On the origin of the relativistic protons observed in heliosphere after some solar flares

Adv. Space Res. Vol. 21, No. 4, pp. 559-562. 1998 0 1998 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

0273-l 177/98$19.00 + 0.00 PII: SO273-I 177(97)00963-O

ON THE ORIGIN OF THE RELATIVISTIC PROTONS OBSERVED IN HELIOSPHERE AFTER SOME SOLAR FLARES A. I. Sladkova* and G. A. Bazilevskaya** *Institute of Nuclear Physics, Moscow State University, 119899, Moscow, Russia **Lebedev Physical Institute, Russian Academy of Sciences, Leninsky Prospect, 53, 117924, Moscow, Russia

ABSTRACT We have analyzed the heliolongitudinal distributions of some features of large solar proton events with peak intensities JLlO cm-%-Isr at 10 MeV proton and reliable flare association on the visible solar disk observed near the Earth in 1970- 1991., We show that the parent flares of large non-GLE events are distibuted uniformely over the solar disk in contrast to ground level enhancements (GLEs) connected mainly to the western flares. In the both groups the >lO MeV proton fluxes exhibit two maxima: one - for well-connected events and another - for the events associated with the central and eastern flares the latter being more pronounced. This is also valid for protons with energies at least up to 500 MeV. This may be an indication of two different modes of particle generation. 01998 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION It is widely known that solar energetic particle (SEP) events can be separated by some features into two classes according to the duration of the parent X-ray flare - impulsive and gradual events. The most significant is the difference in the element abundances and charge states. It is indicative of higher temperatum in the source of impulsive events (-10 MK) in comparison with gradual ones (-2 MK) (Luhn et al., 1985). In addition, after impulsive flares the SEPs are detected in a rather narrow longitude interval (tiO-50”) around the observer’s magnetic footpoint, whereas after gradual events the SEPs are spread over an interval i120” (Cane et al., 1986, 1988, Kallenrode et al., 1992). A hypothesis for two distinct sources of accelerated particles observed in the interplanetary space is now generally accepted (see Reames, 1995 and ref. therein). In impulsive events the particles are accelerated low in the corona during a hot impulsive phase of a flare. Particles generated in gradual events are accelerated by shock waves driven by coronal mass ejections (CMEs) through the corona (Kahler, 1994) and interplanetary space (Cane el al., 1988, 1990). The majority of evidences relate to nonrelativistic SEPs. The generation of relativistic SEPs is most probable in powerful flares which are usually associated with gradual events. However, it is not clear whether a shock can provide the necessary rate of energy growth. It is reasonable to assume that in gradual events a certain fraction of particles is accelerated at the flare site. In this paper we compare some features of solar protons in different energy intervals with a purpose to further our undemtandmg of a relative contribution from different sources to the SEP population. We examine the heliolongitude distributions of the parent flares, average proton intensities and energy spectrum indices. 559

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and G. A Bnnlevskaya

DATA

We used the data on solar protons with energy above 10 MeV observed at the Earth’s orbit in 1970-1991. For 1970-1986 the information was taken from the Catalogues of Solar Proton Events (Bazilevskaya et al., 1986, 1990) based on the SEP observations at the near-Earth spacecraft METEOR and IMP, balloons and neutron monitors. A list of SEP events after 1986 was taken from the unpublished version of the similar Catalogue prepared by the same authors. The intensities of protons with E>SOOMeV are inferred from the neutron monitor observations using the yield functions (Debt-turneret al., 1982)). For analysis we selected the events meeting two criteria: (1) an integral flux of solar protons with energy more than 10 MeV in the maximum of intensity-time profile J exceeded 10 cm-*.s-t-sri-; (2) a reliable association with a flare on the visible solar disk. A value of J was evaluated from the data of all detectors in the adjacent energy range. The data of differential channels were transformed to the integral fluxes, and the integral energy spectrum was fitted by a power law in the energy interval 10 - I 100 MeV. It was the fitted value that was taken in the anaIysis. We considered a first maximum on the profile provided it could be separated unambiguously. In the opposite case we took an absolute maximum of the profile. The source identification is a rather complicated problem, which is described in Akiniyan et al. (1983) in detail. We have chosen only the events with the reliable source association. After verifying the associations we included into consideration 3 events (21 Aug., 1979, 4 April, 1981, and 12 July, 1982) that were not listed in the Catalogues as the isolated ones. The number of events meeting both the criteria (1) and (2) is 129, among them 25 events are ground level enhancements (GLEs). It constitutes 78% of all the events with the peak >lO MeV proton intensity exceeding 10 cm-*s-t& near the Earth in 1970-1991. The rest 36 events were excluded from the analysis as they didn’t meet criterion (2). THE RESULTS AND DISCUSSION We examined separately the events without relativistic protons (i.e. not recorded by neutron monitors) and the GLEs with relativistic SEPs. Hereafter we refer to the first group of events as the nonrelativistic events. Figure 1 displays the longitude distributions of some characteristics of solar protons: on the left for the nonrelativistic events and on the right for GLEs. The upper pane1 of Figure 1 shows the number of events with a parent flare in a given longitude interval. It is well-known (e.g. Bazilevskaya and Sladkova, 1986, Smart and Shea, 1996) and not surprising that GLEs are observed preferably from western flares. However, the distribution of nonrelativistic events is rather uniform (there are slightly more events in the 70-9OW interval, possibly, because some events at the western limb occur actually behind the limb). The uniform longitude distribution for gradual events was discussed by Reames (1994) but for SEPs with particle energies and intensities not indicated in the paper. It was not stressed earlier for relatively large SEP events because GLEs were not isolated. The uniform longitude distribution of nonrelativistic events is an evidence of their relation to the CME-driven shock. As for relativistic protons, they may be associated with shocks but for majority events only shocks closely connected to the flare site. The middle panel of Figure 1 shows the mean values of J averaged over events with the parent flare in a given longitude interval. The nonrelativistic events with the highest intensities come from the eastern longitudes. This is in agreement with an interplanetary shock driven by CME as a source of accelerated >lO MeV protons (Cane et al., 1988, Kallenrode et al., 1992, Reames, 1994, 1995). There is a second relative maximum in the J distribution in the well-connected interval 50-7O”W. It may be indicative of particle population accelerated at the flare site and not spread over large angular distances. A similar maximum is more pronounced for GLE events. This maximum relates to the majority of GLEs originated in the western hemisphere. However, there is a much more intense peak in the distribution for GLEs in the central longitude interval 10 E-10 W (note the broken scale of Y-axis).

