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COSMIC observation and GSM TIP model results
Accepted Manuscript The global morphology of the plasmaspheric electron content during Northern winter 2009 based on GPS/COSMIC observation and GSM TIP model results M.V. Klimenko, V.V. Klimenko, I.E. Zakharenkova, Iu.V. Cherniak PII: DOI: Reference:
S0273-1177(14)00389-5 http://dx.doi.org/10.1016/j.asr.2014.06.027 JASR 11848
To appear in:
Advances in Space Research
Received Date: Revised Date: Accepted Date:
19 February 2014 7 June 2014 18 June 2014
Please cite this article as: Klimenko, M.V., Klimenko, V.V., Zakharenkova, I.E., Cherniak, Iu.V., The global morphology of the plasmaspheric electron content during Northern winter 2009 based on GPS/COSMIC observation and GSM TIP model results, Advances in Space Research (2014), doi: http://dx.doi.org/10.1016/j.asr.2014.06.027
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The global morphology of the plasmaspheric electron content during Northern winter 2009 based on GPS/COSMIC observation and GSM TIP model results
M.V. Klimenko *, V.V. Klimenko, I.E. Zakharenkova, Iu.V. Cherniak
West Department of Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation RAS, Kaliningrad, 236017, Russia, e-mail: [email protected], [email protected], [email protected], [email protected] Correspondence to: M.V. Klimenko ([email protected])
Abstract We studied the contribution of the global plasmaspheric and ionospheric electron content (PEC and IEC) into total electron content (TEC). The experimental PEC was estimated by comparison of GPS TEC observations and FORMOSAT-3/COSMIC radio occultation IEC measurements. Results are retrieved for the winter solstice (January and December 2009) conditions. Global maps of COSMIC-derived IEC, PEC and GPS TEC were compared with Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) results. In addition, we used GSM TIP model results in order to estimate the contribution of plasmaspheric electron content into TEC value at the different altitudinal regions. The advantages and problems of the outer ionospheric/plasmaspheric parameters (O+/H+ transition height, TEC and electron density at height above F2 layer peak) representation by the IRI (International Reference Ionosphere) model are discussed.
Keywords: total electron content; plasmasphere; GPS; FORMOSAT-3/COSMIC; numerical modeling.
1. Introduction Total Electron Content (TEC) is one of the most important parameters used in the ionospheric studies. Nowadays the dense network of the Global Positioning System (GPS) receivers allows the simultaneous coverage of TEC values in global scale with high temporal resolution. However, one of the main limitations of the GPS technique is that the value of GPS TEC has an integral character and it is difficult to determine the precise contribution of the ionosphere to GPS TEC based on GPS measurements only. In fact, the TEC value consists of the ionospheric electron content (IEC) and plasmaspheric electron content (PEC). The plasmasphere is often ignored at analysis and estimation of GPS TEC data due to the arguments that (1) the electron density at plasmasphere are several orders of magnitude less than F region electron density (Gallagher et al., 2000); (2) the TEC and foF2 highly correlated (Liu et al., 1996). Are these arguments right for all longitudes, latitudes and local times? How much does the PEC contribute to TEC at different locations and for different local times? Previous studies show that the plasmaspheric contribution to the TEC is approximately 55–65% during the nighttime and 10–20% during the day-time (Balan et al., 2002; Belehaki et al., 2004; Gulyaeva and Gallagher, 2007; Manju et al., 2008), with the higher percentages occurring near the equator during the winter months (Balan et al., 2002). Most part of the previous investigations, estimated contribution of PEC to the TEC values, were made for the very limited geographic locations by: (1) taking the difference between the electron content measured by the differential Doppler and the Faraday rotation technique using radio beacons on geostationary satellites (Kersley and Klobuchar, 1978); (2) comparisons of GPS TEC data and modeled IEC (Lunt et al., 1999; Balan et al., 2002); (3) comparative analysis of GPS TEC data and IEC derived from the observation of a ground-based ionosonde (Belehaki et al., 2004; Mozert et al., 2007), coherent radio beacon experiment (Manju et al., 2008), and FORMOSAT3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) Radio Occultation (RO) measurements (Zakharenkova et al., 2013). There were only three studies that 2
try to resolve this issue in global scale (Yizengaw et al., 2008; Cherniak et al., 2012; Lee et al., 2013). However there was not even one investigation that analyzes the global first-principal model/data comparisons of PEC contribution to the TEC values. This paper presents first results of the joint analysis based on GPS TEC observations, FORMOSAT-3/COSMIC radio occultation measurements, and results derived by the global numerical model of ionosphere/plasmasphere system. In addition, the comparisons of F region and topside ionospheric electron density derived from numerical model with the IRI (International Refference Ionosphere) output are presented. 2. Observations and brief model description 2.1. Experimental Data – global TEC maps As a source of vertical TEC data we used IGS (the International GNSS (Global Navigation System Service) Service) Global Ionospheric Maps (GIMs) of TEC in the IONEX (IONosphere map
EXchange)
format.
