# Influence of solar flare's location and heliospheric current sheet on the associated shock's arrival at Earth

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A06107, doi:10.1029/2006JA012205, 2007 Click Here for Full Article Influence of solar flare’s location and heliospheric current sheet on the associated shock’s arrival at Earth Xinhua Zhao,1,2 Xueshang Feng,2 and Chin-Chun Wu3 Received 3 December 2006; revised 9 March 2007; accepted 15 March 2007; published 5 June 2007. [1] We study the source locations of 130 solar flare-type II radio burst events with the associated interplanetary shocks observed by L1 spacecraft (type A events) and 217 flare-type II events without such shocks observed at L1 (type B events) during February 1997–August 2002. In particular, we investigate the relative positions between the flare sources, the heliospheric current sheet (HCS), and the Earth. We found the following results: (1) Solar flares are usually distributed within [S30°, N30°] in heliographic latitude and [S30°, N30°] [E10°, W30°] is the predominant source region on the solar disk that includes the majority of geoeffective solar flares. (2) The shocks with the associated flares located near the HCS would have a lower probably of reaching the Earth. For the Earth-encountered shocks, their initial speeds are distinctly higher when their associated flares are located near the HCS. (3) The angular distance from the flare source to the Earth (defined as Y below) also contributes to the probability of the associated shock being observed at the Earth. The shock arrival probability decreases with the increment of Y and the mean initial shock speed increases with Y for those Earth-encountered shocks. (4) The so-called ‘‘same-opposite side effect’’ of the HCS is confirmed to exist. That is, the shocks whose associated flares are located on the same side of the HCS as the Earth (called as ‘‘same side events’’) have a greater chance of reaching the Earth than those shocks with their associated flares on the opposite side (‘‘opposite side events’’). Here for the first time, a comprehensive sample of solar transient events of both arriving and nonarriving ones (at Earth) is used to testify to the same-opposite side effect. These results would be valuable in understanding the solar-terrestrial relations, and helpful for space weather prediction. Citation: Zhao, X., X. Feng, and C.-C. Wu (2007), Influence of solar flare’s location and heliospheric current sheet on the associated shock’s arrival at Earth, J. Geophys. Res., 112, A06107, doi:10.1029/2006JA012205. 1. Introduction produce corresponding geoeffects [e.g., Cane et al., 2000; Park et al., 2002; Wang et al., 2002; Cid et al., 2004; [2] It is widely accepted nowadays that nonrecurrent Yermolaev et al., 2005; Zhao et al., 2006]. Many factors interplanetary (IP) and geomagnetic disturbances come would affect the probability of a solar transient encountering from solar transient activities such as solar flares, erupting the Earth, such as its source location, strength, angular prominences, type II radio bursts, coronal mass ejections width, propagation direction, speed, trajectory, surrounding (CMEs) and so on. Arrival at the Earth of these transient coronal/interplanetary plasma and magnetic environment, disturbances usually indicates the possible initiation of and also its interaction with other transients and/or back- geomagnetic storms if sufficiently long and sufficiently ground solar wind as being pointed out by Cane et al. large magnitude southward components of the interplane- [2000], Wang et al. [2004, 2006], and Zhao et al. [2006]. tary magnetic field exist following the interplanetary dis- [3] The source location of a solar transient is perhaps one turbances [Dryer, 1994]. On the other hand, statistical of the most important factors. Webb et al. [2000] found that studies have demonstrated that not all the solar transients halo CMEs associated with surface activity within 0.5 Rs of happening at the frontside of the Sun hit the Earth and the Sun center appear to be an excellent indicator of increased geoactivity several days later. Cane et al. [2000] 1 Solar-Interplanetary-Geomagnetic Weather Group, State Key Labora- pointed out that most of the interplanetary ejecta observed at tory for Space Weather, Center for Space Science and Applied Research, Earth are associated with halo CMEs within [E40°, W40°] Chinese Academy of Sciences, Beijing, China. in longitude. Especially, for the transients that produce 2 College of Earth Sciences, Graduate University of the Chinese strong geomagnetic disturbance at Earth, their solar source Academy of Sciences, Beijing, China. distribution has the east-west asymmetry in longitude. The 3 Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Alabama, USA. great magnetic storms are usually caused by the solar transients coming from the Western Hemisphere [e.g., Wei Copyright 2007 by the American Geophysical Union. et al., 1985; Wei and Deng, 1987; Wang et al., 2002; Cane 0148-0227/07/2006JA012205$09.00 A06107 1 of 9

