Hello, I have some questions please regarding the "fold" of the electron density models and I didn't find resources to explain it. 

1. What is meant by "fold" in the context of the electron density models?  

2. How can I include the "fold" value in the equation of any electron density model (e.g. Newkirk, Saito)? 

3. How can I choose the right value for the "fold" with respect to the solar activity (e.g. Rising, Max, and declining phases of the solar cycle)?