John T. Steinberg

Magnetopause location under extreme solar wind conditions

During the solar wind dynamic pressure enhancement, around 0200 UT on January 11, 1997, at the end of the January 6-11 magnetic cloud event. the magnetopause was pushed inside geosynchronous orbit. The LANL 1994-084 and GMS 4 geosynchronous satellites crossed the magnetopause and moved into the magnetosheath. Also, the Geotail satellite was in the magnetosheath while the Interball 1 satellite observed magnetopause crossings. This event provides an excellent opportunity to test and validate the prediction capabilities and accuracy of existing models of the magnetopause location for producing space weather forecasts. In this paper, we compare predictions of two models: the Petrinec and Russell [1996] model and the Shue et al. [1997] model. These two models correctly predict the magnetopause crossings on the dayside; however. there are some differences in the predictions along the flank. The Shue et al. [1997] model correctly predicts the Geotail magnetopause crossings and partially predicts the Interball 1 crossings. The Petrinec and Russell [1996] model correctly predicts the Interball 1 crossings and is partially consistent with the Geotail observations. We further found that some of the inaccuracy in Shue et al.s predictions is due to the inappropriate linear extrapolation from the parameter range for average solar wind conditions to that for extreme conditions. To improve predictions tinder extreme solar wind conditions, we introduce a nonlinear dependence of the parameters on the solar wind conditions to represent the saturation effects of the solar wind dynamic pressure on the flaring of the magnetopause and saturation effects of the interplanetary magnetic field B z on the subsolar standoff distance. These changes lead to a better agreement with the Interball 1 observations for this event.

Geotail observations of the Kelvin‐Helmholtz instability at the equatorial magnetotail boundary for parallel northward fields

For several hours on March 24, 1995, the Geotail spacecraft remained near the duskside magnetotail boundary some 15 RE behind the Earth while the solar wind remained very quiet (V=330 km s−1, n=14–21 cm−3) with a very steady 11-nT northward magnetic field. Geotail experienced multiple crossings of a boundary between a dense (n= 19 cm−3), cool (Tp=40 eV), rapidly flowing (V=310 km s−1) magnetosheath plasma and an interior region characterized by slower tailward velocities (V=100 km s−1), lower but substantial densities (n=3 cm−3) and somewhat hotter ions (220 eV). The crossings recurred with a roughly 3-min periodicity, and all quantities were highly variable in the boundary region. The magnetic field, in fact, exhibited some of the largest fluctuations seen anywhere in space, despite the fact that the exterior magnetosheath field and the interior magnetosphere field were both very northward and nearly parallel. On the basis of an MHD simulation of this event, we argue that the multiple crossings are due to a Kelvin-Helmholtz instability at the boundary that generates vortices which move past the spacecraft. A determination of boundary normals supports Kelvin-Helmholtz theory in that the nonlinear steepening of the waves is seen on the leading edge of the waves rather than on the trailing edge, as has sometimes been seen in the past. It is concluded that the Kelvin-Helmholtz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward interplanetary magnetic field.

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.

Extremely high speed solar wind: 29-30 October 2003

[1] On 29-30 October 2003 the Solar Wind Electron Proton Alpha Monitor (SWEPAM) instrument on the Advanced Composition Explorer (ACE) spacecraft measured solar wind speeds in excess of 1850 km/s, some of the highest speeds ever directly measured in the solar wind. These speeds were observed following two large coronal mass ejection (CME) driven shocks. Surprisingly, despite the unusually high speeds, many of the other solar wind parameters were not particularly unusual in comparison with other large transient events. The magnetic field reached -68 nT, a large but not unprecedented value. The proton temperatures were significantly higher than typical for a CME in the solar wind at 1 AU (>10 7 K), but the proton densities were moderate, leading to low to moderate proton beta. The solar wind dynamic pressure was not unusual for large events but, when coupled with the large negative B z , was sufficient to cause intense geomagnetic disturbances.

Evidence for water ice near the lunar poles

Improved versions of Lunar Prospector thermal and epithermal neutron data were studied to help discriminate between potential delivery and retention mechanisms for hydrogen on the Moon. Improved spatial resolution at both poles shows that the largest concentrations of hydrogen overlay regions in permanent shade. In the north these regions consist of a heavily cratered terrain containing many small (less than ∼10-km diameter), isolated craters. These border circular areas of hydrogen abundance ([H]) that is only modestly enhanced above the average equatorial value but that falls within large, flat-bottomed, and sunlit polar craters. Near the south pole, [H] is enhanced within several 30-km-scale craters that are in permanent shade but is only modestly enhanced within their sunlit neighbors. We show that delivery by the solar wind cannot account for these observations because the diffusivity of hydrogen at the temperatures within both sunlit and permanently shaded craters near both poles is sufficiently low that a solar wind origin cannot explain their differences. We conclude that a significant portion of the enhanced hydrogen near both poles is most likely in the form of water molecules.

