APOD: Northern Lights from the Stratosphere (2024 Jan 18)

[APOD Robot]

Northern Lights from the Stratosphere

Explanation: Northern lights shine in this night skyview from planet Earth's stratosphere, captured on January 15. The single, 5 second exposure was made with a hand-held camera on board an aircraft above Winnipeg, Canada. During the exposure, terrestrial lights below leave colorful trails along the direction of motion of the speeding aircraft. Above the more distant horizon, energetic particles accelerated along Earth's magnetic field at the planet's polar regions excite atomic oxygen to create the shimmering display of Aurora Borealis. The aurora's characteristic greenish hue is generated at altitudes of 100-300 kilometers and red at even higher altitudes and lower atmospheric densities. The luminous glow of faint stars along the plane of our Milky Way galaxy arcs through the night, while the Andromeda galaxy extends this northern skyview to extragalactic space. A diffuse hint of Andromeda, the closest large spiral to the Milky Way, can just be seen to the upper left.

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[johnnydeep]

Despite this being yet another Earth based scene, I think the photo is impressive in that a hand-held camera was able to keep the stars - and Andromeda! - in focus for a long 5 second exposure. But I'm not a photographer, so perhaps this is easier than I imagine.

[Chris Peterson]

Despite this being yet another Earth based scene, I think the photo is impressive in that a hand-held camera was able to keep the stars - and Andromeda! - in focus for a long 5 second exposure. But I'm not a photographer, so perhaps this is easier than I imagine.

I doubt it was a single 5-second exposure. That's how the camera presented it externally, but inside, it was taking a series of much shorter shots and aligning and combining them. That's how the night vision mode works on most cameras and phone cameras.

[johnnydeep]

Despite this being yet another Earth based scene, I think the photo is impressive in that a hand-held camera was able to keep the stars - and Andromeda! - in focus for a long 5 second exposure. But I'm not a photographer, so perhaps this is easier than I imagine.

I doubt it was a single 5-second exposure. That's how the camera presented it externally, but inside, it was taking a series of much shorter shots and aligning and combining them. That's how the night vision mode works on most cameras and phone cameras.

Very interesting. So these modern digital cameras (only those in smart phones but not perhaps in larger dedicated 35 mm digital cameras?) don't have "shutters" that can open for variable periods of continuous time? Is every exposure length just a multiple of the only available shortest exposure length? But perhaps I'm not understanding this very well.

[Chris Peterson]

Despite this being yet another Earth based scene, I think the photo is impressive in that a hand-held camera was able to keep the stars - and Andromeda! - in focus for a long 5 second exposure. But I'm not a photographer, so perhaps this is easier than I imagine.

I doubt it was a single 5-second exposure. That's how the camera presented it externally, but inside, it was taking a series of much shorter shots and aligning and combining them. That's how the night vision mode works on most cameras and phone cameras.

Very interesting. So these modern digital cameras (only those in smart phones but not perhaps in larger dedicated 35 mm digital cameras?) don't have "shutters" that can open for variable periods of continuous time? Is every exposure length just a multiple of the only available shortest exposure length? But perhaps I'm not understanding this very well.

Phone cameras and most point-and-shoots have electronic shutters. Most DLSR/mirrorless cameras still have mechanical shutters, but that is starting to change. But even with a mechanical shutter, multiple exposures can be made while the shutter is open. And most cameras with the ability to take multiple shots and combine them will pick the number of shots and the subexposure time based on some internal algorithm that is assessing the conditions and presumed intent.

[johnnydeep]

I doubt it was a single 5-second exposure. That's how the camera presented it externally, but inside, it was taking a series of much shorter shots and aligning and combining them. That's how the night vision mode works on most cameras and phone cameras.

Very interesting. So these modern digital cameras (only those in smart phones but not perhaps in larger dedicated 35 mm digital cameras?) don't have "shutters" that can open for variable periods of continuous time? Is every exposure length just a multiple of the only available shortest exposure length? But perhaps I'm not understanding this very well.

