(6) Aeronomy

Geophysics 153Introduction to Geophysics and

Planetary Physics

What is Aeronomy?

Aeronomy is the science of the upper atmosphere – where dissociation and ionization becomes important.

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If we look at the structure of the atmosphere in terms of composition, this will primarily happen in the Heterosphere and in the Exosphere (picture credit: M. L. Salby).

Spheres

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In terms of the average temperature profile we have to consider the Thermosphere (and also the Mesosphere) – and by definition – the Ionosphere (if we look at the atmosphere in terms of electric properties (picture credit: Thomson).

Some (most) aspects have already been covered in the lectures by Günter Kargl.

Plasmasphere

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Above the Ionosphere (which is partly ionized) we find (above ~1000 km) the Plasmasphere, which is almost entirely ionized (mainly hydrogen), and – largely – co-rotates with the Earth (Picture credit: NESTA).

Mesosphere

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Mean zonal winds in December (NRL): The Mesosphere is characterized by strong (geostrophic) zonal winds, reaching 70 m/s and more, with westerlies in the winter-, and easterlies in the summer hemisphere.

Mesopause

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

Noctilucent Clouds

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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)).

Polar Mesospheric Clouds

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

Anthropogenic Clouds

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

Transient Luminous Events

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

Red Sprites and Elves

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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.

Elves 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. Though 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.

Gigantic Jets and Blue Jets

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Gigantic Jets also appear above thunderclouds, but without associated lightnings (Credit: Nature (l), H. T. Su et al. (r)). They propagate upwards – and are still not really understood.Blue Jets are optical ejections from the top of the electrically active core regions of thunderstorms, but not directly associated with cloud-to-ground lightning. They typically propagate upward in narrow cones of about 15 degrees.

Aurora

Particle precipitation (from the plasma sheet) along magnetic field lines (Birkeland currents) causes ionization and excitation of atmospheric constituents. 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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Aurora

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

Aurora Borealis

Northern Lights „Photo of the Year“ from 2001 (upper left), 2002 (upper right), 2003 („The Bird“, left) and 2004 (right). www.northern-lights.no These spectacular events happen almost exclusively at high latitudes, where the magnetic field lines are close to vertical.

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Aurora Borealis

Aurora Borealis over Andoya Island, Iceland (above, credit: Frank Olsen), and over Austnesfjorden, Norway (left, credit: Max Rive).

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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), UV images from „Dynamics Explorer – 1“, Nov 11, 1981 (left) and Nov 8, 1981 (credit: Univ. of Iowa).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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Magnetic and Geomagnetic Pole

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 (2015) at 80.4 °N, 72.6 °W, in Ellesmere Island (picture: NSIDC).

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Magnetic and Geomagnetic Pole

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

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Southern Lights

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

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Aurora-Ovals are not just present on Earth (left, Quelle: Univ. of Iowa), but also on Jupiter und Saturn (above, Hubble Space Telescope). Saturn is a rare case, where rotation and magnetic axis are perfectly parallel.

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Planetary Aurorae

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“ (Credit: Univ. of Iowa).

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Geomagnetic Storms

Extreme Coronal Mass Ejections on 28. 10. 2003 caused auroras at unusually low latitudes during the following nights: New York (a), Houston (b), Münster (c), Oklahoma (d), Wien (e).

Auroras at mid latitudes are extremely rare, they are usually unstructured and red, since they generate at high altitudes (300 – 700 km) – not like their polar counterparts (like in Alaska (f) for comparison).

(Credit: Aurora Gallery, www.spaceweather.com.)

(a) (c)(b)

(d)

(e)

(f)

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Polar Lights?

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Polar Lights?

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.

Geomagnetic storms in October 2003 (picture credit: ZAMG). The total intensity decreases during the main phase of each event. During event 3 aurorae could be observed in Austria.

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Geomagnetic Storms

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.

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Airglow

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

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Airglow

Airglow over Tibet (Credit: Jeff Dai) with structures caused by gravity waves.

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Airglow

Phenomena in the high atmosphere (Credit: Les Cowley).

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Lights in the High Atmosphere

Aurora on Saturn

Aurora Oval on Saturn (Picture: Hubble Space Telescope).

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