Apex Altitude Research Articles

Effects of electrical coupling on equatorial ionospheric plasma motions: When is the F region a dominant driver in the low‐latitude dynamo?

Previous studies of the low‐latitude atmospheric dynamo have concluded that the E region dynamo is the principal source of electric fields during the day, with the F region dynamo being of significant importance in the postsunset period. By computing field line integrals of the Hall and Pedersen conductivities and wind‐driven currents from a sophisticated F region model, we find that the F region may provide a significant contribution to the dynamo electric field at all local times in a limited altitude and latitude region. Though the peak value of the Pedersen conductivity in the E region is larger than the peak value of the F region Pedersen conductivity, the field line integrated Pedersen conductivity of the F region can be significantly larger than the integrated Pedersen conductivity of the E region. This effect is most pronounced for low‐latitude flux tubes with apex altitudes slightly above the F peak. This is due to the unique geometry of the equatorial geomagnetic field in which the field line path length through the F region is much greater than the field line path length through the E region. This effect decreases with increasing latitude as the field lines become more vertical and the path lengths through the appropriate parts of E and F regions become more comparable. We examine the consequences of this effect upon the low‐latitude drifts derived from three dynamo circuit assumptions and compare the results to average drifts observed at Jicamarca, Peru.

Images of transequatorial bubbles based on field‐aligned airglow observations from Haleakala in 1984‐1986

Images in the 6300‐Å and 7774‐Å plasma recombination emissions have been made that show the development and dynamics of field‐aligned bubbles of low density plasma in the tropical nighttime F region. An intensified charge coupled device detector was used with an eight‐filter camera to view a field‐aligned section of the F region that mapped to magnetic apex heights from 400 to 900 km above the dip equator and up to 700 km in east‐west extent from a ground‐based observation site on Mount Haleakala, Hawaii. Field‐aligned bubbles were observed to grow from near‐stationary wavelike perturbations with horizontal scale sizes of about 100 km at the lower apex altitude regions during the evening hours. The bubbles were seen to elongate toward higher apex levels as they accelerated eastward, sometimes reaching eastward velocities of over 100 m/s. In general, field‐aligned depletions were observed to occur over all portions of the night. A characteristic shape was often observed with a rather sharp vertical wall bounding the depletion on the eastward side and with several tilted branches on the westward side. The structures were often observed to develop westward tilts as shear motions yielded eastward drifts that decreased with increasing apex altitude. During periods of extreme levels of shear, isolated depletions were seen to drift separately from neighboring depletion structures.

Low‐latitude zonal and vertical ion drifts seen by DE 2

Horizontal and vertical ion drift data from the DE 2 spacecraft have been used to determine average zonal and vertical plasma flow (electric field) characteristics in the ±26° dip latitude region during a time of high solar activity. The data base consists of over 800 equatorial passes. Horizontal ion drift data were projected along magnetic field lines to the magnetic equator, using a dipole model to yield their apex height dependence. The “average data” local time profile for an apex height bin centered at 400 km indicates westward plasma flow from 0600 to 1900 solar local time (SLT) with a maximum westward velocity of 80 m/s in the early afternoon. There is a sharp change to eastward flow at approximately 1900 hours with an early evening peak of 170 m/s. A secondary nighttime maximum exists at 0430 SLT preceeding the reversal to westward flow. This profile is in good agreement with Jicamarca radar measurements made under similar solar maximum conditions. Harmonic analysis indicates a net superrotation which is strongest at lower apex altitudes. The diurnal term is dominant, but higher order terms through the quatradiurnal are significant. Vertical ion data taken at low (±5°) dip latitudes, while possessing greater scatter, generally agree with Jicamarca values.