1. SMALL SEP EVENTS WITH METRIC TYPE II RADIO BURSTS
Pertti Mäkelä1,2, Nat Gopalswamy2, Sachiko Akiyama1,2, Seiji Yashiro1,2
1The Catholic University of America, Washington, DC, USA
2NASA Goddard Space Flight Center, Greenbelt, MD, USA

2. Small SEP Event
Large (major) SEP event: GOES >10 MeV peak integral flux ≥ 10 pfu.
pfu = particle flux unit
1 pfu = 1 particle per (cm2 s1 sr1)
GOES measurements are not good for detecting small SEP events due to high background levels.

3. Filament Eruption SEPs
Kahler et al. (1986; ApJ 302)
Gopalswamy et al. (2015; ApJ 806) identified four filament eruptions (FEs) outside active regions (ARs) that were associated with major (GOES >10 MeV flux ≥ 10 pfu) SEP events and interplanetary type II radio bursts (no metric type II bursts except during one event).
Spectral index in the 10–100 MeV range typically >4 for the FE-SEP events.
Time-of-Maximum (TOM) spectra

4. Fluence Spectral Index
Gopalswamy et al. (2016; ApJ 833) used GOES and SAMPEX observations to study the fluence spectra of ground level enhancement (GLE) events, well-connected major SEP events and FE-associated SEP events during cycles 23 and 24.
Cycle 23
Cycle 24
Cycles 23 and 24
FE SEP
4.72
5.41
4.89
Regular SEP
3.85
3.78
3.83
GLE
2.70
2.51
2.68
Fluence spectra
Soft
Intermediate
Hard
Systematic increase in γ as one goes from the GLE events to regular SEP events and to FE SEP events

5. CME Kinematics
Hierarchical relationship between CME kinematics and the spectral index of SEP events
In GLE events the shock forms close to the Sun—about half a solar radius above the solar surface.
Particles accelerated efficiently to GeV energies (hard spectrum) because of the high ambient magnetic field near the Sun.
The low shock-formation height implies impulsive CME acceleration (initial acceleration ∼2 km/s2 ).
In FE SEP events, the shock forms at much larger heights—either in the outer corona or in the interplanetary medium (Mäkelä et al. 2015; ApJ 806).
Particles are not accelerated to high energies (soft spectrum; Gopalswamy et al. 2015; Prog EP&S 2).
The regular major SEP events show intermediate behavior in shock-formation height, initial acceleration, and spectral hardness.

6. Type II Bursts
Gopalswamy et al. (2017; JPCS, Proc 16th AIAC) found that the average starting frequencies of type II radio bursts associated with the GLE, FE SEP and major SEP events have a hierarchy, although the actual distributions overlap.
The shock formation heights are also organized accordingly.
The average starting frequencies:
107 MHz (GLE event)
81 MHz (regular major SEP event)
22 MHz (FE SEP event)
The average shock formation heights:
1.51 Rs (GLE event)
1.71 Rs (regular major SEP event)
5.38 Rs (FE SEP event)

7. Event Selection
Motivation: Check if the small SEP events follow the same hierarchy as GLEs, major SEP and FE SEP events.
Starting from a list of metric type II radio bursts in cycle 24.
Searched for metric type II bursts associated a small SEP event observed by SOHO/ERNE (GOES >10 MeV peak flux must be below 10 pfu).
The solar source is in the western hemisphere or right behind the western limb.
Found a total of 16 good events.
Calculated for each event the power-law index at energies above 13.8 MeV using SOHO/ERNE/HED data (fluence spectra). Number of data points varied from 5 to 7 depending on the maximum energy of particles.

