On equatorial spread F: Establishing a seeding hypothesis
TL;DR: In this paper, a seeding hypothesis for the equatorial spread F (ESF) was proposed, based on the discovery that a direct link exists between regions of deep convective activity in the troposphere, where atmospheric gravity waves (GWs) are spawned, and the occurrence frequency of ESF during solstices.
Abstract: [1] A comprehensive explanation for the complex climatology of the so-called equatorial spread F (ESF) has eluded researchers for more than 70 years. Recently, however, a seeding hypothesis has been proposed, which appears to provide the final major piece of this puzzle. The hypothesis is based on the discovery that a direct link exists between regions of deep convective activity in the troposphere, where atmospheric gravity waves (GWs) are spawned, and the occurrence frequency of ESF during solstices. The objective here is to answer two questions that may impede the general acceptance of this hypothesis. We first show why seed plasma perturbations should develop from GW-driven neutral-wind perturbations, but only when the GW source region is located very close to the magnetic dip equator. We then reexamine this relationship using a data set on GW source regions that is better matched (in time and longitudinal coverage), than that used previously, to the data set on ESF activity used by Tsunoda (2010a). We conclude that seeding is indeed playing an important role in the development of ESF.
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TL;DR: In this article, the authors model the coupling of both circular (local) and plane wave (nonlocal) gravity waves to the bottomside F layer as a mechanism for triggering equatorial plasma bubbles, and support the hypothesis that nonplane gravity waves can more strongly couple to the F layer than plane gravity waves.
Abstract: [1] The Naval Research Laboratory three-dimensional simulation code SAMI3/ESF is used to study the response of the postsunset ionosphere to circular gravity waves. We model the coupling of both circular (local) and plane wave (nonlocal) gravity waves to the bottomside F layer as a mechanism for triggering equatorial plasma bubbles. Results support the hypothesis that nonplane gravity waves can more strongly couple to the F layer than plane gravity waves. Results also show that the coupling of the seed wave to the F layer depends on the (nonlocal) growth rate and the local electron density at the position of the seed wave.
831 citations
TL;DR: In this article, a new numerical algorithm was developed to automatically detect TEC bite-outs that are produced by the transit of equatorial plasma bubbles, which was applied to TEC values measured by the Low Latitude Ionospheric Sensor Network (LISN) and by receivers that belong to 3 other networks that exist in South America.
Abstract: [1] This paper presents for the first time regional plots of total electron content (TEC) depletions derived from GPS observations over the South American continent with a coverage of over 45° longitude (i.e., 35°W to 80°W). We introduce a new numerical algorithm that has been developed to automatically detect TEC bite-outs that are produced by the transit of equatorial plasma bubbles. This algorithm was applied to TEC values measured by the Low Latitude Ionospheric Sensor Network (LISN) and by receivers that belong to 3 other networks that exist in South America. The general characteristics of the TEC depletions are provided along with their temporal length, local time distribution and depletion depth. The regional day-to-day and seasonal variability of the TEC depletions are also presented for 2008, a year of low solar activity. The regional day-to-day variability of TEC depletions is highly dynamic, but their seasonal distributions follow the longitudinal characteristics of plasma bubbles presented by other authors. During the equinoxes, TEC depletions are mainly observed on the west coast of South America, and during the December solstice they mostly occur on the east side of the continent. However, in all seasons, we observe days when depletions extend all over the continent. We place these new results in the context of theories of plasma bubble seeding.
273 citations
Cites background or result from "On equatorial spread F: Establishin..."
...This is the period when the ITCZ moves farther away from the magnetic equator and no convective systems can trigger gravity waves (GWs) [Tsunoda, 2010a]....
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...…Figures 10a and 10b demonstrate that more TEC depletions develop in the western side of South America during the Equinoctial month of March and in the eastern side during December, all in agreement with satellite observations [Burke et al., 2004] and theoretical predictions [Tsunoda, 2010a]....
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...Based on this argument, several authors have concluded that the ESF morphology could be controlled by the seasonal migration of the ITCZ in latitude [Röttger 1977, 1981; McClure et al. 1998, Tsunoda, 2010b]....
