Astro Bob: Night of the bloody red arc / Bullseye for DART!
Monday night's aurora displayed a rare, deep-red SAR arc. We also recap NASA's successful mission to nudge an asteroid.
Yesterday evening NASA successfully plowed into the asteroid Dimorphos, one of the most exciting things I've ever watched from my humble computer. With images arriving in near-real time I felt like a passenger aboard the DART spacecraft hurtling toward the center of the bullseye. NASA's aim was precise, missing the moonlet's center by only 56 feet (17 meters) at a distance of 7 million miles (11 million km) from Earth. The agency plays a mean game of billiards.
During the final minutes, Dimorphos expanded from a featureless spot to an egg-shaped rubble pile 560 feet long. Look at all those rocks strewn higgledy piggledy on its surface. The object was likely assembled from the shattered remains of previous asteroid collisions. A spacecraft smacking it head-on at 14,000 mph? No big deal. It's been there before.
Within minutes of the impact, telescopes on Earth recorded a dust cloud expanding away from the binary asteroid. Now we watch and wait.
Dimorphos regularly passes in front of its larger partner and eclipses it, causing the system to brighten and fade. In the coming days and weeks, astronomers will carefully time those eclipses to see if the interval between them has shortened. That would be proof that the impact changed the moonlet's orbit. The mission will be considered a success if the impact shaved off at least 73 seconds. Some scientists think it'll be closer to 10 minutes.
That much excitement would normally suffice for a day, but the sun had other ideas. Although only low auroral activity was forecast for that night, the planet got a solid hit from a combination of gusty particle winds from a coronal hole and a coronal mass ejection (CME). The CME sideswiped us but bulldozed gobs of protons and electrons into Earth's magnetic field. These sparked a modest display of northern lights in the lower part of the northern sky early in the evening starting around 8-8:30 p.m. CDT.
I had planned to look at comets with my telescope but noticed some glow in the north during a quick scan of the sky. So I drove to an open field and discovered the aurora was active. I took a few photos and was ready to pack up but paused and re-extended the tripod legs to photograph a scene of the reclining Big Dipper with the northern lights.
Checking the replay on the back of the camera, I noticed a lot of red haze around the Dipper, so I opened the lens wide to capture a bigger chunk of the sky to see what was up. Bingo! The ruddy haze was part of a deep-red arc that spanned the entire northern sky. It started at the northwestern horizon, passed roughly midway across the north and continued to the eastern horizon.
It jumped out on my camera's back screen, but I strained to see any trace of it with the naked eye. Our eyes are most sensitive to green and yellow and least to red. That said, you can still see the glow of red in northern lights even if you can't make out the color. This was different. It wasn't just colorless but essentially invisible except with the camera and a time-exposure. At best I detected only the faintest swath of "haze" above the Little Dipper. I've been telling my friends it's the biggest thing I've never seen.
What I and other photographed was Stable Auroral Red (SAR) arc. Until Monday night I'd never seen or pictured one before despite observing the aurora over many years. According to one source SARs arcs weren't discovered until 1956 . Not surprising given that they're nearly invisible.
SARs arcs form about 250 miles (400 km) high, well above the aurora, which averages between 56 and 100 miles (90-150 km) altitude. Excited oxygen atoms emit a specific red wavelength of light that give the arcs their deeply luminous red color. Sometimes they're accompanied by blobs of green aurora, but at the time I observed it, the SAR arc appeared well above the visible aurora.
SARs occur during geomagnetic storms, when the sun slings material at the Earth to initiate the aurora. But whereas auroras come from bursts of energetic particles speeding down from distant regions of Earth's magnetic field (called the magnetosphere), SAR arcs form from heat trickled down from the Van Allen radiation belts that move closer to Earth during large geomagnetic storms.
The Van Allen Belts are two distinct belts filled with electrons and protons trapped from the solar wind, the constant flow of particles streaming from the sun. The inner belt starts about 250 miles (400 km) up, and the outer one extends from 10,000 miles (16,100 km) up to about 50,000 miles (80,500 km).
Intense bombardment by solar particles heats up the outer belt, which transfers that energy down to the upper atmosphere primarily over mid-northern and mid-southern latitudes. Oxygen atoms at that altitude are heated to around 5,000° (2,760° C). Duly excited, they radiate a deep-red glow.
Naturally, most of us look north during an aurora, but more and more we're learning to check other parts of the sky for related activity like STEVE . You can now add the haunting SAR arc to that list and watch for it again on Tuesday night, Sept. 27, when the aurora may return for observers in the northern border states from nightfall to about 1 a.m. CDT.
To learn more the the red arc, go to bit.ly/3SFBOBV .