Phone cameras and most point-and-shoots have electronic shutters. Most DLSR/mirrorless cameras still have mechanical shutters, but that is starting to change. But even with a mechanical shutter, multiple exposures can be made while the shutter is open. And most cameras with the ability to take multiple shots and combine them will pick the number of shots and the subexposure time based on some internal algorithm that is assessing the conditions and presumed intent.

Ok, but do any of these digital cameras take "true" long exposures with the shutter open, constantly allowing photons to "accumulate" in the CCD cells (as opposed to summing up multiple shorter exposures made during a prolonged open shutter)?

[Chris Peterson]

Very interesting. So these modern digital cameras (only those in smart phones but not perhaps in larger dedicated 35 mm digital cameras?) don't have "shutters" that can open for variable periods of continuous time? Is every exposure length just a multiple of the only available shortest exposure length? But perhaps I'm not understanding this very well.

Phone cameras and most point-and-shoots have electronic shutters. Most DLSR/mirrorless cameras still have mechanical shutters, but that is starting to change. But even with a mechanical shutter, multiple exposures can be made while the shutter is open. And most cameras with the ability to take multiple shots and combine them will pick the number of shots and the subexposure time based on some internal algorithm that is assessing the conditions and presumed intent.

Ok, but do any of these digital cameras take "true" long exposures with the shutter open, constantly allowing photons to "accumulate" in the CCD cells (as opposed to summing up multiple shorter exposures made during a prolonged open shutter)?

Sure. All dedicated cameras with mechanical shutters (or even the few newest with only electronic shutters) can take ordinary long exposures, or single exposures made from just one frame. The multiple exposure modes are for night vision, for "best of" shots (like making sure everyone's eyes are open), and for HDR.

[johnnydeep]

Phone cameras and most point-and-shoots have electronic shutters. Most DLSR/mirrorless cameras still have mechanical shutters, but that is starting to change. But even with a mechanical shutter, multiple exposures can be made while the shutter is open. And most cameras with the ability to take multiple shots and combine them will pick the number of shots and the subexposure time based on some internal algorithm that is assessing the conditions and presumed intent.

Ok, but do any of these digital cameras take "true" long exposures with the shutter open, constantly allowing photons to "accumulate" in the CCD cells (as opposed to summing up multiple shorter exposures made during a prolonged open shutter)?

Sure. All dedicated cameras with mechanical shutters (or even the few newest with only electronic shutters) can take ordinary long exposures, or single exposures made from just one frame. The multiple exposure modes are for night vision, for "best of" shots (like making sure everyone's eyes are open), and for HDR.

Ok, thanks. And I assume there must be good reasons for doing it that way, which result in better images in those special case situations.

[Chris Peterson]

Ok, but do any of these digital cameras take "true" long exposures with the shutter open, constantly allowing photons to "accumulate" in the CCD cells (as opposed to summing up multiple shorter exposures made during a prolonged open shutter)?

Sure. All dedicated cameras with mechanical shutters (or even the few newest with only electronic shutters) can take ordinary long exposures, or single exposures made from just one frame. The multiple exposure modes are for night vision, for "best of" shots (like making sure everyone's eyes are open), and for HDR.

Ok, thanks. And I assume there must be good reasons for doing it that way, which result in better images in those special case situations.

Well, the only way to do handheld night vision is to take short enough exposures to avoid motion blur, and enough of them to add up to a long total exposure time. And the only way to pick a "best" shot is to have a burst sequence. And HDR is achieved by combining two or more shots made with different exposure times to get more dynamic range (the longer shots properly expose shadows, the shorter ones properly expose highlights).

[johnnydeep]

Sure. All dedicated cameras with mechanical shutters (or even the few newest with only electronic shutters) can take ordinary long exposures, or single exposures made from just one frame. The multiple exposure modes are for night vision, for "best of" shots (like making sure everyone's eyes are open), and for HDR.