A06107 ZHAO ET AL.: INFLUENCE OF SOLAR FLARE LOCATION AND THE HCS A06107 and Richardson, 2003; Zhang et al., 2003; Zhao et al., 2006]. Zhao et al. [2006] used a large data set to statistically study the characteristics of solar flares associated with IP shock or nonshock events at L1 and found that the most probable location for flares associated with IP shocks at Earth is W20° in longitude. Our results also indicated that the flare-associated shocks originating from the longitudinal range of [E10°, W30°] would have higher probability to reach the Earth. [4] The effect of the heliospheric current sheet (HCS) on the transmission and geoeffects of solar transients is another interesting topic. Wei et al. [1990, 1991] studied the source distribution of 277 flare-shock events on the solar synoptic maps and found that the same side events whose sources are located on the same side of the HCS as Earth are more prone to arrive at Earth than the opposite side events, which is derived from the fact that the number of the same side events is much larger than that of the opposite side events among the total events. The idea of Wei et al. [1990, 1991] has been demonstrated and applied by others [Dryer, 1994; Figure 1. Heliographic latitude as a function of time for Odstrcil et al., 1996; Tokumaru et al., 2000; Zhao and the whole 347 solar flares. Feng, 2005]. [5] In this paper, we follow the previous work of Zhao et al. [2006] and continue to search for the difference presence of an associated interplanetary shock observed at between two kinds of solar disturbances, i.e., those asso- L1 spacecraft about 1 – 5 days later, which are marked as ciated with/without IP counterparts observed at near Earth type A events. The other 217 nonshock events without the space. We lay emphasis on the latitudinal distribution this associated IP shock observed at the near Earth space are time. Especially we take into account the location and shape marked as type B. The whole data set covers an uninter- of the HCS, and calculate quantitatively the angular distance rupted period of about 5 years during the rising and from flare source to the HCS. This latitudinal regulation maximum phase of solar cycle 23, and they can serve as combined with the longitudinal restrictions obtained by an unbiased database for statistically studying the solar/ Zhao et al. [2006] would give a more explicit depiction interplanetary transient disturbance and their geoeffects. of the impact of solar sources on the presence of the The detailed information about these events can be found associated IP disturbances at Earth. We utilize the same in space weather ‘‘Fearless Forecast’’ of Fry et al. [2003] data set as Zhao et al. [2006] and use solar flare as a proxy and McKenna-Lawlor et al. [2006]. for solar transient activity. It is well known that after a long debate on the controversial ‘‘solar flare myth’’ controversy, people begin to know that CMEs and flares are closely 3. Statistical Analysis correlated solar transient phenomena regarded as the solar 3.1. Heliographic Latitude source of the IP disturbances. CMEs and flares are probably [7] The heliographic coordinate system is widely used in two phenomena in one process just as recognized by many describing the location of a point on the solar surface. previous studies [e.g., Dryer, 1996; Harrison, 1996; Zhao Figure 1 gives the heliographic latitude variation versus et al., 2006]. occurrence time for these 347 solar flares including both type A and type B events. It can be seen from this figure 2. Data Set that solar flares do not appear randomly on solar surface but are concentrated in two latitude bands on either side of the [6] We use the data set of 347 solar flare-type II radio equator. At solar activity minimum years, solar flares are burst events during February 1997 – August 2002 collected mainly concentrated near ±30°; as solar activity increases, by Zhao et al. [2006]. The near Sun observations of flares emerge closer and closer to the equator in the course these events include: (1) the start time of the event denoted of a cycle and reach about ±15° at solar maximum years of by the beginning of the metric type II burst; (2) solar 2001 and 2002. The distribution straddles the solar equator, activity location provided by optical Ha flare observations; and looks like two wings of a butterfly. Such a latitudinal (3) initial coronal shock speed estimated by the metric radio