Average Local Time Research Articles

Combining atrial activation maps for the big picture of simulated atrial fibrillation mechanisms

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – EU funding. Main funding source(s): This work was supported by PersonalizeAF project. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 860974. Background Pulmonary vein isolation (PVI) alone is not sufficient for all persistent atrial fibrillation (AF) patients. Ablation of localized AF drivers might be a promising adjunctive ablation strategy but detecting such drivers is limited by the small field of view of current mapping catheters. Purpose We propose a technique to detect localized AF drivers that combines sequential mapping of repetitive activation patterns into a larger composite map. The performance of this technique was evaluated in several simulated scenarios of known mechanisms during AF. Methods Representative repetitive conduction patterns that may occur in AF were simulated in a detailed volumetric 3D model of the atria: a large peripheral wave, stable and meandering re-entries, and single and multiple ectopic foci. Endocardial electrograms were sequentially generated from overlapping regions in the left posterior wall (4x4 grid, 3mm spacing, 0.5-1.5 seconds), and repetitive activation patterns were detected. Overlapping areas between sequential maps were used to align the repetitive patterns. Local patterns were then combined into larger composite maps (Fig. 1A). Reconstruction performance was quantified by the average local activation time (LAT) difference between original and reconstructed activation patterns. The relation between electrode overlaps and the quality of reconstructions were assessed at different degrees of spatial overlap (approximately 10% to 25% of the grid area, corresponding to 1.6 ± 0.6 and 4.1 ± 1.2 overlapping electrodes, respectively). Results All simulated patterns could be reconstructed from sequential recordings (Fig. 1B). LAT errors in recordings with 25% overlap were low in all cases: 5±2 msec for the peripheral wave, 5±2 and 3±1 msec for focal and multifocal activities and 6±2 and 11±2 & 17±3 msec for stable, meandering and highly meandering re-entries respectively (Fig. 2). Composite maps of meandering and highly meandering re-entries presented significantly higher reconstruction errors than other more stable patterns (p<0.01). Reducing spatial overlap to 10% did not significantly increase the errors, except for the highly meandering re-entrant activity (p<0.05). Conclusion We developed a novel methodology to combine high-density sequential recordings with limited coverage into larger composite activation maps. Using this method, we reconstructed highly accurate activation maps for multiple simulated AF mechanism scenarios. Results were promising even with a small overlap between sequential recording sites and relatively unstable patterns, providing a proof of principle to apply this approach in clinical recordings of human AF.

Estimation of Personalized Minimal Purkinje Systems From Human Electro-Anatomical Maps.

The Purkinje system is a heart structure responsible for transmitting electrical impulses through the ventricles in a fast and coordinated way to trigger mechanical contraction. Estimating a patient-specific compatible Purkinje Network from an electro-anatomical map is a challenging task, that could help to improve models for electrophysiology simulations or provide aid in therapy planning, such as radiofrequency ablation. In this study, we present a methodology to inversely estimate a Purkinje network from a patient's electro-anatomical map. First, we carry out a simulation study to assess the accuracy of the method for different synthetic Purkinje network morphologies and myocardial junction densities. Second, we estimate the Purkinje network from a set of 28 electro-anatomical maps from patients, obtaining an optimal conduction velocity in the Purkinje network of 1.95 ± 0.25 m/s, together with the location of their Purkinje-myocardial junctions, and Purkinje network structure. Our results showed an average local activation time error of 6.8±2.2 ms in the endocardium. Finally, using the personalized Purkinje network, we obtained correlations higher than 0.85 between simulated and clinical 12-lead ECGs.

