T. J. Fuller-Rowell

Global Assimilation of Ionospheric Measurements (GAIM)

Abstract : Our primary goal is to construct a real-time data assimilation model for the ionosphere-plasmasphere system that will provide reliable specifications and forecasts. A secondary goal is to validate the model for a wide range of geophysical conditions, including different solar cycle, seasonal, storm, and substorm conditions.

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

[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.

Storm-time changes in the upper atmosphere at low latitudes

Abstract A three-dimensional coupled model of the thermosphere, ionosphere, plasmasphere and electrodynamics has been used to investigate the dynamic and electrodynamic response at low latitudes during a geomagnetic storm. A storm was simulated at equinox and high solar activity, and was characterized by a 12-h enhancement of the high-latitude magnetospheric electric field and auroral precipitation. The deposition of energy at high-latitudes heats the thermosphere and drives equatorward wind surges, and changes the global circulation. The first wave arrives at the equator, 3.5 h after storm onset. The change in the global circulation drives downwelling at low latitudes, which decreases molecular species, and causes a slight positive ionospheric phase. By far the dominant driver of the low latitudes is due to the changes in electrodynamics. The dynamo effect of the altered wind circulation opposes the normal diurnal variation, with downward ion drift during the day and upward drift at night. On the dayside, the equatorial ionization anomaly becomes weaker, the ionospheric F-region peak height is lowered, and the eastward zonal winds are reduced. At night the anomaly is strengthened, the ionosphere is raised, and zonal winds accelerate. The global electrodynamic changes are consistent with earlier results, but the speed of the response was unexpected. The model results showed an equatorial response within 2 h of the storm onset, well before the first gravity waves arrived at the equator. The dynamo action of the mid-latitude wind surges drive an F-region dynamo that can cause charge buildup at the terminators, producing electric fields that immediately leak to the equator. The meridional winds act as the driver of the low-latitude storm response by changing the dynamo action of the winds. In contrast, the zonal winds respond to the redistribution of charge brought about by the electrodynamic changes, rather than acting as a driver of the change.

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

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.

The “thermospheric spoon”: A mechanism for the semiannual density variation

A mechanism is proposed to explain the cause of the global, semiannual thermospheric density variation. It is suggested that the global-scale, interhemispheric circulation at solstice acts like a huge turbulent eddy in mixing the major thermospheric species. The effect causes less diffusive separation of the species at solstice, which tends to raise molecular nitrogen and oxygen densities and reduce atomic oxygen density, compared with equinox. The increased mean mass, at solstice, reduces the density scale height at a given altitude. This “compression” of the atmosphere at solstice can explain the mean amplitude of the semiannual density anomaly. Since ionospheric loss rates are affected by neutral composition, the proposed mechanism also leads to a similar ionospheric density variation.

On the importance of E‐field variability for Joule heating in the high‐latitude thermosphere

Joule heating is known to be one of the major energy sources of the upper atmosphere. Knowledge of the magnitude of this source is fundamentally important to a thorough understanding of the regions physics. However, Joule heating is currently one of the largest sources of uncertainty in the thermospheres energy budget. In numerical models the distribution of Joule heating is generally computed using mean or average convection patterns, which evolve on a relatively long time scale in response to changes in solar wind conditions. The convection patterns represent average electric potential distributions, and thus the resulting amount of Joule heating is proportional to the square of the average E-field. That method ignores the important component of Joule heating due to rapid or small-scale fluctuations in E-field or ion drifts. However, E-field fluctuations are known to exist on a variety of temporal and spatial scales, and the actual amount of Joule heating in the thermosphere is proportional to the average of the square of the E-field. The computation of the average of the square of the E-field requires knowledge of the statistical characteristics of E-field variability; thus knowledge not available at present. In this paper we assess, on the bases of theoretical considerations, the importance of E-field variability as an upper-atmosphere energy source. We show that the inclusion of E-field variability in the high-latitude convection model can significantly increase the amount of Joule heating for a given pattern.

