ELSEVIER Chinese Astronomy and Astrophysics 38 (2014) 439–447

CHINESEASTRONOMYAND ASTROPHYSICS

Analysis of the Properties of Super SolarProton Events and Associated Phenomena† �

CHENG Li-bin1 LE Gui-ming2,3� LU Yang-ping4,2 CHEN Min-hao4,2

LI Peng4,2 YIN Zhi-qiang3

1School of Physics and Electronic Information, Shangrao Normal College, Shangrao 3340012National Center for Space Weather, China Meteorological Administration, Beijing 100081

3National Astronomical Observatories, Chinese Academy of Sciences, Beijing 1000124College of Mathematics and Statistics, Nanjing University of Information Science and

Technology, Nanjing 210044

Abstract The solar flares, the speeds of shocks propagated in the solar-terrestrialspace and driven by coronal mass ejections (CMEs), the heliographic longitudesand Carrington longitudes of source regions, and the geomagnetic storms, whichare accompanied by the super solar proton events with a peak flux equal to orexceeding 10 000 pfu, have been studied by using the data of ground-based andspace observations. The results show that the heliographic longitudes of sourceregions of super solar proton events distributed in the range from E30◦ to W75◦.The Carrington longitudes of source regions of super solar proton events dis-tributed in the two longitudinal belts, 130◦∼220◦and 260◦∼320◦, respectively.All super solar proton events were accompanied by major solar flares and fastCMEs. The averaged speeds of shocks propagated from the sun to the Earthwere greater than 1 200 km/s. Eight super solar proton events were followed bymajor geomagnetic storms (Dst≤−100 nT), except that one super solar protonevent was followed by a geomagnetic storm with the geomagnetic activity indexDst=−96 nT, a little smaller than that of major geomagnetic storms.

Key words: sun: flares, sun: coronal mass ejections (CMEs), sun: particleemission, sun: solar-terrestrial relations

† Supported by National Natural Science Foundation (41074132, 41274193), National 973 Project(2012CB957801), and Special Project for Public Welfare Profession of Quality Inspection (200710123)

Received 2013–11–01; revised version 2013–11–12� A translation of Acta Astron. Sin. Vol. 55, No. 3, pp. 203–210, 2014� Legm@cma.gov.cn

0275-1062/01/$-see front matter c© 2014 Elsevier Science B. V. All rights reserved.PII:

0275-1062/14/$-see front matter © 2014 Elsevier B.V. All rights reserved.doi:10.1016/j.chinastron.2014.10.010

440 Cheng Li-bin et al. / Chinese Astronomy and Astrophysics 38 (2014) 439–447

1. INTRODUCTION

When the solar activity caused the intensity of proton flows with the proton energy ≥10MeV on the geostationary orbit to reach or exceed 10 pfu continuously for 15 minutes, sucha phenomenon is called the solar proton event. It is a very complicated phenomenon ofspace weather, associated not only with the solar flare, the particle acceleration of coronalshock driven by the coronal mass ejection (CME), but also with the particle accelerationof interplanetary shock, and the particle propagation in the solar-terrestrial space. Hence,the information contained in the phenomena of solar proton events is very rich. Since thesolar proton events are harmful to astronauts and spacecraft, and ionospheric disturbancesmay appear in the polar regions during the solar proton events, which will affect the short-wave communication in polar regions, so solar proton events are very important targetsfor the early warning of space weather. According to the intensity of solar proton events,the National Space Weather Monitoring and Fore-warning Center of China MeteorologicalAdministration has set a professional standard for the classification of solar proton events[1].In this paper, a solar proton event with the peak flux ≥104 pfu is called the super solar protonevent. What features exist in super polar proton events? How special is the distribution oftheir source regions on the solar surface? What kind of geomagnetic storms are accompaniedby super solar proton events? How long does it take that the associated shocks propagatefrom the corona to the earth? These questions will be investigated in this paper. Section2 shows the data analysis of super solar proton events, and the discussions and conclusionswill be given in Section 3.

