5.2 Helioseismic probing of subsurface emerging flux

UpdateJump To The Next Update Information There have been several local helioseismology studies looking for wave speed perturbations and plasma flow signatures that may be produced by subsurface rising flux tubes in emerging active regions (e.g. Kosovichev and Duvall Jr, 2008Jump To The Next Citation PointKomm et al., 2009Jump To The Next Citation Point). So far there have been no definitive detections of any significant perturbations or signatures associated with the emerging flux prior to the appearance of the flux at the visible surface.

Time-distance helioseismology analysis of a large emerging active region (AR 10488) observed by SoHO MDI show that the onset of wave speed perturbations within the top 20 Mm layer correlates well with the growth of the magnetic flux at the surface, with no significant perturbations prior to the growth of magnetic flux at the surface (Kosovichev and Duvall Jr, 2008). This suggests that the magnetic flux emerges very rapidly through the top 20 Mm layer, consistent with the estimate based on the thin flux tube model given above, such that there is no significant time difference (on the scale ≳ a few hours, which is the temporal resolution for the measured wave speed perturbation) between the onset of the sound-speed perturbation associated with the emerging active region and the appearance of the photospheric magnetic flux. On the other hand, time-distance helioseismology measurement of subsurface flows in the same region of flux emergence show that there is possibly an onset of diverging horizontal flow (and the associated upflow) at the depth range of 1 – 6 Mm, a few hours prior to the growth of the photospheric magnetic flux. However the signal fluctuates. There also appears to be a localized shear flow forming at the depth of 2 Mm below the photosphere at the location of the first magnetic field signal a few hours before the appearance of the magnetic flux. These interesting signatures of the horizontal flows need to be further studied for more emerging active regions to examine their significance.

By analyzing about five years of GONG high-resolution Doppler data with ring-diagram analysis, Komm et al. (2009) have studied the temporal variation of subsurface horizontal flows of 788 active regions and 978 quiet regions in the depth range of 0 – 16 Mm, during their disk passage within 60° CMD. A subsurface vertical velocity component is also derived from the divergence of the measured horizontal flows using the requirement of mass conservation, and assuming the flows are subsonic. The regions are sorted based on the variation of their unsigned flux during their disk passage into five subsets of equal size ranging from emerging-flux to decaying-flux subsets. It is found that the average vertical flows of the emerging-flux subset are systematically shifted toward upflows (or diverging horizontal flows) compared to the grand average values of the complete data set, whereas the average flows of the decaying-flux subset show much more pronounced downflows (or converging flows). A study of the averaged cross-correlation between the temporal variation of the unsigned flux and the vertical velocity for the emerging-flux subset suggests that flows in the near-surface and the deeper layers might change about one day before flux emerges at the surface. Thus the change in the divergence of the subsurface horizontal flows might be a precursor of the flux changes. Clearly further observational studies are needed to determine whether significant subsurface horizontal flow signatures associated with emerging flux tubes can be detected prior to the appearance of active region flux at the surface.


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