Coronal mass ejections (CMEs) are an important factor in coronal and interplanetary dynamics.

Transcription

1 1 INTERPLANETARY ASPECTS OF CMES David F. Webb 1. Introduction Coronal mass ejections (CMEs) are an important factor in coronal and interplanetary dynamics. They can inject large amounts of plasma and magnetic elds into the heliosphere causing major geomagnetic storms and interplanetary shocks, a key source of solar energetic particles. Much of the plasma observed in a CME is entrained on expanding magnetic eld lines which can be modeled as a ux rope. During this reporting period, studies have improved our knowledge of the coronal properties of CMEs and their source regions, their characteristics in the solar wind, and their importance to space weather. Halo CMEs are of special interest for space weather because they suggest the launch of a geoeective disturbance toward Earth. Since early 1996 the SOHO LASCO coronagraphs have provided the most recent white light observations of CMEs, with coverage now through the rst half and maximum (2000) of cycle 23. During this period, the LASCO observations of CMEs have been complemented by other SOHO coronal instruments, especially EIT, UVCS and CDS, and the TRACE and Yohkoh spacecraft. Studies have alsoinvolved in-situ observations of CMEs in the interplanetary medium, called ICMEs (Burlaga and Russell, 2001), from the ACE and Wind spacecraft near Earth and Ulysses, NEAR, Nozomi and Voyager 2 outside of Earth's orbit. All these missions were launched before mid A new solar wind mission, Genesis, was launched in August The Yohkoh satellite failed in Dec after 10 years of operation. The scope of this review is limited to ICMEs and space weather. Other topics related to CMEs, such as their near-sun aspects, modeling and shock waves, solar energetic particles, and interplanetary scintillation observations, are covered elsewhere in this volume. 2. Workshops, meetings and reviews During this period many workshops and other meetings involving ICME topics were held and some of the Proceedings published. Some of the most important, with their Proceedings in() were: the IUGG/IAGA General Assembly in Birmingham, UK (Schmieder et al., 2000), Chapman Conf. on Space Weather in Clearwater, FL (Song et al., 2001), Intl. Conf. on Solar Eruptive Events in Washington, DC (Gopalswamy 2001), IAU Symp. No. 203 in Manchester, UK (Brekke et al., 2001), First S-RAMP Conf. in Sapporo (Omura 2002), Intl. Solar Cycle Studies meeting in Longmont, CO, Chapman Conf. on Storm-Substorm Relationships in India, SOHO-11 in Davos (Wilson 2002), Solar Wind X in Pisa, Italy, and the annual SHINE Workshops. Recent review papers discussing ICMEs and space weather include Crooker (2000), St. Cyr et al. (2000), Gosling (2000), Webb (2000a, b, 2002), Russell (2001), Webb et al. (2001) and Plunkett et al. (2002). 3. General solar wind signatures of CMEs The coronal magnetic elds and plasma from CMEs can be detected by remote sensing and in-situ spacecraft observations. A single spacecraft will see certain distinctive signatures, but with a great degree of variation from event toevent (e.g., Crooker 2000 Gosling 2000). These signatures include transient interplanetary shocks, depressed proton temperatures, cosmic ray depressions, ows with enhanced helium abundances, unusual compositions of ions and elements, and magnetic eld structures consistent with ux rope topologies. A widely used single-parameter signature of ejecta is the occurrence of counterstreaming suprathermal electrons, usually interpreted as signatures of closed eld lines and, thus, a good proxy for ICMEs. An important multiple-parameter signature of an ICME,

