An overview and synthesis of plasma irregularities in equatorial spread F

A unified picture of plasma irregularities in equatorial spread F is developed from the analysis of satellite, sounding rocket, and coherent scatter radar observations. The coherent scatter data are analyzed using a new in-beam radar imaging technique that permits direct comparison between radar data, in situ data, and computer simulations of the irregularities. Three varieties of irregularities, all produced by ionospheric interchange instabilities, are found to occur. Thin bottom-type layers are composed of waves with primary transverse wavelengths less than about 1 km and with significant parallel wavenumbers. These exist on magnetic flux tubes controlled by the E region dynamo and drift westward in the postsunset ionosphere. A nonlocal analysis is used to calculate their linear growth rate. When the F region dynamo takes control of the flux tube, bottomside irregularities can emerge. These are more robust irregularities with longer primary wavelengths and which exhibit greater vertical development. Nonlinear analyses explain the appearance of steepened structures in rocket observations and solitary waves in satellite observations of bottomside layers. The one-dimensional spectra of these irregularities obey power laws but are anisotropic and have variable spectral indices and spectral breaks. Very strong polarization electric fields can eject large regions of deeply depleted plasma through the F peak and form topside irregularities. Theoretical calculations supported by satellite data show that ion inertia may become important for topside irregularities. The one-dimensional spectra of irregularities in the inertial regime obey a k^−5/3 power law, but strong plasma inhomogeneity implies that Kolmogorov weak turbulence is not the explanation. Topside depletions are shown to bifurcate and also to pinch off from the bottomside.     

Long term studies of equatorial spread F using the JULIA radar at Jicamarca

Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.     

Longitudinal statistics of plasma bubbles observed as He+ density depletions at altitudes of the topside ionosphere

This work presents a new examination of the hypothesis regarding the equatorial origin of low He⁺ density plasma depletions (or subtroughs). For this purpose, we have conducted a detailed comparative analysis of longitudinal variations in the occurrence probabilities of subtroughs in both hemispheres and variations in the occurrence probabilities of equatorial F-region irregularities (EFIs), equatorial spread F (RFS and ESF), and equatorial plasma bubbles (EPBs). Taking into consideration the seasonal dependence and some peculiarities of magnetic field variations in different hemispheres, a conclusion has been reached regarding the similarity between longitudinal statistical occurrences of subtroughs and equatorial ionospheric F-region irregularities. In addition, another piece of evidence in favor of the similarity of the nature of the above-mentioned phenomena has been obtained. We have got a confirmation once again that low He⁺ density depletions (or subtroughs) can be rightfully considered as equatorial plasma “bubbles,” which can be observed at altitudes of the topside ionosphere as depletions in the He⁺ density.     

Equatorial spread-F (ESF) and vertical winds

The Equatorial Spread-F (ESF) phenomenon is recorded in ionograms as a hierarchy of plasma instabilities in the F-layer of the equatorial ionosphere. The ESF is characterized by irregularities in the plasma (electron and ion) density and electric field distributions perpendicular to the Earth’s magnetic field. Large scale irregularities are generated by a primary plasma instability that develops in electric fields and plasma densities. Other secondary instabilities then develop and generate irregularities at several scale sizes that often produce a plasma ‘hole’ or ‘bubble’ that rises up with high E×B velocities. The ESF/plasma bubble phenomenon has been studied extensively with experimental techniques and modeling, which revealed important features. In the bottom side F-layer, near sunset, when the vertical density gradient steepens as the layer is supported by the horizontal (North–South) Earth’s magnetic field lines against the omnipresent Earth’s gravitational acceleration (g), the plasma conditions can give rise to Rayleigh–Taylor (RT) type instability. But the observed day to day variability of the ESF occurrence suggested that other agencies may also be involved in generating the instability. Sekar and Raghavarao (1987) with linear theory, and Raghavarao, Sekar and Suhasini (1992), with non-linear numerical modeling, suggested that vertical downward (upward) winds in the ambient gas have the potential to cause (inhibit) the ESF/bubble phenomenon. The presence of downward winds near the equator was reported earlier. In this paper, we show evidence for the presence of downward winds collocated with irregularities in electric fields and plasma densities as revealed by an unique combination of highly accurate measurements with instruments onboard the DE-2 satellite. The observations reported here are also consistent with the notion that the build-up of the equatorial ionization anomaly (EIA) prior to local sunset is important for the ESF instability.     

