By Miles Hatfield
NASA’s Goddard Space Flight Center
On Aug. 20, 2018, a Coronal Mass Ejection — an explosion of hot, electrically charged plasma erupting from the Sun — made its way towards Earth. By Aug. 26 it had hit — and aurora were visible as far south as Montana and Wisconsin in the United States.
NOAA’s DSCOVR satellite (short for Deep Space Climate Observatory) watched it all go down. DSCOVR’s measurements track magnetic field strength and direction – two aspects of a CME that determine how much it will affect Earth. These data, and the unique view of a CME that they provide, are why DSCOVR is such a useful tool for NASA’s space weather forecasters, detecting CMEs between 15 minutes to an hour before they strike Earth. Here’s a plot showing what DSCOVR saw before the CME hit it, while it was passing over, and after it passed.
- The total magnetic field strength, a combined measure of the magnetic field strength in the north-south, east-west, and towards-Sun vs. away-from-Sun directions; and
- The north-south magnetic field strength on its own.
(The units are in nano-Tesla — named after Nikola Tesla, the famous physicist, engineer and inventor).
(You might be wondering: Why single out the north-south magnetic field strength (red) if it’s included in the total magnetic field strength (blue)? Because Earth’s magnetic field also runs along the north-south direction — and this leads to a very special interaction that can make CMEs especially dangerous. More on that below.)
Let’s walk through what happened.
- Before CME hitsBefore the CME hits, DSCOVR is surrounded by the solar wind. Compared to a CME, the solar wind’s magnetic fields tend to be a little more chaotic — note the squiggly lines to the center-left of the graph. (Further to the left you can see traces of a weak CME that passed by DISCOVR on Aug 24).
Some CMEs leave the Sun so fast that they create a shock: a pile-up of solar wind plasma at their front end that creates jagged lines in DSCOVR data, like a Richter scale during an earthquake. This CME was moving comparatively slowly, so no real shock is apparent.
- The CME hitsAs the CME hits, DSCOVR’s total magnetic field readings (blue) get stronger. The north-south component (red) starts to plunge below zero — not all CME’s have a strong negative north-south component like this one did.
When the red line is above zero, that means that the magnetic field hitting DSCOVR is heading primarily in the northward direction, the same as Earth. No problem there — the incoming magnetic energy simply slides right along with Earth’s own fields. But if the red line goes below zero — the magnetic field direction heads south — then the incoming magnetic field is oppositely aligned to Earth’s. And you probably remember from childhood that opposite magnetic poles attract. If a CME has a strong southward magnetic field it can create havoc with Earth’s magnetic fields, peeling back the outward layers like taking the skin off of an orange. This allows particles to sneak past the magnetosphere’s boundary and rain down toward Earth.
The beginning of a CME looks very similar to the regular solar wind, so in real-time, just as it starts, space weather forecasters check plots of plasma density and speed (not shown here) to help determine if they’re really seeing a CME.
- DSCOVR is inside the CMEOnce inside the CME, the magnetic fields become stronger and in this case the north-south component stays largely negative (remember, that means the magnetic field is directed south and the CME can more easily disturb Earth’s fields). A CME is like an intact chunk of the Sun that has exploded outwards, taking its structured magnetic fields with it; the solar wind is more like shrapnel. Towards the end (right side) of the CME, DSCOVR was hit with a high-speed stream of solar wind — you can see that the magnetic fields start looking a little messier.
- The CME dissipates
Once the CME has passed, it leaves gusty, turbulent plasma in its wake before petering out into the solar wind. Magnetic field readings return to their normal levels.
Depending on DSCOVR’s observations and further simulations, warnings may be sent out to agencies that operate satellites. The Aug. 20 CME (it hit Earth on the Aug. 25, but CMEs are labeled based on when they left the Sun) was not fast enough to warrant these alerts. However, its strong north-south component was strong enough to generate a geomagnetic storm: on the 0 – 9 Kp scale, which measures the disturbance in Earth’s magnetic fields, this one clocked in at a 7. News outlets and blogs reported on it, and aurora sightings right after the event were documented on Aurorasaurus — NASA’s aurora-detecting citizen science collaboration, where real-time aurora sightings are scraped from the web via Twitter or reported directly on their website. Here’s a photo from one user in Fairbanks, Alaska, posted just after midnight on August 26.