摘要:

Calculations have been carried out to determine the neutron flux induced in the earth's atmosphere by galactic protons and alpha particles at solar minimum for a geomagnetic latitude of 42°N. Neutron flux spectra in the energy range ∼10−8to ∼105Mev at various depths in the atmosphere were calculated by using Monte Carlo and discrete ordinates methods, and various comparisons with experimental data are presented. The magnitude and shape of the calculated neutron leakage spectrum at the particular latitude considered support the theory that the cosmic ray albedo neutron decay (Crand) mechanism is the source of the protons trapped in the inner radiatio

ISSN:0148-0227    

DOI:10.1029/JA078i016p02715

年代:1973

数据来源: WILEY

摘要:

The time dependent worldwide distribution of atmospheric fast neutrons has been determined in balloon and aircraft measurements from 1964 to 1971. The 1‐ to 10‐Mev neutron spectrum was measured with a phoswich detector employing seven channels of pulse height analysis. Solar modulation effects were greatest near the high‐latitude transition maximum, where the flux varied by more than a factor of 2 from solar minimum to the deepest Forbush decrease. Near solar maximum and during Forbush decreases the relation between the neutron flux in the upper atmosphere and the counting rate of the Deep River neutron monitor deviated from a single‐valued function. The differential neutron spectrum between 1 and 10 Mev can be represented, within the resolution of the detector, by a power lawN(E) =AE−n, wheren= 1.17−0.20+0.12near the transition maximum,n= 1.08−0.20+0.13at 3‐ to 5‐g/cm² atmospheric depth, andnat sea level is larger than these values and dependent on terrain. The spectral index remains the same to ±0.1 over the solar cycle at fast neutron energies. The fast neutron data are self‐consistent to ±7% from 2 to 300 g/cm² over the range of cutoff rigidity and solar cycle variations. The characteristics of the fast neutrons as outlined here serve as a basis for checking a Monte Carlo calculation of the entire neutron distri

ISSN:0148-0227    

DOI:10.1029/JA078i016p02727

年代:1973

数据来源: WILEY

摘要:

We have calculated the global distribution of atmospheric neutrons and their products by a Monte Carlo simulation of nucleon transport, in the internuclear cascade followed by neutron transport below 19 Mev. First, we present the results generated by monoenergetic primary protons and alpha particles entering the top of the atmosphere. Second, the kernels derived from the monoenergetic cases are used to determine the spatial and energy distributions of neutrons and their products from the protons and alpha particles in the cosmic radiation; solar modulation effects are included. The calculation is compared, in the 1‐ to 10‐Mev region, with the results of our fast neutron experiment; the agreement is within the uncertainties of the primary spectrum and of the experimental results over most of the atmosphere. The calculation is then normalized to the experiment in the fast neutron region. The results of the normalized calculation include the steady state neutron spectrum, the neutron production rates, the radiocarbon production rates, the neutron leakage rates from the top of the atmosphere, and the production rates of other nuclides. The normalized calculation reproduces experimentally observed slow neutron densities and the observed neutron flux and spectrum above 1 kev, and it predicts features of the atmospheric neutron morphology not yet observed. The points of agreement and divergence with earlier calculations are discussed, including the radiocarbon production rates and the neutron leakage rates during solar cycle 20, which is near the mean of the last 10 solar cyc

ISSN:0148-0227    

DOI:10.1029/JA078i016p02741

年代:1973

数据来源: WILEY

摘要:

During solar particle events from 1968 to 1971 we observed increases in the fast neutron flux at high latitude and at 55‐ to 75‐g/cm² atmospheric depth. The increases correlated with the variations in the solar proton fluxes; the neutron yield per incident proton, above threshold, increased by a factor of 100 with increasing hardness of the proton spectrum. Within a factor of 2 the neutron specific yield fell on a smooth curve versus the spectral parameterP0, where the values ofP0were based on the SPME (solar proton monitor experiment) data from Explorer 34 and 41. The neutron yield from solar particle events was calculated from a Monte Carlo simulation of neutron production and transport in the atmosphere. We compare the observed fast neutron flux with that calculated using the solar proton spectra reported at the times of the measurements; the causes for variation among the reported proton spectra and between the calculated and the observed fast neutron flux are discussed. The calculation reproduced the results of experiments by others with moderated slow neutron counters in and above the atmosphere. We calculate that the contribution of solar particle fluxes to the production rates of neutrons, to the production rates of radiocarbon, and to the leakage rates of neutrons from the top of the atmosphere are 2–3 orders of magnitude below the galactic cosmic ray contribution during solar cy

ISSN:0148-0227    

DOI:10.1029/JA078i016p02763

年代:1973

数据来源: WILEY

摘要:

Satellite observations of UV solar irradiance in selected wavelength bands between 1200 and 3000 A have been made continuously by photometers consisting of broad band sensors operated on Nimbus 3 and 4, which were launched in April 1969 and April 1970, respectively. In addition, on Nimbus 4, spectrophotometer measurements of solar irradiance have been made with a dispersive instrument at 12 selected wavelengths from 2550 to 3400 A with a 10‐A band pass. Variations of solar irradiance associated with the solar rotational period have been observed since the launch of Nimbus 3. These variations are apparently associated with two source regions separated by about 180° in solar longitude. The change in irradiance with solar rotation was found to increase with decreasing wavelengths. Different types of observed variations in UV solar irradiance can be classified in accordance with characteristic times; in order of increasing periods, they are (1) flare‐associated enhancements, (2) 27‐day variations due to solar rotation, (3) a possible biennial effect, and (4) long‐term variations associated with the 11‐year s

ISSN:0148-0227    

DOI:10.1029/JA078i016p02779

年代:1973

数据来源: WILEY

摘要:

Jupiter's electron and proton radiation belts are analyzed, with particular reference to the sweeping effect of its five inner satellites, located deep within its magnetosphere. The characteristics of trapped electrons and protons with a magnetic moment of 50 Mev/G, considered typical at Jupiter, are calculated. The assumption is then made that a particle would be removed from the radiation belt if, in its normal motion, it would happen to impact a satellite. The mean absorption time before impact is calculated for particles located at the radial distance of each of the satellites. This average lifetime is found to be of the order of a few days. A characteristic diffusion time near each satellite was calculated, assuming violation of the third invariant due to magnetic fluctuations associated with fluctuations in the solar wind. This diffusion time is found to be long as compared with the absorption lifetimes at Europa (L= 9.5), Io (L= 6.0), and Amalthea (L= 2.5). If magnetic fluctuations are the dominant diffusing force, these three satellites would act as a barrier to the inward diffusion of solar wind particles, and fluxes of trapped protons would be sharply reduced just inside the orbit of Europa; very few could diffuse past Io or Amalthea. The existence of energetic trapped electrons nearL= 2, however, indicates either that (1) perturbations in the electric or magnetic field near a satellite enable electrons to slip past without impacting the satellite, (2) diffusion of electrons in the region 2.5

ISSN:0148-0227    

DOI:10.1029/JA078i016p02793

年代:1973

数据来源: WILEY

摘要:

The photoelectron impact contribution to the Jupiter dayglow is estimated for an atmospheric model with and without helium. Primary photoelectron production rates are calculated at specific altitudes for zero solar zenith angle and a 10.7‐cm flux of 150. Extensive excitation, dissociation, and ionization cross‐section data for H2, He, and H are used to model the energy deposition of photoelectrons produced locally. Volume emission rates are calculated from the direct excitation rates by applying appropriate corrections for cascading and quenching. Finally, estimates are made of intensities above the atmosphere as viewed near normal incidence with multiple scattering and absorption neglected. Rather strong emissions are predicted for the Werner and Lyman bands and the triplet continuum (a³Σg+‐b³Σu+) of H2. The photoelectron impact contribution to the most intense emissions from He and H is found to be relatively small while strong resonant scattering contributions seem r

