We are used to the sun creating shadows when it hits other objects, but it casts its own when viewed through a cosmic ray detector. Now it seems monitoring this shadow could help us to prepare for solar bursts.
Cosmic rays are high-energy, charged particles that constantly stream through the solar system, but are blocked in part by the magnetic field of the sun's corona. The result is a patch that is low in cosmic rays – a shadow – that appears on any cosmic ray detector on Earth.
Michihiro Amenomori of Hirosaki University in Japan and colleagues wondered if the shadow might vary with the solar cycle. They used a cosmic ray detector in Tibet to track changes in the sun's shadow between 1996 and 2009, slightly longer than the sun's full 11-year cycle of activity.
The shadow's intensity varied with the solar cycle, and was most pronounced during the solar minimum, when the sun is at its least energetic. The team conclude that when the sun peaks, the coronal magnetic field is more active too and starts deflecting cosmic rays towards Earth, rather than just blocking them.
Space weather forecast
With the help of the shadow, other variations in the coronal magnetic field might be detectable, such as those that cause coronal mass ejections.
These outbursts can fry satellites, and electronics on Earth, but are tough to predict using the light emitted by the corona – although astronomers can track the sun's surface magnetic field this way. "As the coronal magnetic field observed optically is weak compared with that from the solar surface, one needs to go close to the sun," says team member Masato Takita of the University of Tokyo, Japan. "However, a spacecraft would be burned out before approaching the coronal region."
A detailed understanding of the coronal magnetic field could provide an early warning system for Earth and orbiting spacecraft, says Mathew Owens at the University of Reading, UK, who was not involved in the research. "You can potentially get three or four days' warning," he says, though he adds the information from the sun's shadow is not yet complete enough for space weather predictions.
Journal reference: Physical Review Letters, doi.org/m5m
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