摘要:

About 30 Forbush‐type decreases occurring from 1972 to 1984 at 1 AU for which data were available from the IMP spacecraft (Pmedian of ∼1.7 GV) and the Mt. Washington neutron monitor (Pmedian of ∼5 GV) were examined to determine the characteristic recovery times of the events. The characteristic recovery timet0was found to be the same for rigidities of ∼1.7 GV and ∼5.0 GV. Characteristic recovery times for additional Forbush decreases, including some in 1957–1963, were determined, using the Mt. Washington neutron monitor and monitors at cutoff rigidities of ∼12 GV, and were also found to be the same. Hence the characteristic recovery time of Forbush decreases at 1 AU is independent of energy (or rigidity) over a factor of at least 10. The average recovery timet0at 1 AU is ∼5 days but varies from ∼3 to ∼10 days, and the recovery in most cases is well represented by an exponential. No significant difference in the average recovery time is observed when the solar magnetic field reversed in 1980 nor during the decrease and recovery portion of the solar modulation cycle. A physical model is proposed to ex

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02851

年代:1986

数据来源: WILEY

摘要:

We compare the solar modulation of galactic cosmic ray helium and electrons at 1 AU, within the 600‐ to 1000‐MV magnetic rigidity interval, during a period covering 20 years, from the solar minimum of 1965 through 1984. Most of the data were obtained by University of Chicago cosmic ray telescopes on board balloons and the IMP series and ISEE 3 satellites. Comparison of the time‐intensity variations during the two solar maxima around 1970 and 1981 shows that after 1970 the intensity of helium recovers earlier than that of the electrons, whereas after 1981 the intensity of electrons recovers earlier than that of helium. Within the measured magnetic rigidity interval the flux ratio of helium to electrons (He/e) undergoes a major increase during 1969–1971 and a major decrease during 1979–1983, periods that include the times of reversal of the polarity of the solar polar magnetic fields. A conventional model of solar modulation can describe the modulation of protons and helium from 1965 through 1984, but its predictions depart significantly from the observed modulation of electrons around and after the 1981 solar maximum. The difference in particle velocity between nuclei and electrons in the rigidity interval under study cannot alone account for the observations. The above experimental findings are consistent with a dependence of the solar modulation of galactic cosmic rays on the sign of the partic

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02858

年代:1986

数据来源: WILEY

摘要:

Simultaneous magnetic field and charged particle measurements from the Voyager spacecraft with heliographic latitude separations of>10° are used to investigate the distribution of ∼1‐GeV galactic cosmic ray protons with respect to the heliospheric current sheet in the outer solar system. By comparing the ratio of cosmic ray flux at Voyager 1 to that at Voyager 2 during periods of relatively quiet interplanetary conditions when the spacecraft are either both north or both south of the heliospheric current sheet, we derive an average latitude component of the gradient of the cosmic ray flux on opposite sides of the current sheet under restricted interplanetary conditions of −0.22±0.03%/deg, equivalent to a decrease of ∼1%/AU away from the current sheet at ∼12 AU. Our results for these limited periods are in qualitative agreement with propagation models incorporating part

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02867

年代:1986

数据来源: WILEY

摘要:

We discuss the role which empirical determinations of the latitudinal variation of cosmic rays with respect to the current sheet may have in illuminating the importance of the cross‐field drift of particles in the large‐scale heliospheric magnetic field. UsingKcoronameter observations and measured solar wind speeds, we have determined the latitudinal gradients with respect to the current sheet for cosmic rays in four rigidity ranges. Gradients vary between approximately −2 and −50% per astronomical unit. The rigidity dependence of the decrease of cosmic ray flux with distance from the current sheet lies betweenP−0.72andP−0.86, with the exact dependence being determined by the definition used for the median rigidity of e

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02879

年代:1986

数据来源: WILEY

摘要:

In this paper we report on a quantitative comparison between theoretical predictions and observations of the intensity of galactic cosmic rays near the interplanetary current sheet. In the present work, comparison of our model calculations is made with a statistical analysis of observations of galactic cosmic rays at the earth and the simultaneous position of the current sheet. Since the observations were made over a period of several years, the inclination of the current sheet varied considerably, and comparison with any one model calculation referring to a given inclination would not be appropriate. In this paper we report the expected values from our model, using an ensemble of different current sheet inclinations, in order to make the analysis of the computations approximate the method used to analyze the data. We find agreement between theory and observation at energies of the order of a few GeV.

