Ionospheric Patches Research Articles

Towards a Reliable Ionospheric Polar Patch Climatology With Ground-Based GNSS

We present a new algorithm of ionospheric patch detection with ground-based GNSS data and its application to statistical investigations of polar activity in the northern hemisphere. The patch detection is performed with relative STEC values derived from a time series of geometry-free linear combination of GNSS signals. The approach allows distinguishing between the structures and monitoring polar ionosphere with a high temporal resolution. The results, covering the period between January and May 2014, demonstrate the effectiveness of the proposed methodology and show good agreement with ISR and SWARM datasets. Furthermore, a strong seasonal dependence of polar activity characterized simultaneously by a mutable daily pattern was proven. During the winter months, the minimum and maximum of patch occurrence correspond to pre- and postnoon UT hours, whereas in March the pattern is reversed. This modification is fully explained by a corresponding variation of the ionospheric gradient between subauroral and polar regions. The support of this parameter also clarifies the seasonal effects in patch occurrence as well as the strength of structures observed for particular months. Accordingly, the most severe patches correlate well with the highest ionospheric gradient near the spring equinox, while the suppressed polar activity observed in May 2014 corresponds to the opposite conditions. We also confirmed a strong dependence of patch occurrence on IMF Bz and a close relationship between the enhancement of structures observed during southward orientation and ionospheric gradient.

Sharpening our thinking about polar cap ionospheric patch morphology, research, and mitigation techniques

Since polar cap patches were discovered, their nature, physics, and impact on navigation and communication signals has been repeatedly addressed. Both terminology and inference of physical processes from diverse instruments have introduced confusion. Poleward moving auroral form is a morphological descriptor but cannot be equated to an island of high‐density plasma. Particle precipitation produces low‐density patches, but high‐density patches derive from solar produced plasma. The challenge of patches is finding the dominant mechanism for chopping entering ionization into islands (∼100–1000 km in size). Velocity‐dependent recombination physics is valid in principle but not relevant to patches formed in at least the European sector. Most patches are formed by transient magnetic reconnection events. While the plasma is at too high an altitude for the strong velocity shears to erode plasma densities, the shears become the dominant plasma structuring mechanism until the initial magnetic tension force is relaxed. Initial patch structuring is not by gradient drift as believed for decades but rather by the shear driven instability, impacting mitigation techniques. Large‐scale shears, driven to 2–3 km/s, impact satellite drag through thermospheric heating. The study here is intended to sharpen understanding of patches for future research and development of techniques for mitigation of their effects on navigation and other systems dependent on receiving radio frequency signals from satellites and understanding reconnection driven impact on thermospheric density and satellite drag.

Modeling of scintillation effects on high‐latitude transionospheric paths using ionospheric model (UAF EPPIM) for background electron density specifications

This paper addresses the effects of scintillation on high‐latitude paths of propagation. To specify the high‐latitude ionosphere environment as a time‐dependent three‐dimensional distribution of the electron density, the first‐principles ionospheric model UAF EPPIM (University of Alaska Fairbanks Eulerian Parallel Polar Ionosphere Model) is utilized. For the specification of time‐varying small‐scale irregularities, superimposed onto the background EPPIM, the full three‐dimensional inverse power law spectrum is chosen. It is augmented with the introduction of the irregularities' aspect ratio for two mutually perpendicular directions: transverse (to the geomagnetic field) direction and for the longitudinal direction to the transverse one. This full description of the propagation environment, comprised of the “background” and “irregularities” components, is further merged with the propagation scintillation model to set up the computational suite, capable of modeling the scintillation effects. The parameters of the spectrum of the electron density fluctuations, as well as the models of meso‐scale local polar structures (e.g., patches) are chosen empirically. The integrated simulator enables realistic predictions of the scintillation effects for high‐latitude transionospheric propagation for a number of different propagation scenarios for GPS L‐band and other UHF transionospheric signals. The specific scintillation effects, caused by ionospheric patches, are simulated.

Role of neutral atmospheric dynamics in cusp density and ionospheric patch formation

Ionospheric patches are islands of plasma transiting the polar cap, with density at least double the surrounding background plasma. Since their discovery 25 years ago, many mechanisms for their production have been proposed and examined. However all of these mechanisms consider only electric fields and charged particles as candidate mechanisms for patch formation, particularly transient changes in these electrodynamic terms. Here for the first time, we call attention to the role of thermospheric dynamics in Patch formation. We show that the thermospheric response to transient heating events near the cusp must significantly increase exospheric densities over the cusp, and also that a plausible doubling of these densities near and above 400 km altitude leads to a new patch production mechanism. The underlying processes must drive a solar cycle variation of how many hours UT experience strong polar patches and scintillation. These processes must be added to present thinking and modeling.

