Atmospheric expansion through Joule heating by horizontal electric fields

Incoherent scatter measurements made along a magnetic field line into aurora during a period of high electric field in the recovery phase of a substorm show (1) considerably increased electron densities well above the normal F-region maximum, and (2) field-aligned plasma drifts that increase with altitude. A model invoking atmospheric expansion through Joule heating by the horizontal electric field driving the auroral electrojet is used to explain the observations. From this study it is concluded that during magnetically disturbed periods (1) Joule heating by the auroral electrojet raises the neutral temperature and density in the auroral zone ionosphere at F-region heights, (2) ionization formed by the aurora is transported upward by the expanding atmosphere, at times producing an appreciable increase in lower exospheric plasma densities on the field lines containing the aurora, and (3) combined satellite, radar, and optical observations during periods of aurora and high electric field could provide measured F-region collision frequencies.     

The high latitude circulation and temperature structure of the thermosphere near solstice

The neutral gas temperature and circulation of the thermosphere are calculated for December solstice conditions near solar cycle maximum using NCAR's thermospheric general circulation model (TGCM). High-latitude heat and momentum sources significantly alter the basic solar-driven circulation during solstice. At F-region heights, the increased ion density in the summer hemisphere results in a larger ion drag momentum source for the neutral gas than in the winter hemisphere. As a result there are larger wind velocities and a greater tendency for the neutral gas to follow the magnetospheric convection pattern in the summer hemisphere than in the winter hemisphere. There is about three times more Joule heating in the summer than the winter hemisphere for moderate levels of geomagnetic activity due to the greater electrical conductivity in the summer E-region ionosphere.

The results of several TGCM runs are used to show that at F-region heights it is possible to linearly combine the solar-driven and high-latitude driven solutions to obtain the total temperature structure and circulation to within 10–20%. In the lower thermosphere, however, non-linear terms cause significant departures and a linear superposition of fields is not valid.

The F-region winds at high latitudes calculated by the TGCM are also compared to the meridional wind derived from measurements by the Fabry-Perot Interferometer (FPI) and the zonal wind derived from measurements by the Wind and Temperature Spectrometer (WATS) instruments onboard the Dynamics Explorer (DE−2) satellite for a summer and a winter day. For both examples, the observed and modeled wind patterns are in qualitative agreement, indicating a dominant control of high latitude winds by ion drag. The magnitude of the calculated winds (400–500 m s⁻¹) for the assumed 60 kV cross-tail potential, however, is smaller than that of the measured winds (500–800 m s⁻¹). This suggests the need for an increased ion drag momentum source in the model calculations due to enhanced electron densities, higher ion drift velocities, or some combination that needs to be further denned from the DE−2 satellite measurements.     

Median ionospheric height variations over a sunspot cycle in the Australian-Japanese longitudinal sector

The monthly median virtual height (h′F) of the F-region was studied for a period of 6 years (1980–1985) from sunspot maximum to minimum, using data from 11 ionosonde stations in the Japanese-Australian longitudinal sector, in an invariant latitude range: 37°N to 54°S. The night-time maximum in the median height progressively decreases equatorwards, particularly in the local winter and spring, while a reverse weak tendency is observed in summer. The median height reaches peak in both hemispheres from 1 to 2 years after sunspot maximum then decreases towards sunspot minimum. A second diurnal maximum in h′F, preceded by a well-defined minimum, was consistently observed over the solar cycle close to the sunrise time at the F-region, mainly at low invariant latitudes (9–20°). The second maximum has a distinct seasonal variation, being most pronounced in winter and diminishing in summer. It is envisaged that the second peak in h′F is associated with the wave disturbance generated by the supersonic motion of the sunrise terminator. Possible effects of the background height variations on the propagation of the magnetic storm-induced travelling ionospheric disturbances are discussed.     

