Effects of convection electric field on the thermal plasma flow between the ionosphere and the protonosphere

Dynamic behavior of the coupled ionosphere-protonosphere system in the magnetospheric convection electric field has been theoretically studied for two plasmasphere models. In the first model, it is assumed that the whole plasmasphere is in equilibrium with the underlying ionosphere in a diurnal average sense. The result for this model shows that the plasma flow between the ionosphere and the protonosphere is strongly affected by the convection electric field as a result of changes in the volume of magnetic flux tubes associated with the convective cross-L motion. Since the convection electric field is assumed to be directed from dawn to dusk, magnetic flux tubes expand on the dusk side and contract on the dawn side when rotating around the earth. The expansion of magnetic flux tubes on the dusk side causes the enhancement of the upward H⁺ flow, whereas the contraction on the dawn side causes the enhancement of the downward H⁺ flow. Consequently, the H⁺ density decreases on the dusk side and increases on the dawn side. It is also found that significant latitudinal variations in the ionospheric structures result from the L-dependency of these effects. In particular, the H⁺ density at 1000 km level becomes very low in the region of the plasmasphere bulge on the dusk side. In the second model, it is assumed that the outer portion of the plasmasphere is in the recovery state after depletions during geomagnetically disturbed periods. The result for this model shows that the upward H⁺ flux increases with latitude and consequently the H⁺ density decreases with latitude in the region of the outer plasmasphere. In summary, the present theoretical study provides a basis for comparison between the equatorial plasmapause and the trough features in the topside ionosphere.     

Thermal proton flow in the plasmasphere: The morning sector

Vertical profiles of electron density obtained in the vicinity of the plasmapause using the Alouette-II topside sounder have been analyzed to assess the presence of H⁺ flow in the topside ionosphere. The observations in the midnight sector show clearly the presence of the plasmapause; i.e. there is a sharp boundary separating the poleward regions of polar wind H⁺ flow and the more gentle conditions of the plasmasphere where light ions are present in abundance. In contrast, in the sunlit morning sector upwards H⁺ flow is deduced to be present to invariant latitudes as low as 48° (L = 2·2) in the regions normally known to be well inside the plasmasphere. The upwards H⁺ flux is sufficiently large (3 × 10⁸ ions cm^?2 sec^?1) that the plasmapause cannot be seen in the latitudinal electron density contours of the topside ionosphere. The cause for this flow remains unknown but it may be a result of a diurnal refilling process.     

A family of ionospheric models for different uses

Empirical models of three dimensional electron density distributions in the ionosphere have been constructed for global as well as regional use. The models differ by their degree of complexity and calculation time and therefore have different uses. All are based on “ionogram parameter” (critical frequencies foE, foF1, foF2 and the F2 region transfer parameter M(3000)F2). The models allow the use of global or regional maps for foF2 and M(3000)F2 and use built-in formulations for foE and foF1. Update (instantaneous mapping / nowcasting) versions exist which take foF2 and M(3000)F2 or F2 region peak height and electron density as input. The ground to F2 layer peak part of the profile is identical for all three models and is based on an Epstein formulation. The “quick calculationr” model NeQuick uses a simple formulation for the topside F layer, which is essentially a semi-Epstein layer with a thickness parameter which increases linearly with height. The “ionospheric model” COSTprof is the model which was adopted by COST 251 in its regional “monthly median” form. Its topside F layer is based on O⁺-H⁺ diffusive equilibrium with built-in maps for three parameters, namely the oxygen scale height at the F2 peak, its height gradient and the O⁺-H⁺ transition height. The “ionosphere-plasmasphere” model NeUoG-plas uses a magnetic field aligned “plasmasphere” above COSTprof Typical uses of the models and comparison among them are discussed.     

Helium ions in the mid-latitude plasmasphere

An initial study of the behaviour of He⁺ ions in the mid-latitude plasmasphere is carried out by solving the time-dependent equations of continuity and momentum. Starting with a low He⁺ tube content, results are obtained for a period of 8 days. In the topside ionosphere there is an upflow of He⁺ during the day and downflow at night, for the sunspot maximum conditions considered. The downflow at night differs from the behaviour of H⁺ for these atmospheric conditions. However, little of the He⁺ produced in the daytime is lost by recombination at night; it is suggested that the supply of He⁺ to the mid-latitude plasmasphere is, in effect, an escape process for neutral helium.     

Comparison of the topside ionosphere during magnetic storms of various types

During the initial phase of magnetic storms with steep onset, the topside ionosphere shows enhancement of ionization above and depletion below a zone of unchanged ionization. During storms with a smoothly initiated disturbance, no enhancement of ionization is observed and depletion takes place at all altitudes.     

