Characteristics of four SPE groups with different ... - Wiley Online Library

Journal of Geophysical Research: Space Physics RESEARCH ARTICLE 10.1002/2015JA021280 Key Points: • We found that if the proton acceleration starts from a lower energy, a SPE tends to be a strong event • When the proton acceleration starts from the higher energy, a SPE tends to be a relatively weak event • The SPEs with the simultaneous acceleration in whole energy range within short time tend to be very weak events in spite of strong associated eruptions

Correspondence to: R.-S. Kim, [email protected]

Citation: Kim, R.-S., K.-S. Cho, J. Lee, S.-C. Bong, A. D. Joshi, and Y.-D. Park (2015), Characteristics of four SPE groups with different origins and acceleration processes, J. Geophys. Res. Space Physics, 120, 7083–7093, doi:10.1002/2015JA021280.

Received 1 APR 2015 Accepted 18 JUL 2015 Accepted article online 24 JUL 2015 Published online 4 SEP 2015

©2015. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

KIM ET AL.

Characteristics of four SPE groups with different origins and acceleration processes R.-S. Kim1,2 , K.-S. Cho1,2 , J. Lee3 , S.-C. Bong1,2 , A. D. Joshi1 , and Y.-D. Park1 1 Korea Astronomy and Space Science Institute, Daejeon, Korea, 2 University of Science and Technology, Daejeon, Korea, 3 Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul, Korea

Abstract Solar proton events (SPEs) can be categorized into four groups based on their associations with flare or CME inferred from onset timings as well as acceleration patterns using multienergy observations. In this study, we have investigated whether there are any typical characteristics of associated events and acceleration sites in each group using 42 SPEs from 1997 to 2012. We find the following: (i) if the proton acceleration starts from a lower energy, a SPE has a higher chance to be a strong event (> 5000 particle flux per unit (pfu)) even if its associated flare and/or CME are not so strong. The only difference between the SPEs associated with flare and CME is the location of the acceleration site. (ii) For the former (Group A), the sites are very low (∼ 1 Rs ) and close to the western limb, while the latter (Group C) have relatively higher (mean = 6.05 Rs ) and wider acceleration sites. (iii) When the proton acceleration starts from the higher energy (Group B), a SPE tends to be a relatively weak event (< 1000 pfu), although its associated CME is relatively stronger than previous groups. (iv) The SPEs categorized by the simultaneous acceleration in whole energy range within 10 min (Group D) tend to show the weakest proton flux (mean = 327 pfu) in spite of strong associated eruptions. Based on those results, we suggest that the different characteristics of SPEs are mainly due to the different conditions of magnetic connectivity and particle density, which are changed with longitude and height as well as their origin.

1. Introduction Releases of the solar eruptive energy, such as flares and coronal mass ejections (CMEs), can accelerate normal particles in the solar atmosphere to energetic level ranging from tens of keV to GeV in a short time scale. When the number of incoming energetic (>10 MeV) protons exceeds 10 particle flux unit (pfu) (1 pfu = 1 proton per cm2 sr s) at geosynchronous satellite altitudes, we call this a solar proton event (SPE), or a solar radiation storm, which leads to severe radiation hazards to spacecrafts and astronauts [Siscoe, 2000; Lanzerotti, 2001]. Since the extremely high speeds of energetic protons correspond to their high energies, SPEs arrive at the Earth within only several tens of minutes to several hours after the eruptions. Only few flares or CMEs can produce SPEs, and the forecast of solar radiation storms is still quite challenging. In this issue, understanding how and where the protons are accelerated is very essential for getting clues for the forecast of solar radiation storms with high accuracy. Based on solar radio bursts, Wild et al. [1963] suggested that electrons are accelerated to produce type III radio bursts, and protons are accelerated at shock waves seen as type II bursts, although both radiations are emitted by electrons. These two sources were later classified into impulsive and “long-enduring,” soft X-ray events by Pallavicini et al. [1977]. Recently Reames [2013] reviewed the two physical mechanisms involved in the acceleration of SPEs or solar energetic particles (SEPs). One is the SEP acceleration via magnetic reconnection during solar flares [Ohsawa and Sakai, 1988; Mori et al., 1998; Bombardieri et al., 2008], and the other is the acceleration by shocks formed during CMEs’ propagation [Klein and Trottet, 2001; Roussev et al., 2004; Cane et al., 1986]. Several theoretical models for proton acceleration during flares show a rapid proton acceleration to high energies within a time scale as short as observed at hard X-ray light curves [Miller et al., 1990, 1996]. In the CME-driven shock, the proton acceleration is considered to be gradual, in which energetic particles are dominant in the initial stage, and low-energy particles arise later as the shock propagates further into the solar wind [Zank et al., 2000]. However, as various conditions and mechanisms are entangled in the proton acceleration, our physical understanding of the origin and mechanism of SPEs still remains incomplete [Kallenrode, 2003; Trottet et al., 2015]. CHARACTERISTICS OF FOUR SPE GROUPS

