The Neutral Hydrogen Gun: A New Solar Flare Phenomenon

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In 2006, one of the largest solar flares observed for 30 years erupted, saturating X-ray cameras on board observatories orbiting Earth. The December 5th event was a powerful X-ray flare, registering “X9” on the scale of powerful “X-class” flares. Even though flares weighing in at X20+ have been observed, the X9 is a rare event all the same. However, this 2006 flare is fast becoming known not only for its energetic characteristics. Shortly after the flare, solar astronomers expected to see a flood of interplanetary ions being ejected by the Sun. However, they detected something else; not only a particle they weren’t expecting, but a particle that shouldn’t be there…

When a blast the size of a hundred million nuclear bombs detonates, you wouldn’t expect anything to be intact at ground-zero, would you? In the case of solar flares, a huge amount of magnetic energy is unleashed through a process known as reconnection, quickly accelerating and heating solar plasma. Depending on the conditions, different solar flare energies are possible, but in the case of the Dec. 5th 2006 flare, solar plasma was rapidly and violently accelerated, unleashing X-ray radiation. At the flare site, within the knotted and twisted magnetic flux, plasma temperatures can soar to 10-20 million Kelvin (occasionally, for the biggest flares, 100 million Kelvin). In these conditions, nothing stays intact. Any atoms in the local area become stripped of their electrons, leaving an energetic soup of ionized particles (like protons and helium nuclei) and electrons.

So you can imagine the surprise of a group of solar physicists using data from the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft orbiting the Sun (one ahead of the Earth’s orbit, and one behind), when they detected a jet of pure neutral hydrogen atoms emanating from the flare.

“We’ve detected a stream of perfectly intact hydrogen atoms shooting out of an X-class solar flare,” says Richard Mewaldt of Caltech,. “What a surprise! These atoms could be telling us something new about what happens inside flares.”

“No other elements were present, not even helium (the sun’s second most abundant atomic species). Pure hydrogen streamed past the spacecraft for a full 90 minutes.”

Measurements of radio emissions indicated that a shock wave had been generated low in the solar atmosphere during the flare, revealing the interaction of incoming solar ions. Physicists waited for an hour for the incoming ions (the time calculated for ions to travel from the Sun to the STEREO spacecraft), but instead the stream of neutral atoms arrived. The stream of hydrogen lasted for 90 minutes, and then it went quiet for 30 minutes only for the expected ions to flood the sensors as predicted.

At first glance, the impossible had been achieved; a solar flare had somehow manufactured, then sorted the neutral hydrogen from the soup of plasma and shot it into space. But this produced a very perplexing puzzle: neutral hydrogen, lots of it, has been detected as a result of a solar flare, and yet these atoms cannot exist in the extreme environment surrounding the flare site. What gives?

Actually, these hydrogen atoms were not generated inside the flare, they formed after the flare as the products from the explosion spiralled into interplanetary space.

“We believe they began their journey to Earth in pieces, as protons and electrons,” said Mewaldt. “Before they escaped the sun’s atmosphere, however, some of the protons recaptured an electron, forming intact hydrogen atoms. The atoms left the sun in a fast, straight shot before they could be broken apart again.”

The reason why these neutral atoms appeared at STEREO faster than the ion cloud is because the neutral hydrogen did not get influenced (slowed down) by the Sun’s magnetic field; the atoms shot out, in a straight line, rather than being deflected by magnetic flux. And how did they form? Physicists believe the protons “recaptured” the free electrons in the space between the flare and detector through the well known mechanisms radiative recombination and charge exchange.

Now, solar physicists want to replicate these findings to see whether these hydrogen jets are a common feature of solar flares… but they might have to wait a while, the Sun is still enjoying its quiet spell...

Source: NASA