Comparison of solutions to the thirteen-moment and standard transport equations for low speed thermal proton flows

We have compared solutions obtained from the general 13-moment system of transport equations with those obtained from the standard collision-dominated transport equations for conditions corresponding to low speed thermal proton flow in the topside ionosphere in the vicinity of the plasmapause. In general, the solutions obtained from the 13-moment system of equations, which allows for different species temperatures parallel and perpendicular to the geomagnetic field and non-classical heat flows, are different from those obtained from the standard transport equations, which account for isotropic temperatures and classical collision-dominated heat flows. Within the plasmasphere, where the electron density is high, the differences between the 13-moment and standard solutions are typically small. However, outside the plasmasphere where the electron density is lower and in the ionosphere above SAR-arcs, where substantial electron and proton heat flows occur, there can be significant differences between the 13-moment and standard solutions. Generally, the differences are much larger for the protons than for the electrons. Our 13-moment solutions indicate that the proton and electron distributions are anisotropic with the difference between parallel and perpendicular temperatures approaching 4000 K for the protons and 2500 K for the electrons in the ionosphere above SAR-arcs. Also, above SAR-arcs the 13-moment heat flow equations yield proton heat flows as much as a factor of 10 lower and electron heat flows as much as a factor of 2 lower than those predicted by the classical collision-dominated heat flow expressions for the same boundary conditions.     

Plasma transport in the topside venus ionosphere

We have studied the extent to which certain transport processes affect ion composition and heat flow in the daytime, topside Venus ionosphere. Particular attention is given to the conditions that prevailed during the Mariner 5 measurements, at which time the topside Venus ionosphere appeared to be in a state of diffusive equilibrium. We have found that the ion composition is sensitive to the ion temperature, the ion temperature gradient, and to relative drifts between the ion species of a few $\text{m}\text{sec}$. The electron density, on the other hand, is very insensitive to these parameters. As a consequence, ionospheric models of the topside Venus ionosphere are not likely to yield definitive information about the ion composition, the thermal structure or the flow conditions, since at present only electron density profiles are available for testing model predictions. We have also found that a relative drift between the ion species of a few $\text{m}\text{sec}$ induces an ion heat flow that is equivalent to a $\text{1}\text{K}\text{km}$ temperature gradient. This induced heat flow could influence the energy balance in the topside Venus ionosphere.     

Numerical solutions to the problem of charged particel flow around an ionospheric spacecraft

The interaction of a conducting body moving through the ionosphere with the surrounding plasma is treated numerically. The Poisson and Vlasov equations are solved using computer techniques to give information about the redistribution of charged particles in the wake behind the body and the perturbation of the electric potential sheaths around the body. Three cases of interest are studied: body size less than, equal to, and greater than the Debye length in the surrounding plasma. A range of body potentials and ion Mach numbers are considered which are typical of conditions found in the ionosphere. Wake features, such as ion-free wake lengths and angles of propagation of disturbances in the wakes, are investigated for these conditions. Physical pictures of the mechanisms of wake formation behind a plate and a disc are built up for the three classes of body size, and differences due to geometry or size are explained. The smaller bodies are comparable in size to instrument booms, diagnostic probes, antennae, etc. and the larger bodies approach the dimensions of ionospheric satellites and space probes.     

Ion transition heights from topside electron density profiles

Theoretical electron density profiles are calculated for the topside ionosphere to determine the major factors controlling the profile shape. Only the mean temperature, the vertical temperature gradient and the $\text{O}{}^{\mathrm{+}}\text{H}{}^{\mathrm{+}}$ ion transition height are important. Vertical proton fluxes alter the ion transition height but have no other effect on the profile shape. Diffusive equilibrium profiles including only these three effects fit observed profiles, at all latitudes, to within experimental accuracy.Values of plasma temperature, temperature gradient and ion transition height h_tT were determined by fitting theoretical models to 60,000 experimental profiles obtained from Alouette l ionograms, at latitudes of 75°S–85°N near solar minimum. Inside the plasmasphere h_T varies from about 500 km on winter nights to 850 km on summer days. Diurnal variations are caused primarily by the production and loss of O⁺ in the ionosphere. The approximately constant winter night value of h_T is close to the level for chemical equilibrium. In summer h_T is always above the equilibrium level, giving a continual production of protons which travel along lines of force to aid in maintaining the conjugate winter night ionosphere. Outside the plasmasphere h_T is 300–600 km above the equilibrium level at all times. This implies a continual near-limiting upwards flux of protons which persists down to latitudes of about 60° at night and 50° during the day.     

