normal-incidence multilayer-coated optics selects spectral emission lines from Fe IX/X (171 A ), Fe XII (195 A ), Fe XV 1.

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1 THE ASTROPHYSICAL JOURNAL, 51:46È465, 1999 January 1 ( The American Astronomical Society. All rights reserved. Printed in U.S.A. INJECTION OF Z1 MeV PROTONS IN ASSOCIATION WITH A CORONAL MORETON WAVE JARMO TORSTI, LEON G. KOCHAROV, MATTI TEITTINEN, AND BARBARA J. THOMPSON Space Research Laboratory, Department of Physics, Turku University, FIN-214, Finland; and NASA Goddard Space Flight Center, Greenbelt, MD 2771 Received 1998 May 2; accepted 1998 August 6 ABSTRACT We report extreme-uv observations of the coronal Moreton wave and concurrent observations of D1È1 MeV protons. Observations are carried out with the Extreme-UV Imaging Telescope and the Energetic and Relativistic Nuclei and Electron instrument on board the SOHO spacecraft. We study the proton events associated with coronal mass ejections (CMEs) centered near the central meridian. Observations reveal the initial injection of Z1 MeV protons during the period when the coronal Moreton wave was traversing the western hemisphere of the Sun, this being an early signature of the CME launch. Acceleration of the CME-associated protons starts during the CME lifto, while the main proton production occurs several hours later, when the CME expands in the interplanetary medium. Between the Ðrst proton production and the maximum intensity time, a spectral softening is observed. We analyze in detail the 1997 September 24 event. Development of the event indicates that the spectral softening may be due to a change in the acceleration regime, so the proton production starts with the less intensive but hard-spectrum injection and then moves to the more intensive but soft-spectrum injection farther from the Sun. Subject headings: interplanetary medium È Sun: corona È Sun: particle emission È waves 1. INTRODUCTION It is generally agreed that energetic protons can be rapidly accelerated during a Ñare at the Sun or, on much longer timescales, by the coronal mass ejectionèdriven shock waves traveling in the interplanetary medium (Lee & Fisk 1982; Jones & Ellison 1991; Kahler 1992; Reames 1996; Reames, Kahler, & Ng 1997 and references therein). Continual acceleration at intermediate time and spatial scales cannot be ruled out either (Klein et al. 1996). Traveling interplanetary shocks are frequently associated with enhancements in solar energetic particles. While these observations clearly indicate that shocks are capable of energetic proton production, the scenario and other properties of the acceleration are not completely understood (Kallenrode 1997). In particular, interplanetary shock acceleration may not be efficient enough to accelerate ions from the solar wind (Lim et al. 1995; Boberg, Tylka, & Adams 1996) and to generate a power-law energy spectrum (Savopulos, Quenby, & Bell 1995). Potentialities of coronal mass ejections (CMEs) to produce energetic particles in the interplanetary medium may also crucially depend on the previous evolution of solar eruption at heliocentric heights of [2 R (Torsti et al. 1998). A coronal seed population is probably _ needed for e ective proton production at shocks traveling in the interplanetary medium (Tan et al. 1989; Tylka et al. 1995; Boberg et al. 1996). If so, more attention should be paid to the early phase of CME-associated solar particle events. In a number of previous papers, chromospheric Moreton waves have been linked to the start phase of solar energetic particle (SEP) events. In particular, the observed velocity of the wave was used to put an upper limit to the heliocentric distance of the Ðrst proton acceleration in the 199 May 24 event, ¹2 R (Torsti et al. 1996). It was observed in some events that SEPs _ may have a rapid access to coronal longitudes that are well removed (greater than 1 ) from the Ñare center, and such an extreme propagation ÏÏ occasionally corresponds to the visible chromospheric 46 Moreton waves (Cliver et al. 1995). However, the Ha observations probably provide a less sensitive diagnostic of the wave as compared with the extreme-uv (EUV) observations employed in our present study. Accelerated protons associated with coronal mass ejections and interplanetary shocks may be detected on board a spacecraft and, in the strongest events, can even produce a signal in ground-based detectors. Here we present observations on board the spacecraft SOHO, which is orbiting around the Sun-Earth L1 (Lagrangian) point not far from the Earth. We analyze development of CME-associated proton events detected with the Energetic and Relativistic Nuclei and Electron (ERNE) instrument and corresponding Extreme-UV Imaging Telescope (EIT) observations of solar eruptions. This study is