On the Open Solar Magentic Flux Problem

The solar magnetic fields emerging from the photosphere are comprised of a combination of “closed” and “open” fields. The closed magnetic field lines are defined as those having both ends rooted in the solar surface and not extending beyond the critical point, while the “open” field lines are those having one end that extends out into interplanetary space with the other end rooted at the Sun’s surface. Of course, there are no truly “open” magnetic field lines, and those that are dragged out into interplanetary space by the solar wind eventually close in the far reaches of the outer heliosphere. Since the early 2000’s, the amount of total unsigned open magnetic flux estimated by coronal models have been in significant disagreement with in situ spacecraft observations, especially during solar maximum, with factors of two or more differences not uncommon. Estimates of total open unsigned magnetic flux using coronal hole observations (e.g., using EUV or He 10830) are in general agreement with the coronal model results and thus are in similar disagreements with in situ observations. Several possible sources producing these discrepancies have been postulated over the years such as problems with the photospheric magnetic field measurements, underestimates of the polar field strengths, coronal mass ejection (CME) magnetic fields that are still closed but counted as open, the time-dependent nature of the magnetic field (i.e., the opening and closing of magnetic fields), and the manner in which in situ observations are used to estimate total open unsigned magnetic flux. This paper provides a brief review of the problem and some of the proposed explanations to account for the discrepancies. It concludes with compelling new evidence that appears to largely resolve the problem.