STATISTICAL PROPERTIES OF FAST FORWARD TRANSIENT INTERPLANETARY SHOCKS AND ASSOCIATED ENERGETIC PARTICLE EVENTS: ACE OBSERVATIONS

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1 STATISTICAL PROPERTIES OF FAST FORWARD TRANSIENT INTERPLANETARY SHOCKS AND ASSOCIATED ENERGETIC PARTICLE EVENTS: ACE OBSERVATIONS D. Lario (1), Q. Hu (2), G. C. Ho (1), R. B. Decker (1), E. C. Roelof (1), C. W. Smith (3) (1) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. (2) Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521, USA (3) Institute for Earth Oceans and Space, University of New Hampshire, Durham, NH 03824, USA ABSTRACT/RESUME We investigate 191 fast forward transient shocks observed by the ACE spacecraft from 1998 February 1 to 2003 October 28 and classify the energetic particle response associated with the passage of these shocks. We were able to compute the parameters of about 150 shocks. We present the frequency distributions of the angle between the upstream magnetic field and the normal to the shock, the Alfvenic Mach number, the shock speed along the normal to the shock in the frame of reference of the ambient plasma, V S, and the plasma density and magnetic field ratios. We observe a trend for more quasi-perpendicular shocks. The Alfvenic Mach numbers show values lower than about 7, the shock speeds V S lower than about 350 km/s, and the density and magnetic field compression ratio values below 4.5. A few exceptions above these values were also observed. There is a trend for faster and stronger shocks to have greater effects on the energetic particle intensities. However, the parameters of the shock do not determine unequivocally the characteristics of the energetic particle event observed at the passage of the shock. 1. INTRODUCTION In situ observations of interplanetary shocks and energetic particles provide a unique opportunity to test theories of particle acceleration. Energetic particle signatures associated with the passage of interplanetary shocks range from spike events (i.e., a rapid rise of proton intensities at the time of the shock lasting a few (~10) minutes) to classical energetic storm particle (ESP) events (i.e., a slow rise of particle intensities beginning several hours before the shock passage and flat time-intensity profiles after of the shock). Other particle signatures include step-like post-shock intensity increases and spike events superimposed on gradual intensity increases. The most common energetic (~50 kev) proton signatures observed in association with the passage of interplanetary shocks are either no variations at all above the pre-existing intensity level or irregular time-intensity profiles with multiple impulsive intensity variations (Tsurutani and Lin, 1985; Lario et al., 2003). Theories of particle acceleration mechanisms at shocks were developed in the eighties to explain some of the observed shock particle events. From a theoretical perspective, classic ESP are thought to result mainly from first-order Fermi acceleration at nearly parallel shocks, whereas spike events are thought to result mainly from shock drift acceleration at nearly perpendicular shocks (Decker, 1981; Lee, 1983; Sanderson et al., 1985). In general, however, both acceleration mechanisms will contribute at oblique shocks, with the relative contribution of each depending on the properties of the shock (Jokipii, 1982). Shock parameters such as shock speed, compression ratio, Alfvenic Mach number, and the angle between the upstream magnetic field and the normal to the shock q Bn, are all factors that can influence the relative contributions of particle acceleration mechanisms in producing the observed shocks events. In this paper we analyze the parameters of 191 interplanetary shocks observed by the Advanced Composition Explorer (ACE) and compare the characteristics of the associated particle events with the shock parameters. We see that the shock parameters do not determine unequivocally the properties of the energetic particle event. 2. SHOCK ANALYSIS From 1998 February 1 to 2003 October 28 the ACE spacecraft, at the Sun-Earth Lagrangian point ~230 R E upstream of the Earth, detected near 300 interplanetary shock waves. A preliminary list of these shocks is available at www-ssg.sr.unh.edu/mag/ace/acelists/obs- _list.html. Among these shocks, we have selected 191 fast forward interplanetary shocks with evidence of either being driven by, or else related to, the passage of, interplanetary counterparts of coronal mass ejections (CMEs). Thus we have explicitly excluded reverse shocks, slow shocks and shocks associated with other interplanetary structures such as magnetic holes or corotating interaction regions (CIRs). Because of possible instrument anomalies such as high count rate saturation and high-energy particle contamination we have also excluded a total of 10 shocks associated with the most intense solar energetic particle (SEP) events (such as, for example, the Bastille Day 2000 event and the November 2000 and 2001 SEP events). The 191 shocks were analyzed in terms of energetic particle signatures and shock characteristics. We used both high-time resolution (64 s) level-2 plasma data Proc. Solar Wind 11 SOHO 16 Connecting Sun and Heliosphere, Whistler, Canada June 2005 (ESA SP-592, September 2005)

