Geoelectric field coast effect anomalous and geomagnetic pulse anti-phase near 35°N at mid-latitude during aurora onset

During the geomagnetic storm on 24 April 2023, the rare auroras broke up and expanded in the northern part of the Chinese mainland, allowing the auroras to extend to mid-latitudes. It is an important opportunity to study the ground-based electromagnetic pulses response of geomagnetic storms and substorm onset in mid-latitude regions. This work focused on the coastal effects and pulse response in the mid-latitude based on large-scale ground-based geoelectric field (GEF) and geomagnetic field observations. Then it combines satellite data to interpret the mid-latitude geomagnetic pulse anti-phase anomaly and the distortion of the GEF due to the coast effect. The results indicate that: (1) there is a significant pulse disturbance when the main phase of the storm coincides with the lowest point of the H component. At this point, the pulse phase reverses around 35°N, becoming negative in the northern hemisphere and positive in the southern hemisphere; (2) the maximum amplitude of pulses in the NS component of the GEF also occurs near 35°N. The coastal effect on normal GEF pulses becomes disordered, causing the GEF vectors—which typically point inland near the coastline—to behave more erratically; (3) data from the Swarm satellites confirm an anomaly in ionospheric electron concentration near 35°N, along with the presence of a weak temperature zone in the upper atmosphere. These responses reflect the synchronous effect of prompt penetration electric fields on the mid-latitude midday ionosphere disturbance and the midnight substorm onset, which proves that the source of the mid-latitude electromagnetic pulse response anomaly during the auroral expansion is the correlation of multi-sphere layers and multi-physics fields coupling.