摘要:

In a previous paper it was shown that spatial changes in the high‐altitude magnetospheric electric fieldEwith ▽ ·E<0 can result in the generation of large‐scale ‘inverted V’ regions of auroral electron precipitation. In the present paper it is shown that smaller‐scale precipitation regions, as are required to account for discrete aurora, result from the basic analysis in the previous paper if appropriate structure is introduced in the high‐altitude electric field distribution. Using observations of electric fields and precipitating electrons from a rocket flight over discrete aurora the required structure in the electric field is inferred to exit at high altitudes along magnetic field lines connected to the aurora. By using this inferred high‐altitude electric potential distribution the ionospheric current continuity equation is solved for the ionospheric potential. The analysis assumes that the field‐aligned current is governed by single‐particle motion along field lines. The solution gives ionospheric potentials and precipitating electron energy fluxes in good quantitative agreement with those observed throughout the auroral rocket flight. The results in this and the previous paper imply that the ‘inverted V’ scale size (order of 200 km in width) is a natural result of the current versus electric potential relations along auroral field lines and in the ionosphere. This scale size need not be imposed by structure in the high‐altitude electric field distribution. Smaller‐scale (tens of kilometers) discrete auroral precipitation regions require the same current versus electric potential relations, but the scale size is imposed by structure in the high‐altitude electric potential distribution. The cause of this structure is not cons

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00001

年代:1981

数据来源: WILEY

摘要:

The characteristics of intensity modulations of quasi‐periodic (QP) VLF emissions and their relationships to magnetic variations are examined by using data obtained from Syowa and Mizuho Stations in Antarctica and at Husafell in Iceland, which is located near the geomagnetic conjugate point of Syowa. As shown in the previous studies, QP emissions can be classified into two types, according to whether emissions are closely associated with magnetic pulsations or not. It is found that the frequency of intensity modulations of type 2 QP's, without corresponding magnetic pulsations, is more stable that of type 1 QP's, associated with magnetic pulsations. In most cases magnetic variations during type 2 QP events, which occur under magnetically quiet condition, have no spectral peak corresponding to the peak in QP's spectrum. However, a small but significant peak in pulsation spectrum is occasionally noted, when the type 2 QP event occurs in a moderately disturbed condition. The peak value of magnetic spectrum, in such a case, is generally two orders of magnitude less than the maximum power of background magnetic fluctuations. The amplitude of magnetic pulsations is in the order of 0.01 nT/s. The H component of magnetic pulsations tends to be more correlated with the VLF intensity variations than the D component. These properties are observed at conjugate pair stations with good conjugacy. It is suggested that weak magnetic pulsations, correlated with the type 2 QP, are produced by ionospheric conductivity changes associated with wave induced particle precipitation

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00009

年代:1981

数据来源: WILEY

摘要:

Frequency‐time (f‐t) spectra of quasi‐periodic (QP) ELF‐VLF emissions in the period range 5–200 s are examined by using a few hundred QP events observed at high latitudes. These f‐t spectra are classified into five types: nondispersive, rising tone, falling tone, mixed, and burst. Statistical characteristics of QP emissions in each type are examined by using f‐t spectra of ELF‐VLF emissions recorded continuously for 50 days in Iceland. A phenomenological model to explain the f‐t spectra is proposed; that is, the rising tone type of QP emission is generated by compressional mode Pc 3 magnetic pulsations which propagate in a radial direction toward the earth, while the nondispersive type is generated by standing oscillations of the magnetic fields which have effective compres

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00019

年代:1981

数据来源: WILEY

摘要:

It is assumed that the nonthermal radio continua in the terrestrial and Jovian magnetospheres result from conversion of observed upper hybrid waves into o mode waves. The only favorable conversion mechanism for the escaping terrestrial continuum is coalescence with low‐frequency waves, and the most favorable waves are ion‐cyclotron waves, which should be generated under conditions similar to those required for generation of upper hybrid waves. It is suggested that the Jovian continuum is in a steady state in which the conversion process is balanced by the inverse process, and it is shown that the implied steady state level is reasonably consistent with observations. To account for the observed spectrum of the Jovian continuum requires that the plasma pressure should be approximately constant over a range ne≈ 10²m−3to 105m−3. It is suggested that the terrestrial continuum may escape in bubbles of radiation enclosed by plasma downstream from the m

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00030

年代:1981

数据来源: WILEY

摘要:

Very low frequency lpar;VLF) waves recorded at Siple, Antarctica (L ∼4; 84°W, geographic), are compared with low‐energy (5‐keV electrons detected at the synchronous altitude, and its generation region is inferred to be outside the plasmapause. The chorus upper cutoff frequency increases with time in a characteristic manner, consistent with the expected adiabatic motion of injected electrons in cyclotron resonance with the waves. The second type of chorus, which we refer to as ‘plasmaspheric chorus,’ occurs inside the plasmapause, has no apparent relationship to particle injection at the synchronous altitude and shows clear evidence of being triggered by whistlers, power line radiation, and other signals. The two different types of chorus are readily distinguishable in freq

