T. A. Potemra

Current disruptions in the near-Earth neutral sheet region

Observations from the Charge Composition Explorerin 1985 and 1986 revealed fifteen current disruption events in which the magnetic field fluctuations were large and their onsets coincided well with ground onsets of substorm expansion or intensification. These events are of short durations locally (∼1–5 min). They are mostly confined to within ∼0.5 RE of the neutral sheet and 1 hour local time from the magnetic midnight. Over the disruption interval, the local magnetic field can change by as much as a factor of ∼7. In general, the stronger the current buildup and the closer to the neutral sheet, the larger the resultant field change. There is also a tendency for a larger subsequent enhancement in the AE index with a stronger current buildup prior to current disruption. For events with good pitch angle coverage and extended observation in the neutral sheet region we find that the particle pressure increases toward the disruption onset and decreases afterward. Just prior to disruption, either the total particle pressure is isotropic, or the perpendicular component (P⊥) dominates the parallel comment (P∥), the plasma beta is seen to be as high as ∼70, and the observed plasma pressure gradient at the neutral sheet is large along the tail axis. The deduced local current density associated with pressure gradient is ∼27–80 nA/m² and is ∼85–105 mA/m when integrated over the sheet thickness. We infer from these results that just prior to the onset of current disruption, (1) an extremely thin current sheet requiring P∥ > P⊥ for stress balance does not develop at these distances, (2) the thermal ion orbits are in the chaotic or Speiser regime while the thermal electrons are in the adiabatic regime and, in one case, exhibit peaked fluxes perpendicular to the magnetic field, thus implying no electron orbit chaotization to possibly initiate ion tearing instability, and (3) the neutral sheet is in the unstable regime specified by the cross-field current instability. Subsequent to the disruption onset, enhancement of magnetic noise over a broad frequency range, magnetic field aligned counterstreaming electron beams, ion energization perpendicular to the magnetic field, and current reduction in the amount similar to that of current buildup during the growth phase are observed. These features seem to be compatible with the predicted development of the cross-field current instability.

A comparison of ULF fluctuations in the solar wind, magnetosheath, and dayside magnetosphere: 1. Magnetosheath morphology

“Upstream waves,” generated in the solar wind upstream of a quasi-parallel bow shook, are believed to be a major source of the Pc 3-4 pulsation activity observed in the dayside magnetosphere. In an attempt to better understand the means by which “upstream wave” energy is transmitted from the solar wind into the magnetosphere, we compared simultaneous data from ISEE 1 and 2 in the upstream solar wind, AMPTE IRM in the subsolar magnetosheath, and AMPTE CCE in the dayside magnetosphere. Our observations indicate that dayside magnetospheric Pc 3-4 pulsation activity and low IMF cone angles are associated with increased turbulence in the subsolar magnetosheath magnetic field (with large amplitude fluctuations both parallel and transverse to the average field direction), and with increased and highly variable levels of energetic magnetosheath particles. Fourier analysis of the magnetic field fluctuations shows broadband increases in wave power from 0.01 Hz to at least 0.5 Hz, but with peak power at Pc 3-4 frequencies; there is no evidence in our data set of narrow-band magnetic field variations in the magnetosheath at these times. Purely compressional waves, which are at times observed in the subsolar magnetosheath, have a somewhat narrower frequency distribution, but are associated with neither upstream wave activity nor magnetospheric pulsations.

