scholarly journals Equatorial ionosphere bottom-type spread F observed by OI 630.0 nm airglow imaging

2010 ◽  
Vol 37 (3) ◽  
pp. n/a-n/a ◽  
Author(s):  
H. Takahashi ◽  
M. A. Abdu ◽  
M. J. Taylor ◽  
P.-D. Pautet ◽  
E. de Paula ◽  
...  
Keyword(s):  
Equatorial Ionosphere ◽  
Bottom Type ◽  
Spread F

scholarly journals Bottom-type scattering layers and equatorial spread <i>F</i>

2004 ◽  
Vol 22 (12) ◽  
pp. 4061-4069 ◽  
Author(s):  
D. L. Hysell ◽  
J. Chun ◽  
J. L. Chau
Keyword(s):  
Large Scale ◽  
Coherent Scattering ◽  
Plasma Waves ◽  
Equatorial Ionosphere ◽  
Bottom Type ◽  
Radar Images ◽  
F Region ◽  
Type Layer ◽  
Spread F ◽  
Gradient Drift

Abstract. Jicamarca radar observations of bottom-type coherent scattering layers in the post-sunset bottomside F-region ionosphere are presented and analyzed. The morphology of the primary waves seen in radar images of the layers supports the hypothesis of kudeki+bhattacharyya-1999 that wind-driven gradient drift instabilities are operating. In one layer event when topside spread F did not occur, irregularities were distributed uniformly in space throughout the layers. In another event when topside spread F did eventually occur, the irregularities within the pre-existing bottom-type layers were horizontally clustered, with clusters separated by about 30km. The same horizontal periodicity was evident in the radar plumes and large-scale irregularities that emerged later in the event. We surmise that horizontal periodicity in bottom-type layer irregularity distribution is indicative of large-scale horizontal waves in the bottomside F-region that may serve as seed waves for large-scale Rayleigh Taylor instabilities. Key words. Ionosphere (equatorial ionosphere; ionospheric irregularties; plasma waves and instabilities)

scholarly journals Coherent backscatter radar imaging in Brazil: large-scale waves in the bottomside F-region at the onset of equatorial spread <I>F</I>

2008 ◽  
Vol 26 (11) ◽  
pp. 3355-3364 ◽  
Author(s):  
F. S. Rodrigues ◽  
D. L. Hysell ◽  
E. R. de Paula
Keyword(s):  
Large Scale ◽  
Imaging Technique ◽  
Radar Imaging ◽  
Shear Instability ◽  
Wide Field ◽  
Bottom Type ◽  
F Region ◽  
Type Layer ◽  
Spread F ◽  
Coherent Backscatter

Abstract. The 30 MHz coherent backscatter radar located at the equatorial observatory in São Luís, Brazil (2.59° S, 44.21° W, −2.35° dip lat) has been upgraded to perform coherent backscatter radar imaging. The wide field-of-view of this radar makes it well suited for radar imaging studies of ionospheric irregularities. Radar imaging observations were made in support to the spread F Experiment (SpreadFEx) campaign. This paper describes the system and imaging technique and presents results from a bottom-type layer that preceded fully-developed radar plumes on 25 October 2005. The radar imaging technique was able to resolve decakilometric structures within the bottom-type layer. These structures indicate the presence of large-scale waves (~35 km) in the bottomside F-region with phases that are alternately stable and unstable to wind-driven gradient drift instabilities. The observations suggest that these waves can also cause the initial perturbation necessary to initiate the Generalized Rayleigh-Taylor instability leading to spread F. The electrodynamic conditions and the scale length of the bottom-type layer structures suggest that the waves were generated by the collisional shear instability. These results indicate that monitoring bottom-type layers may provide helpful diagnostics for spread F forecasting.

scholarly journals Semimonthly oscillation observed in the start time of equatorial Spread-F

2019 ◽  
Author(s):  
Igo Paulino ◽  
Ana Roberta Paulino ◽  
Ricardo Yvan de la Cruz Cueva ◽  
Ebenezer Agyei-Yeboah ◽  
Ricardo Arlen Buriti ◽  
...  
Keyword(s):  
Radar Data ◽  
Ionospheric Irregularities ◽  
Start Time ◽  
Bottom Type ◽  
Equatorial Spread F ◽  
Complete Cycle ◽  
Plasma Bubbles ◽  
Spread F ◽  
Using Data ◽  
Coherent Backscatter

