Victor J. Pizzo

Latitudinal variation of solar wind corotating stream interaction regions: Ulysses

Ulysses‧ initial transit to high heliographic latitudes at a heliocentric distance of ∼5 AU has revealed systematic effects in the latitudinal evolution of corotating interaction regions (CIRs). At a latitude corresponding roughly to, but slightly less than, the inferred tilt of the coronal streamer belt and embedded heliospheric current sheet, the strong forward shocks commonly associated with CIRs at lower latitudes disappeared almost entirely; however, the reverse shocks associated with these CIRs persisted to latitudes ∼ 10° above the streamer belt. Systematic meridional flow deflections observed in association with the forward and reverse waves bounding the CIRs demonstrate that the above effect is the result of the fact that the forward waves propagate to lower latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a three-dimensional model of corotating solar wind flows that originate in a tilted dipole geometry back at the Sun.

A new class of forward‐reverse shock pairs in the solar wind

A new class of forward-reverse shock pairs in the solar wind has been discovered using Ulysses observations at high heliographic latitudes. These shock pairs are produced by expansion of coronal mass ejections, CMEs, that have internal pressures that are higher than, and speeds that are comparable to, that of the surrounding solar wind plasma. Of six certain CMEs observed poleward of S31°, three have associated shock pairs of this nature. We suggest that high internal CME pressures may exist primarily for events that have high speeds close to the surface of the Sun.

Improved Method for Specifying Solar Wind Speed Near the Sun

We have found an improved technique for empirically specifying solar wind flow speed near the Sun (∼0.1 AU) using a set of three simple inter‐linked coronal/solar wind models. In addition to magnetic field expansion factor, solar wind speed also appears to be influenced by the minimum angular distance that an open field footpoint lies from a coronal hole boundary. We conduct our study using polar field corrected Mount Wilson Solar Observatory Carrington maps from 1995. During this period, the Sun was in the declining phase of the solar cycle and the solar wind had relatively simple global structure.

A forward‐reverse shock pair in the solar wind driven by over‐expansion of a coronal mass ejection: Ulysses observations

A previously unidentified type of solar wind forward-reverse shock pair has been observed by Ulysses at 4.64 AU and S32.5°. In contrast to most solar wind forward-reverse shock pairs, which are driven by the speed difference between fast solar wind plasma and slower plasma ahead, this particular shock pair was driven purely by the over-expansion of a coronal mass ejection, CME, in transit from the Sun. A simple numerical simulation indicates that the overexpansion was a result of a high initial internal plasma and magnetic field pressure within the CME. The CME observed at 4.64 AU had the internal field structure of a magnetic flux rope. This event was associated with a solar disturbance in which new magnetic loops formed in the corona almost directly beneath Ulysses ∼11 days earlier. This association suggests that the flux rope was created as a result of reconnection between the “legs” of neighboring magnetic loops within the rising CME.

A two-dimensional simulation of the radial and latitudinal evolution of a solar wind disturbance driven by a fast, high-pressure coronal mass ejection

Using a hydrodynamic simulation, we have studied the two-dimensional (symmetry in the azimuthal direction) evolution of a fast, high-pressure coronal mass ejection (CME) ejected into a solar wind with latitudinal variations similar to those observed by Ulysses. Specifically, the latitudinal structure of the ambient solar wind in the meridional plane is approximated by two zones: At low latitudes (< 20°) the solar wind is slow and dense, while at higher latitudes the solar wind is fast and tenuous. The CME is introduced into this ambient wind as a bell-shaped pressure pulse in time, spanning from the equator to 45° with a speed and temperature equal to that of the high-latitude solar wind. We find that such an ejection profile produces radically different disturbance profiles at low and high latitudes. In particular, the low-latitude portion of the ejecta material drives a highly asymmetric disturbance because of the relative difference in speed between the fast CME and slower ambient solar wind ahead. In contrast, the high-latitude portion of the same ejecta material drives a much more radially symmetric disturbance because the relative difference in pressure between the CME and ambient background plasma dominates the dynamics. The simulations reveal a number of other interesting features. First, there is significant distortion of the CME in the interplanetary medium. By ∼ 1 AU the CME has effectively separated (in radius as well as latitude) into two pieces. The radial separation is due to the strong velocity shear between the slow and fast ambient solar wind. The latitudinal separation arises from pressure gradients associated with rarefaction regions that develop as the CME propagates outward. Second, there is significant poleward motion of the highest-latitude portion of the CME and its associated disturbance. The main body of the CME expands poleward by ∼ 18°, while the forward and reverse waves (produced by the overexpanding portion of the CME) propagate all the way to the pole. Third, the simulations show that the high-pressure region, which develops at low latitudes as the fast CME ploughs through the slow ambient solar wind, penetrates significantly (∼ 10°) into the high-latitude fast solar wind. We compare the simulation results with a CME-driven interplanetary disturbance observed at both low and high latitudes and find that the simulation reproduces many of the essential features of the observations.

