What is the white line near the horizon on the backup attitude indicator?

While connected to an external device that provides AHRS data, you will see a white line near the horizon. This white line represents a flat horizon relative to your altitude.  As you climb higher, the white line will appear to sit higher above the terrain.   

Image 1.  White line below mountain peaks at lower altitude.  

 

Image 2.  White line above terrain at high altitude.