Jurgen Buchau

Response of the polar cap F region convection direction to changes in the interplanetary magnetic field: Digisonde measurements in northern Greenland

Results of ionospheric drift measurements with a Digisonde 256 digital ionospheric sounder located at Qaanaaq, Greenland (87°N, corrected geomagnetic latitude), are presented. Digisonde drift data have been related to the interplanetary magnetic field (IMF) measured by the IMP 8 satellite for 32 days during 1986, 1987, and 1988. Extremely good statistical agreement between these measurements and convection directions derived from satellite instrumentation is demonstrated when the IMF z component is negative. For Bz negative and By negative the daily average convection direction is centered on −12° (anticlockwise) from the antisunward direction. When Bz is negative but By is positive, the convection direction is centered on +36°. These directions differ by 3° and 9°, respectively, from the models of Heppner and Maynard (1987). The variation about these values is ∼ ±20°. The excellent agreement between the Digisonde measurements and models derived from satellite measurements demonstrates the utility of the Digisonde for making ground-based measurements of the convection direction in the polar cap F region when Bz is south. The convection directions under conditions of positive Bz have also been examined, and we have measured three types of temporal variation in azimuth, namely, an ordered and slowly (OS) varying change in direction, an ordered and quickly (OQ) varying change in direction, and disordered (D) variations in direction. The latter are believed to result from a breakdown of the analysis technique due to velocity shears in the vicinity of polar cap arcs, and we estimate that they account for ∼25% of the measurements when Bz > 0. When Bz is positive and By is negative, our small subset of OS measurements supports the distorted two-cell model of Heppner and Maynard (1987). The remainder of the measurements show no well-defined daily average convection direction or diurnal variation. Likewise for Bz positive and By positive, no well-defined convection direction can be discerned, nor can any diurnal variation. The existence of OQ variations when Bz > 0 suggests that meaningful average statistical convection patterns may be much harder to synthesize than similar patterns when Bz < 0.

AIRBORNE IONOSPHERIC AND OPTICAL MEASUREMENTS OF NOONTIME AURORA.

Abstract The results are reported of a series of three local noon flights by the AFCRL Flying Ionospheric Laboratory through the latitude of the auroral oval. The flights, which were performed in December 1968 over the Arctic Ocean, maintained local magnetic noon while scanning back and forth in latitude for durations of 5–10 hr each for a total of 16 latitude scans and 20 hr all in darkness and at nearly constant K p (2- to 3-). Coordinated in these airborne measurements were an ionospheric sounder, all sky cameras and photometers. Parameters analyzed are f 0 E, h′E, h′E s , f min ; the presence of discrete aurora overhead; the intensities of N 2 + 4278 A, OI 5577 A and OI 6300 A measured by a photometric technique which discriminates between discrete and non-discrete aurora. An auroral form which is spatially continuous and slowly varying in time is detected simultaneously by the ionospheric sounder as an E -layer and by the photometer as a non-discrete component of N 2 + , the intensity varying as the fourth power of E -layer critical frequency. This continuous aurora: 1. (1) extends in an unbroken band typically 5° equatorward from the instantaneous position of the discrete auroral oval which falls between 75° and 78° corrected geomagnetic latitude; 2. (2) produces an E -layer the virtual height of which corresponds to precipitating electrons whose particle energies decrease from 3 keV at the low latitude edge to 1 keV at the high latitude edge where the E -layer merges with a band of enhanced non-discrete 6300 A emission at the same location as the discrete aurora; 3. (3) terminates at high latitude under the discrete aurora where the E -layer transforms to sporadic E at the same virtual height; 4. (4) has maximum precipitation energy flux of 0.08 ergs/cm 2 sec which decreases with increasing latitude; 5. (5) has a particle flux of 2 × 10 7 electrons/cm 2 sec; 6. (6) frequently overlaps near the low latitude edge with a zone of D -region ionization which extends equatorward at least 7° from 74°; and 7. (7) together with the discrete aurora and non-discrete 6300 A band constitutes the ‘soft’ zone of electron precipitation measured by satellite, the D -layer zone corresponding both to the satellite ‘hard’ zone and to the auroral absorption zone.

