Plasma effects on transionospheric propagation of radio waves II R. Leitinger
General remarks Reminder on (transionospheric) wave propagation Reminder of propagation effects GPS as a data source Some electron content examples Second lecture: Modern applications TEC TEC mapping, adaptive modelling, (data ingestion into models, data assimilation) Inversion Tomography 3D and 4D imaging (filling voxels with data) Ionospheric Radio Occultation The importance of assessment studies
GPS as a data source The classical TEC sources provided or fixed geometry and high temporal resolution (GEO Faraday beacons) or nearly constant Local Time and high spatial (latitudinal) resolution (LEO Diff. Doppler beacons) true signal amplitude and phase GPS (MEO orbit; 12 hour period, inclination 53 degrees) mixes temporal and spatial dependence (diurnal curves of TEC for a given location are always composites, etc.) Needs complicated calibration procedures (CODE = Group Delay needs knowledge of transmitter and receiver delays and cannot provide sufficient resolution for science applications, PHASE = Diff. Doppler suffers from 2 ambiguity CODE and PHASE combination needed for all science applications) Amplitude and phase are reconstructed in the receiver GPS-sTEC is the sum of ionospheric plus plasmaspheric content Therefore: TEC mapping, adaptive modelling, data ingestion into models,
Example for Faraday observations on the signals of the geostationary satellite METEOSAT-2 made at Firenze on 25 and 26 May, 1991. TEC in units of 10 15 m 2 vs. Universal Time in hours. Diurnal curve; strong day to day variations; very strong TID activity on 25 May.
Example for a strong positive storm effect in ionospheric electron content: Sudden Commencement of the geomagnetic storm (SSC) on 4 Februrary, 1983, 16:15 UT, Dst-minima: -159 nt on 4 Feb., 20 UT, -169 nt on 5 Feb., 11 UT. Faraday effect on geostationary satellite signal received at Hamilton, MA, USA (400 km ionosph. point: 39 N, 289 E; geomagn. lat. 50 ). Ordinate: electron content in units of 10 15 m 2, abscissa: Universal Time (UT). 3 February; quiet reference day (Ap=7). Intermediate level of solar activity (monthly mean sunspot number R=51).
Example for a strong negative storm effect in ionospheric electron content: Sudden Commencement of the geomagnetic storm (SSC) on 11 April, 1981, 13:39 UT, Dst-minima: -167 nt on 12 April, 6 UT, -291 nt on 13 April, 7 UT. Faraday effect on geostationary satellite signal received at Madrid, Spain (400 km ionosph. point: 37 N, -4 E; geomagn. lat. 41 ). Ordinate: electron content in units of 10 15 m 2, abscissa: Universal Time (UT). 11 April was a slightly disturbed day (Ap=39). The negative storm effect was pronounced in the longitude zone around 0 only. High solar activity (monthly mean R=156).
Example for latitudinal profiles of vertical electron content from NNSS observations made at Graz demonstrating the potential of combined observations for times of high solar activity. Data of 1 November, 1990. Electron content in units of 10 15 m 2 vs. geogr, latitude in deg. N (N to S). Upper panel: traces of the ionospheric points (in 400 km height) in a geographic system. Even for mid latitudes the equatorial anomaly is important!
Example for latitudinal profiles of vertical electron content from NNSS observations made at Graz demonstrating the potential of combined observations for times of high solar activity. Afternoon/evening of 2 November, 1990. Electron content in units of 10 15 m 2 vs. geogr, latitude in deg. N (N to S). Upper panel: traces of the ionospheric points (in 400 km height) in a geographic system. In the early evening the effect of the equatorial anomaly is even more pronounced!
Latitude dependence of TEC from Diff. Doppler on the 150/400 MHz signals of NNSS. Receiving stations Graz (47.1 N, 15.5 E) and Uppsala (59.8 N, 17.6 E). Typical equjinox/winter night profile. Main trough with minimum around 60 N. Indication of TEC increase towards the equatorial anomaly in the South. 18 October, 1989 around 21:10 UT. TEC in units of 10 15 m 2 vs geographic latitude. Upper panel: traces of the ionospheric points (in 400 km height) in a geographic system. High solar activity (monthly mean R=159, monthly mean 10.7 cm solar radio flux: 209 units).
Very high values of TEC Latitude dependence of TEC from Diff. Doppler on the 150 / 400 MHz signals of NNSS. Receiving station Dionysos (38.1 N, 23.0 E) and Uppsala (59.8 N, 17.6 E). TEC in units of 10 16 m 2 vs geographic latitude. 8 April, 1981, around 11:40 UT. Monthly mean R=156, monthly mean 10.7 cm solar radio flux: 223 units). Geomagnetically nearly quiet (Ap=11).
