Earth, Planets and Space

Recent far-ultraviolet (FUV) observations of Earth have shown the remarkable spatial correspondence between the amplitude of non-migrating atmospheric tides originating in the troposphere and the density and morphology of the nighttime equatorial ionospheric anomaly (EIA). This is likely a result of the modulation of the E-region dynamo electric field in daytime by the tidal winds. FUV observations around the time of the vernal equinox of 2002 show that the signature of tidal influence, the wave-4 periodicity in the separation and density of the two EIA bands, itself exhibits significant temporal variability. Here, we seek to understand this variability, and whether (or not) it is linked to variations in the strength of the upward-propagating tides. This study relies on tidal measurements provided by the global observations from the TIMED-SABER instrument that measures the temperature variations in the MLT associated with the upward-propagating tides. TIMED-GUVI provides F-region density measurements concurrent to the MLT temperature retrievals. It is found that the atmospheric and ionospheric zonal wave-4 signatures very nearly covary over a 30-day period, strongly supporting the theory that the influence of the the diurnal eastward 3 (DE3) tide originating in the troposphere extends to the F-layer of the ionosphere. Additionally, a 6-day periodicity in the power of the ionospheric wave-4 signature is found that may originate with the tide’s interaction with longer period planetary waves.

We studied the assimilation of high-resolution precipitable water vapor (PWV) data derived from a hyper-dense global navigation satellite system network around Uji city, Kyoto, Japan, which had a mean inter-station distance of about 1.7 km. We focused on a heavy rainfall event that occurred on August 13–14, 2012, around Uji city. We employed a local ensemble transform Kalman filter as the data assimilation method. The inhomogeneity of the observed PWV increased on a scale of less than 10 km in advance of the actual rainfall detected by the rain gauge. Zenith wet delay data observed by the Uji network showed that the characteristic length scale of water vapor distribution during the rainfall ranged from 1.9 to 3.5 km. It is suggested that the assimilation of PWV data with high horizontal resolution (a few km) improves the forecast accuracy. We conducted the assimilation experiment of high-resolution PWV data, using both small horizontal localization radii and a conventional horizontal localization radius. We repeated the sensitivity experiment, changing the mean horizontal spacing of the PWV data from 1.7 to 8.0 km. When the horizontal spacing of assimilated PWV data was decreased from 8.0 to 3.5 km, the accuracy of the simulated hourly rainfall amount worsened in the experiment that used the conventional localization radius for the assimilation of PWV. In contrast, the accuracy of hourly rainfall amounts improved when we applied small horizontal localization radii. In the experiment that used the small horizontal localization radii, the accuracy of the hourly rainfall amount was most improved when the horizontal resolution of the assimilated PWV data was 3.5 km. The optimum spatial resolution of PWV data was related to the characteristic length scale of water vapor variability.

We investigate the theoretical background for the retrieval of the tsunami Green’s function from the cross-correlation of continuous ocean waves. Considering that a tsunami is a long-wavelength ocean wave described by 2-D linear long-wave equations, and that the sea-bottom topography acts as a set of point-like scatterers, we use a first-order Born approximation in deriving the tsunami Green’s function having coda waves. The scattering pattern is non-isotropic and symmetrical with respect to the forward and backward directions. We indicate a retrieval process which shows that the derivative of the cross-correlation function of wavefields at two receivers with respect to the lag time gives the tsunami Green’s function when point noise sources generating continuous ocean waves are distributed far from, and surrounding, the two receivers. Note that this relation between the cross-correlation and the Green’s function is different from the case in which uncorrelated plane-wave incidence from all directions is assumed to be continuous ocean waves. The Green’s function retrieved from continuous ocean waves will be used as a reference to examine the validity of the Green’s function obtained by numerical simulations.

A possible driver of precipitation of magnetospheric energetic electrons in the high-latitude atmosphere is represented by electromagnetic ion-cyclotron (EMIC) magnetospheric waves. The precipitating particles produce variations, by collision, in the ionized component of the atmosphere, altering its chemistry and electrical conductivity, with a significant impact on the atmospheric processes. In this framework, it would be significant to find experimental evidence of a correspondence between ionospheric electron density irregularities and the occurrence of Ultra-Low-Frequency (ULF) Pc1 geomagnetic pulsations, i.e., the ground signatures of EMIC waves, at high latitudes. In this work, we face this subject by considering a specific case study occurred on 22 February 2007 during quiet magnetospheric conditions. The study is based on the analysis of simultaneous ULF geomagnetic field and Total Electron Content (TEC) measurements recorded at Mario Zucchelli Station in Antarctica. We show that Pc1 pulsations occur in correspondence to solar wind pressure increases and that, at the same time, the ionosphere is characterized by the presence of ionospheric irregularities. We suggest that a possible link between the Pc1 activity and the ionospheric irregularities may be energetic electron precipitations, driven by EMIC waves generated in the compressed magnetosphere, which produce density variations in the ionized component of the atmosphere.

