The Week in Space and Physics: The Solar Wind

On the origins of the solar wind, gamma ray bursts, solar power beamed from space and a hint of the first stars

Image credit: NASA

The Parker Solar probe is a remarkable spacecraft. Every few months it barrels through the Sun’s atmosphere, daring to travel closer to our star than any other spacecraft in history. When it next does so, on Thursday this week, it will hit speeds of almost six hundred thousand kilometres per hour while facing temperatures of over one million degrees.

That, of course, is perilous. Few spacecraft would have the strength to resist the Sun’s heat or radiation. In order to survive, the Parker Solar Probe is protected by a special heat shield; one so effective that the internal temperatures of the probe rarely rise above room temperatures. Each flyby is also mercifully short, lasting only a few weeks before the spacecraft swings back towards the orbit of Venus.

All that effort is worth it. The probe has allowed scientists to explore the Sun in closer detail than ever before, helping them to answer long standing questions about its outer atmosphere - the solar corona - and its magnetic fields. These, scientists now believe, explain the origins of the solar wind, a constant stream of charged particles flying through the solar system.

On Earth this solar wind is responsible for the aurora, or northern lights. As the wind strikes our planet, some of its particles get trapped in our magnetic field, spiralling in loops high above the surface. When they later encounter the upper atmosphere, they begin to glow, creating the colourful displays sometimes seen over northern countries. Solar storms - sudden upswings in the solar winds - can push those displays further south, sometimes, in the most extreme storms, close to the equator.

Though the presence of the solar wind is well established, its origins are not. We know, of course, that it comes from the Sun, but solar scientists have long struggled to pick out the processes that create it. New research from the Parker Solar Probe may finally have solved this mystery.

Specifically, researchers focused on the fast solar wind, which streams out from the Sun at hundreds of kilometres per second. In the past researchers have found that it comes from coronal holes: areas where the Sun’s magnetic field lines extend deep into space, rather than quickly looping back to its surface. The slow solar wind, by contrast, moves more sedately and seems to come from the Sun’s equator.

Historically, two ideas have been invoked to explain how the fast solar wind is generated. One suggested that magnetic waves flowing across the Sun could be responsible, while the other speculated that reconnecting magnetic field lines might be accelerating particles outwards.

Recent data from the Parker Solar Probe seems to favour the second explanation. The probe saw short bursts of high speed particles accelerating outwards from the Sun. This, researchers think, is evidence of reconnection taking place. Both the solar wind and the Sun’s magnetic waves are probably the result of this process, they say, neatly explaining several of the Sun’s mysteries at once.

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GRBs on Camera

In October last year, telescopes around the world recorded a powerful surge of energy sweeping through the solar system. At blame was a gamma ray burst, a sudden pulse of radiation probably generated by a collapsing star. Rather fortunately, it turned out we had a telescope pointed right at the burst as it took place.

LHAASO, an observatory in the west of China, watched the burst for almost two hours. Within that time, it detected tens of thousands of high energy photons striking the Earth. Analysis of those photons, and the energy they carried, has now helped astronomers figure out what caused the burst.

Unlike other gamma ray bursts we’ve seen, last October’s event was both unusually bright and unusually long. Its brightness can partly be explained by its distance. While it probably took place two billion light years away, that’s far closer than normal. Most that we see, indeed, are at least ten billion light years away.

But its brightness is also down to the way such bursts focus their energy. Rather than exploding in all directions into space, gamma ray bursts shoot out energy in two narrow jets. Only when one of those jets is pointed directly at Earth do we see them, otherwise their power misses us altogether.

Most of the time, these jets are also rather short. Yet the event in October lasted for hours, far longer than expected. Data from LHAASO shows the jet brightening rapidly, with the initial surge in power taking less than five seconds. Subsequently, however, the jet brightened and then faded more slowly. 

Models suggest this can be explained if the jet came from a supernova. Initially the jet would have shot out from the collapsing star unhindered, but then, within a few seconds, debris from the supernova would have interfered. That would have weakened the jet, reducing its power. After a few minutes, the LHAASO data shows, the debris was enough to cause the jet to start fading away.

All this gives us the most detailed view ever obtained of such a burst. Astronomers have long been curious about them, especially as they seem to be some of the most violent events in the universe. Last year’s explosion was undoubtedly rare, but that only makes it all the more fortunate it took place at a time when we were ready to study it in detail.

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Beaming Solar Power to Earth

Space based solar power took a step closer to reality last week, as an experimental satellite beamed solar power from space to Earth for the first time. The satellite, MAPLE, deployed a small array of solar panels in orbit. Once it started generating energy, operators first demonstrated that the satellite could transmit power wirelessly, beaming energy across a short distance in space in order to light a pair of LEDs.

With that done, the satellite then pointed a beam of microwave energy towards Earth. Receivers on the ground picked up the beam and converted it back into electrical energy. Reports don’t say how much energy was received - one can assume, therefore, that it was not much - but the experiment did demonstrate, for the first time, the possibility of collecting solar power in space and beaming it back to Earth.

Some hope that this experiment will pave the way to a future of almost unlimited solar energy beamed from space. They envision placing vast arrays of solar panels into orbit, positioning them so they can efficiently catch the Sun’s energy, and then beaming all that power back to Earth. In theory, at least, this approach could generate huge amounts of clean energy.

That prospect is tempting. Space agencies around the world have started taking the idea more seriously in past years, with both China and Europe carrying out studies to assess its feasibility. The United States, too, seems to be getting interested - Congress last week passed an amendment asking NASA to investigate the topic.

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The Search for the First Stars

Has the James Webb Space Telescope spotted signs of the universe’s first stars? Claims along these lines have been gradually ramping up over the last year, as researchers use its powerful gaze to survey the edges of the observable universe.

Two new studies point to evidence of extremely early stars in far distant galaxies. In one, researchers detected signs of helium that could come from such stars. If true, those stars would be enormous - hundreds of times larger than the Sun. Yet caution is advised: the helium could also have come from other places, like early black holes.

The second paper looked at a small and distant galaxy. It seems to contain very little in the way of heavier elements - which suggests the stars within it belong to one of the first generations to form. Yet here too more study is needed to be sure. Spotting the first stars is still difficult, even with the power of the James Webb.