2 from the SWEPAM instrument (McComas et al., 1998) and level-2 magnetic field data from the MAG instrument (Smith et al., 1998), averaged to the same time basis as the plasma data. We then selected upstream and downstream intervals before and after the shock that show relatively stable evolution of the solar wind and magnetic field parameters (i.e., we exclude those data points included in the foot and overshoot regions usually observed on either side of the shock). We employ the non-linear least-square technique of Szabo (1994) to simultaneously solve the complete set of Rankine-Hugoniot relations for pairs of upstream and downstream data points. This algorithm determines the shock normal, the shock speed and asymptotic fluid states, i.e., the plasma density, temperature, velocity (3 components) and magnetic field (3 components) on both sides of the shock. Limitations of this analysis are the neglect of alpha particles and the use of a constant, yet typical, value for the electron temperature at each data point upstream and downstream of the shock due to the lack of actual electron temperature measurements. The results of the shock parameters are found by an iterative non-linear least square fit technique that minimizes the norm of the residuals c 2 (see details in the work of Szabo, 1994). Since non-linear least square techniques do not generally provide a unique minimum, a mapping of the c 2 function is used to find all local minima and thus investigate all likely solutions, removing those that do not represent a valid realistic shock and then checking that the calculated asymptotic parameters are in good agreement with the observations (see e.g., Viñas and Scudder, 1986). For 156 shocks we have been able to determine both the shock speed along the normal to the shock in the upstream frame of reference, V S, and the density compression ratio, r n. For 162 shocks we have been able to determine the angle between the upstream magnetic field and the direction normal to the shock, q Bn, as well as the magnetic field compression ratio r b (because of plasma data limitations we used the magnetic coplanarity theorem to determine q Bn in 10 of these 162 shocks). The Alfvenic Mach number computed as M A = Vs /[ B (m 0ÿ r) -½ ] was determined for 152 shocks (r is the mass density, B is the magnetic field vector upstream of the shock determined from the Rankine- Hugoniot fitting). To compare with the density compression ratio, r n, calculated from the asymptotic state, we have also computed the upstream to downstream proton density ratio (H) by simply averaging the plasma density observed four minutes before and after the passage of the shock (avoiding those data points in the foot and overshoot regions at each side of the shock). Fig.1 shows the correlation between the parameters r n and H. With the exception of those events with either r n 4 or H 4, both parameters compare well (correlation coefficient=0.8). Figure 1. Correlation between r n and H 3. SHOCK STATISTICAL PROPERTIES We have assembled the parameters derived from the analysis of the shocks into a series of probability distributions or histograms. Fig. 2 contains histograms for r n and H. Most of the shocks (91%) have compression ratios (for both r n and H) below 3.5. The medians of both distributions are 2.3 and 2.0 for r n and H, respectively. The most probable value of both distributions are in the interval [1.5,2.0). The deficit of shocks in the bin [1.0,1.5) suggests that a certain number of weak shocks may have gone undetected (see discussion in section 5). Figure 2. Frequency distributions of r n and H Fig. 3 shows the frequency distributions of V S and M A. The medians of both distributions are 127 km s -1 for V S and 2.2 for M A. We find that about 60% of all Alfvenic Mach numbers are less than 2.5. Only 15 events were observed with M A 4. On the other hand, only one shock was observed with V S >350 km s -1.