These
data
are
accessible
at
the
ftp
server:
ftp://cddis.gsfc.nasa.gov/pub/gps/products/ionex. The GIMs are generated routinely by the IGS community with resolution of 5° longitude and 2.5° latitude and temporal interval of 2 hours; one TEC unit (TECU) is equal to 10 16 electrons/m2. Currently, there are three types of IGS GIMs: the final, rapid and predicted respectively. There are four IGS Associate Analysis Centres (IAACs) for the final and rapid ionospheric products: CODE, ESA/ESOC, JPL and gAGE/UPC. Detailed description of IGS GIMs computation and validation can be found in HernandezPajares et al. (2009). The IAACs provide ionosphere maps computed with independent methodologies that use GNSS data from different set of GPS stations. These maps are uploaded to the IGS Ionosphere Product Coordinator, who computes the official IGS combined products. During the period of more than 10 years of continuous IGS ionosphere operation, the techniques used by the IAACs and the strategies of combination have improved in such a way that the combined IGS GIMs are now significantly more accurate and robust. The combined product generated from GIMs maps, provided by CODE, JPL, ESA and UPC, are now very accurate over 3
regions with regular GNSS permanent networks, but of course it is less accurate over oceans, deserts and at the high latitudes. In this study, the final product of IGS combined GIMs produced by GRL/UWM were used to calculate the global maps of monthly median of GPS TEC values. We calculate median TEC maps with 2 h resolution for January 2009, based on 31 days observations. 2.2. Experimental Data – COSMIC We used COSMIC RO measurements and products that are provided by the COSMIC Data Analysis and Archive Center (CDAAC, http://www.cosmic.ucar.edu/cdaac/). At CDAAC, the ionospheric profiles are retrieved by use of the Abel inversion technique from TEC along Low Earth Orbiting (LEO) satellites-GPS satellites rays. Detailed description of CDAAC data processing and the Ne profile retrieval method can be found in Kuo et al. (2004). We used second level data provided by CDAAC – “ionprf” files containing information about ionospheric electron density profiles (EDPs). It should be mentioned that there is a need to analyze the data quality and to fix and remove bad or questionable data – electron density profiles with large and irregular spikes and data gaps. Special processing routine analyses the shape of each profile, rate of Ne change, its comparison with confidence limit. All unsuitable RO profiles were removed from our analysis. In order to estimate the ionospheric electron content (IEC) the integration of COSMIC RO data was done. For the present study the upper limit of the ionosphere has been taken to be at 700 km (altitude of COSMIC satellites). All selected COSMIC RO EDPs were integrated up to the height of 700 km, in that way estimates of IEC, which correspond to the altitudinal range of 100–700 km, were retrieved. For each month IEC values were accumulated and then they were divided into 12 data sets by intervals with 2 h duration. For the global representation of IEC estimates a spherical harmonics expansion up to degree and order 15 was carried out. The resulted maps illustrated monthly median distribution of IEC; the final outputs are at the same of IONEX format. So, for the considered month 12 IEC median maps (2 h resolution) with spatial 4
