A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid-latitudes. The transport of angular momentum by these winds produces a subrotation of the mid-latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid-latitude current is found to close partly via lower latitudes, resulting in an ‘anti-Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet-day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet-day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm-time ionospheric electric fields at middle and low latitudes.

The ionospheric disturbance dynamo

An integrated treatment of observations and models of the undisturbed equatorial electrojet is presented. Previous reviews have concentrated on particular aspects of the electrojet phenomenon, such as ground magnetic variations, the counterelectrojet, or E region electron density irregularities and associated plasma instabilities. Here accomplishments during the past 15 years involving all aspects of the phenomenon are covered and, where possible, interrelated. Observational and theoretical studies during recent years have emphasized the local time, longitude, vertical, and latitudinal structure of electric fields and currents in the electrojet; this review similarly places its emphasis along these lines and by necessity involves a detailed examination of electrodynamic coupling with the neutral atmosphere tidal oscillations in temperature, composition, and winds. The review concludes with an assessment of current knowledge concerning the equatorial electrojet and suggestions for the future direction of observational and theoretical research on electrodynamical processes in the equatorial ionosphere.

The equatorial electrojet

The dynamo theory of Sq variations is reexamined to see if it can be reconciled with recent observations of ionospheric winds and electric fields. Dynamo simulations are performed by using steady winds and both diurnal and semidiurnal tidal winds, observational and theoretical evidence being used to specify the amplitude and phase variations of the winds with altitude. We find that the first negative diurnal tidal mode, present in the upper E and lower F regions of the ionosphere, is capable of accounting for most of the Sq currents. The structure of this tide, which we call the (1, −2)* mode, is significantly influenced by the ion drag force. Lower E region winds, most notably the semidiurnal (2, 4) tidal mode, also contribute somewhat to the Sq currents. Our simulated electric fields using the combined (1, −2)* and (2, 4) modes are in fair agreement with observations; uncertainties in the ionospheric winds as well as errors in obtaining quiet day mean electric fields from the observations can easily account for the remaining discrepancies. Day-to-day variations in the daytime electric fields are probably related to variability of the E region winds. A simulation of electric currents and fields caused by hypothesized magnetospheric sources at quiet times shows that such sources alone cannot account for most of the observed middle- and low-latitude currents and fields, at least during daytime. Clarification of remaining problems requires further observational and theoretical work.

On the production mechanism of electric currents and fields in the ionosphere

[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 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

Data obtained with the Applied Physics Laboratory HF radar located in Goose Bay, Labrador, have been used to determine the characteristic features of high-latitude ionospheric irregularities at decameter wavelengths. In this paper, we describe a set of four events exhibiting particular characteristics. These observations took place in the postmidnight sector at E region altitudes. The scanning capabilities of the radar indicated that arclike regions of irregularities were moving approximately along L contours with a drift velocity of the order of 200 m/s or less. For periods of a few minutes to a few tens of minutes, localized regions of irregularities exhibiting high Doppler velocities (350 to 650 m/s) and large signal to noise ratios appeared within the radar arcs. Among the high Doppler velocity signals, two distinct types have been identified. Both types can be present simultaneously. One type is distributed between 320 and 550 m/s and has an average value of 445 m/s, while the other is distributed between 500 and 650 m/s and has an average value of 580 m/s. If one assumes the lower of the high-velocity signals to be the ion acoustic velocity C/sub s/, the higher velocity can be interpreted as electrostatic ion cyclotron (EIC)more » waves produced by NO/sup +/ ions. These EIC waves follow perfectly the dispersion relation established from the fluid approximation ..omega../sub r/ = (..cap omega../sup 2//sub i/+k/sup 2/ C/sup 2//sub s/)/sup 1/2/. The radar echoes with low Doppler velocities are associated with irregularities produced by the gradient drift instability which presumably was operative at the top of the E layer.« less

J. Gagnepain

Papers

Comparison of equatorial electrojet models

Theory of longitudinal gradients in the equatorial electrojet