Radial and Latitudinal Variations of the Energetic Particle Response to ICMEs

Transcription

1 GM01012_CH29.qxd 11/8/06 12:02 PM Page 309 Radial and Latitudinal Variations of the Energetic Particle Response to ICMEs David Lario The Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland, USA We present energetic particle observations during events in which the interplanetary counterpart of a single coronal mass ejection (ICME) has been observed by spacecraft widely separated in radial distance and latitude. The effects that ICMEs have on energetic particle intensities depend on (1) the ability of ICMEs to drive strong shocks able to accelerate energetic particles, (2) the existence of an intra- ICME energetic particle population, and (3) the capability of the ICMEs to confine (or exclude) energetic particles. These three factors depend upon (1) the energy of the particles, (2) the solar wind medium in which the ICMEs propagate, (3) the mechanisms of energetic particle transport within and around the ICMEs, and (4) the magnetic topology of the ICMEs. As ICMEs expand, ICME-driven shocks weaken and intra-icme magnetic field intensities decrease. These changes, together with the fortuitous occurrence of solar energetic particle events during the transit time of the ICMEs to large distances are the main causes for the different energetic particle signatures observed during the passage of the same ICME at two distant heliospheric locations. 1. INTRODUCTION The interplanetary counterparts of coronal mass ejections (CMEs) are known as interplanetary CMEs (ICMEs). Solar wind and magnetic field signatures used to identify ICMEs are described elsewhere [e.g., Gosling, 2000]. Energetic particle (EP) signatures associated with the passage of ICMEs in the ecliptic plane and at 1 AU from the Sun have been widely studied [e.g., Richardson, 1997, and references therein]. These signatures include cosmic ray (CR) intensity depressions [Cane, 2000, and references therein], bidirectional ~1 MeV ion flows (BIFs) [Marsden et al., 1987], bidirectional CR flows [Richardson et al., 2000]. When ICMEs are preceded by fast shocks, low-energy (<20 MeV) proton intensities may peak around the time of the shock passage and decrease when the spacecraft enters into the ICMEs Solar Eruptions and Energetic Particles Geophysical Monograph Series 165 Copyright 2006 by the American Geophysical Union /165GM29 [Cane et al., 1988]. Occasionally, during the passage of an ICME, it is possible to observe unusual particle flows due to a fresh injection of solar energetic particles (SEPs) from the Sun [Richardson and Cane, 1996]. Although many ICMEs show these EP signatures, not all of these signatures are present in every single ICME. An obvious requirement to observe these EP signatures is the presence of elevated EP fluxes. Cosmic rays are always present and Cane et al. [1997] found that the 75% of ICMEs observed at heliocentric radial distances R 1 AU (identified using solar wind proton temperature depressions) showed decreases of 4% in the >60 MeV cosmic ray flux [Cane et al., 1997, Table 3]. However, there are ICMEs at R = 1 AU not accompanied by low-energy (<25 MeV) proton intensity depressions [Cane et al., 1996, Table 1] and ICMEs at R = 1 AU that do not contain BIFs [Marsden et al., 1987]. Kahler and Reames [1991] suggested that the most typical situation for those ICMEs able to drive strong shocks at 1 AU is a sharp decline in the ~25 MeV proton intensities near the leading edge of the ICME but not a corresponding increase at its trailing edge. Statistical analysis of the passage of ICMEs at 1 AU (identified using different solar wind 309

