Interplanetary magnetic flux rope observed at ground level by HAWC

TL;DR

This study reports ground-level detection of a Galactic Cosmic-Ray flux enhancement caused by a magnetic flux rope associated with a slow CME, highlighting the role of magnetic field configuration and alignment in cosmic-ray modulation.

Contribution

It provides the first ground-based observation of a magnetic flux rope at Earth and analyzes its impact on cosmic-ray flux, emphasizing the importance of magnetic alignment.

Findings

GCR flux enhancement lasted about 17 hours.

Magnetic flux rope preserved its configuration during transit.

Alignment between MFR axis and observer's asymptotic direction is key.

Abstract

We report the ground-level detection of a Galactic Cosmic-Ray (GCR) flux enhancement lasting $\sim$ 17 hr and associated with the passage of a magnetic flux rope (MFR) over the Earth. The MFR was associated with a slow Coronal Mass Ejection (CME) caused by the eruption of a filament on 2016 October 9. Due to the quiet conditions during the eruption and the lack of interactions during the interplanetary CME transport to the Earth, the associated MFR preserved its configuration and reached the Earth with a strong magnetic field, low density, and a very low turbulence level compared to the local background, thus generating the ideal conditions to redirect and guide GCRs (in the $\sim$ 8 to 60 GV rigidity range) along the magnetic field of the MFR. An important negative $B_Z$ component inside the MFR caused large disturbances in the geomagnetic field and a relatively strong geomagnetic storm. However, these disturbances are not the main factors behind the GCR enhancement. Instead, we found that the major factor was the alignment between the MFR axis and the asymptotic direction of the observer.