S. J. Bame

Geomagnetic activity associated with earth passage of interplanetary shock disturbances and coronal mass ejections

Coronal mass ejection events (CMEs) are important occasional sources of plasma and magnetic field in the solar wind at 1 AU, accounting for approximately 10% of all solar wind measurements in the ecliptic plane during the last solar activity maximum. Previous work indicates that virtually all transient shock wave disturbances in the solar wind are driven by fast CMEs. Using a recently appreciated capability for distinguishing CMEs in solar wind data in the form of counterstreaming solar wind electron events, this paper explores the overall effectiveness of shock wave disturbances and CMEs in general in stimulating geomagnetic activity. The study is confined to the interval from mid-August 1978 through mid-October 1982, spanning the last solar activity maximum, when ISEE 3 was in orbit about the L1 Lagrange point 220 Re upstream from Earth. We find that all but one of the 37 largest geomagnetic storms in that era were associated with Earth passage of CMEs and/or shock disturbances, with the large majority of these storms (27 out of 37) being associated with interplanetary events where Earth encountered both a shock and the CME driving the shock (shock/CME events). Although CMEs and/or shock disturbances were increasingly the cause of geomagnetic activity as the level of geomagnetic activity increased, many smaller geomagnetic disturbances were unrelated to these events. Further, approximately half of all CMEs and half of all shock disturbances encountered by Earth did not produce any substantial geomagnetic activity as measured by the planetary geomagnetic index Kp. The geomagnetic effectiveness of Earth directed CMEs and shock wave disturbances was directly related to the flow speed, the magnetic field magnitude, and the strength of the southward (GSM) field component associated with the events. The initial speed of a CME close to the Sun appears to be the most crucial factor in determining if an earthward directed event will be effective in exciting a large geomagnetic disturbance.

Observations of reconnection of interplanetary and lobe magnetic field lines at the high‐latitude magnetopause

Measurements made with the Fast Plasma Experiment on ISEE 2 in the vicinity of the high-latitude, dusk magnetopause near the terminator plane, at a time when the local magnetosheath and tail lobe magnetic fields were nearly oppositely directed, provide direct evidence for the reconnection of the open field lines of the tail lobes with the interplanetary magnetic field (IMF). The evidence consists primarily of observations of accelerated magnetosheath plasma flowing both tailward and sunward within the high-latitude magnetopause current layer. Observed speed changes at the magnetopause were of the order of twice the magnetosheath Alfven speed and were quantitatively consistent with the predictions of reconnection models. At times when the newly entering magnetosheath plasma observed at ISEE 2 was accelerated sunward a secondary beam of ions, presumably mirrored at low altitudes, was occasionally present. At times when the newly entering magnetosheath plasma observed by ISEE 2 was accelerated tailward a secondary beam of largely unaccelerated mantle plasma was occasionally present. Small plasma accelerations observed on reconnected field lines in the magnetosheath were associated with the presence of ions reflected at the magnetopause and moving at a speed of approximately twice the Alfven speed relative to the remainder of the magnetosheath plasma. Although previous work has anticipated that the re-reconnection of the open field lines of the tail lobes with the IMF would be associated with northward IMF in the magnetosheath, the present reconnection event was associated with a local magnetosheath IMF which had a small southward component.

Magnetospheric plasma analyzer for spacecraft with constrained resources

A light‐weight, low‐power, plasma analyzer is described that can be used for measuring the plasma environments of spacecraft with constrained resources. A unique system using a single electrostatic analyzer coupled to a single array of channel electron multipliers allows measurement of the three‐dimensional energy per charge distributions of both ions and electrons over E/q ranges from ∼1 eV/q to ≳40 keV/q. Particles selected by the analyzer are post‐accelerated into the multipliers to maintain sensitivity for the lowest energy particles. An instrument using this concept called the magnetospheric plasma analyzer (MPA) is described. Presently, three MPAs are in geosynchronous orbits (GEO) aboard spacecraft with International Designators of 1989‐046, 1990‐095, and 1991‐080. The MPA and its response characteristics are described, and examples of on‐orbit data showing some of the MPA capabilities are presented.

Ulysses solar wind plasma observations from pole to pole

We present Ulysses solar wind plasma data from the peak southerly latitude of −80.2° on 12 September 1994 through the corresponding northerly latitude on 31 July 1995. Ulysses encountered fast wind throughout this time except for a 43° band centered on the solar equator. Median mass flux was nearly constant with latitude, while speed and density had positive and negative poleward gradients, respectively. Solar wind momentum flux was highest at high latitudes, suggesting a latitudinal asymmetry in the heliopause cross section. Solar wind energy flux density was also highest at high latitudes.

ISEE-1 and ISEE-2 Fast Plasma Experiment and the ISEE-1 Solar Wind Experiment

Identical fast plasma experiment (FPE) systems were placed on the ISEE-1 and ISEE-2 spacecraft. The FPE consists of three high efficiency 90° spherical section electrostatic analyzers using large secondary emitters and discrete dynode multipliers to detect analyzed particles. Two of them, viewing in opposite directions, produce complete 2D velocity distribution measurements of both protons and electrons every spacecraft revolution. A third FPE analyzer with a divided emitter measures 3D distributions at a slower rate. ISEE-1 also carries a solar-wind experiment (SWE) to measure solar-wind ions with high resolution. The SWE is composed of two 150° spherical section analyzers using the same set of plates. The two acceptance fans are tilted with respect to each other so that 3D characteristics of the ion distributions can be derived.

