Forward-reverse Shock Pairs Research Articles

Observation of a Fully-formed Forward–Reverse Shock Pair due to the Interaction between Two Coronal Mass Ejections at 0.5 au

We report direct observations of a fast magnetosonic forward–reverse shock pair observed by Solar Orbiter on 2022 March 8 at the short heliocentric distance of 0.5 au. The structure, sharing some features with fully-formed stream interaction regions, is due to the interaction between two successive coronal mass ejections (CMEs), never previously observed to give rise to a forward–reverse shock pair. The scenario is supported by remote observations from extreme ultraviolet cameras and coronagraphs, where two candidate eruptions compatible with the in situ signatures have been found. In the interaction region, we find enhanced energetic particle activity, strong nonradial flow deflections, and evidence of magnetic reconnection. At 1 au, well radially aligned Wind observations reveal a complex event, with characteristic observational signatures of both stream interaction region and CME–CME interaction, thus demonstrating the importance of investigating the complex dynamics governing solar eruptive phenomena.

Stream Interaction Regions at High Heliographic Latitudes During Ulysses12/22/2004 6:25PM Second Polar Orbit

Ulysses observed well-defined stream interaction regions, SIRs, associated with solar wind stream structure up to a latitude of S65° and shocks to at least a latitude of S71° during the second polar orbit. These SIRs and shocks produced a substantial heliospheric processing of the solar wind. Only a subset of the SIRs recurred on successive solar rotations and only about half of the well-defined SIRs observed poleward of S9.8° were bounded by forward-reverse shock pairs. The majority of the SIRs had local magnetic topologies and azimuthal orientations similar to, but meridional tilts different from, those observed in the first polar orbit when most SIRs corotated with the Sun. The irregular meridional tilts presumably were a consequence of a complex coronal geometry and the temporally evolving nature of the solar wind flow at this time. A lack of reverse shocks poleward of S54° (with one exception) and a lack of well defined SIRs poleward of S65° is evidence that SIRs develop more slowly with distance at high latitudes.

Solar wind stream interfaces in corotating interaction regions: New SWICS/Ulysses results

We have analyzed data from the solar wind ion composition spectrometer (SWICS)/Ulysses instrument taken between August 1996 and May 1997. In this period the Ulysses spacecraft traveled from 28° to 11° N and encountered a highly regular pattern of high‐speed streams alternating with slow solar wind. Heliocentric distance varied between 4.3 and 5.1 astronomical units (AU). Using proton and alpha‐particle kinetic parameters (density, speed, and kinetic temperatures) as well as charge state and elemental composition data, we identify the stream interfaces in the corotating interaction regions (CIRs) observed in this period. As Wimmer‐Schweingruber et al. [1997] previously reported for a similar period in 1992/1993, stream interfaces are the sites of compositional changes between values typical of the slow solar wind and values typical of high‐speed streams. During that period, Ulysses traveled from 13° to 34° S and from 4.5 to 5.4 AU. In spite of the similarity of the heliospheric regions probed during 1992/1993 and 1996/1997 the corotating interaction regions observed in 1996/1997 are quite different from those observed in 1992/1993. We observe fewer (11) CIRs in 1996/1997 than in the previous period (15); the CIRs are less evolved, i.e., the kinetic signatures at the stream interface are less clear; and mainly, only 3 out of the 11 CIRs are bounded by forward‐reverse shock pairs, whereas in 1992/1993, 13 out of 15 were. This may have important consequences for particle acceleration in CIRs in the inner heliosphere.

Compressible fluctuations in an equatorial pulsar wind and a scenario for wisps in the central Crab nebula

A pulsar wind is expected to be inhomogeneous and intermixed with disturbances originating from the central pulsar in terms of dynamic, electromagnetic and thermal properties. We examine, in this paper, basic properties of two-dimensional compressible perturbations in a magnetized equatorial pulsar wind. Asymptotic solutions at large radii for both fast and slow disturbances in a relativistic magnetohydrodynamic (RMHD) wind are derived analytically. We describe the process of forming RMHD shocks in a pulsar wind with inhomogeneous wind streams emanating from the spinning pulsar and with interlaced, alternately reversed, spiral magnetic field stripes resulting from a pulsar whose rotation and magnetic axes misalign. Spiral bands of forward-reverse shock pairs are expected to appear frequently in a magnetically striped pulsar wind within a certain latitude range about the rotation equator. In highly compressed regions between forward and reverse shocks, magnetized pulsar wind flow could become turbulent so that extremely relativistic particles can be produced in profusion and be injected into the surrounding nebula. Based on our analysis and physical considerations, we propose a scenario for the appearance of wisps in the inner Crab nebula, in which the pattern of inward propagating RMHD fast waves and/or shocks (relative to the pulsar wind) gives rise to the quasi-stationary structure of wisps in the inner Crab nebula, while outward propagating RMHD fast and slow disturbances appear as transients in the sidereal reference frame. The fine wisp region ( ~ 1016 cm) across a strong RMHD reverse shock can squeeze ~ 106_107 magnetic stripes with opposite polarities; magnetic reconnections proceed turbulently therein and lead to dramatic enhancement of relativistic particle acceleration at the expense of flow and magnetic energies continuously supplied from upstream. These processes could also contribute to the production of high-energy cosmic rays.

