Langmuir waves associated with discontinuities in the solar wind: a statistical study

We have reported previously that short bursts of Langmuir waves are frequently observed in the solar wind in isolated local depressions in the magnetic field magnitude, which are called ”magnetic holes”. The magnetic depression regions are often bounded by directional discontinuities (DDs). To study the relation between the waves and the discontinuities, we have examined 402 magnetic field discontinuities in 11 days of Ulysses data which cover a range of radial distances from the Sun (1.8 to 5.2 AU) and of heliographic latitudes (−1.9◦ to −80◦). It is found that Langmuir waves occur at only ∼ 9% of the discontinuities identified. About 75% of the DDs which have waves are associated with magnetic depressions. The waves have a higher occurrence rate at tangential discontinuities with magnetic depressions than at rotational discontinuities with little change in the field magnitude. The results suggest that the reduction in the magnetic field strength (a holelike structure) is important for the excitation of the observed Langmuir waves. The adiabatic focusing of suprathermal electrons passing through the magnetic holes may be a source of free energy for this process.