Large-scale traveling ionospheric disturbance observed by superDARN Hokkaido HF radar and GPS networks on 15 December 2006

Authors


Abstract

[1] On 15 December 2006, during the main phase of a relatively large storm, Doppler velocity data from the Super Dual Aural Radar Network (SuperDARN) Hokkaido radar, together with total electron content (TEC) data from the GPS Earth Observation Network (GEONET), recorded daytime large-scale traveling ionospheric disturbances (LSTIDs). We studied two disturbances propagating southward and one disturbance propagating northward between 0000 and 0600 UT on 15 December 2006. The former disturbances were LSTIDs typical of those reported in many previous studies, whereas the latter was confirmed as an LSTID propagating from the Southern into the Northern Hemisphere, reported in a few past studies. From comparisons of SuperDARN Hokkaido radar Doppler velocity and GEONET TEC, we found a positive correlation between downward ionospheric motion and increasing TEC. This relationship is consistent with results of model calculation. This is the first observation of LSTIDs ranging from high to low latitude combining simultaneous SuperDARN HF radar and GPS network observations.

1. Introduction

[2] Large-scale traveling ionospheric disturbances (LSTIDs) have a horizontal scale of more than 1000 km and a period of 30–180 min [Hunsucker, 1982]. They are generally recognized as ionospheric manifestations of the passage of atmospheric gravity waves generated at high latitudes by energy input from the magnetosphere to the auroral ionosphere. Most LSTIDs in the Northern Hemisphere propagate southward. Research into the generation and propagation of LSTIDs can clarify part of the system of energy flow from the magnetosphere to the low-latitude ionosphere.

[3] The imaging technique using the multipoint GPS network has been applied to ionosphere dynamics studies [e.g., Saito et al., 1998, 2002; Afraimovich et al., 2000, 2002]. GPS network global coverage and continuous operation can provide global maps of total electron content (TEC) used in LSTID studies [Ho et al., 1996, 1998; Tsugawa and Saito, 2004; Tsugawa et al., 2006; Ding et al., 2007, 2008]. Tsugawa and Saito [2004] performed a statistical study of LSTID occurrence rates using GPS Earth Observation NETwork (GEONET) data. The occurrence rate increased as the Kp value increased: 1% at Kp = 4 and 75% at Kp = 9. Tsugawa et al. [2006] indicated that LSTIDs are not connected electromagnetically through the geomagnetic field lines between the two hemispheres but are generated by atmospheric gravity waves propagating independently toward the equator. Shiokawa et al. [2002] investigated prominent LSTIDs during a geomagnetic storm using both 630 nm airglow images and GEONET in Japan.

[4] Super Dual Auroral Radar Network (SuperDARN) HF radar measures backscatter echoes from decameter-scale irregularities in the E and F regions of the ionosphere, as well as echoes backscattered from the ground/sea [e.g., Greenwald et al., 1985]. Three parameters can be derived from the Doppler spectra: echo power (signal-to-noise ratio), Doppler velocity (corresponding to line-of-sight plasma velocity or proportional to the vertical motion of the ionosphere), and Doppler spectral width.

[5] Although several LSTID studies have used GPS data, very few LSTID studies using SuperDARN HF radar data have appeared to date. Bristow et al. [1994] and Stocker et al. [2000] used the SuperDARN radar data to discuss the characteristics of traveling ionospheric disturbances (TIDs) and their relation to atmospheric gravity waves, but they reported only medium-scale TIDs. Karhunen et al. [2006], using SuperDARN radar data, demonstrated the existence of LSTIDs with periods longer than 60 min by applying spectral analysis to the variations in skip distance data. However, these LSTIDs were embedded in medium-scale TIDs with shorter periods, and identifying the temporal sequence of each disturbance was not possible. In this paper we present detailed characteristics of ionospheric disturbances observed by the SuperDARN Hokkaido HF radar and GEONET, from which we obtain more information on LSTIDs.

