On the Nature of Magnetic Shadows in the Solar Chromosphere

Abstract

Recent multi-instrument spacecraft studies of the solar photosphere and chromosphere have uncovered a feature—a shadow—not previously discussed in the literature. A region of the midchromosphere neighboring, but not within, a network magnetic element exhibits a suppression of both the mean UV line/continuum intensity and the characteristic 3 minute oscillation that is clearly observed elsewhere in apparently similar internetwork regions. The clearest cases appear to occur rarely, and their properties stand in obvious contrast to the well-known aureoles of enhanced variability seen surrounding some plage regions. It is imperative to understand more clearly the nature of the shadow region, not least because the suppressed atmospheric heating within it has implications for heating processes elsewhere in the chromosphere that are dependent on, or at least related to, the 3 minute oscillation. Based on the measured photospheric magnetic field, its upward extrapolation, and the appearance of spectral features formed above the midchromosphere, we suggest that a shadow occurs when magnetic structures, in a relatively weak background field, close locally within the chromosphere, suppressing the upward propagation of magnetoatmospheric waves into the chromosphere.