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TL;DR: In this paper, a comprehensive analysis and discussion of the processes of ESF development, suppression or disruption under different phases of a storm activity sequence is presented, and the consequences for ESF occurrence from undershielding and over-shielding penetration electric fields as well as from the disturbance winds and wind dynamo electric field occurring in different local time sectors of the night, as also the irregularity dynamics and longitude extension.
121 citations
TL;DR: In this article, the GPS total electron content (TEC) fluctuations obtained from a global GPS network and spread F in ionograms from Jicamarca (283°E, 12°S, Dip 1°N) in the American longitude sector and Kwajalein (167°E and 9°N, Dip 4°N).
Abstract: [1] Postmidnight equatorial F region irregularities (EFIs) are known to develop mainly during the solstitial months. However, it is not well understood whether they occur at all longitudes and what process causes their occurrence at different longitude sectors. In this study, we use the GPS total electron content (TEC) fluctuations obtained from a global GPS network and spread F in ionograms from Jicamarca (283°E, 12°S, Dip 1°N) in the American longitude sector and Kwajalein (167°E, 9°N, Dip 4°N), Bac Lieu (106°E, 9°N, Dip 2°N), and Chumphon (99°E, 11°N, Dip 3°N) in the Pacific and Asian longitude sectors during 2000–2009, to investigate the EFI characteristics during June solstice. Results from global TEC fluctuations show that at solar maximum, the occurrence rate of postmidnight EFIs is high in African and Pacific regions, moderate in the Southeast Asian region, and low in the Peruvian region and that most postmidnight EFIs are the continuation of postsunset EFIs. During solar minimum the postmidnight EFIs were rarely observed in TEC but were very frequent in ionograms. The latter had more frequent postmidnight onsets over Peru, whereas they were initiated during late postsunset hours in Pacific and Southeast Asian longitudes. In both longitudes the postsunset layer rise occurred with some delay. The layer rise was more prominent on spread F nights over Jicamarca and less so over Pacific longitudes. The results showing different degrees of association at the different longitudes between the postsunset/postmidnight EFIs and F layer heights highlight the influence of other factors in the late-night EFI development. Perturbation seeds and plasma drifts/neutral winds, in particular, are discussed as strong candidates for causing these irregularities in the June solstitial months of solar minimum years.
81 citations
Cites background from "On equatorial spread F: Establishin..."
...There is evidence that equatorial spreadF is not solely controlled by the F layer height rise; seeding by atmospheric gravity waves is beginning to emerge as an important contributor [Tsunoda, 2010b]....
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...However, amplification of existing LSWS can probably continue through the interchange instability [Tsunoda, 2010b]....
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TL;DR: In this paper, the authors reported on the longitudinal, local time and seasonal occurrence of equatorial plasma bubbles (EPBs) and L band (GPS) scintillations over equatorial Africa.
Abstract: . We report on the longitudinal, local time and seasonal occurrence of equatorial plasma bubbles (EPBs) and L band (GPS) scintillations over equatorial Africa. The measurements were made in 2010, as a first step toward establishing the climatology of ionospheric irregularities over Africa. The scintillation intensity is obtained by measuring the standard deviation of normalized GPS signal power. The EPBs are detected using an automated technique, where spectral analysis is used to extract and identify EPB events from the GPS TEC measurements. Overall, the observed seasonal climatology of the EPBs as well as GPS scintillations in equatorial Africa is adequately explained by geometric arguments, i.e., by the alignment of the solar terminator and local geomagnetic field, or STBA hypothesis (Tsunoda, 1985, 2010a). While plasma bubbles and scintillations are primarily observed during equinoctial periods, there are longitudinal differences in their seasonal occurrence statistics. The Atlantic sector has the most intense, longest lasting, and highest scintillation occurrence rate in-season. There is also a pronounced increase in the EPB occurrence rate during the June solstice moving west to east. In Africa, the seasonal occurrence shifts towards boreal summer solstice, with fewer occurrences and shorter durations in equinox seasons. Our results also suggest that the occurrence of plasma bubbles and GPS scintillations over Africa are well correlated, with scintillation intensity depending on depletion depth. A question remains about the possible physical mechanisms responsible for the difference in the occurrence phenomenology of EPBs and GPS scintillations between different regions in equatorial Africa.
