A Comparative Study of Magnetic Fields in the Solar Photosphere and Chromosphere at Equatorial and Polar Latitudes

TL;DR

This study compares magnetic field properties in the solar photosphere and chromosphere at different latitudes using five years of observational data, revealing differences in field inclination and expansion that impact solar magnetic modeling.

Contribution

It provides a detailed analysis of magnetic field inclinations and distributions in the photosphere and chromosphere, highlighting differences and implications for solar magnetic models.

Findings

Photospheric fields are mostly less than 12° inclined east-west.

Chromospheric fields expand significantly in all directions.

Polar photospheric flux increases poleward, reaching 5-6 G.

Abstract

Besides their own intrinsic interest, correct interpretation of solar surface magnetic field observations is crucial to our ability to describe the global magnetic structure of the solar atmosphere. Photospheric magnetograms are often used as lower boundary conditions in models of the corona, but not data from the nearly force-free chromosphere. National Solar Observatory's (NSO) Synoptic Optical Long-term Investigations of the Sun VSM (Vector Spectromagnetograph) produces full-disk line-of-sight magnetic flux images deriving from both photospheric and chromospheric layers on a daily basis. In this paper, we investigate key properties of the magnetic field in these two layers using more than five years of VSM data. We find from near-equatorial measurements that the east-west inclination angle of most photospheric fields is less than about 12{\deg}, while chromospheric fields expand in all directions to a significant degree. Using a simple stereoscopic inversion, we find evidence that photospheric polar fields are also nearly radial but that during 2008 the chromospheric field in the south pole was expanding superradially. We obtain a spatially resolved polar photospheric flux distribution up to 80{\deg} latitude whose strength increases poleward approximately as cosine(colatitude) to the power 9-10. This distribution would give a polar field strength of 5-6 G. We briefly discuss implications for future synoptic map construction and modeling.