High-latitude ionospheric irregularities: differences between ground- and space-based GPS measurements during the 2015 St. Patrick’s Day storm

Global navigation satellite systems (GNSSs), such as GPS and GLONASS, have become a very powerful and well-proved tool for ionosphere diagnostics, both in quiet and in disturbed geomagnetic conditions, because of their global, continuous and permanent measurements and expanding networks of ground-based receivers. As ionospheric plasma irregularities produce phase and amplitude scintillations of a received radio signal, the GNSS/GPS technique offers a relatively cheap and high-resolution method for tracking these irregularities and permanently monitoring them on a regional and global scale (e.g., Pi et al. 1997; Aarons and Lin 1999; Valladares et al. 2004; Jakowski et al. 2012; Cherniak et al. 2014).

The most intense ionospheric irregularities in the high-latitude ionosphere are caused by plasma processes associated with auroral activities, attributed to auroral particle precipitation, and dynamical processes including high-speed plasma convection (e.g., Phelps and Sagalyn 1976; Fejer and Kelley 1980; Keskinen and Ossakow 1983). The steep ionospheric density gradients and irregularities associated with auroral and cusp precipitation in the high latitude cause phase and amplitude fluctuations of GPS signals and have extensively been studied using ground-based GPS data (Skone and Cannon 1995; Mitchell et al. 2004; Alfonsi et al. 2011; Watson et al. 2011; Jiao et al. 2013; Prikryl et al. 2013, 2014; Tiwari et al. 2013; Cherniak et al. 2014; Jacobsen and Andalsvik 2016). However, there are still several unresolved limitations associated with ground-based GPS monitoring of ionospheric irregularities. Firstly, there is a relative scarcity of GPS stations, particularly in the Southern Hemisphere, over oceans, and difficult terrain such as the mainland of Greenland or Antarctica. Secondly, if we use only ground-based GPS data, it is difficult to determine the altitudinal range where the observed ionospheric irregularities occur and develop. These limitations mean that ground-based GPS observations cannot be used to answer the questions of when and where high-latitude topside ionospheric plasma irregularities are developed, or the physical mechanisms of their origin.

One possible solution to these problems is to combine ground-based GPS data with space-borne measurements. Observations from a GPS receiver with a zenith-looking antenna placed onboard a low earth orbit (LEO) satellite offer an opportunity to separate the ionospheric irregularities registered above a LEO from that on the ground. Earlier, Noja et al. (2013), and Zakharenkova and Astafyeva (2015) demonstrated that space-borne GPS measurements onboard a solo-satellite mission (CHAMP) could successfully be used to study the occurrence of high-latitude topside ionospheric irregularities. In this paper, we demonstrate the advantages of using LEO multi-satellite missions for monitoring high-latitude ionospheric irregularities. Our focus was on the successful European Space Agency (ESA) Swarm mission, launched in November 2013, and currently in orbit for more than 2 years. Swarm is a constellation of three identical satellites that orbit at two altitudes of ~460 and ~510 km; the upper satellite (Swarm B) slowly separates in local time from the tandem satellites (Swarms A and C). We analyze the differences in the spatial distribution of ionospheric irregularities detected by the ground-based permanent GPS network with that of the topside GPS measurements onboard the Swarm constellation, using a case study of the 2015 St. Patrick’s Day storm. Additionally, we include GPS measurements from Gravity Recovery and Climate Experiment (GRACE) and TerraSAR-X (SAR—synthetic aperture radar) satellites that orbit at similar altitudes to Swarm.

The 2015 St. Patrick’s Day storm occurred on March 17, 2015, and caused a dramatic response in the ionosphere–plasmasphere–magnetosphere system. To date, it is the strongest geomagnetic storm in 2 years of Swarm operation. The space weather conditions during the event are detailed in several recent papers (Kamide and Kusano 2015; Liu et al. 2015a; Zhang et al. 2015; Cherniak et al. 2015; Jacobsen and Andalsvik 2016). Figure 2 shows the variations in selected interplanetary magnetic field (IMF) and geomagnetic parameters during March 15–20, 2015. The sudden storm commencement (SSC) was registered at ~0445 UT, and then, there was a rapid drop in the SYM-H index to −226 nT at ~2300 UT, with a couple of local minima of −93 and −164 nT at ~0940 and 1740 UT, respectively.

The behavior of the IMF Bz component is shown in Fig. 2 (top panel). After the SSC, the northward IMF Bz component reached a value of ~25 nT. At ~0530 UT, the IMF Bz turned southward and reached the first minimum value of −18 nT at 0615 UT. Then, the IMF Bz turned sharply northward and switched extensively between north and south over ~8 h. After ~1340 UT, the Bz turned southward again and remained southward until the end of the day.

