ICRC Solar activity: minor ion enrichment, particle acceleration and transport, and extreme events

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1 Invited, Rapporteur, and Highlight papers of ICRC 2001: 103 ICRC 2001 c Copernicus Gesellschaft 2002 Solar activity: minor ion enrichment, particle acceleration and transport, and extreme events J. E. Mazur Space Sciences Department, The Aerospace Corporation, El Segundo, CA Abstract. As we approached the maximum of solar cycle 23 in late 2000, many ground-based and on-orbit resources began to offer unprecedented insights into the energetic photon and particle output of the Sun. The sessions on Solar and Heliospheric phenomena, Sun and Corona ( ) at the 27 th ICRC presented new measurements as well as measurements and theories based on previous notable events. This summary of the sessions highlights three major results, synthesized from a few of the 116 papers in Sh : 1. The processes that enrich minor ions in solar flares (especially 3 He and trans-iron) also occurs at other astrophysical sites, thus differentiating the surrounding medium from a stellar composition. 2. How many seed populations and acceleration mechanisms do we need to explain solar energetic particle events? Probably more than one and the same applies to galactic cosmic rays. 3. Solar energetic particle intensities may have varied by more than a factor of 10 in the last 400 years. 1 Introduction and scope The always changing Sun is the engine that drives the phenomena discussed in the ICRC sessions , Solar and Heliospheric phenomena, Sun and Corona. The papers in these sessions described measurements, both existing and planned, as well as theoretical investigations that ultimately address the Sun s energy output. Even though the topics of each of the SH sessions were diverse, their underlying theme was the solar engine addressed with widely different measurement and theoretical techniques. Central to the Sun s variability is the 11-year solar cycle. Figure 1 shows the timing of the most recent cosmic ray conferences in relation to solar activity as measured with sunspot number. As the solar activity varies, so does the probability of the most intense photon and particle acceleration events that are the topic of many ICRC papers. Correspondence to: J. E. Mazur (joseph.mazur@aero.org) Fig. 1. ICRC meeting dates and sunspot numbers for the last 2 solar cycles. At the last ICRC in Salt Lake City in 1999, Cliver (1999) summarized the same sessions as reviewed here and noted several points about the state of investigations into particle acceleration. Cliver pointed out that the relative contributions of flares and shocks to solar particle events was the current frontier, and this question was still debated at the 27 th ICRC. At this conference, several new measurements showed that the seed populations are important in the question of the origin of solar energetic particles. Finally, Cliver noted the prospect of large solar events in the advance to solar maximum. Indeed, several intense flares did occur since the Salt Lake City meeting in The flare and associated particle event that occurred on 14 July 2000 (known as the Bastille Day event) is an example of a widely publicized event investigated with a number of techniques. Figure 2 shows an ultraviolet image of the Sun during the flare on 14 July At least 11 papers at the 27 th ICRC focused on the Bastille day event. In this paper, each of the following sections summarizes the highlights from sessions SH1.2 to SH1.7. As shown in Table 1, the large number of SH papers prevented a thorough review of every study and their links to one another.

