M. L. Parkinson

Thermospheric neutral winds at southern mid‐latitudes: A comparison of optical and ionosonde hmF2 methods

During the first 6 days of March 1995, measurements of the ionospheric electron density were made with a digisonde, and thermospheric winds were measured with a Fabry-Perot interferometer. This was a period of low solar activity and moderate to high magnetic activity. The ionograms have been scaled and the traces inverted to obtain the electron density profile and the peak height of the F2 layer (hmF2). Modeling has been employed to derive equivalent thermospheric neutral winds at hmF2. The derived neutral winds are in very good agreement with the measured optical winds most of the time. The winds follow a strong diurnal pattern with poleward winds during the day, weak winds near dawn and dusk, and strong equatorward winds peaking near local midnight. On most nights the peak equatorward wind speed was around 200 m s−1, but on March 1 it did not exceed 110 m s−1. For these magnetic and solar activity conditions the wind at the F2 peak altitude (∼350 km) from the HWM93 empirical wind model[Hedin et al., 1996] did not exceed 90 m s−1 at any time but was in generally good agreement with the hmF2 wind during the day and with both measured winds on the nights of March 1 and 2. The good agreement between the optical and hmF2 winds was obtained by using the recommended Burnside factor of 1.7 to multiply the O+-O collision frequency, but better agreement was obtained either by using a Burnside factor of 2.0 or by increasing the atomic oxygen density by 20%. Recent suggestions of much lower Burnside factors could be tolerated only if there were large systematic errors in the measurements or large electric fields.

Behavior of the ionosphere and thermosphere at a southern midlatitude station during magnetic storms in early March 1995

During the first six days of March 1995, measurements of the ionospheric electron density were made near Melbourne, Australia, with a digisonde and thermospheric winds, temperatures, and 6300-A emission rates were measured with a Fabry-Perot interferometer. The ionograms were inverted to obtain electron density profiles and peak heights of the F2 layer (h m F 2 ). This paper compares modeled and measured electron densities, airglow emission rates, and neutral temperatures. The measured peak electron density shows strong negative effects from magnetic storms and rapid recovery to normal levels afterward. The model daytime peak density is in good agreement with the measurements on the undisturbed days and also shows negative phases at the appropriate times. However, the model negative effects are not as strong as the measured negative effects. A new algorithm is introduced to bring the measured and modeled peak electron density into better agreement by adjusting the exospheric Tn and atomic oxygen density in the MSIS model. The modified Tn at 300 km altitude agrees reasonably well with both the standard MSIS model and the measured Tn before midnight local time. However, the modified Tn, like the measured Tn, has a tendency to increase after about 2200 LT. At night, adjustments to the MSIS model exospheric temperature and atomic oxygen density are generally small. There is reasonably good agreement between the relative variations of the measured and modeled emission rates. However, there is much more variability in the model intensities than in the measured intensities as a result of rapid movements in the height of the F2 layer.

Digital ionosonde measurements of the height variation of drift velocity in the southern polar cap ionosphere: Initial results

