摘要:

C+pickup ions were discovered with the solar wind ion composition spectrometer flying on Ulysses. Whereas the other nonlocally occurring pickup ions are produced from the interstellar gas penetrating deep into the heliosphere, C+comes from an “inner source” which is located at a solar distance of a few AU and extends over all heliospheric latitudes investigated so far. The total production of C+, N+, and O+by this inner source is of the order of 10−3relative to the total production of O+from the interstellar gas in the heliosphere. Thus the inner source does not significantly contribute to oxygen or nitrogen in the anomalous cosmic rays (ACR), but its contribution to ACR carbon may not be negligible. We propose that the inner source material is carbon compounds evaporating from grains. At this time, the evidence points to interstellar grains as the major source, but we do not want to exclude yet a contribution from grains of solar system o

ISSN:0148-0227    

DOI:10.1029/95JA03051

年代:1995

数据来源: WILEY

摘要:

The existence of the nongyrotropic velocity distribution of newborn ions is a very important feature for comet Grigg‐Skjellerup (P/G‐S); this kind of distribution may be a reservoir of free energy and play a rather important role in the generation of plasma waves observed at P/G‐S, especially close to the comet. In this paper we predict the distribution function of the newborn ion population, taking into account the source term for this population (implantation from neutrals) and the loss term (pickup, i.e., velocity diffusion leading to assimilation in the solar wind). We integrate the equation backward along the solar wind plasma flow line modified by the ion gyration motion using the observations of the Giotto spacecraft at P/G‐S on July 10, 1992. We obtain the nongyrating zero‐order nongyrotropic velocity distribution of the newborn ions which depends on the average pickup time of implanted ions and on the distance from the comet. The stability of the above equilibrium is then studied by adding linear perturbations and solving numerically the inferred dispersion equation. We show that this kind of distribution function triggers a left‐handed instability which can be called “nongyrotropic ring instability” and which arises from the coupling between the two transverse electromagnetic modes and the longitudinal ele

ISSN:0148-0227    

DOI:10.1029/95JA01915

年代:1995

数据来源: WILEY

摘要:

During its south polar passage in 1994, the Ulysses spacecraft continuously sampled the properties of the solar wind emanating from the south polar coronal hole. At latitudes poleward of ∼−60°, the solar wind speed had an average value of 764 km/s and a range of 700–833 km/s. The principal variations in the vector velocity were associated with either outward propagating Alfvén waves with periods up to about half a day or with longer‐period high‐ or low‐speed “microstreams.” The microstreams had an amplitude of ∼40 km/s and a mean half width of 0.4 days, and they recurred on timescales of 2–3 days (power spectral peaks at 1.9 and 3.3 days). The density and temperature profiles showed the expected evidence of pileup and compression on the leading edges of high‐speed microstreams, although no forward or reverse shocks were observed. The particle fluxes were nearly the same for both the fast and slow microstreams. The higher‐speed microstreams had higher proton temperatures and higher alpha‐particle abundances than did the slower microstreams. The absence of latitude variations in the thickness or the recurrence rate suggests that the microstreams are caused by temporal rather than long lived (>a few days) spatial variations in the source region at the Sun. Some speculations are made about the possibl

ISSN:0148-0227    

DOI:10.1029/95JA02723

年代:1995

数据来源: WILEY

摘要:

Occurrences of solar wind plasma with abnormally low proton temperatures (Tp) have long been associated with the interplanetary manifestations (which we term “ejecta”) of coronal mass ejections (CMEs). We survey the National Space Science Data Center Omni solar wind database for 1965–1991, and data from the Helios 1 and 2 spacecraft for more limited periods, to identify plasma in whichTpis less than the temperature expected (Tex) from the well‐established correlation between the solar wind speed andTpfor normal solar wind expansion. The occurrence rate of low‐temperature plasma (specifically withTp/Tex≤ 0.5) is shown to be correlated with solar activity levels. Individual low‐temperature regions have durations from 1 to ∼80 hours with a mean of ∼10 hours. Around one third are encounters with the heliospheric plasma sheet (HPS). These events are observed most frequently during the increasing phase of solar activity when the HPS lies closer to the ecliptic. The abnormally low temperatures may be intrinsic to the HPS or may provide support for the proposal that the coronal streamer belt underlying the HPS is a frequent source of ejecta. The remaining events have an occurrence rate which shows a particularly clear correlation with solar activity levels and with the CME rate at the Sun (when CME observations are available), consistent with an association with ejecta. These events also show greater than chance associations with other ejecta signatures. We suggest that the Omni plasma data can provide information on the rate of ejecta passing the Earth, and hence give an indication of the CME rate, for a period commencing before spacecraft coronagraph CME observations became available in the early 1970s. Our findings suggest thatTpdepressions may provide a more comprehensive indication of the presence of ejecta than other ejecta signatures, such as bidirectional solar wind electron heat fluxes and energetic ion flows, which alone do not id

