Engineering topics

Schunk, R. W.

Publications and source records attributed to Schunk, R. W..

At least 19 records

NASA NTRS2017

CEDAR-GEM Challenge for Systematic Assessment of Ionosphere/Thermosphere Models in Predicting TEC During the 2006 December Storm Event

In order to assess current modeling capability of reproducing storm impacts on total electron content (TEC), we considered quantities such as TEC, TEC changes compared to quiet time values, and the maximum value of the TEC and TEC changes during a storm. We compared the quantities obtained from ionospheric models against ground-based GPS TEC measurements during the 2006 AGU storm event (14-15 December 2006) in the selected eight longitude sectors. We used 15 simulations obtained from eight ionospheric models, including empirical, physics-based, coupled ionosphere-thermosphere, and data assimilation models. To quantitatively evaluate performance of the models in TEC prediction during the storm, we calculated skill scores such as RMS error, Normalized RMS error (NRMSE), ratio of the modeled to observed maximum increase (Yield), and the difference between the modeled peak time and observed peak time. Furthermore, to investigate latitudinal dependence of the performance of the models, the skill scores were calculated for five latitude regions. Our study shows that RMSE of TEC and TEC changes of the model simulations range from about 3 TECU (total electron content unit, 1 TECU = 1016 el m2) (in high latitudes) to about 13 TECU (in low latitudes), which is larger than latitudinal average GPS TEC error of about 2 TECU. Most model simulations predict TEC better than TEC changes in terms of NRMSE and the difference in peak time, while the opposite holds true in terms of Yield. Model performance strongly depends on the quantities considered, the type of metrics used, and the latitude considered.

Shim, J. S.↗

NASA NTRS2011

CEDAR Electrodynamics Thermosphere Ionosphere (ETI) Challenge for Systematic Assessment of Ionosphere/Thermosphere Models: NmF2, hmF2, and Vertical Drift Using Ground-Based Observations

Objective quantification of model performance based on metrics helps us evaluate the current state of space physics modeling capability, address differences among various modeling approaches, and track model improvements over time. The Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Electrodynamics Thermosphere Ionosphere (ETI) Challenge was initiated in 2009 to assess accuracy of various ionosphere/thermosphere models in reproducing ionosphere and thermosphere parameters. A total of nine events and five physical parameters were selected to compare between model outputs and observations. The nine events included two strong and one moderate geomagnetic storm events from GEM Challenge events and three moderate storms and three quiet periods from the first half of the International Polar Year (IPY) campaign, which lasted for 2 years, from March 2007 to March 2009. The five physical parameters selected were NmF2 and hmF2 from ISRs and LEO satellites such as CHAMP and COSMIC, vertical drifts at Jicamarca, and electron and neutral densities along the track of the CHAMP satellite. For this study, four different metrics and up to 10 models were used. In this paper, we focus on preliminary results of the study using ground-based measurements, which include NmF2 and hmF2 from Incoherent Scatter Radars (ISRs), and vertical drifts at Jicamarca. The results show that the model performance strongly depends on the type of metrics used, and thus no model is ranked top for all used metrics. The analysis further indicates that performance of the model also varies with latitude and geomagnetic activity level.

Shim, J. S.↗

NASA NTRS1995

What is the source of observed annual variations in plasmaspheric density?

Plasmaspheric densities have been observed previously to be higher in December than in June, with the ratio varying between 1.5 and 3.0 and with larger variations at lower L shells. In order to search for the cause of the observed annual variations, we have modeled plasmaspheric density, using a time-dependent hydrodynamic model. On an L = 2 field line with geomagnetic longitude equal to 300 deg, the modeled plasmaspheric densities were a factor of 1.5 times higher in December than in June. The modeled December to June density ratio was found to increase slightly with L shell, in contrast to observations; this discrepancy may be due to the fact that outer plasmaspheric flux tubes are never completely full. In addition, for an L = 2 field line with geomagnetic longitude equal to 120 deg, the modeled plasmaspheric density was higher in June than in December by a factor of about 1.2. Various numerical tests were also performed in order to examine the sensitivity of plasmaspheric density to various parameters. In particular, a large vertical neutral wind was applied in order to raise the O(+) profile which had the effect of raising plasmaspheric density by a factor of 6. This in conjunction with a theoretical analysis suggests that plasmaspheric density levels are very sensitive to O(+) levels in the upper ionosphere. We conclude that annual variations in plasmaspheric density are due to similar variations in ionospheric O(+).

