57 research outputs found
A Conversation with Dr. Roderick Heelis
No full textA Conversation With...Dr. Roderick Heelis, the Cecil and Ida Green Honors Professor of Physics and director of the William B. Hanson Center for Space Sciences at UT Dallas.
Dr. Heelis is a fellow of the American Geophysical Union, an international scientific society devoted to the research of the Earth and space.
He joined the UT Dallas Center for Space Sciences in 1973, after graduating from the University of Sheffield (England) with a Ph.D. in applied and computational mathematics.
Heelis is an expert on “space weather,” the phenomenon of disturbances that occur in the ionosphere, the gaseous band of charged particles that surround the Earth.
Space weather “storms” can disrupt GPS signals and wreak havoc on navigation systems for planes, trucks, ships and even missiles.
Heelis’ work may lead to a system that can predict when ionospheric storms are brewing, allowing crucial navigation systems time to adapt.
He is a passionate advocate for space science research and inspiring the next generation of students seeking answers from above the atmosphere
Modeling the daytime energy balance of the topside ionosphere at middle latitudes
No full textRecently reported measurements from the Defense Meteorological Satellite Program (DMSP) indicate that the O⁺ temperature in the topside ionosphere is dependent on the fractional H⁺ density. This finding indicates that the mass-dependent energy exchange rate between O⁺ and H⁺ plays an important role in the thermal balance of the topside ionosphere. In this study we utilize the SAMI2 model to retrieve both T_{H⁺} and T_{O⁺} and verify the previously observed dependence of ion temperature on ion composition. The model shows that in the topside at middle latitudes when a single ion is dominant, O⁺ or H⁺ is heated by electron collisions and cooled by conduction as expected. However, in the intervening altitude region where both O⁺ and H⁺ are present, O⁺ is heated by collisions with H⁺ and cooled by conduction, while H⁺ is heated by collisions with electrons and cooled by collisions with O⁺.NASA Grant NNX10AT02G.School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Changes in thermospheric temperature induced by high-speed solar wind streams
Get PDFDuring high-speed stream (HSS) events the solar wind speed increases, and the cross polar cap potential increases, leading to increased Joule heating at high latitudes. The heat input at high latitudes heats the polar regions, which then conducts to lower latitudes, producing global heating. The heating occurs during the risetime of the cross polar cap potential and throughout the period of high cross polar cap potential as seen in our simulation. These simulations are performed using the Utah State University global thermosphere model driven by Joule heating rates that are consistent with electric fields observed by DMSP-15 observations of HSS events. Cooling occurs as the cross polar cap potential decreases and continues for several days after the cross polar cap potential has returned to background values. Polar cap ionospheric observations are compared to model simulations of heating and cooling, providing evidence that the thermospheric model is capturing the HSS energy input and the post-HSS multiday return to pre-HSS conditions. The HSS heating can be as high as 100 K (as seen from both the model and the data) at high latitudes, with a corresponding, but lower, global increase in thermospheric temperature
Measurement of Individual H <sup>+</sup> and O <sup>+</sup> Ion Temperatures in the Topside Ionosphere
No full textPlasma temperatures in the ionosphere are associated with both the dynamics and spatial distribution of the neutral and charge particles. During the daytime, temperatures are determined by solar energy inputs and energy exchange between charged and neutral particles. Plasma transport parallel to the magnetic field adds another influence on temperatures through adiabatic processes that are most evident during the nighttime. Previous observations suggest that the topside H⁺ temperature (T_{H⁺+}) should reside between the O⁺ temperature (T_{O+}) and the electron temperature (T_e), and further calculations confirm the preferential heat transfer from the electrons to H⁺ in the topside. In this work we implement a more sophisticated analysis procedure to extract individual mass-dependent ion temperatures from the retarding potential analyzer measurements on the DMSP F15 satellite. The results show that the daytime T_{H+} is a few hundred degrees higher than T_{O+} at all longitudes. The nighttime temperature difference between T_{H+} and T_{O+} is indicative of mass-dependent adiabatic heating and cooling processes across the equatorial region. The ion temperatures and measured plasma flows present clear longitudinal variations that are associated with magnetic declination.This work is supported by NASA grant NNX10AT02G and by AFOSR MURI grant FA9559‐16‐1‐0364.School of Natural Sciences and Mathematic
