摘要:

The changes in the cosmic ray intensity>70 MeV/nucleon observed by Voyager 1 (hereinafter called V1) near 30° heliographic latitude and by Voyager 2 (hereinafter called V2) near 0° heliographic latitude from 1986 to 1990 were closely related to the large‐scale fluctuations in the magnetic field strength at V1 and V2, respectively. The decreases in the cosmic ray intensity observed by V1 and V2 from 1986 to 1990 were generally related to merged interaction regions (MIRs). An exception to this relation was observed from about day 45 to day 110, 1987 at V1. The MIRs are of three general types: “global MIRs” (GMIRs), which extend around the Sun and to high latitudes, whose effects persist over several solar rotations; “corotating MIRs” (CMIRs), which are quasi‐periodic structures generated by the interaction of corotating streams lasting several solar rotations; and “local MIRs” (LMIRs), which are nonperiodic and have limited latitudinal and/or longitudinal extent, whose effects last less than one solar rotation. The cosmic ray intensity shows a net increase with time when the MIRs are weak and the magnetic field strength is relatively low, for example, from ∼day 120 to ∼day 45, 1987 at V1 and from ∼120, 1986 to ∼195, 1987 at V2. The cosmic ray intensity fluctuates about a constant level when CMIRs and rarefaction regions are dominant. This is the case from ∼230, 1987, to ∼275, 1988, at V1 and during some intervals from ∼245, 1987, to ∼60, 1989, at V2. The cosmic ray intensity decreases locally during the passage of a LMIR. Such a local decrease was observed at both V1 and V2 near day 100, 1986. Local decreases in the cosmic ray intensity were observed at V2 but not V1 beginning near days 240 and 320, 1987, and near day 275, 1988. A global, broad, steplike change in the cosmic ray intensity was observed at all of the spacecraft during the passage of a GMIR, beginning approximately near day 125, 1989, at V1 and near day 65, 1989, at V2 and extending for approximat

ISSN:0148-0227    

DOI:10.1029/92JA01979

年代:1993

数据来源: WILEY

摘要:

MeV/amu ion enhancements associated with corotating high‐speed solar wind streams in 1978–1986 during pre‐solar maximum to near solar minimum conditions are studied using ISEE 3/ICE, IMP 8, and Helios 1 data. Around 50% of corotating streams contain energetic ion increases. These increases extend to ∼25 MeV/amu, where they merge into the galactic cosmic ray background, and are most evident approaching solar minimum. Sunward ion streaming in the solar wind frame (first‐order anisotropy ∼20%) and positive radial intensity gradients (∼400%/AU) are consistent with acceleration in the outer heliosphere at corotating shocks followed by streaming into the inner heliosphere. The spectra and intensities show little solar cycle variation. The spectra of ions from protons to Fe at ∼2–20 MeV/amu are approximated equally well by exponentials in momentumdJ/dP≈ exp (−P/P0),P0= 11–16 MeVc−1amu−1, or by distribution functions ƒ ≈ exp (−υ/υ0), υ0= 0.18–0.25 (MeV/amu)1/2, with equivalent power law in energy slopes in the range ∼ −3 to −4. Ion abundances are correlated with the stream peak solar wind speed. In slower corotating streams (maximum solar wind speed<600 km/s), mean abundance ratios are protons/4He = 43 ± 18;4He/O = 54 ± 23; C/O = 0.62 ± 0.06; Mg/O = 0.19 ± 0.03, and Fe/O = 0.14 ± 0.02. These show some similarity to the corresponding ratios for “solar energetic particles” (SEP) (protons/4He = 70 ± 10;4He/O = 55 ± 3; C/O = 0.48 ± 0.02; Mg/O = 0.21 ± 0.01 and Fe/O = 0.16 ± 0.02) which are typically accelerated by shocks passing through slow solar wind. In corotating events in higher‐speed streams, these ratios become protons/4He = 19 ± 5;4He/O = 130 ± 35; C/O = 0.89 ± 0.05; Mg/O = 0.14 ± 0.01, and Fe/O = 0.10 ± 0.01 and more closely resemble the corotating event abundance ratios measured in high‐speed streams during the mid‐1970s solar minimum (protons/4He = 17 ± 7;4He/O ∼ 160 ± 50; C/O = 0.89 ± 0.1; Mg/O = 0.13 ± 0.03, and Fe/O = 0.096 ± 0.05). Solar wind plasma may also show similar variations in composition with solar wind speed (based on the limited solar wind composition measurements available) so that the energetic ion compositions are consistent with the acceleration of corotating event ions and SEPs from the solar wind. The ordering of corotating event and solar wind abundances by first ionization potential and their variation with solar wind speed suggest that conditions in the ion‐neutral fractionation region in the upper chromosphere determine the abun

