Download I. What are Auroras?

by Martina Stumpp
Overview
I.
II.
III.
IV.
V.
What are Auroras?
What causes them?
Auroral Zone
Colors
Auroras on other
planets
I. What are Auroras?
• natural light displays in the sky
• usually observed at night and in polar regions
• occur in the ionosphere
Names
• in northern latitudes:
aurora borealis
– Aurora = Roman goddess
of dawn
– Boreas = Greek name for
north wind
• in southern latitudes:
aurora australis
– Australis = Latin word for
“of the South”
• northern lights, polar
lights
II. What causes them?
• origin: the sun
• sun’s energy comes from
its interior
– the temperature exceeds
15 million degrees K
– hydrogen is transformed
into helium
• solar activity varies over
periods of approx. 11
years
• latest solar maximum:
2001-2002
• next is expected:
2011-2012
Sunspots
• created by strong
magnetic fields on the
surface of the sun
• appear darker
 cool compared to
surroundings
• visual indications of
the process that
sends charged
particles into space
Solar Wind
• gas of electrons and positive ions
emitted from the sun
• velocity: around 300-1000 km/h
• density: 5 ions/cm^3
• carries out sun’s magnetic field  IMF
• magnetic field intensity: 2-6 nT
• during magnetic storms: faster & stronger
Formation
• similar to the mechanism
of a generator with a
moving conductor in a
magnetic field
• protons and positive ions
are diverted to the left,
electrons to the right
– they form the pos. & neg.
pole of the generator
• the magnetosphere is
filled with plasma which
allows electricity to flow
between the poles
– positive pole, ionosphere,
negative pole, polar region
• accelerated electrons collide with atoms & molecules in
the atmosphere
– collision energy between solar particles and gas molecules is
emitted as a photon
– many collisions form the aurora
III. Auroral Zone- “Auroral Oval”
• seen from space, auroras
appear as a ring shaped
region around the poles
• seen most often and with
greatest intensity
• altitude: 100km – 1000km
• latitude: 60° – 75° north /
south
• at midnight the oval is nearly
twice as wide and twice as
far from the poles than at
midday
• shapes and locations of the
ovals vary with solar activity
• the reason of an aurora occurring within this zone is due to
the slant of the IMF lines
• when IMF & geomagnetic field are anti-parallel, IMF can
partially cancel Earth’s magnetic field (“southward Bz”)
– solar wind can reach Earth’s atmosphere
• Earth’s magnetic dipole axis is most closely aligned with
the IMF in April & October
Where and when can we observe
Auroras?
• the light can be observed
only from the inside of the
oval ring, NOT on the
pole itself
• oval around the north
pole stays in the areas of
–
–
–
–
–
Nordic countries
Greenland
Alaska
Canada
Russia
• aurora “season”
– late September/early October to late March
– not in the summer  the sun never sets
• auroras can be predicted three days in advance
– a magnetometer measures the effect that solar particles will
have on Earth’s magnetosphere
Forecast for Tuesday,
January 27, 2009
IV. Colors
• auroral spectrum is not continuous
– composed of a series of spectral lines
in the visible, the UV, the infrared and
x-ray ranges
• electrically charged particles excite
the atmospheric gas
– electron is bumped up into a higher
orbit
• the excited particle is unstable
– electron falls back to its original state
– excess energy is released by emitting
light
• a particular gas emits
photons of a fixed
wavelength
– aurora = atmosphere’s
fingerprint
• atmosphere consists of
nitrogen and oxygen
– atomic oxygen:
green & red
– molecular nitrogen &
nitrogen ions:
red & blue/violet
• the altitude affects auroral
color
• level of solar wind activity
influences the color
Auroral Intensity
• weak: diffuse band of light, brightness of the
Milky Way
• medium-strength: brighter than most stars
• strong: comparable to light of the moon
Forms and Structures
• diffuse surfaces
– large, occur in the mornings,
difficult to observe
• spiral structures
– occur during disturbed
conditions, visually impressive
• auroral pulsations
– occur often, resemble puffs or
flames
• active & quiet forms
– active: ray structures
“curtains” with rapid variation
– quiet: homogeneous arcs and
bands, diffuse surfaces
V. Auroras on other Planets
• they occur on celestial
bodies that have an
atmosphere and a
magnetic field
– Jupiter, Saturn, Uranus,
Neptune
• Venus, Mars and our
moon do not have their
own magnetic fields
• Io doesn’t have an
atmosphere, but active
volcanoes which
temporarily create one
Sources
• www.northern-lights.no
• http://en.wikipedia.org/wiki/Aurora_(astron
omy)
• Metzler Physik, J.Grehn, J. Krause