SOHO – the Solar and Heliospheric Observatory – doesn’t have quite the profile of the likes of Hubble and Spitzer, but we have two recent reminders of its value to science.

First the US Naval Research Laboratory (NRL) announced that it had identified the 4000th comet discovered by SOHO since its launch in 1995: not bad for a mission not designed to find comets nor to last anywhere near that long.

A joint venture between NASA and the European Space Agency, SOHO is looking at the Sun, in part via the onboard Large Angle Spectrometric Coronagraph (LASCO) instrument. The comet watching is done largely by citizen scientists through the Sungrazer project, and they’ve now discovered well over half of all known comets.

“This is exciting for many reasons, but perhaps mostly because LASCO is discovering comets that are otherwise completely unobservable from Earth due to their proximity to the Sun,” says NRL’s Karl Battams.

The second development is closer to core business. SOHO, via its Michelson Doppler Imager, provided a good chunk of the data recently used by German scientists to map the flow of the magnetic field that drives the Sun’s activity.

Writing in the journal Science, a team led by Laurent Gizon from the Max Planck Institute for Solar System Research describes its use of helioseismology – probing the Sun’s interior with acoustic oscillations, much as seismology uses of earthquakes to investigate Earth’s interior.

“Seeing the geometry and the amplitude of motions in the solar interior is essential to understanding the Sun’s magnetic field,” Gizon says.

Over each 11-year solar cycle, the Sun’s magnetic activity comes and goes. During solar maximum, large sunspots and active regions appear on its surface, loops of hot plasma stretch throughout its atmosphere and eruptions of particles and radiation shoot into interplanetary space. During the solar minimum, things calm down considerably.

A striking regularity appears in the so-called butterfly diagram, which describes the position of sunspots in a time-latitude plot, and solar physicists have suspected this is linked to the deep magnetic field being carried toward the equator by a large-scale flow.

To test this, Gizon and colleagues analysed helioseismology data from 1996 to 2019 provided by SOHO and the Global Oscillation Network Group (GONG), which combines six ground-based solar telescopes in the US, Australia, India, Spain and Chile.

They found, they say, that the flow is equatorward at just 15km/hr at the base of the convection zone (at a depth of 200 thousand kilometres) but poleward at up to 50km/hr at the surface. The overall picture is that the plasma goes around in one gigantic loop in each hemisphere.

The time taken for the plasma to complete the loop is approximately 22 years, providing the physical explanation, Gizon says, for the 11-year cycle. And sunspots emerge closer to the equator as the solar cycle progresses, as is seen in the butterfly diagram.

“All in all, our study supports the basic idea that the equatorward drift of the locations where sunspots emerge is due to the underlying meridional flows,” says co-author Robert Cameron.