Interplanetary Discontinuities Research Articles

Rapid evolution of magnetic decreases (MDs) and discontinuities in the solar wind: ACE and Cluster

An experiment was conducted to identify and measure the same magnetic decreases (MDs) and interplanetary discontinuities at two different points in space separated by ∼0.01 AU along the radial distance from the sun. The ACE and Cluster satellite data were used in this study. Solar wind speeds measured at Cluster were used to calculate the approximate (earlier) times of arrival at ACE. The field component changes at both spacecraft were intercompared to ensure that the same discontinuities were detected. Out of 7 MDs/discontinuities selected, 6 were identified at ACE. The MDs/discontinuities were identified within 7 s to 35 s of the convection times. The MD thicknesses at Cluster were substantially smaller than at ACE. The ratio of thicknesses varied from 0.21 to 0.045. One possible interpretation of this observation is that the Alfvén wave front (rotational discontinuity) is steepening by a factor of greater than 15 per wavelength propagated. This may have important consequences for the generation of high frequency turbulence in interplanetary space. It is noted that the 6 discontinuities transited the ∼0.01 AU distance from ACE to Cluster with essentially the solar wind convection speed. A possible mechanism contributing to this feature is discussed.

Four‐point discontinuity observations using Cluster magnetic field data: A statistical survey

Using magnetic field data from the four Cluster spacecraft between February and May 2001, a detailed statistical analysis of interplanetary discontinuities is accomplished. In order to find the surface normals, we apply three different methods: (1) minimum variance analysis (MVA), (2) cross‐product method, and (3) triangulation, the latter being the only one that needs information from all four spacecraft. Whereas the cross‐product normals are always almost the same at the position of the four spacecraft, strong deviations between the MVA normals are observed. The cross‐product normals also agree fairly well with the triangulation normals, which mostly point in a direction approximately perpendicular to the magnetic field. The scatter of the MVA normals around the triangulation normal can be reduced by triggering the parameters controlling the MVA accuracy, i.e., λ2/λ3L, the lower limit of the MVA eigenvalue ratio, and ωL, the lower limit of the spreading angle. Our analysis allows to compare the impact of these two parameters on the MVA normals. The error analysis shows that both parameters have strong impact on the MVA accuracy; however, λ2/λ3L is more important. In order to ensure reliable normal estimates, MVA should only be applied if ωL > 60°. The MVA error analysis also enables us to provide a new criterion to distinguish rotational discontinuities (RDs) from tangential discontinuities. However, the triangulation results show that there are no clearly identified RDs in our data set. We will discuss implications for the theory of solar wind discontinuity generation processes. A practical consequence is that the cross‐product method presumably yields better normal estimates than MVA when Bn is small. The colinerarity of the cross‐product normals at the positions of the four spacecraft demonstrate that the discontinuities are one‐dimensional structures on the Cluster separation scale.

Cluster observations in the magnetotail during sudden and quasiperiodic solar wind variations

A clear bipolar (negative/positive) signature in the Ey component was observed by three spacecraft on Cluster in the magnetotail during the passage of a solar wind discontinuity on October 11, 2001 (day 284), which caused a sudden commencement (sc) on the ground. The positive Ey perturbation was accompanied by the northward/dawnward plasma flow and the Bx enhancement, which is the dominant magnetic field component. The estimated Ey from the plasma flow and magnetic field was in good agreement with the observed Ey. Thus the positive Ey perturbation can be interpreted to be the signature of inward plasma motions, corresponding to the compression of the magnetopause. During the interval of the negative Ey perturbation, the magnetic field increase was mainly due to the increase in By and Bz, not in Bx. Unlike the positive Ey perturbation, the negative Ey perturbation was not consistent with the estimated Ey. Therefore the negative Ey signature does not seems to have a direct connection with plasma motions. The observed field and flow variations may be associated with a deformation of the magnetotail due to the solar wind discontinuity moving tailward. The direction and shape of the sc front propagating tailward can be confirmed by three spacecraft of Cluster. We also observed quasiperiodic geomagnetic perturbations at the low‐latitude ground station Kakioka (L = 1.25) following the sc event. They were highly correlated with the magnetic field perturbations at Cluster in the magnetotail (Xgse = ∼12 RE). We show that the source of these perturbations is the quasiperiodic solar wind variations superposed on the increased solar wind pressure behind the interplanetary discontinuity.

