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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A00I90, doi:10.1029/2010JA015732, 2011

GPS TEC technique for observation of the evolution of substorm particle precipitation C. Watson,1 P. T. Jayachandran,1 E. Spanswick,2 E. F. Donovan,2 and D. W. Danskin3 Received 28 May 2010; revised 3 June 2011; accepted 15 July 2011; published 7 October 2011.

[1] One of the signatures of magnetospheric substorms is the precipitation of high energy particles into the high latitude ionosphere. In this paper, we introduce a new method of tracking substorm particle precipitation using GPS Total Electron Content (TEC) and provide some preliminary observations of precipitation signatures from application of this method. Using TEC measurements from several GPS receivers, we examined particle precipitation signatures associated with two separate substorm events (4 October 2008 and 29 October 2008) and monitored the expansion of the high energy precipitation regions with a higher temporal and spatial resolution than previously available. For each event we have observed TEC signatures associated with substorm particle precipitation along 20 to 25 separate GPS raypaths from up to 7 GPS receivers located in the Canadian Arctic. This is in addition to particle injection signatures found in CLUSTER satellite data and precipitation signatures in ground based riometer data. Signature timing on different raypaths from different stations indicates a mainly northward (tailward) expansion of the precipitation (injection) region with a smaller westward (azimuthal) component for the events studied. By applying a triangulation method, we also calculated propagation velocity of the precipitation boundary in regions covered by our GPS receivers. For each substorm, expansion velocity ranged from 0.3–2 km/s northward and 0–1 km/s westward, and tended to decrease in magnitude at higher latitudes. Citation: Watson, C., P. T. Jayachandran, E. Spanswick, E. F. Donovan, and D. W. Danskin (2011), GPS TEC technique for observation of the evolution of substorm particle precipitation, J. Geophys. Res., 116, A00I90, doi:10.1029/2010JA015732.

1. Introduction [2] Signatures of substorm expansion include precipitation of high energy particles into the ionosphere that is associated with injection of high energy particles in the magnetosphere [Arnoldy and Chan, 1969; McIlwain, 1974], and dipolarization [e.g., Liou et al., 2002]. Substorm injections are often dispersionless [McIlwain, 1974; Mauk and Meng, 1987] and are contained within a localized injection region characterized by a well‐defined injection boundary at its edges [Reeves et al., 1991]. It has been shown that dipolarization and injection signatures are localized at substorm onset, and expand azimuthally [Thomsen et al., 2001] and radially [Spanswick et al., 2009] during the substorm’s expansion phase. Azimuthal and radial expansion of the dipolarization fronts have been studied in detail using geo‐ synchronous magnetic field data [Liou et al., 2002; Watson and Jayachandran, 2009]. Little is known about the evo-

1 Physics Department, University of New Brunswick, Fredericton, New Brunswick, Canada. 2 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada. 3 Geomagnetic Laboratory, Geological Survey of Canada, Ottawa, Ontario, Canada.

Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JA015732

lution of the particle injection/precipitation associated with substorms, primarily due to low spatial coverage of in situ satellite instruments capable of detecting these injections. Since substorm injections/precipitation materialize as enhanced ionization in the lower ionosphere, ground‐based methods of substorm injection/precipitation study are feasible by analyzing ionospheric effects on radio wave propagation during a substorm. These methods are advantageous and a valuable compliment to in situ measurements due to availability of dense arrays of ground‐based instruments and relative ease of higher resolution analysis than satellite‐ based studies. One such instrument is the riometer, a common tool for observing fluctuations in auroral zone radio absorption during substorms [Fjordheim and Henriksen, 1974; Baker et al., 1982; Collis et al., 1984; Rosenberg and Dudeney, 1986; Spanswick et al., 2005]. Riometer absorption measurements are sensitive to enhanced D and lower E region electron density associated with the high particle precipitation [Hargreaves et al., 1979]. In this study, we apply a method of tracking high energy precipitation associated with substorm injection using an array of riometers, and extend this method to ionospheric data obtained from the Global Positioning System (GPS). Availability of GPS receivers in Arctic regions [Jayachandran et al., 2009a] combined with multisatellite data at each ground receiver allows for high resolution space/time study of precipitation signatures as the substorm injection region evolves.

