³He-rich Solar Energetic Particles from Sunspot Jets

Figure 1 shows the energetic particle and solar wind plasma measurements for two examined ³He-rich SEP events and the associated radio and soft X-ray activity. Figure 1(a) displays Wind/WAVES radio spectrograms and the GOES  X-ray fluxes. Figure 1(b) shows 1 hr ACE/SIS ³He, ⁴He intensities at 5.2 and 10.5 MeV nucleon⁻¹. ⁴He intensities below ∼10⁻⁵ should be regarded as upper limits. Figure 1(c) presents Wind/3DP electron intensities at different energy channels (EESA-H 1.3–19 keV and SST 27–182 keV). Figure 1(d) shows the solar wind speed and magnetic field magnitude.

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The 2011 February 18 event occurred during the decay phase of the gradual SEP event of 2011 February 15 and near the passage of an IP fast forward shock on 2011 February 18 00:49 UT. Figure 1 indicates that after the shock passage the ³He intensity starts to increase, while ⁴He intensity decreases. The opposite time-intensity profiles suggest that the event onset near the shock passage was most likely coincidental. The shock on Wind has been reported at Harvard-Smithsonian Center for Astrophysics IP Shock Database (https://www.cfa.harvard.edu/shocks/). The corresponding IP coronal mass ejection (ICME) can be found in the list of the Near-Earth ICMEs (http://www.srl.caltech.edu/ACE/ASC/DATA/level3/icmetable2.html). To confirm the solar origin of the 2011 February 18 event we examined the ULEIS/SIS He mass spectrograms of individual ions in the energy ranges 0.4–1.0 and 7.6–16.3 MeV nucleon⁻¹, available at the ACE science center (http://www.srl.caltech.edu/ACE/ASC/DATA/level3/sis/heplots). The spectrograms indicate the velocity dispersive onset for the 2011 February 18 event. In 0.4–1.0 and 7.6–16.3 MeV nucleon⁻¹ energy ranges the first ³He ions appeared at ∼05–06 UT and ∼02–03 UT, respectively. The time lag between the onsets (3 hr) is consistent with the propagation time difference (257–63 minutes) for ions in these two energy intervals traveling from Sun to L1 along the nominal Parker field line. The 2011 February 18 event is included in a statistical study of Fe-rich SEP events in 1995–2013 (Reames et al. 2014) with Fe/O = 1.34 ± 0.20 (at 3–4 MeV nucleon⁻¹). Note that the intensity of Z > 2 ions, including Fe, at energy >10 MeV nucleon⁻¹ on SIS was below the measurable level for both events. The ULEIS confirms that at low energies both events are Fe-rich with Fe/O ∼ 1 (see Table 1). Soft energy spectra and low Z > 2 abundances (relative to He) in ³He-rich events frequently preclude SIS detection of these elements.

Table 1.  ³He-rich SEP Event Properties

SEP	Daysa	3He/4Hea	3He Fluxa	3He/4Heb	Fe/Ob	Type-IIIc	Flared			CMEe
Start			(×10−5)			Start	Class	Start	Loc.	Speed	Width
2011 Feb 18	049.17–050.13	${2.33}_{-0.17}^{+0.22}$	${5.26}_{-0.38}^{+0.49}$	0.12 ± 0.01	1.46 ± 0.13	Feb 17 21:32	C1.1	21:30	S19W46	490	82
2015 Aug 24	236.33–237.42	${1.61}_{-0.07}^{+0.09}$	${5.78}_{-0.27}^{+0.33}$	0.24 ± 0.11	1.16 ± 0.74	Aug 24 06:16	⋯	⋯	S14E01	⋯	⋯
						Aug 24 06:20	C2.3	06:20	"	⋯	⋯
						Aug 24 07:22	C1.3	07:22	S14W00	⋯	⋯
						Aug 24 07:30	M5.6	07:26	"	272	88

Notes.

