ISSN:0148-0227    

DOI:10.1029/93JA00195

年代:1993

数据来源: WILEY

摘要:

The axis of the solar dipole magnetic field is aligned to within 30° of the solar rotational axis for up to 2 years during solar minima. Coronal mass ejections (CMEs) during those periods arise from the equatorial streamer belts and should share the magnetic orientation of the dipole field. If those field orientations are maintained in interplanetary space, CMEs producing geomagnetic storms should be characterized by southwardBzduring minima when the fields point outward in the northern solar hemisphere and by northwardBzat alternate minima when the solar dipole is reversed. Since southwardBzis an important factor in producing geomagnetic storms, we should expect that storms during minima characterized by southwardBzare significantly larger than those during the alternate minima. Storm data from 10 solar minima are used to test this hypothesis. The test yields a null result

ISSN:0148-0227    

DOI:10.1029/92JA02565

年代:1993

数据来源: WILEY

摘要:

We consider parametric instabilities of a circularly polarized Alfvén wave propagating along the background magnetic field. The perturbations too are assumed to propagate along the background field. The new feature of this work is the presence of a second ion species (He++) which drifts relative to the protons. Even though its abundance is small, the He++modifies the dispersion relation of the “pump” Alfvén wave and introduces a new sound wave (alpha sound) in addition to the usual sound wave carried primarily by the electrons and protons. These features modify the wave couplings leading to instability and introduce new wave couplings which lead to several new instabilities which may be of interest in the solar wind. In particular, we will find instabilities which are close to the He++gyroresonance. This may provide a means of directly transferring Alfvén wave energy to the alpha particles, if the alphas are able to resonantly extract energy from the unstable waves, without quenching the instability altogether. We will also find instabilities which are close to the alpha particle sound speed. The alpha particles in this case will tend to absorb energy via Landau damping, and this may again represent a mechanism of transferring Alfvén wave energy to the alpha particles, if the Landau damping does not suppress the instability. Finally, we will find new instabilities close to the proton cyclotron resonance, which may serve as a new mechanism for transferring wave energy into the protons. Some similar results have been recently obtained independently by Goldstein

ISSN:0148-0227    

DOI:10.1029/92JA02347

年代:1993

数据来源: WILEY

摘要:

We have determined the momentum spectrum and charge ratio of muons in the region from 250 MeV/c to 100 GeV/c using a superconducting magnetic spectrometer. The absolute differential spectrum of muons obtained in this experiment at 600 m above sea level is in good agreement with the previous measurements at sea level. The differential spectrum can be represented by a power law with a varying index, which is consistent with zero below 450 MeV/c and steepens to a value of −2.7 ± 0.1 between 20 and 100 GeV/c. The integral flux of muons measured in this experiment span a very large range of momentum and is in excellent agreement with the earlier results. The positive to negative muon ratio appears to be constant in the entire momentum range covered in this experiment within the errors and the mean value is 1.220 ± 0.044. The absolute momentum spectrum and the charge ratio measured in this experiment are also consistent with the theoretical expectations. This is the only experiment which covers a wide range of nearly 3 decades in momentum from a very low momen

ISSN:0148-0227    

DOI:10.1029/92JA02672

年代:1993

数据来源: WILEY

摘要:

A study has been made of energetic particle data, obtained from IMP 8, in conjunction with solar wind field and plasma data at the times of reported magnetic clouds. It is shown that magnetic clouds can cause a depression of the cosmic ray flux but high fields are required. A depression of 3% in a neutron monitor requires a field of about 25 nT. Such high fields are found only in a subset of coronal ejecta. The principal cause for Forbush decreases associated with energetic shocks is probably turbulence in the postshock region, although some shocks will be followed by an ejecta with a high field. Each event is different. The lower‐energy particles can help in identifying the dominant processes in individual event

ISSN:0148-0227    

DOI:10.1029/92JA02479

年代:1993

数据来源: WILEY

摘要:

Eight solar energetic proton events observed in the inner heliosphere on the spacecraft Helios 1 or 2 are studied. Particle pitch angle diffusion coefficients and mean free paths are derived from a power spectral analysis of the turbulence of the magnetic field during the time of the events. Quasi‐linear theory of wave‐particle interaction is applied strictly throughout in combination with the slab model of fluctuations. Several points receive special attention: (1) the large difference in scattering conditions from one event to the other, which requires an individual analysis of each event; (2) the selection of the exact stretch of magnetic field data to be analyzed and the influence of this choice on the result; (3) the exclusion of data periods that contain discontinuities and other nonoscillatory structures; and (4) short‐scale variations of the overall field fluctuation level, which can be considerable. As a consequence, the mean free paths of the particles are derived from time series of the varying momentary power density spectra as an appropriate average of the scattering conditions (as an average mean free path). These results from the magnetic field spectra are then compared with results of fits of propagation models to the particle time‐intensity and time‐anisotropy profiles observed at the same time. Of eight such comparisons, half of the cases exhibit for the first time an approximate agreement between the results of the two approaches, particle analysis and magnetic field analysis. The others, however, still show the discrepancy that is cited in the literature. Among the events with good agreement are the two extreme cases in the whole Helios data set, the event of April 11, 1978, Helios 2, with a mean free path from quasi‐linear theory of 0.01 AU, and the events of June 7, 1980, Helios 1, with a mean free path from quasi‐linear theory of about 0.8 AU at a rigidity of 0.1 GV. This shows not only that quasi‐linear theory in a wave and slab model framework is indeed capable of producing correct results under certain conditions if an appropriate case‐by‐case analysis is conducted, but also that these cases can encompass a wide range of turbulence strengths. Several possibilities for the source of the discrepancy that remains for the other cases, which the theory and the assumptions applied in this work obviously do not explain, are discussed. However, at the present time no definite solution to the

ISSN:0148-0227    

DOI:10.1029/92JA02546

年代:1993

数据来源: WILEY

摘要:

Using 20 years of solar magnetograph and in‐ecliptic interplanetary magnetic field (IMF) measurements, we verify that the radial IMF component (Br) can be approximated by a superposition of the Sun's inclined magnetic dipole moment and a current sheet normal to the dipole axis. The net field is found empirically to be ∼3 times stronger along the dipole axis than near the current sheet, whose effect is to redistribute flux toward the dipole equator. The radial IMF intensity at a given latitude and phase of the sunspot cycle is determined by the changing strength and inclination of the dipole, which attains its maximum amplitude when its axis is aligned with the solar poles near sunspot minimum. The model predicts that over the sunspot cycle, |Br| should undergo the least variation near the heliographic equator and the greatest variation above the Sun's poles, where it decreases by a factor of 10 between sunspot minimum and sunspot maximum. The latitudinal gradients inBrare expected to be steepest near sunspot minimum and flattest near maximum. The model suggests that Ulysses will encounter very strong fields when it flies over the solar poles during the declining phase of sunspot cycle

ISSN:0148-0227    

DOI:10.1029/92JA02396

年代:1993

数据来源: WILEY

摘要:

This paper analyzes the radial, latitudinal, and temporal variations of the magnetic field in the outer heliosphere, using the observations from Voyagers 1 and 2 and Pioneers 10 and 11. The radial variation of the magnetic field strength measured during 1973‐1989 from 1 to 19 AU is compared with that predicted by Parker's spiral field model. The speed deficit (V‐V1)/V1 between 1 and 19 AU is (+0.0027 ± 0.0026)/AU for the Voyager data and (0.0099 ± 0.0024)/AU for the Pioneer 11 data. Thus the Voyager data are consistent with no radial variation of the speed, as assumed in deriving Parker's equations for the spiral field, but the Pioneer data are not. The magnetic flux deficit in the outer heliosphere relative to 1 AU is measured by the quantityD≡ (A‐A1)/A1 versusR. For the Voyager data the 1 AU intercept of the best fit line to these data is (0.029 ± 0.031), which is consistent with zero, as required by the definition of D. For the Pioneer 11 data the corresponding 1 AU intercept is (−0.068 ± 0.044), which implies a flux deficit of (7% ± 4%) at 1 AU, suggesting either a systematic error in the data or statistical uncertainties greater than we estimated. The magnetic flux deficit is given by the slope the best fit line toDversusR. The Voyager data give a slope −(0.0027 ± 0.0036)/AU, which is consistent with zero, indicating no flux deficit in the Voyager data. The Pioneer data give a slope (0.0040 ± 0.0042)/AU, which is also consistent with zero. Since the best fit line to the Voyager observations ofDversusRgives a value ofDat 1 AU consistent with zero, we may setD(1) = 0 and obtainD(R) = [(0.0001 ± 0.0021)/AU] × (R‐ 1)(AU), consistent with no flux deficit between 1 and 19 AU from 1977 through 1985. For the years 1986 and 1987 the Voyager data give a nonzero flux deficit equal to (−0.264 ± 0.075)/AU and (−0.171 ± 0.070)/AU, respectively. The Pioneer 11 data show no significant flux deficit during 1986 (+0.19 ± 0.22)/AU. The flux deficit observed by Voyager 2 during these 2 years near solar minimum might be related to the presence of a heliospheric vortex street at this time. The magnetic field strength measured by Voyager 1 at high latitudes is smaller than the magnetic field strength measured by Voyager 2 near the heliographic equator beyond 21 AU. During 1986, when the heliospheric vortex street wrapped field lines in spirals around the axes of the vortices, the source magnetic field strength at 27°N was greater than the source field strength at ∼1°N. During 1988 and 1989, when the vortex street was no longer present, the source field strength at ∼30°N was less than that at ∼3°N to 4°N. The topology of the spiral interplanetary magnetic field is determined by a defect in the heliosphere, the rotating Sun. The language of group theory can describe this and other more complex defe

