Showing papers by "Arthur D. Richmond published in 2005"

Interaction between direct penetration and disturbance dynamo electric fields in the storm-time equatorial ionosphere

Abstract: [1] The direct penetration of the high-latitude electric field to lower latitudes, and the disturbance dynamo, both play a significant role in restructuring the storm-time equatorial ionosphere and thermosphere. Although the fundamental mechanisms generating each component of the disturbance electric field are well understood, it is difficult to identify the contribution from each source in a particular observation. In order to investigate the relative contributions of the two processes, their interactions, and their impact on the equatorial ionosphere and thermosphere, the response to the March 31, 2001, storm has been modeled using the Rice Convection Model (RCM) and the Coupled Thermosphere-Ionosphere-Plasmasphere-Electrodynamics (CTIPe) model. The mid- and low-latitude electric fields from RCM have been imposed as a driver of CTIPe, in addition to the high latitude magnetospheric sources of ion convection and auroral precipitation. The high latitude sources force the global storm-time wind fields, which act as the driver of the disturbance dynamo electric fields. The magnitudes of the two sources of storm-time equatorial electric field are compared for the March 2001 storm period. During daytime, and at the early stage of the storm, the penetration electric field is dominant; while at night, the penetration and disturbance dynamo effects are comparable. Both sources are sufficient to cause significant restructuring of the low latitude ionosphere. Our results also demonstrate that the mid- and low-latitude conductivity and neutral wind changes initiated by the direct penetration electric field preferentially at night are sufficient to alter the subsequent development of the disturbance dynamo.

Theoretical study of the low- and midlatitude ionospheric electron density enhancement during the October 2003 superstorm: Relative importance of the neutral wind and the electric field

Abstract: [1] During magnetic storms the ionospheric total electron content (TEC) at low- and midlatitudes often shows great enhancements, which may be associated with mechanisms producing midlatitude storm-enhanced density (SED). The TEC enhancements may result from different ionospheric drivers such as electric fields, neutral winds, and neutral composition effects. To study the importance of the ionospheric drivers in producing the TEC enhancement, we perform numerical simulations for the 29–30 October 2003 superstorm period in the American longitude sector (∼ −70°W) using the Sheffield University Plasmasphere Ionosphere Model (SUPIM) with values for the neutral wind, temperature, and composition provided by the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere General Circulation Model (TIEGCM). Various numerical experiments were run to identify the relative importance of the storm-time ionospheric drivers. For carrying out the storm-time SUPIM simulation, the storm-time upward/poleward E × B drifts are derived from ROCSAT-1 satellite measurements at low and equatorial latitudes and input to SUPIM, while the storm-time neutral wind and composition disturbances are obtained from TIEGCM run. The simulation results presented in this paper, mainly during the evening period, show that the enhanced upward E × B drifts due to storm-time eastward penetration electric field can expand the low-latitude equatorial ionization anomaly (EIA) to higher latitudes and produce the TEC enhancement. However, by the effect of penetration electric fields alone, the TEC enhancement is less than by combining the storm-generated equatorward neutral winds and the penetration electric fields. Disturbance neutral composition effects decrease the plasma density at higher latitudes and increase it at low and equatorial latitudes. However, the composition effects do not produce a density increase as large as that produced by the neutral-wind and electric-field effects. Our simulations suggest that the storm-generated equatorward neutral winds play an important role in producing the TEC enhancement at low- and midlatitudes, in addition to the eastward penetration electric field.

Theoretical effects of geomagnetic activity on low‐latitude ionospheric electric fields

Abstract: [1] The influence of geomagnetic activity on middle- and low-latitude thermospheric winds and ionospheric electric fields is investigated using model results from the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model. Model runs are made for different levels of geomagnetic activity. Model results show that the equatorward ionospheric currents produced by disturbance winds develop positive charge accumulation at low latitudes that maximizes in the premidnight sector. The local time of maximum electric potential perturbation depends significantly on universal time so that the local time of reversal of the equatorial zonal perturbation electric field varies with longitude by 2 to 3 hours, depending on the intensity of geomagnetic activity. The westward perturbation electric field in the postsunset period indicates that stronger geomagnetic activity will produce a lower driven height of the evening F region. After geomagnetic activity ceases, model results show that the zonal disturbance winds can last for many days in the postrecovery period, while the meridional disturbance winds decay more rapidly. The long-lasting zonal winds, through the Pedersen currents they drive, help maintain meridional disturbance potential drops that decay much more slowly than the zonal disturbance potential drops after the activity ceases.