Ok, thanks. And I assume there must be good reasons for doing it that way, which result in better images in those special case situations.

Well, the only way to do handheld night vision is to take short enough exposures to avoid motion blur, and enough of them to add up to a long total exposure time. And the only way to pick a "best" shot is to have a burst sequence. And HDR is achieved by combining two or more shots made with different exposure times to get more dynamic range (the longer shots properly expose shadows, the shorter ones properly expose highlights).

Ok, cool - thanks again!

[Roy]

Nobody wants to talk about the picture, just about the camera? Why is the red fluorescence higher than the green fluorescence? Are we seeing different excited atoms, or the same atoms at different levels of ionization? Air is 78% nitrogen, 21% oxygen, 1% argon, 4 hundredths of a percent CO2, maybe a little water vapor. So what is so stunningly visible?

[Chris Peterson]

Nobody wants to talk about the picture, just about the camera? Why is the red fluorescence higher than the green fluorescence? Are we seeing different excited atoms, or the same atoms at different levels of ionization? Air is 78% nitrogen, 21% oxygen, 1% argon, 4 hundredths of a percent CO2, maybe a little water vapor. So what is so stunningly visible?

The color that oxygen emits when it is ionized depends on its concentration and on the energy of the particles that ionize it. Higher energy particles trigger red emissions at higher altitudes, while lower energy particles descend deeper into the atmosphere and create green emissions.

[Skypointer]

Ok, thanks. And I assume there must be good reasons for doing it that way, which result in better images in those special case situations.

Well, the only way to do handheld night vision is to take short enough exposures to avoid motion blur, and enough of them to add up to a long total exposure time. And the only way to pick a "best" shot is to have a burst sequence. And HDR is achieved by combining two or more shots made with different exposure times to get more dynamic range (the longer shots properly expose shadows, the shorter ones properly expose highlights).

Ok, cool - thanks again!

The image is a true 5s exposure with a Canon EOS-R7 a Canon 0.71x Speedbooster and a Tamron SP 15-30mm f/2.8 lens.

Pressing the camera against the flight deck window and supprting my elbow on window frame, in connection with the camera IBIS and the stabilized lens, helped to capture a few reasonably sharp exposures out of a sequence of over a dozen. The stars in the sharpest exposure were still were slightly elongated, but deconvolution was able make them pinpoint.

[Chris Peterson]

Well, the only way to do handheld night vision is to take short enough exposures to avoid motion blur, and enough of them to add up to a long total exposure time. And the only way to pick a "best" shot is to have a burst sequence. And HDR is achieved by combining two or more shots made with different exposure times to get more dynamic range (the longer shots properly expose shadows, the shorter ones properly expose highlights).

Ok, cool - thanks again!

The image is a true 5s exposure with a Canon EOS-R7 a Canon 0.71x Speedbooster and a Tamron SP 15-30mm f/2.8 lens.

Pressing the camera against the flight deck window and supprting my elbow on window frame, in connection with the camera IBIS and the stabilized lens, helped to capture a few reasonably sharp exposures out of a sequence of over a dozen. The stars in the sharpest exposure were still were slightly elongated, but deconvolution was able make them pinpoint.

Cool, thanks for the info. Stabilized lenses are wonderful.

[johnnydeep]

Ok, cool - thanks again!

The image is a true 5s exposure with a Canon EOS-R7 a Canon 0.71x Speedbooster and a Tamron SP 15-30mm f/2.8 lens.

Pressing the camera against the flight deck window and supporting my elbow on window frame, in connection with the camera IBIS and the stabilized lens, helped to capture a few reasonably sharp exposures out of a sequence of over a dozen. The stars in the sharpest exposure were still were slightly elongated, but deconvolution was able make them pinpoint.

Cool, thanks for the info. Stabilized lenses are wonderful.

