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SCIENCE DIRECT.

doi: lO.l016/SO273-1177(03)00355-7

PEAK PROTON INTENSITIES AND COMPOSITION VARIATIONS OF HEAVY IONS DURING LARGE SOLAR ENERGETIC PARTICLE

EVENTS: ULEIS OBSERVATIONS

G. C. Hoi, G. M. Mason2, E. C. Roelof l, R. E. Goldl, and J. E. Mazur3

‘The Johns Hopkins University Applied Physics Liboratory, Laurel, MD 20723, USA 2Department of Physics and I.P.S.T., University of Maryland, College Park, MD 20742, USA

3Aerospace Corporation, El Segundo, CA 90245, USA

ABSTRACT

We have examined low energy proton and heavy ion intensities in large solar energetic particle (SEP) events from 1997 to the end of 2001 using the ULEIS instrument on ACE spacecraft. We selected 33 events that are isolated and show clear onsets at 1 AU. The optical and radio signatures for all of our events are identified and where data are available we also located the associated Coronal Mass Ejection (CME) data from instruments on SOHO. We found at low energy (0.23 - 0.45 MeV), the peak proton intensities and the associated CME speeds are not correlated. In addition, we also found the measured ambient pre-event intensities are not at all related to the event peak intensities. These results are contrary to what was found by Kahler (2001) for ions greater than 10 MeV in 71 large SEP events. At low energy, the ambient particle intensity that we measured before the event onset may not be a good measure of what is later being accelerated in these large SEP events. The suprathermal particles at a few times the solar wind energy (not measurable with our instrument) may play a bigger role in the low energy ion intensities in large solar energetic particle events in our study. We also examined the event averaged abundance ratio of Fe/O as a function of solar longitude and CME speed. No obvious correlation is evident. However, the mean Fe/O ratio for magnetically well-connected (W6 lo - W70’) events has a statistically larger value (0.64) than those from other longitudes. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

INTRODUCTION

Large solar energetic particle (SEP) events are believed to be associated with Coronal Mass Ejection (CME) driven shocks. As the shock travels from the Sun to inner heliosphere, an observer at Earth will sample different part of the shock-to-field surfaces. Hence a different energetic particle time-intensity profile will be observed for shock originated at different solar longitudes (Cane et al., 1986). A positive correlation between the high energy (>lO MeV) SEP peak ion intensities and associated CME speeds has been found by Kahler (2001), further strengthening the close relationship between CMB and long duration SEP events at high energies. However, it was also pointed out by Kahler (2001) that the ambient pre-event particle intensity and particle spectral shape play a key role in the observed peak intensity and CME speed correlation for the high energy particle intensities. At low energy (cl MeV), the ambient particles would be more important in large SEP events because of the relative abundances and energization involved. Lario et al. (2000) found that transient events dominate the inner heliosphere during solar maximum, and the low energy ion intensity level hardly ever reached instrument background level. Mason et al. (1999) measured an enhanced %e/‘!He ratio in a large SEP event and suggested that remnant material from impulsive SEP events can fill up a portion of the inner heliosphere and could be later accelerated by shocks associated with large SEP events. Hence, correlations of CME characteristics and low energy ion intensities need to be further explored.

Adv. Space Res. Vol. 32, No. 4, pp. 561-566.2003 8 2003 COSPAR. Published by Elsevier Ltd. All rights reserved Printed in Great Britain 0273-l 177/$30.00 + 0.00

562 G. C. Ho et al.

In this paper, using composition data from the Ultra-Low Energy Ion Spectrometer (ULEIS) on the Advanced Composition Explorer (ACE) spacecraft (Mason et al., 1998), we examine the low energy (230-450 keV/nucleon) SEP and their associated CME speed in 33 large SEP events (including those examined by Kahler (2001)).

OBSERVATIONS

We selected 33 large SEP events in the AClXJLEIS data set from November 1997 to December 2001. The criteria for our selection are as follows: (1) the event onset at 0.23 - 0.45 MeV/nucleon is isolated from other increases (necessary to determine pre-eventintensity); (2) there are Solar and Helisopheric Observatory (SOHO) observations from the Large Angle and Spectrometric Coronagraph. Experiment (LASCO) to provide us an estimated launch location and time of the associated CME back at the ‘Sun (except for events in September 1998, and February 1999 when SOHO was out of science operations); (3) positive identification on the associated Ha flare location; (4) no compound events (for event averaged study).