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WATSON ET AL.: GPS TEC OBSERVATION OF SUBSTORMS

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Figure 1. Map of our collocated GPS receiver and riometer stations (geomagnetic coordinates).

[3] Sudden absorption increase events associated with substorms have been extensively studied using riometer absorption data from auroral regions [Hargreaves, 1974; Ranta et al., 1981; Hajkowicz, 1990; Ranta and Yamagishi, 1997; Spanswick et al., 2009]. The sharp rise in absorption is interpreted as the sudden onset of high energy particle precipitation into the ionosphere due to sudden enhancement of high energy particle flux in the plasma sheet associated with the substorm expansion phase. This relationship was well documented by Spanswick et al. [2007] using CRESS MEB in situ electron flux data in conjunction with riometer absorption data during substorm particle injections. For a number of events, they found that a sharp rise in plasma sheet electron flux for energies >30 keV (dispersionless injection) was often accompanied (under certain conditions) by a sharp rise in riometer absorption due to sudden enhancement of ionization in the lower ionosphere. If multiple riometers pick up this ionospheric signature of high energy particle precipitation, then tracking the movement of these signatures can give insight into the spatial and temporal evolution of the substorm injection region [Berkey et al., 1974; Liang et al., 2007; Liu et al., 2007; Spanswick et al., 2009]. In this study, we extend this ground‐based method of identifying substorm precipitation signatures to GPS TEC, assuming that enhanced electron density due to high‐energy particle precipitation will produce detectable TEC enhancements. Similar to riometer absorption we find that high‐energy precipitation signatures associated with substorm injection appear as a sharp rise in TEC, signifying the injection onset. Due to availability of multiple satellite ray paths at each ground receiver, GPS TEC provides a relatively high resolution method for tracking signatures of a substorm particle precipitation region as it evolves. For two separate substorm events (4 October 2008 and 29 October 2008) we have used timing of precipitation signatures in TEC data to monitor the evolution of the precipitation region and calculate the velocity of the precipitation boundary during its expansion over the Canadian Arctic. We note that this method of identifying high energy particle precipitation signatures associated with substorm injection

has not been tested at the high latitudes covered by our receivers (>72° magnetic latitude).

2. Data and Method of Analysis [4] Riometer data in this study is provided by Canadian GeoSpace Monitoring (CGSM) and Natural Resources Canada (NRCan) riometer networks. GPS data is from the Canadian High Arctic Ionospheric Network (CHAIN) [Jayachandran et al., 2009a]. Figure 1 shows a geomagnetic map of collocated GPS receivers and riometers used in this study. [5] The riometers are broad beam (60°) antennas and receivers that provide ionospheric attenuation measurements of ∼30 MHz cosmic radio noise (CRN) at 1 s resolution (5 s resolution for Taloyoak). Variations in ionospheric ionization rates due to particle precipitation affect the amount of received CRN power at the ground riometer, where this data is translated to ionospheric absorption. The 30 MHz operating frequency allows riometers to measure CRN absorption occurring in the D region ionosphere, where enhanced ionization is mainly a result of precipitating electrons of energies exceeding 30 keV [Baker et al., 1981]. [6] The CHAIN GPS receivers are GPS Ionospheric Scintillation and TEC Monitors (GISTMs) model GSV4000B [Van Dierendonck and Arbesser‐Rastburg, 2004]. In summary, a GISTM consists of a NovAtel OEM4 dual frequency receiver with special firmware specifically configured to measure amplitude and phase scintillation derived from the L1 frequency GPS signals and ionospheric total electron content (TEC) derived from the L1 and L2 frequency GPS signals. This receiver is capable of tracking and reporting scintillation and TEC measurements from up to ten GPS satellites in view. Phase and amplitude data are sampled and logged, either in raw form or detrended, at a rate of 50 Hz. Nine of the ten receivers are currently fed by a NovAtel GPS‐702 antenna, the exception being the Qikiqtarjuaq receiver which shares an Ashtech ASH701945E_M antenna with a pre‐existing Natural Resources Canada GPS receiver through a splitter. At most times, 8 to 10 GPS satellites are

