The atomic interstellar medium ( ISM ) is observed to be full of linear structures , referred to as “ fibers ” .
Fibers exhibit similar properties to linear structures found in molecular clouds , termed striations .
Suggestive of a similar formation mechanism , both striations and fibers appear to be ordered , quasi-periodic and well-aligned with the magnetic field .
The prevailing formation mechanism for striations involves the excitation of fast magnetosonic waves .
Based on this theoretical model , and through a combination of velocity centroids and column density maps , Tritsis et al .
( 2018 ) developed a method for estimating the plane-of-sky ( POS ) magnetic field from molecular cloud striations .
We apply this method in two H I clouds with fibers along the same line-of-sight ( LOS ) towards the ultra-high-energy cosmic-ray ( UHECR ) hotspot , at the boundaries of Ursa Major .
For the cloud located closer to Earth , where Zeeman observations from the literature were also available , we find general agreement in the distributions of the LOS and POS components of the magnetic field .
We find relatively large values for the total magnetic field ( ranging from \sim 10 to \sim 20 ~ { } \upmu G ) and an average projection angle with respect to the LOS of \sim 50 ^ { \circ } .
For the cloud located further away , we find a large value for the POS component of the magnetic field of 15 ^ { +8 } _ { -3 } ~ { } \upmu G .
We discuss the potential of our new magnetic-field tomography method for large-scale application .
We consider the implications of our findings on the accuracy of current reconstructions of the Galactic magnetic field and on the propagation of UHECR through the ISM .