Abstract. Using data from airglow an all sky imager and a coherent backscatter radar deployed at São João do Cariri (7.4° S, 36.5° W) and São Luís (2.6° S, 44.2° W), respectively, the start time of equatorial Spread-F were studied. Data from a period of over 10 years was investigated from 2000 to 2010. The semimonthly oscillations were clearly revealed in the start time of plasma bubbles from Oi6300 airglow images during three periods (September 2003, September–October 2005, November 2005 and January 2008). Since the airglow measurements are not continuous in time, more than one cycle of oscillation in the start time of plasma bubbles cannot be observed from these data. Thus, coherent backscatter radar data appeared as an alternative to investigate the start time of the ionospheric irregularities. Semimonthly oscillation were observed in the start time of plumes (November 2005) and bottom type Spread-F (November 2008) with at least one complete cycle. Technical/climate issues did not allowed to observe the semimonthly oscillations simultaneously by the two instruments, but from September to December 2005 there was a predominance of this spread-F start time oscillation over Brazil. The presence of this oscillation certainly contribute to the day-to-day variability of spread-F.

scholarly journals Concurrent study of bottomside spread F and plasma bubble events in the equatorial ionosphere during solar maximum using digisonde and ROCSAT-1

2005 ◽  
Vol 23 (11) ◽  
pp. 3473-3480 ◽  
Author(s):  
C.-C. Lee ◽  
S.-Y. Su ◽  
B. W. Reinisch
Keyword(s):  
Drift Velocity ◽  
Geomagnetic Activity ◽  
Equatorial Ionosphere ◽  
Driving Mechanism ◽  
Plasma Bubble ◽  
Geomagnetic Effect ◽  
Vertical Drift ◽  
Spread F ◽  
Abstract Data ◽  
Concurrent Study

Abstract. Data from the Jicamarca digisonde and the ROCSAT-1 satellite are employed to study the equatorial ionosphere on the west side of South America during April 1999-March 2000 for the concurrent bottomside spread F (BSSF) and plasma bubble events. This study, using digisonde and ROCSAT-1 concurrently, is the first attempt to investigate the equatorial spread F. Results show that BSSF and plasma bubble observations appear frequently respectively in the summer (January, February, November, and December) and in the equinoctial (March, April, September, and October) months, respectively, but are both rarely observed in the winter (May-August) months. The upward drift velocity during the concurrent BSSF and bubble observations has been determined to study the driving mechanism. This analysis shows that large vertical drift velocities favor BSSF and bubble formations in the equinoctial and summer months. Conversely, the smaller upward velocities during the winter months cause fewer BSSF and bubble occurrences. For the geomagnetic effect, the BSSF/bubble occurrence decreases with an increasing Kp value in the equinoctial months, but no such correlation is found for the summer and winter months. Moreover, the anti-correlations between Kp and dh'F/dt are apparent in the equinoctial months, but not in the summer and winter months. These results indicate that in the equinoctial months the BSSF/bubble generations and the pre-reversal drift velocity can be suppressed by geomagnetic activity, because the disturbance dynamo effects could have decreased the eastward electric field near sunset. However, BSSF and bubble occurrences may not be suppressed by the geomagnetic activity in the summer and winter months.

scholarly journals Rocket and radar investigation of background electrodynamics and bottom-type scattering layers at the onset of equatorial spread <i>F</i>

2006 ◽  
Vol 24 (5) ◽  
pp. 1387-1400 ◽  
Author(s):  
D. L. Hysell ◽  
M. F. Larsen ◽  
C. M. Swenson ◽  
A. Barjatya ◽  
T. F. Wheeler ◽  
...  
Keyword(s):  
Electric Fields ◽  
Large Scale ◽  
Layer Structure ◽  
Horizontal Structure ◽  
Bottom Type ◽  
Chemical Release ◽  
Spread F ◽  
Density Vector ◽  
Plasma Depletions ◽  
Electron Density Profiles

