Impact of Mascarene High variability on the East African ‘short rains’

The interannual variability of East African ‘short rains’ (EASR) and its link with the Mascarene High (MH) variation are explored, using observations and reanalysis data. Correlation and composite analyses for flood and drought events reveal that the EASR variability is strongly linked to the MH zonal displacement, in particular, the zonal movement of the MH eastern ridge. When the MH eastern ridge is anomalously displaced to the west (east) of its normal position, the south east (SE) trade winds over the South Indian Ocean (SIO) anomalously strengthen (weaken). This enhances (reduces) the relatively cool and dry SE trade winds and induces cold (warm) sea surface temperature anomaly in the SIO. As a result, convection over the western equatorial SIO is suppressed (enhanced) and leads to rainfall deficits (excess) over East Africa. Droughts in East Africa are associated with a westward migration of the MH eastern ridge, while the relationship is less clear for flood events and their link to an eastward migration of the MH. Therefore, the zonal migration of the MH eastern ridge provides a novel indicator for the EASR extremes especially droughts. This revelation has immense social application for rainfall forecast over East Africa where rainfall deficits have become more prevalent against the background of deteriorating conventional forecasts for EASR droughts.     

Radiation characteristics of quasi-periodic radio bursts in the Jovian high-latitude region

Ulysses had a “distant encounter” with Jupiter in February 2004. The spacecraft passed from north to south, and it observed Jovian radio waves from high to low latitudes (from +80° to +10°) for few months during its encounter. In this study, we present a statistical investigation of the occurrence characteristics of Jovian quasi-periodic bursts, using spectral data from the unified radio and plasma wave experiment (URAP) onboard Ulysses. The latitudinal distribution of quasi-periodic bursts is derived for the first time. The analysis suggested that the bursts can be roughly categorized into two types: one having periods shorter than 30 min and one with periods longer than 30 min, which is consistent with the results of the previous analysis of data from Ulysses’ first Jovian flyby [MacDowall, R.J., Kaiser, M.L., Desch, M.D., Farrell, W.M., Hess, R.A., Stone, R.G., 1993. Quasi-periodic Jovian radio bursts: observations from the Ulysses radio and plasma wave. Experiment. Planet. Space Sci. 41, 1059-1072]. It is also suggested that the groups of quasi-periodic bursts showed a dependence on the Jovian longitude of the sub-solar point, which means that these burst groups are triggered during a particular rotational phase of the planet. Maps of the occurrence probability of these quasi-periodic bursts also showed a unique CML/MLAT dependence. We performed a 3D ray tracing analysis of the quasi-periodic burst emission to learn more about the source distribution. The results suggest that the longitudinal distribution of the occurrence probability depends on the rotational phase. The source region of quasi-periodic bursts seems to be located at an altitude between 0.4 and 1.4 Rj above the polar cap region (L>30).     

Occurrence and source characteristics of the high-latitude components of Jovian broadband kilometric radiation

Ulysses had a “distant encounter” with Jupiter when it was within 0.8 AU of the planet during February, 2004. The passage of the spacecraft was from north to south, and observations of the Jovian radio waves were carried out for a few months from high to low latitudes (+80° to +10°) of Jupiter. The statistical study performed during this “distant encounter” event provided the occurrence characteristics of the Jovian broadband kilometric radiation (bKOM), including the high-latitude component as follows: (1) the emission intensity of bKOM was found to have a sinusoidal dependence with respect to the central meridian longitude (CML), showing a broad peak at ∼180°, (2) bKOM was preferably observed in the magnetic latitudinal range from ∼+30° to +90°, and the emission intensities at the high latitudes were found to be two times larger than that at the equatorial region, and (3) the emission intensity was controlled possibly by the sub solar longitude (SSL) of Jupiter. The intensity had a sharp peak around SSL ∼210°. A 3D ray tracing approach was applied to the bKOM in order to examine the source distribution. It was suggested that: (1) the R-X mode waves generated through the Cyclotron Maser Instability process would be unable to reproduce the intense high-latitude component of the bKOM, (2) the L-O mode, which was assumed to be generated at frequencies near the local plasma frequency, was considered to be the dominant mode for past and present observations at mid- and high-latitudinal regions, and (3) the high-latitude component of bKOM was found to have a source altitude of 0.9-1.5 Rj (Rj: Jovian radii), and to be distributed along magnetic field lines having L>10.     

