On the source and acceleration of energetic He + : A long-term observation with ACE/SEPICA

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A10, 8040, doi: /2003ja009938, 2003 On the source and acceleration of energetic He + : A long-term observation with ACE/SEPICA H. Kucharek, 1 E. Möbius, 1 W. Li, 1 C. J. Farrugia, 1 M. A. Popecki, 1 A. B. Galvin, 1 B. Klecker, 2 M. Hilchenbach, 3 and P. A. Bochsler 4 Received 15 March 2003; revised 3 July 2003; accepted 23 July 2003; published 23 September [1] We report on a systematic study of the He + /He 2+ abundance ratio in the energetic population during the years For the investigation we have used data in the energy range MeV/n from SEPICA on board ACE and from CELIAS STOF on SOHO in the energy range MeV/q. The ratio is quite variable, with values up to 1. Over the entire time period the integral abundance ratio of the energetic population is 0.06, which exceeds the abundance of He + in the solar wind and corona by several orders of magnitude and even surpasses substantially the average relative contribution of He + pickup ions. This requires preferential injection and acceleration of He + over He 2+. In a case study of a CME with plasma rich in He +, which drives a shock and is being overtaken by another shock, the largest enhancement is found near the driven shock way ahead of the ejecta. In addition, CMEs which are He + rich are very rare. This implies that the source of the energetic He + is primarily interstellar pickup ions and not ejecta material. However, signatures of large-scale structures in the interstellar source, such as the gravitational focusing cone, have not been identified in this survey. Enhanced He + /He 2+ ratios are associated with interplanetary structures in the solar wind, such as stream-stream interfaces and interplanetary traveling shocks. In fact, more than 90% of all shocks and stream interfaces show ratios that exceed our lower limit of This association points to local acceleration of interplanetary He +, whose injection and acceleration efficiency may vary substantially from event to event, thus probably washing out large-scale structures of the source strength. While the enhancements tend to be very localized at interplanetary traveling shocks, they are much more prolonged in the case of CIRs. INDEX TERMS: 2102 Interplanetary Physics: Corotating streams; 2109 Interplanetary Physics: Discontinuities; 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; 2152 Interplanetary Physics: Pickup ions; 2139 Interplanetary Physics: Interplanetary shocks; KEYWORDS: pickup ions, interplanetary disturbances, energetic particles Citation: Kucharek, H., E. Möbius, W. Li, C. J. Farrugia, M. A. Popecki, A. B. Galvin, B. Klecker, M. Hilchenbach, and P. A. Bochsler, On the source and acceleration of energetic He + : A long-term observation with ACE/SEPICA, J. Geophys. Res., 108(A10), 8040, doi: /2003ja009938, Introduction [2] The first study of the helium charge state abundance and its relation to solar cosmic-ray events was performed by Hovestadt et al. [1984], using the UltraLow Energy Z-E-Q analyzer (ULEZEQ) on ISEE 3, which was located near L1 point upstream of the Earth. They found large enhancements of the He + /He 2+ abundance ratio above expected solar wind and coronal values especially during periods of relatively low interplanetary particle flux. The ratio was between Department of Physics and Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire, USA. 2 Max-Planck-Institut fuer extraterrestrische Physik, Postfach, Garching, Germany. 3 Max-Planck-Institut fuer Aeronomie, Katlenburg-Lindau, Germany. 4 University of Bern, Bern, Switzerland. Copyright 2003 by the American Geophysical Union /03/2003JA and 1.0 for several days during the years (near maximum solar cycle 21). No obvious association with optical flare events could be detected for events with high He + abundance. Assuming that the solar wind could be the only source for He + where normal He + /He 2+ ratios would be of the order of 10 6 [Kozlovsky, 1968], the observed values in the energetic particle population are enormous. On rare occasions, however, solar wind plasma in CMEs (Coronal Mass Ejections) has been observed to have the significantly enhanced He + /He 2+ values of [Schwenn et al., 1980; Gosling et al., 1980; Zwickl et al., 1982, Burlaga et al., 1998]. As the most likely source of these ions an admixture of cold solar material was suggested. However, heavy ions during solar energetic particle events with high He + abundance showed no indication of low ionic charge states [Hovestadt et al., 1984]. Thus a mixture of cold and hot solar material emitted from the Sun could not explain the origin of the unusually high He + abundance in the energetic particle population. LIS 15-1

