Occurrence Of Equatorial Spread Research Articles

On equatorial spread F occurrence: A multi-dimensional quantitative assessment

The present study investigates the role of gravity wave induced seed perturbations in the occurrence of Equatorial Spread F (ESF) under the influence of the post sunset background conditions modulated by prevailing electrodynamics and neutral wind. Ionospheric foF2 data sets over geomagnetic equatorial station Trivandrum (8.5°N, 77°E and magnetic dip 0.68°N-corresponding to the period of study) corresponding to vernal and autumnal equinoctial periods encompassing high, low and moderate solar activity years, are used for the study. Meridional wind data is obtained either from ESA’s sun-synchronous satellite GOCE (Gravity field and steady-state Ocean Circulation Explorer) or derived using ionosonde h’F (base height of ionosphere at 2.5 MHz) data from Trivandrum (TVM- 8.5°N, 77°E and magnetic dip 0.68°N) and Sriharikota (SHAR−13.7°N, 80.2°E and magnetic dip 6.9°N-for period of study). This particular study is carried out for geomagnetically quiet days of Vernal Equinox (VE) and Autumnal Equinox (AE) seasons, which are most favoured for ESF occurrence over Indian longitudes. Considering thermospheric wind, ion-neutral collisions, and electric field effects in association with gravity wave seed, threshold curve is generated, which clearly demarcates ESF and NSF (Non spread F) days. Previous studies have addressed ESF variability in electrodynamical domain alone (wherein the layer is above a threshold level). The present study, for the first time, succeeds in demarcating ESF and NSF days by incorporating effects of electric field, neutral wind, collisional RT instability term, and gravity wave seed perturbations simultaneously irrespective of threshold height.

First climatology of F-region UHF echoes observed by the AMISR-14 system at the Jicamarca radio observatory and comparison with the climatology of VHF echoes observed by the collocated JULIA radar

Coherent backscatter radar observations made at the Jicamarca Radio Observatory (JRO) have contributed significantly to our understanding of equatorial F-region irregularities. Radar observations, however, have been made predominantly at the Very-High Frequency (VHF) band (50 MHz), which corresponds to measurements of 3-m field-aligned irregularities. The deployment of the 14-panel version of the Advanced Modular Incoherent Scatter Radar (AMISR-14) at Jicamarca provided an opportunity for observations of Ultra-High Frequency (UHF - 445 MHz) echoes which correspond to measurements of irregularities with 0.34 m scale sizes. Here, we present what we believe to be the first report describing the quiet-time climatology of sub-meter equatorial F-region irregularities derived from UHF radar measurements. The measurements were made between August 2021 and February 2023 using a 10-beam AMISR-14 mode that scanned the F-region in the magnetic equatorial plane. The results show how F-region sub-meter irregularities respond to variations in season and solar flux conditions. The results also confirm, experimentally, that the occurrence of UHF F-region echoes is controlled by the occurrence of equatorial spread F (ESF). Higher occurrence rates were observed during pre-midnight hours and during Equinox and December solstice. Reduced occurrence rates were observed during June solstice. The results show that an increase in solar flux was followed by an increase in the altitude where noticeable occurrence rates (≳ 10%) start and in the maximum altitude of these occurrence rates. The observations also show that occurrence rates lasted longer (in local time) during low solar flux conditions. Comparisons with collocated VHF radar observations showed that, despite differences in radar parameters, observation days, and the scale size (one order of magnitude) of the scattering irregularities, the two systems show similar climatological variations with only minor differences in the absolute occurrence rates. Finally, the analysis of the occurrence rates for different beams did not show substantial climatological variations over local (within a few 100s of km) zonal distances around JRO. We point out, however, that observations on a single day can show strong local variations in echo detection and intensity within the AMISR-14 field of view due to the intrinsic development and decay of ESF structures.

Multi-process driven unusually large equatorial perturbation electric fields during the April 2023 geomagnetic storm

The low latitude ionosphere and thermosphere are strongly disturbed during and shortly after geomagnetic storms. We use novel Jicamarca radar measurements, ACE satellite solar wind, and SuperMAG geomagnetic field observations to study the electrodynamic response of the equatorial ionosphere to the 23, 24 April 2023 geomagnetic storm. We also compare our data with results from previous experimental and modeling studies of equatorial storm-time electrodynamics. We show, for the first time, unusually large equatorial vertical and zonal plasma drift (zonal and meridional electric field) perturbations driven simultaneously by multi storm-time electric field mechanisms during both the storm main and recovery phases. These include daytime undershielding and overshielding prompt penetration electric fields driven by solar wind electric fields and dynamic pressure changes, substorms, as well as disturbance dynamo electric fields, which are not well reproduced by current empirical models. Our nighttime measurements, over an extended period of large and slowly decreasing southward IMF Bz, show very large, substorm-driven, vertical and zonal drift fluctuations superposed on large undershield driven upward and westward drifts up to about 01 LT, and the occurrence of equatorial spread F irregularities with very strong spatial and temporal structuring. These nighttime observations cannot be explained by present models of equatorial storm-time electrodynamics.

