E. R. de Paula

Average vertical and zonal F region plasma drifts over Jicamarca

The seasonal averages of the equatorial F region vertical and zonal plasma drifts are determined using extensive incoherent scatter radar observations from Jicamarca during 1968-1988. The late afternoon and nighttime vertical and zonal drifts are strongly dependent on the 10.7-cm solar flux. The authors show that the evening prereversal enhancement of vertical drifts increases linearly with solar flux during equinox but tends to saturate for large fluxes during southern hemisphere winter. They examine in detail, for the first time, the seasonal variation of the zonal plasma drifts and their dependence on solar flux and magnetic activity. The seasonal effects on the zonal drifts are most pronounced in the midnight-morning sector. The nighttime eastward drifts increase with solar flux for all seasons but decrease slightly with magnetic activity. The daytime westward drifts are essentially independent of season, solar cycle, and magnetic activity.

Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F

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.

Equatorial ionospheric electric fields during magnetospheric disturbances : local time/longitude dependences from recent EITS campaigns

Data sets collected during a few coordinated Equatorial Ionosphere-Thermosphere System (EITS) observational campaign periods, mainly from the Brazilian and Asian longitude sectors are analysed in this paper. Ionosonde magnetometer and Ionospheric Electron Content (IEC) data from the EITS-I and -2 campaigns (during March and December 1991) are complemented by interplanetary magnetic field and some ground based data sets from other campaigns. The analysis focuses on the response of the equatorial isonospheric heights and ionization anomaly to disturbance electric fields, identified as a direct penetration electric field associated with IMF B, changes and development of the ring current (especially the asymmetric component), and that produced by a disturbance zonal neutral wind. New evidence on the local time and longitudinal dependences of these electric fields constitute the main results of this paper. Especially, a large eastward electric field (associated with the asymmetric ring current) in the dusk-dawn sector causes significant expansion of the EIA in this sector, and amplification of the evening prereversal uplift of the F-layer over Brazil. Significant inhibition of the evening prereversal electric field enhancement seems to be produced by the disturbance zonal wind associated with the magnetic disturbances prevailing several hours earlier. Some tentative evidence on the Brazilian dusk sector disturbance field being larger than that of the Asian dusk sector support the existence of a longitude asymmetry in the intensity of the disturbance electric field.

Ionospheric Plasma Bubble Climatology Over Brazil Based on 22Years (1977-1998) of 630 nm Airglow Observations

Abstract The frequency of occurrence of the large-scale ionospheric plasma depletions, or plasma bubbles, over Cachoeira Paulista—CP (45°W, 33°S, 28°S dip), with solar activity, season, and geomagnetic activity, during the period of 1977–1998, is studied here based on 934 days of scanning photometers and imagers data of the atomic oxygen 630 nm nocturnal airglow. The bubble occurrence for the entire period of study shows maximum rate of 86% in January and 11% in May. The seasonal pattern of the bubble occurrence shows up as broad a maximum and a minimum centered around summer and winter months, respectively. While during the high-solar activity the maximum extends from September to April, for the low-activity period it extends from October to March. The average sunspot number for the solar maximum (minimum) for the data of this work is 129.9 (33.1). Significant increase of bubble occurrence (by ∼80%) from low to high solar activity levels is found to occur in the present analysis only during the equinoctial months of March–April and September–October. The post-sunset plasma bubble occurrence over Cachoeira Paulista is found to decrease with increase of Kp that precedes the sunset by >4 h , suggesting the role of disturbance dynamo electric field to inhibit its development. Increase of Kp during sunset hours can increase the bubble occurrence as a possible effect of prompt penetration electric field. These results are presented and discussed in this paper.

Magnetospheric disturbance effects on the Equatorial Ionization Anomaly (EIA) : an overview

Abstract The Equatorial lonization Anomaly (EIA) development can undergo drastic modification in the form of an anomalous occurrence at local times outside that of its quiet time development and/or inhibition/enhancement at local times of its normal occurrences. This happens for disturbed electrodynamic conditions of the global ionosphere-thermosphere-magnetosphere system, consequent upon the triggering of a magnetospheric storm event. Direct penetration to equatorial latitudes of the magnetospheric electric fields and the thermospheric disturbances involving winds, electric fields and composition changes produce significant alteration in the EIA morphology and dynamics. Results on statistical behaviour based on accumulated ground-based data sets, and those from recent theoretical modelling efforts and from satellite and ground-based observations, are reviewed. Some outstanding problems of the EIA response to magnetospheric disturbances that deserve attention in the coming years are pointed out.

