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This work analysis the blanketing sporadic layers (Es b ) behavior over Sao Luis, Brazil (2° 31′ S, 44° 16′ W, dip: −4.80) which is classified as a transition region between equatorial and low-latitude. Hence, some peculiarities can appear as Es b occurrence instead of the common Es q , which is a non-blanketing irregularity layer. The analysis presented here was obtained using a modified version of a theoretical model for the E region (MIRE), which computes the densities of the metallic ions (Fe + and Mg + ) and the densities of the main molecular ions (NO + , O 2 + , N 2 + ) by solving the continuity and momentum equations for each one of them. In that model, the Es layer physics driven by both diurnal and semidiurnal tidal winds are taken into account and it was extended in height coverage by adding a novel neutral wind model derived from the all-sky meteor radar measurements. Thus, we provide more trustworthy results related to the Es layer formation in the equatorial region. We verified the contribution of each tidal wind component to the Es b layer formation in this equatorial region. Additionally, we compared the Es layer electron density computed by MIRE with the data obtained by using the blanketing frequency parameter ( fb Es) deduced from ionograms. The results show that the diurnal component of the tidal wind is more important in the Es b layer formation whereas the semidiurnal component has a little contribution in our simulations. Finally, it was verified that the modified MIRE presented here can be used to study the Es b layers occurrence over the equatorial region in the Brazilian sector.

This work presents new results about simulations of blanketing sporadic E-layers, Esb, using a modified version of a theoretical model for the E region (MIRE) for a Brazilian region: Cachoeira Paulista, a low latitude station. MIRE computes the densities of the metallic ions (Fe+ and Mg+) and of the main molecular ions (NO+, O2+, N2+) by solving the continuity and momentum equations for each one of them. Additionally, this model includes the physics of Es layer development driven by tidal winds. In this study, we extend MIRE adding a novel neutral wind model derived from the all-sky meteor radar measurements, which provides more trustworthy results related to the Es layer formation in the Brazilian sector. Afterwards, this new model is validated comparing the computed electron density for the Es layer with the electron density obtained using the blanketing frequency parameter (fbEs) deduced from ionograms in the analyzed region. The results show that the values computed by the extended MIRE are in good agreement with the observational data for the Es layers formed by the wind shear mechanism. Therefore, the extended MIRE presented in this work is the only available model that allows simulating Esb layers with a much higher confidence level for the low-latitude in the Brazilian sector.

The ordinary wave critical frequency foEs is measured routinely by ionosondes and used widely in estimating the intensity and occurrence of sporadic E (Es) metal ion layers. Recently, however, it was realized that foEs overestimates Es intensities because it corresponds to the sum of both the layer metal ion plasma and the regular E region plasma densities. To account for this, a new parameter, foμEs, was introduced that corresponds to the Es layer metal plasma density only, and a method was proposed for its calculation. In the present work, an ionosonde data set is analyzed to assess the importance of replacing foEs by foμEs in Es studies. To this objective, the diurnal and seasonal variability of sporadic E is re-examined. It turns out that the Es mean diurnal and seasonal variations estimated by means of using foμEs, differ substantially from those based on foEs, implying that the use of foEs leads to biased results and wrong physical inferences. It is therefore urged that foμEs is adopted and used instead of foEs, in future ionosonde sporadic E studies. The same applies for the case of satellite radio occultation (RO) sporadic E investigations as well, where foμEs should be used, instead of foEs, for calibration and validation purposes. All this is equally valid in the case of the blanketing sporadic E frequency fbEs and the corresponding new parameter fbμEs that needs also to be adopted and used.

A multi-year global comparison between mid-latitude and equatorial sporadic-E ( E s ) height and intensity derived from COSMIC GPS radio occultation (GPS-RO) data and blanketing E frequency (fbEs) from ground-based Digisonde measurements is performed. The E s layer intensity from GPS-RO measurements was estimated using the S2 scintillation index and total electron content (TEC) assuming constant and variable E s cloud thicknesses individually. GPS-RO crossings were limited to within 150 km and 30 min of fbEs occurrences at each Digisonde. Intensities derived from the TEC with constant thickness method agreed more closely with the Digisonde fbEs measurements in terms of relative error and spread. E s path lengths estimated from the variable thickness method were also significantly larger than the average E s cloud length indicating that tube geometry assumptions between thickness and length do not apply to sporadic-E. Furthermore, intensities calculated from the S2 technique generally placed the frequency of fbEs between 2.7 and 5.3 MHz. Mean fbEs predictions from the S2 and constant thickness TEC methods are found to be statistically similar to the mean Digisonde fbEs. Both TEC and S2 methods underestimated the altitude of the fbEs events, but the S2 technique performed better overall. The mean heights calculated from both of the GPS-RO methods show a statistically significant difference with the mean Digisonde calculated heights, suggesting a bias in the height estimates.

This study is to know how the characteristics of sporadic E-layer (E s -layer) affect the generation of spread-F in the nighttime ionosphere near the crest of equatorial ionization anomaly during solar minimum. The data of E s -layer parameters and spread-F are obtained from the Chungli ionograms of 1996. The E s -layer parameters include foE s (critical frequency of E s -layer), fbE s (blanketing frequency of E s -layer), and Δ f (≡foE s −fbE s ). Results show that the nighttime variations of foE s and fbE s medians (Δ f medians) are different from (similar to) that of the occurrence probabilities of spread-F. Because the total number of E s -layer events is greater than that of spread-F events, the comparison between the medians of E s -layer parameters and the occurrence probabilities of spread-F might have a shortfall. Further, we categorize the E s -layer and spread-F events into each frequency interval of E s -layer parameters. For the occurrence probabilities of spread-F versus foE s , an increasing trend is found in post-midnight of all three seasons. The increasing trend also exists in pre-midnight of the J-months and in post-midnight of all seasons, for the occurrence probabilities of spread-F versus Δ f . These demonstrate that the spread-F occurrence increases with increasing foE s and/or Δ f . Moreover, the increasing trends indicate that polarization electric fields generated in E s -layer assist to produce spread-F, through the electrodynamical coupling of E s -layer and F-region. Regarding the occurrence probabilities of spread-F versus fbE s , the significant trend only appears in post-midnight of the E-months. This implies that fbE s might not be a major factor for the spread-F formation.