3 between the two works are not directly comparable. Because of the trend to more quasi-perpendicular shocks and the dependence on q Bn of the expression used by Volkmer and Neubauer (1985), these authors obtained higher Alfvenic Mach numbers with frequency distributions that peak between the values 3 and 4 (for Helios observations between 0.75 and 1.0 AU) and a larger number of shocks with Afvenic Mach numbers above 7 (cf. Fig. 7 in the work of Volkmer and Neubauer, 1985). Figure 3. Frequency distributions of V S and M A Fig. 4 shows the frequency distribution of r b and q Bn. The median of both distributions are 2.0 for r b and 65± for q Bn. Most of the shocks (90%) have r b <3. The q Bn histogram shows that interplanetary shocks may occur at essentially all angles, so that the distribution includes quasi-parallel shocks (q Bn 30±) as well as the more frequently occurring quasi-perpendicular shocks (q Bn 60±). There is a clear tendency for quasiperpendicular shocks. The fraction of shocks with q Bn >60± is 57%, whereas the events with q Bn 30± is only 10.5%. Fig. 5 shows the correlation between r n and r b. As expected, quasi-parallel shocks (open circles) have low r b values, whereas quasi-perpendicular shocks (gray circles) have similar values of r b and r n. Previous studies on the statistical properties of interplanetary shocks were performed by Volkmer and Neubauer (1985) and Smith (1985) using data from the two Helios spacecraft and the ISEE-3 spacecraft, respectively. The trend for more quasi-perpendicular than quasi-parallel shocks was also observed in these two previous studies. Chao and Chen (1985) argued that these distributions result from shocks whose normal directions are distributed about the radial direction (as expected from CME-driven shocks) and magnetic field directions distributed around the nominal Parker spiral. The distribution deduced by Volkmer and Neubauer (1985) for V s as observed by the two Helios spacecraft when they were located at heliocentric distances between 0.75 and 1.0 AU is similar to that shown in the top panel of Fig. 3 (cf. Fig. 5 in the work of Volkmer and Neubauer, 1985). In contrast to our computation of M A, Volkmer and Neubauer (1985) computed the Alfvenic Mach number as the ratio between V S and the normal component of the Alfven speed upstream of the shock. Therefore the frequency distributions of M A Figure 4. Frequency distributions of r b and q Bn Figure 5. Correlation between r b and r n 4. ENERGETIC PARTICLE OBSERVATIONS Phenomenological classification of the energetic particle signatures associated with the passage of interplanetary shocks reveals a rich variety of events. Lario et al. (2003) classified the particle intensity signatures observed by the LEMS120 telescope of the EPAM instrument on board the ACE spacecraft (Gold et

4 al., 1998) into six different types. Type 0 are those events in which the ion intensity does not show any significant variation above the pre-existing intensity level. Type 1 are those events described as classic ESP events in Section 1. Type 2 are spike events coincident with the shock passage and of duration less than ~10 minutes. Type 3 are events with a slow intensity increase and a short duration (~10 minutes) spike at or near the shock superimposed on it (i.e., ESP+spike). Type 4 are step-like post-shock increases. Finally, Type 5 are irregular time-intensity profiles with flux variations not coincident with the shock passage and not fitting into the types above described. Fig. 6 shows six examples of shock-associated particle events corresponding to each of the six types of events. It is important to note that this classification of the events is energy dependent (not all energy channels and particle species present the same time-intensity profiles) and is based on visual inspection of the evolution of the 1- minute spin-averaged time-intensity profiles observed 5 hours prior to and following the shock passage. Additional examples of these types of events can be found in Fig. 1 of Lario et al. (2003). Figure 6. Six examples of shock-associated energetic particle events as observed by the kev ion channel of the LEMS120 of the EPAM instrument on board ACE. Plotted intensities are 1 minute spin-averages. We have compared our phenomenological classification of the particle events with the shock parameters. Fig.7 shows the distribution of the particle events according to their