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This paper presents a review of the progress in the understanding of an additional layer in the low latitude ionosphere, the F3 layer. A brief introduction to the daytime equatorial electrodynamics required for understanding the formation of F3 layer and formation of the usual ionospheric regions and layers are also included for continuity. Numerous papers on various aspects of F3 layer and topside ledge (F3 layer in the topside ionosphere) were published by different research groups using observations and modelling. The first time observations and important new aspects are highlighted and areas for further studies are suggested in this review. The important new aspects include an automated procedure for identifying F3 layer, global map of F3 layer and its longitudinal wave structure, solar activity variation of F3 layer and its long term trend, F3 layer in TEC, F3 layer at sunrise and sunset hours, simultaneous observations of F3 layer and upward ExB drift velocity, simultaneous observations of F3 layer in conjugate summer and winter locations, rapid ascend of F3 layer during geomagnetic storms as an indicator of eastward prompt penetration electric field and mechanism of F4 layer. Main points are summarized at the end.

Following a brief history and progress of ionospheric research, this paper presents a brief review of the recent developments in the understanding of two major phenomena in low and mid latitude ionosphere—the equatorial ionization anomaly (EIA) and involved equatorial plasma fountain (EPF) and ionospheric irregularities. Unlike the easy-to-understand misinterpretations, the EPF involves field perpendicularE×B plasma drift and field-aligned plasma diffusion acting together and plasma flowing in the direction of the resultant at all points along the field lines at all altitudes. The EIA is formed mainly from the removal of plasma from around the equator by the upward E×B drift creating the trough and consequently the crests with small accumulation of plasma at the crests when the crests are within ~±20° magnetic latitudes and no accumulation when they are beyond ~±25° magnetic latitudes. The strong EIA under magnetically active conditions arises from the simultaneous impulsive action of eastward prompt penetration electric field and equatorward neutral wind. Intense ionospheric irregularities develop in the post-sunset bottom-side equatorial ionosphere when it rises to high altitudes, and evolve nonlinearly into the topside. Pre-reversal enhancement (PRE) of the vertical upward E×B drift and its fluctuations amplified during PRE provide the driving force and seed, with neutral wind and gravity waves being the primary sources. At low solar activity especially in summer when fast varying PRE is absent, the slow varying gravity waves including large scale waves (LSW) seem to act as both driver and seed for weak irregularities. At mid latitudes, the irregularities are weak and associated with medium scale traveling ionospheric disturbances (MSTIDs). A low latitude minimum in the occurrence of the irregularities at March equinox predicted by theoretical models is identified. The minimum occurs on the poleward side of the EIA crest and shifts equatorward from ~25° magnetic latitudes at high solar activity to below 17° at low solar activity.

In this paper we carry out a comparative study of the daytime (7–18 LT) behavior of the near-equatorial ionospheric F region at the end of the long deep solar minimum (2009) with respect to that of the previous normal solar minimum (1995) in the Indian longitude sector using ionosonde observations of F layer parameters, radar observations of E × B drift, and the IRI-2012 (International Reference Ionosphere-2012) model. We investigate the F2 and F3 layer behaviors separately. The results reveal that the peak frequencies of the F layer (fpeak), F2 layer (foF2), and F3 layer (foF3) in 2009 are consistently lower than those in 1995. Maximum difference in fpeak/foF2/foF3 between 2009 and 1995 observations is found in the equinoxes followed by winter and summer. The annual mean, seasonal mean, and 10 day mean peak electron density (corresponding to fpeak) in 2009 were lower than those in 1995 by as much as 34%, 46%, and 65%, respectively. Solar rotation effect is less conspicuous in 2009 than in 1995, consistent with the solar rotation signature in F10.7. Observations also show considerable amount of equinoctial asymmetry in electron density, which is found to be closely linked with the corresponding asymmetry in the vertical E × B drift. Seasonal mean peak electron densities of the F layer (corresponding to fpeak) and the F2 layer (corresponding to foF2) observed during the deep solar minimum of 2009 were smaller than those corresponding to IRI-2012 model foF2 by as much as 45% and 50%, respectively, underlining the need to incorporate the data collected during the long deep minimum in the IRI model. The unusually weak ionosphere observed in 2009 is discussed in terms of the direct effect of the low solar EUV flux in 2009 as compared to 1995 and its indirect effects on ionospheric electric field, thermospheric composition (or O/N2 ratio), and thermospheric neutral winds.

In this paper, we present model simulations of the Mid-latitude Summer Nighttime Anomaly (MSNA) in the Northern Hemisphere, which is characterized by noon-time dip and evening maximum in the diurnal variation of the ionospheric density. The simulations are carried out using SUPIM (Sheffield University Plasmasphere Ionosphere Model) for solar minimum at 135° E longitude where MSNA is most pronounced in the Northern Hemisphere. The simulations are used to understand the relative importance of electric fields, and zonal and meridional winds in the formation of MSNA. The wind velocities measured by the Middle and Upper atmosphere radar (MU radar) and those obtained from the horizontal wind model (HWM93) are used. The results show that the formation of MSNA is closely related to the diurnal variation of the neutral winds with little contribution from the changes in the electric fields. The observed features of MSNA are better reproduced when MU radar winds are used as model input rather than HWM winds.