Effects of Transendocardial CD34+ Cell Transplantation on Diastolic Parameters in Patients with Nonischemic Dilated Cardiomyopathy

We sought to evaluate the physiological background and the effects of CD34+ cell transplantation on diastolic parameters in nonischemic dilated cardiomyopathy patients (DCM). We enrolled 38 DCM patients with NYHA class III and LVEF < 40% who underwent transendocardial CD34+ cell transplantation. Peripheral blood CD34+ cells were mobilized by G‐CSF, collected via apheresis, and injected transendocardially in the areas of myocardial hibernation. Patients were followed for 1 year. At baseline, estimated filling pressures were significantly elevated (E/e′ ≥ 15) in 18 patients (Group A), and moderately elevated (E/e ′< 15) in 20 patients (Group B). The groups did not differ in age (54 ± 9 years vs. 52 ± 10 years; p = .62), gender (male: 85% vs. 78%; p = .57), or LVEF (31 ± 7% vs. 34 ± 6%; p = .37). When compared to Group B patients in Group A had more segments with myocardial scar (4.9 ± 2.7 vs. 2.7 ± 2.9; p = .03), myocardial hibernation (2.2 ± 1.6 vs. 0.9 ± 1.1; p = .02), and longer average local relaxation time on electroanatomical mapping (378 ± 41 ms vs. 333 ± 34 ms, p = .01). During follow‐up there was an improvement in diastolic parameters in Group A (E/e′: from 24.3 ± 12.1 to 16.3 ± 8.0; p = .005), but not in Group B (E/e′: from 10.2 ± 3.7 to 13.2 ± 9.1; p = .19). Accordingly, in Group A, we found an increase in 6‐minute walk distance (from 463 ± 83 m to 546 ± 91 m; p = .03), and a decrease in NT‐proBNP (from 2140 ± 1743 pg/ml to 863 ± 836 pg/ml; p = .02). In nonischemic DCM, diastolic dysfunction appears to correlate with areas of myocardial scar and hibernation. Transendocardial CD34+ cell transplantation may improve diastolic parameters in this patient cohort. Stem Cells Translational Medicine 2017;6:1515–1521

Definitive radiotherapy and Single-Agent radiosensitizing Ifosfamide in Patients with localized, irresectable Soft Tissue Sarcoma: A retrospective analysis

Background and PurposeStandard therapy for soft-tissue sarcomas remains complete resection. For primary radiotherapy local control rates of 30-45% have been reported. We analyzed retrospectively 11 cases of radiochemotherapy with single-agent ifosfamide in patients with macroscopic soft-tissue sarcomas.Patients and MethodsThe patients were treated in irresectable high risk situations. Radiation therapy was performed with median 60 Gy. During the first and fifth week the concomitant chemotherapy with ifosfamide was added. Two patients received trimodal therapy with additional regional hyperthermia.ResultsThe therapy was completed in 73% of the patients. Average local control time was 91 months, median disease-free-survival/overall-survival was 8/26 months. Five-year rates for local control/disease free survival/overall survival were 70%/34%/34%. The limited prognosis is mainly caused by systemic treatment failure.ConclusionsThe data strongly suggest a better outcome of radiochemotherapy with ifosfamide compared to radiotherapy alone and radiotherapy in combination with other radiosensitizers.

Local time stacking of geomagnetic solar daily variations

Regular geomagnetic solar daily variations, the Sq signal, is used in electromagnetic induction studies to investigate the electrical conductivity structure of Earth's upper mantle. Traditionally, Sq induction studies employ observatory magnetic data from a few quiet days in which the overprint on the regular Sq ionospheric current vortices from various irregular disturbances is minimal. Active days are typically discarded, a procedure which results in a gappy time series that is difficult to analyze. Furthermore, the quiet day selection is largely arbitrary since daily geomagnetic activity levels range continuously from very quiet to very active. This fact precludes the establishment of rigorous quiet/active thresholds. The average local time variation on active days resembles typical quiet day variations except for a multiplicative scaling factor. A multiyear stack of yearlong geomagnetic time series aligned with respect to local time incorporates “regular active” days while greatly attenuating the effects of irregular geomagnetic disturbances. External and internal current systems that rise and fall in step with the Sun's transit across the sky above an observatory contribute coherently to the local time stack. All other current systems are incoherent and tend to stack out. An exact 1‐year‐long stack preserves the seasonal variation. The resulting stacked time series contains no gaps, is dominated by regular solar daily variations, and can be used for improved Sq induction studies and comprehensive geomagnetic field modeling. Hourly geomagnetic values from Niemegk, Boulder, Mbour, and Meanook observatories between 1970 and 2002, along with the ap index, are used in this study.

