Search

We present a case study of unusual spread-F structures observed by ionosondes at two equatorial and low-latitude Brazilian stations – São Luís (SL: 44.2∘ W, 2.33∘ S; dip angle: −6.9∘) and Fortaleza (FZ: 38.45∘ W, 3.9∘ S; dip angle: −16∘). The irregularity structures observed from midnight to postmidnight hours of moderate solar activity (F10.7 < 97 sfu, where 1 sfu = 10−22 W m−2 s−1) have characteristics different from typical post-sunset equatorial spread F. The spread-F traces first appeared at or above the F-layer peak and gradually became well-formed mixed spread F. They also appeared as plasma depletions in the 630.0 nm airglow emissions made by a wide-angle imager located at the nearby low-latitude station Cajazeiras (CZ: 38.56∘ W, 6.87∘ S; dip angle: −21.4∘). The irregularities appeared first over FZ and later over SL, giving evidence of an unusual westward propagation or a horizontal plasma advection. The drift-mode operation available in one of the ionosondes (a digital portable sounder, DPS-4) has enabled us to analyze the horizontal drift velocities and directions of the irregularity movement. We also analyzed the neutral wind velocity measured by a Fabry–Pérot interferometer (FPI) installed at CZ and discuss its possible role in the development of the irregularities.

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.

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.

[1] Indian scientists got the unique opportunity to study the near-Earth environment during a long annular solar eclipse at the end of the last long deep solar minimum, on 15 January 2010. Continuous high time resolution records of the atmospheric electric parameters and meteorological parameters were made at Tirunelveli (8.07°N, 77.08°E, 35 m Above Mean Sea Level (AMSL) and Braemore Hill (8.41°N, 76.59°E, 460 m AMSL) stations where the eclipse was during 11:07:57–15:06:52 IST with maximum obscuration (~90%) at 13:17:09 Indian Standard Time (IST). The recorded values of the parameters show marked deviations from those normally observed on control fair-weather days. The ambient electric field underwent a large drop by up to 65% during the eclipse, and potential gradient showed epochs of enhancements during and after the eclipse until postsunset. The data also seem to reveal the long lasting paradox of conductivity enhancement during eclipse, which may be due to the eclipse induced upsurge of low winds or waves that brings high density of free space charges embedded in air parcels.

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.

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.

We report the detection of H+ ions in the topside ionosphere above the middle and upper atmosphere (MU) radar in Japan, the first such detection by a mesosphere-stratosphere-troposphere radar extended to perform as an incoherent scatter radar. Owing to the very limited signal-to-noise ratios of ∼1% achievable in the topside ionosphere with this radar, long integration times of 45 min and statistical analysis of 10 years of data are employed to yield useful results. This sensitivity restriction limits the top altitude of observation to only ∼400 km during solar-minimum winter but up to ∼650 km during solar-maximum summer. These measurements are possible only at night, when we may assume that the electrons and ions have the same temperature. While the data scatter is large, climatological averages constructed for various solar-activity, seasonal, and magnetic activity cases show less than ∼5% H+ at most under conditions of detect ability, although it is exactly those conditions of undetectability which are expected to correspond to the highest proportions of H+. At solar maximum we see no detectable amounts of H+ up to 650 km altitude, except perhaps for a faint amount in winter. At solar minimum the data are good up to ∼550 km during summer and equinox but to only ∼400 km during winter. The H+ profiles for solar-minimum summer and equinox closely resemble each other and reach ∼5% at 550 km altitude. We see no clearly discernible H+ in winter up to our 400 km altitude limit of detectability; we expect that there is substantial H+ present above 400 km, but the total ion density is so low that we cannot measure it. We see no detectable change in H+ between magnetically quiet and disturbed periods. The H+ proportions seen above the MU radar are much smaller than those seen above Arecibo, which has a similar magnetic latitude but much lower geographic latitude, and are much more comparable to those amounts seen by the higher-latitude radars at Saint Santin and Millstone Hill. Comparisons with models suggest that at these low altitudes, H+ may well be near its charge-exchange-equilibrium value.

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.

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.

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, 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.

[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.

