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 (around 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 which (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 that results into a transfer of energy, mass and momentum of the solar wind flow in the magnetosphere. The strongest connection, with the most dramatic magnetospheric effects happens when the Bz component is tilted heavily southward.
The Interplanetary Magnetic Field is weak near the Earth , variable in strength between 1 to 37nT with an average of 6nt.

B_t 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 around 40nt, but the Bz component must be tilted south.

B_z value

A strong southward B_z 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.

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