摘要:

We present direct ground based observations of the plasma flow sunward and tailward of the nucleus of comet P/Swift‐Tuttle. The observations are long‐slit high resolution spectra of the H2O+emission centered at 6199Å with a velocity resolution of about 7 km s−1(FWHM) and a spatial resolution of about 104km at the comet. Emission is visible from just inside the predicted position of the cometopause on the sunward side of the nucleus out to 5×105km on the tailward side. The deceleration of the solar plasma on the sunward side is clearly observed as is the acceleration of cometary ions into the tail. These observations show the effectiveness of ground based methods for the systematic study of cometary plasmas and point to the need for a better theoretical understanding of their acceleration mec

ISSN:0094-8276    

DOI:10.1029/93GL01123

年代:1993

数据来源: WILEY

摘要:

The propagation characteristics of waves upstream and downstream of quasi‐parallel shocks are investigated by using two‐dimensional hybrid simulations. At low Alfvén Mach numbersMA≲ 2, the shock is initially associated with upstream phase‐standing whistlers. At later times, backstreaming ions excite longer‐wavelength whistlers via the right‐hand resonant ion/ion instability. As anticipated from linear theory, these waves are found to propagate along the magnetic field. Their group velocity is larger than or comparable to the upstream flow speed, and as a result, the waves remain in the upstream region. At higherMA≳3 these waves are convected back into the shock, causing its re‐formation and downstream perturbations. Shock transmitted waves mode‐convert into Alfvén / ion‐cyclotron waves which have a wavevector along the shock normal (pointing upstream) and convect downstream. The two‐dimensional simulation results confirm our earlier suggestion that the upstream waves should be field aligned, and that their convection into the downstream is associated with linear mode conversion into the Alfvé

ISSN:0094-8276    

DOI:10.1029/93GL01125

年代:1993

数据来源: WILEY

摘要:

We present measurements of the ionospheric plasma flow over the range of invariant latitudes 71–76°, observed at 10‐second resolution using both the EISCAT radars, with simultaneous observations of the 630 nm cusp/cleft aurora made by a meridian‐scanning photometer at Ny Ålesund, Svalbard. A major increase in the trans‐auroral voltage from 5 to 40 kV (associated with sunward convection in the early afternoon sector) is found to follow a southward motion of the aurora and coincide with the onset of regular transient auroral breakup events. It is shown that these observations are consistent with recent theoretical work on how ionospheric flows are excited by time‐dependent reconnection at the dayside m

ISSN:0094-8276    

DOI:10.1029/93GL00846

年代:1993

数据来源: WILEY

摘要:

When the magnetosphere is compressed by a sudden change in the solar wind dynamic pressure, the horizontal component of the Earth's magnetic field is increased at low latitudes. Often there is an overshoot associated with this increase in the field, but not always. The overshoot does not appear to be due to induced currents in the interior of the Earth or in the ionosphere. Rather, its magnitude appears to be controlled by both the strength of the ring current, and by local time. We speculate that the overshoot in the horizontal component is due to an overshoot in the compression of the magnetosphere and that when the ring current is strong the compressional wave is damped and the magnetosphere is not set into oscillation by the compressional wave.

ISSN:0094-8276    

DOI:10.1029/93GL01257

年代:1993

数据来源: WILEY

摘要:

In two barium releases made over the Caribbean during the summer of 1991, the ion cloud was observed to last more than 10 hours. Observations were made with a low light level TV imager and a TV spectrograph operated onboard a Russian research vessel conducting ionospheric research and participating in the NASA‐CRRES mission. The two clouds were observed near the release region for 15 to 25 minutes during the twilight following the initial release. One of the releases was made during evening twilight and the ion cloud reappeared the following morning, while the other was a morning release and the late‐time observations made the next evening. The observations are highly surprising since, generally, it has been assumed that the ion cloud would dissipate within a few hours. However, the long life time appears consistant with current knowledge about dissipative processes. The long lifetime of barium ion clouds may provide for the most sensitive experimental testing of ionospheric convection models, a possibility with clear implications for future barium release experime

ISSN:0094-8276    

DOI:10.1029/93GL01348

年代:1993

数据来源: WILEY

摘要:

Data obtained from the WATS (Wind and Temperature Spectrometer) instrument on DE‐2 (Dynamics Explorer) during high solar activity, show new evidence for the presence of vertical winds of a significant magnitude in the equatorial thermosphere. They reveal a latitudinal structure that can be related to the recently discovered phenomena of the Equatorial Temperature and Wind Anomaly (ETWA). In the local evening hours, the vertical winds usually are downward around the dip equator and collocated with the temperature minimum of ETWA. In general, they are upward at about 24° dip latitude away from the dip equator and are collocated with the ETWA temperature crests. The magnitude of the vertical winds is in the 10–40 m/s range. It is proposed that the temperature and pressure ridges, formed by the excess ion drag on the zonal winds around the two crests and ordered by the relatively lower ion drag at the trough of the well known Equatorial Ionization Anomaly (EIA), drive a new wind system in the meridional plane and that the measured vertical winds form part of this wind sy

ISSN:0094-8276    

DOI:10.1029/93GL01253

年代:1993

数据来源: WILEY

摘要:

The 1083 nm airglow emission arising from the resonant scattering of solar photons by metastable He(2³S) atoms is potentially valuable for remote sensing of total photoelectron fluxes and neutral helium densities at altitudes above the F2peak. In this study, we adopt the diffusion picture used in all previous studies and point out that even within this crude approach, intensities consistent with the winter observations (≳1000 R) collected inTinsley and Christensen[J. Geophys. Res., 81, 1253, 1976] are obtained when the correct parameter values for He(2³S) production, loss, and transport are applied and line‐of‐sight geometries are properly taken into

ISSN:0094-8276    

DOI:10.1029/93GL01117

年代:1993

数据来源: WILEY

摘要:

Laboratory experiments have been performed with O and O2in their ground electronic states to study the distribution of all possible ozone isotopes formed. Results show that with respect to48O3the two symmetric molecules17O17O17O and18O18O18O are depleted, in good agreement with standard recombination theory. A large enrichment of about 18% is found in the asymmetric molecule16O17O18O, while all others carry about 2/3 of that. A comparison with past laboratory and stratospheric ozone isotope measurements leads to the following conclusion: There is a standard enrichment which resides in asymmetric molecules only. It will lead to an enrichment of stratospheric49O3and50O3of 8 to 9%; this has been actually observed in recent balloon experiments. Occasionally, the enrichments in the stratosphere are larger, reaching 40% at certain altitudes. Only when ozone was formed in an electric discharge process have larger enrichments been measured in laboratory experiments, affecting both symmetric and asymmetric molecules. The results provide an important connection between numerous laboratory studies and stratospheric measurements.

ISSN:0094-8276    

DOI:10.1029/93GL01080

年代:1993

数据来源: WILEY

摘要:

Absorption cross‐sections are reported for CS2for wavelengths between 187 nm and 230 nm at a temperature of 300K and a spectral bandwidth of 0.20 nm. The results are compared to previous cross‐section determinations for CS2, and to the absorption spectrum of SO2, which exhibits a strongly structured absorption band in the same region of the spectrum. The effect of spectral bandwidth and temperature on the absorption cross‐sections is also disc

ISSN:0094-8276    

DOI:10.1029/93GL00809

年代:1993

数据来源: WILEY

摘要:

During a recent study carried out at an isolated site in the Colorado mountains, a C5alcohol, 2‐methyl‐3‐buten‐2‐ol, was found to be the most abundant volatile organic compound of biogenic origin present in the atmosphere. This finding, if generally characteristic of the natural chemical species present in the atmosphere in forested areas, has important implications. First, the presence in large quantities of a reactive chemical compound at these high levels can significantly influence the local atmospheric chemistry. Secondly, this compound, although previously identified as a pheromone forIps typographus(spruce bark beetle), an insect predator responsible for major forest die‐backs in this region, is strongly correlated with isoprene. Since isoprene is known to be emitted by the local vegetation, the observed 2‐methyl‐3‐buten‐2‐ol appears also to have a vegetative rather than

ISSN:0094-8276    

DOI:10.1029/93GL00247

年代:1993

数据来源: WILEY