are there two distinct solar energetic particle releases in ... - IOPscience

The Astrophysical Journal, 818:169 (6pp), 2016 February 20

doi:10.3847/0004-637X/818/2/169

© 2016. The American Astronomical Society. All rights reserved.

ARE THERE TWO DISTINCT SOLAR ENERGETIC PARTICLE RELEASES IN THE 2012 MAY 17 GROUND LEVEL ENHANCEMENT EVENT? Liu-Guan Ding1, Yong Jiang2, and Gang Li2,3 1

School of Physics and Optoelectronic Engineering, Institue of Space Weather, Nanjing University of Information Science & Technology, Nanjing, Jiangsu, 210044, China 2 College of Math and Statistics, Institue of Space Weather, Nanjing University of Information Science & Technology, Nanjing, Jiangsu, 210044, China 3 Department of Space Science and CSPAR, University of Alabama in Huntsville, AL 35899, USA; [email protected] Received 2015 October 14; accepted 2016 January 13; published 2016 February 17

ABSTRACT We examine ion release times in the solar vicinity for the 2012 May 17 Ground Level Enhancement event using the velocity dispersion analysis method. In situ energetic proton data from Solar and Heliospheric Observatory (SOHO)/Energetic and Relativistic Nuclei and Electron and Geostationary Operational Environmental Satellite are used. We find two distinct releases of Solar Energetic Particles (SEPs) near the Sun, separated by ∼40 minutes. From soft X-ray observations, we find that the first release coincides with the solar flare eruption: the release starts from the flare onset and ends near the peak of the soft X-ray; type-III radio bursts also occur when the release starts. A type II radio burst may also start at the begining of the release. However, the associated Coronal Mass Ejection (CME) only has a height of 0.08Rs from extrapolation of SOHO/LASCO data. At the start of the second release, the CME propagates to more than 8.4Rs in height, and there are signatures of an enhanced type II radio burst. The time-integrated spectra for the two releases differ. The spectrum for the second release shows the common double-power-law feature of gradual SEP events. The spectrum for the first release does not resemble power laws because there is considerable modulation at lower energies. Based on our analysis, we suggest that SEPs of the first release were dominated by particles accelerated at the flare, and those of the second release were dominated by particles accelerated at the associated CME-driven shock. Our study may be important to understand certain extreme SEP events. Key words: Sun: coronal mass ejections (CMEs) – Sun: flares – Sun: particle emission suggested that there may be a prompt flare-associated component and a delayed CME-associated component in many GLE events. However, no clear evidence of these two components were given in Li et al. (2007a, 2007b). In a later work, Li et al. (2013a) studied the 2012 May 17 GLE event using the Velocity Dispersion Analysis (VDA) method and proposed that the electrons in this event were accelerated by solar flare, while the relativistic protons were released later and may have been accelerated by the accompanying CME-driven shock. The VDA is an often used technique for obtaining the solar release time of energetic particles in SEP events. Using this technique, Tylka et al. (2003) and Reames (2009a, 2009b) have found that the SEP release time in impulsive events often coincides well with the hard X-ray emission, while the release time in large GLE events often coincides with CME-driven shock. By correlating the solar release time of SEPs as derived from the VDA method and the extrapolated CME height for a large number of GLE events, Gopalswamy et al. (2012) suggested that the mean heliocentric height of CMEs associated with GLE events is about ∼3Rs when the solar particle release occurs. These authors also pointed out that all GLE events are caused by associated CME-driven shocks. In many VDA practices, the SEPs of different energies are assumed to be released at the same time and location near the Sun, and the first arriving particles are scatter-free. With this assumption, one can obtain both the propagation path length and particle release time from the VDA method. For western events with good magnetic connection, Reames (2009a, 2009b) and Gopalswamy et al. (2012) found that the derived path lengths are consistent with nominal Parker spiral values. The

1. INTRODUCTION Solar Energetic Particles (SEPs) are a major concern of space weather. These high energy particles are accelerated near the Sun through two main processes: solar flares and Coronal Mass Ejections (CMEs). The SEP time intensity profiles for flare accelerated particles are often “impulsive” (Cane et al. 1986) and those for CME accelerated are “gradual” (Reames 1995, 1999). In large SEP events, however, flares and CMEs often occur together. This is particularly true for Ground Level Enhancement (GLE) events. GLE events are large SEP events that can be observed by one or more neutron monitors (NMs) at ground (Forbush 1946). Historically, flares were thought to be the main source of SEPs in GLEs (e.g., Miroshnichenko 2001). However, more recent studies (e.g., Kahler 1994, 2001; Reames 1999, 2009b; Cliver 2006; Gopalswamy et al. 2012; Li et al. 2012, 2013a; Mewaldt et al. 2012) suggest that in large SEP events, especially in GLE events, particle acceleration may take place mainly at the shocks driven by CMEs, rather than in flare active regions (ARs). It is possible that both flare acceleration and CME-shock acceleration can occur in a single SEP event. Cane et al. (2003, 2006) proposed that flare accelerated material may arrive earlier than CME-shock accelerated material and this flare material can be reaccelerated at the shock (Li & Zank 2005). All 16 GLE events in solar cycle 23 are associated with fast CMEs and X-class flares (Li et al. 2012). Obviously, SEPs can be accelerated at both flares and CME-driven shocks in these events. Following Cane et al. (2003, 2006), by using multiple spacecraft observations of the GLEs, Li et al. (2007a, 2007b) 1