Propagation characteristics of Pi 2 magnetic pulsations observed at ground high latitudes

In order to investigate the characteristics of Pi 2 propagation observed in the high‐latitude region, ground magnetometer data obtained at high‐latitude CPMN stations were analyzed. The power of magnetic perturbation, (ΔH)2 + (ΔD)2, were calculated for Pi 2 events observed at four stations from 11 February through 20 April 1996 and for Pi 2s observed at two stations from 1 January through 27 March 1997. The times when the power of Pi 2s reached the maximum and the maximum amplitudes were compared among stations. The results are as follows. Pi 2s observed at Kotel'nyy (KTN: MLAT = 69.94°, MLON = 201.02°) reached the maximum amplitude earlier than those at lower‐latitude station Tixie (TIK: MLAT = 65.67°, MLON = 196.88°), though the amplitudes were smaller at KTN than at TIK on average. The time lag from KTN to TIK has two peaks in its distribution; the primary and the secondary peaks are located around 10 s and 35 s, respectively. The mean value of the whole distribution of the time lag from KTN to TIK is about 20 s. Ultra Violet Image (UVI) data obtained by the Polar satellite was available during the second period; the UVI data indicate that it was when the auroral oval was located equatorward to KTN that the Pi 2 amplitude tended to reach the maximum earlier at KTN than at TIK. This observational result is important because Pi 2 was observed earlier in the polar cap region rather than in the auroral region. That is to say, Pi 2 was observed earlier in the polar cap region, which is not directly connected with the source region of Pi 2 via the magnetic field line. Longitudinal characteristics of Pi 2 propagation were derived by using data from longitudinally separated stations TIK, Chokurdakh (CHD: MLAT = 64.67°, MLON = 212.12°) and Kotzebue (KOT: MLAT = 64.52°, MLON = 249.72°). The result indicates the existence of the longitudinal center of Pi 2 propagation. The average magnetic local time of the center is estimated to have been ∼22.5 MLT; eastward (westward) of the center, Pi 2 exhibited an eastward (westward) propagation. The temporal and spatial developments of the Pi 2 propagation along the auroral zone were derived in an empirical manner. That is, the MLT (set to zero at the propagation center) dependence of the maximum amplitude time and the maximum amplitude itself of Pi 2 were derived in an empirical manner. As a result it is concluded that in the premidnight sector (i.e., around 22.5 MLT), KTN is the most probable location that observes the maximum amplitude of Pi 2 earliest among the CPMN stations located along 210° magnetic meridian. Our results show that the low‐latitude Pi 2, which has often been used as a time indicator of substorm onset, is often delayed from the Pi 2 observed in the premidnight polar cap region. The present results imply that the consideration of high‐latitude Pi 2s in addition to low‐latitude Pi 2s can provide a new insight into the substorm onset timing. Thus it is necessary to consider the global features, especially Pi 2s observed in higher‐latitude region, for studying substorm onset timing issues.

