Browsing by Author "Nicolls, M. J."
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Item Open Access AMISR-14: Observations of equatorial spread F(American Geophysical Union, 2015-06-11) Rodrigues, F. S.; Nicolls, M. J.; Milla, Marco; Smith, J. M.; Varney, R. H.; Strømme, A.; Martinis, C.; Arratia, J. F.A new, 14-panel Advanced Modular Incoherent Scatter Radar (AMISR-14) system was recently deployed at the Jicamarca Radio Observatory. We present results of the first coherent backscatter radar observations of equatorial spread F (ESF) irregularities made with the system. Colocation with the 50 MHz Jicamarca Unattended Long-term studies of the Ionosphere and Atmosphere (JULIA) radar allowed unique simultaneous observations of meter and submeter irregularities. Observations from both systems produced similar Range-Time-Intensity maps during bottom-type and bottomside ESF events. We were also able to use the electronic beam steering capability of AMISR-14 to “image” scattering structures in the magnetic equatorial plane and track their appearance, evolution, and decay with a much larger field of view than previously possible at Jicamarca. The results suggest zonal variations in the instability conditions leading to irregularities and demonstrate the dynamic behavior of F region scattering structures as they evolve and drift across the radar beams.Item Open Access Estimating E region density profiles from radio occultation measurements assisted by IDA4D(American Geophysical Union, 2009-10-23) Nicolls, M. J.; Rodrigues, F. S.; Bust, G. S.; Chau Chong Shing, Jorge LuisThe Gordeyev integral for plasma particles colliding with neutrals is obtained using a particle dynamics formalism in which the collisions are modeled as a discrete Poisson process. The result leads to an electron density fluctuation spectrum model for partially ionized plasmas which is identical with the spectral model obtained from BGK plasma kinetic equations. This isomorphism between the Poisson process and the BGK operator is analogous to a similar relation between the Brownian motion process and the Fokker-Planck operator with constant coefficients. We take advantage of this analogy to derive a collisional ISR spectrum model that takes into account collisions with both neutrals and charged species.Item Restricted Improved electron density measurements at Jicamarca(American Geophysical Union, 2007-11-27) Rodrigues, F. S.; Nicolls, M. J.; Hysel, D. L.Taking into account the effects of electron Coulomb collisions, incoherent scatter (IS) radar cross sections for probing angles close to perpendicular to the magnetic field have been calculated and used to correct backscattered power profiles measured with the Jicamarca incoherent scatter radar so that they accurately represent the height variation of the ionospheric electron density. The corrected power profiles are compared with simultaneous Faraday rotation measurements of electron density. The profiles agree within the measurements uncertainties. This result improves electron density measurements at Jicamarca, since power measurements are less sensitive to clutter and interference than Faraday rotation measurements. The results can be used to correct Jicamarca long‐term IS power measurements.Item Open Access Ion gyroresonance observations at Jicamarca revisited(American Geophysical Union, 2007-07-12) Rodrigues, F. S.; Nicolls, M. J.; Woodman Pollitt, Ronald Francisco; Hysell, D. L.; Chau Chong Shing, Jorge Luis; Gonzáles, S. A.This paper presents recent observations of the proton gyroresonance over Jicamarca. In October 2006, a single-polarization double-pulse experiment was set up to measure the first gyroresonance peak in the incoherent scatter (IS) auto-correlation function (ACF). Despite the clutter caused by Spread-F and artificial satellites, it was possible to measure the first proton gyroresonance peak of the ACF in the topside ionosphere. For the first time, least-squares fits of theoretical IS ACFs to gyroresonance measurements are reported. Theoretical ACFs that best fit the measurements were found using the H+ fraction and temperature (assuming Te = Ti) as fitting parameters. Uncertainties for the estimated fraction of H+ were as low as 12%, while uncertainties for estimated temperatures were around 30%. These are the first successful gyroresonance measurements since the early observations of Farley (1967), and it is the first time measurements of this type have been used to obtain least squares estimates of ion composition and temperatures.Item Restricted Ionospheric and thermospheric variations associated with prompt penetration electric fields(American Geophysical Union, 2012-08-10) Lu, G.; Goncharenko, L.; Nicolls, M. J.; Maute, A.; Coster, A.; Paxton, L. J.This paper presents a comprehensive modeling investigation of ionospheric and thermospheric variations during a prompt penetration electric field (PPEF) event that took place on 9 November 2004, using the Thermosphere‐Ionosphere‐Mesosphere Electrodynamic General Circulation Model (TIMEGCM). The simulation results reveal complex latitudinal and longitudinal/local‐time variations in vertical ion drift in the middle‐ and low‐latitude regions owing to the competing influences of electric fields and neutral winds. It is found that electric fields are the dominant driver of vertical ion drift at the magnetic equator; at midlatitudes, however, vertical