Waves at the electron plasma frequency associated with solar wind magnetic holes: STEREO/Cluster observations

Magnetic depressions are common structures of the interplanetary medium. These magnetic holes can be just isolated dips of the amplitude of the field or they can be associated with discontinuities in the field orientation (tangential or rotational). Electrostatic waves at the plasma frequency (Langmuir waves) are often observed in these magnetic structures. The aim of the present paper is to provide the main characteristics of these waves and to propose a mechanism to explain their formation. The study is based on a statistical analysis of observations performed by STEREO (between March 2007 and August 2009) and Cluster (between 2002 and 2005) when each mission was in the free solar wind. Complementary information is provided by the two missions through the different instrumental configurations. We first provide new characteristics of the waves (polarization, energy, spectrum, occurrence). We then show that the occurrence of Langmuir waves activity inside a hole is closely linked to the presence of a significant electron strahl outside the hole. Finally, we propose a scenario for the generation of the Langmuir waves inside the holes.