J. M. Ruohoniemi

Large‐scale imaging of high‐latitude convection with Super Dual Auroral Radar Network HF radar observations

The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide measurements of the E × B drift of ionospheric plasma over extended regions of the high-latitude ionosphere. With the recent augmentation of the northern hemisphere component to six radars, a sizable fraction of the entire convection zone (approximately one-third) can be imaged nearly instantaneously (∼2 min). To date, the two-dimensional convection velocity has been mapped by combining line-of-sight velocity measurements obtained from pairs of radars within common-volume areas. We describe a new method of deriving large-scale convection maps based on all the available velocity data. The measurements are used to determine a solution for the distribution of electrostatic potential, Φ, expressed as a series expansion in spherical harmonics. The addition of data from a statistical model constrains the solution in regions of no data coverage. For low-order expansions the results provide a gross characterization of the global convection. We discuss the processing of the radar velocity data, the factors that condition the fitting, and the reliability of the results. We present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field (IMF). In the case of a sudden polarity change from northward to southward IMF, the convection is seen to reconfigure globally on very short (<6 min) timescales.

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.

Cross polar cap potentials measured with Super Dual Auroral Radar Network during quasi-steady solar wind and interplanetary magnetic field conditions

[1]xa0We have analyzed Super Dual Auroral Radar Network (SuperDARN) data between February 1998 and December 2000 to determine the statistical characteristics of the total variation in the high-latitude ionospheric electric potential, or cross polar cap potential, ΦPC. Periods are chosen to satisfy the criteria that (1) the solar wind and interplanetary magnetic field (IMF) are quasi-stable for ≥40 min and (2) sufficient SuperDARN data exist to adequately determine ΦPC. A total of 9464 individual 10-min periods satisfying the first criteria are analyzed. A subset of 2721 periods satisfy both criteria, of which 1638 are considered high-confidence periods. The resulting data set shows that for quasi-steady solar wind and IMF, ΦPC (1) is nonlinear in the expression for the effective interplanetary electric field EKL, (2) saturates at high values of EKL, and (3) is highly variable for any given value of EKL. These results indicate that simple formulations involving the upstream solar wind and IMF conditions are inadequate to describe the instantaneous ΦPC and that the inclusion of internal and coupling processes between the magnetosphere and ionosphere may be necessary.

Observations of a detached, discrete arc in association with field line resonances

Data from the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) array in Canada are used to analyze the magnetic fields and auroral structures which were associated with a field line resonance which occurred near local midnight and in the early morning sector. The electric fields of this resonance, which have a frequency of about 1.95 mHz, were observed by the Johns Hopkins University, Applied Physics Laboratory HF radar at Goose Bay and had a maximum at about 70.7° invariant latitude. Effects of the field line resonance were seen in the 5577 A, meridian scanning photometer data and the magnetometer data from the CANOPUS array. The field line resonance was accompanied by precipitating energetic electrons (energies greater than 3 keV) in a narrow auroral arc which was 3°–4° equatorward of the resonance structure seen in the radar data and approximately 1° equatorward of field lines threading the inner boundary of the proton plasma sheet. The electron precipitation in this arc was modulated at the frequency of the field line resonance. The effects of the resonance are also seen as pulsations in the magnetometer data, and the horizontal polarization of the pulsations showed a change in sense of rotation across the arc. The oscillations in the precipitating electrons might have been caused by modulation in the ELF/VLF growth rates due to the presence of the magnetohydrodynamic waves associated with the resonance, by mode conversion to kinetic Alfven waves, or by the formation of electrostatic ion cyclotron waves due to field-aligned currents associated with a second field line resonance collocated with the arc. The evidence presented here suggests that the modulation of ELF/VLF by magnetohydrodynamic waves on field lines near the plasmapause was the most likely cause of the auroral oscillations.

Observations of ionospheric convection from the Wallops SuperDARN radar at middle latitudes

[1]xa0During geomagnetic storms the ability of the Super Dual Auroral Radar Network (SuperDARN) to measure ionospheric convection becomes limited when the radars suffer from absorption and the auroral disturbance expands equatorward of the radar sites. To overcome these shortcomings, it was decided to construct a SuperDARN radar at middle latitudes on the grounds of the NASA Wallops Flight Facility. This paper presents the first comprehensive analysis of Doppler measurements from the Wallops radar, which commenced operations in May 2005. Wallops measurements are compared with the Goose Bay radar during the onset of a geomagnetic storm on 31 August 2005: Goose Bay measured the onset of geomagnetic activity at high latitude while Wallops monitored the expansion of convection to middle latitudes. Average convection patterns binned by the Kp geomagnetic index are also presented. During weak-moderate geomagnetic activity (Kp ≤ 3) the Wallops radar observes ionospheric irregularities between 50° and 60° magnetic latitude drifting westward across much of the nightside. When these measurements are incorporated into the calculation of an average SuperDARN convection pattern, the streamlines of polar cap outflow on the nightside become kinked in a manner reminiscent of the Harang discontinuity. This morphology arises quite naturally when the two-cell convection at high latitudes merges with the prevailing westward convection at middle latitudes. During increased geomagnetic activity (Kp ≥ 3), Wallops is able to measure the expansion of auroral electric fields to middle latitudes and the average SuperDARN cross-polar cap potential is increased by 25%.