Large-scale variations of the low-latitude ionosphere during the October–November 2003 superstorm: Observational results

Abstract: [1] The GPS-derived total electron content (TEC), ion drift measurements from the ROCSAT-1 spacecraft at around 600 km altitude, and far-ultraviolet airglow measured by the Global Ultraviolet Imager (GUVI) carried on board the NASA TIMED satellite are utilized for studying large disturbances of the low-latitude ionosphere during the October–November 2003 superstorm period. Two chains of GPS receivers, one in the American sector (∼70°W) and the other in the Asian/Australian sector (∼120°E), are used to simultaneously observe the daytime equatorial ionization anomaly (EIA) during the entire storm period. It is found from the GPS-TEC measurements that the EIA expanded to very high latitudes with large increases of TEC right after the storm started. The large expansion of the EIA was associated with strong upward E × B drifts measured from the Ionospheric Plasma and Electrodynamics Instrument (IPEI) on board the ROCSAT-1, providing evidence of a penetration electric field and a strong plasma fountain effect. Suppression of the EIA was observed during the storm recovery, associated with downward E × B drifts that were observed by the ROCSAT-1. Significant negative storm effects in the southern hemisphere were also observed in the GPS-TEC during the first day of the recovery phase. The areas of negative storm effects are in good agreement with reductions in the [O]/[N2] density ratio inferred from the ratio of OI (135.6 nm) to LBH emissions measured from GUVI. An enhancement of the EIA was observed on the day, 1 November, that the storm was about to fully recover.

Simulation study of the longitudinal variation of evening vertical ionospheric drifts at the magnetic equator during equinox

Abstract: [1] Quiet-time low-latitude ionospheric electric fields and vertical drifts in the evening sector at equinox are examined as a function of longitude and solar activity, using simulations from the Magnetosphere-Thermosphere-Ionosphere-Electrodynamics General Circulation Model (MTIEGCM). The model inputs are 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 is somewhat larger than observations. It increases with solar activity at a similar rate in all longitude sectors. This rate, about 0.32–0.37 m/s per unit of 10.7 cm solar flux, is quantitatively consistent with observations in the Peruvian sector but is considerably greater than observations in the Indian sector. The simulations 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 are examined, revealing few clear correlations. However, extrema of many of these quantities are noted in the American-Atlantic sector.

Optimal interpolation analysis of high-latitude ionospheric electrodynamics using empirical orthogonal functions: Estimation of dominant modes of variability and temporal scales of large-scale electric fields

Abstract: [1] In this paper the optimal interpolation method, in conjunction with empirical orthogonal function (EOF) bases and the maximum likelihood method for online error covariance parameter estimation, is successfully implemented for the objective analysis of large-scale high-latitude ionospheric electrodynamic variables. This study demonstrates how this methodology can be used to extract information about the temporal and spatial coherence of the large-scale electric field for a magnetic cloud event on 10–11 January 1997. Compared with the temporal persistence of the interplanetary magnetic field (IMF) and solar wind parameters, the timescale of the spatially coherent part of the electric field, on the spatial scale of the EOFs, is shorter. The principal components of the high-latitude electric field are analyzed during two periods when either the IMF BY or BZ component was relatively steady while the other component varied. The first principal component, not surprisingly, generally reflects the change in the ionospheric convection pattern predicted by IMF-dependent empirical models and thus tends to represent changes in the convection that are directly driven by solar wind–magnetosphere interactions. However, it is the second principal component that is more strongly correlated with the westward auroral electrojet, suggestive of a link to substorm phenomena.