Direct observations of injection events of subauroral plasma into the polar cap

While polar cap ionospheric patches have been studied for over two decades, there remains no general agreement to which of many proposed patch‐production mechanisms are important or dominate. An experiment was designed and implemented to search for transient events redirecting subauroral ionospheric plasma from its subauroral flow to transient injection into the polar cap, as would occur for the (Lockwood and Carlson, 1992) mechanism of patch creation. An earlier experiment provided compelling evidence of this mechanism acting within the cusp, with strong but indirect evidence regarding the source‐reservoir for the plasma injected into the polar cap. The work here, for the first time, directly tracks plasma becoming a “patch”, continuously from subauroral latitudes before the event fires, through the cusp and into the polar cap. We conclude this mechanism is a dominant patch generation mechanism and highlight that poleward‐moving‐form research has direct application to polar cap patch generation by magnetopause reconnection.

Ionospheric patch formation: Direct measurements of the origin of a polar cap patch

We present the first direct measurements documenting the origin of an ionospheric patch, emerging into the polar cap from the noon dark cusp region of the dayside auroral oval. A series of such patches were observed. The observed patch‐formation signatures, in plasma density and velocity, ion and electron temperature, and optical transient signatures, are all compared to those required of the Lockwood and Carlson [1992] transient reconnection mechanism for patch formation. Conformity of the observed signatures, to the stringent signatures demanded of these five independent parameters by the tested mechanism, is taken as strong substantiation of the applicability of this mechanism. The data excludes two alternate candidate mechanisms.

ESR mapping of polar‐cap patches in the dark cusp

We present the first ever measurement of the full thermal plasma properties, of an ionospheric patch in full darkness in the noon region where patches are believed to form. Further these data present the first experimental evidence for the Lockwood and Carlson class of mechanisms for forming patches by plasma injection. These data were possible only because of a new measurement capability we had to develop. We introduce the capability here because it crosses the high‐speed threshold that now allows study of a broader class of mesoscale plasma flow‐transients, which are thought to occur over time scales near 2 minutes vice 8–10 minutes. Cumulatively such transients may significantly drive global convection. We demonstrate both the validity of and need for our new measurement capability, by presenting a transient flow reversal sweeping across a 500 by 1000 km area, with initial reversal in 4 minutes, and recovery within 6 minutes.

Gradient drift instability growth rates from global‐scale modeling of the polar ionosphere

The winter polar ionosphere, under southward interplanetary magnetic field (IMF) conditions, experiences irregularity development leading to consequences such as scintillation on transionospheric communication links. These irregularities are associated with antisunward convecting polar ionospheric patches. The gradient drift instability (GDI) has been considered a primary candidate for the generation of these irregularities, or at least the long‐wavelength energy source of the irregularity‐wave cascade process. The Utah State University time‐dependent ionospheric model (TDIM) enables the polar cap ionosphere and its patches to be modeled on a large scale in a time‐evolving manner. Hence, at each point in space and time, the TDIM has adequate information to compute the growth rate of the gradient drift instability. The spatial gradients are based on a 92×92 km TDIM grid resolution, while the time resolution can be as short as 30 s. In this study, we present results of the GDI linear growth rate calculations for the F region. For a first time, snapshots of the instantaneous GDI linear growth rates over the polar ionosphere are shown. These show, in part, the correlation of the GDI with polar cap patches. Then we present the time evolution of the linear growth rate as plasma flux tubes convect. It is shown that a flux tube of plasma can, in fact, change between being unstable and stable to the GDI as the plasma convection pattern switches in response to changing IMF conditions. These convective results are essential information if an assessment of whether or not a plasma flux tube is stable to the gradient drift mechanism is to be made.

Signatures of polar cap patches in ground ionosonde data

The interpretation of localized density structures in the high‐latitude ionosphere as observed by ground ionosondes is a task that is complicated by the complexity of those structures and by the variety of processes that ionosonde waves undergo. To aid interpretation, we have performed a first‐order analysis of the shapes of traces seen in ionosonde displays. This has been done using an idealized model of a particular structure of current interest, a polar ionospheric patch. The patch is modeled as a spheroidal enhancement of density superposed on a horizontally stratified F layer. Ray tracing is used to find O‐ and X‐mode rays that reflect back to a ground ionosonde after total reflection. Theoretical curves are calculated for swept‐frequency (h'ƒ), fixed‐frequency (h't), and Doppler‐frequency (ƒDt) operational modes. The computed h'ƒ curves have U shapes, the h't curves are quasi‐hyperbolic, and the Doppler curves have reverse‐S shapes. By moving the patch through a series of horizontal separations from the ionosonde, the sequence of curves is calculated for a patch passing by an ionosonde. The theoretical traces have been compared with data from the Canadian advanced digital ionosondes (CADIs) at Resolute Bay, Eureka, and Alert. The CADI h'ƒ traces are observed usually to have incomplete U shapes, missing their lower‐frequency arms. In the light of the ray tracing the CADI results are consistent with the hypothesis that HF‐scattering irregularities are found on both the leading and trailing edges of patches but are stronger on the trailing edge. Similar results have been reported for scintillation‐causing irregularities. This points to the dominance of the gradient‐drift instability. The hyperbolic h't curves are asymmetric, having slower downsweeps than upsweeps. This can be caused by horizontal asymmetries in the patch density distribution. Often, polar patches seen by CADI are not single, isolated enhancements. Rather, they appear as composites of substructures, each of which mimics certain aspects of the single patch described by the present theoretical model.