148. Submerged Sources of Flare-Induced Transient Acoustic Emission

Contributed by Charlie Lindsey. Posted on November 12, 2020

C. Lindsey1, J. C. Buitrago-Casas2,3, J. C. Martínez Oliveros3, D. C. Braun1, A. D. Martínez4, V. Quintero Ortega4, B. Calvo-Mozo4, and A.-C. Donea5
1. North West Research Associates, 3380 Mitchell Lane, Boulder, CO 80301, USA
2. Physics Department, University of California, Berkeley, CA 94720-7300, USA
3. Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA
4. Observatorio Astronómico Nacional, Universidad Nacional de Colombia, Cra. 30 # 45-03, 111321, Bogotá, Colombia
5. School of Mathematical Sciences, Monash University, Victoria 3800, Australia

Review
A new study by a team of eight solar physicists in the US, Colombia, and Australia reports the discovery of an apparent source of ultra-impulsive transient acoustic emission that is submerged a thousand kilometers beneath the photosphere of a flaring active region. Transient acoustic emission from flares was discovered by Kosovichev & Zharkova[1]. The signature of the phenomenon, called sunquake, is a pattern of outwardly propagating ripples on the Sun’s surface in the hour following the onset of a flare.

The theme of this nugget is the basic principles of the wave diagnostics that have led to the recognition of an apparent acoustic source submerged 1,000 km beneath the active- region photosphere. Donea, Braun & Lindsey[2] and their progeny have used the helioseismic holography technique to image the acoustic sources of sunquakes, finding compact, highly localized emitters that are sometimes but not always aligned with the disturbances in the flaring outer atmosphere. This has kept the mechanics of the transient acoustic emission from flares a mystery.

The Flare
A survey of 75 M-class flares by Buitrago-Casas et al.[3] led to an ultra-impulsive sunquake released by the flare of 2011-07-30 hosted by NOAA AR11261[4]. The extreme compactness of the 10-mHz sources of this sunquake allowed helioseismic diagnostics of an unprecedented discrimination in depth.

Figure 1| Left frame shows visible continuum intensity of NOAA AR 11261 in amber with He II 304Å emission in the early impulsive phase superimposed in red on 2011-07-30. Right frame shows Doppler ripples driven by waves emitted downward from the flaring region returning to the Sun’s outlying surface 20 min thence. Ripples are amplified by a factor of 3. Click here to see an animation of this figure.

The Diagnostic
The depth diagnostics applied in the helioseismic domain benefit from the familiar depth-of-field (DoF) phenomenon encountered over a vast range of distance scales in electromagnetic optics. On its smallest scales, the DoF effect is encountered in the need to focus a microscope to exact a sharp image of its specimen. Figures 2 and 3 illustrate the parallels in DoF diagnostics between the familiar electromagnetic and solar acoustic domains. In Figure 2, the exercise is microscopy of ragweed pollen in a 200-μm suspension. The image of any single pollen grain comes into sharp focus only as the focal plane of the microscope passes through it. The otherwise attendant defocusing is distantly annoying to the viewer who only wants sharp images, but it makes the depth of the object clearly evident in a way that unconditionally sharp images cannot.

Figure 2| DoF effect in visible-light microscopy of ragweed pollen. When the focal plane of the microscope is 68 μm into the suspension, the pollen grain labeled S is sharply rendered, with a deeper one, D, out of focus. As the focal plane is lowered to 100 μm, D comes into sharp focus, revealing its deeper submergence with respect to S. Click here to see an animation of this figure.

Figure 3, then, shows the helioseismic analogy of Figure 2, in helioseismic holography applied to the 10-mHz spectral component of the ripples imaged in Figure 1. Two strong acoustic signatures are seen when the focal plane is shallow. The one on the right is compound, an oblong component 1,000 km to the upper right (N-W) of its twin. As the acoustic focal plane is drawn inward, beneath the active-region photosphere, the S-E twin contracts to a compact condensation we identify with an apparent source at approximately that depth. Note the difference in scale between Figures 2 and 3, by a factor of ~1.4×1011.

Figure 3| Same diagnostic as that of Figure 2 applied acoustically to the 10-mHz component of ripples imaged in Figure 1. Left frame images acoustic-power density of waves extrapolated downward from said ripples, back into the solar interior and thence back up to a focal plane at the Sun’s surface. Right frame maps same extrapolation, but over a focal plane 1183 km beneath Sun’s surface. Cospatial red arrows fix location of condensation in right frame attributed to a submerged acoustic source. Click here to see an animation of this figure.

Significance of the Discovery
It has been widely suspected that transient acoustic emission is injected into the solar interior from the outer atmosphere. This has been a highly attractive proposition based on the abundance of coronal-magnetic free energy being released in a flare. There are contrivances under which coronal magnetic energy could continue to be the driver of an apparently submerged acoustic source. However, the extreme compactness of the submerged source found, hence the local concentration of acoustic energy momentarily manifested by it, suggests that the driver of the emission in question, while it must necessarily be triggered by disturbances related to the fun taking place in the overlying outer atmosphere, is itself submerged, possibly including the supply of free energy released into the acoustic field thereat. An acoustic source submerged to a depth at which the nominal gas pressure is fifty times that at the base of the photosphere opens the mechanics of transient acoustic emission to a broad array of new possibilities. There could hardly be better designed facility with which to explore these possibilities in the new solar cycle than NASA’s powerful Solar Dynamics Observatory, which has been so crucial in the development of flare seismology in cycle 24.

A complementary science nugget about these results was published here . For more details of this work, please refer to Ref. [5].

References

[1] Kosovichev, A. G., & Zharkova, V. V. 1998, Nature, 393, 317
[2] Donea, A.-C., Braun, D. C. & Lindsey, C. 1999, ApJ Lett., 513,, 143
[3] Buitrago-Casas, J. C., Martínez Oliveros, J. C., Lindsey, C., et al. 2015 Solar Phys., 290, 3151
[4] Martinez, A. D., Ortega, V. Q., Buitrago Casas, J. C., Martinez Oliveros, J. C., Calvo Mozo, B.; Lindsey, C. 2020 ApJ Lett., 895, 19
[5] Lindsey, C., Buitrago Casas, J. C., Martinez Oliveros, J. C. Braun, D. C., Martnez, A. D., Ortega, V. Q., Calvo Mozo, B.; Donea, A.-C. 2020, ApJ Lett. 901, 9

3 comments on “Submerged Sources of Flare-Induced Transient Acoustic Emission

  1. Brian Welsch

    I looked at the paper (which I should have done before commenting), and my alternative hypothesis is perhaps best captured by the paper’s sec. 4.3, which discusses “warpage” of source wave trajectories, which compromises the inverse solution.

    Reply
  2. Brian Welsch

    Great Nugget! Your operating hypothesis is that the apparent, 1000-km-deep source is the actual source — fair enough! I would to like your opinion, please, of the viability of an alternative interpretation: that this apparent source is a manifestation of some type of focusing of waves that originated at shallower depths, over a larger area, earlier in time. This 1-in-75 event presumably has different physics at work than in a generic sunquake. Is it possible that, in this case, a substantial amount of wave power, launched closer to the surface in different places and at different times, just “happened to” converge at the apparent source point, with the waves’ subsequent propagation proceeding as inferred by your helioseismic technique? Perhaps this is equivalent to asking: could nearer-surface sources explain the observations if the waves were propagated farther back in time, earlier than their apparent origin at the submerged source?

    Reply

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