Previous investigations of ionospheric storm-enhanced density (SED) and tongue of ionization (TOI) focused mostly on the behavior of TOI during intense geomagnetic storms. Little attention has been paid to the spatial and temporal variations of TOI during weak to moderate geomagnetic disturbed conditions. In this paper we investigate the source and development of TOI during a moderate geomagnetic storm on 14 October 2012. Multi-instrumental observations including GPS total electron content (TEC), Defense Meteorological Satellite Program (DMSP) in situ measured total ion concentration and ion drift velocity, SuperDARN measured polar ion convection patterns, and electron density profiles from the Poker Flat Incoherent Scatter Radar (PFISR) have been utilized in the current analysis. GPS TEC maps show salient TOI structures persisting for about 5 h over high latitudes of North America on 14 October 2012 in the later recovery phase of the storm when the magnitudes of IMF By and Bz were less than 5 nT. The PFISR electron density profiles indicate that the extra ionization for TEC enhancements mainly occurred in the topside ionosphere with no obvious changes in the bottomside ionosphere and vertical plasma drifts. Additionally, there were no signatures of penetration electric fields in the equatorial electrojet data and upward ion drifts at high latitudes. At the same time, strong subauroral polarization streams with ion drift speeds exceeding 2.5 km/s carried sunward fluxes and migrated toward lower latitudes for about 5° based on the DMSP cross-track drift measurements. Based on those measurements, we postulate that the combined effects of initial build-up of ionization at midlatitudes through daytime production of ionization and equatorward (or less poleward than normal daytime) neutral wind reducing downward diffusion along the inclined filed lines, and an expanded polar ion convection pattern and its associated horizontal plasma transport are important in the formation of the TOI.

In this paper, we analyze daytime observations of the critical frequencies of the F2 (foF2) and F3 (foF3) layers based on ionosonde observations made from Indian low latitudes close to the magnetic equator and study their local time, seasonal, planetary-scale variations (including the solar rotation effect), and solar activity dependence. Given the occurrence of the F3 layer, which has remarkable local time, seasonal and solar activity dependences, variations in foF2 have been evaluated. Local time variations in foF2 and foF3 show noon "bite-out" in all seasons and in all solar activity conditions, which are attributed to vertically upward plasma transport by the zonal electric field and meridional neutral wind. Comparison of observed foF2 with those of the IRI-2012 model clearly shows that the model values are always higher than observed values and the largest difference is observed during noontime owing to the noon bite-out phenomenon. Peak frequency of the F layer (foF2 / foF3), however, is found to have better agreement with IRI-2012 model. Seasonal variations of foF2 and foF3 show stronger asymmetry at the solstices than at the equinoxes. The strong asymmetry at the solstice is attributed to the asymmetry in the meridional neutral wind with a secondary contribution from E × B drifts, and the relatively weak asymmetry observed at the equinox is attributed to the asymmetry in E × B drifts. Variations in foF2 and foF3 with solar flux clearly show the saturation effect when F10.7 exceeds ~ 120 sfu, which is different from that of the mid-latitudes. Irrespective of solar flux, both foF2 and foF3 in summer, however, are found to be remarkably lower than those observed in other seasons. Variations in foF2 show dominant periods of ~ 27, ~ 16 and ~ 6 days. Intriguingly, amplitudes of ~ 27-day variations in foF2 are found to be maximum in low solar activity (LSA), moderate in medium solar activity (MSA) and minimum in high solar activity (HSA), while the amplitudes of ~ 27-day variations in F10.7 are minimum in LSA, moderate in MSA and maximum in HSA. These results are presented and discussed in light of current observational and model-based knowledge on the variations of low-latitude foF2 and foF3.

This thematic series contains 4 papers mostly presented at the 2016 AOGS meeting in Beijing. The four papers investigate four key regions in the magnetosphere–ionosphere coupling process: mid-tail magnetosphere, near-Earth magnetosphere, inner magnetosphere, and the polar ground region. Guo et al. (Geosci Lett 4:18, 2017) study the current system in reconnection region using 2.5D particle-in-cell simulations. Yao et al. (Geosci Lett 4:8, 2017) use conjugate measurements from ground auroral imagers and in situ THEMIS spacecraft to reveal the mechanism for the wave-like auroral structures prior to substorm onset. Zhang et al. (Geosci Lett 4:20, 2017) investigate the profiles of resonance zone and resonant frequency in the Landau resonance between radiation belt electrons and magnetosonic waves and between protons and cyclotron waves. Rae et al. (Geosci Lett 4:23, 2017) determine the relative timing between sudden increases in amplitude, or onsets, of different ultra-low-frequency wave bands during substorms.

We present a detailed investigation of the solar rotation effects on the Martian high-latitude (~63°N–81°N) ionosphere using the electron density (Ne) data measured by Mars Global Surveyor and solar XUV and EUV fluxes measured by SOHO under high (2000–2001), medium (2003), and low (2005) solar activity conditions. A fast Fourier transform spectral analysis method is used to estimate the amplitude of the rotation period in these parameters. This method clearly reveals the presence of solar rotation effects in the Martian ionospheric Ne at all altitudes (90–220 km), peak electron density (NmM2), and total electron content under the three solar activity conditions. These effects are in phase with the solar UV fluxes (corrected for the Martian orbit). The period of rotation effect (~26 days) is the same at all altitudes, though its amplitude is strongest at the ionospheric M2 peak (~135–140 km, ~3.5–6% of the mean values) and has a secondary enhancement at the M1 peak (~110–115 km). The effect of solar rotation on the M2 peak is larger during medium solar activity (2003) than during high solar activity (2000–2001). The effect, however, is absent in the ionospheric peak height (hmM2). The rotation effects on Mars are also compared with those on the Earth. Unlike at Mars, the Earth's high-latitude ionosphere shows no clear solar rotation effect, though the effect is observed clearly at lower latitudes.

[1] Although the basic mechanism of an additional stratification of the daytime equatorial F region, called the F3 layer, has been explained, two observational aspects that remain not so clear are its summer-winter differences and simultaneous occurrences on either side of the geomagnetic equator. In this paper, we address these two aspects using simultaneous observations of the F3 layer and vertical E×B drift velocity made for the first time in an Indian station Gadanki (6.5°N mag. lat) during the low solar activity period 2008–2009, and making model computations using the Sheffield University Plasmasphere Ionosphere Model. The observations confirm the frequent occurrence of the F3 layer in summer months compared to winter months; the observations also reveal the occasional occurrence of clear and distinct F3 layer in winter although such a layer is frequent in summer. In addition, a threshold vertical plasma velocity (involving the E×B drift and neutral wind) and its time integrated value are found to be important for the formation of the F3 layer. The model results qualitatively reproduce the observations; and show that irrespective of season the formation of the F3 layer is centered on that side of the equator where the equatorward neutral wind reduces the downward field-aligned flow of plasma, and the latitude band of the layer can extend to the opposite hemisphere, especially when the vertical plasma velocity is large.

