Chapter 7 Contribution of Different Source and Loss Processes to the Magnetospheric Plasma Content
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Chapter 7 Contribution of different source and loss processes to the magnetospheric plasma content 7.1. Introduction Chapters 2 to 6 review and discuss the source and loss processes for magneto spheric plasma at the inner and outer magnetospheric boundaries. The objective in this chapter is to evaluate the relative importance of these various processes for the plasma content of the entire magnetosphere. The plasma, which is quasi neutral in all situations of interest here (this does not imply that no small charge separations occur), may in terms of number density and flow be determined by ion or electron measurements. As several ion species in the magnetosphere have only one specific source, whereas the origin of the electrons is much more difficult to determine, most experimental data dealt with here concern ions. This chapter, like the earlier ones, does not dwell upon the transfer of energy and momentum. We try to identify the dominant source and loss processes in each region and then consider the macroscopic system as a whole, emphasising the overall balance of sources and losses. This survey allows us to identify significant gaps in our knowledge. We propose methods of remedying these gaps in the next chapter. 7.2. Summary of Source Processes 7.2.1. THE HIGH-LATITUDE IONOSPHERE Some Introductory Considerations The high-latitude ionosphere is an important source of plasma in the magneto sphere fairly close to Earth, as shown in Chapter 2. Solar UV light and precip itating particles from the magnetosphere create an ionosphere with a maximum charged particle density at altitudes of 250 - 350 Ian. We consider latitudes pole ward of the plasmasphere (above about 50° magnetic latitude) where H+ and O+, sometimes with significant fractions of He+ and other ions, can leave the ionosphere, moving upward along the geomagnetic field. 355 B. Hultqvist et al. (eds.), Magnetospheric Plasma Sources and Losses © Springer Science+Business Media Dordrecht 1999 356 CHAPTER 7 The neutral atmosphere is a homogeneous mixture of mainly N2 (80%) and 02 (20%) at altitudes below about 100 km. At higher altitudes there is efficient photo-dissociation of 02 and much less turbulent mixing than at lower altitudes. This makes 0+ (ionised by solar extreme UV radiation) the dominant ion species at altitudes of a few hundred km, where the ionospheric ion density maximises. Characteristic energies are about 0.1 eV. In the main part of the ionosphere ion collisions with neutrals are important, but in the upper part the plasma can be considered collisionless. The plasma density distribution along the high-latitude geomagnetic field tubes will not be smooth and monotonous unless there is some balance between source and loss processes in each height interval of the field tube. An effective expulsion process working in a limited height interval cannot do more than empty that part of the field tube of plasma, if there are no new ions produced by ionisation of neutral atoms, which is believed to be the case in the uppermost ionosphere. The processes that in effect control the number and mass fluxes of ionospheric ions into the magnetosphere are those that work at the lowest altitudes, i.e. in the topside ionosphere (see e.g. Section 2.5). They define the source strength which cannot be surpassed at higher altitudes in a stationary situation and in transient processes only for limited periods. Higher altitude processes are re sponsible for ions achieving energies great enough to overcome the gravitational field and escape out into the magnetosphere. Without such additional high-altitude accelemtion, many heavy ions would fall back into the ionosphere. Source limitations in different altitude layers play important roles for the dy namics of the ion outflow. An effect of the temporal evolution of a perturbation caused by localised transient transverse ion heating is the development of a re gion with low plasma density (plasma cavity) with a density bump just above it. Ionospheric density depletions have been observed on auroral field lines, and a significant part of the polar cap has also been found to be depleted of cold ionospheric plasma. These density depletions thus occur within the high latitude topside ionosphere, which is already relatively low in density compared with the plasmasphere, owing to polar wind outflows. Consequently, the auroral density cavities can be very deep, with densities as low as a fraction of an ion and electron per cm3. Cavity regions with low plasma density are quite common both in the auroral zone and in the polar cap at altitudes well above 1000 km. Some processes energising ions to hundreds or thousands of electron volts are well correlated with such cavity regions. The outflow of hydrogen ions is mainly source limited, which means that the number flux is relatively uninfluenced by inputs of energy