Evidence for a Coronal Shock Wave Origin for Relativistic Protons Producing Solar Gamma-Rays and Observed by Neutron Monitors at Earth

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Nugget
Number: 373
1st Author: Athanasios Kouloumvakos
2nd Author: Gerald Share
Published: 16 March 2020
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Contents

Introduction

There have been several Nuggets discussing what was earlier referred to as Long Duration Gamma-Ray Flares. Based on a detailed study of thirty >100 MeV gamma-ray events observed by Fermi LAT, we decided that Late Phase Gamma-Ray Emission (LPGRE) best describes the characteristics of this phenomenon (Ref 1 in ): 1) the gamma-ray emission is from pion-decays; 2) it can begin close to the onset of the impulsive flare, with a distinctly different time history, or up to two hours later; 3) rise times range from a few minutes to 6 hr and durations from 5 minutes to 20 hr; 4) there are typically ten times more >500 MeV protons producing the LPGRE than the flare gamma-ray emission; 5) 28 of the 30 LPGRE events were associated with fast and broad CME's and 29 of the 30 were associated with Type ll radio emission (see Gopal ref for more information on this association); 6) LPGRE and >100 MeV solar energetic particle (SEP) durations appear to be correlated and the number of >500 MeV protons producing the LPGRE appear to range from 0.1 to 50% of the number in the accompanying SEP with large systematic effects; and 7) accompanying >100 keV impulsive bremsstrahlung suggests the presence of a suprathermal flare proton seed population that is accelerated by a shock.

These characteristics support suggestions, beginning as early as the 1980's, that LPGRE is produced by the same CME-shock acceleration process that accelerates SEPs Ref). However, this premise is not universally accepted (Ref). In this Nugget we discuss a recent study (Ref 1) of the 2017 September 10 solar eruptive event that produced high-energy gammma-ray emission with a complex time history (Ref 2) and a ground level enhancement (GLE). We examine a common shock-acceleration origin for both the LPGRE and GLE72. The gamma-ray emission observed by Fermi-LAT exhibits a weak impulsive phase, consistent with that observed in hard X-and gamma-ray line flare emissions, and what appear to be two distinct stages of LPGRE. From a detailed modeling of the shock wave, we derive the 3D distribution and temporal evolution of the shock parameters, and we examine the shock wave magnetic connection with the visible solar disk. The evolution of shock parameters on field lines returning to the visible disk, mirrors the two stages of LPGRE. The time history of shock parameters magnetically mapped to Earth agrees with the rates observed by the Fort Smith neutron monitor during the first hour of the GLE72 if we include a 30% contribution of flare-accelerated protons during the first ten minutes, having a release time following the time history of nuclear gamma-rays.

Comparison of Gamma-Ray Observations with Results of Shock Modeling

The black data points in Figure 1 (Figure 10 in the paper) show the time history of the >100 MeV fluxes over the whole event and for the first 30 minutes (inset). We compare the time history of this high-energy emission for the first 15 minute with lower energy emissions in Figure 2 (Fig 2 of paper). The 100-300 keV (RhESSI) plot shows the impulsive flare bremsstrahlung that extends to >1 MeV based on fits to the Fermi/GBM (panel b) spectra. These fits also reveal the presence of nuclear de-excitation lines produced by flare-accelerated ions (panel c). There is evidence for this flare component in the first minutes of >100 MeV emission before the rapid rise of the first stage of LPGRE at ~15:58 UT (panel d). The second stage of LPGRE becomes evident after 16:05 UT. Observations at later times indicate that there was a striking decrease in the flux between 16:30 and 18 UT or possibly a third stage of acceleration (Ref 2).

We set out to determine whether what appear to be at least two stages of LPGRE can be explained by CME related shock acceleration of protons onto magnetic field lines returning to the visible solar disk. This study follows the CME mapping and shock parameter procedures adapted in References 3 and 4. We used 3D modelling of the CME expansion into the corona and interplanetery space to model the grid of shock paramenters with time. In Figure 3a (middle column of Fig 3) we plot the density compression ratio along the shock front at 16 UT. This ratio is a measure of the efficiency for particle acceleration. In Fig 3b we plot representative field lines connected to the shock front, color coded with the density compression ratio. In Fig 3c we plot the ratio as projected onto the solar disk by these field lines. At this time, most of particle acceleration appears to occur on closed field lines emanating from the active forming an ellipse of particle impacts on the Sun around the active region. The pink shaded region is not visible from Earth. Similar plots can be made for Alfvenic Mach number and magnetic field direction relative to the shock normal.

Using this technique, we made maps of shock parameters on the visible solar disk as function of time and constructed their temporal evolution. In order to more accurately represent the particle impact at the Sun we took into account the probability that the particles could reach the Sun in the presense of a converging magnetic field. In Figure 1 we plot, as blue and red bands, an empirical parameter that estimates the number of particles reaching the Sun, with this probability and the Alfvenic Mach number at the show front as parameters. The bands are plotted for different critical Mach numbers (Mc) determined by the local conditions at the front. The normalized model mirrors both the narrow first stage peak at 16 UT and the hours-long second stage of emission. The rise in flux after 16:08 UT, shown in the inset, can be explained by the rapid change in shock geometry from quasi-perpendicular (Mc=2.7) to quasi-parallel (Mc=1.5). The apparent discontinuity in flux between ~16:30 and 18:00 UT is explained by the large increase in magnetic repulsion encountered by particles returning to the Sun from locations with low field strengths.

We have not attempted to take into account the effects of magnetic turbulence in this study, but its effects are apparent when we compare the model's broad spatial distribution of particle interactions on the visible disk with the active region location of the centroid of the gamma radiation (Ref). This difference can be explained by the large amount of turbulence, and therefore particle precipitation, near the active region relative to other locations on the visible disk.


Comparison of Neutron Monitor Observations with Results of Shock Modeling

We also investigated whether particles accelerated by shocks crossing field lines reaching Earth can explain the time history of relativistic protons observed early in the GLE observed by the magnetically well connected Ft. Smith neutron monitor. The calculated time histories, corrected for three proton transit times, are shown in Figure 4(Fig 12 in paper), along with 2 and 5 minute neutron monitor rates. The red curve shows the best fit to the neutron monitor data allowing for both shock and flare (using the nuclear line time profile shown in Figure 2) particle components. Flare particles only contributed about 30% to the total rate for the first ten minutes of the event that is dominated by the shock component. The large particle transport delay suggests a significant amount of interplanetary turbulence.


Conclusions

Our analysis of the 2017 September 10 event provides compelling evidence for a common shock origin for protons producing the LPGRE and most of the particles observed in GLE72.

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