Articles | Volume 35, issue 1
Regular paper
05 Jan 2017
Regular paper |  | 05 Jan 2017

Absence of the strahl during times of slow wind

Chris Gurgiolo and Melvyn L. Goldstein

Abstract. It is not uncommon during periods when the solar wind speed is less than 425 km s−1 to observe near 1 AU no evidence of a strahl population in either the electron solar wind or within the foreshock. Estimating the fluid flow within each energy step returned from the Plasma Electron And Current Experiment (PEACE) on board Cluster-2 often finds that in slow wind the GSE spherical flow angles in energies above where there is a clear core/halo signature are often close to radial with no evidence of a field-aligned flow. This signifies the lack of a strahl presence in the electron velocity distribution function (eVDF). When there is no obvious strahl signature in the data, the electrons above the core/halo in energy appear to be unstructured and smeared in angle. This can either be interpreted as due to statistical noise in low counting rate situations or the result of intense scattering. Regions where the strahl is seen and not seen are often separated by a very thin boundary layer. These transitions in the spacecraft frame of reference can be quite rapid, generally occurring within one to two spins (4–8 s).

Short summary
Observations during periods when the solar wind has a speed < 425 km s−1 show that it is not uncommon to find no strahl present in the data. The research was done in response to observations and was performed through a detailed study of the electron velocity distribution functions. The conclusion arrived at is that the absence of the strahl appears to occur within individual flux tubes, which may indicate that the source lies in the solar corona where the strahl is formed.