We have analyzed the behavior of blanketing frequency of the Es layer (fb Es) occurring at an equatorial station covering the days before, during and subsequent to 24 intense and very intense magnetic storms ( Dst ≤ − 100 nT ) that occurred during the solar cycle 23. The fb Es was measured by digital ionosonde over Sao Luis, Brazil (2.33° S, 44.2° W, dip: −4.5°). Our analysis shows that there are significant changes in the fb Es, mainly during the recovery phase of magnetic storms, characterized by occurrence of peaks that exceed the ambient background values. Also, these peaks are associated to other types of sporadic E layer than the Esq (a non-blanketing layer detected due the plasma irregularities in the equatorial electrojet), which in turn means competing mechanisms. The results are discussed in terms of the statistics of the abnormal enhancement taking into account the phase of the magnetic storm.

One of the recent developments in the form of a novel probe for simultaneous in-situ measurements of Electron density and Neutral Wind (ENWi) parameters was extensively made use of during the annular solar eclipse that passed right over the magnetic dip equatorial station, Trivandrum in India. The response of the equatorial ionosphere to the annular solar eclipse of January 2010 is investigated using the data from ENWi and other ground based instruments like digital Ionosonde, HF radar, magnetometer etc. Significant effects on the neutral wind components and electron densities during the peak phase of the eclipse are brought out by the ENWi data. A clear reversal in the zonal neutral wind is observed during the rocket flights at the peak phase of the eclipse as well as in the flight conducted 3 h later. The ground magnetic field measurements revealed the reversal of electrojet during the eclipse alongwith blanketing Es layers vindicating the earlier hypothesis that blanketing Es layers manifest over magnetic equatorial regions only during periods of either weak or reversed electrojet (counter electrojet). The HF radar results independently substantiate the in situ measurements with regard to the location of the irregularities and their Doppler velocities. The regional scale changes that have been brought about by the Solar eclipse both in the neutral and ionized medium and the consequent electrodynamics are highlighted.

Sporadic-E is a problem in high latitude HF communications because of its irregular and as yet unpredictable behaviour. This paper characterises the change in E-layer critical frequency (foE) that occurs in the four hours following a storm sudden commencement (SSC) event which is a precursor to magnetospheric storms which adversely affect ionospheric communications. Following checks for any seasonal or solar activity dependency in the data, further analysis determines the occurrence of full blanketing (screening) Sporadic-E layer formation at high latitudes as seen on vertical ionosondes in Northern Finland (671 Latitude). An appropriate threshold value of foE is proposed that could be used following the commencement of ionospheric storms, indicating that there is a high probability that F-layer screening will occur. As far as the authors are aware this is the first statistical analysis of the onset of Sporadic-E following SSC.

During an equatorial campaign called COPEX (Conjugate Point Equatorial Experiment) ionosondes at a pair of conjugate stations in Brazil were operated for 2 months in 2002. One expected the sporadic-E (E s ) layers to be rather similar at the conjugate locations. However, there were significant differences in the E s , particularly in the part of the E s trace that extends above the blanketing frequency. Considerations of the ambient conditions at the conjugate locations indicate that relatively small differences in parameters such as the tidal winds, electric fields, and background density gradients can significantly affect the structuring of the E s layer.

A RH-560 rocket instrumented with a Langmuir Probe and two pairs of double probes was launched from SHAR (dip 14 ° N) at 2130 hr LT (1ST) on 4 October 1988 during spread-F conditions. As a part of this campaign VHF scintillations at 136.1 MHz were also recorded, using the beacon onboard the geostationary satellite ETS-II (130 ° E). A new digital ionosonde (KEL) installed at SHAR during the first week of September 1988 provided the basic ionospheric information. Ionosonde and scintillation data during this campaign (15 September–4October), along with the Langmuir probe data, are described. The occurrence of scintillations and spread-F during the campaign period was very high, with maximum hourly percentage occurrences of about 80 and 70%, respectively. Generally the onset of spread-F occurred slightly before 1900 hr LT when scintillations in the satellite signal also start appearing. This is followed soon by the range spreading on the ionosonde trace. On days with spread-F the minimum virtual height of the F-layer, h′F, rose to about 400 km in the post-sunset period while on days without spread-F it rose to only about 300–320 km. The electric field reversal, as inferred from the h′F variations, occurred around 1930 hr LT on spread-F days and around 1900 hr LT on days without spread-F. The ionograms, taken at 5 min interval on several nights, were used to determine the vertical drift velocities and these exceeded 50 m/s on spread-F days in the post-sunset period. At the time of the rocket launch, a strong blanketing type of sporadic-E with several multiple reflections appeared around 105 km. The electron density profile obtained from the Langmuir probe data also showed an extremely sharp layer at 105 km and another prominent layer at 130 km, both during the ascent and the descent. The electron density irregularities associated with spread-F were seen in different altitude regions, the most prominent being from 210 to 250 km and from 300 to 320 km. The power spectra of the electron density fluctuations have been obtained both from the in situ probe data and the scintillation data.