Atmosphere, Ionosphere & Magnetosphere(AIM)The Earth's Upper AtmosphereThe Ionosphere, Plasmasphere and Magnetosphere
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Other HAO Sections C & H | LSA | SIV | Facilities | Community |
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The Earth's Upper AtmosphereBasic Structure | Variability and Disturbances | Solar Irradiance Effects | Large-scale Dynamics | Gravity Waves |
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Basic Structure-Sea Surface Temperature Effects on Middle/Upper Atmospheric Structure- Raymond Roble (HAO) contributed to the NCAR interdivisional effort to study the effects of sea surface temperatures on the structure and composition of the Earth's middle and upper atmosphere. Fabrizio Sassi (CGD) led this effort with contributions from Douglas Kinnison (ACD), Byron Boville (CGD), Rolando Garcia (ACD) and Roble. They forced the lower boundary of the Whole Atmosphere Community Climate Model (WACCM) with observed sea surface temperature (SST) for the period 1950-2000. This period contained several instances of anomalously warm/cold tropical SST, identified by the so-called NINO3 index. The WACCM results revealed anomalies in the structure of vertically propagating planetary waves from the troposphere to the mesosphere. Circulation anomalies in the middle atmosphere were accompanied by large temperature anomalies that were of opposite sign in the stratosphere and mesosphere, the stratosphere being warmer during El Niño events, whereas the mesosphere was colder. Near the summer mesopause, an anomalous source of momentum due to parameterized gravity waves resulted in warming during El Niño. Compositional differences during El Niño/La Niña events also occured, with more ozone depletion occurring during La Nina. Detailed analysis showed statistically significant structure anomaly predictions. The model's mean climatology revealed that Rossby-planetary waves affected the zonal mean circulation up to about the stratopause, whereas parameterized gravity waves were more important in the El Niño/La Niña response. -Interferometric Measurements of Upper Atmospheric Winds- Qian Wu (HAO) developed a new Fabry-Perot interferometer (FPI) in collaboration with R. Daniel Gablehouse, Stanley Solomon, and Timothy Killeen (all of HAO). They designed the FPI to measure mesospheric and lower thermospheric winds, tides, and the upper thermosphere polar cap convection pattern using the OH, O 5577 Å and 6300 Å airglow. The wind errors for these emissions are 6 m/s (3 minute integration), 1 m/s (3 minute) and 2-6 m/s (5 minute), respectively. The instrument was tested in Boulder, Colorado and resultant measurements compared favorably with lidar mesospheric neutral wind measurements made nearby in Fort Collins, Colorado and provided by Chiao-Yao She (Colorado State University). Thereafter, the FPI was deployed at Resolute, Canada (75 N), the future site of the National Science Foundation Advanced Modular Incoherent Scatter Radar (AMISR). The neutral wind data obtained at Resolute also demonstrated that the instrument met the design goal and will provide high quality measurements for studies of mesospheric and lower thermospheric dynamics, as well as magnetospheric-ionospheric coupling and upper atmospheric ion-neutral coupling processes in the polar cap. -Thermospheric Neutral Composition Tongue- Alan Burns and Webin Wang (both of HAO) collaborated with Killeen and Solomon to describe the existence of a "tongue" of neutral composition in the upper thermosphere that is similar to the well-known tongue of ionization. The tongue formed in much the same way that the tongue of ionization does; that is, parcels of air, which are rich in O/N2, are drawn from the dayside by the anti-sunward winds associated with the neutral convection pattern and transported across the polar cap towards the night side auroral oval. The O/N2-rich air can only be transported from a small region on the dayside due to the geometry of the transport, so the neutral tongue is narrow like the ion tongue. But, the neutral tongue is different is several ways; it tends to be weaker than the ion tongue, it extends a shorter distance across the polar cap, and it takes longer to form. Once the neutral tongue forms in conjunction with the tongue of ionization, the latter becomes both stronger and longer, but causality in this relationship has yet to be established.
Variability and Disturbances
-Seasonal Variability of the 6.5-Day Planetary Wave-
-Precursors to a Stratospheric Warming Event- In the 2003 ASR, Liu and Roble reported that the TIME-GCM was able to simulate the major stratospheric warming event (hereafter stratwarm) in the Southern Hemisphere in the year-long simulation of 2002. They subsequently analyzed the event further to reveal a likely mechanism associated with the stratwarm. They explored the possible role of the mesosphere in the dynamical processes using data from both the TIME-GCM and NCEP reanalyses. They found that significant changes in the wind and temperature fields occur in the mesosphere first, due to a strong wave 1 event about a month before the major warming in the stratosphere. Thereafter, a series of wave events during the following month erode the polar jet and alter the transmission conditions for planetary waves at progressively lower altitudes. This helps to set up the atmospheric conditions favorable for the upward and poleward propagation of wave energy, not only for wave 1 but also for wave 2 and 3. At the same time, the jet reversal and the planetary wave surf zone also descend from the mesosphere down to the stratosphere. The preconditioning ultimately leads to an extensive breaking of the polar jet and wave 1 in the stratosphere and thus, the major warming.