That Canon EOS-R7 is a certainly a very nice - and expensively professional - camera, but I don't know enough to understand the specs (https://www.usa.canon.com/shop/p/eos-r7) and how they may relate to what you were saying about adding together multiple exposures taken automatically while the shutter remains open. So, based on what Skypointer posted about the camera hardware and features in effect for this image, was this camera doing that in this case?

[Chris Peterson]

The image is a true 5s exposure with a Canon EOS-R7 a Canon 0.71x Speedbooster and a Tamron SP 15-30mm f/2.8 lens.

Pressing the camera against the flight deck window and supporting my elbow on window frame, in connection with the camera IBIS and the stabilized lens, helped to capture a few reasonably sharp exposures out of a sequence of over a dozen. The stars in the sharpest exposure were still were slightly elongated, but deconvolution was able make them pinpoint.

Cool, thanks for the info. Stabilized lenses are wonderful.

That Canon EOS-R7 is a certainly a very nice - and expensively professional - camera, but I don't know enough to understand the specs (https://www.usa.canon.com/shop/p/eos-r7) and how they may relate to what you were saying about adding together multiple exposures taken automatically while the shutter remains open. So, based on what Skypointer posted about the camera hardware and features in effect for this image, was this camera doing that in this case?

That this was a simple 5-second exposure, with the camera mechanically stabilized against the plane window and further stabilization provided inside the camera lens itself, by moving optics.

[johnnydeep]

Cool, thanks for the info. Stabilized lenses are wonderful.

That Canon EOS-R7 is a certainly a very nice - and expensively professional - camera, but I don't know enough to understand the specs (https://www.usa.canon.com/shop/p/eos-r7) and how they may relate to what you were saying about adding together multiple exposures taken automatically while the shutter remains open. So, based on what Skypointer posted about the camera hardware and features in effect for this image, was this camera doing that in this case?

That this was a simple 5-second exposure, with the camera mechanically stabilized against the plane window and further stabilization provided inside the camera lens itself, by moving optics.

Ok, so this was a true single exposure, not many.

[johnnydeep]

Nobody wants to talk about the picture, just about the camera? Why is the red fluorescence higher than the green fluorescence? Are we seeing different excited atoms, or the same atoms at different levels of ionization? Air is 78% nitrogen, 21% oxygen, 1% argon, 4 hundredths of a percent CO2, maybe a little water vapor. So what is so stunningly visible?

The color that oxygen emits when it is ionized depends on its concentration and on the energy of the particles that ionize it. Higher energy particles trigger red emissions at higher altitudes, while lower energy particles descend deeper into the atmosphere and create green emissions.

"Lower energy particles descend deeper"? Surely, that must be higher energy particles, right?

The following link has a lot more detail that was very clearly explicated - clearly enough even for me to understand!

Light Emission in Aurora

When energetic electrons strike an atom or molecule, they slow down and transfer some of their energy to that atom or molecule. The molecules can store this energy only for a very short time, and then radiate the energy away as light. Some molecules get dissociated into atoms in this process, and some molecules and atoms get ionized. At the altitude where aurora occurs, above about 62 miles (100 km), the air is thin enough that oxygen can exist in atomic form, while the air that we breathe contains only molecular oxygen. During the day, the ultraviolet sunlight splits the molecular oxygen into atoms, while at night the aurora continues this process.

When an atom or molecule emits light as a photon, to rid itself of its excess energy, that photon has a wavelength that is characteristic for that atom. We perceive wavelength as color. Laboratory experiments can reproduce these light-emitting processes by forcing a current through an evacuated glass tube that contains a small amount of a selected gas. The study of these light-emitting processes led to the understanding of atoms early in the twentieth century, and to the discovery of quantum mechanics. Because each type of atom or molecule emits colors unique to it, we can use the colors of the aurora to determine the atmospheric composition at the auroral altitude.

