L. E. Wharton

An equatorial temperature and wind anomaly (ETWA)

Data obtained from the WATS (Wind and Temperature Spectrometer) and LP (Langmuir Probe) experiments on board DE-2 (Dynamic Explorer) during high solar activity show evidence of anomalous latitudinal variations in the zonal winds and temperature at low latitudes. The zonal winds exhibit a broad maximum centered around the dip equator, flanked by minima on either side around 25 degrees; while the temperature exhibits a pronounced bowl-shaped minimum at the dip equator which is flanked by maxima. The two minima in the zonal winds and the corresponding maxima in the temperature are nearly collocated with the crests of the well known Equatorial Ionization Anomaly (EIA). The maximum in the zonal winds and the minimum in the gas temperature are collocated with the trough of the EIA. The differences between the maxima and minima in temperature and zonal winds, on many occasions, are observed to exceed 100 K and 100 m/s, respectively. The characteristics of this new phenomenon have eluded present day empirical models of thermospheric temperature and winds. The connection among these variables can be understood from the ion-neutral drag effect on the motions of the neutrals that in turn affect their energy balance.

Neutral temperature anomaly in the equatorial thermosphere - A source of vertical winds

Data obtained from the WATS (Wind and Temperature Spectrometer) instrument on DE-2 (Dynamics Explorer) during high solar activity, show new evidence for the presence of vertical winds of a significant magnitude in the equatorial thermosphere. They reveal a latitudinal structure that can be related to the recently discovered phenomena of the Equatorial Temperature and Wind Anomaly (ETWA). In the local evening hours, the vertical winds usually are downward around the dip equator and collocated with the temperature minimum of ETWA. In general, they are upward at about 24° dip latitude away from the dip equator and are collocated with the ETWA temperature crests. The magnitude of the vertical winds is in the 10–40 m/s range. It is proposed that the temperature and pressure ridges, formed by the excess ion drag on the zonal winds around the two crests and ordered by the relatively lower ion drag at the trough of the well known Equatorial Ionization Anomaly (EIA), drive a new wind system in the meridional plane and that the measured vertical winds form part of this wind system.

The effects of neutral inertia on ionospheric currents in the high‐latitude thermosphere following a geomagnetic storm

Results of an experimental and theoretical investigation into the effects of the time dependent neutral wind flywheel on high-latitude ionospheric electrodynamics are presented. The results extend our previous work [Deng et al., 1991] which used the National Center for Atmospheric Research Thermosphere/Ionosphere General Circulation Model (NCAR TIGCM) to theoretically simulate flywheel effects in the aftermath of a geomagnetic storm. The previous results indicated that the neutral circulation, set up by ion-neutral momentum coupling in the main phase of a geomagnetic storm, is maintained for several hours after the main phase has ended and may dominate height-integrated Hall currents and field-aligned currents for up to 4-5 hours. We extend the work of Deng et al. to include comparisons between the calculated time-dependent ionospheric Hall current system in the storm-time recovery period and that measured by instruments on board the Dynamics Explorer 2 (DE 2) satellite. Also, comparisons are made between calculated field-aligned currents and those derived from DE 2 magnetometer measurements. These calculations also allow us to calculate the power transfer rate (sometimes called the Poynting flux) between the magnetosphere and ionosphere. The following conclusions have been drawn: (1) Neutral winds can contribute significantly to the horizontal ionospheric current system in the period immediately following the main phase of a geomagnetic storm, especially over the magnetic polar cap and in regions of ion drift shear. (2) Neutral winds drive Hall currents that flow in the opposite direction to those driven by ion drifts. (3) The overall morphology of the calculated field-aligned current system agrees with previously published observations for the interplanetary magnetic field (IMF) BZ southward conditions, although the region 1 and region 2 currents are smeared by the TIGCM model grid resolution. (4) Neutral winds can make significant contributions to the field-aligned current system when BZ northward conditions prevail following the main phase of a storm, but can account for only a fraction of the observed currents. (5) DE 2 measurements provide a demonstration of “local” (satellite-altitude) flywheel effects. (6) On the assumption that the magnetosphere acts as an insulator, we calculate neutral-wind-induced polarization electric fields of ∼20-30 kV in the period immediately following the geomagnetic storm.

