摘要:

From an analysis of the time variations during 1968–1971 of the fast neutron flux in the upper atmosphere (mean energy of response to primaries, 1–2 GeV per nucleon) versus those of ground‐based neutron monitors we have identified two classes of transient intensity decrease on the basis of differences in their spectral responses, time histories, and flare associations. Type I events are found to be classic Forbush decreases, sharp declines accompanying a geomagnetic storm sudden commencement, following by 1–3 days a large optical flare with radio noise and energetic particle production, whereas type II events are more symmetric in their time histories and are therefore not associated with a particular flare. There are also differences in the spectral responses of the two types. During a type I decrease the flux change of the lower‐rigidity cosmic ray particles lags the flux change of the high‐rigidity particles both in the decline and in the recovery, tracing out a hysteresis loop. During a type II event, if there is any hysteresis at all, the lower‐rigidity primaries tend to ‘overrecover’ in comparison to the higher‐rigidity primaries. Intercomparison of neutron monitor data for median response rigidities from 10 to 30 GV reveals that the spectral response in type II events is softer on the average for low‐rigidity (10 GV) primaries than that in type I events. Comparison of intermixed sequences of type I and II events with recurrences of active regions reveals an identifiable but complex evolutionary relation between decrease occurrence and active region development. Long‐lived (of the order of days) low‐energy (<1 MeV) proton events occur during all but one of the type II events identified, supporting an association with solar active region transits. We interpret type II events as either a subsequent evolution of type I (Forbush decrease) events or a quasi‐stationary ‘corotating’ spatial structure loosely associated with an active region. Therefore both type I and type II decreases occur in intermixed recurrence

ISSN:0148-0227    

DOI:10.1029/JA080i010p01189

年代:1975

数据来源: WILEY

摘要:

By using the trajectory‐tracing technique, cutoff rigidities for Palestine, Dallas, and Midland, Texas, have been calculated as a function of various zenith and azimuth angles. Extensive analysis of the trajectory calculations shows that there is a systematic uncertainty involved in computing the lowest allowed rigidity, and this uncertainty may be a significant fraction of the penumbral width. Continuation of the trajectory‐tracing process below the Stormer cutoff allows an evaluation of the reentrant albedo, showing that the average invariant latitude of the guiding center of the trajectory at the albedo origin is the same as the average invariant latitude of the guiding center of the particle trajectory at the detection point. No significant difference in the cutoff rigidities for these locations is found when the external magnetic fields present in the magnetosphere are added to the geomagnetic field of internal ori

ISSN:0148-0227    

DOI:10.1029/JA080i010p01202

年代:1975

数据来源: WILEY

摘要:

Recent measurements of the charge states of low‐energy (∼ 100 keV/nucleon) solar cosmic rays at 1 AU are discussed. The measurements are consistent with models involving charge equilibrium with neutral matter at the sun only if the particles lose ∼90% of their energy owing to adiabatic deceleration in the solar wind. Such an energy loss is shown to be possible only if the diffusion coefficient for 1‐MeV/nucleon particles is smaller than 1020cm² s−1. The implications of these results for models of solar cosmic ray acceleration are

ISSN:0148-0227    

DOI:10.1029/JA080i010p01209

年代:1975

数据来源: WILEY

摘要:

The nonlinear oblique interaction between a plane tangential discontinuity (TD) and a plane fast magnetogasdynamic shock is treated generally and then applied to the interplanetary TD‐bow shock interaction. A number of similarity relations are found for the general problem. For the frequent directional TD's the interaction is strongest if the directional change is 90° and one of the magnetic field vectors is parallel to the shock. Bow shock motions of up to several tens of kilometers per second are induced by these TD's. Tangential discontinuities with density jumps held responsible for geomagnetic sudden impulses lead to a much stronger interaction with the bow shock with nonnegligible strength of the newly generated wave. The oblique interaction requires weaker TD's to produce observed bow shock velocities. It is also shown that complex nonlinear wave patterns very often develop, which have not been predicted in the past in this conte

ISSN:0148-0227    

DOI:10.1029/JA080i010p01213

年代:1975

数据来源: WILEY

摘要:

Solar wind disturbances produced by relatively long‐lasting solar flares are simulated numerically by using the single‐fluid magnetohydrodynamic (MHD) equations with negligible dissipation. The computations are confined to the ecliptic plane of a spherically symmetric flow and are begun when an initial disturbance is introduced near the sun in the ambient solar wind. The velocity and density in the disturbances do not differ appreciably from the values obtained by using an ordinary fluid dynamic calculation, but the thermal pressure exhibits a marked decrease near the contact (or ‘piston’) discontinuity. A comparison with several MHD similarity solutions shows the advantages of the present analysis over the latter theory to be the following: (1) it yields a quantitative prediction of the azimuthally induced plasma flow, (2) it provides for removal of the nonphysical zero pressure at the contact surface, and (3) it allows more flexibility in the specification of initial conditions, which gives the present analysis greater utility in predicting plasma and magnetic data. Alternative initial disturbances, such as pulses in radial velocity or thermal pressure, produce multiple nonlinear variations of the density, thermal pressure, and magnetic field. Spatial and temporal variations of the thermal and magnetic energy densities are thereby in

