摘要:

The possible existence of energetic disturbances in the corona significantly before the associated surface events has profound implications for the location and mechanisms of preevent coronal energy storage. Precursor activity associated with coronal mass ejections (CMEs), if it exists, would reflect the evolution and magnitude of the energy release or failure of magnetic equilibrium characterizing the interval before and during the events. Jackson and Hildner (JH) studied 21 CMEs observed with the Skylab white‐light coronagraph, and found a low‐density plateau rimming each event for which good coverage was available. They concluded that this “forerunner” material could not be explained by mere translation of the overlying coronal plasma. Jackson further inferred from the Skylab data that the forerunner must start moving significantly before the onset of the associated CME, thus identifying this phenomenon as a form of precursor activity. We have performed a systematic search for forerunners using the white‐light coronagraph observations obtained with the Solwind instrument on board the P78‐1 satellite. Forty‐four bright, well‐observed events were selected and analyzed, employing selection criteria and analysis methods similar to those used by JH. Approximately half of these events either do not exhibit low‐density plateaus in front or are questionable (e.g., a frontal plateau only appears intermittently). Based on our analysis of the remaining CMEs, we conclude that identification of the forerunner as a distinct entity probably is not warranted, and that the low‐density plasma is an integral par

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07227

年代:1987

数据来源: WILEY

摘要:

Contemporaneous coronal and interplanetary data sets which span the first full solar cycle of in situ space observations illuminate the solar origins of corotation and recurrence in the solar wind. Because solar rotation is differential, comparisons of solar rotation and interplanetary recurrence periods suggest latitudes of origin for recurrent space phenomena. The 27‐day recurrence period of geomagnetic activity and solar wind streams is associated with lower latitudes in the 1.5RScorona than in the 1.1RScorona. If we associate the recurrence period with coronal tracers at the lower height, we determine an observational upper limit of 20° on typical solar wind motions in latitude over 1 AU. However, if we suppose that magnetic field lines which close just beneath the source surface at the equator are anchored typically 20° away from the equator, stream recurrence periods approximate high‐altitude equatorial rotation periods, in which case, no meridional motion of solar wind over 1 AU is implied. Plausible coronal sources of IMF corotation periods can be found for most intervals, but we discuss one interval in which solar magnetic tracers may not reflect field line rotation at the base of the solar

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07235

年代:1987

数据来源: WILEY

摘要:

Pioneer 10 observations suggest that the mean (longitudinally averaged) solar wind azimuthal field strength,Bϕ, near the ecliptic plane falls off more rapidly with heliocentric distance than would be expected in a classic Parker expansion, showing a deficit of 10‐20% (as compared to the projected 1‐AU value) by 10 AU. Though this observational interpretation has been challenged by subsequent analyses of Voyager data, it has nevertheless stimulated efforts to explain the inferred deficit on the basis of systematic north‐south magnetic pressure gradients generated by the differential spiral wrapping of magnetic field lines in interplanetary space. We reexamine this issue from the theoretical perspective using a three‐dimensional MHD nonlinear numerical model for steady, corotating flow. For realistic solar wind parameters we find that a purely axisymmetric expansion is capable of producing sizable magnetic flux deficits only when there are substantial meridional gradients in mean flow conditions localized about the ecliptic plane near the sun. Even then the match between plausible flow states and significant meanBϕdeficit is achieved over such a limited parameter range that it is unlikely this mechanism alone can produce deficits of the magnitude inferred from the Pioneer data. We present calculations on three‐dimensional corotating flows which demonstrate that latitudinal transport of magnetic flux by stream interactions may be an important consideration. For streams having a tilted‐dipole geometry we find that deficits in the meanBϕof the order of 10% or more at 10 AU are readily produced over a broad range of tilt angles and flow conditions. Both the axisymmetric and stream interaction mechanisms should operate most efficiently and be most prominent observationally in the late declining and minimum phases of the solar cycle, when the solar wind flow is systematically organized with a narrow band of slow flow about the solar equator and fast flow toward the poles. However, it is unlikely that either mechanism would be capable of producing significant deficits under the global flow conditions of the solar cycle epoch (that centered about solar maximum) to which the published Pioneer and Voyager analyses refer. It is furthermore emphasized that deficits inBϕdo not necessarily imply equivalent transport of magnetic flux and that a substantial portion of apparentBϕdeficits may be due to the global, nonlinear averaging by stream interactions of the large‐scale, correlated variations in flow param

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07241

年代:1987

数据来源: WILEY

摘要:

A kinetic model is presented for the generation of proton heat flux tails and double beams by Coulomb collisions in the solar wind. The combined action of the large‐scale interplanetary magnetic field (mirror effect) and of the collisional scattering (runaway) is shown to be able to produce variously skewed velocity distributions. All intermediate cases between the exospheric and isotropic adiabatic expansion can be simulated as a function of the collisionality of the plasma. Good qualitative agreement is achieved between in situ measurements and model results in the collisional domains of the solar win

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07255

年代:1987

数据来源: WILEY

摘要:

Solar wind proton double streams and proton‐alpha differential streaming may provide sufficient free energy to drive field‐aligned magnetosonic waves unstable. We carry out a statistical analysis of this electromagnetic beam instability based on measured Helios ion distributions. Observational evidence is indeed found for unstable distributions, whereby the largest growth rates are obtained in the collisionless high‐speed wind. Alpha particles by themselves are not able to excite these waves but mostly tend to stabilize an existing proton double stream configuration by enhancing the cyclotron damping of the main proton popul

