D. G. Sibeck

Comprehensive study of the magnetospheric response to a hot flow anomaly

We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (∼7 min) move outward some 5 RE beyond its nominal position and engulf Interball-1. A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in high-latitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

Observation of IMF and seasonal effects in the location of auroral substorm onset

We use Polar ultraviolet imager (UVI) and Wind observations to study the location of 648 well-defined Northern Hemisphere auroral breakups (substorm onsets) in response to interplanetary magnetic field (IMF) orientation and season. The most likely onset location is at 2230 MLT and 67° Λm with half-maximum widths of 3 hours of MLT and 2° Λm, respectively. The onset latitude depends primarily on IMF Bz, but also Bx: the onset latitude decreases for Bx > 0 or Bz 0. The onset longitude depends on season and IMF By. In summer, substorms tend to occur in the early evening at ∼2200 MLT, whereas in winter they tend to occur near midnight at ∼2300 MLT. The average summer-winter difference in the onset location is ∼1 hour of MLT. Large By effects on the onset longitude occur only when Bx and By are small. Onset locations shift toward earlier local times for By > 0 and toward midnight for By 0 in summer and latest (2330 MLT) for By 0 the onset location shifts toward dusk when By > 0 but toward dawn when By < 0; the sense of this shift reverses for Bx < 0. An implication of the results is that auroral breakup is not conjugate.

Magnetopause motion driven by interplanetary magnetic field variations

We use previously reported observations of hot flow anomalies (HFAs) and foreshock cavities to predict the characteristics of corresponding features in the dayside magnetosheath, at the magnetopause, and in the outer dayside magnetosphere. We compare these predictions with Interball 1, Magion 4, and GOES 8/GOES 9 observations of magneto-pause motion on the dusk flank of the magnetosphere from 1800 UT on January 17 to 0200 UT on January 18, 1996. As the model predicts, strong (factor of 2 or more) density enhancements bound regions of depressed magnetosheath densities and/or outward magnetopause displacements. During the most prominent event, the geosynchronous spacecraft observe an interval of depressed magnetospheric magnetic field strength bounded by two enhancements. Simultaneous Wind observations indicate that the intervals of depressed magnetosheath densities and outward magnetopause displacements correspond to periods in which the east/west (By) component of the interplanetary magnetic field (IMF) decreases to values near zero rather than to variations in the solar wind dynamic pressure, the north/south component of the IMF, or the IMF cone angle.

The Plasmaspheric Plume and Magnetopause Reconnection

We present near-simultaneous measurements from two THEMIS spacecraft at the dayside magnetopause with a 1.5 h separation in local time. One spacecraft observes a high-density plasmaspheric plume while the other does not. Both spacecraft observe signatures of magnetic reconnection, providing a test for the changes to reconnection in local time along the magnetopause as well as the impact of high densities on the reconnection process. When the plume is present and the magnetospheric density exceeds that in the magnetosheath, the reconnection jet velocity decreases, the density within the jet increases, and the location of the faster jet is primarily on field lines with magnetosheath orientation. Slower jet velocities indicate that reconnection is occurring less efficiently. In the localized region where the plume contacts the magnetopause, the high-density plume may impede the solar wind-magnetosphere coupling by mass loading the reconnection site.

Solar wind preconditioning in the flank foreshock: IMP 8 observations

We use IMP 8 plasma, magnetic field, and energetic ion observations within the Earths foreshock from January through August 1995 to determine the effects of energetic ion bursts on the ambient solar wind for comparison with model predictions. Owing to the spiral interplanetary magnetic field orientation, the events are far more common upstream from the prenoon than postnoon bow shock. Pressures associated with the energetic ions depress foreshock magnetic field strengths and plasma densities. The magnitude of the depression is proportional to the intensity of energetic ions. The excavated plasma and magnetic field sometimes pile up in narrow regions of enhanced plasma densities and magnetic field strengths, but depressed flow velocities, just outside the foreshock cavities. Typical amplitudes of the depressions and enhancements at IMP 8 far upstream from the bow shock are far less (20% and 10%, respectively) than those seen in past case studies of events observed just outside the bow shock. The cavities occur preferentially during high-speed solar wind streams but show no clear dependence upon other solar wind parameters. The distribution of burst durations resembles those for interarrival times for interplanetary magnetic field discontinuities, magnetopause motion, and flux transfer events, suggesting causal relationships between these phenomena.