-Estimates for Sources of Thermospheric Heating- Delores Knipp (United States Air Force Academy) worked with W. Kent Tobiska (Space Environment Technologies), and Barbara Emery (HAO) to estimate the thermospheric heating sources over 29 years for solar cyles 21-23. They reported that the major heating source (~78%) is solar EUV radiation, as specified by the updated SOLAR2000 model. Geomagnetic power included the effects of solar particles, including auroral electrons as filtered through the magnetosphere, and Joule heating. The particle precipitation, estimated using newly intercalibrated hemispheric power indices provided 5-8% of the total power under all conditions. The Joule heat source was estimated from the Thule Polar Cap (PC) index and the Disturbed electrojet (Dst) index using Assimilative Mapping of Ionospheric Electrodynamics (AMIE) Joule heat. The Joule source provided ~16% of the total power most of the time, but ~45% for the top 1% of power events, and up to 70% in some of the top 20 events over the 29 years studied. The most variable power source was Joule heating. -Solar Cycle Variations in Geomagnetic Storm Reponses- Burns, Killeen, Wang, and Roble completed their study, which quantifies the relationship between solar cycle variations in the background thermosphere and ionosphere and thermospheric-ionospheric responses to geomagnetic storms. Their findings include: larger increases of N2 mass mixing ratio at much lower winter geomagnetic latitudes during solar minimum than solar maximum due to comparatively more important horizontal advection during solar minimum; less molecular rich high-latitude air during solar minimum with stronger equatorward winds because the pressure gradients driven by solar EUV heating are weaker, rendering the circulation driven by high-latitude Joule heating relatively more important. In contrast to the compositional variations, storm-time absolute temperature increases are greater during solar maximum than during solar minimum. The initial thermal recovery from storm-time perturbations is faster during solar maximum, but then recovery rates are roughly equivalent after the first 12 hours of the recovery period. The initial compositional recovery also occurs much more rapidly at solar maximum than at solar minimum.
Solar Irradiance Effects-Initial Results from the Solar Extreme-Ultraviolet Experiment (SEE)- Tom Woods, Frank Eparvier (both of University of Colorado), Scott Bailey (University of Alaska), Phillip Chamberlain (University of Colorado), Judith Lean (Naval Research Laboratory), Gary Rottman (University of Colorado), Solomon, Tobiska, and Don Woodraska (University of Colorado) reported on the initial results from the solar extreme-ultraviolet (EUV) experiment (SEE) on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite. Thereafter, Solomon and Liying Qian (HAO) developed a method for applying solar EUV and soft X-ray spectra measured by SEE and by the X-ray photometer system on the Solar Radiation and Climate Experiment (SORCE) satellite to the NCAR TIE-GCM. Qian, Solomon, Roble, and Woods made comparisons between upper thermosphere densities calculated using index-based proxy-model solar inputs and the actual solar measurements. Together with Frank Marcos (Air Force Research Laboratory), these collaborators subsequently reported comparisons with measurements of upper thermosphere densities using the decay rates of orbiting satellites. Solomon and Qian also led the effort to study the upper atmospheric impact of the first full spectral measurements of major solar flares in the extreme-ultraviolet range made by the SEE on the TIMED satellite. Solomon, Gang Lu (HAO), Qian, Art Richmond (HAO), and Woods reported on the ionization, temperature, and density enhancements associated with the flares as modeled by the NCAR TIE-GCM when the SEE measurements were included as inputs to the model.
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Solar spectrum in the ultraviolet and X-ray spectral region measured by TIMED/SEE for the X-17 flare on 28 October 2003. Blue line - before flare. Red line - near peak of the flare. Note that while the spectrum longward of 27 nm is measured by a spectrometer and is highly accurate, the spectrum shortward of 27 nm is measured by photometers and there are significant uncertainties in the magnitude and spectral distribution of the irradiance enhancement.
