Scientists examine sunquakes to identify flare energy source, maybe forecast flare intensity.
Solar flares are violent surges on the sun that fling out high-energy charged particles, often towards Earth, where they interfere with interactions and threaten satellites and astronauts.
But as researchers found in 1996, flares can likewise develop seismic activity — sunquakes — launching spontaneous acoustic waves that permeate deep into the sun’s interior.
While the relationship in between solar flares and sunquakes is still a secret, brand-new findings recommend that these “acoustic transients” — and the surface area ripples they produce — can inform us a lot about flares and might sooner or later assist us anticipate their size and intensity.
A group of physicists from the United States, Colombia and Australia has actually discovered that part of the acoustic energy launched from a flare in 2011 originated from about 1,000 kilometers below the solar surface area — the photosphere — and, therefore, far below the solar flare that activated the quake.
The results, reported in The Astrophysical Journal Letters on September 21, 2020, originated from a diagnostic strategy called helioseismic holography, presented in the late 1900s by French researcher Franoise Roddier and thoroughly established by U.S. researchers Charles Lindsey and Douglas Braun, now at NorthWest Research Associates in Boulder, Colorado, and co-authors of the paper.
Helioseismic holography permits researchers to examine acoustic waves activated by flares to penetrate their sources, much as seismic waves from megaquakes on Earth permit seismologists to find their centers. The strategy was very first used to acoustic transients launched from flares by a college student in Romania, Alina-Catalina Donea, under the guidance of Lindsey and Braun. Donea is now at Monash University in Melbourne, Australia.
“It‘s the first helioseismic diagnostic specifically designed to directly discriminate the depths of the sources it reconstructs, as well as their horizontal locations,” Braun stated.
“We can’t see the sun’s inside directly. It is opaque to the photons that show us the sun’s outer atmosphere, from where they can escape to reach our telescopes,” stated co-author Juan Camilo Buitrago-Casas, a University of California, Berkeley, doctoral trainee in physics from Colombia. “The way we can know what happens inside of the sun is via seismic waves that make ripples on the solar surface similar to those caused by earthquakes on our planet. A big explosion, such as a flare, can inject a powerful acoustic pulse into the sun, whose subsequent signature we can use to map its source in some detail. The big message of this paper is that the source of at least some of this noise is deeply submerged. We are reporting the deepest source of acoustic waves so far known in the sun.”
How sunquakes produce ripples on the sun’s surface area
The acoustic surges that trigger sunquakes in some flares radiate acoustic waves in all instructions, mainly downward. As the downward-traveling waves move through areas of ever-increasing temperature level, their courses are bent by refraction, eventually heading back up to the surface area, where they develop ripples like those seen after tossing a pebble in a pond. The time in between the surge and the arrival of the ripples has to do with 20 minutes.
“The ripples, then, are not just a surface phenomenon, but the surface signature of waves that have gone deep beneath the active region and then back up to the outlying surface in the succeeding hour,” Lindsey stated. Analyzing the surface area ripples can identify the source of the surge.
“It has been widely supposed that the waves released by acoustically active flares are injected into the solar interior from above. What we are finding is the strong indication that some of the source is far beneath the photosphere,” stated Juan Carlos Martínez Oliveros, a solar physics scientist at UC Berkeley’s Space Sciences Laboratory and a local of Colombia. “It seems like the flares are the precursor, or trigger, to the acoustic transient released. There is something else happening inside the sun that is generating at least some part of the seismic waves.”
“Using an analogy from medicine, what we (solar physicists) were doing before is like using X-rays to look at one snapshot of the interior of the sun. Now, we are trying to do a CAT scan, to view the solar interior in three dimensions,” included Martínez Oliveros.
The Colombians, consisting of trainees Ángel Martínez and Valeria Quintero Ortega at Universidad Nacional de Colombia, in Bogotá, are co-authors of the ApJ Letters paper with their manager, Benjamín Calvo-Mozo, associate teacher of astronomy.
“We have known about acoustic waves from flares for a little over 20 years now, and we have been imaging their sources horizontally since that time. But we have only recently discovered that some of those sources are submerged below the solar surface,” stated Lindsey. “This may help explain a great mystery: Some of these acoustic waves have emanated from locations that are devoid of local surface disturbances that we can directly see in electromagnetic radiation. We have wondered for a long time how this can happen.”
A seismically active sun
For more than 50 years, astronomers have actually understood that the sun resounds with seismic waves, similar to the Earth and its constant hum of seismic activity. This activity, which can be identified by the Doppler shift of light originating from the surface area, is comprehended to be driven by convective storms that form a patchwork of granules about the size of Texas, covering the sun’s surface area and continuously rumbling.
