Physics
Scientific paper
Aug 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001icrc....9.3612d&link_type=abstract
Proceedings of the 27th International Cosmic Ray Conference. 07-15 August, 2001. Hamburg, Germany. Under the auspices of the Int
Physics
Scientific paper
We have surveyed the 0.5-2.0 MeV nucleon-1 ion composition of 56 interplanetary shocks (IP) observed with the Ultra-Low-Energy Isotope Spectrometer (ULEIS) on board the Advanced Composition Explorer (ACE) from 1997 October 1 through 2000 November 30. Our results show the first ever measurement (25 cases) of 3 He ions being accelerated at IP shocks. The 3 He/4 He ratio at the 25 shocks exhibited a wide range of values between 0.00140.24; the ratios were enhanced between factors of ~3-600 over the solar wind value. During the survey period, the occurrence probability of 3 He-rich shocks increased with rising solar activity as measured in terms of the daily occurrence rates of sunspots and X-ray flares. The 3 He enhancements at IP shocks cannot be attributed to rigidity dependent acceleration of solar wind ions and are better explained if the shocks accelerate ions from multiple sources, one being remnant impulsive solar flare material enriched in 3 He ions. Our results also indicate that the contribution of impulsive flares to the seed population for IP shocks varies from event to event, and that the interplanetary medium is being replenished with impulsive material more frequently during periods of increased solar activity. 1. Introduction Enhancements in the intensities of energetic ions associated with transient interplanetary (IP) shocks have been observed routinely at 1 AU since the 1960's (e.g., Reames 1999). It is presently believed that the majority of such IP shocks are driven by fast coronal mass ejections or CMEs as they propagate through interplanetary space (e.g., Gosling 1993), and that the associated ion intensity enhancements are due to diffusive shock acceleration of solar wind ions (Lee 1983; Jones and Ellison 1991; Reames 1999). However, the putative solar wind origin of the IP-shock accelerated ions is based on composition measurements associated with a very limited number of individual IP shocks (Klecker et al. 1981; Hovestadt et al. 1982; Tan et al. 1989; Tylka et al. 1999). Although Klecker et al. (1981), Hovestadt et al. (1982), and Tan et al. (1989) interpreted their results in terms of shock acceleration of ions left over from prior solar energetic particle (SEP) events, the general consensus is that IP shock-accelerated ions originate from the solar wind (e.g., Reames 1999; Klecker et al. 2000). Recently, however, in surveying energetic (~1 MeV nucleon-1 ) ion measurements obtained during quiet periods in between the so-called large gradual SEPs observed at the Advanced Composition Explorer (ACE) spacecraft, Mason et al. (1999a) found that residual 3 He and Fe ions from impulsive SEPs or solar flares (e.g., Temerin and Roth 1992) can fill a substantial volume (>50%) of the inecliptic interplanetary medium during periods of high solar activity. Mason et al. (1999a) then suggested that such remnant impulsive material could be an important constituent of the seed population that is available for acceleration at coronal or IP shocks driven by CMEs. Thus, according to this hypothesis, IP shocks should accelerate 3 He ions provided that they encounter remnant flare material en route to 1 AU. The main goal of this paper is to search for 3 He ions accelerated locally at IP shocks observed at ACE. We discuss our findings in terms of characteristics of the source population that is available for acceleration at IP shocks and highlight new challenges for current ideas regarding the origin and acceleration of energetic particles at IP shocks.
Desai Mihir I.
Dwyer Joesph R.
Koug R. M.
Mason Glenn M.
Mazur J. E.
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