The Solar Energetic Particle Event of 14 December 2006
The solar energetic particle event on 14 December 2006 was observed by several near-Earth spacecraft including the Advanced Composition Explorer (ACE), STEREO A and B, SOHO and Wind. An interesting feature of this event is a series of unusual fluctuations in the particle intensity that occurred during the first few hours. These fluctuations were observed inside a magnetic cloud that originated in a solar event on 13 December and show both similarities and variations at the different spacecraft. Interestingly, the most striking difference is between observations at the two closely-separated STEREO spacecraft. In particular, large fluctuations in the proton intensity were seen by the High Energy Telescope (HET) on STEREO A, and to a lesser extent at Wind and ACE, but not by the STEREO B HET. We conclude that the differences in intensity-time profiles were caused by anisotropies in the particle distribution and the different viewing directions of the individual particle telescopes. The intensity/anisotropy variations suggest that flux tubes with different particle propagation conditions existed within this magnetic cloud despite the absence of local magnetic field signatures associated with these regions. The intensity fluctuations are similar to those occasionally seen in impulsive particle events. There were also spacecraft-to-spacecraft differences during the onset of the particle event. An initial rapid onset of energetic (> 40 MeV) protons was observed by the STEREO A and B spacecraft outside the magnetic cloud, but not by spacecraft such as SOHO that were already inside the magnetic cloud at this time. The latter spacecraft observed a slower, lower intensity increase. Evidently, energetic proton propagation from the solar event to the vicinity of Earth was inhibited within the magnetic cloud compared to outside.
Additional Information© 2009 The Author(s). This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. Received: 2 January 2009. Accepted: 31 March 2009. Published online: 17 April 2009. We thank Janet Luhmann of the University of California, Berkeley, for her steadfast support as the Principal Investigator of the STEREO/IMPACT investigation. The work at GSFC, Caltech and JPL was supported by NASA (Caltech and JPL were funded under subcontract SA2715-26309 from the University of California, Berkeley, under NASA Contract NAS5-03131). The ACE observations used in this paper are from the ACE Science Center (http://www.srl.caltech.edu/ACE/ASC/), and the STEREO magnetic fields were obtained from the STEREO archive at UCLA (http://www-sc.igpp.ucla.edu/forms/stereo/). The SOHO/ERNE data were obtained from the Space Research Laboratory, University of Turku (http://www.srl.utu.fi/erne_data/).
Published - art_10.1007_s11207-009-9353-6.pdf