2 LIS 15-2 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + [3] The detection of interstellar pickup He + in the inner heliosphere [Möbius et al., 1985] introduced another potential source for particle acceleration in interplanetary space. These ions had been suggested earlier by Fisk et al. [1974] as the source of the anomalous component of cosmic rays. Pickup ions as a source for energetic particles were first confirmed for a corotating interaction region (CIR) at 4.5 AU by Gloeckler et al. [1994]; using data from Ulisses/SWICS, they identified interstellar pickup He + as the major contributor to the suprathermal He + in CIR s for energies up to 60 kev. He + was also found as part of the suprathermal CIR population at 1 AU up to 0.2 MeV/q with SOHO/STOF [Hilchenbach et al., 1999] and with Wind/ STICS [Chotoo et al., 2000]. These observations led to the suggestion that pickup ions may constitute a generally important source of suprathermal ions for further acceleration at interplanetary shocks [Gloeckler, 1999]. For a recent series of unusual CIRs close to solar maximum 2000, Möbius et al. [2002] have reported a substantial fraction (10 30%) of He + at MeV/n, which they attributed to interstellar pickup ions. For the same CIR events, Morris et al. [2001] showed that the He + /He 2+ ratio increases as time elapses from the beginning of the CIR. They interpret this observation with the increase of the amount of interstellar He + pickup ions increases over solar wind He 2+ as a function of the distance from the Sun. [4] In this paper we will present a survey of the energetic He + /He 2+ ratio in the energy range MeV/n as determined with ACE/SEPICA during the 3-year period We will identify the sources and the interplanetary configurations associated with enhancements of the ratio. The paper is organized as follows: After a brief introduction on the particle sensors, spacecraft, and analysis method used in this investigation, we present a survey of the energetic He + /He 2+ ratio. We then investigate the possible sources of the He + and the potential causes for the variability of the helium abundance ratio. By using examples of interplanetary configurations which emerge as the most representative for the He + /He 2+ ratio enhancements in this survey, we discuss evidence that He + pickup ions are the main source of the energetic component and that these ions are locally accelerated in interplanetary space. Finally, we discuss the implications for injection and the acceleration of He + pickup ions at interplanetary shocks. 2. Particle Sensors and Spacecraft [5] The Advanced Composition Explorer (ACE) was launched in August 1997 into a halo orbit around L1. The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) is the main instrument on ACE for determining the ionic charge states of solar and interplanetary energetic particles in the energy range 0.2 MeV/n to 5 MeV/q using three independent fan-shaped telescopes. The charge resolution is achieved by focusing the incoming ions through a multislit mechanical collimator, deflecting them in an electrostatic analyzer with a voltage up to 30 kv, and measuring the impact position in the detector system. The nuclear charge (element) and energy of the incoming ions are determined by a combination of thin-window flow-through proportional counters with isobutane as counter gas and ion implanted solid state detectors in a E (energy loss) versus E (residual energy) telescope configuration. For more details about the sensor we refer the reader to the instrument description [Möbius et al., 1998b]. Magnetic field data are obtained from the ACE/MAG instrument [Smith et al., 1998]. Solar wind parameters are provided by ACE/SWEPAM [McComas et al., 1998] and by the Proton Monitor (PM) [Ipavich et al., 1998] on board the SOlar and Heliospheric Observatory (SOHO), also located at L1. Data from the STOF sensor, which is also on SOHO, extend our survey of the energetic He + /He 2+ ratio to lower energies ( MeV/q). The STOF sensor consists of a curved plate energy/charge analyzer, a time-of-flight system, and a solid-state detector to determine the energy, mass, and ionic charge of the incoming ions [Hovestadt et al., 1995]. 3. Data Analysis [6] For this study we have determined the He + /He 2+ ratio by using the pulse height analysis (PHA) data of SEPICA for the time period We use the resulting charge state distribution of the helium ions in order to determine the He + /He 2+ ratio by fitting a double Gaussian function to the distribution. Figure 1 shows a typical charge state distribution for helium as derived from the PHA data over a 12-hour time period on DOY (day-of-year) 120, The open diamonds represent the observations, and the solid line represents the best fit. As a quality check on the fit, we calculate the total number of counts by integrating the fitted function and compare this number with the total number of the actual counts in the distribution. We require the difference between these two numbers to be less than 1%. Another criterion for the goodness of the fit is the position of the peaks in the charge state distribution. For this analysis we have only used distributions whose calculated maxima of He + and He 2+ falls within Q 0.3 of their nominal positions. [7] For the survey we have used the full energy range of MeV/n. The instrument clearly separates the charge states of helium in the energy range of our study, and without further correction of the original calibration the peaks for both charge states normally fall within 5% of the nominal values. The width of each peak represents the charge state resolution of the