LAUSR

Using the global navigation satellite system (GNSS) data from the Hong Kong region, this study comprehensively investigates the ionospheric irregularities responses to strong geomagnetic storms over the past two solar cycles 2001–2020. Based on the geomagnetic index Dst, a total of 64 strong storms are confirmed during 2001–2020. Statistical results indicate that for the total 64 strong storms, only 20 storms are considered to trigger irregular occurrences. When the occurrence local time (LT) of the minimum dDst (dDst $_{\mathrm {min}}$ ) is in 10:00–14:00 LT, no ionospheric irregularities occurred at nighttime although there is a total of 14 strong storms, while that of dDst min is in the nighttime of 18:00–21:00 LT, ionospheric irregularities are detected in ten out of 12 strong storms. For the two special storms on 19 April 2002 (dDst min occurred at 21:00 LT) and 23 May 2002 (dDst min occurred at 20:00 LT), they did not trigger the generation of ionospheric irregularities although their dDst min occurred in 18:00–21:00 LT. Based on vertical total electron content (VTEC) derived from global positioning system (GPS) measurements, it is found that the westward electric fields during two storms should play a vital role to inhibit the nighttime ionospheric irregularities (NIIs) occurrence. This study suggests that caution should be taken when the dDst min determined LT is used to decide the occurrence of nighttime irregularities.