Geophysical Research Letters

Strauss, H. R.

doi: 10.1029/GL016i003p00219pmid: N/A

Presently favored mechanisms of coronal heating ‐ current sheet dissipation and Alfvén wave resonant heating ‐ deposit heat in thin layers. Classical thermal conduction cannot explain how heat is transported across the magnetic field. If heating occurs in thin layers, large pressure gradients can be created, which can give rise to ballooning modes. These instabilities are caused by the pressure gradient and the curvature of the magnetic field, and are stabilized by magnetic tension. The modes are broad band in wavelength and should produce turbulence. A mixing length expression for the turbulent heat transport shows that it is more than adequate to rapidly convect heat into much broader layers. Furthermore, the turbulent resistivity implies that heating occurs over most of the width of these broadened layers. The broadening also implies that much shorter time scales are required for heating. The β values in the corona suggest that 1 ‐ 10 turbulent layers are formed in typical loop or arch structures.

Gazis, P. R.; Mihalov, J. D.; Barnes, A.; Lazarus, A. J.; Smith, E. J.

doi: 10.1029/GL016i003p00223pmid: N/A

The Pioneer 10, 11 and Voyager 2 spacecraft are well suited for exploring spatial gradients in the distant solar wind. Between 1984 and 1986 Pioneer 11 and Voyager 2 were located at nearly the same heliocentric distance (≃20 AU) and longitude but were widely separated in latitude; Pioneer 11 was at a heliographic latitude of ≥15° while Voyager 2 was near the solar equator. Pioneer 10 was located near the solar equator but at a considerably greater heliocentric distance (≃40 AU). IMP observations at 1 AU provide an inner heliosphere baseline. Prior to mid‐1985, Pioneer 11 and Voyager 2 observed nearly identical solar wind structures. The characteristics and arrival times of these structures were similar at both spacecraft. After day 150 of 1985 this situation changed dramatically. The velocity structure at Voyager 2 became flatter. The velocities seen at Voyager 2 also fell with respect to Pioneer 11; the difference in velocities observed by the two spacecraft grew to 200 km s−1 by late 1986. In late 1985 Pioneer 10 observed a change in the solar wind similar to that observed at Voyager 2. These changes in the solar wind at low latitudes may be related to a change which occurred in the coronal hole structure in early 1985.

Nishida, Atsuhiro

doi: 10.1029/GL016i003p00227pmid: N/A

We consider consequences of the reconnection that occurs randomly on the dayside magnetopause. Open field lines produced by such reconnection events often intersect with each other, and they do not necessarily show some of the features that would be expected from the reconnection if this mutual interaction were absent. Some of the randomly produced open field lines would re‐reconnect and result in closed field lines. The solar wind plasma streams into the magnetosphere along these field lines as long as they are open, however, and stays there after the field lines become closed again. We propose that this is the mechanism of the plasma supply into the low‐latitude boundary layer. We will show that many of the observed signatures of the LLBL are consistent with this model.

Summers, Danny; Thorne, Richard M.; Mei, Yi

doi: 10.1029/GL016i003p00231pmid: N/A

We construct radial profiles for the Jovian radiation belt flux‐tube content Y* from the reported phase‐space density of energetic particles obtained from the Voyager 1 LECP data over the range L = 6 to L = 9. These experimental profiles are compared with theoretical solutions for Y* from our interchange‐diffusion model of the coupled radiation belt and Iogenic ion populations, which incorporates the pressure gradient of the radiation belt ions and spatially‐varying forms for the precipitation loss‐rate of the radiation belt ions and the concomitant height‐integrated Pedersen ionospheric conductivity. Subject to certain limitations of the Voyager 1 data, the model solutions are found to be consistent with the data for a variety of input parameters. Model solutions are also found corresponding to radiation belt ions in the energy range 1 (MeV/G) ≤ µ ≤ 10(MeV/G) (which was not sampled by Voyager) that are expected to be mainly responsible for the auroral energy input. Comparison of our theoretical profiles with the data implies that the energetic radiation belt ions should have a peak loss rate within a factor of three of that for strong diffusion scattering.

Moorcroft, D. R.

doi: 10.1029/GL016i003p00235pmid: N/A

Auroral structures are often accompanied by tilts to the electron density contours on the underside of the E‐region. By including the effects of these tilts on radio wave refraction and reflection it is possible to understand many of the previously puzzling observations of VHF auroral backscatter at large angles away from perpendicularity to the earth's magnetic field. It is shown that many of the characteristics of echoes from the VHF Canadian Bistatic Auroral Radar System (BARS) are consistent with this process. For BARS observations for which the tilt hypothesis is correct, the resulting Doppler velocities can be interpreted in the same way as those from STARE, and used to give electric fields and convection patterns over the BARS field of view.

