  The sources of ultrahigh-energy cosmic rays (UHECRs) have been difficult to
catch. It was recently pointed out that while sources of UHECR protons exhibit
anisotropy patterns that become denser and compressed with rising energy,
nucleus-emitting-sources give rise to a cepa stratis (onion-like) structure
with layers that become more distant from the source position with rising
energy. The peculiar shape of the hot spots from nucleus-accelerators is
steered by the competition between energy loss during propagation and
deflection on the Galactic magnetic field (GMF). Here, we run a full-blown
simulation study to accurately characterize the deflections of UHECR nuclei in
the GMF. We show that while the cepa stratis structure provides a global
description of anisotropy patterns produced by UHECR nuclei en route to Earth,
the hot spots are elongated depending on their location in the sky due to the
regular structure of the GMF. We demonstrate that with a high-statistics sample
at the high-energy-end of the spectrum, like the one to be collected by NASA's
POEMMA mission, the energy dependence of the hot-spot contours could become a
useful observable to identify the nuclear composition of UHECRs. This new
method to determine the nature of the particle species is complementary to
those using observables of extensive air showers, and therefore is unaffected
by the large systematic uncertainties of hadronic interaction models.
