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ACE News #202 - May 23, 2019
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(a) 1-hr ACE/SIS 5.2 and 10.5 MeV/nucleon 3He, 4He intensities. The upper (lower) black curve shows the lower (higher)-energy 4He intensities. The colors correspond to the two 3He energies. Solid vertical lines mark the event-associated type-III radio bursts. (b) He-mass histograms at 10.5 MeV/nucleon. (c) Two-color composite images at start time of the type-III bursts. The SDO/AIA 304 Å EUV images correspond to green and the SDO/HMI line-of-sight magnetic field (scaled to ±200 G) to red/black. The arrows point to the event-associated jets.
Solar sources of suprathermal (<1 MeV/nucleon) 3He-rich solar energetic particles (SEPs) have been commonly associated with jets originating in small, compact active regions at the periphery of near-equatorial coronal holes (ACE News #96, #169). Sources of relatively rare, high-energy (>10 MeV/nucleon) 3He-rich SEPs remain unexplored although these events were detected for the first time at 10-100 MeV/nucleon ~50 years ago.
We have examined two of the most intense 3He-rich (3He/4He > 1) SEP events of the solar cycle 24 (2008 through 2017) measured on the Advanced Composition Explorer (ACE) at energy >10 MeV/nucleon on 2011 February 18 and 2015 August 24 (panels a, b). Although 3He shows high intensities, Z >2 ions, including Fe, were below the measurable level at energy >10 MeV/nucleon in the Solar Isotope Spectrometer (SIS) for both events. The ACE Ultra-Low-Energy Isotope Spectrometer (ULEIS) confirms that at low energies (~0.5 MeV/nucleon) both events are Fe-rich (Fe/O >1). The events are accompanied by type-III radio bursts, but type-II emission is missing, as is typically the case for suprathermal 3He-rich SEPs.
The corresponding solar sources were analysed using high-resolution, extreme-ultraviolet (EUV) imaging and photospheric magnetic field observations on the Solar Dynamics Observatory (SDO). We find the sources of these events are associated with jets originating at the boundary of large sunspots with complex and less common betagammadelta magnetic configuration (panel c). Thus, details of the underlying photospheric field (such as sunspot area and magnetic field distribution) apparently are important to produce 3He to high energies in the examined events. For further details see Bucik et al. ApJL 869, L21, 2018 (https://doi.org/10.3847/2041-8213/aaf37f).
This item was contributed by R. Bucik [Max-Planck-Institut für Sonnensystemforschung (MPS)], M. E. Wiedenbeck (JPL/Caltech), G. M. Mason (APL/JHU), R. Gomez-Herrero (University of Alcala), N. V. Nitta (LMSAL), and L. Wang (Peking University). Address questions and comments to
Last modified 23 May 2019.