centered mainly on the beginning of the events, where the EIT observations can reveal coronal signatures of the CME lifto and the Ðrst high-energy protons may be detected. We do not attempt to cover in great depth the later development of proton production, when the CME-driven shock expands mainly in a radial direction far away from the Sun. SOHO carries a number of optical and particle instruments. The ERNE instrument can measure protons in the range from D1 to above 1 MeV, plus ions and relativistic electrons (Torsti et al. 1995). Protons above 12 MeV and relativistic electrons are detected with the High Energy Detector (HED) of ERNE. Both silicon detectors and scintillators are used in HED. The geometric factor varies between 36 and 26 cm2 sr, depending on the particle energy, and thus its ability to penetrate into the sensors, and the Ðeld of view is 12. The high sensitivity of HED allows us to study even weak increases in proton Ñux, such as the increases at the very beginning of an event. The EIT on board SOHO provides wide-ðeld images of the corona and transition region on the solar disk and up to 1.5 R above the limb (Delaboudinière et al. 1995). Its _ normal-incidence multilayer-coated optics selects spectral emission lines from Fe IX/X (171 A ), Fe XII (195 A ), Fe XV

2 CME-ASSOCIATED HIGH-ENERGY PROTONS 461 (284 A ), and He II (34 A ) to provide sensitive temperature diagnostics in the range from 6 ] 14 to 3 ] 16 K. For instance, the Fe XII (195 A ) emission corresponds to temperatures D1.5 ] 16 K. The EIT can image the coronal plasma on a global scale as well as many transient details. In particular, it is capable of observing coronal Moreton waves, expanding fronts that appear to be the ground tracks ÏÏ of compressional fronts caused by an impulse delivered by the Ñare and/or CME. Coronal Moreton wave is an early signature of the CME lifto seen on the Sun. 2. OBSERVATIONS We start with the solar eruption observed on 1997 September 24. There were actually three eruptions during late September 23 and the Ðrst half of September 24. The Ðrst CME was observed by the LASCO C2 telescope on board SOHO (Brueckner et al. 1995) at 23:48 UT September 23, but it probably started earlier, somewhere between 22: 2 and 23:48 UT. According to EIT observations, the September 23 event began to evolve between 21:23 and 21:4 UT, and the Ðrst evidence of Ðeld lines opening ÏÏ was between images at 21:4 and 21:57 UT. The corresponding EIT brightening went o primarily southeastward from AR 888 situated on the solar disk around 3 south, 15 east. The eruption was accompanied by the class C1.5 soft X-ray Ñare (start time is 21:36 UT). There were no Type II or IV radio bursts (SGD 1997). No associated proton event was observed by HED. The closest proton event started to rise around 4: UT, September 24, and was associated with the next solar eruption, a partial-halo event observed by the SOHO instruments LASCO and EIT around 3: UT on this day. This event was associated with the class M5.9 impulsive solar Ñare (31 south, 19 east; start time is 2:43 UT) and with Type II and IV radio busts starting at 2:48È 2:49 UT (SGD 1997). The third eruption occurred around 11: UT and was accompanied by the M3. Ñare and Type II and IV radio bursts. However, we are not able to resolve particles from the last eruption, probably because the previous solar particle event, associated with the M5.9 Ñare, was not over at that time. Figure 1 shows a full-disk EIT Fe XII j195 image of the Sun obtained at 2:49:2 UT on 1997 September 24. The corresponding occurrence time at the Sun is 2:41:1 UT. Close to this time, at 2:4 UT (this and all following times are shown for an occurrence at the Sun only), the M5.9 soft X-ray Ñare reached its maximum intensity (SGD 1997). In Figure 1, bright Ñare loops and an expanding coronal Moreton wave are seen clearly. Expansion of the coronal Moreton wave is illustrated in Figure 2, where a running di erence technique has been applied to contrast the brightness development. We estimate the speed of the coronal Moreton wave at 2:41 UT as approximately 325 km s~1. At 2:55 UT, the coronal Moreton wave arrived at the western hemisphere, and at 3:15 UT the leading edge was already beyond the meridian of 5 west. The bright front, labeled Moreton wave front ÏÏ in Figure 1, has experienced an estimated density increase of approximately 1%, decreasing to nearly 15% as it reaches the west limb. This can be used as an estimate of the magnitude of the perturbation that the CME has exerted on the ambient corona. Figure 3 shows the general view of smoothed-out proton intensity-time proðles in di erent energy bands detected by ERNE/HED on 1997 September 24. In Figure 4, we also show the unsmoothed proðles and development of the event in the relativistic particle channel (E1). In the beginning of the event, around 4: UT, a velocity dispersion was observed: the higher energy channels rose Ðrst, because protons with higher energy have less transit time from the Sun. The observed velocity dispersion allows one to deduce the time, t, when the initial accelerated particles were injected at the root of the Earth-connected interplanetary magnetic Ðeld line. The time t can be determined by relat- ing the intensities in di erent energy channels as functions of distance traveled, s \ (t [ t )V, where V is the average proton velocity for the channel. Adjusting the injection onset time, t, we determine that all energy channels demonstrate a rise for the same value of s. Another way to determine the onset time is by examining the relativisticparticle channel, assuming that the Ðrst particles traveled at the speed of light along the nominal spiral interplanetary magnetic Ðeld line. Both estimations result in the initial particle injection time at the Sun, t \ 3:1È3:2 UT, which is about 3 minutes after the end of the observed soft X-ray Ñare at the Sun. At that time, the leading edge of the coronal Moreton wave was between 5 and 7 west, which is not very far from the nominal root of the Earthconnected interplanetary magnetic Ðeld line, and the occurrence of the magnetic connection between a disturbed portion of the solar corona, the lowest part of which had been swept by the Moreton wave, and SOHO was possible. This Ðrst proton production (represented in Fig. 3 with a dark yellow pedestal ÏÏ at 4È8 UT) was followed by the second increase of injection, which was observed as a new rise in the 12È63 MeV proton channels at 8:3È9:3 UT on 1997 September 24 (seen as a red mountain range ÏÏ in Fig. 3). The new rise also revealed a velocity dispersion (see the upper panel of Fig. 4), that is, a signature of its solar origin. After 8:3 UT, the separation of intensity-time proðles of di erent energy channels increased distinctly, indicating the occurrence of the fast softening of the proton spectrum (Fig. 4). No second increase was seen in the relativistic particle channel E1. The ratios of second to Ðrst proton intensities are very di erent for di erent energy channels. At high energies, D7 MeV, the second-to-ðrst intensity ratio is much smaller than the ratio observed at lower energies, D2 MeV. This implies that the second injection spectrum is softer than that produced in the beginning of the event. To estimate the injection spectrum in a more quantitative manner, one has to correct near-earth spectra for the velocity dispersion. To remove the e ect of velocity dispersion at the beginning of the event, we plot spectra for the equal distances traveled, s (series 1 and 2 in Fig. 5). It is seen that, after the correction, the spectrum of the Ðrst protons is still extremely hard, almost Ñat at least up to B1 MeV. For comparison, we also plot the energy spectrum at the intensity maximum for the 2 hr period around 1:15 UT (Fig. 5, upper curve). The latter is representative for the second production period, starting at D9: UT on September 24. The di erence between the energy spectra for the Ðrst and second periods of proton production is really very large. As a result of the examination of the available SOHO data, we conclude that we have observed two di erent periods of proton production during the event. The major (second) production period was preceded by another period, when the injection was less intensive but of a harder energy spectrum. Injection of the Ðrst period starts during the phase of the coronal Moreton wave expansion over the solar surface, while the second period of proton production

3 462 TORSTI ET AL. Vol. 51 FIG. 1.ÈImage of coronal Moreton wave front observed by EIT at 2:49:2 UT on 1997 September 24. West is on the right, north on top. corresponds to the CMEÏ s expansion in the interplanetary space. Qualitatively similar patterns were observed in the other CME-associated Z1 MeV proton events as well. Coronal Moreton waves were also observed by EIT on 1997 April 7 and May 12. Both coronal Moreton waves were initiated by eruptions near the central meridian, at 19 east and 7 west, respectively. In both events, the Ðrst protons observed by ERNE were released well after the soft X-ray Ñare maximum, during or shortly after the Moreton wave passage through the corona over the western hemisphere of the Sun. On April 7, the EIT Moreton wave was traversing the solar disk during 13:52È14:32 UT, while the Ðrst protons were injected at the Earth-connected interplanetary magnetic Ðeld (IMF) line root at 14:3È14:5 UT. The rise of the proton intensity proceeded in two steps similar to the September 24 event. On May 12, the Moreton wave passage period was 4: 26È5: 32 UT, while the Ðrst protons were injected at 5:3È5:5 UT. In comparison with the 25È49 MeV channel, the lower energy (12È25 MeV) channel count rate rose faster and decayed slower, indicating a softening of the proton spectrum in the course of the event. In all three events, the proton spectra revealed a softening during the Ðrst hours of the event, exhibiting thereafter a soft-spectrum phase during which the proton Ñux reached its maximum value. 