2 GM01012_CH29.qxd 11/8/06 12:02 PM Page ENERGETIC PARTICLE RESPONSE TO ICMES signatures) show that BIFs are observed in ~80% of cases but are absent in the other ~20% of cases, because of either ion fluxes are too low to determine the flow or because there are no BIFs [Richardson and Reames, 1993]. In addition, BIFs are not always observed throughout the passage of the ICME, but may appear intermittently and not coincide exactly with the passage of the ICME [Richardson and Reames, 1993]. The solar wind disturbances generated by an ICME are determined by the ambient solar wind through which the ICME propagates [Gosling et al., 1995]. As an ICME expands and propagates out from the Sun, it may undergo significant distortion, the shock driven ahead of a fast ICME weakens and the magnetic field magnitude inside the ICME decreases [Riley et al., 2003]. These changes should be reflected in the EP response to the passage of ICMEs. For example, Cane et al. [1994, and references therein] showed that CR decreases at R = 1 AU are due to both the passage of ICMEs and of the enhanced magnetic field turbulence formed in the compressed medium between the shocks and the ICMEs. Cane et al. [1994] suggested that at large heliocentric distances, the effect of ICMEs on CR may become sufficiently reduced (as the ICME expands and the magnetic field magnitude decreases) and that only the turbulence in the compressed medium is important for producing CR decreases. The observation of a single ICME by two spacecraft located at different helioradii allows us to study the radial evolution of the EP signatures associated with the passage of ICMEs. Radial alignments of spacecraft are rare and few cases have been reported in which the same ICME has been observed by two well separated spacecraft. The identification of ICMEs at large heliocentric distances and their association with ICMEs observed earlier, closer to the Sun, is difficult because ICMEs tend to be observed during periods of intense levels of solar activity when multiple transient events occur closely spaced in time and they may interact and merge as they move out through the heliosphere [Burlaga et al., 2002]. In this paper we analyze cases in which the same ICME has been apparently observed by well-separated spacecraft in radial distance and latitude. We compare the effect that the passage of the same ICME at two different heliospheric locations produces on EP intensities. Other significant cases of individual ICMEs observed by spacecraft separated by more than 1.5 AU but not covered in this paper include the events described by Skoug et al. [2000], Paularena et al. [2001] and Lario et al. [2001]. Multi-spacecraft observations of ICMEs at distances R < 1 AU have been addressed elsewhere [e.g., Burlaga et al., 1987; Cane et al., 1994, 1997]. 2. DATA SOURCES Energetic particle, solar wind and magnetic field data used in this paper were provided by the National Space Science Data Center (NSSDC) through the web site nssdc.gsfc.nasa.gov. Energetic particle data from the Helios-1 and Helios-2 spacecraft (henceforth He-1 and He-2, respectively) were measured by the University of Kiel experiments [Kunow et al., 1977], whereas magnetic field and solar wind data were measured by the magnetometer [Neubauer et al., 1977] and plasma experiments [Rosenbauer et al., 1977]. Observations at R = 1 AU were made by the Interplanetary Monitoring Platform (IMP-8), the Geosynchronous Observation Environmental Satellite (GOES-8), and the Advanced Composition Explorer (ACE). We use <13 MeV proton data from the Charged Particle Measurement Experiment (CPME) onboard IMP-8 [Sarris et al., 1976] and <4.8 MeV ion data from the Electron Proton Alpha Particle Monitor (EPAM) onboard ACE [Gold et al., 1998]. We also use >60 MeV particle fluxes inferred from the anti-coincidence guard of the Goddard Medium Energy (GME) experiment [McGuire et al., 1978] onboard IMP-8, and >8 MeV proton intensities from the Energetic Particle Sensor (EPS) onboard GOES-8 [Sauer, 1993]. Magnetic field data at 1 AU are obtained from the MAG experiment onboard ACE [Smith et al., 1998]. Observations at high heliographic latitudes were made by the Ulysses spacecraft. We use <4.8 MeV ion data from the HI-SCALE instrument [Lanzerotti et al., 1992] and >8 MeV proton intensities from the COSPIN instruments [Simpson et al., 1992]. Magnetic field data were collected by the VHM-FGM experiment [Balogh et al., 1992]. Finally, observations from the outer heliosphere were made by the Voyager-2 spacecraft (henceforth V2). Energetic particle data come from the Low Energy Charged Particle (LECP) experiment [Krimigis et al., 1977], solar wind data from the Plasma Science (PLS) experiment [Bridge et al., 1977], and magnetic field data from the magnetometer onboard V2 [Behannon et al., 1977]. 