Distribution of Treatment Time in Physiotherapeutic Application of Ultrasound

Background and Purpose In physiotherapy, the ultrasound transducer is moved across the treated area. Because of the movement, some points in the tissue are exposed to soundwaves for less than the total treatment time. In the literature, some authors use the total treatment time, while others use an average local exposure time to describe the temporal part of the exposure. It is unknown to what degree the total treatment time and the planned average local exposure time actually represent the local exposure. The reproducibility and sources of variation in spatial and temporal distribution of local exposure time across the tissue were investigated in this work. Method Ideal scanning patterns were constructed mathematically, and manual application patterns from 22 therapists were sampled using a digitiser. One transducer model and various sizes of tissue models were used. From these application patterns the distributions of local exposure were calculated. The calculation model consisted of a circle, representing the cross-section of the beam, moving across a plane surface, the tissue. By relating the beam position to tissue co-ordinates, the local exposure periods were found. Results Constructed applications showed an excellent match between planned and resulting average exposure. The therapists produced median local exposure times that were 0.3-0.9 times the planned average local exposure time. Considerable variation was found, where the calculated maxima were 0.4-3.8 times the planned average values. Conclusion Part of the variation is inherent in the nature of scanning application. Careful planning, concentration and guidelines drawn around the lesion increased the reproducibility of manual application. While the total treatment time alone was inadequate as a time parameter, the planned average local exposure time predicted the central tendency of the exposure, when the application was confined to the designated area. The investigation indicated that clinical testing of different application strategies with equal average exposure is necessary. In physiotherapy, the ultrasound transducer is moved across the treated area. Because of the movement, some points in the tissue are exposed to soundwaves for less than the total treatment time. In the literature, some authors use the total treatment time, while others use an average local exposure time to describe the temporal part of the exposure. It is unknown to what degree the total treatment time and the planned average local exposure time actually represent the local exposure. The reproducibility and sources of variation in spatial and temporal distribution of local exposure time across the tissue were investigated in this work. Ideal scanning patterns were constructed mathematically, and manual application patterns from 22 therapists were sampled using a digitiser. One transducer model and various sizes of tissue models were used. From these application patterns the distributions of local exposure were calculated. The calculation model consisted of a circle, representing the cross-section of the beam, moving across a plane surface, the tissue. By relating the beam position to tissue co-ordinates, the local exposure periods were found. Constructed applications showed an excellent match between planned and resulting average exposure. The therapists produced median local exposure times that were 0.3-0.9 times the planned average local exposure time. Considerable variation was found, where the calculated maxima were 0.4-3.8 times the planned average values. Part of the variation is inherent in the nature of scanning application. Careful planning, concentration and guidelines drawn around the lesion increased the reproducibility of manual application. While the total treatment time alone was inadequate as a time parameter, the planned average local exposure time predicted the central tendency of the exposure, when the application was confined to the designated area. The investigation indicated that clinical testing of different application strategies with equal average exposure is necessary.

Observation of IMF and seasonal effects in the location of auroral substorm onset

We use Polar ultraviolet imager (UVI) and Wind observations to study the location of 648 well‐defined Northern Hemisphere auroral breakups (substorm onsets) in response to interplanetary magnetic field (IMF) orientation and season. The most likely onset location is at 2230 MLT and 67° Λm with half‐maximum widths of 3 hours of MLT and 2° Λm, respectively. The onset latitude depends primarily on IMF Bz, but also Bx: the onset latitude decreases for Bx &gt; 0 or Bz &lt; 0 and increases for Bx &lt; 0 or Bz &gt; 0. The onset longitude depends on season and IMF By. In summer, substorms tend to occur in the early evening at ∼2200 MLT, whereas in winter they tend to occur near midnight at ∼2300 MLT. The average summer‐winter difference in the onset location is ∼1 hour of MLT. Large By effects on the onset longitude occur only when Bx and By are small. Onset locations shift toward earlier local times for By &gt; 0 and toward midnight for By &lt; 0. The average onset local time is earliest (2200 MLT) for By &gt; 0 in summer and latest (2330 MLT) for By &lt; 0 in winter. These dependencies coincide with those previously reported for the evening sector ionospheric zonal flow reversal in response to IMF By and season, indicating that auroral breakups are most likely in regions of large velocity shears. A weak dependence of the MLT onset location on the IMF Bx is identified: for Bx &gt; 0 the onset location shifts toward dusk when By &gt; 0 but toward dawn when By &lt; 0; the sense of this shift reverses for Bx &lt; 0. An implication of the results is that auroral breakup is not conjugate.

