A Swarm case study - Wiley Online Library

Geophysical Research Letters RESEARCH LETTER 10.1002/2014GL062590

Special Section: ESA’s Swarm Mission, One Year in Space

Observation of polar cap patches and calculation of gradient drift instability growth times: A Swarm case study A. Spicher1 , T. Cameron2 , E. M. Grono2 , K. N. Yakymenko3 , S. C. Buchert4 , L. B. N. Clausen1, D. J. Knudsen2 , K. A. McWilliams3, and J. I. Moen1 1 Department of Physics, University of Oslo, Oslo, Norway, 2 Department of Physics and Astronomy, University of Calgary,

Key Points: • New technique to assess GDI/polar cap plasma structuring using Swarm • Internal kilometer-scale structures persist as patches convect across the polar cap • The GDI can act quickly on several kilometer-scale gradients within polar cap patches

Correspondence to: A. Spicher, [email protected]

Citation: Spicher, A., T. Cameron, E. M. Grono, K. N. Yakymenko, S. C. Buchert, L. B. N. Clausen, D. J. Knudsen, K. A. McWilliams, and J. I. Moen (2015), Observation of polar cap patches and calculation of gradient drift instability growth times: A Swarm case study, Geophys. Res. Lett., 42, 201–206, doi:10.1002/2014GL062590.

Received 21 NOV 2014 Accepted 15 DEC 2014 Accepted article online 18 DEC 2014 Published online 20 JAN 2015

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

SPICHER ET AL.

Calgary, Alberta, Canada, 3 Institute of Space and Atmospheric Studies, Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Canada, 4 Swedish Institute for Space Physics, Uppsala, Sweden

Abstract

The Swarm mission represents a strong new tool to survey polar cap patches and plasma structuring inside the polar cap. In the early commissioning phase, the three Swarm satellites were operated in a pearls-on-a-string configuration making noon-midnight transpolar passes. This provides an unparalleled opportunity to examine the potential role of the gradient drift instability (GDI) process on polar cap patches by systematically calculating GDI growth times during their transit across the pole from day to night. Steep kilometer-scale gradients appeared in this study as initial structures that persisted during the approximate 90 min it took a patch to cross the polar cap. The GDI growth times were calculated for a selection of the steep density gradients on both the dayside and the nightside. The values ranged from 23 s to 147 s, which is consistent with recent rocket measurements in the cusp auroral region and provides a template for future studies. Growth times of the order of 1 min found both on the dayside and on the nightside support the existing view that the GDI may play a dominant role in the generation of radio wave scintillation irregularities as the patches transit the polar cap from day to night.

1. Introduction It is well known that the high-latitude F region is populated by field-aligned irregularities (FAIs) that can disrupt communication and navigation systems [e.g., Hey et al., 1946; Basu et al., 1988, 1990; Kintner et al., 2007; Prikryl et al., 2011]. The study of these irregularities is thus important for space weather applications. Two macroinstabilities are thought to be the dominant mechanisms for the development of irregularities in the high-latitude F region [e.g., Tsunoda, 1988; Basu et al., 1990, 1994; Hosokawa et al., 2013]: the gradient drift instability (GDI) and the Kelvin-Helmholtz instability (KHI). The GDI is caused by a differential drift of the ions and the electrons perpendicular to a plasma density gradient [Keskinen and Ossakow, 1983]. Density perturbations are driven unstable if the bulk plasma motion is parallel to the preexisting plasma density gradient [Tsunoda, 1988]. The KHI occurs in the presence of velocity shear flows in the direction perpendicular to the magnetic field [Keskinen et al., 1988]. FAIs ranging from tens of kilometers to a few meters are often embedded in larger-scale structures referred to as polar cap patches [Cerisier et al., 1985; Basu et al., 1990, 1994]. Traditionally, polar cap patches are defined as islands of F region plasma density which are several hundred kilometers in size and with density more than double that of the background [Crowley, 1996]. After their creation, the patches follow the polar convection pattern across the polar cap. Observations by incoherent scatter radars presented by Lockwood and Carlson [1992] show patches as islands of enhanced plasma density with sharp leading and trailing edges. Based on these observations, Gondarenko and Guzdar [2004] performed three-dimensional numerical simulations of the structuring of polar cap patches. They concluded that for a constant velocity across the polar caps, small-scale irregularities would be initiated at a single trailing edge and advance towards the leading edge, structuring the patch from back to front. These numerical results are in accordance with observational findings by Weber et al. [1984] who showed that the strongest irregularities are often observed on the trailing edge of the patch. The instrument packages on the new Swarm satellites allow the investigation of internal structuring throughout polar cap patches. GDI linear growth times have been calculated in the past with a combination of satellite or rocket plasma density data and ground-based plasma drift velocity data [Moen et al., 2012]. ©2014. The Authors.