kev ion time-intensity profiles as a function of (from top to bottom) r n, V S, M A, r b, and q Bn. As expected, Type 0 events are more abundant in those bins corresponding to weak and slow shocks. However, there are clear exceptions. For example, Fig. 6a shows one of the relatively fast (V S =228 km s -1 ) shocks with high Alfvenic Mach number (M A =6.3) but with no significant intensity increase. Note that the intensity increase observed just after the shock passage is a very small enhancement (less than a factor of 1.1), and several other small intensity increases are also observed with no clear association with the shock. Fig. 7 shows that no clear distinction is made between the different classes of events showing kev ion intensity enhancements (Type>0) and any specific shock parameter. Of particular interest is the distribution of events in terms of q Bn. Except for the bins 0± q Bn 10± and 40±<q Bn 50±, the frequency of events with kev ion intensity enhancement (Type>0) and without (Type 0) is similar in each bin and does not seem to correlate with q Bn. Classical ESP events (Type 1) are observed in all the q Bn bins. Comparatively, events with spikes (Types 2 or 3) are more frequent for quasi-perpendicular shocks although occasionally they are also observed for quasi-parallel shocks (see for example the events shown in Figs. 6c and 6d). 5. DISCUSSION We have investigated 191 shock events observed by ACE from 1998 February 1 to 2003 October 28. The

5 Figure 7. Distribution of shock events based on timeintensity classification as a function of (from top to bottom) r n, Vs, M A, r b, and q Bn. vast majority of the shocks has low Alfvenic Mach numbers (M A <4), low normal shock speeds (V S <300 km s -1 ) and low compression ratios (r n, H<3.5; r b <3). The deficit of events with 1.0<r n, H<1.5 (compared to those with 1.5 r n, H <2.0) may result from the difficulty of detecting weak shocks or from the fact that weak shocks do not reach 1 AU, or both. By studying interplanetary shocks observed by the Helios spacecraft at different radial distances, Volkmer and Neubauer (1985) found no radial dependence of the magnetic field, density and pressure jumps across the shocks. Only the normal shock velocity was shown to be decreasing with the radial distance, indicating that shocks decelerate as they move away from the Sun. If the strengths of weak shocks diminish with distance faster than the strengths of strong shocks, the radial independence of the parameters r n and r b found by Volkmer and Neubauer (1985) may also result from weak shocks being difficult to detect at all heliocentric distances. The frequency distribution of the angle q Bn shown in the bottom panel of Fig. 4, which is similar to results of previous studies (Volkmer and Neubauer, 1985; Smith, 1985), is a consequence of the interplanetary magnetic field direction rather than an anomalous or biased shock orientation (Chao and Chen, 1985). The preponderance of quasi-perpendicular shocks should lead to a more frequent observation of a specific type of energetic particle event. Specifically, shock-drift acceleration under weak-scattering conditions at quasiperpendicular shocks would tend to produce particle events of Types 2 and 5 (Decker, 1981; 1990). That is, assuming that the shock-drift mechanism is predominant at quasi-perpendicular shocks near 1 AU, we should expect, according to the frequency distribution of q Bn, to observe a large number of either shock-coincident regular-spike events or alternatively multiple irregular-spike events. However, our phenomenological classification reveals a great variety of particle events from spikes to classic ESP events. The most common are those events that do not display any intensity increase. Also common are those that display multiple impulsive intensity bursts well away from the shock passage, in either the upstream or downstream regions, or both. Such irregular events can result from remote connection of the spacecraft to regions on the shock where conditions for injection and/or acceleration at the shock, and subsequent escape from the shock, are optimum. Models of such events have invoked large-scale directional variations in the solar wind magnetic field (Balogh and Erdos, 1985; Decker, 1991) or on the shock surface (Decker, 1993). The weak correlations between shock parameters and the types of energetic particle events indicate that there is no unique shock parameter that determines the type of particle event. Whereas Type 0 events are more frequent for weaker shocks, strong and fast shocks may also show no significant increase in particle intensities (e.g. Fig. 6a), suggesting that the presence or absence