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07263

年代:1987

数据来源: WILEY

摘要:

The solar cycle evolution of solar wind speed structure was studied for the years from 1973 to 1985 on a basis of interplanetary scintillation observations using a new method for mapping solar wind speed to the source surface. The major minimum‐speed regions are distributed along a neutral line through the whole period of a solar cycle: when solar activity is low, they are distributed on the wavy neutral line along the solar equator; in the active phase they also tend to be distributed along the neutral line, which has a large latitudinal amplitude. The minimum‐speed regions tend to be distributed not only along the neutral line but also at low magnetic intensity regions and/or coronal bright regions which do not correspond to the neutral line. As the polar high‐speed regions extend equatorward around the minimum phase, the latitudinal gradient of speed increases at the boundaries of the low‐speed region, and the width of the low‐speed region decreases. One or two years before the minimum of solar activity, two localized minimum‐speed regions appear on the neutral line, and their locations are longitudinally separa

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07269

年代:1987

数据来源: WILEY

摘要:

We investigate the flow pattern of low‐energy protons (35–1600 keV) associated with interplanetary shocks observed by ISEE‐3 between August 1978 and April 1980. The analysis of the shape of the distribution function in the solar wind frame and its temporal evolution indicates that the low‐energy protons can behave as a coherent, independent population of particles in the solar wind. Ahead of the shock this population propagates along the magnetic field in the same direction as the solar wind flow, while after the passage of the perturbed region associated with the shock, it propagates in the opposite sense. The behavior of the flow pattern of this population through the shock front is discussed for the 17 largest events observed in this

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07280

年代:1987

数据来源: WILEY

摘要:

The structure and variability of ion concentrations, velocities, and electric and magnetic fields in the ionosphere of Venus result from a combination of physical and chemical processes driven by the interaction of the solar wind with the atmosphere of Venus. In this paper we describe these processes and formulate a model describing their interrelationships in a self‐consistent manner. We show that the consequences of this model are in agreement with Pioneer Venus observations in the Venus ionosphere. It is shown that the magnetic field configuration of the Venus ionosphere is a steady state global structure which results from a combination of a large‐scale “open” current system connecting through the ionopause to the postshock solar wind and two small‐scale “closed” current systems entirely confined within the ionosphere itself and generated by ionospheric flows and shears. A previously detected ionospheric layer in O2+near 170 km altitude is shown to be a new type of ionospheric layer resulting from transition of solar‐wind‐driven vertical flow to velocities approaching Mach 1 (sonic flow). Accordingly, this layer has been named the “sonic flow layer” or “S layer.” It is suggested that this model should apply to other nonmagnetic bodies in magnetized plasma flows, such as com

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07289

年代:1987

数据来源: WILEY

摘要:

The frequency with which the total ion density Niand constituent densities O+, H+, and molecular ion density O2++M29+(= [O2+] + [NO+] + [CO+] + [N2+]) fall between equal increments of the logarithm of the density is derived. The individual measurements are first normalized by dividing them by their median value at the altitude of measurement. This process removes the known altitude variation from the sets. The resulting frequency functions are assumed to result from temporal variations alone. The data used in deriving the functions are restricted to 120°–180° solar zenith angle (SZA) and altitudes between 140 and 900 km. The frequency function derived for Niis found to be insensitive to division of the parent set into two approximately equal subsets representing division into dawn and dusk hemispheres, an upper and lower altitude interval, or two SZA intervals. The Nifunction is found to be reasonably well peaked near the median value, and 78% of the normalized values fall within a factor of 2.8 of the median value. The frequency of occurrence decreases more rapidly on the high side of the median than on the low side. The frequency functions for the constituent ions are similar to that for Ni, with the percentage of normalized densities falling within a factor of 2.8 of the median value ranging from 82% for O2++ M29+to 71% for H+. The similarities and dissimilarities between the frequency function for O+and those for H+and O2++ M29+are discussed in relation to the role of O+in producing the H+and O2++ M29+constitue

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07308

年代:1987

数据来源: WILEY

摘要:

The behavior and time evolution of the large‐scale magnetic fields and ionospheric plasma of the dayside Venus ionosphere were studied using a one‐dimensional model. The coupled continuity, momentum, and Maxwell's equations were solved simultaneously for three ions (O+, O2+, H+) and the magnetic field. The CO2+ion was included photochemically. The calculated magnetic field profiles are in good agreement with observations made by the Pioneer Venus orbiter magnetometer. The magnetic field structure is found to be quasi‐steady for slow changes of the solar wind dynamic pressure. The peak of the magnetic field at 165 km is maintained by the downward convection from higher altitudes. The time scale for the decay of magnetic field by the pure one‐dimensional vertical diffusion/convection process is several hours unless the magnetic field flux is resupplied from the top of the ionosphere. Good agreement was obtained between the calculated and observed plasma densities for altitudes below 250 km including the electron density “ledge” near 190 km in magnetized ionospheres. However, the significant disagreement between the calculated and observed plasma densities at high altitudes suggests that under magnetized conditions, horizontal transport processes become important in removing the plasma and/or the magnetic field even in the subs

ISSN:0148-0227    

DOI:10.1029/JA092iA07p07317

年代:1987

数据来源: WILEY