The Wind magnetic cloud and events of October 18–20, 1995: Interplanetary properties and as triggers for geomagnetic activity

Late on October 18, 1995, a magnetic cloud arrived at the Wind spacecraft ≈ 175 RE upstream of the Earth. The cloud had an intense interplanetary magnetic field that varied slowly in direction, from being strongly southward to strongly northward during its ≈ 30 hours duration, and a low proton temperature throughout. From a linear force free field model the cloud was shown to have a flux rope magnetic field line geometry, an estimated diameter of about 0.27 AU, and an axis that was aligned with the Y axis(GSE) within about 25°. A corotating stream, in which large amplitude Alfven waves of about 0.5 hour period were observed, was overtaking the cloud and intensifying the fields in the rear of the cloud. The prolonged southward magnetic field observed in the early part of the cloud produced a geomagnetic storm of Kp = 7 and considerable auroral activity late on October 18. About 8 hours in front of the cloud an interplanetary shock occurred. About three-fourths the way into the cloud another apparent interplanetary shock was observed. It had an unusual propagation direction, differing by only 21° from alignment with the cloud axis. It may have been the result of the interaction with the postcloud stream, compressing the cloud, or was possibly due to an independent solar event. It is shown that the front and rear boundaries of the cloud and the upstream driven shock had surface normals in good agreement with the cloud axis in the ecliptic plane. The integrated Poynting flux into the magnetosphere, which correlated well with geomagnetic indices, jumped abruptly to a high value upon entry into the magnetic cloud, slowly decreased to zero near its middle, and again reached substantial but sporadic values in the cloud-stream interface region. This report aims to support a variety of ISTP studies ranging from the solar origins of these events to resulting magnetospheric responses.

Proton temperature anisotropy constraint in the solar wind: ACE observations

The electromagnetic proton cyclotron anisotropy instability may arise in collisionless plasmas in which the proton velocity distribution is approximately bi-Maxwellian with T⊥ p /T ||p > 1, where ⊥ and || denote directions relative to the background magnetic field B o . Theory and simulations predict that enhanced field fluctuations from this instability impose a constraint on proton temperature anisotropies of the form T⊥ p /T ||p - 1 = S p /β ||p αp where β ||p ≡ 8πn p k B T ||p /B o 2 , and the fitting parameters S p ≤ 1 and α p ≃ 0.4. Plasma and magnetic field observations from the ACE spacecraft reported here show for the first time that this constraint is statistically satisfied in the high speed solar wind near 1 AU, and magnetic power spectra provide evidence that this instability is the source of the constraint.

A simple simulation of a plasma void: Applications to Wind observations of the lunar wake

Previously, the formation of the lunar wake was considered from a magnetohydrodynamic perspective. However, recent Wind particle and field observations suggest the lunar wake may be formed by kinetic processes: those microphysical processes not considered in an MHD formalism. Unfortunately, a full multidimensional and self-consistent kinetic simulation of the lunar wake is beyond current means. However, some elements of the kinetic structure can be simulated via a simple one-dimensional electrostatic particle-in-cell simulation. We present a self-consistent simulation of a cross-sectional element of a plasma void. Essentially, wakeward directed ion beams are formed at the flanks of the simulated void, consistent with the Wind observations of counterstreaming ion beams in the wake region. These wakeward directed beams are generated by ambipolar electric fields formed at the wake edges. Other structures observed by Wind are also seen in the simulation, including an electrostatically turbulent central wake region that causes the wake to fill-in and a rarefaction wave emanating outward from the wake.

Geotail observations of substorm onset in the inner magnetotail

On April 26, 1995, while Geotail was in the near-equatorial magnetotail at 13 RE and 2300 LT, a substorm onset occurred that was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit. Although Geotail was initially outside a greatly thinned current sheet, plasma sheet thickening associated with the substorm dipolarization quickly caused Geotail to move into the plasma sheet where it observed field-aligned earthward moving ions with velocities of 400 km/s. During the subsequent few minutes as the magnetic field became more northward, the velocities increased with particles moving increasingly into the energy range of the energetic particle experiment. These flows culminated with 1-min worth of earthward flow of 2000 km/s that was perpendicular to the northward B field. Such flow, probably the largest ever detected at 13 RE, was confirmed by the observation of an intense dc electric field of 50 mV/m (0.3 megavolts/RE). This large field is probably inductive, caused by reconnection that occurred tailward of the spacecraft, and related to the acceleration processes associated with particle injection at geosynchronous orbit. Energy and magnetic flux conservation arguments suggest that this rapid flow has a small cross-tail dimension of the order of 1 RE. The data appear to support a simulation of Birn and Hesse [1996] which showed rapid earthward flows from a reconnection line at 23 RE that caused a tailward expansion of a region of dipolarized flux. Subsequent to the onset, Geotail observed plasma vortices with typical velocities of 50–100 km/s that occurred in a high-beta plasma sheet with a 15-nT northward magnetic field. The vortices were punctuated by occasional flow bursts with velocities up to 400 km/s, one of which was accompanied by a violently varying magnetic field where north/south field components were as large as 30 nT and as small as −8 nT.

The solar wind helium abundance: Variation with wind speed and the solar cycle

We investigate the helium abundance in the solar wind and variations thereof on a time scale of years. Data from the WIND-SWE experiment gathered between the end of 1994 and early 2000 are analyzed. In agreement with similar work for previous solar cycles, we find a clear dependency of the He/H ratio in the solar wind on the solar cycle. In the slow solar wind, the average He/H rises from a minimum of less than two percent around solar minimum to about 4.5% in early 2000. The solar cycle dependency is stronger the lower the speed that is used to sort the data. We observe the strongest dependency of the He/H ratio on the solar wind speed around solar minimum and it weakens as the solar activity increases. We speculate that the expansion factor of the magnetic field close to the Sun changes over the solar cycle and thereby changes the efficiency of the Coulomb drag. Inefficient Coulomb drag leads to a low helium abundance in the solar wind.