Measuring solar wind velocity with spacecraft phase scintillations

Abstract

THE discovery of interplanetary scintillations (IPS) of natural radio sources¹ has been followed by extensive multiple-station or spaced-receiver IPS measurements of the solar wind velocity in regions not yet probed by direct spacecraft^2–8. Few measurements have, however, been obtained within 40 R_⊙ because amplitude or intensity scintillations saturate when they are strong as is the case near the Sun. This is unfortunate because the scarcity of wind velocity measurements in the acceleration region close to the Sun is one of the reasons why the construction of a unique picture of solar wind dynamics has not yet been possible, despite numerous theoretical studies and direct spacecraft observations farther out^9,10. The demonstration of the measurement of spacecraft phase scintillations with a coherent dual-frequency radio system¹¹ paves the way for making solar wind velocity measurements with multiple-station phase scintillations. Although similar in principle to the IPS technique, several factors make this new tool a more powerful and promising one.