Tuesday, May 21, 2024

Fuzzy Glows: See Zodiacal Light, Gegenschein, Airglow, and Aurorae – Sky & Telescope

AstronomyFuzzy Glows: See Zodiacal Light, Gegenschein, Airglow, and Aurorae - Sky & Telescope


The zodiacal light is the pointed pillar of light that extends from lower right to upper left. Composed of comet, asteroid, and Mars dust it glows in the western sky for an hour or more starting at the end of evening twilight. If you can see the Milky Way — top right in this fisheye image made on February 4, 2024 — the zodiacal light will also be visible. It resembles the Milky Way but has a smoother texture and distinctly tapered shape. The bright dot is the planet Jupiter.
Bob King

Some things are just too big to see in a telescope. As we transition from winter to spring, the zodiacal light, its cousin the gegenschein, airglow, and aurorae wanly light the way. All are best sought from dark skies save for the aurorae, which occasionally break loose during major solar storms and airbrush the heavens over metropolitan areas.

Ecliptic and zodiacal light
The ecliptic defines the plane of the solar system and meets the dusk horizon at a steep angle from late winter through mid-spring. The extra tilt lifts the zodiacal light into good view. The same situation occurs before dawn in the fall months. At other times of year, when the ecliptic encounters the horizon at a shallow angle, atmospheric extinction makes it much harder to see.
Stellarium with additions by Bob King

Although visible year-round from the tropics, the zodiacal light is best seen from mid-northern latitudes at the end of dusk from late winter through early spring. This is when the ecliptic — which represents the plane of the solar system — is most steeply tilted upward from the western horizon just after sunset. The faint dust that comprises the zodiacal light cloud lies within the same plane and straddles the ecliptic. From late February through April, it stands head and shoulders above the horizon haze, greatly enhancing its visibility.

Mars zodiacal light
The dust responsible for the zodiacal light is primarily concentrated between the Earth and just beyond Mars in bands centered on the ecliptic.
NASA, Goddard

Each zodiacal mote is about the same size as typical pollen grain and has long been assumed to originate from comet dust and the finer detritus of asteroid collisions. However, these sources are only part of the story. During its voyage to Jupiter from 2011 to 2016, cameras on NASA’s Juno probe recorded multiple flashes from impacts of interplanetary dust particles — the stuff of zodiacal light — on the craft’s solar arrays. Orbital calculations confirm they originated from Mars or possibly its moons, but how the material could achieve escape velocity from the planet remains unknown. A much smaller fraction consists of even smaller interstellar dust snatched by the Sun as it orbits about the center of the galaxy at more than 800,000 kilometers per hour. All told, there’s an estimated 20 trillion tons worth!

Seeing breath forward scattering
When lit from behind condensed breath forms a bright cloud. The Sun illuminates interplanetary dust in a similar way when we look westward after dusk and see the zodiacal “breath.”
U.S. Air Force / Airman 1st Class Nathanael Callon, public domain

Dust is a good reflector of sunlight especially when it’s backlit (forward scattered) by the Sun and viewed against the dark background of outer space — like seeing someone’s breath on a cold night in the glow of a streetlight (above). It’s even more obvious when the light source is blocked from view. This is exactly what happens when we look in the general direction of the Sun to see the zodiacal light. The horizon blocks direct sunlight and the dust lights up against the dark sky. The nearer the Sun our line of sight passes the more intense the forward scattering and the brighter the zodiacal light. That’s why it appears wide and bright low in the western sky until horizon extinction finally wipes it from view. As we gaze up and away from the Sun forward scattering falls off and the cone of light narrows and fades.

Zodiacal light in March
The zodiacal light extends well past the Pleiades shortly after dusk on March 25, 2019, from near Duluth, Minnesota. If you have trouble seeing it, slowly turn your head from side to side from south to north to better see the contrast between the dark sky and the dusty glow.
Bob King

This season I first noticed the zodiacal light in early February, towering in the western sky at the end of evening twilight. Its shape resembled a stuck-out tongue, wider at the bottom and gently narrowing to a blunt, rounded tip at the Pleiades star cluster. Others have compared it to a cone, wedge, or pointed finger. You’ll see it best starting right at the end of dusk (about 75 minutes after sunset) for about an hour.

A sweet window of Moon-free darkness begins on February 28th and continues through about March 11th. Moonlight interferes until a second window opens from March 27th through April 10th. That means lots of chances to see this beast lurking in plain sight, though it will probably mean a drive to the country. To find the darkest skies in your region check out lightpollutionmap.info. There you can click-and-drag and zoom in to find your city. Yellow and red colors indicate heavy light pollution; blues and grays are what you’re looking for.

