Spectroscopic Studies of the Solar Corona – V. Physical Properties of Coronal Structures

Spectra around the 6374 Å [Fex] and 7892 Å [Fexi] emission lines were obtained simultaneously with the 25-cm coronagraph at Norikura Observatory covering an area of 200 ×500 of the solar corona. The line width, peak intensity and line-of-sight velocity for both the lines were computed using Gaussian fits to the observed line profiles at each location (4 ×4 ) of the observed coronal region. The line-width measurements show that in steady coronal structures the FWHM of the 6374 Å emission line increases with height above the limb with an average value of 1.02 mÅ arc sec^–1. The FWHM of the 7892 Å line also increases with height but at a smaller average value of 0.55 mÅ arc sec^–1. These observations agree well with our earlier results obtained from observations of the red, green, and infrared emission lines that variation of the FWHM of the coronal emission lines with height in steady coronal structures depends on plasma temperatures they represent. The FWHM gradient is negative for high-temperature emission lines, positive for relatively low-temperature lines and smaller for emission lines in the intermediate temperature range. Such a behaviour in the variation of the FWHM of coronal emission lines with height above the limb suggests that it may not always be possible to interpret an increase in the FWHM of emission line with height as an increase in the nonthermal velocity, and hence rules out the existence of waves in steady coronal structures.     

Electrical Structure in Marine Tectonic Settings

This review paper presents recent research on electrical conductivity structure in various marine tectonic settings. In at least three areas, marine electromagnetic studies for structural exploration have increasingly progressed: (1) data accumulations, (2) technical advances both for hardware and software, and (3) interpretations based on multidisciplinary approaches. The mid-ocean ridge system is the best-studied tectonic setting. Recent works have revealed evidence of conductive zones of hydrothermal circulation and axial magma chambers in the crust and partial melt zones of the mid-ocean ridge basalt source in the mantle. The role of water or dissolved hydrogen and its redistribution at mid-ocean ridges is emphasized for the conductivity pattern of the oceanic lithosphere and asthenosphere. Regions of mantle upwelling (hotspot or plume) and downwelling (subducting slab) are attracting attention. Evidence of heterogeneity exists not only in the crust and the upper mantle, but also in the mantle transition zone. Electrical conductive zones frequently overlap seismic low-velocity zones, but discrepancies are also apparent. Some studies have compared conductivity models with the results of seismic and other studies to investigate the physical properties or processes. A new laboratory-based conductivity model for matured oceanic lithosphere and asthenosphere is proposed. It takes account of both the water distribution in the mantle as well as the thermal structure. It explains observed conductivity patterns in the depth range of 60–200 km.     

The 1998 Hida Mountain, Central Honshu, Japan, earthquake swarm: Double-difference event relocation, frequency–magnitude distribution and Coulomb stress changes

By using the double-difference relocation technique, we have determined the fine structure of seismicity during the 1998 Hida Mountain earthquake swarm. The distribution of seismic activity defines two main directions (N–S and E–W) that probably correspond to the regional stress pattern. The detailed structure of seismicity reveals intense spatio-temporal clustering and earthquake lineations. Each cluster of events contains a mainshock and subsequent aftershock activity that decays according to the Omori law. The seismicity and the b-value temporal and spatial patterns reflect the evolution of the static stress changes during the earthquake swarm. About 80% of the swarm's best-relocated events occur in regions of increased ΔCFF. The smaller value of b found in the northern part of the swarm region and a larger b-value observed to the south, for the same period of time, could be well explained by the static stress changes caused by the larger events of the sequence. We argue that the state of stress in the crust is the main factor that controls the variation of b-value.     

Stray-light correction in magnetograph observations using the maximum entropy method

We have developed a method of stray-light correction which is applicable to filter-based magnetograph observations. Stray-light-corrected Stokes images are obtained by performing the deconvolution of observed Stokes images by the point spread function which is determined from the Stokes I image. For image deconvolution, the maximum entropy principle is used to guarantee that intensity should be positive and polarization degrees should be less than unity. We present an iterative algorithm for the maximum entropy method, which seeks the solution in Fourier space and thus accomplishes fast convergence. We find that our method is effective in correcting stray light which has a spread angle greater than the full width at half maximum of the point spread function. We also discuss the effect of stray light on magnetograph calibration.     

High temperature creep of single crystal gadolinium gallium garnet

High temperature creep of single crystal gadolinium gallium garnet (GGG) was studied in the temperature range of 1723–1853 K (0.86–0.94 T_m, T_m: melting temperature) and strain rate from 9 · 10^?7 s^?1 to 2 · 10^?5 s^?1. The compression tests were made along the 〈100〉 and 〈111〉 orientations. We have performed both constant strain-rate and stress-dip tests. For the 〈100〉 orientation, deformation occurs via the 〈111〉 slip systems. For the 〈111〉 orientation, both the 〈100〉 {010} and the 〈111〉 slip systems can be activated. GGG garnet is very strong under these conditions: σ/μ=(1–3)×10^?3 (σ: creep strength, μ: shear modulus). The creep behavior is characterized by a power law with stress exponent n=2.9–3.3 and high activation energies E^*=612–743 kJ/mol (E^*～45×RT_m, at zero stress which decrease with the increase of stress). Stress-dip tests suggest a small internal stress (σ_i/σ～0.62; σ_i: internal stress, σ: applied stress) compared to other materials. These results suggest that the high creep strength of GGG is mainly due to difficulty of dislocation glide rather than dislocation climb.     

