1. Introduction and Motivation
Numerical modeling of equatorial spread F using observed neutral wind profiles
A. Kiene1, M. F. Larsen1, and D. L. Hysell2
1Department of Physics and Astronomy, Clemson University
2Department of Earth and Atmospheric Sciences, Cornell University
Abstract
Model Inputs
Discussion
The neutral wind in the F region ionosphere near sunset is known to be a key driver of the nighttime F region plasma instabilities known as Equatorial Spread-F (ESF). Eastward winds drive vertical Pedersen currents that, in turn, drive the Rayleigh-Taylor mechanisms responsible for the large plumes of plasma depletion often seen in ESF events. Recent sounding rocket chemical release studies have shown large westward winds and wind shears in the equatorial F region during the period near sunset. Westward winds and shears of these magnitudes are unexpected based on current wind models, which show eastward neutral flow with very small vertical gradients above 200 km. It is typical for ionospheric models to use wind inputs from the Horizontal Wind Model (HWM14) in order to model the development of ESF; however, the zonal wind profiles observed in these experiments are not consistent with this approximation during the transition hours near sunset. In this study, we apply the observed chemical release neutral wind profiles to an existing ESF model in order to investigate the effects of such large westward winds and shears during the transitional period near sunset on the subsequent development of ESF plumes.
The EVEX wind field caused quite different results than typical model winds. Fig. 5, above, shows two recent simulation runs for the Peruvian longitude sector that also used HWM14 winds. The ionospheric parameters were initialized using data taken on the same night. The plume morphology of the high activity night (right) is very similar to that of the HWM14 case shown in Fig. 3. Simulation time is 75 min.
When replacing HWM winds with the EVEX winds, we see smaller and much less turbulent plumes. These plumes resemble those presented by Aveiro and Hysell [2010] for the case where CSI is suppressed, resulting in only GRT growth.
On the eastern side of the EVEX simulation, the plumes are less well-defined. This is likely a result of the neutral wind profile shown in Fig. 2, where the eastern side transitions to a more typical HWM-like wind profile sooner than the westward side. This would facilitate CSI growth more on the eastern side than on the western side, leading to some signs of CSI showing on the eastern edge.
This study shows the effect that neutral winds near sunset can have on the subsequent development of ESF. Vertically-resolved F region neutral wind measurements are very scarce. The winds shown in Fig. 1 represent only three nights of data (two of them consecutive). To what extent the vertical shear observed is a common feature remains to be seen, but further studies on the relationship between daily variability in the neutral winds and the occurrence of ESF.
Various models are used to initialize the simulation. For the sake of space, we will discuss only those that are changed from the defaults discussed in Hysell et al. [2015].
The first is the Horizontal Wind Model (HWM14). We ran the model both with the default HWM winds and with a wind field generated based on the EVEX results (see Fig. 2, right).
The background electric fields were initially specified based on vertical plasma drifts from the model established by Scherliess and Fejer [1999], scaled with altitude. This is a valid assumption, provided that the neutral wind is unchanging with altitude (as in the HWM case), but that is not the case for the EVEX and Guara data. Because the zonal wind is the primary driver of the evening vertical drifts, we have used the winds directly to estimate the electric fields.
ux (m/s)
ux (m/s)
Introduction and Motivation
Figure 5: Numerical simulation results for two nights in Dec. 2014, one with low ESF activity (left) and one with high activity (right). The top row shows plasma density with red, green, and blue tones representing molecular, atomic, and protonic ion abundance, respectively. The bottom row shows current density in nA/m2. The white lines are equipotentials, and the vertical electric field profile is plotted to the right. Figures from Hysell et al. [2015]
One of the primary end goals of research into equatorial spread F (ESF) is the isolation of driving factors and the subsequent prediction of disruptive ionospheric irregularities. This has led to a rapidly developing field of ESF modeling. Thus far, most modeling work has focused on correlating model output with experimental observations of ESF and the ionospheric parameters measured near the same time. There are many potential seeding mechanisms for ESF instabilities, and each of them has its own driving parameters that must be fully specified in order for a model to produce realistic output. These parameters, particularly those of the neutral atmosphere, are difficult to measure.
ux (m/s)
Figure 2: Neutral wind field based on the EVEX experiment, shown for (top) release time, (middle) 20 min. after release, and (bottom) 40 min. after release.
A neutral wind profile generated from the Horizontal Wind Model (HWM14) is typically used in numerical models, since there have not previously been direct, vertically-resolved neutral wind measurements. HWM and other empirical models have, by necessity, incorporated only satellite and ground-based data that produce single-altitude measurements in the F region, extending them vertically to cover the entire altitude range.
