Abstract
Observational studies of magnetic fields are crucial. We introduce a process “ground state alignment” as a new way to determine the magnetic field direction in diffuse medium. The alignment is due to anisotropic radiation impinging on the atom/ion. The consequence of the process is the polarization of spectral lines resulting from scattering and absorption from aligned atomic/ionic species with fine or hyperfine structure. The magnetic field induces precession and realign the atom/ion and therefore the polarization of the emitted or absorbed radiation reflects the direction of the magnetic field. The atoms get aligned at their low levels and, as the life-time of the atoms/ions we deal with is long, the alignment induced by anisotropic radiation is susceptible to extremely weak magnetic fields (\(1\,\mathrm{G} \gtrsim B \gtrsim 10^{-15}\) G). In fact, the effects of atomic/ionic alignment were studied in the laboratory decades ago, mostly in relation to the maser research. Recently, the atomic effect has been already detected in observations from circumstellar medium and this is a harbinger of future extensive magnetic field studies. A unique feature of the atomic realignment is that they can reveal the 3D orientation of magnetic field. In this chapter, we shall review the basic physical processes involved in atomic realignment. We shall also discuss its applications to interplanetary, circumstellar and interstellar magnetic fields. In addition, our research reveals that the polarization of the radiation arising from the transitions between fine and hyperfine states of the ground level can provide a unique diagnostics of magnetic fields in the Epoch of Reionization.
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- 1.
Here interstellar is understood in a general sense, which, for instance, includes refection nebulae.
- 2.
Radiative pumping is much slower than magnetic mixing. Radiation was chosen as the quantization axis, nevertheless, which inevitably would lead to the nonzero coherence components. They were neglected in Hawkins (1955), however.
- 3.
Modern theory of dust alignment, which is a very powerful way to study magnetic fields (see Lazarian 2007 and ref. therein) is also appealing to anisotropic radiation as the cause of alignment.
- 4.
In quantum physics, quantum coherence means that subatomic particles are able to cooperate. These subatomic waves or particles not only know about each other, but are also highly interlinked by bands of shared electromagnetic fields so that they can communicate with each other.
- 5.
To remind our readers, The Stokes parameters Q represents the linear polarization along e 1 minus the linear polarization along e 2; U refers to the polarization along \((\mathbf{e}_{\mathbf{1}} + \mathbf{e}_{\mathbf{2}})/\sqrt{2}\) minus the linear polarization along \((-\mathbf{e}_{\mathbf{1}} + \mathbf{e}_{\mathbf{2}})/\sqrt{2}\) (see Fig. 5.1, right).
- 6.
To clarify, we do not distinguish between pumping by optical lines or UV lines, and name them simply “optical pumping”.
- 7.
There are no energy splittings among them, the effect is only to provide more projections of angular momentum (see Yan and Lazarian 2007).
- 8.
Only if hyperfine structure can be resolved, polarization can occur.
- 9.
Since there is no alignment on the ground state and we can choose the direction of radiation as the quantization axis, α = θ.
- 10.
Incidentally, these studies induced a local revolution in understanding of the solar spectra. We expect even deeper impact of the GSA studies. Indeed, the domain of the applicability of the GSA is really extensive and the consequences of the magnetic field and abundance studies are extremely important.
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Acknowledgements
HY acknowledges the support from 985 grant from Peking University and the “Beyond the Horizons” grant from Templeton foundation as well as the visiting professorship at the International Institute of Physics (Brazil). AL’s research is supported by the NSF AST 1109295 and the NSF Center for Magnetic Self-Organization (CMSO). He also acknowledges the Humboldt Award and related productive stay at the Universities of Bochum and Cologne.
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Yan, H., Lazarian, A. (2015). Magnetic Field Measurement with Ground State Alignment. In: Lazarian, A., de Gouveia Dal Pino, E., Melioli, C. (eds) Magnetic Fields in Diffuse Media. Astrophysics and Space Science Library, vol 407. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-44625-6_5
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