The Astrophysical Journal, 507:L185–L188, 1998 November 10 q 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.
ISOTOPIC COMPOSITION OF SOLAR WIND NITROGEN: FIRST IN SITU DETERMINATION WITH THE CELIAS/MTOF SPECTROMETER ON BOARD SOHO R. Kallenbach,1 J. Geiss,1 F. M. Ipavich,2 G. Gloeckler,2,3 P. Bochsler,4 F. Gliem,5 S. Hefti,3,4 M. Hilchenbach,6 and D. Hovestadt7 Received 1998 August 12; accepted 1998 September 14; published 1998 September 29
ABSTRACT Using the high-resolution Mass Time-of-Flight (MTOF) spectrometer of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO), we have determined the solar wind isotope abundance ratio 14 N/15 N 5 200 5 55 (1 j error), suggesting that the relative abundance of 15N in the terrestrial atmosphere is lower than in solar matter. This result is compatible with the hypothesis that terrestrial N (14 N/15 N 5 272) and also N found in lunar surface material are a mixture of a heavy component that is identical to solar N and an unspecified light component. The large variations of 14N/15N in solar system matter is caused by special isotope enrichment processes, as in the case of Mars, as well as by varying contributions of isotopically different components. Subject headings: solar system: formation — solar wind — Sun: abundances the first steps of heating, usually interpreted to represent relatively recent solar wind particles, is isotopically heavier than terrestrial N. Kerridge (1975, 1993) as well as Becker & Clayton (1977) interpreted the variations they found in the N isotopic composition as being caused by a secular variation in solar wind composition and especially in the composition of solar energetic particles, where N isotopes can be accelerated selectively (Bochsler & Kallenbach 1994). However, presentday fluxes of the solar energetic particles are not sufficient to account for all isotope abundance variations observed in lunar N. Geiss & Bochsler (1982) proposed that the N trapped in lunar soils and breccias is a mixture of two components: (1) heavy lunar nitrogen (HLN), consisting of ambient solar wind particles, and (2) a light lunar nitrogen (LLN) of unspecified lunar, planetary, or cometary origin. In this work, we present the first direct measurement of the isotopic composition of N in the solar wind.
1. INTRODUCTION
Since the launch of the WIND and Solar and Heliospheric Observatory (SOHO) spacecraft, a large body of data on the solar wind isotope abundances has been collected. These data confirm that, within the limits of experimental error, there is complete agreement between the average meteoritic and solar isotopic composition of refractory elements such as Mg, Si, Ca, and Fe (Bochsler et al. 1997; Wimmer et al. 1998; Kallenbach et al. 1998a; Ipavich et al. 1998a); i.e., both are representative of the composition of the protosolar nebula. The remaining variations are on the order of a few percent observed and can be explained by weak fractionation processes in the solar wind source region (Bochsler et al. 1997; Bodmer & Bochsler 1998; Kallenbach et al. 1998b). For the volatile element neon, which is depleted by many orders of magnitude in its terrestrial abundance, the results of the Apollo solar wind collection experiments (Geiss et al. 1972) were reproduced (Kallenbach et al. 1997), indicating that 22Ne is enriched relative to 20Ne in the terrestrial atmosphere by 40% 5 4% compared with its solar abundance. Hitherto there has not been any direct determination of N isotope abundances in the solar wind. The N isotopes have only been studied in planetary system samples such as the atmospheres of Earth, Mars, and Venus, various meteorite classes, and lunar surface samples. The N trapped near the surfaces of the grains in lunar soils and breccias is unique inasmuch as it shows much larger isotopic variations than the noble gases (Kerridge 1975; Becker & Clayton 1977; Frick, Becker, & Pepin 1988). In relatively young lunar soils, the N released in
2. DATA ANALYSIS
The Mass Time-of-Flight (MTOF) sensor (Hovestadt et al. 1995) of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the SOHO spacecraft is an isochronous time-of-flight (TOF) mass spectrometer with a resolution M/DM of about 100 that allows for the resolution of the various isotopes of almost all solar wind elements in the range of 3–60 amu. The instrument detects ions at solar wind bulk velocities of 300–1000 km s21, corresponding to energies of about 0.3–3 keV amu21. Details on the instrument calibration, the format of the data transferred by the telemetry, and the data reduction for Ne, Mg, Si, and Ca solar wind isotopes are described in detail elsewhere (see, e.g., Kallenbach et al. 1997). For N, TOF spectra during the time period from day 21 in 1996 to day 365 in 1997 have been accumulated. Time periods with instrumental isotope fractionations larger than 515% have been disregarded. The instrument fractionation 15 1 14 1 N / N has been calculated as the folding of the solar wind distribution functions with the calibrated acceptance and efficiency functions of the MTOF sensor. The solar wind bulk velocity of N, v N, is approximated by the bulk velocity of O, which can be derived from the proton bulk velocity v H (Hefti 1997). The kinetic temperatures Tkin, 14 N and Tkin, 15 N are assumed to be proportional to Tkin, H, where the proportionality
1 International Space Science Institute, Hallerstrasse 6, CH-3012 Bern, Switzerland;
[email protected]. 2 Department of Astronomy, University of Maryland, College Park, MD 20742. 3 Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI 48109-2143. 4 Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland. 5 Institut fu¨r Datenverarbeitungsanlagen, Technische Universita¨t, D-38106 Braunschweig, Germany. 6 Max-Planck-Institut fu¨r Aeronomie, Postfach 20, D-37189 KatlenburgLindau, Germany. 7 Max-Planck-Institut fu¨r extraterrestrische Physik, Postfach 1603, 85740 Garching bei Mu¨nchen, Germany.
