摘要:

We present here for the first time a Lyman‐α image of the north polar region of Jupiter obtained with the Faint Object Camera (FOC) on board the Hubble Space Telescope a few hours after the encounter of the ULYSSES spacecraft with Jupiter. The presence of high latitude regions of enhanced emission is clearly observed. A comparison with the location of the “UVS oval”, the Io (L = 6) and high‐latitude field‐line footprints shows that the best agreement is obtained with the L ≥ 15 footprint and the UVS oval which are close to each other for the particular longitudinal sector (30°<λIII<210°) observed. These two L‐shells correspond to two possible sources of precipitation: particles originating respectively from the region of the plasma torus of Io in a distorted magnetic field or particles from the distant magnetosphere by analogy with the terrestrial aurora. The first direct determination of the latitudinal extent of the oval and of its intensity is made and compared with p

ISSN:0094-8276    

DOI:10.1029/92GL02104

年代:1992

数据来源: WILEY

摘要:

We have used a large number of auroral magnetograms to identify four isolated substorms and estimate their onset times. At the onsets, ISEE‐1 was in the vicinity of magnetic midnight at radial distances of 15.6 to 21.8 Reand very near the outer boundary of the plasma sheet. We find that, for each event, the plasma sheet expanded, and the magnetic field dipolarized at the inferred onset time. Our most definitive event occurred while ISEE was at a geocentric radial distance of 21.8 Re. This result conflicts with previous understanding, though further verification of the result is required. Our observations show very similar characteristics to those observed at synchronous orbit, and they are consistent with an extension of a portion of the substorm current wedge to the radial distance of the satellite. If this explanation is correct, ISEE must have been within the longitude range of the substorm current wedge at the onset

ISSN:0094-8276    

DOI:10.1029/92GL02108

年代:1992

数据来源: WILEY

摘要:

Field line resonances with frequencies in the range 1 to 4 mHz have recently been observed by the JHU/APL HF Doppler radar during quiescent geomagnetic conditions. These structures are observed to have stable frequencies for durations of several hours, leading us to the conclusion that they may be standing waves (in the radial direction, as opposed to standing waves along a field line) of the magnetosphere driven by the solar wind. Using this premise, the propagation of fast mode ULF waves in the magnetosheath and the near earth magnetosphere are examined in an ideal, linearized MHD context. A model is presented in which fast waves propagate in the equatorial plane between the flanks of the bow shock and a turning point deep within the magnetosphere. Due to the magnetic field gradient near the earth, a field line resonance develops between the turning point and the plasmapause. Using a realistic set of magnetospheric parameters, it is possible to reproduce the set of observed frequencies and the respective positions of their field line resonances within the ionosphere (assuming a dipole mapping). However, because the model cavity frequencies are sensitive to magnetosheath parameters, this model does not explain the extreme stability with respect to geomagnetic conditions of the observed frequencies.

ISSN:0094-8276    

DOI:10.1029/92GL01802

年代:1992

数据来源: WILEY

摘要:

Lightning‐induced heating of the nighttimeDregion leads to the excitation of a number of lines of O, O2, N2, at intensities of 10–109Rayleighs (R) for vertical observations. For example, the 5577 Å emission from O has intensity ∼60 R lasting for ∼350 ms while the 1st and 2nd positive bands of N2are at ∼109R but last

ISSN:0094-8276    

DOI:10.1029/92GL02106

年代:1992

数据来源: WILEY

摘要:

In the spring of 1991 the Antarctic lower stratosphere was characterized by a layer of volcanic aerosol from the Cerro Hudson eruption. This aerosol layer was observed from McMurdo Station, Antarctica, with both lidar and balloonborne particle counters beginning around 10 September. After 20 September the aerosol was observed daily between 9 and 13 km. In this layer homogeneous nucleation of new aerosol was observed with concentrations>6000 cm−3. Comparisons of scattering ratio calculated from measured particle size distributions agree best with the lidar measurements when a real index of refraction near 1.5 is used. In the past 5 years of measurements, ozone below 13 km has been relatively unchanged during the annual Antarctic ozone depletion; however, in 1991 ozone below 13 km decreased at a rate of 4 – 8 ppb day−1over 30 days. This change began shortly after the appearance of the volcanic aerosol, providing direct measurements correlating volcanic aerosol and ozone depl

