E. W. Hones

ISEE 3 observations of traveling compression regions in the Earth's magnetotail

A traveling compression region (TCR) is a several-minute long compression of the lobe magnetic field produced by a plasmoid as it moves down the tail. They are generally followed by a longer interval of southward tilting magnetic fields. This study reports the first comprehensive survey of TCRs in the distant magnetotail. A total of 116 TCRs were identified in the ISEE 3 magnetic field observations. Of this population, 37 TCRs were observed to be separated by 30 min or more from any other TCR and are termed “isolated” events. “Paired” events are defined as two TCRs separated by less than 30 min. There were 36 such TCRs corresponding to 18 paired events. “Multiple” events were also observed in which more than two TCRs occurred in a series without a gap between TCRs of more than 30 min. The 11 multiple events identified in this study had an average of about four traveling compression regions each for a total of 43 TCRs. The mean amplitude, ΔB/B, and duration, ΔT, for all TCRs were found to be 7.6% and 158 s, respectively. TCRs occurring as isolated events were the largest (ΔB/B = 8.8% and ΔT = 218 s) and those associated with multiple events were the smallest (ΔB/B = 5.6% and ΔT = 84 s). The mean duration of the period of southward tilting Bz following isolated TCRs was 12.3 min. This time interval was found to be quite similar to the average spacing between TCRs in paired and multiple events, 11.2 and 10.2 min, respectively. TCR amplitude and duration were found to be independent of location within the tail lobes suggesting that the plasmoids which cause the TCRs maintain approximately constant volume and shape as they move down the tail. Mean plasmoid dimensions estimated from TCR duration and amplitude under the assumption of a quasi-rigid magnetopause are 35 RE (length) × 15 RE (width) × 15 RE (height). Utilizing auroral kilometric radiation, the AL index, Pi 2 pulsations at two ground stations, and energetic particle data from three geosynchronous spacecraft, it is found that over 91% of the TCR events identified in this study followed substorm onsets or intensifications. The number of TCR events identified in this study are consistent with their release in association with a new substorm onset every 4-6 hrs. The results of this study strongly suggest that the release of plasmoids down the tail near the time of expansion phase onset is an integral step in the substorm process and an important element in the substorm energy budget.

Plasma flow and magnetic field characteristics near the midtail neutral sheet

Using IMP 6, 7, and 8 magnetic field and plasma data, we have determined statistical occurrence properties of bulk flow and magnetic field orientation near the midtail neutral sheet. Characteristics of bulk plasma flow and magnetic field significantly change according to the radial distance down the tail. High-speed flow events (V > 300 kin/s) are essentially restricted to the region tailward of X = -25 RE and are predominantly sunward or tailward. The low-speed flows were nearly equally likely to be in any direction, with the occunnce rate of duskward and sunward flow being larger than that of tailward and dawnward flow. Duskward flow occurrence is highest in the region Earthward of X = -25 RE, while sunward flow occurrence is highest in the region tailward of X = -25 RE. The significance of the dawn-to-dusk flow in the near-Earth region obtained in our study supports the idea that there exists a very effective mechanism to accelerate ions in the dawn-to-dusk direction and hence the relief of pressure buildup in the near-Earth region. During high-speed flow events the relationship between BZ polarity and plasma flow direction is largely consistent with that expected from the magnetic reconnection processes associated with substorms. There are also significant numbers of negative BZ events that are not associated with tailward flow. Mechanism other than substorm neutral line should therefore also taken into account to explain general BZ polarity in the midtail region.

On open and closed field line regions in Tsyganenko's field model and their possible associations with horse collar auroras

Using the empirical Tsyganenko (1987) long model as a prime example of a magnetospheric field model, we have attempted to identify the boundary between open and closed field lines. We define as “closed” all field lines that are connected with the Earth at both ends and cross the equatorial plane earthward of x = −70RE, the tailward validity limit of the Tsyganenko model. We find that the form of the open/closed boundary at the Earths surface, identified with the polar cap boundary, can exhibit the arrowhead shape, pointed toward the Sun, observed in “horse collar auroras” (Hones et al., 1989). The “polar cap” size in the Tsyganenko model increases with increasing Kp values, and it becomes rounder and less pointed. The superposition of a net By field, which is the expected consequence of an IMF By, rotates the polar cap pattern and, for larger values, degrades the arrowhead shape, resulting in polar cap configurations consistent with known asymmetries in the aurora. The pointedness of the polar cap shape also diminishes or even completely disappears if the low-latitude magnetopause is assumed open and located considerably inside of the outermost magnetic flux surface in the Tsyganenko model. The arrowhead shape of the polar cap is found to be associated with a strong increase of Bz from midnight toward the tail flanks, which is observed independently, and is possibly related to the NBZ field-aligned current system, observed during quiet times and strongly northward IMF Bz. The larger Bz values near the flanks of the tail cause more magnetic flux to close through these regions than through the midnight equatorial region. Since field lines at the flanks primarily map to the dayside polar regions, it becomes plausible that the closed field line region extends to higher latitudes on the dayside than on the nightside, when the increase of Bz becomes more pronounced. By comparison with a different field model we demonstrate that this association is not unique to the Tsyganenko model. The similarity of the quiet symmetric polar cap pattern to “horse collar” auroras suggests that the bright “bars,” observed at the sides of the arrowhead shaped polar cap, are connected with the separatrix layers (or plasma sheet boundary layers) extending to the distant X line or separator, while the adjacent “web” regions, located between the bars and the main auroral oval, are connected with the low-latitude boundary regions along the flanks of the magnetotail.