distribution characteristic is similar to the well-known type II temporal drift in plasma frequency with the use of a butterfly diagram of sunspots, suggesting that this might coronal density model; (4) piston driving time suggested by be an intrinsic feature of solar active regions. This is also the proxy duration of the GOES spacecraft’s soft X-ray consistent with previous studies [e.g., Maunder, 1904; enhancement due to the flare; (5) flare intensity defined by Byrne et al., 1964; Knoska and Krivsky, 1978; Li et al., the maximum flux of X-rays (of wavelength 0.1 to 0.8 nm) 1998; Mari and Popescu, 2004]. during the flaring process; and (6) background solar wind [8] Figure 2 gives the event number histogram of solar speed. The type II radio burst is interpreted as the signature flares along their heliographic latitude for type A (Figure 2a) of shock wave initiation in the solar corona. Out of the 347 and type B (Figure 2b). The x coordinate in this figure events in our sample, there are 130 shock events with represents the latitude bins of flares, and the y coordinate 2 of 9

A06107 ZHAO ET AL.: INFLUENCE OF SOLAR FLARE LOCATION AND THE HCS A06107 Figure 2. The event number frequency of solar flares plotted versus their heliographic latitude in interval of 10° for (a) type A and (b) type B. The event number on each latitude bin is also shown. represents the event numbers on each latitude bin of 10° activity conditions and topology of the HCS during the from 40° to 40°. The event numbers on each latitude bin time the data were taken. of 10° are also displayed. It can be seen from this figure that solar flares do not occupy the whole solar disk in latitude 3.2. The Relative Locations of the Flare Source, the direction. Most of the flares are distributed within the HCS, and the Earth latitude range of ±30° for both type A and type B events [10] The above discussed heliographic latitude is in fact and rare flares appear in high latitude region over ±30° as the angular distance from flare source to the solar equator. shown by the dotted lines in Figures 2a and 2b. Especially, In order to depict the relative positions between the flare the histogram distribution presents a double-peak charac- source, the heliospheric current sheet (HCS) and the Earth teristic with the maximum located near ±(10° –20°). For the quantitatively, we define new parameters of lS and Y. Here 130 solar flares associated with interplanetary shocks at L1 lS is the angle from the flare source to the HCS, thus of type A, 125 of them are located within the latitude range denotes the latitude relative to the HCS (abbreviated as of ±30°, accounting for 96.2% (125/130); similarly, 208 of LRHCS) of flares; while Y stands for the angular distance the 217 nonshock flares are located within the latitude range from source to the Earth (abbreviated as ADFSE). We use of ±30°, accounting for 95.9% (208/217). Therefore the one event as an example to show how to obtain these two latitude range [30°, 30°] would include the majority of parameters. The type II radio burst of this event started at solar sources of flares associated with/without interplanetary 03:18 UT on 8 August 1998 with an associated M3.0 solar shocks. Considering the conclusion about longitude restric- flare located at N30°E09°; the estimated initial shock speed tion by Zhao et al. [2006], we can see that [S30°, N30°] is 600 km/s, and the corresponding IP shock arrived at the [E10°, W30°] is the predominant source region on solar L1 spacecraft, ACE, at 22:40 UT on 11 August. Figure 3a disk from which nonrecurrent geoeffective solar disturban- gives the spread of the flare source on the contour of the ces most likely originate. This is a closed rectangular region solar source surface magnetic field for CR 1939 located at including the disk center of the Sun. Solar transient dis- 2.5 solar radii from the Wilcox Solar Observatory (http:// turbances originating from this region would have higher wso.stanford.edu/). This Carrington rotation started at probability to reach the Earth and cause relatively intense 09:44 UT on 1 August 1998 and ended at 16:20 UT on geomagnetic activity. Although the region boundary is 28 August, containing the time of this event. The thick red derived from statistical study and is only a coarse estima- line in this figure is the HCS at 2.5 Rs. On the basis of the tion, this region can be used as a helpful guide to roughly start time and flare location, we find the point S for the judge a solar transient’s geoeffectiveness by only basing on source location in Figure 3a. P is the foot of the perpen- its source location. dicular from the point S onto the HCS, meaning that the [9] For type A events in Figure 2a, 71 flares occurred in spherical