2 104 Table 2. The top three results of the SH1.2 to SH1.7 sessions. Highlight Major references 1. The processes that enrich minor ions in solar flares (e.g. 3 He and trans-fe) may also occur in other astrophysical sites, thus differentiating the surrounding medium from the composition of the central body. 2. How many seed populations and acceleration mechanisms do we need to explain solar energetic particle events? Probably more than one and the same may apply to galactic cosmic rays. 3. Solar energetic particle intensities may have varied by more than a factor of 10 in the last 400 years. Roth (2001); Mazur et al. (2001) Mewaldt et al. (2001a, b); Cane (2001); Leske et al. (2001); Desai et al. (2001) McCracken et al. (2001a, b) Fig. 2. Bastille day flare of 14 July 2000 imaged in ultra-violet (image courtesy the SOHO EIT Consortium; SOHO is a joint ESA- NASA program.) Table 1. Tally of solar and heliosphere session S papers from the last two ICRC meetings. Session Topic # papers # papers 27 th 26 th ICRC ICRC 1.2 Energetic photons and electrons Solar neutrons Solar energetic particles: composition, spectra, charge state Particle acceleration on/near Sun 1.6 Interplanetary transport of particles 1.7 Ground-level enhancements total Instead, this review focuses on three major results, synthesized from several papers and listed in Table 2. These highlights are necessarily subjective, but point out the interdisciplinary nature of phenomena we can study within the heliosphere. Lessons we can learn about the Sun and the interplanetary medium within a few AU of the Sun also apply to more distant astrophysical sites. The details of the investigations also point out the often confusing nature of the solar output (e.g. the role of flares in energetic particle acceleration) and our relatively limited picture of the possible Fig. 3. The high-energy portion of the gamma ray spectrum of the 6 November 1997 flare (Yoshimori et al. 2001a). extremes of the solar cycle. 2 Energetic photons and electrons (SH1.2) Two of the six papers in this session (Yoshimori et al. 2001a, 2001b) presented gamma ray measurements from the 6 November 1997 flare, an event that occurred during the ramp-up of solar cycle 23. The interplanetary manifestations of these events have been discussed before (e.g. Mason et al., 1999). As a measure of the interacting particles at the Sun, we show in Fig. 3 how Yoshimori et al. inferred the presence of electrons above 10 MeV and protons above 300 MeV from the bump in the gamma ray spectrum centered near 80 MeV measured on YOHKOH. Such intense gamma ray events let us probe the extremes of the acceleration process at the Sun and place time and energy constraints on that process. 3 Solar neutrons (SH1.3) Several papers discussed the searches for solar neutrons using ground-based detectors. For example, Tsuchiya et al. (2001) presented measurements from the Mt. Chacaltaya neutron monitor during the onset of the 6 November 1997

3 105 Fig. 4. Significance of the increase of the neutron monitor count rate in coincidence with the 6 November 1997 flare (Tsuchiya et al. 2001). flare (Fig. 4). The figure shows not the actual counting rate but the statistical significance of the change in the rate at the time of the flare. Flückiger et al. (2001) searched the global neutron monitor network for the recent and intense candidate flares near the current solar maximum and found no statistically significant signals. This result was consistent with the survey by Miyasaka et al. (2001) of 28 neutron monitors over the period of October 1998 to April 2001 which did not yield any significant signals at the times of interest. Future missions that measure the particle emission closer to the Sun than 1 AU have the potential to reveal more about the acceleration and transport of solar energetic particles. The flux of neutrons would also be higher than at 1 AU, possibly offering a new means of probing the flare acceleration process and conditions at the flare site. These investigations will require instruments of low power and mass. McKibben et al. (2001a) and Kuznetsov et al. (2001) described different concepts for such space-based neutron sensors, both with a mass of approximately 1.5 kg. 4 Energetic charged particles: composition, spectra, and charge state (SH1.4) The large number of papers on solar energetic particles reflected not only the large