Observation of energy-dependent ionic charge states in impulsive solar energetic particle events

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1 Advances in Space Research 38 (2006) Observation of energy-dependent ionic charge states in impulsive solar energetic particle events B. Klecker a, *,E.Möbius b, M.A. Popecki b, L.M. Kistler b, H. Kucharek b, M. Hilchenbach c a Max-Planck-Institut für extraterrestrische Physik, Space Plasma Department, Giessenbachstrasse, Postfach 1312, Garching, Germany b University of New Hampshire, Durham, NH 03824, USA c Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany Received 29 October 2004; received in revised form 1 April 2005; accepted 1 April 2005 Abstract We measured the mean ionic charge state of Fe in several 3 He-rich and heavy-ion rich energetic particle events during the time period We combined the ionic charge measurements of CELIAS/STOF onboard SOHO ( kev/nuc) with the measurements of SEPICA onboard ACE ( kev/nuc) to investigate the ionic charge distribution of Fe over a wide energy range. Whereas the mean ionic charge of Fe, Q m (Fe), increases strongly with energy from at kev/nuc to at kev/nuc, the mean ionic charge at lower energies of kev/nuc was found to be significantly lower (12.5 ± 0.9). A comparison of the results with steady-state models that include impact ionization by protons and electrons in the low corona shows that the measured increase of Q m (Fe) with energy is often steeper than predicted by the model, and other effects, as for example adiabatic deceleration during the propagation, may play an important role. Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Sun; Corona; Energetic particles; Ionic charge states 1. Introduction The ionic charge of solar energetic particles is an important parameter for the diagnostic of the plasma conditions at the source region in the solar corona. Furthermore, the acceleration and transport processes depend significantly on velocity and rigidity, i.e., on the mass and ionic charge of the ions. The early measurements from about 25 years ago revealed for IP-shock related events an incomplete ionization of heavy ions in the range C Fe, with Q m (Fe) 10 14, indicative of source temperatures of about K(Gloeckler et al., 1976; Hovestadt et al., 1981; Luhn et al., 1984). * Corresponding author. Tel.: ; fax: address: berndt.klecker@mpe.mpg.de (B. Klecker). It was also found that the mean ionic charge in 3 Heand heavy-ion-rich events was significantly higher, with Q m (Fe) 20 and Q m (Si) 14 (Klecker et al., 1984; Luhn et al., 1987). This was interpreted as being indicative of a high temperature of 10 7 K in the source region. This characteristic difference of ionic charge states was subsequently used as one of the criteria for the classification of solar energetic particle (SEP) events as ÔgradualÕ or ÔimpulsiveÕ, following a classification of flares, based on the duration of the associated soft X- ray event (e.g., Reames, 1999 and references therein). However, new measurements of ionic charge states over an extended energy range have shown that this picture was oversimplified. In IP-shock related events, the ionic charge of heavy ions was observed to increase in many events with energy (e.g., Oetliker et al., 1997 and references therein; Möbius et al., 1999, Leske et al., /$30 Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi: /j.asr

2 494 B. Klecker et al. / Advances in Space Research 38 (2006) ), with a large event-to-event variability, often showing Q m (Fe) 20 at energies above 10 MeV/nuc (Labrador et al., 2003). For ÔimpulsiveÕ events Möbius et al. (2003) reported recently a systematic increase of Q m (Fe) with energy by several charge units in the energy range kev/nuc. A large increase of Q m (Fe) at E < 1 MeV/nuc can be explained by ionization in a dense environment. If the particles propagate in a sufficiently dense environment in the lower corona during or after the acceleration, a large increase of the mean ionic charge with energy is a natural consequence of the energy dependence of the impact ionization cross-sections. This has been shown for equilibrium charge states by Reames et al. (1999), using an approximation for the mean charge as a function of nuclear charge (Z), ion speed, and plasma temperature, with Q m approaching Z at high energies. At energies <0.1 MeV/nuc the impact ionization cross sections are small and Q m is essentially determined by the temperature of the background plasma. Model calculations including the actual impact ionization cross sections of protons and electrons (Barghouty and Mewaldt, 1999; Kocharov et al., 2000; Kovaltsov et al., 2001, Stovpyuk and Ostryakov, 2001) show that impact ionization by protons of the background plasma is very important and that Q m approaches the energy dependent equilibrium charge states Q eq as N t increases in the acceleration region, where