Midlatitude ionospheric dynamics and disturbances

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Share This Paper. Figures from this paper. Citations Publications citing this paper. Parker , S. Deminov , V. Norman , Brett A. Morphology and dynamics of storm-time ionospheric density structures Evan Grier Thomas. Statistical analysis of the mid-latitude trough position during different categories of magnetic storms and different storm intensities Na Yang , Huijun Le , Libo Liu. Coster , Philip J. Erickson , John C. It was not at first obvious that the mid-latitude ionosphere should be unstable since it is not subject to strong forcing from the magnetosphere as at auroral latitudes.

Nor is the situation like that at equatorial latitudes where nearly horizontal magnetic field lines support the ionosphere against gravity, leaving it prone to certain kinds of plasma interchange instabilities. Two main classes of irregularities have been observed at middle latitudes, however. The other class of irregularities is associated with a phenomenon known as mid-latitude spread F. This spreading was historically the earliest indication of plasma density irregularities in the mid-latitude F region see ref. Quiet-time spread F has been associated with thermospheric gravity waves and related undulations in the F -layer height, so-called medium-scale traveling ionospheric disturbances MSTIDs 22 — It has also been linked by numerous investigators to a particular plasma instability mechanism thought to operate at middle latitudes attributed to Perkins Mid-latitude spread F has furthermore been associated with patchy E s layers, observationally and theoretically, although different perspectives exist regarding the causal relationship 28 — Meter-scale irregularities in the F region are also known to cause coherent scatter 33 , Coherent scatter is a useful tool for identifying plasma instability and ionospheric irregularities but does not measure plasma state variables like incoherent scatter does.

The relationship between coherent scatter and the condition of the ionosphere that produces it is indirect We show here observations of mid-latitude F -region plasma density irregularities occurring in tandem with E s -layer irregularities over Arecibo during a geomagnetically quiet period. The morphology of the irregularities together with quantitative information about the underlying plasma state and dynamics are captured precisely in the Arecibo data.

Previous measurements of mid-latitude spread F at Arecibo and elsewhere have suggested wavelike variations in the height of the F -layer peak. The observations presented here are unique in that they show much deeper instability with significant upwelling and overturning in plasma density extending well into the topside. We also present coherent scatter echoes from a common radar scattering volume in the E region observed nearby from St. The measurements reveal unexpected aspects of the irregularities and provide new clues about the instability mechanisms at work.

We present the results of a numerical simulation of the mid-latitude ionosphere which explains the most important features in the Arecibo observations including the patchy E s layers, MSITDs, bottomside spread F irregularities, and irregularities in the topside. The most important ingredient in the simulations are intense gravity waves propagating upward into the thermosphere. The gravity wave wind fields both form the E s layers and cause them to structure, induce MSTIDs through dynamo action, and drive currents in the F region which lead to the formation of bottomside irregularities.

Most importantly here, the gravity waves induce electric fields in the E region which map along magnetic fields into the topside where they produce irregularities in the topside through convection and advection. The results of this paper are divided into radar observations from Arecibo and numerical simulations meant to help interpret the observations. These are addressed separately. Ionospheric observations were made with the Arecibo Observatory on 30 July , using the incoherent scatter technique and a mode which was described in detail by Hysell et al.

The results are shown in Fig. The figure shows electron number density versus altitude and local time directly over the radar. In processing these data, a single noise estimate for the entire period shown was calculated and subtracted from the receiver power at all times. This procedure introduces a small bias since the noise estimate does not capture the noise variation with sidereal time. However, it greatly reduces the temporal variance of the resulting plasma number density estimate, revealing minute variations that would otherwise be difficult to distinguish.

Electron number density on the evening of 30 July , represented as a function of altitude and local time in grayscale format. The figure shows both the E and F regions on the same scale and the E region in an expanded scale. This is a common ionospheric feature, termed MSTID, which is usually attributed to gravity waves in the thermosphere. The modulation crests have a sawtooth appearance. These irregularities are characteristic of mid-latitude spread F conditions and indicative of plasma instability. Note that the depletions are tilted from vertical.

More plasma irregularities were observed in the E region where an E s layer was seen to erupt into dense patches. Only the third set of patches was coincident in time with mid-latitude spread F , however. The irregularities are more evident in Fig. The irregularities exist throughout the topside and appear to modulate the topside boundary. They are both wavelike and, at least superficially, turbulent.

They both precede the striated depletions in the bottomside F region in time and linger behind them for at least an hour. Enhanced version of the topside ionospheric irregularities from the latter portion of the event. A low-pass filter Gaussian with a standard deviation of pixels was applied to the original image, and the difference between the filtered and unfiltered images was then plotted.

Despite having units of per cubic meter, the quantity being plotted is not strictly electron density, and the grayscale is arbitrary. These irregularities are remarkable in that the topside ionosphere is usually regarded as laminar and stable. The neutral gas as these altitudes is highly viscous and unable to support inertial-range turbulence.

Ion inertia is also usually regarded as negligible in this regime of space, implying that the plasma also cannot support inertial-range turbulence. It is, therefore, somewhat surprising for the topside to exhibit telltale signs of turbulence like this. As noted above, coherent scatter has been observed from the topside mid-latitude ionosphere in the past during spread- F events, signifying plasma instability. What is new here is the observation of superficially turbulent density irregularity and flow in the topside revealed by incoherent scatter.