2 2 is a magnetic cloud, dened as long-duration ows with large-scale rotations of unusually strong magnetic eld accompanied by low ion temperatures. 4. Magnetic structure of ICMEs The magnetic eld data from clouds often provide good ts to ux rope models, e.g., Russell (2001) Lepping and Berdichevsky (2000). The original models assumed that the clouds' elds were force-free with cylindrically symmetric topologies. However, recent models of magnetic clouds relax the force-free condition, and include data from multiple spacecraft suggesting that cloud ropes are more curved and nonsymmetric, wider than they are thick, and more non force-free near their ends (Mulligan et al., 1999 Hidalgo et al., 2000 Mulligan and Russell, 2001). There is evidence of occasional twin ux ropes possibly arising from the same solar event (Lepping et al., 2001 Webb et al., 2000a), or a single ux rope having dual polarities (Osherovich et al., 1999 Vandas and Geranios, 2001). Mulligan et al (2001) (also Crooker, 2000) show that the leading polarity of ICME ux ropes is controlled by the large-scale (dipole) solar eld and their axial orientation is controlled by the orientation of the neutral line at the source surface. These surface neutral lines often parallel laments, large dense, cool structures suspended along the magnetic eld in the corona. These occasionally erupt, becoming the lower, slower part of \3-part" CMEs. Many, if not most CMEs erupt through preexisting streamers (Plunkett et al., 2000), which form the base of the heliospheric current sheet (HCS). Solar laments tend to reect the dominant magnetic helicity in each solar hemisphere and their orientation and helicity can be related to that of associated ICME ux ropes. Sigmoids, best observed in X-rays by Yohkoh, are sinuous surface brightenings associated to some degree with erupting laments and CMEs (Caneld et al., 2000), and with geoeective magnetic clouds (Leamon et al., 2002). Rust (2001) argues that, over their lifetime, laments accumulate helicity and, with CMEs, carry o much of the toroidal magnetic ux and helicity from the Sun built up over the solar cycle. Using an earlier technique with electron heat ux data to determine the\true" polarities of the IMF, Kahler et al. (1999) found that many intervals of counterstreaming electrons lay within magnetic sectors, often carrying the sector boundary. However, these ICMEs were not always near the HCS. Shodan et al. (2000) found a complicating factor, that the degree of counterstreaming within clouds varies from 0% to 100%, i.e., clouds can range from fully closed to partially open to fully open. One new possibility for explaining how CME elds become open is interchange reconnection (Crooker et al., 2002). This mode involves merging between the closed elds in the leg of a CME and adjacent open elds along with disconnection of internal CME elds, this mechanism can explain the lack of ux buildup in the heliosphere due to CMEs. 5. Solar wind composition of ICMEs Another class of ejecta plasma signatures are the abundances and charge state compositions of elements and ions which are systematically dierent in ICMEs compared with other kinds of solar wind (Galvin, 2001). As the corona expands, the electron density decreases so rapidly that the plasma becomes collisionless and the relative ionization states are \frozen in", reecting the conditions of origin in the corona. The charge states of minor ions (Z>2) in ICME ows often imply relatively `hot' coronal source conditions (i.e., >2 MK), especially as enhanced Fe 16+ (Lepri et al., 2001) and O 7;8+ (Henke et al., 2001). However, ICMEs exhibit a large variability in their structure and composition, including the detection of unusually low ionization states of He (e.g., enhanced He + ) and minor ions (Gloeckler et al., 1999 Skoug et al., 1999 Ho et al., 2000 Klecker et al., 2000). This admixture of hot and cool material was most evident inthemay 2-4, 1998 period (Gloeckler et al., 1999 Skoug et al., 1999 Popecki et al., 2000). In many of the events having low ionization states, an erupting lament-halo CME could be associated with either a dense and compact `plug' or an extended ow ofcool

3 plasma in the trailing edge of a magnetic cloud. Six such events had enhancements of the isotope 3 He (Ho et al., 2000). Such material is likely from the lament itself, consistent with near-sun observations showing that erupting laments lag well behind the leading edge of their associated CMEs. In a recent study of Wind magnetic clouds, Lepping and Berdichevsky (2000) nd that half show a signicant increase in density toward the rear of the cloud. Mass fractionation has also been observed in several such events (Wurz et al., 2000). These ICMEs show a strong enhancement ofheavy elements increasing montonically with atomic mass, possibly due to a preferential loss of lighter elements in the preeruptive coronal region. Genesis is a solar wind sample return mission which is at the L1 point collecting and categorizing dierent types of solar wind ows, including ICMEs (Wiens et al., 2002). These samples are to be returned to Earth in late 2004 for high-precision isotopic and elemental analysis. 6. ICME observations beyond 1 AU The Ulysses, Voyager 2 and Nozomi spacecraft have encountered ICMEs beyond 1 AU, helping us to understand the long term evolution of ICMEs. Gosling and Forsyth (2001) summarize the characteristics of ICMEs found at high southern latitudes by Ulysses during its 1st and 2nd polar orbits. During the rst orbit 6 CMEs were observed in the high speed solar wind. All exhibited overexpansion due to their high internal pressure and had no distinct heavy ion compositions. During the recent orbit 20 CMEs were observed, and these were typical of CMEs observed near the ecliptic around solar activity maximum. Ulysses and Voyager 2 observations have been compared for the same ICMEs to better understand how CMEs and their shocks propagate in the outer heliosphere and how merged interaction regions (MIRs) form (Richardson et al., 2002 Wang et al., 2001). At the great distance of Voyager, 58 AU, He abundance enhancements are one of the best indicators of ICME ows. The Japanese Nozomi mission was launched in 1998 and is on its way to Mars. Ihara et al. (2001) discuss observations of the July 12-14, 2000 ares from Nozomi at 1 AU but o the solar east limb as viewed from Earth. These suggest continuing connection to the Sunofthefootofahuge (4 AU) ICME ux rope for at least 2 days. 7. Complex ejecta Based on earlier work, Burlaga et al. (2001) studied \fast ejecta" which were transient, high speed ows containing either a magnetic cloud or complex ejecta. The latter have some ICME signatures but have disorganized magnetic elds, and can last for days. Most were associated with multiple halo CMEs that may have been interacting (Gopalswamy et al., 2001). Such interactions are more likely near maximum and could explain some of the compositional anomalies of ICMEs. 8. ICMEs and space weather Geomagnetic activity tends to track the sunspot activity cycle in amplitude, but with more variability. Geoactivity tends to have two main peaks, near sunspot maximum and during the declining phase of the cycle. The rst peak is considered to be associated with transient solar activity, i.e., CMEs, and the later peak with recurrent high speed streams from coronal holes. Richardson et al. ( ) studied the relative contributions of dierent types of solar wind structures to the aa index from They identied CME-related ows, corotating high-speed streams, and slow ows near the Earth, nding that each type contributed signicantly to average aa at all phases of the cycle. For example, CMEs contribute 50% of aa at solar maximum and 10% outside of maximum, and high speed streams contribute 70% outside of maximum and 30% atmaximum. CMEs, however, are responsible for the most geoeective solar wind disturbances and, therefore, the largest storms. Enhanced solar wind speeds and southward magnetic elds associated with interplanetary shocks and ICMEs are known to be important causes of 3

