In this chapter, we turn our attention to the solar-terrestrial environment. We examine the rudiments of solar structure and processes, the genesis and consequences of solar activity, the solar wind and the interplanetary magnetic field (IMF), elements of relevant magnetospheric processes, and solar-terrestrial relationships. We cover solar activity cycles and indices and their importance in specified prediction systems. Relevant prediction systems and services are covered in Chapter 5.

The first title of this chapter was "The Anatomy of Space Weather", but it was changed for several reasons. First of all, the term anatomy suggests a precise dissection of the various processes on the sun and its environment that lead to space weather phenomena. Our real intent is to provide a relatively short overview of such phenomena to the extent they are relevant to our real objective, that of understanding the impact upon telecommunication systems. Moreover there are numerous books that detail solar physics, and these details need not be repeated here. Secondly, it has been noted that up to 70% of space weather occurs in the ionosphere [Meier, 2000], It certainly appears logical to place the sun and its importance up front in this particular book, but it is also sufficient to present the topic in summary fashion, since we dare not diminish the role of the ionosphere that should be at the heart of any dissertation on space weather. Having dispensed with this rationalization for an abbreviated treatment of the sun and space weather origins, let us proceed.

In recent years, the importance of the corona, including coronal holes and coronal mass ejections (i.e., CMEs), has emerged. Certainly a treatment of these phenomena is needed to grasp the impact of transient phenomena that tend to dominate methods of short-term forecasting. An associated phenomenon of major importance is the geomagnetic storm. This topic is central to the matter of any intermediate ionospheric forecasting system since (i) the geomagnetic activity is strongly coupled to the ionosphere, (ii) the ionospheric response is delayed with respect to flare effects, and (iii) the impact is global and relatively long-lasting. The latter property makes the treatment of geomagnetic storms and their causal mechanisms an invaluable component of performance assessment for telecommunications systems that operate in the ionospheric environment. It is duly noted that an ionospheric storm is the response of the ionosphere to a geomagnetic storm. As suggested above, we shall intentionally limit our treatment of ionospheric storms and related disturbances in the present chapter, leaving these important topics until Chapter 3. In the present chapter we largely restrict our discussion to the origins of space weather. A discussion of new satellite observation systems and other space weather resources will be deferred until Chapter 6. The reader will find many excellent texts that deal almost exclusively with solar and magnetospheric physics, and the author will not attempt to duplicate them either in terms of scope or rigor. For detailed discussions of the sun and the magnetosphere, the bibliography at the end of the chapter should be consulted.

The importance of solar activity as it relates to telecommunication systems is well established. It is recognized that the sun exhibits sudden outbursts of energy that are called solar flares, and that these events may play havoc with the performance of certain radiowave systems including commercial television. However these events are relatively short-lived, typically lasting the order of an hour or less. Other well-known influences of the sun include those changes associated with an intensification in the extent and magnitude of the visible and radio auroras. These events affect high latitude terrestrial and earth-space communication to be sure, but the disturbances are actually global in nature. This is evidenced by the fact that ionospheric storms introduce significant alterations in HF coverage at middle latitudes, as well as enhanced scintillation of signals traversing earth-space paths. These scintillation enhancements are due to a descent of the scintillation boundary, corresponding to an equatorward expansion of the auroral zone. We shall find that solar influences on the ionosphere, may generally be characterized as immediate or delayed, with the long-term occurrence of these categories following an 11-year cycle. A well-known index of solar activity that exhibits the cyclic pattern is the sunspot number. This index roughly characterizes the number of spots on the visible solar disk, and is proportional to that component of solar activity that most severely influences telecommunication systems. The 11-year solar cycle is not subject to precise characterization in terms of onset, duration, or magnitude; and its direct influence on the ionosphere is not always clear. Nevertheless, we use indices of solar activity, or proxies of same, in all current long-term prediction programs. The role of ionospheric models is covered in Chapter 3 and prediction systems are covered in Chapter 5.

The ionosphere owes its existence to the sun, but it would clearly exist even in the absence of the 11-year cycle of sunspots. Indeed, the ionosphere possesses some rather interesting features even during periods of few sunspots. Without the 11-year modulation of activity, the ionosphere would possess a reasonably deterministic variability, which is associated with the local solar zenith angle, including diurnal and seasonal effects that are appreciably controlled by geometry. Moreover, even the benign ionosphere is characterized by relatively unpredictable variations that arise because of the constitution and dynamics of the underlying neutral gas. In fact there are a host of temporal fluctuations, which originate from sources other than the sun including neutral atmospheric weather patterns and turbulence. This, of course, does not suggest a lack of ultimate linkage to the sun. In any case, this residual class of fluctuations poses interesting challenges for ionospheric forecasting specialists who have long concentrated on the more obvious and direct association between the sun and the ionosphere.

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