E. Donovan

Tail Reconnection Triggering Substorm Onset

Magnetospheric substorms explosively release solar wind energy previously stored in Earths magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ∼10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ∼20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

The auroral signature of earthward flow bursts observed in the magnetotail

We present direct evidence that transient Earthward flow bursts in the magnetotail can produce an observable signature in the optical aurora. This signature is north-south aligned auroral structures that are extensions of transient intensifications near the poleward boundary of the auroral oval. Our study focuses on the period from 0500 to 0700 UT on January 7, 1997, during which five distinct flow bursts are observed in the Geotail data. At that time, the spacecraft was located approximately 30 RE downtail on field lines that project down to the CANOPUS array of ground based instruments. We find that each of the flow bursts seen in the Geotail data is associated with an auroral poleward boundary intensification (PBI) observed in the CANOPUS meridian scanning photometer (MSP) data, which appears as a north-south aligned auroral structure in the CANOPUS all-sky imager (ASI) data. Based on these observations we estimate that the fast flows originated between 50 and 100 RE downtail.

Ground‐based optical determination of the b2i boundary: A basis for an optical MT‐index

[1] The equatorward boundary of the proton aurora corresponds to a transition from strong pitch angle scattering to bounce trapped particles. This transition has been identified as the b2i boundary in Defense Meteorological Satellite Program (DMSP) ion data [Newell et al., 1996]. We use ion data from 29 DMSP overflights of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Meridian Scanning Photometer (MSP) located at Gillam, Canada, to develop a simple algorithm to identify the b2i boundary in latitude profiles of proton auroral (486 nm) brightness. Applying this algorithm to a ten year set of Gillam MSP data, we obtain ∼250,000 identifications of the optical b2i, the magnetic latitude of which we refer to as b2i Λ . We intercompare ∼1600 near-simultaneous optical and in situ b2i Λ , concluding that the optical b2i Λ is a reasonable basis for an optical equivalent to the MT-index put forward by Sergeev and Gvozdevsky [1995]. Using ∼17,000 simultaneous measurements, we demonstrate a strong correlation between the optical b2i Λ and the inclination of the magnetic field as measured at GOES 8. We develop an empirical model for predicting the GOES 8 inclination, given theuniversal time, dipole tilt, and the optical b2i Λ , as determined at Gillam. We also show that in terms of information content, the b2i boundary is an optimal boundary upon which to base such an empirical model.

Width and structure of mesoscale optical auroral arcs

Arc widths were calculated for 3126 stable auroral arcs observed by an all-sky camera located in Gillam, Manitoba. The camera is filtered to accept 5577-A emissions and has a single-pixel spatial resolution of 1.7 km at zenith. The measured mean width of stable mesoscale arcs located within ±5° of magnetic zenith is 18 km with a standard deviation of 9 km. The width distribution exhibits a steep cutoff below 8 km; when combined with studies of small-scale auroral structure this cutoff suggests a gap in the occurrence of arcs with widths of order 1 km. This feature of the arc width spectrum argues against the notion of a turbulent cascade of energy from larger to small scales. Residuals from the Gaussian fits are only about 3% of the arc amplitude on average, indicating little sub-structure within arcs at scales down to the measurement resolution.

The temporal variation of the frequency of high latitude field line resonances

The diurnal variation in the frequencies of the continuum of ULF field line resonances has been calculated by using the cross-spectral phase of the north-south components of data from latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. On most days the continuum is seen only during the local daytime, and only a single harmonic with an inverted U-shaped temporal variation in frequency is seen. At 67° geomagnetic latitude (L = 6.6) the general trend is a resonant frequency around 2 mHz near local dawn, increasing up to ∼5 mHz by 0600-0700 local time, followed by a decrease in frequency to 2 mHz by 1500–1600 local time. Near local noon, the fundamental resonant frequency is ∼3 mHz at 71° (L = 11.3), increasing monotonically to 7 mHz at 65° (L = 6.1). The waves appear to be a part of the resonant Alfven mode continuum as opposed to the single-frequency, driven magnetic field line resonances often seen at high latitudes. The cross-phase spectra show evidence of impulsively driven resonances that energize the continuum over the latitudinal range of the CANOPUS magnetometers. The temporal variation in the resonant frequency is modeled by using the Tsyganenko (1987) magnetic field model and cold plasma MHD theory. With the use of the observed resonant frequencies, the plasma density for June 1, 1990, was 4.2 × 106 H+/m³ at L = 6.6 while the data for June 7, 1990, showed densities up to 100×106 H+/m³. These results suggest that observations of the magnetohydrodynamic continuum in the magnetometer data may give a very effective method for ground-based time-dependent mapping of the equatorial plasma density.

