Auroral ionospheric cavities (AICs) are latitudinally narrow, field‐aligned density depletions of the wintertime polar F region ionosphere. AICs have been detected in incoherent scatter radar data from the Sondrestrom radar facility in a 2‐year sample and during two coordinated multi‐instrument campaigns (Doe et al., 1993, 1994). These data suggest the possibility that AICs are created by the ionospheric closure of field‐aligned currents (FACs) in the polar ionosphere. In this scenario, the cavity forms in a region where ionospheric electrons are evacuated upward as charge carriers for a downward FAC. In order to model this process, a two‐dimensional (altitude versus latitude) simulation has been constructed that imposes an oppositely directed FAC pair at the top of a polar ionosphere that is subject to chemical loss and diffusive transport; the pertinent equations are solved for the resultant system of closure currents and localized plasma loss. Electrodynamic evacuation is modeled by solving Ohm's law and ▽ · j = 0 on the ionospheric grid with an imposed constant topside potential. The sensitivity of the modeled ionosphere to modification from chemistry and diffusion alone is evaluated by removing the topside potential, and imposing a region of enhanced ion temperature ion‐neutral (slip) velocity at F region altitudes. This confirms our earlier conclusion that perturbed thermospheric temperatures and velocities alone cannot create AICs on observed time scales. Modeling results from field‐aligned current closure, on the other hand, indicate that FACs are very efficient at modifying the polar ionosphere: modest currents of 0.2 to 0.02 µA m−2 can create cavitylike structure on time scales from 30 to 64 s, respectively.
R•dar observations of field-aligned auroral F region density depletions (cavities) have been identified in a portion of the Sondre Stromfjord incoherent scatter radar (ISR) data base covering the period February 1986 to January 1988. These "auroral cavities" are nightside phenomena with localized, field-aligned F region density depletions of 20 to 70 percent below surrounding values. They occur during moderate to quiet geomagnetic conditions when the poleward edge of the auroral oval is within view of Sondre Stromfjord. Seasonally, they are a wintertime phenomena occurring just poleward of the statistical auroral oval. Unlike the previously reported '•polar hole," the average width of the cavities is less than 100 kin. Case studies show that the cavities closely track the poleward edge of the most poleward auroral arc. Sequential radar scans show that cavities appear on time scales as short as several minutes, suggestive of local electrodynamic formation or rapid transport. Data from January 24, 1987, collected during coordinated optical, radar, and satellite observations spanning an hour of local time, were examined for possible cavity fomation mechanisms. The cavity fomation processes examined herein include locally enhanced chemical loss, vertical diffusion, drifting horizontal gradients in the background plasma, and evacuation as a result of field-aligned currents. The fomation time scales, calculated evacuation fluxes, dose proximity to E region aurora, and field-aligned current signatures seen in magnetometer and radar observations suggest a strong association of the cavities with upward flowing electrons carrying region 1 downward field-aligned currents. INTRODUCTIONThe Sondre Stromfjord incoherent scatter radar has measured electron density structure associated with plasma dynamics by typically searching for enhancements rather than voids [Vickrey ½t al., 1980; Tsunoda, 1988, and references therein]. Within a uniform plasma background, however, the radar can also effectively track the motion and evolution of density depletions within its field of view. This study presents observations between February 1986 and January 1988, of medium-scale field-aligned F region plasma density depletions in radar scans along the magnetic meridian. These observations often indicate a region of depleted plasma density that is confined in latitude, but extended in altitude (along the magnetic field). Most often the depletion is located near the poleward edge of the most poleward E
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.
[1] The equatorial ionosphere is host to the most dramatic and enigmatic plasma instability mechanism in the geospace environment. Equatorial spread F (ESF) was discovered in early ionosonde measurements and interpreted theoretically using Rayleigh-Taylor theory. Subsequent diagnostic and modeling advances have improved substantially our understanding of ESF onset and evolution and its associated effects on the ionosphere throughout the low-latitude domain. The degree to which ESF mechanisms penetrate into the lower midlatitudes is a topic of current study, a reverse of the familiar concept of high-to-low latitude coupling for space weather phenomena. Optical diagnostic systems, first ground based and now space based, reveal the presence of ESF structures via images of airglow depletions that are aligned in the approximately north-south direction spanning the geomagnetic equator. Ground-based all-sky camera systems used to capture the two-dimensional horizontal patterns of airglow depletions are the main source of observations showing that ESF processes intrude to midlatitudes in the L $ 1.5 domain. In this paper we review the process of mapping airglow depletions along geomagnetic field lines to the equatorial plane, hence defining the maximum apex heights achieved. A case study comparison of simultaneous radar backscatter data from Kwajalein with optical data from Wake Island, sites that share common magnetic meridians in the Pacific section, confirms the utility of the approach and its applicability to sites at other longitudes. Modeling studies based on buoyancy arguments using flux tube-integrated mean density values versus L shell apex heights show that instability-induced plasma depletions starting at F layer bottomside heights easily reach altitudes above 2000 km in the equatorial plane, implying that ESF intrusions to lower midlatitudes should be a relatively frequent occurrence.
We present a technique which combines time series of line-of-sight (LOS) velocity and electron density measurements from the Sondrestrom incoherent scatter radar (74.5 ø invariant latitude) to reconstruct the large-scale horizontal structure of the F region ionosphere during polar cap patch events. This reconstruction technique provides a new densitybased means of examining patch morphology. Its wide region of coverage also facilitates comparison of radar measurements with other observational data sets. For two periods when patches were present and convection conditions in the nightside polar cap could be adequately approximated by the simple velocity model used in this initial implementation of the technique, we compare reconstructed radar data montages with in situ ion density data from DMSP satellites and 630.0 nm all-sky images taken at the radar site. The satellite data agree well with the radar reconstructions near the observation site, and show general agreement well beyond the radar field of view (FOV). Data from satellite passes to the east and west of the radar coverage suggest high densities were present over many hours of magnetic local time in the nightside polar cap. Many of the patches observed in the radar data were elongated perpendicular to the convective flow and may have extended well beyond the radar FOV. The characteristics of density enhancements observed by the satellites in the nightside auroral zone were found to be consistent with the creation of auroral zone blobs by distortion of patches seen exiting the polar cap in the radar data. Radar data reconstructions also show reasonable qualitative agreement with instantaneous 630.0 nm all-sky images regarding the location, size, and shape of patch features. In addition to validating the streamline-mapping technique, this result also lends support to previous optical studies of patch occurrence and morphology.
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