7083

Journal of Geophysical Research: Space Physics

10.1002/2015JA021280

Under these circumstances, the statistical analysis might be useful not only for the forecast of solar radiation storms but also for the physical understanding of the particle acceleration. Recently, statistical studies on the origin of SPEs have been performed by several researchers. Trottet et al. [2015] presented the statistical evidence for contributions of flares and CMEs to major SEP events. Dierckxsens et al. [2015] examined the relationship between SEPs and properties of flares and CMEs by statistical analysis of solar cycle 23 events. They showed that both CME speed and flare X-ray flux have good correlations with SEP intensity and occurrence probability. In our previous work [Kim et al., 2014], we refined the SPE classes into four groups according to SPEs onset timings relative to flares and acceleration patterns independently of the conventional classification based on the flux-time profile. According to this classification, approximately one third of SPEs are related to flares and the others are related to CMEs, and for a half of the SPEs, the protons are accelerated from lower energy first and then they are accelerated to higher energy later. In addition to the origins and acceleration patterns, the location of particle acceleration in the solar corona is important in many respects. The longitude of acceleration site gives a clue to predict the trajectories of accelerated particles for space weather usage, since the detection of particles is highly dependent on the magnetic connectivity from the foot point of the interplanetary (IP) magnetic field line to the spacecraft [e.g., Aschwanden, 2006]. Its height is also important, since the temperature and density of the corona change as the function of height [e.g., Rosner and Tucker, 1978]. In this study, we try to grasp the general circumstances of proton acceleration for the respective SPE groups in terms of their origins, acceleration mechanisms, and locations. The paper is organized as follows: Data and methodology of our study are given in section 2. We examine the physical properties of SPEs and related eruptions, such as flare X-ray intensity, propagating speed, and angular width of CME in section 3. We also examine the spatial properties of them including longitudes of flaring regions and heights of CME-driven shocks. A brief summary and discussion are presented in section 4.

2. Data and Methodology 2.1. Four Groups of SPEs In our previous study [Kim et al., 2014], we selected 42 SPEs with clear information of associated flares, CMEs, and IP type II radio bursts from 1997 to 2012 and analyzed the multienergy channel observation [Krucker and Lin, 2000; Kocharov et al., 2007] from the Energetic and Relativistic Nuclei and Electron (ERNE) [Torsti et al., 2004] detector onboard the Solar and Heliospheric Observatory (SOHO) to determine the onset times of SPE in the 10 energy channels from 13 to 130 MeV for each event. In this process, we excluded the case with a high pre-existing background. We applied the velocity dispersion analysis, which is generally used to examine SPEs properties. For example, Vainio et al. [2013] used this method to examine the large SEP events of solar cycle 23, and Malandraki et al. [2012] performed a case study of the particle observations and their associated electromagnetic emissions using this method. We calculated the energy-dependent onset times of a SPE in the solar vicinity by subtracting the energy-dependent travel times from the observation times for 10 respective energy channels. Then we classified SPEs into four groups according to their onset timings and energy-dependent flux enhancements. As shown in Figure 1, each group has its typical acceleration pattern. For the SPEs of group A which are considered as the flare-associated events, the acceleration starts before the flare peak time from the lower energy channel. The other SPEs are considered as CME-associated events and their onset times coincide with the times of the first appearances of CMEs in the Large Angle and Spectrometric Coronagraph (LASCO) [Brueckner et al., 1995] field of view. For those CME-associated events, they could be further classified into three subgroups depending on their temporal patterns of proton flux enhancement: starting earlier at the higher energy channels (group B), the lower energy channels (group C), and all energy channels simultaneously (group D). 2.2. Properties of SPEs and Related Eruptions To investigate whether there are any differences among the characteristics of those four groups, we examine the properties of related flares and CMEs as well as SPEs themselves. The data set can be summarized as Table 1. The first seven columns of the table are the SPEs information from Kim et al. [2014]. The onset time is defined as the time of the first flux enhancement regardless of energy channel between the range of 13 MeV and 130 MeV, and onset times at lower or higher are the times of flux enhancement at the lowest KIM ET AL.

CHARACTERISTICS OF FOUR SPE GROUPS

7084


10.1002/2015JA021280

Figure 1. Examples of four SPE groups. In each panel, onset times of SPEs for 10 energy channels (13 to 130 MeV) are illustrated by red filled circles. Those onset times are retraced by applying the velocity dispersion analysis to SOHO/ERNE observations to compare with onset times of related eruptive phenomena near the Sun. Black solid lines denote scaled GOES X-ray intensity. Black filled circles denote CMEs appearances and their heights in SOHO/LASCO field of view, and blue vertical dotted lines show the onset times of IP type II radio bursts [Kim et al., 2014].

and the highest energy channels, respectively. Uncertainty in the determination of onset time can be large for low-energy protons, while it is relatively small for high-energy protons. Previous theoretical works [Rice et al., 2003; Li et al., 2003, 2012; Li and Zank, 2005] suggested that turbulence generated by streaming protons in the upstream CME-driven shock may help retain low-energy particles and therefore leading to difficulties in deciding the release time of low-energy ions. This adds to the uncertainty of onset times of SPEs in low energies. We thus focus on the trend of onset time difference between high- and low-energy channels, which turns out to be insensitive to the uncertainty. The sixth and seventh columns show the peak flux of SPEs and the types of SPEs. Since the energetic electrons can also be generated in the same acceleration process as the energetic protons, we also list the onset times of type II radio bursts detected by the WIND/WAVE instrument from Coordinated Data Analysis Workshops (CDAW) data center (http://cdaw.gsfc.nasa.gov/CME_list/radio/ waves_type2.html) in the eighth column. Although the type III solar radio burst is another indicator for energetic electrons streaming along solar magnetic lines, we only examine the type II burst in this study. In the ninth through eleventh columns, the flare’s onset, peak, and end times are listed, respectively. We examine the maximum X-ray intensity observed by the Geostationary Operational Environmental Satellite (GOES) and location for the associated flare, which is provided by the Lockheed Martin Solar and Astrophysics Laboratory (http://www.lmsal.com/solarsoft/latest_events_archive.html) as listed in the twelfth and thirteenth columns. We also investigate the properties of associated CMEs, such as the angular width and the linear speed, from the LASCO CME catalog provided by the Coordinated Data Analysis Workshops (CDAW) data center (http://cdaw.gsfc.nasa.gov/CME_list/index.html) as listed in the fourteenth through sixteenth columns. KIM ET AL.