Ionospheric response to the interplanetary magnetic field southward turning: Fast onset and slow reconfiguration

[1] This paper presents a case study of ionospheric response to an interplanetary magnetic field (IMF) southward turning. It is based on a comprehensive set of observations, including a global network of ground magnetometers, global auroral images, and a SuperDARN HF radar. There is a clear evidence for a two-stage ionospheric response to the IMF southward turning, namely, fast initial onset and slow final reconfiguration. The fast onset is manifested by nearly simultaneous (within 2 min) rise of ground magnetic perturbations at all local times, corroborated by a sudden change in the direction of line-of-sight velocity near local midnight and by the simultaneous equatorward shift of the auroral oval. The slow reconfiguration is characterized by the different rising rate of magnetic perturbations with latitudes: faster at high latitude than at lower latitudes. Furthermore, a cross-correlation analysis of the magnetometer data shows that the maximum magnetic perturbation is reached first near local noon, and then spread toward the nightside, corresponding to a dayside-to-nightside propagation speed of ∼5 km/s along the auroral oval. Global ionospheric convection patterns are derived based on ground magnetometer data along with auroral conductances inferred from the Polar UV images, using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. The AMIE patterns, especially the residual convection patterns, clearly show a globally coherent development of two-cell convection configuration following the IMF southward turning. While the foci of the convection patterns remain nearly steady, the convection flow does intensify with time and the cross-polar-cap potential drop increases. The overall changes as shown in the AMIE convection patterns therefore are fully consistent with the two-stage ionospheric response to the IMF southward turning.

Dayside reconnection enhancement resulting from a solar wind dynamic pressure increase

[1]xa0It is well known that the Interplanetary Magnetic Field (IMF) is the major contributor to geomagnetic activity on Earth. Recent studies, however, have shown that solar wind dynamic pressure variations also cause global effects when they encounter the terrestrial magnetosphere. In particular, it has been shown that solar wind dynamic pressure enhancements significantly increase particle precipitation and cause global intensification of the aurora. Further studies using Defense Meteorological Satellite Program (DMSP) measurements have demonstrated that solar wind pressure increases also significantly affect the size of the polar cap and the cross-polar cap potential drop. This implies that the dynamic pressure has an important effect on the coupling efficiency between the solar wind and the Earth’s magnetosphere, which is in addition to that due to the IMF. It was previously suggested, on the basis of the DMSP data, that solar wind dynamic pressure enhancements induce enhanced magnetotail reconnection and magnetospheric convection. We now present Super Dual Auroral Radar Network (SuperDARN) observations for a number of events that demonstrate significantly enhanced ionospheric convection in the dayside ionosphere associated with the impact of solar wind pressure fronts. The enhanced convection extends to the vicinity of the expected location of the dayside separatrix, suggesting that the solar wind dynamic pressure strongly affects dayside reconnection as well as polar-cap convection.

Evolution of ionospheric multicell convection during northward interplanetary magnetic field with |Bz /By | > 1

During northward interplanetary magnetic field (IMF), it is generally believed that ionospheric convection appears as a four-cell structure for |Bz/By| > 1 and as a distorted two-cell structure for |Bz/By| 1 on November 11, 1998. We show in detail the evolution of the convection patterns as |Bz/By| changes. Nearly symmetric four-cell convection, with two reverse cells in the polar cap and two normal cells at lower latitudes, occurs for |Bz/By| ≈ 7. The ionospheric flow associated with the reverse cells is closed almost completely on the dayside. A shifted four-cell convection pattern, with the reverse cells shifted toward earlier magnetic local time (MLT) for negative By and toward later MLT for positive By, is observed for |Bz/By| ≈ 2.3. When |Bz/By| decreases to ∼1.7, the convection appears as a three-cell pattern, with a single reverse cell focused near noon and two normal cells. The normal morning and afternoon cells are focused at quite high magnetic latitudes (between 76° and 80°); the spatial extent of the normal cells is 10°–15° or 1000–1500 km in the latitudinal direction. We also present Defense Meteorological Satellite Program (DMSP) satellite data which show sunward convection over the polar cap in the Southern Hemisphere at the same time as the Northern Hemisphere radar observations. We propose a new model of convection patterns during northward IMF for |Bz/By| > 1 and By < 0 on the basis of the combined observations. In the model the convection appears as a symmetric four-cell structure for |Bz/By| ≥ 3, a shifted four-cell structure for |Bz/By| = 2–3, and a three-cell structure for |Bz/By| = 1–2.

Global ULF disturbances during a stormtime substorm on 25 September 1998

[1]xa0Quasiperiodic perturbations of ionospheric plasma with ∼10 min occurrence periods were observed by incoherent scatter radars (ISR) in high-latitude and midlatitude ionosphere after the onset of a stormtime substorm. The latter occurred during a period of complex dynamical activity and had a number of unusual characteristics. The perturbations indicate ultralow frequency (ULF) electric field enhancements over the Northern Hemisphere. Specifically, strong subauroral electric fields are associated with the ring current-related polarization jet (PJ). These perturbations correlate with magnetic variations in high-latitude and midlatitude regions spanning ∼21 hours in magnetic local time (MLT). The radar and ground magnetometer data are supported by simultaneous observations from the DMSP and geostationary satellites. As a result of the onset, the preexisting PJ shifted equatorward to L ∼ 2.5. Similar periodicities in the solar wind dynamic pressure and interplanetary magnetic field (IMF) appear to be natural sources of the ULF perturbations observed at high and middle latitudes.

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.