Comparison of models of upper thermosphere density at 400 km during August/September 2002 to measurements using the satellite drag technique. Solid black line - measurement. Dotted black line - Mass Spectrometer Incoherent Scatter (MSIS-90) empirical model. Solid red line - TIE-GCM with measured solar inputs. Dotted red line - TIE-GCM with proxy-model solar inputs. |
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Large-Scale Dynamics-Studies of Tides and Planetary Waves in the MLT Region- A series of model and measurement comparisons were coupled with diagnostic studies involving the TIME-GCM and the global-scale wave model (GSWM), providing new insight into the behavior and impact of tides and planetary waves in the MLT. Tides are global-scale waves with periods that are harmonics of a solar day. They are the most striking dynamical features in the MLT region, but they can be modulated by more transient planetary waves. She and Tao Li (Colorado State University), Richard Collins (University of Alaska), Tao Yuan, Bifford Williams (both of Colorado State University), Takuya Kawahara, Joseph Vance, Phillip Acott, David Krueger (all of Colorado State University), Liu and Maura Hagan (both of HAO) reported on the short-term variability of the diurnal and semidiurnal tidal temperature and horizontal winds over Fort Collins, Colorado. They reported modulation of the tidal amplitudes by planetary waves with periods of 3 to 5 days. Michael Taylor, A. K. Taori, D. R. Hatch (all of Utah State University), collaborated with Liu and Roble in an investigation of a semiannual oscillation in mesospheric temperatures observed over Maui, Hawaii. Scott Palo (University of Colorado), Liu, Rolando Garcia (NCAR/ACD), James Russell III (Hampton University), and Martin Mlynczak (NASA Langley) investigated the propagation of the quasi-two-day wave from the middle atmosphere into the thermosphere/ionosphere as observed by the SABER instrument on TIMED and modeled by the TIME-GCM. Wu, Gablehouse, Killeen, and Solomon studied mesospheric neutral wind data from 1995 to 1999 obtained with a Fabry-Perot Interferometer (FPI) stationed at Resolute Bay, Canada (75N, 95W). A 12-hour wave was the most prominent feature in the neutral winds. The wave amplitude exhibited large day-to-day variations as well as inter-annual variability, whereas the phase of the wave appeared largely invariant. A small phase shift occurring between early to later winter was the noticeable exception, which occurred when early winter data were available. Large westward zonal winds were observed during stratospheric sudden warming events. The most pronounced 12-hour wave variability occurred during the 1995/1996 northern winter season, and may be related to non-linear interactions with planetary waves. Wu and Killeen also collaborated with David Ortland (Northwest Research Associates), Solomon, Gablehouse, and Roberta Johnson (also of HAO), Rick Niciejewski, Wilbert Skinner (both of University of Michigan), and Steve Franke (University of Illinois) in a study of the migrating (or Sun-synchronous) diurnal and semidiurnal tides deduced from TIMED Doppler Interferometer (TIDI) neutral wind data. They found clear signs of diurnal and semi-diurnal tides in TIDI meridional winds in the MLT region. The TIDI diurnal tide has a vertical wavelength close to 20 km, with an amplitude peak at ~ 97 km. The GSWM diurnal tidal predictions have similar amplitudes, but they peak above 100 km, and have a longer vertical wavelength (~ 25 km). They also reported that TIDI meridional winds are in good agreement with meteor radar measurements made over Maui, Hawaii. Jens Oberheide (Wuppertal University) led an effort to characterize a second class of diurnal tidal oscillations in the TIDI meridional wind data in collaboration with Wu, Ortland, Killeen, Hagan, and Roble. Their focus was the nonmigrating diurnal components with particular attention to the westward propagating wavenumber 2 tide, the eastward propagating wavenumber 3 tide, and the zonally symmetric standing (wavenumber 0) tide. They reported the analysis of March 2002 to June 2004 TIDI measurements that were made between 85 and 105 km altitude. The 95 km monthly-averaged meridional tidal winds compared favorably with similar tidal component estimates deduced from UARS satellite data at the same altitude but for a different time period (1992-1995). The TIDI nonmigrating diurnal tidal wind speeds reached amplitudes in excess of 30 m/s, emphasizing the important role of nonmigrating tides in coupling the middle and upper atmosphere. The TIME-GCM and the GSWM predictions indicate that both latent heat release in the tropical troposphere and non-linear planetary wave/migrating tidal interactions are important sources of nonmigrating tides in the MLT.
Gravity Waves-Scale Invariance of Temperature and Horizontal Wind Spectra- Liu studied the possible origin and implications of the scale invariance of the vertical wavenumber and frequency spectra of temperature and horizontal wind. The vertical wavenumber and frequency power spectra of the horizontal wind and temperature in the troposphere, stratosphere, and mesosphere often exhibit power law distribution on scales larger than the turbulence inertial range. The spectral slopes are quite independent of altitudes and specific measurement locations, and indicate "universal" scale invariance in the atmosphere. The origin of the universal spectra is yet to be fully understood, though previous studies have suggested possible relation to gravity wave saturation and/or nonlinear interaction of the gravity waves. In this work, Liu demonstrated that 1D (vertical) stochastic systems, subject to adjustment due to convective or shear instability, possess scale invariance similar to that observed in the atmosphere. Such systems can be described by stochastic, nonlinear, diffusive equations, and Liu proved that these equations support scale invariance by applying a dynamical renormalization group (DRG) analysis. The DRG analysis yields scaling indices and provides