The time that a molecule or atom can store the energy that it gained in a collision is very short, typically between 1/1000 and less than 1/1,000,000 of a second. Atomic oxygen is one notable exception, and the excited state that causes the most common auroral emission, the green line, has a lifetime of 0.7 seconds. When an excited atom takes that long to radiate away the internally stored energy, other processes, chemical reactions or collisions, compete with the radiation process for that energy. The denser the air is, the more frequent are the collisions between the atoms and molecules. Below the altitude of about 59 miles (95 km), collisions are so frequent that the green oxygen line has no chance to be emitted. All the energy that is put into the oxygen atom is lost before the 0.7-second lifetime allows radiation. This determines the bottom edge of the green emission in aurora.

However, the auroral electrons sometimes have enough energy to give them the punch to penetrate deeper than that into the atmosphere. When that happens, only emissions with a much shorter lifetime are possible. The most abundant gas is molecular nitrogen, and it radiates promptly in deep blue and red colors. Mixing these together gives purple. The bottom edge of a green auroral curtain gets this purple color when auroral elec-trons are accelerated to very high energy (Figures 7-8).

On occasion the aurora gets a deep red color. This comes from higher altitudes, around 120-180 miles (200-300 km). It is again the oxygen atom that is responsible for this color. The oxygen atom has an excited state for this red line emission with a mean lifetime of 100 seconds, and only at very high altitudes are collisions infrequent enough to allow this radiation to be emitted (Figure 9). Since the long life-time of the oxygen red line also allows the aurora to move before it radiates, the de-tailed structure in auroral curtains is also washed out in these emissions (Figure 10).

[Chris Peterson]

Nobody wants to talk about the picture, just about the camera? Why is the red fluorescence higher than the green fluorescence? Are we seeing different excited atoms, or the same atoms at different levels of ionization? Air is 78% nitrogen, 21% oxygen, 1% argon, 4 hundredths of a percent CO2, maybe a little water vapor. So what is so stunningly visible?

The color that oxygen emits when it is ionized depends on its concentration and on the energy of the particles that ionize it. Higher energy particles trigger red emissions at higher altitudes, while lower energy particles descend deeper into the atmosphere and create green emissions.

"Lower energy particles descend deeper"? Surely, that must be higher energy particles, right?

The following link has a lot more detail that was very clearly explicated - clearly enough even for me to understand!

Light Emission in Aurora

When energetic electrons strike an atom or molecule, they slow down and transfer some of their energy to that atom or molecule. The molecules can store this energy only for a very short time, and then radiate the energy away as light. Some molecules get dissociated into atoms in this process, and some molecules and atoms get ionized. At the altitude where aurora occurs, above about 62 miles (100 km), the air is thin enough that oxygen can exist in atomic form, while the air that we breathe contains only molecular oxygen. During the day, the ultraviolet sunlight splits the molecular oxygen into atoms, while at night the aurora continues this process.

When an atom or molecule emits light as a photon, to rid itself of its excess energy, that photon has a wavelength that is characteristic for that atom. We perceive wavelength as color. Laboratory experiments can reproduce these light-emitting processes by forcing a current through an evacuated glass tube that contains a small amount of a selected gas. The study of these light-emitting processes led to the understanding of atoms early in the twentieth century, and to the discovery of quantum mechanics. Because each type of atom or molecule emits colors unique to it, we can use the colors of the aurora to determine the atmospheric composition at the auroral altitude.

The time that a molecule or atom can store the energy that it gained in a collision is very short, typically between 1/1000 and less than 1/1,000,000 of a second. Atomic oxygen is one notable exception, and the excited state that causes the most common auroral emission, the green line, has a lifetime of 0.7 seconds. When an excited atom takes that long to radiate away the internally stored energy, other processes, chemical reactions or collisions, compete with the radiation process for that energy. The denser the air is, the more frequent are the collisions between the atoms and molecules. Below the altitude of about 59 miles (95 km), collisions are so frequent that the green oxygen line has no chance to be emitted. All the energy that is put into the oxygen atom is lost before the 0.7-second lifetime allows radiation. This determines the bottom edge of the green emission in aurora.

