1. Acidified lakes and streams without, or with impoverished fish populations, occur mainly in areas that receive high levels of acid deposition from the atmosphere and have soils derived from granite or other rocks of similar composition that are resistant to weathering and low in exchangeable elements such as calcium and magnesium. Catchments with thin soils are particularly sensitive with respect to the rate and extent of acidification.

2. Examination of the remains of diatoms and other biological material in lake sediments laid down over centuries has established that many lakes in southern Norway and Sweden and in the U.K. have undergone progressive acidification from ca. 1850 until very recently. The magnitude of this acidification is appreciably greater than any that has occurred in the past 10000 years and has marched in parallel with accelerated industrial development, as indicated by increases in several trace pollutants in the sediments. These changes and the extent of inferred acidification are geographically correlated with the intensity of acid deposition and with the geo-chemical status of the catchment.

3. For a given input of acid deposition, the degree of acidification of lakes and streams is largely determined by the structure and chemistry of the mineral and organic soils, and the pathways that the incoming rainwater takes through the soil.[…]

Introduction

As part of the U.K.—Scandinavian Surface Waters Acidification Programme, catchment basins were selected to study the hydrological and hydrochemical consequences of surface water acidification. The purpose of this paper is to indicate why different catchments have been selected, their geographical location in relation to anthropogenic deposition loadings, and to describe in detail the nature of the collaborative research done and the equipment used.

Four main institutes were involved in the planning and execution of the research programme: they were; the Macaulay Land Use Research Institute (MLURI), the Freshwater Fisheries Laboratory (FFL), the Institute of Hydrology (IH) and Imperial College (IC). Other collaborating institutes were Stirling University (SU) and the Forth River Purification Board (FRPB). The identifiers shown in parentheses will be used throughout the text to identify the specific institute involved in any particular area of field research. The field research programme was initiated in late 1985 and continued until mid-1989 in all the Scottish sites.

Instrumentation and equipment for routine and experimental studies were designed to fulfil the four main research objectives.

1. What are the factors, in addition to pH, that determine the fisheries status of surface waters?

2. What are the biological, chemical and hydrological characteristics of catchments that determine whether the composition of surface waters falls within a range acceptable for fish?[…]

Introduction

The definition of the term ‘upper atmosphere’ is not agreed upon by all scientists. To meteorologists it usually refers to the entire region above the troposphere, where the daily weather evolves. With this interpretation the upper atmosphere would include the stratosphere, mesosphere, and thermosphere, regions identified by their temperature structure, density, composition and the degree of ionization. Figure 1.1.1 illustrates schematically the altitude variation of these atmospheric parameters. Logarithmic scales are used for the abscissae and the ordinate to accommodate the large range of densities and heights.

The same laws of physics apply throughout the atmosphere, of course, but the relative importance of various processes varies widely between regions, accounting for the different behaviour of the characteristic parameters as a function of altitude. Thus, the temperature structure is governed by the absorption of solar radiation, and different wavelength bands are absorbed by various constituents in the region. For example, absorption by O3 specifically accounts for heating in the stratosphere. Radiation transfer and terrestrial albedo also contribute to the temperature structure. Each region identified in Figure 1.1.1 warrants a book length treatment. In this work we study that portion of the upper atmosphere labelled the thermosphere and ionosphere.

For centuries, man's perception of the thermosphere was limited to the splendour of the Aurorae Borealis and Australis. It was the need for the propagation of radio waves over long distances that provided a practical stimulus to investigations of the thermosphere and the physical processes that cause and control the ionosphere.

Measurements of the solar UV flux were made by spectrometers carried by the Atmosphere Explorer satellites over a period of several years, spanning an interval between solar minimum and solar maximum activity. A so-called ‘solar UV reference spectrum’ was acquired with a rocket borne spectrometer on 23 April 1974, identified in the table as day 113 of year 1974: 74/113. Table A2.1 lists the irradiance at the top of the atmosphere in photons (cm2sΔλ)−1 where Δλ = 50 Å. The intensities of several bright lines are listed individually, and the line sources are identified; these intensities are not included in the 50 Å bins. The irradiance obtained on five days between 1974 and 1979 is given in the wavelength region between 50 and 1050 Å which is subject to the largest variability with solar activity. Only the spectrum for day 74/113 is listed in Table A2.2 in the wavelength region 1050–1940 Å.

This text focuses on the physics and chemistry of the Earth's upper atmosphere, bounded at the bottom by the pressure level at which most (though not all) of the incoming ionizing radiation has been absorbed, and bounded at the top by the level at which escape of the neutral gas becomes important. The region defined by these boundaries contains a partially ionized gas. The principal ionization sources are solar ultraviolet radiation and auroral energetic particles. Thus, an ionosphere is embedded in the upper atmosphere.

The plan is to identify the multitude of processes that operate in the upper atmosphere and to relate observed input and output parameters by detailed physical and mathematical descriptions of the governing processes. The properties and behaviour of the atmosphere are a consequence of the interaction of processes that span a wide range of commonly identified disciplines; radiation physics and chemistry, transport phenomena, gas phase chemistry, fluid dynamics, optics and spectroscopy, and others. Basic disciplines are drawn upon in attempting to understand the upper atmosphere. It is hoped that this book will bridge the gap between those texts read by students taking courses in the standard disciplines of physics and the research literature in upper atmosphere physics and chemistry. Research papers all too often assume that the reader already has the background required to appreciate the new development reported in the article.

