Ionospheric Induced Scintillation: A Space Weather Enigma

The effect of scintillation on radio signals whose propagation path involves the Earth’s ionosphere is analogous to the allies of World War II receiving radio messages that had passed through the Enigma machine. In both these cases, man-made information has been encrypted and transmitted via radio. The two encryption methods are shown in Figure 1. The right panel shows a World War II Enigma machine used extensively by German U-boats to convey encrypted messages transmitted by radio [Perera, 2010]. The left panel gives an extreme example of a mapping of ionospheric irregularities at 3 m, which creates very severe scintillation on radio communications through this ionospheric region [Fejer, 1996]. In addition, the task of formally deciphering the encrypted signal is a monumental task as time is of the essence and old information quickly becomes redundant

Model‐Based Properties of the Dayside Open/Closed Boundary: Is There a UT‐Dependent Variation?

The open‐closed boundary (OCB) defines a region of significant transformation in Earth\u27s protective magnetic shield. Principle among these changes is the transition of magnetic field lines from having two foot points, one in each hemisphere, to one foot point at Earth, the other mapping to the solar wind. Charged particles in the solar wind are able to follow these open field lines into Earth\u27s upper atmosphere. The OCB also defines the polar cap boundary. Being able to identify and track the OCB allows study of several components of the geomagnetic system. Among them are the electrodynamics of the geomagnetic field and the reconnection balance between the dayside and nightside of the geomagnetic field. Furthermore, the OCB can provide insights into the precipitation of energetic protons into the ionosphere. Using the Tsyganenko model of the geomagnetic field (T96), we demonstrate a diurnal fluctuation which we call the Universal Time (UT) effect of the OCB. This UT effect is independent of all other inputs. We anticipate this UT effect to have important consequences in modeling the OCB and other polar cap‐associated structures, especially polar cap absorption events that adversely affect high‐frequency radio wave propagation in polar regions

The Global Ionosphere-Polar Wind System During Changing Magnetic Activity

A time-dependent, three-dimensional, multi-ion model of the global ionosphere-polar wind system was used to study the system\u27s response to an idealized geomagnetic storm for different seasonal and solar cycle conditions. The model covered the altitude range from 90 to 9000 km for latitudes greater than 50° magnetic in the northern hemisphere. The geomagnetic storm contained a 1-hour growth phase, a 1-hour main phase, and a 4-hour decay phase. Four storm simulations were conducted, corresponding to winter and summer solstices at both solar maximum and minimum. The simulations indicated the following: (1) O+ upflows typically occur in the cusp and auroral zone at all local times, and downflows occur in the polar cap. However, during increasing magnetic activity, O+ upflows can occur in the polar cap, (2) The O+ upflows are typically the strongest where both Te and Ti are elevated, which generally occurs in the cusp at the location of the dayside convection throat, (3) The upward H+ and O+ velocities increase with Te, which results in both seasonal and day-night asymmetries in the ion velocities, (4) During “increasing” magnetic activity, O+ is the dominant ion at all altitudes throughout the polar region, (5) For solar minimum winter, there is an H+ “blowout” throughout the polar region shortly after the storm commences (negative storm effect), and then the H+ density slowly recovers. The O+ behavior is opposite to this. There is an increase in the O+ density above 1000 km during the storm\u27s peak (positive storm effect), and then it decreases as the storm subsides, and (6) For solar maximum summer, the O+ and H+ temporal morphologies are in phase; but the ion density variations at high altitudes are opposite to those at low altitudes. During the storm\u27s peak, the H+ and O+ densities increase at high altitudes (positive storm effect) and decrease at low altitudes (negative storm effect)

A Theoretical Study of the Global \u3ci\u3eF\u3c/i\u3e Region for June Solstice, Summer Maximum, and Low Magnetic Activity

We constructed a time-dependent, three-dimensional, multi-ion numerical model of the global ionosphere at F region altitudes. The model takes account of all the processes included in the existing regional models of the ionosphere. The inputs needed for our global model are the neutral temperature, composition, and wind; the magnetospheric and equatorial electric field distributions; the auroral precipitation pattern; the solar EUV spectrum; and a magnetic field model. The model produces ion (NO+, O2+, N2+, N+, O+, He+) density distributions as a function of time. For our first global study, we selected solar maximum, low geomagnetic activity, and June solstice conditions. From this study we found the following: (1) The global ionosphere exhibits an appreciable UT variation, with the largest variation occurring in the southern winter hemisphere; (2) At a given time, NmF2 varies by almost three orders of magnitude over the globe, with the largest densities (5 × 106 cm-3) occurring in the equatorial region and the lowest (7 × 103 cm-3) in the southern hemisphere mid-latitude trough; (3) Our Appleton peak characteristics differ slightly from those obtained in previous model studies owing to our adopted equatorial electric field distribution, but the existing data are not sufficient to resolve the differences between the models; (4) Interhemispheric flow has an appreciable effect on the F region below about 25° magnetic latitude; (5) In the southern winter hemisphere, the mid-latitude trough nearly circles the globe. The dayside trough forms because there is a latitudinal gap of several degrees between the terminator and the dayside oval. In this gap, there is no strong ion production source, and the ionosphere decays; (6) For low geomagnetic activity, the effect of the auroral oval on the densities is not very apparent in the summer hemisphere, but is clearly evident in the winter hemisphere; (7) The densities in both the northern and southern polar caps exhibit a complex temporal variation owing to the competition between the various photochemical and transport processes

