Showing papers by "Arthur D. Richmond published in 2001"

An investigation into the influence of tidal forcing on F region equatorial vertical ion drift using a global ionosphere-thermosphere model with coupled electrodynamics

Abstract: A recent development of the coupled thermosphere-ionosphere-plasmasphere model (CTIP) has been the inclusion of the electrodynamic coupling between the equatorial ionosphere and thermosphere. The vertical ion drifts which result are shown to be largely in agreement with empirical data, on the basis of measurements made at the Jicamarca radar and other equatorial sites [Scherliess and Fejer, 1999]. Of particular importance, the CTIP model clearly reproduces the “prereversal enhancement” in vertical ion drift, a key feature of the observational data. Inacurracies in the modeled daytime upward ion motion have been investigated with regard to changing the magnitude and phase of components of the lower thermospheric tidal forcing. The results show that daytime vertical ion motion is highly dependent upon both the magnitude and phase of the semidiurnal tidal component. In addition, the CTIP model shows the prereversal enhancement to be unaffected by changes in tidal forcing, but only for conditions of high solar activity. During periods of low solar activity the form of the prereversal enhancement is clearly dependant upon the magnitude and phase of the semidiurnal tide.

Coexistence of ionospheric positive and negative storm phases under northern winter conditions: A case study

Abstract: The response of the thermosphere and ionosphere to the famous January 10, 1997, geomagnetic storm is simulated using the thermosphere-ionosphere-electrodynamics general circulation model with realistic, time-dependent distributions of ionospheric convection and auroral precipitation as inputs. The simulation results show a dominant positive storm phase of increased F layer electron density over much of the northern winter hemisphere, but a negative storm phase with reduced electron density at middle and low latitudes is also evident in the simulation. The coexistence of both positive and negative storm phases is a result of the complex dynamical and chemical interactions between charged particles and neutral gases. The impulsive magnetospheric energy inputs via auroral precipitation and Joule heating generate traveling atmospheric and ionospheric disturbances (TADs and TIDs) which propagate from the northern auroral zone to lower latitudes and penetrate well into the Southern Hemisphere. The simulation results demonstrate that positive storm phases are caused primarily by enhanced auroral precipitation over high latitudes and by TIDs at middle and low latitudes. Globally speaking, composition changes in terms of enhancements in the N2/O ratio are mainly responsible for negative storm effects. However, although there is some correlation between increases in N2/O and decreases in the F layer critical frequency ƒoF2 in the winter hemisphere during the storm main phase and early recovery phase, the overall changes in ƒoF2 are also determined by other processes, such as the ionization production associated with enhanced auroral precipitation and the variations associated with TIDs. In the low to middle-latitude region changes in ƒoF2 approximately anticorrelate with changes at the height of the F layer electron density peak (e.g., hmF2) at 70°W during the storm main phase as well as its early recovery phase. This is attributed in part to the relation that exists between meridional wind velocity and vertical shear of that velocity for aurorally produced TADs.

Ionospheric electrical conductances produced by auroral proton precipitation

Abstract: Prom incoherent scatter radar observations and space-borne particle detector data, it appears that energetic proton precipitation can sometimes, for some locations, be a major source of ionization in the auroral ionosphere and contribute significantly to the electrical conductances. Here we propose a simple parameterization for the Pedersen and Hall conductances produced by proton precipitation. The derivation is based on a proton transport code for computing the electron production rate and on an effective recombination coefficient for deducing the electron density. The atmospheric neutral densities and temperatures and the geomagnetic-field strength are obtained from standard models. The incident protons are assumed to have a Maxwellian distribution in energy with a mean energy 〈E〉 in the 2–40 keV range and an energy flux Q0. The parameterized Pedersen and Hall conductances are functions of 〈E〉 and Q0, as well as of the geomagnetic-field strength. The dependence on these quantities is compared with those obtained for electron precipitation and for solar EUV radiation. To add the contribution of proton precipitation to the total conductances for electrodynamic studies in auroral regions, the conductances produced by electron and proton precipitations can be combined by applying a root-sum-square approximation.

An investigation of the influence of data and model inputs on assimilative mapping of ionospheric electrodynamics

Abstract: The Geospace Environment Modeling (GEM) substorm challenge event of November 24, 1996, has been used as a test case to investigate the influence of different data and model inputs on the assimilative mapping of ionospheric electrodynamics (AMIE) outputs. During that period the interplanetary magnetic field (IMF) went from northward to southward and then returned to northward. In the later part of the day a moderate substorm with AL ∼ -600 nT took place. The AMIE convection patterns derived from the Super Dual Auroral Radar Network (SuperDARN) data alone are generally similar to those derived using ground magnetometer alone, especially during the relatively stable southward IMF period. However, some differences are found during the northward IMF period and during the substorm; namely, the reversed convection configuration near local noon imaged by SuperDARN is absent in the magnetometer observations while the strong convection on the nightside recorded by the magnetometers during the substorm expansion phase is not seen by the radars. Different conductance models do not seem to have a big effect on the large-scale distributions of ionospheric convection and Joule heating, but they do alter the cross polar cap potential drop and the hemispheric integrated Joule heating rate by nearly a factor of 2. When the AMIE-derived electric potential drop was compared with the in situ measurements along the satellite track, it is found that AMIE underestimated the potential drop by 20 kV, amounting to a 22% underestimation. The analysis of the AMIE results based on the SuperDARN and ground magnetometer data reiterates the view that the response of ionospheric convection to a sudden IMF southward turning is global and nearly simultaneous.

Comparison of the auroral E region neutral winds derived with the European Incoherent Scatter radar and predicted by the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics general circulation model

Abstract: A comparison study of the auroral E region neutral wind has been conducted using the European Incoherent Scatter (EISCAT) radar observations and Thermosphere-Ionosphere-Mesosphere-Electro dynamics general circulation model (TIME GCM) predictions. The daily mean wind data as well as diurnal and semidiurnal tidal wind data, are compared for the three seasons summer, equinox, and winter between 95 and 119 km. Fairly good agreement is found in the altitude profile of the mean zonal wind between the EISCAT observation and the TIME GCM prediction for summer, indicating the parameterization of gravity waves employed in the TIME GCM is adequate for this feature. The meridional mean wind amplitude predicted by the TIME GCM is considerably smaller than that observed by EISCAT, and the predicted wind is slightly northward for all the seasons above 100 km. Generally good agreement is found for the amplitude of the diurnal tide, especially the summer prediction, while disagreements between the model and observational results are found for the corresponding phases. The semidiurnal amplitude predicted by the TIME GCM is much smaller than that observed by EISCAT, and relatively large differences of the semidiurnal phase between the observations and predictions are found for all seasons. These comparison results suggest that further advancements in the gravity wave parameterization, as well as the addition of planetary wave effects, are needed to predict more realistic lower thermospheric winds at high latitude.