Time-lapse series of the July 30, 2011, solar flare observed by NASA’s SolarDynamics Observatory. The left frame programs noticeable light emissions in amber and excess severe ultraviolet emissions in red. The right frame reveals the line-of-sight Doppler speed of the solar surface area emissions. Between 20 to 40 minutes following the spontaneous stage of the flare (IP on timeline), a strong acoustic disruption launched downward into the underlying solar interior has actually refracted back to the far-flung surface area, 10s of countless kilometers from the website of the flare, to generate outwardly propagating surface area ripples (right frame). The motion picture is 200 times faster than actual time; the ripples are enhanced by an element of 3 in the right frame compared to the left. Credit: Charles Lindsey
Amid this background sound, magnetic areas can trigger violent surges launching waves that make the incredible ripples that then appear on the sun’s surface area in the prospering hour, as found 24 years back by astronomers Valentina Zharkova and Alexander Kosovichev.
As more sunquakes have actually been found, flare seismology has actually progressed, as have the methods to explore their mechanics and their possible relationship to the architecture of magnetic flux underlying active areas.
Among the open concerns: Which flares do and don’t produce sunquakes? Can sunquakes happen without a flare? Why do sunquakes originate mainly from the edges of sunspots, or penumbrae? Do the weakest flares produce quakes? What is the lower limitation?
Until now, most solar flares have actually been studied as one-offs, given that strong flares, even throughout times of optimum solar activity, might happen just a few times a year. The preliminary focus was on the biggest, or X-class, flares, categorized by the strength of the soft X-rays they release. Buitrago-Casas, who acquired his bachelor’s and master’s degrees from Universidad Nacional de Colombia, partnered with Lindsey and Martínez Oliveros to carry out an organized study of reasonably weak solar flares to increase their database, for a much better understanding of the mechanics of sunquakes.
Of the 75 flares recorded in between 2010 and 2015 by the RHESSI satellite — a NASA X-ray satellite developed, constructed and run by the Space Sciences Laboratory and retired in 2018 — 18 produced sunquakes. One of Buitrago-Casas’s acoustic transients, the one launched by the flare of July 30, 2011, stood out of undergraduate trainees Martínez, now a college student, and Quintero Ortega.
“We gave our student collaborators at the National University the list of flares from our survey. They were the first ones who said, ‘Look at this one. It’s different! What happened here?’” Buitrago-Casas stated. “And so, we found out. It was super exciting!”
Martínez and Quintero Ortega are the very first authors on a paper explaining the severe impulsivity of the waves launched by that flare of July 30, 2011, that appeared in the May 20, 2020, concern of The Astrophysical Journal Letters. These waves had spectral parts that provided the scientists unmatched spatial resolution of their source circulations.
Thanks to excellent information from NASA’s Solar Dynamics Observatory satellite, the group had the ability to identify the source of the surge that produced the seismic waves 1,000 kilometers listed below the photosphere. This is shallow, relative to the sun’s radius of almost 700,000 kilometers, however much deeper than any formerly understood acoustic source in the sun.
A source immersed listed below the sun’s photosphere with its own morphology and no obvious straight overlying disruption in the external environment recommends that the system that drives the acoustic short-term is itself immersed.
“It may work by triggering a compact explosion with its own energy source, like a remotely triggered earthquake,” Lindsey stated. “The flare above shakes something beneath the surface, and then a very compact unit of submerged energy gets released as acoustic sound,” he stated. “There is no doubt that the flare is involved, it’s just that the existence of this deep compact source suggests the possibility of a separate, distinctive, compact, submerged energy source driving the emission.”
About half of the medium-sized solar flares that Buitrago-Casas and Martínez Oliveros have actually cataloged have actually been connected with sunquakes, revealing that they frequently happen together. The group has actually given that discovered other immersed sources connected with even weaker flares.
The discovery of immersed acoustic sources opens the concern of whether there are circumstances of acoustic transients being launched spontaneously, without any surface area disruption, or no flare, at all.
“If sunquakes can be generated spontaneously in the sun, this might lead us to a forecasting tool, if the transient can come from magnetic flux that has yet to break the sun’s surface,” Martínez Oliveros stated. “We could then anticipate the inevitable subsequent emergence of that magnetic flux. We may even forecast some details about how large an active region is about to appear and what type — even, possibly, what kinds of flares — it might produce. This is a long shot, but well worth looking into.”
“Submerged Sources of Transient Acoustic Emission from Solar Flares” by Charles Lindsey, J. C. Buitrago-Casas, Juan Carlos Martínez Oliveros, Douglas Braun, Angel D. Martínez, Valeria Quintero Ortega, Benjamín Calvo-Mozo and Alina-Catalina Donea, 21 September 2020, The Astrophysical Journal Letters.
“Ultra-impulsive Solar Flare Seismology” by Angel D. Martínez, Valeria Quintero Ortega, J. C. Buitrago-Casas, Juan Carlos Martínez Oliveros, Benjamín Calvo-Mozo and Charles Lindsey, 22 May 2020, The Astrophysical Journal Letters.