a plausible explanation and ranges of the variation of these indices. The calculated indices and the variation ranges compared favorably with the values reported in previous studies. Liu also showed that the renormalized diffusion coefficients in such systems are scale dependent, and derived the scaling relations. -Mesospheric Variability from Stochastic Gravity Wave Sources- Liu, Roble, and Hagan examined mesospheric variability due to stochastic gravity wave sources. As in many other general circulation models, the source of the parameterized gravity wave specified at the lower boundary of the NCAR TIME-GCM is uniform in the longitudinal direction and time, and varies smoothly over latitudes. Such a specification is quite arbitrary, due to the lack of knowledge of the distribution and variability of various gravity wave sources. The source strength and the associated momentum flux are tuned such that the temperature, altitude and strength of the jet reversal in the mesosphere agree with observations. The gravity waves in the real world, evidently, are highly variable spatially and temporally. Liu, Roble, and Hagan explored the impacts of such variability on the dynamics of the mesosphere by using the TIME-GCM and stochastic gravity wave sources in time and in longitude and latitude, while keeping the mean characteristics of the wave sources the same. To isolate sources of variability, no planetary waves (except the thermal tides) were included in the simulation. The resultant mesospheric winds and temperatures displayed increased temporal and spatial variability. The density of the zonal wind power spectrum in the zonal direction at higher wavenumbers was higher than in the control case, and the Nastrom-Gage spectrum was recovered with the spatially random gravity wave sources. This implies that random and local gravity wave forcing in the mesosphere can help distribute energy to smaller scales and contribute to the cascading process of quasi-geostrophic turbulence. Liu, Roble, and Hagan also investigated implications for the tidal variability. -A New Parameterization for Convectively Generated Gravity Waves- NCAR ASP postdoctoral fellow Jadwiga (Yaga) Beres (HAO/ACD/CGD) implemented a new method of specifying the gravity wave spectrum above convection into the WACCM, in collaboration with Garcia, Boville, and Sassi. This parameterization is interactive with the underlying convection, and introduces realistic spatial and temporal distributions of gravity wave activity, as well as yielding gravity wave characteristics related to the underlying wave source. The new scheme improves the structure of the tropical stratospheric and mesospheric semiannual oscillations in WACCM, and gives insight into the wave motions that might be responsible for extratropical mesospheric forcing. The newly implemented scheme also provides a new direction for gravity wave parameterizations, as the gravity wave forcing is no longer fixed and will follow changes in tropospheric climate. This allows for more accurate studies of effects of changing climate on the middle atmosphere and on examining the feedbacks between the troposphere and the middle atmosphere. Beres also continued her collaboration with M. Joan Alexander (Colorado Research Associates), who led the effort to gain a better understanding of wave generation by convection via comparisons between theoretical gravity wave estimates and numerical simulations results. They also used observations to force a mesoscale numerical model. -Mountain Wave Studies with an Extended WRF Model- University of Colorado graduate student, Natalie Rooney, collaborated with Liu, and Jimy Dudhia (MMM) to extend the Weather Research and Forecasting Model (WRF) model into the mesosphere to study the impact of mountain waves in the upper atmosphere. The WRF is a regional, non-hydrostatic model designed for operational weather forecasting, regional climate prediction, air-quality simulation, and idealized dynamical studies. Rooney and colleagues adapted the model and extend it to 90 km. The resultant simulations encountered a numerical instability related to noise introduced by the conventional sigma coordinate. They implemented the hybrid terrain-following vertical coordinate to correct the problem. This change resulted in a more accurate upward integration of the pressure profile in the upper atmosphere, and produced other evidence of improved stability. They performed several tests to verify that the change in the vertical coordinate did not disturb the dynamics of the model. Using their extended version of the WRF model, Rooney, Liu, and Dudhia were able to study the effects of various background wind profiles on the mountain wave propagation. One interesting case study involved a time-varying eastward wind in the troposphere and a constant westward wind in the upper atmosphere. They found that the gravity waves generated by the gusty wind at lower levels can have a wide phase speed spectrum. Some of the transient components were able to penetrate into the upper atmosphere. Since this is a fairly typical summer wind profile, they concluded that it is important to study the effects of these transients on dynamics of the upper atmosphere. |
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The Ionosphere, Plasmasphere, and Magnetosphere Structure and Dynamics | Variability and Disturbances | Coupling Between Geospace Domains |
