The calculated and observed diurnal variation of the ionosphere over Millstone Hill on 23–24 March 1970

A numerical model of current F-region theory is use to calculate the diurnal variation of the mid-latitude ionospheric F-region over Millstone Hill on 23–24 March 1970, during quiet geomagnetic conditions. From the solar EUV flux, the model calculates at each altitude and time step primary photoelectron spectra and ionization rates of various ion species. The photoelectron transport equation is solved for the secondary ionization rates, photoelectron spectra, and various airglow excitation rates. Five ion continuity equations that include the effects of transport by diffusion, magnetospheric-ionospheric plasma transport, electric fields, and neutral winds are solved for the ion composition and electron density. The electron and ion temperatures are also calculated using the heating rates determined from chemical reactions, photoelectron collisions, and magnetospheric-ionospheric energy transport. The calculations are performed for a diurnal cycle considering a stationary field tube co-rotating with the Earth; only the vertical plasma drift caused by electric fields perpendicular to the geomagnetic field line is allowed but not the horizontal drift. The boundary conditions used in the model are determined from the incoherent scatter radar measurements of T_e, T_i and O⁺ flux at 800km over Millstone Hill (Evans, 1971a). The component of the neutral thermospheric winds along the geomagnetic field has an important influence on the overall ionospheric structure. It is determined from a separate dynamic model of the neutral thermosphere, using incoherent scatter radar measurements.The calculated diurnal variation of the ionospheric structure agrees well with the values measured by the incoherent scatter radar when certain restrictions are placed on the solar EUV flux and model neutral atmospheric compositions. Namely, the solar EUV fluxes of Hinteregger (1970) are doubled and an atomic oxygen concentration of at least $\text{10}{}^{11}\text{cm}{}^3$ at 120 km is required for the neutral model atmosphere. Calculations also show that the topside thermal structure of the ionosphere is primarily maintained by a flow of heat from the magnetosphere and the night-time F2-region is maintained in part by neutral winds, diffusion, electric fields, and plasma flow from the magnetosphere. The problem of maintaining the calculated night-time ionosphere at the observed values is also discussed.     

Ion and neutral composition changes in the thermospheric region during magnetic storms

The structural differences of the ion and neutral composition in the thermospheric region are studied by solving a system of basic ionospheric and atmospheric equations. The study shows that the compositional changes during a magnetic storm arise largely as a result of changes in the neutral composition at the turbopause. A decrease in [O]/[N₂] in the lower atmosphere triggers a complex chain of events which results in an increase of the neutral gas temperature, depletion of the O⁺ layer and enhancement of NO⁺. The relative changes in these layers occasionally produce a sequence of electron density profiles giving rise to the so-called G condition. It is shown that, compared to the neutral atmosphere, the ionosphere is much more sensitive to the changes in [O]/[N₂] in the lower thernaospheric region. Since the ionospheric parameters can be measured much more accurately than the atmospheric parameters, it is argued that they should form an integral part of the observational data required to construct the atmospheric models.     

The upper atmosphere as a regulator of subauroral red arcs

The mechanisms for producing a subauroral red arc (SARARC) are studied by solving a system of basic ionospheric and atmospheric equations. It is shown that many of the observed features of a SARARC can be explained within the framework of the two processes generally responsible for the ionospheric behavior during a magnetic storm: these are (1) energy conduction from the magnetosphere to the ionosphere and (2) the changes in neutral composition of the lower atmosphere caused by the increase in turbulent mixing. Both the processes trigger a complex chain of events which ultimately results in the redistribution of both the charged and neutral particles, an increase in the electron, ion, and neutral temperatures, and a decrease in the electron density in the altitude region near the F2 peak. It is shown that both the occurrence and the emission intensity of a SARARC are regulated by the neutral atmosphere, even though conduction of the thermal energy from the magnetosphere to the ionosphere provides the excitation energy fo the optical remission. Recent satellite measurements of the ionospheric parameters have confirmed the validity of these findings and have provided grounds for rejecting several other theories which have been proposed in the literature.     

Evidence for topographic effects on the Martian ionosphere

The Radio occultation experiment on board Mariner 9 has been used to demonstrate that the altitude of the main electron density peak in the Martian ionosphere is closely related to the height of Mars’ surface at the occultation point. This is direct evidence for topographic effects on the Martian ionosphere. Modeling indicates that topographic-induced modulations of the neutral density in the upper atmosphere can account for the observed ionospheric effects. The neutral density modulation is likely to be caused by nonmigrating tides in the Martian thermosphere.     

