Starship Asterisk*

Exceptional Rocket Waves Destroy Sun Dog

Explanation: What created those rocket waves, and why did they destroy that sun dog? Close inspection of the above image shows not only a rocket rising near the center, but unusual air ripples around it and a colorful sundog to the far right. The rocket, carrying the Solar Dynamics Observatory (SDO), lifted off two weeks ago from Cape Canaveral, Florida, USA into a cold blue sky. The SDO is designed to observe the Sun continuously over the next several years, exploring the Sun's atmosphere at high resolution and fast time scales. The air ripples -- seen about one minute after launch -- were unexpected, as was the sudden disappearance of the sundog after the ripples passed. Noticed and recorded by several onlookers, there has been much speculation about the origin of the ripples. An ongoing discussion about them can be joined here in APOD's discussion board the Asterisk. A leading hypothesis holds that the ripples resulted from a sonic boom created as the rocket broke the sound barrier, which then jumbled a thin layer of ice crystals that were aligned to create the sundog. Lingering questions include why other rocket launches don't produce air ripples as noticeable, and why the ripples appeared more prominent above the rocket. If you know of images of any other aircraft or spacecraft that have produced similar air ripples, please post them to the discussion thread -- they may be help create a better understanding of the effect.

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I think these visible white rings are "rings of eddies"

As the bow shock of the missile passes through levels of relatively different pressure/humidity, these eddy rings form as a result. As a snowball gains substance by rolling down a hill, this mass of air pushed along by the rocket gains mass (as does the size of the 'bow shock') as it accelerates and gains altitude.

I suspect that, now that we are looking for them, we will detect such rings consistently.

Based on the operations manual for the Atlas V 401, the average acceleration over the first 75 seconds of flight is 1.4 g; offsetting for the Earth's 1 g, that gives an acceleration of 3.9 m/s^2. So at 75 seconds, when this effect occurs, the rocket is at a height of 11 km, with a speed of 292 m/s. That is precisely Mach 1 at this height. I wasn't able to find any information on the height of the clouds at launch time, but 11 km is right where you'd expect to find cirrus clouds.

What I think we are seeing is the exact moment where the sound barrier is broken, which by coincidence is happening just as the rocket is passing through the thin cirrus layer. At that point, the shock front is almost flat (disc-like) at the top of the rocket, and hemispherical behind it. Right at the moment the shock wave first forms, all sorts of turbulent modes are possible. I think this is causing the multiple waves seen in the image. If the clouds had been just a little higher, the conical shock wave would have been stable and we'd see only a single wave spreading outwards.

The rocket's path is perpendicular to the cloud sheet, so the intersection of the shock wave and the cloud is circular. It appears elliptical in the image because the viewpoint from the ground is to the side of the rocket path.

One other possibility, which I consider less likely, is that this is an acoustic effect caused by the rocket engine. The spacing of the ripples looks to be on the order of 50 m. This corresponds to an acoustic frequency of about 6 Hz. The operations manual only characterizes the acoustics down to 32 Hz, but the plot makes it seem as if there is plenty of energy below this. Perhaps some sort of resonance created the ripples- maybe even an interaction between the engine acoustics and the shock wave.

I'm thinking the ripples aren't really "more apparent above the rocket," but the whole circular shape is behind the rocket and it just appears above due to our viewing angle. (And possibly it would appear as a full circle except there may not be any clouds in the "bottom" half of the circle.)

I couldn't agree more with Chris Petersons explanation. That was really well described, thank you.

I have seen this video so many times since it was posted on the web about a week ago (due to pure fascination on my behalf), and I agree that the Sonic Boom ripples must have coincidentally occurred at the exact height of the aligned ice crystals that were causing the Sundog. This sonic ripple would have been more than enough to knock the precisely aligned ice crystals out of alignment.
It also appears in the video that the linear spread of the waves stays at the same altitude once the rocket has passed that height and continued on upwards, leaving the ripples to continue outward from the point of entry to that particular thin slice of the atmosphere.

To address the further discussion points;
"...why other rocket launches don't produce air ripples as noticeable, and why the ripples appeared more prominent above the rocket..."

I would say that the rate of acceleration and the exact conditions within that layer of the atmosphere at the point at which any spacecraft reaches the speed of sound would play a major role in the occurrence or repeat occurrence of this phenomenon.

Keep up the good work!
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Looking at the form of the ripples I would think this are rather sound waves, probably subsonic, produced by the blast of the rocket. An estimation of the wavelength may help to confirm or contradict this hypothese.
Luc

I suspect that the ripples are not "more prominent above the rocket", merely more apparent closer to the observer.

