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On April 18, 2014 (SOL2014-04-18T13:03) an M-class flare was observed byIRIS. The associated flare ribbon contained a quasi-periodic sawtooth patternthat was observed to propagate along the ribbon, perpendicular to the IRISspectral slit, with a phase velocity of $\sim 15$ km s$^{-1}$. This motionresulted in periodicities in both intensity and Doppler velocity along theslit. These periodicities were reported to be approximately $\pm0.5$ arcsecondsin position and $\pm20$ km s$^{-1}$ in velocity and were measured to be$\sim180^{\circ}$ out of phase with one another. This quasi-periodic behaviorhas been attributed by others to bursty or patchy reconnection and slippingoccurring during three-dimensional magnetic reconnection. While able to accountfor periodicities in both intensity and Doppler velocity these suggestions donot explicitly account for the phase velocity of the entire sawtooth structure,or for the relative phasing of the oscillations. Here we propose that theobservations can be explained by a tearing mode instability occurring at acurrent sheet across which there is also a velocity shear. Using a linear modelof this instability we reproduce the relative phase of the oscillations, aswell as the phase velocity of the sawtooth structure. We suggest a geometry andlocal plasma parameters for the April 18 flare which would support ourhypothesis. Under this proposal the combined spectral and spatial IRISobservations of this flare may provide the most compelling evidence to date ofa tearing mode occurring in the solar magnetic field.

Modeling a Propagating Sawtooth Flare Ribbon Structure as a Tearing Mode in the Presence of Velocity Shear