Fingerprint of Galactic Loop I on polarized microwave foregrounds
  Context: Currently, detection of the primordial gravitational waves by the
B-mode of Cosmic Microwave Background (CMB) is primarily limited by our
knowledge of the polarized microwave foreground emissions. Thus improvements of
the foreground analysis are necessary. As revealed
in~\cite{2018arXiv180410382L}, the E-mode and B-mode of the polarized
foreground have noticeable different properties, both in morphology and
frequency spectrum, suggesting that they arise from different physical
processes, and need to be studied separately.
  Aims: I will study the polarized emission from Galactic loops, especially
Loop I, and mainly focus on the following issues: Does it contribute
predominantly to the E-mode or B-mode? In which frequency bands and in which
sky regions can it be identified?
  Methods: Based on a well known result about the magnetic field alignment in
supernova explosions, a theoretical expectation is established that the loop
polarizations should be predominantly E-mode. In particular, the expected
polarization angles of Loop I are compared with those from the real microwave
band data of WMAP and Planck.
  Results and conclusions: The comparison between model and data shows
remarkable consistency between data and expectation at all bands and for a
large area of the sky. This result suggests that the polarized emission of
Galactic Loop I is a major polarized component in all microwave bands from 23
to 353 GHz, and a considerable part of the polarized foreground is likely
originated from a local bubble associated with Loop I, instead of the far more
distant Galactic emission. The result also provides a possible way to explain
the reported E-to-B excess~\citep{2016A&A...586A.133P} by contribution of the
loops. Finally, this work may also provide the first geometrical evidence that
the Earth was hit by a supernova explosion.
