POST-IMPULSIVE-PHASE ACCELERATION IN A WIDE RANGE OF SOLAR LONGITUDES. 1. Introduction

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1 POST-IMPULSIVE-PHASE ACCELERATION IN A WIDE RANGE OF SOLAR LONGITUDES LEON KOCHAROV, JARMO TORSTI, TIMO LAITINEN and MATTI TEITTINEN Space Research Laboratory, Department of Physics, University of Turku, FIN Turku, Finland (Received 15 September 1999; accepted 22 November 1999) Abstract. We have analyzed five solar energetic particle (SEP) events observed aboard the SOHO spacecraft during All events were associated with impulsive soft X-ray flares, Type II radio bursts and coronal mass ejections (CMEs). Most attention is concentrated on the SEP acceleration during the first 100 minutes after the flare impulsive phase, post-impulsive-phase acceleration, being observed in eruptions centered at different solar longitudes. As a representative pattern of a (nearly) well-connected event, we consider the west flare and CME of 9 July 1996 (S10 W30). Similarities and dissimilarities of the post-impulsive-phase acceleration at large heliocentric-angle distance from the eruption center are illustrated with the 24 September 1997 event (S31 E19). We conclude that the proton acceleration at intermediate scales, between flare acceleration and interplanetary CMEdriven shock acceleration, significantly contributes to the production of 10 MeV protons. This post-impulsive-phase acceleration seems to be caused by the CME lift-off. 1. Introduction A modern paradigm of cause and effect in solar-terrestrial physics emphasizes the role of CMEs in producing major SEP events and de-emphasizes the importance of solar flares (Gosling, 1993; Reames, 1996; and references therein). Flare accelerated particles typically dominate in impulsive events, and those particles must be accelerated in less than several seconds in a flare core (Miller et al., 1997, and references therein). The accelerator acts during the flare impulsive phase only. In contrast, production of interplanetary particles in gradual events typically peaks when a CME is far from the Sun, already at the distance of 5 15 R (Kahler, 1994), which corresponds to the time scale of several hours. Time scales from 10 to 100 min and spatial scales of R seem to be missed in this conventional scheme of particle acceleration. However, a CME lift-off involves a significant disturbance of the solar corona as a large-scale eruption carries up to g of material into interplanetary space (e.g., Harrison, 1995), and the lift-off associated coronal shocks and turbulence could accelerate protons in a region comparable with the size of the Sun. Our goal is studying the proton acceleration at intermediate scales, between solar flare and interplanetary CME. We analyzed seven SEP events observed by the ERNE instrument on board SOHO during July 1996 September All events were associated with CMEs. Two proton events associated only with a CME, without Type II burst and flare, Solar Physics 190: , Kluwer Academic Publishers. Printed in the Netherlands.

2 296 LEON KOCHAROV ET AL. were very weak at 10 MeV. There were no events associated with gradual solar flares. We finally selected the five strongest events. For these reasons, all selected events are associated with both CME and preceding impulsive soft X-ray flare and Type II radio burst. In all selected events patrolled by the EIT/SOHO telescope, a coronal Moreton wave was also observed. We start with the nearly well-connected event of 9 July 1996 (Section 2) and then compare it with the disc event of 24 September 1997 (Section 3). Other events are discussed in Section The 9 July 1996 Event The 9 July :10 UT flare (S10 W30) occurred in the NOAA region 7978 on the western hemisphere of the Sun. The maximum of the impulsive X-ray flare (X2.6) was observed by GOES 8 and 9 at 09:12 UT. Due to a gap in LASCO observations, the CME was observed after it had arrived at 10 R (Pick et al., 1998). A relativistic electron event was recorded by the COSTEP instrument on board the SOHO spacecraft (Bothmer et al., 1997), shown in Figure 1. The energetic proton event was observed by our ERNE instrument on board SOHO (Figure 2). Available count statistics allows us to divide the proton data into 6 energy channels, MeV, 3 6 MeV, 6 12 MeV, MeV, MeV, and MeV, so that energy dependencies may be studied and the energy spectrum can be deduced. In order to study an injection scenario, one should de-convolve the interplanetary transport from the observed intensities with the use of particle anisotropy data. An analysis of the proton anisotropy for this event was presented by Torsti et al. (1997). We apply the same method but use a slightly wider energy channel, MeV, in order to enhance statistics. The analysis is performed for the period until 12:50 UT 9 July 1996, when the spacecraft entered a new magnetic flux tube with very different proton transport conditions (Kocharov et al., 1997). The magnetic field direction was acquired from the MFI instrument on board the WIND spacecraft. A difference of the magnetic field resulting from the angular separation of the spacecraft was accounted (for more details on the fitting procedure see Appendix). Final theoretical fits to proton intensity-time profiles are shown in Figure 2. The deduced proton injection functions are presented in Figure 3. The double peak structure of the event is clearly seen, excluding only the lowest energy channel. For those two peaks, we shall adopt the naming convention of Torsti et al., (1996) and name the 1st and 2nd peaks of injection as p- and d-injections, respectively. We start the plots at the point where the maximum contribution to the flux at Earth distance exceeds the 3σ level above background. The portion contributing only to the period after entering another magnetic tube at 12:50 UT is also left out. The d-peak contribution to the MeV injection rate can t be uniquely deduced and, for this reason, is not shown. The injection maximum of the p-component occurred around 09:58 UT 500 s. The variance of the p- injection maximum in Figure 3 illustrates statistical uncertainties, otherwise there