Changes in the ionospheric F2-layer at a place near the Sq-current focus during geomagnetic storms

In this paper hourly data of maximum electron density and total electron content in a unit column up to the level of peak electron density of the F2-layer at Puerto Rico (magnetic dip 52.5°N) in the American sector are studied to find their DS and D_st variations and to compare them with those of the horizontal component of the Earth's magnetic field for 93SC type geomagnetic storms which occurred during the period September 1957–March 1962. These variations are obtained separately for positive and negative F2-storms and then averaged for all the types. It is found that the positive F2-storms are in a way connected with the equatorial type of DS variation of the H-field and the negative F2-storms with the high-latitude type DS variation of the H-field. The D_st variation of the H-field is practically of the same character for both positive and negative F2-storms. These findings combined with those of others indicate that it is the DS current in the ionosphere that cause the observed changes in the F2-layer through electromagnetic movements; diffusion along the field lines and changes in the loss-rates of electrons may also contribute to the nett effects. A statistical survey shows that while there are equal chances for positive F2-storms in Summer and Winter at Puerto Rico, there is a much larger number of negative F2-storms in Summer than in Winter. At a southern conjugate place, there is a much larger number of positive F2-storms in Winter, but equal number of negative F2-storms in Summer and Winter. More than half the total number of the F2-storms are found to be similar types (33 per cent positive, 23 per cent negative) from the consideration of the F2-changes during individual magnetic storms at the conjugate places. These are discussed in the concluding section of the paper.     

Effects of E-region electric fields on F-region parameters at the magnetic equator

The effects of E-region electric fields on F-region parameters like height of constant electron density contours (h_N) and semi-thickness, at an equatorial station, Trivandrum, are investigated. The E-region east-west electric field (E_y) has been deduced using the horizontal magnetic field values from the ground magnetograms. It is found that the semi-thickness parameter and h_N follow closely the E-region electric field (E_y) variations in the forenoon and noon periods. In the afternoon, it is shown that there is close association between h_N and E_y, when the electric field variations are large. On a day-to-day basis also, h_N and the E-region electric field show very good correlation in the forenoon and noon hours. It is also shown that higher levels are affected more than the lower levels in the F-region by E_y.     

Conjugate effects in the generation of travelling ionospheric disturbances (TIDs) in the F-region

A rapid onset of auroral absorptions was simultaneously recorded by a chain of standard riometers, situated in the northern and southern magneto-conjugate areas, during a period of pronounced substorm activity. The first absorption peak was followed by sequential disturbance patterns in the occurrence of the F-region parameters, virtual height (h′F) and spread - F, as deduced from the standard ionosonde data obtained over a wide range of latitudes in both hemispheres. The disturbances were consistent with the simultaneous occurrences of separate trains of large-scale ionospheric disturbances (TIDs), propagating equatorwards from the southern and northern auroral zones. It is suggested that TIDs were generated by an impulse-like increase in the conjugate particle precipitations, inferred from the riometer records. The precipitation pattern was limited to a high-latitude shell whose equatorwards edge was contained between L-values 5.0 and 5.3. The auroral sources of TIDs appeared to have large linear dimensions, extending at least 17 degrees in longitude.     

An auroral F-region study using in situ measurements by the Atmosphere Explorer-C satellite

On 14 July 1974 the Atmosphere Explorer-C satellite flew through an aurora at F-region altitudes just after local midnight. The effects of the particle influx are clearly evident in the ion densities, the 6300 Å airglow, and the electron and ion temperatures. This event provided an opportunity to study the agreement between the observed ion densities and those calculated from photochemical theory using in situ measurements of such atmospheric parameters as the neutral densities and the differential electron energy spectra obtained along the satellite track. Good agreement is obtained for the ions O₂⁺, NO⁺ and N₂⁺ using photochemical theory and measured rate constants and electron impact cross sections. Atomic nitrogen densities are calculated from the observed [NO⁺]/[O₂⁺] ratio. In the region of most intense electron fluxes (20 erg cm⁻² sec⁻¹) at 280 km, the N density is found to be between 2 and 7 × 10⁷ cm⁻³. The resulting N densities are found to account for approx. 60% of the production of N⁺ through electron impact on N and the resonant charge exchange of O⁺(²P) with N(⁴S). This reaction also provides a significant source of O(¹S) in the aurora at F-region altitudes. In the region of intense fast electron influx, the reaction with atomic nitrogen is found to be the main loss of O⁺(²P).     