2. Instrumentation

[6] Figure 1 is a combined plot of the SuperDARN Hokkaido radar field of view (FOV) and the GEONET receiver distribution. The SuperDARN Hokkaido radar (geographic coordinates: 43.5°N, 143.6°E) is one of the midlatitude SuperDARN radars, located at the lowest geomagnetic latitude (AACGM coordinates: 36.5°, −145.3°). Using the SuperDARN Hokkaido HF radar and GEONET, we can simultaneously observe ionospheric disturbances from high to low latitudes [e.g., Ogawa et al., 2009].

Figure 1.

Combined plot of the Super Dual Auroral Radar Network (SuperDARN) Hokkaido HF radar field of view (FOV) and GPS Earth Observation Network (GEONET) receiver distribution.

[7] The SuperDARN Hokkaido HF radar has 16 beams (beams 0 to 15). Beam 1 points toward geomagnetic north. SuperDARN radars operate in a variety of sounding modes. During the period of interest, the Hokkaido radar was operating in normal mode, with a temporal resolution of 1 min and a range resolution of 45 km.

[8] As in the papers by Bristow et al. [1994] and Stocker et al. [2000], we use mainly ground/sea scatter echoes observed by the SuperDARN Hokkaido radar, which are HF radar wave signals reflected by the ionosphere and backscattered by irregular ground/sea surface. On the contrary, focusing/defocusing effects used for detecting medium-scale TIDs in the paper by Bristow et al. [1994] cannot be utilized in large-scale TIDs because their wavelengths are too long. Instead we use Doppler velocity of the ground/sea scatter echoes observed by the radar, which is proportional to the upward/downward motion of the ionosphere reflection point of the HF radar wave. Note that with the ground/sea scatter echoes we observe the vertical motion of the ionospheric structure, and not that of the irregularities. This technique is the same as that introduced by Ponomarenko et al. [2003] for detecting ULF waves although their temporal scale is shorter. Figure 2 shows the working geometry of the observation.

Figure 2.

Working geometry of the HF radar observing the effect of upward/downward motion of the ionospheric reflection point.

[9] GEONET is a dense, wide-area GPS network in Japan, operated by the Geographical Survey Institute, Japan. The GPS array consists of about 1200 GPS receivers and provides GPS data every 30 s. TEC is measured using the phase difference between two GPS signals with different frequencies [Saito et al., 1998].

3. Observations

[10] Figure 3 shows an example of SuperDARN Hokkaido radar Doppler velocity data and GEONET TEC data. The perturbation components of the TEC values were derived by subtracting a 60 min running average. To show the temporal variation of disturbances, we drew a red arrow along the Hokkaido radar beam 0 in Figure 3, sampled the data along this arrow, and plotted the temporal variation of the data along this sampling line.

Figure 3.

Combined two-dimensional plot of the SuperDARN Hokkaido radar data and GEONET data at 0200 UT. The red arrow passes through beam 0 of the SuperDARN Hokkaido radar and the eastern end of the GEONET FOV. In the SuperDARN FOV blue colors show Doppler velocities toward the radar. In the GEONET FOV, total electron content (TEC) values were subtracted by a 60 min running average. The red color indicates increasing TEC.

[11] Figure 4 shows GEONET TEC data and SuperDARN Hokkaido radar beam 0 Doppler velocity data between 0000 and 0600 UT (0900–0015 LT) on 15 December 2006. This period corresponds to the main phase of a relatively large storm event on 14–16 December 2006, with minimum Dst = −146 nT at 0700–0800 UT on 15 December. In the SuperDARN panel blue colors show Doppler velocities toward the radar, corresponding to downward-moving ionosphere (ground/sea scatter). In the GEONET panel TEC values were subtracted by a 60 min running average and then averaged along the line orthogonal to the red arrow to increase the spatial coverage and resolution of the data. The red color indicates increasing TEC. SuperDARN data were plotted as ground/sea scatter echoes between 40° and 54°N geographic latitude. For ground/sea scatter echoes we plotted echoes in the calculated reflection point because the Doppler velocities were mainly determined by the upward/downward motion of the ionosphere at the radar wave reflection point.

Figure 4.