low-resolution fan of SEPICA, i.e., Q/Q = 0.3 in this energy range. As can be seen from the figure, SEPICA provides excellent counting statistics. [8] The initial time resolution of twelve hours was motivated by similar work by Klecker et al. [2001], who investigated the variation of the suprathermal He + /He 2+ abundance ratio in the energy range of MeV/q with SOHO/CELIAS/STOF for the years 1997 to We use this long integration time to compare our results with those obtained by STOF at lower energies. For the rest of the paper we will use a dynamically adjusted integration time. [9] In order to study the exact timing in relation to interplanetary parameters and evolution of these events, a better time resolution is needed. Therefore we introduce a scheme which provides the highest time resolution for any given particle flux. The integration time for the charge state distribution is chosen such that a minimum counting statistics with a total number of 3000 counts is achieved. The uncertainty is lowest (3%) for the highest abundance ratio of

3 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + LIS 15-3 [14] Finally, we have computed the ratio of the integrated fluxes of He + and He 2+ over the entire time period of our survey, which also includes times with low He + abundance. The result for the integrated ratio is This surprisingly high value implies that He + is indeed a substantial component in the interplanetary energetic particle population and, after H + and He 2+, the third most abundant ion species. The key questions at this point are what are the possible sources of the energetic He + and what causes the observed variability? Figure 1. A typical helium charge state distribution of a twelve hour time period on DOY 120, The symbols are the measured values, while the solid line indicates the fitted curve. He + /He 2+ (about 1) and highest (10%) for lower ratios of 0.1 or less. [10] For ratios less than 0.04, the statistical error is of the order of the height of the He + distribution above the wings of the He 2+ distribution. Therefore we adopt 0.04 as the lower limit for this study. To go to a lower minimum value would require a higher number of total counts for the charge state distributions and this, in turn, would lead to longer integration times. [11] Time periods for which fitting the charge state distribution does not meet all of these criteria are not included in this survey. Figure 2 shows the He + /He 2+ abundance ratio in a logarithmic representation as a function of time from 1998 to 2000 using the analysis method described above. The data gap in 1999 is due to a problem with the pressure control valve for the proportional counter. During this time, SEPICA did not return any useable data. [12] The survey result looks very similar to the earlier result of Hovestadt et al. [1984] using ISEE 3 data and covers approximately the same dynamic range in the He + / He 2+ ratios. However, the survey presented here has much better counting statistics and thus achieves better time resolution, in particular, for events with high He + abundance. [13] The helium abundance ratio can be extremely variable, with the ratio varying over nearly two orders of magnitude and maximum values are of the order of 1. Both the duration as well as the time profiles of the enhancements vary substantially from event to event. Of course, the bin size also varies, due to the dynamic allocation of the integration time, but this is not the cause for the variable time profiles. The intensity of these events seems to change from enhancement to enhancement and at first sight they appear to have no common signatures. 4. Source Population for Energetic He + [15] Two possible sources for He + are plasma from cold solar, most likely chromospheric, material and interstellar pickup ions. In the following we will examine these two sources in more detail Solar Wind [16] The integrated abundance of He + in the energetic distribution is He + /He 2+ = In contrast, solar wind values are of the order of 10 6 [Kozlovsky, 1968] for typical coronal temperatures. Any higher ratios usually indicate a significant contribution of cold material from the Sun with T K. Only on rare occasions have significantly enhanced He + /He 2+ values of up to been observed in the solar wind during CME passages [Schwenn et al., 1980; Gosling et al., 1980; Zwickl et al., 1982; Skoug et al., 1999; Burlaga et al., 1998]. Average solar wind values that include these rare events remain several orders of magnitude below the observed ratio in the energetic population Coronal Mass Ejections [17] Let us now explore whether these rare enhancements indeed contribute to the observed energetic He + population, at least occasionally. In a recent publication, Skoug et al. [1999] reported a prolonged enhancement of He + of more than 24 hours duration during the passage of a CME on 2 4 May (DOY ) 1998 with ACE/SWEPAM. They found a He + /He 2+ ratio that exceeded 0.05 most of the time Figure 2. A logarithmic representation of the He + /He 2+ abundance ratio from 1998 to 2000 determined from measurements from ACE/SEPICA in the energy range of MeV/n. The horizontal line at 0.04 marks the lower limit of He + /He 2+ ratio.

