Time-dependent convection at high latitudes

A fully time-dependent ionospheric convection model, in which electric potentials are derived by an analytic solution of Laplaces equation, is described. This model has been developed to replace the empirically derived average convection patterns currently used routinely in the Sheffield/SEL/UCL coupled thermosphere/ionosphere/plasmasphere model (CTIP) for modelling disturbed periods. Illustrative studies of such periods indicate that, for the electric field pulsation periods imposed, long-term averages of parameters such as Joule heating and plasma density have significantly different values in a time-dependent model compared to those derived under the same mean conditions in a steady-state model. These differences are indicative of the highly non-linear nature of the processes involved.     

Slowness and azimuth at the Uppsala array

Summary Relative arrival times at the Uppsala tripartite seismograph array station are used to determinedT/d and ray azimuth for some 200 compressional phases. Corrections, although very small, are applied for station elevations, telephonic transmission delays and array geometry. The computeddT/d and ray azimuths are further corrected to remove the effect of lateral heterogeneity immediately beneath the array station. Errors indT/d and azimuth are considered from a partly theoretical, partly empirical approach. They amount to about 0.18 sec/deg indT/d and 1.6° in azimuth. Standard deviations in epicentral locations are empirically determined to be about 2° due to slowness error and less than 1.5° due to azimutherror; or about 250 km overall. These figures compare favourably with other, more costly, arrays.     

Role of magnetospheric convection and precipitation in the formation of the “dusk effect” during the main phase of a magnetic storm

This paper studies the role of magnetospheric factors, such as convection and energetic electron precipitation during the formation of positive disturbances in the total electron content under the conditions of the summer evening ionosphere. à numerical model of the ionosphere and plasmasphere, where time variations in the magnetospheric convection velocity and electron precipitation parameters correspond to the main phase of a magnetic storm, has been used for this purpose. It has been indicated that the total electron content sharply increases (the “dusk effect”) in the eastern and western sectors at approximately the same geomagnetic latitudes corresponding to the subauroral zone provided that a sudden storm commencement is registered in the morning hours. local time. This peak of the total electron content is formed as a result of joint reconstruction of the magnetospheric convection pattern and energetic electron precipitation during the main phase of a storm. In this case, magnetospheric convection plays the main role, raising the F2 layer by 40–80 km into the region with a lower recombination rate.     

A study of atmospheric gravity waves and travelling ionospheric disturbances at equatorial latitudes

A global coupled thermosphere-ionosphere-plasmasphere model is used to simulate a family of large-scale imperfectly ducted atmospheric gravity waves (AGWs) and associated travelling ionospheric disturbances (TIDs) originating at conjugate magnetic latitudes in the north and south auroral zones and subsequently propagating meridionally to equatorial latitudes. A fast dominant mode and two slower modes are identified. We find that, at the magnetic equator, all the clearly identified modes of AGW interfere constructively and pass through to the opposite hemisphere with unchanged velocity. At F-region altitudes the fast AGW has the largest amplitude, and when northward propagating and southward propagating modes interfere at the equator, the TID (as parameterised by the fractional change in the electron density at the F2 peak) increases in magnitude at the equator. The amplitude of the TID at the magnetic equator is increased compared to mid-latitudes in both upper and lower F-regions with a larger increase in the upper F-region. The ionospheric disturbance at the equator persists in the upper F-region for about 1 hour and in the lower F-region for 2.5 hours after the AGWs first interfere, and it is suggested that this is due to enhancements of the TID by slower AGW modes arriving later at the magnetic equator. The complex effects of the interplays of the TIDs generated in the equatorial plasmasphere are analysed by examining neutral and ion winds predicted by the model, and are demonstrated to be consequences of the forcing of the plasmasphere along the magnetic field lines by the neutral air pressure wave.     

On the interference between closely-spacedP arrivals recorded at seismic arrays

The arrival at the Yellowknife Array (YKA) of closely-spaced P-waves having slightly different values ofdT/d, azimuth, and amplitude has been simulated using synthetic 1 Hz wavelets. Adaptive-processing determinations of apparent slowness and azimuth show a remarkable pattern of fluctuations with time for varying separations of the interfering pulses. In the critical separation range 0.30 to 0.55 s, these fluctuations greatly exceed the differences in arrival vectors of the primary wavelets themselves. Observation of characteristic interference patterns on real array seismograms might permit the identification of triplications associated with radial velocity anomalies in the lower mantle. Processed YKA seismograms for three events at distances close to 90° are presented, which, despite apparently simple P onsets illustrate the pattern of drift indT/d expected for such a triplication.Contribution from the Earth Physics Branch No. 648.     

