Impact of Alaskan Stream eddies on chlorophyll distribution in the North Pacific

The impact of the Alaskan Stream (AS) eddies on the chlorophyll a (chl-a) distribution in the central subarctic North Pacific was investigated through analysis of chl-a and altimetry data from satellite observations. Altimetry observations provided the locations of mesoscale eddies in time and space within the maps of chlorophyll distributions. The climatological chl-a distributions averaged in the area and time showing presence of AS eddies suggested that AS eddies contributed significantly to the chl-a distribution in the deep-sea region of the subarctic North Pacific. The chl-a distribution was closely related to the AS eddies regardless of whether the eddy was located in or detached from the AS. A combination of two or three AS eddies sometimes formed high chl-a concentration belts that injected chlorophyll and coastal nutrient-rich waters southward from the Aleutian Islands far into the deep-sea region of the subarctic North Pacific. These results indicate that chl-a distribution in the central subarctic North Pacific was strongly impacted by AS eddies.     

Seasonal and spatial variation in the denitrifying activity in estuarine and lagoonal sediments

We have elucidated the seasonal and spatial variation in the potential denitrifying activity in estuarine and coastal lagoonal sediments in Lakes Shinji and Nakaumi, Japan. The denitrifying activity increased from summer through autumn and was positively correlated with the temperature of the overlying water at all sites except one, where the bottom was always more reductive than at the other sites and there was no NO₃ ⁻ as a substrate for denitrification from spring to autumn. Moreover, the relationship between the denitrifying activity and the distance from the sea showed different trends in estuarine and lagoonal sediments. These spatial differences indicate that different factors regulate the denitrification in estuarine and lagoonal sediments. Denitrifying activity in estuarine sediment was regulated by the discharge of freshwater containing NO₃ ⁻ or organic matter, while in the lagoonal sediments the occurrence of nitrification via the intrusion of oxic seawater into the reductive sediment appears to be a key requirement for the process of denitrification. Therefore, the denitrifying activity in the lagoonal sediment appears to be greater near the sea. Water intrusion is one of the key factors controlling denitrification in coastal marine ecosystems by affecting the supply of substrate available for denitrification.     

Bathymetry enhancement by altimetry-derived gravity anomalies in the East Sea (Sea of Japan)

The gravity-geologic method (GGM) was used to enhance the bathymetry of the East Sea (Sea of Japan) with satellite altimetry-derived free-air gravity anomalies and shipborne depth measurements. By comparison with the bathymetry model of Smith and Sandwell’s (SAS) approach (1994), GGM was found to have an advantage with short wavelength (≤12 km) components, while SAS better predicts longer wavelength (≥25 km) components, despite its dependency on density contrast. To mitigate this limitation, a tuning density contrast of 10.25 g/cm³ between seawater and the seafloor was primarily estimated by the downward continuation method and then validated by the check points method with GGM. Similarly, SAS is limited by the “A” value in low-pass part of the Wiener filter, which defines the effective range of the wavelength components on bathymetry. As a final result, we present an enhanced GGM bathymetry model by integrating all available data.     

ASCA observations of the Galactic superluminal sources GRO J1655–40 and GRS 1915+105

We summarize the ASCA observations of the two Galactic superluminal sources GRO J1655–40 and GRS 1915+105, focusing on theabsorption line features.The high spectroscopic capability of ASCA enabled us to detectiron-K absorption line features from both objects. This is direct evidence for the presence of highly ionized plasma in a non-spherical configuration around a black hole, and is considered to be a unique signature of superluminal jet systems. The candidate of the line-absorber would be a part of an accretion disk at a distance of 10^9-11 cm from the central X-ray source.     

Observations of GRS 1915+105 from the USA Experiment on ARGOS

We present first results from a monitoring campaign of GRS 1915+105 undertaken with the USA X-ray timing experiment on the ARGOS satellite. A variety of behaviour has been observed, ranging from low, steady X-ray emission to rapid quasi-periodic flaring on timescales of approximately 10–120 seconds.     

Damped Oscillations of Coronal Loops

A mechanism of damped oscillations of a coronal loop is investigated. The loop is treated as a thin toroidal flux rope with two stationary photospheric footpoints, carrying both toroidal and poloidal currents. The forces and the flux-rope dynamics are described within the framework of ideal magnetohydrodynamics (MHD). The main features of the theory are the following: i) Oscillatory motions are determined by the Lorentz force that acts on curved current-carrying plasma structures and ii) damping is caused by drag that provides the momentum coupling between the flux rope and the ambient coronal plasma. The oscillation is restricted to the vertical plane of the flux rope. The initial equilibrium flux rope is set into oscillation by a pulse of upflow of the ambient plasma. The theory is applied to two events of oscillating loops observed by the Transition Region and Coronal Explorer (TRACE). It is shown that the Lorentz force and drag with a reasonable value of the coupling coefficient (c _d ) and without anomalous dissipation are able to accurately account for the observed damped oscillations. The analysis shows that the variations in the observed intensity can be explained by the minor radial expansion and contraction. For the two events, the values of the drag coefficient consistent with the observed damping times are in the range c _d ≈2 – 5, with specific values being dependent on parameters such as the loop density, ambient magnetic field, and the loop geometry. This range is consistent with a previous MHD simulation study and with values used to reproduce the observed trajectories of coronal mass ejections (CMEs).     

