Meteorological effects observed in the D-region of the equatorial ionosphere

Electron density values were measured during morning hours over Thumba. The results show that electron density in mesosphere is more during summer than during winter for same solar zenith angle. The temperature measurements carried out on the same day during night hours show that mesosphere is hotter in winter and cooler in summer over Thumba. The electron density and temperature are anti-correlated. The results are explained in terms of temperature effects and other meteorological effects.     

Latitudinal influences on the quiet daytime D-region

Four versions of a steady-state quiet D-region model are presented. They differ from each other as a result of latitudinal differences in total neutral particle concentrations, nitric oxide concentrations and cosmic ray ionization rates. The total ion concentration profiles of all four versions have minima near 70 km which range from about 10⁸ m^?3 at high latitudes to 3.5 × 10⁷ m^?3 at equatorial latitudes for a solar zenith angle of 60°. Neutral density differences among the four cases result in important vertical shifts for the respective D-region profiles relative to one another. A “C-layer” is evident for the high and mild-latitude models at large solar zenith angles. The altitude where the negative ion/electron concentrations ratio is unity varies from about 63 to 67 km. The computed results are compared briefly with the extensive data base in the literature.     

A steady-state model for the D- to F-region of the polar cap

For obvious reasons the ionosphere of the polar cap, surrounded by the auroral zone, is only poorly investigated. Even ionosonde data are very scant from geomagnetic latitudes beyond 70°. Since 1997 the European incoherent scatter radar facility EISCAT has an additional installation on Svalbard and has been providing electron density data nearly continuously ever since. These measurements which mainly cover the E- and F-regions are supplemented by rocket data from Heiss Island at a comparable magnetic latitude; these data are more sporadic, but cover lower altitudes and densities. A provisional, steady-state, neural network-based model is presented which uses the data of both sites.     

D-region IRI profiles in relation to radio observations

The D-region IRI profiles are compared with the direct rocket measurements as well as with ground-based radio observations by a variety of techniques. The characteristics of D-region IRI profiles and the dependence of electron density on solar zenith angle, sunspot number, latitude and season are discussed. The sensitivity of certain reflection coefficients on the height distribution of electron density below 70 km is illustrated with a typical example. For D-region modelling, the results show the importance of simultaneous measurement of reflection and conversion coefficients together with polarization phase over a wide frequency range.     

A simplified D-region ion chemistry scheme and its possible use for IRI lower ionosphere modelling

Ion composition of the D region is principally characterized by the existence of two distinct regions of predominant molecular ions and predominant cluster ions, separated from each other by a rather sharp ‘transition height’, which is proposed to be included in the IRI as an additional parameter, supplementing the electron density models. It is possible to predict the position of this ‘transition height’ at a given place and time with the aid of a simplified ion chemistry scheme which is shown to be satisfactorily compatible with experimental ion composition data available in the literature. Our suggested method of this prediction makes use of the (IRI or experimental) electron density profile at the location and season in question, together with an effective clustering rate coeeficient calculated from corresponding temperature and density profiles taken from a suitable reference model of the neutral atmosphere.     

Electric field and electron density measurements in the equatorial E-region

A centaure rocket, with payloads of Langmuir probe and Electric field probe, was launched from Thumba (8° 31'N, O° 47'S dip), India on February 12, 1981 at 1057 Hrs IST. The aim of the experiment was to study the role of localised electric fields in the generation of plasma density irregularities through cross field instability and the two-stream instability mechanism. The rocket was launched at a time when Type I irregularities were observed with VHF radar at Thumba.     

Synopsis of D- and E-region electron densities during the energy budget campaign

Electron density profiles from ground based and rocket borne measurements are presented which were derived at three sites in northern Scandinavia under various degrees of geophysical disturbance. These data are checked against the instantaneous picture of the ionospheric absorption as observed with the dense riometer network.     

Role of neutral winds in generating irregularities in equatorial F-region

Electron density and neutral wind velocity measurements were carried out by rocketborne probes from rocket ranges in India. The experiments were carried out at the time of the onset of spread-F at sunset hours. The results show that a neutral wind velocity in north-south direction greater than 100 m/sec is required to trigger spread-F. It is suggested that spread-F is generated by the interaction of neutral gas with ionospheric plasma.     

Characteristics of nighttime E-region over Arecibo: Dependence on solar flux and geomagnetic variations

Electron concentration (Ne) inferred from Incoherent Scatter Radar (ISR) measurements has been used to determine the influence of solar flux and geomagnetic activity in the ionospheric E-region over Arecibo Observatory (AO). The approach is based on the determination of column integrated Ne, referred to as E-region total electron content (ErTEC) between 80 and 150 km altitude regions. The results discussed in this work are for the AO nighttime period. The study reveals higher ErTEC values during the low solar flux periods for all the seasons except for summer period. It is found that the E-region column abundance is higher in equinox periods than in the winter for low solar activity conditions. The column integrated Ne during the post-sunset/pre-sunrise periods always exceeds the midnight minima, independent of season or solar activity. This behavior has been attributed to the variations in the coupling processes from the F-region. The response of ErTEC to the geomagnetic variability is also examined for different solar flux conditions and seasons. During high solar flux periods, changes in Kp cause an ErTEC increase in summer and equinox, while producing a negative storm-like effect during the winter. Variations in ErTEC due to geomagnetic activity during low solar flux periods produce maximum variability in the E-region during equinox periods, while resulting in an increase/decrease in ErTEC before local midnight during the winter/summer periods, respectively.     