number of events available for study, but also the advanced instrumentation on the SOHO and ACE spacecraft. To put the phenomenon into context of particle measurements at 1 AU, Fig. 5 shows the fluence of oxygen measured with several instruments on the ACE spacecraft (Mewaldt et al. 2001a). Even though the Mewaldt et al. (2001a) study was not part of the sessions discussed here, we show it because of the valuable link it provides to other cosmic ray studies. The total fluence spectrum is a sum of many parts, from solar wind at the lowest energies to galactic cosmic rays above 100 MeV/nucleon. The SH1.4 papers focussed on the highly variable phenomena that contribute to the spectrum between 0.01 and 100 MeV/nucleon, and Fig. 5 shows the individual spectra of a few such solar energetic particle events that contributed to the whole. The measurements show that the composition, ionization states, seed populations, and acceleration mechanisms vary from event to event. We know these facts because the spacecraft are close to or Fig. 5. Oxygen fluence spectrum at 1 AU (Mewaldt et al. 2001a). sometimes within the acceleration region. However, an observer outside the heliosphere (neglecting modification of the spectrum during the transport to the outer boundaries) would only see the smoothed picture of the heliospheric particle output and not the multitude of events that went into making it. In fact, the similarity of the spectra of the most abundant species in this energy range is striking. The same conclusion applies to the galactic cosmic rays that originate outside the heliosphere. Our sample at a distance of galactic cosmic rays must be subject to averaging over many sources and acceleration processes. Below we highlight some specific examples of solar energetic particle variability. Leske et al. (2001) discussed measurements of heavy-ion isotopes made with instrumentation on ACE. Figure 6 shows the averages over many (18) events. Leske et al. (2001) pointed out that the isotopic signatures varied significantly ( factor of 5 or less) from event to event. Their many-event sum had uncertainties that are approaching those of previous solar wind measurements, and the values of the two samples were within 20% of each other. The 60 Ni and 58 Ni relative abundance had never been measured before in solar energetic particles or in the solar wind. Our measurements of the energetic particle environment in the inner heliosphere only include the last few solar cycles. How well do these relatively recent measurements characterize the extremes of the space radiation environment? Mc- Cracken et al. (2001a, b) used ice cores in order to extend the record of solar particle events to times long before the space age. They inferred the particle event frequency from the abundance of nitrates in the ice, normalizing the abundance to recent events for which we have space-based measurements of the proton flux. Figure 7 shows the rate of

4 106 Fig. 8. Radio spectrogram of the 8 November 2000 solar radio burst (Cane 2001). Fig. 6. Solar energetic particle isotopic ratios from a sample of 18 events (Leske et al. 2001). Fig. 7. Solar particle event history from the past 400 years inferred from ice cores (McCracken et al. 2001a). solar particle events so inferred with intensities above 10 9 particles-cm 2 above 30 MeV compared to the solar cycle. They identified 151 events from 1561 to 1950, suggesting that our recent space based measurements are at a relative minimum of the particle event fluence. While the absolute calibration of the method has uncertainties, it represents a means of establishing a historical record of the timing of the most intense particle events. These kinds of historical studies give us an intriguing glimpse of the extremes of solar activity. 