Combined beam 0 Doppler velocity data from the SuperDARN Hokkaido radar and the perturbation component of the TEC values obtained with GEONET, sampled along the red arrow in Figure 3, plotted as a function of UT and geographic latitude. In the SuperDARN map (upper), blue colors show Doppler velocities toward the radar, corresponding to downward-moving ionosphere (ground/sea scatter). In the GEONET map (lower), TEC values were subtracted by a 60 min running average and then averaged along the line orthogonal to the red arrow in Figure 3. The red color indicates increasing TEC. Three major disturbances are visible in each map, denoted events 1–3.

[12] Between 0000 and 0600 UT, GEONET data registered several disturbances propagating both northward and southward. Among them, two disturbances propagating southward (0130–0215 and 0215–0245 UT) and one disturbance propagating northward (0245–0400 UT) had amplitudes higher than 0.3 total electron content unit (TECU; 1 TECU = 1016 el m−2). SuperDARN data showed corresponding signatures, that is, two disturbances propagating southward (0100–0130 and 0145–0215 UT) and one disturbance propagating northward (0330–0415 UT).

[13] Figure 5 shows more clearly the corresponding disturbances observed by the SuperDARN Hokkaido radar, with an expanded Doppler velocity scale. Note that the velocity scale is expanded so that the velocity values of some of the echoes are saturated. To show unsaturated velocity values, Figure 6 displays temporal variations of the Doppler velocities in range 25 (∼48.7° geographic latitute) to range 43 (∼52.3° geographic latitude) along beam 0, with 5 min smoothing applied. Echoes with Doppler velocities beyond ±100 m s−1 have been excluded. We called the two disturbances propagating southward events 1 and 2, and the one disturbance propagating northward event 3, as denoted in Figures 4, 5, and 6. We focus on these events in the following sections.

Figure 5.

Similar to Figure 4, showing beam 0 ground/sea scatter Doppler velocity data from the SuperDARN Hokkaido radar with an expanded scale and the perturbation component of the TEC values obtained with GEONET, sampled along the red arrow in Figure 3, plotted as a function of UT and geographic latitude.

Figure 6.

Line plots of Hokkaido radar Doppler velocities of ranges 25, 27, 29,…, 43 (all for beam 0), with 5 min smoothing applied. Echoes with Doppler velocities beyond ±100 m/s are excluded. The positive velocities are toward the radar.

3.1. Disturbances Propagating Southward (Events 1 and 2)

[14] Events 1 and 2 (southward-propagating ionospheric disturbances) happened between 0100 and 0230 UT. We used the procedure adopted by Tsugawa and Saito [2004] to obtain the period and wavelength of the LSTIDs. From two-dimensional data, the propagation direction was estimated to be south-southeast and south for events 1 and 2, respectively. This direction approximately corresponded to the red arrow line along the beam 0 direction of the SuperDARN Hokkaido HF radar, as shown in Figure 3. We calculated the TEC values averaged along the line orthogonal to the red arrow line, as shown in Figures 4 and 5. Then we found the maximum and minimum TEC value locations in each horizontal distance for events 1 and 2 (plotted as triangles and crosses in Figure 7). Next we obtained the horizontal propagation velocities of events 1 and 2 by derivation from the gradient of the least-squares-fit lines of the maximum values as shown in Figure 7. The periods, T (minutes), of events 1 and 2 were determined using twice the average of the time lag between the peak and the minimum at each horizontal distance. The propagation velocity for events 1 and 2, observed by GEONET, were 600–650 and 800–850 m s−1, respectively. Disturbance periods were about 45 and 30 min. Those disturbances propagating approximately southward can be described as typical LSTIDs [Hunsucker, 1982].

Figure 7.

Peak and minimum locations of TEC values for (a) event 1 and (b) event 2 in horizontal distance versus UT, indicated as triangles and crosses, respectively. Horizontal propagation velocities for events 1 and 2 were derived from the gradient of the least-squares-fit lines of the maximum values by the solid line. Periods T (minutes) of events 1 and 2 were determined by twice the average of the time lag between the maximum and the minimum value at each horizontal distance.