Interplanetary magnetic field variations and slow mode transitions in the Earth's magnetosheath

The event observed on September 17, 1978 on ISEE 1–2, which led to the concept of a stationary slow mode transition region (SMT) in the magnetosheath in front of the magnetopause, is revisited. We establish that the two edges of this SMT have an exogenous origin induced by two discontinuities of the interplanetary magnetic field. The key of our analysis is that the outer edge of the SMT is built up by a tangential interplanetary discontinuity which is observed on ISEE‐3 at a large distance from the Sun‐Earth line and which has an unusual direction. In this SMT the subsolar magnetosheath is entirely downstream of a quasi‐parallel bow shock, while upstream this SMT the subsolar magnetosheath is downstream of a quasi‐perpendicular shock. We identify three effects at the origin of the density enhancement in this SMT. We extend this approach to the original statistical study and we find that any SMT is connected to interplanetary magnetic field variations. This corroborates our hypothesis that SMTs have an exogeneous origin driven by interplanetary magnetic field variations.

Hot diamagnetic cavities upstream of the Martian bow shock

We present Mars Global Surveyor (MGS) observations of hot diamagnetic cavities upstream of the Martian bow shock. The events are characterized by a region of turbulent magnetic field, high electron temperature, and densities at or below the ambient solar wind density. The region of high temperature is flanked by layers of high magnetic field and electron density and is accompanied by a rotation in the interplanetary magnetic field (IMF). We suggest that the events are generated by interplanetary discontinuities interacting with the Martian bow shock, i.e., the events are the Martian counterpart to the hot flow anomalies observed upstream of the Earth's bow shock.

Hybrid Simulation of Space Plasmas: Models with Massless Fluid Representation of Electrons. III. Shockless Discontinuities

This is the third article in a series of reviews on hybrid simulation of low-frequency processes in space plasmas. It deals with shockless (contact, tangential, and rotational) discontinuities. A hybrid model is described with ions represented by particles and electrons by a massless fluid. The Hamiltonian scheme for the numerical implementation of this model is discussed in detail. The first part of the article provides basic background information, which covers MHD models (ideal, resistive, and Hall models) as well as a classification of shockless discontinuities and their main properties. The review part of the article surveys the literature on simulation of the structure and properties of nonpropagating (contact and tangential) discontinuities and the structure of the magnetopause as a complex domain consisting of multiple discontinuities and waves. We also review the literature on hybrid simulation of rotational discontinuities and the structure of the reconnection layer—the configuration of the reconnecting magnetic forcelines in the dayside magnetopause and in the distant magnetotail. The concluding sections examine studies on hybrid simulation of the penetration of plasma inhomogeneities from the solar wind into the magnetosphere. The following issues are considered: the interaction of the bow shock with interplanetary discontinuities; the "hot flow anomaly"; the interaction of pressure pulses with the bow shock; impulsive plasma penetration through a tangential discontinuity (the magnetopause) as a result of a "collision" of a magnetosheath plasmoid (plasma current sheet) with the magnetopause.

