A study of Mg and K isotopes in Allende CAIs: Implications to the time scale for the multiple heating processes

Abstract— The measurements of magnesium and potassium isotopic compositions of refractory minerals in Allende calcium‐aluminum‐rich inclusions (CAIs), 7R‐19–1, HN3–1, and EGG3 were taken by secondary ion mass spectrometry (SIMS). The 7R‐19–1 contains ¹⁶O‐rich and ¹⁶O‐poor melilite grains and define a single isochron corresponding to an initial ²⁶Al/²⁷Al ratio of (6.6 ± 1.3) × 10^?5. The Al‐Mg isochron, O isotope measurements and petrography of melilite in 7R‐19–1 indicate that ¹⁶O‐poor melilite crystallized within 0.4 Myr after crystallization of ¹⁶O‐rich melilite, suggesting that oxygen isotopic composition of the CAI‐forming region changed from ¹⁶O‐rich to ¹⁶O‐poor within this time interval. The ¹⁶O‐poor melilite is highly depleted in K compared to the adjacent ¹⁶O‐rich melilite, indicating evaporation during remelting of 7R‐19–1. We determined the isochron for ⁴¹Ca‐⁴¹K isotopic systematics in EGG3 pyroxene with (4.1 ± 2.0) × 10^?9 (2s?) as an initial ratio of ⁴¹Ca/⁴⁰Ca, which is at least two times smaller than the previous result (Sahijipal et al. 2000). The ratio of ⁴¹Ca/⁴⁰Ca in the EGG3 pyroxene grain agrees within error with the value obtained by Hutcheon et al. (1984). No evidence for the presence of ⁴¹K excess (decay product of a short‐lived radionuclide ⁴¹Ca) was found in 7R‐19–1 and HN3–1. We infer that the CAI had at least an order of magnitude lower than canonical ⁴¹Ca/⁴⁰Ca ratio at the time of the CAI formation.     

Hydrometeorological behaviour of pine and larch forests in eastern Siberia

Seasonal changes in the water and energy exchanges over a pine forest in eastern Siberia were investigated and compared with published data from a nearby larch forest. Continuous observations (April to August 2000) were made of the eddy‐correlation sensible heat flux and latent heat flux above the canopy. The energy balance was almost closed, although the sum of the turbulent fluxes sometimes exceeded the available energy flux (R_n ? G) when the latent heat flux was large; this was related to the wind direction. We examined the seasonal variation in energy balance components at this site. The seasonal variation and magnitude of the sensible heat flux (H) was similar to that of the latent heat flux (λE), with maximum values occurring in mid‐June. Consequently, the Bowen ratio was around 1·0 on many days during the study period. On some clear days just after rainfall, λE was very large and the sum of H and λE exceeded R_n ? G. The evapotranspiration rate above the dry canopy from May to August was 2·2 mm day^?1. The contributions of understory evapotranspiration (E_u) and overstory transpiration (E_o) to the evapotranspiration of the entire ecosystem (E_t) were both from 25 to 50% throughout the period analysed. These results suggest that E_u plays a very important role in the water cycle at this site. From snowmelt through the tree growth season (23 April to 19 August 2000), the total incoming water, comprised of the sum of precipitation and the water equivalent of the snow at the beginning of the melt season, was 228 mm. Total evapotranspiration from the forest, including interception loss and evaporation from the soil when the canopy was wet, was 208–254 mm. The difference between the incoming and outgoing amounts in the water balance was from +20 to ?26 mm. The water and energy exchanges of the pine and larch forest differed in that λE and H increased slowly in the pine forest, whereas λE increased rapidly in the larch forest and H decreased sharply after the melting season. Consequently, the shape of the Bowen ratio curves at the two sites differed over the period analysed, as a result of the differences in the species in each forest and in soil thawing. Copyright © 2003 John Wiley & Sons, Ltd.     

