R. A. Greenwald

Statistical patterns of high-latitude convection obtained from Goose Bay HF radar observations

We have derived patterns that describe the statistical interplanetary magnetic field (IMF) dependencies of ionospheric convection in the high-latitude region of the northern hemisphere. The observations of plasma motion were made with the HF coherent backscatter radar located at Goose Bay, Labrador, over the period September 1987 to June 1993. The area covered by the measurements extended poleward of 65°Λ to a working limit of about 85°Λ. Distributions of electrostatic potential have been derived and expressed as series expansions in spherical harmonics. The patterns are the first derived from direct ground-based observations of ionospheric convection that approach in completeness and level of detail the patterns derived in recent satellite studies [Rich and Hairston, 1994; Weimer, 1995]. We show the dependence of the convection on IMF angle in the GSM y–z plane for three intervals of IMF magnitude in this plane. Except for predominantly northward IMF, the convection is primarily two-cell. The dusk cell is larger in terms of both spatial extent and potential variation/The effect of IMF By is apparent in the global shaping of the cells and the orientation of the overall pattern in MLT; for By + (By−) the dusk (dawn) cell is more round (crescent-shaped) and the pattern more rotated toward earlier MLTs. The By effect on the nightside convection is pronounced and is hemispherically antisymmetric, like the well-known day side By effect. For IMF increasingly northward, the convection trajectories on the dayside become increasingly distorted, evolving through a three-cell to a four-cell circulation. The additional cells appear on either side of the noon meridian and result in sunward flow. The overall agreement with the results of the satellite studies is good and extends to quite fine detail in the case of the comparison with Weimer [1995]. There are significant differences with the statistical patterns derived from magnetometer measurements, which tend to show domination by the dawn rather than the dusk cell.

HF radar signatures of the cusp and low-latitude boundary layer

Continuous ground-based observations of ionospheric and magnetospheric regions are critical to the Geospace Environment Modeling (GEM) program. It is therefore important to establish clear intercalibrations between different ground-based instruments and satellites in order to clearly place the ground-based observations in context with the corresponding in situ satellite measurements. HF-radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis. In this paper we report on an intercalibration study made using the Polar Anglo-American Conjugate Radar Experiment radars located at Goose Bay, Labrador, and Halley Station, Antarctica, and the Defense Meteorological Satellite Program (DMSP) satellites. The DMSP satellite data are used to provide clear identifications of the ionospheric cusp and the low-latitude boundary layer (LLBL). The radar data for eight cusp events and eight LLBL events have been examined in order to determine a radar signature of these ionospheric regions. This intercalibration indicates that the cusp is always characterized by wide, complex Doppler power spectra, whereas the LLBL is usually found to have spectra dominated by a single component. The distribution of spectral widths in the cusp is of a generally Gaussian form with a peak at about 220 m/s. The distribution of spectral widths in the LLBL is more like an exponential distribution, with the peak of the distribution occurring at about 50 m/s. There are a few cases in the LLBL where the Doppler power spectra are strikingly similar to those observed in the cusp.

A new mechanism for polar patch formation

Polar patches are regions within the polar cap where the F-region electron concentration and airglow emission at 630 nm are enhanced above a background level. Previous observations have demonstrated that polar patches can be readily identified in Polar Anglo-American Conjugate Experiment (PACE) data. Here PACE data and those from complementary instruments are used to show that some polar patches form in the dayside cusp within a few minutes of the simultaneous occurrence of a flow channel event (short-lived plasma jets ∼2 km s−1) and azimuthal flow changes in the ionospheric convection pattern. The latter are caused by variations of the y-component of the interplanetary magnetic field. The physical processes by which these phenomena cause plasma enhancements and depletions in the vicinity of the dayside cusp and cleft are discussed. Subsequently, these features are transported into the polar cap where they continue to evolve. The spatial scale of patches when formed is usually 200-1000 km in longitude and 2°-3° wide in latitude. Their motion after formation and the velocity of the plasma within the patches are the same, indicating that they are drifting under the action of an electric field. Occasionally, patches are observed to occur simultaneously in geomagnetic conjugate regions. Since some of these observations are incompatible with the presently-accepted model for patch formation involving the expansion of the high latitude convection pattern entraining solar-produced plasma, further modeling of the effects of energetic particle precipitation in the cusp, the consequences of flow channel events on the plasma concentrations, and the time dependence of plasma convection as a result of interplanetary magnetic field By changes is strongly recommended. Such studies could be used to determine the relative importance of this new mechanism compared with the existing theory for patch formation as a function of universal time and season.

