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tokens: gravitational instabilities (GI) operating in the outer disc, driving material towards the protostar, and the magneto-rotational instability (MRI) operating in the inner disc region around the young protostar when the conditions are appropriate for a sufficient ionization fraction (Armitage et al. 2001 ; Zhu et al. 2009a , b , 2010a ; Zhu et al. 2010b ; Stamatellos et al. 2011 ; Mercer Stamatellos 2017 ). As the disc increases in mass by accreting infalling gas from the envelope it becomes gravitationally unstable, as suggested by the formation of the spiral structures seen in the quiescent phase snapshots in the left column of Fig. 1 . Thus, angular momentum is redistributed outwards efficiently resulting in inward gas flow. The inner disc is hotter and gravitationally stable such that there is no mechanism available to redistribute angular momentum, and material accumulates around the protostar (∼1 au). As more gas flows into this region, compressive, viscous, and protostellar heating raise the local temperature further. Once a threshold temperature of 1400 K is reached, this region is sufficiently ionized for the MRI to be activated (Zhu et al. 2009a , b , 2010a ; Zhu et al. 2010b ). The MRI increases the viscosity (the effective viscosity parameter becomes α MRI = 0.1; Shakura Sunyaev 1973 ), providing a way to redistribute angular momentum and resulting in rapid gas accretion onto the protostar. During the outburst phase, the accretion luminosity increases dramatically (∼10 3 L ⊙ ), heating the disc and disrupting the spiral structure (outbursting phase snapshots in the left column of Fig. 1 ; see also MacFarlane Stamatellos 2017 ). Once the gas from the inner region has been accreted, the protostar returns to the quiescent phase. The process of quiescent to outbursting phase repeats multiple times during the evolution of the YSO (Stamatellos et al. 2012 ). Outbursts happen every ∼1000 yr initially, whereas at later stages (but still during the Class O/I stage) every ∼10 000 yr (see Stamatellos et al. 2011 , 2012 ). This is similar to what is inferred from observations (e.g. Scholz, Froebrich Wood 2013 ; Hillenbrand Findeisen 2015 ; Fischer, Safron Megeath 2019 ; see also Peña, Naylor Morrell 2019 , for longer outburst time-scale estimates for older, Class I/II, YSOs). It is assumed that in the quiescent phase gas accretes onto the protostar at a rate of (this is a free parameter in our calculation) and the protostellar luminosity is then on the order of ∼1 L ⊙ . As a comparison, at the same time the accretion rate onto the outer disc is on the order of ∼10 −5 −10 −4 M ⊙ yr −1 . During the outburst phase the accretion onto the protostar is significantly enhanced. This model is different than the models of Vorobyov Basu ( 2005 , 2006 , 2010 ). In their models the GIs alone are responsible for the observed outbursts: GIs create clumps in the disc of a young stellar object that eventually fall onto the protostar, resulting in outbursts