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

Abstract

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.

Effects of the 10 September 2017 Solar Flare on the Density and Composition of the Thermosphere of Mars

“…Earth's plasmasphere was severely eroded (Obana et al., 2019), and at lower altitudes, a CME shock increased the outflow flux of O + from the ionosphere to three times a baseline value already elevated by the earlier impacts of X‐class flares and another shock (Schillings et al., 2018). The CMEs and their associated SEPs corresponded to decreased intensities of cosmic rays (Forbush decreases) as the changing interplanetary magnetic field deflected a greater proportion away from Earth (Badruddin et al., 2019; Chertok et al., 2018). However, the intensities of high‐energy charged particles at Earth's surface increased (ground level enhancements), as the enhanced fluxes of SEPs augmented the typical background provided by cosmic rays (Cohen & Mewaldt, 2018; Copeland et al., 2018; Matthiä et al., 2018; Mavromichalaki et al., 2018; Mishev & Usoskin, 2018).…”

Forbush-storm classification of the events as a device for the solar wind diagnostics

“…June, [26][27][28] February, 18-20 November, 6-8 Thus, this example shows that in the three classes of the events, the behavior of interplanetary parameters is significantly different. This allows us to consider the Forbush-Storm classification of events as a device for determining how the Earth's trajectory passes through the solar wind flow.…”

“…In papers [24][25][26], Forbush-decreases and geomagnetic storms data are used for the investigation of disturbed solar wind structures, but usually such works focus their attention on events of high intensity. The main distinction of our approach is that we take into account not only intense, but also moderate and weak events [27].…”

The role of extreme geomagnetic storms in the Forbush decrease profile

“…They also pointed that maximum negative Dst and maximum FD amplitude did not occur at the same hour. [Badruddin et al, 2019] performed cross-correlation analysis between cosmic ray intensity variation with that of the geomagnetic index and suggested a delay of the storm by 3-4 hours. This indicates that FD can provide storm information by one to a few hours scale in advance.…”