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Figure 2. Comparison of riometer absorption (gray) and GPS TEC (black) for three separate absorption events: (a) Taloyoak on 7 December, 2008, (b) Iqaluit on 30 October 2008, and (c) Sanikiluaq on 8 September 2008. All three events show enhanced absorption and TEC enhancements due to particle precipitation. visible to a single ground receiver. From GPS data we calculate the total electron content (TEC), a value integrated in a 1 m2 column along the satellite to receiver raypath. TEC is calculated in TEC units (TECu), where 1 TECu = 1016 electrons per square meter. [7] GPS TEC is the main data source for this study, and is used in calculating results for the evolution of the substorm precipitation region. To our knowledge, GPS TEC has not previously been used in identification of particle precipitation signatures associated with substorm injections. At the current stage, the sharp rise in TEC we observe during a substorm cannot be attributed to a substorm injection unless we have simultaneous riometer data showing the same signature. Identification of dispersionless injection using riometer absorption is in the literature [Spanswick et al., 2007] and is the foundation for this study. We note that our GPS receivers are located north of the auroral region, whereas the riometers used in the Spanswick et al. [2007] study were located within auroral latitudes. [8] A survey of riometer absorption events occurring between 1 May 2008 and 31 April 2009 was conducted for all available sites. Concurrent observation of GPS TEC and

riometer absorption revealed that variations in the two often show similar features during absorption events. Survey results revealed that correlation between absorption and TEC during absorption events varied with local time, type of physical process (substorm, polar cap absorption, polar patches) and satellite elevation. Events associated with substorm particle injection showed particularly good correlation, where a sharp rise in riometer absorption corresponding to injection onset was always accompanied by the same sharp rise in TEC. This will be seen later in the paper when we investigate individual substorm events. Vertical TEC (vTEC) enhancements during surveyed substorm injections, which we interpret as E region ionization, were in the range of 1–3 TECu. For comparison, Mendillo [2006] found vTEC enhancements of up to 15 TECu during F region storms at auroral latitudes. [9] Three samples from the survey are shown in Figures 2a–2c: Taloyoak on 7 December 2008, Iqaluit on 30 October 2008 and Sanikiluaq on 8 September 2008, respectively. Riometer absorption is plotted in gray and GPS TEC in black. We note that these absorption events are not necessarily associated with substorms. Each case shows

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Figure 3. Simple diagram describing the 3‐IPP triangulation process applied to TEC precipitation signatures to calculate expansion velocity of the high energy precipitation region (vexp).

pierce point (IPP) for that raypath at onset time. The IPP altitude we have chosen will be addressed shortly. [11] Collecting all onset times and IPP positions, we have developed a position‐time map for each substorm precipitation event, which provides insight into expansion direction of the injection/precipitation region and the spatial resolution at which we can monitor this expansion. From this map we have calculated a profile for expansion velocity of the high energy precipitation region by applying a simple triangulation method to different combinations of IPPs. For a single velocity calculation, three appropriate IPPs are chosen: let IPP1, IPP2 and IPP3 be three pierce points suitable for triangulation (Figure 3). A suitable choice of IPPs means that the three points are reasonably close together such that triangulation is not applied over a long baseline. IPPs from a common GPS station and often one or two neighboring stations are usually sufficiently close. From known distances between IPPs and precipitation onset times we can calculate velocities v1–3 and v2–3, which represent the apparent expansion velocities of the precipitation region along straight paths between IPPs (IPP1 to IPP3 and IPP2 to IPP3, respectively). We can use projections of v1–3 and v2–3 to determine the actual expansion velocity of the precipitation region (vexp): vexp ¼ v13 ^ vexp ¼ v23 ^ vexp

variations in absorption that are similar to those observed in TEC, indicating a clear relationship between the two. Note that the broad‐beam nature of the riometer and localized nature of the GPS raypath create some temporal offsets, as can clearly be seen in Figure 2c. The absorption‐TEC similarities in each of these cases indicate that enhanced ionospheric absorption and increased TEC are caused by the same physical process. One thing to keep in mind is that GPS TEC is an integrated measurement along the raypath. When absorption and TEC variations are similar, TEC variations can logically be attributed to enhanced D and lower E region ionization since the riometer is sensitive to these altitudes. Support for this reasoning in the case of substorm injections can be found in results from Mayer and Jakowski [2009], where the authors used GPS radio occultation to show that an E‐layer dominated ionosphere is a common occurrence in evening, nighttime and morning auroral zones. A closer look at Figure 2 reveals that 1 dB of absorption corresponds to an increase in TEC of 2–4 TECu. This is expected since riometer absorption is proportional to electron density in the lower ionosphere. [10] During two substorms, a 4 October 2008 event and a 29 October 2008 event, we have found signatures of high energy particle precipitation associated with substorm injections in riometer and TEC data. While results from analysis of both substorms are presented, we show only analysis of the October 4th event in detail. For each event we collected all clear signatures of high energy substorm precipitation from absorption and corresponding TEC data. For each precipitation signature we determined a precipitation onset time, taken as the time that an increase in absorption/TEC was first observed. Onset times from TEC were interpreted as the intersection of the boundary of high energy precipitation with GPS raypaths at an altitude to be determined. For each GPS raypath that observed the precipitation, we calculated the position of the ionospheric

