Statistical characteristics of solar energetic proton events from January 1997 to June 2005

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artngse AAcaReceived 6 December 2005; accepted 9 June 2006Available online 7 July 2006II emissions. The source region distribution of solar aresrelated to SPEs produced in previous Solar Cycles wasexamined by many researchers (e.g., [25]). Their resultsbetween 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 thatE-mail address: wangrg@mail.ihep.ac.cnAstroparticle Physics 26 (21. IntroductionSince the rst identication 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 solarares, solar coronal mass ejections (CMEs) and radio typeindicate that the gradual SPEs more likely to associate withsolar ares 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 ares.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 correlationAbstractWe have made a statistical study of 163 solar proton events (SPEs) associated with X-ray ares, coronal mass ejections (CMEs) andradio type II bursts during January 1997June 2005. These SPEs were categorized by the peak uxes of >10 MeV solar protons into threegroups. There are 37 large SPEs with uxes of more than 100 protons cm2 s1 sr1, 34 moderate SPEs with ux ranges of 10100 pro-tons cm2 s1 sr1 and 92 minor SPEs with ux ranges of 110 protons cm2 s1 sr1. To understand the determinant of solar protonevents, we have examined the association of these SPEs with X-ray ares, 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 aresfavorably located at solar western hemisphere and the center of the activity source region tended to shifted to the west with increasingof the solar proton uxes; (2) there seems a longitudinal cuto for each group of SPEs, which also moves toward west with increasing ofthe solar proton ux; (3) each SPE observed at Earth was associated with a fast (average speed 1228 kms1) 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 ares located near central meridian of the Sunand shock waves driven by very fast halo CMEs (vP 1600 kms1). 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 ares; Coronal mass ejections (CMEs); Radio type II burstsStatistical characteristics of solJanuary 1997RuiguaNational Astronomical Observatories, ChineInstitute of High Energy Physics, Chinese0927-6505/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.astropartphys.2006.06.003energetic proton events fromo June 2005Wangcademy of Sciences, Beijing 100012, Chinademy of Sciences, Beijing 100049, Chinawww.elsevier.com/locate/astropart006) 202208investigation of SPEs categorized by the dierent peakuxes is necessary. This also suggests that other factorsshould be considered together, such as the longitude ofthe events, solar source characteristics, are energies andmagnetic eld 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 are blast waves or CME-dri-ven shocks (e.g., [1215]). While DH radio type II bursts(in the 114 MHz range) are favorite the interpretation ofCME-driven shocks [1618]. Recently, Cliver, et al. [19]studied both metric and DH type II bursts association withSPEs. Does a solar proton event actually be produced byare 37 large SPEs with uxes of more than 100 protonscm2 s1 sr1, 34 moderate SPEs with ux ranges of 10100 protons cm2 s1 sr1 and 92 minor SPEs with uxranges of 110 protons cm2 s1 sr1. For each selectedSPE we identied its associated X-ray are, 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 nally identifythe associated metric type IIs with X-ray ares from the list0itudR. Wang / Astroparticle Physics 26 (2006) 202208 203are 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 ares,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 are, CMEs and radiotype II bursts correlated with 163 SPEs during the intervalof January 1997June 2005 and statistically study theircharacteristics. In the next section, we describe the dataselection. In Section 3, separate and integrative analysisof ares, CMEs and radio type II bursts associated withSPEs are presented. A brief summary and discussion isgiven in Section 4.2. Data selectionFrom GOES proton data, we selected 163 SPEs withpeak uxes above one protons cm2 s1 sr1 for >10 MeVsolar protons and divided them by the magnitude of peakuxes into three groups. In the sample of our SPEs, there-80-60-40-20020406080-80 -60 -40 -20 0 20 40 60 80 -80 -60 -40 -200123456aLatitude ( deg. ) -80-60-40-20020406080 bLongFig. 1. Distribution of heliocentric coordinates of solar surface source region onumbers on the grey scale indicate percentage of appearance probability (forof 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 114 MHz range)were observed by the radio and plasma wave experiment(WAVES) [21] on the Wind spacecraft.3. Data analysis3.1. Distribution of related are position and energyTo show clearly the source region location of the ares,we plot the heliocentric coordinates of the solar surfaceregion of the related ares in three panels of Fig. 1. Itcan be seen that the latitude distributions of related solarares is similar for the three groups of SPEs. All the areslocate 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[24]. Moreover, for each group of SPEs there seems tobe a eastern longitudinal cuto within west of whichsolar protons could move along the interplanetary mag-netic led lines. They are E80, E70 and E50. On theother hand, both the longitudinal cuto and the center20 40 60 80 -80 -60 -40 -20 0 20 40 60 800123456789-80-60-40-20020406080024681012ce ( deg. )f related ares. (a) Minor SPEs, (b) moderate SPEs and (c) large SPEs. Theexample, 12 means 12%). Black represents the most probable region.94% (50%) CMEs speeds exceed 1000 kms1 while 48%CMEs speeds exceed 800 kms1 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-rayare peak time and CME time(corresponding to SOHO/LASCO C2) was examined. We plotted their time intervaldistribution shown in Fig. 5, where the time intervaldened as CME time minus peak are 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 ratioPhyof are activity region seem to oset to the west with theincreasing of solar proton uxes. 