Northwest Africa 1950: Mineralogy and comparison with Antarctic lherzolitic shergottites

Abstract— NWA 1950 is a new lherzolitic shergottite recently recovered from Morocco and is the first sample of this group found outside Antarctica. Major constituent phases of NWA 1950 are olivine, pyroxenes, and plagioclase glass (“maskelynite”) and the rock shows a two distinct textures: poikilitic and non‐poikilitic typical of lherzolitic shergottites. In poikilitic areas, several‐millimeter‐sized pyroxene oikocrysts enclose cumulus olivine and chromite. In contrast, pyroxenes are much smaller in non‐poikilitic areas, and olivine and plagioclase glass are more abundant. Olivine in non‐poikilitic areas is more Fe‐rich (Fa_29–31) and shows a narrower distribution than that in poikilitic areas (Fa_23–29). Pyroxenes in non‐poikilitic areas are also more Fe‐rich than those in poikilitic areas that show continuous chemical zoning suggesting fractional crystallization under a closed system. These observations indicate that pyroxene in non‐poikilitic areas crystallized from evolved interstitial melts and olivine was re‐equilibrated with such melts. NWA 1950 shows similar mineralogy and petrology to previously known lherzolitic shergottites (ALH 77005, LEW 88516, Y‐793605 and GRV 99027) that are considered to have originated from the same igneous body on Mars. Olivine composition of NWA 1950 is intermediate between those of ALH 77005‐GRV 99027 and those of LEW 88516‐Y‐793605, but is rather similar to ALH 77005 and GRV 99027. The subtle difference of mineral chemistry (especially, olivine composition) can be explained by different degrees of re‐equilibration compared to other lherzolitic shergottites, perhaps due to different location in the same igneous body. Thus, NWA 1950 experienced a high degree of re‐equilibration, similar to ALH 77005 and GRV 99027.     

Theory for the critical ionization velocity phenomenon

A theoretical consideration is given to the critical ionization velocity. A macroscopic theory is constructed, assuming that (1) the newly born ions have an unstable velocity distribution and (2) the resultant fluctuating electric field heats the electrons. For the critical ionization velocity to operate in the case of $\text{m}{}_{\mathrm{a}}\text{V}{}^2\text{2}\text{> eφ}{}_{\mathrm{<mtext>ion</mtext>}}$, the initial values of high speed ion (or newly ionized ion) density and suprathermal electron energy density should be above a certain threshold. This threshold depends upon the velocity diffusion co-efficient and the collisional loss rate of the newly ionized ion.     

Magnetic equilibria and instabilities

Solar flares are understood as a process of explosive liberation of magnetic energy, coming after a slow phase of energy build-up. The slow evolution of magnetic equilibria may end up with (a) the termination of an equilibrium sequence, or (b) an instability. The distinction between the two can be made by drawing schematic potential curves. Case (a) has been extensively studied in two-dimensional models. The appearance of multiple solutions, or disappearance of a solution takes place as the system evolves away from the current-free configuration. Case (b) can be discussed in terms of ideal MHD or resistive MHD instabilities. A possible route to explosive energy release is suggested by combining these two cases.     

Green's function methods for potential magnetic fields

The Green's function method to calculate potential magnetic field on the Sun, which was first established by Schmidt (1964) in the case that the field component normal to a flat boundary plane is specified, is extended to the following three cases: (a) The field component along the line of sight, which is not generally normal to the flat boundary plane, is specified; (b) the line of sight component on a spherical boundary surface is specified; (c) the normal component on a spherical surface is specified, together with the condition that the field becomes approximately radial on an outer spherical surface (the so-called source surface). Properties of these Green's functions are examined, and the applicability of these methods to solar magnetic data is discussed.On leave of absence from Department of Astronomy, University of Tokyo.     

Effects of solar wind parameters on the development of magnetospheric substorms

Effects of solar wind parameters on the development of substorms during the events of southward interplanetary magnetic field (IMF) lasting more than one hour were studied. Analysis on 175 events with average magnitude of the southward component of IMF larger than l·5γ as observed in July–December 1965 lead to the following results: (1) The total auroral electrojet (AEJ) current associated with the southward IMF event is approximately proportional to the time integral of the magnitude of the southward component. (2) The azimuthal component of IMF also affects the AEJ development. AEJ about twice as intense were observed when IMF was directed duskward than when IMF was directed dawnward. (3) AEJ intensity is strongly affected by the solar wind velocity during the southward IMF events, the intensity being approximately proportional to the square of the velocity. (4) No indication was found that the angle between the Sun-Earth line and the Earth's dipole axis plays any role on the development of substorms if effects of the solar wind parameters as described above are eliminated.     

