J. H. A. Sobral

A new aspect of magnetic declination control of equatorial spread F and F region dynamo

Range spread F (RSF) data for a 12-year period (1978–1990) over the Brazilian low-latitude station, Cachoeira Paulista, are analyzed to determine the fine structure of the seasonal pattern of the irregularity occurrence, which appears consistent throughout an entire solar cycle. The RSF occurrence for one of these years is compared also with that over the equatorial station, Fortaleza, to show that the seasonal pattern discussed here corresponds to that of plasma bubble irregularity developments. A striking result that has come out of the present analysis concerns two secondary minima, during the epoch of broad annual maximum, in the RSF that occur in association with the two nodal points at the magnetically conjugate E layer sunset local times, that is, at the perfect alignment of the sunset terminator with the magnetic meridian of the station. The occurrence of these minima, seen in individual solar minimum and solar maximum years (as also in the overall mean behavior), is interpreted on the basis of a simplified F region dynamo development model that considers also asymmetric conjugate E layer decay conditions. Competing roles of a prereversal zonal electric field enhancement and of its height gradient seem to determine the occurrences of these secondary minima in RSF.

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.

Effects of intense storms and substorms on the equatorial ionosphere/thermosphere system in the American sector from ground-based and satellite data

Equatorial ionospheric responses to magnetospheric storm/substorm-associated electric fields are investigated for a few intense events of the equinoctial months of solar maximum years 1978–1979. All the magnetic storms considered here are the result of the transit at Earth of interplanetary magnetic clouds. The interplanetary magnetic field data Bz from the ISEE 3 satellite, the auroral electrojet activity index AE, and the ring current index Dst are used as indicators of the disturbed magnetospheric conditions, and the ionospheric response features are analyzed using the F layer critical parameters h′F, h′F3, hpF2, and ƒ0F2. Focus is given to identify, when a large number of sequential substorms occurs, (1) the responses to prompt penetration electric field (from individual substorm events) as different from the delayed effect from the disturbance dynamo electric field and (2) the verification of local time dependences of the disturbance electric field polarity as predicted from the existing theoretical models. We have found evidence of near-midnight polarity reversal of prompt penetration disturbance electric field during the course of a developing substorm. Evidence is provided also on the near-midnight polarity reversal for the disturbance dynamo electric field. The prereversal enhancement electric field at sunset, produced by the F layer dynamo, is found to undergo drastic day-to-day variations in the course of a disturbed interval. However, the competing influences of the prompt versus delayed electric fields after a series of substorms could result at times in partial, or even complete, cancellation of the effects, so that the prereversal enhancement in the vertical drift could appear unaffected by the disturbances. There are indications that the disturbance dynamo electric field effects on the equatorial ionosphere last for one more day past the end of the substorm recovery.

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.

Onset conditions of equatorial (range) spread F at Fortaleza, Brazil, during the June solstice

Case studies are made of the ionospheric, thermospheric, and geomagnetic conditions associated with the occasional onset of range spread F at Fortaleza (4°S, 38°W, dip latitude −1.8°, and magnetic declination 21°W), Brazil, in the June solstice, a season in which frequency spread F is typically seen in the postsunset hours (after 2000 LT). It is found that the F layer experiences quite consistently a large vertical drift early in the evening hours (1815–1915 LT) on days of range spread F in comparison to days of only frequency spread F. The anomalously large dusk time vertical plasma drift occurs under both geomagnetically disturbed and quiet conditions. There is no significant change in the pattern of meridional neutral winds (at and prior to the time of onset of range spread F) estimated from the data of F layer peak height (hmax) at the low-latitude station, Cachoeira Paulista (23°S, 45°W, and dip 26.5°S), using a modified form of the servo model. The results strongly suggest that though the meridional (poleward) wind is very effective in inhibiting the growth of Rayleigh-Taylor (R-T) instability and hence range spread F at Fortaleza during the June solstice as shown by Maruyama [1996], its variability does not play an important role in creating favorable conditions for R-T instability on a day-to-day basis. The prerequisite for the occasional occurrence of range spread F at Fortaleza in the June solstice seems to be the presence of an impulsive and large F layer vertical plasma drift, a condition favorable for destabilizing the bottomside F region through R-T instability mechanism because of the high altitude of the layer.

