Two solar observatories have joined forces to witness a rare phenomenon: a solar tsunami.
Immediately after an eruption of a coronal mass ejection in the lower corona (the sun’s multimillion degree atmosphere), observations by NASA’s Solar Dynamics Observatory (SDO) and Japan’s Hinode solar observatory tracked a vast wave blast across the upper plasma layers of the sun. By doing so, scientists were able to accurately measure the strength of the sun’s magnetic field and test a method that may ultimately help space weather forecasters predict the characteristics of coronal mass ejections (CMEs).
This particular tsunami — technically known as an “EIT wave” after the EIT instrument on board the veteran NASA/ESA Solar and Heliospheric Observatory (SoHO) that made their discovery — was clocked speeding at up to 1,000 kilometers (620 miles) per second through the highly magnetized, searingly hot solar plasma.
EIT waves are of huge importance to solar physics. Understanding the nature of the lower corona and the interface between the sun’s atmosphere and interior can be difficult; analysis of this extreme environment is limited by the hot plasma and complex magnetic fields bubbling through the solar surface. You can’t simply send a probe into the lower corona, it would be incinerated. So scientists depend on remote observations by an armada of space observatories to spot rare phenomena such as tsunamis to help gauge the lower coronal environment.
“These EIT waves are quite tricky — they’re very random and they’re relatively rare,” David Long, of University College London (UCL) Mullard Space Science Laboratory and lead investigator of the research, told BBC News. “We need to be in the right place at the right time; this has been a long time coming.” The study has been accepted for publication in the journal Solar Physics.
The SDO was used to keep track of the extreme ultraviolet emissions from the million-degree plasma the wave was propagating through and the EIS instrument on board Hinode tracked the density of the plasma. As the speed and shape of EIT waves is strongly affected by the sun’s magnetic field, these accurate measurements were used to deduce the magnetic environment the wave was traveling through.
“We’ve demonstrated that the sun’s atmosphere has a magnetic field about ten times weaker than a normal fridge magnet,” said Long in a UCL press release. Weaker it may be, but the sun’s magnetic field is extensive, dominating the entire solar system.
These measurements have practical implications for Earth, too.
As we become more and more dependent on technology in our daily lives, civilization is becoming increasingly vulnerable to the sun’s ‘temper tantrums.’ Every 11 years or so, the sun winds up into a highly stressed state; a period known as “solar maximum.” During these periods of intense solar activity (as the sun is now), we can expect an amplification in solar wind ferocity and an increase in frequency of solar flares and CMEs — all of which can interfere with communications and damage satellites, for example.
CMEs are of special concern, as when the huge magnetized bubbles of highly charged particles interact with our planet’s magnetic field they can generate huge currents through the atmosphere, threatening entire power grids. Therefore, accurate prediction and characterization of incoming CMEs is paramount.
“These waves are quite important because they’re associated with CMEs that fire plasma out into the heliosphere, toward the Earth,” added Long. “Generally we see them when there’s a CME coming straight at us — but when it’s coming straight at us then it’s quite difficult to measure how fast it’s coming at us or how strong it is.
“So by looking at these waves, we should be able to infer how powerful these CMEs are going to be.”
Image: Series of observations from the SDO and Hinode showing the propagation of the EIT wave across the sun. Credit: NASA/SDO/JAXA