3. DISCUSSION High-energy solar particles escaping into the interplanetary medium propagate along IMF lines that are of a spiral shape and are rooted in the western hemisphere of the Sun (Parker 1963). The arrival of a CME at the vicinity of the Earth-connected IMF line can give rise to a near-earth particle event. At 3: UT on 1997 September 24, the Earth-connected IMF line was rooted at about 69 west (Fig. 1). At 3:1È3:2 UT, when the Ðrst observed protons were injected, the EIT-observed Moreton wave was already in 2 vicinity of the Earth-connected IMF line root. Because the majority of the emission seen in EIT Fe XII

4 No. 1, 1999 CME-ASSOCIATED HIGH-ENERGY PROTONS 463 FIG. 2.ÈRunning di erences between successive images of the Ðrst eruption on 1997 September 24. Each frame shows the di erence between an 195 A image and the previous image. Times at the Sun are 2:24, 2:41, 2:55, and 3:15 UT for frames from left to right and from top to bottom, respectively. occurs within.2 R (see Fig. 1), we assume that EIT is only imaging the ground _ track ÏÏ of the expanding coronal Moreton wave. However, the actual longitude of the coronal Moreton wave may be height-dependent, and the particles may be accelerated at a farther distance, probably at D.5È1 R from the solar surface, where the Ðeld lines are open and _ extended into interplanetary space. Exact magnetic connections in solar corona are not known, and the near-earth particles are referenced relative to di erent western longitudes around the nominal root of the IMF line. Keeping in mind these uncertainties, a possible less than 2 di erence in longitudes between the EIT-observed Moreton wave and the nominal Earth-connected IMF-line root does not look very dramatic. Anyway, the EIT observations clearly indicate that production of the Ðrst protons is associated with CME lifto and with angular expansion of the disturbed region over the Sun. We consider the EIT Moreton wave to be the signature of an expansion that gave rise to the Z1 MeV proton event detected by ERNE, but we do not imply that the Moreton wave accelerates interplanetary-observed protons below.2 R. Major proton events are usually associated _ with coronal mass ejections, and CME-driven shock waves may be regarded as an accelerator for the particles. In such events,

5 464 TORSTI ET AL. Vol. 51 FIG. 3.ÈProton intensity-time proðles observed by ERNE in di erent energy channels. Intensities are shown as 3 ] 3 point averages. The timescale is linear up to 2. hr and is logarithmic beyond this point. The bright yellow strip indicates arrival of the Ðrst protons if injected from the Sun at 3 : 15 UT. the proton injection peaks occur when the CME is estimated to reach 5È15 R (Kahler 1994). At that and later times, the CME expands_mainly in a radial direction, so the angular extension of the shock is limited by D1 È15 in longitude (Richardson & Cane 1993). Our observations show once again that the main proton injection is delayed from the estimated CME launch time and occurs when the CME-driven shock expands far away from the Sun. However, our observations indicate that protons are also accelerated during an earlier phase of eruption, corresponding to an impulse that the launching CME imparts to the lower corona. Two periods of proton production can be easily separated by the naked eye in Figures 3 and 4, before and after B8 : 3 UT, respectively. The Ðrst-period injection reveals a hard spectrum. The coronal shock wave, being quasi-perpendicular at the Earth-connected IMF line root, might be regarded as a source of this hard component. Ellison, Baring, & Jones (1995) studied acceleration rates and injection efficiencies in oblique shocks. They concluded that in a quasi-perpendicular shock the acceleration rate can be much higher than that seen in a parallel shock, but such a shock is the least capable of injecting thermal particles into the acceleration process. Moreton waves may be considered as a visible manifestation of a moving acceleration region and interpreted as the moving skirts of coronal shock waves (Cliver et al. 1995). To explain the spectral evolution observed by ERNE, both quasi-circumsolar coronal and interplanetary shock waves may be required. In the frame of the shock acceleration mechanism, the observed transition from the hard to the soft proton spectrum may be caused by the change in acceleration regime as a result of the shifting shock geometry from quasi-perpendicular to quasi-parallel. However, before the detailed discussion of acceleration models, the event should be carefully Ðtted to tackle the question as to whether the change of the injection spectrum is continuous or, alternatively, the change is abrupt and corresponding protons can be treated as two components produced in the course of a single eruption. 4. SUMMARY We have studied EIT observations of the coronal Moreton wave and concurrent observations of D1È1 MeV protons on board the SOHO spacecraft during the event of 1997 September 24. The event was caused by a solar eruption above AR 888, which is situated about 88