Anomalous electron temperatures above the South American magnetic field anomaly

With an instrument on board the Japanese satellite EXOS-A electron temperatures of more than 1000 K above the “normal” values have been observed in the night-time topside F-region above the geographic region where the total magnetic field is below 20000 nT. Simultaneously enhanced wave emissions on 45 kHz, 2 MHz and 3 MHz were found and an increase in particle precipitation. The observations are described in detail and several mechanism are discussed which can explain the results.     

A numerical investigation of thermosphere-ionosphere interaction over Millstone Hill

The upper thermosphere and F-region ionosphere system at 43°N is modelled for equinox and moderate solar conditions via a series of iterative calculations employing a thermospheric wind model and a one-dimensional ionospheric model which are mutually coupled. Several feedback loops within the system involving F₂-layer peak height, F₂-layer peak density, zonal wind, meridional wind, and Coriolis force are investigated to better understand the interactive aspect of ionosphere-thermosphere coupling. The interplay of primary importance involves the night-time ascent/descent of the F-layer due to equatorward/poleward neutral winds, the resulting changes in ion drag presented to the meridional and zonal wind fields, and the Coriolis force modification of the ion drag coupling. Wind shear and plasma profile shape are not significantly coupled. For magnetically undisturbed conditions, self-consistent treatment of these effects modifies a non-interactive “control” calculation by 20–35 m s⁻¹ in the wind field. During geomagnetically disturbed periods interactive processes play a more crucial role in determining thermospheric and ionospheric storm responses. Our calculations reveal wind enhancements of up to 100 m s⁻¹ associated with the lifting and negative-phase depletion of the F-region for prolonged magnetic disturbance conditions, the former mechanism accounting for a major portion of the effect.     

Mid-latitude radio-satellite scintillation: The height of the irregularities

The heights of the irregularities in ionospheric refractive index responsible for radio-satellite scintillation have been found from long-term spaced-receiver experiments at mid-latitude and sub-auroral locations using the 40 MHz transmissions of satellite BE-B. The important results relate to the diurnal, seasonal, and latitude variations of the irregularity heights. Night-time scintillation is usually produced in the F-region at all times of the year, and in particular in the top-side of the F-region in Summer. Day-time scintillation is rare at midlatitudes; at sub-auroral latitudes it may be produced at any height. The night-time height distributions show a constancy of height with latitude in Summer and Winter, although there is a slight increase poleward in average Autumn-Spring night-time heights. Only records displaying weak to moderate scintillation at 40 MHz were used in the analysis, and therefore the heights reported are those of irregularities producing such scintillation.     

The winter anomaly in ionospheric absorption and the D-region ion chemistry

A detailed analysis of the D-region ion composition measurements performed by Zbinden et al. (1975), during a winter day of high ionospheric absorption, has been carried out. The study examines the interactive mesosphere-D-region processes which occur in such anomalous conditions and their implication for water cluster ion chemistry. Two clustering regimes for NO⁺ have been observed in the data. Association with N₂ is identified as the dominant process below 76 km. Between 76 and 78 km altitude the effective loss rate of NO⁺ drops by two orders of magnitude. Above 77 km, the three-body reaction NO⁺ + CO₂+M→NO⁺CO₂+M seems to be the main NO⁺ loss. A mesospheric temperature profile could be derived from the ion composition data. This indicates the presence of a strong inversion above 76 km altitude. The wavelike structure obtained, is shown to be consistent with in situ winter temperature measurements. The sharp suppression of the N₂ association reaction could, thus, be explained by an increase in the collisional break-up of the NO⁺N₂ ion because of the enhanced temperature. In conclusion, our study indicates that, besides the increase in the production of NO⁺ and O₂⁺, due to an enhancement in the minor ionizable constituents, an additional thermal mesosphere-D-region interaction seems necessary to explain this winter anomalous ion composition data.     

