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The decade-long Vista Variables in the Via Lactea (VVV) survey has detectednumerous highly variable young stellar objects (YSOs). We present a study of 61highly variable VVV YSOs ($\Delta K_s$ = 1-5 mag), combining near infraredspectra from Magellan and VLT with VVV and NEOWISE light curves to investigatephysical mechanisms behind eruptive events. Most sources are spectroscopicallyconfirmed as eruptive variables (typically Class I YSOs) but variableextinction is also seen. Among them, magnetically controlled accretion,identified by H{\sc i} recombination emission (usually accompanied by COemission), is observed in 46 YSOs. Boundary layer accretion, associated with FUOri-like outbursts identified by CO overtone and H$_2$O absorption, is observedonly in longer duration events ($\ge$5 yr total duration). However, even inlong duration events, the magnetically controlled accretion mode predominates,with amplitudes similar to the boundary layer mode. Shorter (100-700 days)eruptive events usually have lower amplitudes and these events are generallyeither periodic accretors or multiple timescale events, wherein largephotometric changes occur on timescales of weeks and years. We find that theratio of amplitudes in $K_s$ and $W2$ can distinguish between variableaccretion and variable extinction. Several YSOs are periodic or quasi-periodicvariables. We identify examples of periodic accretors and extinction-drivenperiodicity among them (with periods up to 5 yr) though more data are needed toclassify some cases. The data suggest that dynamic interactions with acompanion may control the accretion rate in a substantial proportion oferuptive systems, although star-disc interactions should also be considered.

Analysis of physical processes in eruptive YSOs with near infrared spectra and multi-wavelength light curves