Astronomers have taken the most precise look yet beneath the surface of the mysterious “supergranules” on the Sun – and discovered something surprising. The solar plasma in these tens of thousands of kilometers large convection cells shows a strange gap: About 40 percent more hot plasma flows to the surface than sinks back down. This not only contradicts the current theory but also points to an undiscovered mechanism, as the team reports in “Nature Astronomy“.
The Sun’s outer third resembles a pot of boiling water: Driven by the enormous heat in the star’s interior, hot plasma rises, cools further up, and then sinks back into the depths. These convection currents of various scales play a crucial role in solar heat transport, the magnetic field, and also the Sun’s activity cycle. However, how exactly this convection occurs is known primarily for the largest and smallest plasma flows – in between lies a gap.
This gap mainly affects the so-called supergranules – convection cells on the Sun’s surface that are 30,000 to 40,000 kilometers in size. In these, hot plasma rises from deeper zones, cools down, flows outward, and then sinks back into the depths – at least according to the simplified theory. Typically, such a supercell remains stable for about 24 to 36 hours before dissolving and a new one forming.
How Deep Do the Currents Reach?
However, how deep the currents of the supergranules reach and what structures are hidden beneath their surface remains unclear. “These supergranules are a significant component of solar heat transport, but understanding them is a real challenge,” explains co-author Shravan Hanasoge from New York University in Abu Dhabi. The problem: Astronomers cannot look beneath the Sun’s surface; they rely on indirect methods such as helioseismology and observations of surface phenomena.
These measurements have yielded contradictory results. “According to some models, the adiabatic convection zone of the supergranules reaches about 10,000 kilometers deep, but other studies have found much shallower depths of only 2,500 to 5,000 kilometers,” explains the team led by Hanasoge and first author Chris Hanson from New York University. To bring more clarity, they have now analyzed the currents and evolution of over 23,000 solar supergranules. The basis for this was data from NASA’s Solar Dynamics Observatory (SDO) satellite.
40 Percent of the Downward Flow is Missing
The new analyses provide the first more accurate look into the hidden structure of solar supergranules. According to this, the hot upward flows begin at a depth of about 25,000 kilometers below the Sun’s surface and reach their highest velocity at a depth of about 10,000 kilometers, as the team reports. But already about 7,000 kilometers below the surface, this hot plasma begins to spread laterally, forming a zone of outward-directed plasma flows that extends about 18,000 kilometers outward.
The surprising thing, however: At the outer edge of the super-flow cells, much less cooled plasma sinks into the depths than the current models predict. “We found a clear difference between the amplitudes of the average up and down flows – the latter are about 40 percent lower,” report Hanson and his colleagues. This imbalance suggests that the prevailing theory is insufficient to describe the convective transport of supergranules.
Contradiction to Classical Mixing Length Theory
But what does this mean specifically? As the astronomers explain, the classical mixing length theory assumes that the movement of solar plasma is determined solely by density and temperature differences in relation to its environment: The plasma rises as long as it is hotter than its surroundings and sinks as long as it is colder. At the same time, this theory states that larger convection currents also take up more space laterally and form larger flow cells.
At first glance, this is also the case with the Sun’s supergranules. However, the imbalance of up and down flows discovered by Hanson and his team contradicts the classical picture. “Our results refute assumptions that are central to our current understanding of solar convection,” says Hanasoge. As the astronomers explain, there must therefore be an as yet undiscovered mechanism that can compensate for the too weak downward flows.
Plasma “Rain” Instead of Large Convection Currents?
What this process looks like is still open. However, Hanson and his team suspect that in addition to the large sinking currents, small plasma plumes only about 100 kilometers wide could possibly form at the Sun’s surface. This dense, cool plasma “rain” could then sink particularly quickly and far downwards. “According to the hypothesis, this cool material would undergo a ballistic sinking to the base of the convection zone,” they explain.
Because these mini-currents are too small to be detected with conventional helioseismological methods, they have remained undiscovered so far, according to the assumption. Further research is needed here.
