The Aurora in the Northern or Southern Skies






A Fine Spectacle

 

This southern auroral oval was photographed by the Space Shuttle Discovery astronauts Overmeyer and Dr. Halliman. Courtesy of NASA

        The presence of a magnetic field is revealed by the beautiful auroral displays

    

 

           The auroras are mirrored                                   on opposite poles

 

     Rays tend to align themselves with              the lines of force

 

    The lights appear to swirl and pulse due to electric currents

source: http:windows.ucar.edu/tour/link=/earth/Atmosphere/aurora_il.html

       An important part of researching the activities of the magnetic field has been directed towards studying the auroras

 

            The lights appear when solar wind particles find their way into the ionosphere of Earth

 

         The charged particles collide with Earth's atmospheric gases and a stream                              of photons is emitted

 

 

Oxygen produces a green glow, nitrogen and nitrogen oxides give off red light


   
Curtains over Fairbanks, Alaska Courtesy of Jan Curtis

source: http://windows.ucar.edu/tour/link=/earth/Atmosphere/aurora_il.html

 The occurrence of the aurora in the ionosphere is the most visible evidence for the presence of a magnetic field. These ethereal lights are caused by the ionization of Earth’s upper atmospheric gases when solar wind consisting of mostly of protons is redistributed inside the magnetosphere and is guided by the magnetic field lines towards the polar regions. The energy of these particles ranges between few tens of eV to hundreds of keV (spaceweb@oulu.fi, 1998). As they collide with the neutral atmospheric gases, the their atoms are send into higher energy states. Once these excited atoms return to their regular configuration (electron energy level),   a beautiful spectral line glow is emitted.

Red skies over Japan February 11, 1958

source: http://www-istp.gsfc.nasa.gov/istp/outreach/auroras.html

This storm was described by Sidney Chapman (1964; pg 107) as a sequence of two magnetic storms, a second larger one occuring while the magnetic field had not yet recovered, resulting in the spreading of the aurora towards lower latitude regions and the bright red glow.

Green ribbons at sunset. Such bands tend to line up with the sun

Jan Curtis

source: http://windows.ucar.edu/tour/link=/earth/Atmosphere/aurora_il.html

Ionized gases are good conductors. This leads to the induction of electrical currents by the magnetic field. An Auroral Electrojet index measures such currents. The plot below was produced for January 13th, 2004, a day with a higher than average auroral activity

source: http://swdcwww.kugi.kyoto-u.ac.jp/aedir/ael/q/RAE.040113.html

The data for this index was gathered from eleven AE observatory stations around the northern hemisphere. It was prepared by the WDC Geomag Kyoto

Especially splendid auroras occur as a result of a combination of magnetic storms and substorms. The latter are thought to be caused by violent plasma explosions in the tail of the magnetosphere during or after major magnetic storms. Such events rare as they are produce brilliant, rapidly dissipating auroras that often cover the entire sky (http://www-spof.gsfc.nasa.gov). The most recent incident occurred on November 3th, 2003 when night skies turned red over much of the northern hemisphere extending south as far as Colorado and central Europe.

A vivid animation of the auroral electro jet currents produced during the "Bastille Storm" in July 2000 as seen from space is provided by NASA with the data recorded from the NASA IMAGE Spacecraft (NASA IMAGE Discoveries, 2003).

Lighting up the night

Courtesy of Jan Curtis

source: http://windows.ucar.edu/tour/link=/earth/Atmosphere/aurora_il.html

This image depicts the northern auroral oval as of the early morning hours of April 18th, 2004. The red color indicates the highest activity, blue the lowest. The arrow points at noon. Concurrent auroral activity is available at this site

source: http://www.spacew.com/www/aurora.html Courtesy of NOAA Space Environment Center

Under normal conditions the area over the pole is void of aurora. During a substorm however, the external magnetic field temporarily overprints the forces of the main field.

Heaven's Fire over the city of Bamberg, Germany in 1560. A lithograph

source: http://www-istp.gsfc.nasa.gov/istp/outreach/auroras.html

Red and green boreal arcs

source: http://www.geo.mtu.edu/weather/aurora

Courtesy of Michigan Tech

The color of the aurora depends on the composition of the upper atmospheric gases. Charged particles at a height of 50 to 90 km are mainly nitrogen oxide ions and oxygen anions. The next strata consists of nitrogen oxides and oxygen cations. The uppermost layers at 130 to 600 km contain mostly oxygen cations (Backus et.al, 1996). The ionization of the gases is greatest at a height of about 85 to 130 km (spaceweb@oulu.fi, 1998) where most of the auroras occur. Each gas emits light with a color intrinsic of a certain wavelength. But only if emission spectrum lines are dense will the aurora be vivid and observable from Earth. The color of the lights and their movement reveal much about the physical properties of the geomagnetic field.

For more information and Aurora Forecasts, visit the site of the Geophysical Institute of the University of Alaska.

Rays and arc over Finland

source: http://spaceweb.oulu.fi

Courtesy of the University of Finland