Observations of an enhanced convection channel in the cusp ionosphere

Transient or patchy magnetic field line merging on the dayside magnetopause, giving rise to flux transfer events (FTEs), is thought to play a significant role in energizing high-latitude ionospheric convection during periods of southward interplanetary magnetic field. Several transient velocity patterns in the cusp ionosphere have been presented as candidate FTE signatures. Instrument limitations, combined with uncertainties about the magnetopause processes causing individual velocity transients, mean that definitive observations of the ionospheric signature of FTEs have yet to be presented. This paper describes combined observations by the PACE HF backscatter radar and the DMSP F9 polar-orbiting satellite of a transient velocity signature in the southern hemisphere ionospheric cusp. The prevailing solar wind conditions suggest that it is the result of enhanced magnetic merging at the magnetopause. The satellite particle precipitation data associated with the transient are typically cusplike in nature. The presence of spatially discrete patches of accelerated ions at the equatorward edge of the cusp is consistent with the ion acceleration that could occur with merging. The combined radar line-of-sight velocity data and the satellite transverse plasma drift data are consistent with a channel of enhanced convection superposed on the ambient cusp plasma flow. This channel is at least 900 km in longitudinal extent but only 100 km wide. It is zonally aligned for most of its extent, except at the western limit where it rotates sharply poleward. Weak return flow is observed outside the channel. These observations are compared with and contrasted to similar events seen by the EISCAT radar and by optical instruments.

ULF high- and low-m field line resonances observed with the Super Dual Auroral Radar Network

Numerous field line resonance events have been observed with three HF radars (Saskatoon, Kapuskasing, and Goose Bay) of the Super Dual Auroral Radar Network (SuperDARN). The field line resonances cause oscillations in the F region plasma flows which are detected in the measured line of sight Doppler velocities. After analysis, it was found that the resonances were of two types: those with low azimuthal wave number, low-m, and those with high azimuthal wave number, high-m. The high-m events showed many similarities with high-m pulsations of previous reports including local time of most occurrences (noon-dusk), pulsation frequencies, westward propagation, increase in phase with latitude, and north-south polarization. The low-m events exhibited typical field line resonance characteristics and were found near dusk and dawn with anti-Sunward propagation. The most notable result was the fact that the high- and low-m events shared many common features. They both were found to occur at the same discrete and stable frequencies. The most common frequencies were 1.3, 1.9, and 2.5-2.6 mHz, which have previously been associated with magnetospheric waveguide modes. They also occurred at other less common frequencies, such as 1.5–1.6 mHz. Both types of events were localized in latitude with an inverse relation between frequency and latitude. Both were characterized by a wave packet structure with a duration of approximately 1 hour. The numerous features shared by the high- and low-m resonances strongly suggest that they are caused by the same source mechanism. A dispersive waveguide model as a source for the field line resonances is discussed.

HF radar observations of Pc 5 field line resonances in the midnight/early morning MLT sector

On a number of occasions The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay, Labrador, has observed the effects of field line resonances on the drift velocities of irregularities in the F region of the high-latitude ionosphere. These resonances are seen as quasi-monochromatic oscillations in the Doppler velocities measured by the radar. One of the most interesting sets of resonances occurs near midnight MLT and may be associated with shear in the convective flow in the magnetotail. This paper discusses in detail a particularly clear example which shows field line resonances equatorward of a region of shear flow in the early morning sector. Velocity oscillations were recorded at the three discrete frequencies 1.3, 1.95, and 2.6 mHz over a 3 hour period. The motions were predominantly in the geomagnetic east-west direction, indicating north-south electric fields. As expected of field line resonance pulsations, these oscillations had pronounced peaks in their latitudinal power distribution. For the pulsation at 1.95 mHz, a latitudinal phase shift of 180° was observed across the peaks in all the look directions of the radar, and a longitudinal wavelength corresponding to an m value of about 3 was obtained. For the 2.6-mHz pulsation, the phase shifts across the peaks had a variation with look direction that indicated a significant longitudinal as well as latitudinal variation; for this activity we estimate an m value of about 16. The pulsations could occur simultaneously but remained distinct, as the latitude of peak response was observed to vary inversely with the frequency of the pulsation. We interpret these features in terms of field line resonance theory and discuss the possible sources of the pulsation energy.