3-D simulation of high-latitude interaction regions : comparison with Ulysses results

A three-dimensional (3-D) magnetohydrodynamic (MHD) numerical model is used to simulate the global evolution of a steady, tilted-dipole solar wind flow configuration similar to that prevalent in interplanetary space in 1993. Systematic latitudinal changes in the structure of a corotating interaction region (CIR) near 5 AU is shown to agree well with recent Ulysses observations. The abrupt disappearance of forward shocks and continued persistence of reverse shocks poleward of the latitude where Ulysses crossed the southern edge of the coronal streamer belt is explained as a natural consequence of the 3-D flow geometry.

A CME-driven solar wind disturbance observed at both low and high heliographic latitudes

A solar wind disturbance produced by a fast coronal mass ejection, CME, that departed from the Sun on Feburary 20, 1994 was observed in the ecliptic plane at 1 AU by IMP 8 and at high heliographic latitudes at 3.53 AU by Ulysses. In the ecliptic the disturbance included a strong forward shock but no reverse shock, while at high latitudes the disturbance was bounded by a relatively weak forward-reverse shock pair. It is clear that the disturbance in the ecliptic plane was driven primarily by the relative speed between the CME and a slower ambient solar wind ahead, whereas at higher latitudes the disturbance was driven by expansion of the CME. The combined IMP 8 and Ulysses observations thus provide a graphic illustration of how a single fast CME can produce very different types of solar wind disturbances at low and high heliographic latitudes. Simple numerical simulations help explain observed differences at the two spacecraft.

EVIDENCE OF POSTERUPTION RECONNECTION ASSOCIATED WITH CORONAL MASS EJECTIONS IN THE SOLAR WIND

Using a coupled 2.5-dimensional, time-dependent MHD model of the solar corona and inner heliosphere, we have simulated the eruption and evolution of a coronal mass ejection containing a flux rope all the way from the Sun to 1 AU. Although idealized, we find that the simulation reproduces many generic features of magnetic clouds. In this paper we report on a new, intriguing aspect of these comparisons. Specifically, the results suggest that jetted outflow, driven by posteruptive reconnection underneath the flux rope, occurs and may remain intact out to 1 AU and beyond. We present an example of a magnetic cloud with precisely these signatures and show that the velocity perturbations are consistent with reconnection outflow. We suggest that other velocity and/or density enhancements observed trailing magnetic clouds may be signatures of such reconnection and, in some cases, may not be associated with prominence material, as has previously been suggested.

Solar wind corotating stream interaction regions out of the ecliptic plane: Ulysses

Ulysses plasma observations reveal that the forward shocks that commonly bound the leading edges of corotating interaction regions (CIRs) beyond ∼2 AU from the Sun at low heliographic latitudes nearly disappeared at a latitude of S26°. On the other hand, the reverse shocks that commonly bound the trailing edges of the CIRs were observed regularly up to S41.5°, but became weaker with increasing latitude. Only three CIR shocks have been observed poleward of S41.5°; all of these were weak reverse shocks. The above effects are a result of the forward waves propagating to lower heliographic latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a global model of solar wind flows that originate in a simple tilted-dipole geometry back at the Sun.

The tilts of corotating interaction regions at midheliographic latitudes

Previous analysis of midlatitude corotating interaction regions (CIRs) observed by Ulysses in the southern heliosphere has revealed that the flow downstream of the forward (F) shock (or wave) on the leading edge of a CIR generally turns northward and into the direction of planetary motion (westward), while the flow downstream of the reverse (R) shock (or wave) on the trailing edge generally turns southward and eastward. These systematic flow deflections are a natural consequence of large-scale pressure gradients associated with the CIRs and indicate that the F shocks tend to propagate toward and across the equator with increasing heliocentric distance, while the R shocks tend to propagate toward the pole. Numerical simulations indicate that these effects are a natural consequence of the tilt of the solar magnetic-dipole axis relative to the solar rotation axis. The present work utilizes a variety of techniques to analyze the flow deflections observed within midlatitude CIRs from which we can infer the overall orientations of the CIRs and the speeds and directions of propagation of the waves. Notable results include the following: (1) On the whole, F shocks do propagate equatorward and westward, and R shocks propagate poleward and eastward; (2) shock parameters show a modulation in amplitude, peaking at latitudes roughly equivalent to the inferred dipole-tilt angle; (3) R shocks tend to propagate faster (in the upstream solar wind frame) than their counterpart F shocks; and (4) meridional deflections tend to be larger than azimuth deflections for both F and R shocks. While it is reassuring that most of the CIRs analyzed fit the paradigm, there are a significant number of anomalies. We discuss several mechanisms which could, in principle, cause these apparent contradictions. For example, the present analysis necessarily emphasizes the local structure at the shock front. The agreement improves significantly, however, when the large-scale flow deflections throughout the CIR are taken into account.