S. G. Shepherd

Electrostatic potential patterns in the high‐latitude ionosphere constrained by SuperDARN measurements

The recent addition of two radars to the existing network of six Super Dual Auroral Radar Network (SuperDARN) HF radars in the Northern Hemisphere has significantly extended the area in the high latitude where measurements of convecting ionospheric plasma are made. We show that the distribution of the electrostatic potential Φ associated with the “E × B” drift of ionospheric plasma can be reliably mapped on global scales when velocity measurments provide sufficient coverage. The global convection maps, or the equivalent electrostatic potential maps, are solved using an established technique of fitting velocity data to an expansion of Φ in terms of spherical harmonic functions. When the measurements are extensive, and especially when they span the region between the extrema in the potential distribution, the solution for the global pattern becomes insensitive to the choice of statistical model data used to constrain the fitting. That is, the statistical model data then only guide the solution in regions where no measurements are available, and the details of the model data have little effect on the gross features of the large-scale convection patterns. The resulting total potential variation across the polar cap, ΦPC, is virtually independent of the statistical model. The ability to accurately determine ΦPC and the global potential Φ on the basis of direct measurements is an important step in understanding solar wind-magnetosphere-ionosphere coupling.

Climatological patterns of high‐latitude convection in the Northern and Southern hemispheres: Dipole tilt dependencies and interhemispheric comparisons

[1] Using line‐of‐sight measurements of horizontal plasma drift from the Super Dual Auroral Radar Network (SuperDARN) located in the Northern and Southern hemispheres over a period extending from 1998 to 2002, statistical models of the high‐latitude convection electric field are derived for various ranges of interplanetary magnetic field (IMF) magnitude and orientation and for several ranges of dipole tilt angle. Direct comparison of the corresponding convection patterns in each hemisphere shows that under neutral tilt conditions (dipole tilt angle magnitude <10°) the patterns are most similar. However, a strong dipole tilt angle dependence is observed under northward (Bz+) and By dominated IMF conditions. For IMF Bz+, reverse convection is observed to be much stronger during positive tilt than negative tilt. For IMF By dominated conditions (IMF Bz = 0), the round convection cell is more enhanced for positive tilt than for negative tilt, particularly for IMF By < 0 in both hemispheres. The presence of a lobe cell is a likely cause of this enhancement, although it is not entirely clear why it occurs preferentially under IMF By < 0. In addition, the crescent‐shaped cells are weakened as tilt angle progresses from negative to positive, most likely due to vastly different solar produced conductivities under different tilt angles. For IMF Bz−, asymmetric values of the cross‐polar cap potentials (FPC) are observed between hemispheres, with FPC in the south being systematically larger than FPC in the north. Although neutral tilt patterns are similar enough to be used interchangeably, convection has a strong dipole tilt dependence and a Northern Hemisphere convection model should not be applied to the Southern Hemisphere if dipole tilt angle is not taken into account. When dipole tilt is accounted for, FPC differs between hemispheres by less than 10% on average, but the strength of the convection in the individual cells differs by 15% to 20% on average.

The polarization of auroral radio emissions

Ground level observations using two vertical loop antennas oriented at 90° to each other reveal the sense of polarization of several types of auroral radio emissions in the frequency range 30-5000 kHz. Auroral hiss is observed to be right elliptically polarized (RP) with respect to the local magnetic field, consistent with theoretical expectation for the whistler mode and with earlier measurements. Two less well-understood auroral emissions, are found to be left elliptically polarized (LP). This polarization is inconsistent with their generation in the X-mode as suggested by some theories.

