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Astroparticle Physics 26 (2006) 202–208

Statistical characteristics of solar energetic proton events fromJanuary 1997 to June 2005

Ruiguang Wang

National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China

Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

Received 6 December 2005; accepted 9 June 2006Available online 7 July 2006

Abstract

We have made a statistical study of 163 solar proton events (SPEs) associated with X-ray flares, coronal mass ejections (CMEs) andradio type II bursts during January 1997–June 2005. These SPEs were categorized by the peak fluxes of >10 MeV solar protons into threegroups. There are 37 large SPEs with fluxes of more than 100 protons cm�2 s�1 sr�1, 34 moderate SPEs with flux ranges of 10–100 pro-tons cm�2 s�1 sr�1 and 92 minor SPEs with flux ranges of 1–10 protons cm�2 s�1 sr�1. To understand the determinant of solar protonevents, we have examined the association of these SPEs with X-ray flares, CMEs and radio type II emissions from metric to decamet-ric-hectometric (DH) wave ranges. The primary results from this study are: (1) most SPEs (112/163) corresponded to the solar flaresfavorably located at solar western hemisphere and the center of the activity source region tended to shifted to the west with increasingof the solar proton fluxes; (2) there seems a longitudinal cutoff for each group of SPEs, which also moves toward west with increasing ofthe solar proton flux; (3) each SPE observed at Earth was associated with a fast (average speed �1228 kms�1) and wide (average anglewidth of 266�) CME; (4) the percentage of these SPEs associated with metric (DH) type II burst increased from 54% (42%) to 81%(100%). Overall, The most intensive SPEs are more likely to be produced by major flares located near central meridian of the Sunand shock waves driven by very fast halo CMEs (v P 1600 kms�1). This suggested that CME-driven shock acceleration is a necessarycondition for large SPEs production.� 2006 Elsevier B.V. All rights reserved.

Keywords: Solar proton events (SPEs); Solar flares; Coronal mass ejections (CMEs); Radio type II bursts

1. Introduction

Since the first identification of solar proton events(SPEs) in 1942 [1], observations and subsequent studiesof SPEs have been made for about six Solar Cycles. Inorder to completely understand the solar proton produc-tion and acceleration, all kinds of solar activity phenomenaassociated with SPEs were taken into account, such as solarflares, solar coronal mass ejections (CMEs) and radio typeII emissions. The source region distribution of solar flaresrelated to SPEs produced in previous Solar Cycles wasexamined by many researchers (e.g., [2–5]). Their results

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doi:10.1016/j.astropartphys.2006.06.003

E-mail address: wangrg@mail.ihep.ac.cn

indicate that the gradual SPEs more likely to associate withsolar flares at western sides of the solar disk. Kiplinger [6]also reported a high correlation between the existence of10 MeV solar protons at Earth and a typical pattern ofX-ray spectral evolution for 18 associated flares.

After the detection of CMEs, a correlation between thepeak intensities of E > 10 MeV SPEs observed at Earth andthe associated maximum CME speed was found [7]. Kahler[8] and Gopalswamy et al. [9] have investigated correlationbetween the logs of the peak SPE intensities and the logs ofthe CME speeds. There is a strong indication that gradualSPEs are produced by coronal/interplanetary shocks dri-ven by CMEs [10,11,8]. However, the spread of the peakintensities of SPEs still ranges over several orders of mag-nitude and there are still exceptions. This implies that

R. Wang / Astroparticle Physics 26 (2006) 202–208 203

investigation of SPEs categorized by the different peakfluxes is necessary. This also suggests that other factorsshould be considered together, such as the longitude ofthe events, solar source characteristics, flare energies andmagnetic field structures of CME/ICME.

On the other hand, the association between SPEs nearthe Earth and metric and decameter hectometer (DH) solarradio type II bursts was found. Metric radio type II bursts(typical frequency range of �100 to �20 MHz) are thoughtto be manifestations of either flare blast waves or CME-dri-ven shocks (e.g., [12–15]). While DH radio type II bursts(in the 1–14 MHz range) are favorite the interpretation ofCME-driven shocks [16–18]. Recently, Cliver, et al. [19]studied both metric and DH type II bursts association withSPEs. Does a solar proton event actually be produced byflare blast waves or by CME-driven shocks, or by bothof them? It can still not be determined since the principalmechanism of solar energetic proton production and accel-eration are not well understood.