A Theoretical \u3ci\u3eF\u3c/i\u3e Region Study of Ion Compositional and Temperature Variations in Response to Magnetospheric Storm Inputs

The response of the polar ionosphere to magnetospheric storm inputs was modeled. During the “storm,” the spatial extent of the auroral oval, the intensity of the precipitating auroral electron energy flux, and the plasma convection pattern were varied with time. The convection pattern changed from a symmetric two-cell pattern with a 20-kV cross-tail potential to an asymmetric two-cell pattern with enhanced plasma flow in the dusk sector and a total cross-tail potential of 90 kV. During the storm there were significant changes in the ion temperature, ion composition, and molecular/atomic ion transition height. The storm time asymmetric convection pattern produced an ion temperature hot spot at the location of the dusk convection cell owing to increased ion-neutral frictional heating. In this hot spot there were significantly enhanced NO+ densities and hence molecular/atomic ion transition heights. During the storm recovery phase, the decay of the enhanced NO+ densities closely followed the decrease in the plasma convection speed. During the storm, elevated ion temperatures also appeared at high altitudes in the midnight-dawn auroral oval region. These elevated ion temperatures were a consequence of the storm-enhanced topside O+ densities, which provided better thermal coupling to the hot electrons. This region also contained reduced molecular/atomic ion transition heights. These elevated ion temperatures and reduced transition heights persisted for several hours after the storm main phase ended

A Three-Dimensional Time-Dependent Model of the Polar Wind

A time-dependent global model of the polar wind was used to study transient polar wind perturbations during changing magnetospheric conditions. The model calculates three-dimensional distributions for the NO+, O2+, N2+, N+ and O+ densities and the ion and electron temperatures from diffusion and heat conduction equations at altitudes between 120 and 800 km. At altitudes above 500 km, the time-dependent nonlinear hydrodynamic equations for O+ and H+ are solved self-consistently with the ionospheric equations. The model takes account of supersonic ion outflow, shock formation, and ion energization during a plasma expansion event. During the simulation, the magnetic activity level changed from quiet to active and back to quiet again over a 4.5-hour period. The study indicates the following: (1) Plasma pressure changes due to Te , Ti or electron density variations produce perturbations in the polar wind. In particular, plasma flux tube motion through the auroral oval and high electric field regions produces transient large-scale ion upflows and downflows. At certain times and in certain regions both inside and outside the auroral oval, H+ - O+ counterstreaming can occur, (2) The density structure in the polar wind is considerably more complicated than in the ionosphere because of both horizontal plasma convection and changing vertical propagation speeds due to spatially varying ionospheric temperatures, (3) During increasing magnetic activity, there is an overall increase in Te , Ti and the electron density in the F-region, but there is a time delay in the buildup of the electron density that is as long as five hours at high altitudes, (4) During increasing magnetic activity, there is an overall increase in the polar wind outflow from the ionosphere, while the reverse is true for declining activity, (5) In certain regions, however, localized ionospheric holes can develop during increasing magnetic activity, and in these regions the polar wind outflow rate is reduced, (6) During changing magnetic activity, the temporal evolution of the ion density morphology at high altitudes can be different from, and even opposite to, that at low altitudes

Theoretical Study of Anomalously High \u3ci\u3eF\u3c/i\u3e Region Peak Altitudes in the Polar Ionosphere

During the last solar maximum period several observations of anomalously high F region peak altitudes have been made by the high latitude incoherent scatter radars. The observations indicate that there are several distinctive features associated with these high hmF2 ionospheric profiles: (1) they are observed near midnight with the plasma flowing out of the polar cap, (2) NmF2 ranges from 105 to 106 cm−3, (3) hmF2 ranges from 400 to 500 km, (4) below 300 km the profile is devoid of ionization, and (5) the observations are for solar maximum conditions. In an effort to explain these radar observations, a time-dependent high latitude ionospheric model was used to study transport effects for a wide range of solar cycle, seasonal, magnetic activity, and neutral wind conditions. The model results indicate that high hmF2 values in the midnight sector of the polar region can be generated without the need for ionization due to auroral precipitation. For solar maximum, all of the observed features of the high hmF2 density profiles are reproduced by the model if the neutral wind across the polar cap is greater than 400 m/s. Such wind speeds have been frequently measured during the last solar maximum period. The study also shows general results for the influence of transport in the polar cap for different seasonal and solar cycle conditions. NmF2 and hmF2 are lower for solar minimum than solar maximum. However, the seasonal dependences are strongly coupled with both the strength of the convection and the neutral wind speed

Interdisciplinary Scientists Gather for Plasma Structure Workshop

Two of the most exciting papers presented at the Third Peaceful Valley Workshop were on the nature of plasma structure found in the nighttime midlatitude E and F regions. The first was from a coordinated rocket campaign called Sporadic E Experiment over Kyushu (SEEK) dedicated to the understanding of puzzling quasi-periodic radar echoes that have been detected in association with sporadic E layers. In-situ probes on two rockets measured localized electric fields as large as 20 mV/m, confirming theoretical predictions of strong polarization processes that may result from wavelike distortions imposed on normally stratified sporadic E layers. An unexpected result was the discovery of a large localized wind of 150 m/s at E region altitudes and the associated velocity shear, which was likely to lead to Kelvin- Helmholtz instability