Ionization by cosmic rays of the atmosphere of Titan

Cosmic ray radiation is the main mechanism for ionizing the lower atmosphere of Titan. Their higher penetration power, in comparison with solar photons, allows cosmic rays to penetrate deep into the atmosphere of Titan, ionizing the neutral molecules and generating an ionosphere with an electron density peak, placed at around 90 km, similar in magnitude to the ionospheric peak produced by solar radiation in the upper atmosphere. In the lower atmosphere, the electron density profile, in the absence of a magnetic field, depends mainly on the modulation of cosmic rays by the solar wind and on the nature of the ionizable particles. We present here the first results of a new numerical model developed to calculate the concentration of electrons and most abundant ions in the Titan lower atmosphere. The present knowledge of Titan’s atmosphere permits us to include new neutral and ionic species, such as oxygen derivates, in a more detailed ion-chemistry calculation than previous lower ionospheric models of Titan. The electron density peaks at 90 km with a magnitude of 2150 cm⁻³. The ion distribution obtained predicts that cluster cations and hydrocarbon cations are the most abundant ions below and above the electron density peak, respectively. We also discuss the effect of solar activity at the distance of the Saturn orbit on the spectrum of the cosmic particles. We obtain that from solar minimum to solar maximum the ionization rate at the energy deposition peak changes by a factor of 1.2 at 70 km, and by a factor of 2.6 at altitudes as high as 400 km. The electron density at the concentration peak changes by a factor of 1.1 at 90 km, and by a factor of 1.6 at 400 km.     

Localized ionization patches in the nighttime ionosphere of Mars and their electrodynamic consequences

Using an electron transport model, we calculate the electron density of the electron impact-produced nighttime ionosphere of Mars and its spatial structure. As input we use Mars Global Surveyor electron measurements, including an interval when accelerated electrons were observed. Our calculations show that regions of enhanced ionization are localized and occur near magnetic cusps. Horizontal gradients in the calculated ionospheric electron density on the night side of Mars can exceed 10⁴ cm⁻³ over a distance of a few tens of km; the largest gradients produced by the model are over 600 cm⁻³ km⁻¹. Such large gradients in the plasma density have several important consequences. These large pressure gradients will lead to localized plasma transport perpendicular to the ambient magnetic field which will generate horizontal currents and electric fields. We calculate the magnitude of these currents to be up to 10 nA/m². Additionally, transport of ionospheric plasma by neutral winds, which vary in strength and direction as a function of local time and season, can generate large (up to 1000 nA/m²) and spatially structured horizontal currents where the ions are collisionally coupled to the neutral atmosphere while electrons are not. These currents may contribute to localized Joule heating. In addition, closure of the horizontal currents and electric fields may require the presence of vertical, field-aligned currents and fields which may play a role in high altitude acceleration processes.     

Joule heating of Io's ionosphere by unipolar induction currents

The unexpectedly large scale height of Io's ionosphere (Kliore, A., et al., 1975, Icarus24, 407–410) together with the relatively large molecular weight of the likely principal constituent, SO₂ (Pearl, J., et al., 1979, Nature280, 755–758), suggest a high ionospheric temperature. Electrical induction in Io's ionosphere due to the corotating plasma bound to the Jovian magnetosphere is one possible source for attainment of such high temperatures. Accordingly, unipolar induction models were constructed to calculate ionospheric joule heating numerically. Heating rates produced by highly simplified models lie in the range 10^?9 to 10^?8 W/m³. These heating rates are lower than those determined from uv photodissociative heating models (Kumar, S., 1980, Geophys. Res. Lett.7, 9–12) at low levels in the ionosphere but are comparable in the upper ionosphere. The low electrical heating rate throughout most of the ionosphere is due to the power limitation imposed by the Alfvén wings which complete the electrical circuit (Neubauer, F.M., 1980, J. Geophys. Res.85, 1171–1178). Contrary to the pre-Voyager calculations of Cloutier, P. A., et al. (1978, Astrophys. Space Sci.55, 93–112), our numerical results show that the J × B force density due to unipolar induction currents in the ionosphere is much less than the gravitational force density when the combined mass of the neutral species is included. The binding and coupling of the ionosphere is principally due to the relatively dense (possibly localized) neutral SO₂ atmosphere. In regions where the ions and neutrals are collisionally coupled the ionosphere will not be stripped off by the J × B forces. However at a level above that (to which the ions move by diffusion only) the charged species would be removed. Thus there appears to be no need to postulate the existence of an intrinsic Ionian magnetic field as suggested by Kivelson, M. G., et al. (79, Science 205, 491–493) and Southwood, S. J., et al. (1980, J. Geophys. Res., in press) in order to retain the observed ionosphere.     