Short‐duration convection bays and localized interplanetary magnetic field structures on November 28, 1995

We present ground‐based, plasma sheet, and magnetosheath observations of two subsequent short‐duration (10–20 min) increases of the postmidnight westward electrojet on November 28, 1995. Appearing as though small (150–200 nT) substorms, they were not accompanied by any substorm expansion onset signatures. Auroral breakup, worldwide Pi2 pulsations, and the corresponding plasma sheet activity, such as fast flows, current disruption, and plasmoid generation, were all observed only at the recovery of the second electrojet increase. These convection bays were associated with the equatorward expansion of the auroras and simultaneous magnetic variations in the polar cap and middle latitudes. Growth phase signatures of the lobe field increase and tailward stretching of magnetic field were also observed in the plasma sheet. Bursty bulk flows in the plasma sheet seem to be quenched at the onset of first convection bay and did not resume until the auroral breakup which concluded the second convection bay. A point of interest of this event was the “incomplete” convection/current system with a well‐developed dawn vortex in the absence of well‐defined dusk vortex; instead, a complicated transient activity dominated over the afternoon‐dusk local time sector. We interpret this asymmetry either in terms of the magnetopause encounter with the edge of the solar wind driver, i.e., strong southward IMF, which hits only the dawn part of the magnetosphere, or with an extremely slant interplanetary discontinuity. This unique configuration was inferred from observations of uncorrelated strong southward Bz events by the Wind and IMP 8 spacecraft in the dusk and dawn magnetosheath, respectively, as well as from the directional analysis of the interplanetary discontinuities which form the edges of these structures. We suggest that interaction of the magnetosphere with very slant solar wind discontinuities may bring various specific features to magnetospheric and ionospheric dynamics that have not been reported.

Generation of anomalous flows near the bow shock by its interaction with interplanetary discontinuities

Two‐dimensional hybrid simulations using a curvilinear coordinate system are carried out to study the interaction of the Earth's bow shock (BS) with an interplanetary directional discontinuity. In particular, plasma flow patterns are examined. In the interaction of the bow shock with an interplanetary tangential discontinuity (TD), a bulge of magnetic field and plasma may be present near the intersection between the fronts of the BS and the TD. The bulge expands to the upstream and is embedded in the solar wind. High magnetic field and ion density are present in the boundary regions of the bulge, and the temperature in the bulge is significantly higher than that in the ambient solar wind. A core of low density and sometimes low field is present inside the bulge. The flow speed changes from supersonic in the solar wind to subsonic throughout the bulge. A strong sunward deflection in flow velocity is present both in the bulge and in the magnetosheath behind the bulge. The presence of such hot anomalous flows depends on the direction and symmetry condition of the upstream motional electric field. The formation of the bulge and the associated anomalous flows is found to be mainly due to (1) the reflected ions which are focused to TD under the proper electric field conditions, (2) the unbalanced pressure associated with the geometry change and reformation of the BS which causes the expansion of downstream and the sunward motion of ions, or (3) the occurrence of magnetic reconnection in the current sheet. In the interaction of the BS with an interplanetary rotational discontinuity (RD), the flow may be deflected sunward by the magnetic tension force associated with the resulting rotational discontinuities and slow shocks in the magnetosheath. It is suggested that the observed anomalous flow events upstream of the bow shock may be due to the BS/TD interaction, and both BS/RD and BS/TD interactions may generate strong sunward flow deflections in the magnetosheath.

Interplanetary discontinuities and Alfvén waves at high heliographic latitudes: Ulysses