Local time variation of equatorial temperature and zonal wind anomaly (ETWA)

Abstract We describe the average and apparent local (solar) time (LST) variations in the zonal winds (Z) and temperatures (T) as measured with the Wind and Temperature Spectrometer (WATS) on-board the polar orbiting DE-2 satellite in the altitude range of 300–450 km during a near solar maximum period of 1981–1982. During this time period, the variations in the solar flux (F10.7) and magnetic activity (Ap) contribute significantly to the apparent LST variations in T; while those effects on the LST variations in Z are small. The observations are related to the equatorial ionization anomaly (EIA) as seen in the electron density data obtained from the same satellite with the LANG instrument. The latitudinal variations in Z always reveal a maximum at the dip equator where the EIA trough forms and minima (with velocities reduced by a factor of two) are seen on either side of the equator where the EIA crests form. At 20:00 LST, the largest wind velocities are observed, directed eastward, after changing direction at around 17:00 LST where the largest accelerations occur. We delineate the diurnal variations in the strength of the Equatorial Temperature and Wind Anomaly (ETWA) defined by the differences in the wind velocities (DZ) and temperatures (DT) at the crests and troughs of the ionization. The diurnal variations in DZ are similar to those in Z at the trough. The diurnal variations in DT differ from those apparent in T. Excess temperatures, DT, at the crests show up with the development of the EIA as early as 09:30 LST by which time the zonal wind has attained its westward maximum. But DT continues to increase with the EIA crest until 14:00 LST, followed by a dip at around 16:00 LST in phase with the zero crossing of Z. The highest value of DT is only reached at 20:00 LST when both Z and the crest intensification reach their respective maxima. This demonstrates that the development of the equatorial temperature anomaly critically depends both on the development of the EIA crests and the zonal winds, which clearly establishes the primary role of ion drag in generating the ETWA phenomenon.

An experimental investigation of thermospheric structure near an auroral arc

Observations of thermospheric parameters, made from the Dynamics Explorer 2 (DE 2) spacecraft during three successive orbital crossings of a quiescent dusk sector auroral arc, have been compared with the predictions of three fine-grid auroral arc models. DE 2 measured the ion and neutral winds, electron and neutral temperatures, neutral composition, and energetic auroral electron spectra (5 eV to 32 keV) at ∼320 km altitude. These observations were at high spatial and temporal resolution, suitable for comparisons with the models. The observed zonal and meridional neutral winds near the arc were greater than the model predictions, probably because of the presence of stronger electric fields and higher ion and neutral densities during the observations than were used in the models. The measured vertical winds were also larger than the corresponding model values. The DE 2 composition measurements showed a local increase of the N2/O ratio in the arc, which is interpreted to be a result of the upward motion of N2-rich air. A reduction in the measured neutral temperature of ∼100 K in the arc, relative to temperatures on either side, was found for all three arc crossings. Of the three theoretical models examined, only one shows any tendency for the neutral thermospheric temperature to drop in the auroral arc, and the decrease calculated is significantly less than the observed temperature change. A simple calculation of the adiabatic cooling effect of the observed upward motion yields a temperature drop of ∼160 K, comparable with the observed temperature reduction (∼100 K).