Magnetospheric plasma analyzer: Initial three‐spacecraft observations from geosynchronous orbit

The first three magnetospheric plasma analyzer (MPA) instruments have been returning data from geosynchronous orbit nearly continuously since late 1989, 1990, and 1991. These identical instruments provide for the first time simultaneous plasma observations from three widely spaced geosynchronous locations. The MPA instruments measure the three-dimensional velocity space distributions of both electrons and ions with energies between ∼1 eV/q and ∼40 keV/q. MPA capabilities and observations are summarized in this paper. We use the simultaneous observations from three longitudinally separated spacecraft to synthesize a synoptic view of the morphology of the magnetosphere at geosynchronous orbit over a 6-week interval in early 1992. The MPA observations indicate that the spacecraft encountered seven regions with characteristic plasma populations during this period: (1) the cool, dense plasmasphere (13.1% of the data); (2) a warmer, less dense plasma trough (22.5%); (3) the hot plasma sheet (40.3%); (4) a combination of plasma trough and plasma sheet (18.6%); (5) an empty trough region, devoid of plasma sheet, plasmasphere, or plasma trough populations (4.3%); (6) the magnetosheath and/or low-latitude boundary layer (0.7%); and (7) the lobe (0.3%). The local time distributions of these regions are examined. For example, as suggested by previous authors, we find that at geomagnetically quiet times (Kp < 2) geosynchronous orbit can lie entirely within the plasmasphere while at more active times only the afternoon to evening portions of the orbit are typically within the plasmasphere. We also find that the plasma convection inside the plasmasphere is generally sunward in the corotating (geosynchronous spacecraft) reference frame, independent of activity level, in contrast to previous studies. In addition to such statistical results, the simultaneous data sets at different local times allow us to at least partially separate spatial from temporal variations. In particular, we use these observations to examine the instantaneous shapes of the plasmapause and magnetopause as they pass over geosynchronous orbit. As expected, the plasmapause is found to have a highly variable shape, at various times showing (1) a stable dusk side bulge, (2) a variable bulge which expands, contracts, and moves, (3) an overall expansion and contraction of the plasmasphere, and (4) even more complicated behavior which is best accounted for by large-scale structure of the plasmapause and/or disconnected plasma blobs. During the 6 weeks of data examined, the magnetosheath was encountered on several occasions at synchronous orbit, preferentially on the prenoon side of the magnetosphere. For the first time, simultaneous prenoon and postnoon observations confirm this asymmetry and demonstrate that the magnetopause shape can be highly asymmetric about the Earth-Sun line.

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.

Comet Giacobini-Zinner: Plasma description

A strong interaction between the solar wind and comet Giacobini-Zinner was observed oh 11 September 1985 with the Los Alamos plasma electron experiment on the International Cometary Explorer (ICE) spacecraft. As ICE approached an intercept point 7800 kilometers behind the nucleus from the south and receded to the north, upstream phenomena due to the comet were observed. Periods of enhanced electron heat flux from the comet as well as almost continuous electron density fluctuations were measured. These effects are related to the strong electron heating observed in the cometary interaction region and to cometary ion pickup by the solar wind, respectively. No evidence for a conventional bow shock was found as ICE entered and exited the regions of strongest interaction of the solar wind with the cometary environment. The outer extent of this strong interaction zone was a transition region in which the solar wind plasma was heated, compressed, and slowed. Inside the inner boundary of the transition region was a sheath that enclosed a cold intermediate coma. In the transition region and sheath, small-scale enhancements in density were observed. These density spikes may be due to an instability associated with cometary ion pickup or to the passage of ICE through cometary ray structures. In the center of the cold intermediate coma a narrow, high-density core of plasma, presumably the developing plasma tail was found. In some ways this tail can be compared to the plasma sheet in Earths magnetotail and to the current sheet in the tail at Venus. This type of configuration is expected in the double-lobe magnetic topology detected at the comet, possibly caused by the theoretically expected draping of the interplanetary magnetic field around its ionosphere.

Ulysses' return to the slow solar wind

After ten long years of wandering the uncharted seas, Ulysses returned to his home port of Ithaca. Similarly, after its unprecedented five year odyssey through the previously uncharted regions over the poles of the Sun, the Ulysses spacecraft has returned to the slow, variable solar wind which dominates observations near the ecliptic plane. Solar wind plasma and magnetic field observations from Ulysses are used to examine this return from the fast polar solar wind through the region of solar wind variability and into a region of slow solar wind from the low latitude streamer belt. As it journeyed equatorward, Ulysses encountered a large corotating interaction region and associated rarefaction region on each solar rotation. Due to these repeated interactions, Ulysses also observed numerous shocks, all of which have tilts that are consistent with those expected for shocks generated by corotating interaction regions. Eventually, Ulysses emerged into a region of unusually steady slow solar wind, indicating that the tilt of the streamer belt with respect to the solar heliographic equator was smaller than the width of the band of slow solar wind from the streamer belt.

Ulysses Solar Wind Plasma Observations at High Southerly Latitudes

Solar wind plasma observations made by the Ulysses spacecraft through –80.2� solar latitude and continuing equatorward to –40.1� are summarized. Recurrent high-speed streams and corotating interaction regions dominated at middle latitudes. The speed of the solar wind was typically 700 to 800 kilometers per second poleward of –35�. Corotating reverse shocks persisted farther south than did forward shocks because of the tilt of the heliomagnetic streamer belt. Sporadic coronal mass ejections were seen as far south as –60.5�. Proton temperature was higher and the electron strahl was broader at higher latitudes. The high-latitude wind contained compressional, pressure-balanced, and Alfv�nic structures.