Saturday, August 3, 2013

Sprites 2013: Update #4

Three jets

Last night we got out on the north edge of the storm and ran 'racetrack' paths back and forth. Scientifically, it wasn't a great night. The storm was nice and productive when we arrived on station, but the activity was spread out along a line, rather than focused in a hotspot, which makes pointing the cameras much more difficult - we basically wait to see sprites in the wide angle context cameras, then point in the spot they just happened. Well when the sprites are occurring all over the place that doesn't work so well. We only captured 7 sprites to high speed, and only one had a really usable spectra. After the first two racetracks the storm died off and we didn't really see anything for the next two, so we called it a night and went home.

03aug
Flight path in green.

However, I'm thrilled about last night for other reasons: I managed to capture both jets and sprites with my dSLR! I only know of a couple examples off the top of my head where people have captured sprites in SLRs, and I can't find any examples of jets captured with SLRs - am I the first? Since jets tend to hug the top of the clouds it's understandable that they're more difficult for a ground observer to see/photograph, so it makes sense that being up in a sprite-chasing aircraft would give me a serious advantage. The photo at the top of this post is, IMO, the cream of the crop, despite the strong motion blurring (the pilot would pick that moment to hit a bump...) It shows three jets shooting out of the cloud top just above some lightning, and it's even fairly well composed. Magic. Here is a good shot of a single jet - it looks like a blue butane lighter flame sticking out of the top of the cloud:

Blue jet

And here's the sprite I got, it's the red alien jellyfish on the top right:

Sprite

Since I have a good starfield in both images, and I know we were about 200 kilometers from the electrically productive part of the storm, I can estimate the size of these things... but it's far easier to wait for the astrometry.net solver to give me a degrees/pixel solution than to figure it out myself, so we'll just wait for that.

For the technical aspect (skip this paragraph if you don't care about photography), I was shooting 1 second exposures, from a moving aircraft, handheld. I butted the camera up against the window glass and put my weight on it to get rid of most of the wobblies and light leaks, but the motion of the aircraft itself still showed up, especially when we hit a patch of turbulence  (we are, you know, flying right next to a thunderstorm). The city lights on the ground showed quite a bit more motion trailing than the stars so I cropped them out, but it was interesting to notice. For purposes of focus you think of both the city lights and the stars as being 'at infinity', but for purposes of parallax they really aren't. The instantaneous phenomena, the lightning, jets and sprites, show no motion blur. I used a 35mm f/1.8 lens held wide open, ran the camera at ISO 6400, subtracted a dark frame from all of them to get rid of the systematic noise, then ran strong noise reduction to help with the random noise. Some light dodging on the TLE's to make them a little more visible, but they are otherwise pretty much as-shot.

I was also able to see quite a few jets with my naked eyes! That's a first for me, and I'm always excited to see a new sky phenomenon for myself. I still haven't been able to see a sprite naked-eye, and it impresses me just how difficult that actually is. I'm flying in a private jet, right next to a thunderstorm, for the specific purpose of imaging sprites. I have very good low light eyesight, and I've watched tons of sprites in real time on the context cameras so I know exactly what and where to look. I was watching intently out the window while I snapped these shots, and the camera caught a sprite that I didn't see. Garggghhh! Since a typical sprite only lasts a couple of milliseconds, it's entirely possible it happened during a moment I looked away, or blinked. But still... Takeshi did manage to see one by eye last night, his first, so I'm not giving up yet.

5 comments:

  1. Hi Jason, I have seen several naked-eye sprites from the ground, when there's a decent storm in about the right place, though that hasn't happened yet this summer.

    I happen to live east of Fort Collins, just a few miles south of the Yucca Ridge station (I know Walter's retired now).

    I have wondered what the spectrum of a red sprite is and what it tells us. I'm also interested in the perceived color of sprites. Some are faint enough (due to distance, maybe) that they appear gray due to not stimulating the cone cells; but one I saw was a very strong red, almost like a car's tail-light!

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    1. Well we can see in several color images throughout the years that the sprites tend to turn bluer on the lower edge, so why is that? The spectrum will allow us to quantify that and determine which atmospheric species exactly are being ionized and emitting the light. I'm afraid I can't go into much more detail than that because it's not really my area of expertise.

      I know of a couple other people who've seen sprites in color with their naked eye. I've seen three naked eye sprites this campaign, my first ever, and they all looked grey to me. We were fairly close and above the thickest part of the atmosphere, so I don't think it's the distance that does it, I think it's just how bright the individual sprites are. Most of them probably aren't bright enough to stimulate your color vision so you see grey, but every once in a while you get a big bright jellyfish (and sometimes we see something we've dubber a 'super-jellyfish' which is extraordinarily bright and fast) and it might be bright enough to perceive as red.

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    2. The bright one I saw wasn't quite the classic multiple-fused-carrots shape of a sprite. :-) It was more of a curved blob of light, quite high (angularly) and therefore quite close. Maybe that's a typical shape when seen from (kinda) below? Like all sprites, it was very short-lived.

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  2. Hi Jason, Great captures! I understand your excitement of seeing these for the first time, I was dancing like an idiot when I first saw them earlier this year. Im interested more in how you can work out distances and such from the astrometry.net solver,

    I'd love to hear what you think from this shot I got, http://www.flickr.com/photos/craigdanvers/8465154514/in/set-72157624612760825
    I'll split you, you get the first ever shot of a blue jet taken by dslr from a plane, and I'll take the first ever shot taken by dslr on land :)

    Cheers mate, Craig

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    1. I can only estimate distance because I know we were flying about 200km away from the active hotspot. But I can get the angular size through the astrometry solver because it labels the stars and I can find the angular separation in a program like Stellarium. With the angular size of the sprite and an estimate of distance I can estimate the vertical size of the sprite. Unfortunately the automatic solver was unable to figure out the image, so I'll have to match the stars manually if I want to do that.

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