The 2015 Summer Solstice Storm: one of the major geomagnetic storms of solar cycle 24 observed at ground level

TL;DR

This study analyzes the 2015 geomagnetic storm caused by CMEs from active solar regions, highlighting ground and space observations, and suggests narrow CMEs in the western ecliptic can cause severe storms.

Contribution

It provides a detailed correlation between narrow CMEs and severe geomagnetic storms, emphasizing ground-based cosmic ray observations during the 2015 event.

Findings

Ground level cosmic ray detectors recorded Forbush Decreases during the storm.

Narrow CMEs in the western ecliptic can lead to severe geomagnetic storms.

The 2015 G4 storm was the second largest of solar cycle 24.

Abstract

We report on the 22-23 June 2015 geomagnetic storm. There has been a shortage of intense geomagnetic storms during the current solar cycle 24 in relation to the previous cycle. This situation changed after mid-June 2015 when one of the biggest solar active regions (AR 2371) of current solar cycle 24, close to the central meridian produced several coronal mass ejections (CMEs) associated with M-class flares. The CMEs impact on the Earth's magnetosphere resulted in a moderately-severe G4-class geomagnetic storm on 22-23 June 2015 and a G2 (moderate) geomagnetic storms on 24 June. The G4 solstice storm was the second biggest (so far) geomagnetic storms of cycle 24. We highlight the ground level observations made by New-Tupi, Muonca and the CARPET El Leoncito cosmic ray detectors that are located within the South Atlantic Anomaly (SAA) region. These observations are studied in correlation with data obtained by space-borne detectors and other ground-based experiments. The CME designations are from the Computer Aided CME Tracking (CACTus) automated catalog. As expected, Forbush Decreases (FD) associated with the passing CMEs were recorded by these detectors. We noticed a peculiar feature linked to a severe geomagnetic storm event. The 21 June 2015 CME 0091 was likely associated with the 22 June summer solstice FD event. The angular width of CME 0091 was very narrow and measured 56 degrees seen from Earth. In most cases, only CME halos and partial halos, lead to severe geomagnetic storms. We performed a cross-check analysis of the FD events detected during the rise phase of the current solar cycle 24, the geomagnetic parameters, and the CACTus CME catalog. Our study suggests that narrow angular-width CMEs that erupt in the western region of the ecliptic plane can lead to moderate and severe geomagnetic storms.