N is the plasma density and t is the residence time. The equilibrium charge state Q eq is reached when N t exceeds a certain threshold, which for 0.5 MeV/nuc Fe ions is typically cm 3 s (Kocharov et al., 2000). In this paper, we investigate the mean ionic charge state of iron in the energy range kev/nuc, extending the study by Möbius et al. (2003) to lower energies, using data from the STOF sensor of the CELIAS experiment onboard SOHO. 2. Instrumentation and data analysis Within the in situ experiments onboard SOHO the Suprathermal Time-of-Flight Sensor (STOF) of the CE- LIAS experiment provides the ionic charge distribution of suprathermal ions in the energy range kev/ nuc (for Fe). STOF combines electrostatic deflection with time-of-flight (TOF) and residual energy (E*) measurement to derive from E/Q, TOF, and E* the particle parameters energy (E), mass (M), and ionic charge state (Q). For a detailed description of the sensor see Hovestadt et al. (1995) and for a description of the data analysis method see Bamert et al. (2002). In the energy range above 180 kev/nuc (for Fe) the mean ionic charge state is determined with the SEPICA instrument onboard ACE. A complete description of the SEPICA instrument and its data system may be found elsewhere (Möbius et al., 1998). Both ACE and SOHO are located near the Lagrangian point L1. For a consistency check of mean ionic charge states obtained with STOF and SEPICA we selected the IPshock related event on May 1, 1998 that showed an only small (DQ 1) increase of the mean ionic charge state between the solar wind and 0.5 MeV/nuc (Klecker et al., 2000). We evaluated Q m (Fe) for the 10 h period of the particle intensity increase before the IP-shock passage at SOHO on May 1, 1998, 21:15 UT. In the energy ranges and kev/nuc we obtained for Q m (Fe) 9.7 ± 0.6 (STOF) and ± 0.35 (SEPICA), respectively. This shows that for events with only small energy dependence of Q m (Fe) the results in the lowest SEPICA energy channel used in this analysis are compatible with the measurements obtained with STOF below 100 kev/nuc. We then selected from the list of seven impulsive events of Möbius et al. (2003) three events during the time period September 1998 May 2000, where STOF had sufficient statistics and SEPICA data with updated calibration concerning a known non-linearity of the position measurement were available. This calibration update resulted for events 1 and 3 of Table 1 in only small changes of Q m (Fe) by charge units. For events 2 and 4, the new values of Q m (Fe) are significantly smaller at the highest energy (by 2 charge units). However, the revised calibration did not change the finding reported by Möbius et al. (2003) that ionic charge states in impulsive SEP events are generally increasing with energy. We note, however, that for future detailed modelling the new values of Q m (Fe) reported here should be used. For this reason, we also included one event that occurred during the time period SOHO was not operational (event 1). This event shows the highest mean ionic charge of all events during the Table 1 Selected impulsive events Year Time (DOY hh:mm) (kev/n) (kev/n) STOF Q(Fe) DQ Q(Fe) DQ : : No : : Yes : : Yes : : Yes

3 B. Klecker et al. / Advances in Space Research 38 (2006) time period selected, demonstrating the large range of Q m (Fe) observed in the energy range of kev/ nuc. Note that the seven events were selected by Möbius et al. (2003) on the basis of Q m (Fe) being higher than 14 in the energy range kev/nuc, i.e., significantly higher than usually observed in IP shock related SEP events, and that they have sufficient counting statistics for individual analysis. Table 1 provides the list of events selected for this study and the mean ionic charge state of Fe in the energy ranges and kev/nuc. Table 1 also shows whether STOF data are available. Because of the small collecting power of STOF, the count rate for the events of Table 1 is not sufficient to evaluate the mean ionic charge state for individual events. Therefore we integrated events 2 4 and obtained an average Q m (Fe) = 12.5 ± 0.9 for the energy range kev/ nuc. The contributions from events 2 and 3 to this average are about the same (40% each), with 20% from event 4. Fig. 1 shows Q m (Fe) as a function of energy for the events listed in Table 1, together with the average over events 2 4 obtained with STOF at low energies. Whereas all events show a systematic increase of the mean ionic charge state with energy above 200 kev/ nuc, the average Q m (Fe) at kev/nuc is significantly lower. A large increase of the mean ionic charge state of Fe in the energy range <1 MeV/nuc can be explained in terms of impact ionization by protons and electrons in a dense environment low in the corona. The dashed lines in Fig. 1 show the equilibrium charge state computed with the model of Kocharov et al. (2000), that includes Averge