Equatorial electrojet parameters and the relevance of Electromagnetic Drifts (EMD) over Thumba

The thermal imbalance in theE-region heights is inescapable and an undisputed fact. The equatorial electrojet parameters are being evaluated for an Indian Equatorial station of Thumba, making use of the observed electron density, electron temperature, and current density profiles for two rocket flights on 3 March, 1973 and 7 April, 1972 around local noon, corresponding to low and medium solar active conditions. The computed Joule heating due to Equatorial Electrojet Current (EEC) does not account for the observed difference between the electron and neutral gas temperatures. The discrepancy of about 6 km in the peaks of the observed and computed current density profiles may be attributed to the presence of the Electromagnetic Drifts (EMD). In order to see whether or not EMD plays an important role, the photoionization balance between production and loss rates have been computed by making use of the latest available solar flux and cross sections and chemical reaction rate constants for the appropriate solar epoch conditions including the transport term due to EMD. There is an excellent agreement between the observed and computed electron density profiles indicating its relevance.     

Thermospheric changes shortly after the onset of daytime joule heating

In the first few tens of minutes after the onset of widespread Joule heating, the motion of the ionospheric atmosphere can be approximated by the one-dimensional motion of a gas in a gravity field—a problem that is easily solved because the motion takes place at constant pressure. The solution provides an estimate of time for which the model is applicable to the physical situation. Seasonal variations of the early effects are examined by using ion profiles appropriate to each season. The results show that the atmosphere above 100 km is strongly modified within a few tens of minutes after the onset of widespread heating: the density can double, the temperature can increase several hundred degrees, and the molecular nitrogen concentration can quadruple. Vertical winds exceeding 100 m/sec at 400km altitude are possible for a brief period after the onset of electric fields of 100 mV/m—rare but observed events. In the first few tens of minutes after the onset of a given electric field, the greatest power is deposited in the thermosphere around summer solstice, while the greatest winds occur at 200 km altitude in the summer and at 400km in the winter. These differing seasonal effects show primarily that a given level of change occurs sooner for one season than another, not that long term seasonal differences exist. Once a magnetic storm is in progress, the quiet-day ion profiles change to the non-seasonal storm profile ; for this ion distribution, F-region effects are minimum regardless of season. Joule heating effects in the upper thermosphere are therefore concluded to be self-limiting.     

The absorption of radio waves in the ionosphere with respect to geomagnetic current control and other proposed models—a critical survey

It is shown that the arguments advanced by Shaw⁽¹⁾ to demonstrate that the absorption of radio waves in the ionosphere is controlled by the currents causing geomagnetic variations are unsound. Further the method used by Bandyopadhyay⁽²⁾ in deducing the nondeviative absorption leads to too high a proportion of this absorption in the total. The two D-regions model proposed by Rumi⁽³⁾ is also unsatisfactory in several respects. In all three papers, error arises because of the neglect of the deviative absorption in E-region. The reason for this neglect may be because of the resemblance between the frequency variation of E-region deviative absorption and that of the non-deviative absorption, except in the immediate vicinity of ƒ₀E.     

Thermal structure of the equatorialE-region over Thumba

Photoionization and absorbed energy rates have been computed by making use of the latest available semi-empirical solar XUV fluxes and cross-sections for two rocket flights for electron density measurements on 3 March, 1973 and 7 April, 1972 corresponding to low and medium solar active conditions over Thumba. Various heating and cooling rate profiles have been computed by making use of the latest available information. Various efficiency parameters such as photoelectron heating efficiency, photoionization efficiency, and ultraviolet heating efficiency, that are extremely important in understanding the thermal structure of the ionosphere, are clearly defined and distinguished.     

