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Keywords:

  • equatorial spread-F

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References

[1] The Naval Research Laboratory three-dimensional simulation code SAMI3/ESF is used to study the response of the post-sunset ionosphere to electrified mesoscale traveling ionospheric disturbances (MSTIDs). An MSTID is modeled as an externally-imposed traveling-wave E field with wavelength 250 km and period 1 h that drives vertical E × B drifts of up to ±50 m/s. We find that the coupling between the MSTID at low- to mid-latitudes and the equatorial F layer leads to growth of equatorial plasma bubbles (EPBs). This coupling is strongest when the wave vector is perpendicular to the geomagnetic field. Model results reproduce key features of observed nighttime MSTIDs and associated EPBs.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References

[2] Equatorial spread F (ESF) [Haerendel, 1974; Ossakow, 1981; Hysell, 2000; Makela, 2006] is a post-sunset phenomenon in which the equatorial F-region ionosphere becomes unstable: large-scale (∼10 km) electron density “bubbles” can develop and rise to high altitudes (∼1000 km). These quickly evolve to produce steep plasma density gradients that can scatter radar signals [Booker and Wells, 1938] and disrupt radio waves [Kintner and Ledvina, 2005]. While the circumstances in which ESF is likely to occur are generally well-known, specific triggering mechanisms are still under study. Numerous possible (and possibly inter-related) triggering mechanisms have been identified: gravity waves [Huang and Kelley, 1996; Fritts et al., 2009], circular gravity waves [Tsunoda, 2010], sheared plasma drift velocities [Huang and Kelley, 1996], a combination of velocity shear and a zonal E field [Sekar and Kelley, 1998], a collisional-shear instability [Hysell and Kudeki, 2004], a sporadic E-layer instability [Tsunoda, 2007], and large-scale wave structure in the bottomside ionosphere [Tsunoda, 2005]. Some of these have been observed in association with ESF [Saito and Maruyama, 2007; Fritts et al., 2009]. In this Letter, we use the the Naval Research Laboratory SAMI3/ESF code [Huba et al., 2008] to test a recently hypothesized triggering mechanism for ESF. Specifically, we simulate the effect of a mesoscale traveling ionospheric disturbance (MSTID) on the post-sunset ionosphere as suggested by Miller et al. [2009].

[3] We specifically consider nighttime MSTIDs of the type that are formed at northern midlatitudes with southwest propagating phase fronts. These are often attributed to a Perkins-type plasma instability [Perkins, 1973] and have been observed in images of nighttime airglow on numerous occasions [Miller et al., 1997; Taylor et al., 1998; Makela et al., 2010]. Although there is still controversy on the exact mechanism responsible for the MSTID structures, a consistent observational database has been amassed over the years. They manifest as horizontally-propagating oscillations in airglow and in the height of the electron density peak with typical wave periods 30 min < τ < 90 min and typical wavelengths 150 km < λ < 500 km [Garcia et al., 2000; Shiokawa et al., 2003]. Satellite and radar measurements have demonstrated that MSTIDs have associated polarization E fields [Saito et al., 1995; Kelley et al., 2000; Shiokawa et al., 2003] and field-aligned irregularities [Saito et al., 2008]. The strong fields are collocated with the dark bands seen in 630.0 nm imagery, which are regions of low Pedersen conductivity [Makela and Kelley, 2003]. These electric fields can extend along the geomagnetic field, causing conjugate MSTIDs as observed by Otsuka et al. [2004]. The amplitude of the electric field is such to cause E × B drift amplitudes as large as 100 m/s [Kelley et al., 2000].

[4] In this Letter we describe the imposition of an MSTID on the SAMI3/ESF model ionosphere. Here, an MSTID is an externally-imposed traveling-wave E field with wavelength 250 km and period 1 h that drives vertical E × B drifts of up to ±50 m/s. We will show that the coupling between the MSTID at low- to mid-latitudes and the F layer at the equator leads to growth of equatorial plasma bubbles (EPBs), reproducing key features of observed MSTIDs and associated EPBs. We find that coupling to the F layer is strongest when the wave vector is perpendicular to the geomagnetic field and is removed entirely if the MSTID is at such a high latitude that it does not influence the bottomside F layer.

2. Observations

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References

[5] Imaging observations of the 630.0-nm OI dissociative airglow emission have been conducted nightly during moon-down conditions from Mount Haleakala (geographic: 20.71°N, 203.83°E; geomagnetic 21.03°N, 271.84°E) in Hawaii since the end of 2001 [Kelley et al., 2002]. One of the imaging systems has a narrow field-of-view that permits high spatial resolution observations of field-aligned structure employing the configuration first suggested by Tinsley [1982]. Along with numerous images of equatorial plasma depletions [e.g., Makela and Kelley, 2003; Makela, 2004], this system has detected MSTIDs propagating to very low latitudes, often appearing to initiate equatorial depletions [Miller et al., 2009].