Hello,

Apart from the origin of the waves, I'm not quite sure what makes them visible. Is it cloud being compressed when it's hit by the wave, decompressed in its wake? So we're not looking at 'waves against a background of cloud' (why would you see that?) but rather part of that cloud being actively mangled by the waves? But wouldn't one expect the pattern created to remain for a while longer in that case (if you watch the video it disappears very quickly)?

Or is it impurities in the air so that a denser ring/3D-ring* of it looks different than a less dense one?

cheers ~K


* I don't know what you call these geometrically. I mean something like the full peel of a round fruit (which has nonzero thickness), and the 2D version of that.

Was this a liquid rocket? If so, could pogo oscillations cause enough of an exhaust pressure differential to result in expanding rings behind the rocket?

Are such ripples so unusual?
Certainly after a rocket launch, but ripples in the atmosphere are there all the time, we just don't see them until some effect shows them up. This site http://www.w7ftt.net/sundog1.html shows parallel ripples that are otherwise very similar.

Does the APOD pic demand a supersonic explanation? A thermocline can have a distinct border, with different densities on either side. Just like a water/air interface, some solid object splashing through will cause ripples to spread, just as shown in the APOD.

John

This is just a WAG:

I would be willing to bet that there were two shock-fronts not quite at the same point associated with the rocket passing through the thin layer of super-saturated air. When the wave-fronts of the shock waves added, they didn't do much, but when they subtracted, they decompressed the air enough forcing the dissolved water to precipitate out, leaving behind the pattern of thicker concentric rings. I see sort of in a moire pattern disrupted by the non perfectly flat layer of saturated air and other discontinuities.

The sun-dog was disrupted by the same thing as when the ice was quickly formed by the interference pattern, those crystals did not have the same consistent shape as those formed naturally in a consistent shape, and scattered the light in a more random fashion.

This is a "cloud chamber" in action, leaving a trail of the passing sound/shock waves.

I too agree that Chris Peterson's assessment. I think he made the calculations of the rocket's speed a little more complicated than necessary though, have a look at the last video from the APOD post;
http://www.youtube.com/watch?v=4qvX9ZeSys8
You can clearly see a Prandtl–Glauert cloud form at exactly the same time as the ripples appear indicating that it was indeed travelling about mach 1 at that time. (1:50 in this clip).

So Chris Peterson, are you saying we are seeing a flat 2-D slice of the shock wave? I was thinking we were looking up at an angle through a portion of a cone in 3-D, viewing the inside of the cone.

I noticed a couple of other things in http://www.youtube.com/watch?v=iwRfcrxFVEI. For one, there's some cloud that seems to be "swept away" away by the shock waves. If you look around 1:45, at the top of the frame, much of this visible cloud just vanishes as the shock waves appear.

Also, there's a new cloud structure that forms, slanting upward and to the left in that video. It's like a slanted line of cloud that appears to the left of the rocket, initially somewhat ahead of it. It appears to move to the left as the rocket moves by.

And finally, by 1:55 in that video, when the camera pulls back, you can see the sun dog again. Because of the change in zoom, I can't really tell if this is simply a dimmer version, what was left, or if it was starting to reform already. It also looks like the cloud that was "swept away" is starting to reappear.

So now on to wild guesses - could this caused by ice-crystals in the cloud being temporarily aligned by the shock wave? Suppose the shock wave tended to align flat ice crystals perpendicularly to the cone of the shock wave - in that case, we'd see shock waves in the side of the cone that we were viewing closer to "edge-on", while the farther side of the cone that we're viewing more perpendicularly would tend to disappear.

Edited to add that around 1:55 you do hear the announcer say "vehicle is now supersonic". Just in case there was a need for more proof of it just passing Mach 1.
tom

In a frame from the movie link that jaseoldboss provided there is indeed a new feature of some phenomena that apparently shoots up faster than the rocket just as the ripples form. Very interesting, it occurs at ~10 minutes, right at the end.

Redbone wrote: In a frame from the movie link that jaseoldboss provided there is indeed a new feature of some phenomena that apparently shoots up faster than the rocket just as the ripples form. Very interesting, it occurs at ~10 minutes, right at the end.

