Robert W. Schunk

Global Assimilation of Ionospheric Measurements (GAIM)

Abstract : Our primary goal is to construct a real-time data assimilation model for the ionosphere-plasmasphere system that will provide reliable specifications and forecasts. A secondary goal is to validate the model for a wide range of geophysical conditions, including different solar cycle, seasonal, storm, and substorm conditions.

Parameterized Ionospheric Model: A Global Ionospheric Parameterization Based on First Principles Models

We describe a parameterized ionospheric model (PIM), a global model of theoretical ionospheric climatology based on diurnally reproducible runs of four physics based numerical models of the ionosphere. The four numerical models, taken together, cover the E and F layers for all latitudes, longitudes, and local times. PIM consists of a semianalytic representation of diurnally reproducible runs of these models for low, moderate, and high levels of both solar and geomagnetic activity and for June and December solstice and March equinox conditions. PIM produces output in several user selectable formats including global or regional latitude/longitude grids (in either geographic or geomagnetic coordinates), a set of user specified points (which could lie along a satellite orbital path), or an altitude/azimuth/elevation grid for a user-specified location. The user selectable output variables include profile parameters (ƒ0F2, hmF2, total electron content, etc.), electron density profiles, and ion composition (O+, NO+, and O2+).

Theoretical ion densities in the lower ionosphere

Abstract We have solved the coupled momentum and continuity equations for NO + , O 2 + , and O + ions in the E - and F -regions of the ionosphere. This theoretical model has enabled us to examine the relative importance of various processes that affect molecular ion densities. We find that transport processes are not important during the day; the molecular ions are in chemical equilibrium at all altitudes. At night, however, both diffusion and vertical drifts induced by winds or electric fields are important in determining molecular ion densities below about 200 km. Molecular ion densities are insensitive to the O + density distribution and so are little affected by decay of the nocturnal F -region or by processes, such as a protonospheric flux, that retard this decay. The O + density profile, on the other hand, is insensitive to molecular ion densities, although the O + diffusion equation is formally coupled to molecular ion densities by the polarization electrostatic field. Nitric oxide plays an important role in determining the NO + to O 2 + ratio in the E -region, particularly at night. Nocturnal sources of ionization are required to maintain the E -region through the night. Vertical velocities induced by expansion and contraction of the neutral atmosphere are too small to affect ion densities at any altitude.

A Mathematical Model of the Middle and High Latitude Ionosphere

A time-dependent three-dimensional model of the middle and high latitude ionosphere is described. The density distributions of six ion species (NO+, N2+, N2+, O+, N+, He+) and the electron and ion temperatures are obtained from a numerical solution of the appropriate continuity, momentum and energy equations. The equations are solved as a function of height for an inclined magnetic field atE andF region altitudes. The three-dimensional nature of the model is obtained by following flux tubes of plasma as they convect or corotate through a moving neutral atmosphere. The model takes account of field-aligned diffusion, cross-field electrodynamic drifts, thermospheric winds, polar wind escape, energy-dependent chemical reactions, neutral composition changes, ion production due to solar EUV radiation and auroral precipitation, thermal conduction, diffusion-thermal heat flow and local heating and cooling processes. The model also takes account of the offset between the geomagnetic and geographic poles. A complete description of the ionospheric model is given, including a derivation of the relevant transport equations, formulas for all of the chemical and physical processes contained in the model, a discussion of the numerical technique, and a description of the required model inputs. The effects that various chemical and physical processes have on the ionosphere are also illustrated.

Transport equations for aeronomy

Abstract In this paper we present results for a general system of transport equations appropriate to a multi-constituent gas mixture. This system includes a continuity, momentum, internal energy, pressure tensor and heat flow equation for each species. The results can be applied to both collision dominated and collisionless plasmas with there being explicit limits derived for the validity of the various expressions. In the limit of very frequent collisions the pressure tensor and heat flow equations give the usual Navier-Stokes results for the viscous stress tensor and heat flow vector. Furthermore, the momentum equation includes thermal diffusion and thermoelectric transport coefficients equivalent to the second approximation of Chapman and Cowling. The basic system of equations has been applied to different regions of the ionosphere and neutral atmosphere. It is found that: (1) The viscous stress tensor and heat flow expressions used in previous studies of the neutral thermosphere may not be appropriate; (2) The transport coefficients normally used for mid-latitude F2-region and topside studies seem to be adequate; (3) The high speed flow of plasma in the polar topside ionosphere is likely to be strongly affected by stresses and heat flow; and (4) E- and F-region ionization at high latitudes is substantially affected by stresses and heat flow.

