Canadian High Arctic Ionospheric Network (CHAIN)

Authors


Abstract

[1] Polar cap ionospheric measurements are important for the complete understanding of the various processes in the solar wind-magnetosphere-ionosphere system as well as for space weather applications. Currently, the polar cap region is lacking high temporal and spatial resolution ionospheric measurements because of the orbit limitations of space-based measurements and the sparse network providing ground-based measurements. Canada has a unique advantage in remedying this shortcoming because it has the most accessible landmass in the high Arctic regions, and the Canadian High Arctic Ionospheric Network (CHAIN) is designed to take advantage of Canadian geographic vantage points for a better understanding of the Sun-Earth system. CHAIN is a distributed array of ground-based radio instruments in the Canadian high Arctic. The instrument components of CHAIN are 10 high data rate Global Positioning System ionospheric scintillation and total electron content monitors and six Canadian Advanced Digital Ionosondes. Most of these instruments have been sited within the polar cap region except for two GPS reference stations at lower latitudes. This paper briefly overviews the scientific capabilities, instrument components, and deployment status of CHAIN. This paper also reports a GPS signal scintillation episode associated with a magnetospheric impulse event. More details of the CHAIN project and data can be found at http://chain.physics.unb.ca/chain.

1. Introduction

[2] The creation of the Canadian High Arctic Ionospheric Network (CHAIN) is primarily motivated by the fundamental need for a greater understanding of planetary environments that are affected by the short- and long-term variability of solar output. The Sun-Earth system is a vast coupled system, where many physical processes take place, with solar variability driving the effects felt in the terrestrial and space environments. A complete understanding of this complex system necessitates knowledge about the individual physical processes taking place in the system and in turn necessitates relevant observations. Through the construction and operation of CHAIN, we seek to understand the individual physical processes necessary for the fundamental understanding of the solar-terrestrial coupling and its influence on our planetary environment.

[3] The Earth's upper atmosphere (ionosphere) in the polar region is embedded in the “magnetosphere,” a cavity carved by the interaction of the solar wind and its “frozen-in” magnetic field with the terrestrial magnetic field. Large-scale electric currents are a ubiquitous feature of this environment; they are powered by the solar wind, and continuously deposit electric (Joule) and thermal energy as well as particles in the Earth's upper atmosphere, and thus modify the global upper atmospheric chemistry, circulation pattern, and energy budget. The solar wind is inherently nonsteady, with its magnetic field, density, and flow speed varying on a range of temporal and amplitude scales. Variations in the solar wind modulate the solar-terrestrial coupling processes which drive the transport of mass, energy and momentum into near-Earth space, leading to variations of the magnetospheric particle population, electric currents, and the aurora; such disturbances in the Sun-Earth system are commonly called “space weather.” Space weather effects can damage satellite systems [e.g., Baker et al., 1998], affect communication systems, and cause disruption of power transmissions [Boteler, 1998; Pulkkinen et al., 2005] and cause other phenomena [Hunsucker and Hargreaves, 2003]. Increased ionization and disturbances in the upper atmosphere also affect satellite-based communication systems and terrestrial high-frequency radio communication, and can introduce significant position errors in radio navigation systems such as the Global Positioning System (GPS) [e.g., Lanzerotti, 2001].

[4] For the influence of the Sun on the near-Earth environment to be fully explained, understanding and monitoring the fundamental processes responsible for solar-terrestrial coupling is vital. Monitoring the spatial and temporal development of the global electric field and current systems, transport of energy and momentum across the polar cap, and generation and dynamics of ionospheric irregularities of different scale sizes, and ultimately the role of the ionosphere in the whole solar wind-magnetosphere-ionosphere (SW-M-I) coupling process is essential in the understanding of the Sun-Earth system. Given the size of the SW-M-I system one can only accumulate knowledge over time and try to put together a coherent physical picture. One way to address this problem is to use ionospheric measurements as a “road map” to understand the SW-M-I interaction. This is possible because in the high-latitude regions, the high conductivity along terrestrial magnetic field lines, which have a footprint in the high-latitude ionosphere, forms a closed electric circuit together with the conducting ionosphere. The current/voltage generated through the solar wind magnetosphere dynamo/generator is applied to the high-latitude ionosphere through this circuit.