Response of Low-Latitude Ionosphere to Medium-Term Changes of Solar and Geomagnetic Activity
No full textThe paper presents the medium-term quasi periodic (∼9-27 day) response of middle and low-latitude ionosphere to solar F10.7) and geomagnetic (Kp-index) forcing. The ionospheric response is examined by wavelet analysis of the relative deviations of TEC over Japan for the period of time 2000-2008. It is found that the ∼27-day rTEC oscillations correlate well with the same oscillations of the solar index F10.7 particularly in the solar maximum and its early declining phase (2001-2005). During the declining phase of solar activity (for example, year of 2005) the Kp-index variability exhibits additionally strong oscillations with periods 13.5- and 9-days. Similar oscillations are found in rTEC as well but they do not follow the geomagnetic forcing as faithfully as those associated with F10.7. During solar minimum the quasi periodic rTEC variability is shaped mainly by the recurrent geomagnetic activity. An attempt is made to investigate the latitudinal dependence of the ∼9-27-day rTEC response over Japan as well as the phase relationship between the forcing and response. © 2012. American Geophysical Union. All Rights Reserved
Motions of the Convection Reversal Boundary and Local Plasma in the High‐Latitude Ionosphere
No full textWe present results from a systematic study of multisatellite samplings from the Defense Meteorological Satellite Program F13, F15, F16, F17, and F18 satellites over the period from 2007 to 2015 that describe the motion of the convection reversal boundary (CRB) and the local plasma flow across it. Focusing on the cases with continuous poleward and equatorward CRB motion sampled by three consecutive satellites within 50 min, 45% of the time the CRB motion may deviate from the local plasma motion near dawn and dusk where the reconnection process is unlikely to be present. Differences in the inferred CRB motion and the local plasma motion may arise from apparent motion induced by the local time displacement of consecutive samples across the CRB that is tilted with respect to a line of constant latitude. The presence of a viscous- like interaction across the CRB can also contribute to the difference in the CRB and plasma motion. Accounting for these processes, the CRB motion and the motion of the plasma at the CRB are consistent only if a back and forth motion over a timescale of a few minutes is superimposed on a monotonic migration of the CRB over longer time periods.This work is supported by NASA grant NNX14AF33G and by AFOSR MURI grant FA9559-16-1-0364School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Temporal Characteristic of the Mesoscale Plasma Flow Perturbations in the High‐Latitude Ionosphere
No full textSpatial and temporal characteristics of flow perturbations in the high-latitude ionosphere are important considerations for energy deposition from the magnetosphere. In this study, we examine the temporal characteristics of plasma flow perturbations with spatial scales between 100 and 400 km from two consecutive Defense Meteorological Satellite Program (DMSP) passes that have about the same orbital plane and sample time spacing between a few seconds and 20 min during local summer seasons in 2007-2015. The temporal characteristics of mesoscale flow perturbations are described by rise and saturation times for growth and decay derived from the changes in magnitude of perturbations and the time separation between consecutive samples. Observations suggest that the rise times for both growth and decay are shorter for small spatial scales (1-2 min, 100-200 km) and longer for large spatial scales (3-5 min, 200-400 km). The saturation time for decay is similar to 10 min for small scales and similar to 20 min for large scales. The growth saturation time is about 5-10 min for both scale sizes. These characteristic times for growth are always shorter than the decay times. If the difference in these characteristic times between growth and decay is produced by motion of a perturbation with the background flow through the observed volume, then a longitudinal scale size of 750 km or 1.5 hr of local time is implied.AFOSR MURI grant FA9559-16-1-0364School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Plasma Dynamics Associated With Equatorial Ionospheric Irregularities