6 of a seed particle population plays a key role in determining whether a shock-associated particle intensity increases is or is not observed (Tsurutani and Lin, 1985). We suggest that the combination of all shock parameters, together with the presence of an energetic seed particle population, are the main factors that determine the formation of the associated energetic particle event and its final characteristics. Additionally, the weak correlation found between shock parameters and the type of energetic particle event may also result from the different type of measurements involved in this study. The shock parameters are a local measure of the shock at the position of the observer and at the time when the shock moves past the spacecraft. The particle event results from the superposition of all injections along the field line connecting the observer with the moving shock front where the shock parameters most likely would have been different. Energetic particle measurements, although taken very close to the time of the shock passage, may combine particles produced at various locations on the shock front. The type of energetic particle event is then not only result of the instantaneous acceleration and injection of particles from the shock, but also of the multiple particle injections from the dynamically evolving shock as well as the transport conditions of these particles from the shock up to the observer. 6. ACKNOWLEDGMENTS We acknowledge the use of ACE Level 2 data and thank the ACE Science Center for providing these data. This work was supported under NASA grant NAG REFERENCES Balogh A. and Erdos G., Pitch angle distributions of kev protons at quasi-perpendicular interplanetary shocks, Proc. 19 th Int. Cosmic Ray Conf., Vol. 4, , Chao J. K. and Chen Y. H., On the distribution of q Bn for shocks in the solar wind, J. Geophys. Res., Vol. 90, , Decker R. B., The modulation of low-energy proton distributions by propagating interplanetary shock waves: A numerical simulation, J. Geophys. Res., Vol. 86, , Decker R. B., Particle acceleration at shocks with surface ripples, J. Geophys. Res., Vol. 95, , Decker R. B., The role of magnetic loops in particle acceleration at nearly perpendicular shocks, J. Geophys. Res, Vol. 98, 33-46, Gold R. E., et al., Electron Proton and Aplha Monitor on the Advanced Composition Explorer Spacecraft, Space Sci. Rev., Vol. 86, , Jokipii J. R., Particle drift, diffusion, and acceleration at shocks, Astrophys. J., Vol. 255, , Lario D., et al., ACE observations of energetic particles associated with transient interplanetary shocks, in Solar Wind Ten, ed. M. Velli et al., American Institute of Physics, CP679, , Lee M. A., Coupled hydromagnetic wave excitation and ion acceleration at interplanetary travelling shocks, J. Geophys. Res., Vol. 88, , McComas D. J., et al., Solar Wind Electron Proton Alpha Monitor (SWEPAM) for the Advanced Composition Explorer, Space Sci. Rev., Vol. 86, , Sanderson T. R., Reinhard R., van Nes P. and Wenzel K.-P., Observations of three-dimensional anisotropies of 35- to 1000-keV protons associated with interplanetary shocks, J. Geophys. Res., Vol. 90, 19-27, Smith E. J., Interplanetary Shock Phenomena Beyond 1 AU, in Collisionless Shocks in the Heliosphere: Reviews of Current Research, ed. B. T. Tsurutani and R. G. Stone, Geophysical Monograph 35, AGU, 69-83, Smith C. W., et al., The ACE magnetic fields experiment, Space Sci. Rev., Vol. 86, , Szabo A., An improved solution to the Rankine- Hugoniot problem, J. Geophys. Res., Vol. 99, , Tsurutani B. T. and Lin R. P., Acceleration of >47 kev ions and >2 kev electrons by interplanetary shocks at 1 AU, J. Geophys. Res., Vol. 90, 1-11, van Nes P., Reinhard R., Sanderson T. R. and Wenzel K.-P., The energy spectrum of 35- to kev protons associated with interplanetary shocks, J. Geophys. Res. Vol. 89, , Viñas A.-F, and Scudder J. D., Fast and optimal solution of the Rankine-Hugoniot problem, J. Geophys. Res. Vol. 91, 39-58, Volkmer P. M. and Neubauer F. M., Statistical properties of fast magnetoacoustic shock waves in the solar wind between 0.3 and 1 AU: Helios-1, 2 observations, Annales Geophysicae, Vol. 3, 1-12, 1985.