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

Abstract

The ultraviolet coronagraph spectrometer on Solar and Heliospheric Observatory (SOHO) has provided several lines of evidence strongly suggesting that coronal holes and the high‐speed solar wind are heated by resonant interactions with ion cyclotron waves. Related evidence has also been provided by the 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 that 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 suggested by Axford and McKenzie can in principle be detected using interplanetary scintillation (IPS) techniques. If the ion cyclotron waves are obliquely propagating, they will be compressive, and the corresponding density fluctuations will induce phase, intensity, and Faraday rotation fluctuations on radio signals passing through the corona. Tu and Marsch [1997] and Marsch and Tu [1997] provided some detailed models based on Axford and McKenzie's suggestion, including the wave magnetic power spectrum. From the latter we calculate the associated density power spectrum at 5 RS, which at high wavenumbers turns out to be above the actual generic density power spectrum at 5 RS inferred from IPS 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 fluctuations 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, we offer several reasons why such a conclusion, though we believe it is likely, would be premature. We do suggest, though, that IPS has the potential to verify or refute whether the Sun launches very high frequency waves into the coronal holes.