With these criteria we should minimize effects that could obscure the underlying acceleration and transport mechanism For most events, the Ha associations have been identified in Cane et al. (2002). The CME observations are taken from the LASCO web site at http://cdaw.gsfc.nasa.gov/CMl-list/. Out of 31 events in our study (during two events SOHO data were not available), we have identified 30 associated CMEs based on the timing of our ion intensity and the solar event reports from NOAA Space Environment Center (SEC). Figure 1 shows low energy (0.23 - 0.45 MeV) protons for one of the selected event that was detected on ACE on 1998 day 309. The event is associated with a halo CME and an Ha event detected on W17”. Prior to the event onset, the proton intensity is close to instrument background level (-0.6 particles/[cm’-set-str-MeV]), and we are able to observe the event entirely without any contribution from other SEP events. More than two days after the event onset, an interplanetary shock (indicated by the vertical thin solid line) was also observed on ACE with an associated Energetic Storm Particle (ESP) event. The temporal variations of the He&I and Fe/O within these events have been reported by Ho et al. (2003). For this paper, we study both the low energy proton peak intensity and the average Fe/O variations within these 33 large SEP events.

- 0.23-0.45 MeV

310 312 314 316 318

Day of Year 1998 Fig. 1. Large SEP event detected by ULEIS on day 309 in 1998. The thick solid line is the hourly average proton intensity as measured by ULEIS. The event is associated with an Ha flare at W17” and a halo CME. More than two days after the event onset, an interplanetary shock (vertical thin solid line) and its related ESP event was also detected by ULEIS on day 312.

Peak Intensity Figure 2 shows the 0.23 to 0.45 MeV proton intensities’ for the 30 large SEP events in our study as a function

of their associated Ha flare location. The prompt impulsive peak intensity was used even if the delayed peak of the shock associated ESP event had a higher intensity (as shown in Figure 1). The highest intensity event in our survey had maximum proton intensity of 52500 particles/[cm2-set-str-MeV] at -300 keV which was associated with an HO! flare at W83’, while the lowest intensity event had maximum proton intensity of only 98

Large Solar Energetic Particle Events 563

particles/[cm’-set-str-MeV] that was associated with an Hoi brightening at EO2”. No clear correlation is seen for the low energy peak intensity over a large range of solar longitude.

lo5

lo2

0

0. 0 0

0 0

I 1 I 0.l I 101 I II -40 -20 0 20 40 60 80 100

Solar Longitude (Degree) Fig. 2. Peak proton intensities (excluding the ESP portion of the SEP event) as a function of the event location of the Hcl flare locations.

Ktier. (2001) reported that there is a positive correlation between the peak ion intensity of the high energy SEPs (>lO MeV) and the projected CME speed in large SEP events. We show a similar figure for the low energy (0.23 - 0.45 MeV) proton and the associated,CME speed in our survey in Figure 3. Because of the projection effect of the calculated CME speed, we show only 17 events in our survey that have associated flare locations greater than W45”. The CME speed is taken directly from the SOHOILASCO web site at http://cdaw.gsfc.nasa.gov/CMEJist/. Despite a ong correlation reported by Kahler (2001) at high energy, we do

r not see any obvious relation between the low e ergy proton peak intensity and the associated CME speed. The lack of correlation could L due to the available seed parti& for later acceleration by the CME driven shock. At high energy, Kahler (2001) found good correlation between the ambient high energy SEP intensity and the peak SEP intensity. Hence, we show the ambient proton intensity (0.23 - 0.45 MeV) just before the event onset and the measured peak proton intensity for our 33 events in Figure 4.

I I I I 4-

2- 0 0

lo4 r 8 0 0 4: 0 4- 0 0 0

2- 0 103: 000 .

6: 0 4-

Events > W45D 2-

lo2 - 0 (17 out of 30 SEP Events) -

I I I I

0 500 1000 1500 2000 2500

CME Projected Speed (km/s)

Fig. 3. Peak proton intensities of those events that are associated with flare locations greater than W45” as a function of the CME projected speed.

