Browsing by Author "Alken, Patrick"
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Item Restricted Electric fields and zonal winds in the equatorial ionosphere inferred from CHAMP satellite magnetic measurements(American Geophysical Union, 2007-12-16) Maus, Stefan; Alken, Patrick; Lühr, HermannThe Equatorial Electrojet (EEJ) produces a strong magnetic signal in measurements of the low‐orbiting CHAMP satellite. Six years of data with more than 30,000 dayside equator crossings provide a unique data basis to study this current system. In addition to scalar measurements used in previous studies, we have also inverted vector magnetic field measurements to gain accurate meridional profiles of the eastward current in the equatorial region. By solving the relevant differential equation, the CHAMP‐derived current profiles can be inverted for the driving zonal electric fields and zonal winds. As a result of this feasibility study, we suggest an inversion of the complete set of individual CHAMP EEJ current profiles, which will provide valuable new information on dayside electric fields and thermospheric winds at low latitudes.Item Restricted Electric fields in the equatorial ionosphere derived from CHAMP satellite magnetic field measurements(Elsevier, 2009-03-09) Alken, Patrick; Maus, StefanThe day-time eastward equatorial electric field (EEF) in the 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 (EIA). Due to its importance, there is much interest in accurately measuring the EEF. However, there is a severe lack of high quality data with the notable exception being the JULIA coherent scatter radar in Peru. In this work, we use CHAMP satellite-derived latitudinal current profiles of the day-time EEJ in order to estimate the eastward electric field at all longitudes, seasons, and day-side local times. We have constructed a dataset of over 32,000 EEF estimates based on six years of CHAMP data. Our estimates agree well with JULIA measurements, with an RMS difference of 0.13 mV/m.Item Restricted Evidence for anomalous cowling conductivity in the strongly driven equatorial electrojet and a reconciliation of magnetic field data with theory(Editor no identificado, 2009-08) Kelley, Michael C.; Ilma, Ronald R.; Alken, Patrick; Maus, StefanSeveral days of continuous incoherent scatter radar observations during the November 2004 magnetic superstorm are available, as well as data from the CHAMP [CHAllenging Minisatellite Payload] satellite magnetic fiel d detector. We have found that, for zonal electric fields greater than about 1 mV/m, the linear relationship between CHAMP-deduced electric fields and Jicamarca-measured electric fields breaks down. We find that the observed magnetic field is a factor of two smaller than expected for the largest zonal electric field (3.2 mV/m) observed simultaneously in time and nearby spatially with CHAMP. The fact that similar decreases in the effective conductivity occurred during both normal and counter-electrojet conditions indicates that Farley-Buneman waves must be the source of the physical mechanism for the reduced current. In addition, we provide an explanation for a long standing (more than 35 years) discrepancy between theory and (rocket) experiments, concerning the peak height of the electrojet and the magnetic field perturbation. The correction by a factor of four of the electron-neutral collision frequency thought to explain this problem is not necessary if the field line integrated conductivities are used.Item Restricted Relationship between the ionospheric eastward electric field and the equatorial electrojet(American Geophysical Union, 2010-02-26) Alken, Patrick; Maus, StefanThe equatorial electrojet (EEJ) is a strong ionospheric current along the magnetic equator driven by the day side eastward electric field. Current strength is affected by two‐stream and gradient‐drift instabilities which pose a serious obstacle to quantitative electrodynamic modeling of the equatorial ionosphere. Using highly accurate radar and magnetic field measurements taken over the past decade, we deduce empirical relations between the observed EEJ and the driving eastward electric field. These indicate that the current strength is largely unaffected by instabilities for eastward fields in the range of −0.4 mV/m to 0.07 mV/m. This is followed by a band of moderate current reduction from 0.07 mV/m to 1 mV/m, consistent with predictions for the gradient drift instability. At even stronger eastward fields, a further current reduction is observed, as expected for the two‐stream instability. These non‐linear empirical relations can be used to correct ionospheric electric field strengths inferred from space and ground‐based magnetic field measurements.Item Restricted Spatio-temporal characterization of the equatorial electrojet from CHAMP, Ørsted, and SAC-C satellite magnetic measurements(American Geophysical Union, 2007-09-13) Alken, Patrick; Maus, StefanThe equatorial electrojet (EEJ) is an eastward electric current on the day‐side, flowing in a narrow band along the dip equator in the ionospheric E region. Recent magnetic observations from the CHAMP, Ørsted, and SAC‐C satellites, comprising more than 95,000 dip equator crossings from 1999 to 2006, have provided an unprecedented longitudinal coverage of the EEJ magnetic signature. We have used these data to construct an empirical model of the EEJ current climatological mean and day to day variability as a function of longitude, local time, season, and solar flux. Our model has been successfully verified against vertical drift data from the JULIA radar at Jicamarca. We have also used the EEJ observations to estimate the self‐correlation of the EEJ, confirming short longitudinal correlation lengths of 15° and finding a temporal correlation length of 2.4 h. Our model's predictions of the eastward electric field and its standard deviation may provide useful input to various kinds of ionospheric simulations. Coefficients and software are available online at http://models.geomag.us/EEJ.html and http://www.earthref.org.Item Restricted Swarm SCARF equatorial electric field inversion chain(SpringerOpen, 2013-11-22) Alken, Patrick; Maus, Stefan; Vigneron, Pierre; Sirol, Olivier; Hulot, GauthierThe day-time eastward equatorial electric field (EEF) in the ionospheric E-region plays a crucial 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 (EIA). Due to its importance, there is much interest in accurately measuring and modeling the EEF for both climatological and near real-time studies. The Swarm satellite mission offers a unique opportunity to estimate the equatorial electric field from measurements of the geomagnetic field. Due to the near-polar orbits of each satellite, the on-board magnetometers record a full profile in latitude of the ionospheric current signatures at satellite altitude. These latitudinal magnetic profiles are then modeled using a first principles approach with empirical climatological inputs specifying the state of the ionosphere. Since the EEF is the primary driver of the low-latitude ionospheric current system, the observed magnetic measurements can then be inverted for the EEF. This paper details the algorithm for recovering the EEF from Swarm geomagnetic field measurements. The equatorial electric field estimates are an official Swarm level-2 product developed within the Swarm SCARF (Satellite Constellation Application Research Facility). They will be made freely available by ESA after the commissioning phase.