The interplanetary Bt is a measure of strength in the interplanetary magnetic field (IMF) between the Earth and the Sun. It is a combination of the Bx (toward and away), By (east and west), and Bz (north and south) components. You can read more about this on the SpaceWeatherLive.com site.
The higher this value, the stronger the IMF is. Anything less than 10 nanoteslas (nT) is considered weak, values greater than or equal to 10 nT and less than 20 nT are moderate, values between 20 nT and 30 nT are considered strong, and anything over 30 nT is considered very strong. This strength indicates the potential for increased geomagnetic activity and, thus, an increase in the intensity of the aurora. It is important to note that we can have brilliant auroral displays with values in the weak range.
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The free version of the app gives you approximately two hours and 30 minutes of interplanetary Bt data: the previous 90 minutes plus the next hour. The subscription version lets you go back and look at interplanetary Bt data for the past 36 hours in 6-hour increments.