3 POST-IMPULSIVE-PHASE ACCELERATION 297 Figure 1. The MeV electron intensity profile as observed by COSTEP/SOHO instrument (fluctuating curve). Expected arrival of first electrons from the flare X-ray maximum is shown with line A, the start of the new magnetic tube with line B. The dashed curve represents the best theoretical fit with a constant radial mean free path at a single pulse of solar injection. Curve E1 corresponds to the impulsive injection at 09:26 UT 500 s, curve E2 is due to the second (minor) pulse at 09:58 UT 500 s, the thick line is the sum of these two pulses. A choice of mean free path for the three latter curves (E1, E2 and E1+E2) is explained in Appendix. is no energy dependence of the maximum time. In contrast, the maximum time of the d-injection is clearly energy dependent, being an increasing function of energy. Deduced proton energy spectra at the Sun (amplitudes A 1 and A 2 in Equation (1)) are shown with closed symbols in the left panel of Figure 4. It is seen that the d-component spectrum is essentially steeper than the p-component one. The best power-law fits correspond to the spectral indexes 3.6 and 5.5 for the p- and d-components, respectively. The electron event onset indicates injection of the first electrons probably several minutes after the soft X-ray maximum (expected arrival of the first electrons injected at the X-ray maximum time, 09:12 UT 500 s, is indicated by vertical line A in Figure 1). The kev electron intensity curve has a double peak structure, although the dip between the two sub-peaks is rather shallow. Entering the new magnetic tube, clearly seen in proton data at 12:50 UT, is only marginally observable in electrons (vertical line B in Figure 1). Details of the electron fitting are given in Appendix. The electron event could not be precisely fitted with a single impulsive injection. As a second approximation, we fitted the event with a two pulse injection, E1+E2. However, the very early onset is still left out of the fitting. The main injection E1 occurred at 09:26 UT 500 s, i.e., 14 min after the soft X-ray maximum, and was followed by a minor injection, E2, around

4 298 LEON KOCHAROV ET AL. Figure 2. Observed (points) and theoretical (curves) proton intensities in six energy channels for the 9 July 1996 event. Figure 3. Near-Sun injection rate profiles for protons. The injection rate is in units protons min 1 MeV 1 per solar hemisphere.

5 POST-IMPULSIVE-PHASE ACCELERATION 299 Figure 4. Proton energy spectra of 9 July 1996 (left panel) and 24 September 1997 (right panel). Triangles in both panels are for the first (p-)component of proton production. Circles are for the second (d-)component production. Closed symbols indicate proton injection spectra deduced by the comprehensive fitting of the 9 July 1996 event (Section 2); the injection spectrum unit being protons min 1 MeV 1 per solar hemisphere. Lines indicate best power-law fits to the points. Open symbols are for estimates (Section 3), being presented in units of the near-earth intensity, 1 cm 2 sr 1 s 1 MeV 1 ). Open triangles represent spectra of protons that have traveled not more than 2 AU from the first injection time. For the 9 July 1996 flare, the first injection time is not delayed in respect to the flare maximum time. For the 24 September 1997, the delay is 35 min (Torsti et al., 1999a). Open circles represent the maximum intensity spectra. 09:58 UT 500 s. The latter is close to the maximum time of the first (p-) peak of proton production. 3. Comparison of 9 July 1996 and 24 September 1997 Events As an illustration of post-impulsive-phase acceleration well away from the eruption center, we consider the 24 September 1997 event (Torsti et al., 1999a). This event was associated with a CME and impulsive flare (M5.9, t max = 02:48 UT) centered at S31 E19. A large difference in the longitudes of the parent solar eruptions caused a difference in the onset times of corresponding SEP events. This difference is illustrated in Figure 5. In the upper panel, we present the intensity-time profiles of two events in the proton energy channel of MeV. The time is measured from the maximum time of a flare at the Sun, 09:12 UT 500 s and 02:48 UT 500 s for 9 July 1996 and 24 September 1997, respectively. Relativistic electrons were