[15] SuperDARN Hokkaido radar Doppler velocity data in Figure 4 indicated that the LSTID structure had two components: the front side (48° to 50°N) and the main body (50° to 52°N). The main body corresponded to echos for which the Doppler velocity magnitudes were relatively smaller (≤20 m s−1); the front side was located on the equatorward edge of the main body, where the Doppler velocity magnitudes were larger (50 to 100 m s−1). Although Doppler velocities were higher for the front side, those echoes were most likely ground scatter because of the very low Doppler spectral widths (≤10 m s−1).

[16] Because the amplitude of the Doppler velocity of the main body region was relatively low and the latitudinal extent of the FOV of the main body (∼5°) is much smaller than that of GEONET (∼17°), it was difficult to determine the two-dimensional propagation direction precisely from the HF radar data. Nevertheless, we see from Figures 46 that the main bodies of the LSTIDs observed by the SuperDARN Hokkaido radar moved southward along the red line at 400–600 and 500–800 m s−1. These velocities were approximately equal to the propagation velocities of the LSTID observed by GEONET. The propagation velocities of the front side regions differed. The calculated speeds from the front side of the beam 0 Doppler velocities of the SuperDARN Hokkaido radar data were 300–350 and 200–250 m s−1. The difference probably arose because the echo positions were obtained on the basis of the assumption that the radar wave reflection plane of the ionosphere was parallel to the ground/sea surface. If the ionospheric plane of reflection was tilted, then the true location of the reflection points would be different, and consequently the phase velocities would also differ.

[17] The SuperDARN Hokkaido radar data with beams other than 0 also showed similar signatures: southward propagation of the (mainly positive) Doppler velocity structures (not shown in the figures). However, since these beams are more oblique to the wave propagation vector, it is more difficult to discuss the propagation characteristics of the disturbances.

[18] Comparison between the main body of the echoes observed by the SuperDARN Hokkaido radar and the TEC perturbation observed by the GEONET showed that the positive values in the Doppler velocities of the ground/sea scatter echoes were correlated with an increase in the TEC values, as shown in Figures 4 and 5. This indicates that downward ionospheric motion corresponds to increasing TEC. A detailed discussion of this relationship is given in the next section.

[19] The positive value in the ground/sea scatter echo Doppler velocities observed by the SuperDARN Hokkaido radar corresponds to downward ionospheric motion. Whereas typical Doppler velocity magnitudes were about 20 to 40 m s−1 from Figures 4, 5, and 6, the real vertical speed of the ionosphere is ν/2sinθ (θ is the elevation angle). For example, given a Doppler velocity of 20 m s−1, a radar range of 500 km, and a reflection point height of 250 km, we obtain an elevation angle of 26.57° and a vertical motion speed of 22.36 m s−1.

[20] The SuperDARN Hokkaido radar occasionally observed ionospheric scatter echoes in the auroral zone of 65–75°N geographic latitude (55–65°N geomagnetic latitude), although they are not shown in Figures 4 and 5, with a plasma convection speed as high as 1000 m s−1, and an LSTID might be generated by the Joule heating associated with this intense convection region. However, we found no obvious correspondence between the convection enhancement and LSTID generation (events 1 and 2). This is mainly due to insufficient ionospheric backscatter echoes.

3.2. Disturbance Propagating Northward (Event 3)

[21] Event 3 (northward-propagating ionospheric disturbance) happened between 0230 and 0430 UT. From two-dimensional data the propagation direction was estimated to be north to north-northeast. The propagation speed of event 3 along the red arrow in Figure 3, observed by GEONET, was estimated to be 600–650 m s−1 from Figures 4 and 5 (using the procedure as outlined in the previous section), and the period of the northward-propagating disturbance was about 75 min. These values seemed consistent with typical LSTIDs, although the propagation direction was opposite to the typical direction. In addition, the TEC value amplitude, 2.0 TECU at 45°N, was higher than the amplitudes for events 1 and 2 (0.4 and 1.0 TECU, respectively). From SuperDARN Hokkaido radar Doppler velocity data, the disturbance, identified as the green area, propagated from the equatorward boundary of the radar FOV (46°N) up to 54°N. Although the Doppler velocity amplitudes were relatively low (≤20 m s−1), Figures 46 do show that the region of disturbance observed by the SuperDARN Hokkaido radar propagated northward along the red line with velocities similar to the LSTID observed by GEONET. As for events 1 and 2, the SuperDARN Hokkaido radar data with beams other than 0 also showed similar signatures, that is, southward propagation of the (mainly positive) Doppler velocity structures (not shown in the figures).