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ð1Þ

where ^vexp is a unit vector for vexp. By applying this method to all possible combinations of IPPs, a velocity‐time map of the precipitation region can be developed. [12] To examine the evolution of the substorm precipitation region using TEC data, we must first determine an altitude at which ionization enhancement due to injection‐ associated particle precipitation is most prominent. We have developed a method of estimating this altitude using TEC data. By applying the triangulation technique to TEC precipitation signatures using a range of IPP heights, we can essentially identify the height at which our triangulation results make the most sense. We show results for the 4 October 2008 substorm as an example. From all clear precipitation signatures for the event, we first chose four IPPs that are suitable for triangulation (small baselines), and

Figure 4. Histogram showing distribution of precipitation altitudes, ranging from 60 to 500 km, calculated from triangulation of IPPs for the 4 October 2008 substorm.

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injection height ranging from 60 to 600 km, displayed in a histogram in Figure 4 (10 km bins). Although results cover a broad range of altitudes, there is a clear tendency toward the lower E region with the 100–110 km bin containing the most results. A similar distribution was found for the 29 October 2008 substorm. Looking at these results, a choice of 100 km altitude for GPS IPPs seems reasonable. This choice is also backed up by some simple reasoning: The bulk of the contribution to riometer absorption comes from approximately this altitude (one could argue 90 km), therefore we assume that the bulk of the contribution to the TEC increase due to particle injection/precipitation also comes from this altitude since this is the same increase that is seen in absorption. Also, electron density profiles often show peak production rate in electron density around 100 km due to high energy precipitation [Hunsucker, 1975]. Further arguments pertaining to this altitude choice are available in the discussion.

3. Observations and Results [13] Observations of the 4 October 2008 substorm are analyzed in detail in this section. GOES 11 geosynchronous (6.6 RE) electron flux (>0.6 MeV and >2 MeV) and mag-

Figure 5. Satellite data for 4 October 2008: (a) GOES 11 electron flux at >0.6 MeV and >2 MeV showing electron injection (dotted line) at 6.6 RE; (b) GOES 11 magnetic field inclination in GSM coordinates showing magnetic field dipolarization (dotted line); and (c) CLUSTER 2 RAPID electron flux measurements at >30 keV and >100 keV showing electron injection (dotted line) at 17 RE in the tail region. calculated four velocities for the expanding precipitation region from the four possible choices of IPP triads. Under the assumption that these four velocities should closely match due to their close proximity, we find the height at which these velocities best agree. The appropriate height was usually quite clear since IPP positions change considerably with ionospheric height. By applying this method to all suitable IPP quartets we have obtained 274 results for

Figure 6. Riometer absorption on 4 October 2008 for Hall Beach, Taloyoak, and Pond Inlet, all showing a sharp rise in ionospheric absorption due to high energy precipitation associated with a substorm injection. Onset times are shown and are indicated by dotted lines.

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Figure 7a. TEC signatures of substorm precipitation from Iqaluit for 4 October 2008. High energy precipitation onsets observed by each satellite are displayed and are indicated by a dotted line. Satellite locations are shown by the sky plot at the top (horizon coordinates).