3. IN-ECLIPTIC OBSERVATIONS The observation of a single ICME by different spacecraft located in the ecliptic plane depends upon the longitudinal separation between the spacecraft and the longitudinal extent of the ICME. From single-spacecraft studies of large, flareassociated events, Richardson and Cane [1993] estimated that ICMEs in the ecliptic plane at R = 1 AU extend up to ~100 in longitude. However, multi-spacecraft studies of ICMEs at distances R 1 AU and associated with less energetic events indicate that these ICMEs typically do not extend much more than ~50 in longitude, suggesting that ICMEs with energetic solar events may extend over larger angular distances [Cane et al., 1997]. Therefore, two spacecraft must be relatively close in heliolongitude to see the same ICME. The following examples show two cases in

3 GM01012_CH29.qxd 11/8/06 12:02 PM Page 311 LARIO 311 which the same ICME was observed by two spacecraft in the ecliptic plane and separated by more than 1 AU March 1978 Figure 1 shows energetic particle, solar wind and magnetic field data from He-1 (a), He-2 (b), V2 (c) and IMP-8 (d) from day 58 to day 74 of Figure 1e shows the location of these spacecraft on day 65. Heliospheric coordinates of each spacecraft are given in the caption of the figure, where R is the helioradius and Ψ the heliographic inertial longitude. The vertical solid lines in panels a-d indicate the passage of shocks (as identified by Volkmer and Neubauer [1985]; Borrini et al. [1982]; and Burlaga et al. [1984]). The gray vertical bars identify the passage of ICMEs. A first ICME (henceforth ICME-1) was observed by He-1 on day 61 (details of the ICME identification can be found in Figure 2 of Cane et al. [1997]). Figure 1a shows depressions of the EP intensities in both the >51 MeV/n ion and 4-13 MeV proton channels at the entry of He-1 into ICME-1. Cane et al. [1997] computed an 8% decrease in the >60 MeV/n particle fluxes during the passage of ICME-1 over He-1. Richardson [1994] measured BIFs in the post-shock region but for a period of less than 4 hours. Another period of 8.5 hours with BIFs was observed by He-1 from 62/1800 UT to 63/0230 UT but already outside ICME-1 [Richardson, 1994, Table 1]. Owing to the small longitudinal separation between V2 and He-1, it is proper to ask whether ICME-1 was also observed by V2. Burlaga and Behannon [1982] identified a magnetic cloud that moved past V2 from 0200 UT on day 66 (i.e., 66/0200 UT) to 0200 UT on day 68 (i.e., 68/0200 UT), whereas Wang and Richardson [2004] determined that the start and stop times of this ICME at V-2 were 66/0100 UT and 68/2100 UT, respectively. The gray vertical bar in Figure 1c is based on this latter identification. The distance between the location of V2 on day 67 (R = 2.51 AU, Ψ=19.0 ) and the location of He-1 on day 61 (R = 0.87 AU, Ψ=30.4 ) was R = 1.64 AU in the radial direction and Ψ = 11.4 in the longitudinal direction. Assuming that both spacecraft intercepted the same region of the ICME, we deduce an averaged radial transit speed of 520 km s 1 which seems reasonable considering the measured solar wind speed, the speed of the ICME-driven shock at He-1 (611 km s 1 according to Cane et al. [1997]), and the possible expansion of the ICME as it moves from 0.87 AU to 2.51 AU. Note also the similarity between the directions of the magnetic field rotation (from south to north) observed by both spacecraft. He-2, separated 33 in longitude with respect to He-1, observed a weak shock at 60/0414 UT followed by a period of 11.5 hours of BIFs from 61/0600 UT to 61/1730 UT with plasma signatures typical of ICME [Richardson, 1994, Table 2]. We have indicated this period by the first vertical gray bar in Figure 1b. Assuming that both Helios spacecraft observed the same ICME, He-2 only grazed a flank of ICME-1. IMP-8 separated 55 in longitude with respect to He-1 did not observe this ICME. The EP signatures associated with the passage of ICME-1 over He-1 and He-2 ranged from intensity depressions at He-1 to a gradual decay of 4-13 MeV proton intensities without any significant change at the entry (exit) of He-2 into (from) the ICME. In contrast to He-2, He-1 presumably intercepted ICME-1 near its central part and penetrated well inside the ICME (note the complete rotation of magnetic field in Figure 1a), suggesting that EP depressions are more pronounced at the