ISSN:0094-8276    

DOI:10.1029/92GL01943

年代:1992

数据来源: WILEY

摘要:

A lidar has been installed in Thule, Greenland, by the University of Rome in the fall of 1990. Intensive series of aerosol measurements have been carried out in the period early December 1991 – early March 1992, in the context of the European Arctic Stratospheric Ozone Experiment (EASOE). In the same period, several ozonesondes have been launched by the Danish Meteorological Institute. The arctic stratosphere appears loaded with aerosol following the Mt. Pinatubo eruption of June 1991. Scattering ratios as high as 5 have been observed. Both the aerosol and ozone profiles display a layered structure, with a large variability: the aerosol and ozone contents in the layers frequently appear to be negatively correlated, particularly in the height region around 16 km. Fluctuations of the ozone content as high as 50% of the average value have been observed in levels where aerosol layers are present. These results bear a relation to those obtained in past studies of the Mount Agung and El Chichón eruptions, and in observations of Polar Stratospheric Clouds obtained at South Po

ISSN:0094-8276    

DOI:10.1029/92GL01887

年代:1992

数据来源: WILEY

摘要:

Wind‐profiling Doppler radars measure vertical motions along a vertically‐directed beam. By averaging over long periods it is possible to measure very small mean vertical motions. Long‐term measurements at Christmas Island have been analyzed to determine the diurnal variation in the vertical motion field. The outstanding feature that emerges from the analysis is a pronounced diurnal cycle in the subsiding motion of the main troposphere (4–10 km) and an equally pronounced diurnal cycle in upward motion centered around the altitude of the tropopause. The possibility of radiative forcing of the observed diurnal cycle is considered in the context of the diurnal variation of atmospheric diabatic heating and coolin

ISSN:0094-8276    

DOI:10.1029/92GL02105

年代:1992

数据来源: WILEY

摘要:

The Gray Yearly Genesis Parameter (YGP) is an empirical diagnostic tool used to infer regions in which necessary (but not sufficient) conditions exist for tropical cyclone development. This parameter is used here as a measure of the implied tropical cyclone frequency and area of occurrence in climate simulations generated by a General Circulation Model (GCM). In a simulation of the current climate, the CSIRO9 GCM shows reasonable agreement with the original Gray climatology. The YGP for a doubled CO2simulation is presented.

ISSN:0094-8276    

DOI:10.1029/92GL02149

年代:1992

数据来源: WILEY

摘要:

Sixty‐five percent of the short term variability in southern Mexican precipitation is accounted for by the large‐scale circulation. Empirical relationships between sea level and 500 mb circulation fields, and the local precipitation in Chiapas, Mexico, are derived using a neural net. Although much of the rainfall is a result of convective processes, the neural net captures the onset of the precipitation season, and the phase of individual precipitation events. The analysis indicates that both of these aspects of the precipitation regime are controlled to a large extent by the atmospheric circulat

ISSN:0094-8276    

DOI:10.1029/92GL01423

年代:1992

数据来源: WILEY

摘要:

Using radiosonde data from 50 stations for 1973–1990, we quantify relationships between surface air temperature (Ts) and precipitable water vapor (W) for different time scales. Monthly mean observations are fairly well described by an equation of the formln W=A+B Ts, but the coefficientsAandBdepend on theTsrange considered. At highTs, the relationship is poor. This relationship and relationships between sea surface temperature (SST) andWbased on satellite microwave observations over oceans are in remarkably good agreement over restrictedSSTranges. Monthly and annual anomalies ofWandTsare well correlated only outside the tropics, but on longer time scales, there is some evidence of positive trends in bothWandTsat most of the stations studied. Thus the relationship betweenWandTsdepends on the time scales and geographic region considere

ISSN:0094-8276    

DOI:10.1029/92GL02001

年代:1992

数据来源: WILEY