Gegenschein and Milky Way
From the near-pristine skies of the Oklahoma Panhandle the gegenschein (glow to the left of center) was obvious with the naked eye in Leo. The Milky Way appears at right.
Bob King

I think you’ll be impressed by the zodiacal light’s enormity. It’s also brighter than you may have imagined — the lower half of the cone rivals the summer Milky Way. Near the tip it dims to invisibility where it runs into the Milky Way band in Gemini. But if your skies are eye-poppingly dark the light continues eastward as the uber-faint zodiacal band until it reaches the anti-solar point (the spot 180° opposite the Sun’s position) in the midnight sky. Here the hidden Sun illuminates each dust particle face on as if it were a tiny full moon. Backscattered, shadowless lighting enhances the band’s brightness at this spot, which we see as a small, lens-shaped region about 8°–10° wide by 5° tall called the gegenschein, or counterglow.

Gegenschein locator map
Look for the gegenschein southeast of Regulus in Leo in early March. By month’s end it slides into Virgo. As the Sun moves eastward along the ecliptic, the anti-solar point keeps pace, causing the gegenschein to do the same.
Stellarium with additions by Bob King

Faint and extremely diffuse, the gegenschein requires an even darker sky to spot. I roll my eyes around the location using averted vision to coax it into view. Also, if you wait until near midnight local time (1 a.m. daylight-saving time) it will be on the meridian and well placed for observation. Early spring’s really ideal for those living at mid-northern latitudes because the counterglow rides fairly high on the ecliptic in a relatively star-poor area of the sky. Fewer distractions increase its visibility.

Also remember that the ability to see something in the night sky depends on familiarity. It may take a while to find the gegenschein on your first try. But once you see it and know what to look for it becomes exponentially easier to coax out on subsequent attempts. Want a further naked-eye challenge? Try spotting the airglow. In photos taken from the International Space Station it’s the green “rind” surrounding the Earth seen from orbit.

Airglow and Milky Way
Streamers of green and pink airglow festoon the sky beneath the band of the Milky Way on July 10, 2023, from near Duluth, Minnesota. The streaks were also faintly visible with the naked eye but appeared colorless.
Bob King

Solar ultraviolet (UV) light is the chief instigator of airglow. It excites atoms and molecules in the upper atmosphere, which shed that excess energy as photons of varying wavelengths that we see as different colors. The dominant source of airglow arises from molecular oxygen between 90–100 kilometers altitude. Split apart by solar UV in the daytime the molecules release green light when they recombine at night. Atomic oxygen emission accounts for red airglow, while a layer of excited sodium atoms between 85 and 90 kilometers radiates yellow-orange light; ablating meteoroids provide the source material. In a process known as chemiluminescence, oxygen and nitrogen atoms collide, form nitric oxide and chuck out photons. Excited hydroxyl (OH-) radicals radiate deep red light when they react with oxygen and nitrogen. It’s a veritable university chem lab up there.

Watch different colors and types of airglow appear in this short time-lapse video taken from the International Space Station.
NASA

Cameras set to high ISOs easily record the faint banded streaks of green and red airglow in 30-second time-exposures. But your eyes may struggle to see them. Dark skies are a must, with some nights exhibiting brighter and more extensive displays than others. Scan the entire sky between altitudes of 10° to 40° where your line-of-sight slices through more air (and more airglow). At lower altitudes the feeble light is absorbed by the denser air while at higher altitudes we gaze through too little air for airglow to be obvious. To my eye it looks like faint streaks and patches that you might mistake for feebly illuminated clouds.

We’re in luck the next few years. Studies have shown that airglow is brighter around the peak of the solar cycle, which is rapidly approaching. Bursts of X-rays and UV light from flares heat the outer atmosphere, resulting in more atomic and molecular collisions that enhance the phenomenon.

Corona aurora
A sky-filling corona aurora explodes over northern Minnesota on March 23–24, 2023. My 10-mm fisheye could barely contain it!
Bob King

Airglow is also present in the daytime sky but overwhelmed by sunlight. Of course, no inventory of night glows would be complete without the aurora. March and April are particularly favorable for viewing both the northern and southern lights because Earth’s sideways orientation to the Sun around the equinoxes makes it more receptive to solar particle blasts.

Last year, observers at mid-latitudes were treated to two successive spectacular displays on March 23rd and April 23rd. Activity should pick up as we approach spring. To stay abreast of coming solar storms and their associated auroral outbreaks download the free SpaceWeatherLive app for Android or iPhone.

Auroral oval April 23 2023
During the big storm last April the northern oval, normally confined to the polar regions, expanded deeply into the U.S. This is a screen grab from the 30 Minute Forecast.
NOAA

Find a location with an unobstructed view to the north (south in the Southern Hemisphere) during daylight hours. Then when you get word of a significant aurora check the NOAA 30Minute Forecast, which displays the predicted extent of the north and south auroral ovals, two doughnut-shaped regions where aurorae are active. If the oval spreads over your region, head out! Oftentimes, a display will begin with a simple arc low in the northern sky. But if it brightens, expands, and rises, fasten your seatbelt. You might be in for a great show. Just remember to be patient.

Wishing you happy nights basking in another kind of glow — the one surrounding your success at seeing every apparition described here!



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