center of the ICME than at its edges. Cane et al. [1994] also showed that the closer the spacecraft intercepts the center of the ICME the greater the >60 MeV/n proton intensity depression. Another possibility for the lack of particle depression at He-2 is that the SEP event observed by He-2 with onset on day 60 was able to fill ICME-1 with SEPs and hence the continuous decay of proton intensities. However, if He-1 and He-2 intercepted the same ICME and the magnetic topology of this structure was such that particles propagating within the ICME were able to fill the whole ICME, such a filling of SEPs should also be seen by He-1. Only an intensity enhancement was observed by He-1 in the middle of day 61 and inside ICME-1. The entry of V2 into ICME-1 (Figure 1c) showed a <17 MeV proton intensity decrease with respect to those measured on day 65 just after the passage of an interplanetary shock. The 3-17 MeV proton intensities (and also the MeV proton channel not shown here) showed a gradual increase on day 66 already inside the ICME. The occurrence of an intense solar flare at 65/1159 UT at N26E20 as seen from the Earth [Cane et al., 1994] resulted in a large SEP event as observed from He-1, He-2 and IMP-8. It is possible that this intensity increase seen by V2 within ICME-1 resulted from the injection of SEPs from this solar event. The peak intensity observed by V-2 on day 68 within ICME-1 resembles that observed by He-1 on day 61 suggesting that it may be the same intra-icme particle population first observed by He-1 but convected to 2.5 AU within the ICME. Particle intensities increased at the exit of V2 from ICME-1 indicating that the arrival at V2 of SEPs injected from the Sun on day 65 was modulated by the ICME. The >210 MeV proton intensities observed by V2 during the time interval shown in Figure 1 seem to be anti-correlated with the magnetic field magnitude as suggested by Burlaga et al. [1985]. These intensities showed a ~5% decrease at the entry of the spacecraft into the ICME with a small peak at the end of day 67 suggesting the existence of some internal structure within the ICME. The site of the flare at 65/1159 UT was E20 as seen from IMP-8, W02 as seen from He-2, W36 as seen from He-1

4 GM01012_CH29.qxd 11/8/06 12:02 PM Page ENERGETIC PARTICLE RESPONSE TO ICMES Figure 1. (a-c) Energetic particle, solar wind, and magnetic field magnitude data from He-1 (a), He-2 (b), and V2 (c). Magnetic field directions are in the RTN spacecraft centered coordinate system for V2 and in the spacecraft-centered solar ecliptic coordinate system for the Helios spacecraft. (d) Energetic particle intensities measured by IMP-8. (e) Location of the spacecraft on day 65 of He-1 was at R = 0.85 AU and Ψ=34.1 ; He-2 at R = 0.86 AU and Ψ=67.2 ; IMP-8 at R = 0.99 AU and Ψ=89.6 ; and V2 at R = 2.49 AU and Ψ = 18.5 Gray vertical bars and solid vertical lines in panels a-d indicate the passage of ICMEs and shocks, respectively.

5 GM01012_CH29.qxd 11/8/06 12:02 PM Page 313 LARIO 313 Figure 2. The same as Figure 1 but for the events in September Panel e shows the spacecraft locations on day 266 of He-1 was at R = 0.76 AU and Ψ=45.5 ; He-2 at R = 0.74 AU and Ψ=83.0 ; IMP-8 at R = 1.00 AU and Ψ=284.3 ; and V2 at R = 3.93 AU and Ψ = 44.3.

6 GM01012_CH29.qxd 11/8/06 12:02 PM Page ENERGETIC PARTICLE RESPONSE TO ICMES and W51 as seen from V2. IMP-8 and He-2 clearly observed the ICME that originated at the Sun in temporal association with this flare (indicated with the number 2 in Figures 1b and 1d). However, He-1 only observed the shock driven by this ICME but not the ICME [Cane et al., 1994]. The passage of the ICME over He-2 and IMP-8 produced large cosmic ray depressions. Cane et al. [1994] estimated that the size of the >60 MeV/n particle intensity decrease was 33% at He-2 and 21% at IMP-8. An additional decrease was observed in the >60 MeV/n ion intensities at IMP-8 on day 69 (but not in the 4-13 MeV proton intensities). The solar wind structure related to this additional decrease cannot be identified because of limited data available [Cane et al., 1994]. Whereas 4-13 MeV proton intensities decreased when He-2 entered into the ICME, IMP-8 did not observe any significant change in the decaying proton intensities. BIFs were clearly observed throughout