ISSN:0148-0227    

DOI:10.1029/92JA02555

年代:1993

数据来源: WILEY

摘要:

This paper presents a numerical study of the evolution of a velocity enhancement disturbance in the solar wind in terms of a one‐dimensional, isentropic MHD flow model. It is shown that the disturbance steepens and evolves into a double shock pair while propagating outward away from the Sun. The double shock pair consists of a reverse fast shock, a reverse slow shock, a forward slow shock, and a forward fast shock in order of distance away from the Sun. The formation time of the double shock pair is nearly inversely proportional to the average velocity gradient of the disturbance. When the double shock pair is fully developed, the strength of the fast shocks is essentially determined by the disturbance amplitude, while the slow shocks behave differently. Their strength increases first with the disturbance amplitude but starts to decrease once the disturbance amplitude exceeds a certain value. However, the fully developed slow shocks will retain their identity up to 1 AU and even farther, though their propagation speed in the solar wind frame and the jump in velocity and total pressure across them decrease substantially with heliocentric distance. Theoretically, double shock pairs would occur frequently in the inner heliosphere, since the solar wind there is characterized by various large‐scale structures and disturbances, which provide an appropriate ground for the formation of double shock pairs. Such a prediction remains to be confirmed by observations and data interpretat

ISSN:0148-0227    

DOI:10.1029/92JA02567

年代:1993

数据来源: WILEY

摘要:

We consider propagation of three‐dimensional, adiabatic magnetohydrodynamic (MHD) perturbations in a background radial MHD solar wind which is spherically symmetric. In the linear approximation, three‐dimensional propagation of Alfvén waves decouples from three‐dimensional propagation of MHD fast and slow perturbations in the wind. For Alfvén waves, we extend previous analytical WKB, non‐WKB, and asymptotic solutions at large distances as well as numerical results to explicit three‐dimensional versions; as to three‐dimensional MHD fast and slow perturbations, we generalize the prior perturbation scheme for a purely hydrodynamic solar wind by including a magnetic field. In general, Alfvén wave flux in the solar wind is not conserved; we further study the following second‐order nonlinear effects due to a three‐dimensional propagation of Alfvén waves in the wind, namely, (1) the driving of compressible MHD wind flow and thermodynamic variations; (2) the generation of MHD fast and slow waves; (3) the generation of Alfvén waves; and (4) the formation of steady, incompressible vortex and current structures. Therefore in the absence of direct dissipative mechanisms, the energy flux of Alfvén waves in the solar wind is consumed to produce these nonlinear effects. For typical solar wind parameters in the absence of dissipations, the Alfvén wave flux density at the solar coronal base inferred from that observed at 1 AU is thus 1 to 2 orders of magnitude less than what is needed (5 ∼ 8 × 105ergs cm−2s−1) if a solar wind stream would be entirely driven and maintained by Alfvén waves. We discuss several relevant heliospher

ISSN:0148-0227    

DOI:10.1029/92JA02243

年代:1993

数据来源: WILEY