Assessing the potential of ionosonde for forecasting post-sunset equatorial spread F: an observational experiment in Southeast Asia

The occurrence of equatorial spread F (ESF) has the potential to detrimentally impact space-based technological systems. This study investigates the utility of ionosondes in forecasting the incidence of post-sunset ESF in the zonal direction, utilizing observational data obtained from four ionosondes located near the magnetic Equator in Southeast Asia. Data were collected during the equinox seasons (March–April and September–October) between 2003 and 2020. To establish a relationship between the probability of post-sunset ESF occurrence and the evening vertical plasma drift (v), a logistic regression model was employed. Post-sunset ESF occurrence is defined as the presence of ESF during the time window between 19:00 and 21:00 LT, while v is derived from the average time derivative of virtual heights during the interval from 18:30 to 19:00 LT. Results indicate that the probability of post-sunset ESF occurrence approaches zero, signifying that ESF is unlikely to develop when v is negative. Conversely, when v exceeds 30 m/s, the probability of post-sunset ESF occurrence surpasses 0.87, indicating that ESF occurs almost invariably. The likelihood of post-sunset ESF occurrence reaches 1 when v equals or exceeds 40 m/s. Utilizing this model, the study determined that a single ionosonde positioned at the Equator can effectively forecast the incidence of post-sunset ESF up to a longitudinal distance of 30° from its location. The accuracy of ionosondes in predicting post-sunset ESF occurrence above their respective locations is approximately 0.80, with a 10% decrease in accuracy when forecasting ESF occurrence at longitudinal distances of 30°. In conclusion, this study enhances our understanding of the link between the evening vertical plasma drift and the manifestation of post-sunset ESF by leveraging ionosonde data. Furthermore, it provides valuable insights into the recommended coverage range of ionosondes for predicting post-sunset ESF occurrence in the zonal direction, which can be employed to fortify regional space weather services.Graphical

Temporal and altitudinal variability of the spread F observed by the VHF radar over Christmas Island

Abstract. The goal of this work is to study the time and altitude echo characteristics under different solar and seasonality conditions using the VHF radar range–time–intensity (RTI) images. The occurrence of equatorial spread F depends on the existence of conditions that can seed the Rayleigh–Taylor instability, and these conditions can change with solar flux, seasonality, longitude distributions, and day-to-day variability. So, the equatorial spread F is observed as its time and altitude occurrence. The VHF radar of Christmas Island (2.0∘ N, 157.4∘ W, 2.9∘ N dip latitude) has been operational in the equatorial region for some time, allowing long-term observations. The occurrence of echoes during solar minimum conditions is observed throughout the night since the post-reversal westward electric field is weaker than the solar maximum and the possibilities of the vertical plasma drift becoming positive are larger. On the other hand, echoes during solar maximum will be controlled by dynamics near the time of the pre-reversal peak (PRE). Our results indicate that the peak time occurrence of echoes along this period shows a well-defined pattern, with echoes distributed as closer to local sunset during solar maximum and around/closer to midnight during solar minimum conditions; meanwhile, the peak altitude occurrence of echoes shows a slightly regular pattern with higher-altitude occurrences during solar maxima and lower altitudes during solar minimum conditions.

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

Occurrence of equatorial spread F during intense geomagnetic storms

AbstractEquatorial spread F (ESF) has been observed in response to the prompt penetration of magnetospheric electric field to equatorial latitudes during intense (minimum Dst ≤ −100 nT; Bz ≤ −10 nT for at least 3 h) magnetic storms using global ion density plots of Defense Meteorological Satellite Program (DMSP) over nearly one solar cycle (1996–2005). Geostationary amplitude scintillation observations from Calcutta at VHF and L band for 1996–2005 and GPS amplitude scintillation measurements during 2004–2005 from the Indian Satellite Based Augmentation System Geostationary and GPS Navigation Outlay (GPS Aided GEO Augmented Navigation) network of stations all over India have been used to corroborate the DMSP observations. Subsequent to the time of southward interplanetary magnetic field Bz crossing −10 nT for an intense storm, it has been observed that within 4 h, ESF is generated at a longitude where the local time is dusk.