Signatures of the substorm recovery phase at high‐altitude spacecraft

The substorm recovery phase typically commences ∼30 min after the substorm expansion phase onset and covers a period of roughly 1 hour. Several signatures have previously been associated with the recovery phase such as plasma sheet expansion in the midtail and magnetic field return toward the quiet time configuration. However, the detailed temporal sequence during the recovery phase is still not very well established. A total of 66 events in February-April 1979 have been investigated where ISEE 2 observed a plasma sheet expansion associated with fast earthward flows. Of these, 50 events were clearly associated with the substorm recovery phase as identified in ground magnetic records. For this data set, energetic electron (>30 keV) and proton (>145 keV) observations from two geostationary spacecraft were available in 41 cases. Of the 41 cases, 32 of the midtail plasma sheet recoveries were associated with distinctive ion or electron flux increases at geostationary orbit. These flux increases, often in both protons and electrons, were generally observed from the predusk to the postdawn sector. However, very few enhancements were found near local noon. The lack of large energy dispersion in the flux increases and the simultaneous occurrence of both electron and ion enhancements suggests that the particles do not drift from a more distant location (as in substorm expansion phase onsets) but are accelerated or redistributed locally. These events are suggested to be associated with a large-scale reconfiguration of the near-Earth tail as the neutral line retreats to large distances.

Hydromagnetic vortices. I: The 11 december 1977 event

Abstract Through a synthesis of magnetometer, plasma, energetic particle and electric field data from the ISEE satellite pair, we describe the characteristics of the initial (11 December 1977) magnetotail plasma vortex event reported by Hones et al. (1978). The event is associated with a hot (β ∼ 1) compressional hydromagnetic wave and apparent vortical motion is seen because at two points in the flow cycle the flow is field-aligned. The behaviour of the energetic ions receives special study : when combined with the thermal flow measurements energy dispersion is evident in the field-aligned flow, while the large pitch angle energetic ions reveal the presence of gradients. We argue that these gradients are wave-induced and use the data to determine the perpendicular wave wavelength together with the speed and direction of transverse wave propagation.

Is there a near-Earth neutral line?

Abstract High correlations between southward turnings of the IMF, increases of open flux in the tail lobes, and magnetic activity confirm dayside magnetic reconnection plays a fundamental role in magnetospheric substorms. The absence of a time delay between expansion phase onset and the decrease in tail lobe flux suggest that nightside reconnection is involved in the expansion. At 20–30 R e , the disappearance of the center of the plasma sheet and the appearance of tailward flows threaded with southward fields strongly suggest reconnection occurs earthward of this distance. Recent AMPTE/CCE and DMSP observations suggest that current disruption begins near synchronous orbit on closed field lines and expands tailward. Auroral images from the VIKING spacecraft support this. Reports of a lack of correlation between plasma and field changes at the ISEE spacecraft and onsets has led some researchers to reject the hypothesis that reconnection is the cause of the expansion. To counter these reports we present magnetic field and plasma data from substorms observed on an outbound pass by ISEE-2. We show that the tail field responds in the manner predicted by the near-earth neutral line (NENL) model, growing in strength and tilting earthward in the growth phase, and decreasing and tilting upward in the expansion phase. Near the earth the plasma sheet gradually thins in the growth phase while further away it suddenly thins at expansion onset. In the expansion phase the plasma sheet rapidly recovers close to the earth, but its recovery is delayed until the substorm recovery phase further from the earth. We explain many of the recent reports of disagreement between the NENL model and observations by a combination of factors. These include: Large changes in the location of a spacecraft relative to the neutral sheet as the size of the magnetosphere changes with dynamic pressure, or as the central meridian and latitude of substorm onset change due to inherent variability; Differences between the outbound and inbound portions of the ISEE orbit; Change in shape of successive orbits in GSM coordinates. Most important is multiple substorm onsets. Successive intensifications cause quite different effects at a spacecraft as the disturbances moves relative to the spacecraft. We conclude that rather than one x-line, there are probably multiple, localized pairs of x- and o-type lines formed in the near-earth plasma sheet during the substorm expansion.