7085


10.1002/2015JA021280

Table 1. Characteristics of SPEs and Associated Solar Eruptive Events SPE

Flare pfu

#

Date

(1)

(2)

Type II

Onset Lower Higher (#∕cm2 srs) Group Onset Onset Peak (3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

End (11)

CME

Shock Height

AW Speed SPE (Hp ) Type II (He ) Intensity Location Appearance (∘ ) (km/s) (Rs ) (Rs ) (12)

(13)

(14)

(15)

(16)

(17)

(18)

1

11-04-1997 06:03 06:15

06:10

72

D

06:00 05:52 05:58 06:02

X2

S14W33

06:10

360

785

2.54

2.18

2

11-06-1997 12:18 12:41

12:19

490

B

12:20 11:49 11:55 12:01

X9

S18W63

12:10

360 1556

6.34

6.58

3

04-20-1998 10:37 10:43

11:01

1,700

C

10:25 09:38 10:21 11:18

M1

S43W90

10:07

243 1863

7.61

5.69

4

05-02-1998 13:32 13:32

13:44

150

A

14:25 13:31 13:42 13:51

X1

S15W15

14:06

360

-0.67

6.00

5

05-06-1998 07:45 07:45

08:12

210

A

08:25 07:58 08:09 08:20

X2

S11W65

08:29

190 1099

0.84

3.81

6

04-04-2000 15:29 15:32

15:44

55

A

15:45 15:12 15:41 16:05

C9

N16W66

16:32

360 1188

5.36

6.80

7

06-06-2000 17:28 17:51

17:28

84

B

15:20 14:58 15:25 15:40

X2

N20E18

15:54

360 1119

13.15

0.84

8

06-10-2000 16:56 16:56

17:14

46

A

17:15 16:40 17:02 17:19

M5

N22W38

17:08

360 1108

1.66

3.61

9

938

07-14-2000 10:07 10:07

10:23

24,000

A

10:30 10:03 10:24 10:43

X5

N22W07

10:54

360 1674

-2.55

1.29

10 07-22-2000 11:42 12:10

11:47

17

B

11:45 11:17 11:34 12:02

M3

N14W56

11:54

229 1230

3.27

3.50

11 09-12-2000 12:24 12:40

12:24

320

B

12:00 11:31 12:13 13:13

M1

S17W09

13:31

360 1550

6.12

3.28

12 10-16-2000 07:07 07:07

07:19

15

A

07:10 06:40 07:28 09:11

M2

N04W90

07:27

360 1336

3.30

3.56

13 11-08-2000 22:43 22:43

23:08

14,800

A

23:20 22:42 23:28 00:05a

M7

N05W77

23:06

170 1738

0.67

5.54

14 11-24-2000 05:13 06:02

05:13

940

B

05:10 04:55 05:02 05:08

X2

N20W05

05:30

360 1289

1.43

1.00

15 01-28-2001 16:22 16:55

16:22

49

B

15:45 15:40 16:00 16:24

M1

S04W59

15:54

360

916

4.11

1.29

16 03-29-2001 10:36 10:40

11:20

35

C

10:12 09:57 10:15 10:32

X1

N14W12

10:26

360

942

4.66

2.71

17 04-02-2001 22:08 22:08

22:08

1,110

D

22:05 21:32 21:51 22:03

X20

N18W82

22:06

244 2505

6.63

5.89

18 04-15-2001 13:21 13:21

13:47

951

A

14:05 13:19 13:50 13:55

X14

S20W85

14:06

167 1199

-1.19

4.14

19 04-18-2001 02:30 02:34

02:34

321

D

02:55 02:11 02:14 12:16

C2

S20W90

02:30

360 2465

5.07

10.34

20 09-15-2001 11:51 11:51

12:08

11

C

11:50 11:04 11:28 11:54

M1

S21W49

11:54

130

478

3.53

3.50

21 09-24-2001 10:44 10:49

11:06

12,900

C

10:45 09:32 10:38 11:09

X2

S16E23

10:30

360 2402

5.87

5.94

22 10-19-2001 16:59 17:17

17:00

11

B

16:45 16:13 16:30 16:43

X1

N15W29

16:50

360

901

3.86

2.59

23 11-04-2001 16:18 16:18

16:34

31,700

A

16:30 16:03 16:20 16:57

X1

N06W18

16:35

360 1810

1.36

3.37

24 12-26-2001 05:19 05:19

05:32

779

A

05:20 04:32 05:40 06:47

M7

N08W54

05:30

212 1446

2.44

2.45

25 03-22-2002 11:55 11:55

13:09

16

C

11:30 10:12 11:14 11:52

M1

S20W87

11:06

360 1750

10.71

6.77

26 04-17-2002 08:46 08:46

10:23

24

C

08:30 07:46 08:24 09:57

M2

S14W34

08:26

360 1240

5.92

3.94

27 04-21-2002 01:07 01:07

01:27

2,520

A

01:30 00:43 01:51 02:38

X1

S14W84

01:27

360 2393

-0.84

3.74

28 08-14-2002 01:48 01:48

02:03

26

A

02:20 01:47 02:12 02:46

M2

N09W54

02:30

133 1309

0.22

4.22

29 08-24-2002 01:10 01:10

01:18

317

D

01:45 00:49 01:12 01:31

X3

S08W90

01:27

360 1913

3.57

8.76

30 11-09-2002 14:11 14:20

14:18

404

D

13:20 13:08 13:23 13:36

M4

S12W29

13:31

360 1838

10.20

2.53

31 05-31-2003 02:34 02:44

02:39

27

D

03:00 02:13 02:24 02:40

M9

S07W65

02:30

360 1835

3.21

7.28

32 10-26-2003 17:20 17:20

17:38

466

A

17:45 17:21 18:19 19:21

X1

N02W38

17:54

171 1537

-1.57

1.56

33 10-28-2003 11:21 11:23

11:35

29,500

C

11:10 09:51 11:10 11:24

X17

S16E08

10:54

147 1054

3.94

1.16

34 12-02-2003 11:18 11:18

11:32

86

C

11:00 09:40 09:48 09:54

C7

S19W89

10:50

150 1393

7.55

5.40

35 04-11-2004 04:38 04:44

04:38

35

D

04:20 03:54 04:19 04:35

C9

S14W47

04:30

314 1645

6.20

3.09

36 07-25-2004 15:11 15:38

15:11

2,086

B

15:00 14:19 15:14 16:43

M1

N08W33

14:54

360 1333

4.56

3.28

37 01-15 -005 23:00 23:45

23:00

5,040

B

23:00 22:25 23:02 23:31

X2

N15W05

23:06

360 2861

5.58

5.47

38 05-13-2005 17:10 17:13

17:40

