Broadband plasma waves in the boundary layers

Abstract

Boundary layers are commonly encountered in space and astrophysical plasmas. For example, interaction of solar wind plasma with the planets and comets produces magnetopause and cometopause boundary layers, respectively. Generally, the boundary layers are formed when plasmas with different characteristics interact with each other. The plasma sheet boundary layer in the Earth's magnetotail is formed owing to the interaction of hot dense plasma in the plasma sheet region with the rarefied plasma of the lobe region. Boundary layers are the site where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The purpose of this review is to describe the main characteristics and the possible generation mechanisms of the broadband plasma waves (with frequencies of > 1 Hz) observed in the Earth's magnetopause boundary layer, the Jovian magnetopause boundary layer, the plasma sheet boundary layer, and the Earth's polar cap boundary layer. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufficient precipitated energy flux to the ionosphere to create the dayside aurora at Earth and a weak high-latitude auroral ring at Jupiter. In general, the broadband plasma waves may play an important part in the processes of local heating/acceleration of the boundary layer plasma. Recent exciting high time resolution results on the broadband plasma waves coming from Geotail, Polar, and FAST will be discussed.