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02885

年代:1986

数据来源: WILEY

摘要:

There is now a growing awareness that solar cycle related changes in the large‐scale structure of the heliospheric current sheet may play an important role in the modulation of galactic cosmic rays. To date, attention has been focused on the configuration of the current sheet at times near solar minimum when the current sheet structure is relatively simple. Previous analyses have explored the effect on cosmic ray intensities of a single current sheet which is tilted with respect to the heliographic equator under the assumption that the tilt of the current sheet is a minimum at solar minimum and increases as solar maximum approaches. This paper attempts to extend the previous analyses into the period near solar maximum. Two alternative hypotheses are explored: (1) that the tilt of the current sheet continues to increase as solar maximum approaches, finally becoming vertical and overturning, and (2) that the single sheet structure breaks down near solar maximum and the sun at this time sheds the poloidal flux of the previous cycle and develops a new field structure of the opposite polarity. It is found that both hypotheses lead to variations in cosmic ray intensity comparable to those actually observed over the solar cycl

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02889

年代:1986

数据来源: WILEY

摘要:

We have used data from the cosmic ray experiments on the Voyager and Pioneer 10 spacecraft during the period 1977–1985 to measure the energy spectrum of oxygen with 4–125 MeV/nucleon, with particular emphasis on the energy interval from ∼4 to 30 MeV/nucleon, where the “anomalous” component of cosmic rays is dominant. This time period includes the sunspot minimum period at the beginning of solar cycle 21, the time of the solar magnetic field reversal in 1980, the maximum solar modulation period of 1981–1983, and part of the recovery of the solar cycle through early 1985. We find that the spectrum of anomalous oxygen changes dramatically after the middle of 1980, with the peak or plateau region of the differential spectrum shifting to a higher energy. This change does not appear to be due to the increasing radial distance of the spacecraft in this time period, nor does it appear to be simply correlated with the level of modulation. The energy shift also appears to be significantly greater in the ecliptic plane than at 24° north latitude (the heliographic position of Voyager 1 at the end of the

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02896

年代:1986

数据来源: WILEY

摘要:

A general linear response analysis is presented for investigating cosmic ray transients in interplanetary space. The diffusion equation for energetic particle transport in the solar wind is first linearized about a stationary configuration. The perturbation in the particle omnidirectional distribution function resulting from perturbations to the solar wind velocity, drift velocity, or spatial diffusion tensor corresponding to transient solar wind conditions may then be found by solving an inhomogeneous boundary value problem. As illustrative examples of the general analysis, transient perturbations in the cosmic ray number density due to variations in the spatial diffusion coefficient are computed in a one‐dimensional solar wind based on the convection‐diffusion equation, which disregards cosmic ray energy changes in the solar wind. Two variations are considered, corresponding to (1) a Forbush decrease arising from the passage of an interplanetary traveling disturbance (shock) and (2) the solar cycle variation arising from an 11‐year sinusoidal variation of the spatial diffusion coefficient at the sun which then propagates out through the heliosphere. The predicted solar cycle variation exhibits a hysteresis effect in which high‐energy particles lead low‐energy particles and a time delay which increases with heliocentric distance within the inner heliosphere. The predicted Forbush decrease profiles exhibit precursors, precipitous decreases, and gradual approximately energy‐independent recoveries arising from a decay of the propagating disturbance. The general features of both variations are in accord with o

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02903

年代:1986

数据来源: WILEY

摘要:

The phase of the 11‐year galactic cosmic ray variation, due to a varying rate of emission of long‐lived, propagating regions of enhanced scattering, travels faster than the scattering regions themselves. The radial speed of the 11‐year phase in the quasi‐steady force field approximation is exactly twice the speed of the individual episodic decreases. A time‐dependent numerical solution for 1‐GeV protons at 1 and 30 AU gives a phase speed which is 1.85 times the propagation speed of the individua

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02914

年代:1986

数据来源: WILEY

摘要:

Relations among the relative counting rates of cosmic ray protons and the magnetic fields and flows observed by Helios 1 between 0.3 AU and 1 AU in the period January 1977 to August 1980 are discussed. Isolated corotating or transient flows are associated with short‐term depressions in cosmic ray intensity, but the effects are localized in longitude. Systems of corotating streams do not produce a long‐term modulation effect, although temporary, short‐term depressions in cosmic ray intensity are associated with individual streams and interaction regions. Spectra of fluctuations of the magnetic field and magnetic helicity for systems of corotating flows often do not have the formf−5/3at frequencies ≥10−5Hz, and accordingly, the hydromagnetic fluctuations in corotating streams near the sun do not appear to represent fully developed turbulence at low frequencies. Systems of transient streams containing shocks that might encircle the sun are associated with long‐lasting decreases in cosmic ray intensity. The magnetic field fluctuations in such flow systems are often turbulent, withf−5/3spectra extending from below 10−5Hz to the Nyquist frequency of the data. The turbulence is likely to be confined to sheets with thicknesses of the order of 0.1 AU, possibly associated with shocks. We conclude that long‐term modulation of cosmic rays inside 1 AU is associated with turbulence in transient disturbances that probably encompass a large longitudinal extent around the sun. In mixed flow systems the effects are intermediate between those of corotating and transient systems. The net modulation may be relatively small, with large short‐term variations associated wi

ISSN:0148-0227    

DOI:10.1029/JA091iA03p02917

年代:1986

数据来源: WILEY