Multistatic Incoherent Scatter Measurements of Ionospheric Drift Velocity

Abstract

THE observed behaviour of the ionospheric F-region has proved difficult to understand, and one of the principal difficulties has been that, until recently, there was little reliable information about the true plasma drift velocities. Apparent horizontal drift speeds have been measured for many years by means of the “fading” method¹: interpretation of data from these experiments is complex², and it is now clear that the results give only the drift of large electron density irregularities which, although of considerable interest in itself, is not necessarily the same as the drift of the plasma as a whole. Plasma velocity measurements have been made by plotting the movements of luminous chemical tracers released in the ionosphere from rockets or gun launched projectiles^3,4. Information from this type of experiment is of good quality, but is necessarily limited in quantity. An alternative technique is incoherent scatter radar, which measures the doppler shift of the scattered signal and determines one component of the ion drift velocity; the advantages of this method are that measurements can be pursued systematically as functions of time over a wide range of heights and that systematic errors can be minimized by careful equipment design. Incoherent scatter drift measurements were first made by Carru et al.⁵, who measured one component of the velocity, aligned nearly parallel to the direction of the geomagnetic field. A large body of similar measurements from the French installation now exists, and has been used to study thermospheric dynamics⁶. The incoherent scatter radar at Jicamarca, Peru, has measured the field perpendicular component near the magnetic equator⁷, and the vertical component has been measured by the Millstone Hill radar in Massachusetts⁸.