Ionospheric drift measurements are of significant importance in the monitoring of the upper atmosphere dynamics and an important contributor toward the deeper understanding of the ionospheric electrodynamics. An interesting perspective for operational ionospheric drift measurements comes from the HF radars, such as the Digisondes. Nevertheless, when interpreting velocity measurements obtained from the Digisonde drift analysis method, care must be taken in relating the motion to the effects of photoionization, recombination, gravity waves, and plasma motion. In this paper we investigate the effect of all different mechanisms contributing to the apparent velocity measured by the Athens Digisonde. As a first step, the daily drift pattern over Athens under quiet magnetospheric conditions is preliminarily determined for the equinox months. Moreover, the substorm effect on the extracted pattern is also investigated. Our results provide some evidence for the direct effect of traveling atmospheric disturbances in the vertical component, while in the horizontal plane, the disturbance could be attributed to other substorm-related mechanisms.
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 Modern ionosondes operate essentially as radar systems; that is, they measure radar distances and angles of arrival of the receiving echoes [Reinisch, 1996]. In contrast to a convectional radar system, the ionosonde transmits a wide radio beam that illuminates a large area of several hundred kilometers in diameter in the F region, resulting in echoes returning from many directions in the presence of irregularities. With the introduction of Doppler interferometry to ionospheric sounding [Bibl and Reinisch, 1978], it has become possible to identify the source regions of the spread echoes and therefore to measure ionospheric drift motions. In contrast to incoherent or coherent scatter radar systems, which observe backscattered radio energy, ionosonde sounding is based on total (specular) reflection, which occurs at the level where the wave frequency F is equal to the plasma frequency fp (for the ordinary polarization) [Reinisch et al., 1998]. The main methodology for measuring ionospheric drifts using specular reflection involves the signal transmission by the transmitter antenna and its reception, after its reflection in the ionosphere, by four or more receiver antennas located appropriately one wavelength apart in a triangle configuration. By altering the frequency of the transmitted signal, the reflection is achieved in different layers of the ionosphere, allowing identification of drifts at various heights.
 The signals from each antenna are Fourier analyzed to identify echoes with different Doppler frequencies. Interferometry for each Doppler component then determines the source location of each echo on a high-resolution sky map. The sky map, in geomagnetic coordinates, shows the locations of the reflection points, while negative and positive Doppler shifts are identified by colors (red and blue, respectively). The Doppler frequency distribution of the sky map source of points is used for the estimation of all three components of the drift velocities: the vertical (Vz) and the magnetic meridional (Vn) and zonal (Ve) components [Reinisch, 1986] in a least squares errors approach. The technique is now commonly referred to as the “drift mode” and the velocity so obtained as the “drift velocity.”
 In the ionosphere, particle drift motion involves the superposition of three main drift processes: gradient drifts, electric field drifts, and drifts due to gravity. In addition to the motion imparted to the ionization by drifts one needs to consider the effect on this motion due to the neutral atmosphere. Neutral winds and traveling ionospheric disturbances (TIDs) also cause the ionization to move, and this interaction is basically restricted to the meridional plane because of the spiral of ions along the field lines. Hence the ion flow velocity parallel to the magnetic field is very complicated since many factors contribute to this velocity component, such as the neutral wind, the ion pressure gradient, the gravitational forces, and traveling ionospheric disturbances. The velocity drift component in the zonal direction in comparison is simpler to interpret since it results solely from the effect of the ambient electric field via the relationship V = E × B/B2 [Scali, 1993; Kelley, 1989]. In the case of HF radars such as the Digisonde portable sounder, where motion is deduced from a measure of Doppler shifts on radio waves reflected from the ionosphere, a further distinction needs to be made. The Digisonde measures the apparent velocity “Va,” which is a complicated function of a number of time-varying properties of the radio wave propagation medium. For instance, the Doppler frequency shift is affected by both the motion of the reflecting surface and the time variation of the refractive index along the ray. In the ionosphere, the time changes in the refractive index arise from the ionospheric motions and also from production and loss of ionization. In summary, when interpreting velocity measurements obtained from the Digisonde drift analysis (DDA) method [Scali, 1993], care must be taken in relating the motion to the effects of photoionization, recombination, gravity waves, and plasma motion.