That looks totally different than the structure I saw in the video I posted. But maybe it's the same thing from a different angle, as it does look like some clouds form around that dark mark in your video. Very odd. Also, in your angle, shock waves seem to appear on the sides, but not above the rocket.

tom

Chris Peterson wrote:One other possibility, which I consider less likely, is that this is an acoustic effect caused by the rocket engine. The spacing of the ripples looks to be on the order of 50 m. This corresponds to an acoustic frequency of about 6 Hz. The operations manual only characterizes the acoustics down to 32 Hz, but the plot makes it seem as if there is plenty of energy below this. Perhaps some sort of resonance created the ripples- maybe even an interaction between the engine acoustics and the shock wave.

I like the idea that strong low frequency acoustics generate
a quasi periodic refraction effect in the lower air that
distorts our observation of the cirrus clouds (& sun dog) above.

• http://en.wikipedia.org/wiki/Sound
  
  Rocket launch equipment acoustic tests: L(oudness) ~ 165 Decibels
  
  Acoustic pressure fluctuation ~ 20 µPa x 10^[L/20] ~ 3500 Pa ~ 0.0035 atm
  
  A rocket launch acoustic 0.0035 atm. pressure fluctuation is roughly
  equivalent to a 10ºC temperature density fluctuation.
  
  Ergo: Rocket launch acoustical pressure "mirages" ~ 10ºC temperature fluctuation "mirages"

Only after the quasi periodic lower air refraction effect takes place does
a turbulence shock wave permanently disrupt the aligned cirrus crystals.

The disruption of any atmospheric phenomenon or occurrence seems to be related energy. A rocket engine produces a huge amount of heat. The friction from it passing through the air must generate is sizeable static charge. The decibel level from the rocket blast must be considerable (Sonic Boom comes to mind.). Is it possible anyone or a combination of these may have caused the ripple that disrupted the Sun Dog in question?

Hey guys!

I find all the talk about breaking the sound barrier and sonic booms confusing. If the rocket was going at mach 1 or faster, there would be no shock waves travelling ahead of the rocket - only in a cone shape behind it. Visually, that seems not to be the case.

Take a look at films made from the air of large bomb blasts. You will see ripples of shock waves emanating from the explosion, so this should not be a mystery.

Is it possible that this type of shock wave occurs all the time but just isn't noticable in clear air? Is there any kind of instrument like an ultra-fast infrared camera or sensitve air-wave density tracker (with apologies to all - I'm inventing here) that could always be aimed at every rocket launch to see if this ripple effect could be proven to be the norm - even in clear air?

Thomas A. Fine wrote:So Chris Peterson, are you saying we are seeing a flat 2-D slice of the shock wave? I was thinking we were looking up at an angle through a portion of a cone in 3-D, viewing the inside of the cone.

The leading edge of the shock wave when the rocket is at Mach 1 is a very shallow cone. The trailing edge propagates downwards as a hemisphere. While the shock structure itself is 3D, I think we are only seeing a 2D slice for the simple reason that the cloud layer is just a thin sheet. We only see the shock wave where it interacts with the cloud- what's going on above and below the cloud is invisible.

From the Wikipedia article on sonic booms:

biddie67 wrote:Is it possible that this type of shock wave occurs all the time but just isn't noticable in clear air? Is there any kind of instrument like an ultra-fast infrared camera or sensitve air-wave density tracker (with apologies to all - I'm inventing here) that could always be aimed at every rocket launch to see if this ripple effect could be proven to be the norm - even in clear air?

There are techniques for recording air density variations in natural settings, although Schlieren photography and similar methods are much easier to use in a controlled setting. There are lots of shock wave images made in wind tunnels and test ranges.

gALEXy wrote:Hey guys!

I find all the talk about breaking the sound barrier and sonic booms confusing. If the rocket was going at mach 1 or faster, there would be no shock waves travelling ahead of the rocket - only in a cone shape behind it. Visually, that seems not to be the case.

At the time the ripples appear the rocket is wel behind (seen from the ground) the cirrus clouds. In the accompagning picture one sees the waves and the shadow of the rocket and the contrail it made on the cirrus clouds.

Luc

gALEXy wrote:Hey guys!

I find all the talk about breaking the sound barrier and sonic booms confusing. If the rocket was going at mach 1 or faster, there would be no shock waves travelling ahead of the rocket - only in a cone shape behind it. Visually, that seems not to be the case.

Yes, but by the time this happens, the view is probably more behind the rocket than beside it. So the shock waves that appear to be ahead of the rocket are actually off to the side of the rocket.

tom

Air pressure density ripples, Changing refraction. Slightly . ?



Mark