The effect of photoionization on the cooling rates of enriched, astrophysical plasmas

Radiative cooling is central to a wide range of astrophysical problems. Despite its importance, cooling rates are generally computed using very restrictive assumptions, such as collisional ionization equilibrium and solar relative abundances. We simultaneously relax both assumptions and investigate the effects of photoionization of heavy elements by the metagalactic ultraviolet (UV)/X-ray background and of variations in relative abundances on the cooling rates of optically thin gas in ionization equilibrium. We find that photoionization by the metagalactic background radiation reduces the net cooling rates by up to an order of magnitude for gas densities and temperatures typical of the shock-heated intergalactic medium and proto-galaxies  (10⁴ K ≲ T ≲ 10⁶ K, ρ/〈ρ〉≲ 100)  . In addition, photoionization changes the relative contributions of different elements to the cooling rates. We conclude that photoionization by both the ionizing background and heavy elements needs to be taken into account in order for the cooling rates to be correct to an order of magnitude. Moreover, if the rates need to be known to better than a factor of a few, then departures of the relative abundances from solar need to be taken into account. We propose a method to compute cooling rates on an element-by-element basis by interpolating pre-computed tables that take photoionization into account. We provide such tables for a popular model of the evolving UV/X-ray background radiation, computed using the photoionization package cloudy .     

Io: Thermal models and chemical evolution

A combined thermal and chemical evolution model of Io is presented, outlining limits on the possible starting materials, heating history, chemical history, and present state of Io. Our best scenario starts with Io being accreted from material in a proto-Jovian nebula which condensed between 400–600°K. Radionuclides and tidal heating would lead to large-scale convection within Io and chemical reactions leading to the outgassing of water and methane. Reactions between Fe⁰FeS and water, at least near the surface, go to completion, resulting in all Fe being oxidized with elemental sulfur producing a low-conductivity crust. In the deep interior, these reactions may not completely exhaust Fe metal, and an FeS-rich core may be formed.     

Dissipative structures in the F-region of the equatorial ionosphere generated by Rayleigh-Taylor instability

The non-linear regime of electrostatic perturbations of the equatorial ionospheric F-region generated by Rayleigh-Taylor instability has been discussed, taking into account conductivity along magnetic field lines. A closed non-linear equation has been derived in the stationary limit for the polarization electric field potential. It coincides with the Karman equation of an ideal liquid. To solve the equation, the averaged variational Whitham method has been proposed. Some solutions localized along and across the geomagnetic field, B, as well as quasi-periodic solutions in the transverse direction, have been investigated. Non-linear longitudinal localization of perturbations has been shown to be due to electron-ion collisions.     

Some aspects of Sporadic-E at mid-latitudes

Evidence is presented to suggest that the ƒ₀E_s value tends to be high at positions in the E_s-layer where trough slope-lines and crest slope-lines of F2-layer irregularities meet the E_s-layer. These slope-lines are drawn through the troughs and crests, respectively, of the characteristic kinks in the F2-layer ionization contours, which are associated with F2-layer irregularities.

A rotating-loop direction-finding system has allowed an estimation of the distribution of ionization, which gives rise to Sporadic-E echoes. Analysis of Sporadic-E occurrence, on two occasions, suggests that the reflecting surfaces are frontal in nature, the fronts having a separation from each other of some tens of kilometres, and probably existing as closed curves, with diameters of the order of several hundreds of kilometres. A possible association between these structures and the occurrence of the green line of the airglow, is discussed.

A distribution of ionization, which will give contours showing “clouds” of ionization at some frequencies and a ripple structure at other frequencies, is proposed, in an endeavour to explain the apparent dual nature of Sporadic-E occurrence.

The evidence seems to indicate that the mechanism operating at the E_s-layer level, producing the phenomenon of Sporadic-E, is the same as that which produces the F2-layer irregularities which are responsible for “Spread-F”.     

Observations from space of the solar corona/inner zodical light

Observations, from the Apollo 16 Spacecraft, in lunar orbit, of the total radiance of the K + F corona, from 3 R to 55 R are presented and discussed.

The logarithmic slope of the K + F coronal radiance, in the region r > 20 R, is found to be n = 1.93, slightly less steep than previous determinations. The photometric axis of the radiance is found to be displaced 3 ± 1° north of the ecliptic, for the region r > 20 R, and this displacement is interpreted as an annual variation due to non-coincidence of the ecliptic and the symmetry axis of the zodiacal cloud.