1. Hot Precursor Ejecta and Other Peculiarities of the 2012 May 17 Ground Level Enhancement Event N. Gopalswamy 2, H. Xie 1,2, N. V. Nitta 3, I. Usoskin 4, J. M. Davila 1 (1)NASA Goddard Space Flight Center, (2) The Catholic University of America, (3) Lockheed-Martin, (4) Oulu University, Finland Email: nat.gopalswamy@nasa.gov Introduction Ground level enhancement (GLE) in solar energetic particle (SEP) events represents particles accelerated to GeV energies. When and at what height in the corona they are released are important questions that have not been fully answered. GLE events are associated with the fastest (typical seed ~2000 km/s) coronal mass ejections (CMEs) (Gopalswamy et al. 2012). Typically about a dozen GLE events occur each solar cycle. However, there has been a real dearth of GLE events during cycle 24: even though the Sun has reached its solar maximum phase, it has produced only one GLE event. Over the first 4.5 years of solar cycle 23, there were 5 GLE events, which is roughly a third of all the events of that cycle (Gopalswamy, 2012). The first GLE event of solar cycle 24 occurred on 2012 May 17 at 1:43 UT. It was fairly big event (peak intensity ~19% above the background). Only 4 out the 16 events in cycle 23 had intensity at or above this level. The May 17, 2012 GLE event was associated with a moderate flare (M5.1), well below the median flare size (X3.8) of cycle 23 GLE events. However, the CME speed (~2000 km/s) is consistent with the cycle 23 speeds. We take advantage of the extended field of view of the STEREO mission in obtaining accurate CME kinematics and the height of shock formation (CME/shock height at type II onset) and the shock height at GLE particle release. These values are consistent with the corresponding values obtained for cycle 23 GLEs obtained from SOHO/LASCO.. This paper attempts to explain why the May 17, 2012 event is a GLE event while half a dozen eruptions with similar flare size and CME speed did not produce a GLE event.. CME Height at Type II Onset, Comparison with Cycle 23 Events The type II burst started at 01:31 UT. The EUVI image at this time has a well-defined wave (Fig.4). For an initial spherical expansion, the radius of the circular EUV wave is the height of the CME above the solar surface: 0.31 Rs. So, the CME height at type II onset is 1.31 Rs. The CME heights at GLE particle release and type II onset are similar to the values obtained for cycle 23, where the observations were only from LASCO observations (Figure 5). Summary The May 17, 2012 GLE confirms the low CME height at SPR from STEREO data (2.32 Rs) Extrapolation method (used for cycle 23 events) gives similar value: 2.19 Rs CME height at type II onset is directly derived from STEREO/EUVI image: 1.31 Rs and agrees with the one derived from the acceleration assuming flare onset = CME onset The hot ejecta (>6 MK) preceding the CME might have increased the acceleration efficiency Event Timeline and Associations CME Kinematics The May 17, 2012.GLE event was associated with a near-limb eruption from AR 11476 located at N11W76. The eruption consisted of an M5.1 soft X-ray flare and a fast CME. The LASCO CME had a linear speed of 1582 km/s (see cdaw.gsfc.nasa.gov/CME_list). Fortunately, the eruption was observed by the STEREO coronagraphs and EUVI. On 2012 May 17, STEREO-A was ahead of Earth by 115 degrees, so the eruption was at E39 in STEREO-A view. STEREO-B was 118 degrees behind Earth, so the CME was back-sided in STEREO-B view (source region at W 194), appearing as a full halo. We use STEREO-A data for height-time measurements. We actually fit a flux rope to the CME in COR1 and COR2 FOV and then obtain the deprojected height of the CME as a function of time. The LASCO height-time measurements were not subject to projection effects (limb event). The combined height-time (h-t) plot is shown in Figure 3. The CME can be tracked from very close to the surface in EUVI and COR1 to >20 Rs in LASCO C3. The CME height at 01:40 was directly measured as 2.32 Rs from the COR1 image at this time, giving roughly shock height at the time of the GLE particle release. This is a direct measurement without any extrapolation. The CME speed and acceleration were obtained from height-time (h-t) measurements. The peak speed was 1997 km/s (at 01:40 UT) and the peak acceleration was 1.51 km/s 2 (at 01:35 UT - see Fig 3). Figure 2. GOES soft X-ray flare (left) and the LASCO/C2 image at 01:48 UT at CME first appearance, with the leading edge at 3.57 Rs. This height can be back- extrapolated to the SPR time (01:40 UT) as 2.19 Rs using a constant speed of 1997 km/s attained by the CME around the time of SPR (see next section on CME kinematics). 01:25 - 01:47- 02:14 M5.1 SXR flare onset-peak-end 01:29 – 01:42 – 01:55 Microwave burst (4.9 GHz) 01:31 – 01:41 metric type II burst 01:32 – 02:52 metric type IV 01:33 – 01:37 metric type III 01:43 – GLE onset 01:40 – GLE onset at the Sun (normalized to electromagnetic emissions) Figure 3. CME kinematics of the GLE event and the STEREO COR1 image at the time of GLE particle release at the Sun. The COR1 CME leading edge is at a height of 2.32 Rs (deprojected). 118 o 115 o Figure 1. The GLE event observed by the Oulu Neutron Monitor. The onset time is 01:43 UT (when the intensity is 2% of the peak).For1.2 AU Parker spiral length, the solar particle release (SPR) time can be obtained as 01:40 UT, normalized to electro- magnetic emissions. Interestingly, the GLE release occurs at the time of the peak speed (1997 km/s). After this time the CME travels with a roughly constant speed (a slight deceleration due to coronal drag: -18.4 m/s 2 ). The peak speed also roughly coincides with the time of the soft X-ray flare peak. The CME height at SPR derived by back- extrapolation of the first appearance height in LASCO FOV (2.19 Rs) is only by 5.5% from direct measurement obtained from STEREO COR 1 (see the COR1 image in Fig.3). Assuming flare onset = CME onset, we get the initial acceleration as 2.2 km/s 2 not too different from direct measurement. Figure 4 Precursor Figure 6 The cycle 24 GLE fits well with the longitude distribution of cycle 23 CME height at particle release obtained without inner coronal data. What is special about this GLE? During cycle 24 (2007/12/11-2012/07/31) there were 7 other well-connected (longitude W\55-85) flares with size ≥M5.0. None of them produced a GLE event, even though the speeds were high (average ~1800 km/s for 6 events; one was a confined flare).: SDO images (94 Å) revealed that the GLE CME overtook a hot plasma (> 6 MK) ejected some 40 minutes earlier (Fig. 2) from the same active region, implying the presence of particles already energized, making it easy for the shock to accelerate them to higher energies (see Fig. 6 for a superposition of SDO/AIA 94 Å image on 193 Å image). Figure 5 Precursor Hot Ejecta EUV Wave at Type II onset COR1 A at 01:40 STEREO Location SDO/AIA LASCO/C2 CME May 17, 2012 GLE