Equatorial spread<i>F</i>: Implications of VHF radar observations

Farley, D. T.; Balsey, B. B.; Woodman, R. F.; McClure, J. P.

doi:10.1029/ja075i034p07199

Cited by 440 publications

(245 citation statements)

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“…It is well-known that the height of the F -region ionosphere is the major parameter in controlling the onset of equatorial spread F (ESF) (Farley et al, 1970). This relationship is due to the fact that the growth rate for the generalized RayleighTaylor instability is inversely proportional to the ion-neutral collision frequency, which decreases exponentially with altitude (Kelley et al, 1979a).…”

Section: Introductionmentioning

confidence: 99%

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Observations and modeling of post-midnight uplifts near the magnetic equator

et al. 2006

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Abstract. We report here on post-midnight uplifts near the magnetic equator. We present observational evidence from digital ionosondes in Brazil, a digisonde in Peru, and other measurements at the Jicamarca Radio Observatory that show that these uplifts occur fairly regularly in the post-midnight period, raising the ionosphere by tens of kilometers in the most mild events and by over a hundred kilometers in the most severe events. We show that in general the uplifts are not the result of a zonal electric field reversal, and demonstrate instead that the uplifts occur as the ionospheric response to a decreasing westward electric field in conjunction with sufficient recombination and plasma flux. The decreasing westward electric field may be caused by a change in the wind system related to the midnight pressure bulge, which is associated with the midnight temperature maximum. In order to agree with observations from Jicamarca and Palmas, Brazil, it is shown that there must exist sufficient horizontal plasma flux associated with the pressure bulge. In addition, we show that the uplifts may be correlated with a secondary maximum in the spread-F occurrence rate in the post-midnight period. The uplifts are strongly seasonally dependent, presumably according to the seasonal dependence of the midnight pressure bulge, which leads to the necessary small westward field in the post-midnight period during certain seasons. We also discuss the enhancement of the uplifts associated with increased geomagnetic activity, which may be related to disturbance dynamo winds. Finally, we show that it is possible using simple numerical techniques to estimate the horizontal plasma flux and the vertical drift velocity from electron density measurements in the post-midnight period.

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Section: Introductionmentioning

confidence: 99%

“…Hysell and Burcham (2002) divides the pre-sunrise ESF into two categories. The first, as described by Farley et al (1970) and MacDougall et al (1998), occur just before sunrise near the F peak. The other may be due to "dead bubbles".…”

Section: Equatorial Spread-fmentioning

confidence: 99%

Observations and modeling of post-midnight uplifts near the magnetic equator

et al. 2006

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“…Because the K p , D st , AU, AL indices are defined by the different magnetospheric and ionospheric currents one needs to decide which of these is the best indicator for predicting the equatorial scintillations. Comparing the VLF drift velocities and VHF scatter on the Jicamarca radar, the idea was suggested by Farley et al (1970) of a threshold height above which the F -layer has to go before irregularities associated with spread F can be generated. Basu et al (1977) proposed that the observed scintillation activity at the equator and patches of activity off the equator may perhaps be due to varying thickness of the layers of irregularities in the transionospheric propagation of the signal.…”

Section: Discussionmentioning

confidence: 99%

Role of the magnetospheric and ionospheric currents in the generation of the equatorial scintillations during geomagnetic storms

Biktash

2004

Ann. Geophys.