Abstract. Sounding rocket experiments were conducted during the NASA EQUIS II campaign on Kwajalein Atoll designed to elucidate the electrodynamics and layer structure of the postsunset equatorial F region ionosphere prior to the onset of equatorial spread F (ESF). Experiments took place on 7 and 15 August 2004, each comprised of the launch of an instrumented and two chemical release sounding rockets. The instrumented rockets measured plasma number density, vector electric fields, and other parameters to an apogee of about 450 km. The chemical release rockets deployed trails of trimethyl aluminum (TMA) which yielded wind profile measurements. The Altair radar was used to monitor coherent and incoherent scatter in UHF and VHF bands. Electron density profiles were also measured with rocket beacons and an ionosonde. Strong plasma shear flow was evident in both experiments. Bottom-type scattering layers were observed mainly in the valley region, below the shear nodes, in westward-drifting plasma strata. The layers were likely produced by wind-driven interchange instabilities as proposed by Kudeki and Bhattacharyya (1999). In both experiments, the layers were patchy and distributed periodically in space. Their horizontal structure was similar to that of the large-scale plasma depletions that formed later at higher altitude during ESF conditions. We argue that the bottom-type layers were modulated by the same large-scale waves that seeded the ESF. A scenario where the large-scale waves were themselves produced by collisional shear instabilities is described.

scholarly journals <i>Letter to the Editor</i>: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

1999 ◽  
Vol 17 (8) ◽  
pp. 1111-1115 ◽  
Author(s):  
J. H. Sastri
Keyword(s):  
Solar Activity ◽  
Solar Minimum ◽  
Equatorial Ionosphere ◽  
Instability Mechanism ◽  
Layer Height ◽  
Northern Summer ◽  
June Solstice ◽  
Spread F ◽  
Rayleigh Taylor Instability ◽  
Distinct Increase

Abstract. At dip equatorial stations in the Indian zone, spread-F conditions are known to develop preferentially around midnight during the June solstice (northern summer) months of low solar activity, in association with a distinct increase in F layer height. It is currently held that this onset of spread-F far away from the sunset terminator is due to the generalised Rayleigh-Taylor instability mechanism, with the gravitational and cross-field instability factors (and hence F layer height) playing important roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N; 77.5°E; dip 4°N) for the northern summer months (May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions against which the post-midnight onset of spread-F takes place. A data sample of 38 nights with midnight onset of spread-F and 34 nights without spread-F is used for the purpose. It is found that a conspicious increase in F layer height beginning around 2100 LT occurs on nights with spread-F as well as without spread-F. This feature is seen in the nocturnal pattern of F layer height on many individual nights as well as of average F layer height for the two categories of nights. The result strongly suggests that the F layer height does not play a pivotal role in the midnight onset of spread-F during the June solstice of solar minimum. The implications of this finding are discussed.Key words. Ionosphere (equatorial ionosphere; ionospheric irregularities)

scholarly journals Effect of equatorial ionization anomaly on the occurrence of spread-F

1997 ◽  
Vol 15 (2) ◽  
pp. 255-262 ◽  
Author(s):  
P. T. Jayachandran ◽  
P. Sri Ram ◽  
V. V. Somayajulu ◽  
P. V. S. Rama Rao
Keyword(s):  
Electric Field ◽  
Geomagnetic Field ◽  
Critical Frequency ◽  
Equatorial Ionosphere ◽  
Equatorial Ionization Anomaly ◽  
Spread F ◽  
Field Lines ◽  
A Chain ◽  
Indian Sector

Abstract. The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E-W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (ƒ0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.

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 On the Dynamics of Equatorial Spread F

1974 ◽  
Vol 27 (3) ◽  
pp. 391 ◽  
Author(s):  
Tom Beer
Keyword(s):  
Gravity Waves ◽  
Equatorial Ionosphere ◽  
Multistage Process ◽  
Atmospheric Gravity Waves ◽  
Equatorial Spread F ◽  
Spread F ◽  
Atmospheric Gravity ◽  
Spatial Resonance

A theory is proposed to explain the occurrence of spread F in the equatorial ionosphere. It is envisaged as a multistage process involving: movement of ionization drift with atmospheric gravity waves that results in spatial resonance, consequent enhanceme

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