Multifractal and Chaotic Analysis of Vrancea (Romania) Intermediate-depth Earthquakes: Investigation of the Temporal Distribution of Events

The Vrancea seismic region contains an isolated cluster of events beneath the Carpathian Arc Bend in Romania, dipping to about 200 km depth. Seismic activity mainly occurs at intermediate depths (h > 60 km). The main goal of the paper is to perform an in-depth, complex analysis of the occurrence times of these intermediate-depth events. We also try to show the versatility of the methods used to characterize different aspects of the seismicity evolution and to offer a user-friendly software toolbox to do most of the related computations. The earthquake catalog used in this study spans from 1974 to 2002 and includes only the intermediate-depth events. In the first part of the paper, we analyze the multifractal characteristics of the temporal distribution of earthquakes. The study reveals two distinct scaling regimes. At small scales we found a clear nonhomogeneous, multifractal pattern, while at large scales the temporal distribution of events shows a monofractal, and close to Poissonian (random), behavior. The multifractal behavior at small scales (minutes-hours) is shown to be clearly an effect of the short aftershock sequences that occurred after some major Vrancea earthquakes. In the second part of the paper we analyze whether our temporal series shows a persistent (or anti-persistent) long-term behavior, by using the Detrended Fluctuation Analysis (DFA) method. The results suggest that the analyzed temporal series of Vrancea earthquakes is a non-correlated process. In part three of the paper we seek to determine whether the dynamics of our earthquake system (described by the occurrence time of Vrancea earthquakes) is deterministically chaotic, deriving from a rather simple evolution law, or whether it is stochastic and is generated by a system that possesses many degrees of freedom. The results suggest that our signal is stochastic (probably does not possess an attractor). The limited time-span of the catalog and the analysis performed in this paper cannot rule out the emergence of an interesting, quasi-deterministic and low-dimensional structure in the case of major Vrancea earthquakes.Acknowledgement One of the authors (BE) is grateful to the Japanese Ministry of Education for providing him a Monbusho scholarship for studying in DPRI, Kyoto University. We thank Z.R. Struzik, M Holschneider, J. Mori and D. Kaplan for their useful comments, and acknowledge the support of the staff of DPRI, Kyoto University and the National Inst. for Earth Physics, Bucharest. We thank the two reviewers, M.B. Geilikman and M. Anghel, for their useful suggestions which improved the quality of this work.     

Spectroscopic Studies of Solar Corona VII. Formation of a Coronal Loop by Evaporation

We obtained time-sequence spectroscopic observations in (Fe x) 6374 Å and (Fe xiv) 5303 Å lines successively with the 25-cm coronagraph, and narrow-band and Doppler images in 5303 Å line by the 2-D 10-cm Doppler coronagraph NOGIS at the Norikura Solar Observatory, of a coronal region for about 7 h on 9 19–20, 2001. The raster scans were obtained with a quasi-periodicity of about 14 min and NOGIS obtained the images with an interval of about 1 min. The coronal region observed showed the formation of a coronal loop by a high-speed surge in the 6374 Å line rising from one of the footpoints of the loop. Off the limb spectroscopic observations in the 6374 Å line showed large velocities along the line of sight and vertical to the solar limb at the time of formation of the loop. The 5303 Å line observations showed negligible line-of-sight velocities and low vertical velocities when compared to those in the 6374 Å line. A hump in the intensity plots in 5303 Å with height appears to move up with respect to the solar limb with an average velocity of 4km s^–1. The FWHM of the 6374 Å showed a much smaller value of about 0.7 Å near the foot point as compared to a value of 1.2 Å at larger heights at the beginning of observations. Later as the loop developed, the FWHM of 6374 Å line showed a gradual decrease along the loop up to 70 from the limb, reached a minimum value of about 0.5 Å and then increased with height during the formation of the loop; this trend lasted for about 2 h. About 3 h after the beginning of the formation of the loop, the FWHM of 6374 Å emission line showed normal values and normal rate of increase with height with some fluctuations. The FWHM of the 5303 Å line did not show such variations along the loop and showed normal decrease in FWHM with height found earlier (Singh et al., 2003a). These observations suggest that a relatively cooler plasma at a temperature of about 0.7 MK or less (corresponding to minimum value of FWHM of 0.5 Å) was ejected from the transition region with a large velocity of about 48km s^–1, heated up in the corona by some process and formed a coronal loop with a height of about 200 above the limb that had lifetime greater than 4 h. It appears that the plasma moved from one of the footpoints and the loop was formed by evaporation of chromospheric plasma. No large-scale brightening and H flare were observed in this region during the observational period of 7 h.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.