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ELECTRICAL STRUCTURE OF THE MIDDLE ATMOSPHERE 184 SOURCES OF IONIZATION Figure 13.1 shows typical ion-pair production rates (q) at middle latitudes during daytime. Throughout the stratosphere, galactic cosmic rays provide the principal ionization source, as in most of the lower atmosphere. The cosmic-ray ionization rate does not vary diurnally but does vary with geomagnetic latitude and with the phase of the 11-year solar cycle. Heaps (1978) provided useful relations for computing the rate of ion production at any latitude and time. Roughly, the ion-production rate above 30 km increases by a factor of 10 in going from the geomagnetic equator to the polar caps at sunspot minimum (cosmic-ray maximum) and by a factor of 5 at sunspot maximum. The solar- cycle modulation is near zero at the equator, increasing to a factor of about 2 in the polar caps. The ionization rate above 30 km is approximately proportional to the atmospheric density. These properties are a result of (a) the shielding effect ot the geomagnetic field, which allows cosmic-ray particles to enter the atmosphere at successively higher latitudes for successively lower energies, and (b) the reduction in cosmic-ray flux in the inner solar system as solar activity intensifies. Superimposed on these long-term global variations are brief reductions in cosmic-ray flux known as Forbush decreases, after their discoverer (Forbush, 1938). Forbush decreases occur in coincidence with geomagnetic storms and are of brief (hours) duration. However, as their magnitude can be as large as some tens of percent, they can change global electrical parameters significantly. In the mesosphere the major daytime source of ionization in undisturbed conditions is provided by the NO molecule, whose low ionization potential of 9.25 electron volts (eV) allows it to be ionized by the intense solar Lyman- alpha radiation. The concentration of NO in the mesosphere is not well known and is almost certainly variable (Solomon et al., 1982a) in response to meteorological factors. The production-rate profile in Figure 13.1 is an estimate based on reasonable values for the NO concentration and the solar Lyman-alpha flux, which is itself a function of solar activity (Cook et al., 1980). Figure 13.1 Typical ion-pair production rates in the middle atmosphere. At the upper limit of the middle atmosphere, significant amounts of ionization are produced by solar x rays, forming the base of the E region of the ionosphere, and by ionization of O2 in its metastable1â state, which is a by- product of ozone photodissociation. While these sources are never competitive with the NO source in terms of ionization rates, they give rise to different primary positive-ion species ( and as opposed to NO+), and hence to different chemical reaction chains. A sporadic and intense source of ionization at high latitudes is provided by solar-proton events (SPE) (Reid, 1974), and Figure 13.1 shows an ionization-rate profile calculated for the peak of a major SPE in May 1959. These events are caused by the entry into the atmosphere of particles accelerated during solar flares and traveling fairly directly from the Sun to the Earth. The particles are mostly protons, with much smaller fluxes of heavier nuclei and of electrons, having typical energies of 1 to 100 MeV and considerably less atmospheric penetration power than galactic cosmic rays. As a consequence, their effects are largely confined to high magnetic latitudes ( 60°) and to altitudes well above the lower stratosphere. Solar-proton events typically reach their peak intensity within a few hours of a major solar flare and then decay exponentially over the following day or two. Their occurrence is a strong function of the phase of the solar cycle, as illustrated in Figure 13.2, which shows the distribution in the 1956-1973 period of polar- cap absorption (PCA) events and of ground-level events (Pomerantz and Duggal, 1974). Polar-cap absorption is the name given to the intense radio-wave absorption caused by the enhanced mesospheric ionization during an SPE, while ground-level events are the rare events with a large enough high-energy flux to cause an increase in cosmic-ray neutron monitors at the surface. The frequency of the events is related to the solar-activity cycle, which peaked about 1958 and 1969, but intense events can occur at any time, as evidenced by those of February 1956 and August 1972. Figure 13.1 shows clearly that SPEs cause major alterations in middle-atmospheric ionization rates, and hence in the electrical parameters. Energetic electron precipitation from the radiation