resolution similar to IGS GIMs (2.5/5.0 deg in latitude/longitude) were calculated. In order to retrieve monthly median estimates for COSMIC IEC, we used more than 58000 EDPs that were obtained during January 2009. Joint analysis of GPS TEC and COSMIC IEC data can be resulted in the quantitative differences PEC = TEC – IEC as a measure of the plasmasphere contribution (h > 700 km) to GPS TEC. To compare the global morphology of the IEC, TEC and PEC, the global maps of COSMIC IEC, GPS TEC and derived PEC estimates were constructed in magnetic latitude and local time coordinates. 2.3. GSM TIP Model Description In this work the results of numerical modeling, derived with the use of Global Selfconsistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) are presented. The Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) (Namgaladze et al., 1988; Korenkov et al., 1998) was developed in the WD IZMIRAN (West Department of Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio wave propagation of the Russian Academy of Sciences). This model calculates time-dependent global three-dimensional structure of the near-Earth space environment from 80 km to 15 Earth radii, in particular: (1) the distributions of temperature, composition and velocity vector of neutral gas; (2) the density, temperature, and velocity vectors of atomic and molecular ions and electrons; (3) the two-dimensional distribution of electric field potential both of a dynamo and magnetospheric origin. The GSM TIP model consists of three main blocks: thermospheric block, ionospheric block, and the block of electric fields. In the thermospheric block the global distribution of the neutral gas temperature, the N2, O2, O, NO, N(4S), and N(2D) concentrations as well as the threedimensional circulation of the neutral gas are calculated in the range from 80 to 526 km in a spherical geomagnetic coordinate system. In the vertical dimension, the thermospheric code uses 30 layers, with each layer approximately equal to a half thickness of scale height. The minimum distance between knots is 3 km near the lower boundary and increases to 40 km at 526 km 5
altitude. Ionospheric block consists of two modules. In the first module (the D, E, F1 region module), the three-dimensional density of the N2+, O2+, and NO+, ion temperature and velocities are calculated in the range from 80 to 526 km in the spherical geomagnetic coordinate system. In the second module (the F2 region and protonospheric module), the densities, temperatures, and vector velocities of atomic (O+, H+) ions and electrons are calculated in the magnetic dipole coordinate system from a base altitude 175 km in the northern hemisphere to 175 km in the southern hemisphere or to a maximum distance of 15 Earth’s radii (in case of open geomagnetic field lines). In this case, the ionosphere code for atomic ions does not require the upper boundary condition. This ionospheric part of the code has variable spatial steps along the magnetic field lines. Additionally, the model also provides the two-dimensional electric field potential distribution for dynamo and magnetospheric origin. The calculation of electric fields in the GSM TIP model has recently been modified by Klimenko et al. (2006, 2007). All model equations are solved by the finite difference method. The Earth’s magnetic field is approximated by the central dipole. Thus, discrepancy of geographical and geomagnetic axes is taken into account. The solution of the full system of equations of the model is performed numerically on a global grid with resolutions of 5° in latitude and 15° in longitude as specified in the spherical geomagnetic coordinate system; the time step is 2 min. The transformations between all coordinate systems in the model are given by standard equations. The model inputs are (1) the solar EUV and UV spectra (10–1760 Ǻ); (2) the precipitating electron fluxes; (3) the amplitudes and spatial distribution of Region 1 field aligned currents or a cross-polar cap potential difference and Region 2 field aligned currents. The cross-polar cap potential difference was set at geomagnetic latitudes ±75°, and Region 2 Field-Aligned Currents (R2 FAC), j2, at geomagnetic latitudes ±70°. The cross-polar cap potential difference was set equal to 38 kV according to Feshchenko and Maltsev (2003) and R2 FAC was set equal to 3.5⋅10 -9, A/m2 according to Klimenko et al. (2011) which corresponds to quiet geomagnetic conditions. In the current simulation, the empirical model by Zhang and Paxton (2008) was used for high-energy 6
