Effect of magnetic field inclination on SDO/AIA 1600 Å emission

A strong correlation between the intensity of chromospheric emissions and the (unsigned) photospheric magnetic field strength has been established in several studies. These studies have typically been based on line-of-sight (LOS) observations of the magnetic field, while measurements of the full 3D magnetic vector, which provide the true field strength and the orientation of magnetic field line, have not been studied in this context. Thus, the possible effect of magnetic field inclination on chromospheric emissions has remained hidden so far.

We study here how the inclination of the photospheric magnetic field, as measured by the full 3D magnetic vector from the Solar Dynamics Observatory (SDO) Helioseismic Magnetic Imager (HMI), affects the FUV emission at around 1600 Å from SDO Atmospheric Imaging Assembly (AIA). We analyze 1168 co-temporal observations by the two instruments from 2014 to 2017. We focus on magnetically active regions outside the sunspots (e.g., plages and network) close to the solar disk center.

We find that the AIA 1600 Å emission typically decreases with increasing (more horizontal) inclination. For all inclinations, AIA 1600 Å emission increases with increasing magnetic field to a maximum emission and then slowly decreases for larger field strengths. Maximum emission and the related field strength decrease with inclination. Above this field strength of maximum emission, the emission decouples from the field strength and is mainly governed by inclination. For fixed AIA emission level the associated magnetic field strength decreases with inclination. The difference in the median magnetic field strength can be more than 200 G (about a factor of two) for the same emission level between almost radial (γ_Loc ≈ 15°) and nearly-horizontal (γ_Loc ≈ 60°) fields.

AIA 1600 Å emission and magnetic field inclination are bimodally distributed with constant magnetic field strength below 1000 G. One population has a high AIA emission and a roughly vertical magnetic field, the other a lower emission and a horizontal field. The population consisting of less bright pixels with horizontal field is typically found at the border of active region, while the population with bright pixels with a vertical field occupy the bulk of an active region.

Our results show that the chromospheric FUV emission at around 1600 Å is strongly influenced by the inclination of the magnetic field. These results are important, for example, for models aiming to reconstruct the solar spectral irradiance or the past solar activity based on chromospheric emissions. These models would be more accurate if they took into account the effect of inclination of the magnetic field on FUV emissivity.