大气重力波触发的Rayleigh－Taylor不稳定性的时空演变

对大气重力波触发的赤道电离层Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的时空演变进行了数值模拟．结果表明，重力波能在Ｆ区底部触发Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，这种等离子体不稳定扰动穿过Ｆ峰，到达Ｆ区顶部，形成等离子体泡结构．等离子体泡出现向西倾斜和分岔等特征．当夜Ｆ区较高时，产生等离子体泡的时间仅１９００ｓ．数值模拟结果证实了重力波触发．Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的理论，解释了大量电离层观测现象．     

大气重力波触发的Rayleigh－Taylor不稳定性的时空演变

对大气重力波触发的赤道电离层Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的时空演变进行了数值模拟．结果表明，重力波能在Ｆ区底部触发Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，这种等离子体不稳定扰动穿过Ｆ峰，到达Ｆ区顶部，形成等离子体泡结构．等离子体泡出现向西倾斜和分岔等特征．当夜Ｆ区较高时，产生等离子体泡的时间仅１９００ｓ．数值模拟结果证实了重力波触发．Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的理论，解释了大量电离层观测现象．     

电场产生的赤道扩展F的时空演变

利用计算机模拟研究了非均匀电场产生的赤道扩展Ｆ的时空演变。非均匀电场能在赤道电离层Ｆ区底部触发Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，导致等离子体泡形态。计算中非均匀电场的幅度取为０．２５－１．００ｍＶ／ｍ，所产生的等离子体泡在２０００ｓ以内就能穿过Ｆ峰达到５４０ｋｍ的高度。所得结果阐明了非均匀电场在赤道扩展Ｆ中的作用，指出了产生等离子体泡一种可能的扰动源。由于Ｅ区和Ｆ区电场是相互影响的，从而揭示了Ｅ区和Ｆ区扰动的相互联系。     

Observations of pre-midnight 5-m irregularities in the equatorial F region over São Luís,Brazil: Solar-flux dependence and seasonal variations

The statistics of pre-midnight 5-m irregularities in the equatorial F region over São Luís is presented. The data set ranges from October 2001 to December 2008 and covers maximum solar-flux-to-minimum solar flux epoch. The variabilities in irregularity parameters, namely, height and time of their appearance in the radar echoes, with solar-flux variation are presented. The seasonal variations (combined over all years, irrespective of solar-flux) of occurrence of irregularities, occurrence of bottom-type layer (or bottom-side irregularities without plume) and bottom-side/topside plume (or bottom-side irregularities with plume) are presented. The largest occurrences of bottom-side irregularities without plume and with plume are found on April (equinox) and December (summer) months respectively. The ambient ionospheric conditions namely prereversal evening vertical drift, bottom-side density gradient and off-equatorial E region conductivity are inferred using digisonde measurements during April 2002 and December 2002. Based on these conditions and recent studies on gravity wave climatology over Brazil, it is suggested that shear in zonal plasma drift and low gravity wave activity may account for less occurrence of plume during April as compared to December months. This suggestion is quantified using numerical simulation model of collisional-interchange instability (CII) and plasma bubble.     

Pearl diffuse structures on ionograms of Intercosmos-19 associated with irregularities of the low-latitude ionosphere

Unusual complex ionograms obtained by the Intercosmos-19 satellite are considered, in which four diffuse clouds with a characteristic shape are strung like pearls on the main path of the reflected signal. Ray tracing has been used to show that they are associated with 26 layers of irregularities located at altitudes from hmFs2 up to ~900 km. The sizes of the irregularities range from a few kilometers to 100 kilometers, and the intensity of δNe reaches 100%. The heights of irregular layers increase towards the equator, together with a rise of the F2 layer, and are not associated with magnetic field lines. Complex ionograms have been observed on the outer slope and at the top of the crest of the equatorial anomaly. They are probably caused by the processes occurring in the equatorial ionosphere.     

Numerical modeling of the behavior of super-small-scale irregularities in the ionospheric <Emphasis Type="Italic">F</Emphasis>2 layer

Using a mathematical modeling method, evolutions of super-small-scale irregularities of electron concentration stretched along the geomagnetic field which could be formed in the magnetized ionospheric plasma of the F2 layer both in a natural way and at an artificial impact on it, in particular, during heating experiments, are studied. Evolution in time of the initially formed irregularities of two types having different shape of the cross sections lateral to the magnetic field (types of direct narrow long band and with a circular cross section) is calculated. It is found that such irregularities during times tens of times shorter than the time of the electron free path time spread out and disappear, accomplishing thereby periodic attenuating oscillations. The period of these oscillations can be equal to both the period of Langmuir oscillations of electrons and the period of cyclotron oscillations of electrons depending on the irregularity type and its initial parameters.     