 Concerning all the above complications, it is important to identify the basic contributing mechanisms giving the measured velocities. In this paper we present preliminary results from the assessment of drift measurements selected from Athens Digisonde during the equinox months of the year 2004. The derivation of a preliminary quiet pattern and the study of the substorm effect on this pattern provide us with significant evidence concerning the reliability of the technique applied in Digisondes to calculate drift velocities.
2. Athens Digisonde Drift Observations
 A state of the art infrastructure to monitor the Earth's ionosphere has been operating in the National Observatory of Athens since September 2000. The ionospheric station is a digital portable sounder with four receiving antennas (DPS-4). The various separation distances of the receivers are repeated in six different azimuthal planes. This six-way symmetry is exploited by defining the six azimuthal beam directions along the six axes of symmetry of the array, making the beam-forming computations very efficient. The station can operate in four different modes: (1) scanning ionogram, (2) drift mode, (3) fixed-frequency ionogram, and (4) oblique ionogram. Real-time scanning ionograms and sky maps with the results of their automatic scaling and the history of past soundings are currently available on the Web site of Athens Digisonde (http://www.iono.noa.gr).
 The ionosonde drift is derived from the measured Doppler frequency shift and angle of arrival of ionospherically reflected HF echoes, a method similar to that used by coherent VHF and incoherent UHF scatter radars [Reinisch et al., 2006]. Athens Digisonde performs drift soundings in the F region in autodrift mode. This means that the starting frequency of the sounding is automatically selected from the preceding ionogram. Ionogram soundings begin at 0, 15, 30, and 45 min past the hour, followed by autodrift soundings a few minutes later (at 6, 21, 36, and 51 min past the hour). The control program was configured in order to modify the sounding parameters. Since the height resolution of drift measurements is an important consideration [Parkinson and Dyson, 1998], the height resolution was set equal to 5 km (this corresponds to 33.3 μs for the half-power full width of the available transmitter pulses), and the data collection was at 5 km steps in range. For a slow-moving midlatitude ionosphere it is desirable to multiplex frequencies. Athens Digisonde was set to multiplex four frequencies (with a frequency step of 100 kHz) in O polarization mode. The number of integrated repetitions was set equal to seven, which means that the scan of code/frequency/polarization is repeated 128 (27) times, and 128 samples are recorded. The duration of each concurrent time series is calculated as 4 (frequencies) × 1 (polarization) × 2 (code order) × 128 (samples) × 1/50 s (pulse rate) = 20.48 s. Hence the spectral resolution is 0.049 Hz. The bottom and the top height of the window to be searched for a maximum amplitude is set at 150 and 640 km, respectively, to store measurements from the F region.
 The output of a drift sounding is stored on a Discrete Fourier Transform file. All files are controlled by inspecting the amplitude of each receiver separately, aiming at checking the reliability of each individual measurement. In Figures 1a and 1b we present the performance of the four receiving antennas during two drift soundings on 16 February 2005 at 0852 UT and on 25 March 2005 at 1236 UT. It is obvious that in the first case, antenna 4 was blocked, and the measurement cannot be used for the drift data analysis. In the second case, the problem is recovered.
 For the postprocessing of the drift measurements, the Digisonde drift analysis (DDA) [Scali, 1993] tool provided by the University of Massachusetts-Lowell Center for Atmospheric Research was installed in the Athens Digisonde to automatically measure F region plasma drifts by detecting the Doppler shift and angle of arrival of echoes. The DDA method assumes a uniform velocity. Each sky map generally contains a large number of sources. Any three spatially separated sources within a sky map give the plasma velocity, as long as all three move with the same velocity. This will be the case only when a uniform velocity exists within the view of the sounder, and any three (noncollinear) sources will produce the same or similar results depending on the signal-to-noise ratio. In this situation, the translational velocity can be found by least squares fitting using all the sky map points [Reinisch et al., 1998]. The DDA software is configured in each station taking into account its local characteristics and the geometry of the receiving antenna array. The usual layout of the receiving antenna array consists of four antennas, three of which form an equilateral triangle, while the fourth station is located at its center. In the case of Athens Digisonde, the configuration is different as shown in Figure 2.