Cation diffusion in olivine—III. Mn2SiO4 system

The diffusion coefficients of divalent cations, Ni, Co, Mn, Ca, Sr and Ba in Mn-olivine, Mn₂SiO₄, were determined experimentally by the use of ⁶³Ni, ⁶⁰Co, ⁵⁴Mn, ⁴⁵Ca, ⁸⁵Sr and ¹³³Ba radioactive tracers, respectively. The empirical relationship between the diffusion coefficient and ionic size of diffusional species shows a trend of variation which was similar to that observed in Mg-olivine, Mg₂SiO₄. The similarity in the trend of variation of diffusion in various olivine systems strongly suggests that the trend is characteristic to the crystal structure of olivine. However, the diffusion coefficient of each cation in Mn-olivine was higher than that in Mg-olivine by about two orders of magnitude. It is suggested that this is due to the loose structure of Mn-olivine compared with that of Mg-olivine.     

Cation diffusion in olivine—II. Ni-Mg,Mn-Mg,Mg and Ca

Diffusivities of bivalent cations. Mg, Ni. Mn and Ca, in olivine were determined experimentally. The diffusivities of Ni and Mn in forsterite were determined by couple annealing between Ni₂SiO₄ and Mg₂SiO₄, and Mn₂SiO₄ and Mg₂SiO₄, respectively. The diffusivities of Mg and Ca in forsterite were determined by the use of ²⁶Mg and Ca tracers, respectively. Combined with other published results, the diffusion coefficients for bivalent cation diffusion in pure forsterite along the c crystallographic axis range from 2.45 × 10^?11 to 1.4 × 10^?13 cm² sec^?1 at 1200°C, in the order of Fe > Mn > Co > Ni > Mg > Ca. The results suggest that the diffusivity is governed by at least two factors, i.e. the size of the diffusing ionic species and the change of defect density in the crystal structure which is induced by substitution of diffusing ion for Mg ion.     

Locations of Jovian decametric radiation sources

The location of the Jovian decametric radiation main source is determined to be the south magnetic pole while the location of the early source is found to be near the north magnetic pole, with an equal contribution from a region near the south magnetic pole. The results are based on calculations of the region observable from the Earth (ROE) for Jovian decametric radio waves that are emitted in the direction ± 10° centered on the direction perpendicular to the Jovian magnetic field and based on a Pioneer 11 model of the field at the level of the topside region of the Jovian ionosphere. Ground-based observations of the occurrence frequency of the decametric radiation as a function of Jovian longitude, which indicate a remarkable asymmetry between the early and main sources, agree with the calculated ROE area that varies as a function of CML observed from the Earth. The observations support a recent theory for the origin of the decametric radiation which is based on a wave-mode conversion from plasma waves into electromagnetic waves.     

Study of the structure of silica film by infrared spectroscopy and electron diffraction analyses

Amorphous silicon oxide films have been studied on the basis of electron diffraction (ED) analyses and infrared (IR) spectroscopy in order to elucidate the relationship between the structures. After the heat treatment of the film in air at 300 and 500°C, the ED pattern showed halo rings, and the IR spectra clearly changed. Intensity analysis of the ED pattern provided evidence for the structural change of the amorphous film. It was concluded that the spectral changes in the ranges of 9.2–10.2, 12.5–13.5 and 19.5–22.5 μm were the result of phase transitions of the microcrystallites of α-cristobalite to β-cristobalite, and α- or β-quartz. Astrophysical implications have been discussed.     

Cation diffusion in olivine—I. Cobalt and magnesium

Cobalt and magnesium interdiffusion coefficients in synthetic crystals of olivine have been determined by a method of couple annealing. These coefficients increase with temperature and Co concentration. The coefficients in forsterite along the c crystallographic axis range from 1.13 × 10^?12 to 6.85 × 10^{?11 cm2sec?1} at temperatures ranging from 1150 to 1400°C. The calculated activation energies for Co-Mg interdiffusion in forsterite are 526 kJmol^?1 above approximately 1300°C and 196 kJmol^?1 at lower temperatures. These results indicate that the Co-Mg mobility in olivine is relatively low compared to published results for Fe-Mg interdiffusion.     

Relations between turbulent regions of interplanetary magnetic field and Jovian decametric radio wave emissions from the main source

Jovian decametric radio wave emissions that were observed at Goddard Space Flight Center, U.S.A. for a period from 1 October to 31 December, 1974 and data obtained at Mt Zao observatory, Tohoku University, Japan, for a period from 14 July to 6 December, 1975 have been used to investigate the relationship of the occurrence of the Jovian decametric radio waves (JDW), from the main source, to the geomagnetic disturbance index, ΣK_p. The dynamic cross-correlation between JDW and ΣK_p indicates an enhanced correlation for certain values of delay time. The delay time is consistent with predicted values based on a model of rotating turbulent regions in interplanetary space associated with two sector boundaries of the interplanetary magnetic field, i.e. the rotating sector boundaries of the interplanetary magnetic field first encounter the Earth's magnetosphere producing the geomagnetic field disturbances, and after a certain period, they encounter the Jovian magnetosphere. There are also cases where the order of the encounter is opposite, i.e. the sector boundaries encounter first Jovian magnetosphere and encounter the Earth's magnetosphere after a certain period.