Rocket studies of atmospheric scattering and the aerosol size distribution in the troposphere and lower stratosphere over Thumba

Measurements have been made of the atmospheric scattering in the ultraviolet (λ = 0.31 μm) during the 16 February 1980 solar eclipse rocket campaign. The amplitude of the scattered fluxes as well as the angular distribution could be measured from ground up to an altitude of about 24 km. The Rayleigh scattering component was estimated using standard atmospheric models and the observations have been used to study the scattering due to aerosols at tropospheric and lower stratospheric altitudes.     

Performance of the IRI-2007 model for equatorial topside ion density in the African sector for low and extremely low solar activity

The recent availability of new data sets during the recent extreme solar minimum provides an opportunity for testing the performance of the International Reference Ionosphere in historically undersampled regions. This study presents averages and variability of topside ionospheric densities over Africa as a function of season, local time, altitude, and magnetic dip latitude as measured by the Communication/Navigation Outage Forecast System (C/NOFS) satellite. The results are compared to the three topside model options available in IRI-2007. Overall, the NeQuick model is found to have the best performance, though during the deepest part of the solar minimum all three options significantly overestimate density.     

Geomagnetic storm effects on the occurrences of ionospheric irregularities over the African equatorial/low-latitude region

The study investigated the effects of intense geomagnetic storms of 2015 on the occurrences of large scale ionospheric irregularities over the African equatorial/low-latitude region. Four major/intense geomagnetic storms of 2015 were analyzed for this study. These storms occurred on 17th March 2015 (?229?nT), 22nd June 2015 (?204?nT), 7th October 2015 (?124?nT), and 20th December 2015 (?170?nT). Total Electron Content (TEC) data obtained from five African Global Navigation Satellite Systems (GNSS) stations, grouped into eastern and western sectors were used to derive the ionospheric irregularities proxy indices, e.g., rate of change of TEC (ROT), ROT index (ROTI) and ROTI daily average (ROTI_AVE). These indices were characterized alongside with the disturbance storm time (Dst), the Y component of the Interplanetary Electric Field (IEFy), polar cap (PC) index and the H component of the Earth’s magnetic field from ground-based magnetometers. Irregularities manifested in the form of fluctuations in TEC. Prompt penetration of electric field (PPEF) and disturbance dynamo electric field (DDEF) modulated the behaviour of irregularities during the main and recovery phases of the geomagnetic storms. The effect of electric field over both sectors depends on the local time of southward turning of IMF Bz. Consequently, westward electric field inhibited irregularities during the main phase of March and October 2015 geomagnetic storms, while for the June 2015 storm, eastward electric field triggered weak irregularities over the eastern sector. The effect of electric field on irregularities during December 2015 storm was insignificant. During the recovery phase of the storms, westward DDEF suppressed irregularities.     

Thermosphere density and wind measurements in the equatorial region using a constellation of drag balance nanospacecraft

A mission for in situ thermosphere density and winds measurement is described, based on nanospacecraft equipped with a drag balance instrument (DBI) and a GPS receiver. The mission is based on nanosatellite clusters deployed in three orbital planes. In this study, clusters of 10 nanospacecraft are considered, leading to a mission based on a total of 30 nanospacecraft. The geometry analyzed is a symmetrical one, including an equatorial orbit and two orbits with the same inclination and opposing ascending nodes. The main idea is that, by combining the accurate information on the satellite inertial position and velocity provided by the GPS receiver and the drag acceleration intensity provided by the DBI, due to the orbits’ geometrical configuration, both atmospheric drag and wind can be resolved in a region close to the orbit nodes. Exploiting the Earth oblateness effect, a complete scan of the equatorial regions can be accomplished in the short mission lifetime typical of very low Earth orbit satellites, even in high solar activity peaks, when the expected nanospacecraft lifetime is about 40 days.     

Rocket observations of propagating waves in the night time equatorial F-region over Brazil

A Brazilian SONDA III rocket carrying plasma diagnostic experiments was launched from the Brazilian rocket launching stations in Alcântara (2.31°S, 44.4°W Geog. Lat.) to measure the height profiles of electron density, electron temperature and the ambient electric field. High frequency capacitance probe was used to measure the height profile of the electron density and the Langmuir probe was used to measure the electron density and the spatial structures of plasma irregularities. An electric field double probe was used to measure the electric field fluctuations associated with the F-region plasma irregularities. Spectral analysis of the fluctuations in electron density and electric field indicated the presence of propagating waves in the night time F-region over a large height range. The electron temperatures estimated from the LP data showed abnormally high values in the base of the F-region during the upleg of the rocket and practically normal values in the same height region during the downleg. A brief study of the characteristic features of the spectra of electron density and electric field fluctuations and the associated electron temperature variations are presented and discussed here.     

