  Using the Atmospheric Imaging Assembly 304 A images obtained from the Solar
Dynamics Observatory, we study two jets which occurred during the M5.8 class
flare on 2017 April 3 and the M5.5 class flare on 2016 July 23, respectively.
During the M5.8 class flare, many vortex-like structures occurred in the
upstream and downstream regimes of the associated jet. While the jet was
ejected upwards to the corona, some dark material at its base flowed through a
bright structure with a velocity of 110 km/s. The boundary between the material
and the structure changed from smooth to uneven. Later, the jet material at the
higher atmosphere started to fall down with velocities of over 200 km/s, and
the left boundary of the jet developed into a sawtooth pattern. The vortex-like
structures were formed, and the growth rates of two structures were presented.
During the M5.5 class flare, we also observed many vortex-like structures in
the downstream regime of the jet. At the late stage of the jet, some material
at the south boundary of the jet fell back to the solar surface, and
vortex-like structures at the boundary grew from ripple-like minim into
vortices with diameters of 3.4-5.4 Mm. The growth rates of the vortex-like
structures were calculated. We suggest that the vortex-like structures in the
upstream regime are the manifestations of Kelvin-Helmholtz instability, and
those in the downstream regime are simultaneously driven by Kelvin-Helmholtz
instability and Raleigh-Taylor instability.
