Using ionospheric total electron content (TEC) measured by Global Positioning System (GPS) receivers of the Canadian High Arctic Ionospheric Network (CHAIN) we provide clear evidence for a systematic and propagating temporary TEC enhancement produced by compression of the magnetosphere due to a sudden increase in solar wind dynamic pressure. The magnetospheric compression is evident in THEMIS/GOES satellite data. Application of a GPS triangulation technique revealed that the TEC changes propagated with a speed of 3–6 km/s in the antisunward direction near noon and ∼8 km/s in the sunward direction in the pre-noon lower latitude sector. Characteristics of these TEC changes along with riometer absorption measurements seems to indicate that the TEC change is due to electron density enhancement in the F region and is possibly due to particle precipitation associated with sudden magnetospheric compression.
 The Earth's ionosphere is embedded in the “magnetosphere”, a cavity carved by the interaction of the high-speed solar wind and its “frozen-in” magnetic field with the terrestrial magnetic field. The solar wind is inherently non-steady with its magnetic field, density, and flow speed varying on a range of time and amplitude scales. Variations in the solar wind and its magnetic field are known to be the major driver of changes in high-latitude ionospheric phenomena such as convection, optical auroral intensifications, and current systems [e.g., Sibeck et al., 1999; Jayachandran and MacDougall, 2000, and references therein; Boudouridis et al., 2004]. Recent studies have shown that solar wind dynamic pressure plays a crucial role in the Solar Wind – Magnetosphere – Ionosphere (SW-M-I) coupling process and produces significant changes in the high-latitude ionosphere [e.g., Lyons et al., 2009; Juusola et al., 2010; Yang et al., 2011]. These changes in the ionosphere (density and electric field variations) can significantly influence the performance of Global Navigation Satellite Systems (GNSS) systems such as the Global Positioning System (GPS). The dramatic increase in the number of ground based GPS receivers at high-latitudes has made it possible to study SW-M-I coupling processes with high temporal and spatial resolution [e.g., Watson et al., 2011] and in turn try to mitigate ionospheric effects on GNSS. In this paper, we provide clear evidence of a systematic and propagating enhancement in ionospheric Total Electron Content (TEC) as measured by Global Positioning System (GPS) receivers of the Canadian High Arctic Ionospheric Network (CHAIN) [Jayachandran et al., 2009a]. Satellite observations indicate that the observed TEC enhancement presented in this paper was produced by compression of the magnetosphere caused by a sudden increase in solar wind dynamic pressure.
 For this study we have used measurements from satellite and ground-based instruments. Satellite based measurements include up-stream solar wind and interplanetary measurements from the WIND satellite, and magnetic field measurements from THEMIS (A, D, and E) and GOES (11 and 12) satellites. We have used ground based GPS measurements of total electron content (TEC) from three different sites of the Canadian High Arctic Ionospheric Network (CHAIN) [Jayachandran et al., 2009a]. The event of interest to this paper occurred on April 12, 2010 between 16:00 and 16:30 UT. Locations of THEMIS, GOES, and GEOTAIL satellites during that interval, along with locations of the magnetopause and bow shock, are shown in Figure 1a. It is clear from the figure that all three THEMIS satellites were inside the post-midnight sector of the magnetosphere, while GOES 12 was near noon and GOES 11 was just after dawn. Locations (in Magnetic Local Time – Magnetic Latitude coordinates) of the three CHAIN GPS receivers used in this study are shown in Figure 1b, with two stations (Sanikiluaq and Iqaluit) near noon and Edmonton (lowest latitude station of the three) in the pre-noon sector during the event. Figure 2 shows upstream interplanetary magnetic field (IMF) and solar wind (SW) conditions from OMNIWeb data set combining available solar wind monitor data (mostly from ACE spacecraft) projected to the nose of the Earth's bow shock. The SW plasma data time series observed by WIND are shifted by 85 min. The most noticeable feature is the increase in SW dynamic pressure from ∼2 nPa to ∼8 nPa observed by ACE and more gradual increase at WIND. GEOTAIL that was located in the magnetosheath near the pre-dawn magnetopause observed the pressure pulse at 16:21 UT, about 10 min after the OMNI estimated arrival time (16:11 UT) at bow shock. The IMF Bz, the primary driver of high-latitude ionospheric changes, was predominantly steady and northward before the sudden increase in SW dynamic pressure. IMB By showed a significant change during the dynamic pressure increase. However, effect if IMF By usually is to change the orientation of the high-latitude convection [e.g., MacDougall and Jayachandran, 2001, and references therein]. After the pulse IMF Bz fluctuated between northward and southward before it settled at small southward component. Using the method adopted by Jayachandran and MacDougall , a simple calculation of the SW feature's travel time puts the arrival at the ionosphere at around 16:07 UT. This estimate included propagation delays in the magnetosheath and magnetosphere. However, this simple method contains large uncertainty [Ridley, 2000] and should be used as a coarse estimate. Figure 3 shows variations of the total magnetic field and its three GSM components measured by THEMIS A, D, and E [Angelopoulos, 2008] and GOES 11 and 12 satellites during the interval 16:00–16:30 UT. The dashed vertical line in the plots shows the time of arrival of the SW feature estimated from OMNI data and we believe that this is the actual time of arrival of the change. It can clearly be seen from Figure 3 that the magnetic field in the inner magnetosphere started to change nearly simultaneously to this estimated arrival time. These magnetic field changes are consistent with compression of the magnetosphere caused by the increase in SW dynamic pressure.
