  Penumbral microjets (PMJs) are short-lived, jet-like objects found in the
penumbra of sunspots. They were first discovered in chromospheric lines and
have later also been shown to exhibit signals in transition region (TR) lines.
Their origin and manner of evolution is not yet settled. We perform a
comprehensive analysis of PMJs through the use of spectral diagnostics that
span from photospheric to TR temperatures to constrain PMJ properties. We
employed high-spatial-resolution Swedish 1-m Solar Telescope observations in
the Ca II 8542 Angstrom and H-alpha lines, IRIS slit-jaw images, and IRIS
spectral observations in the Mg II h & k lines, the Mg II 2798.75 Angstrom &
2798.82 Angstrom triplet blend, the C II 1334 Angstrom & 1335 Angstrom lines,
and the Si IV 1394 Angstrom & 1403 Angstrom lines. We derived a wide range of
spectral diagnostics from these and investigated other secondary phenomena
associated with PMJs. We find that PMJs exhibit varying degrees of signal in
all of our studied spectral lines. We find low or negligible Doppler velocities
and velocity gradients throughout our diagnostics and all layers of the solar
atmosphere associated with these. Dark features in the inner wings of H-alpha
and Ca II 8542 Angstrom imply that PMJs form along pre-existing fibril
structures. We find evidence for upper photospheric heating in a subset of PMJs
through emission in the wings of the Mg II triplet lines. There is little
evidence for ubiquitous twisting motion in PMJs. There is no marked difference
in onset-times for PMJ brightenings in different spectral lines. PMJs most
likely exhibit only very modest mass-motions, contrary to earlier suggestions.
We posit that PMJs form at upper photospheric or chromospheric heights at
pre-existing fibril structures.