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Structure and Dynamics-Spatial Structure of the Electron Diffuse Aurora- Margaret Chen (Aerospace Corp.), Michael Schulz (Lockheed Martin), Phillip Anderson (Aerospace Corp.), Lu, Glen Germany (University of Alabama), and Martin Wuest (Southwest Research Institute) studied the spatial structure of the electron diffuse aurora during geomagnetic storms. They found that the distributions of precipitating diffuse aurora depend strongly on variation of the magnetotail electron distributions, electron transport, and electron pitch angle scattering. By using two different scatter rate models (e.g., strong uniform diffusion versus a magnetic local time (MLT)-dependent but relatively weaker scattering rate), they found that the simulated storm-time energy flux with strong diffusion tends to be much more intense in the evening sector and much weaker near dawn than what is statistically observed. The most intense electron precipitation under strong diffusion will occur where the electron drift times from the plasmasheet are on the order of the electron lifetime against strong diffusion. On the other hand, the model and data comparison shows that the MLT-dependent but relatively weaker scattering produces a more realistic electron diffuse aurora than with the strong diffusion. -Predicting the Storm-Time Dst Profile- George Siscoe (Boston University), Robert McPherron (UCLA), Michael Liemohn, Arron Ridley (both of the University of Michigan), and Lu reexamined the so-called Burton et al. equation for predicting the storm-time Dst profile. With the aid of the Ring Current Atmosphere Interaction Model (RAM), they showed that by reformulating the Burton et al. equation such that a quadratic form of the driving term replaces the original linear form, the new prediction algorithm, driven by the polar-cap potential saturation form of the driving electric field, produces an equally good fit to the Dst index as the original Burton et al. equation and the RAM simulations. The form of the quadratic driving term is constructed by specializing a general form given by dimensional analysis. -Objective Analysis of High-Latitude Ionospheric Electrodynamic Variables- Tomoko Matsuo (CGD), Richmond, and Lu implemented the Optimal Interpolation (OI) method, in conjunction with Empirical Orthogonal Function (EOF) bases and the maximum likelihood method for on-line error covariance parameter estimation, for the objective analysis of high-latitude ionospheric electrodynamic variables. This work is an important step in understanding how scale-dependent properties of electromagnetic energy and momentum transfer processes affect the global thermospheric Joule heating estimation. Their study demonstrated how this methodology may be used to extract information about the temporal coherence of large- and meso-scale electric field variability, for a magnetic cloud event on January 10-11, 1997. They found that the time scale of the dominant modes of electric field variability generally decreased with increasing EOF order and decreasing spatial scale. Compared with the temporal persistence of the Interplanetary Magnetic Field (IMF) and solar wind parameters, the time scale of the spatially coherent part of the electric field variability, on the spatial scale of the EOFs, is shorter. They analyzed the principal components of the electric field variability during two periods when either the IMF By or Bz component was relatively steady while the other component varied. The first principal component generally reflected the change in the ionospheric convection pattern predicted by IMF-dependent empirical models, and thus tended to represent changes in the convection that are directly driven by solar wind-magnetosphere interactions. However, they found that the second principal component more strongly correlated with the westward auroral electrojet, suggestive of a link to substorm phenomena. -A Simplified Model for High-Latitude Energy Transfer- At high latitudes the interaction between the ionosphere and the magnetosphere can be expressed by the electromagnetic energy flux. For the static case, the electromagnetic energy flux (Poynting flux) is equivalent to the work done by the Ampere force on the neutral gas and the rate at which the atmosphere is heated by Joule heating. The latter is the dominant part. Numerical models of the upper atmosphere often underestimate the highly varying contribution of Joule heating, since the Joule heat estimated from the model Pederson conductance and model electric fields squared only represents the mean values of the conductance and electric fields, and does not represent their variability. Therefore Astrid Maute (HAO), with the assistance of Jeff France (HAO summer undergraduate student), developed a simplified model of the high latitude energy transfer by analyzing the Poynting flux from the Ion Drift Meter (IDM), the Retarding Potential Analyzer (RPA) data, and the MAGB magnetometer on board the Dynamics Explorer 2 satellite. The simplified model depends on magnetic local time, dipole tilt and the magnetic activity index Ap (or Kp) to be used in the WACCM. -Simulations of Ionospheric Electric Fields and Vertical Drifts- Geeta Jadhav (HAO visitor, Indian Institute of Geomagnetism) and Richmond examined the quiet-time low-latitude ionospheric electric fields and vertical drifts in the evening sector at equinox as a function of longitude and solar activity, using simulations from a global model with interactive thermosphere-ionosphere dynamics and electrodynamics. The model inputs were held fixed with respect to universal time, in order to evaluate the contributions to longitudinal variations of the equatorial electric fields and drifts associated with zonal variations in the geomagnetic field. The simulated upward evening drift increases with solar activity, in agreement with observations in the Indian, Peruvian and Brazilian sectors. The simulations also agree with observations in finding larger vertical drifts in the American-Atlantic sector than in the East Asian-Pacific sector, generally associated with longitudinal variations in the strength of the geomagnetic field. Relations among the longitude variations of the vertical drift, the conductivity, the eastward wind velocity, the geomagnetic declination, and gradients of the wind and declination were examined, revealing few clear correlations. However, extrema of many of these quantities were noted in the American-Atlantic sector. -AMIE-Driven Ionospheric Modeling- Pierre-Louis Blelly, J. Fontanari, and Denis Alcayde (all of the Centre d'Etude Spatiale des Rayonnements; CESR) used results from their 13-moment ionospheric