No full textThe Communication/Navigation Outage Forecasting System satellite was operational from 2008, a period of deep solar minimum, to 2015, a period of moderate solar conditions. The behavior of the vertical plasma drift and the distribution of plasma depletions during the deep solar minimum of 2009 deviated substantially from the behavior that was observed during the solar moderate conditions encountered by the Communication/Navigation Outage Forecasting System satellite in 2014, which are typical of previous observations. Presented here are observations of the vertical drift of plasma depletions and the background plasma in which they are embedded. We find that depletions detected at local times after 2100 hr during solar minimum are typically found in background drifts that are weakly downward compared to the strongly downward background drifts observed during moderate solar activity levels. Additionally, at solar minimum, the drift within the depletions is upward with respect to the background as compared with observations at the same local times during solar moderate conditions for which the depleted plasma more nearly drifts with the background. We note that weak background plasma drifts observed throughout the night during solar minimum promote the continued growth of depletions that may evolve more slowly or be continuously generated to appear in the topside in the postmidnight hours. ©2018. American Geophysical Union. All Rights Reserved.NASA grant NNX15AT31G.School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
Effects of electric field methods on modeling the midlatitude ionospheric electrodynamics and inner magnetosphere dynamics
No full textWe report a self-consistent electric field coupling between the midlatitude ionospheric electrodynamics and inner magnetosphere dynamics represented in a kinetic ring current model. This implementation in the model features another self-consistency in addition to its already existing self-consistent magnetic field coupling with plasma. The model is therefore named as Ring current-Atmosphere interaction Model with Self-Consistent magnetic (B) and electric (E) fields, or RAM-SCB-E. With this new model, we explore, by comparing with previously employed empirical Weimer potential, the impact of using self-consistent electric fields on the modeling of storm time global electric potential distribution, plasma sheet particle injection, and the subauroral polarization streams (SAPS) which heavily rely on the coupled interplay between the inner magnetosphere and midlatitude ionosphere. We find the following phenomena in the self-consistent model: (1) The spatially localized enhancement of electric field is produced within 2.5 < L < 4 during geomagnetic active time in the dusk-premidnight sector, with a similar dynamic penetration as found in statistical observations. (2) The electric potential contours show more substantial skewing toward the postmidnight than the Weimer potential, suggesting the resistance on the particles from directly injecting toward the low-L region. (3) The proton flux indeed indicates that the plasma sheet inner boundary at the dusk-premidnight sector is located further away from the Earth than in the Weimer potential, and a "tongue" of low-energy protons extends eastward toward the dawn, leading to the Harang reversal. (4) SAPS are reproduced in the subauroral region, and their magnitude and latitudinal width are in reasonable agreement with data.NSFC. Grant Numbers: 41574156, 41431071; Los Alamos National Laboratory Directed Research and Development (LDRD) program. Grant Number: DE-AC52-06NA25396.School of Natural Sciences and Mathematic
Mesoscale Plasma Convection Perturbations in the High‐Latitude Ionosphere
No full textAn investigation of flow perturbations with spatial scale sizes between 100 and 500 km in the high-latitude ionosphere is presented. These localized flow perturbations are deviations from the large-scale background convection, expected to give us new insights into the magnetosphere-ionosphere coupling process. Ion drift measurements from the Defense Meteorological Satellite Program F17 are utilized to identify these mesoscale flow perturbations. Our intent is to discover the properties of these perturbations in terms of perturbation flow speeds, location, scale size, and occurrence frequency as well as their dependence on the interplanetary magnetic field (IMF) and underlying large-scale convection pattern. Observation suggests that flow perturbation locations strongly depend on the IMF orientation as does the occurrence frequency of the flow perturbations. For southward IMF, more flow perturbations occur in regions of sunward background flow than in regions of antisunward background flow. For flow perturbations with speeds over 300 m/s, an asymmetry in the preferred direction and scale size is seen for those embedded in sunward and antisunward background flows. Significantly less asymmetry is present for flow perturbations with speeds between 100 and 300 m/s. The flow perturbations exceeding 300 m/s are most likely closed locally with lower magnitude return flows or with adjacent flows across the convection reversal boundary and representing additional sources of frictional heating and momentum transfer to the thermosphere. The perturbation flow speed is almost independent of the scale size and underlying convection speed, but the largest speeds are preferentially seen at scale sizes between 200 and 300 km.AFOSR MURI grant FA9559-16-1-0364.School of Natural Sciences and MathematicsWilliam B. Hanson Center for Space Science
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