[1] This study examines the seasonal and interannual variation of the major migrating tidal components in midlatitude to low-latitude total electron content (TEC) observations from the FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellite constellation from 2007 to 2011. Although the absolute amplitudes of the TEC zonal mean and migrating tidal components show a strong positive relation to the increasing and decreasing phases of the solar cycle, the relative tidal amplitudes following normalization by maximum background values show a more varied response to solar activity levels. Features of ionospheric local time variation produced by individual migrating tidal components are consistent from year to year, with DW1 forming the equatorial daytime peak in TEC, SW2 corresponding to the generation of the equatorial ionization anomaly (EIA) crests, and TW3 contributing to the TEC trough between the EIA crests. Numerical experiments using Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) are also performed to determine the sensitivity of the ionospheric migrating tides to upward propagating migrating tidal components from the neutral mesosphere and lower thermosphere (MLT). Zonal mean TECs decrease when MLT tidal forcing is applied and are particularly sensitive to the MLT DW1. Most of the ionospheric SW2 response is attributable to MLT SW2 forcing, enhancing the EIA crests by amplifying the equatorial fountain. TW3 in the model is generated through both in situ photoionization and nonlinear interaction between DW1 and SW2.

[1] The paper studies the physical mechanisms of the ionospheric storms at equatorial and higher latitudes, which are generally opposite both during the main phase (MP) and recovery phase (RP) of geomagnetic storms. The mechanisms are based on the natural tendency of physical systems to occupy minimum energy state which is most stable. The paper first illustrates the recent developments in the understanding of the mechanisms during daytime MPs when generally negative ionospheric storms (in Nmax and TEC) develop at equatorial latitudes and positive storms occur at higher latitudes, including why the storms are severe only in some cases. The paper then investigates the relative importance of the physical mechanisms of the positive ionospheric storms observed at equatorial latitudes (within ±15°) during daytime RPs when negative storms occur at higher latitudes using CHAMP Ne and GPS-TEC data and Sheffield University Plasmasphere Ionosphere Model. The results indicate that the mechanical effect of the storm-time equatorward neutral winds that causes plasma convergence at equatorial F region could be a major source for the positive storms, with the downwelling effect of the winds and zero or westward electric field, if present, acting as minor sources.

The paper studies the relative importance of penetrating eastward electric field (PEEF) and direct effects of equatorward neutral wind in leading to positive ionospheric storms at low-mid latitudes using observations and modeling. The observations show strong positive ionospheric storms in total electron content (TEC) and peak electron density (N max) at low-mid latitudes in Japan longitudes (≈125°E–145°E) during the first main phase (started at sunrise on 08 November) of a super double geomagnetic storm during 07–11 November 2004. The model results obtained using the Sheffield University Plasmashpere Ionosphere Model (SUPIM) show that the direct effects of storm-time equatorward neutral wind (that reduce poleward plasma flow and raise the ionosphere to high altitudes of reduced chemical loss) can be the main driver of positive ionospheric storms at low-mid latitudes except in N max around the equator. The equatorward wind without PEEF can also result in stronger positive ionospheric storms than with PEEF. Though PEEF on its own is unlikely to cause positive ionospheric storms, it can lead to positive ionospheric storms in the presence of an equatorward wind.

The relative importance of the main drivers of positive ionospheric storms at low-mid latitudes is studied using observations and modeling for the first time. In response to a rare super double geomagnetic storm during 07–11 November 2004, the low-mid latitude (17°–48°N geomag. lat.) ionosphere produced positive ionospheric storms in peak electron density (NmF2) in Japan longitudes (≈125°–145°E) on the day of main phase (MP1) onset (06:30 LT) and negative ionospheric storms in American longitudes (≈65°–120°W) on the following day of MP1 onset (13:00–16:00 LT). The relative effects of the main drivers of the positive ionospheric storms (penetrating daytime eastward electric field, and direct and indirect effects of equatorward neutral wind) are studied using the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The model results show that the penetrating daytime (morning–noon) eastward electric field shifts the equatorial ionisation anomaly crests in NmF2 and TEC (total electron content) to higher than normal latitudes and reduces their values at latitudes at and within the anomaly crests while the direct effects of the equatorward wind (that reduce poleward plasma flow and raise the ionosphere to high altitudes of reduced chemical loss) combined with daytime production of ionisation increase NmF2 and TEC at latitudes poleward of the equatorial region; the later effects can be major causes of positive ionospheric storms at mid latitudes. The downwelling (indirect) effect of the wind increases NmF2 and TEC at low latitudes while its upwelling (indirect) effect reduces NmF2 and TEC at mid latitudes. The net effect of all main drivers is positive ionospheric storms at low-mid latitudes in Japan longitude, which qualitatively agrees with the observations.

Vertical total electron content (GPS-TEC) data obtained from the dual-frequency GPS receiver network (GEONET) in Japan are compared with those calculated using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model is also used to estimate the electron content in the plasmaspheric sections of GPS ray paths for the three seasons of high solar activity (F10.7 = 165) under magnetically quiet conditions. According to the estimates, the plasmaspheric sections of vertical GPS ray paths over Japan at altitudes above the O+ to H+ transition height and above the upper altitude (2500 km) of Faraday rotation contain up to 11 and 9 TEC units (1 TEC unit = 1016 electrons m−2) of free electrons, respectively. The free electrons present above the Faraday rotation altitude can cause propagation errors of up to 4.9 ns in time delay and 1.6 m in range at the GPS L1 (1.57542 GHz) frequency. The plasmaspheric electron content, PEC, changes appreciably with season and latitude and very little with the time of the day. However, the percentage contribution of PEC to GPS-TEC changes most significantly with the time of the day; the contribution varies from a minimum of about 12% during daytime at equinox to a maximum of about 60% at night in winter.