Normal moveout velocity for pure‐mode and converted waves in layered orthorhombic medium

We study the azimuthally dependent hyperbolic moveout approximation for small angles (or offsets) for quasi‐compressional, quasi‐shear, and converted waves in one‐dimensional multi‐layer orthorhombic media. The vertical orthorhombic axis is the same for all layers, but the azimuthal orientation of the horizontal orthorhombic axes at each layer may be different. By starting with the known equation for normal moveout velocity with respect to the surface‐offset azimuth and applying our derived relationship between the surface‐offset azimuth and phase‐velocity azimuth, we obtain the normal moveout velocity versus the phase‐velocity azimuth. As the surface offset/azimuth moveout dependence is required for analysing azimuthally dependent moveout parameters directly from time‐domain rich azimuth gathers, our phase angle/azimuth formulas are required for analysing azimuthally dependent residual moveout along the migrated local‐angle‐domain common image gathers. The angle and azimuth parameters of the local‐angle‐domain gathers represent the opening angle between the incidence and reflection slowness vectors and the azimuth of the phase velocity ψ_phs at the image points in the specular direction. Our derivation of the effective velocity parameters for a multi‐layer structure is based on the fact that, for a one‐dimensional model assumption, the horizontal slowness and the azimuth of the phase velocity ψ_phs remain constant along the entire ray (wave) path. We introduce a special set of auxiliary parameters that allow us to establish equivalent effective model parameters in a simple summation manner. We then transform this set of parameters into three widely used effective parameters: fast and slow normal moveout velocities and azimuth of the slow one. For completeness, we show that these three effective normal moveout velocity parameters can be equivalently obtained in both surface‐offset azimuth and phase‐velocity azimuth domains.     

Dynamics of the plasmasphere in the dusk sector as observed in the outer ionosphere

Summary Direct measurements of the thermal plasma parameters in the topside ionosphere reveal variations of the plasmasphere boundary in the dusk sector. The ACTIVE satellite's near-polar orbits at altitudes of 500 – 1800 km around winter solstice 1989 were used to study the bulge region of the plasmasphere during intervals with different levels of geomagnetic agitation. The narrow, sharply defined trough in electron concentration corresponding to the plasmapause under quiet conditions situated at L = 6 – 7 moved to lower L-values with increasing geomagnetic activity. This narrow trough can be found in all main ion constituents. During periods of moderate geomagnetic activity, following the onset of a weak magnetic storm, a portion of the plasmaspheric bulge region was separated from the main plasmaspheric body. This can be seen in the outer ionosphere as an inner narrow trough at lower L-value. Troughs in light ions need no longer coincide with this in electron concentration. He⁺ is the most sensitive constituent reflecting the dusk sector plasmaspheric situation at this altitude.Dedicated to the Memory of Professor Karel P     

Comparison of the measured and modelled electron densities and temperatures in the ionosphere and plasmasphere during 20/30 January, 1993

