[1] A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all-sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north-south (N-S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N-S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre-existing open-closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward-transport of the new plasma leads to a near-Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere-ionosphere electrodynamic coupling, which may play an important role in the motion of pre-onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre-onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well-developed region 2 current system within the duskside convection cell, and also a high probability of diffuse-appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

The detailed quiet time structure of energetic electrons in the earth's radiation belts is explained on the basis of a balance between pitch angle scattering loss and inward radial diffusion from an average outer zone source. Losses are attributed to a combination of classical Coulomb scattering at low L and whistler mode turbulent pitch angle diffusion throughout the outer plasmasphere. Radial diffusion is driven by substorm associated fluctuations of the magnetospheric convection electric field.

It is shown that discontinuities in the magnetospheric convection electric field E with ▽ · E <0 can generate large‐scale regions (of the order of 100 km in width) of magnetic field‐aligned currents with associated field‐aligned electric potential differences and electron precipitation of the magnitudes and widths observed in auroral regions. Such an electric field discontinuity is known to exist along the evening boundary between sunward and antisunward convection. In addition, such discontinuities may also exist over the polar cap, on account of inhomogeneities in the magnetosheath flow and in regions, such as the Alfvén layer, where drifting trapped particles charge separate. The present analysis assumes that the field‐aligned current is governed by the free particle motion in dc electric and magnetic fields, and nothing is assumed to inhibit this free particle motion.

Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.

The existence of the current sheet and the dawn to dusk electric field in the geomagnetic tail implies there is particle energization in the tail current sheet of the order 2–10% of the total solar wind energy incident upon the dayside magnetopause. In this paper we determine that ion acceleration in a current sheet with a small magnetic field across the sheet, via single‐particle motion which violates the guiding center approximation, can account for this large energization in the tail. We calculate the distribution of accelerated ions which result from the current sheet acceleration and compare the results with distributions of accelerated ions frequently observed flowing earthwards along the outer boundary of the plasma sheet. The comparison indicates that the observed earthward‐flowing ions result from current sheet acceleration. Comparison with measurements of auroral ion precipitation at low altitudes implies that the accelerated ions ejected from the current sheet are also an important source of auroral ion precipitation. In addition, these accelerated ions may be an important source of plasma sheet ions.

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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 .