This paper presents the results of the first statistical study of interplanetary directional discontinuities at both low and high heliographic latitudes measured by the Ulysses magnetometer. There is a gradual decrease in the rate of occurrence of interplanetary discontinuities (ROIDs) with increasing radial distance. From 1 to 5 AU, an e−(r−1)/5 dependence is derived. Much of this decrease is believed to be an artifact due to the discontinuity thickening with decreasing |B|, falling outside the detection criteria. As Ulysses goes from the ecliptic plane to high (−80°) heliographic latitudes, the ROID value increases dramatically. The increase is about a factor of 5 as Ulysses moves from Jupiter at 5 AU to 2.5 AU over the south pole. There is a one‐to‐one correspondence between high ROID values and high‐speed streams. This is particularly dramatic just after the Jovian encounter when there are ∼25.4‐day period corotating streams present. Thus the increase with latitude is primarily due to Ulysses spending an increasing percentage of time within a high‐speed stream emanating from the solar coronal hole. High‐speed streams are characterized by the presence of nonlinear Alfvén waves with peak‐to‐peak transverse fluctuations as large as of 1 to 2. Over the south pole, the normalized transverse wave power can be characterized by P = 2.5 × 10−4ƒ−1.6 Hz−1 and the compressional power 1.8 × 10−4ƒ−1.2 Hz−1 for frequencies between 10−5 and 10−2 Hz. The normalized wave power spectra in different regions of the polar coronal hole streams, from midlatitudes to high heliographic latitudes, appear to be quite similar. The wave power in the ecliptic plane is somewhat lower, perhaps due to contamination from low‐speed streams. The Alfvén waves in the high‐speed stream are found to be propagating outward from the Sun, even at these large heliocentric distances (2.5–5.0 AU). The waves typically have arclike polarizations and conserve field magnitude to first order. Directional (rotational) discontinuities often form the edges of the phase‐steepened Alfvén waves, thus offering a natural explanation for the high ROID rates within high‐speed streams.

Interplanetary discontinuities and Alfv�n waves

The rate of occurrence of interplanetary discontinuities (ROID) is examined using Ulysses magnetic field and plasma data from 1 to 5 AU radial distance from the Sun and at high heliospheric latitudes. It is found that there are two regions in interplanetary space where the ROID is high: in stream-stream interaction regions and in Alfven wave trains. This latter feature is particularly obvious at high heliographic latitudes when Ulysses enters a high speed stream associated with a polar coronal hole. These streams are characterized by the presence of continuous, large-amplitude (\(\Delta \bar B/l BI - 1 - 2\),) Alfven waves and an extraordinarily high ROID value (∼150 discontinuities/day). In a number of intervals examined, it is found that (rotational) discontinuities are an integral part of the Alfven wave: they represent ∼ 90° phase rotation of the wave out of the full 360° rotation of the wave. These large amplitude nonlinear Alfven waves thus appear to be phase steepened. The nonlinear Alfven waves are spherically polarized, i.e., the tip of the perturbation vector resides on the surface of a sphere (a consequence of constant |B|). The best description of this wave plus discontinuity is a “spherical arc polarization”.

Ulysses field and plasma observations of magnetic holes in the solar wind and their relation to mirror‐mode structures

The term “magnetic hole” has been used to denote isolated intervals when the magnitude of the interplanetary magnetic field drops to a few tenths, or less, of its ambient value for a time that corresponds to a linear dimension of tens to a few hundreds of proton gyro‐radii. Data obtained by the Ulysses magnetometer and solar wind analyzer have been combined to study the properties of such magnetic holes in the solar wind between 1 AU and 5.4 AU and to 23° south latitude. In order to avoid confusion with decreases in field strength at interplanetary discontinuities, the study has focused on linear holes across which the field direction changed by less than 5°. The holes occurred preferentially, but not without exception, in the interaction regions on the leading edges of high‐speed solar wind streams. Although the plasma surrounding the holes was generally stable against the mirror instability, there are indications that the holes may have been remnants of mirror‐mode structures created upstream of the points of observation. Those indications include the following: (1) For the few holes for which proton or alpha‐particle pressure could be measured inside the hole, the ion thermal pressure was always greater than in the plasma adjacent to the holes. (2) The plasma surrounding many of the holes was marginally stable for the mirror mode, while the plasma environment of all the holes was significantly closer to mirror instability than was the average solar wind. (3) The plasma containing trains of closely spaced holes was closer to mirror instability than was the plasma containing isolated holes. (4) The near‐hole plasma had much higher ion β (ratio of thermal to magnetic pressure) than did the average solar wind. (5) Near the holes, T⊥/T∥ tended to be either >1 or larger than in the average wind. (6) The proton and alpha‐particle distribution functions measured inside the holes occasionally exhibited the flattened phase‐space‐density contours in ν⊥‐ν∥ space found in some numerical simulations of the mirror instability.