The incoherent scatter technique affords measurements of other state parameters in the ionosphere such as ion composition, temperature, and line-of-sight drifts. There are two feed systems that can probe the ionosphere along two distinct lines of sight. From dual-beam data, estimates of the vector drift velocity of the plasma can be made with the assumption that the flow is spatially homogeneous over the volume probed by the two beams. Vector wind profiles can furthermore be estimated in the E region using statistical inverse methods, invoking the conservation properties of the plasma and assuming that the components of the electric field transverse to the magnetic field in the E and F region are the same.

The methods involved were described in detail by Hysell et al. We highlight the results of panel f which shows estimates of the vector plasma drifts in the F region. The red, green, and blue curves denote drifts parallel to the magnetic field B , perpendicular to B and eastward, and perpendicular to B and northward, respectively.

That the difference between the red and blue curves is generally small signifies the fact that the upward drifts were most often modest, echoing the information in panel b. The average of the red and blue curves gives the northward drift which was large and oscillatory. The green curve gives the eastward drift which was also large and oscillatory.

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State parameters derived from incoherent scatter measurements using dual radar beams. The bottom three panels reflect conditions within just the third patchy E s layer. Panels g and h show estimates of the horizontal winds in the E region. Similar observations led 37 to propose neutral KHI as the mechanism underlying E s layer patchiness.

Midlatitude ionospheric dynamics and disturbances: Introduction

Additional contextual information comes from a small Croix, USVI. This radar detects coherent backscatter from meter-scale plasma density irregularities embedded in the E s layers. The irregularities in question are strongly elongated in the direction of the magnetic field B. The radar is located so as to be able to detect strong Bragg or coherent scatter from E -region irregularities directly over Arecibo. Croix during the mid-latitude spread F event. Images like these are constructed using aperture synthesis methods of the kind used by radio astronomers to image distant radio sources in the cosmos.

Images can be computed at a cadence of once every few seconds. The imaging technique utilized here, which has been described in detail by Hysell and Chau 38 , gives information about the Doppler shift of the echoes as well as the intensity of the backscatter. The velocities indicated by the color scale are the phase velocities of the meter-scale waves in the plasma which is controlled by a number of factors, the proper motion of the E s layer patches being only one of them.

Representative images of radar echoes due to coherent scatter from plasma density irregularities in E s layers near km altitude at The brightness of the image pixels specifies the echo signal-to-noise ratio on a decibel scale. The hues reflect Doppler velocity, with red blue hues denoting drifts away from toward the radar located on St.

Studies have shown that the strongest coherent backscatter generally arises from the densest regions of the patchy E s layer as determined by incoherent scatter. The coherent scatter therefore conveys information about the horizontal configuration of the patches, their proper motion, their propagation, evolution, and lifetimes.

This image suggests that patchy E s layers extended well away spatially from the immediate vicinity of Arecibo. This representative image shows that the E s -layer patches are organized horizontally along fronts. In this case, there are two fronts over Puerto Rico and a third to the northwest.

The two larger fronts are themselves structured, showing evidence of secondary waves along their length. When animated, successive images from this event show that the fronts propagate in space in different directions but most often to the southwest.

This is typical of E s -layer observations made using this instrument. It has been argued by Larsen 37 and Hysell et al. In this scenario, E -region coherent scatter is indicative of strong winds and wind shears in the lower thermosphere.

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We can attempt to reproduce the ionospheric irregularities observed over Arecibo, the F -region and topside irregularities in particular, through numerical simulation. The simulation couples together two models—one of neutral thermospheric dynamics, and another of the ionospheric plasma response. Neutral winds predicted by the former are used to drive ionospheric evolution in the latter.

The numerical simulation is described in more detail in the methods section of this paper. The problem is complex, and a number of compromises have been made in the modeling in order to keep the computations tractable. For example, the neutral dynamics model is cast in two dimensions only. In previous model studies, primary KHIs with two-dimensional 2D symmetry arise and achieve large amplitudes before 2D and three-dimensional 3D secondary instabilities which break the symmetry and go on to affect billow dissipation 40 , By modeling the neutral dynamics in 2D, we will be testing whether the multifaceted ionospheric phenomena we observe can arise from relatively simple 2D neutral wind forcing.

The results are extended into three dimensions by assuming invariance in the unmodeled horizontal dimension. This figure is arbitrary and can be varied without significantly affecting the simulation outcome. Another limitation of the simulation is that it neglects reverse coupling from the ionosphere back to the neutral atmosphere.

The ionospheric plasma is only a minor constituent of the upper atmosphere and represents a small part of the momentum budget. While ion drag alters the neutral atmospheric circulation at low and middle latitudes as well as the temperature and composition, e.

This is long compared to the e-folding time of the plasma instabilities of interest here. We nonetheless cannot fully discount a role for reversed coupling which could include thermal and compositional effects. A third limitation of the simulation is that it does not make provisions for ionized metallic species. Sporadic E layers are known to be composed primarily of metallic ions, the small rates of diffusion and recombination of metallic species being important contributing factors to the intensity of the layers which can rival the F peak in density.

This simulation cannot reproduce very dense E s layers. We are therefore essentially testing in simulation whether dense E s layers are a necessary component of the ionospheric plasma instabilities underlying mid-latitude spread F. The results presented below suggest that they are not. Maruyama, J. Chau, K. Yumoto, A. Bhattacharyya, and S. Maruyama, T. Codrescu, D. Richmond, A.


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