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Swarm’s pearls-on-a-string configuration makes it possible for the first time to calculate GDI growth times using in situ data exclusively.

2. Instrumentation Launched on 22 November 2013, Swarm is a constellation of three satellites whose primary goal is to study Earth’s intrinsic magnetic field, upper atmosphere, and ionosphere. Each satellite has an identical set of instruments [Friis-Christensen et al., 2008]. In this study, we use data from the Electric Field Instrument (EFI), which is comprised of two parts. Langmuir probes measure electron density and temperature at two samples per second. The thermal ion imagers measure ion flow velocity and temperature. The EFI also provides a measurement of ion density with the negatively biased front faceplate of the instrument at 16 samples per second. The ion density is cross calibrated using the electron density data from the Langmuir probes. This study focuses on the plasma density measurements in order to study instability growth. The data presented here are from a 2 month period when Swarm had yet to be maneuvered into its final arrangement. The satellites were flying as pearls-on-a-string in nearly polar circular orbits at an altitude of approximately 490 km. Swarm B was 58 s ahead of Swarm A and 164 s ahead of Swarm C, corresponding to about 435 km and 1230 km, respectively. The maximum longitudinal separation between the satellites was of the order of 10 km in the region of interest. Additionally, data from the SuperDARN (Super Dual Auroral Radar Network) [Greenwald et al., 2006] were used. SuperDARN consists of high frequency (HF) coherent radars sensitive to decameter FAIs. The main product of the SuperDARN is global-scale plasma convection maps. The maps are derived from the Doppler velocity measurements of backscatter echoes using the “map-potential technique” described by Ruohoniemi and Baker [1998]. The echo power data from a single SuperDARN radar located at Rankin Inlet, Canada (62.82◦ N, 93.11◦ W geographic coordinates, 72.96◦ N, 28.17◦ W corrected geomagnetic coordinates (CGM), were also investigated. This radar monitors the dynamics of the polar cap. The Longyearbyen all-sky imager (ASI) located at 78.15◦ N, 16.04◦ E (75.24◦ N, 111.21◦ E CGM coordinates) on Svalbard, Norway, was also used. The ASI is a high-resolution camera that images the aurora in both 557.7 nm and 630.0 nm images. It is ideally positioned to observe polar cap patches convecting toward and into the nightside auroral oval.

