Browsing by Author "Alken, P."
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Item Restricted A quiet time empirical model of equatorial vertical plasma drift in the Peruvian sector based on 150 km echoes(American Geophysical Union, 2009-02-14) Alken, P.Equatorial vertical plasma drift is an important consequence of the E and F region dynamos. Understanding the climatology of vertical drift can provide significant insight into many ionospheric phenomena. In this study we present the first empirical model of vertical plasma drifts at 150 km altitude, observed by the Jicamarca Unattended Long‐term studies of the Ionosphere and Atmosphere (JULIA) coherent scatter radar located in Peru. The model, called JULIA Vertical Drift Model (JVDM), describes the local time, seasonal, and solar flux behavior of the equatorial vertical drifts in the Peruvian sector. The model is valid from 0800 through 1600 local time, which is typically when JULIA makes vertical drift measurements. During very high solar flux conditions, however, the model is unreliable before 1000 local time owing to a lack of JULIA data. The model includes a climatology of the 150 km equatorial vertical drifts as well as an estimate of the day‐to‐day variability, which can be significant. The daytime drifts typically peak between 1000 and 1200 LT and have amplitudes of 25–30 m/s ± 10 m/s. The model has been validated against the global empirical model of Scherliess and Fejer, with a total rms difference of under 4 m/s for 1000 to 1600 LT. This model will allow researchers to study daily variations in the equatorial electric field by subtracting the climatological mean. Model coefficients and software are available online at http://geomag.org/models and http://www.earthref.org.Item Open Access Estimating the daytime Equatorial Ionization Anomaly strength from electric field proxies(American Geophysical Union, 2008-09-10) Stolle, C.; Manoj, C.; Lühr, H.; Maus, S.; Alken, P.The Equatorial Ionization Anomaly (EIA) is a significant feature of the low‐latitude ionosphere. During daytime, the eastward electric field drives a vertical plasma fountain at the magnetic equator creating the EIA. Since the eastward electric field is also the driving force for the Equatorial Electrojet (EEJ), the latter is positively correlated with the EIA strength. We investigate the correlation between the zonal electric field and the EIA in the Peruvian sector and compare the results with correlations of the EEJ versus EIA strength. Analyzing 5 years of Challenging Minisatellite Payload (CHAMP) electron density measurements, plasma drift readings from the Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere (JULIA) radar, and magnetic field observations at Huancayo and Piura, we find the EEJ strength and the zonal electric field to be suitable proxies for the EIA intensity. Both analyses reveal high correlation coefficients of cc > 0.8. A typical response time of the EIA to variations in the zonal electric field is ∼1–2 h, and it is ∼2–4 h after EEJ strength variations. Quantitative expressions are provided, which directly relate the EIA parameters to both proxies. From these relations, we infer that an EIA develops also during weak Counter Electrojets (CEJs), but no EIA forms when the vertical plasma drift is zero. For positive EEJ magnetic signatures to form, a minimum eastward electric field of 0.2 mV/m is required on average. The above‐mentioned delay between EIA and EEJ variations of ∼3 h is further confirmed by the investigation of the EIA response to transitions from CEJ to EEJ, e.g., during late morning hours.Item Restricted Longitudinal and seasonal structure of the ionospheric equatorial electric field(American Geophysical Union, 2013-03-28) Alken, P.; Chulliat, A.; Maus, S.The daytime eastward equatorial electric field (EEF) in the ionospheric E‐region plays an important role in equatorial ionospheric dynamics. It is responsible for driving the equatorial electrojet (EEJ) current system, equatorial vertical ion drifts, and the equatorial ionization anomaly. Due to its importance, there is much interest in accurately measuring and modeling the EEF. In this work we propose a method of estimating the EEF using CHAMP satellite‐derived latitudinal current profiles of the daytime EEJ along with Δ H measurements from ground magnetometer stations. Magnetometer station pairs in both Africa and South America were used for this study to produce time series of electrojet current profiles. These current profiles were then inverted for estimates of the EEF by solving the governing electrostatic equations. We compare our results with the Ion Velocity Meter (IVM) instrument on board the Communication/Navigation Outage Forecasting System satellite. We find high correlations of about 80% with the IVM data; however, we also find a constant offset of about 0.3 mV/m between the two data sets in Africa. Further investigation is needed to determine its cause. We compare the EEF structure in Africa and South America and find differences which can be attributed to the effect of atmospheric nonmigrating tides. This technique can be extended to any pair of ground magnetometer stations which can capture the day‐to‐day strength of the EEJ.Item Restricted Long‐period prompt‐penetration electric fields derived from CHAMP satellite magnetic measurements(American Geophysical Union, 2013-08-22) Manoj, C.; Maus, S.; Alken, P.The prompt penetration of the interplanetary electric field to the equatorial ionosphere is conveniently modeled with a frequency‐dependent transfer function. However, long‐period responses (>3 h) of previously estimated transfer functions differ considerably due to insufficient ionospheric eastward electric field (EEF) data useful for spectral analysis. The EEF derived from the