[1] The correlation between the electron temperature (Te) and electron density (Ne) at 600 km height at magnetic dip latitudes (MLat) less than about ±40° measured by the Hinotori satellite from February 1981 to June 1982 is presented. The results show the well-known negative correlation between daytime Ne and Te when Ne is low. However, when the daytime Ne is significantly high (>106 cm−3), the correlation turns positive irrespective of latitude, longitude, season, solar flux levels, and magnetic activity levels. The positive correlation is most clear during 1100–1500 local time and around the magnetic dip equator (∣MLat∣ < 10°) where high values of Ne with high Te are most abundant. Te also increases with increasing MLat in the same Ne range. Since the cooling through Coulomb collisions increases with the increase of Ne, the results suggest that an additional heat source(s) is involved for the positive correlation between Ne and Te. The additional heat source seems to be related to the integrated Ne along the magnetic field lines from the ground to 600 km altitude in one hemisphere, which is found to decrease with increasing MLat. Although the mechanism for the positive correlation is not well understood, the results imply that the Te in the topside ionosphere is controlled more by the integrated Ne than by in situ Ne or F2 peak Ne.

[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.

[1] The response of the dayside equatorial F2 layer to the main phases of the 22 intense geomagnetic storms (Dst < −150 nT) in 1998–2008 is investigated using the digital ionosonde data from the equatorial stations in Brazilian, Indian, and Australian longitudes together with equatorial electrojet strength and IMF Bz; the storms include 15 superstorms (Dst < −200 nT). The observations show that there is a period during all MPs when the F2 layer peak rises (and falls) rapidly with large peak electron density (Nmax) reduction, the rise velocity strongly correlates with the intensity (Dst) of the storms, and the duration of the Nmax reduction corresponds to that of strong eastward electrojet when IMF Bz remains highly negative. The observations indicate the occurrence of strong eastward prompt penetration electric fields (PPEF) during the rapid F2 layer response. The PPEF drives the F2 layer peak rapidly upward, which reduces Nmax due to vertical expansion and diffusion. The results therefore suggest that the rapid F2 layer response (rapid rise (and fall) of peak height (hmax) with large Nmax reduction) observed by ionosondes can be used to detect the occurrence of the daytime eastward PPEF during intense geomagnetic storms irrespective of season and level of solar activity. The data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and/or prompt penetration. The results are important when radars are not available to monitor the occurrence of the PPEF.

The relative importance of height, vertica.l drift velocity, and electron density gradient of the postsunset bottomside (5.5 MHz) equatoria.l Fregion for the onset ofsprea.d Fisstudied using simultaneous HF Doppler radar and ionosonde observations. The study conducted for the periods January-March of 1984 and 1985 shows that the height of the F layer, determined by the time history of the prereversa.l enhancement of the drift velocity, is the deciding factor for the onset of equatoria.l spread F (ESF) with little contribution from the electron density gradient. Maximum growth rate of linear collisiona.l Rayleigh-Taylor instability occurs at the time of peak height rather than at the time of pea.k velocity confirming that, for the onset of ESF, the layer should attain a threshold height. The threshold (group) height of the 5.5 MHz layer fa.lls from ""450 km in 1984 (mean FlO.7 equa.ls 120) to ",,350 km in 1985 (mean FlO.7 equa.ls 70); the corresponding evening peak upward drift velocities decrease from about 30 m S-l in 1984 to about 20 m S-1 in 1985. The significant fa.ll of the thresholds with the declining solar activity is due to the decrease in the ion-neutra.l collision frequency with declining solar activity; the fa.ll of the thresholds is reflected in large decreases in the intensity and duration of the sprea.d F.

[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.

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.

[1] The paper presents the observations and modeling of an additional layer in the low-latitude ionosphere in Indian longitudes. The signatures of the additional layer are observed as ledges or humps between the equatorial ionization anomaly trough and crest (EIA) in the latitudinal profiles of total electron content (TEC), measured using a single ground-based beacon receiver located at Trivandrum (8.5°N, 77°E, dip 0.5°N) in India. The ground-based ionograms also show the presence of the so-called F 3 layer for a short duration corresponding to these signatures, and the layer is found to drift upward to the topside ionosphere. The study provides first observational evidence that the so-called "humps" in the latitudinal variation of TEC are nothing but the upward propagating F 3 layer. This conclusion is supported by theoretical modeling using the Sheffield University Plasmasphere Ionosphere Model. It is shown that upward ExB drift and strong equatorward neutral wind (perturbed by atmospheric waves) can produce the humps in the latitudinal variation of TEC through the reduction in the downward diffusion of ionization along geomagnetic field lines. The model results also show that the F 3 layer drifts to the topside and forms topside ledges.

