Verification of CHAMP Radio-Occultation Observations in the Ionosphere Using MIDAS

Transcription

1 Verification of CHAMP Radio-Occultation Observations in the Ionosphere Using MIDAS Paolo Spalla 1, Norbert Jakowski 2, Andreas Wehrenpfennig 2, Paul S J Spencer 3, Cathryn N Mitchell 3 1 IFAC-CNR, Florence, Italy 2 DLR, Neustrelitz, Germany 3 University of Bath, Bath, UK. Summary. The MIDAS (Multi-Instrument Data Analysis System) algorithm performs a 4- dimensional mathematical inversion. It is designed to assimilate a number of different measurement techniques, thus allowing the spatial and temporal factors to be accounted for in the inversion. In this study ionospheric data have been collected together from a number of observations in Europe. The MIDAS program incorporates these data into a single 4-D inversion and produces electron concentration maps. CHAMP occultations over Europe have been identified for comparison purposes and the data have been inverted using a newly developed technique. First results indicate excellent agreement between the specification of the ionospheric electron concentration using MIDAS and that calculated from CHAMP. Key words: Ionosphere, occultations, tomography, electron density, linear inversion 1 Introduction The purpose of this short paper is to demonstrate the first results of a validation of the ionospheric radio occultation data of the CHAMP satellite. Comparisons are made between vertical electron-density profiles generated with CHAMP occultation data and those obtained using the MIDAS algorithm from a large number of observations collected all over the northern hemisphere. 2 Data analysis To derive electron density profiles from CHAMP radio occultation data over Europe, a tomographic approach with a spherically layered voxel structure has been applied. The linear equation system describes the measured total electron content along numerous ray paths obtained during the occultation process, as a function of the sum of electron along the ray path elements within the crossed voxels. The solution provides the mean electron density within the spherically layered voxel. Due to the rather low orbit height of CHAMP (about 430 km) that is very close to the F2 layer peak, a model assisted technique has been applied to

2 546 Paolo Spalla et al. overcome the upper boundary problem. For more details of this technique see Jakowski et al. in this issue. The MIDAS (Multi-Instrument Data Analysis System) algorithm performs a 4- dimensional mathematical inversion. It is designed to assimilate a number of different ionospheric measurement types, thus allowing the spatial and temporal factors to be accounted for in the inversion. The data used for the present work are: GPS data from the International GPS Service Peak density and height from ionosondes distributed over Europe NIMS observations from Italy The MIDAS program incorporates these data into a single 4-D inversion and produces electron concentration maps. Another paper describing MIDAS in more detail is provided in this issue by C.N. Mitchell The comparisons of CHAMP and MIDAS profiles at the location of the midpoint defined by the CHAMP occultation are shown in the following figures. The date, time of occultation and the coordinates of each profile are given in the title. Each plot shows the vertical profile from the MIDAS inversion as a solid line and the profile from the CHAMP data as a broken line. These plots also show the peak densities and heights as given by observations from neighboring ionosondes. A key to these is given in the final figure. Different peak density values are due to different locations and local time. 3 Conclusions The CHAMP occultations are often located between 25 o E and 30 o E, which is on the edge of the European region, where other ionospheric information used in the inversion is sparse. This could be the most important reason for any differences. Ionosonde data should (ideally) straddle the image area, which is not generally the case. In addition, ionosonde data give an information about peak heights via ionogram inversions that are always questionable: this is expected to lead to a worse agreement between peak heights than between peak densities. Two very different measurement techniques have been compared here. The MIDAS inversion has used satellite-to-ground data in conjunction with ionosonde data to create maps of electron concentration over Europe. The CHAMP data has been inverted in isolation from this procedure. Despite these differences the agreement is remarkably good, providing a validation of the excellent ionospheric information now provided by CHAMP. The results encourage further studies that will help to validate CHAMP derived electron density profiles. In future work the radio occultation data should also improve the accuracy of the MIDAS reconstruction results.

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