摘要:

We describe a technique for measuring time delays by using the cross spectrum of Pc 1 recorded at different stations. The method is based upon the computation of the coherency coefficient and of the phase of the cross spectrum. It is applied to data which were obtained during a campaign of observations at two pairs of conjugate stations (L=.7 andL=.5). We show that the coherency coefficient between signals recorded at conjugate points may be large (∼1) for structured Pc 1, which implies that the optical path is constant, to within a fraction of a wavelength, over long periods of time (the integration time is of the order of 16 min). Measurements of time delay between the northern and southern hemispheres show that there is generally an asymmetry, the delay being longer for the north to south propagation. This asymmetry is tentatively interpreted in terms of differences between the ionospheric characteristics above the two stations but also in terms of a difference between the two magnetospheric paths. The detailed study of the cross‐spectral power density phase versus frequency variation shows that on some occasions a reamplification process takes place at each bounce of the wave, but only on one of the two trajector

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04115

年代:1980

数据来源: WILEY

摘要:

A numerical prediction is presented for upper bounds on negative floating potentials of shaded spacecraft surfaces, using a local‐current‐balance formulation. Results include the following: (1) By influencing the velocity‐space cutoff boundaries for incident ion fluxes, the spacecraft geometry and sheath potential profile (particularly shaded‐sunlit asymmetries in the latter) have large influences on shaded‐surface potentials, which may exceed ‐ 20 kV in certain circumstances. (2) For electrically isolated surfaces in shaded cavities, negative floating potentials may exceed those on convex surfaces. (3) In some conditions, two distinct floating potentials are predicted. This implies the possibility of ‘bifurcation phenomena’ in which adjacent isolated surfaces made of the same material may follow different charging histories. It also implies that large and relatively sudden changes in surface potentials can be caused by gradual changes in either the external environment or beam em

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04125

年代:1980

数据来源: WILEY

摘要:

Meridian‐scanning photometer (MSP) measurements of atmospheric optical emissions made in Longyearbyen, Svalbard, in the winter of 1978–1979 show two major types of daytime auroras associated with the magnetospheric cusp region. A steady, subvisual arc extending over 1.5° to 5° of invariant latitude characterized by a predominance of 6300‐Å emission is a permanent feature of the cusp region. Additionally, short‐lived, relatively narrow arcs and bands appear occasionally within this region; in these transient auroral forms the [O I] 5577‐Å line and the N2+1 NG bands are the strong optical emissions. Detailed correlation of the cusp optical emission and simultaneous geomagnetic measurements from both day and nightside stations show that the cusp, as defined by the 6300‐Å arcs, moves equatorward during magnetically disturbed conditions. This equatorward drift of the cusp region is directly proportional to the maximum negative excursion of the horizontal disturbance magnetic field in the meridian, which is approximately 180° displaced in longitude from the dayside cusp; (while the local magnetic disturbance on Svalbard is remarkably similar to nightside observations, it is approximately a factor of 5 less intense, making it difficult to correlate cusp position with local magnetic activity during relatively weak magnetic substorms). From three days of simultaneous data it may be seen that an increase in cusp auroral activity is characterized by the appearance of discrete auroral arcs and bands that occur in the prenoon and postnoon periods. The most significant result from these measurements is the absence of any detectable delay between the enhancement in cusp auroral activity and the commencement of magnetic disturbance at a station situated under maximum electrojet current

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04132

年代:1980

数据来源: WILEY

摘要:

This paper describes the morphology of midnight sector and morning sector auroral zone scintillation observations made over a two‐year period using the Wideband satellite, which is in a sun‐synchronous, low‐altitude orbit. No definitive seasonal variation was found. The nighttime data showed the highest scintillation occurrence levels, but significant amounts of morning scintillation were observed. For the most part the scintillation activity followed the general pattern of local magnetic activity. The most prominent feature in the nighttime data is a localized amplitude and phase scintillation enhancement at the point where the propagation vector lies within anLshell. A geometrical effect due to a dynamic slab of sheetlike structures in theFregion is hypothesized as the source of this enhancement. The data have been sorted by magnetic activity, proximity to local midnight, and season. The general features of the data are in agreement with the accepted morphology of auroral zone scintill