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N is mainly fivefold charged. Applying all this information, the total average instrument fractionation 15 N1 /14 N1 is determined to be 1.055 5 0.015 for the data of Figure 1. In the spectra, there is interference from 28 Si21 with 14N1 and 30Si21 with 15N1 because Si leaves the carbon foil in the isochronous spectrometer VMASS as a doubly charged ion also. However, this interference can be estimated quite well from the isolated 29 Si21 peak, the calibrated instrument functions, the solar wind bulk velocity of Si (Hefti 1997), the solar wind charge states of Si, and the solar wind isotopic composition of Si that is already known (Kallenbach et al. 1998b). The errors of the detection efficiencies are not the dominant contribution to the experimental uncertainty. The 1 j error is determined mainly by statistics and by the uncertainty in subtracting the background and the contribution of the 14N1 and 28Si21 main peak to the baseline of the 29Si21 peak and the 30Si21 and 15N1 peak. The sum of the 14N1 and 28Si21 peak and the 16O1 peak is described by the following peak shape function:
( O {O 5
Fig. 1.—Mass spectrum of the MTOF sensor showing the contributions of the isotopes 14, 15N1, 16O1, and 28, 29, 30Si21. The line shape of the 14N1 and 28 Si21 peak is adapted to the line shape of the 16O1 peak. The fit methods M1 and M2 are represented by the solid line in the full channel range and the dashed line in channels 453–470, respectively. With “e.r.,” we mark the wellunderstood instrumental effect of electronic ringing caused by reflections of the electronic start signal. The arrow points to the signature of the analog-todigital conversion (ADC) effect. This effect is observed in any channel where tADC 5 9i5n0 ai2i, with ai 5 0, 1 (the larger n0, the larger the ADC effect). For channels t slightly higher than tADC, the TOF signal is lower; for t slightly lower than tADC, the signal is higher. However, the count number that is totaled over the neighboring channels of tADC remains strictly conserved.
O
factor is the particle mass in atomic mass units (Ogilvie et al. 1980). The proton bulk velocity v H and kinetic temperature Tkin, H are measured every 5 minutes by the proton monitor, which is a subsystem of the MTOF sensor (Ipavich et al. 1998b). The entrance system voltage UW and the postacceleration voltage Vf of the MTOF sensor are also available every 5 minutes from the instrument housekeeping data. Solar wind TABLE 1 Fitted Parameters of the Peak Shape Function (Eq. [1]) for the N and O Spectrum Parameter
Method M1
Method M2
A14 . . . . . . . . . . . . . . . . A16 . . . . . . . . . . . . . . . . p14 . . . . . . . . . . . . . . . . . p16 . . . . . . . . . . . . . . . . . b1 . . . . . . . . . . . . . . . . . . b2 . . . . . . . . . . . . . . . . . . b3 . . . . . . . . . . . . . . . . . . b4 . . . . . . . . . . . . . . . . . . b5 . . . . . . . . . . . . . . . . . . o .................. l ................... q .................. w .................. u .................. r ................... c ...................
399390 1510497 449.868 481.330 0.07028 0.01738 0.007696 0.003471 0.001465 29689 5.847 0.0 1.786 1.906 7.388 0.111
404794 1489937 449.975 481.364 0.06204 0.01573 0.006324 0.001996 0.001034 30174 155.81 23.522 1.822 1.792 7.492 0.120
Note.—The differences in the fitted parameters of methods M1 and M2 lead to different baselines for the 29Si21 peak and the 30Si21 and 15N1 peak, which are taken into account in the error bars of Fig. 2. These errors range from about 150 counts channel21 in the peak minimum at channel 462 to more than 1000 counts channel21 at channels 451–453 on the slope of the 28Si21 and 14N1 main peak.