^a10.5 MeV nucleon⁻¹; flux units—particles (cm² s sr MeV nucleon⁻¹)⁻¹. ^b0.546 MeV nucleon⁻¹. ^c∼10 MHz from Wind/WAVES 1 minute data. ^dGOES X-ray class and start; location in SDO/AIA 304 Å. ^eCoronal Mass Ejection (CME) speed (km s⁻¹), width (°) from SOHO/LASCO catalog (http://cdaw.gsfc.nasa.gov/CME_list).

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Solar energetic electrons were not clearly measured in the 2011 February 18 event due to enhanced background related to the preceding gradual SEP event and the passage of the IP shock. Furthermore, the electron channels at ≥108 keV were contaminated by protons. Note that the SST data were corrected for scattered electrons that leave only a fraction of their energy in the detector (Wang et al. 2006a). An association with type-III radio bursts is rather straightforward as there were no other significant type-III bursts in the 11 hr interval before the event start time. The event-associated type-III burst on February 17 21:32 UT was accompanied by a C3.1 X-ray flare (see Figure 1(a), left panel). The associations are consistent with the analysis by Reames et al. (2014). The 2015 August 24 event was accompanied by two consecutive solar energetic electron events, separated by ∼1 hr. For this reason, the first electron intensity enhancement, notably weaker (measured at <100 keV) than the second, is not well resolved. Each electron event is associated with a double type-III burst; in the first electron event, the type-III bursts are separated by 4 minutes and in the second event by 8 minutes. The type-III bursts on August 24 06:16 and 06:20 UT, related to the first electron event, occurred near a double X-ray flux enhancement (see Figure 1(a), right panel) corresponding to the C3.2 and C2.3 flares. The high-cadence EUV images suggest that the type-III burst at 06:16 UT is likely unrelated to the C3.2 flare (see Section 3.2). The type-III bursts on August 24 7:22 and 7:30 UT, related to the second electron event, were accompanied by C1.3 and M5.6 flares, respectively. The C1.3 flare, not well resolved from elevated X-ray flux during the decay phase of previous stronger flare, was identified with the help of 0.5–4 Å GOES channel. None of the examined events have type-II bursts arising from coronal shocks. The events were accompanied by only slow coronal mass ejections (CMEs; see Table 1).

Figure 2 shows ⁴He, ³He ULEIS, and SIS fluence spectra for the two examined events. The He mass histograms integrated over the duration of the events are embedded in Figure 2. The ⁴He fluence in the February 18 event should be treated as upper limit due to the possibility of a contribution from the preceding event. Note that the ³He intensity increase on August 25 (see Figure 1(b), right panel), probably associated with the same active region as the August 24 event, is not included in the calculation of the event characteristics. Missing ULEIS spectral points (five for ³He and one for ⁴He) for the August 24 event means that the fluence was near the lowest measurable level (zero or one count in the integrated period). It is surprising that below ∼1 MeV nucleon⁻¹ the 2015 August 24 event is at the threshold level, though at >10 MeV nucleon⁻¹ it is one of the most intense ³He-rich events in the solar cycle. The energy spectra are roughly consistent with double power laws where slopes are harder below ∼1.5 MeV nucleon⁻¹.

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Table 1 summarizes the characteristics of the examined events. Column 1 gives the particle event start date, and column 2 shows the ³He-rich period where fluences/abundances were determined. Columns 3 and 4 indicate the average ³He/⁴He ratio and the average ³He flux at 10.5 MeV nucleon⁻¹, respectively. Note the enormous 1 hr ³He/⁴He ratios in the early stage of the 2011 February 18 event, exceeding a value of 10 at 5.2 MeV nucleon⁻¹ (see Figure 1(b), left panel). Columns 5 and 6 give the ³He/⁴He and Fe/O ratio at 0.546 MeV nucleon⁻¹, respectively. Column 7 gives the type-III burst start time as observed by Wind/WAVES. Columns 8 and 9 indicate the GOES X-ray flare class and start time, respectively. Column 10 shows the flare location as observed in SDO/AIA 304 Å channel. Columns 11 and 12 give the CME speed and width, respectively, obtained from the SOHO LASCO catalog.