ion drift driven by disturbance meridional winds exceeds that driven by electric fields. The temporal evolution of the UT‐latitude electron density profile from the simulation depicts clearly a super‐fountain effect caused by the PPEF, including the initial slow‐rise of the equatorial F‐layer peak height, the split of the F‐layer peak density, and the subsequent downward diffusion of the density peaks along magnetic field lines. Correspondingly, low‐latitude total electron content (TEC) becomes bifurcated around the magnetic equator. The O/N2column density ratio, on the other hand, shows very little variations during this PPEF event, excluding composition change as a potential mechanism for the TEC variations. By using realistic, time‐dependent, high‐latitude electric potential and auroral precipitation patterns to drive the TIMEGCM, the model is able to successfully reproduce the large vertical ion drift of ∼120 m/s over the Jicamarca incoherent radar (IS) in Peru, which is the largest daytime ion drift ever recorded by the radar. The simulation results are validated with several key observations from IS radars, ground GPS‐TEC network, and the TIMED‐GUVI O/N2column density ratio. The model‐data intercomparison also reveals some deficiencies in the TIMEGCM, particularly the limitations imposed by its upper boundary height as well as the prescribed O+ flux.Item Restricted Multi-longitude case studies comparing the interplanetary and equatorial ionospheric electric fields using an empirical model(Elsevier, 2007-03) Kelley, M. C.; Nicolls, M. J.; Anderson, D.; Anghel, A.; Chau Chong Shing, Jorge Luis; Sekar, R.; Subbarao, K. S. V.; Bhattacharyya, A.Electric fields have been determined at three longitudes corresponding to Peru, India, and the Philippines. We compare these fields to applying a frequency-dependent linear transfer function (TF) to the dawn-to-dusk component of the interplanetary electric field (IEF). The TF is based on four years of simultaneous observations of the IEF and equatorial data. The model gives good results for the prompt penetrating electric field (PPE) in the case of an oscillatory IEF with a period in the 1–2 h range, when the interplanetary magnetic field remains southward for a long period and, to a lesser extent, when the IEF can be described as a square wave. There is evidence that a disturbance dynamo (DD) effect contributes on the dayside, where it leads to suppression of the normal quiet time pattern. A very strong counter-electrojet was seen at two locations during a time of persistent Bz south and was not predicted by the model or a linear scaling of the IEF. This suggests that suppression (and even reversal) of the E-region dynamo can occur in a large storm. Both the data and the model yielded a long-lived response to a sustained southward interplanetary magnetic field. Previously suggested By effects on equatorial electric fields are confirmed by a sequence of three distinct spikes in the By component of the IMF, one of which had no associated Bz change and yet was reproduced by two independent ground magnetometer-based electric field determinations. The sometimes remarkable agreement of a linear relationship between the equatorial and interplanetary electric fields shown here and elsewhere remains somewhat mysterious for such an apparently complex system.Item Open Access Observations and modeling of post-midnight uplifts near the magnetic equator(European Geosciences Union (EGU), 2006-07-03) Nicolls, M. J.; Kelley, M. C.; Vlasov, M. N.; Sahai, Y.; Chau Chong Shing, Jorge Luis; Hysell, D. L.; Fagundes, P. R.; Becker-Guedes, F.; Lima, W. L. C.We report here on post-midnight uplifts near the magnetic equator. We present observational evidence from digital ionosondes in Brazil, a digisonde in Peru, and other measurements at the Jicamarca Radio Observatory that show that these uplifts occur fairly regularly in the post-midnight period, raising the ionosphere by tens of kilometers in the most mild events and by over a hundred kilometers in the most severe events. We show that in general the uplifts are not the result of a zonal electric field reversal, and demonstrate instead that the uplifts occur as the ionospheric response to a decreasing westward electric field in conjunction with sufficient recombination and plasma flux. The decreasing westward electric field may be caused by a change in the wind system related to the midnight pressure bulge, which is associated with the midnight temperature maximum. In order to agree with observations from Jicamarca and Palmas, Brazil, it is shown that there must exist sufficient horizontal plasma flux associated with the pressure bulge. In addition, we show that the uplifts may be correlated with a secondary maximum in the spread-F occurrence rate in the post-midnight period. The uplifts are strongly seasonally dependent, presumably according to the seasonal dependence of the midnight pressure bulge, which leads to the necessary small westward field in the post-midnight period during certain seasons. We also discuss the enhancement of the uplifts associated with increased geomagnetic activity, which may be related to disturbance dynamo winds. Finally, we show that it is possible using simple numerical techniques to estimate the horizontal plasma flux and the vertical drift velocity from electron density measurements in the post-midnight period.