Observational determination of magnetic connectivity of the geosynchronous region of the magnetosphere to the auroral oval

This is a report of a program to study the magnetic connectivity between the auroral region of the ionosphere and the equatorial geosynchronous region of the magnetosphere. The program uses plasma measurements made with polar orbiting Defense Meteorological Satellite Program (DMSP) satellites and several geosynchronous satellites, seeking time intervals when nearly identical plasma electron spectra (32 eV to 30 keV) indicate magnetic connectivity between a polar/geosynchronous satellite pair. When such signatures of connectivity are found, the locations of the relevant satellite pair are compared with the locations that would be predicted by a magnetospheric model. Here we report results from the initial application of this program that uses data from DMSP F-8, F-9 and F-10 polar satellites and the synchronous satellites 1989-046 and 1990-095 acquired in the 6-day interval March 7–12, 1991. The results are compared with predictions of the T89a model [Tsyganenko, 1989; Peredo et al., 1993]. Orbital calculations predicted 96 close conjunctions among these satellites during the interval. Of those we have made spectral comparisons for 47, finding 20 for which close spectral similarity occurred during few-second intervals. Ionospheric footpoints of the satellite pairs calculated by the T89a model for these intervals revealed eight for which the discrepancy between the DMSP latitude at “best spectral match” and the model projection of the synchronous satellite was smaller than 1°. However, there were five intervals for which the discrepancy was greater than 6°. The latter were all found in the local time interval 0700–0900, perhaps indicating a particular inadequacy of the T89a model in that region. This initial work also supports the view that the auroral oval of discrete arcs is an ionospheric mapping of the full thickness of the plasma sheet (e.g., Feldstein and Galperin, 1985) and not just of the plasma sheet boundary layer (e.g., Eastman et. al., 1985). During just its first few years of concurrent operation this constellation of well-instrumented synchronous and polar-orbiting satellites has provided data for literally thousands of close conjunctions such as we analyze in this report. Thus an algorithm is developed enabling the verification and testing of proposed mappings between the ionosphere and magnetosphere (such as T89a).

Hydromagnetic vortices. II: Further dawnside events

We show that the 11 December 1977 plasma vortex event—the subject of a multi-instrument investigation in Paper I (Saunders et al., 1983)—was neither atypical nor uncommon, by describing the magnetic and plasma characteristics of three further vortices recorded within 3 weeks of, and at similar locations to, the 11 December study. One of the new events has added interest since magnetic pulsations were seen simultaneously on the ground in the vicinity of the satellite magnetic “footprint”.

Relations between bursts of energetic electrons at 17 Earth-radii in the magnetotail and radio absorption events in the ionospheric D-region

Abstract A three-dimensional picture of the auroral phenomena around the Earth, in terms of their energy relations and movements, may be constructed by combining ground-based synoptic observations with measurements in selected satellite orbits. In the magnetotail at seventeen Earth-radii bursts of energetic electrons ( E > 45 keV), presumably the high-energy component of clouds of hot plasma, are frequently observed. The present contribution suggests that these plasma clouds appear at successively greater distances from the Earth, so that they appear to be moving away from the Earth into the magnetotail. The flux of electrons in these bursts is less intense than the associated flux at ionospheric heights deduced from D -region absorption. In these and other respects the results suggest that these two phenomena (magnetospheric electron bursts and D -region absorption) have a common source near midnight, perhaps at about seven Earth-radii geocentric distance.

Energetic ion anisotropies in the geomagnetic tail. 1. A statistical survey

During 1978 and 1979, the medium energy particles experiment on board the ISEE 2 spacecraft (apogee 22.7 RE) collected almost 122 days of data in the central region of the Earths magnetotail. We have surveyed 16-s averages of the energetic ion (E > 25 keV) and plasma measurements in order to establish the statistical properties of high ion anisotropy events (|A| > 1) which are frequently observed in the near-Earth plasma sheet. Our major findings from the analysis of 1944 high-anisotropy samples (HAS) are as follows: (1) Strong energetic ion anisotropies are not maintained over long periods of time but occur in bursts lasting for fractions of a minute up to a few minutes. (2) For almost 80% of the HAS, the anisotropy vector points either earthward or tailward rather than across the tail (3) The ratio of HAS toward the Earth and HAS toward the tail is approximately 4.8:1. (4) The probability of observing high-anisotropy events is well enhanced beyond ≈16 RE downtail distance, on the duskside, and close (within 2–3 RE) to the neutral sheet. (5) Strong tailward anisotropies can be observed anywhere in the near-Earth plasma sheet. However, their occurrence frequency is considerably above average beyond xSE ≈ −19 RE and within 2 RE of the neutral sheet. (6) Both plasma bulk flow and ion anisotropy at higher energy (>115 keV) are generally in the same direction as the energetic ion anisotropy at E > 25 keV. (7) Streaming energetic electron populations in the same direction as the ions are significantly more often observed when the ions are streaming tailward than simultaneously with earthward streaming ions when the electrons are usually isotropic. We conclude that statistically the observation of strong tailward ion streaming indicates the existence of a particle source in the near-Earth magnetotail between the spacecraft and the Earth. Certain features suggest that this source might be identical with a near-Earth magnetic neutral line.