2. CHAIN Instruments, Locations, and Status

[5] CHAIN is a distributed array of radio instruments primarily in the polar cap. The CHAIN instrument components consist of 10 specialized GPS receivers and six digital ionosondes. A unique feature of CHAIN is that 6 of the 10 GPS receivers are collocated with ionosondes. This configuration of instruments (collocated ionosondes and GPS receivers) will have an added advantage in the tomographic imaging of the electron density structures in the polar cap and calibration of the GPS data. New Resolute Bay Incoherent Scatter Radar (AMISR) will also help in the tomographic reconstruction of the ionosphere. Locations of the CHAIN instruments are shown in Figure 1. Ten GPS receivers and three ionsondes are now installed and collecting data routinely. The remaining three ionosondes will be installed during the summer of 2009. Data from these stations are transferred to the University of New Brunswick CHAIN Data Center on a near-real-time basis. Table 1 provides station details, including the instruments located at each site, status of deployment, and a summary of the near-real-time data collection. Brief descriptions of the CHAIN instruments are given below.

Figure 1.

A geographic map showing the locations of the Canadian High Arctic Ionospheric Network (CHAIN) GPS and ionosonde stations.

Table 1. CHAIN Station Details
StationLatitude (°N)Longitude (°E)InstrumentsStatusReal-time Data StatusComplete Data Availability
Eureka79.99274.03GPSInstalledSummary data1 month delay
   IonosondeInstalledCompleteNear real time
Resolute Bay74.75265.00GPSInstalledCompleteNear real time
   IonosondeInstalledCompleteNear real time
Pond Inlet72.69282.04GPSInstalledCompleteNear real time
   IonosondeJuly 2009  
Taloyoak69.54266.44GPSInstalledSummary data1 month delay
Cambridge Bay69.12254.97GPSInstalledSummary data1 month delay
   IonosondeInstalledCompleteNear real time
Hall Beach68.78278.74GPSInstalledCompleteNear real time
   IonosondeJuly 2009  
Qikiqtarjuaq67.56295.97GPSInstalledCompleteNear real time
Iqaluit63.73291.46GPSInstalledCompleteNear real time
   IonosondeJuly 2009  
Sanikiluaq56.54280.77GPSInstalledSummary data1 month delay
Ministik Lake53.35247.03GPSInstalledSummary data1 month delay

[6] The CHAIN GPS receivers are GPS Ionospheric Scintillation and TEC Monitors (GISTMs) model. 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 10 GPS satellites in view. Phase and amplitude data are sampled at a rate of 50 Hz. The GSV4004Bs automatically compute and log the amplitude scintillation index, S4, and phase scintillation index, σΦ, computed over 60 s. Phase and amplitude data, either in raw form or detrended, are also logged at 50 Hz. Nine of the 10 receivers are currently fed by a NovAtel GPS-702 antenna. The exception is the receiver at Qikiqtarjuaq which shares an Ashtech ASH701945E_M antenna with a preexisting Natural Resources Canada GPS receiver through a splitter. Examples of the 50 Hz slant TEC variation (uncalibrated and uncorrected for cycle slips) for one day (5 September 2008) from all 10 CHAIN GPS stations are shown in Figure 2.

Figure 2.

An example of daily variation of slant total electron content derived using 50 Hz measurements from all 10 CHAIN GPS stations for 5 September 2008.

[7] The CHAIN ionosondes are Canadian Advanced Digital Ionosondes (CADIs) [MacDougall and Jayachandran, 2001]. The CADI is a modern digital ionosonde, which is capable of providing ionospheric drift measurements along with conventional ionograms. A CADI employs an antenna array consisting of four short dipoles arranged along the sides of a square of 30 m on each side, and one receiver is dedicated to each antenna. Fixed frequency Doppler samples with 64 data points are obtained at 30s intervals and an ionogram of 95 frequencies is obtained every minute. The convection velocity (speed and azimuth) and other echo properties are derived from the fixed frequency Doppler measurements. Examples of the fixed frequency drift measurements from Eureka, Resolute Bay, and Cambridge Bay for 30 August 2008 are shown in Figure 3.

Figure 3.

An example of Canadian Advanced Digital Ionosonde measurements from three CHAIN stations (Eureka, Resolute, and Cambridge Bay) for 30 August 2008. The plots for each station represent group height at a (top) fixed frequency, (middle) convection azimuth, and (bottom) speed.