 The configuration file that controls the behavior of the DDA program was edited. The maximum half-power zenith angle was set to 40°. This is the threshold of the zenith angle for which sources are chosen. The zenith is measured from the vertical axis. Zenith angles higher than 40° are not considered. The threshold for the calculated vertical velocity was set equal to 200 m/s. If the calculated velocity is higher than this threshold, the value takes the value of the threshold. This threshold is a good approximation to reality as the vertical velocity of the ionosphere in most of the cases is much lower than 200 m/s. The threshold for the evaluated horizontal velocity was set equal to 2000 m/s. This value covers extreme cases of very high ionospheric drift velocity. Concerning the DDA height settings, the upper threshold for which sources are selected is 700 km, which is a quite high value so that the F2 region is included, while the lower threshold of height for which sources are selected is set equal to 90 km. The DDA software is used for postprocessing of drift measurements, and the above settings anticipate measurements in the E region as well. Nevertheless, analysis and study of the E region drifts is beyond the scope of this paper.
 The daily plots of the F region drifts appear on the Web (http://www.iono.noa.gr). Historical drift measurements are available on the archive that is kept on the Athens Digisonde Web site. As an example, the estimated drift velocities for 31 March 2005 are presented in Figure 3, while the sky maps corresponding to the structure observed at 2100 UT are shown in Figure 4. A smoothing operation in the calculated velocities is performed in order to eliminate occasions of very rapid consequent changes, which tend to happen because of errors. The smoothing algorithm is an iterative procedure using a time window which spans ±45 min from the current time. This is translated into at most five measurements falling into the smoothing window (the values at –45 min and +45 min are not included).
3. Data Analysis
 Having collected—and checked for their reliability—observations from three equinox months, the next step is the identification of patterns and regular structures in the motion of the overhead ionosphere. The work done was organized in two steps. First, an attempt was made to determine quiet magnetospheric conditions according to the AE index. The corresponding ionospheric drift measurements obtained from Athens Digisonde at F layer heights during these intervals were statistically treated in order to extract the quiet daily drift pattern. As a second step, the effect of substorms on the quiet time drift pattern was also investigated during days characterized by high auroral activity.
3.1. Derivation of the Quiet Time Ionospheric Drift Pattern for Athens Site
 Aiming at the derivation of the quiet time ionospheric drift pattern over Athens, ionospheric drift measurements obtained from Athens Digisonde during quiet conditions were analyzed. Six quiet days during September and October 2004 were identified in order to investigate the pattern during equinox time. The selection was based on the daily plots of AE index (http://swdcwww.kugi.kyoto-u.ac.jp/aedir/index.html). The selected quiet days were 30 September 2004, 17–18 October 2004, 23 October 2004, 26 October 2004, and 28 October 2004. The quiet pattern was extracted on the basis of observations taken during these days, and therefore the results are valid for the state of the ionosphere over Athens in equinox months. The daily plots of the three velocity components Vz (vertical), Vn (north), and Ve (east) are given in Figure 5. For each vector component, the averaged value is plotted together with the standard deviation.
 The main features of the daily quiet pattern can be summarized as follows: The east-west drifts (Ve) are dominated by a diurnal variation and tend to have a daily mean westward component presenting drastic eastward increase at the time of sunrise and sunset. The northward (Vn) component during quiet conditions is mainly southward oriented and tends to a semidiurnal behavior with its largest amplitude during nighttime hours. The vertical component (Vz) is mainly directed downward with very small amplitude which seldom exceeds 20 m/s. This is expected during quiet conditions under the absence of atmospheric disturbances. In Figure 6 we show the motion of the horizontal drift velocity versus time during the average quiet conditions extracted from the analysis presented above. It is clearly demonstrated that during daytime hours, the orientation and magnitude of the horizontal component is stable. Drastic disturbances in both the orientation and the magnitude of the horizontal component are detected around sunrise and sunset. A possible explanation for these disturbances might be the drastic change of the refractive index due to the rapid change in the photoionization rate at sunrise and sunset.
