High-energy particles were recorded by the near-Earth spacecraft and groundbased neutron monitors (NMs) on 2012 May 17. This event was the first Ground Level Enhancement (GLE) of the solar cycle 24. In present study, we try to identify the acceleration source(s) of solar energetic particles (SEPs) by combining in-situ particle measurements from W IND/3DP, GOES 13, and solar cosmic rays (SCRs) registered by several NMs, as well as the remote-sensing solar observations from SDO/AIA, SOHO/LASCO, and RHESSI. We derive the interplanetary magnetic field (IMF) path length (1.25 ± 0.05 AU) and solar particle release (SPR) time (01:29 ± 00:01 UT) of the first arriving electrons by using their velocity dispersion and taking into account the contamination effects. It is found that the electron impulsive injection phase, indicated by the dramatic change of spectral index, is consistent with the flare non-thermal emission and type III radio bursts. Based on the potential field source surface (PFSS) concept, a modeling of the open-field lines rooted in the active region (AR) has been performed to provide escaping channels for flare-accelerated electrons. Meanwhile, relativistic protons are found to be released ∼10 min later than the electrons, assuming their scatter-free travel along the same IMF path length. Combining multi-wavelength imaging data on the prominence eruption and coronal mass ejection (CME), we obtain some evidence of that GLE protons, with estimated kinetic energy of ∼1.12 GeV, are probably accelerated by the CME-driven shock when it travels to ∼3.07 solar radii. The time-of-maximum (TOM) spectrum of protons is typical for the shock wave acceleration.
[1] Ground-level enhancements (GLEs) are sudden, sharp, and short-lived increases in cosmic ray intensities registered by neutron monitors. These enhancements are known to take place during powerful solar eruptions. In the present investigation, the cosmic ray intensities registered by the Oulu neutron monitor have been studied for the period between January 1979 and July 2009. Over this span of time, increase rates of 32 GLEs have been deduced. In addition, we have studied characteristics of the 32 event-associated solar flares, coronal mass ejections (CMEs), and solar energetic particle (SEP) fluxes. We found that all of the 32 GLEs were associated with solar flares, CMEs, and SEP fluxes. Approximately 82% of the events were associated with X-class flares. Most of the flares that were associated with GLEs of increase rates >10% originated from the active regions located on the southwest hemisphere of the Sun. The average speed (1726.17 km/s) of GLE-associated CMEs was much faster than the average speed (423.39 km/s) of non-GLE-associated CMEs. It also became evident that ∼67% GLEs were associated with very fast (>1500 km/s) CMEs. Although a GLE event is often associated with a fast CME, this alone does not necessarily cause the enhancement. Solar flares with strong optical signatures may sometimes cause GLE. High SEP fluxes often seem to be responsible for causing GLEs as the correlation with SEP fluxes implies.
The flux rate of cosmic rays incident on the Earth's upper atmosphere is modulated by the solar wind and the Earth's magnetic field. The amount of solar wind is not constant due to changes in solar activity in each solar cycle, and hence the level of cosmic ray modulation varies with solar activity. In this context, we have investigated the variability and the relationship of cosmic ray intensity with solar, interplanetary, and geophysical parameters from January 1982 through December 2008. Simultaneous observations have been made to quantify the exact relationship between the cosmic ray intensity and those parameters during the solar maxima and minima, respectively. It is found that the stronger the interplanetary magnetic field, solar wind plasma velocity, and solar wind plasma temperature, the weaker the cosmic ray intensity. Hence, the lowest cosmic ray intensity has good correlations with simultaneous solar parameters, while the highest cosmic ray intensity does not. Our results show that higher solar activity is responsible for a higher geomagnetic effect and vice versa.
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