G. J. Sofko

Global energy deposition during the January 1997 magnetic cloud event

The passage of an interplanetary magnetic cloud at Earth on January 10–11, 1997, induced significant geomagnetic disturbances, with a maximum AE in excess of 2000 nT and a minimum Dst of about −85 nT. We use a comprehensive set of data collected from space-borne instruments and from ground-based facilities to estimate the energy deposition associated with the three major magnetospheric sinks during the event. It is found that averaged over the 2-day period, the total magnetospheric energy deposition rate is about 400 GW, with 190 GW going into Joule heating rate, 120 GW into ring current injection, and 90 GW into auroral precipitation. By comparison, the average solar wind electromagnetic energy transfer rate as represented by the e parameter is estimated to be 460 GW, and the average available solar wind kinetic power USW is about 11,000 GW. A good linear correlation is found between the AE index and various ionospheric parameters such as the cross-polar-cap potential drop, hemisphere-integrated Joule heating rate, and hemisphere-integrated auroral precipitation. In the northern hemisphere where the data coverage is extensive, the proportionality factor is 0.06 kV/nT between the potential drop and AE, 0.25 GW/nT between Joule heating rate and AE, and 0.13 GW/nT between auroral precipitation and AE. However, different studies have resulted in different proportionality factors. One should therefore be cautious when using empirical formulas to estimate the ionospheric energy deposition. There is an evident saturation of the cross-polar-cap potential drop for large AE (>1000 nT), but further studies are needed to confirm this.

Influence of ionospheric electron density fluctuations on satellite radar interferometry

Evidence is presented that auroral zone ionospheric disturbances can influence satellite radar interferometry (SRI) obtained with the RADARSAT, ERS and JERS-1 satellites. Fluctuations in ionospheric electron density can lead to an azimuth shift modulation in synthetic aperture radar (SAR) imagery, which can be detected using SRI. Measurements of azimuth shift in SRI can help to differentiate ionospheric from tropospheric propagation problems, and to understand better the impact of the ionosphere on spaceborne SAR. Further, SRI azimuth shift modulation may be useful in mapping patterns of polar auroral zone ionospheric disturbances over large distances.

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.

Comparison of plasma flow velocities determined by the ionosonde Doppler drift technique, SuperDARN radars, and patch motion

We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3-hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.

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.

Super dual auroral radar network radar imaging of dayside high‐latitude convection under northward interplanetary magnetic field: Toward resolving the distorted two‐cell versus multicell controversy

Data from the Kapuskasing and Saskatoon radars of the evolving Super Dual Auroral Radar Network (SuperDARN) HF radar network have been analyzed to study the two-dimensional structure and dynamics of dayside high-latitude ionospheric convection under northward interplanetary magnetic field (IMF) conditions. A period extending from 1600 to 2030 UT (∼0900-1330 MLT) on January 10, 1994, was examined. During this interval, magnetic field data were available from the IMP 8 satellite and indicated moderately stable northward IMF conditions. For the first few hours of observation the By component of the IMF was positive, reasonably steady, and approximately twice the magnitude of Bz. During this interval, the high-latitude convection images obtained with the SuperDARN radars were very similar to the distorted two-cell convection maps for positive By as presented by Heppner and Maynard (1987). At ∼1840 UT, a decrease in By in association with an increase in Bz, led to an extended period with By ≈ Bz. During this second interval the convection patterns were highly variable and even chaotic. Finally, a sharp decrease in the By component at 1914 UT, probably in association with a rotational discontinuity in the solar wind, led to an extended period with By ≪ Bz. During this third interval, the high latitude convection pattern was again stable and exhibited a single counterclockwise rotating vortex consistent with one of the polar cap merging cells proposed by Dungey (1963), Burke et al. (1979), and Reiff and Heelis (1994). The transition to the counterclockwise rotating vortex occurred over a large spatial area within two radar scans (200 s) and appeared to proceed via an equatorward incursion of the vortex into the previous convection configuration. Once this polar cap merging cell was formed, it remained stable for the remainder of the period (∼40 min) that By remained small. Sunward convection was also observed in the polar cap of the conjugate hemisphere at 1932 UT by the DMSP F11 satellite, confirming that both magnetopause and internal reconnection sites were active at that time.