Local time variation of equatorial temperature and zonal wind anomaly (ETWA)

We describe the average and apparent local (solar) time (LST) variations in the zonal winds (Z) and temperatures (T) as measured with the Wind and Temperature Spectrometer (WATS) on-board the polar orbiting DE-2 satellite in the altitude range of 300–450 km during a near solar maximum period of 1981–1982. During this time period, the variations in the solar flux (F10.7) and magnetic activity (Ap) contribute significantly to the apparent LST variations in T; while those effects on the LST variations in Z are small. The observations are related to the equatorial ionization anomaly (EIA) as seen in the electron density data obtained from the same satellite with the LANG instrument. The latitudinal variations in Z always reveal a maximum at the dip equator where the EIA trough forms and minima (with velocities reduced by a factor of two) are seen on either side of the equator where the EIA crests form. At 20:00 LST, the largest wind velocities are observed, directed eastward, after changing direction at around 17:00 LST where the largest accelerations occur. We delineate the diurnal variations in the strength of the Equatorial Temperature and Wind Anomaly (ETWA) defined by the differences in the wind velocities (DZ) and temperatures (DT) at the crests and troughs of the ionization. The diurnal variations in DZ are similar to those in Z at the trough. The diurnal variations in DT differ from those apparent in T. Excess temperatures, DT, at the crests show up with the development of the EIA as early as 09:30 LST by which time the zonal wind has attained its westward maximum. But DT continues to increase with the EIA crest until 14:00 LST, followed by a dip at around 16:00 LST in phase with the zero crossing of Z. The highest value of DT is only reached at 20:00 LST when both Z and the crest intensification reach their respective maxima. This demonstrates that the development of the equatorial temperature anomaly critically depends both on the development of the EIA crests and the zonal winds, which clearly establishes the primary role of ion drag in generating the ETWA phenomenon.

Spatial distribution of ionospheric plasma and field structures in the high‐latitude F region

Ion density and velocity measurements from the Dynamics Explorer 2 (DE 2) spacecraft are used to obtain the average magnetic local time versus invariant latitude distribution of irregularities in the high‐latitude F region ionosphere. To study the small‐scale structure and its relationship to background conditions in the ionosphere, we have formed a reduced database using 2‐s (≈ 16 km) segments of the ion density and velocity data. The background gradients associated with each 2‐s segment and the spectral characteristics, such as power at 6 Hz (≈ 1.3 km) and spectral index, are among the reduced parameters used in this study. The relationship between the observed plasma structure and its motion is complex and dependent on the externally applied fields as well as locally generated plasma structure. The evolution of plasma structures also depends critically on the conductivity of the underlying ionosphere. Observations indicate an enhancement of irregularity amplitudes in two spatially isolated regions in both the ion density and the velocity. Convective properties seem to play a more important role in winter hemisphere where smaller‐scale structures axe maintained outside the source regions. ΔV irregularity amplitudes are enhanced in the cusp and the polar cap during northward interplanetary magnetic field regardless of season. The power in ΔV is usually higher than that associated with local polarization electric fields, suggesting that the observed structure in ΔN/N is strongly influenced by ΔV structure applied to large density gradients.

Bias in acoustic biomass estimates of Euphausia superba due to diel vertical migration

The diel vertical migration (DVM) of Antarctic krill ( Euphausia superba) can greatly bias the results of qualitative and quantitative hydroacoustic surveys which are conducted with a down-looking sonar and irrespective of the time of day. To demonstrate and quantify these negative biases on both the estimates of biomass distribution and abundance, a time-depth-density analysis was performed. Data were collected, as part of the United States Antarctic Marine Living Resources Program (AMLR), in the vicinities of Elephant Island, Antarctica, during the austral summers of 1992 and 1993. Five surveys were conducted in 1992; two covered a 105 by 105 n.mi. area centered on Elephant Island, two encompassed a 60 by 35 n.mi. area immediately to the north of the Island, and one covered a 1 n.mi. 2 area centered on a large krill swarm to the west of Seal Island. The 1993 data include repetitions of the two small-area and two large-area surveys. Average krill volume densities were calculated for each hour as well as for three daily periods: day, twilight and night. These data were normalized and presented as a probability of daily average density. With spectral analysis to identify the frequencies of migration, a four-term periodic function was fitted to the probability density function of average daily biomass versus local apparent time. This function was transformed to create a temporal compensation function (TCF) for upwardly adjusting acoustic biomass estimates. The TCF was then applied to the original 1992 survey data; the resulting biomass estimates are an average of 49.5% higher than those calculated disregarding biases due to diel vertical migration. The effect of DVM on the estimates of krill distribution are illustrated by a comparison of compensated and uncompensated density maps of two 1992 surveys. Through this technique, high density kril areas are revealed where uncompensated maps indicated low densities.