To localize the solar source of the ³He-rich SEP events, we have examined high-cadence, full-disk SDO AIA images for flaring around the time of the event-associated type-III bursts. Figures 3(a)–(c) show the base-difference images at 193 Å, revealing jets at the type-III burst times. Associated animations show the progress of flaring activity for several minutes around the type-III bursts. Note the relatively large longitudinal (∼45°) separation between the L1 magnetic foot-point and the associated jets in the August 24 event. The foot-point longitude of the Parker spiral connecting L1 was determined from the measured solar wind speed at the type-III burst onset time. In the February 18 event the jet was accompanied by a faint larger-scale front propagating northward; in the August 24 event the jets evolved to a wider eruption.

In order to provide a more detailed view on the event-associated jets and the underlying photospheric magnetic field, Figures 3(d)–(f) show two-color composite images where the AIA 304 Å channel corresponds to green and the HMI line-of-sight magnetic field to red (+black). The black and light-red areas indicate regions with strong magnetic fields of negative and positive polarity, respectively. The images are shown at times of the event-associated type-III bursts (see the animation for other times). In both events, the EUV jets are rooted at the boundary of a large sunspot at the interface with the minor (positive) polarity field. In the February event, the magnetic fields of opposite polarities were separated by about 15''; in the August event, the fields were closely adjacent. The jet in the February event shows a complex helical structure with unwinding counterclockwise motion. The jets in the August event show a structured spire with no obvious rotation. The jet at the onset of the M5.6 flare in the August 24 event was less clearly visible. The animation shows that type-III burst on August 24 06:16 UT coincides with the jet at the sunspot boundary and not with C3.2 flare at 06:13 UT (the first X-ray flux increase in Figure 1(a), right panel) that occurred in the more central part of the sunspot along the closed loops.

Figure 4 (top panels) shows another two-color composite image combining the AIA 171 Å channel and the HMI line-of-sight magnetic field. Here coronal loops, connecting the fields of opposite polarities, and peripheral fan structures are clearly seen. Strikingly, the jets are not co-located with these large-scale coronal structures, which were visible in the sunspots before the events. The jet in the August event appears more complex in 171 Å, but a rotation (as in the February event) is not observed. Figure 4 (bottom) shows the corresponding HMI continuum with over-plotted contours of high (95th percentile) level of intensity from the 171 Å image. The jets in both events appear to be rooted near the sunspot penumbra.

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Active region (AR) 11158, associated with the 2011 February 18 event, emerged in the eastern solar hemisphere on February 11. According to the Solar Event List (ftp://ftp.swpc.noaa.gov/pub/warehouse), AR 11158 produced 1 B-, 56 C-, 5 M-, and 1 X-class flare during its disk transit. AR 11158 consisted of two major emerging bipoles that showed complex sunspot motion and interaction (e.g., Jiang et al. 2012). AR 12403, associated with the 2015 August 24 event, also contains two major bipoles. AR 12403 appeared near the eastern limb on August 17. During its disk transit, AR 12403 produced 34 B-, 84 C-, 11 M-class flares. Both ARs show a less common βγδ magnetic classification.⁸ The βγδ class has been reported in ∼4% of all ARs in 1992–2015 (Jaeggli & Norton 2016). According to USAF/NOAA Sunspot Group Reports (https://www.ngdc.noaa.gov/stp/space-weather/solar-data/solar-features/sunspot-regions/usaf_mwl/), AR 12403 was among the 10 largest ARs in the present solar cycle. The maximum reported area was 1320 millionths of the solar hemisphere. The maximum area of AR 11158 was 670.