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04139

年代:1980

数据来源: WILEY

摘要:

In this paper we model the azimuthal component of the magnetic field at Jupiter. We start by writing a current distribution which is the sum of two currents, one flowing along the magnetic field lines (J=αB) and another injected into the current sheet atr0(∼10RJ). We look at two cases, one in which current flows along the field lines into the equatorial plane and then radially outward and a second case in which the only current is that injected into the current sheet atr0. Each of these two cases results in an azimuthal magnetic field which fits the magnetic field da

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04152

年代:1980

数据来源: WILEY

摘要:

Signals propagating from the Siple, Antarctica, VLF transmitter to the conjugate station at Roberval, Quebec, Canada (L∼4.2), are generally multipath in nature. From previously reported initial observations at Roberval with a tracking receiver/direction finder (TR/DF) it was found that the directional properties of the received signals were stable for only minutes at a time, compared to the tens of minutes that had been expected from earlier dispersion analyses of whistlers. This effect was further investigated in the present study of TR/DF observations of multipath Siple signals in 1977. In a case study from May 20, 1977, the TR/DF consistently obtained directional information on the arrival bearings of multipath signals whose amplitude at the receiver was ∼5 dB or more above that of other signals within the 340‐Hz passband of the frequency‐tracking filter. It was found that the arrival bearing of the strongest signal shifted widely within minutes. The sequence of shifts over a 1‐hour period, coupled with the occasional appearance of power line radiation activity, may possibly be induced by the eastward gradient drift of low‐energy (≃1–2 keV) ‘clouds’ of electrons through a pattern of whistler ducts. This interpretation was suggested by the rapidity (∼10–100 s) with which signal amplitudes rose and fell by greater than ∼10 dB on individual paths. The sudden turn‐ons and turn‐offs are tentatively attributed to the ‘threshold’ effect being reported elsewhere, in which temporal growth of Siple transmitter signals suddenly occurs as the amplitude of the injected waves reaches a particular level. In the present case the phenomenon is believed to occur at constant wave level as the result of changes in particle flux associated with drifting structure in the low‐energy electrons. Other effects observed in the primary case study were a slow northward movement on a time scale of about 30 min of the bearing of the stronger signals. This movement is believed to be associated with quiet‐day convection. Also observed were small‐scale (Δθ ∼10°–20°) fluctuations on a time scale of several minutes in the arrival bearing of signals from a particular path. These fluctuations may be due to a temporal modulation of propagation conditions between the ‘ends’ of magnetosph

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04157

年代:1980

数据来源: WILEY

摘要:

Previous studies of magnetic field perturbations at very high latitude have indicated the presence of an equivalent current system which is strongly modulated by the azimuthal (By) component of the interplanetary magnetic field. This current system is now thought to flow in the portion of the ionosphere to which the polar cusp (or cleft) maps. In this paper we shall show that, while the By component of the interplanetary magnetic field does not significantly change the total current flowing in the magnetosphere, it does redistribute the ionospheric portion of the current flow. This redistribution results in eastward ionospheric current being observed in the prenoon quadrant on the poleward side of the normally present westward electrojet when By>0, whereas only westward current flows in the prenoon quadrant when By<0.

