SSN

17 Jun SSN

The SunSpot Number (SSN) is a commonly used index of solar activity. The daily sunspot number was first introduced in 1848 by the Swiss astronomer, Johann Rudolph Wolf. His method, which is still used today, counts the total number of spots visible on the face of the sun and the number of groups into which they cluster because neither quantity alone satisfactorily measures sunspot activity. The relative sunspot number is an index of the activity of the entire visible disk of the Sun. It is determined each day without reference to the preceding days. Each isolated cluster of sunspots is termed a sunspot group, and it may consist of one or a large number of distinct spots whose size can range from 10 or more square degrees of the solar surface down to the limit of resolution (e.g., 1/25 square degree).

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. They usually appear as pairs, with each sunspot having the opposite magnetic pole to the other.