distance from S to P is the shortest arc. E denotes the Northern Hemisphere but only 59 flares in the Southern the Earth location at the start time of the event, and H is its Hemisphere. The number of northern flares is 20% 7159 foot of the perpendicular on the HCS. The LRHCS of the 59 higher than that of southern flares. While for the type B flare, i.e., lS, is ﬀSOP as shown in Figure 3b. Figure 3b is events in Figure 2b, 107 flares occurred in the Northern the three-dimensional display of Figure 3a. The spherical Hemisphere and 110 flares in the Southern Hemisphere; surface in Figure 3b is the source surface at 2.5 solar radii, their event numbers are nearly equivalent. The reason for and the gridding denotes the heliographic longitude and the preponderance of the northern flares among the total latitude projected onto the source surface. The shape and shock-arriving events may be correlated with the solar location of the HCS, point S, P, E in Figure 3b are the same 3 of 9

A06107 ZHAO ET AL.: INFLUENCE OF SOLAR FLARE LOCATION AND THE HCS A06107 Figure 3. (a) The spread of solar flare source on the source surface magnetic field located at 2.5 solar radii, (b) Its three-dimensional display. The red solid line stands for the HCS at solar source surface; S and E denote the flare source and Earth position projected onto the source surface, P and H are their perpendicular feet, respectively; O is the center of the Sun. as those in Figure 3a. The LRHCS of solar flare lS can be graphic latitude, lS is defined to be positive when the flare obtained from the following equation: lies to the north of the HCS and negative on the other hand. In this figure, Ni (i = 1; 2; . . . ; 5) stands for the number of lS ¼ arccosðcosðqS qP Þ cosðfS fP ÞÞ ð1Þ type A events with their lSs in the bins of [50°, 30°], [30°, 10°],. . ., [30°, 50°], respectively; and ni stands for Here (qS; fS) and (qP; fP) denote the heliospheric longitude the corresponding type B event number. Then the event and latitude of S and P in Figure 3a. Another parameter Y is percentage of type A (Pi) and type B (pi) can be obtained as angle ﬀSOE, that is: follows: Y ¼ arccosðcosðqS qE Þ cosðfS fE ÞÞ ð2Þ Ni Pi ¼ ð3Þ Ni þ ni and (qE; fE) stands for the heliospheric longitude and latitude of E in Figure 3a. The Y, defined in this way, ni presents the angular span from the solar transient’s erupting pi ¼ 1 Pi ¼ ð4Þ normal to the Sun-Earth direction. It will be seen below that Ni þ ni this parameter Y can have an effect on the propagation/ arrival of the associated IP shock. From Figure 3a or 3b, [12] Therefore Pi and pi, derived in this way, reflect the 8 August 1998 flare is located on the opposite side of relative percentage of type A and type B events within fixed the HCS as the Earth. Then this event is called a ‘‘opposite lS interval. The solid line in Figure 4 denotes the percent- side’’ event. Otherwise, the event will be called a ‘‘same age of type A events (Pi) every 20° along lS; and the dashed side’’ event. For the 347 events in our data set, we compute line denotes the corresponding percentage of type B events lS and Y for each event, and divide it into the same side or (pi). The event numbers of Ni and ni in each bin are shown opposite side category. In the following, we try to on the solid/dashed lines together with Vi and vi (in units of investigate the contribution of these new defined parameters km/s), which represents the mean value of initial shock to the associated shock’s arrival probability at the Earth. speed for these Ni type A events (Vi) or ni type B events (vi). These new parameters include lS, Y, and whether the flare Figure 4 indicates that the event percentage is not averagely and the Earth are on the same side or on opposite side of the distributed along solar flare’s lS. The percentage of type A HCS. They are discussed in the following three subsections, events decreases gradually when lS approaches zero, while respectively. that of type B events increases. The percentage of type A is 3.2.1. Latitude Relative to the Heliospheric Current minimum at [10°, 10°] of lS. This means that the shocks Sheet (ls) originating near the HCS have a smaller probability of [11] Figure 4 gives the event number percentage distri- reaching the Earth. The HCS, as we know, is the magnetic bution of type A and type B along lS. Similar to helio- field reversal region embedded within the heliospheric 4 of 9

