Delayed traveling ionospheric disturbances, especially in equatorial regions, following geomagnetic activity

For the Western-Pacific region spread-F has been found to occur with delays after geomagnetic activity (GA) ranging from 5 to 10 days as station groups are considered from low midlatitudes to equatorial regions. The statistical (superposed-epoch) analyses also indicate that at the equator the spread-F, and therefore associated medium-scale traveling ionospheric disturbances (MS-TIDs) occur with additional delays around 16, 22 and 28 days representing a 6-day modulation of the delay period. These results are compared with similar delays, including the modulation, for D-region enhanced hydroxyl emission (Shefov, 1969). It is proposed that this similarity may be explained by MS-TIDs influencing both the F and D regions as they travel. Long delays of over 20 days are also found near the equator for airglow-measured MS-TIDs (Sobral et al., 1997). These are recorded infrequently and have equatorward motions, while normally eastward motions are measured at the equator. Also in midlatitudes D-region absorption events have been shown (statistically) to have similar long delays after GA. It is suggested that atmospheric gravity waves and associated MS-TIDs may be generated by some of the precipitations responsible for the absorption. The recording of the delayed spread-F events depends on the GA being well below the average levels around sunset on the nights of recording. This implies that lower upper-atmosphere neutral particle densities are necessary.

Influence of geomagnetic activity on the occurrence of midlatitude ionogram-recorded spread-F

Delayed spread-F occurrence as recorded by ionograms, following geomagnetic activity (GA) has been investigated using data from 88 stations located around the world. The spread-F occurrence is delayed progressively from one to three days, from subauroral to midlatitude regions. The equatorial latitudes show suppressed activity. An examination of daily spread-F occurrence values relative to the AE index reveals not only a main delay of one day, but also delays of two and three days. These delays involve principally GA occurring around 0600 hrs LT.

A total electron content space weather study of the Nighttime Weddell Sea Anomaly of 1996/97 southern summer with TOPEX/Poseidon radar altimetry

This paper reports on a total electron content space weather study of the nighttime Weddell Sea Anomaly, overlooked by previously published TOPEX/Poseidon climate studies, and of the nighttime ionosphere during the 1996/1997 southern summer. To ascertain the morphology of spatial TEC distribution over the oceans in terms of hourly, geomagnetic, longitudinal and summer-winter variations, the TOPEX TEC, magnetic, and published neutral wind velocity data are utilized. To understand the underlying physical processes, the TEC results are combined with inclination and declination data plus global magnetic field-line maps. To investigate spatial and temporal TEC variations, geographic/magnetic latitudes and local times are computed. As results show, the nighttime Weddell Sea Anomaly is a large (∼1,600(°)2; ∼22 million km2 estimated for a steady ionosphere) space weather feature. Extending between 200°E and 300°E (geographic), it is an ionization enhancement peaking at 50°S–60°S/250°E–270°E and continuing beyond 66°S. It develops where the spacing between the magnetic field lines is wide/medium, easterly declination is large-medium (20°–50°), and inclination is optimum (∼55°S). Its development and hourly variations are closely correlated with wind speed variations. There is a noticeable (∼43%) reduction in its average area during the high magnetic activity period investigated. Southern summer nighttime TECs follow closely the variations of declination and field-line configuration and therefore introduce a longitudinal division of four (Indian, western/eastern Pacific, Atlantic). Northern winter nighttime TECs measured over a limited area are rather uniform longitudinally because of the small declination variation. TOPEX maps depict the expected strong asymmetry in TEC distribution about the magnetic dip equator.

Geodesics without differential equations: general relativistic calculations for introductory modern physics classes

Introductory courses covering modem physics sometimes introduce some elementary ideas from general relativity, though the idea of a geodesic is generally limited to shortest Euclidean length on a curved surface of two spatial dimensions rather than extremal aging in spacetime. It is shown that Epstein charts provide a simple geometric picture of geodesics in one space and one time dimension and that for a hypothetical uniform gravitational field, geodesics are straight lines on a planar diagram. This means that the properties of geodesics in a uniform field can be calculated with only a knowledge of elementary geometry and trigonometry, thus making the calculation of some basic results of general relativity accessible to students even in an algebra-based survey course on physics.

The lattice solid model to simulate the physics of rocks and earthquakes: Incorporation of friction

The particle-based lattice solid model developed to study the physics of rocks and the nonlinear dynamics of earthquakes is refined by incorporating intrinsic friction between particles. The model provides a means for studying the causes of seismic wave attenuation, as well as frictional heat generation, fault zone evolution, and localisation phenomena. A modified velocity-Verlat scheme that allows friction to be precisely modelled is developed. This is a difficult computational problem given that a discontinuity must be accurately simulated by the numerical approach (i.e., the transition from static to dynamical frictional behaviour). This is achieved using a half time step integration scheme. At each half time step, a nonlinear system is solved to compute the static frictional forces and states of touching particle-pairs. Improved efficiency is achieved by adaptively adjusting the time step increment, depending on the particle velocities in the system. The total energy is calculated and verified to remain constant to a high precision during simulations. Numerical experiments show that the model can be applied to the study of earthquake dynamics, the stick-slip instability, heat generation, and fault zone evolution. Such experiments may lead to a conclusive resolution of the heat flow paradox and improved understanding of earthquake precursory phenomena and dynamics. (C) 1999 Academic Press.