An Examination of FORMOSAT-3/COSMIC Ionospheric Electron Density Profile: Data Quality Criteria and Comparisons with the IRI Model

Abstract

In this ar ti cle, we an a lyze the prop er ties of ion o spheric elec tron den sity pro fil ing re trieved from FORMOSAT-3/COS MIC ra dio occultation mea sure ments. Two pa ram e ters, namely, the gra di ent and fluc tu a tion of the top side elec tron den sity pro file, serve as in di ca tors to quan ti ta tively de scribe the data qual ity of the re trieved elec tron den sity pro file. On the ba sis of 8 month data (June 2006 -Jan u ary 2007), we find that on av er age 93% of the elec tron den sity pro files have up per elec tron den sity gra di ents and elec tron den sity fluc tu a tions smaller than -0.02 #/m 3 /m and 0.2, re spec tively, which can be treated as good data for fur ther anal y sis. The same re sults are also achieved for the peak height of the elec tron den sity. Af ter re mov ing the ques tion able data, we com pare the gen eral be hav iors of the elec tron den sity be tween FORMOSAT-3 and the IRI model. It is found that the global dis tri bu tions of the peak height and the peak elec tron den sity for the FORMOSAT-3/COS MIC data are gen er ally con sis tent with those for the IRI model. How ever, a sig nif i cant dif fer ence be tween their scale heights of the top side elec tron den sity pro files is found. It sug gests that the shape of the top side elec tron den sity pro file in the IRI model should be revised accordingly such that it more closely resembles the real situation. Ocean. Sci., 20, 193-206, doi: 10.3319/TAO.2007.10.05.01(F3C) IN TRO DUC TIONRa dio occultation tech nique is an old, but very so phis ticated, method for the re trieval ter res trial at mo sphere parameters (Fjeldbo et al. 1971). The core of this tech nique is (under a num ber of as sump tions) to trans form the bend ing an gle of the ra dio ray path to the at mo spheric re frac tive index, which is trans mit ted from a very sta ble source sit u ated on one side of the Earth and re ceived by a re ceiver lo cated on the op po site side of the Earth (Rocken et al. 1997;Hajj et al. 2000). Once the at mo spheric re frac tive in dex is re trieved, the lower at mo spheric tem per a ture, hu mid ity and ion ospheric elec tron den sity at the tan gent point of the ray path pierc ing through the at mo sphere can be es ti mated in ac cordance with the re la tion be tween the re frac tive in dex n and the pa ram e ters given be low:( 1) where P is pres sure (hpa), T is tem per a ture (k), e is wa ter va por pres sure (hpa), f is ra dio wave fre quency (Hz), and n e is elec tron den sity (#/m 3 ). In the ion o sphere (higher than an al ti tude of about 100 km), the con tri bu tion of T, P and e to the at mo spheric re frac tive in dex is neg li gi ble com pared to the elec tron den sity con tri bu tion. As a re sult, n e can be directly es ti mated from n for given f. Ex cept for the at mospheric re frac tive in dex, un der the straight line as sump tion of the ra dio ray path, the height vari a tion of the ion o spheric elec tron den sity can also be re trieved from cal i brated to tal elec tron con tent (TEC) in ac cor...

Vertical and longitudinal electron density structures of equatorial E- and F-regions

“…A number of factors, such as thermal noise fluctuations of the received signals, sharp gradients of the ionospheric refractive index in vertical and horizontal directions, system bias of the excess phase, inaccurate position of the satellites, and so on, may influence the accuracy and precision of the retrieved bending angle in terms of radio occultation technique, causing physically erroneous inversion results (Schreiner et al, 2007). In addition, the presence of plasma irregularities in the GPS ray path may cause significant fluctuations of the retrieved electron density profile, giving rise to large uncertainty of the estimation and impairment of the data reliability (Yang et al, 2009). In this study, we use the criteria of data quality control developed by Yang et al (2009) to remove questionable and unreliable data from original data set.…”

“…In addition, the presence of plasma irregularities in the GPS ray path may cause significant fluctuations of the retrieved electron density profile, giving rise to large uncertainty of the estimation and impairment of the data reliability (Yang et al, 2009). In this study, we use the criteria of data quality control developed by Yang et al (2009) to remove questionable and unreliable data from original data set. Moreover, the retrieved electron densities for the conditions of geomagnetic Kp index greater than 3 are also not included in the data analysis to limit effects from geomagnetic disturbances.…”

“…To attain these goals, the measurements of global electron density profiles with sufficiently high resolutions in space and time for a long period of time are required. With the aid of low orbit satellites performing GPS radio occultation observations, it has been shown possible to obtain global soundings of the electron density profiles in the height range from lower E-region to topside ionosphere with vertical resolution of about 1 km (Schreiner et al, 2007;Yang et al, 2009). …”

A global survey of COSMIC ionospheric peak electron density and its height: A comparison with ground-based ionosonde measurements

“…Although quality control schemes have been implemented in the COSMIC Data Analysis and Archive Center (CDAAC) located at Boulder, Colorado, USA, to reject bad occultation profiles, some problematic profiles may still pass the quality control to reduce the reliability of the collected occultation data. With the use of slope (or vertical gradient) of topside electron density profile and mean deviation of electron density fluctuations, Yang et al (2009) set up the criteria of data quality control to further remove questionable electron density profiles from CDAAC data base. In this study, the peak electron densities and peak heights of the COSMIC electron density profiles that were passed through the screening of the data quality control are employed to compare with those observed by ground-based ionosondes for the period from July 2006 to February 2007.…”

Developing an Electron Density Profiler over Europe Based on Space Radio Occultation Measurements

“…This violates the requirement for EDP inversion producing a very unrealistic profile. In order to overcome this limitation and concentrate on good quality EDPs a selection process was applied in order to exclude those measurements where the distortion of the profiles was excessive [14]. Figure 3 demonstrates the uniform distribution of locations where electron density measurements have been recorded during one week of RO.…”