Thermal ion flows in the topside auroral ionosphere and the effects of low-altitude,transverse acceleration

Topside ionospheric profiles are used to study the upward field-aligned flow of thermal O⁺ at high latitudes. On the majority of the field lines outside the plasmasphere, the mean flux is approximately equal to the mean polar wind measured by spacecraft at greater altitudes. This is consistent with the theory of thermal light ion escape supported, via charge exchange, by upward O⁺ flow at lower heights. Events of larger O⁺ flow are detected at auroral latitudes and their occurrence is found to agree with that of transversely accelerated ions within the topside ionosphere and the magnetosphere. The effects of low altitude heating of O⁺ by oxygen cyclotron waves, driven by downward field-aligned currents, are considered as a possible common cause of these two types of event.     

The global morphology of irregularities in the topside ionosphere,as measured by the total ion current probe on ESRO-4

The total ion current probe on the satellite ESRO-4 monitored thermal plasma density variations in the range ± 30% of ambient density with a spatial resolution of about 1.5 km. Latitudinal, diurnal, and altitudinal characteristics of density irregularities in the topside ionosphere have been investigated using the 2 × 10⁸ total ion current values recorded during the lifetime of the satellite. Dominating the morphology of topside irregularities is the high-latitude zone evident throughout the day, with the appearance of a distinct sub-auroral zone at night. Significant mid-latitude irregularity occurs at low altitudes during the night. The results reported here provide the most comprehensive study of topside ionospheric irregularities from direct probe measurements, and reveal new evidence on possible irregularity production mechanisms.     

Spectrum,angular distribution and polarization of auroral hard X-rays

The angular variations of elastic and inelastic scattering cross-sections have been calculated accounting for Hartree-Fock atomic model. Using these cross-sections the evolution of electron energy and angular distributions at different heights in the ionosphere have been computed with the help of Monte Carlo technique. Mono-energetic, power law and exponential electron spectra with isotropic and mono-directional incidence starting at an altitude of 300 km have been taken to obtain the angular and energy distribution at certain height intervals. It is found that isotropic distribution incident at the top of the ionosphere becomes anisotropic due to collisions at lower heights. Using Sauter bremsstrahlung cross-section and the calculated electron flux we have computed the spectrum, angular distribution and polarization of bremsstrahlung X-rays at different heights.The emission is found to be peaked at lower angles at higher heights and becomes isotropic with depth of penetration. Polarization is considerable at higher altitudes for mono-directional beams and becomes significant at lower heights for isotropic incidence. It is argued that the study of angular distribution and polarization can yield information about the nature of precipitating electron flux and hence about the acceleration mechanism operating during electron precipitation.     

State studies of Earth's plasmasphere: A review

The plasmasphere sandwiched between the ionosphere and the outer magnetosphere is populated by up flow of ionospheric cold (∼1 eV) and dense plasma along geomagnetic field lines. Recent observations from various instruments onboard IMAGE and CLUSTER spacecrafts have made significant advances in our understanding of plasma density irregularities, plume formation, erosion and refilling of the plasmasphere, presence of thermal structures in the plasmasphere and existence of radiation belts. Still modeling work and more observational data are required for clear understanding of plasmapause formation, existence of various sizes and shapes of density structures inside the plasmasphere as well as on the surface of the plasmapause, plasmasphere filling and erosion processes; which are important in understanding the relation of the process proceeding in the Sun and solar wind to the processes observed in the Earth's atmosphere and ionosphere.     

Plasmaspheric hiss observed in the topside ionosphere at mid- and low-latitudes

Latitudinal characteristics of ELF hiss in mid- and low-latitudes have been statistically studied by using ELF/VLF electric field spectra (50 Hz-30 kHz) from ISIS-1 and -2 received at Kashima station, Japan from 1973 to 1977. Most ISIS ELF/VLF data observed in mid- and low-latitude include ELF hiss at frequencies below a few kHz. The ELF hiss has the strongest intensity among VLF phenomena observed by the ISIS electric dipole antenna in mid- and low-latitudes, but the ELF hiss has no rising structure like the chorus in the detailed frequency-time spectrum. The ELF hiss is classified into the steady ELF hiss whose upper frequency limit is approximately constant with latitude and the ELF hiss whose upper frequency limit increases with latitude. These two types of ELF hiss occur often in medium or quiet geomagnetic activities. Sometimes there occurs a partial or complete lack of ELF hiss along an ISIS pass.Spectral shape and bandwidth of ELF hiss in the topside ionosphere are very similar to those of plasmaspheric hiss and of inner zone hiss. The occurrence rate of steady ELF hiss is about 0.3 near the geomagnetic equator and decreases rapidly with latitude around L = 3. Hence it seems likely that ELF hiss is generated by cyclotron resonant instability with electrons of several tens of keV in the equatorial outer plasmasphere beyond L = 3.Thirty-seven per cent of ELF hiss events received at Kashima station occurred during storm times and 63% of them occurred in non-storm or quiet periods. Sixty-seven per cent of 82 ELF hiss events during storm times were observed in the recovery phase of geomagnetic storms. This agrees with the previous satellite observations of ELF hiss by search coil magnetometers. The electric field of ELF hiss becomes very weak every 10 s, which is the satellite spin period, in mid- and low-latitudes, but not near the geomagnetic equator. Ray tracing results suggest that waves of ELF hiss generated in the equatorial outer plasmasphere propagate down in the electrostatic whistler mode towards the equatorial ionosphere, bouncing between the LHR reflection points in both the plasmaspheric hemispheres.     