Charge of Fe Event 1 Event 2 Event 3 Event 4 STOF-AVG T e = T e = Energy (MeV/nuc) Fig. 1. Mean ionic charge state of Fe as a function of energy for four impulsive events and energy dependence obtained for equilibrium conditions in a charge stripping model (Kocharov et al., 2000). the effects of radiative and di-electronic recombination and impact ionization by protons and electrons. The model results show that above 200 kev/nuc the mean ionic charge state is essentially determined by stripping, whereas at energies <100 kev/nuc the mean ionic charge state is determined by the temperature, i.e., by the distribution of ambient electrons. The two temperatures of 1.2 and K are used to illustrate the temperature range that would be about compatible with the ionic charge state measurements at low energies, assuming a Maxwellian electron distribution. Fig. 1 shows that for three of the four events studied the mean ionic charge state measurements above 180 kev/nuc are systematically above the equilibrium charge states obtained with the model calculation. 3. Discussion and summary We extended previous observations of the mean ionic charge state of Fe in impulsive events by Möbius et al. (2003) into the suprathermal energy range of kev/nuc using data from the STOF sensor onboard SOHO. For the average of the three events in this study, when SOHO data are available, we observe Q m (Fe) = 12.5 ± 0.9 at low energies, consistent with a systematic decrease of the mean ionic charge state of Fe with decreasing energy at energies <550 kev/nuc. The measurements at <100 kev/nuc and the increase of Q m (Fe) with energy are qualitatively consistent with equilibrium charge states computed for T K, using the model of Kocharov et al. (2000). This demonstrates that the ionic charge states in impulsive events at energies above 200 kev/nuc are primarily determined by charge stripping. Single-component equilibrium models with temperatures above 10 7 K would result in Q m (Fe) in the energy range MeV/nuc (see Fig. 3 of Kocharov et al. (2000)), not compatible with the observations. However, multicomponent models with contributions from high temperature regions cannot be excluded and need to be investigated with a more complete model, including non-equilibrium conditions and interplanetary propagation effects (see below). For three of the four events the observed increase of Q m with energy is steeper than computed with the model. A possible explanation for the increase already at lower energies than in the model could be propagation effects in interplanetary space, i.e., energy loss due to adiabatic deceleration. If the particles spend sufficiently long time in interplanetary space, i.e., if the scattering mean free path, k, is sufficiently small, the energy loss can be significant. With a numerical solution of the full transport equation Kocharov et al. (1998) obtained for 2 MeV protons, a solar wind velocity of 370 km/s and k = 0.3 AU an energy loss

4 496 B. Klecker et al. / Advances in Space Research 38 (2006) of 50% in 24 h. Because the fractional rate of energy loss by adiabatic deceleration is independent of the particlesõ energy, mass, and ionic charge state (Roelof, 2000) energy loss by adiabatic deceleration during transport moves the computed equilibrium charge state to lower energies, with a larger change for lower energies than for higher energies. That could qualitatively account for the difference between the simple model and the measurements. If adiabatic deceleration is significant and the injection at the Sun occurs during a short time period, then the ionic charge state as measured at 1 AU should increase with time: observing at fixed energy at 1 AU, later in the event the ions started at higher energies with higher charge states. Thus, the variation of Q m (Fe) with time is another important observational parameter to constrain model calculations. A quantitative model fit to the data is beyond the scope of this paper. A model including the effects of acceleration, charge stripping, and spatial diffusion, convection and adiabatic deceleration in interplanetary space was recently presented by Kartavykh et al. (2004). They were able to reproduce for several events the energy dependence of the ionic charge composition above 200 kev/nuc. It will be interesting in the future to extend these models to the now available lower energies, to allow for multiple components with different temperature, and to include the observation of Q m (Fe) as a function of time during the events to further constrain the model parameters. Thus, the combination of ionic charge state measurements with model calculations may develop into a valuable tool to infer the plasma parameters at the acceleration site. Acknowledgements The CELIAS experiment was supported at MPE by DLR under Contracts 50OC8905 and 50OC9605. 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