4 LIS 15-4 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + Figure 3. The solar wind plasma and magnetic field conditions for 24 April to 5 May From top to bottom the panels show the He + /He 2+ ratio from SEPICA and from SWEPAM, the solar wind speed, the proton density, the thermal speed from SOHO/PM, and the magnetic field components from ACE/MAG. and reached values as high as 1. They concluded that He + is cold solar prominence material which fills a large fraction of the volume of the CME. This is one of the very rare occasions when a CME contains a substantial amount of cold material from the Sun. [18] Figure 3 shows a comprehensive plot of the time period of DOY , 1998, which includes the passage of the CME. Shown from top to bottom are the He + /He 2+ ratio as seen by ACE/SEPICA in the energy range MeV/n, the He + /He 2+ ratio as observed by ACE/

5 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + LIS 15-5 SWEPAM at solar wind energies, and the solar wind velocity, density, and thermal speed as measured by the Proton monitor (PM) on board the SOHO spacecraft. The bottom three panels show the components of the interplanetary magnetic field measured by the magnetometer on the ACE spacecraft. [19] Three shocks and a CME-related magnetic cloud event have been reported in this time period. During DOY 122 through 123 the magnetic field shows a rotation while the plasma kinetic temperature is relatively low. This is a strong indication of a magnetic cloud (indicated by the gray shaded area) passing the spacecraft [Burlaga et al., 1981]. The three shocks encountered in this time period are indicated by vertical lines. A very significant enhancement of the energetic He + /He 2+ ratio is seen at the shock arrival on DOY 120 (30 April). Note that this shock is not associated with the CME. At the second shock, which is driven by the CME [Farrugia et al., 2002], we also see an enhancement of the energetic He +, although it is less pronounced than at the previous shock. It should also be pointed out that the cloud associated with the CME has not yet arrived. In the cloud itself we find a large amount of He + in the SWEPAM data (energy range of about 1 kev/n) but no significantly enhanced values of the energetic He + /He 2+ ratio. Actually the only, very moderate, enhancement occurs in the later part of DOY 123 at the interface of the fast stream presumably overtaking the cloud [Farrugia et al., 2002]. If singly charged helium from cold solar material were to be efficiently accelerated, one would expect the largest enhancement of the abundance ratio to occur at this latter shock. While at this time a small contribution of cold solar material to the energetic He + /He 2+ ratio cannot be excluded, much more prominent enhancements of the ratio are observed at the preceding shock with no cold solar wind material present. Therefore even during this unusual CME with an extraordinary amount of He + in the solar wind, this solar material is not the dominant source of the energetic He + population Interstellar Pickup Ions [20] Apparently, interstellar pickup ions must be the major source of the energetic He + population, which are already known to be preferentially injected and accelerated at shocks. Using SOHO CELIAS STOF/HSTOF data, Bamert et al. [2002] recently suggested pickup ions as a possible nonsolar source of energetic He + as seen in the event discussed above. For the remainder of the paper let us therefore assume that pickup ions are the main source for the energetic He + population Overall He + Pickup Ion Abundance [21] According to Rucinski et al. [1996], the ratio of pickup He + to solar wind He 2+ at 1 AU typically lies in the range between 0.5 and , with the highest ratios reached in the gravitational focusing cone of the interstellar helium flow. However, even the maximum ratio during the cone crossing falls short by a factor of about 40 of the observed average He + /He 2+ ratio in the energetic particle population. Therefore a substantial enrichment in He + has to occur during injection and acceleration of these particles to the observed energies. This huge enhancement of the ratio also suggests that probably a large fraction of the variability in the ratio arises during injection and acceleration Large-Scale Variations [22] Let us first concentrate on the question whether largescale interplanetary structures, such as the gravitational focusing cone [Fahr, 1973; Möbius et al., 1985], are at least partially responsible for the observed temporal changes in the singly charged helium intensity. The Earth passes through the focusing cone in early December every year. The observed increase in the local interstellar helium density is a factor of 5 10 depending on solar activity [Gloeckler and Geiss, 2003]. This could also be reflected in the abundance ratio of the energetic ion population as suggested by Kallenbach et al. [2000]. [23] In order to see whether large-scale structures, such as the gravitational focusing cone of interstellar helium, can be identified as part of the observed variations in the He + abundance, we performed a superposed epoch analysis with all the data of 1998 through Figure 4 shows the energetic abundance ratio for helium with the superimposed 12-hour averages of the He + /He 2+ ratio in the top panel. Because we use 12-hour averages here (with a much better counting statistics), ratios lower than our earlier cut-off of 0.04 are included. As can be seen, the He + /He 2+ ratios vary over more than two orders of magnitude, and no sign of the focusing cone (indicated by the vertical arrows) is evident. The lower panel shows the same data but with a 