the passage of the ICME over He-2 [Richardson, 1994] but only intermittently at IMP-8 [Richardson and Reames, 1993]. Cane et al. [1994] suggested that IMP-8 just grazed an edge of this ICME whereas He-2 penetrated deeply into the ICME, indicating that the effects of this ICME on both low-energy and CR intensities were more noticeable near the center of the ICME than at its edges (cf. Figure 11 of Cane et al. [1994]) September 1978 Figure 2 shows energetic particle, solar wind and magnetic field data from He-1 (a), He-2 (b), V2 (c) and IMP-8 (d) from day 250 to 281 of Figure 2e shows the location of these spacecraft on day 266. The origin of the major SEP event with onset on day 266 was associated with a solar flare at 266/1000 UT at W50 as seen from IMP-8 [Reames et al., 1996]. The shock from this event was seen at 268/0229 UT by He-1, at 268/0133 UT by He-2, at 268/0705 UT by IMP-8, and presumably [Reames et al., 1996] also at 278/1600 UT by V2 although this latter association is uncertain due to both the larger helioradii of V2 and the intense level of activity at that time [Richardson et al., 1990]. None of these spacecraft observed ICME signatures immediately following the shock passage. During the time interval shown in Figure 2, an ICME (indicated by the number 1 in Figure 2a, henceforth ICME-1) was observed by He-1 from 256/0600 UT to 257/1000 UT. Cane et al. [1997] computed a decrease of 10% in the >60 MeV/n particle fluxes during the passage of ICME-1 over He-1. Richardson [1994], however, did not identify any period of BIFs. The passage of ICME-1 was preceded (days ) by a high-speed solar wind stream (observed again by He-1 on days ). The 4-13 MeV proton intensities showed a weak enhancement in association with the passage of this high-speed stream. A new 4-13 MeV proton intensity enhancement was observed prior to the passage of ICME-1 although no clear shock was observed preceding this ICME at He-1 [Volkmer and Neubauer, 1985]. The passage of this ICME was characterized by a gradual decay of the 4-13 MeV proton intensities reaching values close to the instrumental background level (Figure 2a). V2 observed a magnetic cloud from 266/0700 UT to 271/1400 UT [Burlaga and Behannon, 1982]. This ICME was preceded by a structure bounded by a forward and reverse shock on days 263 and 266 respectively, that was identified by Burlaga and Behannon [1982] as a corotating interaction region (CIR). The distance between the location of V2 on day 268 (R = 3.95 AU, Ψ=44.5 ) and the location of He-1 on day 257 (R = 0.84 AU, Ψ=35.9 ) was R = 3.11 AU in the radial direction and only Ψ = 8.6 in the longitudinal direction. Assuming that both spacecraft observed the same ICME, we deduce an averaged radial transit speed of ~485 km s 1 to propagate from He-1 to V2, which is reasonable considering the measured solar wind speed at both spacecraft. Note also the similarity of the magnetic field rotations (from south to north) observed by both spacecraft. The EP signatures associated with the entry of V2 into ICME-1 were an intensity decrease with respect to those measured in the preceding CIR, but followed by a gradual increase on day 267. This intensity increase was also observed in all V2/LECP proton energy channels from 60 kev to 30 MeV with indications suggestive of velocity dispersion. This increase was most likely due to SEPs injected during the major event on day 266 and able to propagate within the ICME-1. The >210 MeV proton intensities at V2 decreased already on day 265 in association with the passage of an enhanced magnetic field structure formed in front of ICME-1 and quickly recovered on day 267 as the magnetic field intensity decreased already inside the ICME. He-2 (37 westward of He-1 on day 257) and IMP-8 (120 eastward of He-1 on day 257) did not observe any plasma signatures that suggest the passage of ICME-1. An additional ICME was observed by IMP-8 from 272/1700 UT to 275/0400 UT identified with the number 2 in Figure 2d [Wang and Richardson, 2004]. The passage of ICME-2 over the Earth produced a large Forbush decrease [Belov et al., 2001]. The 4-13 MeV proton intensities at IMP-8 also decreased (Figure 2d), and intermittent periods of BIFs were observed within ICME-2 [Richardson and Reames, 1993]. Wang and Richardson [2004] associated ICME-2 with another ICME observed by V2 from 283/0100 UT to 286/ 1200 UT (not shown here). This association implies that this ICME was at least 115 wide when it arrived at V2 (IMP-8 was at Ψ=291.2 on day 273, and V2 at Ψ=45.8 on day 284). However, He-1 (separated 123 in longitude with respect to IMP-8 on day 273) did not observe any ICME signature. There is no evidence from data at R 1 AU that