particle precipitation. In this model, the energy and energy flux of precipitating electrons depend on a three-hour Kp-index. The GSM TIP model was described in detail by Namgaladze et al. (1988), and its recent modifications were presented in Klimenko et al. (2006, 2007, 2011). A modified GSM TIP model has already been used to study the F2 region and topside ionospheric structure in quiet conditions and during different space weather events (Klimenko et al., 2008, 2011, 2012, Korenkov et al., 2012). 3. Results The amount of the ionospheric and plasmaspheric contribution into the Total Electron Content (TEC) depends on O+/H+ transition height that is the boundary between ionosphere and plasmasphere (protonosphere). Integral electron content in the column from the lower boundary of the ionosphere up to O+/H+ transition height represents the Ionospheric Electron Content (IEC) and from O+/H+ transition height up to GPS satellite altitude (~20000 km) – the Plasmaspheric Electron Content (PEC). The O+/H+ transition height has been already investigated by means of satellite observations (Miyazaki, 1979; Kutiev et al., 1980; Triskova et al., 1998). Empirical models of O+/H+ transition height have been also developed (Titheridge, 1976; Miyazaki, 1979; Kutiev et al., 2006). For high solar activity the O+/H+ transition height lies much higher than for low solar activity (e.g. MacPherson et al., 1998; Třísková et al., 1998; Truhlík et al., 2005). It is shown that during 2008/2009 the O+/H+ transition height dropped to it lowest ~450 km at night and ~750 km during daytime (Heelis et al., 2009; Hyssel et al., 2009; Balan et al., 2012; Nanan et al., 2012; Hazra et al., 2012; Aponte et al., 2013), that was not observed in the previous minima of solar activity. It follows the changes in the PEC contribution to the TEC during this period. To confirm or deny this statement it is necessary to use the model calculations and measurements of the TEC, PEC and IEC in the same place at the same time. Note that the PEC always contributes to the TEC. This contribution depends on solar activity level, season and local time. At that, the relative PEC contribution into TEC is the most significant at night and decreases during daytime. 7
Figure 1 presents the map of the O+/H+ transition height obtained in GSM TIP model for 12:00 UT on January 15, 2009. It is evident that the transition height: is less in the winter hemisphere than in the summer one; drops below 500 km at mid-latitudes of the Northern Hemisphere in the American longitudinal sector in the morning that is consistent with the observations and model calculations of other authors (Heelis et al., 2009; Balan et al., 2012; Nanan et al., 2012; Hazra et al., 2012; Aponte et al., 2013); reaches the maximum values of ~ 700 km near the equator and about 1000 km at middle latitudes in the afternoon. Thus, the average value of the transition height for the selected date and UT moment amounts to ~ 700 km. In such a way, we can approximately assume that for the given UT moment the ionospheric (IEC) and plasmaspheric (PEC) contribution to the TEC determined at the altitude range below 700 km and from 700 km up to 20000 km, respectively. Figure 2 presents the TEC, IEC and PEC maps (for the chosen moment of time corresponding to
12:00 UT), constructed in magnetic latitude and local time coordinates for January 2009. These results were derived from observations and model results. We limit these maps by magnetic latitude of 50 degree, as both COSMIC EDPs and global TEC GIMs can reveal some problems with data accuracy at the high latitudes.
