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'
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.
We examine ground-based observations of the meridional profile of 6300 ,• atmospheric emission from 67.3 ø to 80.7 ø invariant latitude for the signature of the polar cap boundary, the ionospheric boundary between open and closed magnetic field lines. The openclosed field line boundary is assumed to lie at the boundary between polar rain and plasma sheetprecipitation. We assume that nonprecipitation-dependent sources of 6300 • emission cause a spatially uniform luminosity in the polar cap and that auroral zone luminosity is also spatially uniform. Therefore we determine the location of the polar cap boundary from the auroral emission data at each time by finding the best fit of the observations to a step function in latitude. Thus we produce a time series of the location of the polar cap boundary. We have developed criteria on the step function fit that identify when a reliable boundary identification has been obtained. Generally, where these criteria are not satisfied, the boundary is outside the latitudinal range of the optical observations. We compare the boundary identified from the emissions to the boundary in precipitating particle observations made by DMSP as it passes along a meridian within 1 1/2 hours of local time of the photometer. The latitudes of the two boundaries are highly correlated. During the expansion phase of substorms, however, there are large discrepancies apparently arising from longitudinal structure of the polar cap boundary associated with auroral surges. We conclude that 6300 • emissions provide a good means for monitoring the polar cap boundary continuously with an estimated precision of_+ 0.9 ø invariant latitude.
[1] We present an analysis of the spectral characteristics of 1-hop HF radar ground scatter and ½-and 1½-hop ionospheric scatter as measured by the Super Dual Auroral Radar Network. Our objective is to determine criteria that separate signals scattered from the ground and the ionosphere. We find that for both ground scatter and ionospheric scatter that the probability density function of backscatter Doppler velocity decreases exponentially with velocity, but with significantly different e-folding velocities for the two types of backscatter. We use this observation to separate the total probability density of Doppler velocity and spectral width into two component distributions. This process yields the posterior probability that a signal of given Doppler velocity and spectral width is ground scatter. The resulting criterion for classification of a particular signal as ground scatter, v < 33.1 m/s + 0.139w À (0.00133 s/m)w 2 , significantly reduces the probability that a signal will be erroneously classified as ionospheric scatter, while only moderately increasing the probability that an ionospheric scatter signal will be erroneously classified as ground scatter. Finally, we validate the ground scatter probability function by demonstrating that the backscatter virtual height increases as expected with increasing probability of ground scatter.Citation: Blanchard, G. T., S. Sundeen, and K. B. Baker (2009), Probabilistic identification of high-frequency radar backscatter from the ground and ionosphere based on spectral characteristics, Radio Sci., 44, RS5012,
A technique to measure the magnetotail reconnection rate from the ground is described and applied to 71 hours of measurements from 20 nights. The reconnection rate is obtained from the ionospheric flow across the polar cap boundary in the frame of reference of the boundary, measured by the Sondrestrom incoherent scatter radar. For our measurements, the polar cap boundary is located using 6300 Å auroral emissions and E region electron density. The average experimental uncertainty of the reconnection rate measurement is 11.6 mV m−1 in the ionospheric electric field. By using a large data set, we obtain the dependence of the reconnection rate on magnetic local time, the interplanetary magnetic field, and substorm activity, with much higher accuracy. We find that two thirds of the average polar cap potential drop occurs over the 4‐hour segment of the separatrix centered on 2330 MLT, that the linear correlation between the reconnection electric field and the half‐wave rectified dawn‐dusk solar wind electric field VBs peaks between 1.0 and 1.5 hours, with a maximum linear correlation coefficient of 0.46 at 70 min; and that following substorm expansion phase onset, the reconnection electric field becomes larger than the experimental uncertainty, with an average delay of 23 min. The 70‐min delay of the reconnection rate with respect to VBs is a typical convection time for a flux tube across the polar cap. This result indicates that reconnection in the magnetotail is influenced by the solar wind electric field VBs on the field line being reconnected.
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