Identification of high-latitude acoustic gravity wave sources using the Goose Bay HF radar

Observations of medium-scale acoustic gravity waves using the Goose Bay HF radar are presented. Data were examined for the period of November and December 1991, and days that showed evidence of traveling ionospheric disturbances (TIDs) were selected for analysis. TIDs were identified in the HF radar data by quasi-periodic enhancements of the power in the ground-scattered portion of the radar signal. Various parameters were determined from the data, including the wave frequency, the wavelength, the propagation direction, and the phase velocity. It was found that multiple wave packets from multiple directions are often present simultaneously. Source times and locations were identified from data from 2 days, December 5 and 6, 1991, and these were compared with data from the Greenland magnetometer chain. The source times and locations were found to correspond to sudden magnetometer deflections, indicating the correlation of gravity wave sources with the occurrence of ionospheric electrojet current surges. Finally, the source times and locations were examined in local time and magnetic latitude, and it was found that the sources were clustered in two locations: near the magnetospheric cusp; and near 1600 MLT at about 75° magnetic latitude.

Two substorm intensifications compared: Onset, expansion, and global consequences

We present observations of two sequential substorm onsets on May 15, 1996. The first event occurred during persistently negative IMF B-Z, whereas the second expansion followed a northward turning o ...

A comparison of a model for the theta aurora with observations from Polar, Wind, and SuperDARN

A model is presented according to which theta auroral arcs form after southward turnings of interplanetary magnetic field (IMF) and/or large variations in IMF By, following prolonged periods of northward IMF or very small Bz, with |By| ≳ |Bz|. The arcs start on the dawnside (duskside) of the auroral oval and drift duskward (dawnward) across the polar cap for positive (negative) By in the northern hemisphere and conversely in the southern hemisphere. After the theta aurora has formed, changes in IMF By or Bz readjust the merging configuration and continue the auroral pattern. The transpolar arcs are on closed magnetic field lines that bifurcate two open sections of the polar cap and map to the outer plasma sheet. Four theta auroral events were studied using data from the ISTP/GGS Polar and Wind spacecraft and the ground-based SuperDARN radars. Observations that are correctly predicted by our model include the following: (1) The formation and evolution of theta auroras observed by the visible imaging system are closely related to the IMF patterns measured by the Wind magnetic field investigation. (2) Both electrons and ions in the transpolar arc and poleward part of the night side auroral oval exhibit similar spectral characteristics, identified from the data acquired with Hydra and the comprehensive energetic particle and pitch angle distribution experiment. The low-energy electrons show counterstreaming distributions, consistent with their being on closed field lines that magnetically connect to the boundary plasma sheet in the magnetotail. (3) Ion composition measurements obtained from the toroidal imaging mass-angle spectrograph show cold plasma outflows from the ionosphere and hot, Isotropic magnetospheric ions in the two regions, also indicating transpolar arcs are on closed field lines. (4) Large scale polar cap convection inferred by SuperDARN observations is well correlated with IMF patterns. (5) Plasma convection in the transpolar arcs, inferred from the electric field instrument and the magnetic field investigation measurements, is sunward.

On the seasonal dependence of medium‐scale atmospheric gravity waves in the upper atmosphere at high latitudes

Seasonal dependence of medium-scale atmospheric gravity waves is examined in SuperDARN HF radar data and through analysis of the gravity wave dispersion relation. We found that the probability of gravity wave observation in the HF radar data is highest in winter months and lowest in summer months. The winter probability of observation in a given 2-hour period with sufficient amounts of ground backscatter peaks at about 0.8, while in the summer the probability remains near 0.4. Examination of the dispersion relation shows that there is a seasonal dependence to the altitude profile of the gravity waves vertical wavelength that may lead to a seasonally dependent reflection of the waves resulting from the mesospheric temperature gradient. The dispersion relation predicts that for waves of a given horizontal wavelength, the minimum wave period that may be transmitted through the mesosphere is longer in the summer than in the winter; the longer the wavelength, the longer the minimum period. Thus some waves that are transmitted through the mesosphere in the winter may not be transmitted in the summer. This seasonal dependence predicted from the dispersion relation is consistent with the HF radar observations.