The Astrophysical Journal, 818:169 (6pp), 2016 February 20

Ding, Jiang, & Li

assumption that particles of different energies are released at the same time is perhaps a reasonable one for impulsive events where particles are accelerated within a short period of time. However, for gradual SEP events where particles are continually accelerated at a CME-driven shock, lower energy particles have a later release time than higher energy particles. This is easily seen from simulations in Li et al. (2003, 2005), where the maximum achievable particle energy at the shock front decreases with time. An alternative VDA practice is therefore to assume that the path length is given by that of a nominal Parker spiral and then obtain the release times for particles of different energies. Recently, Kim et al. (2014) took this approach and proposed a new classification scheme of SEP events based on the onset timing and energy-dependent flux enhancement. In their scheme, there is clear distinction between flare-associated SEPs and CME-associated SEPs in terms of the onset timing. Clearly, there are uncertainties for the inferred release times in both practices (see, e.g., Wang & Qin 2015). We also note that the scatter-free assumption needs justification because energetic particles will interact with solar wind MHD turbulence as they propagate from the Sun to 1 au. Nevertheless, the inferred solar release time from the VDA method has provided useful information about the underlying particle acceleration process, especially when correlating with timing of other solar activities (e.g., Kahler 1994; Tylka et al. 2003; Reames 2009a, 2009b; Gopalswamy et al. 2010, 2012; Ding et al. 2014). If two separate acceleration processes occur in the same SEP event, but at different times, there may be two releases. In this work, we report such a case study in the 2012 May 17 GLE event. Using the VDA method and multiple spacecraft data that is mainly from the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on board the Solar and Heliospheric Observatory (SOHO) spacecraft (Torsti et al. 1995; Valtonen et al. 1997), we find two distinct particle release episodes that were separated by ∼40 minutes. By comparing with X-ray and radio observations, we suggest that the first episode, which coincided with the peak of hard X-ray (as obtained from time differential of soft X-ray time intensity profile), was caused by particles accelerated at the flare; the second episode, which occurred at the decay phase of the flare and with an associated CME with a height of more than 8.4Rs, was caused by particles accelerated at the CME-driven shock.

2.1. In Situ Observation Figure 1 shows the energetic proton flux as a function of time from the ERNE instruments on board the SOHO spacecraft (left) and the GOES spacecraft (right panel). ERNE has two detectors: the High Energy Detector (HED) and the Low Energy Detector (LED). The ERNE/HED consists of 10 energy channels from 13 to 130 MeV and the ERNE/LED consists of 10 energy channels from 1.3 to 13 MeV. The HED and LED operate separately. We do not use the LED data from the 2012 May 17 event because the quality was poor. In comparison, because the HED data are clean, we only use HED data from ERNE. Data from eight energy channels of ERNE/ HED are shown in the left panels of Figure 1. Data from six energy channels of GOES are shown in the right panels of Figure 1. In obtaining these onset times, we follow the procedure outlined in Ding et al. (2014). The rising time is decided by f (trising ) = á f ñ + 2s , where á f ñ is the average flux and σ the standard deviation. We also obtain the uncertainties by using σ and 3σ. Two separate episodes of energetic particle arrival at 1 au can be best seen from ERNE/HED observations. The insitu onset times of each release are marked by the red and blue arrows, respectively. Hereafter we use the terms “first onset” and “second onset” to refer to in-situ observations; and “first release” and “second release” to refer to releases of energetic particles at the Sun. For the GOES observations, the first onset can be identified in six energy channels from 6.5 to 433 MeV, as shown in Figure 1. However, there were large fluctuations in the background as well as a data gap for the 6.5 MeV and 11.6 MeV channels. Consequently, the onset times for these two channels contain large uncertainties. Furthermore, the GOES data does not show the second onset as clearly as the ERNE/HED. To better show the second onsets, we plotted the fluxes in linear scale around the second onset in the insets of the right panels of Figure 1. 2.2. Particle Release Near the Sun The onset times obtained for different energy channels in the last section vary with energy. There are uncertainties in determining the onset time, however, much of this difference is because the propagation is energy-dependent. In this section we obtain the solar release time of energetic protons assuming that the first arrival protons (from which the onset time was determined) in all energy channels are basically free-streaming and experience no pitch angle scattering. We perform the VDA of the data obtained from ERNE/HED on board SOHO and from the GOES instrument. For the uncertainty, we select the maximum of all possible measurement uncertainty, such as data resolution, the onset time difference using 2 sigma (the criteria of searching first onset time) from 1 sigma (low limit) or 3 sigma (upper limit), or other possible onset times identified from the time profile (this is mainly for searching the second onset time, e.g., the first times of GOES6.5 and 11.6 MeV, and the second onset times of GOES11.6 and 165 MeV). To obtain the solar release time, we use

