(9) Atmospheric Optics 3

Physics of the Atmosphere II

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Terrestrial Emissions

The Earth usually emits radiation just in the thermal Infrared. Emissions in the visible part of the spectrum require significantly higher temperatures (cf. Wien’s displacement law) – these are e.g., reached in lightnings (Credit: David Rankin).

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Terrestrial Gamma Rays!

One of the biggest (still largely unexplained) surprises during the past decades was, that thunderstorms can even produce gamma rays and anti-matter (field-line parallel, relativistic positrons) (Credit: NASA).

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Lights in the High Atmosphere

Optic phenomena in the high atmosphere (Credit: Les Cowley).

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Spheres

Here we consider the Thermosphere and the Mesosphere (in terms of average temperature profile) – and the Ionosphere (in terms of electric properties) (credit: Thomson).

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Mesopause

The upper atmosphere is very dry, but Polar Mesospheric Clouds (PMCs) often mark the Mesopause – where the atmosphere is coldest (P.M. Heden).

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Noctilucent Clouds

Observers on the ground (not only at polar latitudes) know them as Noctilucent Clouds (NLCs). They frequently show features of Gravity Waves (Credit: D. Weir, NASA (right)).

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There is no emission involved, just reflection of sunlight.

Polar Mesospheric Clouds

The satellite perspective (AIM) reveals the typical spatial distribution, the ISS the (Cirrus-like) appearance (Credit: NASA).

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Anthropogenic Clouds

Shuttle launches (Hydrogen + Oxygen) were a significant water vapor source for the upper atmosphere, producing artificial clouds (K. Thornsley, NASA (r)).

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Transient Luminous Events

The Mesosphere (“Ignorosphere”) is home to (still) strange phenomena, summarized under Transient Luminous Events (Credit: NOAA).

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Red Sprites

Red Sprites (“Kobolde”) are large-scale electric discharges over active thunderstorms, triggered by (rare) positive cloud-to-ground lightnings (Credit: NOAA (l), H. Stenbaek-Nielsen (r)). They typically start at ~70 km altitude, last just milliseconds, propagate downwards at speeds of ~107 m/s – and are still not really understood.

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Elves

Elves (“Elfen” – nicht “Elben”) are rapidly expanding (up to 500 km across) disk-shaped regions of luminosity, lasting less than a millisecond, which occur high above energetic cloud-to-ground lightning of positive or negative polarity. Elves most likely result when an energetic electromagnetic pulse (EMP) propagates into the ionosphere. They can be accompanied by sprites. Elves got their name as an acronym for Emission of Light and Very Low Frequency perturbations due to Electromagnetic Pulse Sources.

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Gigantic Jets and Blue Jets

Gigantic Jets also appear above thunderclouds, but without associated lightnings (Nature (l), H. T. Su et al. (r)), propagate upwards – and are still not understood.Blue Jets propagate upward in narrow cones of about 15° from the top of electrically active core regions of thunderstorms, but not directly associated with lightning.

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Aurora Borealis

Particle precipitation (from the plasma sheet) along magnetic field lines (Birkeland currents) causes ionization and excitation of atmo-spheric constituents (credit: Frank Olsen (above), Max Rive (l)).

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Aurorae

Atomic Oxygen is responsible for green auroras (558 nm wavelength), mainly between 100 km and 180 km altitude, and for red aurorae (630 nm) above 200 km (picture credit: University of Manitoba).

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Aurorae

Green and Read Aurora over White Dome Geyser, Yellowstone National Park (credit: Robert Howell, aurora spectrum: Les Cowley).

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Aurora Borealis

Aurora sequence (30 min) over Fairbanks, Alaska (credit: LeRoy Zimmermann).

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Aurora Australis

“Southern Lights“ – Aurora Australis from the ISS-perspective (NASA).

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Aurora Oval

Polar Lights occur in the Aurora Oval in 10° to 20° distance from the Geomagnetic Pole (M), Note that there are never polar lights at/above the magnetic poles, and they are even very unlikely at the geographic north pole (N).

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UV images from Dynamics Explorer-1, Nov 11, 1981 (left) and Nov 8, 1981 (credit: Univ. of Iowa).

Magnetic and Geomagnetic Pole

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Secular movement of the Magnetic (red) and the Geomagnetic North Pole (blue) (British Geol. Survey).

The compass needle points towards the Magnetic Pole, where the inclination is 90°. The Geomagnetic Poles mark the Axis of the best-fitting magnetic Dipole – currently (2013) at 80.1°N, 72.2°W, in Ellesmere Island (picture: NSIDC).

Magnetic and Geomagnetic Pole

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Secular movement of the Magnetic (red) and the Geomagnetic South Pole (blue) (British Geol. Survey).

Southern Lights

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Star Trails above the geographic South Pole and (rare) “polar” lights (credit: Robert Schwarz).

Geomagnetic Storms

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During Geomagnetic Storms the Aurora Oval expands equatorward within a few hours. Above: Northern Aurora Oval, 2.4. 1982, left: Southern Aurora Oval,13.6. 1984. UV images, Dynamics Explorer–1 (Univ. of Iowa).

Polar Lights?

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A rare “Polar Light” above Schöckl (!) in March 2015 (credit: Herfried Eisler). During the extreme Storm of 1859 (Carrington Event), aurorae have even been observed in Hawaii and Cuba.

Airglow

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Airglow is observed at similar altitudes as Aurorae (left, Miller et al., PNAS) and it has the same colors, since it is also caused by (mostly the same) excited atoms (spectrum: Les Cowley). But here daytime EUV solar radiation leads to chemical excitation.Unlike Aurorae, Airglow therefore appears around the globe – but it is not as bright.

Airglow

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Airglow over Crater Lake, Oregon (Credit: John Moore). Airglow has first been observed in 1868 by Anders Ångström.

Therefore: What’s the most prominent wavelength – in Ångström?. 5577 Å

Airglow

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Airglow over Tibet (Credit: Jeff Dai) with structures caused by Gravity Waves.

Zodiacal Light

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Zodiacal Light is in fact an Astronomical Phenomenon – caused by scattering of sunlight by interplanetary dust in the ecliptic plane (left: Yuri Beletsky, above: Alex Cherney).

Zodiacal Light

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Zodiacal Light can even by observed in Austria – but only in very dark regions, like Reichraming – when fog helps dimming the lights of Vienna (left, 150 km away) and Graz (right, 100 km) (credit: Rudi Dobesberger).

Dark Sky?

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