Polar cap convection for Bz northward

The Digisonde at Qanaq, Greenland (87°Λ) measures the ionospheric convection velocity in the central polar cap. The average local time variation of the velocity for Bz northward under various By conditions is presented. The nightside drifts for Bz northward tend to be antisunward, and resemble those which occur for southward IMF, as pointed out by Heppner and Maynard (1987). However, it is difficult to reconcile the observed diurnal drift variation with the Heppner‐Maynard distorted two‐cell model. Modified forms of the Potemra et al. (1984) four cell model offer a more plausible framework in which to understand the Qanaq data. The polar cap cells are relatively small, and are displaced towards the dayside.

Simultaneous observations of Schumann resonances in California and Australia: Evidence for intensity modulation by the local height of the D region

Observations are presented of the horizontal magnetic component of Schumann resonance intensities as simultaneously measured at locations in California and Western Australia during two separate intervals September 2–17, 1989, and April 14–21, 1990. For both intervals, diurnal variations of the average magnetic power over the lowest three modes of the Schumann resonances showed substantially different temporal profiles at the California and Western Australia stations, with interstation correlations of 0.51 and 0.39, respectively. A method is demonstrated for determining from these observations the average local time variation of the height of the D region. A height variation is obtained that is nearly identical for the respective analysis intervals, with a minimum height occurring at approximately 1300–1400 LT and a maximum‐to‐minimum height difference of roughly 50% of the mean. When corrected for the local D region height, the detailed diurnal intensity profiles over the analysis intervals display a greatly improved similarity, with interstation correlation coefficients increasing from 0.51 to 0.70 and from 0.39 to 0.82, respectively. Substantial agreement between the two stations after correction for D region height suggests that such observations could be used to monitor the global totality and variability of lightning, quantitatively and at time resolutions of the order of 10 min or less, in studies of global change. These results also suggest that with suitable instrumentation, in situ measurement of Schumann resonance intensities could provide a viable method for monitoring the global totality of lightning in other planetary systems capable of supporting the resonances, most notably Venus and Jupiter.

Low‐latitude zonal and vertical ion drifts seen by DE 2

Horizontal and vertical ion drift data from the DE 2 spacecraft have been used to determine average zonal and vertical plasma flow (electric field) characteristics in the ±26° dip latitude region during a time of high solar activity. The data base consists of over 800 equatorial passes. Horizontal ion drift data were projected along magnetic field lines to the magnetic equator, using a dipole model to yield their apex height dependence. The “average data” local time profile for an apex height bin centered at 400 km indicates westward plasma flow from 0600 to 1900 solar local time (SLT) with a maximum westward velocity of 80 m/s in the early afternoon. There is a sharp change to eastward flow at approximately 1900 hours with an early evening peak of 170 m/s. A secondary nighttime maximum exists at 0430 SLT preceeding the reversal to westward flow. This profile is in good agreement with Jicamarca radar measurements made under similar solar maximum conditions. Harmonic analysis indicates a net superrotation which is strongest at lower apex altitudes. The diurnal term is dominant, but higher order terms through the quatradiurnal are significant. Vertical ion data taken at low (±5°) dip latitudes, while possessing greater scatter, generally agree with Jicamarca values.

Stress accumulation and release since 1882 in Ometepec, Guerrero, Mexico: Implications for failure mechanisms and risk assessments of a seismic gap