3,140

C

17:00 16:13 16:57 17:28

M8

N12E11

17:22

360 1689

4.69

3.36

39 07-13-2005 14:56 15:07

14:56

134

B

14:15 14:01 14:49 15:38

M5

N10W80

14:30

360 1423

6.41

1.06

40 08-22-2005 17:36 18:15

17:42

330

B

17:15 16:46 17:27 18:02

M5

S12W60

17:30

360 2378

7.46

3.46

41 12-13-2006 02:37 03:05

02:41

698

B

02:45 02:14 02:40 02:57

X3

S05W23

02:54

360 1774

2.57

3.78

42 03-07-2011 20:37 21:05

20:39

50

B

20:00 19:43 20:12 20:58

M3

N24W59

20:00

360 2125

9.69

2.22

a Next day.

KIM ET AL.


7086


10.1002/2015JA021280

2.3. Heights of Proton and Electron Acceleration We estimate the height of proton acceleration, Hp , and that of electron acceleration, He , from the data of CMEs and type II bursts. We made the following assumptions: (1) the protons are accelerated by the shocks located close to the CME’s leading edges and (2) the time of type II burst appearance indicates initiation of electron acceleration. Under these assumptions we use the location of CME’s leading edge at the onset times of proton flux increase as the height of proton acceleration and that at the time of type II bursts as the height of electron acceleration. The location of CME’s leading edge could be determined by extrapolating the LASCO observation back to the onset time. In Figure 2 we plot the heights of CME’s leading edges (the purple diamonds) as a function of time, together with the second-order polynomial fitting to the CME data (purple line). Now for timing of the onset, we plot the retraced SPEs onset times near the Sun for 10 energy channels (the red closed circles) and choose the earliest one as the onset time of proton acceleration and the time of type II bursts appearance (blue dot) as the onset time of electron acceleration. We read off the height of CME’s leading edges from the right vertical axis at the onset time of proton acceleration (red horizontal dotted line) and at the onset time of electron acceleration (blue horizontal dotted line), respectively. The results of Hp and He are listed in the last two columns in Table 1. Figure 2. The extrapolation of CME’s leading edges (purple line). The horizontal red and blue dotted lines denote estimated acceleration heights of proton, Hp , and electron, He , respectively.

3. Results 3.1. SPEs’ Onset Time Difference Depending on Energy Channels First, we examine the relationship between the proton peak flux and the onset time difference for each group. The onset time difference between the highest and the lowest energy channels, TH − TL , could represent a characteristic for each group. If TH −TL is a positive value, then acceleration started first in lower energy channel and later in higher energy and vice versa. As a matter of fact, in Figure 3, all SPEs of groups A (blue circles) and C (orange circles) have positive values of TH − TL longer than 10 min, while all events of group B (yellow circles) have negative TH − TL less than −10 min as marked by two vertical black dotted lines, since the TH − TL value was one of the criteria of Kim et al. [2014]. SPEs classified as Group D are located in between groups A, C, and B.

Figure 3. The relation of the onset time difference between the highest and the lowest energy channels, TH − TL , and the proton peak flux. Each color represents each groups, and the diamonds represent the mean values for groups.

KIM ET AL.


In Figure 3, it is shown that five strongest SPEs above the horizontal dotted line (104 pfu) have positive TH − TL and are classified into group A or C. As illustrated by diamonds with different colors for the mean values of groups, there is a tendency in case of group A or C, which the acceleration starting from the lower energy has the higher chance to be a stronger event. The exceptional three events of group C as marked by arrows show very weak flux enhancements and long time differences of accelerations between the energy channels. It is notable that the acceleration pattern of group C is the same as that of group A, even though their proton acceleration starts after the flare peak time. And for the 7087


10.1002/2015JA021280

Table 2. Mean Properties of SPEs and Associated Solar Eruptive Events for Each Groupa Characteristics

Group A

Group B

Group C

Group D

Total

(1)

(2)

(3)

(4)

(5)

(6)

SPE

Flare

CME

Acceleration Heights

Proton Flux (pfu)

5824

788

5268

327

3230

TH − TL (min)

17 (0.33)

−27 (−0.51)

35 (−0.56)

−2 (0.65)