Figure 4 shows variations in slant TEC along different ray paths from the three CHAIN stations, during the period 16:00–16:30 UT of 12-04-2010. Figure 4 (left) is the raw data and Figure 4 (right) is the filtered data. For Figure 4 we have removed background TEC and the trend in TEC due to GPS satellite motion using the method adopted by Horvath and Crozier  and Watson et al. . In essence, the variations in TEC are only the short-term fluctuating component. TEC from all stations is sampled at 1 Hz, and we have only used measurements from satellites at >10° elevation angle to avoid multi-path effects. Different ray paths (PRNs) are color coded in each plot. Over the 16:00–16:30 time interval, the Edmonton receiver (Figure 4a) was able to track ten GPS satellites (i.e. ten different ray paths), five of which measured the fluctuation in TEC. The five satellites tracking south of Edmonton did not measure any significant change in TEC and are not shown in Figure 4. All ten satellites tracked by the Sanikiluaq receiver (Figure 4b) and all seven satellites tracked by the Iqaluit receiver (Figure 4c) observed the TEC change. The following two features are clearly evident from Figure 4: 1.) An increase in TEC at all the three stations due to sudden compression of the magnetosphere (maximum variations of ∼0.6 TECu in Edmonton, ∼1.2 TECu in Sanikiluaq, and ∼0.6 TECu in Iqaluit). One TECu = 1016 elec/m2. These TEC enhancements are smaller compared to other features such as tongue of ionization (TOI), storm enhanced density (SED), and polar patches but comparable to TEC changes associated with high-latitude auroral forms [e.g., Jayachandran et al., 2009a] 2.) There is a systematic delay in the onset of TEC signatures at each station and from different ray paths of the same station, indicating that the ionospheric response (increase in TEC) of magnetospheric compression is propagating. Figure 5a shows a map of estimated onset times from ray paths of all three stations, with each dot representing a ray path's Ionospheric Pierce Point (IPP) at 300 km and the color of the dot representing onset time measured by that ray path. IPP is the point at which the GPS ray-path intersects the ionosphere. The color bar below Figures 5a and 5b shows how we relate color to onset time. For this analysis, we take the time of onset for these signatures of magnetospheric compression as the peak in TEC (for easier and less ambiguous identification). For this calculation we have used only short-baseline (data from the same station and not between stations) to avoid TEC changes due to location of the stations and large separation between the IPPs. For Iqaluit and Sanikiluaq stations, the TEC increase was first observed near noon and propagated poleward. Edmonton, the lowest latitude of the three stations, also showed similar behaviour except that the response delay was progressing from dawn towards the noon sector. Since there were several GPS ray paths available, it is possible to calculate the propagation velocity of the response using a GPS triangulation technique described by Jayachandran et al. [2009b] for polar cap arcs. This approach was proven to be a very effective technique as shown by Watson et al.  for the case of substorm expansion. The only assumption we require is the altitude of the IPP of the GPS ray path. Calculated propagation velocities of the ionospheric response, assuming an IPP height of 300 km (F region), are shown in Figure 5b. For this figure we have calculated velocities using all available combinations of GPS ray paths (minimum three required), which results in a remarkably consistent set of velocities around each station. Figure 5c shows the average velocity for each station, which indicates that propagation speeds varied between 3.29 km/s near Iqaluit to 8.52 km/s near Edmonton. Sanikiluaq measurements showed an average speed of 6.38 km/s. Looking at propagation direction, the ionospheric response moved antisunward around Sanikiluaq and Iqaluit (higher latitude stations and near noon) and sunward around Edmonton (lowest latitude station and in the morning sector). This picture is consistent with a classical two-cell convection pattern that is extended all the way to Edmonton. Figure 5d shows the estimated average velocity assuming an IPP height of 120 km (E region). Propagation directions are consistent with estimations using a 300 km IPP height, however speed is significantly reduced as expected. Use of start time of TEC increase instead of the peak yielded slightly higher speeds and directions were not uniform as the case of the directions presented in the paper.