into the flow. That the thermal hydrogen ion outflow is source limited is indicated in Table 2.1 by the total H+ outflow rate (the sum over the polar caps and auroral regions) being essentially independent of the solar cycle phase, although the heating of the upper atmosphere is much higher at solar maximum than at minimum (ionospheric temperature of CONTRIBUTION OF SOURCE AND LOSS PROCESSES 357 order 2000 K instead of 500 K). The independence of H+ ion flow on solar cycle phase is primarily a consequence of an increased altitude at solar maximum of the cross-over from domination of oxygen atoms (below) to domination of hydrogen atoms (above). The outflow of oxygen ions is limited by gravitation and by charge exchange with atomic hydrogen in the uppermost ionosphere (see Section 2.2.1). Rapid outflow through the crossover region mentioned above increases the 0+ content of the outflow. This is because the charge exchange process in the equilibrium state maintains the ratio of ion densities at about the same value as the ratio of the respective neutral atom densities. Slow flow of 0+ ions through the crossover region thus allows equilibrium to be reached and increases H+ density above it at the expense of 0+ ions. To keep its charge an oxygen ion has to flow rapidly through the region. The upflow velocity increases generally with altitude, so pro cesses which increase the altitude of the cross-over region favour the density of 0+ ions in the uppermost ionosphere and at the same time disfavour the H+ density in the outflow. The composition of the upper atmosphere depends strongly on its temperature. Hydrogen atoms can almost escape at ordinary temperatures, so a temperature increase leads to significant outflow of such atoms and decrease of H density, and therefore of H+ density, in the ionosphere. To a lesser degree this is true also for helium and helium ions. For 0+ ions the opposite happens according to the above. Increase of the ionospheric temperature by solar EUY radiation, louIe heating, or particle precipitation leads to expansion of the ionosphere towards greater alti tudes, thereby increasing the altitude of the cross-over region and increasing the cold-ion density at greater altitudes, where non-thermal heating/acceleration pro cesses can operate. The thermal polar wind outflow is hardly affected by increased magnetospheric activity level, as Table 2.1 indicates, but the cold 0+ density at altitudes above the cross-over region is increased and the outflow of energetic H+ and, primarily, 0+ ions in the form of beams and conics increases considerably (see below). Bulk Outflow in the Upper Ionosphere The Polar Wind Ion Outflow. Bulk outflow means that all particles in the pop ulation have a common drift velocity in addition to their thermal motion. The polar wind is an ion outflow that occurs essentially at all times poleward of the plasmasphere and at all altitudes above a lower limit located in the uppermost ionosphere. The velocity of this bulk outflow increases with altitude and, on the average, reaches 1 km S-l at altitudes of about 2000,3000 and 6oo0km for H+, He+, and 0+, respectively. Typical thermal energies are around 0.3 e V. The polar wind outflow does not depend much on magnetospheric activity. as mentioned above. Its dependence on solar activity has not been studied in detail. Dominant physical mechanisms causing the polar wind are believed to be am- 358 CHAPTER 7 bipolar electric fields (due to a slight charge separation between ions and the faster upflowing electrons) and plasma pressure gradients, but centrifugal force along rapidly convecting magnetic field tubes may also be important and the mirror force may contribute. The relative contributions of those mechanisms remain to be quantified. Theoretical results indicate that the two first-mentioned may be intimately coupled. There is still no quantitative theory for the 0+ outflow. The total polar wind outflow is of the order of IOZS ions s-1 both for H+ and 0+ near solar maximum activity, while the outflow of He+ is generally smaller. Some values are given in Table 2.1. Auroral Bulk Upflow. A bulk ion outflow in the 0+ dominated part of the upper ionosphere has been observed at auroral latitudes by means of incoherent scat ter radar facilities and satellites. This bulk flow has been observed from about 400 km altitude in active parts of the auroral region up to above 1000 km. Upflow velocities may be as great as one kms-1 (see Section 2.6.1). The number fluxes are important for the provision of the magnetosphere with plasma. Most of the ions in the bulk outflow may, however, not reach escape velocity unless they are accelerated further at greater altitudes by other mechanisms before they fall down again. In fact, downward flowing heavy ions have been observed over the polar caps by means of spacecraft (see Section 2.2.4) and it has even been found that the difference between what is going up and coming down is consistent with the auroral outflow at higher altitudes.