We found evidence that four episodes of large shallow (h &lt; 30 km) interplate earthquakes ruptured approximately the same segment of the Middle America subduction zone in Ometepec, Guerrero, Mexico, during the last century: in 1890 (M = 7.2), 1937 (MS = 7.5), 1950 (MS = 7.3), and 1982 (MS = 6.9 and 7.0). This segment is about 70 km long, which is near the limit of resolution to define seismic gap dimensions. We synthesized the spatial, temporal, and mechanistic patterns of regional relocated seismicity (mb ≥ 4.0) to investigate the mechanics of rupture within the same seismic gap for the relatively large number of times (four) offered by the Ometepec case history. Despite the similarities in source depths and geometries of the shallow main shocks, we found marked differences in their magnitudes, recurrence intervals, and rupture modes (e.g., degree of source complexity and event multiplicity). The episodic amounts of energy release do not appear to scale with the observed elapsed times between the shallow main shocks (47.1, 13.0, and 31.5 years in chronological order) according to time‐ or slip‐predictable models of earthquake recurrence. We interpret these findings in the context of a variable rupture mode of the Ometepec region. This variable rupture mode suggests that probabilistic forecasts of future large earthquakes in the region, which are based solely on the historic record of main shocks, can be uncertain by at least ±57% of the average local repeat time of 30.5 years and 1/2 unit of magnitude. Specific predictions of future strong ground motions are obviated because earthquakes within the gap do not exhibit a “characteristic” behavior. We also found that all four episodes of interplate main shocks were preceded by large downdip normal fault earthquakes at intermediate depths (M ≥ 6.5, h &gt; 60 km). The time intervals between extensional and subsequent compressional events range from 1.6 to 9.9 years with an average of 6.3±3.7 years. This apparent pattern which initiates at depth was further documented for the final 2/3 of the last seismic cycle. From the spatiotemporal distribution of seismicity during this final period, we identified three general stages of stress release. Stage I, from 1964 to January 1980, was characterized by scattered seismicity. Stage II, from February 1980 to June 6, 1982, showed a significant increase in downdip seismic activity while the shallow zone became quiescent. (There was also a period of apparent seismic quiescence from mid‐1975 to early 1978). Most seismic energy in stage II was released on October 24, 1980, by a downdip normal fault earthquake at intermediate depth (MS = 7.0, h = 65 km). Stage III began on June 7, 1982, with the occurrence of two large interplate thrust fault earthquakes at shallow depths (MS = 6.9, h = 18 km, and MS = 7.0, h = 19 and 23 km). The documentation of stage III through June 1986 shows subsidence of previous downdip activity and an aftershock zone which is spatially coincident with the shallow quiet zone identified in stage II. The spatiotemporal characteristics of the 1982 main shock‐after shock sequence suggests that rupture propagated to the northwest toward the site of the 1957 (MS = 7.5) Acapulco‐San Marcos earthquake. This propagation is of interest because the latter site is presently considered to possess high seismic potential. We suggest that downdip normal fault seismic activity at intermediate depths reflects a symptomatic state of stress associated with some basic mechanism of plate interaction prior to interplate slippage in a large earthquake. This mechanism may have an important bearing on the generation and possible intermediate‐term forecasts of large shallow interplate earthquakes along the subduction zone in Mexico.

Upper-atmosphere features revealed by the orbit of 1980-43A

The satellite NOAA-B (1980-43A) was launched in May 1980 into an orbit with perigee height near 260 km and apogee height 1440 km, at an inclination of 92.2°.The lifetime was 11 months. The orbit has been determined at 40 epochs between October 1980 and May 1981 from about 3000 radar and optical observations. The average orbital accuracy, radial and cross-track, was about 100 m, with rather better accuracy in the final 14 days. The variation of orbital inclination has been analysed to determine two good values of atmospheric rotation rate, namely 1.10 ± 0.10 rev day −1 at 300 km (average local time) and 1.15 ± 0.06 rev day −1 at 225 km (evening). The effect of atmospheric rotation on the precession of the orbital plane of an actual satellite has never previously been detected; it is clearly apparent for 1980-43A in its last days and conforms to the expected theoretical change. The variation of perigee height has been analysed to determine ten values of atmospheric density scale height, for heights of 280–370 km. These values, accurate to about 3%, exceed by 15% the values indicated by the COSPAR International Reference Atmosphere. Solar activity was higher in the years 1980–1981 than at any time since early 1958 and it appears that the CIRA model underestimates the density and density scale height at high levels of solar activity.

