Journal of Atmospheric and Terrestrial Physics最新文献

Temporal variations of the global lightning activity were deduced from long-term Schumann resonance (SR) continuous records. The intensities of the horizontal magnetic field component in the vicinity of the first, second, and third SR modes were monitored at Tottori observatory (35.5°N, 134.33°E) from 1968. Variations of the effective source-observer distance were estimated using the ratios of the intensities of individual modes. This allowed us to obtain average diurnal variations of the global lightning activity for each month over a one-year period. The results show that the distances estimated between the field-site and the effective source are very stable, while the temporal changes of the fields and the global lightning intensity derived demonstrate substantial variability.

Two long mean surface air temperature series are presented for Armagh Observatory; one based on twice daily ‘spot’ temperature readings, 1796–1882, and the other on daily maximum and minimum temperatures, 1844–1992. Our data confirm the correlation of temperature with solar cycle length, first suggested by Friis-Christensen and Lassen (Science254, 698, 1991) and, for this site, extend their result a further 65 years, back to the end of the 18th century.

In this study we consider the phase relationships between the oscillations of various ionospheric signatures associated with Pcl geomagnetic pulsations. Investigations using a simple analytical method and a numerical model, which has proved successful when applied to longer period pulsations, both suggest that Doppler velocity oscillations should be predominantly in anti-phase with oscillations of the rates of change of group range and echo amplitude. However, observations indicate that the Doppler velocity oscillations are in quadrature with the other two types of oscillations. Possible causes for this discrepancy are suggested.

NCAR-TIGCM simulations predict mesoscale cellular structures in the high latitude neutral density at altitudes from 120–350 km. During magnetically active conditions, the density structure at 200 km consists of low-density cells near dawn and dusk and high-density cells near noon and midnight. Mechanisms causing the structured density cells are a result of thermosphere-ionosphere coupling and can be explained in terms of dynamic meteorology. For example, at high latitudes ion drag causes the neutral circulation to flow cyclonically in the dawn sector and anticyclonically in the dusk sector. Low densities are contained within the cyclonic circulation at all altitudes. Below about 170 km, the densities inside the anticyclonic flow are high, while above that altitude densities within the anticyclonic flow are low. While typical dynamic meteorology explains low densities in the centre of cyclonic circulation and high densities inside anticyclonic circulation, the dusk low-density cell in the centre of anticyclonic flow is unexpected. The anticyclonic dusk low-density cell is explained by anomalous antibaric flow due to high-speed winds. 120 km and 200 km altitudes are used to demonstrate the relationship between the high latitude densities and winds as well as the effect of joule heating and auroral particle precipitation on the density structures.

Group delays and Doppler shifts from ducted whistler-mode signals are measured using the VLF Doppler experiment at Dunedin, New Zealand (45.8°S, 170.5°E). Equatorial zonal electric field and plasmasphere-ionosphere coupling fluxes are determined for L ≈ 2.3 at June solstice and equinox during magnetically quiet periods. The general features of the electric field measured at Dunedin agree with those predicted from ionospheric dynamo theory with a (1,−2) tidal component. Some seasonal variations are observed, with the electric field measured during equinox being smaller and predominantly westward during the night. The electric field at June solstice is also westward during the evening and for part of the night, but turns sharply eastward during the pre-dawn and dawn period at the duct entry site. The June electric field appears to follow a diurnal variation whereas the equinox electric field shows a possible 4-hourly periodic variation. Seasonal variations in the neutral wind pattern, altering the configuration of the ionospheric dynamo field, are the probable cause of the seasonal differences in the electric field. The seasonal variation of the coupling fluxes can be explained by the alteration of the E x B drift pattern, caused by the changes in the electric field.

A new high latitude thermospheric neutral density structure has been revealed in NCAR-TIGCM simulations at 120–350 km altitude. The structure consists of density cells above 50° latitude with radii of approximately 1000 km. There are between two to four cells present depending on the altitude and magnetic activity. For example, at 200 km under magnetically active conditions, the density structure consists of four cells: low density cells are located near dawn and dusk and high density cells are located near noon and midnight. Density variations among the cells range from 5 to 50% for magnetically quiet and active conditions respectively. The cells are present at all seasons, for a wide range of magnetic activity levels, and at solar minimum and solar maximum. The density cell morphology is established for equinox solar maximum as a function of altitude and magnetic activity. Departures of the cell structure from this morphology due to seasonal and solar cycles are discussed. The cell morphology provides a new framework in which to interpret lower thermospheric density data. Data to test and confirm the model predictions were provided by the SETA-1 satellite.

A three-dimensional, time-dependent, MHD model of solar-disturbance-caused storms (Wu, 1993; Wu et al., 1996a) is used to predict the turning direction of the interplanetary magnetic field (IMF) at Earth. More explicitly, we examine the polarity of B_z caused by solar disturbances on the Sun. Three manifestations of solar disturbances, as studied by previous workers, are examined. Firstly, twenty-nine kilometric Type II events, associated (Cane, 1985) with geomagnetic storms, are studied within the context of our three-dimensional model. Then, an additional eleven long-duration X-ray events (LDEs) with radio fluxes greater than 100 solar flux units were examined; these events were not associated with interplanetary Type II events but were also associated (Cane, 1985) with geomagnetic storms. Finally, in situ interplanetary phenomena that caused ten large (Dst < −100 nT, the intensification of the storm) geomagnetic storm episodes (Tsurutani et al., 1988) near solar maximum are also studied via the B_z predictions of our 3D MHD model. The accuracy of these B_z turning-direction-predictions is found to be as follows: (1) for the kilometric Type II events, the model's prediction was successful for 26 of the 29 events studied; (2) 10/11 for the LDE events; and (3) 7/9 for the major geomagnetic storm events. The overall prediction accuracy of these three independent data sets is 43/49. Thus, consideration of these three independent data sets strongly suggests that the recipe proposed by the basic 3D MHD model may be valid for a zero-th order prediction scheme.