TRANSCAR model, driven with 10-minute Assimilative Mapping of Ionospheric Electrodynamics (AMIE) outputs provided by Emery, in a comparative analysis with European Incoherent SCATter (EISCAT) radar data provided by Chantal Lathuillere and Jean Lilensten (both of the Laboratoire de Planetologie de Grenoble). The AMIE ion drifts from cross-tail potential patterns underestimated the maximum ion drifts above the EISCAT transmitter at Tromsø by a factor of 2.5. The AMIE drifts were then used to infer magnetic flux tube transport over the polar cap. AMIE auroral electron patterns were used along with AMIE parallel currents as magnetospheric particle and electron heat flux inputs, respectively, to the ionosphere. The resulting simulation of the electron density and ion and electron temperatures between about 100 and 600 km matched the EISCAT observations well, although some differences were found which could be attributed to the preciptation of electrons with very low energies which are not included in AMIE. -Magnetic Reconnection in the Magnetopause- Stefan Eriksson, Scott Elkington (both University of Colorado), Tai-Duc Phan (University of California, Berkeley), Steven Petrinec (Lockheed Martin), Henri Reme (CESR, France), Malcolm Dunlop (Rutherford Appleton Laboratory, UK), Michael Wiltberger (HAO), A. Balogh (Imperial College London, UK), Robert Ergun (University of Colorado), and Mats Andre (Swedish Institute of Space Physics) collaborated to compare detailed observations of flank magnetopause reconnection with results from simulations of the magnetospheric configuration performed using the Lyon-Fedder-Mobarry (LFM) global-scale magnetohydrodynamic (MHD) code. Their results suggest that the Cluster spacecraft were passing through the simulated MHD sash region in the Northern Hemisphere on 30 June 2001, while crossing the magnetopause just equatorward of the Southern Hemispheric sash on 29 May 2001. Plasma jets were detected from a location sunward and mostly poleward of the spacecraft on 30 June, but reaching Cluster from a poleward and somewhat antisunward location relative to sc1 on 29 May. The Walen test confirmed that the observed jets flow at the local Alfven velocity in the deHoffmann-Teller frame, and Eriksson and collaborators interpreted them as velocity enhancements due to magnetic reconnection. The observed plasma jet directions compared well with those expected from antiparallel merging in the simulated MHD sash. The local magnetic field shear between the observed magnetosheath and the geomagnetic fields ranged between 162 and 168 degrees in the estimated tangential magnetopause plane for these two events. A projection of antiparallel merging regions and the tilted component merging line onto the magnetopause on 30 June and 29 May together with these Cluster observations support the view that the IMF controls the global location of magnetic reconnection on the magnetopause in contrast to any randomly occurring reconnection process driven by locally fulfilled conditions. |
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A 3D topological view of the MHD magnetosphere and the magnetic fields surrounding Cluster sc1 (yellow diamond) from the Eriksson et al. 2004: Two cross-sections of the magnetosphere are shown at z = 0 and y = 0 respectively. A selected number of magnetic flux tubes are displayed with red indicating "open-open" fields, blue depicting "closed-closed" fields, and yellow showing "open-closed" flux tubes. |
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-Comparison of Simulated/Observed Field-Aligned Currents- Haje Korth, Brian Anderson (both Johns Hopkins University), Wiltberger, John Lyon (Dartmouth College), and Phillip Anderson (Aerospace Corporation) compared global magnetospheric simulation results of the field-aligned current (FAC) density at low altitudes with global two-dimensional distributions of Birkeland currents at the topside ionosphere, derived from magnetic field data of the Iridium satellite constellation. They studied two events with opposite directions of the IMF By, 23 November 1999, 1400 -1700 UT, By > 0, and 31 March 2000, 1500-1800 UT, By < 0. In both observations and simulations, the configuration of the FACs near noon displayed the expected reversal of sense with latitude corresponding to the sign of By. The total Region-1 current in the simulations (2-3 MA) was larger than that derived from Iridium (1-1.5 MA), consistent with known underestimates from Iridium, but there was a deficit of Region 2 in the simulations. The poleward limit of the simulated Region-1 currents was significantly (< 5°) poleward of the observed currents for standard resolution simulations. Doubling the simulation resolution led to a marked improvement in the agreement with the observed spatial distribution of Region-1 FACs. Moreover, the higher resolution led to the appearance of some Region-2 FACs in the November 23 event. The results imply that in addition to including a ring current module to properly account for the particle drift physics, high simulation resolution and improved nighttime ionospheric conductivities are two factors that must be addressed in order to obtain reliable simulation results. | |
Variability and Disturbances-Photochemical Modeling of the E-Region Ionosphere- Rick Doe, Jeff Thayer (both SRI), and Solomon performed photochemical modeling of the E-region ionosphere for comparison with electron density measurements made by the Sondrestrom incoherent scatter radar facility during geomagnetically quiet, daytime conditions. Comparisons between electron densities calculated by the NCAR Global Airglow (GLOW) model and measured by the radar were performed for an array of solar zenith angles and solar activity levels, showing good correspondence in both dimensions of this variability. Correspondence of the grand average of seven years of radar data and model runs was remarkable, demonstrating that the solar inputs and ion-neutral chemical parameters are in agreement with the lower ionosphere electron density, at least in a mean sense.
