W. A. Bristow

A superposed epoch study of SuperDARN convection observations during substorms

[1] Case studies of flow patterns during substorms have revealed characteristic patterns in the period leading to expansion onset. Velocities in the premidnight region have been observed to become strongly sheared, and the premidnight convection reversal has been observed to extend across the midnight meridian. At expansion onset, the shear decreases and flow at midnight becomes meridional, out of the polar cap. The study presented here superposes flow patterns from 10 substorm intervals to determine the average flow patterns. The study confirms the findings of the case studies and extends them to additional local time sectors. In addition, changes to the flow pattern after expansion onset are examined.

Detailed analysis of substorm observations using SuperDARN, UVI, ground‐based magnetometers, and all‐sky imagers

[1] A case study of a small-magnitude isolated substorm is presented. The substorm was observed by a variety of instruments including the Super Dual Auroral Radar Network (SuperDARN), the Polar Ultra Violet Imager (UVI), CANOPUS magnetometers, the Alaska chain magnetometers, the Poker Flat meridian-scanning photometer, and the Poker Flat all-sky imager. It was determined that the initial brightening was centered over the combined fields of view of the various instruments. Temporal and spatial relationships among plasma flows, auroral luminosity, and magnetometer perturbations are investigated. It is found that the initial substorm signature was observed in the plasma flows, followed by the auroral brightening, and finally followed by the magnetometer perturbation. Enhanced plasma flows were observed in a spatially confined region near the auroral oval for a period of ∼5 minutes prior to the brightening. After the brightness peaked, the plasma flow velocity decreased back to a preenhancement level.

Substorm convection patterns observed by the Super Dual Auroral Radar Network

A study of convection patterns during substorms observed by the Super Dual Auroral Radar Network (SuperDARN) radar network is presented. Specific attention is given to the growth phase and the time immediately following expansion onset. It is found that the growth phase is characterized by the enhancement of the velocity shear near midnight and its extension to low latitudes and to local times across the midnight meridian. The velocity shear was observed to diminish at expansion onset. In addition, the ionospheric flow velocity magnitude was observed to be enhanced for some period of time prior to expansion onset and to decrease at expansion onset. Possible explanations for the establishment of the midnight velocity shear during growth phase and its subsequent disappearance at expansion onset are presented. Also, arguments are presented regarding the decrease of ionospheric flow velocity at expansion onset.

Numerical simulation of the formation of thin ionization layers at high latitudes

A one-dimensional simulation of high latitude sporadic-E layers has been developed to investigate the role of electric field in layer formation. The simulation model computes the ion densities (O+, O2+, N+, N2+, NO+, Fe+) and temperatures as a function of altitude. The stationary state momentum equation and continuity equation is solved for each ion species. Then the energy equation is solved for electrons, neutrals, and a generic ion having the mean ion mass and velocity. The model does not include any photoionization or particle precipitation. Chemical production and loss are included for the five standard ion species, but not for the Fe+ ions. An initial altitude profile is assumed for the ions, and the model computes the altitude distribution as a function of time. The horizontal direction of the electric field is important for determining the layer structure and its altitude. An electric field directed between magnetic west and north results in convergent plasma flow and thus layer formation at about 120–130 km altitude. For fields directed to the west and south layers form at lower altitudes(100–115km). With an assumed dip angle of 75° the layers formed by the N-W quadrant fields form quickly (<10min), are quite thin, and remain stationary in altitude. Layers formed by S-W quadrant fields are initially thick but compress in time (15–20 min). Additionally they move down in altitude as they compress. The large perpendicular electric field (50 mV/m) used in the simulation leads to significant heating of the ions and to a smaller extent the electrons and neutrals. The layer formed by the field directed in the N-W quadrant occurs in the region of significant heating, and the temperature profile shows some enhancement at the layer altitude.