The auroral electrojet index (AE) depicts two intensification peaks at ~09 and 14 UT. Between these two peaks, Bz was observed to turn northward. The mid-latitude magnetic activity index Kp (not shown here) reached a value of 8. The strong disturbance of the geomagnetic field on March 17, 2015, led to intense particle precipitation and an enhancement in substorm activity. It was reported that during March 17–18, 2015, auroras were observed at different locations around the globe, even at mid-latitudes as equatorward as Tasmania and New Zealand in the Southern Hemisphere as well as in the USA, Europe and Japan in the Northern Hemisphere (e.g., Liu et al. 2015b; Kamide and Kusano, 2015; Nishitani et al. 2015; Shiokawa and Otsuka 2015; GUVI TIMED JHU/APL website http://guvitimed.jhuapl.edu, 2015; USGS NGP website http://geomag.usgs.gov/storm/22, 2015 and links therein).

In this study, we have compared the pattern of ionospheric irregularities derived from ground-based GPS measurements with daily ROTI maps constructed from multi-satellite LEO–GPS data. We found major differences between the shapes of irregularities’ structures observed by these techniques. During the main phase of the March 2015 geomagnetic storm, ground-based ROTI maps indicated that the zone of intense ionospheric irregularities was expanded and moved equatorward comparing to quiescent conditions. The oval pattern of ionospheric irregularities occurred due to plasma density gradients caused by storm-induced particle precipitation and subsequent ionization of the neutral components. We found not only oval patterns of ionospheric irregularities, but also radial structures oriented from the local noon sector to midnight. These structures do not relate to the particle precipitation mechanism and cannot be represented by energetic particle precipitation models.

It is known that radial structures oriented in the day–night direction can be associated with the formation of storm-enhanced density (SED) and further evolution of the SED plume to the polar tongue of ionization (TOI), which follows the convection pattern anti-sunward across the polar cap. Figure 3b confirms the presence of TOI structures in the post-noon sector in both hemispheres, but slightly more intensive in the Southern Hemisphere. SED has been described as a region of enhanced plasma density observed in the post-noon and pre-midnight sectors extending from the equatorward edge of the main ionospheric trough to the noontime cusp (Foster 1993; Coster et al. 2007). The dayside source of the TOI is the SED plume transported from lower latitudes in the post-noon sector by enhanced storm-time electric fields of the subauroral polarization streams (SAPS) (Foster and Burke 2002). The SED latitude decreases with increasing local time and disturbance level (Foster et al. 2005). Anti-sunward convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material (Foster et al. 2005).

For more detailed study of the SED/TOI appearance during March 17, 2015, in the context of two-cell plasma convection, we compared Super Dual Auroral Radar Network (SuperDARN) polar potential maps with satellite observations. Here, we consider Swarm Ne measurements and two kinds of LEO–GPS observations, topside TEC and ROTI, derived from Swarm, GRACE and TerraSAR-X POD GPS data. To enable visualization of the SuperDARN polar potential maps in the geographic coordinate system, their CGM coordinates were recalculated back to geographic ones.

Figure 5a (left) shows the SuperDARN polar potential map for the Southern Hemisphere at 18.4 UT, with superimposed Swarm A track showing geo-referenced values of electron density and topside ROTI. We observe an extreme enhancement in the Swarm in situ plasma density. We also have two passes with satellite-to-satellite tracking between Swarm A and two GPS satellites (PRN 19 and 21). ROTI values along these passes depict significant increases close to the region with the observed Ne peak. The right-hand panel of Fig. 5a presents detailed variations of electron density (Ne) along Swarm A satellite track, as well as relative TEC for two GPS pass projections. For both GPS satellites, the relative TEC values show a considerable enhancement in magnitude. We should also note that Swarm C, separated from Swarm A by ~10 s in time, shows the same extreme enhancement in electron density as Swarm A. Geographically, all the enhancements identified are located between lobes of the plasma circulation cells on the dayside part of the convection pattern (Fig. 5a, left). But it was not a single event.

About half an hour later, Swarm B satellite fortunately passes through the same region and registers a multiple-peak structure in plasma density with a peak value of 1.67 × 10⁶ el/cm³. This peak density is even higher than that of Swarm A, even despite the fact that the altitude of Swarm B was ~50 km higher (i.e., smaller density might be expected). As is known, when polar patches in the ionospheric F2 region (Tsunoda 1988) are as dense as 10⁶ el/cm³, they cause severe communication/navigation outages when they intersect radio links between receiver and satellite (e.g., Weber and Buchau, 1981; Basu et al. 1988). We should note that while Swarm B crossed these extreme enhancements, there was observed degradation of GPS performance: The number of tracked GPS satellites suddenly decreased from 8 (the maximal number for Swarm GPS receiver) to 0–1. That is the reason we cannot present any collocated GPS tracks for Swarm B satellites. Regarding GPS performance on the Swarm A (C) satellites, a decrease from 8 to 4 (3) tracked GPS satellites was also observed when Swarm crossed this polar patch/TOI structure at 18.35 UT (18:21 UT). Aside from lost Swarm B data, we consider several links to GPS satellites from GRACE and TerraSAR-X. All these GPS tracks are located close to the central part of the two-cell convection pattern, and high ROTI values indicate that they cross large-scale polar patches traveling already from local noon, which was most closely intersected by Swarm B (Ne track), to midnight direction. Significant increases in the topside TEC along these tracks (Fig. 5b, right) reveal that these extreme enhancements, or polar patches, are well developed in the topside ionosphere above ~500 km altitude.