4 (−0.07)

X-ray Intensity (104 )

2.11 (0.56)

1.61 (0.24)

2.37 (0.73)

3.77 (0.33)

2.29 (0.47)

Longitude (W)

53 (−0.30)

36 (−0.32)

35 (−0.57)

62 (0.40)

45 (−0.32)

Angular Width (∘ )

274 (0.03)

350 (0.41)

274 (−0.05)

337 (−0.41)

308 (−0.06)

Speed (km s−1 )

1444 (0.65)

1573 (0.56)

1423 (0.51)

1855 (0.61)

1548 (0.46)

Hp (Rs )

0.70 (−0.47)

5.74 (−0.16)

6.05 (−0.26)

5.35 (0.47)

4.18 (−0.26)

He (Rs )

3.85 (−0.31)

2.95 (0.42)

4.28 (−0.22)

5.73 (0.25)

3.98 (−0.03)

a The

parentheses show the correlations with SPE intensity and respective parameters. The bold texts show the correlation coefficients higher than 0.5.

event of group D, for which the acceleration takes place simultaneously in all energy channels in a short time less than ±10 min, has the lowest chance to be a strong event compared to other groups. From this result, we speculate that the onset time difference between the higher and the lower energy channels, |TH − TL |, is an important factor for the increment of energetic proton flux. If this time difference between lower and higher energy channels is short, the event may not become a strong SPE, even if it is accompanied by a very strong flare and CME. Above all, for group B, there is a definite tendency for the stronger SPEs to have the more negative onset time differences, TH − TL , which means that the CME keeps accelerating the protons to the higher energy level for longer times before the acceleration of the lower energy level. We emphasize that the correlations of TH − TL and SPEs intensity for different groups as marked by dotted lines with different colors are significantly higher than this for the all events (black solid line) in the figure. Each line shows the polynomial fitting result for each case. The correlation coefficients for groups A, B, C, and D are 0.33, −0.51, −0.56, and 0.65, while this for all events is only −0.07 as listed in Table 2, which summarizes the characteristics of SPEs and associated solar eruptive events for each group. This table shows the mean values for the properties and correlation coefficients with SPE intensity. 3.2. Flare and CME Association To inspect characteristics of associated flare for each group, we examine its maximum X-ray intensity and the source location on the Sun. Figure 4 shows the relation of the flare intensity and SPEs peak flux, which is generally proportional to each other. Each color represents each group as same as Figure 3. As marked by diamonds, the mean values of X-ray intensity are the strongest for group D and the weakest for group B. It is difficult to find any strong group dependence on the X-ray intensity from all over the events (cc = 0.47); however, groups A and C show relatively high correlations of 0.56 and 0.73, while groups B and D show lower correlations of 0.24 and 0.33. This might imply that even though the protons of group C events are accelerated after flare peak times as the protons of groups B and D events are, their acceleration mechanism is different with that of groups B and D.

Figure 4. Relation of the flare X-ray intensity and SPEs peak flux.

KIM ET AL.


The Figure 5 (left) shows the distribution of flaring regions on the Sun. The location distribution of groups A and D, which have relatively short intervals between high- and low-energy channels (TH − TL ), is placed in close to the 7088


10.1002/2015JA021280

Figure 5. (left) Distribution of flare location on the Sun and (right) the relation of SPE peak intensity and heliolongitude of associated flare.

western limb (mean longitude = 53∘ and 62∘ for groups A and D, respectively). In contrast, groups B and C show wider distribution of flaring sites, and their mean longitudes are 36∘ and 35∘ , while the mean value for all events is 45∘ . This tendency confirms that flare associated SPEs are relatively more connected from the foot-point of the interplanetary (IP) magnetic field line to the spacecraft. Figure 5 (right) shows the correlations between the flare location and SPEs peak intensity. As listed in Table 2, only for group C, the correlation is higher than 0.5. We also examine the angular width and speed of associated CMEs. For the events of groups B and D, most of them are associated with full halo CMEs (AW = 360∘ ), and the mean angular widths are 350∘ for group B and 337∘ for group D as shown in Figure 6. Meanwhile, the mean values and correlations with SPE peak intensity for groups A and C are 274∘ , 0.03 and 274∘ , −0.05, respectively. From this result, we can speculate that CMEs can strongly accelerate protons regardless of their angular width, if their acceleration start at the lower energy channel. Figure 7 shows the relation of the SPEs peak intensity and the mean propagation speed of associated CME. The mean speed for group D, which show simultaneous accelerating process for all energy channels of proton, is the fastest among all events with the value of 1855 km s−1 . It is hard to see any systematic tendency from all events; however, it becomes clearer when we calculate the correlations of the CME speed and SPE intensity by dividing the groups. As listed in Table 2, the correlation coefficients of four groups (0.65, 0.56, 0.51, and 0.61 for A, B, C, and D) are much higher than that for all events (0.46). 3.3. Acceleration Heights of Protons and Electrons In Figure 8, we plot the acceleration heights of protons (Hp ) and electrons (He ) as determined using the method described in section 2.3 and listed in Table 1. Here symbols are data points, and the dotted lines represent the solar surface (1 Rs ). Since the height cannot be lower than 1 Rs , any data points below these lines should be regarded as false results. For group A (blue dots), the SPE onset heights are either less than 1 Rs or close to 1 Rs . Since the heights for proton acceleration are inferred from the extrapolation of CME location, any values less than 1 Rs should be regarded as implying no relation of the events with CMEs. This again proves that group A events are mostly associated with flares only. For other groups, Hp tends to be higher than He . The mean heights Figure 6. The relation of SPE peak intensity and angular width of of electron acceleration are 2.95, 4.28, and 5.73 associated CME. KIM ET AL.