A06107 ZHAO ET AL.: INFLUENCE OF SOLAR FLARE LOCATION AND THE HCS A06107 Figure 4. The percentage distribution along solar flare’s latitude relative to the HCS (lS) for type A (solid line) and type B (dashed line) events. Ni and ni are the event number of type A and type B within the fixed lS interval; Vi and vi are their mean initial shock speed, respectively. plasma sheet (HPS) with high density and low solar wind 3.2.2. Angular Distance From Flare Source to the speed. Therefore the interaction of solar transient disturban- Earth (Y) ces with the HCS/HPS would greatly change the propaga- [14] The event number percentage distribution along Y, tion process and signatures of these solar disturbances [e.g., i.e., the angular distance from the flare source to the Earth, Wu and Dryer, 1997; Smith et al., 1998; Hu and Jia, 2001]. is shown in Figure 5. In this figure, Ni (i = 1; 2;. . .; 6) is That is to say, the shocks near the HCS would have larger the type A event number with Y in the bins of [0°, 20°], probability of interacting with the HCS and then to weaken [20°, 40°], . . . , [100°, 120°] respectively; and ni is the or even dissipate before reaching the Earth. corresponding type B event number. The event percentage [13] Figure 4 also shows that the mean value of initial of type A (Pi) and type B (pi) are derived according to shock speed of type A events is larger than that of type B equations (3) and (4) in the same way. The event number of events at all the five lS intervals, i.e., Vi > vi for i = 1; 2; . . . ; Ni and ni together with their mean initial shock speed of Vi 5. The initial shock speed, as we know, can be regarded as a and vi are also labeled on the solid and dashed lines as we proxy for the energy of solar transient events and used as have done in Figure 4. Figure 5 indicates that the percentage input for predicting shock arrival time in the prevalent of type A event decreases with the increment of Y, while ‘‘fearless forecast’’ models. Especially, the ISPM model that of type B increases. This reveals the potential contri- supposed that the total energy of a solar transient event is bution of Y to the IP shock’s arrival ability. As a reasonable expected to be proportional to the kinetic energy flux, i.e., approximation, the flare source can be regarded as the E / V3s [Smith and Dryer, 1990, 1995]. This means that the originating place of the associated shock on solar surface; energy of type A shock-associated events is higher than then the line through the Sun center and the flare source (OS that of type B nonshock events in statistical sense. That is in Figure 3b) is assumed to be the shock’s erupting normal, to say, the shocks of more energy would have larger which can be taken as the main propagation direction of the probability to arrive at Earth before decaying into ordinary shock in interplanetary space; also, the shock intensity is the MHD waves during their passage. This is consistent with strongest along the main propagation direction; therefore, the work of Zhao et al. [2006], where the X-ray peak flux large Y event corresponds to the large deviation from the is used as an indicator for the total energy released in the shock’s main propagation direction to the Sun-Earth line; flaring process. Another thing to mention is that for the the flank of a shock is relatively weak, then the satellites at type A events originating near the HCS (lS within [10°, L1 would have a higher probability to miss the shock in this 10°]), the mean value of their initial shock speed (V3 = case. This is why the shocks associated with solar flares 1012 km/s) is larger than the nearby value of V2 and V4. erupting far away from the Sun-Earth direction would have Therefore we can say that only the higher speed shocks less probability to arrive at Earth. It can also be seen from have possibility to arrive at Earth if the shock originates Figure 5 that the percentage of type A events is larger than near the HCS. Stated differently, this result again indicates that of type B events when Y 40°, but the type A the low probability of arrival for the shocks near the HCS, percentage decreases dramatically when Y exceeds 40°. even for relatively high-speed shocks. As for the initial shock speed, the mean value of type A events (Vi) is higher than that of the corresponding type B events (vi) on each Y interval, which is consistent with 5 of 9

