Two extrapolation models of the solar coronal magnetic field are compared using magnetogram data from the SDO/HMI instrument .
The two models , a horizontal current–current sheet–source surface ( HCCSSS ) model and a potential field–source surface ( PFSS ) model differ in their treatment of coronal currents .
Each model has its own critical variable , respectively the radius of a cusp surface and a source surface , and it is found that adjusting these heights over the period studied allows better fit between the models and the solar open flux at 1 AU as calculated from the Interplanetary Magnetic Field ( IMF ) .
The HCCSSS model provides the better fit for the overall period from 2010 November to 2015 May as well as for two subsets of the period — the minimum/rising part of the solar cycle , and the recently-identified peak in the IMF from mid-2014 to mid-2015 just after solar maximum .
It is found that a HCCSSS cusp surface height of 1.7 R _ { \odot } provides the best fit to the IMF for the overall period , while 1.7 & 1.9 R _ { \odot } give the best fits for the two subsets .
The corresponding values for the PFSS source surface height are 2.1 , 2.2 and 2.0 R _ { \odot } respectively .
This means that the HCCSSS cusp surface rises as the solar cycle progresses while the PFSS source surface falls .