Guiter, S. M.↗

NASA NTRS1994

Ionospheric response to the sustained high geomagnetic activity during the March '89 great storm

A simulation was conducted to model the high-latitude ionospheric to the sustaied level of high geomagnetic activity for the great magnetic storm period of March 13-14, 1989. The geomagnetic and solar activity indices and the Defense Meterological Satellite Program (DMSP) F8 and F9 satellite data for particle precipitation and high-latitude convection were used as inputs to a time-dependent ionospheric model (TDIM). The results of the TDIM were compared to both DMSP plasma density data and ground-based total electron content (TEC) measurements for the great storm period as well as with earlier storm observations. The comparisons show that the overall structure of the high-latitude ionosphere was dominated by an increased convection speed within the polar cap that led to increased ion temperatures. In turn, this enhanced the NO(+) density, raised the atomic-to-molecular ion transition height to over 300 km, decreased N(sub m)F(sub 2), increased h(sub m)F(sub 2), and in places either increased n(sub e) at 800 km or slightly decreased it. The morphology of the ionosphere under these extreme conditions was considerably different than that modeled for less distributed intervals. These differences included the character of the dayside tongue of ionization that no longer extended deep into the polar cap. Instead, as a result of the ion heating and consequent reduction in N(sub m)F(sub 2), a large polar hole occupied much of the polar region. This polar hole extended beyond the auroral oval and merged with the night sector midatitude trough. The limitaions associated with the applicability of the TDIM to the geomagnetic conditions present on March 13 and 14 are discussed. The primary limitations of the TDIM derive from the limited temporal resolution of the model input parameters and the lack of suitably dynamic thermospheric specification for the great storm conditions. These limitations leads to midlatitude ionospheric storm phases that do no follow those observed.

Sojka, J. J.↗

NASA NTRS1994

A Monte Carlo simulation of the effect of ion self-collisions on the ion velocity distribution function in the high-latitude F-region

Non-Maxwellian ion velocity distribution functions have been theoretically predicted and confirmed by observations, to occur at high latitudes. These distributions deviate from Maxwellian due to the combined effect of the E x B drift and ion-neutral collisions. At high altitude and/or for solar maximum conditions, the ion-to-neutral density ratio increases and, hence, the role of ion self-collisions becomes appreciable. A Monte Carlo simulation was used to investigate the behavior of O(+) ions that are E x B-drifting through a background of neutral O, with the effect of O(+) (Coulomb) self-collisions included. Wide ranges of the ion-to-neutral density ratio n(sub i)/n(sub n) and the electrostatic field E were considered in order to investigate the change of ion behavior with solar cycle and with altitude. For low altitudes and/or solar minimum (n(sub i)/n(sub n) less than or equal to 10(exp -5)), the effect of self-collisions is negligible. For higher values of n(sub i)/n(sub n), the effect of self-collisions becomes significant and, hence, the non-Maxwellian features of the O(+) distribution are reduced. The Monte Carlo results were compared to those that used simplified collision models in order to assess their validity. In general, the simple collision models tend to be more accurate for low E and for high n(sub i)/n(sub n).

Barghouthi, I. A.↗

NASA NTRS1994

Plasmaspheric convection with non-closed streamlines

A mechanism is described to drive a continuous outflow of thermal plasma in the plasmasphere without violating Faraday's law. It is based on time-dependent interchange motion. time-dependent interchange motion.

