摘要:

Forbush decreases observed by superneutron monitors are frequently preceded by an almost equally large predecrease. Five such events are studied here, and there are two main observational results: (1) the predecrease represents a cosmic ray depletion in a narrow range of pitch angles (±30°) around the mean interplanetary field direction and (2) the predecrease has an unusual rigidity dependence inasmuch as its amplitude increases with rigidity, going something likeR0.5→1.5up to a limit of about 80 Gv. Both these results can be accounted for by a model of the interplanetary magnetic field that generates the Forbush decrease and the predecrease by way of a propagating disturbance from the sun. Using L. R. Barnden's (1971) work, it appears that such a model requires a cosmic ray mean free path that increases with rigidity according toR

ISSN:0148-0227    

DOI:10.1029/JA078i004p00577

年代:1973

数据来源: WILEY

摘要:

Using calculations in a model geomagnetic field, the influence of nonadiabatic particle motion on the polar cap structure of solar protons is investigated. Breakdown of the first invariant, due to excessive field curvature and gradients near the last closed field line, results in an effect that cannot be differentiated from strong pitch angle scattering using low‐altitude detectors, although no time variations of the magnetic field are considered. The results obtained thus form a basis to which wave‐particle interactions and other scattering effects must be added. In a companion paper, these results are applied to observations of solar protons over the polar caps, and it is shown that the quiettime behavior of these particles is indeed approximately given by the constraints due to the spatial variations of the geomagnetic fi

ISSN:0148-0227    

DOI:10.1029/JA078i004p00588

年代:1973

数据来源: WILEY

摘要:

The development of structure in the flux profiles of low‐energy (∼2–Mev) solar protons at low altitudes over the polar caps is examined in detail for the event beginning on June 9, 1968. The structure is studied in the frame of reference provided by simultaneous measurements of the trapping boundary for electrons of energy>1.2 Mev and calculations of the boundary of the pseudo‐trapping region. The results are interpreted in terms of (1) delayed access of solar protons to the magnetotail, (2) an effect indistinguishable from strong pitch angle scattering near the last closed field lines, owing to breakdown of the adiabatic invariants in the strong curvature of the magnetic field, (3) weak pitch angle scattering on the dayside at low latitudes in the pseudo‐trapping region and in the stable‐trapping region, and (4) inward diffusion and acceleration in the stable‐tr

ISSN:0148-0227    

DOI:10.1029/JA078i004p00597

年代:1973

数据来源: WILEY

摘要:

Lockheed spectrometer data from orbits 1 through 20 of Ogo 5 are used to study proton scattering near the bow shock. Correlative UCLA fluxgate magnetometer data from the satellite are used to complete motion studies of the bow shock when spectrometer data are missing. Thirty‐eight analyzable sets of multiple shock crossings show 358 shock encounters ranging from γ = 10REto γ = 22REand spanning sun‐earth‐satellite angles from 30° to 90°. Gross features of the study show that the location and shape of the bow shock are in good agreement with the hydrodynamic model of the bow shock. On a microscale the protons are slowed and scattered, having concomitant randomization of velocity vectors as they traverse the bow shock layer. A change in directed proton flux is noted, and study of the available spectrometer data shows ac/ωpiscale length for proton scattering, where the Ogo 5 JPL plasma probe density measurements have been used. This scale length is based upon the time required for scattering the protons into the wings of the velocity distribution and may not be representative of the time required for prot

ISSN:0148-0227    

DOI:10.1029/JA078i004p00607

年代:1973

数据来源: WILEY

摘要:

A cross‐correlation study between magnetospheric activity (theAEindex) and the southward‐directed component of the interplanetary magnetic field (IMF) is made for a total of 792 hours (33 days) with a time resolution of about 5.5 min. The peak correlation tends to occur when the interplanetary data are shifted approximately 40 min later with respect to theAEindex data. Cross‐correlation analysis is conducted on some idealized wave forms to illustrate that this delay between southward turning of the IMF and theAEindex should not be interpreted as being the duration of the growth

ISSN:0148-0227    

DOI:10.1029/JA078i004p00617

年代:1973

数据来源: WILEY

摘要:

Balloon‐borne electric field and X ray detectors have been flown from Fort Churchill and from near the foot of the magnetic field line on which the ATS 5 satellite is located in order to obtain satellite and balloon measurements of trapped and precipitating particles and to compare them with balloon measurements of the ionospheric electric field. The approximately 80 hours of coordinated data show that nighttime precipitation events (1) are preceded by enhancements of the westward component of ionospheric electric field about 70% of the time, (2) cause the westward electric field strength to decrease about 70% of the time, (3) are unrelated to variations of the southward component of the ionospheric electric field. The westward electric field enhancement before many precipitation events occurs when the trapped electron flux is at or near the stable trapping limit. Such precipitation events are therefore interpreted as resulting from enhanced pitch angle scattering by whistler mode waves as the trapped electron flux exceeds the stable trapping limit due to the inward convection associated with the enhanced westward field. This model explains the observed diurnal dependence of the phenomenon and the poor correlation between trapped and precipitated electron intensities reported in this paper. The decrease of the westward ionospheric electric field during many precipitation events is interpreted as due to ∼25–kev potential drops along magnetic field lines resulting from the increased currents flowing during precipitation. The geometry of the westward electrojet current flow is such that the westward ionospheric electric field should decrease and little or no variation should occur in the southward component during precipitation, in agreement with the experimental