ISSN:0148-0227    

DOI:10.1029/95JA02684

年代:1995

数据来源: WILEY

摘要:

Coronal hole regions are well‐known sources of high‐speed solar wind; however, to account for the observed properties of the solar wind, a source of energy must be included in addition to heat conduction. Alfvén waves were suggested as the possible source of heating that accelerates the solar wind. We investigate the heating and propagation of the fast and shear Alfvén waves in coronal holes via numerical solution of the time‐dependent, linearized, resistive, low‐β, two‐dimensional MHD equations in slab geometry. The waves are driven at the lower boundary of the coronal hole and propagate into the corona. We find that fast waves are partially reflected at the coronal hole boundary and significant part of the wave energy leaks out of the coronal hole. We compare the calculated wavelengths and the attenuation rate of the fast waves in the leaky waveguide formed by the coronal hole with the analytical ideal MHD solutions forky= 0, wherekyis the perpendicular wavenumber, and find an excellent agreement. Whenky≠ 0 the fast waves couple to the shear Alfvén waves and transfer energy across field lines. Resonance heating layers are found to occur when shear Alfvén waves are driven and a continuous density profile is assumed for the coronal hole. When resonance absorption is considered, the leakage is small compared to the heating rate. The heating is enhanced by phase mixing when coronal hole inhomogeneities (i.e., plumes) are included. We investigate the dependence of the heating rate on the driver frequency and the Lundquist numberSand find a good agreement with the analyticalS1/3scaling of the dissipation length. We find that whenS= 104the low‐frequency Alfvén waves can be a significant source of heating of coronal holes at several solar radii. At larger values ofS, nonlinear effects might reduce the effective dissipation length. We discuss the relation of our results to the observed properties of high‐speed solar w

ISSN:0148-0227    

DOI:10.1029/95JA02222

年代:1995

数据来源: WILEY

摘要:

The nonlinear evolution of the resonant absorption of standing and propagating Alfvén waves in an inhomogeneous plasma is studied via solution of the time‐dependent, three‐dimensional, low‐β, resistive MHD equations over a wide parameter range. When the nonlinear effects become important, the velocities at the dissipation layer were found to be lower than the linear scaling ofS1/3would predict, whereSis the Lundquist number. Highly sheared velocities that are subject to the Kelvin‐Helmholtz‐like instability were found at the narrow dissipation layers. Three‐dimensional Kelvin‐Helmholtz‐like vortices appear at and near the dissipation layers and propagate along the slab of plasma when traveling Alfvén wave solution are considered. The narrow resonant heating layers are deformed by the self‐consistent shear flow. In the solar active regions where the resonant absorption of Alfvén waves is believed to occur, the instability may lead to turbulent enhancement of the dissipation parameters and account for the observed turbulent velocities inferred from the nonthermal broadening of x‐ ray and EUV emission lines. The self consistentJ×Bforce changes significantly the density structure of the loop that leads to a shift in the global mode frequency response of the loop and a subsequent drop in the heating rate. In the solar corona the density evolution of the loop is likely to be dominated by evaporation of material fr

ISSN:0148-0227    

DOI:10.1029/95JA01907

年代:1995

数据来源: WILEY

摘要:

In two or three dimensions, magnetic reconnection may occur at neutral points and is accompanied by the transport of magnetic field lines across separatrices, the field lines (or flux surfaces in three dimensions) at which the mapping of field lines is discontinuous. Here we show that reconnection may also occur in three dimensions in the absence of neutral points at so‐called “quasi‐separatrix layers,” where there is a steep gradient in field line linkage. Reconnection is a global property, and so, in order to determine where it can occur, the first step is to enclose the volume being considered by a boundary (such as a spherical surface). Then the mapping of field lines from one part of the boundary to another is determined, and quasi‐separatrix layers may be identified as regions where the gradient of the mapping or its inverse is very much larger than normal. The most effective measure of the presence of such layers is the norm of the displacement gradient tensor; their qualitative location is robust and insensitive to the particular surface that is chosen. Reconnection itself occurs when there is a breakdown of ideal MHD and a change of connectivity of plasma elements, where the field line velocity becomes larger than the plasma velocity, so that the field lines slip through the plasma. This breakdown can occur in the quasi‐separatrix layers with an electric field component parallel to the magnetic field. In three dimensions the electric fieldE(and therefore the field line velocityv⊥) depends partly on the imposed values ofE(orv⊥) at the boundary and partly on the gradients of the inverse mapping function. We show that the inverse mapping determines the location of the narrow layers where the breakdown of ideal MHD can occur, while the imposed boundary values ofv⊥determine mainly the detailed flow pattern inside the layers. Thus, in general,E(and thereforev⊥) becomes much larger than its boundary values at locations where the gradients of the inverse mapping function are large. An example is given of a sheared X field, where a slow smooth continuous shear flow imposed on the boundary across one quasi‐separatrix produces a flipping of magnetic field lines as they slip rapidly through the plasma in the other quasi‐separatrix layer. It results in a strong plasma jetting localized in, and parallel to

