Temporal Defocusing as a Depth Diagnostic of Submerged Sources of Transient Acoustic Emission from Solar Flares

Helioseismic holography applied to HMI observations of a sunquake associated with the SOL20140207T10:29M1.9 flare hosted by NOAA AR11968 shows the signature of a compact submerged acoustic source. In the 9 – 11-mHz bandpass, this source appears to be at a depth of 2 Mm. This is nearly double the depth of the highly impulsive acoustic transient, referred to as an ultra-impulsive transient source recently found in the SOL20110730T02:09M9.3 flare, emerging from NOAA AR11261 in 2011. The latter source was compound, having multiple surface components overlaying a single submerged component. Many of the sources observed have evidence of being constituted of multiple components, some of which are staggered in depth. The helioseismic source of the flare of 2014-02-07 is distinguished by the apparent absence of a strong shallow compact overlying source component matching the character of those apparent in the flare of 2011-07-30. This suggests that the volume several Mm beneath active-region photospheres could possibly harbor many more ultra-impulsive transient acoustic sources than the rarities that have so far appeared in familiar surface-focal-plane diagnostics. Based on weak surface signatures that appear in place of the strong ones, we find a temporal delay between weak surface and strong submerged sources similar to that found between surface-and-submerged sources in the flare of 2011-07-30. While it remains highly speculative based on the limited statistics we have at this point, this temporal delay supports a model in which the submerged source is perturbed by some presently invisible triggering disturbance that propagates downward from the flaring outer atmosphere at ∼ 5 km s −1 . This is slower than the sound speed anywhere in the 0 – 2-Mm depth range. However, this potentially could be an Alfvèn speed if submerged magnetic flux densities along which the trigger propagates are as high as possible. The standard local-helioseismic diagnostics we have used in the past have been heavily reinforced in this study by a powerful new control resource found in the recognition of temporal defocusing of compact transient sources. In particular, its tight relationship to the standard spatial defocusing upon which helioseismic holography has capitalized from its early advent.