Sunward Strahl: A Method to Unambiguously Determine Open Solar Flux from In Situ Spacecraft Measurements Using Suprathermal Electron Data

M J Owens,M Lockwood,P Riley,J Linker

doi:10.1002/2017ja024631

Abstract

AbstractA fraction of the magnetic flux which threads the photosphere reaches sufficient coronal altitude to be dragged out by the solar wind and form the heliospheric magnetic field (HMF). Directly measuring this “open solar flux” (OSF) component, however, is difficult. While OSF can be extrapolated from photospheric magnetic field measurements, the most direct method is from in situ spacecraft measurements of the HMF. The difficultly is unambiguously distinguishing between HMF which connects directly back to the Sun (the OSF) and that which is locally distorted by waves, turbulence, and near‐Sun reconnection. Suitable temporal filtering of the data can remove such “noise,” but the level of filtering cannot be known a priori and varies with solar cycle, solar wind types, etc. Here we use the suprathermal electron beam, or “strahl,” to distinguish between different HMF topologies. As strahl moves antisunward on global scales, times when strahl is observed to be moving sunward indicate that the HMF is locally inverted. By subtracting the inverted HMF, we compute the OSF without need for arbitrary filtering of the data. We find that the OSF obtained in this manner is slightly larger than the proposed “kinematic correction” based on observed solar wind velocity structure, though in general agreement. Our new OSF estimate agrees with methods based wholly on HMF data, if the data are first used to compute approximately 1 day averages during solar minimum and approximately 3 day averages during solar maximum, stressing the point that the filter method is unreliable because the required characteristics vary.

Highlights

The majority of the magnetic flux which threads the photosphere forms closed magnetic loops in the low corona that do not reach sufficient altitude to be dragged out by the solar wind and so do not, contribute to the heliospheric magnetic field (HMF; Owens & Forsyth, 2013)
The difficultly is unambiguously distinguishing between HMF which connects directly back to the Sun and that which is locally distorted by waves, turbulence, and near-Sun reconnection
While we are primarily interested in macroscale structures, averaging suprathermal electron pitch angle distributions can produce misleading results (e.g., averaging across a HMF polarity inversion could result in equal parallel and antiparallel strahl and the incorrect conclusion that the interval contains counterstreaming electrons and closed flux (Gosling et al, 1987))

Summary

Introduction

The majority of the magnetic flux which threads the photosphere forms closed magnetic loops in the low corona that do not reach sufficient altitude to be dragged out by the solar wind and so do not, contribute to the heliospheric magnetic field (HMF; Owens & Forsyth, 2013). While such measurements are currently only performed from Earth (or from near-Earth space), solar rotation allows a complete synoptic map of the photospheric magnetic field to be constructed every Carrington rotation (i.e., approximately every 27 days), though the polar fields remain poorly observed These “magnetograms” can be extrapolated out through the corona to a source surface, typically assumed to be a sphere at 2.5 solar radii, where the OSF is estimated by integrating the unsigned radial flux. This result is to be expected because close to the Sun, where the plasma beta is low, solar wind flows will be slightly nonradial until the tangential pressure, and the radial magnetic field strength is rendered uniform (Suess & Smith, 1996). In this study we use sunward strahl as a direct method to identify local HMF inversions, quantify the flux they contain, and so deduce the true OSF

Data and Methods

Occurrence and Properties of Sunward Strahl

The Effect of Sunward Strahl on Open Solar Flux Estimates

Findings

Discussion and Conclusions