However, the auroral electrons sometimes have enough energy to give them the punch to penetrate deeper than that into the atmosphere. When that happens, only emissions with a much shorter lifetime are possible. The most abundant gas is molecular nitrogen, and it radiates promptly in deep blue and red colors. Mixing these together gives purple. The bottom edge of a green auroral curtain gets this purple color when auroral elec-trons are accelerated to very high energy (Figures 7-8).

On occasion the aurora gets a deep red color. This comes from higher altitudes, around 120-180 miles (200-300 km). It is again the oxygen atom that is responsible for this color. The oxygen atom has an excited state for this red line emission with a mean lifetime of 100 seconds, and only at very high altitudes are collisions infrequent enough to allow this radiation to be emitted (Figure 9). Since the long life-time of the oxygen red line also allows the aurora to move before it radiates, the de-tailed structure in auroral curtains is also washed out in these emissions (Figure 10).

It's complicated. Higher energy particles can be preferentially absorbed depending on the conditions, so it's perfectly possible for lower energy ones to descend further before they interact.

[johnnydeep]

The color that oxygen emits when it is ionized depends on its concentration and on the energy of the particles that ionize it. Higher energy particles trigger red emissions at higher altitudes, while lower energy particles descend deeper into the atmosphere and create green emissions.

"Lower energy particles descend deeper"? Surely, that must be higher energy particles, right?

The following link has a lot more detail that was very clearly explicated - clearly enough even for me to understand!

Light Emission in Aurora

When energetic electrons strike an atom or molecule, they slow down and transfer some of their energy to that atom or molecule. The molecules can store this energy only for a very short time, and then radiate the energy away as light. Some molecules get dissociated into atoms in this process, and some molecules and atoms get ionized. At the altitude where aurora occurs, above about 62 miles (100 km), the air is thin enough that oxygen can exist in atomic form, while the air that we breathe contains only molecular oxygen. During the day, the ultraviolet sunlight splits the molecular oxygen into atoms, while at night the aurora continues this process.

When an atom or molecule emits light as a photon, to rid itself of its excess energy, that photon has a wavelength that is characteristic for that atom. We perceive wavelength as color. Laboratory experiments can reproduce these light-emitting processes by forcing a current through an evacuated glass tube that contains a small amount of a selected gas. The study of these light-emitting processes led to the understanding of atoms early in the twentieth century, and to the discovery of quantum mechanics. Because each type of atom or molecule emits colors unique to it, we can use the colors of the aurora to determine the atmospheric composition at the auroral altitude.

The time that a molecule or atom can store the energy that it gained in a collision is very short, typically between 1/1000 and less than 1/1,000,000 of a second. Atomic oxygen is one notable exception, and the excited state that causes the most common auroral emission, the green line, has a lifetime of 0.7 seconds. When an excited atom takes that long to radiate away the internally stored energy, other processes, chemical reactions or collisions, compete with the radiation process for that energy. The denser the air is, the more frequent are the collisions between the atoms and molecules. Below the altitude of about 59 miles (95 km), collisions are so frequent that the green oxygen line has no chance to be emitted. All the energy that is put into the oxygen atom is lost before the 0.7-second lifetime allows radiation. This determines the bottom edge of the green emission in aurora.

However, the auroral electrons sometimes have enough energy to give them the punch to penetrate deeper than that into the atmosphere. When that happens, only emissions with a much shorter lifetime are possible. The most abundant gas is molecular nitrogen, and it radiates promptly in deep blue and red colors. Mixing these together gives purple. The bottom edge of a green auroral curtain gets this purple color when auroral elec-trons are accelerated to very high energy (Figures 7-8).