Scattered reflections and multiple traces in the range of 7–10 MHz on ionograms of the interkosmos-19 satellite

The scattered reflections and multiple traces regularly recorded on the topside sounding ionograms of the Interkosmos-19 satellite in the frequency range of 7–10 MHz are considered. The reflected radio signals in this frequency range appear both above and below the critical frequency of the regular layer F2. They are observed at all altitudes of the topside ionosphere from hmF2 to a satellite altitude of 1000 km. It is shown that these phenomena regularly appear at high latitudes (≥60° ILAT) and, less often, in the equatorial region. The scattered reflections indicate the presence of small-scale irregularities, and continuous traces are a consequence of total internal reflection from large-scale irregularities. Small-scale irregularities evidently form within a large-scale irregularity. Ray tracing shows that the size of large-scale irregularities is hundreds of kilometers in height and tens of kilometers in latitude. The appearance of scattered reflections and multiple traces at high latitudes is nearly independent of local time; in the equatorial region, they appear only in the interval of 20–08 LT. All of this agrees well with other observations of irregularities in the ionospheric plasma of different scales.     

Equatorial plasma bubbles at altitudes of the topside ionosphere

A consistent patter, indicating that subtroughs in the He⁺ density and plasma bubbles can be considered as phenomena of the same origin, has been obtained within the scope of the existent model of equatorial plasma bubbles. The study has been performed based on the measurements of the ISS-b satellite, which flew during the period of high solar activity. The conclusion has been made based on a comparative analysis of the characteristics of subtroughs with the parameters of the known equatorial phenomena. (1) The similarity of the LT variations in the latitude of the minimums of subtroughs in the He⁺ density has been revealed. (2) It has been displayed that the variations in the averaged depth of subtroughs change from season to season similarly to the LT variations in the average velocity of the equatorial vertical plasma drift. (3) Good correlation (R = 0.67) between the occurrence probability of subtroughs and equatorial spread F statistics, constructed as the functions of LT and month, has been obtained. (4) The obtained velocity of the possible rise of plasma irregularities (observed as regions depleted in He⁺) is in good agreement with the ionosonde, satellite, and radar measurements of the equatorial plasma bubble velocities of the same period. (5) It has been indicated that plasma irregularities, reaching the altitudes of the topside ionosphere in the low-latitude and midlatitude regions during high solar activity, are most observable as depleted regions (subtroughs) of He⁺ density.     

The mid-latitude F region at the mesoscale: some progress at last

Almost no new theoretical work has been conducted in the area of mid-latitude F-region plasma instabilities since Perkin's (J. Geophys. Res. 78 (1973) 218) linear theory. New experimental data now suggest that the nonlinear development of mid-latitude F-region structures includes large polarization electric fields which dominate the final state. Airglow and radar data show that Σ_P is greatly depleted in some regions, which is in agreement with a polarization hypothesis. We hope these new results will inspire new simulations with some anticipation of solving this perplexing but fascinating problem.     

Equatorial night-time F-region zonal electric fields

Night-time F-region vertical electrodynamic drifts at the magnetic equatorial station, Trivandrum are obtained for a period of 2 years, 1989 and 1990 (corresponding to solar cycle maximum epoch), using ionosonde h^‘F data. The seasonal variation of the vertical drift is found to be associated with the longitudinal gradients of the thermospheric zonal wind. Further, the seasonal variation of the prereversal enhancement of the vertical drift is associated with the time difference between the sunset times of the conjugate E-regions (magnetic field line linked to F-region) which is indicative of the longitudinal gradients of the conductivity (of the E-region). The vertical drifts and the causative zonal electric fields at Trivandrum are compared with those at Jicamarca and F-region zonal electric field models. It is seen that the night-time downward drift (as also the causative westward electric field) at Jicamarca is greater than that at Trivandrum. The prereversal enhancement of the drift is greater at Jicamarca than at Trivandrum during the summer and the equinoxes, whereas during the winter the opposite is the case.     

Spectacular low- and mid-latitude electrical fields and neutral winds during a superstorm

In November 2004, a major magnetic storm occurred, a lengthy portion of which was recorded by the Upper Atmospheric Radar Chain. On the 9th and 10th, the Jicamarca Radar detected the highest magnitude penetrating electric fields (±3 mV/m) and vertical drifts (±120 m/s) ever seen at this premiere facility. These large and variable drifts were highly correlated with the interplanetary magnetic and electric fields and created a double F layer on the dayside and unusual TEC behavior throughout the low-latitude zone. These solar wind-induced drifts both suppressed and generated irregularities at the magnetic equator at different times. Large-scale thermospheric disturbances were generated by high-latitude heating and tracked through the middle- to low-latitude zones where both parallel and perpendicular plasma drifts created major ionospheric changes. The auroral oval was located at a magnetic L shell of about three for many hours.     