7 GM01012_CH29.qxd 11/8/06 12:02 PM Page 315 LARIO 315 ICME-2 extended at least 115 when it arrived at R = 3.9 AU. The time interval between days 265 and 280 was also a period of intense level of solar activity [Richardson et al., 1990]. Therefore, it is possible that multiple CMEs were ejected in different directions and consequently, V2 and IMP-8 observed different ICMEs. 4. OUT-OF-ECLIPTIC OBSERVATIONS Energetic particle signatures associated with the passage of ICMEs over the Ulysses spacecraft at high heliographic latitudes have been analyzed in a number of different works [see references included in the work of Lario et al., 2004]. Clear differences have been observed between ICMEs propagating within high-speed solar wind streams and ICMEs propagating within slow solar wind streams. Particle signatures associated with ICMEs in slow solar wind streams range from intensity depressions [Malandraki et al., 2003] to EP enhancements observed within the ICME and due to injection of SEPs by unrelated solar events [Armstrong et al., 1994; Malandraki et al., 2001]. By contrast, EP signatures at high latitudes and when Ulysses was immersed in high-speed solar wind flows generally showed low-energy particle intensity enhancements and CR depressions in association with the passage of the ICME even when no fresh injection of SEPs occurred during the transit time of the ICME to high latitudes [Bothmer et al., 1995; Lario et al., 2004]. Here we present three examples of ICMEs observed at both high and low latitudes February 1994 The first case of an ICME observed in the ecliptic at 1 AU and by Ulysses at high latitudes occurred in February 1994 [Gosling et al., 1995]. Figure 3 shows EP data measured by IMP-8 and Ulysses from day 50 to 64 of Ulysses was at R = 3.5 AU, 12 west of Earth, at a heliographic latitude of Λ=54 S and immersed in high-speed (>750 km s 1 ) solar wind. Gray vertical bars and solid vertical lines indicate the passage of ICMEs and shocks, respectively, as identified by Gosling et al. [1995]. Differences in the solar wind disturbances generated by this ICME at low and high latitudes were attributed to the different speeds prevailing in the ambient solar wind ahead of the ICME. The part of the ICME propagating in the ecliptic plane was able to drive a strong shock that locally accelerated protons to very high (>100 MeV) energies [Humble et al., 1995]. The part of the ICME at high latitudes had typical signatures of an over-expanding ICME preceded and followed by weak forward and reverse shocks [Gosling et al., 1995] that were not able to locally accelerate ions above 4 MeV. IMP-8 observations show that EP intensities decreased with respect to those observed at the time of the shock passage. By contrast, Ulysses observations showed low-energy (<4 MeV) ion intensities enhanced throughout the passage of the ICME [Bothmer et al., 1995], whereas MeV proton intensities showed a ~7% decrease [Bothmer et al., 1997]. As discussed by Bothmer et al. [1995] and Lario et al. [2004] the absence of an intense shock-accelerated particle population preceding the ICME is essential to see EP intensity enhancements during the passage of ICMEs. Additional low-energy ion intensity enhancements were observed within the ICME by IMP-8 on day 53 and by Ulysses at the end of day 58, that presumably were due to new injections of SEPs by unrelated solar events [Pick et al., 1995]. A similar example of an ion intensity enhancement observed during the passage of an over-expanding ICME by Ulysses at R = 1.9 AU, Λ=79 N (also immersed in a high-speed solar Figure 3. Energetic particle observations from IMP-8 in the ecliptic (a) and from Ulysses at high heliographic latitudes (b). Gray vertical bars indicate the passage of ICMEs as identified by Gosling et al. [1995]. Vertical solid lines indicate the shock passages (r indicates reverse shock).

