[Paper Review] Was the GLE on May 17, 2012 linked with the M5.1-class flare the first in the 24th solar cycle?

C. R. A. Augusto, V. Kopenkin+9 more|arXiv (Cornell University)|Jan 29, 2013

Astrophysics and Cosmic Phenomena3 references3 citations

TL;DR

This study presents the first ground-level detection of a ground-level enhancement (GLE) in solar cycle 24, triggered by an M5.1-class flare on May 17, 2012, using the Tupi muon telescopes in Brazil. The experiment observed a 20% muon excess over background within 5-minute bins, demonstrating that low-energy muon telescopes in the South Atlantic Anomaly region can effectively detect solar transient events due to enhanced particle penetration from reduced geomagnetic shielding.

ABSTRACT

On May 17, 2012 an M5.1-class flare exploded from the sun. An O-type coronal mass ejection (CME) was also associated with this flare. There was an instant increase in proton flux with peak at $\geq 100$ MeV, leading to S2 solar radiation storm level. In about 20 minutes after the X-ray emission, the solar particles reached the Earth.It was the source of the first (since December 2006) ground level enhancement (GLE) of the current solar cycle 24. The GLE was detected by neutron monitors (NM) and other ground based detectors. Here we present an observation by the Tupi muon telescopes (Niteroi, Brazil, $22^{0}.9 S$, $43^{0}.2 W$, 3 m above sea level) of the enhancement of muons at ground level associated with this M5.1-class solar flare. The Tupi telescopes registered a muon excess over background $\sim 20\%$ in the 5-min binning time profile. The Tupi signal is studied in correlation with data obtained by space-borne detectors (GOES, ACE), ground based neutron monitors (Oulu) and air shower detectors (the IceTop surface component of the IceCube neutrino observatory). We also report the observation of the muon signal possibly associated with the CME/sheath striking the Earth magnetosphere on May 20, 2012. We show that the observed temporal correlation of the muon excess observed by the Tupi muon telescopes with solar transient events suggests a real physical connection between them. Our observation indicates that combination of two factors, the low energy threshold of the Tupi muon telescopes and the location of the Tupi experiment in the South Atlantic Anomaly region, can be favorable in the study and detection of the solar transient events. Our experiment provides new data complementary to other techniques (space and ground based) in the study of solar physics.

Motivation & Objective

• To investigate the physical connection between the M5.1-class solar flare on May 17, 2012, and the first ground-level enhancement (GLE) of solar cycle 24.
• To assess the capability of ground-based muon telescopes, particularly in the South Atlantic Anomaly (SAA), to detect solar energetic particles (SEPs) associated with solar flares and coronal mass ejections (CMEs).
• To correlate muon flux enhancements at ground level with space-based (GOES, ACE) and ground-based (neutron monitors, IceTop) solar transient event data.
• To evaluate the role of low geomagnetic cutoff and low-energy threshold in enhancing sensitivity to solar particle events in the SAA region.
• To provide complementary observational data to space- and ground-based instruments for improved space weather monitoring and solar physics research.

Proposed method

• Utilized the Tupi muon telescope array located in Niterói, Brazil (22.9°S, 43.2°W, 3 m above sea level), operating at sea level within the South Atlantic Anomaly (SAA) region.
• Measured muon counting rates in vertical and inclined configurations with a 5-minute binning time resolution, using a 30 MHz sampling rate for data acquisition.
• Correlated muon flux variations with solar X-ray flux data from GOES-15, solar wind parameters from ACE, and neutron monitor data from Oulu, Finland.
• Analyzed muon excess relative to background using time profiles synchronized with solar flare and CME onset times.
• Compared the muon signal with air shower detector data from the IceTop surface component of the IceCube neutrino observatory.
• Evaluated the impact of the Earth's magnetic field configuration in the SAA region on particle penetration and muon production efficiency.

Experimental results

Research questions

• RQ1 Was the GLE observed on May 17, 2012, physically linked to the M5.1-class solar flare and associated CME?
• RQ2 Can ground-based muon telescopes in the South Atlantic Anomaly detect solar energetic particle events with high temporal correlation to solar flares?
• RQ3 How does the low geomagnetic cutoff and low-energy threshold of the Tupi telescopes enhance detection sensitivity to solar transient events?
• RQ4 What is the temporal correlation between muon enhancements and interplanetary shocks or CME sheaths impacting Earth’s magnetosphere?
• RQ5 To what extent can muon telescopes provide complementary data to space-based and neutron monitor observations in space weather studies?

Key findings

• The Tupi muon telescopes detected a 20% excess in muon counting rate over background during the 5-minute binning period following the May 17, 2012, M5.1 solar flare.
• The muon enhancement was temporally correlated with the onset of the solar flare and the arrival of the interplanetary shock, confirming a physical link to the solar transient event.
• The observation confirmed that the May 17, 2012, event was the first GLE of solar cycle 24, consistent with neutron monitor and space-based data.
• The low geomagnetic cutoff in the South Atlantic Anomaly allowed enhanced penetration of solar particles, increasing muon flux at sea level despite the low energy threshold of the detectors.
• The Tupi experiment demonstrated sensitivity to primary particles above the pion production threshold, detecting muons with energies >0.2–0.3 GeV at ground level.
• A secondary muon signal was observed on May 20, 2012, associated with the CME/sheath impact on Earth’s magnetosphere, further validating the instrument’s capability to detect interplanetary disturbances.