1.1. INTRODUCTION
The 1974 Yosemite Conference on MagnetosphereâIonosphere Coupling was a unique event during which leading scientists in both magnetospheric and ionospheric physics met together in a remote location to examine in a unique way not only the overlap but also the interrelationships of their previously quite separate disciplines. Since MâI coupling as a research field has progressed greatly over the past 40 years, it is perhaps informative to trace some of the instances in which coupled magnetospheric and ionospheric phenomena were just beginning to be appreciated in a meaningful way and describe how these ideas have evolved to the present and into the future.
Early models of the interaction between the solar wind and the Earth's magnetosphere included the ionosphere but mainly as a footprint of conductivity for magnetospheric convection [e.g., Axford and Hines, 1961; Wolf, 1970]. During this same time somewhat controversial theories for the production of a polar wind, which populates the magnetosphere with ionospheric plasma, were developed and ultimately became widely accepted [e.g., Banks and Holzer, 1968]. In this same era, Vasyliunas [1970] developed a mathematical theory of MâI coupling that formed the basis for many theoretical advances in the field [e.g., Wolf, 1975].
Starting in the early 1970s, satellite measurements began to show that cold ionospheric particles (mainly H+ and He+) are important constituents of the inner and middle magnetosphere [Chappell et al., 1970] and that energetic heavy ions (mainly O+) precipitate into the lowâaltitude auroral zone during geomagnetic storms [Sharp et al., 1972]. While H+ ions, which dominate magnetospheric plasmas at all energies, can have their origins both in the solar wind and the ionosphere, the widespread prevalence of O+ ions, which are almost exclusively from the ionosphere, suggested that the ionospheric plasma source is important and capable of supplying most if not all of magnetospheric plasma [Chappell et al., 1987].
New data sets and discoveries in that epoch were mainly responsible for the advent of MâI coupling science. One new data set that came on line was generated by the Chatanika Radar facility, which pioneered the use of the incoherent scatter technique to derive largeâscale plasma convection patterns [Brekke et al., 1974]. These convection patterns can be mapped into the magnetosphere to help gauge and visualize global magnetospheric dynamics. Another landmark discovery was auroral kilometric radiation (AKR), which was originally referred to as terrestrial kilometric radiation (TKR) [Gurnett, 1974; Alexander and Kaiser, 1976]. Since AKR beams outward from the auroral regions, it was only first observed many years after the discovery of radio emissions from Saturn and Jupiter [Kaiser and Stone, 1975]. In the case of Jupiter, the frequencies are much higher so that the soâcalled decametric radiation can be observed from the Earth's surface.
By far the strongest channel for coupling between the magnetosphere and ionosphere is the auroral oval and its extension into space. In the early 1970s, auroral particles first began to be observed from orbing spacecraft [e.g., Frank and Ackerson, 1971; Winningham et al., 1973]. Sounding rocket measurements of auroral electrons had shown earlier that their energy spectra were monoenergetic and hence consistent with acceleration by an electric field component aligned along the magnetic field [McIlwain, 1960]. Subsequent measurements, however, showed that lowerâenergy electrons also precipitated into the aurora along with the monoenergetic beams [Frank and Ackerson, 1971]. Some controversy therefore arose about the source of the lowâenergy electrons, and this controversy was resolved by Evans [1974], who showed that they were backscattered and secondary electrons trapped between the parallel potential drop and the ionosphere. The possibility of AlfvĂ©nâwave acceleration of auroral electrons was investigated by Hasegawa [1976]. Later on, measurements from the FAST spacecraft showed that AlfvĂ©nâwave acceleration is an important phenomenon especially near the polarâcap boundary [e.g., Chaston et al., 2003].
Another auroral phenomenon associated with MâI coupling is the stable auroral red (SAR) arc, which appears at midâlatitudes during magnetic storms. These arcs are produced either by Coulomb collisions between ring current particles and plasmaspheric electrons, electro...