Upper-atmosphere zonal winds from satellite orbit analysis

In this paper we review and interpret the values of upper-atmosphere rotation rate (zonal winds) obtained by analysing satellite orbits determined from observations. The history of the method is briefly reviewed, the basic principles are explained, objections to the method are answered, and three examples are given. Existing analyses of the atmospheric rotation rate A are critically reviewed, and, after rejecting some and revising others, we are left with 85 values. These are divided according to local time and season, to give the variation of A with height in nine situations—namely morning, evening and average local time, for summer, winter and average season. These observational results indicate that the value of Λ (in rev/day), averaged over both local time and season, increases from 1.0 at 125 km to 1.22 at 325 km and then decreases to 1.0 at 430 km and 0.82 at 600 km. The value of Λ is higher in the evening (18–24 h), with a maximum value (near 1.4) corresponding to a West-to-East wind of 150 m s −1 at heights near 300 km. The value of Λ is lower in the morning (06–12 h), with East-to-West winds of order 50 m s −1 at heights of 200–400 km. There is also a consistent seasonal variation, the values of Λ being on average 0.15 higher in winter and 0.1 lower in summer than the average seasonal value. No significant variation with solar activity is found, but there is a slight tendency for a greater rotation rate at lower latitudes for heights above 300 km. Unexpectedly, the values for the 1960s are found to be significantly higher than those for the 1970s. Finally, these observational values are compared with the theoretical global model of Fuller-Rowell and Rees: there is complete agreement on the trends, though there are some differences in the mean values.

The plasmapause revisited

Saturation of the dc double-probe instrument on Explorer 45 has been used to identify the plasmapause. Fifteen months of these data resulted in a data base of sufficient size to statistically study the average position of the plasmapause over 14.5 hours of magnetic local time (MLT) under differing magnetic conditions. The afternoon-evening bulge in the L coordinate of the plasmapause versus local time was found to be centered between 20 and 21 hours MLT during magnetically quiet periods and shifted toward dusk as activity increased, but always postdusk. During quiet periods a bulge in the L coordinate near noon was also seen, which disappeared as activity increased. The average local time distribution plasmapause position during high magnetic activity was irregular in the afternoon region, where large-scale convection models predict the creation of plasmatails or detached plasma regions from increases in the solar wind induced convection. The results suggest that solar wind induced convection is partially shielded from the dayside. As the intensity of the convection is increased, it more effectively penetrates the dayside, which shifts the postdusk bulge nearer to dusk and eliminates the quiet time bulge near noon (created by Sq dynamo ionospheric electric fields projecting into the magnetosphere when dayside convection is weak). Correlations of the plasmapause L coordinate with Dst were more ordered than corresponding correlations with Kp or with the maximum Kp over the previous 12 hours.

Comparison of coincident OGO 3 and OGO 4 hydrogen ion composition measurements

Ion composition measurements made on August 8, 1967 and August 28, 1967 by the topside ionospheric polar orbiting satellite OGO 4 and simultaneously by the eccentric orbiting magnetospheric satellite OGO 3 were compared on nightside passes through the plasmasphere. Throughout most of the midlatitude regions sampled, an isothermal diffusive equilibrium model at an ion temperature of 1000°K provides a good approximation for coupling these ionospheric regions. However temperature gradient components directed upwards along the field lines of the order of 1°K/km or greater are required to bring the profiles into agreement near the plasma pause assuming diffusive equilibrium and assuming the magnitude of the temperature gradient does not vary along the field line. Similar gradients are also required along the low latitude field lines traversed on the August 28, 1967 passes. In agreement with previous studies of the average local time asymmetry of the plasmasphere boundary, the plasmapause L coordinates measured by OGO 3 near midnight were greater than the L coordinates associated with the light ion troughs observed near dawn on the coincident OGO 4 pass. In the steady state, if field line preservation is assumed and the convection stream-line pattern in the equatorial plane is known, the H + densities measured at different local times along the OGO 3 trajectory can be transformed into densities at a local time characteristic of the OGO 4 measurements so that the transformed OGO 3 plasma-pause is located on the same field line as the plasmapause determined from the OGO 4 measurements.