564 G. C. Ho et al.

2

lo2 I 1 I I,lll I 1 ,,,I,, 1

6 70 2 3 4‘5678 2 3 45678 1 10 . 100

Ambient Intensity [part/(cm2-s-sr-MeV)] ,-

Fig. 4. Peak proton intensities of the 33 events as a function of ambient proton intensities just prior to the event onset. The instrument background is off scale at -0.6 part/(cm*-s-sr-MeV).

Despite a good correlation that was found by Kahler (2001) at high energy ($ > 10 MeV), no correlation is evident between the ambient intensity and the peak intensity of our large SEP events. Because of our selection criteria, most of the selected events have very low ambient intensity levels.

Event-Averaged ‘katio Figure 5 shows the event averaged Fe/O ratio for all events as a function of their Ha locations. The event

peak proton intensity is also shown in the figure with different size of the circular data points. Larger circles denote higher intensity events. The event with the highest Fe/O ratio in our survey is associated with an Ha flare located at W69”, a magnetically well-connected region. However, the remainders of the events show no clear longitude signature. The mean Fe/O ratio of all, events is 0.30 f 0.26 (0.27 f 0.19 without the largest Fe/O event), about twice of the SEP based coronal value (0.17). However, if we average the Fe/O ratios for those events at W61” - W70”. the mean Fe/O = 0.64 f 0.49 is more than three times the SEP based coronal value. However, the spread of those ratios is also large for the four events in that longitude (Fe/O from 0.1 to 1.3), and is still within the coronal value. At higher energy (12 - 60 MeV/nucleon) von Rosenvinge et al. (2001) have found that Fe/O ratios are higher for magnetically well-connected events.

0 Solar Longitude $&rees)

80

Fig. 5. Fe/O ratios for events with different maximum intensities as a function of solar longitude. Larger circles denote higher intensity events.

Large Solar Energetic Particle Events 565

Figure 6 shows the event averaged Fe/O ratio as a function of the CME projected speed for all 33 events in our study. The peak event proton intensities are indicated by the size of the symbol, with larger circles denoting higher intensity. Again no obvious correlation is evident, except the event that has the largest Fe/O (1.3) was associated with the fastest CME projected speed at over 2000 km/s. However, for the rest of the events, CMEs with speeds of 300 km/s to 1800 km/s produced Fe/O ratios ranging from nominal to five times the coronal values.

-.- 0 5Nl 1000 1500 2000 2500

CME Projected Speed (km/s)

Fig. 6. Event averaged Fe/O for all 33 events in our study as a function of the projected CME speed. The W69” event that we repotted earlier which has the highest Fe/O ratio (1.3) is associated with the fastest CME speed (2221 km/s) in our survey. However, the rest of the event averaged Fe/O ratios have no correlation with its associated CME projected speed. Peak proton intensities are indicated by the size of the symbol.

DISCUSSION AND CONCULSIONS

We selected a set of isolated and clean large SEP events from September 1997 to December 2001 using the low energy composition instrument ULEIS on ACE. Each event’s associated CME and Ha flare location were also identified using the LASCO CME list and reported solar events from NOAA/SEC. No clear correlation is found between the low energy (0.23 - 0.45 MeV) peak proton intensities and the projected CME speed (shown in Figure 3). This is contrary to what was found for ions at greater than 10 MeV, when a statistically significant correlation was found between the peak ion intensities and the CME speed (Kahler, 2001). In addition, Kahler (2001) has noted that the ambient particle intensity plays a major role in spreading the otherwise excellent correlation between the’ observed high energy peak ion intensities and CME speed. Our results, however, indicate the pre-event ambient proton intensities have no correlation with the event peak intensities in the energy that we studied (Figure 4). We rule out the possibility that contribution from impulsive SEP events could mask such correlations because of our selection criteria (1) and (3). However, these different results at low energy may not be totally unexpected. Mason et al. (1999) suggested the suprathermal particles (a few times the solar wind energy) from earlier impulsive SEP events might contribute enough fluence to alter the composition of large SEP event at low energies. Hence, the ambient particle intensity that we measured before the event onset at this energy range may not be a good measure to what is later being accelerated in these large SEP events. Desai et al. (2001) studied the composition of ESP events that are associated with interplanetary shocks and concluded that the ions being accelerated by these shocks originated from either remnant-impulsive SEP material and/or from the suprathermal tail of the solar wind. We consider our results to be consistent with the suprathermal tail of the solar wind being a major source of particles since we found no direct relationship between the observed CME speed and peak intensity. This observed behavior between the lack of correlation between the CME speed and peak particle intensity may be correlated with solar activity. During solar maximum when the number of SEP events (both impulsive and gradual) is high, the inner heliosphere energetic particle population never has time to reach its quiet level as noted by Lario et al. (2000). Hence, the inner heliosphere will be filled with both energetic particle and suprathermal particles all