The relationship between interplanetary discontinuities and Alfvén waves: Ulysses observations

The rate of occurrence of interplanetary discontinuities (ROID) is examined using Ulysses magnetic field and plasma data from 1 to 5 AU radial distance from the Sun and at high heliographic latitudes. We find two regions where the ROID is high: in stream‐stream interaction regions and in Alfvén wave trains. This latter feature is particularly obvious at high latitudes when Ulysses enters a high speed stream associated with a polar coronal hole. These streams are characterized by the presence of continuous, large‐amplitude Alfvén waves and an extraordinarily high ROID value (∼150 discontinuities/day). In a number of intervals examined, it is found that (rotational) discontinuities are an integral part of the Alfvén waves. The nonlinear Alfvén waves are spherically polarized, i.e., the tip of the perturbation vector resides on the surface of a sphere (a consequence of constant |B|). The slowly rotating part of the wave rotates ∼270° in phase. There is a slight arc in the B1‐B2 hodogram, suggesting an almost linear polarization. The phase rotation associated with the discontinuity is ∼90°, lies in the same plane as the slowly rotating part of the Alfvén wave, and therefore completes the 360° phase rotation. The best description of the overall Alfvén wave plus discontinuity is a spherical, arc‐polarized, phase‐steepened wave.

Low‐frequency plasma waves and ion pitch angle scattering at large distances (>3.5 × 105 km) from Giacobini‐Zinner: Interplanetary magnetic field α dependences

The properties of 50‐ to 100‐s low‐frequency plasma waves and the parent 65‐ to 95‐keV H2O group ion pitch angle distributions are presented for spatial regions distant (3.5‐7.0 × 105 km) from comet Giacobini‐Zinner where the cometary ion densities are low and the solar wind interaction is weak. The presence of interplanetary discontinuities in this interval causes the interplanetary magnetic field angle α to vary over a range from ∼35° to 90°, allowing the possible generation of several new wave modes which are theoretically expected, but as yet undetected. It is determined that when x = 45° ± 20° (Parker spiral orientations), the waves are typically elliptically polarized and propagate at 20°‐50° relative to B0. The wave properties are, however, not limited to the above range. They can exhibit even more extreme polarizations and propagation directions. The waves have periods near the H2O group ion cyclotron period and are left‐hand polarized in the spacecraft frame. In the relatively rare instances when wave steepening is detected, the steepened edge is near the trailing portion of the wave. The wave period, sense of polarization, and steepening are consistent with the properties of waves detected closer to the comet and identify them as being generated by the right‐hand resonant instability. The high degree of ellipticity is in sharp contrast to waves detected closer to the comet, however. The energetic ions are not isotropic in pitch angle (in the plasma frame), in general agreement with recent numerical modeling. When α increases to 70°‐90°, the right‐hand resonant mode is observed to cut off, as theoretical studies indicate. However, the left‐hand cyclotron resonant mode is not detected. Although the detected energetic 65‐ to 95‐keV ion fluxes are enhanced during these intervals (because of the instantaneous pickup by the solar wind), particle pitch angle scattering away from the initial ring distribution is very weak, consistent with the observed absence of resonant cyclotron waves.

Tangential discontinuities in the solar wind: Correlated field and velocity changes and the Kelvin‐Helmholtz instability

Three‐dimensional Helios plasma and field data are used to investigate the relative changes in direction of the velocity and magnetic field vectors across tangential discontinuities, (TDs) in the solar wind at solar distances of 0.29–0.50 AU. It is found for tangential discontinuities with both Δv and ΔB/B large that Δv and ΔB are closely aligned with each other, in agreement with the unexpected results of previous studies of tangential discontinuities observed at 1 AU and beyond. It is shown that this effect probably results from the destruction by the Kelvin‐Helmholtz instability of TDs for which Δv and ΔB are not aligned. The observed decrease in the number of interplanetary discontinuities with increasing solar distance may be associated with the growth of the Kelvin‐Helmholtz instability with decreasing Alfvén speed.

