\textit{This is the author’s version of the work. It is posted here by permission of the AAAS for personal
use, not for redistribution. The definitive version was published in \href{http://www.sciencemag.org/content/350/6259/423.abstract}{Science} on
Vol. 350 no. 6259 pp. 423-426 , DOI: 10.1126/science.aac6933}



Internal stellar magnetic fields are inaccessible to direct observations
and little is known about their amplitude, geometry and evolution. We
demonstrate that strong magnetic fields in the cores of red giant stars
can be identified with asteroseismology. The fields can
manifest themselves via depressed dipole stellar oscillation modes,
which arises from a magnetic greenhouse effect that scatters and traps
oscillation mode energy within the core of the star. The \emph{Kepler}
satellite has observed a few dozen red giants with
depressed dipole modes which we interpret as stars with
strongly magnetized cores. We find field strengths larger than
$\sim\! 10^5 \,{\rm G}$ may produce the observed
depression, and in one case we infer a minimum core field
strength of $\approx \! \! 10^7 \,{\rm G}$.
  
  
  
  
  
  
  
  
  
  
  
  
  
  