netic field inclination (GSM coordinates) for 0–8 UT on 4 October are shown in Figures 5a and 5b. A jump in electron flux in both populations is observed starting at 04:02:30 UT, followed closely by magnetic field dipolarization at 04:05:30 UT. Both occurrences are represented by a dotted line. This time discrepancy between dipolarization and auroral brightening (particle injection) is similar to the ones reported by Liou et al. [2002]. CLUSTER 2 RAPID electron flux (>30 keV and >100 keV) for 0–8 UT are shown in Figure 5c. A sudden increase in flux for both electron populations occurs at 04:12:44 UT, indicated by a dotted line in the figure. CLUSTER 2 was located at about 17 RE (∼23 MLT) at this time. Timing of injection signatures observed by GOES 11 and CLUSTER 2 suggests a

tailward expansion of the substorm injection region, from a region earthward of CLUSTER out to 17 RE. [14] Riometer absorption on 4 October 2008 from three separate stations is shown in Figure 6. Sites are plotted in order of substorm precipitation onset time, which is displayed in the figure and represented by a vertical dashed line. Hall Beach is the first to show the injection signature, with a sharp rise in absorption starting at 04:24:51 UT. Taloyoak, west of Hall Beach, shows high energy precipitation starting at 04:27:54 UT, and Pond Inlet, north of Hall Beach, shows injection at 04:35:04 UT. This indicates a northwestward expansion of the precipitation region from Hall Beach. Keep in mind that particle injection occurred at geosynchronous orbit around 04:02:30 UT and that these

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Figure 7b. Hall Beach TEC precipitation signatures for 4 October 2008.

stations are poleward of the geosynchronous magnetic footprint, thus the time delay. [15] Figures 7a–7e show GPS slant TEC from five different ground receivers for the time interval 04:10–05:00 UT, each site observing injection‐associated precipitation signatures (sudden TEC increase) from multiple satellites. Although ten satellites were visible to each receiver, multipath and other errors caused by low satellite elevation contribute to incorrect TEC calculations. As a result, only four to five clear precipitation signatures could be identified for each station. TEC from each station is plotted in order of onset time, which is displayed in the figures and indicated by vertical dashed lines. Sky plots for each site show azimuth and elevation of satellite trajectories in horizon coordinates. The horizon coordinate system uses the local horizon of a GPS receiver as a reference plane, with azimuth as the

angle of the satellite as projected onto the reference plane (measured clockwise from north) and elevation as the angle between the reference plane and the satellite’s position. Satellite PRN numbers indicate starting point of a satellite’s trajectory. Cambridge Bay and Iqaluit TEC data are included for this event despite absence of riometer plots in Figure 2. We use GPS TEC from Cambridge Bay despite unavailability of absorption data since TEC variations and signature timing are consistent with those of Hall Beach, Taloyoak and Pond Inlet. Iqaluit riometer data was available, but due to some missing data and calibration peaks we could not precisely determine onset time. Signature timing in Iqaluit absorption is consistent with Iqaluit TEC observations and absorption signatures from other sites. For Iqaluit in Figure 7a, PRN 24 is first to observe injection at 04:16:45 UT, followed by PRN 10 at 04:16:49 UT, PRN 21

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Figure 7c. Taloyoak TEC precipitation signatures for 4 October 2008.

at 04:17:30 UT, PRN 27 at 04:17:43 UT and finally PRN 7 at 04:18:10 UT. Looking at satellite positions in the sky plot, PRN 24 is located south of the Iqaluit ground receiver and is the southernmost satellite to observe the substorm precipitation. Following other onset times and respective satellite positions, the precipitation region evidently expands from south to north, with PRN 7 the northernmost and last satellite to pick up the precipitation signature. In Figure 7b for Hall Beach, PRN 15 is first to detect precipitation at 04:23:03 UT. From the Hall Beach sky plot, PRN 15 is located south of the ground receiver. PRN 21, northwest of PRN 15 is next to show precipitation at 04:26:19 UT. PRN 27, 16 and 7 all detect precipitation within a matter of seconds, PRN 7 being the last at 04:22:50 UT. PRN 16 is northwest of Hall Beach while PRN 27 and 7 are north, indicating a northwestward expansion of the precipitation region. Taloyoak TEC data in Figure 7c shows precipitation

first for PRN 15 at 04:25:35 UT, the southernmost satellite. Northwest expansion through PRN 24 to PRN 21 at 04:28:22 UT as well as Northeast expansion to PRN 10 at 04:28:58 UT is observed. Figure 7d for Cambridge Bay shows precipitation first at PRN 15 at 04:27:52 UT. This is followed by a northward progression through PRN 26 and 24, reaching the northernmost PRN 10 at 04:30:01 UT. Pond Inlet data in Figure 7e shows the first precipitation onset at 04:31:47 at PRN 26, again the southernmost satellite. A northwestward expansion past PRN 24 to PRN 21 at 04:36:29 UT is observed, as well as a northeastern expansion past PRN 10 to PRN 8 at 04:38:20 UT. A northward progression of the precipitation region is consistently observed at each station. Figure 8 shows the position‐time map (geomagnetic coordinates) of substorm precipitation signatures from all sites for the 4 October substorm. Each dot shows IPP location (at 100 km ionospheric height) for the

