H. Rishbeth

Patterns of F2-layer variability

The ionosphere displays variations on a wide range of time-scales, ranging from operational time-scales of hours and days up to solar cycles and longer. We use ionosonde data from thirteen stations to study the day-to-day variability of the peak F2-layer electron density, NmF2, which we use to define quantitative descriptions of variability versus local time, season and solar cycle. On average, for years of medium solar activity (solar decimetric flux approximately 140 units), the daily fluctuations of NmF2 have a standard deviation of 20% by day and 33% by night. We examine and discuss the patterns of behaviour of ionospheric and geomagnetic variability, in particular the equinoctial peaks. For further analysis we concentrate on one typical midlatitude station, Slough. We find that the standard deviations of day-to-day and night-to-night values of Slough NmF2 at first increase with increasing length of the dataset, become fairly constant at lengths of 10–20 days and then increase further (especially at equinox) because of seasonal changes. We found some evidence of two-day waves, but they do not appear to be a major feature of Sloughs F2 layer. Putting together the geomagnetic and ionospheric data, and taking account of the day-to-day variability of solar and geomagnetic parameters, we find that a large part of F2-layer variability is linked to that of geomagnetic activity, and attribute the rest to ‘meteorological’ sources at lower levels in the atmosphere. We suggest that the greater variability at night is due to enhanced auroral energy input, and to the lack of the strong photochemical control of the F2-layer that exists by day.

On the seasonal response of the thermosphere and ionosphere to geomagnetic storms

Ionosonde observations have provided the data to build a picture of the response of the midlatitude ionosphere to a geomagnetic storm. The particular characteristic of interest is the preference for “negative storms” (decrease in the peak electron density, NmF2) in summer and “positive storms” (increase in NmF2) in winter. A three-dimensional, time-dependent model of the coupled thermosphere and ionosphere is used to explain this dependence. During the driven phase of a geomagnetic storm the two main magnetospheric energy sources to the upper atmosphere (auroral precipitation and convective electric field) increase dramatically. Auroral precipitation increases the ion density and conductivity of the upper atmosphere; the electric field drives the ionosphere and, through collisions, forces the thermosphere into motion and then deposits heat via Joule dissipation. The global wind response is divergent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and carry molecule-rich air to higher levels. Once created, the “composition bulge” of increased mean molecular mass is transported by both the storm-induced and background wind fields. The storm winds imposed on the background circulation do not have a strong seasonal dependence, and this is not necessary to explain the observations. Numerical computations suggest that the prevailing summer-to-winter circulation at solstice transports the molecule-rich gas to mid and low latitudes in the summer hemisphere over the day or two following the storm. In the winter hemisphere, poleward winds restrict the equatorward movement of composition. The altered neutral-chemical environment in summer subsequently depletes the F region midlatitude ionosphere to produce a “negative storm”. In winter midlatitudes a decrease in molecular species, associated with downwelling, persists and produces the characteristic “positive storm”.

How the thermospheric circulation affects the ionospheric F2-layer

Abstract After a historical introduction in Section 1 , the paper summarizes in Section 2 the physical principles that govern the behaviour of the ionospheric F2-layer. Section 3 reviews the physics of thermospheric dynamics at F-layer heights, and how the thermospheric winds affect the neutral chemical composition. Section 4 discusses the seasonal, annual and semiannual variations of the quiet F2 peak at midlatitudes, while Section 5 deals with storm conditions. The paper concludes by summing up the state of understanding of F2-layer variations and reviewing some important principles that apply to ionospheric studies generally.

Electrical coupling of the E- and F-regions and its effect on F-region drifts and winds

Electric currents, generated by thermospheric winds, flow along the geomagnetic field lines linking the E-and F-regions. Their effects on the electric field distribution are investigated by solving the electrical and dynamical equations. The input data include appropriate models of the F-region tidal winds, the thermospheric pressure distribution and the E-and F-layer concentrations. At the magnetic equator, the calculated neutral air wind at 240 km height has a prevailling eastward component of 55 m sec-1 and the west-east and vertical ion drifts agree in their general form with incoherent scatter data from Jicamarca

Cooling of the upper atmosphere by enhanced greenhouse gases — modelling of thermospheric and ionospheric effects

Abstract The concentrations of “greenhouse gases” — carbon dioxide and methane in particular — in the middle atmosphere are expected to double by the mid-21st century. This should lead to “global greenhouse cooling” of the upper atmosphere. Using the NCAR Thermosphere/Ionosphere General Circulation Model, we predict that (depending on location and the phase of the solar cycle) the thermospheric temperature will be lowered by 30–40 K and the air density at heights of 200–300 km will be reduced by 20–40%, thus increasing the orbital lifetimes of satellites. The height of the ionospheric F2-layer peak will drop on average by about 15 km, with some effect on radio propagation, though the F2-layer critical frequency will hardly be affected.

