities, and the F region may be relevant to explain the generation of the MSTIDs.In the present study simultaneous observation from OI 630 nm all-sky airglow imager, GPS-TEC, ionosonde and Meteor radars, are used to investigate the role of E and F region coupling on the generation of MSTIDs .Nighttime MSTIDs observed on three nights (14 March 2020, 23 March 2020 and 28 May 2020) in the OI 630 nm airglow images over Kuehlungsborn (54&#176;07'N; 11&#176;46'E, 53.79N&#160; mag latitude), Germany, are presented. Simultaneous detrended GPS-TEC measurements also shows presence of MSTIDs on these nights. In addition, simultaneous ionosonde observations over Juliusruh (54&#176;37.7'N 13&#176;22.5'E) show spread-F in the ionograms as well as sporadic-E layer occurrence.&#160; Furthermore, we also investigate the MLT region wind variations during these nights. The role of Es-layers and the interplay between the winds and Es-layers role on the generation of the MSTIDs will be discussed in detail in this presentation.&#160;

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

Annales Geophysicae ◽

10.5194/angeo-25-855-2007 ◽

2007 ◽

Vol 25 (4) ◽

pp. 855-861 ◽

Cited By ~ 25

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.

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