A06107 ZHAO ET AL.: INFLUENCE OF SOLAR FLARE LOCATION AND THE HCS A06107 Figure 5. The percentage distribution along the angular distance from flare source to Earth (Y) for type A (solid line) and type B (dashed line) events. Ni, ni, Vi, and vi are defined the same as those in Figure 4. Figure 4; on the other hand, the mean shock speed of type A number is about three times as large as that of the opposite events, i.e., Vi, increases with the increment of Y. This side event. However, all these conclusions are only based on implies that the larger the angular distance is, the more the solar activities for which the related interplanetary energy is needed for the IP shocks to reach Earth. But the counterparts (IP shock or ICME) were observed at Earth. extremely high value of V6 may not have the general The solar events with no such IP manifestations at Earth representative as the event number of N6 is only one. For were not discussed in those works. On the other hand, Hu the type B events, their mean shock speed vi do not change and Jia [2001] presented a one-dimensional analysis and evidently along Y except for the last one, v6. While, the type 2.5-dimensional MHD simulations on the interplanetary B event number in the last interval is also little, i.e., n6 = 4. shock interaction with the HCS and its associated structures; For the same reason, the abnormity of v6 does not have they found that the HCS alone is transparent for any much significance in statistical sense. interplanetary shocks. Therefore a more reliable witness 3.2.3. The Same Side-Opposite Side Effect about the same-opposite side effect should be based on a [15] As the HCS is the most important structure in an comprehensive and unbiased sample of solar transient undisturbed interplanetary medium, its effect on the prop- activities both with and without the associated IP counter- agation of solar disturbances and corresponding geoeffects parts at Earth. has attracted extensive attention both in data analysis and [16] Our data set gives 347 solar flare-type II radio simulation. Much of this kind of work is pursued in terms of burst events covering an uninterrupted time period during the relative positions between the source of solar transients February 1997 –August 2002. For the total events, 212 of and the observer (i.e., the Earth) with respect to the HCS in them are the same side events with flares located on the order to investigate the effect of the HCS on the propagation same side of the HCS as the Earth, and the same side events properties of these transient disturbances. This approach is account for 61% of the total; the other 135 events belong to appropriate because of lack of sufficient observations at the the opposite side ones, which account for 39%. We can see larger (>1 AU) distances in the interplanetary medium. that the same side percentage is larger than the opposite side Henning et al. [1985] pointed out that solar flares located percentage during the total events including both shock- on the same side of the HCS as the Earth would likely associated and nonshock-associated flares. However, the produce larger geomagnetic disturbances than flares located same-opposite side percentages are not equal for type A on the opposite side. Wei et al. [1990, 1991] studied the and B events. For the 130 type A events, 94 of them are the source distribution of 277 flare-shock events under a newly same side events and their same side percentage is 72%, established flare-HCS coordinate system. They found that larger than 61% of the total events. While for the 217 type B the same side event number was larger than the opposite events, only 118 of them are the same side ones; the same side event number and also that the strong shocks and side percentage of type B is 54%, lower than 61%. The corresponding geomagnetic storms were usually associated comparison between our same-opposite side percentages with the same side events. This indicates that the HCS obtained here and those published in previous papers are might have the ‘‘impeding’’ effect on the trans-propagation listed in Table 1. We can see from Table 1 that the same side of shocks in interplanetary space. Recently, the source percentage of type A events (72%) is fairly consistent with distribution of nearly 100 CME-ICME conjugated events previous results [Wei et al., 1990, 1991; Zhao and Feng, have been investigated [Zhao and Feng, 2005; Feng and 2005; Feng and Zhao, 2006] where only solar events with Zhao, 2006], and it is found that the same side event IP manifestations at Earth were discussed. But the previous 6 of 9

A06107 ZHAO ET AL.: INFLUENCE OF SOLAR FLARE LOCATION AND THE HCS A06107 Table 1. Summary and Comparison of the Same Side (S.S.) and Opposite Side (O.S.) Percentages Between the Previously Published Results and This Paper Results (Denoted as *) Event Kind Event Number S.S. Percentage, O.S. Percentage Published Paper a Flare-Disturbances 135 56 44 Henning et al. [1985] Flare-IP Shockb 277 71 29 Wei et al. [1990, 1991] CME-ICMEs 80 70 30 Zhao and Feng [2005] CME-ICMEs 100 74 26 Feng and Zhao [2006] Flare-Type II 347 61 39 * Flare-IP Shockc 130 72 28 * Flare-Nonshockd 217 54 46 * a Here ‘‘disturbance: refers to solar wind disturbance or Dst disturbance associated with a particular flare. b The ‘‘IP shock’’ refers to the interplanetary shocks observed by spacecrafts at near Earth space. c This is the type A events in the paper; and ‘‘IP