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Figure 7d. Cambridge Bay TEC precipitation signatures for 4 October 2008.

indicated satellite PRN at time of precipitation onset. Color of dots represents injection time as shown in the color bar. A systematic progression of the precipitation region from southern latitudes to northern latitudes can clearly be seen in the figure. Satellites at southernmost latitudes around Iqaluit are first to pick up precipitation, PRN 24 at 04:16:45 UT being the earliest. The precipitation region expands northwestward, taking ∼5 min to reach PRN 15 in Hall Beach. Expansion then proceeds northward through Hall Beach, Taloyoak and Cambridge Bay, progressing through the ∼3 degrees latitude containing these IPPs in ∼7 min. Precipitation onsets along similar latitudes have approximately equal onset times indicating a mainly poleward expansion. The precipitation region continues to expand northward toward Pond Inlet, taking almost 1 min over ∼2 degrees latitude to reach PRN 26. Expansion of the precipitation region from PRN 24 in Iqaluit to PRN 8 in Pond Inlet takes

just over 28 min, a total distance of about 1100 km at 100 km ionospheric height. A velocity‐time map of the expanding precipitation region is shown in Figure 9, calculated from the triangulation method outlined earlier. Arrows representing the expansion velocity of the precipitation region are colored based on onset time. An arrow length‐to‐speed scale is also shown in the figure. A consistent northwestward expansion through all latitudes is clearly seen, along with a tendency for speed to decrease with increasing latitude. Northwestward expansion speeds up to 2.1 km/s were calculated around Iqaluit, decreasing to 1–1.5 km/s around Hall Beach, Taloyoak and Cambridge Bay, and reaching a minimum speed of 0.4 km/s around Pond Inlet. [16] Figure 10 is the position‐time map of the precipitation region for the 29 October substorm, from 06:25 UT to 06:50 UT. For this event we observe signatures at Resolute

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Figure 7e. Pond Inlet TEC precipitation signatures for 4 October 2008. and Qiqiktarjuaq in addition to the 5 stations used for 4 October. A consistent northward expansion is seen through all latitudes, with substorm precipitation observed first around Iqaluit and last around Resolute. Figure 11 is the corresponding velocity‐time map for the event, and again shows a poleward expanding precipitation region. Northeast expansion speeds of 0.3 km/s were calculated around Iqaluit, while mainly northward velocities of 0.5–1.5 km/s were found further north.

4. Discussion [17] We have outlined a new and useful technique of tracking the evolution of the precipitation region associated with substorm particle injection using GPS TEC observations. An injection‐associated precipitation signature is first identified in riometer absorption data using the method of Spanswick et al. [2007], followed by identification of the

corresponding signature in the TEC data. A signature of the substorm precipitation is a sudden increase in TEC, similar to absorption signatures. The close correlation between TEC and absorption increase is expected since high‐energy particle precipitation produces enhanced ionization in the lower ionosphere (therefore TEC increase), which is a major source of riometer absorption. [18] Results of our analysis with GPS TEC data showed a mainly northward expansion of the substorm precipitation region in the ionosphere corresponding to a tailward (radial) expansion of the injection region in the magnetotail. The 4 October 2008 results are consistent with onset timings at GOES 11 (6.6 RE) and CLUSTER 2 (17 RE). Particle injection occurred around 04:02:30 UT at geosynchronous orbit and 04:12:44 UT at 17 RE, indicating a tailward expansion. Our lowest latitude station to observe high energy precipitation (Iqaluit) showed the signature at