A greenhouse effect in the ionosphere

Abstract Following a suggestion by Roble and Dickinson that increases in the mixing ratios of mesospheric carbon dioxide and methane will cool the thermosphere by about 50K, this paper examines the consequences for the ionosphere. The cooling and the associated composition changes, as described by Roble and Dickinson, would lower the E- and F2-layer peaks by about 2 km and 20 km respectively, but changes in the E- and F2-layer electron density are small.

The F-region dynamo

This paper reviews the theory of the F-region dynamo which drives about 10–15% of the total mid-latitude ionospheric current by day, and the major part at night (Section 2). Polarization fields associated with the dynamo cause marked effects in the night-time F-region, notably the mean eastward wind (Section 3). The paper also discusses the equipotentiality of geomagnetic field lines (Section 4 and Appendix) and the question of location of Sq and L current systems (Section 5).

Ionospheric F 2 layer seasonal and semiannual variations

An extensive series of computations, using the Coupled Thermosphere-Ionosphere-Plasmasphere model (CTIP), has been undertaken to investigate the semiannual variation in peak noontime electron density, a common feature of the Fa-layer, particularly at low latitudes and in the southern hemisphere at mid-latitudes. Results from the model reveal such a variation, most prominently, at mid-latitudes, in the South American sector. An analysis of this phenomenon shows that it is intimately related to the large offset of the geomagnetic axis from Earths spin axis in the southern hemisphere. Because of this offset, a given geographic latitude in the South American sector corresponds to a lower magnetic latitude than in other sectors and is thus farther from the energy inputs associated with the auroral regions. As a result, the composition changes are much smaller during the winter months than at other longitudes, the mean molecular mass being essentially constant for a 4-month period centered on the winter solstice. This result is understood in terms of the global thermospheric circulation. In the absence of any composition changes, noon ionospheric density is influenced primarily by the solar zenith angle. This angle reaches a maximum at the winter Solstice, leading to diminished ion production, a minimum in N(m)F2, and therefore a semiannual variation overall. On the basis of the model results, the semiannual variation is seen as a feature of the midlatitude ionosphere at geographic longitudes opposite to the location of the geomagnetic pole. This phenomenon is seen in both northern and southern hemispheres, though the effect is much larger in the southern hemisphere as a result of the greater magnetic offset.

F-region storms and thermospheric circulation

Abstract This paper puts together various current ideas on the effects of magnetic storms on the ionospheric F-layer, principally at midlatitudes. A major factor is a large-scale circulation in the thermosphere, with equatorward winds driven by the heating resulting from joule dissipation and particle precipitation, different from the quiet-day pattern associated with the diurnal bulge. At midlatitudes this circulation produces upward drift in the F-region which tends to increase NmF2; but it also transports, from lower heights and higher latitudes, air with an enhanced molecular gas concentration which tends to decrease NmF2. Recovery from the midlatitude storm proceeds as molecular diffusion restores the gas composition to normal. Though in principle the circulation can cause both ‘positive’ and ‘negative’ F2-layer effects at midlatitudes, in practice other factors such as electromagnetic drift and ionosphere-magnetosphere plasma flux have important roles, and special considerations apply at low latitudes. Recent satellite experiments provide an excellent opportunity for investigating the storm circulation theory.

Modelling F2-layer seasonal trends and day-to-day variability driven by coupling with the lower atmosphere

This paper presents results from the TIME-GCM-CCM3 thermosphere–ionosphere–lower atmosphere flux-coupled model, and investigates how well the model simulates known F2-layer day/night and seasonal behaviour and patterns of day-to-day variability at seven ionosonde stations. Of the many possible contributors to F2-layer variability, the present work includes only the influence of ‘meteorological’ disturbances transmitted from lower levels in the atmosphere, solar and geomagnetic conditions being held at constant levels throughout a model year. In comparison to ionosonde data, TIME-GCM-CCM3 models the peak electron density (NmF2) quite well, except for overemphasizing the daytime summer/winter anomaly in both hemispheres and seriously underestimating night NmF2 in summer. The peak height hmF2 is satisfactorily modelled by day, except that the model does not reproduce its observed semiannual variation. Nighttime values of hmF2 are much too low, thus causing low model values of night NmF2. Comparison of the variations of NmF2 and the neutral [O/N2] ratio supports the idea that both annual and semiannual variations of F2-layer electron density are largely caused by changes of neutral composition, which in turn are driven by the global thermospheric circulation. Finally, the paper describes and discusses the characteristics of the F2-layer response to the imposed ‘meteorological’ disturbances. The ionospheric response is evaluated as the standard deviations of five ionospheric parameters for each station within 11-day blocks of data. At any one station, the patterns of variability show some coherence between different parameters, such as peak electron density and the neutral atomic/molecular ratio. Coherence between stations is found only between the closest pairs, some 2500 km apart, which is presumably related to the scale size of the ‘meteorological’ disturbances. The F2-layer day-to-day variability appears to be related more to variations in winds than to variations of thermospheric composition.