The Open Flux Problem: The Need for High Latitude Observations

This is the authors' abstract. We don't add key points for this paper.

The Sun's open magnetic field is a fundamental aspect of coronal and heliospheric physics.The primary source location of the open magnetic field is believed to be coronal holes, usually detected by the absence of EUV and/or X-ray emission.We have observed photospheric magnetic fields remotely and measured interplanetary magnetic fields in situ for over five decades.A long-standing issue is that models based on photospheric magnetic field observations significantly underestimate the open magnetic field inferred from interplanetary measurements, if their open field regions are compatible with coronal hole observations.This is not a model problem, but rather an observed open flux problem: When the open flux is estimated from coronal hole detections superimposed on observatory-based solar magnetic flux maps (entirely eliminating models), the deficit persists or is even larger.A major uncertainty is the strength of the polar magnetic fields, which are poorly observed from the ecliptic plane.Resolving the contribution of polar fields requires line-ofsight measurements of the photospheric field at high heliographic latitude (greater than 65 • ) with corresponding detection of coronal hole boundaries in a coronal emission line, at a time not too far from solar minimum (when polar fields are strongest), for at least a solar rotation.Regardless of the result (strong or weak polar fields), such measurements will have profound implications for our understanding of the structure of the solar corona and inner heliosphere, including CME and SEP propagation, and the formation and sources of the solar wind.