5 Particle acceleration on or near the Sun (SH1.5) The location of the acceleration sites for solar energetic particles in the most intense events was a major topic of the SH1.5 papers. An off-site workshop on the roles of flares and interplanetary shocks held before the ICRC meeting (organized by E. Cliver and M. B. Kallenrode) set the stage for several of the presentations, including an attempt to organize the solar particle abundances and charge states by the height of the acceleration site in the corona (Cane, 2001). For example, Fig. 8 shows a radio spectrogram of the emission associated with an intense particle event that occurred on 8 November The combination of ground and space-based measurements extended the frequency scale from 100 khz to 50 MHz. In this case, Cane (2001) pointed out how the type-3 radio bursts dominated the emission spectrum, overwhelming the small signal of a type-2 drifting feature due to emission at a propagating shock. Cane (2001) noted that such fast-drift radio bursts preceded all the major proton events in the survey, suggesting that interplanetary acceleration of the highest energy particles may not be as significant as acceleration high in the corona related to reconnection. In one class of particle event, those that are less intense than the ones discussed by Cane et al. (2001), the enrichments of heavy ions and 3 He and association with streaming electrons suggest the acceleration site is within a solar flare. Mazur et al. (2001) presented measurements of large (factor of 1000) enrichments of trans-iron elements in 3 He-rich events observed with ACE. Reames (2000) first showed such large enhancements at high mass in impulsive particle events. Mazur et al. (2001) noted that the magnitude of the transiron enrichments was not a simple extrapolation in mass from those that occur from neon to iron. Roth (2001) suggested that the wave-particle resonance

5 107 Fig. 9. Proton intensities at Ulysses and 1AU during the Ulysses south polar pass (McKibben et al. 2001) mechanisms that enrich the 3 He isotope might also occur in other astrophysical sites such as planetary nebulae. The nebulae are of interest because indirect measurements of the 3 He abundance within them are an order of magnitude higher than other sites such as HII regions or the local interstellar medium. We know from energetic particle measurements at 1 AU that the Sun routinely enriches the local, interplanetary medium with 3 He, so Roth suggest a similar resonant mechanism might explain the planetary nebulae abundances without need to reexamine models of stellar evolution to account of for the differences. 6 Interplanetary transport of solar energetic particles (SH1.6) From measurements of solar energetic particles in the ecliptic plane, we have learned that the intensities are often similar on scales of <1 AU after the peak intensity (e.g. Reames 1999). Reames suggested that after the CME-driven shock passes 1 AU, the particles we observe are isotropic and trapped within an expanding magnetic bottle producing a spectral invariant region. The recent pass of the Ulysses spacecraft over the south pole of the Sun near solar maximum afforded the chance to examine the particle transport in three dimensions as well as over 3 AU (Maclennan et al. 2001; McKibben et al. 2001; Zhang et al. 2001; Marsden et al. 2001; Dalla et al. 2001). Maclennan et al. (2001) discussed 1 MeV electron and ion measurements at Ulysses and ACE during the Bastille day event (14 July 2000). In July 2000, Ulysses was located at 62 south heliolatitude. They found similar energetic particle intensities at 1 and 3 AU lasting 10 days. McKibben et al. (2001) expanded the Ulysses and 1 AU comparisons to cover the entire polar pass and found remarkable, rapid particle access to both locations regardless of the flare location and spacecraft separation. They also found the intensities of 30 MeV protons were within a factor of two in after the peak intensities and revisited the idea of an inner heliosphere particle reservoir (Fig. 9). There are continuing studies that aim to relate the measured transport characteristics of ions and electrons to the Fig. 10. Model of the amplitude of interplanetary field line turbulence versus distance (Ruffolo and Matthaeus 2001). state of the interplanetary magnetic field. As an example of recent calculations, Fig. 10 plots a simulation of the amplitude of field line turbulence in 2 dimensions, then convected along field lines to give a 3-dimensional picture of the turbulence (Ruffolo and Matthaeus 2001). In this realization, some regions had more field line mixing while others remained as distinct bundles of flux tubes. Also, this calculation predicted field line separation due to increased divergence of field lines with increasing distance as opposed to models of field line separation due to random walk of the footpoints in the photosphere (e.g. Giacalone et al. 2000). 