Southwestward-propagating medium-scale traveling ionospheric disturbances (MSTIDs) observed over Shigaraki (34.85°N, 136.10°E) in Japan on the night of May 22, 1998 are analyzed in detail. The MSTIDs were detected with a 630.0 nm (OI) all-sky imager at Shigaraki and a large number of GPS (Global Positioning System) receivers distributed around Shigaraki. Each GPS receiver provided total electron content (TEC) between the GPS altitude (20,200 km) and the ground. MSTID amplitudes varied in space and time, and showed decay and enhancement during the southwestward propagation, suggesting that amplitudes of atmospheric gravity waves and the interaction process between gravity waves and F region plasma were highly variable. It is found that spatial and temporal fluctuations of the 630 nm intensity are well correlated with those of GPS-TEC except for a certain period of time. The Scheffield University Plasmasphere Ionosphere Model (SUPIM) is used to obtain theoretical relationships between the 630 nm airglow intensity and GPS-TEC and between their fluctuation amplitudes. The results indicate that the fluctuation amplitudes observed in weak airglow regions are caused by an electron density fluctuation of about ±20% occurring around an altitude of 250 km, where the 630 nm emission rate reaches a maximum, below the F layer peak altitude. Highly enhanced 630 nm intensity and GPS-TEC within a bright airglow region are due to an electron density enhancement of about 150% occurring at altitudes below 300 km.

Analysis of the EISCAT UHF CP1-J data during daytime and nighttime ion frictional heating events indicates that the temperature of heavier ions increases much more than that of lighter ions and the concentration of molecular ions increases while that of atomic ions decreases. These indications are studied through model calculations.

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.

Estudios recientes usando modelos y observaciones ionosféricas han revelado la existencia de una capa adicional en el tope de la ionosfera ecuatorial, la capa F3. En el presente trabajo analizamos los datos de ionosondas de dos estaciones: una de baja latitud y otra ecuatorial en Brasil, durante el año 1995, un periodo de baja actividad solar, con el objeto de estudiar la capa F3 en estas latitudes. Se observa que en la estación magnética de São Luís (2.3° S, 44° W, dip angle -0.5°), la ocurrencia de la capa F3 es mucho menor que en la estación de baja latitud de Fortaleza (4° S, 38° W, dip angle -9°). Los ionogramas de Fortaleza en 1995 muestran la existencia de la capa en el 50% de los días. Dicha ocurrencia es frecuente y distintiva (75%) en verano (mes de diciembre). Los ionogramas también muestran la existencia de la capa en el 66% de los días en invierno (mes de junio) y en el 28% en los equinoccios. La capa empieza a aparecer alrededor de las 0930 hora local y dura desde unos minutos hasta varias horas. Esto significa que la altura de la capa varía desde 570 km en verano hasta 440 km en invierno, aunque día a día la altura puede variar desde 375 km hasta 775 km. El modelo ionosférico SUPIM (Sheffield University Plasmasphere-Ionosphere Model) se usó para simular las características de la capa sobre Fortaleza. El modelo explica bien la ocurrencia de la capa en verano, pero no en invierno, a menos que se modifiquen algunos parámetros clave tales como el campo eléctrico y los vientos. Esto indica que los modelos existentes para el campo eléctrico y los vientos no son representativos de las latitudes bajas brasileñas en el invierno.

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.

The ionograms recorded at the equatorial station Fortaleza in Brazil in 1995 are analysed to study the occurrence and other characteristics of an additional layer, called the F3 layer, that has been predicted to exist in the equatorial ionosphere. At Fortaleza, the F3 layer occurs on 49% of the days, with the occurrence being most frequent 75% in summer as predicted. The occurrence time of the layer varies from about 0800 LT to 1500 LT and the layer lasts from about 15 min. to 6 hours. The peak (virtual) height of the layer ranges from about 375 km to 750 km and the critical frequency of the layer exceeds that of the F2 layer by 0.2 MHz to 2.3 MHz.

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.

The temperature variations in the ionosphere over the middle and upper atmosphere radar at Shigaraki (34.85°N, 136.10°E, magnetic latitude 25°N) in Japan are studied using the electron and ion temperature (Te and Ti, respectively) data measured by the radar during nearly a full solar cycle (1986–1995). A comprehensive picture of the diurnal, seasonal, and solar activity variations of Te and Ti is presented for the altitude range 200–550 km. The temperatures Te and Ti are found to have similar diurnal and altitude variations and different seasonal and solar activity dependence. With season, while daytime Te is highest in summer and lowest in equinox, daytime Ti is highest in equinox and lowest in summer. With solar activity, while daytime Te decreases, the corresponding Ti increases. The diurnal variation of Te is characterized by morning and evening peaks. The occurrence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoelectron heating of the morning and evening electron gas. However, neutral winds play a dominant role in the appearance of the peaks. A poleward wind, which reduces the electron density to a low value before sunset, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in Te are illustrated through model calculations using the Sheffield University plasmasphere-ionosphere model.

A brief description of the Sheffield University plasmasphere ionosphere model (SUPIM) is presented. In the model, time-dependent equations of continuity, momentum, and energy balance are solved along eccentric-dipole magnetic field lines for the densities, field-aligned fluxes and temperatures of the O + , H + , He + , N 2 + , O 2 + and NO + ions, and the electrons. A review of some of the important results from recent studies of the model is presented. The studies show that during daytime, the equatorial plasma fountain can rise to altitudes of around 800 km at the magnetic equator and can cover magnetic latitudes of about ± 30 °. At regions outside the fountain, plasma flows towards the magnetic equator from both hemispheres and leads to the formation of an additional layer, the F3 layer, at latitudes close to the magnetic equator (± 10 °). The peak electron density of the F3 layer can exceed that of the F2 layer for a short period of time near noon when the E × B drift is large. Associated with the enhanced electron densities of the F3 layer are reduced electron temperatures. The modelled electron temperatures and densities are in accord with observations made by the Hinotori satellite at 600 km altitude. Closer agreement in the modelled and observed values is achieved if the phase and magnitude of the meridional wind, as given by the HWM90 thermospheric wind model, are modified in accordance with the observations made by the Japanese MU radar and the AE-E satellite. There is better agreement in the modelled and observed values when the equatorial vertical E × B drift velocity model used by SUPIM has an altitude variation in accord with the observations made by the AE-E satellite and at Arecibo.

