Fluctuations in the interplanetary plasma

Spacecraft based observations of fluctuations in the interplanetary magnetic field and solar wind speed yield dominant spatial scales of the order 10⁶ km, and negligible structure below about 500 km. Earth based observations of the angular broadening and scintillation of cosmic radio sources have been interpreted in terms of electron density scales of order a few hundred km. It is suggested that for scales below a few hundred km, there exists an enhanced level of small scale density fluctuations not accompanied by comparable magnetic variations. This proposal is shown to be consistent with radio observations, the contribution of the much larger electron density irregularities being quite negligible. A physical mechanism that might account for the small scale fluctuations is described.