ISSN:0148-0227    

DOI:10.1029/95JA02740

年代:1995

数据来源: WILEY

摘要:

In this study, wavelet transforms and neural networks are used to analyze the formation and evolution of steepened magnetosonic waves (shocklets) generated in hybrid (fluid electrons, particle ions) simulations. These waves model the shocklets observed upstream of planetary bow shocks and at comets. Specifically, the usefulness of wavelet transforms and neural networks for understanding the nature of the steepening process, and the further evolution of shocklets into less coherent structures is investigated. Previous studies had suggested that the nonlinear steepening process is associated with the excitation of higher frequency waves within the original wave. This hypothesis, however, could not be directly substantiated using Fourier transforms. In order to continue with the analysis of shocklets, it has become necessary to implement new techniques using wavelet transforms and neural networks. Wavelet transforms are tailored to the analysis of localized structures such as shocklets, while neural networks have the ability to model nonlinear dynamical systems. Application of wavelet transforms has verified the presence of higher frequency waves within the steepening shocklet and has identified them as the forward propagating and the backward propagating magnetosonic waves as well as the backward propagating Alfvén ion‐cyclotron mode. The wavelet transform has also located the source of the whistler wave packet attached to the shocklet to be the region of steep magnetic field gradient. In order to understand the further evolution of shocklets, neural networks have been applied in the analysis. Used in conjunction with a translation invariant transform, neural networks have been successfully trained to identify shocklets. This allows for the scanning of large data sets as well as the development of a classification system for shocklets. A multinetwork classification system using various techniques, including wavelet preprocessing, has been developed to analyze the further evolution of shocklets and their components. Identification and classification of neural networks have increased our understanding of shocklet evolution. The techniques involving wavelet transforms and neural networks that have been employed in this study show considerable potential for the study of not only shocklets, but also other wave phenomena in space plasm

ISSN:0148-0227    

DOI:10.1029/95JA01207

年代:1995

数据来源: WILEY

摘要:

The interaction of upstream pressure pulses with the Earth's bow shock and their propagation through the magnetosheath are studied by means of one‐dimensional hybrid (particle ions, massless fluid electrons) simulations. The pulses either are generated self‐consistently as a result of the presence of ions streaming away from the quasi‐parallel shock or are created artificially through density or velocity perturbations in the solar wind some distance upstream of the shock so that interaction of pressure pulses with both the quasi‐parallel and quasi‐perpendicular portion of the bow shock can be investigated. We demonstrate that the pulses tend to perturb the shock structure only for a limited time and then propagate in a coherent manner at the magnetosonic speed for large distances (>5RE) downstream of the shock. Implications for the intersection of such pulses with the magnetopause are also

ISSN:0148-0227    

DOI:10.1029/95JA02856

年代:1995

数据来源: WILEY

摘要:

The highly structured magnetic field and plasma properties observed in the heliospheric extension of the coronal streamer belt have been interpreted as evidence for multiple current sheets originating at coronal helmet streamers. We explore the linear stability of a simple case: a triple current sheet, as would exist above two neighboring helmets of the same polarity. The behavior of the triple current sheet when perturbed by small disturbances can be described (in the incompressible limit) by MHD Orr‐Sommerfeld and Squire equations, which we solve with a Chebyshev‐τ method. We show the velocity and magnetic fields which characterize the three unstable modes and describe the modal dependence on fieldwise wavenumber and current sheet separation. At long wavelengths an unexpected phenomenon occurs: two modes degenerate into unstable traveling modes. We also explore the three‐dimensional behavior and the modal variation with both large and small values of the resistivity and viscosity. We conclude that the magnetic topology in closely packed streamers is susceptible to instabilities with growth times of the order of hours. Our predictions indicate that the resultant plasmoid structures should be observable with the large angle and spectrometric coronagraph (LASCO) and ultraviolet coronagraph spectrometer (UVCS) instruments on the upcoming Solar and Heliospheric Observatory (SOHO) m

ISSN:0148-0227    

DOI:10.1029/95JA02496

年代:1995

数据来源: WILEY