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.     

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.     

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.     

Photoionization balance and ion composition model in the equatorial D-region over Thumba

Positive and negative ion profiles, together with the λ-profile (ratio of negative ions to electrons), have been derived for Thumba (8°54′N) for noon conditions and for the height range 50–90 km, using the latest available input parameters.     

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.     

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.     

Energy distribution of thermal electrons at the height oflower-E-region

Energy distributions of thermal electrons (0.2 – 1 eV) were measured in the lower E region from the height of 90 Km. The distributions of the electrons started deviating from Maxwellian distributions at the height of ～104 Km. The distortion becomes most intense at the height of 107 Km and the electron population apparently consists of two groups at different electron temperatures in addition to excess high energy tail. The distortion almost vanished at the height of ～130 Km, again increased at ～150 Km and finally faded out at the height of ～210 Km. We suggest that both high electron temperature and excess high energy tail are locally generated by some ionospheric-current related mechanism.     

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.     

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.     

Equatorial F-region ionization differences between March and September, 1979

Ionospheric total electron content and the F-region maximum electron density at a number of stations in the equatorial region, during the recent solar activity maximum period 1979 to 1980, show significant differences between the two equinoctial periods. Ionization during the month of March is higher than in September, irrespective of the station location both in northern and southern hemispheres, and in different longitude sectors. The observed pattern is compared with those predicted by different models, in particular with one of the authors which includes processes such as ionization production, loss, electrodynamic drifts, winds and global composition changes involved in the equatorial ionosphere. It is found that a change in the neutral composition is primarily responsible for the observed F-region density differences between March and September.     

Comparison between d- and lower e-region electron density profiles and IRI-79

The D and E-region electron density profiles obtained by different techniques are compared with the IRI-79 model to see how they fit. The rocket data showed good agreement. However discrepancies between the observed and model values were found especially for solar zenith angle greater than 50 degrees.     

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.     

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.     

X-ray observations of high-z radio loud/quiet quasars

It is widely recognized that a knowledge of the X-ray properties of high-z radio loud/quiet quasars (RLQs/RQQs) would have important consequences for probing the conditions of the early Universe, studying the evolution of quasars, and assessing quasar contributions to the CXB. In distant active galactic nuclei (AGN) we can detect only the quasars having high luminosities or collimated beams with current X-ray astronomy satellites. We have also detected more than ten RLQs above z=2 with ASCA. We have observed 8 high z RQQs above z=2, but have not detected the higher z RQQs, above z=2.5. A considerable fraction of the high z RLQs show excess absorption over Galactic column densities, while most high z RQQs do not indicate excess absorption, although the samples are small. The X-ray photon index of RLQs is smaller than that of RQQs. The two RLQs above z=4 are very flat, suggesting that they are blazar-like AGN. A long term observation of RQQ, H1400#10 (z=2.078) did not indicate an Fe Kα emission line, giving a weaker upper limit than a typical line intensity of Seyfert 1 galaxies.     

Equatorial F-region vertical ion drifts during quiet solar maximum

Median values of ionosonde h′F data acquired at Ibadan (Geographic:7.4°N, 3.9°E, Magnetic: dip 6°S, and magnetic declination, 3°W), Nigeria, West Africa, have been used to determine vertical ion drift (electric field) characteristics in the postsunset ionosphere in the African region during a time of high solar activity (average F_10.7 −208). The database spans from January and December 1958 during the era of International Geophysical Year (IGY) for geomagnetic quiet conditions. Bimonthly averaged diurnal variations patterns are very similar, but differ significantly in magnitude and in the evening reversal times. Also, monthly variations of F-region vertical ion drift reversal times inferred from the time of h′F maximum indicates early reversal during equinoxes and December solstice months except for the month of April. Late reversal is observed during the June solstice months. The equatorial evening prereversal enhancement in vertical ion drift (V_zp) occurs largely near 1900 LT with typical values 20–45 m/s. Comparison of Ibadan ionosonde V_zp with the values of prereversal peak velocity reported for Jicamarca (South America), Kodaikanal (India), and Scherliess and Fejer global model show considerable disparity. The changes of postsunset peak in virtual height of F-layer (h′F_P) with prereversal velocity peak V_zp are anti-correlated. Investigation of solar effects on monthly values of V_zp and h′F_P revealed that these parameters are independent of monthly averaged solar flux intensity during quiet-time sunspot maximum conditions.     