3. Scientific Objectives of CHAIN

[8] Most of the time, the polar cap ionosphere, a region of open field lines, is directly coupled to the solar wind and interplanetary magnetic field. Mass and energy derived from the interaction are transported across the polar cap and are directly controlled by the variability of solar input (both particle and electromagnetic). Because of this coupling, the polar cap ionosphere is often composed of ionization and electromagnetic structures [Tsunoda, 1988; Crowley, 1996; Aarons, 1997; Basu and Valladares, 1999; Aarons et al., 2000; Jayachandran and MacDougall, 2001; Foster et al., 2005; MacDougall and Jayachandran, 2007]. Understanding of the polar cap ionosphere will go a long way in understanding the SW-M-I coupling and CHAIN scientific objectives are aimed at understanding the polar cap processes. Since we cannot accommodate here (due to lack of space) a thorough description of individual scientific objectives, just the broad-based scientific objectives of CHAIN may be given. They include the understanding of (1) drivers and variabilities of polar cap convection, (2) generation and dynamics of ionization structures in the polar cap (macroscale, tongue of ionization (>1000 km); mesoscale, polar patches (few hundred kilometers); microscale, scintillation producing structures (few kilometers), and (3) role of ionosphere in M-I coupling.

[9] The basic CHAIN measurements, relevant to these scientific objectives, are the 30s resolution measurement of the convection and one minute resolution measurement of the ionograms at the six CADI locations, and GPS-based measurements of TEC and scintillation at 10 stations (50 Hz sampling). As mentioned earlier, one important and unique factor regarding CHAIN is that the 6 of the 10 GPS receivers are collocated with ionosondes. This configuration will allow us to perform more realistic 4D (three spatial dimensions plus time) tomographic inversions of the ionosphere using the MultiInstrument Data Analysis System (MIDAS) [Mitchell and Spencer, 2003; Bust et al., 2007] or the Global Assimilative Ionospheric Model (GAIM) [Hajj et al., 2004] for addressing some of the scientific objectives. CHAIN along with other instruments such as SuperDARN radars, Resolute Bay Incoherent Scatter Radar (AMISR), and array of optical imagers will form a powerful ground-based suite of instruments to study the high-latitude phenomena and SW-M-I coupling.

4. Observation of GPS Scintillation Associated With a Magnetospheric Impulse Event Using CHAIN Measurements

[10] Ionospheric scintillations are rapid random fluctuations of the phase and field strength (amplitude) of radio frequency signals that transit the ionosphere [Hey et al., 1946]. These signal fluctuations are caused by the presence of small-scale electron density structures (irregularities) in the ionosphere [e.g., Aarons, 1982; Tsunoda, 1988; Aarons et al., 2000]. Most of the early radiowave scintillation studies were based on geostationary satellite signals at lower frequencies [e.g., Aarons, 1982]. The recent explosion of GPS-based ionospheric research and the mushrooming use of GPS signals for navigational purposes has put renewed interest on the nature and cause of radiowave scintillations. (For more on GPS and scintillations, see Kintner et al. [2007].) However, recent scintillation studies based on GPS data were conducted for extreme magnetic storm conditions [Mitchell et al., 2005; Meggs et al., 2008]. In geospace, there are other events that can also produce considerable changes in the ionosphere. One such event is the magnetospheric impulse event (MIE) [Lanzerotti et al., 1990]. MIEs have been historically identified using ground magnetic field measurements and long-period Pc5 ground magnetic pulsations is one of the main signatures of MIEs [e.g., Arnoldy et al., 1996]. These MIEs are mostly externally driven and several mechanisms such as magnetic reconnection [Konik et al., 1994] and the impact of dense solar wind with the magnetopause and its penetration into the magnetosphere [e.g., Heikkila, 1982; Sibeck et al., 1989; Kataoka et al., 2003] are proposed. Since these MIEs produce significant changes in the ground magnetic field, it is expected that they will also produce changes in the ionosphere. Impacts of MIEs on GPS signals are relatively unknown. A case study of an MIE-related scintillation event using CHAIN GPS measurements is presented below.