ISSN:0148-0227    

DOI:10.1029/92JA01837

年代:1993

数据来源: WILEY

摘要:

We consider a system in which a planar shock propagates into a plasma where the magnetic field consists of a uniform background component that is nearly parallel to the shock surface (i.e., the shock is nearly perpendicular) plus a random component that is transverse to the background field. The random component is a superposition of Alfvén waves with a wide range of wavelengths and with amplitudes determined from a power spectral density function that is flat at long wavelengths and a power law at short wavelengths. We investigate the characteristics of superthermal ion (proton) distributions accelerated in this system by integrating along particle orbits. The main difference between results reported herein and those reported previously by us is the major role played by magnetic loops in mediating multiple shock interactions. The transverse magnetic fluctuations are composed of short wavelength components that scatter the ions in pitch angle, and long wavelength components that produce a large‐scale spatial meandering of magnetic field lines. At nearly perpendicular shocks, this meandering results in a distribution of transient magnetic loops of various sizes at the shock. Charged particles are guided by the large‐scale spatial variations and can interact with the shock at many points, becoming temporarily trapped along several such loops, and also being scattered by resonant waves during and between shock interactions. A few particles can acquire large energy gains before the field lines along which they propagate convect through the shock. As a specific application of the model, we investigate the acceleration of a monoenergetic seed proton population for both impulsive and continuous time injection at a shock that is on average perpendicular. In the model the typical energy density of the magnetic fluctuations is 10–20% that of the background field. Characteristic features of the accelerated proton distributions are (1) energy spectra with power law tails extending to at least 1000 times the injection energy, with spectral slopes somewhat steeper than those predicted, for example, by diffusive shock acceleration theory; (2) upstream intensity precursors that grow exponentially towards the shock, withe‐folding scales that increase with energy up to a maximum scale of the order of the spatial scale of the meandering field lines; (3) upstream angular distributions with large field‐aligned bidirectional anisotropies throughout the intensity precursor; (4) downstream angular distributions with weaker anisotropies peaked transverse to the magnetic field within an intensity plateau extending from the shock to the convection boundary, and field‐aligned bidirectional distributions beyond the convection boundary. We develop a simplified model that explains the salient features of the upstream precursors in terms of the statistical properties of the long‐wavelength components that comprise the random field and suggest how electron acceleration can be accommodate

ISSN:0148-0227    

DOI:10.1029/92JA01841

年代:1993

数据来源: WILEY

摘要:

The re‐formation process of more oblique quasi‐parallel shocks is investigated using one‐dimensional hybrid simulations. Several types of simulations have been performed. The simulation of a shock with a magnetic field‐shock normal angle of 30° shows that a more oblique quasi‐parallel shock exhibits reformation cycles with a larger length scale, that is of about 20 ion inertial lengths. This is considerably larger than the distance specularly reflected ions are able to propagate upstream before they are deflected so that their velocity in the shock normal direction is close to zero. These cycles are due to steepening and growth of upstream waves into pulsationlike structures when they are convected into the region of strongly increasing diffuse ion density immediately upstream of the shock. When the steepening wave packet crashes into the shock, the shock ramp dispersively radiates whistler waves into the region between the shock ramp and the approaching wave, while the steepening of the pulsation leads to phase standing whistler waves on the upstream side. Entropy production occurs either at the shock ramp or at the upstream edge of the pulsation when the steepening process has produced a large kink in the magnetic field and is due to nonadiabatic motion of the incident solar wind ions. In order to analyze the wave steepening, upstream waves have been isolated, and their subsequent interaction with a hot, tenuous ion beam representing the diffuse backstreaming ions has been studied. When an upstream wave is convected into or a region with increasing hot beam density, the wave steepens and becomes a pulsationlike wave packet. In order for the wave to grow to a pulsationlike structure the characteristic scale length of the density increase has to be of the same order as the wavelength of the original magnetosonic wave. Similar results are obtained when counterstreaming beam of hot ions is injected into a solar wind which does not initially contain a wave field. In this case the polarization of the pulsations depends on the hot beam temperature. The strong density increase of hot beam ions in these simulations is due to the steady injection of beam ions in a solar wind with an embedded field which is inclined relative to the solar wind direction. In the shock simulation the shock itself is the steady source of the hot backstreaming ions. These simulations suggest that upstream waves, shocklets, and short large‐amplitude magnetic structures are all the same entity in different stages of their development and play a crucial role in re‐forming oblique quasi