7 Ground-level enhancements (SH1.7) Ground-based neutron monitors register signals from the most solar energetic particle events with high fluxes of GeV protons. The measurements at a particular neutron monitor depend on the cutoff rigidity at that location. For example, Vashenyuk et al. (2001) compared the count rate profiles at two neutron monitor stations in two events (Fig. 11). With observations of more events in the ramp up to solar maximum, they suggested the local time of the ground station affects the access through the magnetosphere. Several papers discussed the GLE associated with the Bastille day event. Duldig and Watts (2001) presented a new international GLE database (wwwaadc.aad.gov.au/datasets/gle) that will be useful for quick access to data for the most recent events as well as a repository of historic data. 8 Summary The solar and heliospheric papers at the ICRC took advantage of the increased solar activity as the solar cycle approached its maximum. The opportunities to study the sun and interplanetary phenomena as nearby particle accelerators are of interest not only by themselves, but also because they give us insight into processes that occur in other parts of the

6 and Heliospheric Rapporteur. I also thank the organizers and sponsors of the 27 th ICRC for assembling such a successful meeting, many colleagues for helpful discussions at the meeting, 108 and J. B. Blake for a review of the manuscript. Several papers discussed the GLE associated with the Bastille day event. Duldig and Watts (2001) presented a new international GLE database (www- Fig. 11. Neutron monitor count rates from two stations in two events suggesting an ordering of the particle access by local time (see Fig. Vashenyuk 11. Neutronet monitor al count for details). rates from two stations in two events suggesting an ordering of the particle access by local time (see Vashenyuk et al for details). aadc.aad.gov.au/datasets/gle ) that will be useful for quick access to data for the most recent events as well as a References repository galaxy. of historic Specifically, data. in impulsive solar flares the Sun routinely ejects into the interplanetary medium particles that are Cane, H., Proc. 27 th ICRC (Hamburg), 8, 3231, 2001 References 8. Summary enriched in ultra-heavy ions and 3 He. Thus, the surrounding suprathermal and energetic particle population often has Cliver, Cane, E., AIP H., Conf. Proc. Proc. 27 th ICRC 516, (Hamburg), 103, , 3231, 2001 Dalla et Cliver, al., Proc. E., AIP 27 th Conf. ICRC Proc. (Hamburg), 516, 103, 8, , 2001 The solar and heliospheric papers at the 27 Desai, M. Dalla I. et al., Proc. 27 ICRC took 9, 3612, 2001 a composition much different than the bulk of the Sun, and th ICRC (Hamburg), 8, 3289, 2001 Duldig, Desai, M. L. M. & I. Watts, et al., Proc. D. J., 27Proc. 27 advantage of the increased solar activity as the solar cycle th ICRC (Hamburg), 8, the resonance processes that may account for the enrichments th ICRC (Hamburg), 9, 3612, , Duldig, 2001 M. L. and Watts, D. J., Proc. 27 th ICRC (Hamburg), 8, approached mayits also maximum. occur in sites The such opportunities as planetaryto nebulae. study the Flückiger, 3409, E. O et al., Proc. 27 th ICRC (Hamburg), 8, 3044, sun and interplanetary We also discussed phenomena the relative as roles nearby of flares particle and interplanetary are of shocks interest innot theonly production by themselves, of solar but energetic also parti- Giacalone, Giacalone, J. et al., J. et ApJ al., Letters, ApJ Letters, 532, 532, L75, L75, Flückiger, E. O. et al., Proc. 27 th ICRC (Hamburg), 8, 3044, 2001 accelerators because they cle events give us at the insight conference, into processes as has been that the occur caseinin many Kuznetsov, Kuznetsov, S. al., S. et Proc. al., Proc. 27 th ICRC 27 th ICRC (Hamburg), (Hamburg), 8, 3069, 8, 3069, other parts previous of the conferences. galaxy. Specifically, Tools suchin as impulsive composition, solardown Leske, to Leske, R. et al., Proc. R. et al., Proc. 27 th 27 ICRC th ICRC (Hamburg), 8, 3124, 2001 (Hamburg), 8, 3124, 2001 flares the thesun isotopic routinely level, asejects well asinto the timing the interplanetary of the most energetic Maclennan et al., Proc. Maclennan et al., Proc. 27 th 27 ICRC th ICRC (Hamburg), 8, 3265, 2001 (Hamburg), 8, 3265, 2001 medium particles release that are and enriched the 3-dimensional in ultra-heavy nature ions of the andparticle Marsden et al., Proc. 27 th ICRC (Hamburg), 