ISSN:0148-0227    

DOI:10.1029/JA078i004p00630

年代:1973

数据来源: WILEY

摘要:

A rocket‐borne experiment containing a vector magnetometer and a set of charged‐particle detectors was launched from Poker Flat, Alaska, at 2217 LT on February 13, 1971, over a single auroral arc. Particle data in the energy range 0.5–20 kev, obtained during part of the flight, show a peak incident flux of 4 × 107el/cm2sec ster kev in the energy range 1–2 kev. The pitch angle distributions of these precipitating electrons were found to be fairly isotropic from 0° to 60° and to decrease from 60° to 90°. The results of the vector magnetometer indicate the existence of a system of Birkeland currents with magnitudes of 5 × 10−6amp/m2in the vicinity of the auroral arc. Two different models that fit the data are discussed. In both of these models the current carried by precipitating electrons opposed the net downward current measured by the magnetometer in the same region, this region being located south of that containing the peak auroral luminosity. The net upward current measured by the magnetometer flowed somewhat north of the peak aur

ISSN:0148-0227    

DOI:10.1029/JA078i004p00640

年代:1973

数据来源: WILEY

摘要:

The Chatanika incoherent scatter radar was used on fifteen days during the winter of 1971–1972 to provide data on ionospheric parameters in a meridional cross section of the auroral zone. On three of these days the aurora was quiet and conditions were sufficiently stable that variations during the observations could be ignored. These three cases, supported by the other less well‐defined cases, provide the following results: (1) during one quiet period, the main ionospheric trough extended from 69° down to at least 61° geomagnetic latitude; (2) the poleward edge of the trough was a steep horizontal electron density gradient (essentially a wall of ionization) that is approximately field aligned, and its position appears to be determined not by the concurrent position of the visual aurora but by the most equatorward position of the visual aurora during the preceding hours; and (3) the maximumFregion electron density in the space between the trough and the visual aurora occurs 1 to 2 deg of latitude equatorward of the aurora. These results are interpreted in terms of a qualitative model that invokes the equatorward and upward transport of aurorally produced ionization by collisional coupling with an equatorward neutral wind caused by auroral he

ISSN:0148-0227    

DOI:10.1029/JA078i004p00648

年代:1973

数据来源: WILEY

摘要:

Conjugate auroral all‐sky camera data obtained in 18 flights through the auroral zone near the College, Alaska, magnetic meridian show hemispherical differences in auroral frequency and intensity. Conjugate auroras on the equatorward boundary of the display (the equatorward arc system) are found to be consistently brighter in the northern hemisphere by a factor of approximately 1.3. At higher latitudes (in the poleward arc system) the auroras exhibit varying degrees of conjugacy, but the auroras in the north tend to occur more frequently and to be brighter. The hemispherical difference is attributed to the 8000‐γ difference in magnetic field strength between the conjugate areas. Simplified theoretical calculations of the difference in precipitation at conjugate points from idealized cases of pitch angle diffusion and from a small hemispherical potential difference show that pitch angle diffusion may explain observations in the equatorward arc system but not in the poleward arc system. The evidence for hemispheric differences in auroral particle precipitation adds substantially to earlier indications of variation with longitude of auroral precipitation patt

ISSN:0148-0227    

DOI:10.1029/JA078i004p00659

年代:1973

数据来源: WILEY

摘要:

Whistlers recorded in Antarctica and North America were used to monitor changes in electron concentrations in the dayside plasmasphere during isolated substorm activity. The ‘viewing area’ of the whistler receiver network extended overL∼ 2 to 6 and ∼6 hours in geomagnetic time. Before the substorm, the plasmasphere was relatively full and smooth, and there was no evidence of large longitudinal variations in plasma distribution. Immediately after the substorm onset, parts of the plasmasphere were depleted rapidly and produced large‐scale structures with sharp longitudinal gradients in plasma density. The depletion of the plasmaspheric tubes is interpreted in terms of downward flow of plasma into the underlying ionosphere, and fluxes of the order of 109el/cm² sec across the 1000 km level are inferred. Simultaneous ionosonde records show enhanced critical frequencies in the underlyingF2layer. These results indicate that significant amounts of plasma in the plasmasphere are dumped into the ionosphere during geomagnetic disturbances and that this is an important process in storm phenomena in bo

ISSN:0148-0227    

DOI:10.1029/JA078i004p00672

年代:1973

数据来源: WILEY