On occasion the aurora gets a deep red color. This comes from higher altitudes, around 120-180 miles (200-300 km). It is again the oxygen atom that is responsible for this color. The oxygen atom has an excited state for this red line emission with a mean lifetime of 100 seconds, and only at very high altitudes are collisions infrequent enough to allow this radiation to be emitted (Figure 9). Since the long life-time of the oxygen red line also allows the aurora to move before it radiates, the de-tailed structure in auroral curtains is also washed out in these emissions (Figure 10).

It's complicated. Higher energy particles can be preferentially absorbed depending on the conditions, so it's perfectly possible for lower energy ones to descend further before they interact.

Hmm...

[alter-ego]

"Lower energy particles descend deeper"? Surely, that must be higher energy particles, right?

The following link has a lot more detail that was very clearly explicated - clearly enough even for me to understand!

It's complicated. Higher energy particles can be preferentially absorbed depending on the conditions, so it's perfectly possible for lower energy ones to descend further before they interact.

Hmm...

The energy level diagram says the excitation energy for the green auroral line is ~2x higher than the red, and the green transitions to the lower red excitation level. This diagram makes sense when density is high, collisions quench the red emission based on the 100x faster green radiative half-life.
Also, for auroras, there are no doubt high energy particles reach deep into the atmosphere. The blue N2 emissions, which occur at the lowest altitudes, have an excitation energy of ~19 eV (~4x higher than the green line).

 
I think a few factors may explain the lack of green at high altitudes: In addition to the low oxygen density, t̶h̶e̶ ̶h̶i̶g̶h̶e̶r̶ ̶p̶a̶r̶t̶i̶c̶l̶e̶ ̶e̶n̶e̶r̶g̶i̶e̶s̶ ̶h̶a̶v̶e̶ ̶s̶m̶a̶l̶l̶e̶r̶ ̶i̶n̶t̶e̶r̶a̶c̶t̶i̶o̶n̶ ̶c̶r̶o̶s̶s̶ ̶s̶e̶c̶t̶i̶o̶n̶s̶, the much longer radiative half-life for the red emission all lead to the predominant red emissions at high altitudes.

Edit: Strike the comment about collision cross section. It is not that simple.

[Ann]

It's complicated. Higher energy particles can be preferentially absorbed depending on the conditions, so it's perfectly possible for lower energy ones to descend further before they interact.

Hmm...

The energy level diagram says the excitation energy for the green auroral line is ~2x higher than the red, and the green transitions to the lower red excitation level. This diagram makes sense when density is high, collisions quench the red emission based on the 100x faster green radiative half-life.
Also, for auroras, there are no doubt high energy particles reach deep into the atmosphere. The blue N2 emissions, which occur at the lowest altitudes, have an excitation energy of ~19 eV (~4x higher than the green line).


Energy Levels of Atomic Oxygen.jpg
 
I think a few factors may explain the lack of green at high altitudes: In addition to the low oxygen density, t̶h̶e̶ ̶h̶i̶g̶h̶e̶r̶ ̶p̶a̶r̶t̶i̶c̶l̶e̶ ̶e̶n̶e̶r̶g̶i̶e̶s̶ ̶h̶a̶v̶e̶ ̶s̶m̶a̶l̶l̶e̶r̶ ̶i̶n̶t̶e̶r̶a̶c̶t̶i̶o̶n̶ ̶c̶r̶o̶s̶s̶ ̶s̶e̶c̶t̶i̶o̶n̶s̶, the much longer radiative half-life for the red emission all lead to the predominant red emissions at high altitudes.

Edit: Strike the comment about collision cross section. It is not that simple.

Thanks for pointing me to that paper on the blue N2 emission in auroras. So, a 427.8 nm emission? That would be about this color, ███.

Well, the blue aurora must be very rare, because I hardly ever see blue in any aurora photos. I googled "red green blue aurorae" and found very few auroras that looked blue at all. (Perhaps there is a hint of purple at the "left edge" of the aurora in the APOD, but that was not enough to make me interested or impressed.)

Ond of the best pictures I could find when I googled was this one:

Source: https://slideplayer.com/slide/1484800/

---

Couldn't find a photographer for this picture, and I don't know if it is legitimate.