During the late austral summer of 1995-1996 we operated an HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E, -80.8° corrected geomagnetic (CGM) latitude), in an experimental drift mode with the aim of resolving the height variation of drift velocity in the polar cap ionosphere. We devised control programs for a Digisonde Portable Sounder 4 to collect data at separate frequency-range gates corresponding to the E and F regions to investigate the differences in their motions. During a 4-day campaign commencing March 11, 1996, the mode values of the drift perpendicular to the magnetic field ( V⊥ ) were 85 m s -1 in the E region and 485 m s -1 in the F region (using 10 m s -1 bins and echoes from all heights in each region). Vertical profiles of drift velocity were obtained by sorting echoes into 10-km group-height bins. For measurements obtained within ±3 hours of magnetic noon the average profile showed that in the lower E region V⊥ increased approximately exponentially with true height. The corresponding velocity scale height was 46.7 m s -I km -1 . The mean value of V⊥ leveled off to about 700 m s -1 above 120 km, where it remained up to the F region peak height. The vertical gradient was caused by the increase in collision frequencies at the lower heights. The F region field-aligned component of drift (V∥) showed a strong diurnal variation, with mean values of -30 m s -1 near noon and +60 m s -1 during the night at a height of 180 km. The average over the whole day reveals a net upward drift of 30 m s -1 . This behavior is attributed to the interaction between the meridional components of the generally antisunward neutral wind (U N ) and perpendicular drift ( V⊥ S ) moving plasma down the field lines during the day and up the field lines during the night, with U N and V⊥ s having net equatorward values when averaged over all day. While the E region drift direction tended to be aligned with the basic antisunward convection which dominates the F region above Casey, it also tended to show greater temporal variability in direction, suggesting a smaller-scale size and lifetime for the E region structures giving rise to the echoes. There were events lasting over 2 hours during which the drifts in the two regions were clearly resolved into different azimuths (by nearly 180° for two events). These transient directional shears show the time variability in the phase transition between an F region collisionless, magnetized plasma driven by the E X B/B 2 convection to an E region collisional, unmagnetized plasma driven by E and irregular neutral winds.

Analysis of direction‐of‐arrival aliasing for MF/HF Doppler‐sorted interferometry measurements of ionospheric drift

The determination of accurate direction of arrival (DOA) of echoes is paramount when performing Doppler-sorted interferometry (DSI) at MF/HF to measure the drift of ionospheric plasma. Important factors affecting the accuracy include inadequate signal-to-noise ratio (SNR) and coherency of ionospheric echoes. As the SNR deteriorates, there is an increase in the errors in the phases measured at the individual antennas of an array, and this leads to ambiguities in DOA equivalent to directional aliasing. In this paper we model this effect, the effects of Doppler-frequency aliasing, and the finite resolution of Doppler spectra. We thereby show the importance for vertical incident sounders of rejecting echoes with large off-vertical angles, and the choice of appropriate signal-processing parameters for the accurate estimation of ionospheric motions. As an example, we consider the situation for vertical incident ionosondes and show that a low SNR can produce spurious echoes at relatively large zenith angles, which can, however, be avoided or minimized by the suitable choice of operating parameters.

Application of the Dopplionogram to Doppler‐sorted interferometry measurements of ionospheric drift velocity

The Dopplionogram was developed as a method of displaying Doppler shifts along the frequency axis of ionograms recorded using B-mode soundings of the Dynasonde, an early type of HF digital ionosonde. The basic idea of recording Doppler shifts in an ionogram format is applied and extended to the Doppler velocity mode of the Digisonde Portable Sounder-4 (DPS-4), a related and more recent type of digital ionosonde. In order to describe our mode of operation a Dopplionogram is redefined to mean a set of stepped-frequency soundings that yields a set of ionospheric Doppler shifts particular to the chosen transmission frequencies. Extension of the technique to include Doppler-sorted interferometry (DSI) analysis of the Doppler spectra facilitates a detailed analysis of ionospheric velocity variations in time and group height. This revitalized approach to DSI should prove useful for the study of ionospheric dynamics for which knowledge of the height profile of electric currents, drift velocity, and neutral winds is required. The technique is demonstrated using measurements of polar cap plasma winds obtained with a DPS-4 located at Casey, Antarctica (66.3°S, 110.5°E).

Signatures of the ionospheric cusp in digital ionosonde measurements of plasma drift above Casey, Antarctica