7089


10.1002/2015JA021280

for groups B, C, and D, respectively, whereas the mean heights of proton acceleration are 5.74, 6.05, and 5.35 Rs for groups B, C, and D, respectively. Our brief interpretation is as follows. The result that Hp is generally greater than He for groups B and C would imply that the proton acceleration comes later than electron acceleration for these groups. It could be associated with the fact that CME speed can attain super Alfven speed only above some critical height. This tendency is more obvious for group B events where higher energy protons are detected earlier than lower energy protons. Group C events can be explained in terms of any acceleration mechanism that is initiated from lower energies. This further requires that the acceleration Figure 7. The relation of SPEs peak intensity and mean mechanism effective for higher energy propropagation speed of associated CME. tons (responsible for group B) is switched on later than some other acceleration mechanism effective for lower energy protons (responsible for group C). Which acceleration mechanisms can produce such energy-dependent onset times suitable for groups B and C is beyond the scope of the present study and is open to future works. Group D events show a broad distribution of the proton acceleration height. 3.4. Characteristics of Four SPE Groups We summarize the mean values of those characteristics for the different groups in Table 2. We list the correlation coefficients of associated eruptive properties and SPE intensity in the parentheses, and also, we mark the higher correlations than 0.5 by bold texts. It is found that SPE intensity is related with different properties for different groups, while CME speed is the most effective parameter for all groups. For group A, it highly relates with flare intensity, and it is reasonable since the events in group A are considered as the flare associated events. For group B, the onset time difference between the higher and the lower energy channels, TH − TL , plays an important role in increasing the SPEs intensity. For the events categorized as group C, they show the best correlations with SPE intensity and flare properties, even though their late onset times. And in the events in group D, they only show weak SPE intensities caused by short TH − TL . To see different dependencies of SPE groups on the flare and CME properties in more detail, we draw charts as shown in Figure 9. In each chart, black dotted equilateral octagon with closed circles in the middle denote the normalized average values using 42 SPEs as summarized in the sixth column of Table 2. The outer boundary of the circle represents the maximum value, and the center of the circle represents the minimum value for each property.

Figure 8. Relation between acceleration heights of proton and electron inferred from the extrapolation of CME leading edge and onset times of SPEs and type II bursts.

KIM ET AL.


Group A shows the strong SPE intensity. Its CME properties of speed and angular width are also weaker than average values as they locate inside of the octagon. Also, it has low acceleration height, Hp , and western-biased mean longitude. Group B shows the CME-biased properties, which are weak flare and SPE intensities but wide angular width and fast speed with higher Hp and wider longitude of flaring site. 7090


10.1002/2015JA021280

Figure 9. Different dependencies of SPE intensities according to four groups. In each chart, from the top in clockwise direction are the SPE’s intensity and onset time difference, TH − TL , the flare’s intensity and longitude of source region, CME’s angular width and speed, and the acceleration heights of proton, Hp , and electrons, He . The black dotted equilateral octagons with closed black circles denote the normalized average values using 42 SPEs. The outer boundary of circle represents the maximum value, and the center of the circle represents the minimum value for each property. In the case of flare longitude, the maximum value means western limb.

This could be a clear evidence supporting that the SPEs in group B are associated with CMEs. Meanwhile, group C shows a strong SPE intensity and higher Hp and He in spite of weak flare and CME. And SPEs in group D are associated with very strong flare and CME, and the acceleration sites of proton and electron are relatively high. However, their proton intensities are very weak.

4. Summary and Discussion We have investigated different characteristics of SPEs depending on their association with flare and CME. For this we first examine the properties of the four groups of SPEs classified by Kim et al. [2014] and then estimate the acceleration heights of SPEs. Our findings are as follows: 1. Five strongest SPEs (> 104 pfu) have positive TH − TL and are classified into groups A and C. This means that if the acceleration starts from the lower energy, then it has a higher chance of being a stronger SPE even if the associated flare and CME are not so strong. In contrast, when the proton acceleration starts from the higher energy as like group B, a SPE tends to be a relatively weak event (< 1000 pfu), in spite of its associated CME being relatively strong. The events of group D, which their accelerations take place in all energy channels simultaneously within short time scale less than 10 min, have the lowest chance to be a strong event, even though it is generally associated with strong eruption. 2. We have confirmed that flare associated SPEs are relatively better connected from the foot point (∼ 1Rs ) of the IP magnetic field line to the spacecraft near the Earth, since only the flares occurring near the west limb can produce earthward energetic protons, while the solar sources of CME associated SPEs are located in wider longitude of the higher corona (mean = 6.05 Rs ). 3. The heights of proton acceleration for CME-associated SPEs are not so much different between the groups as their mean values are in between 5 to 6 Rs . This result is consistent with the result from Aschwanden [2006]. We have found that the proton acceleration heights are lower than electron acceleration height in case of all flare associated SPEs, while for the most of CME associated SPEs in groups B and C, the proton acceleration heights are higher than electron acceleration heights. 4. Group A shows the strongest SPE flux. Its CME properties of speed and angular width are weaker than average values. And it is clear that the events in group A are associated with flares due to their low acceleration height and their solar sources near western limb area. Group B shows the CME-biased properties, which are associated with weak flares and SPE intensities but wide angular width and fast speed with higher proton KIM ET AL.