2. DATA ANALYSIS

Large and slow-varying solar energetic particle events are always accompanied by solar flaresand CMEs. The super solar proton event just means the extremely large slow-varying solarenergetic particle event. The super solar proton events are often accompanied by very intenseexplosive events of CMEs. The solar energetic particles may be accelerated by the coronaland interplanetary shocks driven by CMEs, and cause the intensity of solar proton events toincrease continuously. When the shocks driven by CMEs reach the earth’s magnetosphere,the intensity of solar proton events becomes maximum, meanwhile the sudden storm com-mencements (SSCs) will appear before the geomagnetic storms to be observed. The Dstindex is often used to describe the intensity of geomagnetic storms internationally. Becausethe time resolution of Dst index is only one hour, which is not enough to reflect the fineevolutionary processes of geomagnetic storms, also not enough to measure the arrival timesof shocks to the earth’s magnetosphere. The studies of Wanliss et al. found that the SYM-Hindex with the time resolution of 1 min can be considered as the Dst index of high temporalresolution[2]. Thus, in the following we take a super solar proton event as an example toanalyze the solar-terrestrial phenomena associated with the super solar proton events. AnM9.4 flare occurred in the solar active region AR6555 (S26E28 on the solar surface) at 22:45UT of 22nd March 1991, and companied with the meter-wave type II burst, which meansthat the coronal shock forms during the explosion of CME. Afterwards, an enhancement ofintensity of the protons of energy ≥10 MeV was observed by the GOES satellite (Geosta-tionary Operational Environmental Satellite). After the CME entered the interplanetary

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space, it became an interplanetary CME (ICME), the interplanetary shocks driven by theICME can still accelerate the particles, to cause a continuous enhancement of the intensityof solar proton event as observed by the GOES satellite. When the shocks driven by theICME reached the earth’s magnetosphere, the SSC was triggered, and the intensity of solarproton event reached its maximum. Fig. 1 gives the solar-terrestrial events accompanied bythis typical super solar proton event, as shown by the variations of solar soft X-ray flux,intensity of solar proton event and geomagnetic SYM-H index with the time. When theinterplanetary shock driven by the ICME reached the earth’s magnetosphere, there was asudden enhancement of the SYM-H index (i.e., the SSC caused by the interplanetary shockwhen it reached the earth’s magnetosphere, the arrival time is marked by the vertical dashedline in Fig. 1), while the intensity of solar proton event reached its maximum. The shockand the followed ICME triggered an extraordinarily large magnetic storm with the mini-mum Dst index of -298 nT, this is a very typical major solar-terrestrial event. Such kind ofsolar-terrestrial events are the space weather events to which much attention should be paidto, , and they are also the very important events concerned with the space weather studyand prediction. It is noticed that during this solar proton event, because of the malfunctionof satellite-borne instruments in the interplanetary space, the solar wind data were failed toobtain, hence, the data of solar wind were missing in this event. Using the sudden decreaseof cosmic ray intensity caused by the ICME when it reached the earth’s magnetosphere, Leand Han analyzed the arrival times when the ICME and the shocks driven by it arrived inthe earth’s magnetosphere[3].

Fig. 1 The super solar proton event that began on 23rd March 1991 and the associated geomagnetic

storms. From top to bottom, they are the solar soft X-ray flux, the logarithm of the flux of E ≥10 MeV

protons, and the SYM-H index, respectively. The vertical dashed line corresponds to the time when the

interplanetary shock reached the earth’s magnetosphere.

