3-NIRVANA-user-guide/3.1-Code-basics.md

@@ -9,6 +9,8 @@ CONTENTS:
 
 ## Functionality overview
 
+#### Physics system
+
 NIRVANA is a multi-physics, multi-species, multi-scale simulation code.
 It implements the following physics system:
 
@@ -116,6 +118,34 @@ assumed edge-centered analog to the electric field.
 *Note: The TESTFIELDS infrastructure does not implement any numerical
 method to solve induction equations for testfield fluctuations.*
 
+#### The Boris correction
+
+NIRVANA supports the so-called Boris correction for reducing the Alfven speed,
+avoiding a too small timestep in MHD simulations with strong magnetic field
+and/or extremely low gas density.
+See the physics guide
+([PDF](https://gitlab.aip.de/ziegler/NIRVANA/-/tree/master/doc/pdf/PhysicsGuide.pdf)).
+for more information on the underlying idea and equations.
+The use of the Boris correction is controlled
+by setting corresponding macros in the user interface
+[nirvanaUser.h](3.2-User-interfaces#user-controllable-macros)).
+
+#### Magnetic field splitting
+
+NIRVANA implements a magnetic field splitting strategy. In this strategy the
+magnetic field is decomposed into an imposed background field and a residual field.
+By solving for the (presumably weaker) residual field accuracy of the solution
+can be increased since the background field is discarded
+in the time evolution of the total energy density.
+For details on the modified MHD equations which are solved consult the
+physics guide
+([PDF](https://gitlab.aip.de/ziegler/NIRVANA/-/tree/master/doc/pdf/PhysicsGuide.pdf)).
+Use of the magnetic field splitting is controlled
+by setting the corresponding macro in the user interface
+[nirvanaUser.h](3.2-User-interfaces#user-controllable-macros)
+and by the definition of the background magnetic field in the user interface
+[sourceB0User.c](3.2-User-interfaces#user-defined-background-magnetic-field)).
+
 ## Dynamic arrays
 
 The NIRVANA infrastructure implements its own proprietary routines for