Although interstellar polarization has been known for almost 70 years (and emission polarization, in the FIR, since 1982), it is only relatively recently that a quantitative, empirically tested, theory has been developed. This “Radiative Alignment Torque (RAT) theory” predicts that paramagnetic dust grains align if exposed to an anisotropic radiation field, with wavelengths less than the grain diameter. For strong radiation fields, RAT theory predicts that the alignment can become aligned with the radiation direction. Observations also show that – with the possible exception of the very smallest grains - the long-standing “Davis-Greenstein” alignment mechanism is not viable as an explanation for ISM polarization. A number of specific, observationally testable, predictions follow from RAT theory many of which have now been confirmed. With a full testing of the theory and quantification of its parameters, polarimetry has the promise to not only allow efficient and reliable tracing of interstellar and interplanetary magnetic fields, but also to provide new and unique probes of the dust and the interstellar environment with SOFIA and other observatories. I will review grain alignment and its observational testing, and discuss some of the probes of ISM environmental parameters and dust that the verified theory allows.
Interstellar Dust Grain Alignment - Current Status
NASA Ames, SOFIA Science Center