Space Is Just a Little Bit Closer Than Expected

SpuriousLogic points out a BBC story which begins "The upper reaches of Earth's atmosphere are much lower than expected, a US Air Force satellite has found. Currently, the ionosphere — a layer of charged particles that envelopes the planet — is at an altitude of about 420km, some 200km lower than expected. The behaviour of the ionosphere is important because disturbances in its structure can upset satellite communications and radar."

Re:below 30MHz

[Anonymous Coward]

Yes, HF propagation is affected -
primarily by the dearth of sunspots...

20m was kind of lousy on Sunday, 'tho I did hear a weak ZS6 here in SoFla.

It will be interesting to see if they repeat these ionospheric measurements regularly, say monthly, over the next 2-3 years as the next cycle starts (to start) up... maybe...

Re:Wow

[megamerican]

I blame HAARP [google.com] and the Air Force [af.mil]

For a given value of ionosphere, and of Space

[DynaSoar]

I've never seen a definition of "space" that was based on the altitude of the ionosphere before. I've never seen a claim that the ionosphere was at a certain altitude, rather than a range with upper and lower bounds before. Most articles I see give about a 500 to 600 km altitude range, such as http://www.dcs.lancs.ac.uk/iono/ionosphere_intro/ [lancs.ac.uk]

Still, that's the ionosphere, not "space", and it's subject to wide variations of many different periods. TFA fails to show whether the result is a permanent feature or simply the measurement they found. It can hardly be anything other than the latter because there have been many, many measurements of the ionosphere, starting with numerous sounding rockets during the International Geophysical Year, 1957-58. TFA fails to account for their one results being at odds with many others.

And by "space" they mean "outer space", ie. outside the earth's atmosphere. If they meant simply "space", it could be the simple Euclidian definition of 3 extent dimensions. As such, we all exist in "space".

GPS

[Detritus]

An accurate model of the ionosphere is also important for GPS. GPS works by measuring the propagation delays of radio waves, which are affected by the Earth's atmosphere.

Re:WTF???

[FailedTheTuringTest]

It's changed. From the fine article: "We are in the depths of a very low solar minimum right now and as a result the ionosphere is lower and less dense than, we believe, at any other time in the history of the space age..."

Atmospheric depth and temperature

[RobotWisdom]

Even textbooks on this topic don't usually spell out the very simple dependence between atmospheric depth and surface temperature: when you warm the Earth, air molecules 'bounce' higher, so the atmosphere gets deeper. When you cool it, they bounce less high. The higher they fly, the slower they move, unintuitively termed 'adiabatic cooling'.

A small percentage of the highest bouncers can be reheated by the Sun near the top of their bounces, and I assume the reported lower ionosphere is more due to a decline in this factor than to any global cooling.

Re:GPS

[digitig]

It doesn't have to be that accurate a model for GPS. Yes, the atmosphere introduces errors into the propagation delays, but those errors are measured and accounted for. Ionospheric delays vary with the square of the frequency, so now two GPS frequencies are available for civilian use it's easy to adjust for ionospheric delays moment-by-moment, wherever the ionosphere happens to be at the time. Tropospheric delays are more of a problem, and are a significant part of the residual GPS error, but if the accuracy matters to you then you can use differential GPS and measure those delays too, moment by moment, and correct for them, whatever the troposphere happens to be doing at the time.

Re:For a given value of ionosphere, and of Space

[Doctor_Marc]

Actually the choice of that wording was determined by the folks helping us with the press conference at AGU. Most folks don't even know what an ionosphere is, so we had to go with something that would at least give the average reader (not the Slashdot reader) a concept to start with. The BBC article did a good job of explaining the science and the concepts once you get past the headline.

There is no universally agreed-upon definition of "where space begins." What we were reporting is that the "transition height" or "topside" in the ionosphere, the altitude where the density of O+ and the density of the light ions (H+ and He+) are equal, is lower than we have ever seen before. Here's a link for the definitions of these layers:

http://www.ngdc.noaa.gov/stp/IONO/ionostru.html [noaa.gov]

Note that above this level we don't have much interference with radio communications, so the practical interest is with the ionosphere below this altitude. Also this site points out that the topside rarely is below 500 km on the nightside, but the C/NOFS results show that it's currently almost always below that height (down to 400 km in places) .

Here's a link to the press release that went with this press conference that gives a bit more information and a nice graphic of the topside measured by the CINDI instrument on C/NOFS.

http://www.utdallas.edu/news/2008/12/16-001.html [utdallas.edu]

(Full disclosure: I am a member of the CINDI-C/NOFS project.)

Re:how could we not have already known this?

[Quantumstate]

It does seem surprising given that the ionosphere was used to bounce radio waves around the earth when satellites were not available.