Jonas R. Souza

Equatorial spread F statistics and empirical representation for IRI: A regional model for the Brazilian longitude sector

The empirical representation of the equatorial spread F (ESF) statistics in the IRI scheme requires well established distribution statistics of ESF occurrence and intensity as a function of local time, season/month, latitude and solar and magnetic activity levels. We present here a regional model for the quiet-time spread F distribution in the Brazilian longitude sector. In view of the well known dependence of spread F occurrence on magnetic declination angle, and the fact that the declination angle varies rapidly from the west coast (Peruvian sector) to the east coast (Brazilian sector) of south America the present model can be said to be valid for the latter sector. 13 years of spread F data simultaneously collected (during 1978–1990) over the equatorial site Fortaleza and low latitude site Cachoeira Paulista are used in this model. Only spread F data that is believed to be related to plasma bubble developments is used in the model. The data were first grouped into solar flux range bins representing low, medium, and high solar activity levels, represented, respectively, by F10.7 ≤ 100; 100 < F10.7 < 180; and F10.7 ≥ 180. Spread F percentage occurrence as a function of (nocturnal) local time for each of the 12 months in each solar flux range was calculated, and form the database for the model. Cubic-B spline fits of the data in local time, latitude, season/month and solar flux nodes constitute the structure of the present model. The model confirms many characteristic features of the spread F statistics already known as well as brings out some new outstanding features for the Brazilian sector. Among the results to be highlighted are: The spread F onset and peak occurrence get delayed in local time with increasing distance from the equator, indicating the plasma bubble origin for the low latitude ionogram spread F traces; The plasma bubble occurrence as well as the vertical rise velocity increase with the increase in solar flux; They attain larger values in summer months (centered around December) than in equinoctial months (March and September). The latitudinal variation in spread F, though based only on two-station data sets in this study, looks compatible with the latitude variation of ion density fluctuations observed by the AE-E satellite. The model will be made available to interested users.

Determination of vertical plasma drift and meridional wind using the Sheffield University Plasmasphere Ionosphere Model and ionospheric data at equatorial and low latitudes in Brazil: Summer solar minimum and maximum conditions

The F region critical frequency foF2 and peak height hmF2, measured simultaneously at the equatorial location Fortaleza (4°S, 38°W, magnetic latitude = 3.5°S) and at the low-latitude location Cachoeira Paulista (22°S, 45°W, magnetic latitude = 15°S), are compared with their values calculated by the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) to determine the vertical (E × B) drift velocity at the equator and the magnetic meridional wind velocity over the two locations. The calculated and observed values of foF2 are then matched at both Fortaleza and Cachoeira Paulista to obtain the magnetic meridional winds over their respective conjugate locations. To account for the observed foF2 diurnal variation pattern over Cachoeira Paulista, it was found necessary to include a small source of ionization, attributable to energetic particle precipitation in the South Atlantic anomaly region. The vertical drift velocity and magnetic meridional wind velocity derived for summer months during both solar minimum and solar maximum are compared with their values given by other published models. While the diurnal variation of the modeled vertical drift velocity shows general agreement with the values based on Jicamarca radar measurements (the exception being during the sunset-midnight period at solar maximum and between 2000–2300 LT at solar minimum), the magnetic meridional wind shows significant differences with respect to the Horizontal Wind Model 1990 (HWM90) [Hedin et al., 1991] during both solar minimum and solar maximum at Fortaleza and at locations conjugate to Fortaleza and Cachoeira Paulista.

Modeling the equatorial and low‐latitude ionospheric response to an intense X‐class solar flare

We have investigated the ionospheric response close to the subsolar point in South America due to the strong solar flare (X2.8) that occurred on 13 May 2013. The present work discusses the sudden disturbances in the D region in the form of high-frequency radio wave blackout recorded in ionograms, the E region disturbances in the form of the Sq current and equatorial electrojet intensifications, and the enhancement and decay in the ionospheric total electron content (TEC) as observed by a network of Global Navigation Satellite Systems receivers, the last of these manifestations constituting the main focuses of this study. The dayside ionosphere showed an abrupt increase of the TEC, with the region of the TEC increase being displaced away from the subsolar point toward the equatorial ionization anomaly (EIA) crest region. The decay in the ΔTEC following the decrease of the flare EUV flux varied at a slower ratio near the EIA crest than at the subsolar point. We used the Sheffield University Plasmasphere-Ionosphere Model to simulate the TEC enhancement and the related variations as arising from the flare-enhanced solar EUV flux and soft X-rays. The simulations are compared with the observational data to validate our results, and it is found that a good part of the observed TEC variation features can be accounted for by the model simulation. The combined results from model and observational data can contribute significantly to advance our knowledge about ionospheric photochemistry and dynamics needed to improve our predictive capability on the low-latitude ionospheric response to solar flares.