The Sheffield University Plasmasphere Ionosphere Model (SUPIM) has been used to study the effects of neutral winds of the north-south asymmetries in the ionospheric equatorial anomaly at longitudes 120 °E, 200 °E, 283 °E and 330 °E. The model results obtained for magnetically quiet equinoctial conditions at solar maximum produce north-south asymmetries in the equatorial anomaly in agreement with the observations. The study shows that the neutral wind causes the north-south asymmetries and that the asymmetries have longitudinal variation in accord with the longitudinal variations in the displacement of the geographic and geomagnetic equators and in the magnetic declination angle. At longitudes 120 °E and 283 °E, where the magnetic declination angle is small and the magnetic equators are located in opposite geographic hemispheres, the asymmetries in the anomaly are caused mainly by the asymmetries in the meridional wind. On the other hand, at longitudes 200 °E and 330 °E, where the geographic and geomagnetic equators are almost coincident and the magnetic declination angles are eastward and westward, respectively, the asymmetries in the anomaly arise mainly from the zonal wind. During daytime, in the hemisphere of stronger poleward wind, the crest values in TEC are weaker than in the conjugate hemisphere at all longitudes considered. The values of NmF2 can be stronger or weaker, depending on the competition between the effects of increased chemical loss rate and the downward flow of plasma from the plasmasphere caused by the stronger poleward wind. At night, after the magnetic meridional wind has changed direction and has been in that direction for some time, the stronger crests in both TEC and NmF2 occur in the hemispheres of stronger equatorward wind.

Calculations using the Sheffield University plasmasphere ionosphere model have shown that under certain conditions an additional layer can form in the low latitude topside ionosphere. This layer (the F3 layer) has subsequently been observed in ionograms recorded at Fortaleza in Brazil. It has not been observed in ionograms recorded at the neighbouring station São Luis. Model calculations have shown that the F3 layer is most likely to form in summer at Fortaleza due to a combination of the neutral wind and the E×B drift acting to raise the plasma. At the location of São Luis, almost on the geomagnetic equator, the neutral wind has a smaller vertical component so the F3 layer does not form.

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.

Calculations using the Sheffield University plasmasphere ionosphere model have shown that under certain conditions an additional layer can form in the low latitude topside ionosphere. This layer (the F3 layer) has subsequently been observed in ionograms recorded at Fortaleza in Brazil. It has not been observed in ionograms recorded at the neighbouring station, Sao Luis. Model calculations have shown that the F3 layer is most likely to form in summer at Fortaleza due to a combination of the neutral wind and the E × B drift acting to raise the plasma. At Fortaleza the horizontal wind moves the plasma up the inclined field lines; at Sao Luis the field lines are nearly horizontal and the wind cannot raise the plasma further.

High electron temperature in the equatorial ionization anomaly region detected first by Kyokko satellite and later observed frequently by Hinotori satellite are found to be closely associated with the ionization crests of the anomaly. This phenomenon, called the equatorial electron temperature anomaly, is found to occur predominantly in the equinoctial months and to become enhanced with the increase in solar activity. It is mainly a nighttime phenomenon and shows maximum temperature enhancement at around 2100 LT. Using a theoretical model, a mechanism for its occurrence is presented. The mechanism is based on the plasma transport in the evening equatorial ionosphere resulting from the sunset electrodynamical processes.

The importance of diffusion, electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) for the ionosphere above Jicamarca (77°W) under magnetically quiet ( Ap = 4) equinoctial conditions (day 264) at medium solar activity ( F 10.7 = 145). The study also investigates the effects of the fountain, which include the equatorial anomaly. The F -region vertical E × B drift velocity measured at the equatorial station Jicamarca is used to represent the electrodynamic drift. The neutral wind is obtained from the HWM90 thermospheric wind model. As expected, the F -region electrodynamic drift generates the plasma fountain and the anomaly, which are symmetric with respect to the equator. The neutral wind makes the fountain and the anomaly asymmetric, with larger plasma flow (towards the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. The paper also addresses many important (some new) features which are related to the fountain. The features are: (1) the possibility of existence of an additional layer (called the G-layer) in the equatorial ionosphere, (2) the reverse plasma fountain, (3) the equatorial anomaly in vertical ionospheric electron content (IEC), (4) the presence (in Nmax) and absence (in IEC) of noon bite-out, (5) the occurrence of nighttime increase in ionization, and (6) plasma bubbles and spread- F .

The equatorial plasma fountain and equatorial anomaly in the ionospheres over Jicamarca (77°W), Trivandrum (77°E), and Fortaleza (38°W) are compared under magnetically quiet (Ap = 4) equinoctial conditions (day 264) at high solar activity (F10.7 = 178). The comparison, carried out using the Sheffield University Plasmasphere-Ionosphere Model with the same perpendicular E×B drift velocity at all locations, is aimed to investigate the modulating effect of neutral wind. The neutral wind is found to produce north-south asymmetries in the plasma fountain and anomaly, with larger plasma flow and stronger anomaly crests occurring in opposite hemispheres at all locations. The north-south asymmetry in the plasma fountain is stronger at locations of larger displacement between the geomagnetic and geographic equators than at locations of larger magnetic declination angle. The vertical E×B drift velocities at the magnetic equators of Trivandrum and Fortaleza are also derived by matching the measured and model values of the peak height hmax. The derived velocities (which correspond to the ionospheric peak) are compared with that measured at Jicamarca using the incoherent scatter radar.