Dynamics of the low-latitude thermosphere: Quiet and disturbed conditions

Abstract Low-latitude dynamics, electrodynamics, and plasma density structure are closely linked. Dynamically driven electric fields initiate the equatorial ionization anomaly. Between the latitudes of the anomaly crests, steep gradients in ion density span more than three orders of magnitude. Zonal winds accelerate in response to the severe deficit of plasma, and reduced ion drag, at the dip equator. Zonal winds give rise to a vertical polarization field, causing plasma to drift with the neutrals and further diminish ion drag. Signatures of neutral temperature are associated with the winds; cooling appears in the zonal jet itself and there is slight warming on either side. Chemical heating is suggested as the mechanism responsible for the temperature feature, but this has yet to be confirmed. During geomagnetic disturbances, large-scale waves propagate efficiently from the remote high-latitude source region. The strength of the waves and the circulation changes depend on local time; the strongest and most penetrating waves arise on the nightside, where they are hindered least by drag from the low ion densities. The rapid arrival of waves to low latitudes may be the cause of the electrodynamic drift that has been observed to follow a rise of geomagnetic activity within four hours. Winds at low latitudes respond to sources from both polar regions. The changes are manifest by the arrival and interaction of a series of waves from high latitudes that propagate well into the opposite hemisphere. Lower altitudes, below the F-region, respond more slowly because propagation speeds are limited in the cooler, dense lower thermosphere. Finally, during solstice, bulges enriched in molecular nitrogen migrate, over a period of a day or so, from their high latitude source to low latitudes. Characteristic negative phases can result, depleting the ionosphere and further feeding electrodynamic change. The timing of low-latitude electrodynamic signatures in response to geomagnetic disturbances is, at least in part, closely connected to global dynamical time scales. Numerical models are used to illustrate the response of the upper atmosphere during quiet and magnetically disturbed conditions, and are used to elucidate the important physical processes.

Electric field variability associated with the Millstone Hill electric field model

Joule heating that is generated at high latitudes in the thermosphere because of the magnetospherically imposed electric potential is proportional to the average of the square of the electric field (E field). Most theoretical Joule heating computations use only average electric fields, resulting in heating that is proportional to the square of the average E field. The computation of the average of the square of the E field requires knowledge about the statistical characteristics of E field variability associated with the average electric field model. In this paper we present the variability associated with the Millstone Hill bin-averaged empirical E field model [Foster et al. 1986] and discuss the implications of variability as an upper atmosphere energy source. We rebinned the radar plasma drift measurements from Millstone Hill, Massachusetts, in magnetic latitude and local time as a function of auroral activity and calculated the average electric fields and the variability associated with them as reflected in the bin standard deviations. We present the E field patterns and the associated variability for both quiet and disturbed geomagnetic conditions for the four seasons. We show that for an electric field model with a Gaussian distribution of small-scale variability around the mean, the average field and the variability have equal contributions to Joule heating generation.

Modelling composition changes in F-layer storms

A coupled thermosphere-ionosphere-plasmasphere model CTIP is used to simulate storm changes in the ionosphere. The simulations cover a period of 72 hours, starting with imposed high-latitude energy inputs (particle precipitation and electric fields) that represent a moderately severe geomagnetic storm (Kp 5) lasting for 12 h. Equinox and solstice conditions are studied. We give particular attention to comparing changes in peak electron density, NmF2, to those of the [ON2] concentration ratio of the neutral air. During the first few hours of the storm, large perturbations are produced by strong meridional winds. After that initial phase, we find that the changes of NmF2 and of [ON2] ratio correspond closely, the composition changes being produced by the thermospheric “storm circulation”, as in the “composition bulge” theory of Fuller-Rowell el al. (1994). The simulations reproduce the general form of the seasonal variations in the changes of NmF2 at mid-latitudes as derived from worldwide ionosonde data. Some storm effects at sub-auroral latitudes are caused by movement and infilling of the ionospheric trough. We conclude that the composition change theory accounts for the major features of F-layer storm behaviour at midlatitudes.

Global variations of thermospheric winds and temperatures caused by substorm energy injection

Two numerical simulations of the thermospheric response to magnetospheric energy injection have been performed using a zonally averaged, time-dependent model of neutral composition, dynamics, and energy budget. The simulations are distinguished by the duration of the source. The first simulation has an energy injection of 1 hour, representative of substorm type forcing, and the second one has a 12-hour energy injection, representative of main storm type forcing. They were performed under the condition of equinox at solar minimum. In the first simulation, large-scale atmospheric gravity waves (AGWs), generated by the substorm energy via Joule heating of ionospheric currents, are clearly identified in the wind-field in a meridional plane as well as in the temporal and spacial variations of the total energy density of air above about 130 km height. These waves reach the equator after about 3 hours and propagate into the opposite hemisphere. The horizontal propagation speed is close to the speed of sound (for example, roughly 440 m/s at about 150 km altitude and 670 m/s at about 260 km altitude). Snapshots of the wind system affected by the substorm energy injection show a “four-cell” pattern between the poles. Above 260 km, the cells have the opposite rotational direction to those below. These small-scale features in the wind system are indicative of the internal atmospheric gravity waves with the vertical phase propagation. From a term analysis of the energy conservation equation, it is identified that the dominant energy process associated with the propagation of AGWs is adiabatic compressional heating and/or expansive cooling process. It can be concluded that the energy oscillations at middle and low latitude are mainly produced by AGWs propagating from high latitude during the substorm. The second simulation indicated that horizontal and vertical advections, vertical heat conduction, and infrared radiative cooling by nitric oxide are important in addition to adiabatic compressional heating and/or expansive cooling. It is suggested that short-duration energy injection preferentially generates AGWs which dominate the energy oscillations at low latitudes through adiabatic heating and cooling. Long-duration energy injection is more effective in generating a meridional circulation which transfers energy by both advective and adiabatic processes.