Evaluating (and Improving) Estimates of the Solar Radial Magnetic Field Component from Line-of-Sight Magnetograms

Abstract

Although for many solar physics problems the desirable or meaningful boundary is the radial component of the magnetic field B r , the most readily available measurement is the component of the magnetic field along the line-of-sight to the observer, B los . As this component is only equal to the radial component where the viewing angle is exactly zero, some approximation is required to estimate B r at all other observed locations. In this study, a common approximation known as the "µ-correction", which assumes all photospheric field to be radial, is compared to a method which invokes computing a potential field that matches the observed B los , from which the potential field radial component, B pot r is recovered. We demonstrate that in regions that are truly dominated by radially-oriented field at the resolution of the data employed, the µ-correction performs acceptably if not better than the potential-field approach. However, it is also shown that for any solar structure which includes horizontal fields, i.e. active regions, the potential-field method better recovers both the strength of the radial field and the location of magnetic neutral line.

The presence of a systematic error in SDO/HMI data

“…Note that the results presented in Figure 3 and (2) with coefficient (3) explain well Figure 4, b from [Leka et al, 2017], which shows the los r B dependence on r B in the polar region. The statistical distribution of these values exhibits a clear, nearly twofold decrease in the former value relative to the latter (the same as in our case for r~1 (Figure 3, a).…”

Extended statistical analysis of emerging solar active regions

“…The reliability of the estimation of the radial magnetic field B r from the LOS component B los by the µcorrection is related to how radial the real magnetic field is. This issue was analysed by Leka, Barnes, and Wagner (2017) who compared different technique of B r approximation from B los . They considered B r derived from the SDO/HMI vector magnetic field measurements to be a true radial field.…”

“…The authors found that the µ-correction provides the best estimation of the total unsigned flux in ARs from the B los component for cos(µ) > = 0.5 (see Fig. 11 in Leka, Barnes, and Wagner 2017). In early stages of magnetic flux emergence, the emerging site is comprised mostly of plage areas in which the magnetic field is close to radial.…”

Possibilities and problems of solar magnetic field observations for space weather forecast

“…The polarity structure at this level is set fixed and is carried out by the solar wind radially to the outer heliospheric regions. The raw observational material for the calculations are synoptic maps of the radial field derived from daily magnetograms, in which the field is recorded along the line of sight (longitudinal field), by recalculating in the approximation of predominantly vertical direction of the magnetic field in the photosphere (see discussion of this issue in the recently published paper by Leka et al [2017).…”