Though there are many studies individually on flares,CMEs and radio type II bursts associated with SPEs, acomprehensive study of determinant characteristics ofthem is rare. Many observations and studies suggested thatall possible factors related to the solar proton productionand acceleration should be involved. In the present study,we will form a database of X-ray flare, CMEs and radiotype II bursts correlated with 163 SPEs during the intervalof January 1997–June 2005 and statistically study theircharacteristics. In the next section, we describe the dataselection. In Section 3, separate and integrative analysisof flares, CMEs and radio type II bursts associated withSPEs are presented. A brief summary and discussion isgiven in Section 4.

2. Data selection

From GOES proton data, we selected 163 SPEs withpeak fluxes above one protons cm�2 s�1 sr�1 for >10 MeVsolar protons and divided them by the magnitude of peakfluxes into three groups. In the sample of our SPEs, there

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are 37 large SPEs with fluxes of more than 100 protonscm�2 s�1 sr�1, 34 moderate SPEs with flux ranges of 10–100 protons cm�2 s�1 sr�1 and 92 minor SPEs with fluxranges of 1–10 protons cm�2 s�1 sr�1. For each selectedSPE we identified its associated X-ray flare, CME andradio type II burst. We collected the observed propertiesof the related CME using the observations of Large Angleand Spectrometric Coronagraph on board of Solar andHeliospheric Observatory (LASCO) [20]. The source infor-mation was obtained from the on-line solar geophysicaldata (SGD) as well as the data from other inner coronalimages such as the Extreme-ultraviolet Imaging Telescope(EIT) on board SOHO and Yohkoh soft X-ray telescope(SXT). Referring to the reports in SGD, we finally identifythe associated metric type IIs with X-ray flares from the listof metric type II bursts. We considered reports from all sta-tions with frequency range of �100 to �20 MHz and alldurations except for those events marked as UE (uncertainemission). The DH type II bursts (in the 1–14 MHz range)were observed by the radio and plasma wave experiment(WAVES) [21] on the Wind spacecraft.

3. Data analysis

3.1. Distribution of related flare position and energy

To show clearly the source region location of the flares,we plot the heliocentric coordinates of the solar surfaceregion of the related flares in three panels of Fig. 1. Itcan be seen that the latitude distributions of related solarflares is similar for the three groups of SPEs. All the flareslocate within a latitude strip of ±40�. However, the longi-tudinal distribution is asymmetric with a large fraction of69% (112/163) SPEs having western hemisphere origin.This result is on the whole consistent with early results[2–4]. Moreover, for each group of SPEs there seems tobe a eastern longitudinal ‘‘cutoff’’ within west of whichsolar protons could move along the interplanetary mag-netic filed lines. They are E80�, E70� and E50�. On theother hand, both the longitudinal ‘‘cutoff’’ and the center

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Fig. 2. Histogram of SPEs with different flare classes. Yellow for minorSPEs, green for moderate SPEs and red for large SPEs. (For interpretationof the references in colour in this figure legend, the reader is referred to theweb version of this article.)

204 R. Wang / Astroparticle Physics 26 (2006) 202–208

of flare activity region seem to offset to the west with theincreasing of solar proton fluxes. The most probable longi-tude regions also shift from nearby central meridian of thesun to western hemisphere. In early study, Wang and Wang[22] have obtained a most probable longitude of betweenW60� and W70� for 13 ground level enhancement (GLE)events during the current solar cycle. It is agreement withthe result that particles accelerated at the Sun at a longi-tude of �W60� will propagate easily to the Earth due tothe solar spiral magnetic field [23]. Of all SPEs, 69%(112/163) related flares locates in western hemisphere while31% (51/163) flares locates in eastern hemisphere. There isa strong tendency for the solar active region responsible toa solar proton event to be located at westward solar longi-tudes. It is note that the magnitude of peak proton fluxdetected from the Earth orbit does not necessarily meanthe difference in physical condition of the solar sourceregions, but may mostly reflect the effects of particlepropagation.

Except for solar flare location, solar flare energy isanother important factor for solar energetic proton pro-duction and acceleration. Since flare eruption in low coro-nal may provide seed particles for CME-driven shocks inhigh coronal and makes a pre-acceleration, the solar pro-ton peak intensity is, in general, associated with flareenergy. Gopalswamy et al. [9] examined 42 SPEs withintensity of >10 MeV protons exceeding 10 pfu andobtained a weak correlation (r = 0.41) between the X-rayflare energies and solar proton intensities. For these SPEsthe peak intensities still cover about four orders of magni-tude. To make an exhaustive study, here we divided theSPEs into three groups, the minor, the moderate and thelarge SPEs. To show the correlation of solar proton peakintensity and flare energy, we plotted a histogram of theSPEs percentage against the flare levels of C, M and X,as shown in Fig. 2. For all SPEs or the sum of the largeand moderate events, there also is a week correlationbetween the X-ray flare energy and SPE intensity, whichis consistent with the result obtained by Gopalswamyet al. [9]. However, It is obvious that there is a good corre-lation for the large SPEs although the poor correlationsstill exist for minor and moderate SPEs. This suggests thatviolent flares seem to produce more seed particles andrelease more energy for particle acceleration. It is alsofound from Fig. 2 that a majority (57%) of moderate andminor SPEs is contributed by the solar flare of class M.

