Forbush Decreases and Geomagnetic Storms During a Highly Disturbed Solar and Interplanetary Period, 4–10 September 2017

Features and peculiarities of the cosmic ray intensity (CRI) and the geomagnetic activity, along with several solar plasma and interplanetary magnetic field, during the period 4–10 September 2017 are studied. The period was characterized by strong solar activity: Several solar flares occurred; several halo coronal mass ejections (CMEs) were ejected in space. In the near‐Earth interplanetary space, the CMEs driving shock(s) and sheath(s) were identified. At the Earth, strong Forbush decreases in CRI and geomagnetic storms were observed. Several large solar flares, one of them of very high X‐ray importance (X9.3) and three halo CMEs were detected in the solar atmosphere. Two shock‐associated interplanetary CMEs were observed during that interval in near‐Earth space; the latter and faster one arrived even as the ejecta of the earlier one was still crossing. Variations in interplanetary plasma and field parameters during, before, and after the Forbush decreases and geomagnetic storms that occurred during the considered period were examined. A detailed time‐lagged correlation analysis using data at three different temporal resolutions (hourly, 5‐min, and 1‐min) was also performed. Cross correlations of time series of CRI with geomagnetic activity during the period 4–10 September 2017 are computed. This cross‐correlation analysis between CRI variability (defined as the difference of the CRI count rate between the current and the previous time step) and the Dst indicates a delay of Dst by 3–4 hr. Key Points Forbush decreases and geomagnetic storms in the interval 4–10 September 2017 are studied Simultaneous high‐resolution data of cosmic rays, Dst index, and solar wind are analyzed Time lag correlation analysis is performed, and possibility of its use in space weather forecast is indicated