According to the list of solar proton events observed by the GOES satellites and pro-vided by the Space Environment Service Center (SESC) of USA (http://www.swpc.noaa.gov/ftpdir/indices/SPE.txt), there have been totally 9 super solar proton events since 1976. We

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have analyzed the solar flares, geomagnetic storms accompanied by these super solar protonevents and the arrival times of coronal shocks from the sun to the earth magnetosphere,hence, we have obtained the information about the 9 super solar proton events, the asso-ciated magnetic storms, and the arrival times of coronal shocks from the sun to the earth,which is listed in Table 1. The first column of Table 1 shows the date of solar flare event,the second column gives the intensity of solar soft X-ray flare, the third column refers to thetime of peak solar soft X-ray flux, the fourth and fifth columns are respectively the positionand ordinal number of the active region where the flare occurred, the sixth column showsthe property of active region (SAR and NSAR mean the super active region and non-superactive region, respectively), the seventh column is the maximum area of active region inunits of one millionth of solar hemisphere area (μh), the eighth column is the Carringtonlongitude of active region, the ninth column is the peak intensity of solar proton event, thetenth column gives the initial time of the meter-wave type II burst excited by the shocksthat driven by the CME associated with the solar proton event (TII), the eleventh columnmeans the time of SSC caused by the interplanetary shocks that driven by the ICME andreached the earth’s magnetosphere, the twelfth column shows the intensity of geomagneticstorm after the ICME and the shocks driven by it arrived in the earth’s magnetosphere, thethirteenth and fourteenth columns give the time and averaged speed that the shocks drivenby the CME propagate from the sun to the earth’s magnetosphere, respectively. We considerthe initial time of meter-wave type II bursts to be the initial time of coronal shocks, thus thedifference between the initial time of meter-wave type II bursts and the time of SSC, whichmarks the arrival of interplanetary shocks in the earth’s magnetosphere, can be consideredas the propagation time of shocks in the solar-terrestrial space. It is assumed that the CMEspropagate along straight lines from the sun to the earth, then we can calculate the averagedpropagation velocity of interplanetary shocks in the solar-terrestrial space. However, it isnoticed that the CMEs do not propagate along straight lines in the interplanetary space,the propagated distance in the interplanetary space should be larger than 1 AU, hence, thereal averaged velocity should be larger than that given in Table 1.

Table 1 The list of super solar proton events

SPEDate X-ray Time Location No. PAR Smax CL (E>10 MeV) TII SSC Dst �t V shock

peak (hh:mm) /μh /(◦) Peak/pfu (dd-hhmm) (dd-hhmm) /nT /h /(km/s)1989-10-19 X13.0 12:55 S27E10 5747 SAR 1160 210 40000 19-1249 20-0916 -268 20.5 2032.51991-03-22 X9.4 22:45 S26E28 6555 SAR 2530 188 43000 22-2228 24-0342 -298 29.2 1425.31994-02-20 M4.0 01:41 N09W02 7671 NSAR 450 188 10000 20-0156 21-0901 -144 31.1 1340.52000-07-14 X5.7 10:24 N22W07 9077 SAR 1010 310 24000 14-1017 15-1437 -301 28.3 1470.62000-11-08 M7.4 23:28 N10W75 9213 NSAR 250 270 14800 08-2251 10-0628 -96 31.6 1317.92001-09-24 X2.6 10:38 S16E23 9632 NSAR 790 272 12900 24-1045 25-2025 -102 33.7 1237.62001-11-04 X1.0 16:20 N06W18 9684 NSAR 550 136 31700 04-1610 06-0156 -292 33.7 1235.22001-11-22 M9.9 23:30 S15W34 9704 NSAR 620 271 18900 22-2231 24-0556 -221 31.4 1326.32003-10-28 X17.0 11:10 S16E08 10486 SAR 2610 284 29500 28-1017 29-0611 -353 19.9 2039.8

Note: PAR means the properties of active regions, SAR and NSAR respectively mean the super andnon-super active regions.

