What is space weather? The US Department of Defense, in its implementation plan [OSD, 2000], indicates, "space weather refers to adverse conditions on the sun, the solar wind, and in the earth's magnetosphere, the ionosphere, and the thermosphere." Indeed this definition portrays those aspects of space weather that are generally of most concern, namely the more pathological elements. But space weather, and ionospheric weather, in particular, can be turbulent or benign. All aspects of space weather should be included in the definition. Indeed, from a telecommunications perspective, it can be safely stated that quiet conditions are not always good and disturbed conditions are not always bad. From the NRL Plasma Physics Division web site comes a rather crisp definition: "space weather refers to the state of the magnetosphere and ionosphere which is determined by the solar wind." The National Space Weather Program [NSWP, 1995] has defined space weather as representing "conditions on the sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems." This definition is more appropriate for our purposes in this book. On the NOAA/SEC web site, it is indicated "space weather describes the conditions in space that effect Earth and its technological systems." It goes on to say, "space weather is a consequence of the behavior of the sun, the nature of Earth's magnetic field and atmosphere, and our location in the solar system". This is also a useful definition.
In this monograph, the vantage point of the telecommunications specialist drives our view of space weather. Specifically we treat ionospheric and plasmaspheric weather as the most important from a nowcasting perspective. On the other hand, we treat the space weather generated within the magnetosphere and the extra-magnetosphere as primary in the context of forecasting and prediction services. I want to emphasize this point, not only because the effects on various communication systems derive from near-space regions just above the troposphere, but because it identifies the hierarchy of very important terrestrial sensors that have played (and continue to play) a significant role in our knowledge of the ionosphere. These sensors are a major part of the space weather remote sensing "network" and are basically ionospheric diagnostic instruments.
Space weather phenomena have been affecting legacy communication systems involving longwave and shortwave signaling since the dawn of the radio communication era. The impact of the ionosphere on radio transmission is well known through the consideration of the Appleton-Hartree expressions that detail the relationship between plasma and signals that propagate within that medium. Over the 20th Century, a wide range of telecommunication systems have been developed, and many have been fielded for operational use. While many of the systems received their impetus through military necessity, the utility of telecommunications is evident in virtually all aspects of human activity. Space weather, a relatively new terminology, loosely defines the hierarchy of all phenomena within the earth-sun environment that may impact biology and systems that reside within that environment.
Earthbound telecommunication practitioners would be tempted to use a more restrictive term such as the geoplasma environment, to include the magnetosphere and ionosphere, as an appropriate definition of the primary region of interest since that is the focus of effects that can be observed or calculated. Indeed, only the closest geoplasma region, the ionosphere, is the primary focus for nowcasting and assessment of effects on many telecommunication systems. We will focus principally upon the ionosphere and its interactions with communication systems, especially in Chapters 3-5. However it is obvious that space weather is the real driver of pertinent properties of the ionosphere. Hence a treatment of the hierarchy of effects on telecommunication systems is really a more general space weather problem.
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