3.2. Auroral Activity Effect
 The effect of magnetospheric substorms on the quiet daily pattern of ionospheric motions observed over Athens is investigated by examining two characteristic cases where auroral activity launched traveling atmospheric disturbances (TADs) accompanied by traveling ionospheric disturbances (TIDs) over Athens. At the onset of magnetospheric substorms, acoustic gravity waves (AGWs) are generated at both hemispheres as a result of Joule heating and Lorentz forces associated with the auroral electrojet and localized heating of the atmosphere by intense precipitation of charged particles [Hajkowicz, 1990]. Heating at high latitudes causes expansion of the neutral atmosphere. The expansion may cause upwelling which results in departures from diffusive equilibrium and increases in the mean molecular mass [Rishbeth et al., 1987]. The expansion also results in pressure gradients which modify the global thermospheric circulation. Enhanced equatorward winds transport the composition changes to lower latitudes so that one sees a “composition disturbance zone” of increased mean molecular mass reaching from high to middle latitudes [Prölss, 1987]. The equatorial winds often take the form of equatorial surges or traveling atmospheric disturbances when the heating events are impulsive. The large-scale acoustic gravity waves can penetrate to low latitude or even to the opposite hemisphere. They manifest themselves in the ionosphere as large-scale traveling ionospheric disturbances, which have been seen as sequential rises in hmF2 along north-south chains of ionosondes [Hajkowicz, 1990, 1991].
 For the purposes of this investigation, 2 days have been selected in which substorm activity caused large-scale traveling ionospheric disturbances over Athens. On 14 September 2004, a multiple-onset substorm event has been recorded by the AE indices. Three main onsets at 0130, 0930, and 1520 UT generated waves that modified the global thermospheric circulation. The resulting equatorial winds caused increase of the ionization observed in the foF2 parameter and uplifting of the F2 layer seen in the hmF2 parameter. These are typical characteristics of the TIDs, reported by many authors in the past [e.g., Buonsanto, 1999]. Similar conditions occurred on 25 October 2004, which is the second day chosen to examine auroral activity effects on the estimated drift velocities over Athens. In the top plots of Figure 7 we present the Athens Digisonde observations and estimated drift velocities for the two selected days, 14 September 2004 and 25 October 2004. The black lines correspond to the monthly median values for the case of foF2 and hmF2, while for the case of the drift velocity components, the black lines correspond to the quiet pattern shown in Figure 5. In the bottom plots of Figure 7 we present the percentage deviations of the foF2 (Δf%) and hmF2 (Δh%) from their median conditions and the difference of the current Vz (ΔVz) from the average quiet conditions.
 During both days, the substorm activity caused uplifting of the F2 layer clearly seen in the hmF2 by an increase that exceeds 20%, and significant daytime positive effect of short duration reaching 40% in ionization increase, features that characterize the effect of traveling atmospheric disturbances over Athens. During daytime, the most affected velocity component is the vertical one (Vz). As can be seen in the bottom plots of Figure 7, its orientation is changed upward during most daytime hours. From the bottom plots of Figure 7 we can extract a possible correlation between hmF2 and Vz positive variations during daytime as the result of traveling atmospheric disturbances over Athens, seen by the inspection of Δh% and ΔVz daily plots. In contrast, the two components of the horizontal velocity seem to have their major disturbance during nighttime, while during daytime they are not much affected by the traveling atmospheric disturbances generated by the auroral activity. This provides us with some evidence that the Vn and Ve components are mainly influenced by other mechanisms dominating at night, possibly substorm related. Indeed, the three-dimensional plot of the motion of the horizontal velocity with time, for the two magnetospherically disturbed days presented in Figure 8, shows that the daily pattern in general is kept, compared to the quiet condition presented in Figure 6. The variations during daytime are again smoother than those recorded during the nighttime. The northward orientation of the drift velocity observed around noon during both disturbed days is less likely to be attributed to traveling atmospheric disturbances since these propagate in the southward-westward direction. However, significant deviations from the quiet pattern are detected during all 24 hours, and these might be attributed to other mechanisms related to the recorded auroral activity.