The response of the high-latitude ionosphere to IMF variations

The convection of plasma in the high-latitude ionosphere is strongly a2ected by the interplanetary magnetic 3eld (IMF) carried by the solar wind. From numerous statistical studies, it is known that the plasma circulation conforms to patterns that are characteristic of particular IMF states. Following a change in the IMF, the convection responds by recon3guring into a pattern that is more consistent with the new IMF. Some early studies reported that the convection 3rst begins to change near noon while on the dawn and dusk 5anks and on the nightside it remains relatively una2ected for tens of minutes. Work by Ridley et al. (J. Geophys. Res. 103 (1998) 4023– 4039) and Ruohoniemi and Greenwald (Geophys. Res. Lett. 25 (1998) 2913–2916) that was based on measurements with more global sets of instruments challenged this view. A debate ensued as to the true nature of the convection response. We follow the arguments of Lockwood and Cowley (J. Geophys. Res. 104 (1999) 4387– 4391) and Ridley et al. (J. Geophys. Res. 104 (1999) 4393– 4396) by reviewing recent results on the timing of the onset of the convection response to the changed IMF. We discuss the timing problem from the perspectives of observations and modeling. In our view, the onset of the ionospheric response to changed IMF is globally simultaneous on time scales of a few minutes. A physical basis for the rapid communication of e2ects in the dayside convection to the nightside has been demonstrated in magnetohydrodynamic simulations. We also o2er some cautionary notes on the timing of convection changes and the use of global assimilative techniques to study local behavior. c � 2002 Elsevier Science Ltd. All rights reserved.

Testing the Hill model of transpolar potential with Super Dual Auroral Radar Network observations

We use a data set consisting of periods for which the transpolar ionospheric potential ( pc) is well-determined by Super Dual Auroral Radar Network (SuperDARN) data to test the Hill model. The Hill model, as formulated by Siscoe et al. [2002], specifies pc as a function of solar wind speed and ram pressure, the interplanetary magnetic field, the reconnection electric field (Er), and the ionospheric conductance ( ). The periods used in our study were identified as times when the interplanetary electric field was quasistable and SuperDARN coverage was sufficient to determine pc. SuperDARN-determined pc ( SDc ) is compared to pc determined using the Hill model ( Hillc ) for 1317 10-min periods. A minimum in the root-mean-square difference between SDc (Er) and Hillc (Er) is achieved when = 23 S and a constant potential, 0 = 17 kV, are used. Some aspects of the data agree very well for these values of and 0, including the mean value of pc(Er) and that both data sets clearly indicate saturation at higher values of Er. The ram pressure dependence of Hillc , however, is inconsistent with that of SD pc and suggests that should be lower than 23 S. There is also significantly more variability in SDc for all values of Er than the

Altitude‐adjusted corrected geomagnetic coordinates: Definition and functional approximations

Analysis of the functional approximations used to transform between geo- graphic and Altitude-Adjusted Corrected Geomagnetic (AACGM) coordinates reveals that errors of >50 km can occur in the auroral and polar regions. These errors are the result of efforts to better approximate AACGM coordinates near the magnetic equator and the South Atlantic Anomaly. In these regions AACGM coordinates are not defined and alternate coordinates have been used. This augmentation and emphasis on the so- lution in regions near the equator result in spherical harmonic approximating functions that are less accurate than need be in the auroral and polar regions. In response, a new set of spherical harmonic coefficients have been derived that better represent AACGM coordinates in these regions. These new AACGM coefficients are limited to below 2000 km in altitude in order to ensure accuracy. For altitudes above 2000 km, a magnetic field- line tracing solution is recommended. A software package developed to take advantage of the new AACGM coefficients provides the capability of tracing magnetic field lines at any altitude, for improved accuracy. In addition, linear interpolation between 5-year epochs is used to produce coordinates that vary smoothly over the entire period from 1965-present. The intent of this work is to provide a more accurate procedure for de- termining AACGM coordinates in the auroral and polar regions for the study of mag- netospheric and ionospheric processes.