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04167

年代:1980

数据来源: WILEY

摘要:

We have obtained steady state solutions of the coupled continuity, momentum, and energy equations for He+, O+, and electrons for conditions appropriate to the daytime high‐latitude topside ionosphere. Our study was limited to subsonic He+outflows, which result in He+being either a major ion or an important minor ion over the altitude range from 500 to 2500 km. This study therefore complements the previous He+studies, which were limited to situations where He+was a minor ion at all altitudes. In addition to He+outflow velocity, we studied the effect on the ion densities and temperatures of convection electric fields, different electron temperature distributions, and different assumed He+and O+heat fluxes at high altitudes. The latter effect was not considered in previous studies of the high‐latitude topside ionosphere. Some of the more important results of our study are the following: (1) the absence of topside He+and O+heat fluxes results in closely coupled He+and O+temperatures at all altitudes for low to moderate He+outflow speeds; (2) a downward topside He+heat flux results in elevated He+temperatures at all altitudes above about 800 km and a large He+temperature gradient over a significant altitude range; (3) the thermal diffusion associated with the steep He+temperature gradient has a significant effect on the He+and O+density profiles for low to moderate He+outflow speeds but a much smaller effect on these profiles for more substantial He+outflow speeds; thermal diffusion acts to drive the He+ions downward toward cooler regions and the O+ions upward toward hotter regions; (4) because of thermal diffusion the change in the ‘diffusive equilibrium’ He+density profile with an increase in the topside He+heat flux is very similar to the change obtained for ‘dynamic equilibrium’ with an increase in the topside He+escape flux; (5) in contrast with previous results, the He+‐O+frictional heating that occurs when He+is in a state of outflow can in some cases raise the He+temperature above the O+temperature at high altitudes; this new result is due in part to the use of different ionospheric parameters and in part to the fact that the diffusion‐thermal heat flow opposes ordinary He+thermal conduction, thereby increasing the relative importance of He+‐O+frictional heating; and (6) basically, a converting ionosphere and a nonconvecting ionosphere exhibit a similar variation with regard to changes in the topside He+escape flux and in the topside He+and O+heat fluxes. The main differences stem from the additional heating that occurs at low altitudes owing to the frictional interaction between

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04177

年代:1980

数据来源: WILEY

摘要:

Airglow meridional intensity gradients (MIG) that moved from east to west with the antisolar meridian have been observed during times of considerable magnetic activity. The magnitude of the gradient is around 0.1 R/km or more. We believe the MIG phenomenon develops as a result of a velocity gradient that is produced by the meeting of opposing meridional neutral winds. The northern wind system arises from the auroral heating at high latitude that produces meridional winds penetrating to low latitudes in the summer hemisphere. The southern wind system is produced by the equatorial midnight pressure bulge that is formed by the thermal tides of the lower atmosphere combined with the in situ forcing by EUV radiation absorption in the daytime. In a magnetic storm the merging of the two wind systems produces a sharp front over a latitudinal width of several hundred kilometers. North of the front the ionosphere is elevated, and the airglow is weak. South of the front the ionosphere is depressed, and the airglow is enhanced with a surface brightness of the order of 100 R. The conceptual model that evolved from these observations accounts for the overall behavior of the MIG as well as the normal midnight collapse. A simple calculation shows that the MIG can be used to estimate the meridional velocity gradient as well as the magnitude of the winds in the midnight pressure bulge.

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04191

年代:1980

数据来源: WILEY

摘要:

A detailed description and analysis is presented of the Lagopedo II results. The rocket was launched on September 11, 1977. Prior to an explosive chemical release a rocket‐borne ion mass spectrometer and dual‐frequency beacon measured the ion composition and electron content of the undisturbedFregion above Kauai, Hawaii. These results are compared to a detailed calculation of ionospheric ion densities. Considerable H2O outgassing produced measurable 18+(H2O+) and 19+(H3O+) currents which together with O+current were used to determine the H3O+/H2O+dissociative recombination rate ratio. The explosive event at 283 km swept clean a 1‐km‐radius region. The boundary of the ionic void was characterized by a steep gradient in the ion density. Results for the first 60 s after the event are presented which show changes in ambient ion species 14+, 16+, 30+, and 32+, new reactant species 15+, 17+, 18+, 19+, 27+, and 46+, and evidence of ionic depletion by s

ISSN:0148-0227    

DOI:10.1029/JA085iA08p04205

年代:1980

数据来源: WILEY