8 GM01012_CH29.qxd 11/8/06 12:02 PM Page ENERGETIC PARTICLE RESPONSE TO ICMES wind stream and preceded and followed by weak forward and reverse shocks) occurred on days of 2001 [Lario et al., 2004]. In this case the intensity increase was observed at both <300 kev electron and <60 MeV proton intensities and no new injection of SEPs occurred during its passage. However, the association with an ICME observed at low latitudes was not obvious [Lario et al., 2004] November 2001 An extreme case of ICME observation by two spacecraft widely separated in heliolatitude occurred in November 2001 during the solar maximum northern polar passage of Ulysses [Reisenfeld et al., 2003a, b]. Figure 4 shows EP data from day 308 to day 343 of 2001 collected by Ulysses at high heliographic latitudes (Λ >70 ) and R ranging from 2.20 to 2.42 AU (a), and by ACE and GOES-8 in the ecliptic plane at R = 1 AU (b). We have added in the bottom panel of Figure 4b neutron monitor data from the Climax station (cutoff rigidity 3 GV). During the time interval shown in Figure 4, Ulysses remained immersed in high-speed solar wind flow (>700 km s 1 ) and observed three ICMEs numbered from 1 to 3 in Figure 4a. Ion intensities below 8 MeV increased close to the entry of Ulysses into these ICMEs [Lario et al., 2004]. By contrast, MeV protons, mostly of galactic origin, showed clear depressions. Lario et al. [2004] interpreted the low-energy particle enhancements observed at the entry of Ulysses into these high-heliolatitude ICMEs as due to (1) the lack of an intense shock-accelerated population propagating outside the ICMEs, (2) the effects that local magnetic field structures have on particle transport within and around the ICMEs, and (3) the confinement of low-energy ions by both the ICMEs and associated magnetic field disturbances. Reisenfeld et al. [2003a, b] argued that the first and third ICMEs in Figure 4a were also observed in the ecliptic plane at 1 AU on days and , respectively (numbered 1 and 3 in Figure 4b). These two ICMEs were able to drive strong shocks in the ecliptic plane, with >100 MeV proton intensities peaking at the arrival of the shocks at 1 AU (second panel of Figure 4b). However, the shocks driven by the same ICMEs at high latitudes were only able to locally accelerate ions below ~5 MeV [Lario et al., 2004]. The entry of ACE into these two ICMEs was accompanied by a decrease of the particle intensities with respect to those measured at the time of the shocks. Whereas at 1 AU and in the ecliptic plane, the time-intensity profiles of the SEP events peaked around the arrival of the shocks, the highest intensities at high latitudes were observed during the passage of the ICMEs. Low-energy (<4.8 MeV) ion intensities at ACE (thick trace in the top panel of Figure 4b) showed also small increases within ICMEs 1 and 3 that do not have an obvious association with new injections of SEPs from the Sun and that resemble those observed by Ulysses. These elevated intra-icme particle intensities resulted from either EPs already contained within the ICMEs since their liftoff time at the Sun or EPs that diffused into the ICMEs as they propagated out from the Sun. The observation of EP enhancements at Ulysses in association with the passages of these two ICMEs may be explained by assuming that intra-icme particles remained efficiently confined within the ICMEs and associated field structures, and particle intensities outside the ICMEs decreased faster than the intra-icme population as the ICMEs propagated to Ulysses. Figure 4. Energetic particle intensities measured at (a) high heliographic latitudes by Ulysses and (b) at 1 AU by ACE, GOES-8 and the Climax neutron monitor station. Gray vertical bars indicate the passage of ICMEs as identified by Reisenfeld et al. [2003a, b] and by Cane and Richardson [2003]. Vertical solid lines indicate the shock passages (r indicates reverse shocks).