Miles, T.; Grose, W. L.

doi: 10.1029/GL016i003p00239pmid: N/A

Synoptic‐scale zonal eddies (longitudinal disturbances) confined to polar latitudes in the southern hemisphere (SH) upper stratosphere have been observed in Fourier analyses of temperature measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment during April‐May 1979. Eddy activity at 60°S, 1 mb is characterized by a Fourier zonal wavenumber 5 (wavelength ∼ 4,000 km) disturbance of amplitude 2‐3 K which exhibits intermittent eastward propagation, substantial meridional phase tilt, and little vertical phase tilt. These wave properties, along with evidence of pronounced latitudinal variations in the upper stratosphere zonal wind distribution, suggest that the synoptic‐scale modes may arise from a latitudinal wind shear (barotropic) instability of the SH polar jet stream. This analysis apparently represents the first identification of polar upper stratosphere synoptic‐scale disturbances from satellite‐based information, heretofore only seen in rocketsonde measurements in northern hemisphere (NH) polar regions and also predicted by linear model studies. The ability to discern these disturbances in satellite temperature data sets offers an opportunity for future studies of the interhemispheric and interannual variability of synoptic‐scale eddy activity in the stratosphere. However, an unambiguous determination of dominant wavelength and phase speed in such studies will be precluded by the fact that satellite data always will suffer from a certain degree of aliasing if disturbances exist with scales unresolved by the asynoptic satellite sampling.

Granier, C.; Jegou, J. P.; Megie, G.

doi: 10.1029/GL016i003p00243pmid: N/A

Resonant lidar detection of iron atoms have been performed at the Observatoire de Haute Provence (France) during 4 nights in november 1986 and in april 1987. The average iron atoms abundance is 3.6 × 109 cm−2 in november, and 2.6 × 109− cm−2 in april. Iron atoms density profiles are compared to the atomic sodium ones, obtained simultaneously by lidar and indicating an average sodium abundance of 4 × 109 cm−2. The relative abundance of the sum of the atomic and ionic forms is compared for several metallic species with their abundance ratios in the incoming meteorites. Finally, similar comparisons using lidar and mass spectrometer data are made for the ratio of the atomic and the ionic forms in the atmosphere.

Hameed, Sultan; Currie, Robert G.

doi: 10.1029/GL016i003p00247pmid: N/A

The agent that generates and maintains the 14‐month Chandler wobble of the solid earth about its rotation axis has remained unresolved for a century with first the atmosphere, later earthquakes, and more recently the earth's fluid core proposed as candidates. Here we report that surface air pressure calculated in a coupled ocean‐atmosphere general circulation model (GCM) displays a 14.7 month signal, whose amplitude is similar to that found by Maksimov (1960) in station data; we identify it as the atmospheric Chandler wobble. This result indicates that changes in atmospheric mass distribution excite and maintain the wobble of the solid earth, and that neither earthquakes nor the fluid core are significant contributors. Another result is that in the GCM the amplitude of the wobble at high latitudes is a substantial fraction of the annual cycle, and thus is an important factor in climate formation as Maksimov (1960) suggested.

Moriceau, Christophe; Fleitout, Luce

doi: 10.1029/GL016i003p00251pmid: N/A

The geoid over the Pacific Ocean has been filtered in order to enhance wavelengths of the order of 200 km. As noticed by previous authors, lineations elongated in the direction of present plate motion are observed in the East‐Central Pacific. Elongated features as clearly visible with a larger amplitude and a totally different direction are also present south of the Hawaiian chain and west of Baja California. The dominant directions visible on the filtered geoid map have been confirmed by spectral analyses. Near the Californian coast, the NE‐SW lineations seem to be related to extensional tectonics. South of Hawaii, the NW‐SE directions correspond to the various directions of the Pacific plate prior to 40 Ma. Magmas implaced in the lithosphere over fixed convective plumes may be the source of these geoid lineations.

Hyndman, R. D.; Klemperer, S. L.

doi: 10.1029/GL016i003p00255pmid: N/A

Seismic refraction velocities provide an important constraint on the composition of the lower continental crust. However, valid inferences on the composition may require allowance for the effect of porosity. Many refraction velocities obtained for the lower crust are characteristic of laboratory values for intermediate composition rocks, in disagreement with other estimates of a primarily mafic lower crust composition. The discrepancy is particularly large in Phanerozoic areas where most velocities are less than 7.0 km s−1, and where non‐seismic estimates such as from xenoliths are almost exclusively mafic. A mafic composition can be reconciled with the refraction data if the insitu velocities are reduced by porosity of a few percent. Support for the model is given by magnetotelluric measurements which have shown the lower crust to be commonly very electrically conductive. Lower crustal porosities of up to several percent are inferred, particularly in areas where the resistivity is less than 100 ohm‐m.