Onset Time of the GLE 72 Observed at Neutron Monitors and its Relation to Electromagnetic Emissions

Abstract

We present an overview of the ground-level enhancement (GLE 72) of the cosmic-ray intensity associated with the recent powerful solar flare SOL2017-09-10 (X-ray class X8.9) based on the available neutron monitor (NM) network observations and on data from the satellite GOES 13. The maximum increase at high-latitude near-sea-level NMs was ${\approx }\,6\,\mbox{--}\,7\%$ (2-min averages), greater with better time resolution. A scatter plot of the maximum increase of the GLE versus solar energetic-particle (SEP, proton) flux ${>}\,100~\mbox{MeV}$ shows one of the softest spectra among GLEs relative to the SEP fluxes. However, at two high-mountain middle-latitude NMs the increase was ${\approx }\,1\%$ , indicating the possibility of proton acceleration up to 6 GeV. Among the analyzed NM data the Fort Smith (FSMT) NM shows the earliest and the rather high increase between 16:06 – 16:08 UT. This indicates an anisotropy in the first phase of the GLE event. We calculate the acceptance cones of several NM stations at high latitudes and contours of pitch angles corresponding to the interplanetary magnetic field (IMF). When employing the available data we find that pion-decay $\gamma $ -ray emission onset is in accordance with the time of the main flare energy release. The observed time interval of the impulsive burst of ${>}\,100~\mbox{MeV}$ $\gamma $ -ray emission probably corresponds to the time of a turbulent current sheet creation. The observed location of the impulsive burst pion-decay emission source coincides with the active region and the cusp-shaped structure. It seems that models assuming sub-relativistic proton production beginning in a turbulent reconnecting current sheet are consistent with the observations. If these particles were released from the Sun during a type III emission with a pion-decay maximum at $16{:}00{:}30\pm 30~\mbox{UT}$ , we get a plausible path length equal to $1.5\pm 0.3~\mbox{AU}$ of the particles responsible for the onset of the SEP event and GLE. The time lag of GLE 72 corresponds to the most probable interval of the time difference between GLE onset and main flare energy release. Although other scenarios are not excluded we attribute the protons that create the pion-decay emission and the protons responsible for the GLE and SEP event onset to a general population of accelerated particles.