[22] We need to consider the possible origin of generating the northward-propagating disturbance. It is possible that event 3 was generated in the Southern Hemisphere and then penetrated into the middle and high latitudes of the Northern Hemisphere. To examine this possibility, we checked GPS TEC data at seven stations in the Northern and Southern Hemispheres (Figure 8). The seven stations were Yuzhno-Sakhalinsk, Tsukuba, Chichijima, Guam, Jabiru, Alice Springs, and Ceduna. We saw the disturbance propagating northward in the Northern Hemisphere (dashed circles). If we assume that the disturbance came from the Southern Hemisphere at Jabiru at a constant velocity of 600–650 m s−1, the perturbation denoted by the solid circle in Figure 8 would correspond to the origin of the disturbance. This suggests that the disturbance was first generated in the Southern Hemisphere, then penetrated into the Northern Hemisphere. Alice Springs and Ceduna did not observe the corresponding perturbation, probably because of the smaller daytime background TEC in the Southern (summer) Hemisphere than in the Northern (winter) Hemisphere and the consequent smaller TEC perturbations in the Southern Hemisphere.

Figure 8.

(a) GPS TEC data derived by subtracting a 60 min running average at seven stations. Different curves for each station were derived from different satellite-receiver paths. The vertical separation between neighboring station baselines was set to be proportional to the latitudinal difference between them. (b) Distribution of GPS receivers at the seven stations.

4. Discussion

[23] We have described ionospheric disturbances propagating both southward and northward observed with GPS TEC and SuperDARN Hokkaido HF radar data. The data reveal a close relationship between downward ionospheric motion and increasing TEC. This is inconsistent with the traditional idea of ionospheric storms, where downward motion in the ionosphere should push the plasma downward to a higher recombination region and eventually decrease the plasma density [e.g., Fuller-Rowell et al., 1994]. Note that LSTIDs have shorter time scales than ionospheric storms, so the relative importance of the chemical processes and other processes, such as concentration of plasma density due to gravity waves, might be different. In this section the relationship between ionospheric motion and TEC variation associated with LSTIDs is investigated using a model calculation.

[24] LSTIDs have been considered to be caused by atmospheric gravity waves [Hines, 1960; Hooke, 1968]. The linear dispersion relation of the gravity waves is given as [Hines, 1960]

equation image

where m, N, c, k, and H are the vertical wave number, Brunt-Väisälä frequency, phase velocity, horizontal wave number, and scale height, respectively. Typical values of N = π/12 min and H = 45 km are taken in this model calculation. Using the observed phase velocity of 630 m s−1 and period of 45 min, the vertical wavelength λz = π/m is estimated to be 849 km from the above dispersion relation of gravity waves.

[25] Neutral wind perturbations due to the gravity waves move ions in the F region along the geomagnetic field lines through the neutral-ion collisions. Since the characteristic time scale for the LSTIDs reported in this paper (30 to 75 min) is much longer than that for the neutral-ion collisions in the F-region ionosphere, the velocity of the macroscopic plasma motion along the geomagnetic field is approximately the same as that of the neutral motion along the geomagnetic field [e.g., Song et al., 2005]. On the contrary, the ion motion across the magnetic field line is restricted because the ion gyrofrequency is much higher than the ion-neutral collision frequency. Hooke [1968] first formulated the linear theory that describes the relationship between the magnitude and phase of perturbations of the neutral gas caused by gravity waves and those of the resulting plasma density perturbations, that is, TIDs. The model is based on the continuity equation of the ionospheric plasma. Effects of variations induced by gravity waves in the rates of ionization motions, photoionization, and chemical loss are considered. Above the F2 peak altitude, however, production and loss rates of the plasma are lower than the frequency of the LSTIDs. Therefore, assuming that the effects of the chemical processes such as production and loss of the plasma are neglected, the plasma density perturbation Ne, which is caused by the oscillating neutral wind velocity u, is given by

equation image

where i = equation image, ω is the angular frequency of the gravity wave, ub and kb are the projection of u and k in the direction of the geomagnetic field, respectively, I is the dip angle of the geomagnetic field, and Ne is the ambient plasma density. The perturbation quantities are assumed to be described by a plane wave of the form Ne, u ∝ exp{i(ωtk · x)}, where k is the wave vector of the gravity wave. The first and second terms in this equation represent the plasma density perturbations caused by vertical motions of the F-layer plasma and by convergence and divergence of the plasma along the field line, respectively.