High resolution OI (630 nm) image measurements of F‐region depletion drifts during the Guará Campaign

A high performance, all-sky, imaging system has provided data on the evolution and drift motions of F-region depletions above the magnetic dip equator at Alcântara, Brazil, (2.3°S, 44.5°W). Monochromatic images of depletions in the OI(630 nm) nightglow were recorded on eight nights during 1-16 October, 1994, as part of the Guara campaign. The drift motions of the depletions were typically 80–100 m/s eastward prior to local midnight and reduced to a minimum of ∼30–50 m/s in the morning hours, in accord with previous observations. However, on October 2–3 and 12–13 the depletions were observed to reverse direction for ∼60–90 min, achieving westward speeds of ∼30 m/s before the motion reverted to eastward around 0100 LT and accelerated to 35–45 m/s near dawn. Magnetic activity and other evidence suggests that these reversals in the motion of the airglow depletions probably result from reversals in the F-region dynamo rather than from shifts in the altitude of the shear in the nighttime F-region plasma drift.

Equatorial disturbance dynamo electric field longitudinal structure and spread F: A case study from GUAR/EITS Campaigns

Digisondes/ionosondes, an HF Doppler radar and magnetometers were operated in Brazil and India during the September/October 1994 GUARA/EITS campaigns. Analysis of the data for the two disturbed intervals, 2–4 October and 25–27 September provided evidence of a longitudinal structure in the disturbance dynamo (DD) electric field at low latitudes. The DD electric field which is westward in the evening, inhibited the developments of the equatorial prereversal electric field and postsunset ESF, whereas the simultaneous eastward field in the predawn sector, did not lead to ESF, contrary to what is normally expected. This nondevelopment of ESF is suggested as evidence of the stabilizing effect of transequatorial neutral winds associated with stormtime circulation. The campaign observations clearly demonstrate that the DD electric fields could often mask the low latitude ionospheric responses to prompt penetration electric fields in the course of sustained substorm activity.

Variability of an additional layer in the equatorial ionosphere over Fortaleza

The day-to-day variations (or the weather) of an additional layer, called the F 3 layer, that has been predicted to exist at altitudes above the F 2 peak in the equatorial ionosphere are studied through ionosonde observations and theoretical modeling. The ionograms recorded in 1995 at the equatorial station Fortaleza (4°S, 38°W; dip angle 9°S) in Brazil show the occurrence of the F 3 layer during daytime from 0800 to 1630 LT, with the duration of occurrence ranging from 15 min to 6 hours. Although the layer occurs most frequently (75% of the days) in local summer as previously predicted, there are consecutive and individual magnetically quiet and disturbed days when the layer does not occur. There are also days when the layer reoccurs. The model results, obtained using the Sheffield University plasmasphere-ionosphere model, show that the day-to-day variations of the F 3 layer arise from the corresponding variations of the vertical plasma velocity. The layer occurs when the time-cumulative vertical velocity displaces the daytime F 2 peak to high altitudes, to form the F 3 layer, while the normal F 2 layer develops at low altitudes. Sudden displacements result in more distinct F 3 layers than gradual displacements. Model results also show that the plasma temperature within the F 3 layer decreases as the plasma density increases, and, like the plasma density, the plasma temperature also undergoes large day-to-day variations.

Equatorial spread-F occurrence statistics in the American longitudes: Diurnal, seasonal and solar cycle variations

Abstract Incorporation of equatorial spread-F (ESF) occurrence statistics into the IRI requires first a detailed characterization, as a function of longitude, of the spatial and temporal distribution of the different types of ESF irregularity activity indices based on data from the different available techniques, ionosondes/digisondes, scintillation and satellite in-situ measurements. In this paper we present and discuss the distribution statistics of ESF events, mainly from ionograms, using the long term data sets available from equatorial and low latitude stations in Brazil and Argentina. Significant longitudinal dependence in the ESF occurrence is observed. The results are discussed on the basis of the longitude dependent magnetic declination angle that characterizes this region and on relevant thermospheric parameters.