The Sheffield University Plasmasphere Ionosphere Model (SUPIM) has been used to study the importance of diffusion, vertical E × B drift, and neutral wind on the generation and modulation of the equatorial plasma fountain and the equatorial anomaly of the Earth's ionosphere. The study has been carried out for the longitude of Jicamarca under magnetically quiet equinoctial conditions at medium solar activity. The model results indicate that the behaviour of the fountain closely follows the variation of the F region vertical E × B drift. During daytime the fountain rises to a maximum altitude of about 800 km at the equator and covers magnetic latitudes of about ±30°. At regions outside the fountain plasma flows towards the equator and this leads to the formation of an additional layer during the prenoon hours between latitudes ±10°; we call this layer the G layer. The maximum plasma concentration of the G-layer can be greater than that of the F layer for a short period of time just before noon. In the evening, soon after the drift turns downward, the fountain becomes a reverse fountain with supply of ionization from the region outside the fountain. The neutral wind modulates and introduces latitudinal asymmetries into both the fountain and anomaly.

The importance of diffusion, perpendicular electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University plasmasphere-ionosphere model for the ionosphere above Jicamarca under magnetically quiet equinoctial conditions at medium solar activity. The effects of the fountain, which include the equatorial anomaly, are also investigated. As expected, the F region electrodynamic (E×B) drift generates the plasma fountain and the anomaly, which are symmetric with respect to the magnetic equator. The neutral wind introduces asymmetries, with larger plasma flow (toward the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. During daytime, when the drift is upward, the fountain rises to about 800 km altitude at the equator and covers about ±30° magnetic latitude ; outside the reach of the fountain, plasma flows toward the equator from both hemispheres. This convergence of plasma leads to the formation of an additional layer (called the G layer) within ±10° of the magnetic equator during the prenoon hours when the drift is large. At the magnetic equator, the maximum plasma concentration of the G layer can be greater than that of the F layer for a short period of time just before noon and when the drift starts to decrease. In the evening, soon after the drift turns downward, the fountain becomes a reverse fountain with supply of ionization from both hemispheres from regions outside the fountain. The reverse fountain acts as the main source for the nighttime increase in ionization at equatorial anomaly latitudes, with some contribution from the prereversal strengthening of the forward fountain. The importance of the prereversal strengthening of the forward fountain, which rises to about 1000 km altitude at the equator, and the following reverse fountain on the generation and propagation of plasma bubbles and spread F irregularities is discussed. It is also shown that the equatorial anomaly in the vertical ionospheric electron content need not be as pronounced as the interpretation of the observations suggests.

Hourly values of IEC and of f 0 F 2 (critical frequency) for a low latitude station, Hawaii (21.2°N, 157.7°W), during the solar maxima (1969 and 1981) and minima (1965 and 1985) years of two consecutive solar cycles, 20 and 21, are used to study the day to day variabilities of the ionospheric parameters IEC and NP. It is found that there is good correspondence in the day to day variations of IEC and NP from one solar cycle to the other for both solar maximum and minimum years in the two solar cycles. Depending on solar phase and season, while the mean daytime IEC and NP variations range from about 20% to 35%, the mean night time values vary from about 25% to 60%. The mean daytime variations in NP for the solar minimum phase are remarkably higher in all the three seasons compared to the solar maximum phase. However, no such increase is observed in the mean daytime IEC variations, indicating the highly variable nature of the daytime ionospheric F region compared to the topside during solar minimum for this low latitude station. The winter night time IEC also seems to be a relatively stable parameter during the solar minimum. The short term day to day variabilities of the day time peak values of IEC and NP (ie IEC max and NP max ) are not closely associated with the variations in F 10.7 solar flux. Contrary to the common expectation, the variabilities in both the parameters, particularly in NP max , are somewhat reduced during the solar maximum (when the variability in F 10.7 solar flux is much higher compared to the solar minimum) which is more evident in the stronger 21 solar cycle. A larger number of significant components are seen in the spectra of the percentage variation of both IEC max and NP max during both solar phases of the two solar cycles compared to the corresponding F 10.7 solar flux spectra. The number of additional components for both the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density ( N max) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and N max, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes. At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and N max (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F -region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and N max and stronger daytime N max in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.

[1] The strong westward electrojet and simultaneous upward drift of the equatorial ionospheric peak observed over South-East Asia and Indian equatorial regions during the prolonged Dst minimum phase of an intense geomagnetic storm during 14–15 December 2006 are investigated for the altitudinal variation of zonal electric field polarity using ground based and space-borne observations. The results show first observational evidence for simultaneous existence of daytime westward and eastward zonal electric fields at equatorial E and F region altitudes, respectively, in a wide longitude sector. While the westward electric fields at E region altitudes cause westward electrojet, at the same time, the eastward zonal electric fields atFregion altitudes cause the upward drift of the equatorial ionospheric peak and reinforcement of the equatorial ionization anomaly (EIA) even in the topside ionosphere (∼660 km). The reversal of the electric fields is found to occur at ∼280 km height. A clear bifurcation of F region plasma at ∼280 km is evident in the iso-electron density contours due to these oppositely polarized zonal electric fields, which manifests as an unusually deep cusp between F1 and F2 layers on equatorial ionograms.

Nighttime enhancements in ionospheric electron content (IEC) observed at conjugate stations in the equatorial anomaly and mid-latitude regions under moderate to high solar activity conditions are studied. The observations at equatorial anomaly latitudes show that when an enhancement occurs in one hemisphere then an enhancement usually occurs in the conjugate hemisphere. The enhancement characteristics (frequency of occurrence, time of occurrence, amplitude, and duration) and their seasonal and solar activity variations are in agreement with the fact that the primary source of an enhancement is the post-sunset increase in the equatorial fountain. It is suggested that the north-south differences in the enhancement characteristics, e.g. the enhancement being more frequent and stronger in the southern hemisphere than in the northern hemisphere, are due to the north-south differences in the neutral air wind velocity. At mid-latitudes, on the other hand, when an enhancement occurs in one hemisphere then either no enhancement or only a weak enhancement occurs in the conjugate hemisphere; on no observed night does a strong enhancement occur in both hemispheres. The occurrence and other characteristics of the enhancements demonstrate that the primary source for the nighttime enhancements in IEC at mid-latitudes (i.e. the downward flow of plasma from the protonosphere to the ionosphere) is asymmetric; a strong downward flow occurs in only one hemisphere on any one night.