[26] Figure 9a shows the result of a model calculation simulating plasma density perturbation in the altitude from 160 to 400 km for the case of a southward-propagating gravity wave. TEC, which is integration of the plasma density in an altitude range from 160 to 400 km, is shown at the bottom of Figure 9a. The background altitude profile of the plasma density is taken from the IRI-95 model [Bilitza, 1997]. In this calculation, f0F2 is set at 8.5 MHz. This f0F2 was observed by an ionosonde at Wakkanai (45.38°N, 141.68°E), Japan, at 0100 UT (1000 LT) on 15 December 2006. The inclination I is assumed to be 60°. The wave period is set to be the period of the event 1 LSTID (45 min). The amplitude of the neutral wind perturbations (u = 60 m s−1) is chosen to reproduce the amplitude of the observed TEC perturbations (0.3 TECU). At the bottomside of the F layer, the isosurface of the plasma density is tilted from upper south to lower north (from upper north to lower south) at horizontal distances of 0–800, 1500–2500, and 3200–4000 km (1000–1400 and 2700–3100 km). The distance range of the isosurface tilted from upper south to lower north is larger than that of the isosurface tilted from upper north to lower south. This is the result of the downward phase velocity of the gravity wave propagating upward. The TEC enhancement can be seen at the position where the bottomside plasma density has a phase front elongated from upper south to lower north. Because the gravity wave propagating southward is simulated in this case, the phase front of the plasma density perturbation also moves southward and downward. The Doppler velocity observed by the HF radar corresponds to the motion of the isosurface of the plasma density. Consequently, this model calculation reproduces the observational result that the TEC enhancement coincides with the downward Doppler velocity observed by the HF radar.

Figure 9.

Results of ionospheric plasma density calculations for disturbances propagating (a) southward and (b) northward, using Hooke's [1968] equation. The top plots show ionospheric plasma density and the bottom plots show the TEC value integrated over altitude.

[27] Figure 9b shows a result of the model calculation for the case of a northward-propagating gravity wave. For this calculation the horizontal phase velocity (630 m s−1) and period (75 min) of the observed LSTID are adopted. From the dispersion relation of the gravity wave, the vertical wavelength is estimated to be 788 km. Figure 9b shows that the TEC enhancement coincides with a phase of the plasma density perturbation elongated from upper north to lower south. Since the gravity wave propagates northward, the TEC enhancement region corresponds to the downward motion of the isosurface of the plasma density. This is also consistent with our observational result that the downward Doppler velocity coincided with the TEC enhancement.

[28] According to equation (2), the amplitude of the plasma density perturbations (Ne) is proportional to ub. Since neutral particle oscillation parallel to B is larger for gravity waves propagating equatorward than those propagating poleward, southward-propagating gravity waves could cause a higher amplitude of TEC perturbations than gravity waves propagating northward. However, the amplitude of the observed TEC enhancement caused by the northward-propagating LSTID is much higher than that caused by the southward-propagating LSTID and, also, quite larger than the background TEC. To consider the plasma density perturbations caused by gravity waves according to the theory of Hooke [1968], it is necessary to assume that the plasma density perturbations and neutral wind perturbations are adequately smaller than the background values. Therefore, nonlinear treatments could be needed to reproduce the observed TEC enhancement in the case of the northward-propagating LSTID.