Radar chain study of the May, 1995 storm

We summarize the main features of the ionospheric F region as observed bythe Sondrestrom, Millstone Hill, Arecibo, and Jicamarca incoherent scatter radars during the 1–5May, 1995 CEDAR Storm Study interval. This paper apparently represents the first study of amajor storm interval using the current incoherent scatter radar chain supported by the U.S.National Science Foundation. We focus most attention on 2–3 May, and include additional datafrom IMP-8, the St. Johns magnetometer, SuperDARN, and global total electron content (TEC)maps from GPS. Three intervals of likely penetration of magnetospheric electric field from high tolow latitude are identified on 2 May. A unique feature of this storm are the strong daytimeequatorward wind surges in the neutral meridional wind observed at Millstone Hill. The first ofthese (at 14 UT on 2 May) is apparently due to a travelling atmospheric disturbance launched byintense frictional and Joule heating as observed at Sondrestrom. An evening enhancement in NmF₂ (the dusk effect) is typically seen only on the first day of a geomagneticstorm. However, during this storm a strong dusk effect is seen at Millstone Hill on 2, 3, and 4May, associated with the equatorward wind surges. A penetrating eastward electric field alsocontributed to the dusk effect on 2 May. A large rise in hmF₂ at Arecibo near0000 UT on 3 May is due to the same eastward electric field, which penetrates to the equator,causing a strong upward plasma drift at Jicamarca. This apparently results in a polewardexpansion of the equatorial anomaly zones as seen in GPS total electron content, and an increasein NmF₂ at Arecibo to the largest value seen at midnight in several years.     

Long term trends in the frequency of occurrence of the F3 layer over Fortaleza,Brazil

Recent studies using model calculation and ionospheric observations have revealed the existence of an additional layer in the topside equatorial ionosphere, the F₃ layer. The observations using bottomside ionograms from locations close to the magnetic equator in Brazilian region have shown that the occurrence of the layer is very high from December to February (local summer) and from June to August (local winter). In fact, for the year 1995 the occurrence of the F₃ layer is >75% during the months of January, February and December, and it is >65% for the period of June, July and August (Geofisica Int. 39 (2000) 57). In this work, we use 25 years of data for the months of January and August to investigate how the layer occurrence varies with the magnetic dip angle and solar activity.     

Some aspects of electrostatic coupling between E and F regions relevant to plasma irregularities: A review based on recent observations

This paper provides a review on some of the electrostatic coupling effects relevant for generating/modifying plasma irregularities during nighttime in the low latitude ionosphere based on recent observations. Emphasis is given to the role of large polarization electric field associated with an unstable region affecting another region remotely located. Recent radar observations on valley region and E region irregularities from low latitudes show convincing evidence in support of effective electrostatic field coupling along the magnetic field line for their manifestation. Interestingly, the low latitude observations clearly show the ineffectiveness of plasma bubble related fringe fields in generating low latitude valley region irregularities unlike that over the dip equator. Velocity perturbations associated with the unstable low latitude E region relevant for studying the seeding of equatorial spread F are also shown. These new observations have been critically examined in the light of existing experimental knowledge and current understanding of the electrostatic coupling effects for the generation/modification of plasma irregularities in a remote region.     

Spectral analysis of plasma drift measurements from the AE-E satellite: evidence of an inertial subrange in equatorial spread F

Plasma drift data from the AE-E satellite are spectrally analyzed to investigate the characteristics of the flow in the topside equatorial F region ionosphere during strong spread F conditions. Plasma flow around rapidly rising depletions is thought to exhibit behavior similar to two-dimensional Kolmogorov turbulence, but only on flux tubes with sufficiently small integrated ion–neutral collision frequencies. We find that one-dimensional spectra computed from vertical plasma drift measurements made in such depletions on such flux tubes tend to display a −5/3 spectral index over scale sizes from about 1 to $\text{100}\text{km}$, suggesting the operation of an inverse energy cascade. This universal spectral form is evidence of an inertial regime of the underlying ionospheric interchange instability.     

Waveguide radio-wave propagation in the equatorial ionosphere according to the Interkosmos-19 data

Additional strongly remote (up to 2000 km) radio-signal reflection traces on Intercosmos-19 ionograms obtained in the equatorial ionosphere have been considered. These traces, as a rule, begin at frequencies slightly lower than the main trace cutoff frequencies, which indicates that an irregularity with a decreased plasma density exists here. The waveguide stretched along the magnetic-field line is such an inhomogeneity in the equatorial ionosphere. The ray tracing confirm that radio waves propagate in a waveguide and make it possible to determine the typical waveguide parameters: ?δN _e ≥ 10%, with a diameter of 15–20 km. Since the waveguide walls are smooth, an additional trace is always recorded distinctly even in the case in which main traces were completely eroded by strong diffusivity. Only one additional trace (of the radio signal X mode) is usually observed one more multiple trace is rarely recorded. Waveguides can be observed at all altitudes of the equatorial ionosphere at geomagnetic latitudes of ±40°. The formation of waveguides is usually related to the formation of different-scale irregularities in the nighttime equatorial ionosphere, which result in the appearance of other additional traces and spread F.