Estimation of projection effects in the polar magnetic field measurements from the ecliptic view

TL;DR

This paper assesses how projection effects from ecliptic view impact the accuracy of polar magnetic field measurements, using MHD simulations to estimate systematic errors and suggest multi-viewpoint observations for improvement.

Contribution

It introduces a method to quantify projection-induced errors in polar magnetic flux measurements using MHD simulations and proposes multi-viewpoint observations as a solution.

Findings

Projection effects can cause significant under- or overestimation of polar magnetic flux.

Simulated observations reveal the extent of systematic errors due to viewing angles.

Multi-viewpoint observations can mitigate measurement inaccuracies.

Abstract

The distribution and evolution of the magnetic field at the solar poles through a solar cycle is an important parameter in understanding the solar dynamo. The accurate observations of the polar magnetic flux is very challenging from the ecliptic view, mainly due to (a) geometric foreshortening which limits the spatial resolution, and (b) the oblique view of predominantly vertical magnetic flux elements, which presents rather small line-of-sight component of the magnetic field towards the ecliptic. Due to these effects the polar magnetic flux is poorly measured. Depending upon the measurement technique, longitudinal versus full vector field measurement, where the latter is extremely sensitive to the SNR achieved and azimuth disamiguation problem, the polar magnetic flux measurements could be underestimated or overestimated. To estimate the extent of systematic errors in magetic flux measurements at the solar poles due to aforementioned projection effects we use MHD simulations of quiet sun network as a reference solar atmosphere. Using the numerical model of the solar atmosphere we simulate the observations from the ecliptic as well as from out-of-ecliptic vantage points, such as from a solar polar orbit at various heliographic latitudes. Using these simulated observations we make an assessment of the systematic errors in our measurements of the magnetic flux due to projection effects and the extent of under- or over estimation. We suggest that such effects could contribute to reported missing open magnetic flux in the heliosphere and that the multi-viewpoint observations from out-of-the-ecliptic plane together with innovative Compact Doppler Magnetographs provide the best bet for the future measurements.