30-day sliding average applied to the combined He + and He 2+ counts before taking the ratio. We chose 30 days because this time period spans over more than one solar rotation and is still comparable to the half width of the focusing cone [Möbius et al., 1995]. Through this procedure any largescale structure should become more clearly visible. However, even in the averaged abundance ratios large jumps remain visible, which stem from individual time periods when the abundance ratios are high. At the same time, the data show no signature of the focusing cone. [24] Apparently, at higher energies the signature from this primary source variation is washed out and other processes are more important. For instance, pickup ion fluxes themselves vary substantially on many time scales. Second, the acceleration of pickup ions depends on several factors, such as the seed particle distribution and the acceleration efficiency of the accelerator. Indeed, if all shocks were identical in their injection and acceleration properties of pickup ions, an enhanced seed particle population would likely produce more energetic He +. However, the acceleration depends strongly on the shock strength, the shock normal angle, and the plasma conditions in the ambient plasma. Therefore the energetic particle intensity at this energy range, which is far beyond the injection energy, can vary by orders of magnitude and it no longer reflects the injection conditions. All these effects conspire to produce tremendous variations of the energetic He + /He 2+ ratio, as is indeed visible in the fluctuations in the top panel of Figure 4. These considerations make it plausible that the signature of the gravitational focusing cone is no longer visible Small-Scale Variations [25] Indeed, large variations of pickup He + fluxes and their energy spectra in the solar wind have been observed on many different time scales. The reasons for some of the variations have been identified, such as depletion of pickup ions in the antisunward part of the distribution during time

6 LIS 15-6 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + Figure 4. He + /He 2+ abundance ratio for the years shown as a function of time on a logarithmic scale. The first 35 days of each year are repeated after the end of the year (dashed line). The top panel shows the superposed 12-hour averages and the lower panel shows the ratios of 30-day sliding averages for the He + and He 2+ counts. The arrows indicate the enter position of the gravitational focusing cone in early December. solar wind speed. It might well be that the strong dependence of the ratios on the solar wind speed at low energies reflects a strong variability of the pickup ion density with the solar wind velocity or, more likely, it might be related to the variability of the thermal distribution of the solar wind helium and of the distribution of pickup ions injected into an acceleration process. [28] The variability of the abundance ratio of helium at higher energies could be due to the different injection efficiency of pickup ions compared to solar wind alpha particles into an acceleration process, such as diffusive shock acceleration. Interstellar pickup ions exhibit a shelllike distribution with a diameter of twice the solar wind speed in the solar wind frame. In contrast to pickup ions, solar wind alpha particles have a Maxwellian distribution with a width of the kinetic temperature. Therefore pickup ions are already suprathermal compared with solar wind ions, and suprathermal ions can be preferentially injected into an acceleration process [Jokipii and Giacalone, 1996; Scholer and Kucharek, 1999]. [29] At higher energies, however, dispersion and transport processes might be effective, and the signature from the injection process washed out. Apparently, in the variability of the abundance ratio of helium at higher energies there is neither an indication of a variability caused by the solar wind nor of the variation of the pickup ion density due to large-scale structures in interplanetary medium, such as the gravitational focusing cone. [30] The example of a CME with related shocks has shown that the He + enhancements are connected with periods of radial interplanetary magnetic fields [Gloeckler et al., 1995; Möbius et al., 1998a], variations in the ionization rate, variations in the cutoff of the He + spectra due to the nonzero inflow speed of neutral helium [Chalov and Fahr, 2000; Möbius et al., 1999], and variations due to solar wind compression regions [Saul et al., 2002]. However, large variations of the pickup He + fluxes still remain unexplained. Certainly, these variations of the source must feed into the final variations of the energetic particle population Injection Conditions [26] A systematic study of the He + /He 2+ ratio at lower energies ( MeV/q) for the years by Klecker et al. [2001], who used 12-hour averages of SOHO/ CELIAS/STOF data, showed an inverse correlation of the He + /He 2+ abundance ratio with the solar wind speed. A dependence on solar wind speed at lower energies could reflect a variation of the pickup densities with solar wind speed or it could be related to a variability of the thermal distribution of solar wind He 2+ which, in turn, would control the injection process or pickup ions injected into the acceleration process. [27] However, as shown in Figure 5, applying the same analysis to the ACE/SEPICA data at MeV/n, we found no significant correlation between the ratio and the Figure 5. He + /He 2+ abundance ratio plotted versus the solar wind velocity. The top panel refers to data from ACE/ SEPICA whereas the bottom panel refers to data from SOHO/CELIAS/STOF.