6 300 LEON KOCHAROV ET AL. Figure 5. Comparison of the MeV proton (upper panel) and MeV electron (lower panel) intensity time profiles of the two events. The time is measured from the flare maximum time at the Sun. The thick curves represent the 9 July 1996 event, the thin curves are for 24 September Expected arrival times of flare-injected 32 MeV protons and 0.5 MeV electrons after traveling 1.2 AU are marked with corresponding vertical lines A. Delays in respect to this line indicate that particles were injected after the flare. also observed in the 24 September 1997 event. We use the same energy channel as for 9 July 1996 to compare these two events (lower panel of Figure 5). It is seen that the onset of the 24 September SEP event is delayed, by about 20 min with respect to the 9 July one. The studies of the 24 September 1997 event did not include direct fitting of particle injection and interplanetary transport. The onset spectrum introduced by Torsti et al. (1999a) was obtained by assuming an impulsive injection of protons at 03:23 UT 500 s. Under this assumption, one can estimate the injection spectrum in the very beginning of the event by selecting particles that have traveled equal distances at different energy channels. Such a spectrum may not exactly coincide with the source injection spectrum deduced by the direct fitting of a prolonged injection event, while identical methods should be employed for a comparative study of different events. For this reason, we have additionally estimated the spectrum of first protons in the 9 July 1996 event using exactly the same method as that employed by Torsti et al. (1999a). The result is shown in Figure 4. The best power law fit to the onset energy spectrum in the 9 July event corresponds to a spectral index of 3.1. This is close to but slightly less than the value of 3.6 deduced for the p-component protons in Section 2. Similar to Torsti et al. (1999a), we also plot the maximum intensity spectrum. The 9 July maximum spectrum has been picked up before entering the new magnetic tube at 12:50 UT. The abrupt change

7 POST-IMPULSIVE-PHASE ACCELERATION 301 in magnetic environment might affect the determination of the maximum intensity in the two lowest energy channels, because a full maximum might be not reached before 12:50 UT. Thus the corresponding maximum intensities are shown as lower limits. In both events, spectra of the first protons are essentially harder than corresponding spectra at the intensity maximum, a spectral softening was observed between the first proton injection and the maximum intensity time. The period of the hard-spectrum injection in the 24 September 1997 event was associated with production of relativistic electrons, much like in the 9 July 1996 event. Torsti et al. (1999a) reported a coronal Moreton wave (EIT wave) associated with the 24 September 1997 flare and CME. The speed of the wave was estimated to be 325 km s 1. At 03:23 UT 500 s, when the first SOHO-observed particles were injected near the Sun, the leading edge of the wave had already passed the 50 longitude, approaching to the root of the nominal SOHO-connected interplanetary magnetic field line. The EIT wave is considered as a visual signature of an expansion in solar corona that gave rise to the proton event. The angular expansion of the disturbed region in solar corona, being associated with EIT wave and CME, is regarded as a physical reason for the delay in the SEP event onset. However, the EIT-observed region may not exactly coincide with the acceleration site, but most likely is situated beneath it. The 9 July 1996 flare was missed by both EIT and LASCO instruments on board SOHO, so that images are available for a period > 3 h after the flare. Pick et al. (1998) carefully studied projected displacement of selected CME features in the 9 July 1996 event as observed with LASCO coronographs. The LASCO data are available for the period after 12:28 UT, and an extrapolation is needed to deduce position of the CME during the first three hours. According to Pick et al. (1998), the extrapolated CME launch time was within the first hour following the flare start. The velocity of the CME was not high, 400 km s 1. At the peak time of the p-component proton production, the CME could not be farther than 2 R from the flare site (see Figure 8 by Pick et al., 1998). These estimates indicate that the p-component particles were accelerated below 2 R in association with the CME lift-off. 4. Discussion and Conclusions Observational evidences for the post-impulsive phase proton acceleration were previously reported for the outstanding high-energy neutron event of 24 May 1990 (Shea, Smart, and Pyle, 1991; Debrunner, Lockwood, and Ryan, 1993; Kocharov et al., 1994; Torsti et al., 1996; Kocharov et al., 1996, and references therein). The X9.3 flare (N33 W78) started with a short pulse of X- and γ -ray emissions. A simultaneous impulsive production of high-energy neutrons was a signature of highenergy protons precipitating into the solar chromosphere. This pulse of nuclear interactions was followed by a more prolonged production of neutrons. Concurrent