Equatorial Ionization Anomaly: The Role of Thermospheric Winds and the Effects of the Geomagnetic Field Secular Variation

The vertical plasma drift is the well known driver of the equatorial ionization anomaly (EIA). The latitudinal distribution of ionization in the EIA is determined also by thermospheric meridional wind whose precise role can only be evaluated through the use of theoretical models because it depends not only upon the local configuration of the wind, but is a complex function of its distribution along the entire magnetic field line. Besides, in the Brazilian region, the magnetic field secular variations are fast enough for their effects on the ionosphere to be observed in the time span of the order of a solar cycle. In this work we use the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) to investigate the role of the vertical plasma drift, thermospheric meridional wind and of the magnetic field secular variations in the changing trend of the EIA over the Brazilian region.

Comparison of low latitude F region peak densities, heights and equatorial E×B drift from IRI with observational data and the Sheffield University plasmasphere ionosphere model

Observed values of the critical frequency and real height of the ionospheric F-region peak (foF2 and hmF2, respectively) over Fortaleza (4° S, 38° W) and Cachoeira Paulista (22.5° S, 45° W) near solar minimum are compared with their values calculated by the International Reference Ionospheric Model (IRI). A comparison between the equatorial E×B drift velocities represented by IRI and those obtained by a method that uses the Sheffield University Plasmasphere Ionosphere Model (SUPIM) has also been carried out. In general, the values of foF2 and hmF2 calculated by IRI using the International Radio Consultive Committee (CCIR) maps show better agreement with the observed values than do those obtained using the International Union of Radio Science (URSI) maps for both stations, Fortaleza and Cachoeira Paulista. Equatorial E×B drift velocities calculated by IRI/Scherliess and Fejer (1999) have presented values different from those calculated by SUPIM mainly during evening and nighttime for December solstice and equinox conditions.

Disturbance zonal and vertical plasma drifts in the Peruvian sector during solar minimum phases

In the present work, we investigate the behavior of the equatorial F region zonal plasma drifts over the Peruvian region under magnetically disturbed conditions during two solar minimum epochs, one of them being the recent prolonged solar activity minimum. The study utilizes the vertical and zonal components of the plasma drifts measured by the Jicamarca (11.95°S; 76.87°W) incoherent scatter radar during two events that occurred on 10 April 1997 and 24 June 2008 and model calculation of the zonal drift in a realistic ionosphere simulated by the Sheffield University Plasmasphere-Ionosphere Model-INPE. Two main points are focused: (1) the connection between electric fields and plasma drifts under prompt penetration electric field during a disturbed periods and (2) anomalous behavior of daytime zonal drift in the absence of any magnetic storm. A perfect anticorrelation between vertical and zonal drifts was observed during the night and in the initial and growth phases of the magnetic storm. For the first time, based on a realistic low-latitude ionosphere, we will show, on a detailed quantitative basis, that this anticorrelation is driven mainly by a vertical Hall electric field induced by the primary zonal electric field in the presence of an enhanced nighttime E region ionization. It is shown that an increase in the field line-integrated Hall-to-Pedersen conductivity ratio ∑H∑P, which can arise from precipitation of energetic particles in the region of the South American Magnetic Anomaly, is capable of explaining the observed anticorrelation between the vertical and zonal plasma drifts. Evidence for the particle ionization is provided from the occurrence of anomalous sporadic E layers over the low-latitude station, Cachoeira Paulista (22.67°S; 44.9°W)—Brazil. It will also be shown that the zonal plasma drift reversal to eastward in the afternoon two hours earlier than its reference quiet time pattern is possibly caused by weakening of the zonal wind system during the prolonged solar minimum period.

