Exploring the heliospheric open flux problem from multiple perspectives

Open magnetic flux (OMF) emanating from the Sun permeates the entire interplanetary space and plays an important role in all physical processes throughout the heliosphere that involve magnetic fields. It has been a topic of investigation based on both observational and numerical model analysis. And yet, there are still unresolved debates surrounding the OMF, considered to be among the big open questions in the field of solar and space physics. One of these is the “missing” open flux problem, according to which photospheric open flux estimates do not match measurements made in situ at 1 au. These photospheric estimates are obtained based on two different methods. According to the first, extreme ultraviolet (EUV) observations of coronal holes (CHs), that are considered as primary sources of OMF, are overlaid over global magnetic maps (Carrington maps), and the magnetic flux they enclosed is summed up. The second method is based on areas of open flux determined by coronal models and the summation of the magnetic flux they enclose. However, regardless of the complexity of coronal models, current research has shown that modelled open flux strongly underestimates that determined by the first method, and both underestimate the flux measure in situ at 1 au, by at least a factor of 2. These comparisons with values measured at 1 au are based on the conclusion made by Ulysses’ observations of the latitudinal invariance of the magnitude of the radial interplanetary magnetic field, which lead to the consensus that the total heliospheric open flux can be calculated by a single point in situ measurements. The aforementioned discrepancies have raised many questions. Are observational limitations responsible for the missing open flux? Are model limitations, such as the complexity of the model, the numerical implementation, and uncertainties in input data, contributing to the problem? How can we constrain and validate coronal models? Do we fully understand the sources of open flux? During this presentation we will navigate through research contributing to answering these questions and the direction of current and future efforts both in modelling and observations.