8 302 LEON KOCHAROV ET AL. injection of protons into the interplanetary medium during 1.5 hr was responsible for the first peak in the SEP flux. The first proton production was directly preceded by a Moreton wave (BBSO observations) and accompanied by Type II and IV radio burst. The observed velocity of the Moreton wave was used to estimate the heliocentric distance of the first proton acceleration, 2 R. About 2 hr later, a second injection maximum occurred. A CME-driven bow shock acceleration is a generally accepted source of this second interplanetary proton component. The observed interplanetary proton intensity-time profile in that event was qualitatively similar to the profile of the 9 July 1996 event, while the magnitudes of proton fluxes were very different (Kocharov et al., 1999). On the other hand, it was observed in some events that SEPs may have a rapid access to coronal longitudes that are well-removed (>100 ) from the flare center, and such an extreme propagation occasionally corresponds to the visible chromospheric Moreton waves (Cliver et al., 1995). It was even suggested that the coronal shocks may be essentially circumsolar in azimuth. However, the Hα observations probably provide a less sensitive diagnostic of the wave as compared with the EUV observations of EIT on board SOHO. We have examined in detail the 9 July 1996 event and find clear evidences for a two component proton production after the flare impulsive phase: (i) the double-peak structure of the > 6 MeV proton event; (ii) a change in the electronto-proton ratio; and (iii) a change in the proton energy spectrum. The 9 July 1996 SEP event was associated with the LASCO-observed CME (Pick et al., 1998). The CME is most likely to be launched during the period of the post-impulsivephase acceleration. Launch of a CME implies a huge impulse imposed on solar corona. We suggest that, if the impulse has been imposed during a sufficiently short time, a coronal wave is produced. The wave traverses an extended coronal region and gives rise to the post-impulsive-phase proton acceleration in a wide range of solar longitudes. Observations of the angle-distant event of 24 September 1997 revealed the initial injection of >10 MeV protons during the period when the coronal Moreton wave was traversing the western hemisphere of the Sun, being an early signature of the CME launch. Acceleration of the CME-associated protons starts during the CME lift-off, while the main proton production occurs several hours later, when the CME expands in the interplanetary medium. Between the first proton production and the maximum intensity time, a spectral softening is observed. Qualitatively similar patterns were also observed in the angle-distant events of 7 April, 12 May 1997, and August 1996 (Torsti et al., 1998, 1999b). These observations indicate that the first acceleration starts near the Sun in a wide range of solar longitudes. The major proton production, however, occurs several hours later, when the CME expands in the interplanetary medium. Thus the post-impulsive-phase proton acceleration, taking place in solar corona during 100 min after the impulsive phase, is associated with CME lift-off. Properties of this acceleration, however, differ from properties of the delayed CMEdriven interplanetary shock acceleration. A physical mechanism of the post-impul-

