University of Minnesota
School of Physics & Astronomy

Spotlight

The Spell of the Northern Lights

Professor Robert Lysak
Bob Lysak
Richard Anderson
                                                       

The aurora borealis, or Northern Lights have long been a subject of fascination in folklore and for those of us lucky enough to see them in northern latitudes. Professor Bob Lysak is a theoretician who has been studying the unique physics of aurora since the 1970’s. The classic model of the Aurora is that electrons trapped in the Earth’s magnetosphere are accelerated by quasi-static electric fields that form parallel to the Earth’s magnetic field and cause the light display.

Lysak has created an alternative model of the Aurora which insists that Alfven Waves, an oscillation that occurs along the lines of the Earth’s magnetosphere, play a part in delivering energy to the Aurora. Lysak’s model was borne out by data from the FAST satellite, which showed that patterns of acceleration that were more consistent with the Alfvén wave picture than what was predicted by the standard model of aurora physics.

Lysak explains that part of the problem with constructing a model of the physics of the Aurora is that the magnetosphere and ionosphere are not uniform. The ionosphere is so named because solar-radiation causes particles within it to lose electrons, that is, to become ionized. Due to the Earth’s gravitational field, the ionosphere has layers that vary in density at different altitudes. The Alfvén waves move through this plasma more slowly when it is more dense and more quickly where it is less dense. When plasma density falls off rapidly it creates a cavity where packets of Alfvén waves form, leading to the particle acceleration. The speed of Alfven waves increases exponentially above the ionosphere and can sometime hit relativistic speeds. Lysak has created a mathematical model to predict this ionospheric "weather."

Certain other space "weather" conditions such as the solar maximum, a cycle of Sun storms that peaks every eleven years, enable physicists and aurora enthusiasts alike to be able to see the strongest aurora in high northern latitudes, and correspondingly at high southern latitudes as well. Although electrons and ions hit the Earth’s atmosphere continuously, the large energies required to form a visible aurora are only kicked up when a storm on the Sun hits the Earth’s magnetosphere and disturbs it. Most aurora are diffuse and appear in the sky as a single colored haze. Occasionally more spectacular aurora will be visible that appear like a changing multi-hued light show in the sky. Lysak describes especially dynamic or “dancing” aurora lights as being Alfvenic, because the Alfven wave picture predicts more dynamic aurora than the standard static model. Lysak says that although we are now in the trough of the Sun storm cycle, the solar minimum, physicists believe that solar activity is again increasing and Northern Minnesota will once again be a good place to observe the Northern Lights.