The Interplanetary Magnetic Field
4-day predictions
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The magnetic field of the Sun permeates the solar system. Out among the planets it's called the Interplanetary Magnetic Field, or IMF.

When the IMF near Earth tilts south, it partially cancels our planet's own magnetic field and "opens the door" for solar wind energy to penetrate the space near Earth. South pointing IMFs often trigger geomagnetic storms. The north-south component of the IMF is also known as Bz (pronounced "B sub Z").

Now we have enough information to understand the 4-day predictions posted on spaceweather.com.

Bz 4-day Forecast  

Today

+1

+2

+3

days

N

N

N

S

"Today" is April 12

This table displays the expected average direction of Bz for four consecutive days beginning "today," which is the date given just below the table. These predictions are derived from NOAA's Wang-Sheely model of the Sun's magnetic field. On days when Bz is expected to be north (N), geomagnetic storms are less likely. On days when Bz is expected to be south (S), sky watchers should be alert for auroras.

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Bz 4-day Forecast  

Today

+1

+2

+3

days

N

N

N

S
"Today" is
explanation | more data 

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