Lemaire, J. F.↗

NASA NTRS1994

Patches in the polar ionosphere: UT and seasonal dependence

The seasonal and Universal Time (UT) dependencies of patches in the polar ionosphere are simulated using the Utah State University time dependent ionospheric model (TDIM). Patch formation is achieved by changing the plasma convection pattern in response to temporal changes in the interplanetary magnetic field (IMF) B(sub y) component during periods of southwrd IMF. This mechanism redirects the plasma flow from the dayside high-density region, which is the source of the tongue of ionization (TOI) density feature, through the throat and leads to patches, rather than a continuous TOI. The model predicts that the patches are absent at winter solstice (northern hemisphere) between 0800 and 1200 UT and that they have their largest seasonal intensity at winter solstice between 2000 and 2400 UT. Between winter solstice and equinox, patches are strong and present all day. Patches are present in summer as well, although their intensity is only tens of percent above the background density. These winter-to-equinox findings are also shown to be consistent with observations. The model was also used to predict times at which patch observations could be performed to determine the contributions from other patch mechanisms. This observational window is +/- 20 days about winter solstice between 0800 and 1200 UT in the northern hemisphere. In this observational window the TOI is either absent or reduced to a very low density. Hence the time dependent electric field mechanism considered in this study does not produce patches, and if they are observed, then they must be due to some other mechanism.

Sojka, J. J.↗

NASA NTRS1994

Effect of high-latitude ionospheric convection on Sun-aligned polar caps

A coupled magnetospheric-ionospheric (M-I) magnetohydrodynamic (MHD) model has been used to simulate the formation of Sun-aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fields. The formation process involves launching an Alfven shear wave from the magnetosphere to the ionosphere where the ionospheric conductance can react self-consistently to changes in the upward currents. We assume that the initial Alfven shear wave is the result of solar wind-magnetosphere interactions. The simulations show how the E region density is affected by the changes in the electron precipitation that are associated with the upward currents. These changes in conductance lead to both a modified Alfven wave reflection at the ionosphere and the generation of secondary Alfven waves in the ionosphere. The ensuing bouncing of the Alfven waves between the ionosphere and magnetosphere is followed until an asymptotic solution is obtained. At the magnetosphere the Alfven waves reflect at a fixed boundary. The coupled M-I Sun-aligned polar cap arc model of Zhu et al.(1993a) is used to carry out the simulations. This study focuses on the dependence of the polar cap arc formation on the background (global) convection pattern. Since the polar cap arcs occur for northward and strong B(sub y) IMF conditions, a variety of background convection patterns can exist when the arcs are present. The study shows that polar cap arcs can be formed for all these convection patterns; however, the arc features are dramatically different for the different patterns. For weak sunward convection a relatively confined single pair of current sheets is associated with the imposed Alfven shear wave structure. However, when the electric field exceeds a threshold, the arc structure intensifies, and the conductance increases as does the local Joule heating rate. These increases are faster than a linear dependence on the background electric field strength. Furthermore, above the threshold, the single current sheet pair splits into multiple current sheet pairs. For the fixed initial ionospheric and magnetospheric conditions used in this study, the separation distance between the current pairs was found to be almost independent of the background electric field strength. For either three-cell or distorted two-cell background convection patterns the arc formation favored the positive B(sub y) case in the northern hemisphere.

Sojka, J. J.↗

NASA NTRS1994

Model study of multiple polar cap arcs: Occurrence and spacing

A new scenario for the formation of multiple polar cap arcs is proposed based on the results from a time-dependent electrodynamic model of polar cap arcs developed by Zhu et al. (1993). The results suggest that the appearance of multiple polar cap arcs may not be due to multiple structures in the magnetospheric source region, but instead, may primarily be determined by the coupled magnetosphere-ionosphere system in which the ionosphere plays an active role. It was found that with the same magnetospheric driver, a strong ionospheric background convection and an ionospheric background Hall conductance in the range of from 0.5 to 1.5 mho favor the occurrence of multiple polar cap arcs. It was also found that the spacing between individual arcs is highly correlated with the ratio of the enhanced Hall and Pedersen conductances (i.e., the hardness of the primary magnetospheric precipitation), which is allowed to vary in the model runs. A harder precipitation causes a wider spacing. In the modelings, the spacing ranged from about 20 kilometers to about 60 kilometers when the precipitation hardness increased.

