摘要:

Audiofrequency noise bands of continuous and triggered types that are evidently associated with the lower hybrid resonance frequency of the ionospheric medium have been observed with Dartmouth's whistler receiver using a 9‐ft electric dipole antenna on the OGO 2 spacecraft at heights up to 1500 km (apogee) and at frequencies up to 18 kHz (upper cutoff of the broadband receiver). Previous reports of observations of such bands have all been from the Alouette satellites that also carry whistler receivers equipped with electric dipole antennas. Although the electric dipole on OGO 2 is much shorter than the antennas of Alouette, our results are similar to the Alouette observations. A direct comparison is made of records obtained simultaneously on OGO 2 by Stanford's VLF experiment, which is connected to a loop antenna, and it is shown that triggered LHR emissions are seen almost exclusively on our experiment with the electric dipole antenna, pointing to an electric character of the waves. A new observation made by our experiment is that the upper cutoff frequency of the lower hybrid resonance noise bands triggered by fractional‐hop whistlers occasionally displays an envelope that has the shape of an Eckersley whistler. Whereas the results of the experiment are consistent with the interpretation that the lower cutoff frequency of noise bands triggered by whistlers is the lower hybrid resonance frequency of the ionosphere in the vicinity of the satellite, there is at present no satisfactory explanation of the upper cut

ISSN:0148-0227    

DOI:10.1029/JA074i001p00141

年代:1969

数据来源: WILEY

摘要:

A photometric latitude survey in the northern hemisphere was made from a jet aircraft to study atmospheric emissions. No diffuse, particle‐excited emissions were detected above 80° invariant latitude at night during January through March 1968, and an upper limit on the energetic‐particle energy influx during this period is therefore calculated to have been ∼2 × 10−3erg cm−2sec−1(for both protons and electrons). Soft electrons with energies ∼0.6 kev precipitate in a zone that is located poleward of the normal auroral zone, resulting in a zone of enhanced λ6300 O I emission. Averaged latitudinal distributions for Hβ, λ4278 N2+, λ5577 O I, λ6300 O I, and λ5890–96 Na between 13°N and 87°N geom. are presented, and their significance relevant to direct satellite measurements of par

ISSN:0148-0227    

DOI:10.1029/JA074i001p00153

年代:1969

数据来源: WILEY

摘要:

The time‐dependent flux of equatorial electrons (0.4 Mev ≲E≲ 2.2 Mev) observed on ATS 1 often exhibits drift‐periodic structures that can be traced back to a sudden expansion or compression of the magnetosphere. The magnetospheric disturbance redistributes the energetic electrons among drift shells in an asymmetrical manner, and this spatial asymmetry between noon and midnight reveals itself as a temporal variation in particle flux as the electrons subsequently drift past the observer. A mathematical model for the dynamics of this azimuthal bunching of energetic electrons reveals the microstructure of the redistribution of electrons among drift shells and indicates qualitatively the relation of this microstructure to the diffusion of particles

ISSN:0148-0227    

DOI:10.1029/JA074i001p00159

年代:1969

数据来源: WILEY

摘要:

Geomagnetically trapped electrons having mirror points in the auroral ionosphere may experience temporally periodic forces exerted by magnetosonic oscillations occurring at discrete frequencies there. The effects of these forces build up systematically with time for those particles having bounce periods that are integer multiples of an oscillation period. As a result of this bounce‐resonant scattering, particles tend to accumulate in bunches along the field line, i.e., to become bunched in bounce phase, as the mirror points are driven toward the dense atmosphere. The phase bunching may contribute to the formation of auroral microbursts, and the scattering mechanism may enhance mirror‐point diffusion of the resonant partic

ISSN:0148-0227    

DOI:10.1029/JA074i001p00169

年代:1969

数据来源: WILEY

摘要:

The density gages on the Explorer 32 satellite have measured local variations in the atmospheric density that confirm that waves propagate in the neutral atmosphere. The waves were observed in the northern hemisphere over the altitude range from 286 km (satellite perigee) to at least 510 km. The waves are most prevalent at the higher latitudes near the auroral zone (orbit inclination is 65°) and were observed most frequently in the late evening and early morning hours, but were not limited to these latitudes and times. The apparent vertical half‐wavelengths of the waves increase with altitude from 1 km at 286‐km altitude to 70 km at 510‐km altitude. Apparently four integrally related wavelengths have been observed, a ‘fundamental’ and the second, third, and fourth harmonics. The wave density perturbation half‐amplitudes range from the limit of detectability to the largest observed of 50% of the smoothed density profile value. These waves are interpreted as free internal gravity waves propagating predominately north‐south, or south‐north, with maximum horizontal wavelengths between 130 and 520 km. The altitude dependence of the apparent vertical half‐wavelengths results from the satellite moving with varying vertical velocity through a slowly propagating wave pattern with nearly vertical phase planes. This interpretation is consistent with observations of large‐scale traveling ionospheric disturbances

ISSN:0148-0227    

DOI:10.1029/JA074i001p00183

年代:1969

数据来源: WILEY

摘要:

Ion temperature measurements below theFpeak have revealed the presence of a fine structure in the ionospheric temperature. The measurements were made with an Ion Energy Analyzer carried aboard a low‐altitude polar‐orbiting satellite. The temporal resolution of the Ion Energy Analyzer was 3 seconds corresponding to a horizontal spatial resolution of approximately 23 km. At the altitudes of these measurements the ions and neutral species of the atmosphere have a common temperature; thus the temperature structure present in the ion temperature must also be present in the neutral atmosphere as well. The temperature structure is observed to have a wave‐like character with wavelength of the order of 200 km. The fractional temperature variation is typically 13%. Much of the observed wave structure is interpreted as internal atmospheric gravity waves, and the results obtained here are compared with the existing theory. A large portion of the wave structure observed cannot be explained on the basis of the simple linearized gravity wave theory. This wave structure is either the result of some as yet unspecified phenomenon or is the result of gravity waves in a real atmosphere where the idealized conditions and restriction from linearization are re

ISSN:0148-0227    

DOI:10.1029/JA074i001p00197

年代:1969

数据来源: WILEY

摘要:

A model of hydromagnetic‐wave (hm‐wave) amplification in the magnetosphere is presented that permits a consistent interpretation of several general and specific features of Pc 1 micropulsation activity (0.2 Hz ≲ ƒ ≲ 5 Hz). It has been found that (1) micropulsations with 0.2 Hz ≲ ƒ ≲ 5 Hz originate preferentially in the magnetospheric regions of maximum amplification; (2) the complete frequency range is amplified in the plasmasphere whereas the plasma trough amplification is limited to the 0.3 Hz ≲ ƒ ≲ 1 Hz range; (3) the μ = 10–100 Mev/gauss protons amplify only the 0.2 Hz ≲ ƒ ≲ 5 Hz primary hm waves. Several properties of the μ = 10–100 Mev/gauss protons are implied on the basis of micropulsation activity. The amplification mechanism is cyclotron resonance between left‐hand polarized hm waves and the energetic component of the magnetospheric proton population. Spatial and energy distributions of the magnetospheric proton population and a model of the bounded geomagnetic field that are consistent with experimental observations are util

ISSN:0148-0227    

DOI:10.1029/JA074i001p00205

年代:1969

数据来源: WILEY

摘要:

Data from the IMP 2 magnetic experiment obtained in interplanetary space are analyzed to study the character of the fluctuations in the magnetic field from the magnetohydrodynamic point of view. The fluctuation in interplanetary space is quite anisotropic relative to the steady magnetic field, with the fluctuations transverse to the magnetic line of force much larger than the longitudinal component of fluctuations. This anisotropy gradually decays as the amplitude of the fluctuation increases and tends to become rather isotropic. These fluctuations may be related to the thermal anisotropy of solar wind ions, with the conversion of the transverse fluctuations into the longitudinal ones occurring by means of a nonlinear effect when the amplitude of the fluctuation grows above a certain level. Frequency dependence of the mode of fluctuations is also investigated.

ISSN:0148-0227    

DOI:10.1029/JA074i001p00225

年代:1969

数据来源: WILEY

摘要:

Afternoon radio aurora was observed on 26 days out of 54 during the period May 1965 to January 1966, with the Millstone Hill, Massachusetts, radar. The completely steerable 84‐foot‐diameter antenna provided a 0.6° half‐power beamwidth at 1295 MHz. Echoes were obtained at azimuths within ±30° of the magnetic meridian (345°T) and at elevation angles up to 13°. They were returned from a layer‐like region having a thickness of 5–20 km and located at an average height near 110 km. The echoes are strongly aspect sensitive, and none were obtained from regions where the aspect angle was greater than about 3°. Both the occurrence of auroral echoes and the southernmost limit of the echo region were found to correlate with the magnetic indexKp. The Doppler spectra of the echoes show predominantly approaching velocities to the east of the magnetic meridian and receding velocities to the west. The magnitude of the Doppler offset and the variation of offset with azimuth favor the two‐stream plasma instability theory of Farley as an explanation of auroral echoes. The direction of plasma wave motion together with the variation of electrojet current with altitude may explain the presence of echoes at the magnetic meridian (not expected from the Farley theory for an e

ISSN:0148-0227    

DOI:10.1029/JA074i001p00231

年代:1969

数据来源: WILEY

摘要:

Energetic plasma drifting toward the earth from the plasma sheet in the earth's magnetospheric tail is terminated by an inner boundary (the Alfvén layer). Sheets of field‐aligned current (Birkeland currents) flow down to the ionosphere from this plasma boundary. The mechanism for generating the current is essentially that originally proposed by Alfvén in 1939. A certain amount of charge separation develops in the Alfvén layer because the geocentric drift paths of ions and electrons are not coincident within the layer. The charge separation produces an electric field that is perpendicular toBand is transmitted along lines of force to the ionosphere. Currents flow across magnetic field lines in the ionosphere and along the magnetic field lines to the Alfvén layer. The electric field is reduced by these currents, which flow to neutralize the charge separation. The position of the Alfvén layer is such as to produce a current sheet that enters the ionosphere at auroral latitudes. The Birkeland current necessary to neutralize the Alfvén layer is calculated to be about 106amps, which is sufficient to produce the transverse magnetic disturbances of about 100 γ that have been observed by satellites in auroral latitudes; the thickness of the Alfvén layer corresponds to the width of the zone of these transverse magnetic disturbances. Finally, the Birkeland current is related to a larger current system that includes theDScurre

ISSN:0148-0227    

DOI:10.1029/JA074i001p00247

年代:1969

数据来源: WILEY