566 G. C. Ho et al.

dependent on recent solar activities (Roelof et al., 1992; Droge, Mtiller-Mellin, and Cliver, 1992). When a CME driven shock propagates through this “disturbed” medium, at low energy, the particle intensity will be dependent on the seed population as well as the strength of the traveling shock

We examined the event averaged low energy (-300 keV/nucleon) abundance ratio of Fe/O ratio as a function of longitude, event maximum intensity and project CME speed. No obvious correlation is evident. This is again consistent with more than one source of seed particles for these large SEP events when such large variations of Fe/O are observed in an apparently incoherent manner. However, the mean Fe/O ratio for those magnetically well- connected events (W61” - W70’) has a larger value (0.64) than those from other longitudes. But the spread of those ratios is also large for the four events in that longitude (Fe/O from 0.1 to 1.3), and is still within the coronal value. A similar result at higher energy (12 - 60 MeV/nucleon) was also reported by von Rosenvinge et al. (2001).

ACKNOWLEDGMENTS I

The work at JHU/APL is supported under NASA contrac;,N@-97271, task order 009.

REFERENCES .

Cane, H. V., R. E. McGuire, T. T. von Rosenvinge, Two classes of solar energetic particle events associated with impulsive and long-duration soft x-ray flare, Astrophys. J., 301,448-459, 1986.

Cane, H. V., W. C. Erickson, N. P. Prestage, Solar flares, type III radio bursts, coronal mass ‘ejections, and energetic particles, J. Geophys..Res., 107(AlO), 1315, 10.1029/2001JA000320,2002.

Desai, M. I., G. M. Mason, J. R. Dwyer, et al., Acceleration of 3He nuclei at interplanetary shocks, Astrophys. J., 553, L89-L92,2001.

Droge, W., R. Mtiller-Mellin, and E. W. Cliver, Superevent - Their origin and propagation through the heliosphere from 0.3 to 35 AU, Astrophys. J, 387, L97-LlOO, 1992.

Ho, G. C., E. C. Roelof, G. M. Mason, et al., Composition variations during large solar energetic particle events, in Solar Wind 10, edited by M. Velli, AZP Confi Proc., in press, 2003.

Kahler, S. W., The correlation between solar energetic particle peak intensities and speeds of coronal mass ejections: Effects of ambient particle intensities and energy spectra, J. Geophys. Res., 106, 20947-20956, 2001.

Lario, D., R. G. Marsden, T.~ R. Sanderson, et al., Energeticproton -observations at 1 and 5 AU, 1, January- September 1997, J. Geophys. Res., 105, 18235-18250,200O.

Mason, G. M., J. E. Mazur, J. R. Dwyer, 3He enhancements in large solar energetic particle events, Astrophys. J., 525, L133-L136,1999.

Mason, G. M., R. E. Gold, S. M. Krimigis, et al., The Ultra-Low-Energy Isotope Spectrometer (ULEIS) for the ACE spacecraft, Space Sci. Rev., 86,409-448, J99S.

Roelof, E. C., R. E. Gold, G. M. Simnett, et al., Low-energy solar electrons and ions observed at ULYSSES February-April, 1991&e inner heliosphere as warticle reservoir, Geophys. Rq. L&t., 19, i243-1246,1992.

von Rosenvinge, T. T., C. M. S. Cohen, E. R. Cl&&an, ‘et al., Time variations in elemental abundances in solar energetic particle events, in Solar and Galactiu Composition, edited by R. F. Wimmer-Schweingruber, AlP Con$ Proc., 598, pp. 343-348,200l.

E-mail address of G.C. Ho george.ho@jhuapl.edu Manuscript received 19 October 2002; revised 28 January 2003; accepted 28 January 2003