Long-lasting disturbances in the mid-latitude sub-ionospheric VLF radio signals due to the super geomagnetic storm of 17 March 2015

•Effect of super geomagnetic storm on the sub-ionospheric VLF radio signals.•Long-lasting VLF signal disturbances observed in the mid-latitude ionosphere.•West-east and east-west propagation paths showed different disturbance patterns.•Enhanced electron density in the D-region follows the precipitated electron flux.•Wave-like oscillations with periods in the range of 15–80 min have been observed.