Effects on the plasmasphere of a time-varying convection electric field

The time evolution of the plasmasphere has been investigated theoretically, using simple computational models. The magnetic field is assumed to be dipolar and time-independent, but the convection electric field is allowed to vary in time. For purposes of comparison, various spatial distributions of the magnetospheric electric field are considered. Plasmasphere flux tubes are assumed to be filled by diffusion of plasma upwards from the dayside ionosphere. Following a reduction in the convection field, the bulge of the original plasmasphere develops into a long tail that gradually wraps itself around the main plasmasphere. Periodic gusts in a spatially uniform convection field produce extremely complicated fine structure that depends strongly on both local time and universal time. Each large gust produces a distinct tail of cold plasma that stretches from the main body of the plasmasphere to the magnetopause, and causes a peak in density, outside the main plasmapause; similar features have been observed by OGO satellites. The calculations indicate that a periodic gusty field has a major effect on the size of the plasmasphere if the field has large Fourier components close to the drift period of cold plasma near the plasmapause. Gusts occurring randomly, at an average rate of several a day, can also cause substantial reduction in the size of the plasmasphere. The assumption that the convection field is spatially uniform, but gusty, leads to better agreement with the observed average shape of the plasmasphere than the assumption of a constant, uniform electric field. The theory indicates that the thickness of the plasmasphere boundary should be inversely correlated with magnetic activity, in general agreement with OGO 5 observations.     

Diffusion coefficients for three major ions in the topside ionosphere

Published experimental data on ion composition in the topside ionosphere are examined. For certain features (the light ion trough, the high-latitude trough, the high-latitude hole and the mid-latitude total ion concentration trough) it is pointed out that the number of major ions present may be 3 or more. Transport equations derived by Schunk and co-workers are extended to include the case of three major ions in the topside ionosphere. Specific calculations are made for the O⁺, H⁺ and He⁺ ions and the behaviour of the diffusion coefficients is discussed. From a model of the high-latitude ionization hole, described by Heelis et al., representative concentration and temperature profiles are obtained. These profiles are used to demonstrate further the behaviour of the ion diffusion coefficients.     

Monitoring the Dynamics of the Ionosphere–Plasmasphere System by Ground-Based ULF Wave Observations

Cross-spectral analysis of ULF wave measurements recorded at ground magnetometer stations closely spaced in latitude allows accurate determinations of magnetospheric field line resonance (FLR) frequencies. This is a useful tool for remote sensing temporal and spatial variations of the magnetospheric plasma mass density. The spatial configuration of the South European GeoMagnetic Array (SEGMA, 1.56 <  L <  1.89) offers the possibility to perform such studies at low latitudes allowing to monitor the dynamical coupling between the ionosphere and the inner plasmasphere. As an example of this capability we present the results of a cross-correlation analysis between FLR frequencies and solar EUV irradiance (as monitored by the 10.7-cm solar radio flux F10.7) suggesting that changes in the inner plasmasphere density follow the short-term (27-day) variations of the solar irradiance with a time delay of 1–2 days. As an additional example we present the results of a comparative analysis of FLR measurements, ionospheric vertical soundings and vertical TEC measurements during the development of a geomagnetic storm.     

On existence conditions of dust-acoustic solitary waves in a plasma with positively charged dust

Dust acoustic (DA) solitary wave existence conditions are investigated for positively charged dust particles in the presence of nonthermal electrons. Once Sagdeev pseudo-potential derived through fluid equations, for large amplitude DA waves, the lower limit on Mach number is calculated analytically using the necessary condition for the solitary waves existence. The double layers conditions provides the upper limit on Mach number. This allowed us to numerically investigate the effect of the temperature, density and nonthermal parameters on the solitary waves’ characteristics. The present study is devoted to a complex plasma subject to ultraviolet radiations such as the one in the lower earth’s ionosphere.     

Equivalent thickness of the outer ionosphere obtained by evaluation of Faraday records

With an appropriate system of higher order approximations the Faraday-effect is calculated for instantaneous ionospheric models the data of which are derived from the results of vertical soundings. Only one parameter which corresponds to the thickness of the outer ionosphere is left open; it is determined by global comparison with simultaneous Faraday-observations of satellite emissions. The thickness parameter shows various forms of variation with latitude; an increase towards north is the rule for quiet daytime conditions. Comparison with local methods shows considerable differences due to the indetermination of the absolute number of rotations.