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 eld [23]. Of all SPEs, 69%(112/163) related ares locates in western hemisphere while31% (51/163) ares 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 uxdetected from the Earth orbit does not necessarily meanthe dierence in physical condition of the solar sourceregions, but may mostly reect the eects of particlepropagation.Except for solar are location, solar are energy isanother important factor for solar energetic proton pro-duction and acceleration. Since are 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 areenergy. 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-rayare 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 are energy, we plotted a histogram of theSPEs percentage against the are 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 are 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 ares 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 are of class M.3.2. Distribution of related CME speed and widthTo 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 (vP 700 kms1) [10,8]. Inour study we constructed the histograms of the speed distri-bution in Fig. 3 for three groups of CMEs which are SPE-204 R. Wang / Astroparticleassociated. We can see that the speeds averagely rise whensolar proton intensity increasing. The average speeds of thethree groups of CMEs are 830 kms1, 1110 kms1 and1745 kms1, respectively for minor SPEs, moderate SPEsand large SPEs. For all CMEs the maximal speed arrivesup to 3387 kms1 and minimal speed is 138 kms1 witha mean speed of 1228 kms1. For large (moderate) SPEs,020406080100CLarge SPEsModerate SPEsMinor SPEs Flare classesPercentage of SPEsM XFig. 2. Histogram of SPEs with dierent are classes. Yellow for minorSPEs, green for moderate SPEs and red for large SPEs. (For interpretationof the references in colour in this gure legend, the reader is referred to theweb version of this article.)sics 26 (2006) 202208of about 1:1.3:1.5 (211:269:318) are all comparable for thethree groups of SPEs. This implies not only that fast CMEsMPhy0510152025Mean 830.6aNo. of events024681012bR. Wang / Astroparticleneed shorter travel time than slow CMEs from cradlelandto observation site on average, but also that solar aresand 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 burstRadio 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 plasma500 1000 1500 2000 2500 3000 500 1000 1500SpeedFig. 3. Distribution of CME speed. (a) Minor S05101520253035400 90 180 270 360 0 90 18Mean 211.0aNo. of events0246810121416182022 MbWidthFig. 4. Distribution of CME angular width. (a) Min05101520253035-300 -200 -100 0 100 200 300Mean 27.39aNo. of events0246810121416-300 -200 -100MbTime interFig. 5. Distribution of time intervals between CME and X-ray ean 1110.012345678910 Mean 1745.csics 26 (2006) 202208 205frequency. Since radio type II bursts were found to be clo-sely associated with are 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 1.5R10R).As a result, we are able to associate 64% (105/163) of themetric type II bursts with X-ray ares, which is lower thanthe association obtained by Dodge ([25]) that 79% Type IIbursts correlated with Ha ares. For a list of DH type IIs,we also obtained a same association rate (65%) usingWind/WAVES list of preliminary type II/IV bursts. Of the 1632000 2500 3000 500 1000 1500 2000 2500 3000 (km/s)PEs, (b) moderate SPEs and (c) large SPEs.0 900 270 360ean 269.1051015202530180 270 360Mean 318.6c (deg.)or SPEs, (b) moderate SPEs and (c) large SPEs.0 100 200 300ean 13.120246810121416-300 -200 -100 0 100 200 300Mean 8.788cvals (min.)are. (a) Minor SPEs, (b) moderate SPEs and (c) large SPEs.exceeding 103 pfu (total 17 events). However, so low asso-ciation of SPEs having over 103 pfu uxes with metric typeII bursts is not occasional.3.4. Distribution of solar proton uxes for large SPEsSince major solar proton events have a big impact onterrestrial environments, it is essential to learn as muchas possible about the determinant factors of solar protonuxes in order to accurately predict these events occur-rence and severity. In general, the large SPEs are morelikely to aect the near-Earth space than moderate andminor SPEs. Solar proton uxes of some large SPEs inour database rise up to above 105 pfu. What is the determi-nant for solar proton intensity? For ares the longitudinal20406080100Metric type II burstsDH type II burstsMetric and DH type II burstsWithout Metric and DH type II burstsPercentage of type II radio bursts206 R. Wang / Astroparticle Physics 26 (2006) 202208SPEs, 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 the01 10 10 2 10 3 10 4 10 5 Peak flux of SPEs (cm-2 sr-1 s-1)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).percentage 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 uctuation when uxFig. 