R. J. Fitzenreiter

A magnetic cloud containing prominence material: January 1997

This work discusses the relations among (1) an interplanetary force-free magnetic cloud containing a plug of cold high-density material with unusual composition, (2) a coronal mass ejection (CME), (3) an eruptive prominence, and (4) a model of prominence material supported by a force-free magnetic flux rope in a coronal streamer. The magnetic cloud moved past the Wind spacecraft located in the solar wind upstream of Earth on January 10 and 11, 1997. The magnetic field configuration in the magnetic cloud was approximately a constant-α, force-free flux rope. The 4He++/H+ abundance in the most of the magnetic cloud was similar to that of the streamer belt material, suggesting an association between the magnetic cloud and a helmet streamer. A very cold region of exceptionally high density was detected at the rear of the magnetic cloud. This dense region had an unusual composition, including (1) a relatively high (10%) 4He++/He+ abundance (indicating a source near the photosphere), and (2) 4He+, with an abundance relative to 4He++ of ∼1%, and the unusual charge states of O5+ and Fe5+ (indicating a freezing-in temperature of (1.6–4.0) × 105 °K, which is unusually low, but consistent with that expected for prominence material). Thus we suggest that the high-density region might be prominence material. The CME was seen in the solar corona on January 6, 1997, by the large angle and spectrometric coronagraph (LASCO) instrument on SOHO shortly after an eruptive prominence. A helmet streamer was observed near the latitude of the eruptive prominence a quarter of a solar rotation before and after the eruptive prominence. These observations are consistent with recent models, including the conceptual model of Low and Hundhausen [1995] for a quasi-static helmet streamer containing a force-free flux rope which supports prominence material and the dynamical model of Wu et al. [1997] for CMEs produced by the disruption of such a configuration.

Ionospheric mass ejection in response to a CME

We report observations of a direct ionospheric plasma outflow response to the incidence of an interplanetary shock and associated coronal mass ejection (CME) upon the earths magnetosphere. Data from the WIND spacecraft, 185 RE upstream, document the passage of an interplanetary shock at 23:20 UT on 24 Sept. 1998. The polar cap plasma environment sampled by the POLAR spacecraft changed abruptly at 23:45 UT, reflecting the compressional displacement of the geopause relative to the spacecraft. POLAR left the polar wind outflow region and entered the mantle flows. Descending toward the dayside cusp region, POLAR later returned from the mantle to an enhanced polar wind flux dominated by O+ plasma and eventually containing molecular ions. The enhanced and O+− dominated outflow continued as the spacecraft passed through the high altitude cleft and then the southern cleft at lower altitude. Such a direct response of the ionosphere to solar wind dynamic pressure disturbances may have important impacts on magnetospheric dynamics.

Observations of the lunar plasma wake from the WIND spacecraft on December 27, 1994

On December 27, 1994, the WIND spacecraft crossed the lunar wake at a distance of 6.5 lunar radii ( RL ) behind the moon. The observations made were the first employing modem instruments and a high data rate. The SWE plasma instrument on WIND observed new aspects of the interaction between the solar wind and unmagnetized dielectric bodies. The plasma density decreased exponentially from the periphery of the wake towards its center as predicted by simple theory. Behind the moon two distinct cold ion beams were observed refilling the lunar cavity. The ions were accelerated along the direction of the magnetic field by an electric field of the order 2 × 10−4 volts/m. The region of plasma depletion was observed to extend beyond the light shadow, consistent with a rarefaction wave moving out from the wake into the undisturbed solar wind.

Structure and properties of the subsolar magnetopause for northward interplanetary magnetic field: Multiple‐instrument particle observations

The structure and properties of the subsolar magnetopause for northward interplanetary magnetic feild (IMF) are studied with measurements from 10 different instruments for three ISEE crossings. Data show that the overall structure and properties are similar for the three crossings, indicating the magnetopause is relatively well determined in the subsolar region for strongly northward IMF. The measurements from different instruments are consistent with each other and complementary based on the current knowledge of space plasma physics. The combined data set suggests that the magnetopause region is best organized by defining a sheath transition layer and steplike boundary layers. The sheath transition layer contains mostly magnetosheath particles. The magnetosheath, magnetospheric, and ionospheric populations are mixed in the interior boundary layers. This result, which is consistent with previous studies, is now supported by observations of a much broader spectrum of measurements including three-dimensional electron, energetic particle, heavy ion and plasma wave. Some new features are also found: even for quiet subsolar magnetopause crossings, transient or small-scale structures still occur sporadically; slight heating may occur in the boundary layers. Some outstanding issues are clarified by this study: the electron flux enhancements in the lowest energies in the boundary layers and magnetosphere are ionospheric electrons and not photoelectrons from the spacecraft; for northward IMF, they are photoelectrons, but for southward IMF they may be secondary electrons; and the density measurements from differential and integral techniques are similar, leaving no room for a significant “invisible” population.

Counterstreaming electrons in magnetic clouds

Two widely used signatures of interplanetary coronal mass ejections are counterstreaming suprathermal electrons, implying magnetic structures connected to the Sun at both ends, and magnetic clouds, characterized by large-scale field rotations, low temperature, and high field strength. In order to determine to what extent these signatures coincide, electron heat flux data were examined for 14 magnetic clouds detected by ISEE 3 and IMP 8 near solar maximum and 34 clouds detected by Wind near solar minimum. The percentage of time during each cloud passage that counterstreaming electrons were detected varied widely, from 6 clouds with essentially no counterstreaming to 8 clouds with nearly 100% counterstreaming. All of the former but less than half of the latter occurred near solar minimum, suggesting a possible solar cycle dependence on the degree of magnetic openness. The counterstreaming intervals were distributed randomly throughout the clouds, with a median length of 2.5 hours. A plot of counterstreaming percentages against cloud diameter for 33 clouds modeled as cylindrical flux ropes indicates a linear dependence of the percentage of closed flux on cloud size, with the largest clouds being the most closed. Overall the results are consistent with the view that although magnetic field lines within a magnetic cloud can form a large-scale, coherent structure, reconnection in remote regions of the structure, presumably near the Sun, sporadically alters its topology from closed to open until the cloud assimilates into the ambient solar wind.