We present a comparison of the electron density and temperature behaviour in the ionosphere and plasmasphere measured by the Millstone Hill incoherent-scatter radar and the instruments on board of the EXOS-D satellite with numerical model calculations from a time-dependent mathematical model of the Earths ionosphere and plasmasphere during the geomagnetically quiet and storm period on 20/30 January, 1993. We have evaluated the value of the additional heating rate that should be added to the normal photoelectron heating in the electron energy equation in the daytime plasmasphere region above 5000 km along the magnetic field line to explain the high electron temperature measured by the instruments on board of the EXOS-D satellite within the Millstone Hill magnetic field flux tube in the Northern Hemisphere. The additional heating brings the measured and modelled electron temperatures into agreement in the plasmasphere and into very large disagreement in the ionosphere if the classical electron heat flux along magnetic field line is used in the model. A new approach, based on a new effective electron thermal conductivity coefficient along the magnetic field line, is presented to model the electron temperature in the ionosphere and plasmasphere. This new approach leads to a heat flux which is less than that given by the classical Spitzer-Harm theory. The evaluated additional heating of electrons in the plasmasphere and the decrease of the thermal conductivity in the topside ionosphere and the greater part of the plasmasphere found for the first time here allow the model to accurately reproduce the electron temperatures observed by the instruments on board the EXOS-D satellite in the plasmasphere and the Millstone Hill incoherent-scatter radar in the ionosphere. The effects of the daytime additional plasmaspheric heating of electrons on the electron temperature and density are small at the F-region altitudes if the modified electron heat flux is used. The deviations from the Boltzmann distribution for the first five vibrational levels of N₂(v) and O₂(v) were calculated. The present study suggests that these deviations are not significant at the first vibrational levels of N₂ and O₂ and the second level of O₂, and the calculated distributions of N₂(v) and O₂(v) are highly non-Boltzmann at vibrational levels v > 2. The resulting effect of N₂(v > 0) and O₂(v > 0) on NmF2 is the decrease of the calculated daytime NmF2 up to a factor of 1.5. The modelled electron temperature is very sensitive to the electron density, and this decrease in electron density results in the increase of the calculated daytime electron temperature up to about 580 K at the F2 peak altitude giving closer agreement between the measured and modelled electron temperatures. Both the daytime and night-time densities are not reproduced by the model without N₂(v > 0) and O₂(v > 0), and inclusion of vibrationally excited N₂ and O₂ brings the model and data into better agreement.     

Formation mechanisms of the midlatitudinal <Emphasis Type="Italic">NmF2</Emphasis> semiannual anomaly under daytime quiet geomagnetic conditions at low solar activity

The F-region peak electron densities NmF2 measured during daytime quiet geomagnetic conditions at low solar activity on January 22, 2008, April 8, 1997, July 12, 1986, and October 26, 1995, are compared. Ionospheric parameters are measured by the ionosonde and incoherent scatter radar at Millstone Hill and calculated with the use of a 1D nonstationary ionosphere–plasmasphere model of number densities and temperatures of electrons and ions at middle geomagnetic latitudes. The formation of the semiannual anomaly of the midlatitudinal NmF2 under daytime quiet geomagnetic conditions at low solar activity is studied. The study shows that the semiannual NmF2 anomaly occurs due to the total impact of three main causes: seasonal variations in the velocity of plasma drift along the geomagnetic field due to the corresponding variations in the components of the neutral wind velocity; seasonal variations in the composition and temperature of the neutral atmosphere; and the dependence of the solar zenith angle on a number of the day in the year at the same solar local time.     

Unstable steam-water convection as possible trigger to earthquakes

Convection occurs when two water reservoirs, the overlying and the underlying, are connected by a narrow channel and the fluid in the lower reservoir is heated to the stage of phase transition into steam. The laboratory study of the properties of unstable steam-water convection showed that under favourabieP-T conditions the convection can be the triggering mechanism of seismicity. This type of convection causes a sudden fall of pressure in the lower reservoir and in the connecting channel, the impulsive mechanical disturbances, and cyclicity. The point of initiation of this phenomenon can be located at a depth of 5–7 km from the earth's surface with subsequent propagation of the process of instability to larger and smaller depths. This model of the natural terrestrial conditions can account for the earthquake cyclicity in the same focal zones, the rise of temperature and of the level of ground waters during earthquakes, the enhancement of seismicity while filling the water storage basins, the effect of floating up of hypocenters of aftershocks and the greater intervals between them.     

Two-dimensional electric field measurements in the ionospheric footprint of a flux transfer event

Line-of-sight Doppler velocities from the SuperDARN CUTLASS HF radar pair have been combined to produce the first two-dimensional vector measurements of the convection pattern throughout the ionospheric footprint of a flux transfer event (a pulsed ionospheric flow, or PIF). Very stable and moderate interplanetary magnetic field conditions, along with a preceding prolonged period of northward interplanetary magnetic field, allow a detailed study of the spatial and the temporal evolution of the ionospheric response to magnetic reconnection. The flux tube footprint is tracked for half an hour across six hours of local time in the auroral zone, from magnetic local noon to dusk. The motion of the footprint of the newly reconnected flux tube is compared with the ionospheric convection velocity. Two primary intervals in the PIFs evolution have been determined. For the first half of its lifetime in the radar field of view the phase speed of the PIF is highly variable and the mean speed is nearly twice the ionospheric convection speed. For the final half of its lifetime the phase velocity becomes much less variable and slows down to the ionospheric convection velocity. The evolution of the flux tube in the magnetosphere has been studied using magnetic field, magnetopause and magnetosheath models. The data are consistent with an interval of azimuthally propagating magnetopause reconnection, in a manner consonant with a peeling of magnetic flux from the magnetopause, followed by an interval of anti-sunward convection of reconnected flux tubes.     

