Relativistic Proton Production during the 2000 July 14 Solar Event: The Case for Multiple Source Mechanisms

Abstract

Protons accelerated to relativistic energies by transient solar and interplanetary phenomena caused a ground-level cosmic ray enhancement on 14 July 2000, Bastille Day. Near-Earth spacecraft measured the proton flux directly and ground-based observatories measured the secondary responses to higher energy protons. We have modelled the arrival of these relativistic protons at Earth using a technique which deduces the spectrum, arrival direction and anisotropy of the high-energy protons that produce increased responses in neutron monitors. To investigate the acceleration processes involved we have employed theoretical shock and stochastic acceleration spectral forms in our fits to spacecraft and neutron monitor data. During the rising phase of the event (10:45 UT and 10:50 UT) we find that the spectrum between 140 MeV and 4GeV is best fitted by a shock acceleration spectrum. In contrast, the spectrum at the peak (10:55 UT and 11:00 UT) and in the declining phase (11:40 UT) is best fitted with a stochastic acceleration spectrum. We propose that at least two acceleration processes were responsible for the production of relativistic protons during the Bastille Day solar event:(1) protons were accelerated to relativistic energies by a shock, presumably a coronal mass ejection (CME).(2) protons were also accelerated to relativistic energies by stochastic processes initiated by magnetohydrodynamic (MHD) turbulence.

Analysis of the Ground-Level Enhancements on 14 July 2000 and 13 December 2006 Using Neutron Monitor Data

“…A detailed description of the method is given elsewhere (Mishev, Kocharov, and Usoskin, 2014). The method is similar to that used by Shea and Smart (1982); Humble et al (1991); Cramp et al (1997); Bombardieri et al (2006); Vashenyuk et al (2006bVashenyuk et al ( , 2008.…”

“…The fractional increase of the count rate of a NM station represents the ratio between the NM count rates due to SEPs and GCR averaged over 2 hours before the event's onset. In our model we assume a modified power law rigidity spectrum of SEP similarly to Cramp, Humble, and Duldig (1995); Cramp et al (1997); Bombardieri et al (2006); Vashenyuk et al (2008):…”

Numerical model for computation of effective and ambient dose equivalent at flight altitudes

“…Details for the analysis of GLE events based on data from several NM stations are given in (Shea & Smart 1982;Cramp et al 1997;Bombardieri et al 2006;Vashenyuk et al , 2008. Detailed description of the method used in this study is given by .…”

“…For the GCR we apply the described above force field model and reconstructed solar modulation parameter according to Usoskin et al (2011). A modified power law rigidity spectrum of SEP is assumed in the analysis similarly to Cramp et al (1997); Bombardieri et al (2006);Vashenyuk et al (2008):…”

Assessment of spectral and angular characteristics of sub-GLE events using the global neutron monitor network

“…Shea and Smart, 1982;Cramp et al, 1997;Bombardieri et al, 2006;Mishev et al, 2014). According to the generally accepted common definition, a GLE event is registered when there is a simultaneous statistically significant increase of the count rate of at least two differently located NM stations accompanied with a statistically significant increase of the SEP flux directly observed by a spaceborne instrument(s), accordingly a sub-GLE event is registered when there are simultaneous statistically significant increase of the count rates of at least two differently located high-altitude NMs with corresponding enhancement in the proton flux measured by a space-borne instrument(s), but no statistically significant enhancement in the countrates of NMs near to the sea level .…”