On the origin of narrow non‐ion‐acoustic coherent radar spectra in the high‐latitude E region

Many types of coherent radar spectra have a width in Doppler velocity units that is less than the ion acoustic speed of the medium. In spectra labeled as type 1 the mean Doppler shift of these narrow spectra matches the ion acoustic speed of the medium. There also exist narrow high-latitude spectra for which the mean Doppler shift is either markedly less or markedly more than the ion acoustic speed. We propose that electron density gradients with scale lengths as small as 100 m are at the origin of a large fraction of these narrow spectra near 50 MHz. The sharp density gradients in that case are created in regions of discrete auroral precipitation associated either with multiple narrow arcs or with sharp edges of broader features. Using the same principle at radar frequencies in the 10- to 20-MHz range, we find that gradient scales from 20 to 30 km in size create a combination of fast and slow phase velocities closely resembling the spectral characteristics expected from NO+ ion cyclotron waves. However, gradients are not always responsible for slow narrow spectra; a detailed analysis of available observations has led us to conclude that the high-latitude E region cannot always be considered as fully turbulent even when appreciable coherent echo returns are registered by the radars. In particular, slow narrow spectra at 50 MHz are at times produced under gradient-free weakly turbulent conditions. In addition, at lower radar frequencies (10 to 20 MHz) the narrow spectral width of slowly moving waves and the morphology of these waves both suggest that the irregularities are generated indirectly via mode coupling of linearly unstable modes and that these “secondary” waves are themselves not coupling efficiently. This implies that processes other than mode coupling are contributing to the overall wave energy budget. In that case we suggest that the convective properties of the slowly growing modes are an important factor in removing wave energy, even for waves as small as a few meters in wavelength. We also propose that there may be two distinct generation mechanisms for secondary waves at 10 MHz, each with its own mean Doppler shift behavior.

Ionospheric refraction effects in slant range profiles of auroral HF coherent echoes

The theory of auroral coherent echoes developed for VHF scattering by Uspensky et al. (1988, 1989) is applied to the interpretation of intensity and Doppler velocity slant range profiles of HF radar aurora. The theoretical model includes the effects of irregularity aspect sensitivity, ionospheric refraction of the radar beam, and the reception of signals from different heights. The predicted profiles of HF radar aurora are compared with Schefferville HF radar observations in the frequency interval of 9–18 MHz. Satisfactory agreement is found between theory and experiment for the intensity profiles. However, there are significant discrepancies for the Doppler velocity profiles. We discuss this lack of agreement in light of other recent observations.

Solar‐wind‐driven pulsed magnetic reconnection at the dayside magnetopause, Pc5 compressional oscillations, and field line resonances

A series of observations by satellites of the solar wind and magnetosphere, by HF radars of the F region, and by ground-based magnetometers of E region currents are presented to show the correlation between quasiperiodic fluctuations in the solar wind, magnetosphere, and ionosphere on January 24, 1996. The Iceland Stokkseyri SuperDARN radar observed quasiperiodic convection flow bursts in or near the ionospheric footprint of the cusp, identified by large spectral widths. Sometimes called flow channel events (FCEs), these enhanced convection flows are generally believed to be the ionospheric signatures of flux transfer events (FTEs) in the cusp. Initially, the flows pulsed with a periodicity of ∼12 min. When the “radar” cusp shifted to lower latitudes, possibly as a result of a series of FTEs, FCEs occurred every 10 min or less. The Wind spacecraft observed linearly polarized interplanetary magnetic field (IMF) oscillations with dominant periodicities of about 12 and 20 min (∼1.3 and 0.8 mHz) that were superposed on long-period IMF fluctuations on the scale of a few hours. The pure state power spectra of the IMF components also showed several subsidiary peaks at higher frequencies (e.g., ∼1.7, 2.0, 2.4, and 2.9 mHz) or shorter periods (∼10, 8.5, 7, and 6 min). Similar oscillations were observed in the solar wind dynamic pressure which was anticorrelated with the IMF magnitude and included a strong oscillation with a period of ∼15 min (1.1 mHz). About 20 min later, in the postnoon magnetosheath, Geotails magnetic and electric field sensors observed an ULF wave packet showing similar discrete spectra. The compressional MHD waves in the solar wind applied oscillating magnetic/electric fields and dynamic pressure on the dayside magnetopause driving multifrequency compressional oscillations in the magnetosphere. Discrete frequencies similar to those found in the solar wind and magnetosheath data were identified in the fast Fourier transform spectra of the ground-based and space-borne (GOES 8 and 9) magnetograms. A series of solar wind dynamic pressure pulses caused compressions and rarefactions of the magnetosphere observed by GOES 8 and GOES 9 magnetometers at geostationary orbit. The dynamic pressure driven magnetic perturbations, including those observed on the ground, propagated antisunward. In the noon sector the compressional waves coupled to shear modes resulting in propagation delays of the ground/ionospheric response. The convection flow bursts in the ionospheric footprint of the cusp were closely correlated with the low-frequency oscillations of the IMF BZ and magnetosheath duskward electric field suggesting that the ULF waves in the solar wind modulated the magnetic reconnection at the dayside magnetopause into pulses. The higher-frequency components of the IMF oscillations and/or sudden IMF changes excited field line resonances (FLRs) on the magnetic shells adjacent to the noon magnetopause. As the relative power of these higher-frequency oscillations in the solar wind IMF increased and the cusp shifted to lower latitudes, the FCE rate also increased. A possibility of a reverse coupling (feedback) from the resonating magnetic shells to the reconnection region at the dayside magnetopause is suggested.

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.