Compressibility of ion cyclotron and whistler waves: Can radio measurements detect high-frequency waves of solar origin in the corona?

The ultraviolet coronagraph spectrometer on Solar and Heliospheric Observat ory (SOHO) has provided several lines of evidence strongly suggesting-that coronal holes and the high-speed solar wind are heated by resonant interact ions with ion cyclotron waves. Related evidence has also been provided by t he solar ultraviolet measurements of emitted radiation instrument on SOHO. However, the source of the waves is still unclear. Hollweg [1986], Hollweg and Johnson [1988], and Isenberg [1990] developed models in which the high- frequency waves are the result of a turbulent cascade from lower-frequency waves that are launched by the Sun. Axford and McKenzie [1992] suggested th at solar reconnection events launch the high-frequency waves directly; the frequencies of these waves must be in the kHz range if they are to resonate with the coronal protons. In this paper we point out that the waves sugges ted by Axford and McKenzie can in principle be detected using interplanetar y scintillation (IPS) techniques. If the ion cyclotron waves are obliquely propagating, they will be compressive, and the corresponding density fluctu ations will induce phase, intensity, and Faraday rotation fluctuations on r adio signals passing through the corona. Tu and Marsch [1997] and Marsch an d Tu [1997] provided some detailed models based on Axford and McKenzie's su ggestion, including the wave magnetic power spectrum. From the latter we ca lculate the associated density power spectrum at 5 R-S, which at high waven umbers turns out to be above the actual generic density power spectrum at 5 Rs inferred from IFS by Coles and Harmon [1989]. The predicted spectrum is even farther above an inferred density spectrum in coronal holes based on Coles et al. [1995]. It is tempting to conclude that the density fluctuatio ns implied by the models of Tu and Marsch are not present and thus that the postulated ion cyclotron waves of solar origin are not present. However, w e offer several reasons why such a conclusion, though we believe it is like ly, would be premature. We do suggest, though, that IFS has the potential t o verify or refute whether the Sun launches very high frequency waves into the coronal holes.