Simultaneous MLT (mesosphere-lower thermosphere) and thermospheric F region (upper thermosphere and ionosphere together) observations conducted using the middle and upper atmosphere (MU) radar (35 degrees N, 136 degrees E) in alternate meteor and incoherent scatter modes in October 2000 and March 2001 are presented. The continuous observations, each lasting more than a week, provide simultaneous zonal and meridional wind velocities at MLT altitudes (80-95 km), meridional wind velocity in the upper thermosphere (220-450 km), and electron density and peak height in the ionosphere with a time resolution of 1.5 hours. The data seem to suggest that the upper atmospheric regions could be dynamically coupled through mean winds, tides, and waves. Diurnal (24-hour) and semidiurnal (12-hour) tides and waves of periods 16-20 hours and 35-55 hours coexist at MLT and upper thermosphere altitudes, and the waves become stronger than tides at mesopause (approximate to 88 km) in both October and March. The data in these equinoctial months also show large differences in mean winds, tides, and waves in the MLT region. The amplitudes and phases of the 24-hour and 12-hour tides at MLT altitudes are compared with those predicted by the global scale wave model (GSWM). The model qualitatively predicts the observed growth of the tides with altitude but does not predict the 12-hour tide becoming stronger than the 24-hour tide at altitudes above mesopause in October.

On 18April 2002 the Cluster spacecraft crossed through the northern outer magnetospheric cusp region during 16:25-17:55UT when the solar wind dynamic pressure was rather low (Bz was more negative than IMF By. The Cluster data from the FGM, CIS, PEACE, EFW, WHISPER and STAFF instruments reveal that the cusp is structured with three anti-sunward ion flow events of durations ≈1.5, 17.5 and 19.0 min, with bulk plasma flow roughly parallel to the magnetopause toward north. The ion and electron densities within the events are much greater than those outside. The zonal electric field in the ion flow events turns eastward as expected from V×B effect. The sharp inward boundaries of the ion flow events cross the four spacecraft in one time sequence, and the outward boundaries of the events cross the spacecraft in the reverse time sequence. The observations studied using magnetosphere and magnetopause models suggest that the structured cusp is a temporal feature that arises due to three inward and outward movements of the magnetopause by about 1.5RE so that Cluster, while crossing through the cusp, happened to be in the magnetosheath (ion flow event) and cusp alternately. The magnetopause moved due to the changes in the solar wind dynamic pressure by up to 100%.

[1] Simultaneous mesosphere/lower thermosphere (MLT) and thermospheric F region observations made during geomagnetic storms are presented using the MU radar (35°N, 136°E) in alternate meteor and incoherent scatter (IS) modes, MF radar (45°N, 136°E), optical mesosphere thermosphere imagers (OMTI) (35°N, 136°E) and over 1000 GPS receivers in Japan. The observations were made during one of the MTEC-S (mesosphere-thermosphere experiments for coupling studies) campaigns conducted during 23 March to 2 April 2001 when a major storm with Dst reaching −358 nT and a moderate storm with Dst reaching −112 nT occurred and solar activity varied from high to very high (F10.7 = 220 to 270). The campaign has provided zonal and meridional wind velocities at MLT altitudes (80–95 km) over the MU and MF radars; meridional wind velocity in the upper thermosphere (220–450 km) and ionospheric electron density (150–600 km), peak height and plasma drift velocity over the MU radar; all-sky airglow intensities at 557.7 nm, 630.0 nm, and 777.4 nm wavelengths over the MU radar location; and vertical GPS-TEC all over Japan. Study of the data indicates that following the major storm, the meridional wind in the upper thermosphere over the MU radar (35°N) becomes weak (less poleward) during daytime and its diurnal amplitude reduces. The winds and tides at MLT altitudes show changes during both nonstorm times and storm times, but this study is not able to confirm any statistically significant changes related to the storms. In the ionosphere, following the onset of the major morning storm, the electron density increases at altitudes near and above the ionospheric peak, which after about 5 hours is followed by large increase in density at all altitudes above about 200 km. On the following night, though the density is generally low, there is a large increase in density in the bottomside for about 3 hours centered at 0230 LT. During this time, a rare intense optical activity has occurred at all wavelengths (557.7 nm, 630 nm, and 777.4 nm) over the low midlatitude location (35°N), and an associated large-scale traveling ionospheric disturbance (LSTID) has propagated from north to south at F region altitudes.

