LATITUDE DISTRIBUTION OF POLAR MAGNETIC FLUX IN THE CHROMOSPHERE NEAR SOLAR MINIMUM

摘要

The distribution of magnetic flux elements as a function of latitude in the polar solar caps at a time close to solar cycle minimum is explored. High-sensitivity line-of-sight magnetograms acquired by the Synoptic Optical Long-term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM) in the chromospheric line Ca n 854.2 nm are utilized. The area density distribution of the magnetic flux averaged over months is found to decrease close to the solar poles. This trend is more pronounced when considering only flux elements with relatively large size (larger than ≈5″ × 5″). The flux density of the latter is relatively flat from the edge of the polar cap up to latitudes of 75°-80° and decreases by more than 50% up to the solar pole. The density of smaller flux features is more uniformly distributed. Although the density decrease is still present, it is less pronounced. Hence, we believe there are two populations of flux elements in the polar caps. The small flux elements are probably produced uniformly across the polar area, in contrast to the large flux elements that are the product of magnetic fields of decaying active regions. The latter are concentrated by solar convection and carried up to high latitudes by differential rotation and meridional circulation. This result is important in studying meridional flows that transport the magnetic flux from lower to higher solar latitudes resulting, in particular, to the solar-cycle-related polar reversal. It is also of importance in studying polar structures contributing to the fast solar wind, such as polar plumes.