We analyse, theoretically and experimentally, the nature of solitonic
vortices (SV) in an elongated Bose-Einstein condensate. In the experiment, such
defects are created via the Kibble-Zurek mechanism, when the temperature of a
gas of sodium atoms is quenched across the BEC transition, and are imaged after
a free expansion of the condensate. By using the Gross-Pitaevskii equation, we
calculate the in-trap density and phase distributions characterizing a SV in
the crossover from an elongate quasi-1D to a bulk 3D regime. The simulations
show that the free expansion strongly amplifies the key features of a SV and
produces a remarkable twist of the solitonic plane due to the quantized
vorticity associated with the defect. Good agreement is found between
simulations and experiments.Comment: 6 pages, 4 figure