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00037

年代:1981

数据来源: WILEY

摘要:

The Kelvin‐Helmholtz instability in the magnetopause‐boundary layer region is studied on the basis of an idealized model which consists of three uniform plasma regions: the magnetosheath, the boundary layer, and the magnetosphere. There are two unstable modes in the magnetopause‐boundary layer region: one is excited at the magnetopause (the magnetopause mode) and the other is excited at the inner surface of the boundary layer (the inner mode). The inner mode is found to be unstable most of the time, while the excitation of the magnetopause mode depends on the magnetic field in the magnetosheath. The observed variation of the boundary layer thickness can be attributed to the unstable inner mode. Possible relationships between the Pc 3–5 geomagnetic pulsations and the surface waves excited on the magnetospheric boundary are also di

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00054

年代:1981

数据来源: WILEY

摘要:

At high latitudes during solstice the ray path through the atmosphere to about 400‐km altitude near midnight is either so small that production of O+, He+, and H+continues at all times or so large that production of these species is negligible. The latter condition will produce an ionospheric ‘hole’ if the plasma stagnates or circulates in the nightside winter pole. Near equinox, however, the shadow height near midnight may ensure that photoproduction of O+is negligible, while the ray path at higher altitudes may be short enough that significant production of He+and H+takes place. Observations made by Atmosphere Explorer D and calculations suggest that an ionospheric hole should also be observable at equinox but will not be so pronounced as that observed during winter. However, the dominance of He+in the hole between 500 and 1000 km is unique to equinoctial conditions. We note that stagnation of the plasma rather than slow passage across the polar cap is required, since at equinox the field tubes must reach the nightside before the shadow height ensures that the photoproduction of O+ceases. Stagnation times between 3 and 6 hours are required for He+to be dominant at altitudes around 6

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00059

年代:1981

数据来源: WILEY

摘要:

The great spatial and temporal variability of auroral ionospheric conductivity significantly influences the ionospheric closure path for high‐latitude, field‐aligned currents. Because these closure paths can extend to low latitudes, changes in auroral zone conductivity can influence the global electric field distribution. In this paper, synoptic Chatanika radar observations of auroral zone conductivity that cover ∼62° to 68° geomagnetic latitude are presented. They are representative of quiet winter, active winter, and equinoctial conditions. During the daytime the solar contribution to the height‐integrated conductivity is well represented by ΣP,H≃ (5, 10) cos1/2(χ), where χ is the solar zenith angle. The nighttime, height‐integrated Pedersen and Hall conductivities (ΣPand ΣH) in the electron density trough are, at times, below our detection threshold of ∼0.5 mho. Following magnetic substorm onset, enhanced conductivity regions move southward and intensify. As the recovery phase begins, the conductivity pattern recedes northward and diminishes. The onset and cessation of precipitation associated with these events can be as abrupt as a few minutes. In one example the behavior of ΣPand ΣHare examined in the vicinity of the Harang discontinuity, which was quite sharp (≲30 min) in local time. At the Harang discontinuity on that day, the ratio of ΣHto ΣPdecreased, indicating a softening of the precipi

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00065

年代:1981

数据来源: WILEY

摘要:

Auroral kilometric radiation (AKR) appears on the Isis 1 topside sounder ionograms as intense noise bands between the electron cyclotron frequency and 700 kHz. A variable gap occurs between the cyclotron frequency and the lowest AKR frequency. As Isis 1 traverses the source region, the gap narrows, and the AKR signals at higher frequencies weaken. This signature suggests that the AKR waves are generated directly in the extraordinary mode at frequencies just above the local cutoff frequency and that the radiation is initially perpendicular to the magnetic field.

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00076

年代:1981

数据来源: WILEY

摘要:

Measurements from electrostatic analyzers aboard the polar‐orbiting S3‐3 satellite have been tabulated to form a synoptic picture of the occurrence of upgoing 90eV to 3.9keV auroral ions. In this survey, a distinction is made between ion distributions having peak fluxes alongB(beams) and those exhibiting flux maxima that are not field‐aligned (conics). It is shown that both beams and conics are common auroral phenomena, whose frequencies of occurrence in latitude, local time, and altitude have a marked dependence on magnetic activity. During quiet conditions (Kp≲3) conical ion distributions are observed with constant frequency in altitude above 1000 km and appear to be associated with the daytime polar cusp region. In contrast, quiet time ion beams have a maximum occurrence frequency in the premidnight sector. Ion beams are observed primarily above 5000 km, with a frequency increasing with altitude up to the satellite apogee at 8000 km. During disturbed times, ion beams are a persistent phenomenon, mainly confined to the dusk sector, while conical distributions are observed uniformly in local time with a frequency that increases steadily in altitude. The results of this study, which are shown to be consistent with previous surveys of upward flowing ions if no distinction is made between conical and field‐aligned distributions, provide information relating to auroral ion acceleration

ISSN:0148-0227    

DOI:10.1029/JA086iA01p00083

年代:1981

数据来源: WILEY