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

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

    II emissions. The source region distribution of solar aresrelated to SPEs produced in previous Solar Cycles wasexamined by many researchers (e.g., [25]). Their results

    between 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.cn

    Astroparticle Physics 26 (21. Introduction

    Since 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 type

    indicate 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 correlationAbstract

    We 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 1997

    Ruigua

    National Astronomical Observatories, Chine

    Institute 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 2005

    Wang

    cademy of Sciences, Beijing 100012, China

    demy of Sciences, Beijing 100049, China

    www.elsevier.com/locate/astropart

    006) 202208

  • investigation 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 by

    are 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 list

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    R. 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 selection

    From 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

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    LongFig. 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 analysis

    3.1. Distribution of related are position and energy

    To 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 center

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    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