7 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + LIS 15-7 Figure 6. Solar wind plasma parameters and the total magnetic field for DOY 200, 1999 to DOY 200, From top to bottom, panels show the He + /He 2+ ratio from SEPICA, the solar wind speed and the proton density from ACE SWEPAM, and the magnetic field magnitude from ACE MAG. The onsets of all major He + enhancements with ratios exceeding 0.2 are marked by vertical lines. interplanetary discontinuities such as shocks. It is here where the actual injection and acceleration take place and where one should study the dependence of the He + /He 2+ ratio on interplanetary conditions. 5. He + Enhancements and Interplanetary Discontinuities [31] There are two principal reasons that may be responsible for the observed variations of the ratio: First, the possible sources for these ions, such as the Sun, the solar wind, and interstellar pickup ions, may provide a varying amount of He + and He 2+, as discussed in the previous section. Second, interplanetary disturbances such as shocks, corotating streams or coronal mass ejections (CMEs), which appear to be the possible origin of these enhancements, may vary in their injection and acceleration efficiencies. Therefore we continue with a survey of interplanetary disturbances, followed by a few case studies for selected events from this survey, in order to identify the interplanetary configurations which are associated with such enhancements Survey of Disturbances [32] In Figure 6 we show from top to bottom the He + / He 2+ ratio, the solar wind speed and density from ACE/

8 LIS 15-8 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + [36] Of the shocks and CIRs, 57 were not seen by our instrument because of data gaps. This leaves us with 91 shocks and CIRs. Only seven of these disturbances show a He + /He 2+ ratio lower than our minimum value of This means that 92% (84 events) of these disturbances are associated with enhanced He + abundance. We then sorted them according to the value of the helium abundance ratio. The result of this statistical study is shown in Figure 7 as a bar graph showing the fraction of all disturbances which exceed a certain level for the He + /He 2+. About 85% of the shocks/ CIRs in the list are associated with an enhancement that exceeds 0.1. An abundance ratio of more than 0.2 is found in 33% of the events, and in 19% of the events, the ratio exceeds 0.3. Within this latter sample we have found 10 shocks and seven CIRs/TIRs. In the next section we will study in more detail some examples of this last sample, which are typical for disturbances that produce the highest ratios. Figure 7. Fractions of He + /He 2+ abundance ratios above these different thresholds associated with all interplanetary shocks/cir s during as obtained from the list by C.W. Smith (available at chuck/ace/acelists/obs_list.html). SWEPAM, and the total magnetic field from ACE/MAG for the time period from DOY 165, 1999, to DOY 200, [33] Vertical lines indicate the onsets of events for which the abundance ratio exceeds the value of 0.2. We have chosen this value because all major abundance ratio enhancements, including the CIR episodes, are included and because this value is well above our lower limit of 0.04 for this analysis. As one can see in Figure 6, all of these events are associated with interplanetary disturbances. For every major increase one can find a related discontinuity in solar wind speed, density, and interplanetary magnetic field. However, there seems to be no obvious quantitative correlation/anticorrelation between He + /He 2+ ratio in this energy range with any of the solar wind parameters or the strength of the IMF. A detailed correlation study with these parameters, and combinations thereof, did not return conclusive results. [34] To further investigate the association of He + enhancements with interplanetary disturbances we have compared the timing of these enhancements with the ACE list of disturbances and transients, which is publicly available on the world wide web (C.W. Smith, ACE list of disturbances and transients, available at udel.edu/chuck/ace/acelists/obs_list.html). Here 36 CMEs are tabulated for 1998, and 148 shocks are tabulated for the time period [35] CMEs were identified by intervals of counterstreaming heat flux electrons in this list [see Gosling, 1990]. Because of data gaps or low quality data we miss 10 events which are identified as CMEs. Sixteen of the remaining 26 CME events show no enhancements or a He + /He 2+ ratios lower than or equal to 0.1. We do see higher enhancements (0.1 < He + /He 2+ < 0.2) during five CME type events. During four events we found a ratio higher than 0.2 but lower 0.3. One event showed a ratio even larger than 0.3. However, all of the events with an abundance ratio which exceeds 0.2 were associated with shocks Corotating Interaction Regions [37] We start our case-by-case study with known results on CIRs. Here unusually high abundance ratios at suprathermal energies have been observed on Ulysses at 4.5 AU by Gloeckler et al. [1995], on WIND by Chotoo et al. [2000], on SOHO by Hilchenbach et al. [1999], and at high energies with ACE/SEPICA [Morris et al., 2001]. Morris et al. investigated six episodes of recurrent enhancements of the He + /He 2+ ratio at the end of 1999 (DOY 284, 312, 338) to the beginning of 2000 (DOY 001, 028, 055). Using 12- hour averages for the abundance ratio, they found a continuous increase across every recurrence of the corotating stream. This increase of the ratio is explained in terms of pickup ions [Morris et al., 2001]. The number density of solar wind alpha particles decreases quadratically with heliospheric distance whereas the number density of pickup ions decreases only linearly. Thus the ratio of the number densities of pickup ions over solar wind alpha particles increases linearly with the distance from the Sun. Therefore after each recurrence, the shock which is associated with the corotating stream will propagate into a region of higher pickup density compared to alpha particles. The seed particle population available for further acceleration will contain more pickup ions at larger distances, and this, in turn, will lead to the observed increase of the He + /He 2+ ratio at higher energies. This series of corotating events can clearly been seen in Figure 6. A closer look at the time span between these enhancements shows a time period of 27 days. All of these enhancements are correlated with a high-speed stream (second panel) originating from a coronal hole close the equator of the Sun (not shown in this paper). After DOY 82, 2000, the coronal hole disappears, as does the associated high-speed stream. After that, the He + /He 2+ enhancement does not recur. [38] Below we will concentrate on the strongest enhancements, indicated by the segments A, B, C, and D. It should be pointed out here that these enhancements are the most prominent and the most permanent in the entire survey. This is a new result because none of the previous investigations have made a comparative analysis of interplanetary disturbances and their assocation with the He + /He 2+ ratio within a survey. New in this investigation is also the high time resolution, which is important for a detailed study of the time profiles, as we will see below.