An ascending liquid-fuel rocket is known to make a hole in the ionosphere, or localized electron depletion, by leaving behind large amounts of neutral molecules (e.g. water) in the exhaust plume. Such a hole was made by the January 24, 2006 launch of an H-IIA rocket from Tanegashima, Southwestern Japan, and here we report its observation with a dense array of Global Positioning System receivers as a sudden and temporary decrease of total electron content. The observed disturbances have been compared with a simple numerical model incorporating the water diffusion and chemical reactions in the ionosphere. The substantial vanishing of the ionosphere lasted more than one hour, suggesting its application as a window for ground-based radio astronomical observations at low frequencies.

We present preliminary results of an electromagnetic survey over a region of hydrothermal circulation in the spreading axis of the central Mariana Trough. The Magnetometric Resistivity (MMR) method is used to determine the electrical resistivity structure of the oceanic crust around the hydrothermal system. 1-D electrical resistivity structure is calculated from the relation between the amplitude of magnetic field and source-receiver separation. The amplitudes of magnetic field variation of ocean bottom magnetometers (OBMs) placed on axis are larger than those of OBMs about 700 m away from the spreading axis, for source-receiver separations larger than 400 m. We estimated two resistivity structure models: one for the axial OBMs, and another for the off-axis OBMs. A region of lower resistivity between depths of 100–300 m is identified in the on-axis model. The hydrothermal source of the Alice Springs Field probably exists beneath the spreading axis, and the size of this source is smaller than 700 m. Lower resistivities at a depth of 50 m are identified in both the on-axis and the off-axis models, suggesting that the porosity in the uppermost oceanic crust is largest at this depth.

The ~30-km-long surface ruptures associated with the M w 7.0 (M j 7.3) earthquake at 01:25 JST on April 16 in Kumamoto Prefecture appeared along the previously mapped ~100-km-long active fault called the Futagawa-Hinagu fault zone (FHFZ). The surface ruptures appeared to have extended further west out of the main FHFZ into the Kumamoto Plain. Although InSAR analysis by Geospatial Information Authority of Japan (GSI) indicated coseismic surface deformation in and around the downtown of Kumamoto City, the surface ruptures have not been clearly mapped in the central part of the Kumamoto Plain, and whether there are other active faults other than the Futagawa fault in the Kumamoto Plain remained unclear. We produced topographical stereo images (anaglyph) from 5-m-mesh digital elevation model of GSI, which was generated from light detection and ranging data. We interpreted them and identified that several SW-sloping river terraces formed after the deposition of the pyroclastic flow deposits related to the latest large eruption of the Aso caldera (86.8–87.3 ka) are cut and deformed by several NW-trending flexure scarps down to the southwest. These 5.4-km-long scarps that cut across downtown Kumamoto were identified for the first time, and we name them as the Suizenji fault zone. Surface deformation such as continuous cracks, tilts, and monoclinal folding associated with the main shock of the 2016 Kumamoto earthquake was observed in the field along the fault zone. The amount of vertical deformation (~0.1 m) along this fault associated with the 2016 Kumamoto earthquake was quite small compared to the empirically calculated coseismic slip (0.5 m) based on the fault length. We thus suggest that the slip on this fault zone was triggered by the Kumamoto earthquake, but the fault zone has potential to generate an earthquake with larger slip that poses a high seismic risk in downtown Kumamoto area.

The present paper proposes a technique for low-frequency and trend compensation of broadband seismograms, which involves frequency-band broadening using digital filtering and background-trend compensation based on a Heaviside-type tilt signal estimated using a stochastic trend model. Applying this method to the east-west component of broadband seismograms, recorded using VSE355G3 broadband seismometers at the KSN site of F-net for the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0), allowed a sharpened velocity waveform to be obtained and the static displacement associated with the earthquake to be determined.