Challenging Minisatellite Payload (CHAMP) satellite provides a new opportunity to reliably estimate the long‐period transfer function for the first time. Our objectives in this paper are twofold: first, we analyze the frequency spectra of the equatorial ionospheric eastward electric field for periods greater than 6 h; second, we test the hypothesis that the average prompt‐penetration effect lasts less than 3 h after an initial perturbation in the interplanetary electric field (IEF). We find that atmospheric sources dominate the EEF at diurnal frequencies, and its subharmonics and magnetospheric sources dominate the EEF for other frequencies. The CHAMP‐derived transfer function has smaller errors than the previous estimates, and we confirm that the average prompt‐penetration response of the equatorial ionospheric electric field to a 1 mV/m change in the IEF is negligible after 3 h and up to the maximum analysis period of 32 h. We update our transfer function model with the new data sets and make the filter coefficients available to the scientific community. The transfer function prediction matched reasonably well with the EEF observation in both the South American and Indian sectors. A transfer function model of the prompt‐penetration effects, driven by the interplanetary electric field, can be highly beneficial to the real‐time specification of the ionosphere.Item Restricted Penetration characteristics of the interplanetary electric field to the daytime equatorial ionosphere(American Geophysical Union, 2008-12-23) Manoj, C.; Maus, S.; Lühr, H.; Alken, P.Using 8 years of ionospheric drift measurements from the low‐latitude JULIA (Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere) radar and the solar wind and interplanetary magnetic field data from the ACE (Advance Composition Explorer) satellite, we study the characteristics of the prompt penetration of electric fields to the equatorial ionosphere. A large database allowed us to bring out statistically significant characteristics of electric field penetration as a function of frequency. The coherence between the interplanetary electric field (IEF) and the equatorial electric field (EEF) peaks around a 2‐hour period with a maximum magnitude squared coherence of 0.6. The coherence is slightly higher (0.7) on magnetically active (Ap > 20) days. The cross‐phase spectra between the ACE and JULIA variations, after elimination of the propagation delay, have negligible values. Correspondingly, the time shift between IEF and EEF is less than 5 minutes at all periods. We also find that the penetration efficiency is highest during local noon, as compared with that of morning and evening hours. The coherence is lower for days with high solar flux values. We find that the penetration of electric fields into the equatorial ionosphere has no significant dependence on season and on the polarity of IMF Bz. We propose a transfer function between IEF and EEF, which was validated on synthetic as well as observed IEF data. The use of this transfer function decreases the misfit of a climatological model with the measured equatorial electric field by 27%.Item Restricted Swarm equatorial electric field chain: First results(American Geophysical Union, 2015-01-13) Alken, P.; Maus, S.; Chulliat, A.; Vigneron, P.; Sirol, O.; Hulot, G.The eastward equatorial electric field (EEF) in the E region ionosphere drives many important phenomena at low latitudes. We developed a method of estimating the EEF from magnetometer measurements of near‐polar orbiting satellites as they cross the magnetic equator, by recovering a clean signal of the equatorial electrojet current and modeling the observed current to determine the electric field present during the satellite pass. This algorithm is now implemented as an official Level‐2 Swarm product. Here we present first results of EEF estimates from nearly a year of Swarm data. We find excellent agreement with independent measurements from the ground‐based coherent scatter radar at Jicamarca, Peru, as well as horizontal field measurements from the West African Magnetometer Network magnetic observatory chain. We also calculate longitudinal gradients of EEF measurements made by the A and C lower satellite pair and find gradients up to about 0.05 mV/m/deg with significant longitudinal variability.Item Restricted The influence of nonmigrating tides on the longitudinal variation of the equatorial electrojet(American Geophysical Union, 2008-08-22) Lühr, H.; Rother, M.; Häusler, K.; Alken, P.; Maus, S.The climatological model of the equatorial electrojet, EEJM‐1, derived from Ørsted, CHAMP and SAC‐C satellite measurements provides the opportunity to investigate the longitudinal variation of the current strength in detail. Special emphasis is put in this study on the effect of nonmigrating tides. We have found that the influence of the diurnal eastward‐propagating mode with wavenumber‐3, DE3, is particularly strong. In polar orbiting satellite observations the DE3 tidal signal appears as a four‐peaked longitudinal structure. We have put special emphasis in our analysis to isolate the DE3 contribution from other sources contributing to the wavenumber‐4 structure in satellite data. The amplitude of the DE3 signature in the EEJ not only peaks during equinox seasons, but is also strong around the June solstice. When looking at the modulation of the EEJ intensity the DE3 accounts for about 25% during the months of April through September. It is thus the dominant cause for longitudinal variations. During December solstice months the influence of DE3 is negligible. A secondary three‐peaked longitudinal pattern emerges during solstice seasons when the DE3 influence is removed. From the data available it is, however, not clear whether this pattern is related to any tidal drivers.