Mean winds, tides, and quasi‐2 day wave in the polar lower thermosphere observed in European Incoherent Scatter (EISCAT) 8 day run data in November 2003

Abstract: [1] Simultaneous European Incoherent Scatter (EISCAT) radar observations using two EISCAT radars, the Tromso (69.6°N, 19.2°E) UHF radar and the EISCAT Svalbard radar (Longyearbyen, 78.2°N, 16.0°E), were conducted for 8 consecutive days, from 11 to 19 November 2003, to study the lower thermospheric wind dynamics in the polar region. Altitude profiles of the amplitudes of the diurnal and semidiurnal components at Tromso and Longyearbyen are similar. The semidiurnal amplitudes in the meridional and zonal components exhibit maxima at 105–107 km, with values of ∼70–90 m s−1. The semidiurnal phases vary with roughly a 30 km vertical wavelength between 98 and 110 km. The quasi-2 day wave (Q2DW) was not detected in the lower thermosphere at either Tromso or Longyearbyen during the period, while it was found between 70 and 82 km in the mesosphere with the colocated Tromso MF radar. This observational result suggests that in general, the Q2DW attenuates in the mesosphere and cannot penetrate into the lower thermosphere in winter. Ion drag acceleration of the wind is generally negligible below 107 km at Tromso and below 118 km at Longyearbyen, but significant ion drag acceleration is found above these heights. A comparison of mean wind and tidal amplitudes and phases with National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model predictions for the period shows some agreement.

Application of a fluid-type model for plasmapause and plasmaspheric refilling

Abstract: We have developed a new model that includes both plasmaspheric refilling and the evolution of the plasmapause. The model self-consistently solves the temporal variation of plasma distribution from the ionosphere within the dynamic plasmapause, as well as the refilling of plasma from the ionosphere. In the saturated plasmasphere, we found reasonable agreement between the model and previous measurements of electron densities. Furthermore, by imposing realistic electric fields and a depletion of flux tubes, the model demonstrates the temporal evolution of a sharp gradient in the plasma density distribution, such as seen at the plasmapause from the observations. In this presentation, a detailed analysis of the evolution of the plasmapause gradients is discussed. We also compare refilling time scales with observations.

A study on the ionosphere and thermosphere interaction based on ncar-tiegcm: dependence of the interplanetary magnetic field (imf) on the momentum forcing in the high-latitude lower thermosphere

Abstract: To understand the physical processes that control the high-latitude lower thermospheric dynamics, we quantify the forces that are mainly responsible for maintaining the high-latitude lower thermospheric wind system with the aid of the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM) Momentum forcing is statistically analyzed in magnetic coordinates, and its behavior with respect to the magnitude and orientation of the interplanetary magnetic field (IMF) is further examined By subtracting the values with zero IMF from those with non-zero IMF, we obtained the difference winds and forces in the high-latitude 1ower thermosphere( > 08| |) or negative( |) IMF- conditions, with maximum values appearing around -80 magnetic latitude Difference winds and difference forces for negative and positive have an opposite sign and similar strength each other For positive( > 03125||) or negative( |) IMF- conditions the difference winds and difference forces are noted to subauroral latitudes Difference winds and difference forces for negative have an opposite sign to positive condition Those for negative are stronger than those for positive indicating that negative has a stronger effect on the winds and momentum forces than does positive At higher altitudes(>125 km) the primary forces that determine the variations of tile neutral winds are the pressure gradient, Coriolis and rotational Pedersen ion drag forces; however, at various locations and times significant contributions can be made by the horizontal advection force On the other hand, at lower altitudes(108-125 km) the pressure gradient, Coriolis and non-rotational Hall ion drag forces determine the variations of the neutral winds At lower altitudes( condition the ion drag force tends to generate a warm clockwise circulation with downward vertical motion associated with the adiabatic compress heating in the polar cap region For positive IMF- condition it tends to generate a cold anticlockwise circulation with upward vertical motion associated with the adiabatic expansion cooling in the polar cap region For negative IMF- the ion drag force tends to generate a cold anticlockwise circulation with upward vertical motion in the dawn sector For positive IMF- it tends to generate a warm clockwise circulation with downward vertical motion in the dawn sector

LWS Proposal to Provide Scientific Guidance and Modeling Support for the Ionospheric Mapping Mission

Abstract: A data assimilation system for specifying the thermospheric density has been developed over the last several years. This system ingests GRACE/CHAMP-type in situ as well as SSULI/SSUSI remote sensing observations while making use of a physical model, the Coupled Thermosphere-Ionosphere Model (CTIM) (Fuller-Rowel1 et al., 1996). The Kalman filter was implemented as the backbone to the data assimilation system, which provides a statistically 'best' estimate as well as an estimate of the error in its state. The system was tested using a simulated thermosphere and observations. CHAMP data were then used to provide the system with a real data source. The results of this study are herein.