CCE plasma wave observations during the storm of September 4, 5, 1984

Near 0700 on September 4, 1984 a series of interplanetary discontinuities arrived at earth when the AMPTE Charge Composition Explorer (CCE) was near apogee. During the next few hours the spacecraft passed in and out of the magnetosheath. At the magnetopause boundary, the CCE wave instrument detected strong electron plasma oscillations, weaker electromagnetic waves at the electron plasma frequency, and broadband electrostatic waves. During the subsequent perigee passes on September 4 and 5, the wave observations of upper hybrid resonance emissions, continuum radiation, electrostatic noise bands and unusual low latitude auroral kilometic radiation were used to monitor significant variations in the magnetospheric characteristics as the main storm phases developed.

Helium, hydrogen, and oxygen velocities observed on ISEE 3

The velocities of helium, oxygen, and hydrogen ions have been recorded over a full range of solar wind conditions by the ion composition instrument (ICI) and Los Alamos National Laboratory plasma instrument (LANLPI), respectively, aboard the ISEE 3 spacecraft between August 1978 and December 1979. Interspecie velocity differences were observed frequently. For solar wind velocities between 300 and 400 km s−1 the helium velocity exceeded the hydrogen velocity by 5 km s−1 on the average. For solar wind velocities between 400 and 500 km s−1 the average difference was 14 km s−1; however, no evidence was found for a systematic nonzero average velocity difference between helium and oxygen ions even at the higher velocities. Velocity differences were examined in a number of streams and across a number of interplanetary shocks. Helium‐hydrogen velocity differences are generally bounded by the Alfvén speed. Velocity differences show abrupt changes across interplanetary discontinuities, presumably tangential. Differences between the speeds of differently charged minor ions appear also to result from the electrostatic potential differences across the interplanetary shocks. The potential difference, calculated from the energy jump condition for a perpendicular hydromagnetic shock, is of the correct magnitude to produce the observed effects.

Interplanetary discontinuities: Temporal variations and the radial gradient from 1 to 8.5 AU

Interplanetary discontinuities have been investigated at heliocentric distances between 1 and 8.5 AU by using Pioneer 10 and 11 vector helium magnetometer observations. The principal purpose of the study was to investigate a possible dependence of the rate of occurrence and properties of the discontinuities on radial distance. This objective required a separation of spatial and temporal variations and used the simultaneous nearly continuous data from both spacecraft. Discontinuities were identified by using carefully developed criteria that were shown to be comparable to those used in earlier studies but which are still applicable in the weak magnetic fields that are typical of large radial distances. Special attention was given to the identification of relatively thick discontinuities in the expectation that discontinuities might grow progressively thicker with distance. The statistics associated with the rate of occurrence of discontinuities have been shown to be well approximated by a Poisson distribution. The rate of occurrence of discontinuities undergoes large variations, from day to day and from one solar rotation to another, which are well outside the deviations to be expected on the basis of statistical fluctuations alone. Temporal changes in the rate of occurrence averaged over Bartels solar rotations were well correlated at Pioneer 10 and 11, which were separated by a distance of ≃2 AU. The time variations consisted of a slow modulation of the rate of occurrence such that successive increases and decreases persisted for several months at a time, presumably as a result of changing solar conditions. The correlation over widely separated distances is most simply interpreted by a model in which the discontinuities originate inside 1 AU, probably near the sun, and are convected outward by the solar wind. Further support for this model is provided by the statistical properties of the discontinuities at 1 and 5 AU, which were found to be very similar. Clear evidence of a decrease in the rate of occurrence ρ with distance has been obtained. The simultaneous rates from the two spacecraft reveal that this decrease is well approximated on the average by the function ρ=50e−(R−1)/4 and imply a radial gradient of 25% per astronomical unit. This gradient may be apparent and does not necessarily imply that discontinuities actually occur less frequently at large radial distances. The decreased rate of occurrence may be associated with an increasing thickness of the discontinuities such that they no longer satisfy the identification criteria. Possible evidence of a dependence of ρ on heliographic latitude was sought, but no statistically significant dependence was found. The results of this study are inconsistent with previous inferences of a very large radial or latitudinal gradient on the basis of Pioneer 8 data (Mariani et al., 1973). The reason for this earlier result was probably the influence of time variations which occurred on a scale of several months, as was found in our study, and masqueraded as spatial variations.

