While the Sun’s periods of intense activity often capture our attention, even during its quiescent phases, our star can subtly reconfigure its internal dynamics.
A recent in-depth analysis of decades of observational data reveals that the quiescent periods within the Sun’s 11-year activity cycle are not uniform. Measurable alterations in its internal vibrational patterns were observed during the most profound solar minimum recorded in recent history.
“This marks the first instance where we’ve been able to precisely quantify the modifications in the Sun’s internal architecture from one solar cycle minimum to the subsequent one,” stated astronomer Bill Chaplin, affiliated with the University of Birmingham in the United Kingdom.
“The Sun’s outer envelopes undergo subtle transformations across its activity cycles, and our findings indicate that periods of deep quiescence leave a discernible imprint on its internal characteristics.”
Our Sun is far from being a static, unchanging celestial furnace. A prominent manifestation of its dynamic nature is the solar cycle, which is intrinsically linked to the reversal of its magnetic poles. Approximately every eleven years, the Sun reaches a zenith of activity, termed solar maximum, before receding into a period of reduced activity known as solar minimum.
Activity escalates as the Sun approaches solar maximum, characterized by an increase in sunspots, solar flares, and coronal mass ejections. During this phase, the Sun’s magnetic poles undergo a reversal. No two solar cycles are identical, with some maxima exhibiting greater intensity than others. Conversely, solar minima, at least from an observational standpoint, tend to appear remarkably similar.
Utilizing the Birmingham Solar-Oscillations Network (BiSON), a global network comprising six telescopes, a research group spearheaded by astrophysicist Sarbani Basu from Yale University meticulously examined four consecutive solar minima, covering the transitions from cycles 21 through 25 (we are presently within solar cycle 25).
The team focused on the analysis of acoustic oscillations within the Sun – effectively trapped sound waves that propagate through the solar plasma, causing minute fluctuations in the brightness of the Sun’s surface.
Much in the same way that seismic waves traversing the Earth provide insights into its interior, sound waves within the Sun can illuminate conditions beneath its surface. This analytical methodology is referred to as helioseismology.
The researchers investigated two primary indicators. The first is termed the helium glitch. Beneath the Sun’s visible surface lies a stratum where helium undergoes ionization due to electron loss. This alteration in its energetic state leaves a distinct “signature” within the oscillation data, observable just below the luminary surface.
The second indicator is the velocity of sound propagation within the Sun. The speed at which sound travels through a medium is dependent on its intrinsic properties, such as temperature and density. Any subtle modification in the Sun’s internal structure will consequently alter the speed of sound, thereby influencing the vibrational frequencies.
The research group also correlated these measurements with computational models of solar behavior, which were predicated on slightly varied internal conditions.
The observed minima occurred in 1985 (between cycles 21 and 22), in 1996 (between cycles 22 and 23), between 2008 and 2009 (between cycles 23 and 24), and between 2018 and 2019 (between cycles 24 and 25).
Notably, the solar minimum experienced between 2008 and 2009 was among the most protracted and tranquil on record. It exhibited the most pronounced internal shifts among the four periods examined. The helium glitch signal was more pronounced than in the other three minima, and the speed of sound was measurably faster within the Sun’s outer layers.
These observations suggest that gas pressure was elevated, temperatures were slightly warmer, and magnetic fields were less intense in specific regions of the Sun during this particular minimum.
“Understanding how the Sun behaves beneath its surface during these quiescent interludes is significant because this behavior profoundly influences the escalation of activity observed in subsequent cycles,” Basu remarked. Indeed, solar cycle 24 was characterized by unusually low activity, featuring one of the weakest maxima ever documented.
The prediction of solar behavior is notoriously challenging, partly because its underlying mechanism remains concealed beneath the surface. This dynamic, rotating, magnetized plasma sphere is such that even minor internal perturbations can propagate outward, resulting in substantial variations in activity levels.
This newly published research demonstrates that solar activity, which appears uniform on the surface, can originate from subtly divergent internal conditions. This suggests an inherent variability that current solar models may need to incorporate with greater precision.
“Our investigation effectively showcases the efficacy of sustained stellar seismic observation,” Chaplin commented.
“With upcoming missions such as the European Space Agency’s PLATO observatory, the techniques employed in this study can be extended to other stars similar to our Sun. This will enhance our comprehension of how their activity fluctuates and the impact they exert on their surrounding cosmic environments, including any exoplanets they might host.”
