J. T. Steinberg

Solar wind control of density and temperature in the near-Earth plasma sheet: WIND/GEOTAIL collaboration

A statistical survey of GEOTAIL observations reveals the following properties of the near-Earth plasma sheet (−15 < XGSM′ < −50 Re): During the periods when the northward IMF dominates, (1) the plasma sheet becomes significantly cold and dense, (2) the best correlations between the plasma sheet and the IMF parameters occur when the latter quantities are averaged over 9−4+3 hours prior to the plasma sheet observations, and (3) temperatures diminish and densities increase near the dawn and dusk flanks of the plasma sheet. We suggest that during prolonged northward IMF periods (∼ several hours) there is a slow diffusive transport of the plasma from the solar wind into the plasma sheet through the the magnetotail flanks.

On the Relationship Between Coronal Mass Ejections and Magnetic Clouds

We compare the substructures of the 1997 February 07 coronal mass ejection (CME) observed near the Sun with a corresponding event in the interplanetary medium to determine the origin of magnetic clouds (MCs). We find that the eruptive prominence core of the CME observed near the Sun may not directly become a magnetic cloud as suggested by some authors and that it might instead become the ”pressure pulse” following the magnetic cloud. We substantiate our conclusions using time of arrival, size and composition estimates of the CME-MC substructures obtained from ground based, SOHO and WIND observations.

Differential flow between solar wind protons and alpha particles: First WIND observations

Alpha particle and proton measurements in the solar wind made using the SWE Faraday cup detectors on the WIND spacecraft are reported. Some overall trends observed confirm past observations : the ratios of alpha particle to proton density N α /N P , thermal speed W α /W P , as well as the differential velocity V α -V P (hereafter V αP ) are generally correlated with bulk solar wind flow speed. The detailed WIND measurements enable us to investigate instances when the alpha-proton differences deviate from these overall general trends. Occasionally, difference velocities as large as 80 km/s were seen, with the ratio of |V αP | to the Alfven speed V A near unity, characteristics more typical of observations at solar distances less than I AU. An example is presented where |V α |-|V P | reverses sign while |V αP | stays nearly constant. Comparison of the vector velocities and the magnetic field suggests that the speed reversal is associated with a localized kink in the magnetic field. Finally we show an instance where |V αP | exceeds the observed wave speed for Alfvenic fluctuations (V wave =B 0 ΔV/ΔB) resulting in alpha particle velocity fluctuations that anti-correlate with the wave. Though this phenomenon has been previously reported in high-latitude measurements beyond I AU, it is shown here to also occur at 1 AU in the ecliptic.

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 ...

Electrostatic instability in the central lunar wake: A process for replenishing the plasma void?

We report on the WIND observations and subsequent analysis of electrostatic ion beam driven waves in the central lunar wake. Previous reports indicate that ion beams are present in the central lunar wake, these generated by an ambipolar electric field near the wake flanks. We demonstrate that these ion beams are inherently unstable, and thus they are capable of generating the observed broadband electrostatic noise. The beam disruption process may be a dominate mechanism to replace ions in the void trailing the moon. This picture of the wake, with an extended electrostatic tail in the central region is distinctly different from the MHD wake picture discussed previously.

Extremely cold electrons in the January 1997 magnetic cloud

The 3D Plasma and Energetic Particle (3DP) instrument on the WIND spacecraft detected extremely cold solar wind electrons in the January 10–11, 1997 magnetic cloud, ∼4 times lower temperature than any previously reported measurement. Detailed fits to the electron distributions show that the core electron temperature, Tec, generally ranged from ∼1 to 4.5 eV through the cloud, dropping to a low of ∼0.7 eV in an unusually dense (n>∼150 cm−3) region in the trailing portion of the cloud. Due to the extremely low average electron temperature Te and high density, the ions and electrons are collisionally coupled as confirmed by Te=Tp, the first such observation in the solar wind. For most of the cloud the halo density is very low (neh<∼0.1 cm−3), implying magnetic disconnection from the Sun. Correlations in Te and Tp are observed throughout the cloud and are particularly evident when neh is low. We suggest the diminished halo density has reduced a significant heat source to the core population, thus allowing the electrons to cool more thoroughly. Furthermore, we show that Coulomb collisions are a significant mechanism of energy transfer between the halo and core distributions under normal solar wind conditions.

