A Brief Explanation of the Aurora

About 90% of the time, the light from the aurora comes from an altitude of 100-250 km. Electrons from space follow Earth’s magnetic field lines down to Earth’s upper atmosphere near the magnetic poles. These electrons are sped up by electrical forces and move very fast. When the electrons collide with the gas in the upper atmosphere, mostly N2, O2, and O, they excite the gas. When the gas relaxes, it gives off light. This is the light of the aurora we can see. This light is found in an oval band around Earth’s magnetic poles. Sometimes this oval moves and changes shape in a specific pattern, known as an auroral substorm.

Aurora and city lights were photographed by the US Air Force DMSP satellite on November 6, 2003, with a map drawn over the image in yellow. Note the sharp southern edge, defining the equatorward edge of the auroral oval as it passes over Montana, the Great Lakes, and New England. Observers as far south as Georgia saw a spectacular display looking north to the aurora, located at an elevation of 100 km. (Click to enlarge)
The space shuttle crew photographed aurora from above Earth. The height of the aurora reaching above 100 km in altitude is plainly seen in this oblique view with the stars shining above Earth’s surface. Also note the color changes from green-blue at lower altitudes, to reddish at highest altitudes. (Click to enlarge)
Arches and arcs are long ribbons of light extending from horizon to horizon without structure, often from the east to west. Sometimes these arcs turn in forms called folds. Arcs are often green, sometimes with red above the green and sometimes with purple below the green. Arcs mostly remain motionless in the sky and are always present, usually at very high latitudes (such as far north). This is the type of aurora one can usually see during the quiet phase of the aurora. During the growth phase of an auroral substorm, this arc slowly drifts sideways in the direction of the equator. For an observer in the north, this would mean the arc drifts slowly towards the south. From an image, it is hard to tell if aurora is in a quiet phase of the aurora, or a growth phase of a substorm. If it is very bright, it is more likely to be in the growth phase.

Bands are arcs with structure. When seen on the horizon, bands often have rays and are called curtains or drapes. An auroral arc curling in its length-wise direction from horizon to horizon causes these curtains. These curls can be seen from below. Sometimes shooting rays will quickly flow down the curtain to lower altitudes, and “trains” of these rays may flow eastwards or westwards along the curtain. Other times, bands will rapidly move across the sky. Bands are often green, sometimes with red above the green and sometimes with purple below the green. These forms make their appearance during the substorm expansion phase, also known as the substorm break-up phase. The transition from a slowly moving arc in the growth phase to quickly moving aurora in the form of curtains and rays is known as substorm onset.

Auroras appear as coronas during the expansion phase of a substorm whenever an active, rayed curtain passes over the observer’s zenith. Geometric perspective effects make it look as though the auroral rays are coming from a “vanishing point” and flowing to the horizon in all directions. The top of an auroral curtain may be over 250 km above the ground, and the rays are only a few kilometers wide, giving a spectacular visual effect.

Diffuse glows are the most common at the end of an auroral display, during the substorm recovery phase. At high latitudes, the displays are usually greenish. At lower latitudes during intense storms, red diffuse aurora are common. Diffuse auroras are often hard to observe with one’s eyes because they are very faint.

Above images courtesy Jan Curtis (Click to enlarge) http://climate.gi.alaska.edu/Curtis/aurora/aurora.html

For Northern Hemisphere observers, an auroral substorm display begins with the appearance of an auroral arc above the northern horizon or in the zenith. Over the course of 30 minutes during substorm growth phase, this arc will grow in brightness exceeding that of the full moon, with a white or pale green color. It will also drift across the sky towards the zenith or towards the equator. The auroral arc often will resolve itself into folds of light extending from the eastern horizon to the western horizon in the northern sky. From the ground, the substorm onset begins suddenly within seconds.

This is followed by the substorm expansion phase (also known as the break-up phase). In the expansion phase, the auroral arcs or bands break into many moving bands, which dance wildly across the sky both to the north and south. Within minutes, rays of light will start streaming down the developing curtains, which often surge westwards. From space, the expansion or break-up phase is seen as a thickening of the auroral oval along its north-south extent. The auroral oval’s substorm brightening region begins to expand westward, dissolving into numerous individual structures, which we see from the ground as individual curtains. As a curtain passes directly overhead, the rays flowing down the curtain create an auroral corona, which from a central point look like a meteor shower raining down the sky – an effect of perspective.

Within 30 minutes, the activity begins to slow and fade, and retreat northwards during the recovery phase. From space, the recovery phase is seen as a thick oval, often with two bright regions to the north and south. From the ground, pulsating aurora will be visible, which are large patches of diffuse aurora that brighten and fade with periods of seconds. The aurora slowly becomes a diffuse glow. An uninterrupted recovery phase lasts several hours. At any time during this sequence, the aurora may erupt into another auroral display if a new magnetic substorm event is triggered by disturbances in Earth’s magnetotail region.

Observers from many countries claim, rather steadfastly, that they sometimes hear sounds from aurora, such as crackling or swooshing. This subject is an interesting one for students to explore. Are people simply imagining the sound because auroras LOOK so much like fires in the sky? For decades, scientists have looked into this subject, but have not been able to capture any sound on tape.

One thing we do know is that direct observations show that auroras never occur closer than about 70 km above the ground, where the air is nearly a vacuum. There is no gas to carry pressure waves that could make our ears sense sound. This has caused some scientists to look into indirect “sympathetic” causes. Some of these are psychological. Others may involve powerful electrical currents flowing in the ground that cause electric “crackling” discharges on sharp objects near the observer (pine needles, etc). These currents are well-known to exist at the latitudes where sounds are reported.