[1] A prominent large-scale traveling ionospheric disturbance (LSTID) was observed in Japan during the major magnetic storm (Dst ∼ −358 nT) of 31 March 2001. It was detected as enhancements of the 630-nm airglow and foF2, GPS-TEC variations, and a decrease in F-layer virtual height at 1700–1900 UT (0200–0400 LT). It moved equatorward with a speed of ∼600 m/s. The decrease in the F-layer height was also detected by the MU radar at Shigaraki. Thermospheric wind variations were observed by the MU radar through ion drift measurement and by a Fabry-Perot interferometer (FPI) through a Doppler shift of the 630-nm airglow line at Shigaraki. The wind data show a turn of the meridional wind from −94 m/s (equatorward) to +44 m/s (poleward) during the LSTID, indicating that an intense poleward wind in the thermosphere passed over Shigaraki as an atmospheric gravity wave and caused the observed ionospheric features of the LSTID. Intense poleward wind was also detected at mesospheric altitudes (95–100 km) by the MU radar (through meteor echoes) and by the FPI (through the 558-nm airglow) with a delay of ∼2 hours from the thermospheric wind, indicating downward phase progression of the wave. Generation of the observed poleward wind in the auroral zone was investigated using magnetic field data and auroral energy input estimated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. We suggest that simple atmospheric heating and/or the Lorentz force in the auroral zone do not explain the observed poleward wind enhancement.

enhancement of GPS total electron content (� 1.0 � 10 16 m � 2 ). Multipoint and imaging observations of these parameters show that the LSTID moved equatorward over Japan with a velocity of � 400–450 m/s. From a comparison with the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) we conclude that an enhancement (250–300 m/s) of poleward neutral wind (that is propagating equatorward) caused these observational features of the LSTID at midlatitudes. To investigate generation of the LSTID by auroral energy input, we have used auroral images obtained by the Polar UVI instrument, magnetic field variations obtained at multipoint ground stations, and the empirical Joule heating rate calculated by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. Intense auroral energy input was observed at 0800–1100 UT (4–6 hours before the LSTID), probably causing equatorward neutral wind at lower latitudes. It is likely that the poleward wind pulse that caused the observed LSTID was generated associated with the cessation of this equatorward wind. The effect of Lorentz force is also discussed. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 2427 Ionosphere: Ionosphere/atmosphere interactions (0335); 2435 Ionosphere: Ionospheric disturbances; 2437 Ionosphere: Ionospheric dynamics; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: large-scale traveling ionospheric disturbance, thermosphere–ionosphere coupling, magnetic storm, airglow imaging, GPS network, ionosonde

[1] The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135°E, 35°N) incoherent scatter observations and Sheffield University plasmasphere-ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200–600 km altitudes measured by the radar under low solar activity (F10.7 ≤ 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (Uθ) and northward perpendicular plasma drift velocity (V⟂) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS-86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio); the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to ∼400 km and shows comparatively weak annual and semiannual variations.

Modeling studies are carried out using the Sheffield University plasmasphere-ionosphere model to investigate the relative importance of neutral winds, neutral densities, and solar EUV fluxes in leading to the saturation of ionospheric ionization observed during the intense solar cycle 21. Values of mean daytime (1100-1700 LT) ionospheric electron content, peak electron density (Nmax), and peak height (hmax) computed for four midlatitude stations for the month of October 1980-1985, when the 10.7-cm solar activity index (F10.7) varied from 66 to 303, increase nonlinearly with F10.7 with saturation for high values of F10.7. Neutral winds, neutral densities, and solar EUV fluxes (obtained from empirical models based on observed data) used in the model computations also undergo similar nonlinear increase with F10.7. The study reveals that (1) the nonlinear increase of neutral winds and neutral densities has no net effect on the saturation of ionospheric ionization, and (2) the saturation of ionization is caused by the saturated production of ionization due to the nonlinear increase of the solar EUV fluxes. The model values of ionospheric height saturate mainly due to the nonlinear increase of the neutral densities determined by solar EUV and UV radiations. The study concludes that the ionosphere (and atmosphere) responds linearly to the solar EUV (and UV) inputs and nonlinearly to F10.7 because the expected linear relationship between the EUV (and UV) fluxes and F10.7 breaks down during solar maximum. Thus it is recommended that the F10.7 proxy, which has conventionally been used as an index of solar activity, must be replaced or be used carefully, particularly during solar maximum.

