Mysterious geomagnetic response to minor solar wind disturbance: Observations

On 15 April 2022, the Kiruna magnetometer detected an isolated geomagnetic spike of 400 nT with rising time of 2 minutes. This is on the same level of large sudden commencements (historically largest one is about 1000 nT in Kiruna), but this event was not followed by any magnetic storm or substorm.  In this sense, the observed 400 nT spike is unique in the history of Kiruna magnetometer (more than 30 years of digital data). At the same time, the Kiruna riometer detected a strong absorption with short rise time, indicating a sudden increase of the electron density.  The world-wide geomagnetic observations available at IMAGE, SuperMAG and INTERMAGNET geomagnetic networks, show isolated localised geomagnetic spikes in the dawn sector in both hemispheres, but not in the dusk sector, gradually moving toward midnight with decreasing intensity.  Detailed analyses of geomagnetic deviation in the northern hemisphere indicates strong shear in the ionospheric Hall current with the sense of downward field.  Considering its location and electron density increase, this field-aligned current is most likely caused by the ring current particles, as is indicated by DMSP data. The solar wind velocity is constant with no specific variation that can cause such a unique event.  However, multi-spacecraft observations by SOHO, DSCOVR, ACE, Cluster and MMS suggest the possibility of a very localized IMF structure.  We thank magnetic stations of IMAGE, SuperMAG and INTERMAGNET network, and SOHO, DSCOVR, ACE, Cluster, DMSP and MMS team for providing data.