F(t) 5
Ai
i514, 16
j50
[
# exp 2
[
bj exp 2
(t 2 pi 1 r) 2 2u 2
(t 2 pi 2 4j) 2 1c 2w 2
]})
]
1 o 1 l(t 2 t 0 ) 1 q(t 2 t 0 ) 2. (1)
The first term with the amplitudes A i describes the main peaks centered at TOF channels t 5 pi, where A i and pi are free-fit parameters. The shape of the 14 N1 peak is matched to the essentially interference-free 16 O1 peak with its asymmetric “tail” to higher TOF channels. This tail is approximated by a sum of six Gaussians. The coefficients bj determine the shape of the tail; the fixed b 0 5 1 belongs to the peak maximum, whereas the other fitted bj’s describe the side peaks that approximate the tail with decreasing amplitude centered at a distance of 4j TOF channels to the peak maximum. The fitted w is the width of a single Gaussian of the multiple peak. The second term describes the well-understood “electronic-ringing” peak (Kallenbach et al. 1997) with an amplitude c relative to the main amplitude A i, a width u, and centered at a distance r to the main peak, where c, u, and r are also free-fit parameters. The background represented in the third term of equation (1) has been described by two methods, M1 and M2, a first-degree and a second-degree polynomial with fitted coefficients o, l, and q (q 5 0 for method M1), centered at the TOF channel t 0 5 462. The shape of the 14N1 and 28Si21 main peak is exactly the same as the shape of the 16O1; only the total peak amplitude A14 and the position p14 are different. This is important because the baseline for estimating the number of 29Si21 counts and 30 Si21 and 15N1 counts can be derived from the shape of the 16 1 O peak that contains a high count number and is free of interference, except for the negligible 17 O1 and 34S21 peak and the very small 18O1 peak (Collier et al. 1998) that are outside the relevant range. The two fitted peak shape functions for both methods, M1 and M2, are shown in Figure 1, and the values for the fit parameters are displayed in Table 1. The average of the two fit functions has been subtracted from the total count numbers, leaving a spectrum that contains the 29Si21 counts and the 30Si21 and 15N1 counts (Fig. 2). A fit of the two remaining peaks, where the two peak amplitudes are the only free param-
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Fig. 2.—The fitted peak shape functions shown in Fig. 1 are subtracted from the measured counts, thus leaving the 29Si21 peak and the 30Si21 and 15 1 N peak. The different data symbols represent method M1 (filled squares), method M2 (open squares), and the average of methods M1 and M2 (crosses). The uncertainty in the difference spectrum is due to the purely statistical counting error that arises from the full count numbers, including the background, as well as the systematic uncertainty determined from the deviations in the fitted parameters of methods M1 and M2 (Table 1). In channels 456–457 and 464–465, the ADC effect is visible. The signal has been corrected by assuming that the sum of counts in two neighboring channels remains essentially conserved; the uncorrected signal in these channels is shown with open circles. Only the largest ADC effects with n0 5 3, 4 have been corrected. In fact, each pair of channels shows the effect to some extent. The fit assumes the same width for the 29Si21 peak and the 30Si21 and 15N1 peak as for the 28 Si21 and 14N1 peak and the 16O1 peak.
eters, yields 52,092 counts for 29Si21, with an uncertainty of 1615 in the x 2 fit, and 43592 counts for 30Si21 and 15N1, with an uncertainty of 1264 (all uncertainties here and in the following correspond to 1 j errors). The number of 30Si21 counts is estimated by referring to the measured solar wind 30Si/29Si ratio of 0.68 5 0.03 (Kallenbach et al. 1998b). This presumably does not deviate from the meteoritic ratio 0.664 by more than 2% because of the fractionation processes in the source region of the solar wind, so we can assume an error of 0.013. Based on the calibration data, we determined that 30Si21 is detected more efficiently than 29 Si21 by a factor of 1.04 5 0.014, leading to 30Si21 5 (0.71 5 0.017) # 29Si21 5 (36985 5 1308). This results in 6607 5 1819 counts of 15N1 in the spectrum. There are 2,232,693 5 1494 counts in the 14 1 N and 28Si21 main peak, with 980,892 5 30,408 counts
belonging to 28Si21, which is estimated from the observed 29 Si21 counts and the calibrated instrument fractionation 29 Si21/28Si21 5 1.07 5 0.014. This gives 1,251,801 5 30,445 counts of 14N1. Taking into account that the detection efficiency for 15N1 is higher by a factor of 1.055 5 0.014, this finally leads to 14N/15N 5 200 5 55 in the solar wind. To estimate the systematic uncertainty of this result, we evaluated the data separately with methods M1 and M2; we obtained 14 1 15 1 N / N 5 179 and 227, respectively. We also noted that the peak shape function displayed in Figure 2 does not match the data very well in the peak valley between the 29Si21 peak and the 30Si21 and 15N1 peak. Therefore, we derived the N isotope abundance ratio by simply adding the counts between channels 452 and 462 in Figure 2 to obtain the number of 29Si21 counts and between channel 462 and 472 for the number of 30Si21 and 15 1 N counts. This resulted in 14N1/15N1 5 165 and 210 for methods M1 and M2, respectively. We estimated that the overall systematic uncertainty in determining the N isotope abundance ratio is 28. This implies that the total experimental error of 55 is predominantly statistical and must be considered as a 1 j error. We estimate the probability to be only about 9% that the solar wind 14N/15N ratio is equal to or larger than the terrestrial ratio of 272. 3. DISCUSSION AND CONCLUSIONS