Equatorial temperature anomaly during solar minimum

We show evidence for the occurrence of the equatorial temperature anomaly (ETA) during solar minimum by analyzing the temperature and total ion density data from the Neutral Atmosphere Temperature Experiment (NATE) and the Cylindrical Electrostatic Probe (CEP), respectively, on board the Atmospheric Explorer-E satellite. The chosen data refer to a height of ∼254 km in the African and Asian longitude sector (340.1°E–200°E) during a summer season in the Southern Hemisphere. As during the solar maximum period, the spatial characteristics of the ETA are similar to those of the equatorial ionization anomaly (EIA). A minimum in the gas temperature is collocated with the minimum in the ion density at the dip equator, and a temperature maximum on the south side of the equator is collocated with the density maximum of the EIA. The daytime behavior of ETA formation is about the same as that of EIA as both of them are clearly present at around 1300 and 1400 local solar time (LST) only. At 1400 LST the difference between the temperatures at the crest and the trough (ETA strength) reaches a maximum value of about 100°K which is ∼14% of the temperature at the trough. Like the EIA, the ETA also suddenly disappears after 1400 LST. Thus the EIA appears to be a prerequisite for the ETA formation. During the premidnight time (2200 LST), however, while the EIA is nonexistent, the temperature distribution forms a pattern opposite to that at 1400 LST in the daytime. It shows a maximum around the dip equator and a broad minimum at the daytime crest region where the postsunset cooling also is faster and occurs earlier than at the dip equator. This nighttime maximum appears to be related to the signature of the midnight temperature maximum (MTM). Mass Spectrometer Incoherent Scatter (MSIS) model temperatures, in general, are higher than the observed average temperatures for the summer season and in particular for the region around the dip equator around noon hours.

Equatorial spread-F (ESF) and vertical winds

Abstract The Equatorial Spread-F (ESF) phenomenon is recorded in ionograms as a hierarchy of plasma instabilities in the F-layer of the equatorial ionosphere. The ESF is characterized by irregularities in the plasma (electron and ion) density and electric field distributions perpendicular to the Earth’s magnetic field. Large scale irregularities are generated by a primary plasma instability that develops in electric fields and plasma densities. Other secondary instabilities then develop and generate irregularities at several scale sizes that often produce a plasma ‘hole’ or ‘bubble’ that rises up with high E × B velocities. The ESF/plasma bubble phenomenon has been studied extensively with experimental techniques and modeling, which revealed important features. In the bottom side F-layer, near sunset, when the vertical density gradient steepens as the layer is supported by the horizontal (North–South) Earth’s magnetic field lines against the omnipresent Earth’s gravitational acceleration ( g ), the plasma conditions can give rise to Rayleigh–Taylor (RT) type instability. But the observed day to day variability of the ESF occurrence suggested that other agencies may also be involved in generating the instability. Sekar and Raghavarao (1987) with linear theory, and Raghavarao, Sekar and Suhasini (1992) , with non-linear numerical modeling, suggested that vertical downward (upward) winds in the ambient gas have the potential to cause (inhibit) the ESF/bubble phenomenon. The presence of downward winds near the equator was reported earlier. In this paper, we show evidence for the presence of downward winds collocated with irregularities in electric fields and plasma densities as revealed by an unique combination of highly accurate measurements with instruments onboard the DE-2 satellite. The observations reported here are also consistent with the notion that the build-up of the equatorial ionization anomaly (EIA) prior to local sunset is important for the ESF instability.

Empirical orthogonal functions of monthly precipitation and temperature over the United States and homogeneous stochastic models

The monthly mean precipitation and temperature at p = 62 stations over the United States and Canada for N = 91 years (1900-1990) are analyzed in terms of empirical orthogonal functions (EOFs) and their variances. The eigenvalues and eigenfunctions are compared with a succession of stochastic noise models : (1) uncorrelated noise, having eigenvalues depending on the ratio p/n, with n = N - 1 ; (2) homogeneous noise having spatial correlations which are fit to the observations ; and (3) homogeneous noise having both spatial and temporal correlations fit to the observations. Individual monthly data for January and July were analyzed as well as a combined data set of all months. The eigenvalue spectra of the homogeneous noise models are found to be in close agreement with the observed spectra even when time correlation is excluded from the model. Time correlations only slightly affect the results for temperature and have less impact for precipitation. The EOF patterns of the noise models contain inhomogeneities due only to the distribution of stations, the common correlation length, and the limited sample but are nevertheless in good agreement with the observed patterns, whose inhomogeneities may also be affected by secular trends and physical inhomogeneities such as orography. The observed EOF eigenvectors also show identifiable deviations from the homogeneous EOFs. Further work will be needed to see if these deviations can be convincingly associated with true physical inhomogeneities.