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Average electron density profile from seven years of incoherent scatter radar measurements at Sondrestromfjord, Greenland. |
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-A Statistical Study of High-Latitude Joule Jeating- M. Geoffrey McHarg, Francis Chun (both United States Air Force Academy) worked with Knipp, Lu, and Emery to carry out a statistical study of the high-latitude Joule heating in response to the IMF. They used approximately 9000 Northern Hemisphere (NH) AMIE Joule heat patterns to characterize the Joule heating in terms of the IMF components. They found that during extreme northward IMF conditions, Joule heating is restricted to the high-latitude dayside. During strongly southward IMF condition, Joule heating is located predominantly in the auroral region, with increased heating in the morning sector compared to the evening sector. They also found only a weak IMF By effect in the Joule heating distributions. -Analysis of Peterson/Hall Conductance Variations- Working under the direction of Maute, Linde Clark (HAO summer undergraduate student) used fourteen years of EISCAT data to derive height-integrated Pedersen and Hall conductivities. The conductances were binned by UT, season, solar cycle, and polar cap index, and their variations with these parameters were analyzed and qualitatively compared with conductances derived from the IRI 90 model. Both the Pedersen and Hall conductances show peaks during the night (LT=UT+1), corresponding to times of significant particle precipitation. The Hall conductance exhibits more UT variation than the Pedersen conductance because the Hall conductance is more sensitive to hard particle precipitation. During solar minimum, the conductances show the most UT variation and have the largest values in the autumn. The large nighttime conductance levels in autumn could be attributed to ion velocities. -Modification of the Ionospheric Dynamo by Penetration Electric Fields- HAO postdoctoral fellow Naomi Maruyama examined the relative importance between direct penetration and disturbance dynamo electric fields in the storm-time equatorial ionosphere. This study was undertaken in collaboration with Timothy Fuller-Rowell, Mihail Codrescu (both NOAA Space Environment Center and University of Colorado), Richmond, George Millward (University College, London), Stanislav Sazykin, Frank Toffoletto, and Bob Spiro (all Rice University). They utilized the Coupled Thermosphere-Ionosphere-Plasmasphere-Electrodynamics (CTIPe) model for their study, and imposed electric fields as specified by the Weimer electric field model and the Rice Convection Model (RCM). They found dominant penetration electric field effects during daytime and at the early stage of the storm (see accompanying figure, results at 05 UT on March 31) associated with the sudden enhancement of the polar cap potential. As a consequence of the enhancement of the upward drift, the electron density peak is lifted and moved poleward, and the density is increased in the equatorial ionosphere. They found comparable penetration and disturbance dynamo effects at night. The disturbed drift showed a reversal from the downward to the upward direction. Their results demonstrate that the direct penetration electric field can modify the ionospheric dynamo by changing the neutral wind and conductivity in the F-region ionosphere. This modification is preferentially observed during nighttime since the nighttime ionospheric electric field is mainly determined by the F-region dynamo, in contrast to the daytime E-region dynamo driven mainly by the tidal forcing propagating from the lower atmosphere.
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An example of the vertical ExB drifts at the magnetic equator obtained from CTIPe for 36 hours starting from 12UT on March 30 2001, at a longitude of 127 degrees. The RCM electric field is imposed from 00UT on March 31 2001. At this longitude sector, 00UT corresponds to 8.47LT. Four different CTIPe simulations are shown. The black dash line is the quiet time reference. The black solid line is the simulation with disturbance dynamo effect only. The blue solid line is the simulation with the penetration electric field only. The red solid line is the simulation including both effects of the disturbance dynamo and penetration electric fields. The disturbance electric field is dominated by the penetration effect at around 05UT on March 31 during daytime and at the early stage of the storm, while the nighttime disturbance observed at around 16UT is determined by the combination of both the penetration and disturbance dynamo electric fields. |
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-Comparison of Simulated Storm-time and Isolated Substorms- Wiltberger led an effort to compare LFM global magnetohydrodynamic (MHD) simulations of isolated substorms to those occurring during magnetic storms. His collaborators in this study were Elkington, Timothy Guild (Boston University), Daniel Baker (University of Colorado), and Lyon. They found that the August 27, 2001 substorm was fairly typical and isolated as monitored by an extensive set of satellite and ground-based observations. The LFM simulation results for the interval agreed with some of these observations. The simulation results also showed that thin current sheets developed during the growth phase of substorms and were disrupted, in part, by flows originating in the mid-tail region prior to ionospheric signatures of substorm onset. Detailed analysis of the energy partitioning in the magnetotail showed that the transfer of energy from the lobes into the plasma sheet occured a few minutes before substorm onset. The magnetic storm, which began on March 31, 2001, was one of the largest magnetic storms during the current solar cycle. A very large substorm occurred during this interval with an onset at 0630 UT. Results from the LFM simulation of this period agree with some observations made by the constellation of spacecraft that monitored this substorm. Comparisons between the LFM results for these two events indicate that storm-time and isolated substorms share the same essential features, including the development of intense thin current sheets during growth phase, the disruption of this current sheet linked to activity in the mid-tail region, and rapid recovery phases. | |