6 No. 1, 1999 CME-ASSOCIATED HIGH-ENERGY PROTONS 465 FIG. 4.ÈDevelopment of 1 minute average proton intensities observed on 1997 September 24 in energy channels 12È19, 19È32, 32È46, 46È63, and 63È85 MeV (upper panel, consecutive curves from top to bottom, respectively) and 1 minute count rate in the relativistic particle channel E1 (lower panel). Bars marked with s ÏÏ and m ÏÏ illustrate periods for calculation of energy spectra (Fig. 5) for the beginning and the maximum phase of the event, respectively. Period s ÏÏ corresponds to the distance of 1È3 AU traveled by 2 MeV protons (for di erent energy channels, the time intervals are di erent according to the velocity dispersion). eastward from the nominal root of the Earth-connected interplanetary magnetic Ðeld line. We conclude the following: 1. The Ðrst injection of high-energy protons at the root of the Earth-connected magnetic line started about 3 minutes after the soft X-ray Ñare, during the period when the coronal Moreton wave was traversing the western hemisphere of the Sun and a Type IV radio burst was in progress. 2. Between the Ðrst proton injection and the maximum intensity time, which occurred about 6 hr later, a spectral softening was observed. FIG. 5.ÈProton energy spectra at the beginning of the event vs. distance traveled, s \ (t [ t )V. The traveled distances are s \ 1È2 and2è3 AU for series 1 and 2, respectively. The lines represent the best-ðt powerlaw spectra. Series 3 represents the proton spectrum at the intensity maximum (see Fig. 4). 3. The period of the hard-spectrum injection is associated with the CME lifto and the corresponding expansion of the coronal Moreton wave in longitude and latitude on the solar surface. Qualitatively similar patterns were also observed in the 1997 April 7 and May 12 events. These observations indicate that the Ðrst acceleration of the CME-associated protons starts near the Sun in a wide range of solar longitudes concurrently with the coronal Moreton wave expansion. The major proton production, however, occurs several hours later, when CME expands in the interplanetary medium. Observed spectral development may be a signature of the change in the acceleration regime during the rise phase of the event. We thank the LASCO team for the coronagraph data available in the SOHO archive. The ERNE team was Ðnancially supported by the Academy of Finland. SOHO is an international cooperation project between ESA and NASA. Boberg, P. R., Tylka, A. J., & Adams, J. H., Jr. 1996, ApJ, 471, L65 Brueckner, G. E., et al. 1995, Sol. Phys., 162, 357 Cliver, E. W., Kahler, S. W., Neidig, D. F., Cane, H. V., Richardson, I. G., Kallenrode, M.-B., & Wibberenz, G. 1995, Proc. 24th Int. Cosmic Ray Conf. (Urbino), 257 Delaboudinie` re, J.-P., et al. 1995, Sol. Phys., 162, 291 Ellison, D. C., Baring, M. G., & Jones, F. C. 1995, ApJ, 453, 873 Jones, F. C., & Ellison, D. C. 1991, Space Sci. Rev., 59, 259 Kahler, S. W. 1992, ARA&A, 3, 113 ÈÈÈ. 1994, ApJ, 428, 837 Kallenrode, M.-B. 1997, J. Geophys. Res., 12(A1), Klein, K.-L., Trottet, G., Aurass, H., Magun, A., & Michou, Y. 1996, Adv. Space Res., 17(4/5), 247 Lee, M. A., & Fisk, L. A. 1982, Space Sci. Rev., 32, 25 Lim, T. L., Quenby, J. J., Reuss, M. K., Keppler, E., Kunow, H., Heber, B., & Forsyth, R. J. 1995, Proc. 24th Int. Cosmic Ray Conf. (Rome), 4, 353 REFERENCES Parker, E. N. 1963, Interplanetary Dynamical Processes (New York: Interscience) Reames, D. V. 1996, in AIP Conf. Proc. 374, High Energy Solar Physics, ed. R. Ramaty, N. Mandzhavidze, & X.-M. Hua (New York: AIP), 35 Reames, D. V., Kahler, S. W., & Ng, C. K. 1997, ApJ, 491, 414 Richardson, I. G., & Cane, H. V. 1993, J. Geophys. Res., 98(A9), Savopulos, M., Quenby, J. J., & Bell, A. R. 1995, Sol. Phys., 157, 349 SGD. 1997, Solar-Geophysical Data (Boulder: NOAA Space Enviroment Center) Tan, L. C., Mason, G. M., Klecker, B., & Hovestadt, D. 1989, ApJ, 345, 572 Torsti, J., et al. 1995, Sol. Phys., 162, 55 ÈÈÈ. 1998, Geophys. Res. Lett., 25, 2525 Torsti, J., Kocharov, L. G., Vainio, R., Anttila, A., & Kovaltsov, G. A. 1996, Sol. Phys., 166, 135 Tylka, A. J., Boberg, P. R., Adams, J. H., Jr., Beahm, L. P., Dietrich, W. F. & Kleis, T. 1995, ApJ, 444, L19