We explore for the first time effects of the magnetic field on the escape of $^{22}$Na positrons and on the flux evolution of annihilation 511 keV line in novae. It is shown that for the white dwarf magnetic field of $sim 10^6$ G the field of the expanding nova shell is able to significantly impede positrons escape and increase the time of the nova emission in 511keV up to hundreds days.
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