What is polarity of self induced emf?
Your indication of the voltage polarity after the switch opens is correct, however your understanding of the magnetic field at that point is not.

The magnetic field existed before the switch opened and at that moment remains the same, meanwhile, the current still wants to flow through the inductor. 

The magnetic field it will decay very quickly as the current decays in inductor.







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Until it does, and as you can see from the image above, the only current path for the inductor to cycle that current, is through the parasitic impedance of the air around it. The voltage drop across that impedance drops the left side of the inductor to a negative value compared to the right side.

This is why, when switching inductors, we use fly-back diodes. The voltage on the left side of the diode in the image below will drop to the point where the diode turns on and allows the current in the inductor, and the associated magnetic field, to safely decay.







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