Convective response to a transient increase in dayside reconnection

Measurements with five of the northern hemisphere Super Dual Auroral Radar Network (SuperDARN) radars have yielded a detailed temporal and spatial view of the evolution of a dayside convection enhancement, which we associate with a transient increase in dayside reconnection. The convection enhancement was located in and immediately poleward of the ionospheric footprint of the cusp. During the enhancement, both the cusp and the region of enhanced flow shifted equatorward by ∼2°. As the flow enhancement diminished, the cusp footprint moved poleward to its original position. The entire event had a duration of ∼18 min and was associated with a transient 29 kV increase in the cross polar cap potential. We estimate that ∼3.2 x 10 7 Wb of magnetic flux was opened at the dayside magnetopause during the most active 12 min of this event. The length of the reconnection line on the dayside magnetopause is estimated to have reached 19,000 km. The characteristics of the dayside ionospheric response are very similar to those predicted by Cowley and Lockwood [1992] in their expanding and contracting polar cap model. These are the first observations that have provided an extended spatial and temporal view of the responses of dayside convection and the cusp to a transient reconnection event.

Mesoscale dayside convection vortices and their relation to substorm phase

Measurements made with the first two pairs of the northern hemisphere component of the Super Dual Auroral Radar Network (SuperDARN) have revealed the intermittent existence of a mesoscale convection vortex in the high-latitude postnoon ionosphere. The vortex is a feature of the substorm growth phase and is typically centered between 1430 and 1530 MLT and 75° and 80° invariant latitude. It has a diameter ranging from a few hundred to ∼1000 km and an associated potential drop of 5–10 kV. The vortex is centered on a filamentary upward field-aligned current with an estimated magnitude approaching 3 μA/m2. The vortex is centered near the sunward end of the dusk convection cell just poleward of the sunward convecting plasma and just duskward of the region where the sunward convecting plasma rotates sharply poleward and enters the polar cap. As the plasma convects poleward, it passes through an irregularity zone that has been associated with the ionospheric footprint of the cusp. A remarkable feature of the vortex is that it disappears concurrently with the onset of a substorm expansion phase in the midnight sector. Several magnetospheric source mechanisms, including the Kelvin-Helmholtz and tearing mode instabilities, flux transfer events, and macroscale current systems, have been considered for the vortex. The best explanation appears to be that the vortices are associated with filamentary field-aligned currents that are driven by the cross polar cap potential and close as Pedersen currents through the cusp region. The disappearance of the vortex following the onset of an expansion phase is attributed to a redirection of magnetospheric closure currents as a consequence of the significant increase in nightside conductivity during a substorm expansion.

SuperDARN observations of ionospheric heater–induced upper hybrid waves

Observations are reported of an undocumented HF scattering mode detected by the Kodiak SuperDARN radar during an O-mode heating experiment recently conducted at the HAARP facility near Gakona, AK. The main features of these observations include enhancements to the Kodiak radar spectrum at frequencies equal to the radar plus the heater frequencies (f r + f H ) as well as somewhat weaker enhancements at (f r + f H ) ± 85 and (f r + f H ) ± 108 kHz. Observations were recorded for ∼20 minutes and enhanced spectra from the heated volume above HAARP were consistently observed by the Kodiak radar during times when the HAARP heater was transmitting. We suggest that the spectral enhancements indicate the presence of perpendicularly propagating upper hybrid (UH) waves which are believed to exist near the UH resonance layer.