ISSN:0148-0227    

DOI:10.1029/92JA01875

年代:1993

数据来源: WILEY

摘要:

During solar cycle 20, the first full cycle with measurements of solar wind parameters, geomagnetic activity measured byApwas found to correlate with the square of solar wind speedV, and activity measured byDstwas found to correlate with the product ofVand the southward component of the interplanetary magnetic field,Bs. Both of these correlations break down during cycle 21. In the case ofAp, the much stronger variation ofBsin cycle 21 compared to cycle 20 makes clear that theBscontribution to activity is important on yearly as well as shorter time scales. The productBsV² gives an excellent correlation withApover both cycles. In the case ofDst, the stronger variation ofBsin cycle 21 causes a stronger variation inBsV, which is not reflected inDst, perhaps becauseDstalso depends upon solar wind dynamic pressure in a nonlinear way

ISSN:0148-0227    

DOI:10.1029/92JA01978

年代:1993

数据来源: WILEY

摘要:

The relationships between the plasma parameters downstream and upstream of plane wave rotational discontinuities (RDs) are reanalyzed for the case of an isotropic plasma. The analysis is carried out in the rest frame of the RD using a Cartesian coordinate system with axes labeledn, j,andz. TheNJZsystem is oriented in such a way that thenaxis is parallel to the RD surface normal vector and thejcomponent ofBis the same on both sides of the RD. In thisNJZsystem the magnitude of each of the downstream components ofB, U,andEis equal to the magnitude of the respective upstream components, and the sign ofBz, Uz,andEnchanges across the discontinuity. In theNJZsystem the angle through which the tangential component of the magnetic field (Bt) rotates is not arbitrary. It depends explicitly on the angle between the upstreamBtand thejaxis. The above jump relations can be used to simplify initial conditions used in theoretical and simulation studies of RD structure. The jump relations can also be used, in principle, to provide an additional method for determining the direction of the RD surface normal vector from spacecraft data.

ISSN:0148-0227    

DOI:10.1029/92JA02344

年代:1993

数据来源: WILEY

摘要:

Particle observations from the dayside magnetosphere of Jupiter have shown that the plasma subcorotales beyond a radial distance of ≈20RJ. However, no information is available on the azimuthal velocity of plasma on the nightside of Jupiter owing to the unfavorable viewing geometries of the Voyager spacecraft. In this paper, we follow the approach of Vasyliunas (1983) to calculate the torque applied by the magnetic field on the plasma and calculate the resulting plasma angular velocity for various assumed outflow rates. Thus for a given value of the outflow rate we can calculate the sector‐averaged angular velocity of plasma from the observed values of the azimuthal component of the magnetic field (Bϕ) in the lobes and the normal component (Bz) in the plasma sheet. We develop new techniques to determine the normal and azimuthal components of the magnetic field in a plasma sheet coordinate system. From the Voyager 2 data, it appears that the corotation in the postmidnight quadrant of the magnetosphere can be maintained up to a radial distance of ≈50RJif the outflow rate in that quadrant does not exceed 2.5×1029amu/s. For outflow rates exceeding 2.5×1029amu/s in that quadrant partial corotation would result. Our conclusions differ from those of Vasyliunas because we have reanalyzed the Voyager data and have improved the estimates ofBϕandBz. A comparison of results from Voyager 1 and 2 is