Comparison between two theoretical mechanisms for the formation of the plasmapause and relevant observations

The plasmapause formation physical mechanisms are recalled: (i) the MHD convection mechanism, based on the original idea that the plasmapause coincides with the last closed equipotential (LCE) of the magnetospheric convection electric field or with the last closed streamline (LCS) of the equatorial plasma, and (ii) the interchange mechanism, which is based on peeling off the plasmasphere as a result of substorm associated enhancements of the night side convection velocity, leading to an enhanced centrifugal acceleration in the outermost layers of the plasmasphere. The plasmapause positions, predicted by these alternative theories, were numerically determined for two different magnetospheric empirical electric field models: (i) the Volland-Stern-Maynard-Chen (VSMC) and (ii) McIlwain E5D models, both of which are Kp-dependent. The predicted positions and overall shape of the equatorial plasmapause cross-sections are confronted to those derived from decades of whistler and satellite observations including the EUV observations during the substorm of June 27, 2001. It is found that the VSMC electric field model and the LCS plasmapause formation theory less correspond to whistler measurements and in-situ satellite observations than the E5D model and the interchange plasmapause formation mechanism. Published in Russian in Geomagnetizm i Aeronomiya, 2008, Vol. 48, No. 5, pp. 579–597. The article was translated by the authors.     

A note on autoregressive spectral estimates for frequency-wavenumber analysis of strong-motion array data

Frequency-wavenumber (f-k) spectra of seismic strong-motion array data are useful in estimating back-azimuth and apparent propagation velocity of seismic waves arriving at the array. Such estimates are required to model wave passage effects while studying spatial variability of strong ground motion. Although periodogram-based spectral estimates are commonly used, practical applications based on them encounter limitations, such as, lack of objective criteria for selecting a proper smoothing window and its associated bandwidth, and relatively large variance of the estimated spectral quantities. We present an alternative spectral estimate based on parametric time series modelling approach. The well-known autoregressive (AR) time series model is used in a system-based approach to estimate the spectral matrix of auto- and cross-spectral densities. Such spectral estimates are found to be smoother than the windowed periodogram estimates, and can directly be used in f-k spectral analysis. We present an example application of the proposed technique using strong-motion data recorded by the SMART-1 array in Taiwan during the January 29 1981 $M_{L}$ 6.3 earthquake. Our results, in terms of back azimuth and apparent propagation velocity, are found to be in excellent agreement with those reported in the literature.     

Global plasmaspheric TEC and its relative contribution to GPS TEC

The plasmaspheric electron content is directly estimated from the global positioning system (GPS) data onboard JASON-1 Satellite for the first time. Similarly, the ground-based GPS total electron content (TEC) is estimated using about 1000 GPS receivers distributed around the globe. The relative contribution of the plasmaspheric electron content to the ground-based GPS TEC is then estimated globally using these two independent simultaneous measurements; namely ground-based GPS TEC and JASON-1 GPS TEC. Results presented here include data from 3 months of different solar cycle conditions (October 2003, May 2005, and December 2006). The global comparison between the two independent measurements was performed by dividing the data into three different regions; equatorial, mid- and high-latitude regions. This division is essential as the GPS raypaths traverse different distances through the plasmasphere at different latitudes. The raypath length through the plasmasphere decreases as latitude increases. The relative contribution of the plasmaspheric electron content exhibits a diurnal variation that depends on latitude with minimum contribution (10%) during daytime and maximum (up to 60%) at night. The contribution is also maximum at the equatorial region where the GPS raypath traverses a long distance through the plasmasphere compared to its length in mid- and high-latitude regions. Finally, the solar cycle variation of plasmaspheric contribution is also reported globally.     