Variations of ionospheric ionization (represented by ionospheric electron content (IEC)) and related solar fluxes with the 10.7-cm solar flux index (F10.7) are studied for the intense solar cycle 21 when F10.7 was as high as 367. The IEC data collected at several stations during 1980-1985, the solar EUV (50-1050 A) fluxes obtained from the EUV91 solar EUV flux model, and the measured values of Lyman α (1216 A) flux and He I (10,830 A) equivalent width (EW) are used for the study. It is shown that daily values of diurnal maximum IEC (IECmax) saturate (remain constant) when F10.7 (or its 81-day running average) exceeds a threshold (approximately 160-200) which depends slightly on season and latitude. Variations of the model values of the solar EUV fluxes reveal that when F10.7 exceeds the threshold: (1) the integrated solar EUV (50-1050 A) flux increases at a very low rate, and (2) the fluxes of the important (for thermospheric heating) chromospheric lines and intervals generally saturate (remain constant), while those of the coronal lines and intervals increase at a reduced rate. Lyman α flux and He I EW, which are used as input data in the solar EUV flux model, also increase at a very low rate when F10.7 exceeds the threshold. The saturation of ionospheric ionization, observed for high values of F10.7 during the last three solar cycles 19-21, is the result of the nonlinear variation of the solar EUV and Lyman α fluxes with F10.7. IECmax increases linearly with the integrated solar EUV flux, Lyman α flux and He I EW.

The relationship between nighttime enhancements in total electron content (TEC) and VHF amplitude scintillations has been studied using the ATS 6 satellite data obtained from two closely located equatorial stations (Ootacamund and Thumba) for the period October 1975 to July 1976. The results show that the occurrence and various other characteristics of the TEC enhancements are correlated to corresponding characteristics of the scintillation index SI. The peak value of the SI index is found to increase linearly with that of the TEC enhancement until it reaches the saturation value of nearly 1 for the peak TEC enhancement of about 0.6×1017 m−2. Both phenomena start mostly prior to midnight with the TEC enhancement being delayed by 1 to 1½ hours with respect to the scintillations. The half-amplitude duration of the scintillation enhancement is greater than that of the TEC by a factor of 1 to 2. The intensity and duration of both phenomena are found to be maximum at equinox and minimum in summer. From the results it is clear that the TEC enhancements cannot be regarded as a source for the small-scale irregularities but rather they both seem to depend on the postsunset vertical drifts in the F layer.

Using the ATS 6 satellite radio beacon data collected over a chain of stations in the Indian zone, a study has been made of the equatorial anomaly in total electron content (TEC) and its relation to the E region currents. Two parameters, ΔHd, the difference in the magnetic field perturbation between the equator and a low-latitude station, and Vd, the backscatter radar measurement of the draft velocity of the electrojet irregularities, are used to represent the equatorial current strength. The results, presented in the form of regression plots relating the strength of the anomaly and that of the current, indicate that the anomaly dependence on the current strength is about the same for all the three seasons. It is also noted that, regardless of the season, the anomaly crest moves toward a farther latitude from the equator with increasing current strength. The total ionization inventory, Σ TEC, of the anomaly belt is found to increase linearly with the strength of the equatorial current and this result seems to bear some significance for the correlation observed between TECmax and ΔHdmax at the equator.

Hf Doppler observations of vertical plasma drifts in the evening F region of the equatorial ionosphere at Trivandrum (8.5{degrees}N, 77{degrees}E; dip 0.9{degrees}S) are presented. The observations reveal a consistent pattern characterized by a post-sun-sent enhancement with peak drift velocity in the range 30-60 m s{sup {minus}1} for quiet conditions. Under moderately disturbed conditions, the enhancement is found to be down by as much as a factor of 2, while no such appreciable change is noted for highly disturbed conditions. The pattern of drift velocity dependence on magnetic activity is seen to be consistent with that of spread F occurrence. A striking feature of the observations is the presence of a significant fluctuating component in the drift velocity with quasiperiods ranging from a few minutes to a few tens of minutes during both quiet and disturbing periods.

Latitudinal variations of the various characteristics of nighttime anomalous enhancements in total electron content (TEC) are presented by considering TEC data from the ATS series of satellites for a 2-month period from a number of stations in the northern hemisphere extending from 2° to 63° dip latitude. The latitudinal variations of the various TEC enhancement characteristics are found to be strikingly similar, and they reveal a pattern of cyclic variation with two distinct minima, one at 30°N and the other at 60°N (dip latitude). The TEC enhancements show a strong positive correlation with geomagnetic activity at middle and high latitudes and no significant correlation at low latitudes. The observed pattern of the latitudinal vaiations of the TEC enhancements is discussed in the light of the potential source mechanisms.