3.2. Distribution of related CME speed and width

To realize particles acceleration, a CME must be fastenough to drive an MHD shock. Gradual SPEs have beenthought to be originate in coronal and interplanetaryshocks driven by fast CMEs (v P 700 kms�1) [10,8]. Inour study we constructed the histograms of the speed distri-bution in Fig. 3 for three groups of CMEs which are SPE-associated. We can see that the speeds averagely rise whensolar proton intensity increasing. The average speeds of the

three groups of CMEs are �830 kms�1, �1110 kms�1 and�1745 km s�1, respectively for minor SPEs, moderate SPEsand large SPEs. For all CMEs the maximal speed arrivesup to 3387 km s�1 and minimal speed is 138 km s�1 witha mean speed of �1228 kms�1. For large (moderate) SPEs,94% (50%) CMEs’ speeds exceed 1000 kms�1 while 48%CMEs’ speeds exceed 800 kms�1 in minor SPEs.

The CME width is another important parameter inunderstanding the association between CMEs and SPEs.Wang and Wang [24] found a average CME angle widthof 317� in their investigation of 13 GLEs during solar cycle23. Fig. 4 contains the distributions of CME angular widthfor three groups of SPEs. We can see that most CMEsassociated with SPEs appearers to have large angularwidths. The average widths of CMEs correlated with thethree groups of SPEs are 211�, 269� and 318�, respectively.More than half of the CMEs in the later two classes ofSPEs is halo (82% in the large SPEs and 63% in the mod-erate SPEs), while number of halo CMEs of minor SPEs isa fraction of 40% (37/92).

On the other hand, the relationship between the X-rayflare peak time and CME time(corresponding to SOHO/LASCO C2) was examined. We plotted their time intervaldistribution shown in Fig. 5, where the time intervaldefined as CME time minus peak flare time. It was foundthat the average time intervals became short with theincreasing of solar proton intensities. It is interesting thatthe average CME speed ratio of about 1:1.3:2.1(830:1110:1745), the average time interval ratio of about1:1.4:3.1 (8.8:13.1:27.4), and the average CME width ratioof about 1:1.3:1.5 (211:269:318) are all comparable for thethree groups of SPEs. This implies not only that fast CMEs

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Fig. 3. Distribution of CME speed. (a) Minor SPEs, (b) moderate SPEs and (c) large SPEs.

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Fig. 4. Distribution of CME angular width. (a) Minor SPEs, (b) moderate SPEs and (c) large SPEs.

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Fig. 5. Distribution of time intervals between CME and X-ray flare. (a) Minor SPEs, (b) moderate SPEs and (c) large SPEs.

R. Wang / Astroparticle Physics 26 (2006) 202–208 205

need shorter travel time than slow CMEs from cradlelandto observation site on average, but also that solar flaresand solar CMEs have a close connection in SPE produc-tion. This result also indicates that our analysis was consis-tent each other.

3.3. Association with radio type II burst

Radio type II emission is currently interpreted as plasmawaves generated by electrons accelerated in the MHDshock front, which get converted into electromagnetic radi-ation at the fundamental and harmonic of the local plasma

frequency. Since radio type II bursts were found to be clo-sely associated with flare eruptions and/or CME shocks,they were regarded as one of the basic signatures of strongsolar activity. We investigate the association of SPEs withmetric and DH type II bursts that covers frequency rangefrom �100 to 1 MHz (corresponding to �1.5R�–�10R�).As a result, we are able to associate 64% (105/163) of themetric type II bursts with X-ray flares, which is lower thanthe association obtained by Dodge ([25]) that 79% Type IIbursts correlated with Ha flares. For a list of DH type IIs,we also obtained a same association rate (65%) using Wind/WAVES list of preliminary type II/IV bursts. Of the 163

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Fig. 6. Percentage association of SPEs with radio type II bursts as afunction of SPEs peak intensity. Association with metric type IIs (solidline), with DH type IIs (dashed line), with both metric and DH type IIs(dotted line) and non-association with both metric and DH type IIs(strokes and dots line).