Equatorial electrojet responses to intense solar flares under geomagnetic disturbance time electric fields

Large enhancement in the equatorial electrojet (EEJ) current can occur due to sudden increase in the E layer density arising from solar flare associated ionizing radiations, as also from background electric fields modified by magnetospheric disturbances when present before or during a solar flare. We investigate the EEJ responses at widely separated longitudes during two X-class flares that occurred at different activity phases surrounding the magnetic super storm sequences of 28–29 October 2003. During the 28 October flare we observed intense reverse electrojet under strong westward electric field in the sunrise sector over Jicamarca. Sources of westward disturbance electric fields driving large EEJ current are identified for the first time. Model calculations on the E layer density, with and without flare, and comparison of the results between Jicamarca and Sao Luis suggested enhanced westward electric field due to the flare occurring close to sunrise (over Jicamarca). During the flare on 29 October, which occurred during a rapid AE recovery, a strong overshielding electric field of westward polarity over Jicamarca delayed an expected EEJ eastward growth due to flare-induced ionization enhancement in the afternoon. This EEJ response yielded a measure of the overshielding decay time determined by the storm time Region 2 field-aligned current. This paper will present a detailed analysis of the EEJ responses during the two flares, including a quantitative evaluation of the flare-induced electron density enhancements and identification of electric field sources that played dominant roles in the large westward EEJ at the sunrise sector over Jicamarca.

An empirical model for the ionospheric electron content at low latitude in Brazil and a comparison with IRI95

Abstract Ionospheric total electron content (TEC) data obtained over Cachoeira Paulista (22.5° S, 45° W, dip angle 28° S) from measurements of Faraday rotation of VHF beacon transmission from signals transmitted by GOES 1, 2 and 3 geostationary satellites during the years 1982, 1984, 1986, 1988 and 1989 are used to develop a local empirical model for the TEC for the Brazilian low latitude. The TEC values, predicted by this local model, are compared with those calculated by the IRI95. Discrepancies between these models are found, mainly near sunrise when the IRI predictions for the morning TEC minimum, and the following sunrise increase, are ∼ 1 hour earlier than observations at all seasons and at all solar activity levels. IRI overestimates the values of TEC for all local times and seasons during solar minimum, except at winter nighttime. Disagreements are also found during afternoon hours near solar maximum with underestimated values of TEC predicted by IRI. Further more, the IRI95 does not predict the near midnight peak in TEC that arises from the equatorial anomaly.

Longitudinal ionospheric effects in the South Atlantic evening sector during solar maximum

[i] Large-scale horizontal gradients in ion density and vertical drift observed by the Atmospheric Explorer E satellite in the South Atlantic region (latitudes 10°S-20°S, longitudes 50°W-10°E) during the June solstice at solar maximum are presented and analyzed. These features occur during the nighttime period. The observations near 450-km altitude show vertically downward ion drift velocities exceeding 120 m s - 1 and depleted regions where the ion density is around 2 x 10 4 cm - 3 . It is shown, using values modeled by the Sheffield University Plasmasphere Ionosphere Model (SUPIM) along the satellite trajectory, that the large ion density depletions appear as a result of large downward ion drifts driven by large southward winds along the magnetic meridian and by diffusion. During others seasons such behavior is not observed by the AE-E satellite, neither by SUPIM results. The roles played by the different physical processes responsible for the large downward drift velocities are investigated. The model results highlight the relationship between longitudinal variation of the ion densities and the location of the equatorial anomaly crest in the South Atlantic region.

An approximate nearest neighbors search algorithm for low-dimensional grid locations

We propose a new algorithm for the problem of approximate nearest neighbors (ANN) search in a regularly spaced low-dimensional grid for interpolation applications. It associates every sampled point to its nearest interpolation location, and then expands its influence to neighborhood locations in the grid, until the desired number of sampled points is achieved on every grid location. Our approach makes use of knowledge on the regular grid spacing to avoid measuring the distance between sampled points and grid locations. We compared our approach with four different state-of-the-art ANN algorithms in a large set of computational experiments. In general, our approach requires low computational effort, especially for cases with high density of sampled points, while the observed error is not significantly different. At the end, a case study is shown, where the ionosphere dynamics is predicted daily using samples from a mathematical model, which runs in parallel at 56 different longitude coordinates, providing sampled points not well distributed that follow Earth’s magnetic field-lines. Our approach overcomes the comparative algorithms when the ratio between the number of sampled points and grid locations is over 2849:1.