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Abstract. The equatorial ionosphere parameters, K p , D st , AU and AL indices characterized contribution of different magnetospheric and ionospheric currents to the Hcomponent of geomagnetic field are examined to test the geomagnetic activity effect on the generation of ionospheric irregularities producing VLF scintillations. According to the results of the current statistical studies, one can predict near 70% of scintillations from Aarons' criteria using the D st index, which mainly depicts the magnetospheric ring current field. To amplify Aarons' criteria or to propose new criteria for predicting scintillation characteristics is the question. In the present phase of the experimental investigations of electron density irregularities in the ionosphere new ways are opened up because observations in the interaction between the solar wind -magnetosphere -ionosphere during magnetic storms have progressed greatly. According to present view, the intensity of the electric fields and currents at the polar regions, as well as the magnetospheric ring current intensity, are strongly dependent on the variations of the interplanetary magnetic field. The magnetospheric ring current cannot directly penetrate the equatorial ionosphere and because of this difficulties emerge in explaining its relation to scintillation activity. On the other hand, the equatorial scintillations can be observed in the absence of the magnetospheric ring current. It is shown that in addition to Aarons' criteria for the prediction of the ionospheric scintillations, models can be used to explain the relationship between the equatorial ionospheric parameters, h F , f oF 2, and the equatorial geomagnetic variations with the polar ionosphere currents and the solar wind.

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“…This rocket experiment along with the observations from a 50 MHz radar, provided the ®rst evidence that the regions of depleted plasma density rise due to a buoyancy force and attain altitudes far above the region unstable to the Rayleigh Taylor (RT ) instability. This partly explained the high altitude spread F, observed by Farley et al (1970), in terms of primary instability operating below the peak in F region electron density (F peak ). Costa and Kelley (1978) analyzed the bottom side electron density data, obtained from the same¯ight, and found that the steepened coherent structures exhibited shallow spectra, with k )2 power law behavior in intermediate (10 km > k > 100 m) and transitional (100 m > k > 10 m) scales instead of a turbulent cascade of structures.…”

Section: Introduction and Literature Reviewmentioning

confidence: 97%

“…Various radio, optical and in situ techniques have, therefore, been employed to understand di erent aspects of irregularities associated with ESF. Ground based observations of coherent VHF radar from Jicamarca (Farley et al, 1970) renewed the interest in this ®eld. The optical techniques which have contributed signi®cantly, include, all sky imagers and scanning photometers for measuring the zonal drift of large-scale irregularities (Mendillo and Baumgardener, 1982;Sinha et al, 1996, Taylor et al, 1997Tinsley et al, 1997, and references therein).…”

Section: Introduction and Literature Reviewmentioning

confidence: 99%

Some new features of electron density irregularities over SHAR during strong spread F

Raizada

Sinha

2000

Annales Geophysicae

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Abstract. An RH-560 rocket¯ight was conducted from Sriharikota rocket range (SHAR) (14°N, 80°E, dip latitude 5.5°N) to study electron density and electric ®eld irregularities during spread F. The rocket was launched at 2130 local time (LT) and it attained an apogee of 348 km. Results of electron density¯uctuations are presented here. Two extremely sharp layers of very high electron density were observed at 105 and 130 km. The electron density increase in these layers was by a factor of 50 in a vertical extent of 10 km. Large depletions in electron density were observed around 175 and 238 km. Both sharp layers as well as depletions were observed also during the descent. The presence of sharp layers and depletions during the ascent and the descent of the rocket as well as an order of magnitude less electron density, in 150±300 km region during the descent, indicate the presence of strong large-scale horizontal gradients in the electron density. Some of the valley region irregularities (165±178 km), in the intermediate scale size range, observed during this¯ight, show spectral peaks at 2 km and can be interpreted in terms of the image striation theory suggested by Vickrey et al. The irregularities at 176 km do not exhibit any peak at kilometer scales and appear to be of new type. The growth rate of intermediate scale size irregularities, produced through generalized Rayleigh Taylor instability, was calculated for the 200± 330 km altitude, using observed values of electron density gradients and an assumed vertically downward wind of 20 ms )1 . These growth rate calculations suggest that the observed irregularities could be produced by the gradient drift instability.

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Equatorial spreadF: Implications of VHF radar observations

Cited by 440 publications

References 34 publications

Observations and modeling of post-midnight uplifts near the magnetic equator

Observations and modeling of post-midnight uplifts near the magnetic equator

Role of the magnetospheric and ionospheric currents in the generation of the equatorial scintillations during geomagnetic storms

Some new features of electron density irregularities over SHAR during strong spread F

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