A possible explanation for rapid, large-scale ionospheric responses to southward turnings of the IMF

We have examined the dayside high-latitude convection response to a sudden southward turning of the Interplanetary Magnetic Field (IMF) and found that the response is nearly instantaneous (<2 min) over a spatial region extending from ∼75° to 85° and from ∼9 to 16 MLT. Observations of the magnetic field were made with the WIND spacecraft in the solar wind and the GEOTAIL and IMP8 spacecraft in the magnetosheath. In the high-latitude ionosphere, the HF radars of the Super Dual Auroral Radar Network (SuperDARN) were used to monitor the convection. Based on the magnetosheath flow of the gas dynamic approximation, the field lines of the new IMF state were draped over a large portion of the dayside magnetopause when the first significant indication of convection response was measured in the ionosphere. Significant magnetic field line draping accompanied by extended reconnection on the dayside magnetopause may help explain the rapid, large-scale response of ionospheric convection.

Propagation of medium frequency (1–4 MHz) auroral radio waves to the ground via the Z‐mode radio window

Recent ground-based observations of auroral radio waves have identified narrowband emissions near 2 and 3 times the lower ionospheric electron cyclotron frequency (fce) known as auroral roars. In this paper the propagation of these waves in the auroral ionosphere is investigated by means of a ray-tracing technique. We model one particular scenario in which a large-scale (tens of kilometers) horizontal density structure, based on density structures observed with the Sondrestrom radar at times of auroral roar emissions, plays a crucial role in both guiding the waves to the ground and enabling mode conversion. The location and the mode characteristics of the initial waves are determined on the basis of local stability properties, which suggests that Z-mode wave excitation is favored near 2fce. However, since Z-mode cannot propagate to the ground they must first undergo a mode conversion to one of the free-space modes (X and O). It is found that for a narrow range of frequencies and initial wave phase angles the trapped Z mode can be converted to O mode via the Ellis radio window. This finding is consistent with the fact that auroral roar emissions are nearly 100% O-mode polarized. However, it is important to note that the evaluation of the damping of the Z-mode waves along the ray path is not considered within the context of this preliminary study and will be critical for eventually determining the exact physical scenario of the auroral roar generation mechanism.

Ionospheric structure and the generation of auroral roar

Ionospheric electron density data from the Sondrestrom incoherent scatter radar (ISR) have been used to characterize the structure of the F region ionosphere during ground-based LF/MF/HF receiver observations of natural ionospheric radio emissions known as auroral roar. In five out of six cases, the F region ionosphere has significant horizontal Ne gradient scale lengths ( , measured with 23–137 km spatial resolution). In three of these cases, localized F region auroral ionospheric cavities, with horizontal scales ∼50 km, are observed. In one of six cases, the ionosphere lacks either of these features, and a laminar, mostly unstructured, F region is observed. The data suggest that auroral roar events may occur for a range of large-scale (>30 km) ionospheric conditions. Some theories for the generation of auroral roar require that the relationship between the electron plasma frequency (ƒpe) and the electron gyrofrequency (ƒce) in the source region is , where n is the harmonic number of the observed emission. Comparisons between observed auroral roar emission frequencies, ISR observations of electron density, and the IGRF model for the magnetic field show that this frequency-matching condition holds somewhere in the ionosphere in 16 out of 18 cases studied and in all 3 cases of ISR elevation scans capable of measuring a source located directly overhead.

Further investigation of auroral roar fine structure

In April 1996, a downconverting receiver was operated in Churchill, Manitoba, Canada, to increase the statistics about the recently discovered fine structure of auroral roar emissions. Auroral roar is found to be both structured and unstructured. A wide variety of previously unknown tonal features drifting in a complicated manner were recorded. These structured features can be classified according to their duration, frequency drift, and grouping with like features. Typically, 95% of the structured features last less than 1 s. The slope of drifting features is more commonly negative than positive with a magnitude typically less than a few kHz s -1 and a maximum of ∼ 800 kHz s -1 . The minimum bandwidth of features is 6 Hz or less, and typical separation between similar features is ∼ 400 Hz. These measurements form a basis for reviewing proposed generation mechanisms of auroral roar including a localized source model and laser cavity mechanism.