2 Methodology of Magnetic Field Measurements
Magnetic fields are not directly visible. Their effects on the observable world are manifold, but all we can
hope to measure are the consequences the presence of a magnetic field has on any properties that are
accessible to observation. A particularly useful indicator of stellar magnetic activity, for example, is the
non-thermal emission generated through magnetic heating (for a review, see Hall, 2008
). Non-thermal
emission is an example for indicators of magnetism that I will call indirect in the following.
Indirect indicators require an additional mechanism to provide evidence for magnetic fields, and it
is often difficult to entirely rule out alternative mechanisms as a source for its presence. For
example, non-thermal emission may be generated by acoustic heating mechanisms so that the
detectability cannot be translated into a magnetic field strength without further knowledge
(Narain and Ulmschneider, 1996). Nevertheless, there is ample evidence that indirect indicators
like chromospheric Ca or coronal X-ray emission are reliable tracers of magnetic flux density
at least in sun-like stars (Schrijver et al., 1989; Pevtsov et al., 2003). For the following, an
observable is called direct if its detection or amplitude necessarily implies the presence of a magnetic
field.
The most successfully employed mechanism for direct detection of stellar magnetic fields is the Zeeman
effect (Zeeman, 1897). Different approaches to use the Zeeman effect for magnetic field measurements and
resulting field determinations are discussed and build the main part of the following chapters. Another
mechanism that appears similar to the Zeeman effect and has been used in solar magnetic field
measurements is the Hanle effect (Hanle, 1924). The Hanle effect describes how selective level population
can be modified by a magnetic field (Landi Degl’Innocenti and Landolfi, 2004; Trujillo Bueno, 2006). It can
be used to measure tangled, very small-scale fields of up to a few hundred Gauss but requires a very
detailed understanding of atomic level population and three-dimensional scattering processes
(Trujillo Bueno et al., 2004). This level of detail cannot yet be achieved in stellar observations
and the Hanle effect could so far not be used to detect magnetic fields in stars other than the
Sun.
Observations of magnetically induced emission, i.e., indirect magnetic field diagnostics, provide a wealth
of information on stellar magnetic activity that is often easier accessible than direct field measurements. For
reviews on observations of coronal emission, chromospheric emission, and starspots the reader is referred to
the reviews by Güdel (2002, 2004), Berdyugina (2005), and Hall (2008). In this article, the main
consequences of indirect observations for our picture of stellar magnetism will be discussed only briefly in
Section 2.3.
