A two-fold analysis of electromagnetic core tokamak instabilities in the framework of the gyrokinetic theory is presented. First principle theoretical foundations of the gyrokinetic theory are used to explain and justify the numerical results obtained with the global electromagnetic particle-in-cell code ORB5 whose model is derived from the Lagrangian formalism. The energy conservation law corresponding to the ORB5 model is derived from the Noether theorem and implemented in the code as a diagnostics for energy balance and conservation verification. An additional Noether theorem based diagnostics is implemented in order to analyse destabilising mechanisms for the electrostatic and the electromagnetic ion temperature gradient instabilities in the core region of the tokamak. The transition towards the Kinetic Ballooning Modes at high electromagnetic beta is also investigated.