[![enter image description here][1]][1]

I have a plot (above) which is magnetic field (y-axis) over time (x-axis), one measure each hour (Dst - Disturbance Storm-Time). 

From time to time there is a negative peak, which indicates a magnetic storm. The more it goes down the bigger the storm. 

What's the physical meaning of the integral of it (nT x h)?


  [1]: https://i.sstatic.net/G0drq.png