Evidence for multiple ejecta: April 7–11, 1997, ISTP Sun‐Earth connection event

Evidence is presented that the enhanced geomagnetic activity, on April 10–11, 1997, was caused by one of two ejecta that left the Sun at ≈ 14 UT on April 7. This ejecta was not directly detected at the Earth. The evidence for this interpretation is based on WIND spacecraft observations in the solar wind (SW). It is consistent with: (i) measured velocities of the coronal mass ejections from the SOHO coronagraph; (ii) the initial propagation speed of the shock generated in this event, estimation from type II radio burst observations from the WAVES instrument on WIND, and (iii) the time profile of energetic ions observed by EPACT on WIND. This locally unobserved ejecta (moving at 600 to 700 kms−1) generated a fast shock which accelerated ions to several tens of MeV/amu. The inferred passage of the first ejecta close to Earth (on April 10 to 11) is based on the observation of an interplanetary shock (IS) ahead of a field and plasma compressional region where the draping of the SW flow and possibly the changes in the direction of the IMF are consistent with a location northward of a faster ejecta. This ejecta was responsible for disturbed SW conditions including approximately ten hours of southward orientation of the interplanetary magnetic field (IMF) and a ram pressure many times above normal. The slower moving ejecta was directed toward Earth and was observed with WIND from about 0550 until 1500 UT on April 11. It had a strong northward IMF and produced density enhancements which elevated the ram pressure to more than four times above normal.

A two‐stream, four‐sector, recurrence pattern: Implications from WIND for the 22‐year geomagnetic activity cycle

Continuous solar wind data from WIND reveals a new recurrence pattern which implies that speed variations contribute to the 22-year cycle of geomagnetic activity. Near December 1994 solstice, in keeping with expectation, a four-sector interplanetary magnetic field pattern was accompanied by four streams. As the season advanced toward March equinox, however, the streams in the two sectors with away polarity diminished, leaving a strikingly unusual two-stream, four-sector pattern until late April. Since the magnetic field pointed toward the sun in both streams, the polarity effect of Russell and McPherron [1973] combined with the high-speed flow resulted in a recurrent pattern of sustained geomagnetic activity during these sector passages. The solar wind pattern is consistent with Earths excursion to southern heliographic latitudes at March equinox enabling WIND to sample high-speed flow from only the southern coronal hole. The WIND data imply that the 22-year variation in geomagnetic activity results not only from longer immersion in toward sectors in March and away sectors in September during even solar cycles, as proposed by Russell and McPherron [1973], but also from higher flow speeds in those sectors.

Magnetic cloud‐bow shock interaction: WIND and IMP‐8 observations

An interplanetary magnetic cloud of typical magnetic field flux rope structure and of diameter 0.23 AU was observed in WIND magnetic field and plasma data of February 8, 1995. Starting at about 1 hour later it was also observed at IMP 8, where the bow shock reached at least 39 RE from Earth, on the dusk side, for brief periods within the cloud. This expanded bow shock changed from a perpendicular to pulsation type at IMP-8 as a result of the magnetic field smoothly changing direction within the passing cloud, and its surface normal was nearly, and unexpectedly, invariant during the expansion. It is inferred that, within measurement error, the bow shocks shape expanded almost ‘isotropicly’ and that the magnetosheath became unusually wide.

Bifurcated cusp ion signatures: Evidence for re‐reconnection?

Toroidal Imaging Mass-Angle Spectrograph (TIMAS) ion composition measurements during a pass of the POLAR spacecraft through the Earths magnetospheric cusp show a bifurcated ion signature. High energy (several keV/e) solar wind ions are observed simultaneously with a low energy (up to several hundred eV/e) component. The high energy component exhibits an energy-latitude dispersion consistent with magnetic reconnection at the dayside magnetopause and velocity filtering in the cusp. The low energy component exhibits no such dispersion. Simultaneous observations of the solar wind ion composition from the WIND spacecraft indicate that the high and low energy components in the cusp have a higher He2+/H+ density ratio than that in the solar wind. Additional solar wind data show that the IMF was southward and the solar wind plasma and magnetic field were relatively steady. These POLAR and WIND observations are interpreted as evidence for continuous re-reconnection of magnetospheric field lines that thread the high latitude cusp.