Interplanetary shock waves: Ulysses observations in and out of the ecliptic plane

Between its launch in October 1990 and the end of 1993, approximately 160 fast collisionless shock waves were observed in the solar wind by the Ulysses space probe. During the in-ecliptic part of the mission, to February 1992, the observed shock waves were first caused mainly by solar transient events following the solar maximum and the reorganisation of the large scale coronal fields. With the decay in solar activity, relatively stable Corotating Interaction Regions (CIRs) were observed betwen 3 and 5.4 AU, each associated with at least one forward-reverse shock pair. During the out-of-ecliptic phase of the orbit, from February 1992 onwards, CIRs and shock pairs associated with them continued to dominate the observations. From July 1992, Ulysses encountered the fast solar wind flow from the newly developed southern polar coronal hole, and from May 1993 remained in the unipolar magnetic region associated with this coronal hole. At latitudes beyond 30°, CIRs were associated almost exclusively with reverse shocks only. A comprehensive list of shock waves identified in the magnetic field and solar wind plasma data from Ulysses is given in Table 1. The principal characteristics were determined mainly from the magnetic field data. General considerations concerning the determination of shock characteristics are outlined in the Introduction.

CME associated forward-reverse shock pairs

For some coronal mass ejections (CMEs), their interaction with the ambient solar wind can produce a forward-reverse shock pair. The high-speed mass ejecta compresses the plasma near the top of the CME on both sides of the tangential discontinuity which separates the CME plasma from the ambient solar wind plasma. The front of the compressed CME plasma propagates in the reverse direction relative to the ejecta flow, it may steepen to from a reverse slow shock. The front of the compressed solar wind plasma also propagates in the forward direction relative to the ambient solar wind and it may steepen to form a forward shock. The forward-reverse shock pair associated with CMEs moves outward in interplanetary space and evolves into a pair of fast shocks. The interplanetary manifestation of some CMEs is pictured as a magnetic cloud accompanied by a shock pair: a forward shock precedes the cloud and a reverse shock either within or behind the cloud.

Forward‐reverse shock pairs associated with transient disturbances in the solar wind at 1 AU

While relatively common in the outer heliosphere, forward‐reverse shock pairs occur rarely in the solar wind at 1 AU. Using the combined solar wind plasma and magnetic field data from ISEE 3 for the interval from August 1978 through February 1980 we have identified several examples of such shock pairs at 1 AU. Two of these shock pair events were clearly associated with transient disturbances rather than with corotating streams. Strong bidirectional electron heat flux events, which we identify as the coronal mass ejecta (CME) driving the shocks, were detected during both of these disturbances. In the stronger of the two disturbances the reverse shock was found within the CME and was separated from the forward shock by ∼0.2 AU; the pressure was nearly constant between the two shocks. On the other hand, in the weaker of the two disturbances the reverse shock had propagated entirely through the CME and trailed the forward shock by ∼0.3–0.4 AU; the pressure declined substantially and monotonically between the two shocks. The overall disturbance profiles for these two events compare favorably with the results of previously published numerical simulations. On the basis of those simulations we suggest that these two events are prototypes of “driven” and “blast” disturbances, wherein the time scale of the initial velocity perturbation near the Sun is, or is not, short compared to the transit time to 1 AU. Our observations support the notion that reverse shocks in transient disturbances are most likely to be found near the central axes of such disturbances.

Solar wind stream evolution at large heliocentric distances: Experimental demonstration and the test of a model

A stream propagation model which neglects all dissipation effects except those occurring at shock interfaces, was used to compare Pioneer-10 solar wind speed observations, during the time when Pioneer 10, the earth, and the sun were coaligned, with near-earth Imp-7 observations of the solar wind structure, and with the theoretical predictions of the solar wind structure at Pioneer 10 derived from the Imp-7 measurements, using the model. The comparison provides a graphic illustration of the phenomenon of stream steepening in the solar wind with the attendant formation of forward-reverse shock pairs and the gradual decay of stream amplitudes with increasing heliocentric distance. The comparison also provides a qualitative test of the stream propagation model.

Effects of solar flare duration on a double shock pair at 1 AU

Numerical solutions of the time-dependent equations of motion for spherically symmetric flow are obtained to study the propagation and modification of the forward-reverse shock pair in the region between the sun and 1 AU. For cases where the duration of the disturbance is long compared to T, the shock transit time to 1 AU, a double shock pair that resembles those predicted by similarity theory occurs at 1 AU. In cases where the duration of the solar disturbance is less than about 0.45T, the shock pair structure is appreciably altered by a rarefaction, initiated at the end of the solar disturbance, which has caught up with the shock pair. In cases where the duration is less than about 0.1T, the rarefaction completely destroys the reverse shock, leaving a single shock. This analysis indicates that a forward-reverse shock pair will not be observed at 1 AU unless the time duration of the solar disturbance is greater than about 5 hours. Since solar flares that last this long are quite rare, it is concluded that observation of a flare-associated forward-reverse shock pair at 1 AU is unlikely.

Shock waves in the interplanetary medium

A similarity solution is given for spherically-symmetric, radial flow due to a point source, which is an idealization of the disturbance produced in the interplanetary medium by a sudden expansion of the solar corona following a flare. The solution illustrates the formation of forward-reverse shock pairs, essentially as postulated by Sonett and Colburn. The distortion of the interplanetary magnetic field by the disturbance is also discussed briefly.