80 citations
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Journal Article•
2,620 citations
"On equatorial spread F: Establishin..." refers methods in this paper
...Interpolation, both spatial and temporal, was performed to fill the array, as described by Liebermann and Smith [1996] ....
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TL;DR: In this paper, the proper interpretation of irregular motions in the upper atmosphere has been investigated by a variety of techniques, but their proper interpretation has yet to be established. But their proper meaning has not yet been established.
Abstract: Irregularities and irregular motions in the upper atmosphere have been detected and studied by a variety of techniques during recent years, but their proper interpretation has yet to be established...
1,886 citations
TL;DR: In this paper, the authors used radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F.
Abstract: We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts In the early night sector, the bottomside of the F layer is almost always unstable The evolution of the unstable layer is controlled by the history of the vertical drift velocity When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux
656 citations
"On equatorial spread F: Establishin..." refers background in this paper
...And, indeed, the strength of the PSSR has been shown to play a major role in ESF development [e.g., Fejer et al., 1999 ]....
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TL;DR: In this paper, the authors present fundamental climatological characteristics of the intertropical convergence zone (ITCZ) in a simple concise manner using the highly reflective cloud (HRC) dataset.
Abstract: This paper presents fundamental climatological characteristics of the intertropical convergence zone (ITCZ) in a simple concise manner using the highly reflective cloud (HRC) dataset. This satellite-derived dataset uses both visible and infrared observations to measure the frequency of occurrence of large-scale convective systems over the global tropics at a 1 deg spatial resolution. These dataset characteristics make the HRC particularly well suited for climatological analysis of the ITCZ because the dataset is based on estimates of organized deep convective cloud systems rather than observations of clouds as a whole, and it provides the spatial resolution needed to identify these large-scale convective structures. Furthermore, the dataset covers a time period extending nearly two decades, which provides for a fairly robust climatology and the opportunity to examine seasonal and interannual variability of both the convection and the latitude of the ITCZ.
538 citations
"On equatorial spread F: Establishin..." refers background or methods or result in this paper
...The OLR maps indicate broad regions of deep convective activity, similar to those found in HRC data presented by Waliser and Gautier [1993] ....
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...Estimates of the mean ITCZ location, on the other hand, was made using averages of highly reflective cloud data, obtained over a 17 yr period [ Waliser and Gautier, 1993 ]....
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...In comparison, the highly reflective cloud (HRC) data used by Waliser and Gautier [1993] include only daytime measurements....
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...(A zonally propagating plane wave is a reasonable approximation for a GW that is in the far field of its source region, which is located near the dip equator.) [8] Because most GWs are spawned from mesoscale convective cells (MCCs), which are centers of deep tropospheric convection found in the ITCZ [e.g., Waliser and Gautier, 1993 ], it seemed to follow that seeding should occur when the ITCZ is located near the dip equator....
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TL;DR: In this paper, a test of the generally accepted Rayleigh-Taylor (R-T) instability mechanism for equatorial spread F (ESF) is derived following the formalism of Haerendel (preprint, 1973) which takes into account the variations of physical parameters along geomagnetic flux tubes.
Abstract: In a test of the generally accepted Rayleigh-Taylor (R-T) instability mechanism for equatorial spread F (ESF) a linear instability growth rate γ RT is derived following the formalism of Haerendel (preprint, 1973) which takes into account the variations of physical parameters along geomagnetic flux tubes. The resulting form of γ RT extends the results of previous work by including direct dependencies on transequatorial neutral winds, zonal electric fields, vertical and horizontal ionospheric density gradients, the presence of an E region, and chemical recombination. Realistic atmospheric and ionospheric density model inputs are used for the first time to make quantitative calculations of R-T growth rates for a range of geophysical conditions. The key result of this study is that time/altitude domains having positive calculated instability growth rates are found to coincide with observed time/altitude patterns of ESF occurrence over both a monthly and a yearly time frame. This success in being able to model the climatological occurrence of ESF lends support to the physical model adopted for the instability mechanism and opens up new avenues of research into ESF predictability on a night-to-night and even an hour-to-hour basis.
490 citations
"On equatorial spread F: Establishin..." refers background in this paper
...Most attempts to explain this behavior have been in terms of variations in the linear growth rate of the generalized Rayleigh‐Taylor (GRT) instability (gGRT) [e.g., Sultan, 1996 ]....
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