Synoptic Mapping of Chromospheric Magnetic Flux

Abstract

We used daily full-disk Ca ii 854.2 nm magnetograms from the Synoptic Optical Long Term Investigations of the Sun (SOLIS) facility to study the chromospheric magnetic field from 2006 April through 2009 November. We determined and corrected previously unidentified zero offsets in the SOLIS magnetograms. By tracking the disk passages of stable unipolar regions, the measured net flux densities were found to systematically decrease from the disk center to the limb by a factor of about two. This decrease was modeled using a thin flux tube model with a difference in signal formation height between the center and limb sides. Comparison of photospheric and chromospheric observations shows that their differences are largely due to horizontal spreading of magnetic flux with increasing height. The north polar magnetic field decreased nearly linearly with time during our study period while the south polar field was nearly constant. We used the annual change in the viewing angle of the polar regions to estimate the radial and meridional components of the polar fields and found that the south polar fields were tilted away from the pole. Synoptic maps of the chromospheric radial flux density distribution were used as boundary conditions for extrapolation of the field from the chromosphere into the corona. A comparison of modeled and observed coronal hole boundaries and coronal streamer positions showed better agreement when using the chromospheric rather than the photospheric synoptic maps.

Solar Magnetism in the Polar Regions

“…VSM and SP observations saturate at ± 30 G, and Hinode filtergtaph (FG) observation saturates at ±0.006 in circular polarization. Image reproduced with permission from Jin et al (2013), copyright by AAS.…”

“…When Petrie and Patrikeeva (2009) applied this method to SOLIS Ca ii 6542Å chromospheric field measurements, these fields had evolved too much to produce well-defined line-of-sight flux functions of latitude, except during 2008 when the north polar field was approximately radial and the south polar field was expanding towards the observer, i.e., super-radially (see also Jin et al, 2013).…”

“…Jin et al (2013) took such an approach. They developed a method for producing synoptic maps for the chromospheric radial field component based on full-disk chromospheric Ca ii line-of-sight magnetograms taken by the SOLIS/VSM between April 2006 and November 2009.…”

“…The center-to-limb variation of polar field measurements cannot be investigated directly because we can only observe the polar fields from the ecliptic plane with large viewing angles. Instead, Jin et al (2013) investigated the center-to-limb variation of nearly unipolar low-latitude regions, which are believed to be similar to polar fields. They tracked 20 unipolar regions, of unsigned flux density > 8 G and ratio of major-polarity flux to minor-polarity flux > 3.…”

“…In practice, Jin et al (2013) were able to derive useful estimates of the meridional and zonal components. They then derived estimates for the radial component by exploiting the fact that the meridional field component is dominated by the radial component at low latitudes whereas at polar latitudes the north-south component is significant.…”