M. A. Abdu

Equatorial plasma fountain and its effects over three locations: Evidence for an additional layer, the F 3 layer

The equatorial plasma fountain and equatorial anomaly in the ionospheres over Jicamarca (77°W), Trivandrum (77°E), and Fortaleza (38°W) are presented using the Sheffield University plasmasphere-ionosphere model under magnetically quiet equinoctial conditions at high solar activity. The daytime plasma fountain and its effects in the regions outside the fountain lead to the formation of an additional layer, the F3 layer, at latitudes within about plus or minus 10° of the magnetic equator in each ionosphere. The maximum plasma concentration of the F3 layer, which occurs at about 550 km altitude, becomes greater than that of the F2 layer for a short period of time before noon when the vertical E × B drift is large. Within the F3 layer the plasma temperature decreases by as much as 100 K. The ionograms recorded at Fortaleza on January 15, 1995, provide observational evidence for the development and decay of an F3 layer before noon. The neutral wind, which causes large north–south asymmetries in the plasma fountain in each ionosphere during both daytime and nighttime, becomes least effective during the prereversal strengthening of the upward drift. During this time the plasma fountain is symmetrical with respect to the magnetic equator and rises to over 1200 km altitude at the equator, with accompanying plasma density depletions in the bottomside of the underlying F region. The north–south asymmetries of the equatorial plasma fountain and equatorial anomaly are more strongly dependent upon the displacement of the geomagnetic and geographic equators (Jicamarca and Trivandrum) than on the magnetic declination angle (Fortaleza).

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.

Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere

A physical mechanism and the location and latitudinal extent of an additional layer, called the F3 layer, that exists in the equatorial ionosphere are presented. A statistical analysis of the occurrence of the layer recorded at the equatorial station Fortaleza (4°S, 38°W; dip 9°S) in Brazil is also presented. The F3 layer forms during the morning-noon period in that equatorial region where the combined effect of the upward E×B drift and neutral wind provides a vertically upward plasma drift velocity at altitudes near and above the F2 peak. This velocity causes the F2 peak to drift upward and form the F3 layer while the normal F2 layer develops at lower altitudes through the usual photochemical and dynamical effects of the equatorial region. The peak electron density of the F3 layer can exceed that of the F2 layer. The F3 layer is predicted to be distinct on the summer side of the geomagnetic equator during periods of low solar activity and to become less distinct as the solar activity increases. Ionograms recorded at Fortaleza in 1995 show the existence of an F3 layer on 49% of the days, with the occurrence being most frequent (75%) and distinct in summer, as expected. During summer the layer occurs earlier and lasts longer compared to the other seasons; on the average, the layer occurs at around 0930 LT and lasts for about 3 hours. The altitude of the layer is also high in summer, with the mean peak virtual height being about 570 km. However, the critical frequency of the layer (f0F3) exceeds that of the F2 layer (f0f2) by the largest amounts in winter and equinox; f0F3 exceeds f0F2 by a yearly average of about 1.3 MHz.

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.

Long term trends in the frequency of occurrence of the F3 layer over Fortaleza, Brazil

Abstract Recent studies using model calculation and ionospheric observations have revealed the existence of an additional layer in the topside equatorial ionosphere, the F3 layer. The observations using bottomside ionograms from locations close to the magnetic equator in Brazilian region have shown that the occurrence of the layer is very high from December to February (local summer) and from June to August (local winter). In fact, for the year 1995 the occurrence of the F3 layer is >75% during the months of January, February and December, and it is >65% for the period of June, July and August (Geofisica Int. 39 (2000) 57). In this work, we use 25 years of data for the months of January and August to investigate how the layer occurrence varies with the magnetic dip angle and solar activity.

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 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].

A plasma temperature anomaly in the equatorial topside ionosphere

A study of the thermal structure of the low-latitude (30°N to 30°S) ionosphere under equinoctial conditions at low, medium, and high solar activity has been carried out using the Sheffield University plasmasphere-ionosphere model (SUPIM) and Hinotori satellite observations. The study reveals the existence of an anomaly in the plasma (electron and ion) temperature in the topside ionosphere during the evening-midnight period. The anomaly, called the equatorial plasma temperature anomaly (EPTA), is characterized by a trough around the magnetic equator with crests on either side. The trough develops before the crests. The model results show that the anomaly occurs between 1900 and 0100 LT at altitudes between 450 and 1250 km; the strongest anomaly occurs around 2130 LT at 950 km altitude during high solar activity. The temperature trough of the anomaly arises from the adiabatic expansion of the plasma and an increase in plasma density caused by the prereversal strengthening of the upward vertical E × B drift. The temperature crests arise from the combined effect of the reverse plasma fountain and nighttime plasma cooling. The electron temperature measured by the Hinotori satellite near 600 km altitude during medium and high solar activity periods shows the existence of the EPTA with characteristics in close agreement with those obtained by the model. The model also reproduces the occurrence of a daytime temperature bulge in the electron temperature in the bottomside ionosphere; the ion temperature shows no bulge.

Annual variations of the ionosphere: A review based on MU radar observations

A review of the annual variations of the ionosphere, which focuses on the physical mechanisms causing the well-known seasonal anomaly and equinoctial asymmetry, is presented. In this review, the electron density (Ne), electron and ion temperatures (Te and Ti), and field-parallel and field-perpendicular plasma velocities (V∥ and Vt), measured by the MU radar in the 180–600 km altitude range during 1986–1994, are analysed to study the altitude dependence of the seasonal anomaly and equinoctial asymmetry. The meridional component of the thermospheric neutral wind velocity (Uθ) derived from V∥ and neutral densities obtained from MSIS-86 are used to investigate the relative importance of the chemical and dynamical processes causing the anomaly and asymmetry. The review concludes that, although the anomaly and asymmetry involve chemical and dynamical processes, the dynamical processes (mainly through the neutral wind) predominate in the asymmetry while the chemical processes predominate in the anomaly.