Coupling Between Geospace Domains-Riometer Observations of Sawtooth Particle Injections- HAO postdoctoral fellow Andrew Kavanagh, Lu, Eric Donovan (University of Calgary, Canada), Geoff Reeves (Los Alamos National Laboratory), F. Honary (Lancaster University, UK), J. Manninen (Sodankyla Geophysical Observatory, Finland), and Thomas Immel (University of California, Berkeley) investigated simultaneous riometer observations in the nightside and dawn-noon sectors during sawtooth particle injections (which are characterized as a series of 2-3 hour, quasi-periodic, energetic particle flux sudden enhancements, followed by gradual decay), measured by the geostationary satellites during the geomagnetic storm on 18 April 2002. They found a good agreement between the nightside riometer absorption in the low-altitude ionosphere and the expected propagating discrete flux enhancement during the four substorms associated with sawtooth injections in the equatorial magnetosphere. -Inner Magnetosphere-Radiation Belt Coupling During Geomagnetic Storms- Elkington, Baker, and Wiltberger studied the coupling between the inner magnetosphere and the radiation belts during a geomagnetic storm that began on March 31, 2001. This storm was characterized by high solar wind speeds and long intervals of strongly southward interplanetary magnetic field. These conditions led to high levels of magnetospheric convection and significant distortion in the inner magnetosphere. During a period of particularly intense driving by the solar wind, a substorm onset was observed to inject energetic particles into low L-values in the premidnight region of the inner magnetosphere. Particle simulations of keV protons, injected from the plasma sheet into the inner magnetosphere, driven by fields taken from the LFM global- scale MHD simulation, show qualitative agreement with global observations of the event. These results suggest that a weak field region in the near-Earth tail may have served as a source region for the more energetic particles injected into the inner magnetosphere. -Analysis of Auroral Electron, Ion, and Total Hemispheric Powers- Barbara Emery and Weibin Xu (both of HAO) combined twenty-six years of low-energy auroral electron and total hemispheric power indices from 21 NOAA and DMSP satellites to produce both hourly and daily median and average composite indices for the Southern and Northern Hemispheres. The SEM-2 NOAA satellites also provided estimates of the auroral ion hemispheric power over the last 6 years. Daily median intersatellite correlations exceeded 70% most of the time after contamination or degradation issues were addressed, and the median daily values usually agreed within 5% overall, after making baseline adjustments that ranged within a factor of two. Initial corrections were made to eliminate sunlight contamination, data dropouts over the auroral oval, the degradation of sensors over time, high spurious count rates, and increased noise at the end of a satellite lifetime. Adjustments were also made in most satellites so that the ratio of the south to north electron or total hemispheric power was approximately one over a year. The first accompanying Figure shows the adjusted 27-day median of daily median Hp values for the Southern Hemisphere for all satellites. The ratio of the concurrent median daily hemispheric power from both hemispheres in the second Figure shows that the winter values are almost 30% higher than the summer values during solar maximum, and about 5% higher in solar minimum conditions. The winter/summer concurrent ratios of the ion hemispheric power are opposite to those of the electrons, showing increases in the concurrent summer to winter ratios of about 27%, independent of solar cycle as seen in the third Figure. Ions contribute most to the total in quiet conditions. Kp and Ap are strongly correlated with the electron and the ion daily hemispheric power, with correlation coefficients of about 0.8 for the electrons, and about 0.6-0.7 for the ions. The Hp adjustments will be revised with new estimates using the latest DMSP calibration corrections in FY2005.
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 Southern/Northern Hemisphere (SH/NH) composite median, 27-day total, or electron hemispheric power from daily composite medians from 21 NOAA and DMSP satellites. |
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 Southern (SH), Northern (NH), and ratio (SH/NH) median 27-day ion hemispheric power from daily medians from three NOAA satellites from ions < 20 keV after the adjustments for ion hemispheric power are applied. |
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-Initial Results from the CMIT Model- Wiltberger and Wang worked with Burns, Solomon, Lyon, and Charles Goodrich (Boston University) to report on initial results with the new Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model for the Center for Integrated Space Weather Modeling (CISM). CMIT combines the LFM model with the Thermosphere-Ionosphere Nested Grid (TING) model. The LFM uses the ideal MHD equations to model the interaction between the magnetospheric plasma and the solar wind. The CMIT magnetosphere-ionosphere interaction requires the conservation of current flowing between the magnetosphere and the ionosphere. TING solves the mass, momentum, and thermodynamic energy equations for the global thermosphere and ionosphere. The TING model results replace the simple 2D ionosphere within the LFM, while the LFM results replace the empirical cross-cap potential drop and field-aligned currents in TING to produce the CISM CMIT Model. Comparisons between the CMIT results for a series of steady IMF with the results from the stand-alone LFM and TING models show that the component models have been successfully coupled. The CMIT ionospheric conductances show a more realistic distribution driven by EUV radiation, as well as a more clearly defined auroral oval. In addition, the CMIT simulation resulted in Joule heating enhancements that affected the neutral atmosphere by producing temperature increases and stronger winds.
-Analysis of Doppler Spectral Width Parameter Behavior-
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