A new model of the ion composition at 75 to 1000 km for IRI

Ion composition of the ionosphere is an important parameter of any ionospheric model. The International Reference Ionosphere-1979 includes a program for the relative ion composition computation. The program was constructed on the basis of the Danilov and Semenov /1/ empirical model, which averaged 42 rocket measurements of the ion composition at middle latitudes below 200 km, on “AEROS” satellite measurements, and on Taylor's data /2/ above that altitude.     

Plasma transport process in the equatorial/low-latitude ionosphere

The behaviour of the equatorial/low-latitude ionosphere and the transport processes during magnetic disturbed and quiet periods of a high solar activity year, 2014, in the American sector are investigated. Parameters used include vertical drift (Vz), transport term (W), NmF2, hmF2 and scale-height (H). The F2 plasma variations followed the diurnal local solar pattern, being higher at daytime. The sunset maximum and sunrise minimum peaks of hmF2 were directly opposite to the scale height (H) pattern. The plasma distribution was basically controlled by combined actions of the electrodynamic convection/thermospheric composition, which is geomagnetic activity dependent. The annual, semi-annual and winter-anomalies of the F2 parameters were higher at the dip equator in comparison with the low-latitude. The Vz pre-reversal peak magnitude coincided with hmF2 peak and the effects are more pronounced during geomagnetic disturbed conditions. The transport term pattern was similar to that of the scale height and it is suggested as a proxy parameter for quantifying low-latitude plasma irregularities and distribution of thermospheric composition.     

Practical method for routine analysis of the valley parameters between E- and F-region of the ionosphere

Measurement of the virtual height and the frequency of the minimum of the F-region extraordinary trace in digital and analog ionograms can provide a world-wide survey of the main parameters for the valley between E- and F-region ionization. The two added quantities establish also the starting point for the true height analysis of the F-region ionization.     

Variation of ionospheric electron and ion temperatures during periods of minimum to maximum solar activity by the SROSS-C2 satellite over Indian low and equatorial latitudes

To study the variation of ionospheric electron and ion temperatures with solar activity the data of electron and ion temperatures were recorded with the help of Retarding Potential Analyzer payload aboard Indian SROSS-C2 satellite at an average altitude of ∼500 km. The main focuses of the paper is to see the diurnal, seasonal and latitudinal variations of electron and ion temperatures during periods of minimum to maximum solar activity. The ionospheric temperatures in the topside show strong variations with altitude, latitude, season and solar activity. In present study, the temperature variations with latitude, season and solar activity have been studied at an average altitude ∼500 km. The peak at sunrise has been observed during all seasons, in both electron and ion temperatures. Further, the ionospheric temperatures vary with latitude in day time. The latitudinal variation is more pronounced for low solar activity than for high solar activity.     

Comparison of ion composition from the goddard comprehensive ionosphere database with the international reference ionosphere model

We compared the IRI values of T_e, N_e, T_i, O⁺, H⁺, He⁺, O₂₊, and NO⁺ with AE-C values, obtained from the Goddard Comprehensive Ionosphere Database (GCID), composed of data from the satellites, AE-B, OGO-6, ISIS-2, AE-C, AE-D, and AE-E. O⁺ - H⁺ transition levels were derived from the IRI and AE-C altitude profiles. Some discrepancies were found between IRI and the AE-C data. The IRI electron density was found to be about a factor of 2 higher than the data. The H⁺ composition agreed best among the IRI ions, with an average AE-C/IRI ratio of 1.05. The temperatures of both electrons and ions agreed quite well: the average ratios of AE-C/IRI was found to be .99 for electrons and 1.17 for ions.     

Pre-storm behaviour of NmF2 and TEC (GPS) over equatorial and low latitude stations in the Indian sector

The pre-storm behavior of NmF₂ and TEC over an equatorial station, Trivandrum (8.47°N, 76.91°E, dip 0.6°S) and a low latitude station, Waltair (17.7°N,83.3°E, dip 20°N) has been studied for a total of 18 strong geomagnetic storms with DST ? −100 nT. The simultaneous measurements of GPS-TEC and NmF₂ over Trivandrum and Waltair during the period 2000–2005 have been considered for the present study. It is found that there is a substantial increase in NmF₂ and TEC before the onset of the storm over Waltair, while the increase is not present at Trivandrum. The origin of pre-storm enhancements in electron density still remains unresolved owing to several conditions in their potential sources and occurrence mechanisms. In the present study an attempt is made to identify the possible mechanisms responsible for such enhancements in electron density of the F-region.