7091


10.1002/2015JA021280

acceleration site and wider longitude of flaring site. This supports that the SPEs of group B are associated with CMEs. Meanwhile, group C shows strong SPE intensity in spite of weak flare and CME. And SPEs in group D are associated with very strong flare and CME, and the acceleration sites of proton and electron are relatively higher. However, the intensity of proton flux is very weak. Based on those results, we could clearly characterize four SPE classes and suggest that the different characteristics of four groups, including the strength of SPE, are mainly due to the different mechanisms governing the acceleration pattern at the different heights of the acceleration. For example, the acceleration of SPEs in group B is more efficient at higher energies; therefore, the diffusive shock acceleration mechanism [Zank et al., 2000] may be more appropriate to explain their flux enhancement. The events in group C seem to undergo a different acceleration mechanism that is more effective at lower energies with a limiting high energy even though the acceleration takes place in the CME-driven shock. The finite electric field can be a cause of this kind of accelerations. Our results well support Reames’s [2013] suggestion for two different physical mechanisms for acceleration of SEPs. We also note that the onset time difference between the higher and the lower energy channels, |TH − TL |, is an important factor for the energetic proton flux. If the onset time difference between low and high energies is small, the event may not become a strong SPE event. This rule holds even when the event is accompanied by very strong flare and CME. This is in line with the result of Trottet et al. [2015], which the SEPs peak intensity shows better agreement with the fluence than the peak intensity of X-ray emission from associated flare. In addition, the acceleration height also seems to control the SPEs intensity, since the acceleration efficiency is related to the condition of the IP medium.

Acknowledgments The data for this paper are available at Space Research Laboratory of Turku University (http://www.srl.utu.fi/ erne_data/) for multichannel observation of energetic protons, Coordinated Data Analysis Workshops (CDAW) data center (http://cdaw.gsfc. nasa.gov/CME_list/radio/waves_type2. html) for type II radio bursts information detected by the WIND/WAVE instrument, the Lockheed Martin Solar and Astrophysics Laboratory (http://www.lmsal.com/solarsoft/ latest_events_archive.html) for flare properties, and the Coordinated Data Analysis Workshops (CDAW) data center (http://cdaw.gsfc.nasa.gov/ CME_list/index.html) for the properties of associated CMEs. We thank all the teams for the data available on the Internet. This study is supported by the “Planetary system research for space exploration” from KASI and a grant from the U.S. Air Force Research Laboratory, under agreement “FA 2386-14-1-4078”. J.L. was supported by the Brainpool Program of KOFST and the BK21 Plus Program (21A20131111123) of the Ministry of Education (MOE, Korea) and National Research Foundation of Korea (NRF). Yuming Wang thanks Olga Malandraki and one anonymous reviewer for their assistance in evaluating this paper.

KIM ET AL.

Our statistical results show clear differences among the four groups and might be improved if the condition of IP medium is considered in further studies. Recent research works show the importance of seed population in IP medium. Ding et al. [2013] presented the observation of the correlation between the presence of preceding CMEs and large SPE events. On the other side, Kahler and Vourlidas [2014] suggested that the large SPEs with preceding CMEs may not be due primarily to CME interactions but are explained by a general increase of both background seed particles and more frequent CMEs during times of higher solar activity. The SPE’s association with type III solar radio bursts also need to be examined in future works with information on the magnetic condition around the flaring site from the type III burst [Klein et al., 2010, 2011]. The small values of |TH − TL | might be observed when an interplanetary structure is crossing over the spacecraft that can greatly modulate the energetic particle profiles and influence the particle propagation [Malandraki et al., 2007; Lario et al., 2008]. More detailed examinations of this effect should be performed for those events with simultaneous onset times in all energy channels. We hope that this study may provide a clue for understanding particle acceleration mechanism and even further for the forecast of SPEs and solar radiation storms in the space weather context.

References Aschwanden, M. J. (2006), The localization of particle acceleration sites in solar flares and CMEs, Space Sci. Rev., 124, 361–372, doi:10.1007/s11214-006-9095-9. Bombardieri, D. J., M. L. Duldig, J. E. Himble, and K. J. Michael (2008), An improved model for relativistic solar proton acceleration applied to the 2005 January 20 and earlier events, Astrophys. J., 682, 1315–1327. Brueckner, G. E., et al. (1995), The Large Angle Spectroscopic Coronagraph (LASCO): Visible light coronal imaging and spectroscopy, Sol. Phys., 162, 357–402. Cane, H. V., R. E. McGuire, and T. T. von Rosenvinge (1986), Two classes of solar energetic particle events associated with impulsive and long-duration soft X-ray flares, Astrophys. J., 301, 448–459. Dierckxsens, M., K. Tziotziou, S. Dalla, I. Patsou, M. S. Marsh, N. B. Crosby, O. Malandraki, and G. Tsiropoula (2015), Relationship between solar energetic particles and properties of flares and CMEs: Statistical analysis of solar cycle 23 events, Sol. Phys., 290(3), 841–874. Ding, L., Y. Jiang, L. Zhao, and G. Li (2013), The Twin-CME scenario and large solar energetic particle events in solar cycle 23, Astrophys. J., 763, 30, doi:10.1088/0004-637X/763/1/30. Kahler, S. W., and A. Vourlidas (2014), Do interacting coronal mass ejections play a role in solar energetic particle events?, Astrophys. J., 784, 47, doi:10.1088/0004-637X/784/1/47. Kallenrode, M. B. (2003), Current views on impulsive and gradual solar energetic particle events, J. Phys. G: Nucl. Part. Phys., 29, 965–981, doi:10.1088/0954-3899/29/5/316. Kim, R.-S., K.-S. Cho, J. Lee, S.-C. Bong, and Y.-D. Park (2014), A refined classification of SPEs based on the multienergy channel observations, J. Geophys. Res. Space Physics, 119, 9419–9429, doi:10.1002/2014JA020358. Klein, K.-L., and G. Trottet (2001), The origin of solar energetic particle events: Coronal acceleration versus shock wave acceleratios, Space Sci. Rev., 95, 215–225. Klein, K.-L., G. Trottet, and A. Klassen (2010), Energetic particle acceleration and propagation in strong CME-less flares, Sol. Phys., 263, 185–208. Klein, K.-L., G. Trottet, S. Samwel, and O. Malandrakiet (2011), Particle acceleration and propagation in strong flares without major solar energetic particle events, Sol. Phys., 269, 309–333.