Introduction

The subject of this chapter is plural because the neutral gas, the ions and the electrons are generally all at different temperatures in the upper atmosphere. Although each component has energy sources and sinks the temperatures are not independent since, in a collision dominated, partially ionized gas, energy is shared amongst the various constituents. We focus principally on the kinetic temperatures of various species but note that excitation of internal energy can be characterized by vibrational and rotational temperatures in molecules. Rotational relaxation is rapid, and the distribution of rotational lines in a band may be used to infer the kinetic temperature of the gas. Vibrational relaxation is relatively slow so that the vibrational temperature of a molecular species may differ from the kinetic temperature of the gas.

Temperature is the observable parameter in the energy balance of the thermosphere. It is important, therefore, to understand the physical processes that underlie the energetics of the region and thereby control the altitude profiles of the several temperatures. In addition, temperature influences processes other than those associated with energetics. We have already pointed out in Chapter 5 that several reaction rate coefficients are temperature-dependent, so that the composition of the thermosphere and the ionosphere is influenced by the thermal structure.

Partial energy level diagrams of OI, OII, NI and NII are presented and the transitions leading to emissions that have, so far, been identified with reasonable certainty in the airglow and aurora are indicated by their wavelength (only one wavelength is shown for a multiplet). Inconclusive identifications are excluded, even though some may appear in the spectra reproduced from the research literature and shown in Chapter 7.

For the molecular species N2, O2, NO and their ions, fairly complete potential curves are given.

Introduction

This book deals with the outermost shell of the collisionally dominated gaseous envelope of the Earth, the thermosphere and ionosphere. In the preceding chapters collision cross sections have appeared prominently in the physical descriptions and in the equations that govern the interaction of energetic photons, ions and electrons with the atmosphere. We learned, for example, that the optical depth (Equation (2.2.1)) is proportional to the column density of absorbing molecules, with the proportionality factor identified as the absorption cross section. Photoionization (Section 2.3) involves ionization cross sections and electron transport (Section 3.2) depends on elastic and inelastic collision processes and the associated collision cross sections. It is therefore appropriate to assess our understanding of cross sections and expand upon the part which they play in physical and chemical processes in the atmosphere.

It has been found convenient to classify collisions into several types: elastic, inelastic and reactive. When two particles collide and only kinetic energy and linear momentum are exchanged (and the total of each is conserved) then the collision is elastic. If one or both of the collision partners undergoes a change in internal energy then the collision is inelastic. Reactive collisions are those that involve the production of new species; such collisions are also inelastic.

Empirical models are constructed from a large data base of observations made in situ by satellite borne mass spectrometers and remotely from the ground. Minimum perigee for long-lived satellites is at about 150 km, depending on the available on-board propulsion system. Data points below this altitude are therefore inferred from incoherent scatter radar measurements, which also serve to follow the temporal variation of several parameters at one location. Satellites provide the global coverage.

One such empirical model is based on decades of accumulated measurements acquired by several low-altitude orbiting satellites and radar measurements. It has been updated periodically and the most recent version is the MSIS-86. The large number of individual measurements allowed binning by geographic position and universal time, solar zenith angle, solar activity and geomagnetic activity.

The parameters computed by the empirical model as a function of altitude are the concentrations of N2, O, O2, H, He, Ar, N, the mass density and the neutral temperature. The data sets provided here correspond to the solar minimum (July 1976) and solar maximum (February 1979) periods used as examples in Chapter 2, as well as day 23 April, 1974 on which the solar irradiance reference spectrum for the Atmosphere Explorer satellite was acquired by a rocket payload.

Solar UV radiation is the principal cause for the existence of the ionosphere and the physical processes that underlie the production of photoelectrons are presented in the preceding chapter. Since photoelectrons are produced within the atmosphere the term commonly applied to this population is an embedded source of ionization. We have not avoided the observational aspect of photoelectrons at this point, rather, a production rate is not a measurable quantity. Detectors measure the intensity or the flux, the number of particles or photons passing through or absorbed by unit area per unit time and solid angle. Photoelectron intensities will be derived in this chapter but, first, an additional source of ionization is discussed, a source that is due to energetic charged particle precipitation of solar and magnetospheric origin, associated principally with the aurora. Unlike photoelectrons, primary auroral electrons are a source external to the atmosphere and their intensity is a measurable quantity, analogous to the solar photon flux at the top of the atmosphere. Carrying the analogy one step further, the primary auroral electrons ionize the atmospheric gases producing secondary electrons that are the equivalent of the photoelectrons produced by photoionization.

In this chapter we first survey briefly the characteristics of energetic electron fluxes that have been measured by rocket and satellite borne detectors. We next analyse the processes that operate when a stream of energetic electrons penetrates into the atmosphere from the tenuous regions of the magnetosphere.