ISSN:0148-0227    

DOI:10.1029/JA078i016p02812

年代:1973

数据来源: WILEY

摘要:

Theoretical O I 1304‐ and 1356‐A intensities for Venus are presented, based on values of the excitation parameters consistent with estimates derived by Strickland et al. (1972) for Mars. Appropriate intensities are compared with the rocket data for Venus obtained by Moos and Rottman (1971) and are found to disagree seriously. Additional sources of photons at 1304 A and 1356 A are sought, but none are established as important. Alternatively, the values of the excitation parameters relative to the calibration may be in error. For a factor of 4 error, or less, the results indicate that more atomic oxygen is present in the thermosphere of Venus than Mars. This is not consistent with the interpretation of the Venera 4 1304‐A data by Kurt et al. (1968). From the present analysis it is found that no useful upper limit to the atomic oxygen concentration can be estimated from the Venera

ISSN:0148-0227    

DOI:10.1029/JA078i016p02827

年代:1973

数据来源: WILEY

摘要:

Following a southward shift of the interplanetary magnetic field, which implies enhanced reconnection at the nose of the magnetosphere, the magnetopause shrinks from its Chapman‐Ferraro equilibrium position. If the convective return of magnetic flux to the magnetopause equaled the reconnection rate, the magnetopause would not shrink. Consequently, there is a delay in the development of magnetospheric convection following the onset of reconnection, which we ascribe to line tying by the polar cusp ionosphere. A simple model relates the dayside magnetopause displacement to the currents feeding the polar cap ionosphere, from which the ionospheric electric field, and consequently the flux return rate, may be estimated as a function of magnetopause displacement. Flux conservation arguments then permit an estimate of the time scale on which convection increases, which is not inconsistent with that of the substorm growth phas

ISSN:0148-0227    

DOI:10.1029/JA078i016p02837

年代:1973

数据来源: WILEY

摘要:

Time‐dependent magnetospheric electric fields have been computed, including the effects of Birkeland currents from the inner edge of a sheet of ions that moves under the influence of the computed electric fields. Ionospheric currents are also taken self‐consistently into account with the use of a time‐independent model of ionospheric conductivity that includes day‐night asymmetry and auroral enhancements but neglects electric fields parallel to the magnetic field. Ion precipitation and neutral winds are also neglected. The behavior and effects of the ion sheet are studied in a series of model calculations, with the following results. (1) In agreement with the conclusions of E. T. Karlson, L. P. Block, V. M. Vasyliunas, and D. W. Swift, Birkeland currents from the Alfvén layer (inner edge of the ion sheet) are found to reduce the electric field earthward of the Alfvén layer to a small value by the time a steady state is reached; however, different parts of the electric field earthward of the Alfvén layer are eliminated at different rates; one component of the nightside field relaxes to near its low asymptotic value in a few minutes, whereas the dayside field takes hours to relax. (2) If the ion sheet is brought in from the tail by a large cross‐tail electric field that stays large, the inner edge of the sheet generally touches the magnetopause boundary layer; however, a decrease in the cross‐tail field can cause the Alfvén layer to contract and form a complete ring. (3) For the parameters used, a cross‐tail potential of 134 kv will bring a sheet of ring current protons in to aboutL= 4; the results of the model calculations support the idea that convection electric fields bring low‐energy ions in from the tail to form the storm time ring current; the minimum geocentric distance to which convection fields can bring the ions is found to be roughly proportional to (ημ/Φ0σρ)1/3, where η is the number of ions per unit magnetic flux, μ is the ion magnetic moment, Φ0is the cross‐tail potential, and σρis an average height‐integrated Pedersen conductivity on the dayside. (4) The nightside Alfvén layer naturally produces a dividing line near local midnight, such that particles arriving west of the line drift to the west, whereas those arriving east of the line drift east; this characteristic is often observed in motions

ISSN:0148-0227    

DOI:10.1029/JA078i016p02852

年代:1973

数据来源: WILEY