[1] The statistics of occurrence of the geomagnetic storms, and ionospheric storms at Kokubunji (35.7°N, 139.5°E; 26.8°N magnetic latitude) in Japan and Boulder (40.0°N, 254.7°E; 47.4°N) in America are presented using the Dst and peak electron density (Nmax) data in 1985–2005 covering two solar cycles (22–23) when 584 geomagnetic storms (Dst ≤− 50 nT) occurred. In addition to the known solar cycle and seasonal dependence of the storms, the statistics reveal some new aspects. (1) The geomagnetic storms show a preference for main phase (MP) onset at around UT midnight especially for major storms (Dst ≤− 100 nT), over 100% excess MP onsets at UT midnight compared to a uniform distribution. (2) The number of positive ionospheric storms at Kokubunji (about 250) is more than double that at Boulder, and (3) the occurrence of the positive storms at both stations shows a preference for the morning‐noon onset of the geomagnetic storms as expected from a physical mechanism of the positive storms. (4) The occurrence of negative ionospheric storms at both stations follows the solar cycle phases (most frequent at solar maximum) better than the occurrence of positive storms, which agrees with the mechanism of the negative storms.

[1] The neutral mass density N and electron density Ne at 400 km height measured by CHAMP during nine intense geomagnetic storms bring out some new aspects of the thermospheric and ionospheric storms. The thermospheric storms (increase of N) develop with the onset of the main phases (MP) of the geomagnetic storms and reach their peak phases before or by the end of the MPs. The ionospheric storms (change of Ne) in general undergo an initial negative phase (with the equatorial ionization anomaly (EIA) crests shifting poleward) before turning positive, and the positive storms reach their peak strengths (or phases) centered at ±25°–30° magnetic latitudes; in some (4) cases the positive storms develop without an initial negative phase and with the EIA crests shifting equatorward; in all cases the positive storms reach their peak phases before the end of the MPs and turn to conventional negative storms by the end of the MPs. The observations agree with the different aspects of a physical mechanism of the positive storms. The observations also reveal that the Halloween storms of 30 October 2003 with a short MP without fluctuations produced the strongest positive ionospheric storms through impulsive response, and there is strong equinoctial asymmetry in the ionosphere and thermosphere during geomagnetic storms.

A physical mechanism of the positive ionospheric storms at low and mid latitudes reported recently is reviewed, and comapred with the positive ionospheric storms observed during a super storm. In addition, the possible variations of the mechanism with the strength of the equatorward winds, intensity of prompt penetration electric field (PPEF), local time and season are discussed. According to the mechanism, the mechanical effects of the equatorward wind (1) reduce (or stop) the downward diffusion of plasma along the geomagnetic field lines, (2) raise the ionosphere to high altitudes of reduced chemical loss, and hence (3) accumulate the plasma at altitudes near and above the ionospheric peak centered at ±15°–30° magnetic latitudes. The daytime eastward PPEF, if occurs, also shifts the EIA crests to higher than normal latitudes. The positive ionospheric storms are most likely in the longitudes of morning-noon onset of the geomagnetic storms. The mechanism agrees with the positive ionospheric storms observed during the super storm of 07–08 November 2004.

Results obtained from an analysis of the ionospheric electron content (IEC) data collected at several stations in the Northern Hemisphere during December 1980–December 1985, when the 10.7 cm solar flux index ( F 10.7 ) varied from 66 to 303, are presented. Diurnal maximum IEC value (IEC max ) increases linearly with ( F 10.7 ), as expected, for values of ( F 10.7 ) less than about 200; at higher values of ( F 10.7 ), contrary to expectation, IEC max saturates at all stations. The observed variation of IEC max is interpreted as convincing ionospheric evidence for a nonlinear relationship between solar e.u.v. and 10.7 cm fluxes during intense solar cycles. Variation of the e.u.v. flux obtained from the latest version of the SERF2 solar e.u.v. flux model for the intense solar cycle 21 agrees with this interpretation. Lyman-α (1216 A) and He I (10,830 A) equivalent width data, measured during the same period as the IEC observations and used as independent data sets in the solar e.u.v. flux model, also show nonlinear variations with ( F 10.7 ).

[1] A physical mechanism of the positive ionospheric storms at low latitudes and midlatitudes is presented through multi-instrument observations, theoretical modeling, and basic principles. According to the mechanism, an equatorward neutral wind is required to produce positive ionospheric storms. The mechanical effects of the wind (1) reduce (or stop) the downward diffusion of plasma along the geomagnetic field lines, (2) raise the ionosphere to high altitudes of reduced chemical loss, and hence (3) accumulate the plasma at altitudes near and above the ionospheric peak centered at around ±30° magnetic latitudes. Daytime eastward prompt penetration electric field (PPEF), if it occurs, also shifts the equatorial ionization anomaly crests to higher than normal latitudes, up to approximately ±30° latitudes. The positive ionospheric storms are most likely in the longitudes where the onset of the geomagnetic storms falls in the ionization production dominated morning-noon local time sector when the plasma accumulation due to the mechanical effects of the wind largely exceeds the plasma loss due to the chemical effect of the wind. The mechanism agrees with the multi-instrument observations made during the supergeomagnetic storm of 7–8 November 2004, with 18 h long initial phase (IP) and 10 h long main phase (MP). The observations, which are mainly in the Japanese-Australian longitudes where the MP onset was in the morning (0600 LT, 2100 UT), show (1) strong positive ionospheric storms (in Ne, Nmax, hmax, Global Positioning System–total electron content (GPS-TEC), and 630 nm airglow intensity) in both Northern and Southern hemispheres started at the morning (0600 LT) MP onset and lasted for a day, (2) repeated occurrence of strong eastward PPEF events penetrated after the MP onset and superposed with westward electric field started before the MP onset, and (3) storm time equatorward neutral winds (inferred from 1 and 2). Repeated occurrence of an unusually strong F3 layer with large density depletions around the equator was also observed during the morning-noon MP.