Electron content (TEC) maps global (bi-hourly) regional (30 minutes) (somehow) based on models global maps need data interpolation (directly or indirectly by model) International GPS Service (IGS): 6 global maps (by their regional centers) + one official product (= average!!) La Plata maps DLR Neustrelitz regional maps (based on adaptive model)
Ionospheric Radio Occultation tomography occultation inversion combined space-ground tomography
Occultation rays from the GNS to the LEO. Slant electron content (stec) consists of three parts: ionosphere (P to P ), uppermost ionosphere (P to U in 2000 km height), plasmasphere (U to G). Each ray can be characterised by its perigee (tangent point). In reality the rays are not co-planar and the location of their perigees forms a 3D curve. Its projection to the surface of the Earth is the sink path. Useable ( good ) occultations have a ionospheric sink path length of < 10 (ground occultation point to perigee of the highest possible ray).
Ionospheric use of Radio Occultation Primary information are (uncalibrated) GNS to LEO electron content values I s usually gained by means of carrier phase differences. Subtraction of the electron contents of uppermost ionosphere and plasmasphere leads to the ionospheric electron content which is subjected to inversion I I = I s - I U - I P Refraction (ray bending) is negligible straight line propagation is assumed continuous solutions are unique even if refractivity is not increasing / decreasing monotonously (very important because of electron density profiles with F2 maximum (and E maximum). discrete problem has straightforward triangular design matrix (consists of path length elements) ( onion peeling ) solutions are inherently stable with error accumulation in the bottom of the electron density profile
Ionospheric use of Radio Occultation We should distinguish between a) high LEO orbit (h LEO >700 km) (HL) and b) low LEO orbit (h LEO <700 km) (LL) cases LL experience from CHAMP (CHAllenging Minisatellite Payload): h LEO > h mf2 is a necessary condition for ionospheric inversion! HL: Model assistance of advantage in removing I U and I P LL: Model assistance and/or high elevation electron content data (LEO to additional GNS) necessary in removing I U and I P Model assistance can strongly improve the reliability of inversion results in the vicinity of ionisation ridges (maxima in the TEC distribution) and troughs (minima in the TEC distribution) An other type of model assistance was very successful in CHAMP inversion (N. Jakowski et al. at DLR Neustrelitz) (adaptive models for the ionosphere [and plasmasphere] above the CHAMP orbit)
Extensive calculations were carried out on the basis of the three dimensional and time dependent ionosphere/plasmasphere model NeUoG-plas (Hochegger and Leitinger, 2000). The following four parameters of electron density height profiles were investigated: - peak height hmf2, (best error <20 km in more than 50% of the cases) - peak electron density NmF2, (second for far more than 50% of the cases error <15%. - topside scale height or topside slab thickness, - a shape parameter for the bottomside ionosphere (not really good). The low latitudes exception region: vicinity of one of the crests of the equatorial anomaly and (to a lesser degree) the region around the dip equator model assistance can improve situation substantially
Necessity of Assessment studies on the basis of realistic models Calculate artificial transmitter to receiver electron contents (stec) Subject stec series to retrieval / inversion method Compare retrieval / inversion results with model truth
Electron density from NeUoG-plas 3 GNS to LEO rays: ground occultation (tangent height h t = 0 km), near F layer peak (h t = 300 km), high ray (h t = 300 km).
height profiles of electron density; black: given profile, blue: direct inversion, red: model assisted inversion. Horizontal distribution of N max (contours over geographic systems) in the occultation region; blue low values, red high values; occultation rays (thin red) and occultation path (thick red). N max distribution along the ground occultation ray bottom: symmetric (red) and antimetric (light blue) part. Data base: artificial stec values, calculated by means of NeUoG-plas with one GPS to LEO scenario shifted in geographic latitude, season and local time to encounter crest and valley conditions.
height profiles of electron density; black: given profile, blue: direct inversion, red: model assisted inversion. Horizontal distribution of N max (contours over geographic systems) in the occultation region; blue low values, red high values; occultation rays (thin red) and occultation path (thick red). N max distribution along the ground occultation ray bottom: symmetric part (red) and model for it (blue). Data base: artificial stec values, calculated by means of NeUoG-plas with one GPS to LEO scenario shifted in geographic latitude, season and local time to encounter crest and valley conditions.
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