4 4 storms. Strong southward elds can occur either in magnetic clouds or in the preceding postshock regions, or both. For example, Wu and Lepping (2002) found that the geoeective southward eld in>80% of Wind magnetic clouds occurred in either or both of these regions (see also Webb et al., 2000b). Interacting streams and their shocks can also cause enhanced southward elds and lead to intense magnetic storms (Gonzales et al., 2002). Slower CMEs not driving shocks are probably associated with many smaller storms. Since the launch of SOHO, halo CMEs have been used to study the inuence of Earthdirected CMEs on geoactivity. Analyses of the relation between halo CMEs and geomagnetic storms were carried out by Webb et al. (2000a), Cane et al. (2000) and St. Cyr et al. (2000) and indicate a high degree of correlation near solar minimum and a decreased association near the current maximum. Webb et al. (2000a) compared the onset times of frontside halo events in early 1997 with storms at Earth (peak Dst < {50nT) 2{5 days later, nding that all the frontside halo CMEs with surface sources within 0.5 R s of Sun center were associated with magnetic cloud-like structures at 1 AU and moderate-level storms. St. Cyr et al. (2000) found similar, but weaker associations between halo CMEs and storms. They studied halo CMEs from 1996 to mid 1998, and concluded that 83% (15 of 18) of intense storms, were preceded by frontside halo CMEs. However, 25 of the frontside halo CMEs did not produce such large storms and were, therefore, false alarms. Cane et al. (2000) found that only about half of frontside halo CMEs were associated with ejecta (ICMEs) at Earth and their geoeectiveness depended on the strength of the ejecta southward eld. Ejecta with no preceding halo CMEs were not very geoeective. All these prior studies included partial halo CMEs. Webb (2002) summarized statistical studies of only full (360 ) halo events, those most likely to be directed along the Sun-Earth line. In a study of 89 frontside full halos observed from , he found that 70% of the halos were associated with shocks and/or counterstreaming electrons or other ICME signatures at 1AU. Magnetic clouds were involved with 60% of the halos. The average travel time from the onsets of the halo CMEs to the onsets of the storms at Earth was 3.3 days. From solar activity minimum to maximum the average CME rate and speeds increased and the travel time from the Sun to 1 AU decreased, as expected for more energetic events. Although the degree of association between the full halo CMEs and moderate or greater storms decreased from 1997 through 1999, it increased in 2000, yielding an overall average of 65%. Conrming previous studies, Webb (2002) found that 70% of the most intense storms (Dst <{150nT) of this cycle were associated with one or more halo CMEs. References Brekke, P., et al. (eds.) 2001, IAU Symp. 203, Recent Insights into the Physics of the Sun and Heliosphere (San Francisco: ASP Conf. Ser). Burlaga, L.F. and C.T. Russell 2001, EOS, Transactions Am. Geophys. Union, 82, 433. Burlaga, L.F. et al. 2001, JGR, 106, 20,957. Cane H.V., I.G. Richardson and O.C. St. Cyr 2000, Geophys. Res. Lett., 27, Caneld, R.C., H.S. Hudson, and A.A. Pevtsov 2000, in IEEE Trans. on Plasma Science: Special Issue on Space Plasmas, 28, Crooker, N.U. 2000, JASTP, 62, Crooker, N.U., J.T. Gosling, and S.W. Kahler 2002, JGR, 107, /2001JA Galvin, A.B. 2001, IAU Symp. 203, Recent Insights into the Physics of the Sun and Heliosphere (San Francisco: ASP Conf. Ser), p Gloeckler, G. et al. 1999, Geophys. Res. Lett., 26, 157. Gonzalez, W.D., B.T. Tsurutani, R.P. Lepping, and R. Schwenn 2002, JASTP, 64, 173. Gopalswamy, N. 2001, in Special Section: Solar Eruptive Events, JGR, 106, 25,153. Gopalswamy, N. et al. 2001, Ap. J., 548, L91. Gosling, J.T. and R.J. Forsyth 2001, Space Sci. Rev., 97, 87.