7092


10.1002/2015JA021280

Kocharov, L., O. Saloniemi, J. Torsti, E. Riihonen, J. Lehti, K.-L. Klein, L. Didkovsky, D. L. Judge, A. R. Jones, and R. Pyle (2007), High-energy protons associated with liftoff a coronal mass ejection, Astrophys. J., 659, 780–787. Krucker, S., and R. P. Lin (2000), Two classes of solar proton events derived from onset time analysis, Astrophys. J., 542, L61–L64. Lanzerotti, L. J. (2001), Space weather effects on technologies, in Space Weather, edited by P. Song, H. J. Singer, and G. L. Siscoe, AGU, Washington, D. C., doi:10.1029/GM125p0011 Lario, D., R. B. Decker, O. E. Malandraki, and L. J. Lanzerotti (2008), Influence of large-scale interplanetary structures on energetic particle propagation: September 2004 event at Ulysses and ACE, J. Geophys. Res., 113, A03105, doi:10.1029/2007JA012721. Li, G., and G. Zank (2005), Mixed particle acceleration at CME-driven shocks and flares, Geophys. Res. Lett., 32, L02101, doi:10.1029/2004GL021250. Li, G., G. P. Zank, and W. K. M. Rice (2003), Acceleration and transport of energetic particles at CME-driven shocks, Adv. Space Res., 32, 2597–2602. Li, G., A. Shalchi, X. Ao, G. Zank, and O. P. Verkhoglyadova (2012), Partocle acceleration and transport at an oblique CME-driven Shock, Adv. Space Res., 49, 1067–1075. Malandraki, O. E., R. G. Marsden, C. Tranquille, R. J. Forsyth, H. A. Elliott, L. J. Lanzerotti, and A. Geranios (2007), Energetic particle observations by Ulysses during the declining phase of solar cycle 23, J. Geophys. Res., 112, A06111, doi:10.1029/2006JA011876. Malandraki, O. E., et al. (2012), Scientific analysis within SEP server—new perspectives in solar energetic particle research: The case study of the 13 July 2005 event, Sol. Phys., 281(1), 333–352. Miller, J. A., N. Guessoum, and R. Ramaty (1990), Stochastic Fermi acceleration in solar flares, Astrophys. J., 361, 701–708. Miller, J. A., T. N. LaRosa, and R. L. Moore (1996), Stochastic electron acceleration by cascading fast mode waves in impulsive solar flares, Astrophys. J., 461, 445–464. Mori, K., J. Sakai, and J. Zhao (1998), Proton acceleration near an X-type magnetic reconnection region, Astrophys. J., 494, 430–437. Ohsawa, Y., and J. Sakai (1988), Prompt simultaneous acceleration of proton and electrons to relativistic energies by shock waves in solar flares, Sol. Phys., 332, 439–446. Pallavicini, R., S. Serio, and G. Vaiana (1977), A survey of soft X-ray limb flare images—The relation between their structure in the corona and other physical parameters, Astrophys. J., 216, 108–122. Reames, D. V. (2013), The two sources of solar energetic particles, Space Sci. Rev., 175, 53–92. Rice, W. K. M., G. P. Zank, and G. Li (2003), Particle acceleration and coronal mass ejection drived shocks: Shocks of arbitrary strength, J. Geophys. Res., 108(A10), 1369, doi:10.1029/2002JA009756. Rosner, R., and W. H. Tucker (1978), Dynamics of the quiescent solar corona, Astrophys. J., 220, 643–665. Roussev, I. I., I. V. Sokolov, T. G. Forbes, T. I. Gombosi, M. A. Lee, and J. Sakai (2004), A numerical model of a coronal mass ejection: Shock development with implications for the acceleration of GeV protons, Astrophys. J., 605, L73–L76. Siscoe, G. (2000), The space-weather enterprise: Past, present, and future, J. Atmos. Sol. Terr. Phys., 62, 1223–1232. Torsti, J., E. Riihonen, and L. Kocharov (2004), The 1998 May 2-3 magnetic cloud: An interplanetary highway for solar energetic particles observed with SOHO/ERNE, Astrophys. J., 600, L83. Trottet, G., S. Samwel, K.-L. Klein, T. Dudok de Wit, and R. Miteva (2015), Statistical evidence for contributions of flares and coronal mass ejections to major solar energetic particle events, Sol. Phys., 290, 819–839. Vainio, R., et al. (2013), The first SEP server event catalogue ∼68 MeV solar proton events observed at 1 AU in 1996-2010, J. Space Weather Space Clim., 3, A12. Wild, J. P., S. F. Smerd, and A. A. Weiss (1963), Solar bursts, Annu. Rev. Astron. Astrophys., 1, 291–366. Zank, G. P., W. K. M. Rice, and C. C. Wu (2000), Particle acceleration and coronal mass ejection driven shocks: A theoretical model, J. Geophys. Res., 105(A11), 25,079–25,095.

KIM ET AL.


7093