A comparison of the N isotopic composition measured in the solar wind, planetary atmospheres, meteorites, and lunar surface materials is shown in Table 2. The closest approximations to protosolar N ought to be found in the solar wind and on Jupiter. Isotopic fractionation in the solar wind is very small (Bodmer & Bochsler 1998; Kallenbach et al. 1998b), with the exception of 3He/4He, and Jupiter has probably undergone the least elemental and isotopic fractionation among the planets. Jovian N is found to be isotopically equal to or heavier than terrestrial N (Tokunaga et al. 1979; Drossart, Encrenaz, & Combes 1985), which is in agreement with the result of our direct solar wind measurement, 14N/15N 5 200 5 55, and with the more precise but indirect measurement of the solar wind component in young lunar soils (Becker & Clayton 1977; Becker & Pepin 1994; Kim, Kim, & Kerridge 1995), yielding 14N/15N values between 225 and 260. Thus, it becomes increasingly evident that the N in the terrestrial at-
TABLE 2 Variations in Solar System N Compositions 14
Source
N/15N
Reference 14
15
Values Approximating the Protosolar N/ N Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar wind component in young lunar soils . . . . . . HLN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solar wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
170 5 100 1251145 275 242 225 260 240 200 5 55
Tokunaga et al. 1979 Drossart et al. 1985 Becker & Clayton 1977 Becker & Pepin 1994 Kim et al. 1995 Geiss & Bochsler 1982 This work
Other Solar System Values Earth atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mars atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Venus atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C1 and C2 chondritesa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Apollo 11/15 lunar microbrecciasa . . . . . . . . . . . . . . . . Apollo 16 lunar drill corea . . . . . . . . . . . . . . . . . . . . . . . . . LLN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a
Bulk samples.
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272 155 5 30 270 5 55 233–266 260–320 243–300 ≥325
Nier, McElroy, & Yung 1976 Hoffman et al. 1979 Kung & Clayton 1978 Thiemens & Clayton 1980 Becker & Clayton 1977 Geiss & Bochsler 1982
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mosphere is isotopically lighter than the solar and protosolar N. Also, the N in C1 and C2 chondrites, the least chemically fractionated meteorites, is isotopically heavier than terrestrial atmospheric N. Solar system bodies that are strongly depleted in volatiles have N isotopic compositions deviating from the protosolar composition. The N of the Martian atmosphere is presumably enriched in 15N because of the preferred dynamical escape of 14N following dissociation by electron impact or photodissociation of N2 in the exosphere (McElroy, Yung, & Nier 1976). We note that terrestrial Ne and Ar are isotopically heavier than the solar and protosolar Ne and Ar (cf. Benkert et al. 1993), whereas terrestrial N appears to be isotopically lighter than protosolar N. This suggests (cf. Geiss & Bochsler 1982) that the difference between terrestrial atmospheric N and protosolar N is not caused by an isotope fractionation process but is due to the contributions of two components. The distribution of N isotopes trapped in lunar surface material also suggests
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that the variability of the 14N/15N ratio is not only due to isotope fractionation but also, at least in part, to the contributions of at least two components. This work was supported by the Swiss National Science Foundation, by the PRODEX program of ESA, by NASA grant NAG5-2754, and by DARA, Germany, with grants 50 OC 89056 and 50 OC 9605. CELIAS is a joint effort of five hardware institutions under the direction of the Max Planck Institute for Extraterrestrial Physics, Garching, Germany, (prelaunch) and the University of Bern, Switzerland (postlaunch). The University of Maryland was the prime hardware institution for MTOF, the University of Bern provided the entrance system, and the Technical University of Braunschweig, Germany, provided the Data Processing Unit. The authors would like to thank Rainer Wieler from the Swiss Federal Institute of Technology for stimulating discussions.
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