ISSN:0148-0227    

DOI:10.1029/JA080i010p01223

年代:1975

数据来源: WILEY

摘要:

The spatial extent and energy distribution function of the magnetosheath electron population at lunar distance have been analyzed by using measurements from the Apollo 14 charged particle lunar environment experiment (CPLEE). The magnetotail is shown to be approximately 52REin diameter. The mean bow shock surface has a cross section of about 91RE. The average aberration angle measured is about 3°. The electron distribution function reveals two distinct particle populations. The first, at low energies (40 eV ≤E≤ 200 eV), is well characterized by a nearly isotropic Maxwellian distribution, with temperatures in the range 15–25 eV. Densities calculated from fitted functions range from 4–8 cm−3at the bow shock to 1–2 cm−3near the magnetopause. The high‐energy portion of the spectrum (200 eV ≤E≤ 2000 eV), however, is generally anisotropic and is generally denser and more energetic in the dawn magnetosheath than in the dusk magnetosheath. Separate Maxwellian fits to the high‐energy population yield densities near 0.02 (0.10) cm−3and temperatures near 200 (250) eV for the dusk (dawn) magnetosheath. It is argued that these particles originate at the bow shock rather

ISSN:0148-0227    

DOI:10.1029/JA080i010p01232

年代:1975

数据来源: WILEY

摘要:

We present the results of an extensive analysis of plasma and magnetic field data from Pioneer 7 taken in the geomagnetic tail approximately 1000REdownstream from earth. The principal observations are (1) measurable fluxes of protons in the tail, flowing away from earth, sometimes with a double‐peaked velocity distribution; (2) field reversal regions in which the field changes from radial to antiradial by a vector rotation in the north‐south plane; and (3) general characteristics of the tail similar to those observed near earth with good correlation between taillike magnetic fields and pla

ISSN:0148-0227    

DOI:10.1029/JA080i010p01238

年代:1975

数据来源: WILEY

摘要:

Two intermingled streams of protons have been observed in the distant geomagnetic tail. The number densities of the two streams are comparable, and their velocity difference tends to lie along the field direction. The lower‐velocity stream is probably composed of magnetosheath protons which have diffused through the boundary of the distant tail. The higher‐velocity stream appears to originate in the field reversal reg

ISSN:0148-0227    

DOI:10.1029/JA080i010p01245

年代:1975

数据来源: WILEY

摘要:

During intervals of a steady state condition of the interplanetary magnetic field the geomagnetic variations at the polar cap have been examined as a function of the direction of the interplanetary field in theY‐Zplane of the geocentric solar magnetospheric coordinate system. A prominent feature of the equivalent current patterns during a positiveBzsteady state is the total vanishing of the well‐known two‐cell current system, which is generally considered to be related to the convection across the polar cap. Another significant effect for a northward interplanetary magnetic field is a poleward shift of theByassociated currents at the day side of the polar cap, with no decrease of the current strength. The existence of these currents even during the absence of the two‐cell current system indicates that the explanation proposed by Heppner (1972) of the currents' being due to a shift toward the dawn or the dusk of the polar cap convection is not exhaustive. Rather, the magnetic perturbations for a positiveBzmay correspond to merging of the northward interplanetary magnetic field with the open geomagnetic field lines at the poleward boundary of the cusp as suggested by Burch (1973). A pronounced decrease of theByrelated magnetic perturbations from summer to winter compared to the seasonal variation of theBzrelated disturbances at the polar cap indicates a different origin of the associated c

ISSN:0148-0227    

DOI:10.1029/JA080i010p01248

年代:1975

数据来源: WILEY

摘要:

Disturbances in total magnetic field magnitude ΔBare generally positive from 2200 to 1000 MLT and negative from 1000 to 2200 MLT. The maximum disturbances from the positive and negative regions of ΔB(BpandBn, respectively) are investigated with respect to their correlation with (1) the average north‐south componentBz, (2) the average angle θ with respect to the solar magnetospheric equatorial plane, (3) the variance σi, and (4) the magnitudeBiof the interplanetary magnetic field. These quantities were averaged over a periodTranging from 20 min to 8 hours prior to the measurement ofBporBn. In the correlation withBztwo methods were used: (1) direct correlation withBzand (2) correlation with the sumBΣ= ∑Bz/T, whereBz<0. The best correlation was shown byBpversusBΣ, −0.79 forT= 120 min. ForBnversusBΣ, data were divided into an equinoctial period and a winter period. Maximum correlations of 0.63 atT= 200 min and 0.70 atT= 60 min were obtained for equinox and winter, respectively. In all cases the peak in the correlation coefficient, when it was plotted againstT, was very broad. Correlations with all other parameters were smaller than the correlation forBΣ. Examination ofBpas a function of averageBzshows that an approximately linear relation holds forBz0,Bpis relatively constant. Significant magnitudes of ΔBare found to occur in both the positive and negative regions even when the interplanetary field has been northward for over an hour. Comparison of the occurrence of disturbance in ΔBwith the occurrence ofDP2 fluctuations indicates no correlation between th

ISSN:0148-0227    

DOI:10.1029/JA080i010p01261

年代:1975

数据来源: WILEY