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).…”

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).…”

An Observational Test of Solar Plasma Heating by Magnetic Flux Cancellation

“…The radial component B r of the magnetic field to the solar surface is derived from the LOS component B LOS , assuming the magnetic field is radial at the photosphere: i.e., B r = B LOS /cos(θ), where θ is the heliocentric angle of the given observed location (i.e., the angle between the LOS and the local normal to the surface) -here cos(θ) = 0.85 -0.89. This assumption is consistent with plage field structure as observed by HMI, and more appropriate than other correction options given the nature of the field and the location (refer to Leka et al 2017).…”

Deep-learning Reconstruction of Sunspot Vector Magnetic Fields for Forecasting Solar Storms

“…In the case of absolute net current helicity, attributions correspond to regions with "mixed" magnetic fields of the positivenegative polarities closely located. The appearance of a spurious magnetic field polarity inversion line (PIL) is a known artifact in the LOS magnetograms whenever the magnetic field inclination relative to the LOS exceeds 90° ( Leka et al 2017). We find that in many cases (e.g., HARPs 407, 3291, and 3311) when the PIL artifact exists for magnetic fields within penumbrae, it wrongly constitutes an important attribution.…”

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).…”

An Observational Test of Solar Plasma Heating by Magnetic Flux Cancellation

“…The radial component B r of the magnetic field to the solar surface is derived from the LOS component B LOS , assuming the magnetic field is radial at the photosphere: i.e., B r = B LOS /cos(θ), where θ is the heliocentric angle of the given observed location (i.e., the angle between the LOS and the local normal to the surface) -here cos(θ) = 0.85 -0.89. This assumption is consistent with plage field structure as observed by HMI, and more appropriate than other correction options given the nature of the field and the location (refer to Leka et al 2017).…”

Deep-learning Reconstruction of Sunspot Vector Magnetic Fields for Forecasting Solar Storms

“…In the case of absolute net current helicity, attributions correspond to regions with "mixed" magnetic fields of the positivenegative polarities closely located. The appearance of a spurious magnetic field polarity inversion line (PIL) is a known artifact in the LOS magnetograms whenever the magnetic field inclination relative to the LOS exceeds 90° ( Leka et al 2017). We find that in many cases (e.g., HARPs 407, 3291, and 3311) when the PIL artifact exists for magnetic fields within penumbrae, it wrongly constitutes an important attribution.…”

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).…”

An Observational Test of Solar Plasma Heating by Magnetic Flux Cancellation

“…The radial component B r of the magnetic field to the solar surface is derived from the LOS component B LOS , assuming the magnetic field is radial at the photosphere: i.e., B r = B LOS /cos(θ), where θ is the heliocentric angle of the given observed location (i.e., the angle between the LOS and the local normal to the surface) -here cos(θ) = 0.85 -0.89. This assumption is consistent with plage field structure as observed by HMI, and more appropriate than other correction options given the nature of the field and the location (refer to Leka et al 2017).…”

Deep-learning Reconstruction of Sunspot Vector Magnetic Fields for Forecasting Solar Storms

“…In the case of absolute net current helicity, attributions correspond to regions with "mixed" magnetic fields of the positivenegative polarities closely located. The appearance of a spurious magnetic field polarity inversion line (PIL) is a known artifact in the LOS magnetograms whenever the magnetic field inclination relative to the LOS exceeds 90° ( Leka et al 2017). We find that in many cases (e.g., HARPs 407, 3291, and 3311) when the PIL artifact exists for magnetic fields within penumbrae, it wrongly constitutes an important attribution.…”