Modeling the midlatitude F-region ionospheric storm using east-west drift and a meridional wind

Under magnetically quiet conditions, ionospheric plasma in the midlatitude F-region corotates with the Earth and relative east-west drifts are small compared to the corotation velocity. During magnetic storms, however, the enhanced dawn-to-dusk magnetospheric convection electric field often penetrates into the midlatitude region, where it maps into the ionosphere as a poleward electric field in the 18:00 LT sector, producing a strong westward plasma drift. To evaluate the ionospheric response to this east-west drift, the time-dependent O⁺ continuity equation is solved numerically, including the effects of production by photoionization, loss by charge exchange and transport by diffusion, neutral wind and $\text{E}\text{×}\text{B}$ drift. In this investigation only the neutral wind's meridional component and east-west $\text{E}\text{×}\text{B}$ drift are included. It is found that an enhanced equatorward wind coupled with westward drift produces an enhancement in the peak electron density (NMAX(F2)) and in the electron content (up to 1000 km) in the afternoon sector and a subsequent greater-than-normal decay in ionization after 18:00 LT. These results agree in general with midlatitude F-region ionospheric storm observations of NMAX(F2) and electron content which show an afternoon enhancement over quiet-time values followed by an abrupt transition to lower-than-normal values. Westward drift appears to be a sufficient mechanism in bringing about this sharp transition.     

A snapshot of the polar ionosphere

This paper presents a picture of the north polar F layer and topside ionosphere obtained primarily from three satellites (Alouette 2, ISIS 1, ISIS 2), that passed over the region within a time interval of ca. 50 min on 25 April 1971, a magnetically quiet day. The horizontal distribution of electron densities at the peak of the F layer is found to be similar to synoptic results from the IGY. Energetic particle and ionospheric plasma data are also presented, and the F layer data are discussed in terms of these measurements, and also in terms of electric field and neutral N₂ density measurements made by other satellites on other occasions. The major features observed are as follows: A tongue of F region ionization extends from the dayside across the polar cap, which is accounted for by antisunward drift due to magnetospheric convection. In the F layer and topside ionosphere, the main effect of auroral precipitation appears to be heating and expansion of the topside. A region of low F layer density appears on the morning side of the polar cap, which may be due to convection and possibly also to enhanced N₂ densities.     

Structure and time variations of the Jovian ionosphere

The problem of the ionospheric formation in the Jovian upper atmosphere is examined. By adopting two plausible atmospheric models, we solve coupled time-dependent continuity equations for ions H₂⁺, H₅⁺, H⁺, H₃⁺ and H_eH⁺ simultaneously. It is shown that both radiative and three body association of H⁺ to H₂ are important for the determination of the structure of the Jovian ionosphere. The maximum electron density in the daytime is found to be about 10⁵ cm^?3. It is also shown that diurnal variation with large-amplitude can exist in the Jovian ionosphere.     

Ion temperature troughs induced by a meridional neutral air wind in the night-time equatorial topside ionosphere

A mathematical model has been developed to calculate consistent values for the O⁺ and H⁺ concentrations and field-aligned velocities and for the O⁺, H⁺ and electron temperatures in the night-time equatorial topside ionosphere. Using the results of the model calculations a study is made to establish the ability of F-region neutral air winds to produce observed ion temperature distributions and to investigate the characteristics of ion temperature troughs as functions of altitude, latitude and ionospheric composition. Solar activity conditions that give exospheric neutral gas temperatures 600 K, 800 K and 1000 K are considered.It is shown that the O⁺-H⁺ transition height represents an altitude limit above which ion cooling due to adiabatic expansion of the plasma is extremely small. The neutral atmosphere imposes a lower altitude limit since the neutral atmosphere quenches any ion cooling which field-aligned transport tends to produce. The northern and southern edges of the ion temperature troughs are shown to be restricted to a range of dip latitudes, the limiting dip latitudes being determined by the magnetic field line geometry and by the functional form of the F-region neutral air wind velocity. Both these parameters considerably influence the interaction between the neutral air and the plasma within magnetic flux tubes.     

