摘要:

We use full‐year and seasonal averages ofFregion electron density and ion velocity measurements made by the Japanese middle and upper atmosphere (MU) radar in Shigaraki, Shiga, Japan, to compute the neutral meridional winds for this location. The average winds are highly diurnal in nature. The all‐year average winds are poleward by day and equatorward by night. Maximum daytime winds are reached near 10 LT, and maximum nighttime winds are reached at midnight. This is consistent with the accepted picture of global circulation consisting of winds driven basically around the Earth from the dayside subsolar point toward the nightside antisubsolar point. Dependence on ion drag is seen in the form of larger nighttime winds and the prenoon daytime velocity maximum. In winter the winds are strongly northward for the entire daylight period, while in the summer the winds become northward for only a few hours before noon and then turn southward for the remainder of the day. The MU radar winds are consistent with the majority of other such wind results reported in the literature for other mid‐latitude locations concerning the phases and times of wind direction turnings. The amplitudes of the MU radar winds, however, are generally smaller than the amplitudes reported elsewhere by a factor of 1.5–2. We see no trace of an abatement in the southward wind at midnight as seen at lower latitudes or of large winds exceeding 200 m/s in the nighttime as reported for locations near the polar

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07683

年代:1990

数据来源: WILEY

摘要:

In the late fall and early winter, The Johns Hopkins University HF radar at Goose Bay, Labrador, observes the effects of atmospheric gravity waves on radar transmissions that are obliquely reflected from the ionosphere and subsequently backscattered from the Earth's surface. The waves exist under a wide variety of geomagnetic conditions; how ever, they are particularly noticeable under quiet conditions (0 ≤ Kp ≤ 1 +). The clearest signatures of the waves are spatially localized enhancements in the backscattered power and quasi‐periodic fluctuations in the backscattered powers, Doppler velocities, and reflection heights. The waves are generally observed during daylight hours and propagate equatorward from regions of high‐latitude ionospheric backscatter that are located near the ionospheric convection reversal boundary. The gravity waves appear to be generated just equatorward of the dayside flow‐reversal boundary in the vicinity of the auroral electrojet at altitudes of 115 to 135 km and propagate approximately perpendicular to the boundary along azimuths ranging from 156° to 180°. The waves propagate obliquely downward through the lower atmosphere until they are reflected by the Earth's surface back into the upper atmosphere. The frequencies associated with these gravity waves cover the range of 0.3 to 0.6 mHz, with wavelengths of 300 to 500 km, and with average phase velocities of 110 to 180 m/s. The maximum phase speeds are 270 to 300 m/s, which is slightly less than the speed of sound in the lower atmosphere. Poleward‐propagating gravity waves are sometimes observed under disturbed conditions when the polar cap and convection reversal boundary have expande

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07693

年代:1990

数据来源: WILEY

摘要:

We have undertaken a series of investigations of excitation and decay processes in molecular nitrogen in pulsed electric discharges under conditions relevant to those of the aurora. Previously, we had observed the continuous evolution of the spectrum of the N2(B³Πg) → N2(A³Σu+) (1 PG) with time and under the influence of collisions after the initial direct electron excitation. In the present investigation, we find that there are progressive changes in the condition of the gaseous medium which occur during the time interval between pulses. At all but the very lowest repetition rates, the sample atmosphere is found to be “preconditioned” at the beginning the excitation pulse by virtue of its previous excitation‐deexcitation history. One possible component of a preconditioned medium is the presence of electronically excited metastables. Another is the retention of excitation in the ground state vibrational manifold. Our observations indicate N2ground state vibrational temperatures of greater than 3000 K just prior to the beginning of the excitation pulse when conditions for preconditioning are favorable. In addition, our model results indicate that currently accepted N2(B) quenching coefficients are not adequate to explain our observations during the afterglow following the discharge. Ground‐based and rocket measurements of the auroral and airglow spectra have revealed alterations in the distribution of nitrogen molecular band emission intensities only very recently. We are led to believe that most of these phenomena being uncovered in the laboratory are likewise occurring in the aurora, to varying extents depending on conditions and will become evident as observational technology advances. We include a discussion of some conditions under which deviations in the distribution of excited N2band intensities have been observed

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07711

年代:1990

数据来源: WILEY

摘要:

Images of the entire auroral oval at carefully selected wavelengths contain information on the global energy influx due to energetic particles and some information on the characteristic energy of the precipitating particles. In this paper we investigate the sensitivity of selected auroral emissions to changes in the neutral atmosphere. In particular, we examine the behavior of OI 1356 Å and two Lyman Birge Hopfield (LBH) bands and their ratios to each other with changing atmospheric composition. The two LBH bands are selected so that one lies in the region of strong O2absorption (1464 Å) and one lies at a wavelength where O2absorption is effectively negligible (1838 Å). We find that for anticipated average uncertainties in the neutral atmosphere (factor of 2 at auroral altitudes), the resultant change in the modeled intensities is comparable to or less than the uncertainty in the neutral atmosphere. The smallest variations, for example, are for I 1838 (approximately 10 to 20%) while the largest variation is seen in the OI 1356 Å emission which is linear with [O] to within 20%. We have also investigated the dependence of these intensities, and their ratios, to much larger changes in the composition (i.e., [O]/[N2]) such as might be encountered in large magnetic storms, or over seasonal or solar cycle extremes. We find that the variation in the I 1356/I 1838 ratio over the equivalent of a solar cycle is less than 50%. The summer‐to‐winter changes are approximately a factor of 2. The I 1356/I 1838 ratio is a very sensitive indicator of the charactenstic energy, showing a change of 13 over the energy range 200 eV to 10 keV. The corresponding change in the LBH long‐to‐short wavelength ratio is much less (about a factor of 3). However, the latter is insensitive to changes in the neutral atmosphere (<20% changes in LBH emission ratio for large changes in N2). The three emissions therefore potentially provide a most valuable diagnostic of particle characteristic energy and

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07725

年代:1990

数据来源: WILEY

摘要:

A photochemical equilibrium daytime model is used to study the ionosphere between 110 and 180 km at mid‐latitudes. The model includes the latest photoionization and photoabsorption cross sections, extreme ultraviolet (EUV) fluxes in 37 wavelength bands, and all reactions believed to be important in this region. Model results are compared with (1) noon‐timeElayer critical frequency (foE) at Boulder and Wallops Island over a full solar cycle; (2) Millstone Hill incoherent scatter radar observations of electron density at 180 km (N180) for a wide variety of seasons and solar geophysical conditions; (3) selected Millstone Hill incoherent scatter profiles of electron density between 110 and 180 km which includedE‐F1valley minima; and (4) the ratio of the molecular ion concentration to the total ion concentration at 180 km for noon throughout the solar cycle as given by both the IRI‐86 ion composition model and the semiempirical ion composition model of Oliver. Best agreement between the photochemical model documented in this paper and the observations and ion composition models is generally obtained if (1) the EUV fluxes in the photochemical model are increased by 25–30% above values derived from published reference spectra; (2) neutral densities used in the photochemical model are decreased by 25% below those given by MSIS‐86 at equinox, with larger decreases in winter, and smaller or no decreases in summer. The results show that this region of the ionosphere can be modeled with reasonable success given our current state of knowledge. Modeling this region of the ionosphere is important for resolving ambiguities in true height analysis of ionograms and reduction of incoherent scatter spectra. Improved modeling requires more accurate values of aeronomical parameters, i.e., ionizing fluxes, cross sections, reaction rates, composition and

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07735

年代:1990

数据来源: WILEY

摘要:

A critical velocity ionization experiment was carried out with a heavily instrumented rocket launched from Wallops Island at dawn on May 13, 1986. Two neutral barium beams were created by explosive shaped charges released from the rocket and detonated at 48° to B at altitudes near 400 km and below the solar UV cutoff. Critical velocity ionization was expected to form a detectable ion jet along the release field line, but instead an ion cloud of fairly uniform intensity was observed stretching from the release field line across to where the neutral barium jet reached sunlight. The process creating these ions must have been present from the time of the release and the efficiency is estimated to be equivalent to an ionization time constant of 1800 s. This ionization is most likely from collisions between the neutral barium jet and the ambient atmospheric oxygen, and if so, the cross section for collisional ionization is 9×10−18cm². A critical velocity ionization process may have been present during the first few tenths of a second after release, but its efficiency cannot have exceeded an equivalent ionization time constant of about 18

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07749

年代:1990

数据来源: WILEY

摘要:

Experiments in charge control on the AF/NASA P78‐2 (SCATHA) satellite were conducted with a plasma/ion source in the inner magnetosphere. These experiments were monitored with plasma wave instruments capable of high temporal and frequency resolution in the 0–6 kHz frequency range. Ion gun experiments revealed two distinct classes of behavior. Nonneutralized ion beam operation at 1 mA, 1 kV resulted in arcing signatures (spiky in time, broad frequency range), coincident with induced satellite potentials of −600 to −900 V. This signature disappeared when the accelerating voltage was switched off or the beam was neutralized (at which time the satellite body approached potentials of a few volts). The signal is attributed to arcing between differentially charged surfaces. An additional feature was noted in the 100‐kHz channel of the wave receiver. During emission of dense, low‐energy plasma, a signal is generated which may be at the upper hybrid, or plasma frequency for the l

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07759

年代:1990

数据来源: WILEY

摘要:

During a rocket flight to high altitude (585 km) a narrow band signal on an electric antenna was sometimes observed, whose frequency varied as the rocket turned. Such a signal cannot be natural, but apparently must be generated by the interaction of the rocket‐antenna system with the ambient plasma. Conditions for development of the instability are investigated. Maximum oscillation amplitude occurs when the antenna is aligned with the Earth's magnetic field. Similar observations have been reported by Gurnett and Mosier [1969]. Several attempts are made to understand the nature of this interaction, but without success. The instability treated by Fiala, due to the interaction of an inductive antenna impedance with stray capacitance to a phase shifted point in the preamplifier, can be ruled out. It appears that a negative antenna resistance due to interaction with waves Doppler‐shifted through zero is an unlikely explanation. The rocket velocity seems too small to give such an anomalous Doppler shift, and even if the observations of plasma density and temperature are stretched, the positive sheath resistance is larger than calculated negative resistances. Ion transit time instability in the sheath would only work at 10 times higher frequency. Interaction of the flowing plasma with sheath waves around the antenna is suggested and appears promising but the theory is not sufficiently developed for meaningful compari

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07773

年代:1990

数据来源: WILEY

摘要:

We discuss the mathematical representation of the electric current density and the magnetic field in equilibrium anisotropic magnetized plasma for which the particle motions parallel to the magnetic field are adiabatic. The current density is the sum of contributions due to plasma‐pressure currents and force‐free currents. The plasma pressure contribution can be expressed in terms of a vector field that is a function of the equatorial parallel pressure, the equatorial pitch angle distribution, and the magnetic field geometry. In the absence of force‐free currents it is possible to introduce a magnetic scalar potential in the presence of anisotropic magnetized plasma. The second‐order partial differential equation governing the scalar potential expresses the full coupling between the plasma and the magneti

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07789

年代:1990

数据来源: WILEY

摘要:

Satellite in situ measurements made by the Dynamics Explorer 2 (DE 2) satellite were utilized to describe the nature of plasma structuring at high latitudes caused by the gradient drift instability process. Specifically, by using noon‐midnight and dawn‐dusk orbits of the DE 2 satellite it was found possible to study the simultaneous density and electric field spectra of convecting large‐scale (approximately hundreds of kilometers) plasma density enhancements in the polar cap known as “patches”) in directions parallel and perpendicular to their antisunward convection. Distinct differences were noted in the behavior of the ac and dc electric field structure and short‐scale (<125 m) density irregularities in these two mutually orthogonal directions perpendicular to the geomagnetic field. However, since these two orthogonal directions were not sampled simultaneously, the observed differences cannot be unequivocally related to the direction of convection. Structured plasma density enhancements in the auroral oval (known as “blobs”) were found to have considerable power spectral density at these short scales in the presence of significant Pedersen and Hall conductances in the 10‐ to 20‐mho range. While density irregularity amplitudes (ΔN/N)rmswere found to be as large as 15–20% using 8‐s samples of the DE 2 data, the corresponding dc electric field fluctuation ΔEwas found to be less than a few millivolts per meter for both patches and blobs. This (ΔN/N)RMSvis‐a‐vis ΔEbehavior for the gradient drift process provided a fairly dramatic contrast with velocity shear driven processes where the ΔEmagnitudes were found to be at least an order of magnitude larger for the same levels of density irregularities. The electric field spectra for the moderate shear category discussed by Basu et al. (1988a) were also found to have a significantly different spectral index as compared to such spectra associated with the gradient drift process. The results of this paper together with those of Basu et al. (1988a) provide fairly conclusive evidence for the existence of at least two generic classes of instabilities operating in the high‐latitude ionosphere: one driven by large‐scale density gradients in a homogeneous convection field with respect to the neutrals and the other driven by the structured convection field itself in an ambient ionosphere where

ISSN:0148-0227    

DOI:10.1029/JA095iA06p07799

年代:1990

数据来源: WILEY