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Figure 8. Position‐time map for TEC substorm precipitation onsets on 4 October 2008, where dots represent IPP position (100 km altitude) and color indicates onset time (UT) for each precipitation signature. IPPs are labeled with corresponding satellite PRN numbers and coordinates are geomagnetic. ∼04:16:45 UT, indicating that precipitation first occurred south of Iqaluit. A simple mapping using Tsyganenko 89c [Tsyganenko, 1989] shows that the CLUSTER 2 magnetic footprint was located south of Iqaluit (around our Sanikiluaq station) during the substorm. These facts are all consistent with tailward expansion of the particle injection from geosynchronous orbit. Our results are also consistent with a recent event study by Spanswick et al. [2009], which identified an injection region that expanded radially outward from geosynchronous orbit for a 27 August 2001 substorm. Due to high spatial density provided by GPS measurements

we are able to obtain a more detailed picture of the expanding injection region, although we are restricted to monitoring the poleward expansion due to the high latitude locations of our receiver stations. [19] The two events studied in this paper showed consistent poleward results for expansion velocity. For each substorm, most velocities ranged from 0.3–2 km/s northward and 0–1 km/s westward. Studies using riometer networks in Canada and Europe to monitor the expansion of sharp auroral absorption onsets associated with substorms have found similar results. Ranta et al. [1981] estimated

Figure 9. Velocity‐time map (geomagnetic coordinates) for expansion of the precipitation region associated with substorm injection calculated from Figure 8. Arrows are velocity vectors for the precipitation boundary (scale is shown) and are colored based on onset time of TEC signatures. 11 of 15

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Figure 10. Position‐time map for TEC substorm precipitation onsets on 29 October 2008, where dots represent IPP position (100 km altitude) and color indicates onset time (UT) for each precipitation signature. IPPs are labeled with corresponding satellite PRN numbers and coordinates are geomagnetic. poleward expansion speeds of 0.58–2.2 km/s during a substorm event on 22 May 1976 from five riometer stations between 69° and 76° magnetic latitude. The highest speed was observed near the lowest latitude stations, while higher latitude results were all less than 0.7 km/s. Berkey et al. [1974] found poleward expansion speeds of 0.3–1.9 km/s from observations of 26 substorm events at stations between 57° and 76° magnetic latitude. A tendency for speeds to decrease above 71° magnetic latitude was also reported. Imaging riometers have also been used to track the propagation of these sharp absorption onsets on small spatial

scales in localized regions [Ranta and Yamagishi, 1997; Ranta et al., 1999], where northward propagation speeds were consistently found in the 1–3 km/s range. [20] Additional development of our TEC method for identifying substorm injections (2‐D cross correlation) can lead to high resolution study of substorm onset as well as precise details on the magnetospheric location of injection onset and consequent injection expansion. With installation of more GPS receivers in and around auroral latitudes, a more complete picture of the expanding particle injection region can be formed. This includes tracking the radial and

Figure 11. Velocity‐time map (geomagnetic coordinates) for expansion of the precipitation region associated with substorm injection calculated from Figure 10. Arrows are velocity vectors for the precipitation boundary (scale is shown) and are colored based on onset time of TEC signatures. 12 of 15