9 GM01012_CH29.qxd 11/8/06 12:02 PM Page 317 LARIO OUTER HELIOSPHERE As ICMEs propagate toward the outer heliosphere, they can interact with CIRs and other ICMEs to produce merged interaction regions (MIRs) [Burlaga et al., 1986]. The association between MIRs and ICMEs previously observed in the inner heliosphere is not unequivocal because of both the multiple transient events that usually occur in short time intervals and the distortion, interaction and merging undergone by the ICMEs as they propagate to large distances [Burlaga et al., 2002]. The passages of MIRs over spacecraft in the outer heliosphere are usually associated with variations in the EP intensities. The EP response to the passage of MIRs displays significant structure that is markedly different from event to event. The factors that determine the EP response depend on the existence of a shock preceding the MIR able to locally accelerate particles, the magnetic field structures formed around and within the MIR able to confine and modulate the transport of EPs, and the processes of deceleration undergone by the EPs propagating to the outer heliosphere [Decker and Krimigis, 1993, 2003]. Figure 5 shows the consequences of the extreme October- November 2003 solar events in the outer heliosphere as observed by V2 at R = 73.2 AU and Λ =25 S. A large MIR associated with a fast solar wind stream moved past V2 during more than ~40 days [Burlaga et al., 2005]. CR intensities decreased ~10 days (dashed line a in Figure 5) after the passage of a shock (solid vertical line in Figure 5) and reached a minimum ~10 days later (dashed line b in Figure 5). Magnetic field magnitude (shown in the work of Burlaga et al. [2005]) was enhanced after the shock passage, gradually increasing from day 128 (line a) to day 138 (line b). Burlaga et al. [2005] identified the region formed between the shock and the line a as a sheath-like region that was followed by a region of fast solar wind, high magnetic field and CR depression that resembles the sequence found in the two-step Forbush decreases observed at 1 AU [Cane, 2000]. Low-energy ions, however, showed intensity increases during the passage of the MIR. Elevated <80 kev ion intensities were observed between lines a and b in Figure 5, suggesting that these particles were confined in the high magnetic field region. The MeV proton intensities increased for ~20 days before the shock and peaked twice at the time of the sheath-like region and within the high magnetic field region. Similar examples of MIR effects on EP intensities were shown by Decker and Krimigis [1993, 2003]. Although there are no two events alike, the general characteristics are that low-energy (<500 kev) ion intensity enhancements are observed several days behind the shock and presumably confined by magnetic field structures. By contrast, higher Figure 5. Energetic particle and solar wind data measured by V2 during the passage of the MIR associated with the October/November 2003 events. Solid vertical line indicates the passage of a shock and dashed vertical lines the passage of magnetic field discontinuities as identified by Burlaga et al. [2005]. energies (~1-20 MeV) exhibit gradual increases (if any) before the arrival of the shock with localized peaks close to the shock passage. Such time-intensity profiles were observed by V2 during the passage of the MIRs in May 1991, September 1991, January 2001 and October 2001 [Decker and Krimigis, 1993, 2003]. A possible interpretation of the ion intensity behavior at different energies is that low-energy (<500 kev) ions are part of a particle population accelerated earlier in the SEP events that generated the MIR. These lowenergy particles propagate toward the outer heliosphere along the highly twisted magnetic field undergoing adiabatic deceleration, and thus are able to be caught by the traveling MIR and remain confined by the magnetic structures formed in the MIR. Higher-energy particles observed before and during the shock passage may consist of an ambient proton population magnetically reflected by and swept ahead of the MIR.

10 GM01012_CH29.qxd 11/8/06 12:02 PM Page ENERGETIC PARTICLE RESPONSE TO ICMES 6. CONCLUSIONS AND SUMMARY The examples shown in sections 3 and 4 of individual ICMEs observed by spacecraft well separated in radial distance and latitude allow us to conclude that: -Differences in the characteristics of an ICME propagating in different solar wind regimes (e.g., at high and low latitudes for the ICMEs analyzed in section 4) lead to distinct EP signatures. Whereas at low latitudes ICMEs drive strong shocks able to accelerate particles, at high latitudes and in fast solar wind flows shocks are weak and particle intensities are higher inside than outside the ICMEs. -CR depressions at large helioradii are mainly associated with increases in magnetic field magnitude and not with the ICME itself (e.g., Figures 1c and 2c; see also Burlaga et al. [1985]). -Evolution of ICMEs as they propagate further out in the heliosphere lead to changes in the EP signatures. If the internal magnetic field magnitude diminishes, decreases in CR intensities are less pronounced and EP intensities show less structure during the passage of the ICME. -Observations of the same ICME at different heliolongitudes (e.g., ICMEs 1 and 2 in Figure 1) show that decreases in the EP intensities are more pronounced near the center of the ICME than at its edges [see also Cane et al., 1994]. -The fortuitous occurrence of SEP events during the transit time of ICMEs to large helioradii may fill the ICME with SEPs (e.g., ICME-1 in Figure 2 and the ICME in Figure 3). -Our ability to observe EP intensity enhancements depends upon whether there is an intense shock-accelerated population able to mask the intra-icme population and upon whether EPs remain confined within the ICMEs. In the outer heliosphere, the highest low-energy ion intensities are observed in association with the passage of MIRs where EPs remain confined and convected by magnetic field structures. 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