Browsing by Author "Farley, D. T."
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Item Restricted Dependence of equatorial F region vertical drifts on season and solar cycle(American Geophysical Union, 1979-10-01) Fejer, B. G.; Farley, D. T.; Woodman Pollitt, Ronald Francisco; Calderón, C.Vertical drift measurements have been made at Jicamarca for more than half a solar cycle. The data from periods of high and low activity are appreciably different. Daytime drift velocities during sunspot minimum are usually larger than during the maximum, while the opposite is true for nighttime periods. The evening reversal occurs earlier during sunspot minimum than during the maximum, but the morning reversal is not altered. The period of eastward electric field (upward drift) is thus shortest during sunspot minimum and local winter. By integrating the drift velocity data with respect to time, one can obtain a measure of the total potential drop between reversal points (near the terminators). This drop is largest at solar maximum. There is also a pronounced seasonal variation, with a minimum in mid-December during both solar minimum and maximum. The general features of the data cannot be explained solely on the basis of tidal winds driving an E region dynamo; polarization fields related to the F region dynamo are of major importance, particularly in helping to explain the enhancement of the daytime upward drift which often occurs shortly before the drift reverses to downward in the evening. In order to account quantitatively for the observed variations, however, numerical models considerably more sophisticated than those presently available are needed.Item Open Access Electron energy balance in the F region above Jicamarca(American Geophysical Union, 1999-05-01) Aponte, N.; Swartz, W. E.; Farley, D. T.Incoherent scatter measurements from Jicamarca, Peru, show that current models and cross sections account quite well for the heating and cooling of F region electrons, in marked contrast to earlier similar studies at low and middle latitudes. The latter showed discrepancies of the order of a factor of 2 between the calculated energy input and loss rates. The equatorial F region ionosphere provides the simplest configuration for such studies because the horizontal magnetic field eliminates vertical photoelectron transport and thermal conduction. We based our estimates of electron heating on photoelectron energy spectra computed from recently developed solar flux models and new absorption and ionization cross sections and included the additional energy source clue to quenching of the 2D metastable state of nitrogen. This extra source is sometimes significant. Electron and ion temperatures and densities measured with the Jicamarca incoherent scatter radar were used to complete the calculations of the heating and cooling rates. We present here data from 2 days, one with low solar activity and one with moderate activity, over the altitude range 220–325 km. The heating/cooling rates ranged from about 500 to 6000 eV cm−3 s−1. Over this entire range the calculated heating and cooling rates differed by 10% or less when the data quality was good.Item Restricted Equatorial electric fields during magnetically disturbed conditions 1. The effect of the interplanetary magnetic field(American Geophysical Union, 1979-10-01) Woodman Pollitt, Ronald Francisco; Kelley, M. C.; Farley, D. T.; Fejer, B. G.; Gonzáles, S. A.Radar measurements of E and F region drift velocities have been used to look for correlations between changes in equatorial electric fields and the interplanetary magnetic field (IMF). The east-west component of the IMF appears to be unimportant, but the north-south component has some effect; rapid reversals from south to north are sometimes correlated with reversals of the equatorial east-west electric field during both daytime and nighttime. This is not always true, however, the IMF may reverse without any apparent effect at the equator. Furthermore, large equatorial field perturbations are sometimes observed when the IMF Bz is large and southward but not varying drastically. In this latter case the equatorial perturbations start nearly simultaneously with the onset of auroral substorms, while in the previous case they usually correlate with the onset of the substorm recovery phase. These observations indicate that the IMF does not affect the equatorial electric fields directly. Rather, it is changes in the magnetospheric electric fields and the auroral zone electric field and conductivity distribution (which may or may not be triggered by IMF changes) which alter the worldwide ionospheric current flow and electric field pattern, of which the equatorial observations are an indication.Item Restricted F region east-west drifts at Jicamarca(American Geophysical Union, 1981-01-01) Fejer, B. G.; Farley, D. T.; Gonzales, C. A.; Woodman Pollitt, Ronald Francisco; Calderon, C.F region east-west drifts have been measured at Jicamarca for almost 10 years, using incoherent scatter. The drifts are westward during the day and eastward at night. The daytime drift velocities are about 50 m/s and change very little with season or solar cycle. The evening reversal occurs at about 1600 local time throughout the solar cycle. The maximum nighttime eastward drifts are about 105 and 130 m/s during solar minimum and maximum, respectively. The daytime and nighttime drifts show very litle variation with magnetic activity. These Jicamarca incoherent scatter results (especially the reversal times) differ appreciably from results obtained using other techniques, but there appear to be fairly simple explanations for the apparent disagreements.Item Open Access Magnetic aspect sensitivity of 3‐m F‐region field‐aligned plasma density irregularities over Jicamarca(American Geophysical Union, 2011-10) Hysell, D. L.; Hedden, R. B.; Swartz, W. E.; Farley, D. T.; Chau Chong Shing, Jorge Luis; Milla, MarcoThe magnetic aspect angle sensitivity of 3‐m plasma density irregularities in the F‐region ionosphere over Jicamarca has been measured during the passage of a radar plume in an equatorial spread‐F event. The measurement technique utilizes radar interferometry with a number