Central polar cap convection response to short duration southward Interplanetary Magnetic Field

Central polar cap convection changes associated with southward turnings of the Interplanetary Magnetic Field (IMF) are studied using a chain of Canadian Advanced Digital Ionosondes (CADI) in the northern polar cap. A study of 32 short duration (1 h) southward IMF transition events found a three stage response: (1) initial response to a southward transition is near simultaneous for the entire polar cap; (2) the peak of the convection speed (attributed to the maximum merging electric field) propagates poleward from the ionospheric footprint of the merging region; and (3) if the change in IMF is rapid enough, then a step in convection appears to start at the cusp and then propagates antisunward over the polar cap with the velocity of the maximum convection. On the nightside, a substorm onset is observed at about the time when the step increase in convection (associated with the rapid transition of IMF) arrives at the polar cap boundary.     

Disturbances in the topside ionosphere during solar flares

The results of studying the ionospheric response to solar flares, obtained from the data of the GPS signal observations and incoherent scatter radars and as a result of the model calculations, are presented. It is shown that, according to the GPS data, a flare can cause a decrease in the electron content at altitudes of the topside ionosphere (h > 300 km). Similar effects of formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, with the Arecibo incoherent scatter radar. The mechanism by which negative disturbances appear in the topside ionosphere during solar flares has been studied in this work based on the theoretical model of the ionosphere-plasmasphere coupling. It has been indicated that the formation of the electron density negative disturbance in the topside ionosphere is caused by an intense removal of O⁺ ions into the overlying plasmasphere under the action of an abrupt increase in the ion production rate and thermal expansion of the ionospheric plasma.     

Shear Wave Velocity Estimates through Combined Use of Passive Techniques in a Tectonically Active Area

We made an attempt to assess the shear wave velocity values V _S and, to a lesser extent, the V _P values from ambient noise recordings in an array configuration. Five array sites were situated in the close proximity to borehole sites. Shear wave velocity profiles were modeled at these five array sites with the aid of two computational techniques, viz. spatial autocorrelation (SPAC) and H/V ellipticity. Out of these five array sites, velocity estimates could be reliably inferred at three locations. The shear wave velocities estimated by these methods are found to be quite consistent with each other. The computed V _S values up to 30 m depth are in the range from 275 to 375 m/s in most of the sites, which implies prevalence of a low velocity zone at some pocket areas. The results were corroborated by evidence of site geology as well as geotechnical information.     

Quasilinear heating of electrons in the earth's plasmasphere

In the framework of a quasilinear theory we examine the interaction between thermal electrons and ion-cyclotron waves (ICW) in the outer plasmasphere of the earth. For this type of wave-particle interaction, a simplification of the quasilinear diffusion integral in a magneto-active plasma under plasmaspheric conditions is given. Under the assumption of a Maxwellian distribution of electrons we have calculated the collision frequency and the heating source as the electrons are scattered by ICW. The obtained values of intrinsic parameters of the outer plasmasphere may exceed greatly, accordingly, the Coulomb frequency of collisions and the heating source due to suprathermal electrons. ICW-heating causes the ionosphereward thermal flux to increase, and this must lead to an increase in electron temperature in lower-lying plasmaspheric regions and in the subauroral ionosphere. A quantitative estimation of the electron temperature for the hot zone, made in this paper, is consistent with available experimental data.     

Subduction of the Pacific plate under Kamchatka: Seismic velocity of underthrust

The results of studying the process of underthrusting in the Pacific plate under Kamchatka are presented, and the phenomena associated with this process are described. The seismic component of the velocity of the plate underthrust is estimated on the basis of (1) data from the CMT (Centroid Moment Tensor) catalog and (2) the sequence of the strongest Kamchatka earthquakes. A flat site with a strike azimuth of 217° and a dip angle of 25° located at depths of 30–70 km is assumed to be the interface between the plunging plate and Kamchatka. From CMT focal mechanisms, the underthrust velocity is estimated at V = 0.7 cm/yr for the southern zone (located south of Cape Shipunski) and at V = 1.1 cm/yr for the central zone (from Cape Shipunski to Cape Kronotski). From pairs of the strongest earthquakes that have occurred successively since 1737, the underthrust velocity for the southern zone is estimated at V = 6.6–7.1 cm/yr (from two pairs) and for the central zone, at V = 6.6 cm/yr. The creep portion of the underthrust amounts to 5–15% of the total velocity (the velocity of motion of the Pacific plate is 8 cm/yr).