Zhu, L.↗

NASA NTRS1994

Dynamics of three-dimensional plasma clouds with coupling to the background ionosphere

A three-dimensional, time-dependent model with a two-grid system was developed to study the expansion of a plasma cloud in the F region and topside ionosphere. The model maintains an adequate resolution for the released cloud motion and its interaction with the immediate environment, and it includes the effect due to the coupling with the distant part of the ionosphere (i.e., E region). Simulations were performed using realistic background ionospheric density profiles in both the E and F regions. The results show that the cloud coupling to the underlying E region affects the perpendicular cloud motion the most. The distant coupling acts to reduce the perturbation potential and perpendicular velocity and delays or eliminates the striations. These simulation results are consistent with simple analytical approximations. The simulation results also show that the distant coupling has a very small effect on 'localized' phenomena, such as the cloud expansion along the B RIGHT ARROW field and the electrostatic snowplow. The cloud-induced electric potential is attenuated in the lower E region. The electrons flow along the B RIGHT ARROW field, carrying the current to the E region and back to the cloud. The current closure is demonstrated in three dimensions for the first time for such a problem. The perpendicular current flowing through the plasma cloud is closed by the field-aligned electron current and the background perpendicular (mainly Pedersen) current in both the E and F regions. The 'image cloud' formation in t he E region is also clearly demonstrated. The variation of the density change in the 'image cloud' along the B RIGHT ARROW field and the features of the image cloud are shown.

Ma, T.-Z.↗

NASA NTRS1994

A multi-ion generalized transport model of the polar wind

The higher-order generalizations of the equations of standard hydrodynamics, known collectively as generalized transport theories, have been used since the early 1980s to describe the terrestrial polar wind. Inherent in the structure of generalized transport theories is the ability to describe not only interparticle collisions but also certain non-Maxwellian processes, such as heat flow and viscous stress, that are characteristic of any plasma flow that is not collision dominated. Because the polar wind exhibits a transition from collision-dominated to collisionless flow, generalized transport theories possess advantages for polar wind modeling not shared by either collision-dominated models (such as standard hydrodynamics) or collisionless models (such as those based on solving the collisionless Boltzmann equation). In general, previous polar wind models have used generalized transport equations to describe electrons and only one species of ion (H(+)). If other ion species were included in the models at all, it was in a simplified or semiempirical manner. The model described in this paper is the first polar wind model that uses a generalized transport theory (bi-Maxwellian-based 16-moment theory) to describe all of the species, both major and minor, in the polar wind plasma. In the model, electrons and three ion species (H(+), He(+), O(+)) are assumed to be major and several ion species are assumed to be minor (NO(+), Fe(+), O(++)). For all species, a complete 16-moment transport formulation is used, so that profiles of density, drift velocity, parallel and perpendicular temperatures, and the field-aligned parallel and perpendicular energy flows are obtained. In the results presented here, emphasis is placed on describing those constituents of the polar wind that have received little attention in past studies. In particular, characteristic solutions are presented for supersonic H(+) outflow and for both supersonic and subsonic outflows of the major ion He(+). Solutions are also presented for various minor ions, both atomic and molecular and both singly and multiply charged.

Demars, H. G.↗

NASA NTRS1993

Comparison of generalized transport and Monte-Carlo models of the escape of a minor species

The steady-state diffusion of a minor species through a static background species is studied using a Monte Carlo model and a generalized 16-moment transport model. The two models are in excellent agreement in the collision-dominated region and in the 'transition region'. In the 'collisionless' region the 16-moment solution contains two singularities, and physical meaning cannot be assigned to the solution in their vicinity. In all regions, agreement between the models is best for the distribution function and for the lower-order moments and is less good for higher-order moments. Moments of order higher than the heat flow and hence beyond the level of description provided by the transport model have a noticeable effect on the shape of distribution functions in the collisionless region.

Demars, H. G.↗

NASA NTRS1993

Modeling polar cap F-region patches using time varying convection

Creation of polar cap F-region patches are simulated for the first time using two independent physical models of the high latitude ionosphere. The patch formation is achieved by temporally varying the magnetospheric electric field (ionospheric convection) input to the models. The imposed convection variations are comparable to changes in the convection that result from changes in the B(y) IMF component for southward IMF. Solar maximum-winter simulations show that simple changes in the convection pattern lead to significant changes in the polar cap plasma structuring. Specifically, in winter, as enhanced dayside plasma convects into the polar cap to form the classic tongue-of-ionization the convection changes produce density structures that are indistinguishable from the observed patches.