Two point observation of high‐latitude reconnection

Reconnection on the high-latitude magnetopause can produce sunward plasma flow in a portion of the lobes. This plasma should precipitate into the polar cap, poleward of the locations where magnetosheath plasma is normally found. We present two point INTERBALL observations of reconnection which is qualitatively consistent with the antiparallel merging model. Reconnection creates a plasma layer about 1R E thick adjacent to the magnetopause (MP). Timing considerations suggest tailward motion of the reconnection site with a velocity ∼ 20 km/s and a reconnection site with dimensions on the order of 1 R E .

Plasma and Energetic Particle Behaviors During Asymmetric Magnetic Reconnection at the Magnetopause

The factors controlling asymmetric reconnection and the role of the cold plasma population in the reconnection process are two outstanding questions. We present a case study of multipoint Cluster observations demonstrating that the separatrix and flow boundary angles are greater on the magnetosheath than on the magnetospheric side of the magnetopause, probably due to the stronger density than magnetic field asymmetry at this boundary. The motion of cold plasmaspheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, which are probably accelerated by reconnection in the diffusion region near the subsolar magnetopause, the colder ions are simply entrained by E×B drifts at high latitudes on the recently reconnected magnetic field lines. This indicates that plasmaspheric ions can sometimes play only a very limited role in asymmetric reconnection, in contrast to previous simulation studies. Three cold ion populations (probably H+, He+, and O+) appear in the energy spectrum, consistent with ion acceleration to a common velocity.

Concerning the location of magnetopause merging as a function of the magnetopause current strength

We start from an assumption that merging occurs in regions of the magnetopause where current strengths are greater than some threshold value which corresponds to the total jump in the field across the magnetopause greater than 50 nT. Because time and cost constraints preclude running numerical simulations for a wide variety of interplanetary magnetic field (IMF) orientations to determine these locations, we adopt an analytical model based on previously derived formulations for magnetospheric and magnetosheath magnetic fields. The magnetospheric magnetic field is confined within a paraboloid. The magnetosheath magnetic field is derived from that in the solar wind and lies between the magnetopause and a paraboloid bow shock. We allow a slight diffusion of the magnetosheath magnetic field into the magnetosphere. The results of the model show that during periods of due southward IMF orientation, merging occurs (as expected) in a wide region centered on the subsolar magnetopause. During periods of northward IMF, connection continues near the subsolar point but also poleward of the cusps. Magnetic energy is only released to the plasma in the latter regions. During periods of strongly northward IMF (By = 0), reconnection ceases on the subsolar magnetopause but continues poleward of the cusp. If the IMF points northward but By is nonzero, reconnection continues near the subsolar point and poleward of the cusps. During periods of sunward IMF orientation, merging nearly ceases on the northern hemisphere (except in the vicinity of the subsolar point) but continues outside the southern lobes. Dawnward and duskward IMF orientations produce tilted patches of enhanced current densities in the subsolar region. We compare the results of our model with previous predictions of the “component” and “antiparallel” merging models.

Magnetosheath response to the interplanetary magnetic field tangential discontinuity

We present a multipoint observational study of the magnetosheath response to the interplanetary magnetic field tangential discontinuities which form hot flow anomaly-like (HFAs) structures. We identify these structures in the vicinity of the bow shock as well as deeper in the magnetosheath. Two or more points of simultaneous observations allow us to describe the gradual evolution and propagation of these HFAs through the magnetosheath. From tens of events recorded by INTERBALL-1, we present two cases. In the first, GEOTAIL identified HFAs in the solar wind near the bow shock, and INTERBALL-1 and MAGION-4 observed related events in the magnetosheath. During the second interval, all three spacecraft observed the HFA features in the magnetosheath. Our analysis of reported events suggests the negligible evolution of these structures in the magnetosheath. A survey of the INTERBALL-1 data has shown that magnetosheath HFAs are observed predominantly during periods of fast solar wind.

Two‐point measurements of the magnetopause: Interball observations

The impulsive penetration model predicts that blobs of solar wind plasma penetrate the magnetosphere by becoming detached from the magnetopause. If so, negative radial density gradients should be common near the magnetopause. We present the results of case and statistical studies of simultaneous Interball 1 and Magion 4 omnidirectional plasma sensors (VDP) plasma observations during magnetopause passes. The spacecraft observe very similar features when their interspacing is small. By contrast, the spacecraft located farther radially outward invariably observes densities greater than or equal to those seen at the spacecraft nearer Earth when the interspacing increases. The observations are consistent with the standard interpretation in which a boundary layer of plasma with intermediate densities often lies sandwiched between the magnetosheath and the magnetosphere proper. Blobs of magnetosheath or boundary layer plasma rarely, if ever, become detached from the magnetopause.