8. m-T II SEP γ CME Vsk Vsp W LOC Flare Size Start Peak GOES EUV Wave
[UT]
SEP γ
CME
Vsk
[km/s]
Vsp W
[°]
LOC
Flare
Size
Start
Peak
GOES
[pfu]
EUV Wave
2009/12/22 04:57
22/05:30
2.56
05:54
318
506 47
S26W46
C7.2
04:50
04:56 -
Yes
2010/06/12 00:57
12/01:45
3.14
01:31
486
691
119
N23W43
M2.0
00:30
00:57
1.21
2010/08/18 05:51
18/07:00
4.46
05:48
1471
1498
184
N17W101
C4.5
04:45
3.35
2011/05/11 02:27
11/02:10
3.27
02:48
745
975
225
N19W51
B8.1
02:23
02:43
0.52
2011/08/02 06:40
02/06:40
2.79
06:36
712
939
268
N14W15
M1.4
05:58
06:19
1.66
2013/04/28 20:18
28/22:20
3.03
20:48
497
703 91
S18W37
C4.4
20:10
20:17
2013/05/02 05:06
02/09:10
3.20
671
894 99
N10W26
M1.1
04:58
05:10
2013/08/17 18:56
17/20:45
4.74
19:12
1202
1418
360
S05W30
18:49
19:33
2013/12/07 07:24
07/11:45
3.62
07:36
1085
1165
S16W49
M1.2
07:17
07:29
2014/06/12 22:00
12/23:40
3.18
22:12
684
908
186
S20W55
M3.1
21:34
22:16
0.47
2014/08/25 15:08
25/16:50
3.45
15:36
555
697
N05W36
14:46
15:11
1.00
2015/09/20 18:16
20/18:35
3.60
18:12
1239
1458
S20W24
M2.1
17:32
18:03
2.29
2015/11/04 13:43
04/15:30
2.86
14:48
578
987
N09W04
M3.7
13:31
13:52
0.43
2016/02/11 20:35
12/01:15
3.74
21:17
719
1174
N11W07
C8.9
20:18
21:03
2016/03/16 06:45
16/07:35
2.99
07:00
592
154
N12W88
C2.2
06:34
06:46
1.29
2016/04/18 00:30
18/02:05
4.12
00:48
1084
1213
162
N12W62
M6.7
00:14
00:29
0.53

9. 2010 June 12 Event
Metric type II with the highest frequency ~400 MHz (harmonic emission) observed by Hiraiso Radio Spectrograph (HiRAS).
GOES M2.0 flare has a short duration ~ 8 mins.
EUV wave observed by SDO/AIA
Metric type II radio burst started when the leading edge of the wave was at the limb in projection
(actual height ~1.20 Rs).
Empirical formula: f = r – 0.14 gives: r≈1.13 Rs for f = 200 MHz
(formula from Gopalswamy et al. 2013, AdSpR 51)
(Gopalswamy et al. 2017, JPCS, Proc 16th AIAC)

10. 2010 June 12 CME
Type II burst start: CME leading edge speed of ~900 km/s.
CME speed increased to a maximum value of ~1275 km/s and dropped back to ~1000 km/s when the type II ended.
No IP type II burst. Shock died or was too weak to produce type II emission.
Type II start and end time
SEP release time
(Gopalswamy et al. 2017, JPCS, Proc 16th AIAC)

11. 2010 June 12 SEP
SOHO/ERNE 13.8 – 16.9 MeV energy channel 0.3 particles per (cm2 s sr MeV) .
GOES >10 MeV channel peak 1.21 particles per (cm2 s sr)
The SEP event started ~01:30 UT (~34 minutes after the type II onset).
Assuming Parker spiral length of 1.2 AU gives the travel time of ~31 minutes for 50 MeV protons.
The particle release time at the Sun is 00:59 UT, which after correcting for the travel time of the electromagnetic emissions becomes 01:07 UT.
Type II burst 00:56-01:10 UT, so the SEP acceleration took ~ 10 mins, which is a typical value.
The fluence spectrum resembles those of regular SEP events. The spectral index (3.14) lies between the 2001 April 12 regular SEP event and the 2004 April 11 FE SEP event.
(Gopalswamy et al. 2017, JPCS, Proc 16th AIAC)

12. Spectral Index Comparison
Small SEP events
GLEs, regular major SEP and FE SEP events
Gopalswamy et al. (2016; ApJ 833)

13. Onset Frequency Comparison
Type IIs with a clear F-H structure
(Gopalswamy et al. 2017, JPCS, Proc 16th AIAC)

14. Conclusions
The average power-law index of fluence spectra in small SEP events is similar to that in major SEP events.
The average onset frequency of type II radio bursts is also similar to that of major SEP events.
The small SEP events show no deviations from the hierarchy of spectral index or the hierarchy of the starting frequency of type II radio bursts.