7. Solar proton ux of large SPEs as a function of solar longitude and X-rawell as CME speed and X-ray are class (right panel). The labels of 10, 20respectively.region 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, are position and energy. To understand thesekey factors contribution to the solar proton intensity, weexamine solar proton ux which is a function of CMEspeed, are longitudinal position and energy.Fig. 7 shows the two dimensional histograms of solarproton uxes of large SPEs versus solar longitude and X-ray are class (left panel), solar longitude and CME speed(middle panel), as well as CME speed and X-ray are class(right panel), where ux means the ux value of a SPE orthe sum of many SPEs ux and the labels of 10, 20 and30 in are class axis represents X-ray are classes of C, Mand X, respectively. From Fig. 7 we can see that the SPEswith high intensity associated with M or X level areslocated in a longitude west of E30 and with halo CMEstheir speed exceeding1300 km s1, and that the SPEs withthree highest uxes associated with X level ares locatedwithin a latitude strip between E100 and N200 and with haloCMEs their speed exceeding 1600 kms1. 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 are originated in central meridian of they are class (left panel), solar longitude and CME speed (middle panel), asand 30 in are class axis represents X-ray are classes of C, M and X,witH4685PhySun, with halo CMEs with speed above 1600 kms1 andwith metric and DH type II bursts.4. Summary and discussionBefore getting into the discussion, we extracted someinformation of ares, 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 ares located at western hemisphere(WH), number of ares with dierent energy class X are(XF), class M are (MF) and class C are (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 40in latitude and western hemisphere of 69% (112/163)events in longitude. A longitudinal cuto for eachgroup of SPEs was found.2. With solar proton uxes increasing, the longitudinalcuto 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 ux is generally high when the solar areenergy increases. Especially for large SPEs there is agood correlation between solar proton intensity andare energy.4. SPEs are accompanied by fast wide CMEs. AverageCME speed is 1745 km s1 and average CME width is318 for large SPEs; 1110 kms1 and 269 for moderateSPEs; 830 kms1 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-Table 1Extracted information of ares, CMEs and radio type II bursts correlatedNo. WH (%) XF (%) MF (%) CF (%)Minor 92 60 21 63 16Moderate 34 79 18 70 12Large 37 83 61 31 8Total 163 69 29 57 14R. Wang / Astroparticlesions, 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 uxesassociated with class X are located in central meridianof the Sun, fast halo CMEs (vP 1600 kms1) and met-ric and DH type II bursts.Our observations indicated that the ares, 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 are explosion in lowercoronal and upward to result in CME in higher coronalwhen energy stored in twisted magnetic elds is suddenlyreleased.X-rays is electromagnetic radiations emitted as conse-quence of the acceleration of the thermal/nothermal elec-trons during solar are. 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 are location on the vis-ible disk of the Sun, the observed solar particle ux is astrong function of the are longitudinal position withrespect to the detection point in space. Particle events asso-ciated with ares occurring near the footpoint (W60) ofthe interplanetary magnetic eld line connecting the Sun tothe Earth generally have a rapid rise time and maximalintensity. Particles from the ares and CMEs far fromthe footpoint have a reduced intensity since they have todiuse across the interplanetary magnetic eld 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 are-generated blast wavepicture. One of the early ideas is that solar eruption canresult in two shocks, a blast wave from the are and a pro-ceeding CME-driven shock due to the CME [28]. Accordingto this idea, the metric type II bursts should be caused by theare 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 observedh SPEsCME (%) M (%) DH (%) MDH (%) NMDH (%)0 54 42 30 343 74 88 68 62 81 100 81 04 64 65 50 20sics 26 (2006) 202208 207by the WAVES/RAD2 coving frequencies of 114 MHzoriginate about 310 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 bothares 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 are locatednear 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 ares located near central meridian ofthe Sun and accelerated by shock waves driven by very fasthalo CMEs (vP 1600 kms1). Flares, CMEs and radiotype II emissions are fundamental indication of strongsolar activity accompanied by SPEs, while sudden releaseof energy from twisted magnetic elds should be the sourceof these solar active phenomena.AcknowledgementsWe 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.References[1] S.E. Forbush, Phys. Rev. 70 (1946) 771.[2] K.G. McCracken, J. Geophys. Res. 67 (1962) 447.[3] K.A. Anderson, R.P. Lin, Phys. Rev. Lett. 16 (1966) 1121.[4] H.V. Cane, D.V. Reames, T.T. von Rosenvinge, J. Geophys. Res. 93[7] S.W. Kahler, N.R. Sheeley Jr., R.A. Howard, D.J. Michels, et al., J.Geophys. 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Astrophys. 120 (1983) 136.Statistical characteristics of solar energetic proton events from January 1997 to June 2005IntroductionData selectionData analysisDistribution of related flare position and energyDistribution of related CME speed and widthAssociation with radio type II burstDistribution of solar proton fluxes for large SPEsSummary and discussionAcknowledgementsReferences

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