Observations of electron velocity distribution functions in the solar wind by the WIND spacecraft : High angular resolution strahl measurements

The WIND Solar Wind instrument (SWE) includes a sensor especially configured to measure the solar wind strahl. The strahl is the excess electron (≥100 eV) halo component of the solar wind most closely aligned with the magnetic field. Strahl electrons originate in the inner corona and move freely out to 1 A.U., providing information on the state of the corona. Electron data acquired during seven solar rotations show that the electron velocity distributions are most anisotropic and the strahl flux most intense during high speed streams. The strahl angular width is smallest when the solar wind velocity is largest, approximately 5° in width at 600 eV, and becomes much wider when the velocity is low. Coincident with the strahl is a small reverse electron flux which does not form a beam but fills the field of view. This variability of the flux and angular distribution of the strahl and anti-strahl suggests that a pitch angle scattering process may be acting to broaden the strahl and that the anti-strahl may be due to backscattering of the strahl.

Upstream ULF waves and energetic electrons associated with the lunar wake : Detection of precursor activity

We present observations of precursor ULF wave activity and energetic electron flows detected by the WIND spacecraft just prior to entry of the lunar wake on 27 December 1994. This activity occurs upstream of the wake on field lines directly connected to the wake penumbra region. The activity ceases near the penumbra entrance. The observations of upstream ULF wave activity and solar wind counterstreaming electron flows is similar to observations made upstream of collisionless bow shocks. Analogously, the wake precursor region is characterized by thermalization and information propagation ahead of the wake structure.

A uniform-twist magnetic flux rope in the solar wind

We describe magnetic field, proton, electron, and α-particle observations made by WIND on 24–25 October, 1995 of a structure consisting of a magnetic flux rope containing a relatively low beta plasma. While the flux rope structure was inferred from the magnetic field data, the particle behavior corroborates the inference. Minimum variance analysis of the magnetic field data indicates an axis highly inclined to the ecliptic plane and pointing away from the Sun-Earth line. The diameter of the flux rope is estimated as 0.07 AU. Despite a pronounced overpressure, the structure is not expanding but is rather being convected passively with the ambient flow. An intense antisunward field-aligned flow of heat flux electrons indicates that the flux rope is connected at one end to the Sun. The field variation is suggestive of a magnetic flux rope of constant field line twist, and a least-squares fit of this model to the data confirms this to a good approximation. The field line twist per unit length is estimated as ...

Wave properties near the subsolar magnetopause: Pc 3–4 energy coupling for northward interplanetary magnetic field

Strong slow mode waves in the Pc 3–4 frequency range are found in the magnetosheath close to the magnetopause. We have studied these waves at one of the ISEE subsolar magnetopause crossings using the magnetic field, electric field, and plasma measurements. We use the pressure balance at the magnetopause to calibrate the Fast Plasma Experiment data versus the magnetometer data. When we perform such a calibration and renonnalization, we find that the slow mode structures are not in pressure balance and small scale fluctuations in the total pressure still remain in the Pc 3–4 range. Energy in the total pressure fluctuations can be transmitted through the magnetopause by boundary motions. The Poynting flux calculated from the electric and magnetic field measurements suggests that a net Poynting flux is transmitted into the magnetopause. The two independent measurements show a similar energy transmission coefficient. The transmitted energy flux is about 18% of the magnetic energy flux of the waves in the magnetosheath. Part of this transmitted energy is lost in the sheath transition layer before it enters the closed field line region. The waves reaching the boundary layer decay rapidly. Little wave power is transmitted into the magnetosphere.

Near-simultaneous bow shock crossings by WIND and IMP 8 on December 1, 1994

Near‐simultaneous dawn‐side bow shock crossings by WIND and IMP 8 on December 1, 1994 are analyzed to determine shock location and shape and to examine the changes in shock structure and the foreshock MHD wave properties with increasing downstream distance. The WIND and IMP 8 crossings took place at sun‐Earth‐spacecraft angles of 64.7° and 115.3°, respectively. The solar wind speed and interplanetary magnetic field magnitude were near their long‐term average values. However, the orientation of the IMF was unusual in that it rotated from an angle of ∼50–60° to the sun‐Earth line at the beginning of the interval of shock crossings to less than 20° just after the final crossings. The ratio of the downstream to upstream components of the magnetic field tangential to the shock decreases from 4.1 at WIND to 3.1 at IMP 8 in general agreement with theory. In addition, the overshoot in the shock magnetic ramp observed at WIND is greatly diminished by the downstream distance of IMP 8. In the foreshock, MHD waves with periods of 10–20 s and amplitudes of 3–6 nT were observed at both spacecraft. However, at WIND they have a strong compressional component which is much weaker farther downstream at IMP 8. Unexpectedly, the radial distance of the shock at both spacecraft is only ∼80–85% of that predicted by recent models. Motivated by this event, we have statistically analyzed a larger data set of bow shock crossings which took place under quasi‐field‐aligned flow conditions. On this basis it is suggested that magnetosheath thickness may decrease by ∼10% as the IMF becomes increasingly flow aligned.