A study on ionospheric scintillation near the EIA crest in relation to equatorial electrodynamics

AbstractEquatorial electrojet (EEJ) data, which are considered as a proxy index of equatorial electric field, are analyzed in conjunction with equatorial ionosonde, total electron content (TEC) and scintillation data near the equatorial ionization anomaly (EIA) crest for the equinoctial months of high solar activity years (2011–2012) to identify any precursor index of postsunset evolution of equatorial electron density irregularities and subsequent occurrence of scintillation near the northern EIA crest. Only geomagnetically quiet and normal electrojet days are considered. The diurnal profiles of EEJ on the scintillation days exhibit a secondary enhancement in the afternoon to presunset hours following diurnal peaks. A series of electrodynamical processes conducive for generation of irregularities emerge following secondary enhancement of EEJ. Latitudinal profile of TEC exhibits resurgence in EIA structure around the postsunset period. Diurnal TEC profile near the EIA crest resembles postsunset secondary enhancement on the days with afternoon enhancement in EEJ. Occurrence of equatorial spread F and postsunset scintillation near the EIA crest seems to follow the secondary enhancement events in EEJ. Both the magnitude and duration of enhanced EEJ are found to be important for postsunset intensification of EIA structure and subsequent occurrence of equatorial irregularities. A critical value combining the two may be considered an important precursor for postsunset occurrence of scintillation near the EIA crest. The results are validated using archived data for the years 1989–1990 and explained in terms of modulation effects of enhanced equatorial fountain.

Low‐latitude Es capable of controlling the onset of equatorial spread F

AbstractTo investigate their role on the occurrence of equatorial spread F (ESF), the low‐latitude E region in the evening hours has been characterized in terms of sporadic E (Es) activity based on ionosonde observations from Sriharikota, a low‐latitude station in India. While prior to the sunset, either Chapman type E layer, or Es, or a combination of the two was observed, after sunset, when observed, the E layer was exclusively Es. Es, however, could be either blanketing or nonblanketing type. Observations revealed that while the occurrence of ESF had a broad relationship with the type of Es, a detailed investigation suggested that (1) ESF occurred (did not occur) irrespective of Es type if the virtual height of the F layer base (h'F) was greater than ~360 km (less than ~290 km), and (2) ESF occurrence depended strongly on the type of Es if 290 km < h'F < 360 km. Model computation shows that low‐latitude Es can significantly influence the growth of the Rayleigh‐Taylor instability for the cases corresponding to category 2 by increasing the total field‐line integrated Pedersen conductivity, consistent with the observations. The results presented in this paper highlight under what circumstances the low‐latitude Es would play a critical role in the occurrence of ESF, which can be used to understand the day‐to‐day variability of ESF.

On seeding, large-scale wave structure, equatorial spreadF, and scintillations over Vietnam

[1] Understanding the day-to-day variability in occurrence of equatorial spread F (ESF) remains as a high-priority objective in space weather research. A major difficulty has been an inability to resolve the roles being played by large-scale wave structure (LSWS) and the post-sunset rise (PSSR) of the equatorial F layer, in the production of ESF. In this paper, we show conclusively that total electron content (TEC), measured as a function of latitude and longitude, provides clear, routine descriptions of LSWS. Then, together with ionosonde data, we show, for the first time, that while a seed for LSWS can occur in the late afternoon, its amplification takes place mostly during the PSSR. Implications of these findings are discussed in light of existing theories.

Day‐to‐day variability in equatorial spread F: Is there some physics missing?

Attempts continue to be made, without notable success, to identify the source of day‐to‐day variability in occurrence of equatorial spread F (ESF). Most seek to uncover a controlling factor in one of the parameters that describe the linear growth rate of the Rayleigh‐Taylor (RT) instability. It is possible, however, that some physics is still missing in that description of ESF generation. Consideration is given here to an F‐region response to a large‐scale polarization electric field that is generated by a sporadic‐E layer instability (Cosgrove and Tsunoda, 2002) and mapped to the bottomside of the F2 layer. Results indicate that the large‐scale wave structure, the most reliable precursor for ESF, may be initiated by this process at the base of the F2 layer, where plasma drift is westward.