Approximate analysis for relative motion of satellite formation flying in elliptical orbits

This paper studies the relative motion of satellite formation flying in arbitrary elliptical orbits with no perturbation. The trajectories of the leader and follower satellites are projected onto the celestial sphere. These two projections and celestial equator intersect each other to form a spherical triangle, in which the vertex angles and arc-distances are used to describe the relative motion equations. This method is entitled the reference orbital element approach. Here the dimensionless distance is defined as the ratio of the maximal distance between the leader and follower satellites to the semi-major axis of the leader satellite. In close formations, this dimensionless distance, as well as some vertex angles and arc-distances of this spherical triangle, and the orbital element differences are small quantities. A series of order-of-magnitude analyses about these quantities are conducted. Consequently, the relative motion equations are approximated by expansions truncated to the second order, i.e. square of the dimensionless distance. In order to study the problem of periodicity of relative motion, the semi-major axis of the follower is expanded as Taylor series around that of the leader, by regarding relative position and velocity as small quantities. Using this expansion, it is proved that the periodicity condition derived from Lawden’s equations is equivalent to the condition that the Taylor series of order one is zero. The first-order relative motion equations, simplified from the second-order ones, possess the same forms as the periodic solutions of Lawden’s equations. It is presented that the latter are further first-order approximations to the former; and moreover, compared with the latter more suitable to research spacecraft rendezvous and docking, the former are more suitable to research relative orbit configurations. The first-order relative motion equations are expanded as trigonometric series with eccentric anomaly as the angle variable. Except the terms of order one, the trigonometric series’ amplitudes are geometric series, and corresponding phases are constant both in the radial and in-track directions. When the trajectory of the in-plane relative motion is similar to an ellipse, a method to seek this ellipse is presented. The advantage of this method is shown by an example.     

Coronal Holes and Solar Wind High-Speed Streams: II. Forecasting the Geomagnetic Effects

We present a simple method of forecasting the geomagnetic storms caused by high-speed streams (HSSs) in the solar wind. The method is based on the empirical correlation between the coronal hole area/position and the value of the Dst index, which is established in a period of low interplanetary coronal mass ejection (ICME) activity. On average, the highest geomagnetic activity, i.e., the minimum in Dst, occurs four days after a low-latitude coronal hole (CH) crosses the central meridian. The amplitude of the Dst dip is correlated with the CH area and depends on the magnetic polarity of the CH due to the Russell – McPherron effect. The Dst variation may be predicted by employing the expression Dst(t)=(−65±25×cos λ)[A(t ^*)]^0.5, where A(t ^*) is the fractional CH area measured in the central-meridian slice [−10°,10°] of the solar disc, λ is the ecliptic longitude of the Earth, ± stands for positive/negative CH polarity, and t−t ^*=4 days. In periods of low ICME activity, the proposed expression provides forecasting of the amplitude of the HSS-associated Dst dip to an accuracy of ≈30%. However, the time of occurrence of the Dst minimum cannot be predicted to better than ±2 days, and consequently, the overall mean relative difference between the observed and calculated daily values of Dst ranges around 50%.     

ISA accelerometer onboard the Mercury Planetary Orbiter: error budget

We have estimated a preliminary error budget for the Italian Spring Accelerometer (ISA) that will be allocated onboard the Mercury Planetary Orbiter (MPO) of the European Space Agency (ESA) space mission to Mercury named BepiColombo. The role of the accelerometer is to remove from the list of unknowns the non-gravitational accelerations that perturb the gravitational trajectory followed by the MPO in the strong radiation environment that characterises the orbit of Mercury around the Sun. Such a role is of fundamental importance in the context of the very ambitious goals of the Radio Science Experiments (RSE) of the BepiColombo mission. We have subdivided the errors on the accelerometer measurements into two main families: (i) the pseudo-sinusoidal errors and (ii) the random errors. The former are characterised by a periodic behaviour with the frequency of the satellite mean anomaly and its higher order harmonic components, i.e., they are deterministic errors. The latter are characterised by an unknown frequency distribution and we assumed for them a noise-like spectrum, i.e., they are stochastic errors. Among the pseudo-sinusoidal errors, the main contribution is due to the effects of the gravity gradients and the inertial forces, while among the random-like errors the main disturbing effect is due to the MPO centre-of-mass displacements produced by the onboard High Gain Antenna (HGA) movements and by the fuel consumption and sloshing. Very subtle to be considered are also the random errors produced by the MPO attitude corrections necessary to guarantee the nadir pointing of the spacecraft. We have therefore formulated the ISA error budget and the requirements for the satellite in order to guarantee an orbit reconstruction for the MPO spacecraft with an along-track accuracy of about 1 m over the orbital period of the satellite around Mercury in such a way to satisfy the RSE requirements.