Equatorial anomaly effects on GPS scintillations in brazil

In a collaborative study, INPE and Cornell University have installed several Global Positioning System (GPS) based scintillation monitors over the Brazilian territory in order to study L Band scintillation. These scintillation monitors were developed by Cornell University to measure the amplitude scintillation observed at L1 (1.575 GHz) GPS signal and are sensitive to ionospheric irregularities of about 400 meters scale size. This paper describes some characteristics of the intensity of scintillations observed at three observation sites in Brazil: (1) Sao Luis (2.33 ° S, 44 ° W, dip latitude 1.3 ° S), located at magnetic equator, (2) Sao Jose dos Campos (23.21 ° S, 45.86 ° W, dip latitude 17.8 ° S), located under the equatorial anomaly peak and (3) Cuiaba (15.33 ° S, 56.46 ° W, dip latitude 6.1 ° S), an intermediate observation site located in between the magnetic equator and the equatorial anomaly peak. Analysis of data from January to March of 2000 showed that the occurrence percentage as well as the magnitude of the L Band scintillation increase with latitude from the magnetic equator to the equatorial anomaly crest as previously reported by Basu et al. (1988). Strong scintillation with S4 index exceeding 0.5 only has been observed under equatorial anomaly peak while at magnetic equator scintillation intensity (S4 index) did not exceed 0.3. Such studies from the network of stations set up by INPE and Cornell University in Brazil, where the effect of large declination controls the ESF statistics, will be very useful for developing a regional scintillation model for use in IRI.

Equatorial ionospheric vertical plasma drift model over the Brazilian region

Comparison between equatorial inospheric F region vertical plasma drift from satellite measurements [Fejer et al., 1995], for the Brazilian longitude sector, and the drifts derived from ionosonde measurements around sunset shows significant differences on the prereversal peak behavior during solstices of high solar activity periods. Using ionosonde measurements around sunset and satellite measurements at other local times, we constructed an ionospheric vertical plasma drift model that is representative of the equatorial region over the Brazilian longitudes, where the magnetic declination is around −20°. The so derived drift model, here called IDM (ionosonde drift model), is used as an input to the Sheffield University plasmasphere-ionosphere model (SUPIM). It is shown that the F layer heights given by SUPIM with IDM are in good agreement with ionosonde measurements over the Brazilian longitudes and that IDM better simulates the F layer heights than the averaged drifts given by the satellite drift model [Fejer et al., 1995].

Ionospheric irregularity zonal velocities over Cachoeira Paulista

Abstract We have studied the zonal drift velocity of nighttime ionospheric irregularities from Cachoeira Paulista (22.41°S,45°W, dip latitude −17.43°), a station under the Equatorial Anomaly, from December 1998 to February 1999 using L1 band GPS receivers and OI 630 nm all-sky images. The average decimetric solar flux index for this period of increasing solar activity was about 145 and magnetically quiet days with ΣKp 55 m in the magnetic east–west direction and probed small scale plasma structures (scale size about 400 m ) at altitudes near 350 km . The zonal irregularity drift velocities measured by this technique were eastward with values of about 160 m / s at 20 LT, about 140 m / s around midnight, and decreased further in the post-midnight sector. The variability of these drifts decreased significantly after midnight. The zonal velocities of large scale plasma structure were obtained using OI 630 nm all-sky images from a region located about 24.1°S and 45°W at a nominal height of 250 km which corresponds to the bubble projection along the magnetic field lines to 350 km over Cachoeira Paulista. These all-sky imager derived zonal drifts are also eastward, but have magnitudes smaller than the spaced GPS eastward drifts, particularly in the pre-midnight sector. We will discuss these two drift measurement techniques and the interpretation of our results.

Equatorial electrojet irregularities investigations using a 50 MHz back-scatter radar and a digisonde at São Luís: some initial results

Abstract A new 50 MHz coherent back-scatter radar has recently become operational at the Equatorial Space Observatory of INPE at Sao Luis (2.33°S, 44.2°W, dip: −0.5°), in Brazil, located close to the center of the equatorial electrojet. Preliminary results from a 12-day campaign in December 1999 involving this radar and a nearby digisonde are presented in this paper. The range–time–intensity (RTI) maps of electrojet irregularities show short-period ( ∼5 min or larger) fluctuations and significant day-to-day variability. A notable characteristic is the midday descent in the altitude of the RTI pattern observed on some days when the dominant Doppler spectra are of type-2. This feature is similar to the diurnal variation of the base height of the q-type sporadic-E-layer (h′Es−q) simultaneously observed by the digisonde. Quasi-periodic fluctuations in the RTI and type-1 and type-2 spectral intensities seem to indicate electric field modulation from winds associated with typical gravity wave periods. Large surges in the westward irregularity drift velocity, presenting a higher velocity tail to the Doppler spectral distribution, is observed at sunset concurrent with the evening F-layer vertical uplift as observed by the digisonde. Some implications of these results on the E- and F-region electrodynamics are discussed.

Total electron content at low latitudes and its comparison with the IRI90

Abstract We used total electron content (TEC) data measured by Faraday rotation technique over Cachoeira Paulista (22.5°S, 45°W), in Brazil, to study the TEC variations with the solar flux at 10.7 cm (F10.7) and to compare the results with the IRI90 predictions. The data were divided into summer, equinox and winter. During the analysed period F10.7 varied from 66 up to 330. Our data shows that the observed TEC at 1600 LT (around the diurnal maximum) and at 0500 LT (around the diurnal minimum) increases with F10.7 until saturation is reached which occurs at F10.7≈210 to 220 for equinox and summer, and at F10.7≈180 for winter months. Comparison with the IRI90 predictions shows that IRI overestimates the TEC at 0500 LT for all solar flux values. At 1600 LT, IRI overestimates the observed TEC for low solar flux but underestimates it for high solar flux values.