The quiet nighttime low-latitude ionosphere as observed by TIMED/GUVI

In this paper, we examine the nighttime ionosphere climatology structure in the low latitude region and discrepancies between Global Ultraviolet Imager (GUVI) observations and the IRI model predictions using (1) the magnetic zonal mean of electron number density as a function of altitude and magnetic latitude, (2) vertical electron density profiles at various levels of F10.7 index, (3) nighttime descent and magnitude decrease of the ionosphere, (4) point-to-point comparisons of F-peak height (hmF2) and density (NmF2), and (5) the magnetic longitudinal variations of hmF2 and NmF2. The data collected from the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) mission since its launch in December 2001 have provided great opportunities for many scientific investigations of the ionosphere. In this analysis, we investigate the climatology of the nighttime low-latitude ionosphere under low geomagnetic activity (kp ? 4) using the electron density profiles inferred from the airglow measurements obtained by the GUVI aboard the TIMED spacecraft and compared with the results obtained from IRI (International Reference Ionosphere) model-2001. The observed climatology is an essential tool for further understanding the electrodynamics in the low-latitude region and improving the model’s prediction capability. The time range of the GUVI data used in this study is from 2002 (day 053) to 2006 (day 304), and the IRI model predictions were produced at every GUVI location. The ionosphere observed is generally of greater density than what IRI predicts throughout the night for all four seasons for low and moderate solar activity while the model over-predicts the electron density near the F-region peak at high solar activity before midnight. Observations show that the height of the F-region peak has a steep descent from dusk to midnight and near midnight the height of layer is insensitive to solar conditions, significantly different than what is predicted by IRI. Longitudinal features shown in GUVI data are present in the low-latitude ionosphere after sunset and continue through to midnight after which the low-latitude ionosphere is largely zonally symmetric.     

On the nature of coronal loops above the quiet sun network

The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity. Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different. We investigate a model of an active region for two sunspots surrounded by magnetic field patches comparable in magnetic flux to the sunspots. The model results in emission from the model corona being concentrated in loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV and X-ray emission outline the magnetic field, following the general paradigm. However, in our model, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field lines. This result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network.     

Latitudinal variation of the topside electron temperature at different levels of solar activity

A database of electron temperature (T_e) measurements comprising of most of the available satellite measurements in the topside ionosphere is used for studying the solar activity variations of the electron temperature T_e at different latitudes, altitudes, local times and seasons. The T_e data are grouped into three levels of solar activity (low, medium, high) at four altitude ranges, for day and night, and for equinox and solstices. We find that in general T_e changes with solar activity are small and comparable in magnitude with seasonal changes but much smaller than the changes with altitude, latitude, and from day to night. In all cases, except at low altitude during daytime, T_e increases with increasing solar activity. But this increase is not linear as assumed in most empirical T_e models but requires at least a parabolic approximation. At 550 km during daytime negative as well as positive correlation is found with solar activity. Our global data base allows to quantify the latitude range and seasonal conditions for which these correlations occur. A negative correlation with solar activity is found in the invdip latitude range from 20 to 55 degrees during equinox and from 20 degrees onward during winter. In the low latitude (20 to −20 degrees invdip) F-region there is almost no change with solar activity during solstice and a positive correlation during equinox. A positive correlation is also observed during summer from 30 degrees onward.     

Solar wind origins in coronal holes and in the quiet Sun

Coronal hole (CH) and the quiet Sun (QS) are considered to account for sources of fast and slow solar wind streams, respectively. The differences between the solar wind streams flowing out from the CH and the QS are thought to be related with different plasma generation and acceleration mechanisms in the respective source regions. Here we review recent studies on the solar wind origin in the CH and the QS, compare the possible flow geometries and magnetic structures in these two kinds of solar regions, and summarize the physics associated with two different origin scenarios.