8, 3310, 2001 Marsden et al., Proc. 27 th ICRC (Hamburg), 8, 3310, 2001 Mason, G. M. et al., Geophys. Res. Lett. 26, 141, He. Thus, access the in surrounding the inner heliosphere, suprathermal all show and the energetic complexity Mason, of G. M. et al., Geophys. Res. Lett. 26, 141, Mazur, J. et al., Proc. 27 particle population the processes often that has give a composition rise to an event. much The different same conclusion applies to galactic cosmic rays, wherein a multitude McCracken, of K. et al., Proc. 27 th ICRC (Hamburg), 8, 3090, 2001 Mazur, J. et al., Proc. 27 th ICRC (Hamburg), 8, 3090, 2001 McCracken, K. et al., Proc. 27 th ICRC (Hamburg), 8, 3205, 2001a than the bulk of the Sun, and the resonance processes that McCracken, K. et al., Proc. th 27 ICRC th ICRC (Hamburg), (Hamburg), 8, 8, 3209, 3205, 2001b sources, seed populations, and acceleration events yields 2001a a may account for the enrichments may also occur in sites McKibben, R. et al., Proc. 27 th ICRC (Hamburg), 8, 3281, 2001a smoothed galactic cosmic ray spectrum that we observe at McCracken, a K. et al., Proc. 27 th ICRC (Hamburg), 8, 3209, such as planetary nebulae. McKibben, R. et al., Proc. 27 th ICRC (Hamburg), 8, 3062, 2001b distance. 2001bMewaldt, R. et al., Proc. 27 We also discussed the relative roles of flares and th ICRC (Hamburg), 10, 3984, 2001a We also gained some insight into the frequency of solar McKibben, Mewaldt, R. et R. al., et al., Proc. Proc th th ICRC (Hamburg), 8, 8, 3132, 3281, 2001b interplanetary shocks in the production of solar energetic particle events over the past 400 years using ice cores. 2001aMiyasaka, H. et al., Proc. 27 th ICRC (Hamburg), 8, 3050, 2001 particle events These kinds at the of conference, studies remind as has us that been we the have case measured in the McKibben, Reames, R. et D. al., V., 1999, Proc. Space 27 th Sci. ICRC Rev., (Hamburg), 90, 413 8, 3062, many previous Sun s output conferences. for a relatively Tools short such time. as composition, The extremes of 2001b its Reames, D. V., ApJ Letters, 540, L111, 2000 down to the isotopic level, as well as the timing of the Mewaldt, R. et al., Proc. 27 th ICRC (Hamburg), 10, 3984, activity are truly unknown. Roth, I., Proc. 27 th Int. Cosmic Ray Conf., 8, 3219, 2001 most energetic particle release and the 3-dimensional nature 2001a Ruffolo, D. and W. H. Matthaeus, Proc. ICRC (Hamburg), 8, 3289, of the particle Acknowledgement. access in the I am inner grateful heliosphere, to Wolfgang all Dröge show and thereinhard Mewaldt, R et al., Proc. 27 th ICRC (Hamburg), 8, 3132, 2001b complexity Schlickeiser of the processes for the invitation that give to serve rise as to the an Solar event. and The Heliospheric Miyasaka, Tsuchiya, H. et al., H. et Proc. al., Proc. 27 th ICRC 27 th ICRC (Hamburg), 8, 3050, 8, 3040, Reames, D. V., 1999, Space Sci. Rev., 90, 413 same conclusion Rapporteur. applies I alsoto thank galactic the organizers cosmic rays, and sponsors wherein ofathe 27 th Vashenyuk, E. V. et al., Proc. 27 th ICRC (Hamburg), 8, 3383, 2001 Reames, D. V., ApJ Letters, 540, L111, 2000 multitude ICRC of sources, for assembling seed such populations, a successful and meeting, acceleration many colleagues Yoshimori. M. al., Proc. 27 th ICRC (Hamburg), 8, 3029, 2001a for helpful discussions at the meeting, and J. B. Blake for a review Roth, I., Yoshimori. Proc. 27 th M. Int. et Cosmic al., Proc. Ray 27 th Conf., ICRC (Hamburg), 8, 3219, ,, 3025, 2001b events yields a smoothed galactic cosmic ray spectrum that of the manuscript. Zhang et al., Proc. 27 th ICRC (Hamburg), 8, 3302, 2001 we observe at a distance. We also gained some insight into the frequency of solar particle events over the past ~400 years using ice cores. These kinds of studies remind us that we have measured the Sun s output for a relatively short time. The extremes of its activity are truly unknown. Acknowledgements. I am grateful to Wolfgang Dröge and Reinhard Schlickeiser for the invitation to serve as the Solar Ruffolo, D. & W. H. Matthaeus, Proc. ICRC (Hamburg), 8, 3289, 2001 Tsuchiya, H. et al., Proc. 27 th ICRC (Hamburg), 8, 3040, 2001 Vashenyuk, E. V. et al., Proc. 27 th ICRC (Hamburg), 8, 3383, 2001 Yoshimori. M. et al., Proc. 27 th ICRC (Hamburg), 8, 3029, 2001a Yoshimori. M. et al., Proc. 27 th ICRC (Hamburg), 8,, 3025, 2001b Zhang et al., Proc. 27 th ICRC (Hamburg), 8, 3302, 2001