shock’’ refers to the interplanetary shocks observed by L1 spacecrafts. d This is the type B events in the paper for which no interplanetary shocks are observed by L1 spacecrafts. papers exclude those solar events without IP disturbances. depends on the relative locations between the flare source, Here our analysis is based on solar transient events with and the HCS, and the Earth. Then the angular distance from the without IP disturbances and shows the opposite side per- flare source to the Earth location (Y) is the most related centage of those nonshock-flares (type B) to be higher than parameter to this kind of sorting. Generally speaking, the that of the shock-associated flares (type A) revealing the flare has a high chance of lying on the opposite side of the underlying ‘‘impeding’’ effect of the HCS on the trans- HCS from the Earth for the large Y events. Specific propagation of IP shocks when the flare source is located on calculations show that the averaged Y for these 212 same the opposite side of the HCS as the Earth. side events is 44°, less than the averaged value of 58° for [17] To explore further the effect of the HCS on the those 135 opposite side events. Unfortunately, Y has evident arrival of the shock at the Earth, we compare the shock contribution to the associated shock’s arrival probability as arrival probabilities for the same side (S.S.) and opposite discussed previously. In order to exclude the contribution of side (O.S.) events in a more direct way. The total same side Y, the same-opposite side effect of the HCS is explored event number is 212, and 94 of them have the associated using event with equivalent angular distances of Y. Figure 6 shock being observed at Earth (accounting for 44.3%). gives the percentage distribution of type A (solid line) and While among the total 135 opposite side events, only 36 type B (dashed line) events along Y, where the same side of their shocks arrived at Earth (26.7%). However, we and opposite side events are treated separately. The same should keep in mind that this same-opposite side sorting side distribution is displayed on the right-hand half of the Figure 6. The distribution of type A (solid line) and type B (dashed line) percentage along Y for the same side and opposite side events. This figure is similar to Figure 5, but different from it in dealing with the same-opposite side events separately, as shown in the right and left half panel, respectively. NSi (i = 1; 2; 3) denotes the type A event number of the same side events in Y intervals of [0°, 40°], [40°, 80°], [80°, 120°] and VSi denotes their mean initial shock speed corresponding; nSi and vSi are those for the type B events of the same side events. The definitions of NOi, VOi, nOi, and vOi are in the same way but for the opposite side events. 7 of 9

A06107 ZHAO ET AL.: INFLUENCE OF SOLAR FLARE LOCATION AND THE HCS A06107 figure, while the opposite side distribution is on the left- the same side events still exists among the same-opposite hand half of the figure. The x coordinate is symmetrical side events with the same angular distance of Y. In this about the dotted line in the middle. In the right half of the context, the ‘‘impeding’’ of the HCS turns into the exclusive figure, NSi (i = 1; 2; 3) is the type A event number of the explanation if other factors are neglected. same side events with their Y in intervals of [0°, 40°], [40°, 80°], [80°, 120°], respectively; VSi is the mean initial shock [19] Acknowledgments. We are appreciated to the Wilcox Solar speed of them; nSi and vSi are those for the type B events of Observatory (http://wso.stanford.edu/) for its free data policy. This work is jointly supported by National Natural Science Foundation of China the same side events. In the left half, the definitions of NOi, (40621003, 40536029, 40336053, and 40523006), 973 program under VOi, nOi, and vOi are used for the opposite side events. We grant 2006CB806304 and the CAS International Partnership Program for can see from Figure 6 that the percentage distribution is not Creative Research Teams. symmetrical about the central dotted line. The solid line [20] Zuyin Pu thanks Elizabeth Lucek and another reviewer for their assistance in evaluating this paper. within each Y bin in the right half is higher than that within the relevant Y bin in the left half; while the dashed line References distribution is on the contrary as required. This means that Byrne, F. N., M. A. Ellison, and J. H. Reid (1964), A survey of solar flare for a group of solar flares with similar angular distances phenomena, Space Sci. Rev., 3, 319. (including both same side and opposite side), the IP shocks Cane, H. V., I. G. Richardson, and O. C. 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