Elena G. Broock¹, Angel D. Martínez Cifuentes², Alina-Catalina Donea², and Charles Lindsey³

1 Instituto de Astrofisica de las Canarias, Tenerife, Spain
2 School of Mathematical Sciences, Monash University, Victoria 3800, Australia 4
3 North West Research Associates, 3380 Mitchell Ln, Boulder, CO 80301, USA

Strong Acoustic Scatterers
A powerful diagnostic in solar seismology applied to observations of large sunspots by the Helioseismic and Magnetic Imager (HMI) aboard NASA’s Solar Dynamics Observatory (SDO) shows the seismic signature of clusters of compact acoustic anomalies nestled from 500 to 1000 km beneath the photospheres of large sunspots. These anomalies, we now call “strong acoustic scatterers,” appear in helioseismic phase-correlation maps of p-modes, low-frequency acoustic waves reverberating throughout the solar interior, that have scattered off of the shallow subphotospheres of large sunspots. This discovery is reported in a study by Broock et al. (2005) in Ref. [1].

Figure 1. Acoustic phase map (left frame) of a small, isolated sunspot is made by applying phase-correlation helioseismic holograhy, to a 24-hour timeseries of helioseismic observations of NOAA AR12289 centered at 2014-11-10.0 by SDO/HMI. Right frame shows a concurrent co-spatial HMI continuum intensity snapshot of the same. Red reference fiducials locate one of the anomalies in the southern penumbra of the sunspot. A more detailed depiction of how helioseismic holography works is available here .

Figure 1 shows how these anomalies appear in an isolated, moderate sized sunspot. Red fiducials mark a single arbitrarily selected strong acoustic scatterer that appears in this map, one located in the southern penumbra of the sunspot. Strong acoustic scatterers are not confined to large sunspots. They abound in isolated compact magnetic poles far from large sunspots (Figure 2).

Figure 2. Helioseismic map (left) of the magnetic environment of NOAA AR 11944. Red fiducials mark a compact pore embedded in a small, irregular penumbra.

It must seem at least a bit suspect that such a distinctive structurality as arrays of strong acoustic scatters would be pressed so sharply up against a sunspot photosphere from below, yet far enough beneath it to be completely hidden from the view of sunspots we have had since the Renaissance. In fact, just this kind of structure was actually predicted in a series of studies by legendary solar physicist E. N. Parker nearly half a century ago^[2,3].

The Sunspot Mystery
Parker’s studies concluded at the outset that the magnetic flux that seamlessly papers the photospheres of sunspot umbrae must be separated into multiple strands within a few hundred kilometers beneath their photospheres, with hot ambient gas filling the gaps (Figure 3). This was a consequence of the crushing vertical gradient in the pressure of the ambient gas surrounding the sunspot with increasing depth. The hidden dynamical consequences of this structure were tectonic: Pockets of hot gas became trapped in the gaps between the magnetic strands beneath the cool overlying magnetic monolith. This is a radically unstable hydrostatic configuration, one susceptible to rapidly growing “overstable oscillations.” In Parker’s model, these oscillations drive downwardly propagating waves that are so intense that they carry most of the thermal energy sunspots owe to their surfaces deep beneath them, depositing it as heat therein, heat that takes weeks to find its way back to the surface. The ergonomics of this conversion of differential thermal energy to wave-kinetic energy are essentially those of Carnot’s¹ celebrated “heat engine,” cyclically converting excess thermal energy in the hotter gas in the non-magnetic gaps to mechanical work, using the cool overlying magnetic monolith and interplanetary space above it as a heat dump for the renewal of each cycle. A more detailed elaboration of this mechanics based on elementary thermodynamics can be found here.

Figure 3. Diagram of Parker’s model of fragmentation of magnetic flux beneath the opaque photospheres of sunspots. Hot nonmagnetic gas trapped between magnetic flux strands and under a cool overlying magnetic monolith are unstable to vertical oscillations (two-way vertical arrows). The oscillating hot columns punch momentary holes in the seams of the magnetic monolith, releasing a fraction of their energy into space. Most of the rest is converted to acoustic waves that propagate downward, carrying the blocked energy, somewhat ironically, downward, an unknown distance back into the solar interior, where it is dissipated into heat, and from whence it takes weeks, possibly longer, for it to return to the surface by convection. Blue and red curves represent to scale the species of incoming (blue) p-mode waves and their outgoing echos (red), whose surface signatures observed by the SDO, outside of the sunspot domain, are reconstructed and correlated to give us the acoustic maps shown in Figures 1 and 2, above.

Why Is Fragmentation of Magnetic Flux beneath Sunspot Photospheres Important?
The role of strong acoustic scatterers in sunspot structure must certainly become a major component of our understanding of the sunspot phenomenon. But, their true significance might lie more essentially in their functionality in active region evolution. The thermal income they are proposed to manage is enormous, a deluge of heat flux due to which, if not for its continuous expedient disposal, all sunspots would quickly die of thermal obesity before they were born. As the thermal gluons that bring disparate magnetic poles of like polarity together to flux densities many times that of the active-region-band mean, the role of strong acoustic scatterers is a potentially elemental driver of active-region evolution at large. In centuries to come, the MHD architecture of which strong acoustic scatterers are a signature will remain recognized as a fundamental component of sunspot structure, our knowledge of which is a uniquely distinctive child of NASA-SDO parentage. Their discovery now stands to open an ample landscape of applications to a burgeoning array of powerful observational and analytical facilities, to the further development of which the SDO itself remains an immanently prospective resource.

¹Nicholas Léonard Sadi Carnot (1796–1832)

References

[1] Broock, E. G., Martínez Cifuentes, A., Donea, A.-C. & Lindsey, C. 2025 ApJ Lett., 986, L20
[2] Parker, E. N. 1979, ApJ, 230, 905.
[3] Parker, E. N. 1979, ApJ, 230, 914.