Electron density structures in the polar F region

Abstract Arctic campaigns conducted over the last decade, using digital ionospheric sounders and all-sky imaging photometers have allowed an ordering of the typically very irregular behavior of the polar cap ionosphere, and a preliminary definition of the solar cycle effects. Under quiet magnetic conditions the polar cap is populated with sun-aligned F layer arcs. Under magnetically active conditions (Bz

Polar cap convection for Bz northward

The Digisonde at Qanaq, Greenland (87°Λ) measures the ionospheric convection velocity in the central polar cap. The average local time variation of the velocity for Bz northward under various By conditions is presented. The nightside drifts for Bz northward tend to be antisunward, and resemble those which occur for southward IMF, as pointed out by Heppner and Maynard (1987). However, it is difficult to reconcile the observed diurnal drift variation with the Heppner-Maynard distorted two-cell model. Modified forms of the Potemra et al. (1984) four cell model offer a more plausible framework in which to understand the Qanaq data. The polar cap cells are relatively small, and are displaced towards the dayside.

Digital ionosonde observations of the polar cap F region convection

Ground-based drift observations of the winter polar cap F-region show that the magnetospherically induced ionospheric convection can be measured for the bottomside ionosphere. A digital ionosonde with four spaced receiving antennas operated at Thule, Greenland (86° CGL) in the Doppler-drift mode. A number of 24-hour measurements indicate that the drift direction changes linearly as a function of time in accordance with the predicted antisunward convection pattern. The drift velocities vary from 300 to 900 m/s. Measurements at a subauroral station (Goose Bay, Labrador, 65° CGL) with the same spaced-antennas-Doppler-drift technique show a steady westward drift until local magnetic midnight and a fast switch-over at that time to an eastward drift. We conclude that the observed subauroral drifts are the sunward return flows of the polar plasma convection, and the switch-over occurs when the station rotates from the dusk cell into the dawn cell.

The dynamic ionospheric polar hole

At a time when most polar cap ionospheric studies are focused on electron density enhancements caused by patches or arcs, this paper discusses an F region depletion known as the ionospheric polar hole. Statistically, polar holes tend to develop on the nightside from 2100–0600 MLT between 70°–80°Λ. Electron concentrations as small as 2×102 cm−3 at altitudes near 300 km have previously been observed in polar holes during solar minimum winter conditions. Under magnetically quiet conditions, polar holes are thought to form due to slow convection of plasma across the polar cap. The electron concentration is depleted by normal recombination processes during many hours spent in the dark polar cap. In contrast with previous studies, which have tended to be statistical in nature, this paper describes the temporal and spatial development of a single polar hole. During the Geospace Environment Modelling Pilot Program of January 16,1990, the development of the polar hole was monitored by the DMSP F8 and F9 satellites, a digital ionosonde and a 250-MHz scintillation receiver. The data reveal that the polar hole is a very dynamic phenomenon, and its location can change dramatically within several hours. This is the first study of the polar hole using ground based instruments. The observations presented here are of interest not only because they contribute to a morphological understanding of the polar hole, but also because they illustrate the influence of the polar hole on radio signals propagating in the ionospheric medium. The results are of direct relevance to applications and systems involving ionospheric and transionospheric propagation. The data set is sufficiently complete that it will provide rigorous constraints for future modeling studies and will thus contribute to a better physical understanding of the polar hole.

Observations of Plasma Structure and Transport at High Latitudes

Radio and optical diagnostics from the AFGL Airborne Ionospheric Observatory are used to study the structure and motion of regions of enhanced F-region density at high latitudes. Plasma flow can be tracked from the poleward edge of the dayside cusp, across the polar cap and into the nightside auroral zone. Simultaneous satellite amplitude and phase scintillation measurements define the degree of structuring or intensity of sub-kilometer ionospheric irregularities within these regions. The combined measurements are used to track large scale plasma flow, and to infer plasma source regions.

Determining the F-region critical frequency from satellite-borne noise measurements

Abstract A means of determining the F2-region critical frequency using noise measurements made on-board satellites orbiting above the F2-peak is investigated. Values of foF2 observed on iomograms recorded by the ISIS 2 satellite were compared against corresponding records of the ISIS AGC voltage trace superimposed on the ionograms. The difference between foF2 and the frequency at which the AGC trace displayed a continuous enhancement above the cosmic background level, was taken as a measure of how well noise measurements can be used to infer the sub-satellite value of foF2. It is seen that above regions of the globe where ground-based HF noise is high and the ionospheric structure does not display severe horizontal gradients, measurements of noise breaking through the ionosphere can be used to determine foF2 generally to within one MHz. Comparison of daytime and nighttime measurements over the ocean shows a considerably smaller difference between foF2 and noise breakthrough at night. It is postulated that this feature is due to multihop propagation modes that are attentuated less at night than during the day.