Fig. 4From: Thermospheric wind variations observed by a Fabry–Perot interferometer at Tromsø, Norway, at substorm onsetsX-component of 12-h ground-based magnetometer data collected at Bear Island station for all event times, using the same format in Fig. 3. Panels a–d indicate the measurements for events R1–R4, respectively. Local substorm onsets can be identified in bBack to article page