Effects of the Solar Wind

From the beautiful aurorae to magnetic storms, the solar wind plays a large part in what goes on in the Earth's magnetosphere.  Here we will look a little more closely at how the solar wind affects the Earth.

Magnetospheric substorms happen when increased energy flows from the solar wind into the magnetosphere near the poles of the Earth.  They last about a half hour.  They could be a result of a coronal mass ejection or some other type of solar activity.  Magnetic storms develop from these substorms when they become strong, prolonged and intense and they take place over the entire Earth.

One better-known effect of the magnetospheric substorm is called the aurora, also known as the northern lights.  The aurora takes place when the solar wind collides with the Earth's ionosphere.  Atmospheric particles become excited during the collision.  They need to release energy in order to return to their normal state.  This energy is seen by us as bursts of colored lights in the sky.  When a substorm develops into a full magnetic storm, the aurora are closer to the Earth's surface and more intense.
 

Here is is an image of the aurora taken from Earth...

         ...and here is one taken from space.
 
 

The aurorae are most often seen in the auroral ovals, which are centered on the north and south poles.  They are rarely seen in areas closer to the equator, except during times of high magnetic activity, such as during a magnetic storm.  Two classes of aurorae can be seen in the auroral ovals:

Diffuse auora--Seen closer to the equator and has no clear structure

Discrete aurora--Seen farther north, made of long east-west bands and tiny north-south extensions

The auroral substorm is the form in which the aurora most often shows itself. It has three stages that take place in the ovals:

Onset--Sudden brightening closer to the equator.
 

Expansion phase--Dynamic auroral disturbances form a bulge and this is the interval of time in which the bulge grows

Westward-traveling surge--the bulge develops a kink that appears to move west

Omega bands--torchlike forms appear, and drift east

Recovery phase--Activity slows down, but a pulsating aurora appears for some time
 
 

Here are four images in sequence that illustrate the phases of an auroral substorm:
 
 

Onset                                                             Expansion--the bulge covers the sky

Expansion--westward-traveling             Recovery

surge and omega bands

Auroral height from Earth is related to the average energy of the particles colliding with the atmosphere. The particles with more energy are moving faster, causing them to plow further into the atmosphere before they excite the atmospheric particles, making the aurora closer to Earth.  The height is also related to the strength of the magnetic field, as energetic particles are harder for a weaker magnetic field to catch.  This can affect the level within the ionosphere that the particles reach.

The auroral colors are often yellow-green and red but sometimes appear to be grey when they are below the color threshold of the eye.  The color may also vary with altitude, where blood-red dominates above 200 kilometers and magenta 100 kilometers and below.  This makes sense because the blood-red appears farther from Earth than the magenta, and red is at the lower energy end of the spectrum than magenta.  Therefore, the particles that cause the blood-red do not get as close to the Earth as those that cause the magenta.  Aurorae seen during the day are caused by particles with less energy and therefore make the aurora show up farther from Earth.