 The main contributor to F region ionospheric drift motions is the ionospheric electric field. This field at middle and low latitudes on magnetically quiet conditions is believed to be produced mainly by the dynamo action of thermospheric winds [e.g., Richmond, 1979] and causes the ionospheric and plasmaspheric plasmas to drift perpendicular to the geomagnetic field [Richmond, 1976]. Richmond et al.  in an empirical study used seasonally averaged quiet day (Kp < 3) F region ionospheric E × B drift observations (horizontal drift vector components) from the Millstone Hill, St. Santin, Arecibo, and Jicamarca incoherent scatter radars to produce a model of the middle- and low-latitude electric field for solar minimum conditions. This model was designed to serve as a reference standard of electrodynamic drifts over middle- and low-latitude ionosphere for studies requiring this information. According to Richmond et al.  model drifts for equinox time and middle to low latitudes, the upward/poleward drift tends to a semidiurnal behavior but with small amplitudes. This result is quite reasonable since it is fairly well established that the semidiurnal atmospheric tide dominates in the midlatitude region [Chapman and Lindzen, 1970]. Thus the results are fairly consistent with a tidal E region source, in particular for the daytime zonal electric field (meridional drift component). The meridional electric field component (zonal drift component) seems dominated by a diurnal variation. The east-west drifts tend to have a daily mean westward component at higher latitudes and a daily mean eastward component at low latitudes. It seems curious that one component exhibits a semidiurnal modulation and the other a diurnal one. It may be that two dynamos (E and F region) conspire to yield a diurnal pattern for the meridional electric field (zonal drift) at middle latitudes [Kelley, 1989].
 In this work the quiet daily drift velocity pattern established on Athens Digisonde observations was derived. It should be noted that the pattern of daily variation of drifts is only preliminary because it is based on data from a few days only and is valid for equinox months. The quiet days were selected on the basis of the auroral activity as expressed by the AE index, and the behavior of each of the three velocity components (Ve, Vn, Vz) was considered independently during equinox months. According to our analysis, the daily pattern identified for the Vn and Ve components presents some common characteristics with the theoretical drift pattern of Richmond et al. , indicating that during quiet periods, the Athens Digisonde detects drifts due to a semidiurnal zonal electric field and a diurnal meridional electric field. On the other hand, under the absence of atmospheric disturbances during quiet conditions, it is reasonable to have a daily pattern for the Vz component mainly directed downward with a very small amplitude. Indeed, during disturbed conditions with dominant traveling atmospheric disturbances over Athens, this component is the most disturbed one, changing its orientation upward during the daylight hours, almost simultaneously with the increases of the hmF2 parameter, indicating consistent observations of the F2 layer uplifting. In contrast, during intervals of enhanced auroral activity, the horizontal component is less affected. During daylight hours, the northward deviation of the velocities suggests the effect of a mechanism other than traveling atmospheric disturbances since these propagate southward-westward. During nighttime, the horizontal component is more affected, and this could be related to physical processes related to the substorm evolution.
 The above findings are consistent with the argument that the vertical component shows ionization motions due to a large number of factors, such as gravity gradients, neutral winds, ion pressure gradients, and traveling ionospheric disturbances, and therefore appears to have a rather random behavior. Indeed, during periods of enhanced auroral activity, this component seems to be the most disturbed one. There is also some evidence presented that in the horizontal plane, other mechanisms than traveling atmospheric disturbances contribute to the measured drifts, and this deserves further investigation.
 The above results give preliminary evidence that the Athens Digisonde drift observations may be used in the monitoring of the ionospheric drift motions. The systematic analysis of drift observations could lead to significant results giving a deeper understanding of the ionospheric electrodynamics.
 This work is supported by EOARD grant FA8655-04-1-3019.