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Figure 12. Riometer absorption versus change in vTEC for high latitude absorption events observed from 1 May 2008 to 31 April 2009. Three distinct TEC‐absorption relationships are evident: one for 5–30 keV electron precipitation (blue), another for >30 keV electron precipitation (red), and a third for substorm injection (green). Electron energy spectra were determined from in situ satellite measurements (THEMIS, GOES, CLUSTER and DMSP). Representative points for the 4 October 2008 event are indicated by black stars. azimuthal expansion of the injection region from its very beginning. These further developments should be highly valuable in full understanding of substorm physics. [21] One issue regarding interpretation of the TEC data is whether enhancements due to high energy particle precipitation associated with substorm injections can be distinguished from TEC enhancements due to lower energy auroral precipitation. Figure 12 shows results from our riometer absorption event survey discussed in section 2. For this survey, we have calculated the increase in GPS vTEC associated with absorption events observed at Figure 1 stations between May 2008 and April 2009, where satellite data was available to determine precipitation characteristics. Precipitation energies were determined from in situ measurements of THEMIS, GOES, CLUSTER and DMSP satellites. As in Figure 12, a plot of absorption versus corresponding vTEC enhancements for all events reveals three clear absorption‐vTEC relationships associated with three different sources of ionization: Small absorption increases (30 keV and >100 keV) for the 4 October 2008 substorm when the magnetic footprint of CLUSTER2 is located just south of our Iqaluit receiver. Four minutes later, the first high energy precipitation signature is observed in Iqaluit TEC data, followed by all other TEC signatures in a matter of minutes. This evidence, along with the evidence of tailward propagation, indicates that the sharp rises in TEC are due to flux increases in the same high energy electron populations observed by CLUSTER2. Second, the magnitude of TEC enhancement associated with these substorm events is significantly smaller when compared to TEC enhancements from F region ionization. For example, Mendillo [2006] shows vTEC enhancements of up to 15 TECu during F region storms, whereas the vTEC enhancements that we interpret as E region contributions are in the 1–5 TECu range. This can be explained by simple logic. A look at ionization profiles produced by particles of various energy levels will yield the following [Hunsucker and Hargreaves, 2003]: In the lower ionosphere, the structures associated with high energy precipitation are narrow (in the vertical direction), whereas the ionization structures associated with low energy precipitation cover a wider range of altitudes. This is because of the vertical distribution of neutral density and collisional cross sections. A GPS raypath will spend a short time traveling though narrow structures, producing smaller TEC variations, whereas the GPS ray will spend a longer time traveling through wider structures, producing larger TEC variations. This is also evident in Figure 12, where lower energy auroral precipitation produces larger TEC enhancements than precipitation associated with substorm injections. A calculation of the vertical thickness of an ionization layer based on GPS data can be found in work by Jayachandran et al. [2009b]. Last, the distribution of precipitation heights we calculated in Figure 4 shows that TEC signatures we are looking at are due to enhanced E region ionization. [23] One other factor we would like to further address here is the choice of 100 km altitude for assumed precipitation depth in the ionosphere. Usually particle precipitation associated with injections is of high‐energy, and the maximum rate of ionization for particles with energy > 5 keV is around 100 km [Rees, 1963]. It’s also known that riometer absorption measurements are sensitive to D/lower E region altitudes and high energy particle precipitation. Since we have attributed TEC signatures to ionization from the same electron population (similar variations observed in absorption and TEC), we have taken 100 km as the height at which the substorm precipitation region evolves. We have also investigated injection height for the 4 October 2008 event by applying our triangulation method to Hall Beach, Taloyoak and Pond Inlet riometer data using precipitation

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onset times from Figure 6. Assuming 100 km altitude, we calculated an average expansion velocity of 0.71 km/s at 4.8° W of N (0.70 km/s N and 0.06 km/s W) over the region spanning these sites. We applied this calculation to a range of altitudes spanning the entire ionosphere, which resulted in, at most, a ± 0.05 km/s change in speed and no change in direction. This riometer‐derived velocity is consistent with TEC‐derived velocities (using 100 km altitude) in the same region. TEC‐derived velocities change significantly with assumed altitude due to the slant of GPS raypaths. For example, a ± 50 km change in altitude results in about a ± 50% change in calculated speed depending on chosen IPPs. Taking these arguments into account, in addition to the evidence discussed in section 2 regarding the calculated altitude distribution (Figure 4), our assumption of 100 km precipitation height seems reasonable.

5. Conclusions [24] We have used GPS TEC data during the substorm expansion phase for two events to monitor the evolution of the precipitation region associated with substorm injection as it expands over the Canadian arctic. In both cases, precipitation region signatures were observed first at lower latitudes followed by mainly north or northwestward expansion. Expansion speeds ranged from 0.3 km/s to 2.1 km/s, while speeds for the 4 October 2008 event tended to decrease over time and with increasing latitude. Combined GOES 11 and Cluster 2 as well as GPS TEC all indicate tailward expansion of the precipitation region for 4 October. [25] Due to availability of multisatellite data at each site, GPS TEC has proven to be a useful tool in studying evolution of injection‐associated substorm precipitation, even in these preliminary stages. High spatial density of TEC data allows for calculation of expansion velocity and a reasonably clear picture of expansion direction. Future study will include lower latitude stations and 2D cross correlation for better understanding of injection region onset as well as the dynamics of the expanding injection region. [26] Acknowledgments. Infrastructure funding for CHAIN was provided by the Canada Foundation for Innovation and the New Brunswick Innovation Foundation. CHAIN operation is conducted in collaboration with the Canadian Space Agency. Science funding is provided by the Natural Sciences and Engineering Research Council of Canada. The GOES and CLUSTER data was obtained from CDAWeb. [27] Robert Lysak thanks the reviewers for their assistance in evaluating this paper.

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