  We analyse high temporal and spatial resolution time-series of spectral scans
of the Halpha line obtained with the CRisp Imaging SpectroPolarimeter (CRISP)
instrument mounted on the Swedish Solar Telescope. The data reveal highly
dynamic, dark, short-lived structures known as Rapid Redshifted and Blueshifted
Excursions (RREs, RBEs) that are on-disk absorption features observed in the
red and blue wings of spectral lines formed in the chromosphere. We study the
dynamics of RREs and RBEs by tracking their evolution in space and time,
measuring the speed of the apparent motion, line-of-sight Doppler velocity, and
transverse velocity of individual structures. A statistical study of their
measured properties shows that RREs and RBEs have similar occurrence rates,
lifetimes, lengths, and widths. They also display non-periodic, non-linear
transverse motions perpendicular to their axes at speeds of 4 - 31 km/s.
Furthermore, both types of structures either appear as high speed jets and
blobs that are directed outwardly from a magnetic bright point with speeds of
50 - 150 km/s, or emerge within a few seconds. A study of the different
velocity components suggests that the transverse motions along the
line-of-sight of the chromospheric flux tubes are responsible for the formation
and appearance of these redshifted/blueshifted structures. The short lifetime
and fast disappearance of the RREs/RBEs suggests that, similar to type II
spicules, they are rapidly heated to transition region or even coronal
temperatures. We speculate that the Kelvin-Helmholtz instability triggered by
observed transverse motions of these structures may be a viable mechanism for
their heating.