9 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + LIS 15-9 Figure 8. Four episodes of recurring enhancements of the He + /He 2+ ratio (indicated by A, B, C, and D). The top panels show the abundance ratio and the lower panel the solar wind speed (black line) as well as the proton density (grey line). Note the density has been multiplied by a factor of 30. [39] The four enhancements of the He + /He 2+ ratio DOY 365, 1999 (A), DOY 27 (B), DOY 55 (C), and DOY 82 (D), 2000, of this corotating stream are shown in more detail in Figure 8. For each case, the top panel shows the He + /He + ratio, and the bottom panel shows the solar wind bulk speed as well as the plasma density (grey line). (The solar wind density has been multiplied by a factor of 30 in this plot.) The temporal profiles of interplanetary parameters of all four time periods are very similar, and they are typical of a CIR at 1 AU. Before the sudden rise of the solar wind velocity a density increase occurs as a signature of the compression region where a fast solar wind stream overtakes a slower stream. After the sharp density rise at the leading edge of the corotating stream, the He + /He 2+ ratio starts to increase linearly and peaks towards the end of the compressed fast solar wind stream. In the fast wind the He + / He 2+ ratio stays high but the integration time increases, which is due to a decrease of the helium flux in the fast wind. It should be pointed out here that CIRs produce the largest He + enhancement within the survey. In addition, these enhancements have a very specific time profile with a rather long duration compared to any other observed enhancements, such as those at isolated interplanetary traveling shocks (as we will see later in this paper). All the CIR-related enhancements show an almost linear increase with time as the structure passes the spacecraft, and the ratio stays high in the high-speed solar wind. By the end of the last recurrence the He + /He 2+ is about 0.8. [40] Transient interaction regions (TIR) have the same topology as corotating interaction regions [Burlaga, 1986]. A high-speed solar wind stream runs into a region of slow solar wind and forms a shock at larger heliospheric distances. In the survey we found several enhancements in He + /He 2+, for example at DOY 293, 2000, which are

10 LIS KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + Figure 9. He + /He 2+ abundance ratio during a passage of an interplanetary traveling shock. From top to bottom we show the He + /He 2+ abundance ratio, the plasma density, the solar wind bulk speed as well as the thermal speed. The vertical line indicates the shock arrival at the spacecraft. associated with transient streams (not shown in this paper). TIRs also show an extended time profile similar to CIRs Interplanetary Traveling Shock [41] In contrast to the prolonged episodes of enhancements in CIRs and TIRs, a very sharp peak with a ratio of 0.6 can be seen on DOY 97. This time period will be discussed in detail in this section. Figure 9 shows a detailed view of this time period. From top to bottom we show the He + /He 2+ ratio, the total helium flux from ACE/ULEIS, the plasma density, and the solar wind bulk speed as well as the thermal speed. The plasma parameters and the total field show very clear signatures of an interplanetary fast forward shock. Right at the shock arrival, which is marked with a vertical line, we see a sharp increases of the solar wind speeds, and sharp increases in density, thermal speeds, and in the total magnetic field. [42] During the entire time period, the helium flux is high. Enhanced flux is observed already on DOY 96 when the CME left the Sun, and this is probably caused by acceleration of ambient material closer to the Sun [Tan et al., 1989], most probably rich in He 2+. Before the arrival of the shock the particle flux increases, and then it falls off by one order of magnitude when the shock has passed the spacecraft. [43] Because of the high helium flux, the integration time for the He + /He 2+ ratio can be made very short. In this time period the temporal resolution is about 6 min. This allows for a very detailed time profile of the abundance ratio. The abundance ratio of helium peaks at 0.6 right at the time of the shock arrival. Downstream of the shock, the helium abundance ratio drops off very quickly, which is in contrast to the temporal behavior of the enhancements at CIRs regions discussed in the previous section. Since the abundance ratio of helium peaks right at the time of shock arrival, the He + is most likely being preferentially injected and accelerated locally at the shock ICME Without Shocks [44] If shocks associated with interplanetary disturbances cause an enhancement of energetic helium population, a CME which is not driving a shock should not show an enhanced He + /He 2+ ratio. Clear evidence of this expectation is given by the example shown in Figure 10. The top panel shows the He + /He 2+ ratio as well as the total helium flux from ACE/ULEIS for DOY , During DOY a CME is passing the spacecraft, which does not drive an observable shock, as is evident from the blowup in the lower panels of this figure where the interplanetary conditions are shown for DOY (from top to bottom: solar wind speed, plasma density, kinetic temperature, total magnetic field and its components). [45] The dashed lines delimit the time when the cloud is passing over the spacecraft. During this time period a gradual decrease of the solar wind velocity, a low kinetic temperature and a rotation of the magnetic field are recorded which serve as a clear indication of a magnetic cloud [Burlaga et al., 1981]. However, there is no evidence of