Simultaneous observation of the poleward expansion of substorm electrojet activity and the tailward expansion of current sheet disruption in the near-Earth magnetotail

In this paper we present observations of a substorm that occurred on June 7, 1985. The data consist of energetic ion and magnetic field data from AMPTE/CCE, magnetometer data from the ground stations of the EISCAT magnetometer cross, STARE radar data, and Pi 1 data from Sodankyla. CCE was in the Finnish local time sector, and the model field line threading CCE came down near the center of the EISCAT magnetometer cross. Furthermore, CCE was located near the neutral sheet and observed the disruption of the cross-tail current in situ. At 2209 UT, Sodankyla registered a Pi 1 burst and the EISCAT stations recorded the onset of a negative bay. Simultaneously, CCE observed the onset of current sheet disruption and ion energization. The energetic ions observed during this initial burst had gyrocenters earthward of CCE. An intensification of activity occurred at 2212 UT, and it was observed essentially simultaneously on the ground and in space. It consisted of an intensification of Pi 1 activity, an intensification of the westward electrojet poleward of the initial breakup latitude (as determined from magnetometer and radar data), and magnetic turbulence and a burst of energetic ions at CCE in the magnetotail. Significantly, this second burst was composed only of ions with gyrocenters tailward of the satellite, and the magnetic turbulence was considerably weaker than that observed during the initial current disruption. This strongly suggests that the current disruption region had moved tailward of CCE. The event provides a direct observational link between the tailward expansion of active regions in the near-Earth magnetotail and the poleward expansion of ionospheric electrojet activity during substorms.

The correlation of VLF propagation variations with atmospheric planetary-scale waves

Abstract Variations in the received daytime phase of long distance, cesium-controlled, VLF transmissions are compared to the height variations of the 10 mb isobaric surface during the first 3 months of 1965 and 1969. The VLF phase values are also compared to height variations of constant electron densities in the E -region from Brown and Williams (1971) and to variations of ƒ- min from Deland and Friedman (1972) which have been shown to be well correlated with planetary-scale variations in the stratosphere by Deland and Cavalieri (1973). The VLF phase variations show good correlation with these previous ionospheric measurements and with the 10 mb surfaces. The VLF variations appear to lag the stratospheric variations in the same area by about 4 days during the 1965 period, but lead the latter by about 4 days during the 1969 period. The planetary scale waves in the stratosphere are shown to be travelling eastward on the average in 1965 and westward in 1969. The above correlations are interpreted as due to the propagation of travelling planetary scale waves with westward tilted wave fronts. Upward energy transport due to the vertical structure of those waves is also discussed. These correlations provide further evidence for the coupling between the lower ionosphere at about 70 km altitude (the day-time VLF reflection height) and the stratosphere, and they demonstrate the sensitivity and usefulness of VLF transmissions for studies of planetary wave phenomena.

Observations of solar wind pressure initiated fast mode waves at geostationary orbit and in the polar cap

Abstract This study involves a multi-satellite investigation of magnetic field perturbations in the magnetosphere due to solar wind dynamic pressure variations. The case study uses data acquired on 3 August 1986 from IMP-8, GOES-5, GOES-6, VIKING, and ground magnetograms. As expected, dramatic solar wind dynamic pressure variations, recorded at IMP-8, produced compressions and rarefactions of the magnetosphere as seen in GOES-5 and GOES-6 magnetic field observations at geostationary orbit. These same compressions and rarefactions were also recorded by the VIKING magnetic field experiment over the polar cap at mid-altitudes. The magnetic field perturbations are interpreted in terms of fast mode waves which were generated near the magnetopause and propagate anti-sunward into the inner magnetosphere. In addition, we compare the amplitude of the magnetic field perturbations recorded at geostationary orbit and VIKING. The wave amplitudes recorded by GOES and VIKING were nearly identical, suggesting a low damping rate of the waves in the magnetosphere.

An ionospheric travelling convection vortex event observed by ground-based magnetometers and by VIKING

A transient ionospheric travelling convection vortex (ITCV) event was recorded by the EISCAT magnetometer cross in northern Scandinavia on April 21, 1986 around 8:40 MLT. Simultaneously, the near-c ...