Sojka, J. J.↗

NASA NTRS1993

A time-dependent model of polar cap arcs

In the present 2D time-dependent model of polar cap arcs, the electrodynamics of the polar cap arcs are treated self-consistently in the frame of the coupled magnetosphere-ionosphere system. The focus of this work is a description of the features of the spatial structure and temporal evolution of the polar cap arcs. Results indicate that the time constant for the formation of the polar cap arcs is around 10 min; an initial single-arc precipitation pattern associated with a polar cap arc tends to split into multiple precipitation regions, yielding a multiple polar cap arc structure. Strong downward field-aligned currents can develop near the intensive upward field-aligned currents and form a pair structure of the field-aligned current in the polar cap arcs. The model predicts plasma flow crossing the polar cap arcs, but its amplitude is small and its characteristic time scale is much larger than the time constant for the formation of the polar cap arcs. While polar cap arcs are developing, the associated current system tends to become further localized.

Zhu, L.↗

NASA NTRS1993

Parameterized study of the ionospheric modification associated with sun-aligned polar cap arcs

The local ionospheric modification that is due to a generalized steady state solar aligned (SA) arc structure is addressed. For a representative set of SA arc parameters which includes both convection and precipitation, emphasis is placed on the modification by SA polar cap arcs upon the F region electron density and the height integrated conductivity. At low fluxes and low characteristic energies, SA polar cap arcs have the most pronounced relative effect at F region altitudes in darkness for winter solar minimum conditions. The absolute enhancement in summer solar minimum and winter solar maximum is equivalent to that of winter solar minimum, but the higher ambient densities make the relative enhancement less. The TEC enhancement associated with an SA arc may be used to indicate the degree of plasma cross flow across the arc.

Crain, D. J.↗

NASA NTRS1993

Influence of horizontal inhomogeneity in the ionosphere on the reflection of Alfven waves

A study of the reflection of Alfven waves at a horizontally inhomogeneous ionosphere has been carried out. In this study, the Alfven speed above the ionosphere is assumed to be uniform and the ionosphere is treated as a height-integrated conducting slab. Analytical and numerical results indicate that the horizontal nonuniformity of the ionospheric conductivity can lead to a rotation of the reflected wave fields and cause field-aligned currents that originate in the ionosphere. A strong conductivity nonuniformity on the direction perpendicular to the incident wave field, large Hall to Pedersen conductivity ratios, and low conductivity values lead to a large rotation of the reflected wave field in the range from a few to 40 deg. The implications of the theoretical results for some ionospheric phenomena are presented, including comments on the Harang discontinuity and sun-aligned arcs.

Zhu, L.↗

NASA NTRS1993

Ionization and expansion of barium clouds in the ionosphere

A recently envelope 3D model is used here to study the motion of the barium clouds released in the ionosphere, including the ionization stage. The ionization and the expansion of the barium clouds and the interaction between the clouds and the background ions are investigated using three simulations: a cloud without a directional velocity, a cloud with an initial velocity of 5 km/s across the B field, and a cloud with initial velocity components of 2 km/s both along and across the B field.

Ma, T.-Z.↗

NASA NTRS1992

High voltage spheres in an unmagnetized plasma - Fluid and PIC simulations

The basic physics involving the interaction of a high voltage sphere with a LEO-Plasma was investigated via computer experiments. Two approaches were used: the fluid description and the more general particle pushing method. Both techniques displayed qualitatively similar features. Thus, the initial time-dependent response of the plasma predicted by the fluid model, including the initial current surge and sheath formation, the formation of ion and electron density shells about the sphere with a double layer potential structure, and the subsequent propagation of the density shells away from the sphere. The PIC results also displayed an oscillatory character and slightly different time constants for the growth and decay mechanisms involved.

Thiemann, H.↗