2. OBSERVATIONS The 2012 May 17 GLE event was reported as a longduration M5.1 X-ray flare by NOAA, and was associated with a high-speed CME (1582 km s−1). The event was examined earlier by Shen et al. (2013) and Li et al. (2013a). Shen et al. (2013) identified two ejections that were separated by only three minutes. They suggested that these two interacting shocks could play an important role in the underlying particle acceleration process. In this study, we focus on in-situ observations of energetic particles. Data from both the ERNE instrument on board SOHO and the Geostationary Operational Environmental Satellite (GOES) instrument are used. We also use radio observations from the Learmonth and BIRS ground stations, as well as the WAVES instrument on board the WIND spacecraft.

to = tr + td = tr + L v (E ) ,

(1 )

where tr is the particle release time at its source region near the Sun, td=L/v(E) is the travel time for a proton of energy (E), L is the path length, and v(E) is the speed of energetic proton with 2

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Ding, Jiang, & Li

Figure 1. Energetic proton flux time series detected by SOHO/ERNE (left panel) and GOES (right panel). The red arrows indicate the onset times of the first release of SEPs and the blue arrows indicate the onset times of the second release. The second onset is best seen from the SOHO/ERNE observation. The insets of the the GOES observations show the flux around the second onsets with a linear scale, which helps to better identify the second onsets.

energy E. We assume that particles at all energies travel the same path length (L). We also assume that the path length is L=1.188 au, corresponding to a solar wind speed of 366 km s−1. To compare with radio observations, we add 8.3 minutes (i.e., the travel time of light from the Sun to the Earth) to the release time tr. Release times are obtained using Equation (1). For the first release episode, the release times tr1 at all valid energy channels between 6.5 and 433 MeV in SOHO and GOES range from ∼01:15 UT to ∼01:46 UT. For the second release episode, the release times tr2 range from ∼02:26 UT to ∼02:50 UT. The proton release time tr as a function of energy E is shown in Figure 2. The dots in the left shaded area correspond to the first releases of the energetic protons, and those in the right shaded area correspond to the second ones. As shown in the figure, there is a clear gap between the first release and the second release, which is about ∼40 minutes. The green solid line in Figure 2 denotes the soft X-ray flux intensity in the 1–8 Å channel detected by GOES. We can see that the initial phase of solar flare is detected at ∼00:50 UT, and then the soft X-ray flux increases suddenly at ∼01:25 UT and reaches its peak at ∼01:47 UT. The impulsive phase of solar flare coincides nicely with the first release of the energetic protons. The blue dots in Figure 2 (with arbitrary unit) show the derivatives of the soft X-ray flux intensity in the 1–8 Å channel. This derivative is often used as a proxy for the hard X-ray emissions. Again, the release time of the first episode in this event is very close to the peak of the hard X-ray eruption. Furthermore, a clear signature of type-III radio bursts (denoted by the dot dash lines in Figure 2) was observed at this time. Similar to hard X-rays, the appearance of Type III radio bursts is interpreted as electron acceleration at the flare site. The fact that the inferred solar release time for the first episode of the energetic ions coincides with both the radio observation and

Figure 2. SEP release times near the Sun and related eruptive phenomena including flare, CME, and type-III radio bursts. The shaded areas indicate the first and the second releases. The green line shows the soft X-ray flux in 1–8 Å. The dashed line is the derivative (arbitrary unit) of the soft X-ray time flux, which is a proxy of hard X-rays. The brown line with crosses is the fit of the heliocentric height of the associated CME as a function of time.