Filamentary current structures in the postnoon sector: Observations from UARS

During an intense geomagnetic storm (Kp 7+) that began at {approximately} 1830 UT on October 1, 1991, the UARS satellite encountered the dayside postnoon auroral oval. On two consecutive crossings of the northern hemisphere between 2040 and 2240 UT, the vector magnetometer detected region 1 and 2 Birkeland and ionospheric currents in the postnoon sector. Low-energy electron events were observed near 1400 MLT within a narrow portion of the region 1 current system. Simultaneous magnetic field measurements revealed the presence of intense ({approximately} 20 {mu}A/m{sup 2}) bipolar filament current structures embedded in the auroral oval. The upward-directed currents were associated with the more concentrated region of precipitating electrons. Ions associated with the more intense flux of low-energy electrons exhibited a dispersion signature typical of an ion velocity filter. The dispersion, aligned along the orbit, exhibited higher-energy ions at lower latitudes and earlier local times. The colocation of filament currents and ion dispersion signatures at such late postnoon local times is not consistent with typical E x B {open_quotes}cusp{close_quotes} dispersions. These features more likely result from dayside boundary wave phenomena. 34 refs., 9 figs.

Magnetic-field fluctuations from 0 to 26 Hz observed from a polar-orbiting satellite

Data from the Viking magnetic-field experiment are described which demonstrate the diverse types of magnetic fluctuations which may be observed from a polar-orbiting satellite. The Viking observations provide an opportunity to observe Pc 1 waves at middle-latitudes above the ionosphere and to determine the spectral structure and polarization of the waves. ULF/ELF broadband noise represents a second type of magnetic fluctuation measured by Viking. This type of magnetic fluctuation was observed at high latitudes near the polar cusp and may be useful in the identification of polar-cusp boundaries. Thirdly, electromagnetic ion-cyclotron waves have also been observed in the polar-cusp region. These waves occur only during an unusually high level of magnetic activity and appear toe be generated locally. >

The Projection of the Magnetospheric Boundary Layers to Mid-Altitudes

Observations made by the Viking spacecraft above the auroral oval in the postmidnight to pre-noon local time sector are used to study how the various magnetospheric plasma regimes project to the polar ionosphere. Under specific consideration is the projection of the lowlatitude boundary layer (LLBL). In the present study the term LLBL will be used to designate the total magnetospheric boundary region adjacent to the magnetopause and it only excludes the part connected to the tail lobes (plasma mantle, high-latitude boundary layer). It is shown that the ionospheric projection of the LLBL has a much wider extent both in latitude and MLT as previously assumed. Furthermore, during quiet geomagnetic conditions the LLBL plasma flow drives the large-scale downward field-aligned current (R1 current) on the morning side. It is only during disturbed periods that processes associated with the inner magnetospheric tail or internal tail boundary layers have a major impact on the dawn to pre-noon auroral oval.

Boundary Determinations from Low Frequency Magnetic Field Measurements

Objects in the interplanetary medium are subject to interactions with the solar wind. As the solar wind encounters magnetized or unmagnetized objects, a shock wave forms upstream of the object and the solar wind field is distorted and draped over the obstacle. For magnetic bodies, this interaction produces three distinct regions of space: the solar wind, the magnetosphere of the body, and the magnetosheath or shocked solar wind between the solar wind and the magnetosphere. Accurate characterization of magnetic bodies therefore depends on identification of crossing from the solar wind or magnetosheath into the magnetosphere of the object. Magnetic measurements in the < 1Hz to 100’s Hz range (denoted AC) have proven extremely useful in determining characteristic regions within the Earth’s magnetosphere. At low altitudes, magnetic fluctuation levels are particularly useful in identifying field-aligned currents of the ionosphere auroral zones. These currents are a necessary consequence of the solar wind and will occur at other bodies as well, connecting the various altitude regions of magnetospheres. At high altitudes, the AC levels have proven indicative of magnetosheath and magnetopause current layers. Magnetosheath regions are subject to ion cyclotron and mirror mode local instabilities which do not penetrate the magnetopause. Furthermore, the magnetopause boundary for large magnetic shear is the site of intense magnetic noise. Magnetic fluctuation levels may be used to assist in the identification, at various altitudes, of the boundaries between the regions of magnetic influence of the solar wind and interplanetary bodies.