5 Gosling, J.T. 2000, in 26th Intl. Cosmic Ray Conf., CP516, eds. B.L. Dingus et al. (New York: AIP), p. 59. Henke, T. et al. 2001, JGR, 106, 10,597. Hidalgo, M.A., C.C.J. Medina, and A.F. Vinas 2000, Solar Phys., 194, 165. Ho, G.C. et al. 2000, Geophys. Res. Lett., 27, 309. Ihara, A. et al. 2001, in Proc. of ICRC 2001 (Copernicus Gesellschaft: Katlenburg-Lindau, Germany), p Kahler, S.W., N.U. Crooker and J.T. Gosling 1999, JGR, 104, Klecker, B. et al. 2000, in Acceleration and Transport of Energetic Particles Observed in the Heliosphere, eds. R.A. Mewaldt et al. (Melville, NY: AIP), p Leamon, R.J., R.C. Caneld, and A.A. Pevtsov 2002, JGR, 107, in press. Lepping, R.P. and D. Berdichevsky, Recent Res. Devel. Geophys., 3, 77. Lepping, R.P. et al. 2001, Solar Phys., 204, 287 Lepri, S.T. et al. 2001, JGR, 106, 29,231. Mulligan, T. et al. 1999, JGR, 104, 28,217. Mulligan, T. and C.T. Russell 2001, JGR, 106, 10,581. Mulligan, T. et al. 2001, GRL, 28, 891. Osherovich, V.A., J. Fainberg, and R.G. Stone 1999, Geophys. Res. Lett., 26, 401. Omura, Y. (ed.) 2002, in STEP-Results, Applications and Modelling Phase (S-RAMP), JASTP, 64, 113. Plunkett, S.P. et al. 2000, Solar Phys., 194, 371. Plunkett, S.P. et al. 2002, Adv. Space Res., 29 (10), Popecki, M.A. et al. 2000, in Acceleration and Transport of Energetic Particles Observed in the Heliosphere, eds. R.A. Mewaldt et al. (Melville, NY: AIP), p Richardson, J.D. et al. 2002, JGR, 107, /2001JA Richardson, I.G., E.W. Cliver, and H.V. Cane 2000, JGR, 105, 18,203. Richardson, I.G., E.W. Cliver, and H.V. Cane 2001, Geophys. Res. Lett., 28, Russell, C.T. 2001, Space Weather, GM 125 (Washington, DC: AGU), 73. Rust, D.M. 2001, JGR, 106, 25,075. Schmieder, B. et al. (eds.) 2000, in Special Issue of JASTP, 62, Shodan, S. et al. 2000, JGR, 105, 27,261. Skoug, R.M. et al. 1999, Geophys, Res. Lett., 26, 161. Song, P. et al. (eds.) 2001, Space Weather, GM 125 (Washington, DC: AGU). St. Cyr, O.C. et al. 2000, JGR, 105, 18,169. Vandas, M. and A. Geranios 2001, JGR, 106, Wang, C., J.D. Richardson, and L.F. Burlaga Solar Phys., 204, 413. Webb, D.F. 2000a, in Encyclopedia of Astronomy and Astrophysics (Bristol, UK: Inst. of Physics Publ.). Webb, D.F. 2000b, in IEEE Trans. on Plasma Science: Special Issue on Space Plasmas, 28, Webb, D.F., E.W. Cliver, N.U. Crooker, O.C. St. Cyr and B.J. Thompson 2000a, JGR, 105, Webb, D.F. et al. 2000b, JGR, 105, 27,251. Webb, D.F., N.U. Crooker, S.P. Plunkett, and O.C. St. Cyr 2001, Space Weather, GM 125 (Washington, DC: AGU), 123 Webb, D.F. 2002, in Proc. SOHO 11 Symp., From Solar Min to Max: Half a Solar Cycle with SOHO, SP-508, ed. A. Wilson (Noordwijk, Netherlands: ESA), 409. Wiens, R.C. et al. 2002, EOS, Transactions Am. Geophys. Union, 83,

6 6 Wilson, A. (ed.) 2002, Proc. SOHO 11 Symp., From Solar Min to Max: Half a Solar Cycle with SOHO, SP-508 (Noordwijk, Netherlands: ESA). Wu, C-C and R.P. Lepping 2002, JGR, in press. Wurz, P. et al. 2000, JGR, 105, 27,239.