Ionosphere-plasmasheet field-aligned currents and parallel electric fields

Two kinetic models for the auroral topside ionosphere are compared. The collisionless plasma distributed along an auroral magnetic field line behaves like a non-Ohmic conducting medium with highly non-linear characteristic curves relating the parallel current density to the potential difference between the cold ionosphere and the hot plasmasheet region. The (zero-electric current) potential difference, required to balance the current carried by the precipitating plasmasheet particles and the current transported by the outflowing ionospheric particles, depends on the ratio n_ps.e/n_th.e and T_ps.e/T_th.e of the plasmasheet and ionospheric electron densities and temperatures. When in the E-region the magnetic field lines are interconnected by a high conductivity plasma the resulting field-aligned currents driven by the magnetospheric potential distribution are limited by the integrated Pedersen conductivity of the ionospheric layers. These currents are not related to the parallel electric field intensity as they would be in Ohmic materials. The parallel electric field intensity is necessarily determined by the local quasi-neutrality of the plasma.     

Ionospheric warming by neutral winds

The effect of frictional heating by means of neutral winds on the ion and electron temperature in the undisturbed ionosphere is studied theoretically by solving a system of basic ionospheric and atmospheric equations. The study shows that both the electron and ion temperatures are increased in the night-time ionosphere through friction. In the region between 150 and 200 km T_i may exceed T₆ by as much as 130°. The increase of T_i due to friction amounts to about 100–200°, depending on the atmospheric model employed in calculating the neutral wind velocity. It is illustrated that frictional heating may be very important for the determination of the neutral temperature from measured ion temperature values.     

Coupled ion and neutral rotating model of Titan's upper atmosphere

A one-dimensional composition model of Titan's upper atmosphere is constructed, coupling 36 neutral species and 47 ions. Energy inputs from the Sun and from Saturn's magnetosphere and updated temperature and eddy coefficient parameters are taken into account. A rotating technique at constant latitude and varying local-time is proposed to account for the diurnal variation of solar inputs. The contributions of photodissocation, neutral chemistry, ion-neutral chemistry, and electron recombination to neutral production are presented as a function of altitude and local time. Local time-dependent mixing ratio and density profiles are presented in the context of the TA and T₅ Cassini data and are compared in detail to previous models. An independent and simplified ion and neutral scheme (19-species) is also proposed for future 3D-purposes. The model results demonstrate that a complete understanding of the chemistry of Titan's upper atmosphere requires an understanding of the coupled ion and neutral chemistry. In particular, the ionospheric chemistry makes significant contributions to production rates of several important neutral species.     

The effect of realistic conductivities on the high-latitude neutral thermospheric circulation

The dynamics of the high latitude thermosphere are dominated by the ion circulation pattern driven by magnetospheric convection. The reaction of the neutral thermosphere is influenced by both the magnitude of the ion convection velocity and by the conductivity of the thermosphere. Using a threedimensional, time-dependent, thermospheric, neutral model together with different ionospheric models, the effect of changes in conductivity can be assessed. The ion density is described by two models: the first is the empirical model of Chiu (1975) appropriate for very quiet geomagnetic conditions, and the second is a modified version of the theoretical model of Quegan et al. (1982). The differences in the neutral circulation resulting from the use of these two ionospheric models emphasizes the need for realistic high latitude conductivities when attempting to model average or disturbed geomagnetic conditions, and a requirement that models should couple realistically the ionosphere and the neutral thermosphere. An attempt is made to qualitatively interpret some of the features of the neutral circulation produced at high latitudes by magnetospheric processes.     

Detection of Titan's Ionosphere from Voyager 1 Radio Occultation Observations

Evidence for a marginal detection of the Titan ionosphere has been obtained from a new analysis of the dual-frequency Doppler data recorded during theVoyager 1occultation in 1980. The original report by Lindalet al.(1983,Icarus53,348–363) gave only upper bounds on the peak electron density of 3000 cm⁻³during ingress (evening terminator) and 5000 cm⁻³during egress (morning terminator). The dual-frequency ingress data imply a maximum electron density of 2400 ± 1100 cm⁻³for Titan's upper ionosphere at an altitude of 1180 ± 150 km. The egress data were determined to be of limited use for this analysis because the X-band signal was received for only a few seconds. Nevertheless, a distinct ionospheric peak is revealed in the S-band data for both ingress and egress. The height and peak density of this ionized layer are in good agreement with expectations from numerical models that invoke photoionization and energetic electron impacts.     

Ionospheric winds in the F-region and their effects on the limiting periods of gravity waves

Spectrum analyses of ionospheric electron density and content fluctuations show periods with a lower limit near 5 min. Interpretation of this cut off in terms of gravity waves in a windless atmosphere leads to unacceptably low thermospheric temperatures near 180°K. It is concluded that neutral winds reduce the apparent cut-off period in the ionosphere. The maximum horizontal wind speed obtained from cut-off data is about 100 m/sec.     

