The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay often sees F‐region drifts or electric fields which are associated with field line resonances in the Earth's magnetosphere. These resonances are seen in the interval from local midnight to morning, and have discrete, latitude‐dependent frequencies at approximately 1.3, 1.9, 2.6–2.7, and 3.2–3.4 mHz. We show that these frequencies are compatible with MHD waveguide modes, with antisunward propagation and reflection at the magnetopause and at turning points on dipolar field lines.

Abstract. Poleward boundary intensifications are nightside geomagnetic disturbances that have an auroral signature that moves equatorward from the poleward boundary of the auroral zone. They occur repetitively, so that many individual disturbances can occur during time intervals of-1 hour, and they appear to be the most intense auroral disturbance at times other than the expansion phase of substorms. We have used data from three nightside conjunctions of the Geotail spacecraft in the magnetotail with the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based array in central Canada to investigate the relation between the poleward boundary intensifications and bursty plasma sheet flows and to characterize the bursty flows associated with the disturbances. We have found a distinct difference in plasma sheet dynamics between periods with, and periods without, poleward boundary intensifications. During periods with identifiable poleward boundary intensifications, the plasma sheet has considerable structure and bursty flow activity. During periods without such poleward boundary intensifications, the plasma sheet was found to be far more stable with fewer and weaker bursty flows. This is consistent with the intensifications being the result of the mapping to the ionosphere of the electric fields that give rise to bursty flows within the plasma sheet. Two different types of plasma sheet disturbance have been found to be associated with the poleward boundary intensifications. The first consists of plasma sheet flows that appear to be the result of Speiser motion of particles in a localized region of thin current sheet. The second, seen primarily in our nearest-to-the-Earth example, consists of energy-dispersed ion structures that culminate in bursts of low-energy ions and isotropic low-energy electrons and are associated with minima in magnetic field and temperature and maxima in ion density and pressure. Both types of plasma sheet disturbance are associated with localized regions of enhanced dawn-to-dusk electric fields and appear to be associated with localized enhanced reconnection. Our analysis has shown that poleward boundary intensifications are an important aspect of geomagnetic activity that is distinct from substorms. In addition to their very distinct auroral signature, we have found them to be associated with a prolonged series of ground magnetic Pi 2 pulsations and ground X component perturbations, which peak at latitudes near the ionospheric mapping of the magnetic separatrix, and with a series of magnetic B z oscillations near synchronous orbit. Like substorms, the tail dynamics associated with the poleward boundary intensifications can apparently extend throughout the entire radial extent of the plasma sheet. Color versions of figures are available at http ://www' atmøs'ucla'edu/-larry/geøtail'html'

In summer 1969 a line of 7 magnetic stations was set up in western Canada between the geomagnetic latitudes of 59 ø and 77øN and within •2 ø of 302øE corrected geomagnetic longitude. This paper concerns the analysis of the horizontal components of quasi-monochromatic, geomagnetic micropulsations recorded at these stations over 3 days. Both the amplitude spectra and the sense of polarization of the micropulsations exhibit marked latitude dependence, thus implying that much of the micropulsation energy is distributed in the torotrial mode of eigenoscillations of the geomagnetic lines of force. A switch in the sense of polarization around 1200-1400 LT strongly suggests that Pc micropulsations are generated through the development of Kelvin-I-Ielmholtz instabilities at the magnetopause. An intriguing aspect of geomagnetic micropulsations is that they often occur as a series of almost sinusoidal oscillations lasting up to several hours. Although micropulsations were in the past classified according to morpholo•cal, correlative, and genetic characteristics [Jacobs, 1970], it is often interesting to consider and compare characteristics common to quasi-sinusoidal micropulsations as a group. This paper deals with experimentally determined characteristics of quasi-sinusoidal or quasi-monochromatic micropulsations in the frequency range 1 to 20 mItz. The quasi-monochomatic restriction specifies that the micropulsations have a spectral peak of mean frequency v with a peak width Av (measured between half power points) such that A•/• << 1. By determining the morphological features of these long-period micropulsations, it should be possible to infer something about the character of their energy sources and wave vector fields. Numerous sources of energy have been proposed for quasi-monochromatic micropulsations. Some workers suggested that long-period continuous micropulsatlons, Pc 4's and Pc 5's (see Jacobs et al. [1964] for the classifications of geomagnetic micropulsations), derive their energy from a Kelvin-Itelmholtz instability at

Abstract. To understand the magnetospheric substorm, it is necessary to determine whether its onset is externally triggered by the interplanetary magnetic field (IMF). We analyze the relationship between the IMF and the onset of classical substorms with well-defined onset times. A classical substorm is one that has auroral brightening and electrojet formation at onset, followed by poleward expansion of the region of bright aurora. Substorms meeting these criteria are identified using Canadian Auroral Network for the OPEN Program United Study ground photometer data. We find that a clear IMF trigger (a northward turning or a reduction in the magnitude of the y component) can be identified for 14 of the 20 substorms used in our study. All but one of the identified triggers are northward turnings. We develop a rigorous set of criteria that represents these triggers. By applying the criteria to a large set of IMF data, we find that it is essentially impossible for the observed association between triggers and substorms to happen by chance. Tl•is demonstrates that substorm triggering is a real phenomenon and not the result of the requirement that the IMF be southward before but not at•er a substorm. We also find that spatial structure in the plane perpendicular to the Earth-Sun line critically affects whether or not a trigger is observed from a particular IMF monitor; the probability of seeing a trigger for the substorms in our study is 89% for monitors that are < 30 R•: from the Earth-Sun line but only 50% for monitors 30 R•,: to 56.7 R•c from the Earth-Sun line. Thus a well-defined IMF trigger is associated with most of substorms considered here, and the probability of trigger identification is a strong function of IMF monitor distance from the Earth-Sun line. Given this limitation of trigger identification due to spatial structure, our observations imply that a large majority of classical substorms are triggered by the IMF. We also obtain estimates of-9 min for the mean time delay between magnetopause contact of an IMF trigger and substorm onset and -•64-72 min for the median growth-phase period of southward IMF that precedes triggered classical substorms.

Mechanics and Materials Technology Center: Evaluation and characterization of new materials: metals, alloys, ceramics, polymers and their composites, and new forms of c;u-bon; development and analysis of thin films and depositi m techniques; nondestructive evaluation, component failure analysis and reliability; fracture mechanics and stress corrosion; development and evaluation ':,f hardened components; analysis and evaluation of materials at cryogenic and elevated temperatures; launch vehicle and reentry fluid mechanics, heat trans fer and flight dynamics; chemical and electric propulsion; spacecraft structural mechanics, spacecraft survivability and vulnerability assessment, contamination, thermal and structural control; high temperature thermomechanics, gas kinetics and radiation; lubrication and surface phenomena.Space and Envirunment Technology Center: Magnetospheric, auroral and cosmic ray physics, wave -particle interactions, magnetospheric plasma waves; atmospheric and ionospheric physics, density and composition of the upper atmosphere, remote sensing using atmospheric radiation; solar physics, infrared astronomy, ;nfrared signature analysis; effects of solar activity, magnetic storms and nuclear expiusions on the earth 's atmosphere, ionosphere and magnetosphere; effects of electromagnetic and particulate radiations on space systems; space instrumentation; propellant chef. istry, chemical dynamics, environmental Lhemistry, trace detection; atmospheric chemical reactions, atmospheric optics, light scattering, state-specific cherr ical reactions and radiative signatures of missile plumes, and sensor out-offield-of-view rejection.. . ,^.,..2:Yw ^n .