Origin of Relativistic Solar hotons

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Fig. 1. Longitude distributions for 1970-1991 of the number of events with J(> 10 MeV) ~10 p/cm&r (roppunel); the average intensities(middlepanel), and spectral indexes in the range of lo-100 MeV (boffompanef). Left - the nonrelativistic events, right - GLEs

Actually, in this interval there are only 2 events: 4 August, 1972, and 19 October, 1989. However, looking at the GLEs before 1970 we can add the events 28 February. 1942, and 12 November, 1960. Both are known as very intense, so this maximum is hardly unreal. Thus, there are a few GLEs with very high intensity of >lO MeV protons that were accelerated by interplanetary shocks. The more numerous GLEs from western flares have lower > 10 MeV proton intensity. The lower panel of Figure 1 presents the power law spectrum index in the energy range 10 - 1100 MeV. It is seen that energy spectrum in the range is harder for GLEs as it was obtained earlier in Bazilevskaya and Sladkova (1987). There is a tendency to the growth of spectral index for the eastern events, especially for the nonrelativistic events. This is probably also a signature of shock acceleration. 10 q-

b 9

103

ii -% .g

I0

z

10-l i”a

t I 10

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)

100

1000

I 10000

Energy, MeV Fig. 2. The average intensity in two longitude intervals VS. proton energy in GLEs. Crosses for 50E-10W;point.s for 30-70W.

To estimate the energy limit for shock acceleration in GLEs we calculated the average intensities for solar protons of different energies in two longitude intervals: 50E- 1OW (presumably representing the events caused by interplanetary shocks) and 30-70W (representing particle acceleration near the flare site). They are shown VS. energy in Figure 2. The difference in the intensities from the two intervals diminishes only at high (GeV) energies. It is clear that interplanetary shocks give much higher intensities of protons with energies at least up to 500 MeV.

562

A. 1. Sladkova

and G. A. Bazilcvskayr

coNcLusioN

The results of this work argue that generation of solar protons observed at the Earth’s orbit is dependent on flare sites. The parent flare longitude distribution is rather uniform for nonrelativistic events, but the average proton intensity demonstrates 2 maxima: one for the well-connected western flares and another for the eastern flares. If the particles &e due to interplanetary shocks one could expect the monotonous intensity increase from the well-connected to the eastern longitudes. The presented results allow us to suggest that there are 2 different main modes of generation. The majority of relativistic events is associated with the well-connected western events. However, the average proton intensity for relativistic events has also 2 maxima corresponding to 2 optimal longitude intervals for particles of different origin. The interplanetary waves may be much more effective for proton acceleration up to 500 MeV than another agent operating near the flare site. The rare observations of GLE events outside the wellconnected longitude interval demonstrate that the conditions for acceleration of GeV protons by shocks are rather unique; it is possible that a system of waves is necessary for such acceleration. Acknowledgement This work was partly supported by the Russian Foundation for Basic Research (grant no. 96-02-l 6657a). REFERENCES

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