Most of the large and super large geomagnetic storms[4−5], and most of the solar flaresabove the X5 class[6] all occurred in the descent stage of solar cycles. The halo CMEs inthe 23rd solar cycle also appeared mainly in the descent stage of this solar cycle[7−8]. Fig. 2shows the time intervals of the super solar proton events appeared in the different solar cycles,from which it is found that all super solar proton events appeared in the descent stage ofsolar cycles. This means that most extremely strong events of space weather occurred in thedescent stage of solar cycles. It is found from Fig. 2 that the numbers of super solar proton

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events in the 22nd and 23rd solar cycles are 3 and 6, respectively. Because there was no anysuper solar proton event in the 21st solar cycle, only the time intervals of super solar protonevents in the 22nd and 23rd solar cycles are plotted in Fig.2. Since the maximum values ofsmoothed monthly mean sunspot numbers in the 21st, 22nd and 23rd solar cycles are 164.5,158.5, and 120.8, respectively, and the numbers of super solar proton events in the threesolar cycles are respectively 0, 3, and 6, which means that the number of super solar protonevents is not related to the intensity of solar cycle that described by the smoothed monthlymean sunspot numbers.

Fig. 2 Solar cycle distribution of super solar proton events. From top to bottom, it shows the variations

of the smoothed monthly mean sunspot number (SMMSN) and the flux of solar proton event with the

time, respectively. The vertical dashed lines indicate the times when the SMMSNs reached their maxima

in the different solar cycles.

Fig. 3 gives the feature of the Carrington longitude distribution of super solar protonevents. It is found from Fig. 3 that the super solar proton events occurred mainly in twoCarrington longitude belts, i.e., 130◦∼220◦and 260◦∼320◦. These two regions are basicallycoincident with two important Carrington regions of ground level enhancement (GLE)[9].This means that a particular attention should be paid to these two regions, i.e., very strongparticle events were mainly concentrated in these two longitude regions. It not only hasan important significance for predicting the space weather, i.e., the active regions in thesetwo longitude belts should be particularly noticed, but also proposes a new subject for theresearch of solar physics, i.e., why very strong solar explosive events happened mostly inthese two longitude belts?

3. DISCUSSIONS AND CONCLUSIONS

In spite of that the source regions in the eastern part of the solar disk are far away from thesolar-terrestrial magnetic connecting line, but the CMEs are the large-scale structures, theirexplosion may cause the global reconstruction of magnetic field lines[10−11], thus to make

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the energetic particles produced by the flare and CME explosions in the eastern part of thesolar disk still able to be observed by GOES satellites. Reames indicated that the shock atthe nose position in the very front of interplanetary shock is the strongest[12], hence, thedurative intensity enhancement of solar proton events observed by the GOES satellite justmeans that the shock front magnetically connected with the GOES satellite moves graduallyfrom a relatively far place to a nearer position in respect to the very front of shock, or theposition of shock front of accelerated particles does not change very much.

Fig. 3 The Carrington longitude distribution of super solar proton events

It is found from Fig. 4 that in the present 9 super solar proton events, the sourcepositions of the solar proton events with a peak flux exceeding 20 000 pfu are distributed inthe heliographic region of E30◦ <Longitude≤W20◦. An M7.4 class solar flare exploded inthe solar active region AR9213 (N10W75) at 23:28 UT of 8th November 2000, this flare wasaccompanied by a fast CME that caused a super solar proton event with the peak flux of14 800 pfu. It should be indicated that the time when the super solar proton event reachedits peak flux was 16:00 UT of 9th November, while, the time when the shock driven bythe ICME reached the earth’s magnetosphere was 06:28 UT of 10th November, as shown inFig. 5. This means that after 16:00 UT of 9th November, the capacity of particle accelerationof the shock in the position magnetically connected with the GOES satellite was graduallyreduced, which caused the intensity of solar proton event to start decreasing continuously.Because the longitude of source region was W75, hence, the nose of the shock front driven bythe CME started connecting the satellite very well, the flux of accelerated particles enhancedvery fast, and reached its maximum, then the flank of shock front was connected with thesatellite, and the capacity of particle acceleration gradually decreased. When the ICMEreached the earth’s magnetosphere, it was also swept by the flank, hence, the producedgeomagnetic storm was relatively weaker, but still was a stronger one.