From Fig. 4, we saw that intensity of polar patches in both hemispheres was quite different and Swarm Ne data depicted the dominance of the most intense polar patches in the Southern Hemisphere. Figure 5c shows an example of topside plasma enhancement observed at 18 UT over the Northern Hemisphere. It is clearly seen that observed effects in Ne and TEC are much smaller in magnitude than in the Southern Hemisphere. The Swarm A track crossed the two-cell convection pattern in a different direction, and more rapid and intense Ne fluctuations are observed close to exit to the midnight side. Here, maximal values of plasma density enhancements inside the polar cap are ~0.2 × 10⁶ el/cm³, which are quite comparable with the surrounding background density. LEO TEC enhancement observed with Swarm A, GRACE and TerraSAR-X measurements is much lower than in Fig. 5a, b. These topside plasma enhancements are accompanied by strong topside irregularities detected by the LEO ROTI measurements.

We examined all other temporal intervals until 06 UT of March 18 (not shown here) and did not reveal such extreme enhancements in plasma density over the Northern Hemisphere as observed in the Southern. This interhemispheric asymmetry can be explained by a strong positive storm effect observed in the Southern Hemisphere in comparison with a negative storm over the North America region (e.g., Astafyeva et al. 2015). The storm-induced plasma enhancement at equatorial and mid-latitudes of South America could support formation of much higher plasma densities in SED/TOI structures (compared with that observed in the Northern Hemisphere) transported toward the Southern polar cap.

Analysis of the combined ground-based GPS and LEO measurements (daily ROTI maps from ground and LEO, 2 h resolution geographical ROTI maps, LEO TEC and in situ electron density) demonstrates that during this particular storm large-scale plasma density enhancements were observed in the topside polar ionosphere of both hemispheres. These enhancements could be most probably associated with SED development and further plasma transportation by subauroral polarization streams in the form of TOI.

We should note that, to date, SED/TOI observations during the March 2015 storm have been reported only for the Northern Hemisphere. Here, we present the first observations of the much more intense SED/TOI structures in the Southern Hemisphere, which were detected mainly due to a fortunate Swarm position.

Another important feature is that the ionospheric SED observed in the Northern Hemisphere during the March 2015 storm was mainly registered above the F2 layer peak (Liu et al. 2015a). Liu et al. (2015a) found that this SED event was identified as a SED in TEC but not in F2 layer peak density NmF2. From Millstone Hill Incoherent Scatter Radar (ISR) measurements, they reported a large increase in electron densities above the F2 peak (>500 km) between 18 and 21 UT, corresponding to the SED passage in TEC over the North American sector. Zhang et al. (2015) studied neutral wind disturbances in subauroral and mid-latitudes during this storm. They observed SED plumes in TEC and ISR electron density enhancements over the northeast US prior to 21 UT before passage of the mid-latitude trough. A strong westward ion drift (500–750 m/s), identified as SAPS, had developed during 21–02 UT, as it was observed by the Millstone Hill ISR. It is worth noting that these SED/TOI structures were well observed in ground-based GPS TEC maps constructed with high spatial resolution (e.g., Liu et al. 2015a; Cherniak et al. 2015).

There are several physical mechanisms for the formation of ionospheric irregularities in the polar ionosphere during TOI development, in particular Kelvin–Helmholtz and gradient-drift instabilities (e.g., van der Meeren et al. 2014). Sojka et al. (1998) discussed one such mechanism related to plasma motion based on numerical simulation results by the Utah State University Time-Dependent Ionospheric Model. They considered the gradient-drift instability (GDI) as a primary candidate for the generation of these irregularities and showed correlation of GDI with TOI and polar cap patches. They suggested that this mechanism of generating plasma irregularities in the polar ionosphere caused scintillation of transionospheric communication links.

For the March 17, 2015 storm, both factors of the fast neutral and plasma flows, confirmed by the Millstone Hill ISR measurements together with topside plasma enhancement (derived from Swarm LP and topside GPS TEC measurements), support that favorable conditions for GDI development caused the occurrence of the plasma density irregularities in the topside ionosphere.

Recently, Spicher et al. (2015) used Swarm in situ measurements to observe the persistence of more than 90-min kilometer-scale gradients (day and nighttime polar patches) in the Northern polar region. They identified the presence of GDI and obtained GDI growth times using along-track velocities for a test case of December 29, 2013. The maximum magnitude of electron density inside the polar patches was less than 1.4 × 10⁵ el/cm³. Goodwin et al. (2015) reported on polar patch observation in Swarm Ne data with a top magnitude of ~6.0 × 10⁴ el/cm³. For the case of the March 2015 storm, we revealed polar patches in the Southern Hemisphere with peak Swarm Ne measurements at least 10 times higher than was observed previously in Swarm data.

We conclude that an analysis of concurrent ground-based and LEO–GPS measurements offers a possible way to separate high-latitude ionospheric irregularities caused by direct particle precipitation from those developed due to dynamic plasma processes in the polar ionosphere of both hemispheres. Routine LEO–GPS measurements can provide unprecedented opportunities to further explore this phenomenon and effectively support radar data for solving components of SED/TOI structure modeling.