A comparison of the temperature and density structure in high and low speed thermal proton flows

Abstract The continuity, momentum and energy hydrodynamic equations for an H + -O + topside ionosphere have been solved self-consistently for steady state conditions similar to those found outside the plasmasphere. Results are given for undisturbed and trough conditions with a range of H + outflow velocities yielding subsonic and supersonic flow. In the formulation of the equations, account was taken of the velocity dependence of ion-neutral, ion-ion and ion-electron collision frequencies. In addition, parallel stress and the nonlinear acceleration term were retained in the H + momentum equation. Results computed from this model show that, as a result of Joule (frictional) heating, the H + temperature rises with increasing outflow velocity in the subsonic flow regime, reaching a maximum value of about 4000 K. For supersonic flow other terms in the H + momentum equation become important and alter the H + velocity profile such that convection becomes a heat sink in the 1000–1500 km altitude range. This, together with the reduced Joule heating resulting from the high-speed velocity dependence of the H + collision frequencies, results in a decrease in the H + temperature as the outflow velocity increases. However, for all outward flows the H + temperature remains substantially greater than the O + temperature. With identical upper boundary velocities, the H + flow velocity is higher at low altitudes for trough conditions compared with non-trough conditions, but the H + temperature in the trough is lower. The form of the H + density profiles for supersonic flow does not in general differ greatly from those obtained with wholly subsonic flow conditions.

Modeling Polar Cap F-Region Patches Using Time Varying Convection

Here the authors present the results of computerized simulations of the polar cap regions which were able to model the formation of polar cap patches. They used the Utah State University Time-Dependent Ionospheric Model (TDIM) and the Phillips Laboratory (PL) F-region models in this work. By allowing a time varying magnetospheric electric field in the models, they were able to generate the patches. This time varying field generates a convection in the ionosphere. This convection is similar to convective changes observed in the ionosphere at times of southward pointing interplanetary magnetic field, due to changes in the B[sub y] component of the IMF.

Ionosphere-Thermosphere Space Weather Issues

Abstract Weather disturbances in the ionosphere-thermosphere system can have a detrimental effect on both ground-based and space-based systems. Because of this impact and because our field has matured, it is now appropriate to develop specification and forecast models, with the aim of eventually predicting the occurrence, duration, and intensity of weather effects. As part of the new National Space Weather Program, the CEDAR community will focus on science issues concerning space weather, and this tutorial/review is an expanded version of a tutorial presentation given at the recent CEDAR annual meeting. The tutorial/review provides a brief discussion of weather disturbances and features, the causes of weather, and the status of weather modeling. The features and disturbances discussed include plasma patches, boundary and auroral blobs, sun-aligned polar cap arcs, the effects of traveling convection vortices and SAID events, the lifetime of density structures, sporadic E and intermediate layers, spread F and equatorial plasma bubbles, geomagnetic storms and substorms, traveling ionospheric disturbances (TIDs), and the effects of tides and gravity waves propagating from the lower atmosphere. The tutorial/review is only intended to provide an overview of some of the important scientific issues concerning ionospheric-thermospheric weather, with the emphasis on the ionosphere. Tutorials on thermospheric and magnetospheric weather issues are given in companion papers.

Storm-Time Density Enhancements in the Middle Latitude Dayside Ionosphere

[1] Enhancements of the total electron content (TEC) in the middle-latitude dayside ionosphere have often been observed during geomagnetic storms. The enhancements can be as large as a factor of 2 or more, and many sightings of such structures have occurred over the United States. Here we investigate the effectiveness of an expanded convection electric field as a mechanism for producing such ionospheric enhancements. As a test case, we examine the storm period of 5-7 November 2001, for which observations from the DMSP F 13 are used to drive the Time Dependent Ionospheric Model (TDIM). Our findings indicate that at favorable universal times, the presence of the expanded electric field is sufficient to create dayside TEC enhancements of a factor of 2 or more. The modeled enhancements consist of locally produced plasma; we do not find it necessary to transport high-density plasma northward from low latitudes.

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 systems 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 T e and T i 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 T e , 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 storms 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 storms peak, the H + and O + densities increase at high altitudes (positive storm effect) and decrease at low altitudes (negative storm effect).