11 KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + LIS Figure 10. He + /He 2+ abundance ratio as well as the total helium flux from ULEIS for DOY (top panel) and the interplanetary conditions for DOY of The panels show, from top to bottom, the solar wind speed, the proton density, the ion temperature, the total magnetic field, and its components. an interplanetary shock driven by this ejection. Although the helium flux is increased at this time, there is no significant enhancement in the He + /He 2+ ratio. This negative result reemphasizes that it must be the local acceleration at associated shocks that is responsible for He + enhancements Selective Injection and Acceleration Condition [46] As demonstrated by Scholer [1999] and Scholer and Kucharek [1999] in numerical simulations, pickup ions, which are essentially suprathermal ions in the frame of the solar wind, are preferentially injected into an acceleration process, when compared with the cold solar wind. Therefore these simulation results also support the idea that the source of energetic He + population must be interstellar pickup ions rather than cold solar material. They are also consistent with our results that the He + abundance in the energetic population is substantially enhanced over the source, which requires a preferential injection and acceleration of He + over

12 LIS KUCHAREK ET AL.: SOURCE AND ACCELERATION OF ENERGETIC HE + He 2+. It should be noted that shock drift acceleration is most likely not the major acceleration mechanism for the higher energies observed here. It is the major acceleration mechanism when the ions are injected and form a seed particle population for further acceleration, and the energy gain of ions by shock drift acceleration is usually limited to a factor of 4 10 above their energy before their first shock encounter, i.e., up to several tens of kev/n. Diffusive shock acceleration is thought to be the process which accelerates particles to higher energies [Hudson, 1965]. 6. Conclusions [47] We have extended the work by Hovestadt et al. [1984] with a comprehensive survey of the ionic charge state of energetic helium for the years in the energy range MeV/n. Compared with the previous work, the collecting power of ACE SEPICA yields much better statistics, resulting in substantially improved time resolution. During times with high fluxes, the time resolution can be as good as 6 min instead of daily averages, which permits a detailed study of the temporal evolution of events. As in the previous study, the abundance ratio of He + and He 2+ is highly variable and reaches values of the order of 1. The average value for the energetic abundance ratio over the entire time period is 0.06, and thus He + constitutes, after H + and He 2+, the third most abundant ionic species in the energetic particle population in the inner heliosphere. [48] This result shows clearly that the solar wind cannot be the major source for the energetic He + population because the He + /He 2+ ratios under normal solar wind conditions are of the order of Although much higher ratios have been found as contributions from cold solar plasma in CMEs, it is known that such He + -rich magnetic clouds are very rare, while He + enhancements in our survey are frequent. Also, the observed high average value for the abundance cannot be explained by such rare events. In addition, we have found in one of these rare CME events that no substantial enhancement of the energetic He + / He 2+ ratio occurred, although it is associated with a very strong He + seed population in the CME/cloud. The only significant enhancements in the energetic ions are found at shocks driven by the CME outside of the cloud that contained the He + -rich material. Therefore cold solar material does not seem to play a significant role, even if it is abundant, and we conclude that the source population for the energetic He + must be interstellar pickup ions. [49] However, even the average abundance of pickup He + at 1 AU during passage through the gravitational focusing cone ( of the solar wind He + ) falls substantially short of the observed average ratio of 0.06 in the energetic population. This suggests a preferential acceleration of He + to higher energies. Furthermore, the variation of the abundance ratio cannot be explained by the variation of the pickup ion density in the interplanetary medium. We have found no signature of the gravitational focusing cone in the data set and the variations in the energetic population are even larger than the observed fluctuations in the pickup population. Therefore injection and acceleration must be responsible for the lion s share of these variations. [50] We have identified mostly two types of interplanetary structures at which the most prominent He + increases occur, namely solar wind stream-stream interfaces and traveling shocks. However, no obvious correlation of the observed abundance was found with solar wind or other interplanetary parameters that could control the injection conditions for solar wind and pickup ions. The most prolonged and most intensive enhancements within this survey are found at the leading edge of high-speed streams associated with corotating interaction regions. Similar enhancements of the energetic abundance ratio of helium are also observed at interplanetary structures with similar topology, so-called TIRs. In addition, interplanetary traveling shocks produce significant enhancements, but with a quite different time profile. Here, the abundance ratio peaks right at the shock arrival, which is a clear indication of local acceleration. [51] All these important results provide many new opportunities for further investigations. With pickup He + and solar wind He 2+ we have two species with a mass-percharge ratio different by a factor of two and distinctly different velocity distributions. Making use of the database of He + rich events discussed here we hope to exploit these differences in future investigations on acceleration efficiencies and their relation to shock conditions and geometry. 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