Results – HWM14 Winds
It is likely that the difference is due to the sheared westward wind suppressing CSI growth. The CSI growth rate is proportional to the integral over a flux tube of the quantity (u – v0) where u is the zonal neutral wind and v0 is the zonal ion velocity [Hysell et al., 2006]. Typically, the profile of v0 is positively sheared with altitude (see Fig. 6), and thus shows the opposite trend to the initial EVEX zonal wind profile. This causes a suppression of the CSI growth rate, leading to smoother, weaker plumes.
log ne (m-3)
The first model case that we studied used the default HWM14 wind profile. Shown to the right in Fig. 3 are log(ne) plots from three simulation times: t = 0, 30 min., and 60 min. The ESF plumes that develop are somewhat turbulent and extend to 550 km in some instances. This type of structure is common in ESF events and results from a combination of CSI and GRT instability (see discussion).
log ne (m-3)
Recently, Kiene et al. [2015] reported sounding rocket observations (shown in Fig. 1) that showed significant vertical shear in the zonal wind previously unexpected by models. Because the zonal wind is a primary driver of ESF instabilities, particularly near sunset, the implications of such sheared winds upon ESF development are of great interest. This study investigates the effect of such winds when compared with winds derived from HWM14 though a numerical model of the evening ionosphere.
Figure 6: Zonal plasma drift profile measured on the high activity night shown in Fig. 5. From Hysell et al. [2015]
log ne (m-3)
Figure 3: log(ne) (in m-3) plots from the simulation using HWM winds, shown for simulation times of (top) 0 min., (middle) 30 min., and (bottom) 60 min.
Fig. 1: (top) Neutral wind profiles derived from the EVEX campaign, and comparison to HWM output from the time of release; (bottom) zonal neutral winds from the Guara campaign.
Results – EVEX winds
The second case that we studied was the EVEX neutral wind case. For this simulation, we used the wind field shown in Fig. 2, above. Electron density plots from t = 0, 30 min., and 60 min. are shown in Fig. 4 to the right. The plumes that develop are much less turbulent and do not rise to altitudes as high as those in the HWM case.
That plumes develop in spite of the westward wind is certainly an interesting result. While the winds do rapidly shift toward a more uniform eastward flow, the plumes that result are significantly different in shape and strength when compared to the HWM case.
Numerical Model
Aveiro and Hysell [2010] developed a fully three-dimensional ESF model that incorporated the collisional shear instability (CSI) first proposed by Hysell and Kudeki. [2004]. The model solves the potential equation without assuming equipotential field lines. It is run on a grid that is 159 x 133 x 189 points wide in (p, q, ϕ), which are magnetic dipole coordinates. This translates to an altitude extent from ~90 to ~570 km, a longitudinal extent of ~10o and a latitudinal extent of ~30o, centered on the geomagnetic equator. The model used here is an updated version, most recently described by Hysell et al. [2015].
One of the principal advances of this model is the incorporation of CSI. The large plumes of plasma depletion that are commonly associated with ESF events are a result of the generalized Rayleigh-Taylor (GRT) instability; however, the GRT process is driven by other, faster-growing seed instabilities that eventually give way to GRT growth. CSI is one such instability, arising from shear in the vertical profile of the zonal plasma drifts, where the plasma drift is retrograde to that of the zonal neutral wind.
log ne (m-3)
Acknowledgements
The work at Clemson was supported by NASA grant NNX15AL02G and NSF grant AGS
log ne (m-3)
References
Aveiro, H. C. and D. L. Hysell (2010), J. Geophys. Res. 115, A11321.
Hysell, D. L. and E. Kudeki (2004), J. Geophys. Res. 109, A11301.
Hysell, D. L., M. F. Larsen, C. M. Swenson, A. Barjataya, and T. F. Wheeler (2006), J. Geophys. Res. 111, A11317.
Hysell, D. L., M. A. Milla, L. Condori, and J. Vierinen (2015), J. Geophys. Res. Space Physics 120,
Kiene, A., M. F. Larsen, and E. Kudeki (2015), J. Geophys. Res. Space Physics 120,
Kudeki, E., B. G. Fejer, D. T. Farley, and H. M. Ierkic (1981), Geophys. Res. Lett. 8, 377.
Scherliess, L., and B. G. Fejer (1999), J. Geophys. Res. 104,
log ne (m-3)
Figure 4: Same as Fig. 3, but for the simulation using EVEX winds.