3. Observations On 29 December 2013, Swarm completed 14 crossings of the northern polar cap, of which two consecutive crossings were selected for this study. During both passes, Swarm crossed the northern polar cap from noon to midnight. Figure 1a depicts the intersection of Swarm’s trajectory through the field of view (FOV) of the Rankin Inlet SuperDARN in magnetic latitude (MLAT) and magnetic local time coordinates. The satellites’ flight path is shown as a green dashed line through the color-coded HF radar backscatter power obtained by SuperDARN at 17:57:00 universal time (UT). The direction in which the satellites were moving is shown with green arrows. Poleward of a broad region of high backscatter power in the near ranges, the radar observed two regions of enhanced backscatter power (highlighted by two red arrows in Figure 1a). Subsequent SuperDARN scans show that these structures convected poleward along the convection streamlines. As the Swarm satellites crossed these moving regions of strong radar backscatter power, they observed two plasma density enhancements up to 8 times larger than the background density inside the polar cap. These two plasma density enhancements are marked by a horizontal blue line in Figure 1b, showing the plasma density measurements made by Swarm with respect to geomagnetic latitude. This part of the orbit is also plotted in blue in Figure 1a. The antisunward displacement in latitude of the plasma density enhancements between subsequent satellites passes indicates a poleward movement consistent with the SuperDARN observations. In this study, focus is placed on the second plasma density enhancement, which has distinguishable leading and trailing sides and was encountered by Swarm A between ∼ 84.1◦ MLAT and ∼ 86.5◦ MLAT and between 17:56:47 UT and 17:57:32 UT. This second plasma density enhancement had a sharp leading edge and a significantly structured trailing side; the latter being collocated with strong backscatter power (see Figure 1a). SPICHER ET AL.

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Figure 1. (a) Rankin Inlet SuperDARN backscatter power at 17:57:00 UT for the first Swarm pass on the dayside, with the Swarm A trajectory superimposed as a green dashed line. The direction of motion of the satellites is represented by green arrows. The blue portion identifies the patch region. Red arrows indicate the position of two regions of high backscatter power. (b) Plasma density measurements of the dayside pass for Swarm A, B, and C in blue, black, and red, respectively. The density enhancements are underlined in blue. The dayside patch analyzed in this study was encountered by Swarm A between 17:56:47 UT and 17:57:32 UT, corresponding to ∼ 84.1◦ and ∼ 86.5◦ geomagnetic latitude. (c) Longyearbyen ASI frame from the nightside at 19:37:00 UT of the second Swarm pass with the Swarm C trajectory superimposed as a green dashed line. The direction of motion of the satellites is represented by a green arrow. The blue portion identifies the patch region. (d) Plasma density measurements of the nightside pass for Swarm A, B, and C in blue, black, and red, respectively. The nightside patch is marked by a horizontal blue line and was encountered by Swarm A between 19:35:45 UT and 19:37:27 UT.

About 90 min after the Northern Hemisphere crossing shown in Figures 1a and 1b, Swarm crossed the FOV of the Longyearbyen ASI on the nightside near local midnight. Figure 1c shows the satellite’s trajectory as a dashed green line within the FOV of the 19:37 UT ASI image. The direction in which the satellites were flying is shown with a green arrow. During this pass, the imager observed a patch of airglow moving equatorward toward the auroral oval. As the satellites crossed the airglow patch, which had a luminosity of about 600 R, they encountered a plasma density enhancement about 6 times larger than the polar cap background (see Figure 1d, showing plasma density with respect to magnetic latitude). Again, the plasma density enhancement is marked by a blue line. Swarm A encountered the patch between 19:35:45 UT and 19:37:27 UT. The sequential plasma density measurements of Swarm B, A and C, as well as ASI observations, indicate that the enhancement was traveling antisunward. Like on the dayside, the enhancement featured a sharp leading edge and an elongated trailing side with many kilometer-scale density structures. The plasma density enhancements encountered by the satellites on the dayside during the first orbit and on the nightside during the second orbit were polar cap patches emerging from the cusp and convecting across the northern polar cap.

4. Discussion Two polar cap patches were identified in consecutive Swarm passes over the Northern Hemisphere, one on the dayside and one on the nightside. The dayside patch was collocated with strong backscatter from the Rankin Inlet SuperDARN HF radar. The presence of radar backscatter indicates the existence of decameter-scale irregularities which are likely linked to the kilometer-scale gradients observed in situ by the Swarm satellites. The Longyearbyen ASI observed airglow propagating toward the auroral oval as Swarm crossed its FOV. The airglow is the signature of the nightside polar cap patch, as confirmed by in situ SPICHER ET AL.