9 POST-IMPULSIVE-PHASE ACCELERATION 303 sive-phase acceleration is not known. Coronal shocks seem to be the most plausible candidate for the acceleration because of Type II radio bursts and coronal Moreton waves observed. We conclude the following: (1) There is a post-impulsive-phase acceleration between impulsive flare acceleration and delayed CME-driven interplanetary shock acceleration. (2) Post-impulsive-phase acceleration produces a harder proton spectrum than that produced by the delayed acceleration. (3) Relativistic electrons are also accelerated, and the electron-to-proton ratio is higher for the post-impulsive-phase acceleration than for the delayed acceleration. (4) Post-impulsive-phase acceleration regions may stretch up to above 90 in each direction from the eruption center. (5) Being observed at a large angular separation from the eruption center, the acceleration delay seems consistent with coronal Moreton wave transit time. The conclusions have been drawn for SEP events associated with impulsive soft X-ray flare, Type II radio burst and CME. Acknowledgements We are grateful to Dr R. P. Lepping for permission to use the magnetic field data of MFI/WIND. We thank COSTEP team for the electron data available in the SOHO archive. SOHO is an international co-operation project between ESA and NASA. Present investigation was supported by the Academy of Finland. Appendix. Injection and Transport Model for the 9 July 1996 Event Because of a double-peak structure of the observed proton event, we fit the intensitytime profiles with two injection components. Thus the injection function used is in the form q(t,e) = A 1 e 1(t) + A 2 e 2(t), (1) i (t) = D i (t t i ) at 0 <t<t i, i (t) = (t t i )/τ i at t>t i. (2) The fitting parameters, D i, τ i and t i, rule the injection rise, decay and the maximum injection time, respectively. We fit each energy channel separately in order to obtain the energy dependence of the injection. The normalization factors A i determine the injection spectra for the p- an d-components, i = 1, 2, respectively. The proton anisotropy data, in a form of pitch-angle distribution, are shown in Figure 6. The cosmic-ray background of protons cm 2 sr 1 s 1 MeV 1,

10 304 LEON KOCHAROV ET AL. Figure 6. Proton anisotropy measurements for the first (10:14 12:50 UT) period of the 9 July 1996 event (points). Also two theoretical fits to the data: contribution of a large-angle scattering f = 0.85 and f = 0 for the solid and dotted curves, respectively. Insertion: an illustration of finding the best fits (f = 0.85, 0 = 1.1 AUandf = 0, 0 = 5.4 AU) to the ERNE anisotropy measurements. The χ 2 for the first period proton angular distribution is plotted as a function of the near-earth radial mean free path for the standard model, f = 0(dotted curve), and also for the composite model with varied contribution of a large angle scattering, 0.05 <f <0.9(solid curve). determined from the flux 8 hr before the event, has been subtracted. A very high anisotropy implies that a proton mean free path in the interplanetary medium exceeds 1 AU. For this reason, adiabatic deceleration of > 1.6 MeV protons during their passage from Sun to near Earth has been neglected. In every other respect, our initial transport code was similar to that described by Kocharov et al. (1998). We use for parameterization the near-earth radial mean free path of 20 MeV protons, 0. As a first attempt to fit the data, we had suggested that the radial mean free path is constant throughout the interplanetary medium, r = constant, and scattering had been modeled as a pitch angle diffusion in the conventional slab model with spectral index of interplanetary turbulence q = 1.5 (Wanner and Wibberenz, 1993; Kallenrode, 1993; the Anisotropic small-angle Scattering or AAS model by Kocharov et al., 1998). However, after fitting the MeV electron intensity time profile (see below) we slightly corrected spatial dependence of the mean free path for both electrons and protons, so that we finally adopted a constant radial mean free path beyond the Earth, but a constant parallel mean free path inside the Earth s orbit, = 0 / cos 2 constant, being the magnetic field tilt angle at

