Diurnal variation of the dayside, ionospheric, mid-latitude trough in the southern hemisphere at 800 km: Model and measurement comparison

Our high latitude ionospheric model predicts the existence of a pronounced “dayside” trough in plasma concentration equatorward of the auroral oval in both the Northern and Southern Hemispheres for solar maximum, winter, and low geomagnetic activity conditions. The trough in the Southern Hemisphere is much deeper than that in the Northern Hemisphere, with the minimum trough density at 800 km being 2 × 10³ cm⁻³ in the Southern Hemisphere and 10⁴ cm⁻³ in the Northern Hemisphere. The dayside trough has a strong longitudinal (diurnal) dependence and appears between 11:00 and 19:00 U.T. in the Southern Hemisphere and between 02:00 and 08:00 U.T. in the Northern Hemisphere. This dayside trough is a result of the auroral oval moving to larger solar zenith angles at those universal times when the magnetic pole is on the antisunward side of the geographic pole. As the auroral ionization source moves to higher geographic latitudes, it leaves a region of declining photoionization on the dayside. For low convection speeds, the ionosphere decays and a dayside trough forms. The trough is deeper in the Southern Hemisphere than in the Northern Hemisphere because of the greater offset between the geomagnetic and geographic poles. Satellite data taken in both the Northern and Southern Hemispheres confirm the gross features of the dayside trough, including its strong longitudinal dependence, its depth, and the asymmetry between the Northern and Southern Hemisphere troughs.     

Auroral ion velocity distributions for a polarization collision model

We have calculated the effect that convection electric fields have on the velocity distribution of auroral ions at the altitudes where the plasma is weakly-ionized and where the various ion-neutral collision frequencies are much smaller than the ion cyclotron frequencies, i.e. between about 130 and 300 km. The appropriate Boltzmann equation has been solved by expanding the ion velocity distribution function in a generalized orthogonal polynomial series about a bi-Maxwellian weight factor. We have retained enough terms in the series expansion to enable us to obtain reliable quantitative results for electric field strengths as large as 90 mV m^?1. Although we have considered a range of ion-neutral scattering mechanisms, our main emphasis has been devoted to the long-range polarization interaction. In general, we have found that to lowest order the ion velocity distribution is better represented by a two-temperature or bi-Maxwellian distribution than by a one-temperature Maxwellian, with there being different ion temperatures parallel and perpendicular to the geomagnetic field. However, the departures from this zeroth-order bi-Maxwellian distribution become significant when the ion drift velocity approaches (or exceeds) the neutral thermal speed.     

Effect of diffusion-thermal processes on the high-latitude topside ionosphere

We have studied the extent to which diffusion-thermal heat flow affects H⁺ temperatures in the high-latitude topside ionosphere. Such a heat flow occurs whenever there are H⁺?O⁺ relative drifts. From our study we have found that at high-latitudes, where H⁺ flows up and out of the topside ionosphere, diffusion-thermal heat flow acts to reduce H⁺ temperatures by 500–600 K at altitudes above about 900 km.     

Theoretical ion densities in the lower ionosphere

We have solved the coupled momentum and continuity equations for NO⁺, O₂⁺, and O⁺ions in the E- and F-regions of the ionosphere. This theoretical model has enabled us to examine the relative importance of various processes that affect molecular ion densities. We find that transport processes are not important during the day; the molecular ions are in chemical equilibrium at all altitudes. At night, however, both diffusion and vertical drifts induced by winds or electric fields are important in determining molecular ion densities below about 200 km. Molecular ion densities are insensitive to the O⁺ density distribution and so are little affected by decay of the nocturnal F-region or by processes, such as a protonospheric flux, that retard this decay. The O⁺ density profile, on the other hand, is insensitive to molecular ion densities, although the O⁺ diffusion equation is formally coupled to molecular ion densities by the polarization electrostatic field. Nitric oxide plays an important role in determining the NO⁺ to O₂⁺ ratio in the E-region, particularly at night. Nocturnal sources of ionization are required to maintain the E-region through the night. Vertical velocities induced by expansion and contraction of the neutral atmosphere are too small to affect ion densities at any altitude.     

Behaviour of ion velocity distributions for a simple collision model

For application to studies of the high latitude ionosphere, we have calculated ion velocity distributions for a weekly-ionized plasma subjected to crossed electric and magnetic fields. An exact solution to Boltzmann's equation has been obtained by replacing the Boltzmann collision integral with a simple relaxation model. At altitudes above about 150 km, where the ion collision frequency is much less than the ion cyclotron frequency, the ion distribution takes the shape of a torus in velocity space for electric fields greater than 40 mV m^?1. This shape persists for 1–2 hr after application of the electric field. At altitudes where the ion collision and cyclotron frequencies are approximately equal (about 120 km), the ion velocity distribution is shaped like a bean for large electric field strengths. This bean-shaped distribution persists throughout the lifetime of ionospheric electric fileds. These highly non-Maxwellian ion velocity distributions may have an appreciable affect on the interpretation of ion temperature measurements.     

Response of the ionosphere–thermosphere system to magnetospheric processes

The magnetosphere–ionosphere–thermosphere system at high latitudes is strongly coupled via electric fields, particle precipitation, plasma and neutral outflows, and field-aligned currents. Although the climatology of the coupled system is fairly well established, our understanding of the variability of the disturbed state (weather) is rudimentary. This variability is associated with magnetic storms and substorms, nonlinear processes that operate over a range of spatial scales, time delays, and feedback mechanisms between the different domains. The variability and resultant structure of the ionosphere can appear in the form of propagating plasma patches and polar wind jets, pulsing ion and neutral polar winds, auroral and boundary blobs, and ionization channels associated with polar cap arcs, discrete auroral arcs, and storm-enhanced densities (SEDs). The variability and structure of the thermosphere can appear in the form of propagating atmospheric holes, neutral gas fountains, neutral density patches, and transient neutral jets. In addition, during periods of enhanced plasma convection, the neutral winds can become supersonic in relatively narrow regions of the polar cap. The spatial structure in the ionosphere–thermosphere system not only affects the local environment, but the cumulative effect of multiple structures may affect the global circulation and energy balance. A focused topical review of recent results in our modeling the variability and structure of the high-latitude ionosphere–thermosphere system is presented. This review was given at the Greenland Space Science Symposium (May 2007).     

Peat extraction, conservation and the rural economy in Northern Ireland

Northern Ireland has a tradition of peat cutting; 78 per cent by area of remaining lowland bogs and 46 per cent of blanket bogs have been cut in the past. Since the early 1980s, machine peat extraction for fuel and horticulture has increased, against a background of changing economic support for agriculture, high rural unemployment and agricultural underemployment. Cutting fuel peat can reduce household fuel costs and some peat fuel is sold to gain income. Horticultural peat is cut mainly for sale. Expansion of machine extraction, with possible environmental impacts, occurred as interest in peatland conservation increased. Potential conflict arises between local economic needs and conservation. Machine cutting for fuel is widespread, but affects only 2.6 per cent of the blanket peat area; horticultural extraction is more localized and is only one-fifth of the extent of fuel extraction. Altitude, distance from roads, land ownership and rights, turbary, religious affiliation, local economic needs and the role of government departments all contribute to explaining the distribution and extent of extraction.     

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.