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measurements of the Swarm satellites. This fortuitous arrangement provides an unprecedented opportunity to study polar cap patch evolution using in situ measurements supported by ground-based observations. Traditionally, newly created patches have been considered as plasma density enhancements with a steep leading edge and a steep trailing edge [e.g., Tsunoda, 1988; Weber et al., 1984; Gondarenko and Guzdar, 2004]. In contrast, the present case study reveals that the trailing side is already composed of several steep gradients even near its origin on the dayside. This provides conditions for the GDI to operate at more than one location in the patch. Kelley et al. [1982] suggested that soft electron precipitation in the cusp could be able to create such steep structures within a polar cap patch already at the time of their creation. He estimated that the lifetime of these structures would be in the order of hours, and hence, they could serve as the base for the GDI process when patches are transported across the winter polar cap. Carlson et al. [2007] proposed that shear-driven processes such as the Kelvin-Helmholtz instability (KHI) could rapidly prestructure the polar cap patch entering the polar cap. The GDI would then feed off these initial structures. However, the initialization of plasma structures within the cusp formation region is beyond the scope of this letter. The latitudinal displacement of the nightside patch observed by the three Swarm satellites and their temporal separation provides an estimate of 400 m/s for the along-track velocity of the patch. Using the temperature data from the EFI (not shown), the location of the open-closed boundary was estimated to be at about 82◦ MLAT. The nightside patch being encountered at ∼ 78◦ MLAT on the nightside, its travel distance in the polar cap was ∼ 20◦ in latitude. Assuming a constant velocity, its travel time was about 90 min. According to the modeling studies of the nonlinear evolution of the GDI by [Gondarenko and Guzdar 2004], the GDI starting on a steep trailing edge on the dayside progresses through the patch while it is convected across the polar cap. When the instability reaches the leading edge, the scale size of the structures on both edges become similar, reducing the asymmetry between the leading and trailing side. An important feature of their simulations is that in the nonlinear late-time phase, the patch is not disintegrated and maintains some robustness, even though it is fully structured. As expected, the trailing edge of the nightside patch observed by Swarm was significantly structured. However, the patch was not yet fully structured at the time of observation on the nightside: it still featured a sharp leading edge and an elongated shallow trailing side where many kilometer-scale gradients persisted. This may indicate that the GDI was still active, generating smaller-scale structures that can produce radio wave scintillation irregularities. Due to the pearls-on-a-string configuration, Swarm was able to measure both density and along-track velocity. This allows GDI growth times to be calculated using purely in situ data. The growth time 𝜏GDI in the high-latitude F region with k ⋅ B = 0 (flute mode) is given by [Tsunoda, 1988] 𝜏GDI =

L v0

(1)

where v0 is the plasma velocity relative to the neutral atmosphere and L is the gradient length scale. The gradient length scale is given by L = N0

(

ΔN Δx

)−1

(2)

N0 represents the background density, and ΔN∕Δx is the horizontal density gradient. No measurements of the neutral wind were available, so the velocity of the neutrals was neglected, as done by Moen et al. [2012] and Oksavik et al. [2012]. Heelis et al. [2002] reported that the neutral wind speed could reach 60% of the plasma bulk speed during intervals of strong ionospheric convection, which would increase the growth times for density gradients moving in the same direction as the convection flow. In that case, a neutral wind of 60% of the plasma bulk speed would increase the GDI growth times by a factor of 2.5. In order to obtain a better estimate of the growth time, the neutral velocity measured by Swarm’s accelerometer (not available at the time of writing) should be included in future calculations.

The density gradients selected for the growth time calculation are highlighted in Figure 2. These gradients were chosen because they were observed by all three Swarm satellites, and this was essential for determining the along-track velocity. For each gradient, the along-track velocity was calculated separately, as well as the growth times for the three satellites. The average growth time was 46 s for the dayside patch, SPICHER ET AL.