[29] As mentioned in the previous section, the disturbance propagating northward (event 3) was first generated in the Southern Hemisphere and then propagated into the Northern Hemisphere. This is consistent with Lei et al. [2008], who used simulation results from the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model on 15 December 2006 and showed that the disturbance denoted as event 3 in this paper propagated from the Southern Hemisphere into the Northern Hemisphere (Figure 10). As can be seen from the East Asia region in Figure 10, during 0030–0400 UT, a vertical downward neutral wind launched from the Southern Hemisphere penetrated to the high latitudes of the Northern Hemisphere. This is consistent not only with the observations by the GPS network and GEONET as reported by Lei et al. [2008], but also with the observations by the SuperDARN Hokkaido HF radar, in which the LSTID traveled through their fields of view with approximately the same time sequence as the simulation result. Lei et al. [2008] also have reported that the northward neutral wind is of primary importance in producing the LSTID and that electric fields and chemical reaction of the plasma density also contribute to the plasma density enhancement. This might explain some of the nonlinear effects to reproduce the observed TEC enhancement in the case of the northward-propagating LSTID.

Figure 10.

Vertical neutral wind distribution calculated from the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model for 15 December 2006 [Lei et al., 2008]. The figure shows the maps of the vertical neutral wind flowing across a pressure surface at about 300 km during 0000–0430 UT. The blue color indicates downward flow. During 0030–0400 UT, a vertical downward neutral wind launched from the Southern Hemisphere penetrated to high latitudes in the Northern Hemisphere.

[30] Note that the simulation by Lei et al. [2008] did not reproduce some of the disturbances observed by the SuperDARN Hokkaido radar and GEONET (events 1 and 2). Nevertheless, we found the relationship between the downward motion of the ionosphere and the GPS TEC increase, as in another event (event 3). Although this might be because the spatial scale for events 1 and 2 are too small to be reproduced by their model because of the 5° horizontal resolution, they succeeded in reproducing southward-propagating LSTIDs for other periods with similar spatial scales, so there could be other factors. The present result could provide some clues to the possible generation mechanisms of LSTIDs that were not reproduced by the simulation that uses solar wind parameters as input. This is a subject for future studies.

5. Conclusions

[31] In this paper we have demonstrated the capability of the SuperDARN Hokkaido HF radar and GEONET to simultaneously monitor ionospheric disturbances ranging from 55° to 25° geographic latitude. During the main phase of a relatively large storm on 15 December 2006, Doppler velocity data from the SuperDARN Hokkaido radar, together with TEC data from GEONET, recorded daytime LSTIDs. Analysis of two disturbances propagating southward using GPS data confirmed that those disturbances could be described as typical LSTIDs. From the spatial/temporal distribution of the Hokkaido radar Doppler velocity data, the main body of the ground/sea scatter echoes traveled at approximately the same velocity as LSTIDs observed by GEONET. In addition, we found a positive correlation between the downward Doppler velocity observed by the SuperDARN Hokkaido HF radar and an increase in TEC recorded by GEONET. This is consistent with results of calculations using Hooke's [1968] equation.

[32] SuperDARN Hokkaido radar and GEONET observed one disturbance propagating northward. The disturbance propagating northward was probably generated in the Southern Hemisphere and propagated from the Southern Hemisphere up to the high latitudes of the Northern Hemisphere, judging from the TEC value of the Southern Hemisphere. This is consistent with the CMIT model simulation result of Lei et al. [2008], whereas their simulation result did not reproduce some of the disturbances propagating southward that were identified by the SuperDARN Hokkaido radar and GEONET.

[33] To the best of our knowledge, this is the first SuperDARN radar observation of an LSTID including detailed discussion of temporal development. The next step is to find more examples in similar events and to investigate the statistical relationship between TEC variation and vertical motion in the ionosphere.

Acknowledgments

[34] We would like to thank all the staff who contributed to the HF radar experiment at Hokkaido. The GEONET GPS data were provided by the Geographical Survey Institute of Japan. The Kyoto University GEONET-TEC database is supported by the Japan Society for the Promotion of Science (JSPS) with a grant-in-aid for publication of scientific research results. Other GPS data were provided by the International GPS Service. Dst geomagnetic index data were provided by the World Data Center for Geomagnetism, Kyoto University. This work was supported by Grant-in-Aid for Scientific Research 19340141 and, also, Special Funds for Education and Research (Energy Transport Processes in Geospace) of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

[35] Zuyin Pu thanks the reviewers for their assistance in evaluating the manuscript.

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