206 R. Wang / Astroparticle Physics 26 (2006) 202–208

SPEs, the breakdown is as follows: 105 with metric type IIs;106 with DH type IIs; 81 with both metric and DH type II;33 without both metric and DH type II. Fig. 6 shows thepercentage association of SPEs with metric and DH typeII bursts versus solar proton peak intensity. It can be seenthat minor SPEs are more likely to be associated with met-ric type IIs than with DH type IIs and that the associationof SPEs with DH type II bursts increases rapidly with solarproton intensity. Corresponding curve of the association ofSPEs with both metric and DH type II bursts is similar. Onthe contrary, the non-association of SPEs with both metricand DH type II bursts decreases rapidly up to zero withincrease of solar proton intensity. Since small statisticsthe associations may have large fluctuation when flux

Fig. 7. Solar proton flux of large SPEs as a function of solar longitude and X-rawell as CME speed and X-ray flare class (right panel). The labels of 10, 20respectively.

exceeding 103 pfu (total 17 events). However, so low asso-ciation of SPEs having over 103 pfu fluxes with metric typeII bursts is not occasional.

3.4. Distribution of solar proton fluxes for large SPEs

Since major solar proton events have a big impact onterrestrial environments, it is essential to learn as muchas possible about the determinant factors of solar protonfluxes in order to accurately predict these events’ occur-rence and severity. In general, the large SPEs are morelikely to affect the near-Earth space than moderate andminor SPEs. Solar proton fluxes of some large SPEs inour database rise up to above 105 pfu. What is the determi-nant for solar proton intensity? For flares the longitudinalregion and class level should be important, and for CMEsthe most representative feature should be their speed. Afterindividual analysis, we make a synthetical investigation ofCME speed, flare position and energy. To understand thesekey factors’ contribution to the solar proton intensity, weexamine solar proton flux which is a function of CMEspeed, flare longitudinal position and energy.

Fig. 7 shows the two dimensional histograms of solarproton fluxes of large SPEs versus solar longitude and X-ray flare class (left panel), solar longitude and CME speed(middle panel), as well as CME speed and X-ray flare class(right panel), where flux means the flux value of a SPE orthe sum of many SPEs flux and the labels of 10, 20 and30 in flare class axis represents X-ray flare classes of C, Mand X, respectively. From Fig. 7 we can see that the SPEswith high intensity associated with M or X level flareslocated in a longitude west of E30� and with halo CMEstheir speed exceeding�1300 km s�1, and that the SPEs withthree highest fluxes associated with X level flares locatedwithin a latitude strip between E100 and N200 and with haloCMEs their speed exceeding �1600 kms�1. In addition, allthese SPEs correlated with DH type II bursts and most ofthem correlated with metric type II bursts. Thus, we are ableto get a suggestion that intensive SPEs more likely to asso-ciate with class X flare originated in central meridian of the

y flare class (left panel), solar longitude and CME speed (middle panel), asand 30 in flare class axis represents X-ray flare classes of C, M and X,

Table 1Extracted information of flares, CMEs and radio type II bursts correlated with SPEs

No. WH (%) XF (%) MF (%) CF (%) HCME (%) M (%) DH (%) MDH (%) NMDH (%)

Minor 92 60 21 63 16 40 54 42 30 34Moderate 34 79 18 70 12 63 74 88 68 6Large 37 83 61 31 8 82 81 100 81 0Total 163 69 29 57 14 54 64 65 50 20

R. Wang / Astroparticle Physics 26 (2006) 202–208 207

Sun, with halo CMEs with speed above �1600 kms�1 andwith metric and DH type II bursts.

4. Summary and discussion

Before getting into the discussion, we extracted someinformation of flares, CMEs and radio type II bursts corre-lated with these SPEs, listed in Table 1. From left to rightcolumns designate the name of SPEs, number of SPEs(No.), number of flares located at western hemisphere(WH), number of flares with different energy – class X flare(XF), class M flare (MF) and class C flare (CF), number ofhalo CMEs (HCME), number of SPEs associated withmetric type IIs (M), DH type IIs (DH), both metric typeand DH type IIs (MDH), and number of SPEs un-associ-ated with both metric type and DH type IIs (NMDH).Except for number of SPEs, other values were expressedin unit of percentage that equals to the number of this itemovers the number of SPEs corresponding to this item.

Some results need to be emphasized.

