Abundance Enhancements in Impulsive Solar Energetic-Particle Events with Associated Coronal Mass Ejections

Abstract

We study the abundances of the elements He through Pb in Fe-rich impulsive solar energetic-particle (SEP) events with measurable abundances of ions with atomic number Z>2 observed on the Wind spacecraft, and their relationship with coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). On an average the element abundances in these events are similar to coronal abundances at low Z but, for heavier elements, enhancements rise as a power law in the mass-to-charge ratio A/Q of the ions (at coronal temperatures of 2.5 – 3 MK) to a factor of 3 at Ne, 9 at Fe, and 900 for 76≤Z≤82. Energy dependences of abundances are minimal in the 2 – 15 MeV amu⁻¹ range. The 111 of these Fe-rich impulsive SEP events we found, between November 1994 and August 2013 using the Wind spacecraft, have a 69 % association rate with CMEs. The CMEs are narrow with a median width of 75^∘, are characteristically from western longitudes on the Sun, and have a median speed of ≈ 600 km s⁻¹. Nearly all SEP onsets occur within 1.5 – 5 h of the CME onset. The faster (> 700 km s⁻¹), wider CMEs in our sample are related to SEPs with coronal abundances indicating hot coronal plasma with fully ionized He, C, N and O and moderate enhancements of heavier elements, relative to He, but slower (< 700 km s⁻¹), narrower CMEs emerge from cooler plasma where higher SEP mass-to-charge ratios, A/Q, yield much greater abundance enhancements, even for C/He and O/He. Apparently, the open magnetic-reconnection region where the impulsive SEPs are accelerated also provides the energy to drive out CME plasma, accounting for a strong, probably universal, impulsive SEP-CME association.

Appendix

Table 2 lists properties of 111 Fe-rich impulsive SEP events. Columns in the table are the event number, the SEP onset time, the SEP event duration (Dur.), the abundance ratios of Fe/O, Ne/O, and He/O at 3 – 4 MeV amu⁻¹, the ratio (50≤Z≤56)/O at 3 – 10 MeV amu⁻¹, the associated CME speed, V, the central position angle (CPA) and width (W) of the CME, the delay between the CME onset and the SEP onset, the UT onset time of the DH type III burst (* indicates previous day), the presence of a DH type II radio burst, and the location of associated flare or other source. NA in the CME speed indicates that no LASCO coronagraph data were available, dash indicates that data were available but no CME was listed.

Table 2 Properties of the Fe-rich SEP events and associated CMEs.