Signatures of the ionospheric cusp in HF digital ionosonde measurements of plasma drift made at the polar cap station Casey, Antarctica (−80.8° geomagnetic latitude), are investigated. Measurements recorded during the campaign interval February 13–17, 1996, are considered in this case study because the summer dipole tilt effect, and an interplanetary magnetic field (IMF) northward condition on February 16, were favorable for the detection of the cusp at a higher than usual latitude. On February 14 and 15 the magnitude of the IMF was about 4–6 nT, and the station probably passed just poleward of the cusp. The most general signatures of the cusp were enhanced electric field and electric field turbulence shown by increased drift velocity and velocity scatter in the convection throat, respectively. Broadband, unstructured fluctuations in the geomagnetic field measured by magnetometers near to noon are well known to be a signature of cusp currents and were associated with the intervals of enhanced convection turbulence. A major cusp event occurred above Casey on February 16, when the magnitude of the IMF increased from about 5 to > 10 nT. Cusp signatures during this event included the drift velocity surging to large values just before and after an interval during which the F region echoes were lost because of an absence of F region ionization and the formation of electron density patches. The loss of echoes was only partly explained by increased absorption and scatter of the transmitted radio waves. Although the spectral width of Doppler peaks increased, this, by itself, was not a unique signature of the cusp because the obliquity of echoes also controlled the spectral width in our near-vertical interferometry. However, signatures of the cusp were easily recognized in digisonde data, and the cusps location and dynamics can be monitored using digisondes.

On the temporal evolution of midlatitude F region disturbance drifts

[1]xa0Superposed Epoch Analysis (SEA) is used to examine a 5-year (1999 to 2003) database of Digisonde drift measurements made at Bundoora (145.1°E, 37.7°S geographic, 49°S magnetic), Australia, to determine the temporal evolution of midlatitude F region electric fields associated with the magnetospheric (lifetimes of about an hour) and ionospheric disturbance (lifetimes of a few to several hours) dynamos. The magnetospheric effects are qualified using AE “step-up” and “step-down” temporal filters and the SEA results reveal features fairly consistent with under- and over-shielding conditions described by the Rice Convention Model (RCM). The disturbance dynamo effects are qualified using onset times of AE- and Dst-defined storms. These onset times are further subdivided into three categories: short-, medium- and long-duration storms. We find there are no noticeable changes in ionospheric electric fields near Bundoora during short-duration AE-defined storms. In contrast, the SEA responses for medium-duration AE-defined storms and short- and medium-duration Dst-defined storms are in good agreement with the ionospheric disturbance dynamo predictions. The SEA results associated with long-duration AE or Dst-defined storms indicate that the electric field perturbations agree with the effects of the high-latitude two-cell convection pattern expanding to the latitude of the station (49° magnetic) for up to 20 h after t = 0 h. Overall, the perturbation drifts are predominantly westward with largest amplitudes in the dusk to midnight sector and continued for nearly 50 h in storm time. These enhancements are also consistent with the influence of the sub-auroral polarization stream (SAP) extending to the latitude of the station.

Pilot Ionosonde Network for Identification of Traveling Ionospheric Disturbances

Travelling Ionospheric Disturbances (TIDs) are the ionospheric signatures of atmospheric gravity waves (AGWs). Their identification and tracking is important because the TIDs affect all services that rely on predictable ionospheric radio wave propagation. Although various techniques have been proposed to measure TID characteristics, their real-time implementation still has several difficulties. In this contribution, we present a new technique, based on the analysis of oblique Digisonde-to-Digisonde (D2D) “skymap” observations, to directly identify TIDs and specify the TID wave parameters based on the measurement of angle-of-arrival, Doppler frequency, and time-of-flight of ionospherically reflected high-frequency (HF) radio pulses. The technique has been implemented for the first time for the Net-TIDE project with data streaming from the network of European Digisonde DPS4D observatories. The performance is demonstrated during a period of moderate auroral activity, assessing its consistency with independent measurements such as data from auroral magnetometers and electron density perturbations from Digisondes and GNSS stations. Given that the different types of measurements used for this assessment were not made at exactly the same time and location, and that there was insufficient coverage in the area between the AGW sources and the measurement locations, we can only consider our interpretation as plausible and indicative for the reliability of the extracted TID characteristics. In the framework of the new TechTIDE project (European Commission H2020), a retrospective analysis of the Net-TIDE results in comparison with those extracted from GNSS TEC-based methodologies is currently being attempted, and the results will be the objective of a follow up paper.