  The recent confirmation of the bipolarity of the eROSITA bubbles suggests
that the well-known North Polar Spur (NPS)/Loop I probably is a 10 kpc sized
relic in the Galactic halo and not a small local structure near the Sun. By
virtue of multiwavelength observations of the NPS, unprecedentedly precise
parameter constraints on the cosmic-ray electrons (CRes) and magnetic field in
the post-shock halo medium can be provided. The parameters of the CRes and the
magnetic field can be derived independently by modeling the gamma-ray and the
radio data of the NPS via inverse Compton scattering and synchrotron emission,
respectively. Our main results are (1) that the energy density of the CRe is
(3-6)$\times 10^{-14}$ erg cm$^{-3}$, and the spectral index is $p\simeq 2.0\pm
0.1, $ below the cooling break energy of about 5 GeV; (2) that the magnetic
field strength is 3 $\mu$G; and (3) that the shock acceleration efficiency of
the CRe is (1-2)%. Given the Mach number of 1.5, the high acceleration
efficiency and flat spectrum of the CRe suggest that preexisting relativistic
electrons may be reaccelerated in the NPS. Alternatively, these CRes could be
accelerated by an evolving shock in the early epoch when its Mach number is
high, and efficiently diffuse throughout the post-shock halo gas. In addition,
the cooling break energy suggests that the cooling timescale is $10^7$ yr,
which agrees with the age of the eROSITA bubbles.