[11] The event of interest occurred between 1400 and 1600 UT on 4 January 2008. Variations of solar wind (SW) parameters using the OMNI database are shown in Figure 4. Figure 4a shows the variations of the SW flow pressure, Figure 4b shows number density, and Figure 4c shows the X component of the SW velocity. It can be clearly seen from Figure 4 that SW pressure increased (primarily due to the increase in the SW density) suddenly around 1445 UT. One point to keep in mind is that the OMNI database has already been corrected for the propagation of the SW variations from the monitor to 1 AU. Temporal variation of the X component of the ground magnetometers for the interval 1400–1600 UT is shown in Figure 5. The magnetometer data used are from four Canadian Array for Realtime Investigations of Magnetic Activity (CARISMA) [Mann et al., 2008] stations, namely, TALO (69.54°N, 266.45°E); RANK (62.82°N, 267.89°E); GILL (56.38°N, 265.36°E); and MSTK (53.35°N, 247.03°E) and they are arranged in decreasing latitudes from top to bottom in Figure 5. It can be seen from Figure 5 that there was a simultaneous magnetic field change (change in polarity between TALO and other stations due to their locations) detected at all these stations around 1450 UT with a maximum change in the magnetic field around 1508 UT. The presence of periodic oscillations (∼15 min periodicity) in the magnetic field following the initial signature can also be noted. These magnetic field variations are typical signatures of MIE [Lanzerotti et al., 1990]. By looking at Figures 4 and 5, one can obviously conclude that the increase in the SW dynamic pressure is the cause of this MIE as suggested by Sibeck et al. [1989].

Figure 4.

Variation of solar wind (a) pressure and (b) number density parameters, and (c) X component of the speed between 1400 and 1600 UT of 4 January 2008. These data are from the Coordinate Data Analysis Web OMNI database, so the time is corrected for the transit time from the monitor to 1 AU.

Figure 5.

Variations of the X component of the magnetic field from four Canadian Array for Realtime Investigations of Magnetic Activity stations between 1400 and 1600 UT on 4 January 2008 in order of decreasing latitude.

[12] Only 2 of the 10 GPS receivers were deployed and providing data during this event. These stations were Cambridge Bay (69.12°N, 254.97°E) and Ministik Lake (53.35°N, 247.03°E–one of the magnetometer stations). Figure 6 shows the carrier-to-noise-density ratio (a measure of the received power), expressed in dB-Hz, of the GPS signals from satellites at two stations (PRN 22 for Cambridge Bay and PRN 26 for Ministik Lake) along with the S4 index (derived from the 50 Hz data). We have selected data above 10 degrees elevation and the elevation angle varied between 20 and 15 degrees for PRN 22 and between 30 and 25 degrees for PRN 26. One can clearly see the start of scintillation activity in the GPS signal around ∼1450 UT at these two stations (along certain raypaths) associated with the MIE. Prior to this, there was no scintillation activity in the GPS signals (a low S4 index). For this event, eight satellites were visible from each of these stations and scintillation activity is observed only on the raypaths in the N–W direction of the station and analysis of the raypath along with a statistical location of the plasmapause showed that the scintillation signature is only seen on the raypath skimming the plasmapause. One point to note here is that we have looked at only one event for this study and in order to make a valid connection between MIE and scintillation and to identify the generation mechanism of scintillations, we have to look at more events and a detailed study is already in progress.

Figure 6.

L1 carrier-to-noise-density ratio (C/N0 in dB-Hz) and calculated amplitude scintillation index (S4) of the GPS signal for (a and b) Cambridge Bay and (c and d) Ministik Lake between 1400 and 1600 UT for 4 January 2008. Vertical lines indicate the onset time of magnetospheric impulse event.

5. Summary

[13] CHAIN is a network of 10 dual frequency high data rate GPS receivers and six CADIs. Six of the 10 GPS receivers are collocated with CADIs providing a unique instrument configuration for detailed high-latitude ionospheric studies. Broader scientific objectives of CHAIN are the understanding of (1) drivers and variabilities of polar cap convection, (2) generation and dynamics of ionization structures (of different temporal and spatial scales), and (3) role of ionosphere in M-I coupling. Ten GPS receivers and three CADIs are now in operation providing near-real-time data. More details of the CHAIN project and data can be found at http://chain.physics.unb.ca/chain. A case study of GPS signal scintillation showed the onset of scintillation associated with a MIE.

Acknowledgments

[14] Infrastructure funding for CHAIN was provided by the Canada Foundation for Innovation and the New Brunswick Innovation Foundation. CHAIN and CARISMA operations are conducted in collaboration with the Canadian Space Agency. Science funding is provided by the Natural Sciences and Engineering Research Council of Canada. We acknowledge NASA's Coordinate Data Analysis Web (CDAWEB) for providing the OMNI solar wind data.

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