of antenna baselines with different lengths and orientations. Several corrections are applied to the data to reduce experimental biases. The RMS aspect angle half widths (the square root of the angular variance, the standard deviation) were found to be 0.01 ± 0.005° in a bottomside layer and near the top of the plume and 0.02 ± 0.005° in the central channel of the plume near F‐peak altitudes. In the frequency domain, the magnetic aspect width was narrowest at small Doppler shifts and broader in the wings of the spectra when wings existed. These findings appear to be reasonably consistent with theoretical predictions, although questions remain.Item Restricted Oblique VHF radar spectral studies of the equatorial electrojet(American Geophysical Union, 1975-04-01) Fejer, B. G.; Farley, D. T.; Balsley, B. B.; Woodman Pollitt, Ronald FranciscoA new narrow‐beam antenna at the Jicamarca Observatory permits oblique (zenith angle, 25°) radar spectral studies of the electrojet with an altitude resolution down to 1.1 km. Only daytime observations are possible presently, however. The general altitude variations in spectral shape observed are consistent with linear instability theory, if the effect of recombination is included. The height at which the mean Doppler shift of the echo maximizes, however, is about 4 km higher than one would expect on the basis of electrojet models. An increase in the assumed collision frequency would remove the discrepancy. Other data presented strongly suggest that at least some of the variations observed in the type 1 echo phase velocity are due to changes in the temperature of the E region.Item Restricted Radar observations of two‐dimensional turbulence in the equatorial electrojet, 2(American Geophysical Union, 1976-01-01) Fejer, B. G.; Farley, D. T.; Balsley, B. B.; Woodman Pollitt, Ronald FranciscoObservations with an altitude resolution of about 1 km were made with the large, vertically directed 50‐MHz radar system at the Jicamarca Radar Observatory during the day, when the electrojet was strong. Type 1 (‘two stream’) echoes were seen in a limited range of altitudes, with Doppler shifts corresponding to upward and downward motion at the acoustic velocity. Most of the radar returns were due to type 2 echoes, however. The direction of motion of the type 1 waves sometimes reversed in as little as 1 s, supporting a turbulent model of the electrojet region. During the day the upgoing type 1 waves were observed to be somewhat more easily excited than the downgoing waves, an effect similar to an east‐west asymmetry noted in earlier studies. The correspondence between the Jicamarca radar observations and rocket measurements in India is discussed.Item Restricted Radar studies of anomalous velocity reversals in the equatorial ionosphere(American Geophysical Union, 1976-09-01) Fejer, B. G.; Farley, D. T.; Balsley, B. B.; Woodman Pollitt, Ronald FranciscoRadar observations made at Jicamarca show that the equatorial electrojet current and the E and F region electric fields can reverse from their normal direction during the day or night and during magnetically quiet or disturbed conditions. The nighttime reversals can only be detected by such radar measurements. The observations support most of the current hypotheses concerning the electrojet plasma instabilities. The rapid reversals sometimes seen during disturbed conditions indicate that high‐latitude currents and electric fields associated with substorm activity strongly perturb the dynamo current system at all latitudes.Item Restricted The equatorial E-region and its plasma instabilities: a tutorial(European Geosciences Union, 2009-04-02) Farley, D. T.In this short tutorial we first briefly review the basic physics of the E-region of the equatorial ionosphere, with emphasis on the strong electrojet current system that drives plasma instabilities and generates strong plasma waves that are easily detected by radars and rocket probes. We then discuss the instabilities themselves, both the theory and some examples of the observational data. These instabilities have now been studied for about half a century (!), beginning with the IGY, particularly at the Jicamarca Radio Observatory in Peru. The linear fluid theory of the important processes is now well understood, but there are still questions about some kinetic effects, not to mention the considerable amount of work to be done before we have a full quantitative understanding of the limiting nonlinear processes that determine the details of what we actually observe. As our observational techniques, especially the radar techniques, improve, we find some answers, but also more and more questions. One difficulty with studying natural phenomena, such as these instabilities, is that we cannot perform active cause-and-effect experiments; we are limited to the inputs and responses that nature provides. The one hope here is the steadily growing capability of numerical plasma simulations. If we can accurately simulate the relevant plasma physics, we can control the inputs and measure the responses in great detail. Unfortunately, the problem is inherently three-dimensional, and we still need somewhat more computer power than is currently available, although we have come a long way.Item Restricted Vertical structure of the VHF backscattering region in the equatorial electrojet and the gradient drift instability(American Geophysical Union, 1975-04-01) Fejer, B. G.; Farley, D. T.; Balsley, B. B.; Woodman Pollitt, Ronald FranciscoRadar measurements made with high spatial resolution and large dynamic range at the Jicamarca Radar Observatory near the time of reversal of the electrojet current provide further proof that the gradient drift instability is in fact responsible for the type 2 irregularities. Echoes are received over a much wider range of altitudes at night than during the day partly because of the change in character of the background electron density profile and partly because of recombination effects, which can be important during the day. It is also shown that one must be cautious, particularly at night, in associating the mean Doppler shift of oblique radar echoes with the maximum east‐west electron drift velocity.