by Martina Stumpp

Overview I.What are Auroras? II.What causes them? III.Auroral Zone IV.Colors V.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: next is expected:

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 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 omy) Metzler Physik, J.Grehn, J. Krause