The Sun is the only star close enough to study in detail, and it is the one that most directly affects us. It is also, in several respects, poorly understood — with a couple of longstanding problems that have resisted explanation for decades.
A fleet of spacecraft is now watching it continuously, from several vantage points at once.
Why constant monitoring
The Sun is not a steady lamp. Its magnetic field is generated by the churning motion of ionised gas in its interior, and it reverses polarity on a roughly eleven-year cycle. Activity — sunspots, flares, eruptions — rises and falls with it.
The consequences reach Earth. Solar flares release radiation that arrives in eight minutes and can disrupt radio communication and navigation. Coronal mass ejections launch billions of tonnes of magnetised plasma that take one to three days to arrive and can induce currents in power grids, damage satellites, and pose radiation risks to astronauts.
Modern infrastructure is far more sensitive to this than infrastructure was a century ago. The largest recorded geomagnetic storm, in 1859, mainly affected telegraph systems. An equivalent event now would be considerably more disruptive, which is why space weather forecasting has become an operational service rather than a research curiosity.
The layers being studied
Understanding the observations requires knowing what part of the Sun each instrument targets.
- The core, where fusion converts hydrogen to helium, releasing the energy that powers everything else.
- The radiative and convective zones, through which that energy slowly works outward — a journey that takes photons on the order of a hundred thousand years.
- The photosphere, the visible surface, at around 5,500 degrees Celsius. This is where sunspots appear as cooler regions where strong magnetic fields suppress convection.
- The chromosphere, a thin layer above it.
- The corona, the outer atmosphere, visible during total eclipses as a pearly halo.
The corona is over a million degrees. The surface below it is around 5,500. Heat flowing from cooler to hotter is exactly backwards, and explaining it is one of solar physics’ central open problems.
The coronal heating problem has candidate explanations — dissipation of magnetic waves, or vast numbers of small reconnection events called nanoflares — but no settled answer. Distinguishing between them requires measurements from inside the corona itself.
The current fleet
Several missions approach the problem from complementary angles.
A spacecraft in a highly elliptical orbit has flown repeatedly through the outer corona, closer to the Sun than anything before it, protected by a carbon composite heat shield. Flying inside the region where the solar wind is accelerated allows direct sampling of plasma and magnetic fields rather than inference from a distance.
Another mission, a European-led collaboration, combines remote sensing with in-situ measurement and has used Venus gravity assists to raise its orbital inclination — giving humanity its first views of the Sun’s polar regions, which are important because the polar magnetic field is central to the solar dynamo.
An observatory in geosynchronous orbit has provided continuous high-resolution imaging of the full solar disk across multiple wavelengths for over a decade, producing a data set of remarkable consistency for tracking how active regions develop.
Older spacecraft continue operating, including a long-serving observatory at the L1 point between Earth and the Sun, and a pair of probes placed in orbits ahead of and behind Earth to allow stereoscopic views.
On the ground, a new generation of solar telescopes resolves features on the photosphere at scales of a few tens of kilometres, revealing convection cells and magnetic structure in unprecedented detail.
What has come out of it
Close-in observations have found magnetic field reversals in the solar wind — sharp S-shaped kinks nicknamed switchbacks — that were not anticipated and are still being interpreted. They may be connected to the process that accelerates the wind in the first place.
The first images of the solar poles have begun to constrain dynamo models that previously relied on extrapolation.
Continuous full-disk monitoring has substantially improved the ability to identify active regions likely to produce eruptions, though flare prediction remains probabilistic rather than deterministic.
The forecasting gap
Space weather prediction is roughly where terrestrial weather forecasting was several decades ago: useful, improving, and frequently wrong about specifics.
The fundamental limitation is warning time. A coronal mass ejection’s effect on Earth depends heavily on the orientation of its magnetic field, and that is not reliably measurable until the cloud passes a spacecraft at the L1 point — about a million and a half kilometres upstream, which translates to something like thirty to sixty minutes of notice.
Extending that lead time is a significant motivation for missions that would monitor the Sun from off the Sun-Earth line, viewing eruptions from the side rather than head-on. For grid operators, the difference between thirty minutes and several hours is the difference between reacting and preparing.