[1] Predawn ionospheric temperature has been known to increase with conjugate sunrise. This paper presents the onset time of predawn ionospheric heating and heating rate at around 600 km height globally using Hinotori electron temperature data for magnetically quiet (Kp −50 nT) medium to high solar activity conditions. The analysis of the data shows that the onset of predawn ionospheric heating occurs at nearly the same solar zenith angle (SZA) of the conjugate point at low latitudes where the geomagnetic field line is shorter than about 5000 km. However, at higher latitudes with longer field lines, the conjugate SZA decreases with increasing field line length. In addition, the heating rate decreases with increasing field line length until the field line becomes about 5000 km long, and the rate remains nearly constant for longer field lines. The conjugate SZA increases with increasing solar activity (F10.7) until F10.7 reaches about 200, and the conjugate SZA remains nearly constant for higher F10.7. The observations indicate that the photoelectron flux causing predawn ionospheric heating is attenuated by scattering in the high-altitude (>600 km) ionosphere and plasmasphere.

[1] The role of electrodynamics in producing additional stratifications in the equatorial F region (F 3 layer) at dawn and dusk during geomagnetic storms is discussed. Two cases of F 3 layer at dawn (0600-0730 LT on 5 October 2000 and 8 December 2000) and one case of F 3 layer at dusk (1600-1730 LT on 5 October 2000) are observed, for the first time, by the digital ionosonde at the equatorial station Trivandrum (8.5°N; 77°E; dip ∼ 0.5°N) in India. The unusual F 3 layers occurred during the geomagnetic storms and are associated with southward turning of interplanetary magnetic field B z , suggesting that eastward prompt penetration electric field could be the main cause of the F 3 layers. The dawn F 3 layer on 5 October is modeled using the Sheffield University Plasmasphere-Ionosphere Model by using the E x B drift estimated from the real height variation of the ionospheric peak during the morning period. The model qualitatively reproduces the dawn F 3 layer. While the existing F 2 layer rapidly drifts upward and forms the F 3 layer and topside ledge, a new layer forming at lower heights develops into the normal F 2 layer.

[1] Relative importance of diffusion, electric field, and neutral wind on equatorial plasma fountain and equatorial ionization anomaly (EIA) during a strong daytime eastward prompt penetration electric field (PPEF) event are evaluated using the Sheffield University Plasmasphere Ionosphere Model and the recorded PPEF during the super geomagnetic storm of 9 November 2004. The fountain rapidly develops into a super fountain during the PPEF event. The super fountain becomes strong with less poleward turning of the velocity vectors in the presence of an equatorward wind that reduces (or stops) the downward velocity component due to diffusion and raises the ionosphere to high altitudes of reduced chemical loss. The EIA crests in peak electron density and total electron content shift rapidly to higher than normal latitudes during the PPEF event. However, the crests become stronger than normal only in the presence of an equatorward wind. The results suggest that the presence of an equatorward neutral wind is required to produce a strong positive ionospheric storm during a daytime eastward PPEF event. The equatorward neutral wind need not be a storm time wind though stronger wind can lead to stronger ionospheric storms.

[1] The occurrence of an additional layer, called F3 layer, in the equatorial ionosphere at American, Indian, and Australian longitudes during the super double geomagnetic storm of 7–11 November 2004 is presented using observations and modeling. The observations show the occurrence, reoccurrence, and quick ascent to the topside ionosphere of unusually strong F3 layer in Australian longitude during the first super storm (8 November) and in Indian longitude during the second super storm (10 November), all with large reductions in peak electron density (Nmax) and total electron content (GPS-TEC). The unusual F3 layers can arise mainly from unusually strong fluctuations in the daytime vertical E × B drift as indicated by the observations and modeling in American longitude. The strongest upward E × B drift (or eastward prompt penetration electric field, PPEF) ever recorded (at Jicamarca) produces unusually strong F3 layer in the afternoon hours (≈1400–1600 LT) of PPEF, with large reductions in Nmax and TEC; the layer also reappears in the following evening (≈1700–1800 LT) owing to an unusually large downward drift. At night, when the drift is unusually upward and strong, the F region splits into two layers.

A study has been designed specifically to investigate the dependence of the ionospheric response on the time of occurrence of sudden commencement (SC) and the intensity of the magnetic storms for a low- and a mid-latitude station by considering total electron content and peak electron density data for more than 60 SC-type geomagnetic storms. The nature of the response, whether positive or negative, is found to be determined largely by the local time of SC, although there is a local time shift of about six hours between low- and mid-latitudes. The time delays associated with the positive responses are low for daytime SCs and high for night-time SCs, whereas the opposite applies for negative responses. The time delays are significantly shorter for mid-latitudes than for low-latitudes and, at both latitudes, are inversely related to the intensity of the storm. There is a positive correlation between the intensity of the ionospheric response and that of the magnetic storm, the correlation being greater at mid-latitudes. The results are discussed in the light of the possible processes which might contribute to the storm-associated ionospheric variations.

and Bz changed from highly negative to positive up to 25 nT. The responses of the magnetosheath-cusp region during the unusual event are presented using Cluster and ground-based (EISCAT VHF radar; 69.6N, 19.2E) observations. The unusual Cluster crossing (compared to the usual midaltitude cusp crossing at this time of the year) occurred owing to a large compression of the magnetosphere. Cluster, which was in the southern magnetospheric lobe, suddenly found itself in the magnetosheath at the arrival of the CME at 1524:45 UT. Cluster then crossed through the compressed magnetosheath (highly compressed after the discontinuity in the CME) for about 1.5 hours (1525– 1655 UT), magnetopause with strong signatures of lobe reconnection (1655 UT), stagnant but compressed exterior cusp for about an hour (1700–1802 UT), and then entered the dayside magnetosphere. The observations also show strong signatures of magnetosphere-ionosphere coupling through a late afternoon (17 MLT) cusp during the first 40 min (1525–1605 UT) of the event when IMF Bz remained negative. Strong magnetic waves are also generated in the magnetosheath-cusp region.