ISSN:0148-0227    

DOI:10.1029/92JA01890

年代:1993

数据来源: WILEY

摘要:

The relationship of geomagnetic substorms and plasmoids is examined by determining the correlation of the 366 plasmoids identified by Moldwin and Hughes (1992) with ground auroral zone magnetograms and geosynchronous particle data signatures of substorm onsets. Over 84% of the plasmoid events occurred between 5 and 60 min after a substorm onset. We also find near one‐to‐one correlation between large isolated substorm signatures in the near‐Earth region and signatures consistent with a passing plasmoid in the distant tail (i.e., a traveling compression region, or an actual plasmoid observation). However, there does not appear to be an absolute correspondence of every substorm onset to a plasmoid signature in the deep tail especially for periods of prolonged disturbance that have multiple substorm onsets. A correlation of interplanetary magnetic fieldBzsouth with plasmoid observations was also found. The locations of the near‐ and far‐Earth reconnection sites are estimated using the time of flight of the plasmoids from substorm onset to their observation at ISEE 3. The estimates of the near‐ and far‐Earth reconnection sites are highly variable and range fro

ISSN:0148-0227    

DOI:10.1029/92JA02153

年代:1993

数据来源: WILEY

摘要:

The dynamics of charged particles inx‐dependent magnetotail models is examined, wherexis along the Sun‐Earth direction. An earlier paper showed that particle motion in a class ofx‐dependent Harris‐like equilibrium models can be significantly different from the motion inx‐independent magnetotail field models. In the present paper, it is shown that these Harris‐like equilibrium models have “bulb‐shaped” field lines and that this property leads to the reported differences. Furthermore, it is shown that the scale length of variation of the Harris‐like models in thexdirection,Lx, is comparable to the typical excursion distance, Δ, of particles in thexdirection and to ρnwhich is the gyroradius based on the magnetic field componentBnnormal to the plane of the current sheet. It is argued that neither bulb‐shaped field lines nor scale lengthsLxcomparable to Δ or ρnapply to the Earth's magnetotail. It is suggested that a key criterion for applicability ofx‐dependent models to the terrestr

ISSN:0148-0227    

DOI:10.1029/92JA01528

年代:1993

数据来源: WILEY

摘要:

During 1978 and 1979, the medium energy particles experiment on board the ISEE 2 spacecraft (apogee 22.7RE) collected almost 122 days of data in the central region of the Earth's magnetotail. We have surveyed 16‐s averages of the energetic ion (E>25 keV) and plasma measurements in order to establish the statistical properties of high ion anisotropy events (|A|>1) which are frequently observed in the near‐Earth plasma sheet. Our major findings from the analysis of 1944 high‐anisotropy samples (HAS) are as follows: (1) Strong energetic ion anisotropies are not maintained over long periods of time but occur in bursts lasting for fractions of a minute up to a few minutes. (2) For almost 80% of the HAS, the anisotropy vector points either earthward or tailward rather than across the tail (3) The ratio of HAS toward the Earth and HAS toward the tail is approximately 4.8:1. (4) The probability of observing high‐anisotropy events is well enhanced beyond ≈16REdowntail distance, on the duskside, and close (within 2–3RE) to the neutral sheet. (5) Strong tailward anisotropies can be observed anywhere in the near‐Earth plasma sheet. However, their occurrence frequency is considerably above average beyondxSE≈ −19REand within 2REof the neutral sheet. (6) Both plasma bulk flow and ion anisotropy at higher energy (>115 keV) are generally in the same direction as the energetic ion anisotropy atE>25 keV. (7) Streaming energetic electron populations in the same direction as the ions are significantly more often observed when the ions are streaming tailward than simultaneously with earthward streaming ions when the electrons are usually isotropic. We conclude that statistically the observation of strong tailward ion streaming indicates the existence of a particle source in the near‐Earth magnetotail between the spacecraft and the Earth. Certain features suggest that this source might be identical with a near‐Earth

ISSN:0148-0227    

DOI:10.1029/92JA01656

年代:1993

数据来源: WILEY