The Interplanetary Magnetic FIeld - IMF
General
During the solar minimum, the magnetic field of the Sun, the Earth also has a magnetic field, looks a bit like an ordinary bar magnet with closed lines close to the Equator and open field lines near the poles. Scientist call those areas a dipole. The dipole field of the Sun is about as strong as a magnet on a refrigerator (arround 50 gauss). The magnetic field of the Earth is about 100 times weaker.
Around solar maximum, when the sun reaches her maximum activity, many sunspots are visible on the visible solar disk. These sunspots are filled with magnetism and large magnetic field lines which run material along them. These field lines are often hundreds of times stronger than the surrounding dipole. This causes the magnetic field around the sun to be a very complex magnetic field with many disturbing field lines.
The magnetic field of our Sun doesn't stay around the Sun itself. The Solar Wind carries it through the Solar System untill it reaches the Heliopause. The Heliopause is the place where the solar wind comes to a stop and where it collides with the interstellar medium. That's why we call the magnetic field of the sun the Interplanetary Magnetic Field or IMF. Because the Sun turns around her axis (once in 27 days) the IMF has a spiral shape (called the Parker Spiral)
The IMF is a vectorquantity with a three axis component, two of witch (Bx and By) are orientated
parallel to the ecliptic. THe third component, the Bz value is perpendicular to the ecliptic and is
created by waves and other disturbances in the solar wind. When the IMF and the geomagnetic field
lines are reverse orientated, or not perpendicular to each other, it fails to reconnect with
each other witch result in a transfer of energy, mass and momentum of the solarwindflow in the
magnetosphere. The strongest connection, with the most dramatic magnetospheric effects happens when
the Bz component is tilted heavely southward.
The Interplanetary Magnetic Field is weak near the Earth , variable in strength between 1 to 37nT
with an average of 6nt.
Bt value
The Bt value on the website is the strength of the Interplanetary Magnetic Field. How stronger it is how higher the chances for viewing the aurora. For Middle latitudes the values are good arround 40nt, but the Bz component must be tilted south.
Bz value
A strong southward Bz value is the most important value if we want to see some aurora. A strong southward orientated IMF gives us nice aurora displays so how lower it goes the better.