Variation of plasmatrough density derived from magnetospheric field line resonances

The diurnal variation of ULF field line resonant frequencies has been calculated using the cross phase of data from the north-south components recorded at seven latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. CANOPUS magnetometers range in latitude from Rankin Inlet (L = 12.4) south to Pinawa (L =4.3). Using cold plasma MHD theory, an R−4 plasma density function, and the T87 magnetic field model, the variation of plasma density in the equatorial region has been calculated from the experimentally determined resonant frequencies. Consecutive, adjacent magnetometer pairs provide six local daytime spatial estimates of the variation in plasma mass density between 4 and 11 RE. Typical values are 1–20 H+cm−3 for the plasmatrough and 50–200 H+cm−3 for the plasmasphere. The data recorded on June 7, 1990, shows an afternoon increase in density near geosynchronous orbit in agreement with convection models of the magnetosphere. The ground-based measurements of plasma mass density have been compared with data from the Los Alamos Magnetospheric Plasma Analyser on board the 1989-046 geosynchronous spacecraft. These comparisons show that the ground-based technique should allow a robust procedure for calculating dayside, time-dependent mappings of the equatorial plasma mass density from the plasmapause to the magnetopause in near real time.

Determination of the substorm initiation region from a major conjunction interval of THEMIS satellites

[1] We investigate in detail the time history of substorm disturbances in the magnetotail observed during a major tail conjunction of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites on 29 January 2008, 0700―0900 UT. During this interval, all THEMIS satellites were closely aligned along the tail axis near midnight and were bracketed in local time by GOES 11 and 12. The radial distance covered ranges from the geosynchronous altitude to ∼30 R E in the tail. This interval consists of three activations detected by the THEMIS satellites with good ground all-sky-camera observations of auroral activity. The first activation is a small substorm with spatially limited disturbance in the tail. The onset arc was equatorward of an undisturbed arc. The second activation is a moderate size substorm with the onset arc also being equatorward of an undisturbed arc. The third activation is an intensification of the substorm with its onset indicated by the second activation. The active auroral arc for this intensification was near the poleward boundary of the auroral oval. Analysis of these observations indicates that the first activation is a small substorm initiated in the near-Earth plasma sheet and does not involve magnetic reconnection of open magnetic field lines. Magnetic reconnection on closed field lines can be ruled out for this substorm because it cannot generate the observed high-speed plasma flow. The second and third activations are part of a moderate size substorm initiated also in the near-Earth plasma sheet, with a subsequent substorm intensification involving activity initiated tailward of ∼30 R E . Overall, the time history of substorm activity for these two substorms is consistent with the near-Earth initiation model.

Day‐night coupling by a localized flow channel visualized by polar cap patch propagation

We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all-sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and Defense Meteorological Satellite Program (DMSP)-17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low-latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open-closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet.

Large-scale vortex dynamics in the evening and midnight auroral zone : Observations and simulations

We use Canadian Auroral Network for the OPEN Program Unified Study All-Sky Imager (ASI) and Meridional Scanning Photometer (MSP) data as the basis for a study of the dynamics of large-scale (hundreds of kilometers) auroral vortices. We consider 28 events corresponding to a range of auroral activity levels. Three of these are presented in detail, one corresponding to growth phase, one to pseudo-breakup and one to expansive phase onset. We show that vortex formation starts from a discrete arc with half thickness δ of the order of 20 km. This arc intensifies near the poleward boundary of enhanced proton aurora, as seen in the Hydrogen β (Hβ) MSP data and becomes azimuthally structured. This structuring is in the form of vortices with wavelength of the order of ∼ 2πδ. The vortices intensify and extend radially, leading to broadening of the initial arc. While the sizes and growth rates of the vortices vary, the overall scenario of vortex evolution is similar for all of the events. Structures that develop during the growth phase saturate at latitudes matching the poleward boundary of Hβ emissions and pseudo-breakup structures saturate further poleward. Expansive phase onset vortices expand poleward in a similar fashion, but we do not observe any saturation stage, presumably due to limitations imposed by the ASI field of view. We present results of shear flow ballooning vortex modeling in which we used initial conditions and parameters consistent with our observations. On the basis of our model results, we speculate that all of these experimentally observed vortices are the result of shear flow ballooning instability in the hot proton region in the near-Earth plasma sheet.

Modeling the Magnetic Effects of Field-Aligned Currents

In this paper, I present a newly developed model of the magnetic effects of field-aligned currents. The magnetic field of a large-scale field-aligned current system is obtained by summing the magnetic fields of a large number of finite length cylindrical current elements. The resulting magnetic field is valid inside of, near and far away from the model field-aligned currents, both near the ionosphere and in the magnetosphere. The modeling technique allows for the closure of the field-aligned currents along any specified path. In order to investigate the effects of field-aligned currents on the nightside magnetospheric magnetic field and on mapping between the night-side magnetosphere and ionosphere, I add the effects of region I and II field-aligned currents to an empirical magnetic field model. Using this model, I demonstrate the following three things: (1) reasonable amounts of field-aligned current can cause the ionospheric footprint of a point at geosynchronous orbit to shift in local time by one hour or more; (2) depending on how they are closed, field-aligned currents can cause Bz in the equatorial plane, near midnight, either to increase or to decrease; (3) CPS field lines can cross the equatorial plane 10 RE or more further from the noon-midnight meridian than they would if field-aligned current effects were not included, a result that also depends on how the currents are closed.