11 POST-IMPULSIVE-PHASE ACCELERATION AU. The near-earth radial mean free path, 0, had been varied, along with above mentioned injection parameters, and the best fit to the observed angular distribution of MeV protons and to their intensity time profile was found. The dotted line in Figure 6 illustrates the best-fit angular distribution in the AAS model, while a dotted line in the insertion shows a sum of normalized squared deviations of the experimental points from the theoretical pitch-angle distributions for different values of the parameter 0, χ 2 ( 0 ). It is seen that, in the conventional pitch-angle diffusion model, the best fit corresponds to a huge value of the radial mean free path 0 = 5.4 AU. However, even at this value of 0, the theoretical curve in Figure 6 misses experimental points at pitch angle cosines µ < 0.7. To fit those points, we have introduced a composite scattering model where conventional pitch angle diffusion is supplemented with a large angle scattering. The latter is modeled as Small time-step Isotropization (SSI-model by Kocharov et al., 1998). A large-angle scattering may be important in wave-particle interactions (Quest, 1988). It can be also suggested that irregularities in the interplanetary magnetic field comprise not only weak Alfvén waves but also other irregularities and discontinuities which might cause a large-angle scattering. We introduce a scattering frequency which comprises two terms, ν = ν D + ν L, where two scattering processes are suggested: (i) pitch angle diffusion with corresponding partial mean free path D {ν D }, and (ii) small-chance isotropizations with corresponding partial mean free path L {ν L }, D = 3V µ 2 ν D dµ, L = V ν L. (3) We have adopted the following parameterization for the partial mean free paths: L = 0 f cos 2, D = 0 (1 f)cos 2, (4) where f designates the fraction of the large-angle scattering. Consequently, 1 0 = 1 D cos L cos 2. (5) The parameter 0 would be a radial mean free path also in the composite model, if no interference between the scattering processes existed, but because of the interference, 0 is slightly less than the radial mean free path r (1AU). However, 0 exactly coincides with r (1AU) if f = 0orf = 1. The pitch-angle distribution shown with solid curve in Figure 6 illustrates the best fit in the composite model of interplanetary scattering. The solid curve in the insertion shows minimum values of χ 2 obtained by varying f at a fixed value of 0. Corresponding best-fit values of f for several selected 0 are also shown. A χ 2 well in Figure 6 enables one to select the best-fit parameter, 0, and corresponding value of the fraction f. We finally adopt the values 0 = 1.1 AUandf = 0.85.

12 306 LEON KOCHAROV ET AL. The corresponding theoretical angular distribution exhibits a kind of wing which fits reasonably well the large pitch angle points. A fitting procedure suggests finding a best fit to both angular distribution and intensity time profile. When fitting intensity time profiles one should take into account that the SOHO is a three-axis stabilized spacecraft and thus ERNE s particle detectors do not cover the whole 4π solid angle, and magnetic field direction is not stable. In order to take this into account we calculate the differential acceptance of the detector as a function of time and pitch angle and convolve it with interplanetary transport function to get a value of the proton intensity being averaged over the instrument acceptance cone. The resulting intensity curves are directly comparable with the experimental intensities. The best-fit curves are plotted in Figure 2. To find the electron injection scenario, interplanetary mean free path should be ascertained also at electron rigidities. However electron anisotropy data are not available, while extrapolation of the mean free path from proton rigidity range to electron rigidities is risky. For these reasons, the following method has been employed. The late phase of the relativistic electron event (Figure 1) looks much like a diffusion decay tail. The observed decay rate, being entirely ascribed to the interplanetary propagation, corresponds to the near-earth radial mean free path parameter (e) 0 = 0.13 AU for 0.4 MeV electrons. This interpretation implies rather isotropic electron distribution after the intensity maximum. By a lucky chance, an abrupt scanning of the interplanetary magnetic field direction gave us an opportunity to verify this point. Interplanetary magnetic field direction abruptly changed at 12:50 UT by more than 70 (Torsti et al., 1997), so that electron sampling into the narrow view cone of COSTEP could be strongly affected if the electron flux was as anisotropic as the proton flux was. However only a minor change in the electron count rate was observed (marked with vertical line B in Figure 1). This implies the electron angular distribution being consistent with our estimate (e) 0 = 0.13 AU. While the simplest model with exactly constant radial mean free path fits the electron event decay, the expected shape of the rising portion of the intensity curve is more rounded than the observed one (see dashed curve in Figure 1). For this reason, we have adjusted interplanetary transport model by choosing a constant parallel mean free path between Sun and Earth (the constant radial mean free path is still employed beyond the Earth s orbit). The same spatial dependence of the mean free path is adopted for protons. A composite angular scattering model for electrons is also identical to protons, i.e. we employ for both electrons and protons f = The thick solid curve in Figure 1 has been calculated using these angular and spatial dependencies of interplanetary scattering. It fits well both rise and decay of the electron event. Calculated rise of the electron event is very sensitive to the choice of the solar injection time t E1, so that estimated uncertainty is only ±2 min. However, determination of the minor injection pulse, E2, is difficult. Estimated uncertainty of t E2 is about 5 min. For protons, highest experimental uncertainties are expected for the low energy channels, MeV and 3 6 MeV. On the lowest energy channel the

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