Crustal structure of the Mizuho Plateau, East Antarctica from geophysical data

Various kinds of geophysical surveys have been carried out in the Mizuho Plateau, East Antarctica by the Japanese Antarctic Research Expeditions (JARE). The correlation between the high-level gravity anomaly and the bedrock elevation is examined along a route where both data are sufficient to permit deriving a crustal model from gravity, radio-echo sounding and explosion seismic data. The bedrock elevation usually correlates well with the high-level gravity anomaly. However, along the traverse route S-H-Z from the Syowa to Mizuho stations, the bedrock elevation has a weak negative correlation with the high-level gravity anomaly. Such a weak negative correlation is attributed to the deeper part of the crust.The crustal structure between the Syowa and Mizuho stations is modeled from the gravimetric data and the radio-echo sounding of bedrock elevations, so as to fit the P-wave velocity structure derived from the data of explosion seismic experiments. Then the structure is extended from Syowa Station seaward across Lützow-Holm Bay and from Mizuho Station southeastwards inland, where only gravimetric data are available. Thus, a crustal section about 600 km long is obtained on a margin of East Antarctica. The depth of the Moho increases by about 7 km from Syowa Station to the point Y200 (71° 46′S, 48° 56′E), about 500 km from the coast. A graben-like structure is obtained along the line across Lützow-Holm Bay. This suggests that both sides of the bay are bounded by faults.     

Geophysical studies of crustal structure of the Ongul Islands and the Northern Mizuho Plateau, East Antarctica

Explosion seismic experiments, gravity measurements and aeromagnetic surveys were made in the northern Mizuho Plateau including the Ongul Islands, East Antarctica, from 1979 to 1982 by the Japanese Antarctic Research Expeditions. The objective of these field operations was to determine the crustal structure along the 300 km-long oversnow traverse route between Syowa and Mizuho Stations. Three big shots were fired; at sea near Syowa Station, in an ice hole near Mizuho Station and in an ice hole between both stations. Twenty-seven temporal seismic stations were set up along the route. Gravity measurements were carried out at 30 points along this route. Aeromagnetic surveys over the area were made four times.In the seismic experiments, clear refracted waves from the Conrad (estimated depth 30 km) and the Moho (estimated depth 40 km) discontinuities were recorded. No layer with a velocity of less than 6 km/s was found in the Ongul Islands nor beneath the ice sheet in the surveyed area. The P-wave velocity in the upper layer varies with depth from 6.0 km/s on the surface to 6.4 km/s at a depth of 13 km. Comparing the observed record section with synthetic seismograms, it was derived that the Conrad was not associated with a sharp velocity discontinuity, but a linear velocity increase of 0.55 km/s in a transition zone of 2.4 km thick. Velocities of P^* and P_n were determined as 6.95 km/s and 7.93 km/s assuming a flat layered structure.Bouguer gravity anomalies could not be calculated along the whole profile because of a lack of data on bedrock topography, so reduced gravity anomalies were calculated. These anomalies indicate no abrupt changes of the bedrock topography.     

Audio frequency magneto-telluric survey of Norikura Volcano in central Japan

Norikura Volcano has not been active during the last 10,000 years in spite of the activity of the surrounding volcanic mountains. To study past volcanic activities, geological studies were carried out extensively. However, quite a few geophysical investigations were conducted to contribute to volcanology. Our objective is to detect the present subsurface structure of Norikura Volcano and to define volcanic stratifications. In the vicinity of Norikura Volcano, geothermal fields are still active. Subsurface volcanic rocks in this area have been exposed to geothermal activity and altered. To comprehend volcanic stratifications of Norikura and geothermal activity, we conducted audio frequency magneto-telluric (AMT) surveys around Norikura Volcano. AMT survey is useful in clearly defining the resistivity structure related to volcanic regions. The AMT data were acquired over a frequency range 10 Hz–10 kHz. Decomposition analysis was applied to the tensor impedance data. Subsequently, apparent resistivity and phase data were inverted using a two-dimensional magneto-telluric (MT) inversion and a model of Norikura was derived. The final model manifests that the surface resistors are in agreement with andesite lava or dacite lava. As for the deeper structure, a horizontal conductor is situated above resistive basements. The alteration of the conductor was weak, while basement rocks were strongly altered and/or heated through the thermal activity. The existence of these layers seems to indicate the degree of thermal activity of Norikura Volcano.     

Latitudinal Distribution of CO2 Fugacity along 175°E in the North Pacific in 1992–1996

In order to examine latitudinal distribution and seasonal change of the surface oceanic fCO₂, we analyzed the data obtained in the North Pacific along 175°E during the NOPACCS cruises in spring and summer of 1992–1996. Except for around the equator where the fCO₂ was significantly affected by the upwelling of deep water, the latitudinal distribution of fCO₂ showed distinctive seasonal variation. In the spring, the fCO₂ decreased and then increased going southward with the minimum value of about 300 µatm around 35°N, while in the summer, the fCO₂ displayed high variability, showing minimum and maximum values at latitudes of around 44° and 35°N, respectively. It was also found that the fCO₂ was well correlated with the SST, but the relationship between the two was different for different hydrographic regions. In the subpolar gyre, the frontal regions between the Water-Mass Front and the Kuroshio bifurcation front, and between the Kuroshio bifurcation front and the Kuroshio Extension current, SST, DIC and TA influenced the seasonal fCO₂ change through seasonally-dependent biological activities and vertical mixing and stratification of seawater. In the central subtropical gyre and the North Equatorial current, the seasonal fCO₂ change was found to be produced basically by changes in SST and DIC. The summertime oceanic fCO₂ generally increased with time over the period covered by this study, but the increased rate was clearly higher than those expected from other measurements in the western North Pacific.