Azimuthal flow bursts in the inner plasma sheet and possible connection with SAPS and plasma sheet earthward flow bursts

We have combined radar observations and auroral images obtained during the Poker Flat Incoherent Scatter Radar Ion Neutral Observations in the Thermosphere campaign to show the common occurrence of westward moving, localized auroral brightenings near the auroral equatorward boundary and to show their association with azimuthally moving flow bursts near or within the subauroral polarization stream (SAPS) region. These results indicate that the SAPS region, rather than consisting of relatively stable proton precipitation and westward flows, can have rapidly varying flows, with speeds varying from ~100 m/s to ~1 km/s in just a few minutes. The auroral brightenings are associated with bursts of weak electron precipitation that move westward with the westward flow bursts and extend into the SAPS region. Additionally, our observations show evidence that the azimuthally moving flow bursts often connect to earthward (equatorward in the ionosphere) plasma sheet flow bursts. This indicates that rather than stopping or bouncing, some flow bursts turn azimuthally after reaching the inner plasma sheet and lead to the bursts of strong azimuthal flow. Evidence is also seen for a general guiding of the flow bursts by the large-scale convection pattern, flow bursts within the duskside convection being azimuthally turned to the west, and those within the dawn cell being turned toward the east. The possibility that the SAPS region flow structures considered here may be connected to localized flow enhancements from the polar cap that cross the nightside auroral poleward boundary and lead to flow bursts within the plasma sheet warrants further consideration.

Climatology of medium‐scale traveling ionospheric disturbances observed by the midlatitude Blackstone SuperDARN radar

A climatology of daytime midlatitude medium-scale traveling ionospheric disturbances (MSTIDs) observed by the Blackstone Super Dual Auroral Radar Network (SuperDARN) radar is presented. MSTIDs were observed primarily from fall through spring. Two populations were observed: a dominant population heading southeast (centered at 147° geographic azimuth, ranging from 100° to 210°) and a secondary population heading northwest (centered at −50° azimuth, ranging from −75° to −25°). Horizontal velocities ranged from 50 to 250 m s−1 with a distribution maximum between 100 and 150 m s−1. Horizontal wavelengths ranged from 100 to 500 km with a distribution peak at 250 km, and periods between 23 and 60 min, suggesting that the MSTIDs may be consistent with thermospheric gravity waves. A local time (LT) dependence was observed such that the dominant (southeastward) population decreased in number as the day progressed until a late afternoon increase. The secondary (northwestward) population appeared only in the afternoon, possibly indicative of neutral wind effects or variability of sources. LT dependence was not observed in other parameters. Possible solar-geomagnetic and tropospheric MSTID sources were considered. The auroral electrojet (AE) index showed a correlation with MSTID statistics. Reverse ray tracing with the HINDGRATS model indicates that the dominant population has source regions over the Great Lakes and near the geomagnetic cusp, while the secondary population source region is 100 km above the Atlantic Ocean east of the Carolinas. This suggests that the dominant population may come from a region favorable to either tropospheric or geomagnetic sources, while the secondary population originates from a region favorable to secondary waves generated via lower atmospheric convection.

Coordinated ionospheric observations indicating coupling between preonset flow bursts and waves that lead to substorm onset

A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm expansion phase onset. Recent Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI) array observations have shown a repeatable preonset sequence, which is initiated by a poleward boundary intensification (PBI) and is followed by auroral streamers moving equatorward (earthward flow in the plasma sheet) and then by substorm onset. On the other hand, substorm onset is also preceded by azimuthally propagating waves, indicating a possible importance of wave instability for triggering substorm onset. However, it has been difficult to identify the link between fast flows and waves. We have found an isolated substorm event that was well instrumented with the Poker Flat incoherent scatter radar (PFISR), THEMIS white-light ASI, and multispectral ASI, where the auroral onset occurred within the PFISR and ASI fields of view. This substorm onset was preceded by a PBI, and ionospheric flows propagated equatorward from the polar cap, crossed the PBI, and reached the growth phase arc. This sequence provides evidence that flows from open magnetic field lines propagate across the open-closed boundary and reach the near-Earth plasma sheet prior to the onset. Quasi-stable oscillations in auroral luminosity and ionospheric density are found along the growth phase arc. These preonset auroral waves amplified abruptly at the onset time, soon after the equatorward flows reached the onset region. This sequence suggests a coupling process where preexisting stable waves in the near-Earth plasma sheet interact with flows from farther downtail and then evolve to onset instability.