Helioseismic Signature of Parker Fragmentation of Magnetic Flux beneath Sunspot Photospheres

Abstract

Standard local helioseismic diagnostics of large sunspots show the signature of anomalously strong compact scatterers of p-modes within about 1 Mm beneath their photospheres. We applied standard “subjacent-vantage phase-correlation helioseismic holography” to the major sunspot umbra and inner penumbra in NOAA AR12192 as it crossed central solar meridian on 2014 October 23. This diagnostic delivers diffraction-limited maps of the phase delay in the echos of p-mode noise in the 2.5–4.5 mHz acoustic spectrum impinging upward from the underlying solar interior into the sunspot photosphere. What we recognize as the nominal sunspot umbra sends its echos back into the solar interior up to about 1.2 minutes ahead of those reflected from the quiet Sun. In addition to this, we find multiple compact regions approximately 5 Mm in horizontal diameter whose echos are expedited by a further 30 s ahead of the nominal umbral echos. These “strong acoustic scatterers” appear in both umbrae and inner penumbrae, but in a sunspot whose umbra is large enough to accommodate several of them, they show a decided affinity for the boundary separating the two. Standard focus–defocus depth diagnostics pin the anomalies to within about 1 Mm of the base of the photosphere. We propose that the strong acoustic scatterers are the helioseismic signature of fragmentation of magnetic flux beneath sunspot photospheres predicted by E. N. Parker 4.5 decades ago as a crucial accommodation for the stability of sunspot magnetic flux subject to warpage by a precipitous ambient vertical pressure gradient.