Magnetic dips in the solar wind

Using magnetic data from the HELIOS-1 fluxgate magnetometer, with a 0.2 s resolution, we have investigated the structure of several interplanetary discontinuities involving magnetic dips and rotations of the magnetic field vector. A minimum variance analysis illustrates the behaviour of the magnetic field through the transition. Using this analysis, quite different structures have been isolated and, in particular, narrow transitions resembling almost one dimensional reconnected neutral sheets. For the thinner cases (scale lengths of the magnetic rotation of the order or smaller than 103 km), we find that the observed structures can be the nonlinear effect of a resistive tearing mode instability having developed on an originally one dimensional neutral sheet at the solar corona.

Bow shock and its interaction with interplanetary shocks

Harbingers of significant magnetospheric motions consist of the interactions of interplanetary discontinuities with the standing bow shock. The most common discontinuity is the tangential discontinuity. Less frequent in occurrence, but of major significance to subsequent magnetospheric dynamics, is the flare‐generated interplanetary shock wave and its initiation of bow shock and magnetopause motion toward Earth. Early studies utilized the ordinary gas‐dynamic analogy of the well‐known Riemann splitting of an initial discontinuity (i.e., bow shock) into a reflected shock (the moving bow shock) and the transmitted shock (the inward‐moving interplanetary shock). It was shown that order‐of‐magnitude pressure increases at the subsolar point of the magnetopause are readily found for typical shock‐on‐shock studies. Phenomenological studies also demonstrated, in agreement with observations, that the magnetopause motion could be predicted on the basis of quasi‐static variation of the solar wind dynamic pressure. Recent hydromagnetic studies (for the simplified case of perpendicular shocks) have extended the theory to predict bow shock and magnetopause velocities which appear to be observed by spacecraft. Such one‐dimensional studies provide upper limits for all average plasma parameters within the region of the Sun‐Earth axis. The general case of the time‐dependent interaction in three dimensions has, as yet, not been done. Nevertheless, the general configurational and plasma details are, in principle, amenable to examination with the use of multiple spacecraft measurements during the interaction and the ensuing dynamic motions on a time scale of tens of min to the several hr required for the magnetopause to move to smaller geocentric distances. The physical processes of various energy transfers from kinetic to thermal and magnetic as predicted by the hydromagnetic theory could then be assessed by study of the plasma average velocity, direction of flow, magnetic field, temperature, and density.The magnetosheath plasma properties following such interactions could, on a more speculative note, provide new boundary conditions (associated with appropriate magnetosheath and magnetosphere magnetic field polarities) which could, in turn, initiate and drive the reconnection process within the magnetopause.

Forbush decreases and interplanetary disturbances

The correlation between perturbations of the interplanetary medium and Forbush decrease events is studied by using interplanetary-magnetic-field and cosmic-ray data for the period March to November 1968, with the purpose of investigating the particular mechanism responsible for this type of modulation of the cosmic-ray intensity. The main results can be summarized as follows: 1) The major and more unequivocal Forbush-like events are associated with interplanetary discontinuities of the type of hydromagnetic shock waves. 2) The decreasing stage of a Forbush decrease takes place entirely while the perturbed region sweeps over the Earth. 3) Irregularities of a scale size comparable to the gyroradius of approximately 10 GeV/c rigidity cosmic-ray particles are enhanced in the perturbed regions. And the Forbush-decrease amplitude seems to be related to the amount of field fluctuations.