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and values ranged from 23 s to 83 s. On the nightside, the mean growth time was 60 s and values ranged from 30 s to 147 s. These values, which are consistent with the 47.6 s reported by Moen et al. [2012] using rocket data, indicate that the GDI can act rapidly on these gradients, both on the dayside and the nightside. The preferred direction of the structures seen in both our dayside and nightside patches as well as the short growth times suggest that the GDI may be a dominant instability mechanism at multiple steep gradients inside every patch. Kivanç and Heelis Figure 2. Density measurements of the dayside and nightside patches [1997] studied 18 patches using the showing the gradients selected for GDI growth time calculations. Only Dynamics Explorer 2 and found the Swarm B is shown, though the same gradient features were used for presence of structures on most of their Swarm A and C. patches regardless of their position with respect to the cusp. They also found that on average the level of structure was comparable on the leading and trailing edges. Jin et al. [2014] studied polar cap patches and GPS scintillations and reported that most of the patches entering the night sector are structured (giving rise to radio scintillations), and sometimes, the entire patch is unstable, while at other times just parts of the patch are unstable, and it could be either or both the leading and the trailing edges that are unstable. Explanations offered for such variability in patch structuring are, for example, that the GDI is associated with changes in the convection pattern [Gondarenko and Guzdar, 2004] or alternatively that plasma gradients may be initialized by cusp particle precipitation [Kelley et al., 1982] and flow shears [Carlson et al., 2007]. In order to obtain ionospheric scintillation forecast models, there is a need for new tools to systematically investigate plasma instabilities, and we have demonstrated the strength of the Swarm mission to further study the potential role of the GDI mechanism.

5. Conclusion Using the Swarm satellites, two polar cap patches were observed in consecutive passes over the Northern Hemisphere, one on the dayside and one on the nightside. This allowed for a first-ever in situ comparison study of a dayside and a nightside patch observed within a time interval of about 90 min. Steep kilometer-scale gradients appeared as initial structures throughout the trailing side of the dayside patch. Similar gradients are also observed in the nightside patch, so these gradients persisted during the approximate 90 min it took the nightside patch to cross the polar cap. Swarm’s pearls-on-a-string configuration provided an unparalleled opportunity for the calculation of the along-track velocities of these gradients, which opens the possibility of systematically obtaining GDI growth times using purely in situ data. With this new technique, GDI growth times were calculated for a selection of steep gradients on both dayside and nightside patches, providing a template for future statistical studies. The technique presented here can easily be automated and used to derive global maps of growth times once the full Swarm data set is available, which will represent an unprecedented opportunity for future scintillation studies. The values found here were of the order of 1 min, indicating that the gradient drift instability was active both on the dayside and the nightside. This supports the existing view that the GDI plays a dominant role in the structuring of polar cap patches and may produce radio wave scintillation irregularities. However, there is a need to revise the traditional perspective on how the GDI operates on polar cap patches and take into account that it may be operating on kilometer-scale structures already from the cusp inflow region as proposed by Kelley et al. [1982] and Moen et al. [2012]. Additionally, results of the new GDI calculation technique can be used as realistic inputs to simulations of plasma patch dynamics. SPICHER ET AL.


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Geophysical Research Letters Acknowledgments Swarm is a European Space Agency (ESA) project. L.B.N.C. and the imager at LYR are supported by the Research Council of Norway under contract 230935. The imager data are available at http://tid.uio.no/plasma/aurora/. The authors acknowledge the use of SuperDARN data. SuperDARN is a collection of radars funded by National Scientific Funding agencies of Australia, Canada, China, France, Japan, South Africa, United Kingdom, and United States of America. Rankin Inlet SuperDARN radar operations are supported by the Canadian Space Agency and the University of Saskatchewan. We thank Jonathan Burchill for critical reading of the manuscript and Rune Floberghagen for helpful advice. This work is related to the 4DSpace Strategic Research Initiative at the University of Oslo. Data used in this study are available from ESA. The work would not have been possible without Canada-Norway mobility funds provided by the International Cooperation in Education (SiU) project NNA-2012/10999. The project has also received economic support from the Research Council of Norway, grant 230996. The Editor thanks Knut Jacobsen and an anonymous reviewer for their assistance in evaluating this paper.

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