X-ray observations suggests that these energetic ions may be dominantly accelerated at the flare site. As a GLE event, this event also triggered the OULU NM. Assuming that the geomagnetic cutoff energy at the OULU station is ∼433 MeV or a rigidity ∼1 GV, we find that an onset time of ∼433 MeV protons is 1:51 UT (intensity plot not shown). Unlike the GOES and ERNE data, there was no sign of a second onset. We added the resulting release time from the OULU NM as a cyan data point in Figure 2. As can be seen 3

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from the figure, the OULU data is consistent with the first release. Also shown in Figure 2 is the CME-height profile. The heliocentric heights of the associated CME at different times are obtained from the Large Angle and Spectrometric Coronagraph Experiment (LASCO) on board SOHO (data are from CDAW website http://cdaw.gsfc.nasa.gov/CME_list/ UNIVERSAL/2012_05/yht/) and are denoted by the brown crosses. The solid brown line is the least-square quadratic fit to the data. If we take the type-III onset time, which is ∼01:32 UT, as the release time of the protons in the first episode, the corresponding heliocentric distance of the associated CME can be estimated to be ∼1.08Rs, as extrapolated from the quadratic fitting. Thus, the height of the associated CME is 0.08Rs. The CME has just lifted off from the surface of the Sun, and at this height, a CME-driven shock may not have formed. However, if no shock exists, then the energetic particles (∼100 MeV) observed must be generated at the flare site. There are uncertainties in the extrapolated CME height, but if the CME-driven shock is responsible for the energetic particles, the acceleration time must be very short because the CME travels about 1Rs in ∼10 minutes. The proton release time for the second episode is much later than the flare onset and the hard X-ray emissions. If the duration of the hard X-ray emission provides a reasonable proxy for the acceleration period, then the protons of the second release are unlikely to be accelerated at the flare. These energetic particles may instead be accelerated at the CMEdriven shock. At the beginning of the second release, ∼02:26 UT, the CME propagated to a heliocentric distance of ∼9.4Rs (a height of 8.4Rs). This is consistent with previous studies by Reames (2009a) and Gopalswamy et al. (2012) who find that the release height of SEPs in large SEP events and GLE events are a few solar radii (2–8Rs). Furthermore the 1 au in-situ time intensity profile for this event shows the typical “gradual” feature as in many other gradual SEP events. We therefore suggest that energetic particles released during the second episode are accelerated by the associated CME-driven shock.

Figure 3. Observed radio bursts of the 2012 May 17 event. Data are from the Learmonth and the BIRS ground stations and the WIND/WAVES instrument.

Newkirk (1961) density model, then the derived shock speeds corresponding to these three type-II radio bursts are 1464, 915, and 2382 km s−1, respectively. While the second and the third type-II radio bursts start later, the first one started as early as 01:32 UT. This implies that the shock driven by the first CME was already formed at the first release. Consequently, it is possible that energetic particles from the first release are accelerated by this shock. However, we note that the height of the first CME is very low at the first release, so even if the shock is formed at 01:32 UT, there may not be enough time for accelerating protons to reach above 400 MeV. We note that the acceleration process of electrons and ions at a shock can be very different. Type II radio bursts are due to >∼20 keV electrons. These electrons may be accelerated quickly at a quasi-perpendicular shock. In comparison, the acceleration timescale for ions >100 MeV can be much longer. Furthermore, as argued in Li et al. (2012), ion acceleration is more efficient at a quasi-parallel shock. There are two episodes of enhanced radio emissions. The first is between 02:00 UT and 02:10 UT (BIRS and WIND/ WAVES), and the second is between ∼02:18 UT and ∼02:22 UT (WIND/WAVES). Such enhanced radio emissions can result from the interaction of two CMEs (e.g., when a faster CME catches a slower CME from behind), or if the shock driven by the second CME goes through the dense core of the first CME (Gopalswamy et al. 2001). In our event, the first enhancement is likely due to the interaction of the two CMEs with speeds 1464 and 915 km s−1, and the second is likely due to the interaction of the two CMEs with speeds of 1464 and 2382 km s−1. The first radio enhancement is stronger, suggesting that the energetic electron population (>∼20 keV) is more intense. However, the CMEs involved are slower than those for the the second radio enhancement. The time of the second enhanced radio emission almost coincides with the beginning of the second proton release of this event. It is therefore possible that the second particle release in this event is due to particle acceleration at a converging shock pair (Zhao & Li 2014).

2.3. Type II Radio Burst We now discuss the associated type-II radio bursts of the 2012 May 17 GLE event. Type II radio bursts were examined by Shen et al. (2013), who were interested in identifying two ejections (very close in time) from the same AR. Here we focus on determining whether there are notable features from type II radio bursts at the second proton release. Figure 3 shows radio observations in the frequency range of 1–180 MHz. These radio data are from the Learmonth (40–180 MHz) and BIRS ground stations (13.8–40 MHz), as well as the WIND/WAVES (