1. Associated solar activity was located at a strip of ±40�in latitude and western hemisphere of 69% (112/163)events in longitude. A longitudinal ‘‘cutoff’’ for eachgroup of SPEs was found.

2. With solar proton fluxes increasing, the longitudinal‘‘cutoff’’ moves toward solar west disk, and the mostprobable longitude regions also shift from nearby cen-tral meridian of the Sun to western hemisphere.

3. Solar proton flux is generally high when the solar flareenergy increases. Especially for large SPEs there is agood correlation between solar proton intensity andflare energy.

4. SPEs are accompanied by fast wide CMEs. AverageCME speed is 1745 km s�1 and average CME width is318� for large SPEs; 1110 kms�1 and 269� for moderateSPEs; 830 kms�1 and 211� for minor SPEs.

5. Sixty-four percent (105/163) SPEs have metric type IIemissions, 65% (106/163) SPEs have DH type II emis-sions, versus 50% (81/163) events with both metric andDH type IIs and only 20% (33/163) events un-associatewith both metric and DH type IIs.

6. The association of SPEs with DH type II bursts increasesrapidly with increasing of the solar proton intensities.For large SPEs this association reaches to 100%.

7. The most intensive SPEs with the highest proton fluxesassociated with class X flare located in central meridianof the Sun, fast halo CMEs (v P 1600 kms�1) and met-ric and DH type II bursts.

Our observations indicated that the flares, CMEs, radiotype II emissions, and SPEs are closely related. Since theyall involve magnetic reconnection as the primary way ofexplosive magnetic energy release, these phenomena shouldbe considered as the manifestation of a strong magneticactivity. A acceptable scenario is that a magnetic distur-bance travels downward to cause flare explosion in lowercoronal and upward to result in CME in higher coronalwhen energy stored in twisted magnetic fields is suddenlyreleased.

X-rays is electromagnetic radiations emitted as conse-quence of the acceleration of the thermal/nothermal elec-trons during solar flare. They can reach the Earth inabout eight minutes due to their travel with a speed oflight. Although X-ray emissions observable at 1 AU isapproximately independent of the flare location on the vis-ible disk of the Sun, the observed solar particle flux is astrong function of the flare longitudinal position withrespect to the detection point in space. Particle events asso-ciated with flares occurring near the ‘footpoint’ (�W60�) ofthe interplanetary magnetic field line connecting the Sun tothe Earth generally have a rapid rise time and maximalintensity. Particles from the flares and CMEs far fromthe ‘footpoint’ have a reduced intensity since they have todiffuse across the interplanetary magnetic field line duringtheir propagation from the Sun to the detection point nearthe Earth orbit.

The origin of metric type II burst has long been a arguingsubject. Some authors (e.g., [26]) hold CME-driven viewwhile others (e.g., [27,14]) favor a flare-generated blast wavepicture. One of the early ideas is that solar eruption canresult in two shocks, a blast wave from the flare and a pro-ceeding CME-driven shock due to the CME [28]. Accordingto this idea, the metric type II bursts should be caused by theflare shocks , while DH type II bursts are due to CME-dri-ven shocks also because of their high degree associationwith CMEs. Metric type II bursts originate from very nearthe solar surface, frequencies of �100 MHz roughly corre-sponding to �1.5 R�, while DH tpye II bursts observedby the WAVES/RAD2 coving frequencies of 1–14 MHzoriginate about 3–10 R�. In the former study we found allground level enhancements (GLEs) in current solar cycleassociated with DH type II bursts [24]. So we have a tenta-tive suggestion that most of SPEs are accelerated by bothflares and CMEs. The former provide seed particles forthe later. However, acceleration by CME-driven shocksplays more important role especially in large SPEs.

The primary result of this study is that large SPEs aremuch more likely to be produced by a major flare located

208 R. Wang / Astroparticle Physics 26 (2006) 202–208

near central meridian of the Sun or more western hemi-sphere, accelerated by a shock driven by a fast wideCME, and accompanied by an metric and DH type IIburst. The most intensive ones of these events may pro-duced by class X flares located near central meridian ofthe Sun and accelerated by shock waves driven by very fasthalo CMEs (v P 1600 kms�1). Flares, CMEs and radiotype II emissions are fundamental indication of strongsolar activity accompanied by SPEs, while sudden releaseof energy from twisted magnetic fields should be the sourceof these solar active phenomena.

Acknowledgements

We would like to thank Dr. J.X. Wang for helpful com-ments and suggestions. We are also grateful to the variouscatalogs available on the internet, especially to the GOESdata, Solar Geophysical Data (SGD), LASCO CME cata-logs, and WAVES data products.

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