[6] Taking the 630.0-nm emission altitude to be approximately uniform across the imager field of view at 250 km permits maps of ionospheric structure to be produced from the airglow images. Figure 1 displays six successive observation maps of the 630.0-nm emission collected on 20 January (020) 2009 Universal Time (UT = HAST + 10). This date is characterized by good sky conditions and relatively quiet geomagnetic conditions (Kp ≤ 2), and is outside of the typical ‘spread-F’ season [Makela, 2004].

image

Figure 1. Sequence of 630.0-nm airglow images collected at Mount Haleakala, Hawaii, on 20 January 2009 showing the growth of (a–c) an MSTID into (d–f) an equatorial plasma depletion. These images have been spatially mapped to geographic coordinates.

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[7] Two structures are identified. The first, seen clearly as bands oriented in the NW-SE direction in Figures 1a1c, is an MSTID band drifting to the southwest. The presence of MSTIDs is highlighted by ovals in these frames; their propagation direction is indicated by a straight arrow in each case. Simultaneous radar observations (not shown) indicate coincident field-aligned irregularities. As time goes on, the MSTID signature approaches the magnetic equator. In Figures 1d1f, another structure appears to grow out of one of the raised MSTID bands. This equatorial plasma depletion grows northward while the MSTID continues to move to the southwest. The growing depletions are highlighted by an oval in Figure 1d. This example is representative of the observations we have of this phenomenon, which is most often observed during the deep solar minimum solstice months from Hawaii around local midnight.

3. The SAMI3/ESF Model

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References

[8] The Naval Research Laboratory SAMI3/ESF code [Huba et al., 2008], which is based on the SAMI2 (Sami2 is Another Model of the Ionosphere) [Huba et al., 2000] and SAMI3 [Huba and Joyce, 2010] ionosphere codes, has been used for numerous studies of ESF [e.g., Huba et al., 2008, 2009; Krall et al., 2009, 2010]. The 2D potential equation in SAMI3/ESF is based on current conservation (∇ · J = 0). For this study of MSTIDs we make the same simplifying assumptions as Huba et al. [2008]. That is, we neglect Hall-conductance terms in the potential equation and drop a low-order term from the RHS of this same equation. Examples of the potential equation without these simplifications are given by Krall et al. [2009] and, in the context of the SAMI3 code, by Huba and Joyce [2010].

[9] As configured for this study, SAMI3/ESF is limited to 8 degrees in longitude with periodic boundary conditions. Periodic boundary conditions–where plasma drifting out of one boundary reappears at the other boundary–work well for computing the potential. For simplicity SAMI3/ESF uses a non-tilted dipole field, so magnetic latitude and geographic latitude are the same, and the geographic longitude is centered on 0°, so universal time and local time are the same. In all cases the geophysical parameters are F10.7 = 150, F10.7A = 150, Ap = 4 and day-of-year 80. As in previous studies, the initial state of the SAMI3/ESF ionosphere is computed using SAMI2 [Huba et al., 2000]. Rather than simulate the specific conditions of the observations, we consider a case previously simulated [Krall et al., 2010] but with a different seed mechanism.

[10] For these runs the MSTID is modeled as an external traveling-wave E field with wavelength 250 km and period 1 h. The amplitude of the E field is set so as to cause vertical E × B drifts of up to 50 m/s. These parameters are chosen based on observed MSTIDs [e.g., Garcia et al., 2000; Shiokawa et al., 2003]. The vertical (p) and horizontal (h) components of these drifts are

  • equation image

where x is a Cartesian coordinate in the longitude direction (leading to a vertical drift), y corresponds to the latitude direction (leading to a horizontal drift), kx = k cos θTID, ky = k sin θTID, UTID is the drift amplitude (set to 50 m/s in the simulations), and θTID is the propagation angle, which ranges from 0 (parallel to the equator with density crests aligned with the geomagnetic field) to 90° (parallel to the geomagnetic field). These model E × B drifts are added to those resulting from the self-consistently computed E field on each time step. Consistent with observations, the direction of propagation of the imposed MSTID wave is southward and westward.

[11] Despite evidence that MSTID E fields extend along geomagnetic field lines [Otsuka et al., 2004], we control the effect of our model MSTID by localizing it in height to altitudes 200 km < z < 400 km, in longitude to between −3° and +3° and in latitude to between 4° and 16°. However, faint MSTID-like density waves in the conjugate ionosphere appear self-consistently in our simulation because the locally-imposed MSTID drifts affect the electron density which, in turn, affects the computed potential field. Because the potential field calculation treats geomagnetic field lines as equipotentials, the resulting disturbance extends along the field leading to an MSTID signature in the conjugate ionosphere, as has been observed [Otsuka et al., 2004]. In our simulations, this conjugate MSTID wave is strongest for θTID ≤ 30° and very weak for θTID ≥ 50°.

4. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References

[12] We performed several runs of the SAMI3/ESF code, each beginning at 19:20 UT, after the F layer has been lifted by the pre-reversal enhancement. In each case there were no winds and no initial perturbation of the ionosphere. We find that as the drifts of the MSTID wave act on the plasma where the wave is localized, the self-consistently computed potential is altered. When the model MSTID affects field lines that correspond to the bottom side of the equatorial F layer, the MSTID wave can trigger EPBs.

[13] For example, Figure 2 shows electron density at constant height 289 km plotted versus longitude and latitude in the northern hemisphere, in the region affected by the model MSTID. Figure 2 (left) shows a run in which the MSTID propagates at an angle of 20° to the magnetic equator. In Figure 2 (right) the angle is 50°; here EPBs occur at a later time. This is consistent with the idea that density perturbations that are better-aligned with the geomagnetic field will couple more strongly with the equatorial ionosphere to trigger EPBs. In each of the upper plots, only the effect of the MSTID wave is apparent; this wave propagates through the system at a speed of λ/τ = 69 m/s. In the lower plots, which correspond to later times, depletions can be seen extending northward from the lower boundary of the plot. These depletions are outlined in each case by a single contour line, indicating the lowest-density region of the image. Typical of past studies of ESF-related airglow depletions [Makela, 2004; Krall et al., 2009], depletions move outward in latitude as the corresponding EPBs move upward in altitude. In this idealized simulation (e.g., there are no winds to mitigate ESF growth), E × B drifts within the EPBs eventually overwhelm imposed MSTID drifts. This suggests that if MSTIDs are driven by E fields, then these fields could be affected by the presence of EPBs.

image

Figure 2. Electron density versus longitude and latitude at height 289 km at various times for MSTID propagation angle (left) 20° and (right) 50°. In the lower panels, a single-valued contour line outlines the lowest-density regions. Later time values in Figure 2 (right) indicate a later ESF onset time.

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[14] We analyzed ESF growth in each case, using the peak vertical E × B drift as a proxy for the ESF amplitude. In Figure 3 we plot the minimum instantaneous e-folding time and the ESF onset time versus θTID. Here the onset time is defined as the time from the beginning of the simulation to the time of the peak growth rate (minimum e-folding time), which occurs during the nonlinear, explosive phase of ESF growth. We find the fastest onset and the fastest growth at θTID = 0. At the other extreme, θTID = 90° (not shown), there appears to be no coupling at all. In this limit, ESF eventually grows from numerical fluctuations, with a weaker growth rate than the MSTID-driven cases.

image

Figure 3. Plots of onset time (h, squares) and minimum ESF e-folding time (min, dots) versus MSTID direction angle. E-folding times correspond to the peak nonlinear growth rates.

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5. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References

[15] We find that vertical drifts associated with our localized model MSTID, which naturally lead to a strong local airglow signature, disturb the non-local ionospheric potential in such a way as to produce a faint MSTID-like density wave in the conjugate ionosphere, similar to those observed [Otsuka et al., 2004]. In our simulations, the effect was strongest for θTID ≤ 30°. We conclude that the process that drives these “electrified” MSTIDs could be local (i.e., confined to one conjugate point).

[16] When the model MSTID affects field lines that correspond to the bottom side of the equatorial F layer, the MSTID wave can trigger ESF. In additional simulations in which the MSTID was confined to higher latitudes (14° to 26°), no coupling between the MSTID wave and the ESF instability was found. Comparing the simulated (Figure 2) and observed (Figure 1) airglow images, we see that ESF depletions are observed within an hour of the appearance of the observed MSTID at latitudes corresponding to those of the simulated MSTID. In the simulations, both the position and the timing of the coincident MSTID and ESF signatures are in good agreement with observation. In simulations with MSTID propagation angles similar to those observed (30–40°), the imposed E × B drifts lead to the development of clear MSTID density signatures during the first hour and to the development of ESF depletions over the second hour. Further investigations are planned in which both MSTID dynamics and associated ESF growth are placed in a global context, similar to Huba and Joyce [2010].

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References

[17] This work was supported by the Office of Naval Research and NASA. Work at the University of Illinois is supported under NSF grant NSF-06-44654. ESM acknowledges support from NSF AGS-0924914.

[18] The Editor thanks Susumu Saito and an anonymous reviewer for their assistance in evaluating this paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. The SAMI3/ESF Model
  6. 4. Results
  7. 5. Discussion
  8. Acknowledgments
  9. References