Number density structures in the inner heliosphere

Abstract

The origins and generation mechanisms of the slow solar wind are still unclear. Part of the slow solar wind is populated by "number density structures", discrete patches of increased number density that are frozen in to and move with the bulk solar wind. In this paper we aimed to provide the first in-situ statistical study of number density structures in the inner heliosphere. We reprocessed in-situ ion distribution functions measured by Helios in the inner heliosphere to provide a new reliable set of proton plasma moments for the entire mission. From this new data set we looked for number density structures measured within 0.5 AU of the Sun and studied their properties. We identified 140 discrete areas of enhanced number density. The structures occurred exclusively in the slow solar wind and spanned a wide range of length scales from 50 Mm to 2000 Mm, which includes smaller scales than have been previously observed. They were also consistently denser and hotter that the surrounding plasma, but had lower magnetic field strengths, and therefore remained in pressure balance. Our observations show that these structures are present in the slow solar wind at a wide range of scales, some of which are too small to be detected by remote sensing instruments. These structures are rare, accounting for only 1\% of the slow solar wind measured by Helios, and are not a significant contribution to the mass flux of the solar wind.

Accepted for publication in A&A

References in corpus (6)

• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
• Examining Periodic Solar Wind Density Structures Observed in the SECCHI Heliospheric Imagers
• The temporal and spatial scales of density structures released in the slow solar wind during solar activity maximum
• Structured Slow Solar Wind Variability: Streamer-blob Flux Ropes and Torsional Alfvén Waves
• Multi-scale Pressure-balanced Structures in Three-dimensional Magnetohydrodynamic Turbulence

Cited by in corpus (4)

• Diagnosing solar wind origins using in-situ measurements in the inner heliosphere
• Connecting the Low to High Corona: Propagating Disturbances as Tracers of the Near-Sun Solar Wind
• Identification of a Single Plasma Parcel during a Parker Solar Probe-Solar Orbiter radial alignment
• Constraints on the variable nature of the slow solar wind with the Wide-Field Imager on board the Parker Solar Probe