A modelling study of the effects of neutral air winds on electron content at mid-latitudes in winter

A modelling study of the effects of neutral air winds on the electron content of the mid-latitude ionosphere and protonosphere in winter has been made. The theoretical models are based on solutions of time dependent momentum and continuity equations for oxygen and hydrogen ions. The computations are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.).It is found that the magnitude of the poleward neutral air wind velocity has a strong effect on the general magnitude of the electron content, but that the daily pattern of electron content variation is relatively insensitive to changes in the magnitude and phase of the wind pattern. These results are in contrast with the behaviour reported previously (Sethia et al., 1983) for summer conditions. However, the night-time electron content is increased by advancing the phase of the neutral air wind and decreased by retarding it. It appears that day-to-day variations in the electron content pattern in winter cannot be explained as effects of changing neutral air winds, which again contrasts with the findings for summer. As in summer, the wind has a major effect on the filling of the protonosphere, but in opposite sense.It is argued that the effect of the neutral air wind on the ionospheric and the protonospheric electron contents depends on the duration of the poleward wind in relation to daylight and on whether or not the wind reverses direction whilst the ionosphere is sunlit.     

Jovian ionospheric models

This paper reports results obtained on ionosphere formation in the Jovian upper atmosphere with special reference to some of the recently available reaction rates, and to recent models of the Jovian neutral atmosphere based on the possibility of a warmer mesopause. We find that the role of the hypothetical radiative association of H⁺ to H₂ to form H₃⁺, as brought to light in our earlier study, is still important, even with a reaction rate as low as 10^?15 cm³sec^?1. In the lower regions of the ionosphere three-body processes leading to the formation of H₃⁺ and H₅⁺ ions, which have very fast dissociative recombination rates, produce a dramatic reduction in the electron density. When no radiative association takes place, and the H⁺ ions are lost by radiative recombination alone, we confirm that the photochemical equilibrium profile is also the diffusive equilibrium profile. However, with collisional-radiative recombination, whose rate becomes altitude-dependent, diffusion tends to bring about some redistribution of the ionization. Inclusion of radiative association enhances the role of diffusion. In this case, diffusion brings about all the expected changes. In particular, the differences in the electron density profile, originated in the lower-middle ionosphere by radiative association, are propagated up to all higher altitudes by diffusion. The rate constant of radiative association is, however, unknown. It is hoped that the critical importance of this reaction for the Jovian ionosphere will be an incentive towards a careful laboratory determination of its rate coefficient. In the older models of the Jovian ionosphere the major ions were H⁺ which were lost only by pure radiative recombination. This led to high electron densities and practically no diurnal change. In contrast, our new models have relatively much smaller electron densities, especially in lower regions, and may be susceptible to significant diurnal variation.     

Photochemistry and Vertical Transport in Io's Atmosphere and Ionosphere

We present an updated model for the photochemistry of Io's atmosphere and ionosphere and use this model to investigate the sensitivity of the chemical structure to vertical transport rates. SO₂is assumed to be the dominant atmospheric gas, with minor molecular sodium species such as Na₂S or Na₂O released by sputtering or venting from the surface. Photochemical products include SO, O₂, S, O, Na, NaO, NaS, and Na₂. We consider both “thick” and “thin” SO₂atmospheres that encompass the range allowed by recent HST and millimeter-wave observations, and evaluate the possibility that O₂and/or SO may be significant minor dayside constituents and therefore likely dominant nightside gases. The fast reaction between S and O₂limits the column abundance of O₂to ∼10⁴less than that calculated by Kumar (J. Geophys. Res.87, 1677–1684, 1982; 89(A9), 7399–7406, 1984) for a pure sulfur/oxygen atmosphere. If a significant source of NaO₂or Na₂O were supplied by the surface and mixed rapidly upward, then oxygen liberated in the chemical reactions which also liberate free Na would provide an additional source of O₂. Fast eddy mixing will enhance the transport of molecular sodium species to the exobase, in addition to increasing the vertical transport rate of ions. Ions produced in the atmosphere will be accelerated by the reduced corotation electric field penetrating the atmosphere. These ions experience collisions with the neutral gas, leading to enhanced vertical ion diffusion. The dominant ion, Na⁺, is lost primarily by charge exchange with Na₂O and/or Na₂S in the lower atmosphere and by diffusion through the ionopause in the upper atmosphere. The atmospheric column abundance of SO, O₂, and the upper atmosphere escape rates of Na, S, O, and molecular sodium species are all strong functions of the eddy mixing rate. Most atmospheric escape, including that of molecular sodium species, probably occurs from the low density “background” SO₂atmosphere, while a localized high density “volcanic” SO₂atmosphere can yield an ionosphere consistent with that detected by the Pioneer 10 spacecraft.