Vertical drift velocities and east‐west electric fields at the magnetic equator
TL;DR: Incoherent scatter observations of vertical drifts taken at Jicamarca (2° dip) are presented in this paper, where the effect of geomagnetic activity is discussed.
Abstract: Incoherent scatter observations of vertical drifts taken at Jicamarca (2° dip) are presented. Vertical drifts are found to be nearly constant as a function of height. These vertical drifts can also be taken as a direct measurement of the east-west electric fields at the magnetic equator. Their daily and seasonal behavior is presented. The effect of geomagnetic activity is discussed.
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TL;DR: In this article, 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.
Abstract: 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.
1,049 citations
TL;DR: In this paper, the results of backscatter observations of the F region irregularities made with the large 50MHz radar at Jicamarca, Peru, during a few days of observations are presented.
Abstract: The paper presents some results of backscatter observations of the F region irregularities made with the large 50-MHz radar at Jicamarca, Peru, during a few days of observations. The results were obtained by using three observational techniques: the modified range-time-intensity technique, the digital power mapping technique, and the digital raw data recording technique. Backscatter intensity maps as a function of altitude and time are presented, which can be interpreted as radar pictures of F region irregularities. A classification of spread F spectral signatures resulting from approximately 30,000 spectra obtained in sets of 64 simultaneous heights under a variety of conditions is also given.
917 citations
TL;DR: In this paper, the authors used radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F.
Abstract: We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts In the early night sector, the bottomside of the F layer is almost always unstable The evolution of the unstable layer is controlled by the history of the vertical drift velocity When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux
656 citations
TL;DR: In this article, the authors present a global empirical model for the F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite.
Abstract: We present the first global empirical model for the quiet time F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite. This analytical model, based on products of cubic-B splines and with nearly conservative electric fields, describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values. Our results indicate that during solar minimum, the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15–20 m/s, is observed only in the American sector during December solstice with amplitudes of about 5–10 m/s, and is absent at all longitudes during June solstice. The solar minimum evening reversal times are fairly independent of longitude except during December solstice. During solar maximum, the evening upward vertical drifts and reversal times exhibit large longitudinal variations, particularly during the solstices. In this case, for a solar flux index of 180, the June solstice evening peak drifts maximize in the Pacific region with drift amplitudes of up to 35 m/s, whereas the December solstice velocities maximize in the American sector with comparable magnitudes. The equinoctial peak velocities vary between about 35 and 45 m/s. The morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle. The daytime drifts have largest amplitudes between about 0900 and 1100 LT with typical values of 25–30 m/s. We also show that our model results are in good agreement with other equatorial ground-based observations over India, Brazil, and Kwajalein.
571 citations
TL;DR: The seasonal average of the equatorial F region vertical and zonal plasma drifts were determined using extensive incoherent scatter radar observations from Jicamarca during 1968-1988.
Abstract: The seasonal averages of the equatorial F region vertical and zonal plasma drifts are determined using extensive incoherent scatter radar observations from Jicamarca during 1968-1988. The late afternoon and nighttime vertical and zonal drifts are strongly dependent on the 10.7-cm solar flux. The authors show that the evening prereversal enhancement of vertical drifts increases linearly with solar flux during equinox but tends to saturate for large fluxes during southern hemisphere winter. They examine in detail, for the first time, the seasonal variation of the zonal plasma drifts and their dependence on solar flux and magnetic activity. The seasonal effects on the zonal drifts are most pronounced in the midnight-morning sector. The nighttime eastward drifts increase with solar flux for all seasons but decrease slightly with magnetic activity. The daytime westward drifts are essentially independent of season, solar cycle, and magnetic activity.
550 citations
References
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TL;DR: VHF radar measurements of scattering by field aligned irregularities associated with equatorial spread are reported in this article, where the authors show that the scattering is caused by field-aligned irregularities.
Abstract: VHF radar measurements of scattering by field aligned irregularities associated with equatorial spread F
407 citations
TL;DR: In this article, an expression for the ionospheric electric field is derived in terms of the magnetic field, velocities of the neutral and ionized clouds, λ* (the ratio of the integrated Pedersen conductivities in the cloud and in the atmosphere), and ki (= gyrofrequency for Ba + /collision frequency).
177 citations
TL;DR: In this article, the analysis of VHF radar echoes from electron density irregularities in the equatorial electrojet indicate that there are two distinct types of irregularities, one of which is generated by the two-stream instability mechanism and consists of planar irregularities which travel at the ion-acoustic velocity.
Abstract: The analysis of VHF radar echoes from electron density irregularities in the equatorial electrojet indicate that there are two distinct types of irregularities in the region. One of these has previously been shown to be generated by the two-stream instability mechanism and consists of planar irregularities which travel at the ion-acoustic velocity. The other type of irregularity moves at about the electron drift velocity, and can exist when the electron drift velocity is insufficient to produce the two-stream type irregularities. Drift observations of this second type of irregularity show that the electrojet current reverses during nighttime periods, the electron drift velocity being comparable to that during the day. Additional observations show that these irregularities exist in large patches which move with the same approximate velocity and retain their identity for many seconds.
167 citations
TL;DR: Vertical ionospheric motions near magnetic equator measured by Doppler shifts of incoherent scatter frequency spectra were measured by doppler shift of coherent spectrum spectra.
Abstract: Vertical ionospheric motions near magnetic equator measured by Doppler shifts of incoherent scatter frequency spectra
161 citations
TL;DR: In this paper, the authors present the results from the French Thomson-scatter bistatic sounder (SaintSantin-Nancay) and interpret these velocities as the sum of two effects: horizontal circulation of the neutral air and ambipolar diffusion of ionization under gravity and under its own partial pressure.
114 citations