It is necessary to point out that among the 9 super solar proton events, only the supersolar proton event on 8th November 2000 has a large difference between the time of peakflux and the time when the interplanetary shock reached the earth’s magnetosphere, thepeak fluxes of the other 8 super solar proton events all occurred at the times when theinterplanetary shocks reached or were closed to the earth’s magnetosphere. The sourcelongitudes of these 8 super solar proton events were distributed in the range of [E30◦W35◦],

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and all the 8 events were associated with strong geomagnetic storms (Dst≤-100 nT). It isfound from Table 1 that the shock speeds driven by the CMEs accompanied by the supersolar proton events were very fast, with the averaged propagation velocities in the solar-terrestrial space exceeding 1 200 km/s, and the propagation times from the sun to the earthnot exceeding 34 hours, i.e., the propagation times from the sun to the earth were shorterthan 1.5 day.

Fig. 4 The heliographic longitude distributions of super solar proton events and of the associatedgeomagnetic storms

The concept of SAR was proposed most early in 1987[13], it indicates such an activeregion which occupies less than 0.5% of total number of active regions of one solar cycle, butin which occurred more than one half of total strong flares in this solar cycle. Afterwards,many researchers made studies on the SARs, but the concept of SAR given by each researcherwas somehow different[14−19]. Chen et al. gave a definition of SAR in their studies, andaccording to this definition, 45 SARs were selected in the 21st and 23rd solar cycles[19]. It isfound from Table 1 that only 4 of the 9 super solar proton events were produced in the SARs,but the other 5 were not, which means that the super solar proton events are not closelyrelated to the SARs. However, over 77% of GLE events were produced in the SARs[9], i.e.,the GLE events are more closely associated with the SARs. Similarly, the solar energeticparticle events of energy higher than 100 MeV are closely related to the SARs[20]. Becausethe main feature of SARs is related to strong flares, hence, it means that the solar energeticparticle events of energy higher than 100 MeV and the GLE events are mainly associatedwith flares, while the solar proton events are mainly associated with shocks.

The maximum areas of active regions corresponding to the 5 events in the 9 super solarproton events were below 1 000 μh, which means that the area of active region is not a keyparameter to determine whether strong explosions occur in an active region , and to causea super solar proton event. The feature of magnetic fields may be the key parameter todetermine the activity level or whether the strong explosions occur in the active region, but

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it is somehow beyond the range of this study.

Fig. 5 The solar proton event began on 8th November 2000 and the associated geomagnetic storms. From

top to bottom, they are the solar soft X-ray flux, the logarithm of the flux of E ≥10 MeV protons, and the

SYM-H index, respectively. The vertical dashed line indicates the time of the peak flux of E ≥10 MeV

protons, while the vertical dotted line indicates the time when interplanetary shocks reached the earth’s

magnetosphere.

We have finally obtained the following conclusions from the above analysis:(1) All the super solar proton events were accompanied by large flares and fast CMEs,

the heliographic longitudes of their source regions were distributed in the range [E30◦W75◦].The averaged speeds of the shocks propagated in the solar-terrestrial space and associatedwith the super solar proton events were greater than 1 200 km/s. The longitudes of sourceregions of the super solar proton events with a peak flux greater than 20 000 pfu weredistributed in the range of [E30◦, W20◦], and all these events were accompanied by stronggeomagnetic storms.

(2) All the super solar proton events appeared in the descent stage of solar cycles, andthe Carrington longitudes of their source regions were distributed in two regions on the solarsurface, one is the region of 130◦∼220◦, and another one is the region of 260◦∼320◦.

(3) The number of super solar proton events in one solar cycle is not related to theintensity of the solar cycle that described by the sunspot numbers.

ACKNOWLEDGEMENT The data of super solar proton events are taken from http://umbra.nascom.nasa.gov/SEP/, the data of smoothed monthly mean sunspot numbers aretaken from http://sidc.oma.be/, and the initiation criterions of coronal shocks are selectedas the initial times of the meter-wave type II bursts, the data of which are taken fromftp://ftp.ngdc.noaa.gov/STP/SOLAR DATA/SGD PDFversion/, and the data of sudden

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storm commencements (SSCs) are taken from the geomagnetic storm report written by theInstitute of Geophysics, China Earthquake Administration.

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