3. Discussion and Conclusion
 This paper presents a case study of a propagating TEC enhancement associated with compression of the magnetosphere (evident in THEMIS and GOES magnetic field data), resulting from a sudden increase in solar wind dynamic pressure. We have used multiple GPS ray paths to estimate the propagation characteristics of the response for two assumed IPP heights (F region and E region). Propagation direction is the same for both IPP height assumptions. However, as expected, speed of propagation is substantially lower for the 120 km assumption. The next obvious question is which IPP height assumption, and hence which calculated velocity, is closest to reality. Figure 6 shows riometer absorption at Sanikiluaq and Iqaluit (co-located with the GPS receivers) during the interval 16:00–16:30 UT on 12-04-2010. Over this time interval, a small but detectable increase (0.1–0.2 dB) in absorption is observed at both stations. These absorption enhancements are similar and concurrent to the TEC enhancements observed at Sanikiluaq and Iqaluit (Figures 4b–4c). These types of absorption-TEC similarities were also reported by Watson et al.  for high-latitude regions. This small increase in absorption is usually associated with F region electron density enhancements [Wang et al., 1994], while a concurrent 0.1–0.2 dB absorption and 1.0–1.5 TECu increase is most likely associated with low energy precipitation at these latitudes [Watson et al., 2011]. If this was an E region electron density enhancement, the absorption level would be much higher than what we have observed. As a result, an F region IPP height is a reasonable assumption and gives the best estimate for propagation velocity of the ionospheric response. Riometer observations also suggest that the TEC enhancement is due to low energy particle precipitation. Observations of optical emission intensification near noon and post-noon sectors associated with a sudden increase in solar wind dynamic pressure have been reported by Sibeck et al.  and recently reported by Yang et al., , are consistent with our observations.
 The other aspect of this study is the unusually high propagation velocity of the enhanced TEC feature associated with the magnetospheric compression. The propagation direction estimated using GPS triangulation mimics the classical two cell convection pattern (antisunward convection near noon in the polar cap, and sunward convection in the pre-noon sector at auroral and lower latitudes). There is also evidence for global convection enhancement in the SuperDARN convection data (figure not shown),which shows an increase in convection velocity from ∼300 m/s at 16:05 UT to ∼1.8 km/s at 16:15 UT. GPS estimated velocities are much higher than the observed convection velocity, and thus a simple explanation based on increased convection may not be adequate. There have been several reports in the literature, based on magnetometer data, of travelling convection vortices (TCVs) produced in the dayside, high latitude ionosphere by sudden changes in the solar wind [e.g., Friis-Christensen et al., 1988; Murr and Hughes, 2003, and references therein]. TCVs are spatially localized, transient convection cells embedded in a large-scale convection pattern, and are characterized by enhanced electric fields, particle precipitation, and an upward/downward field-aligned current pair. They are typically observed in the pre-noon and post-noon sectors at geomagnetic latitudes between 60–75°, while pre-noon sector vortices are usually located at much lower latitudes. These TCVs propagate with velocities of 3–10 km/s in the ionosphere. These properties of TCVs seem to suggest that the observed TEC changes and their propagation characteristics are a result of a morning-side TCV, and that the temporary TEC enhancement is due to precipitation associated with the field aligned current of the TCV. GPS based TEC measurement is a new viable and complementary tool to study TCVs and their dynamics, and to distinguish between the current and conductivity effects. Observed velocity near Edmonton (lowest latitude station) is still puzzling because of the location of the station and needs to be investigated further.
 Infrastructure funding for CHAIN was provided by the Canada Foundation for Innovation (CFI) and the New Brunswick Innovation Foundation (NBIF). CHAIN is conducted in collaboration with the Canadian Space Agency (CSA). Science funding is provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada. We acknowledge NASA contract NAS5-02099 and V. Angelopoulos for use of data from the THEMIS Mission. Specifically: K. H. Glassmeier, U. Auster and W. Baumjohann for the use of FGM data provided under the lead of the Technical University of Braunschweig and with financial support through the German Ministry for Economy and Technology and the German Center for Aviation and Space (DLR) under contract 50 OC 0302.
 The Editor wishes to thank Endawoke Yizengaw and Larry Lyons for their assistance evaluating this paper.