---

Ann

[johnnydeep]

Hmm...

The energy level diagram says the excitation energy for the green auroral line is ~2x higher than the red, and the green transitions to the lower red excitation level. This diagram makes sense when density is high, collisions quench the red emission based on the 100x faster green radiative half-life.
Also, for auroras, there are no doubt high energy particles reach deep into the atmosphere. The blue N2 emissions, which occur at the lowest altitudes, have an excitation energy of ~19 eV (~4x higher than the green line).


Energy Levels of Atomic Oxygen.jpg
 
I think a few factors may explain the lack of green at high altitudes: In addition to the low oxygen density, t̶h̶e̶ ̶h̶i̶g̶h̶e̶r̶ ̶p̶a̶r̶t̶i̶c̶l̶e̶ ̶e̶n̶e̶r̶g̶i̶e̶s̶ ̶h̶a̶v̶e̶ ̶s̶m̶a̶l̶l̶e̶r̶ ̶i̶n̶t̶e̶r̶a̶c̶t̶i̶o̶n̶ ̶c̶r̶o̶s̶s̶ ̶s̶e̶c̶t̶i̶o̶n̶s̶, the much longer radiative half-life for the red emission all lead to the predominant red emissions at high altitudes.

Edit: Strike the comment about collision cross section. It is not that simple.

Thanks for pointing me to that paper on the blue N2 emission in auroras. So, a 427.8 nm emission? That would be about this color, ███.

Well, the blue aurora must be very rare, because I hardly ever see blue in any aurora photos. I googled "red green blue aurorae" and found very few auroras that looked blue at all. (Perhaps there is a hint of purple at the "left edge" of the aurora in the APOD, but that was not enough to make me interested or impressed.)

Ond of the best pictures I could find when I googled was this one:

Source: https://slideplayer.com/slide/1484800/

---

Couldn't find a photographer for this picture, and I don't know if it is legitimate.

---

Ann

I searched for a source for the image for a long time with google image search, and though there are many pages that use it, I have yet to find the original one. One reddit thread had some comments that it was clearly the result of color tweaking, but others disagreeed. That same thread had a post with a link to the claimed source, but the link was stale.

The closest I got was this https://twitter.com/discoverfinland/sta ... 2709574657, which links to a large gallery of aurora pics, but I couldn't find this image! But maybe Chris' search skills are superior to mine.

[johnnydeep]

It's complicated. Higher energy particles can be preferentially absorbed depending on the conditions, so it's perfectly possible for lower energy ones to descend further before they interact.

Hmm...

The energy level diagram says the excitation energy for the green auroral line is ~2x higher than the red, and the green transitions to the lower red excitation level. This diagram makes sense when density is high, collisions quench the red emission based on the 100x faster green radiative half-life.
Also, for auroras, there are no doubt high energy particles reach deep into the atmosphere. The blue N2 emissions, which occur at the lowest altitudes, have an excitation energy of ~19 eV (~4x higher than the green line).


Energy Levels of Atomic Oxygen.jpg
 
I think a few factors may explain the lack of green at high altitudes: In addition to the low oxygen density, t̶h̶e̶ ̶h̶i̶g̶h̶e̶r̶ ̶p̶a̶r̶t̶i̶c̶l̶e̶ ̶e̶n̶e̶r̶g̶i̶e̶s̶ ̶h̶a̶v̶e̶ ̶s̶m̶a̶l̶l̶e̶r̶ ̶i̶n̶t̶e̶r̶a̶c̶t̶i̶o̶n̶ ̶c̶r̶o̶s̶s̶ ̶s̶e̶c̶t̶i̶o̶n̶s̶, the much longer radiative half-life for the red emission all lead to the predominant red emissions at high altitudes.

Edit: Strike the comment about collision cross section. It is not that simple.

I don't quite understand all that, but thank you for the reply. I'm sure it helped others here.