Large‐amplitude transient growth in the linear evolution of equatorial spread F with a sheared zonal flow

The occurrence of equatorial spread F on the bottomside of the Flayer is likely the result of a process often referred to as the collisional interchange instability or generalized Rayleigh‐Taylor instability. The traditional approach to the analysis of this instability with sheared zonal flow has been to calculate the eigenvalues of the linearized system. It is well documented that the introduction of shear has a stabilizing effect on the eigenvalues and significantly increases the wavelength corresponding to the fastest growing eigenmode. In this paper it is argued that the well‐accepted conclusions drawn from eigenvalue analyses are not correct for cases of geophysical interest. A calculation of the ε pseudospectra demonstrates that the system is highly nonnormal and that large‐amplitude transients may exist even when all of the eigenmodes are decaying with time. Transient effects are shown to be of fundamental importance to the evolution of the system over a wide range of horizontal wavelengths. This viewpoint is consistent with some aspects of VHF radar observations of F region gravitational interchange dynamics and may explain the presence of irregularities which appear to be confined to the bottomside of the F layer. The techniques discussed are of general interest to the analysis of nonnormal linear systems.

Nonlinear evolution of equatorial spread F: 4. Gravity waves, velocity shear, and day‐to‐day variability

Production and evolution of plasma bubbles seeded by gravity waves in the equatorial F region are studied under different conditions using a computer simulation. The problem of day‐to‐day variability in the occurrence of equatorial spread F (ESF) is discussed. It is shown that gravity waves over a wide range of amplitude and wavelength are a very effective seed mechanism for production of ESF. However, there is the day‐to‐day variability in the occurrence of ESF even if gravity waves are omnipresent. The maximum height of the F peak arid the bottomside background electron density gradient can significantly influence production and rise of plasma bubbles. It is found also that the timing of the seed gravity wave is critical to ESF generation. If à gravity wave exists in the F region during the upward drifting of the F layer, the gravity wave can initiate the Rayleigh‐Taylor instability and result in topside bubbles. In contrast, gravity‐wave‐induced perturbations in a descending ionosphere evolve into large‐scale, wavelike structures but not bubbles. Gravity waves can propagate in any direction, but only the gravity waves propagating with an azimumal angle range of about 10° relative to the zonal direction are expected to be capable of generating ESF bubbles. These factors are all variable from day to day. Their variation must cause variation of resulting ESF. In addition, we show first that in agreement with linear analysis, a velocity shear in the plasma drift can generate plasma structures in the bottomside F layer. For reasonable ionospheric parameters, the shear‐determined wavelength of the Rayleigh‐Taylor instability is a few tens of kilometers. The plasma structures organized by velocity shear appear as bottomside spread F but cannot evolve into topside bubbles. Velocity shear provides a possible explanation of generation of bottomside spread F and secondary plumelike structures on the west wall of large‐scale plasma upwelling.

Onset conditions for equatorial spread F

The problem of day‐to‐day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two‐night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all‐sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300‐Å airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh‐Taylor instability growth rates. We present evidence to suggest that this picture can be extended to far shorter time scales, namely, that “surges” in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low‐latitude thermospheric winds.

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index ( S 10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread- F ( ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.

On the occurrence of equatorial spread- F in the evening hours

By comparing electron drift velocities at Jicamarca with corresponding ionograms and VHF radio scintillation records at Huancayo it has been shown that the day-to-day variability in the occurrence of equatorial spread- F irregularities in the post-sunset period depends critically on the time of reversal of the Sq electric field. The field reversal before sunset does not produce any spread- F in the evening hours, while the continuation of the day-time electric field for a couple of hours after sunset at normal strength is a favourable condition for generating spread- F.

Geomorphology of spread F and characteristics of equatorial spread F

Between magnetic latitudes 20°N and 20°S there is a well defined region where spread F is a normal occurrence on magnetically quiet days. Equatorial spread F is a night-time phenomenon that begins between 1900 and 2200 by a characteristic doubling of the layer and an increase in virtual height, indicating a vertical velocity. Later in the night, after 2300, the records show group retardation and no stratification. The occurrence of equatorial spread F is decreased by magnetic activity. Radio-star scintillations in equatorial regions are correlated strongly with spread F and have the same diurnal variation and the same anticorrelation with magnetic activity. The phenomenon of ‘flutter’ (rapid fading of long-distance, high-frequency stations) shows the same variations. Measurements using the spaced receiver method indicate that, in the equatorial region, the irregularities in the F layer are greatly elongated along the lines of the earth's magnetic field.