The main morphological features of observed and modeled TEC, IEC and PEC maps are very similar. It is evident that the PEC contribution to the TEC values has a maximum near to geomagnetic equator. It follows that the equatorial ionization anomaly in TEC is weaker than in IEC due to reduction of the depth of the equatorial trough. The percentage contribution of the plasmasphere in TEC has maximum at the equator and reaches 85% at night and about 40% during the daytime. It is necessary to emphasize that our results revealed higher values of PEC contribution in comparison to all previous studies (e.g., Balan et al., 2002; Belehaki et al., 2004; Gulyaeva and Gallagher, 2007; Manju et al., 2008) that might be due the fact that our PEC estimates are related to the greater height interval (700–20200 km) then above-mentioned studies (1100–20200 km)
8
due to the lowest O+/H+ transition height during 2008-2009 deep minimum of solar activity. Another contributing factor is the small value of TEC during very deep solar minimum that leads to the fact that the relative contribution of the plasmasphere to GPS TEC is the largest, especially during night-time. The obtained results are in a good agreement with plasmasphere contribution estimates corresponded to the Northern Hemisphere mid-latitudes and low solar activity (Cherniak et al., 2012). Another interesting objective of our paper is the estimation of which altitudinal intervals of plasmasphere have the most significant contribution to the TEC value. Why this is important? Remember that most of empirical and first-principle models have the upper boundary lower than GPS satellite orbits. Therefore, these models are usually underestimating TEC values observed using GPS satellites. For examples: IRI model has the upper boundary at the height of 2000 km; Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) has the upper boundary at the height of ~ 700–800 km (Roble and Ridley, 1994); a whole Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) has the upper boundary at the height of ~ 3000 km (Jin et al., 2012). The GSM TIP model provides the unique opportunity to investigate in details the PEC contributions into TEC values in order to estimate the TEC underestimation obtained by different models, associated with the neglect of the PEC values from the model’s upper boundary up to the GPS satellite orbits. We divided PEC into four different altitudinal parts: (1) 700 – 2000 km, (2) 2000 – 3300 km, (3) 3300 – 10000 km, (4) 10000 – 20000 km and calculated the contribution of each PEC parts into the TEC. The results of these calculations are shown in Fig. 3. It is seen that the PEC contribution in TEC decreases with increasing altitude. Estimation of contribution of (1), (2) and (3) regions into TEC reveals rather close values to each other – each region has about 12-15% of TEC at daytime and ~ 2030% at night. At that the electron content in region (1) gives a largest contribution to TEC. The contribution to TEC decreases with altitude that is evident from comparison between contributions of (1) and (2) regions which have the same altitudinal scale. The electron content 9
contribution of (3) region is just a little greater than the same one of (2) region due to a larger altitudinal range of this region. Region (4) represents the smallest contribution of its electron content to TEC - about 3% at night and 2% at daytime. This information is of the high importance for interpretation of differences between TEC maps derived with use of the different ionospheric models that have different plasmaspheric extensions. Knowledge of separate contribution of the ionosphere and plasmasphere to TEC is essential for improvement of the ionospheric data assimilation (e.g. Thompson et al. (2009)), empirical (IRI) and first principles (TIME-GCM and GAIA) models and to correct interpretation of TEC variability during quiet and disturbed conditions. Figure 4 presents the global maps of electron density obtained in GSM TIP and IRI models for 03:00 UT on December 22, 2009 at heights of 300 and 1000 km. The selected date and UT moment correspond to the most favorable conditions for the formation of such large-scale ionospheric features of the electron density distribution as Main Ionospheric Trough (MIT) in the northern hemisphere (local winter) and Weddell Sea Anomaly (WSA) in the southern hemisphere (local summer). In general, Fig. 4 shows a good qualitative agreement between the calculated results obtained using the GSM TIP and IRI models. In particular, this agreement refers to the Equatorial Ionization Anomaly (EIA), WSA in the Southern Hemisphere and MIT in the Northern Hemisphere at a height of 300 km and to manifestations of WSA and MIT at a height of 1000 km. The EIA in the IRI model results appears on all altitudes above the F2 layer peak and, in particular, as it is clear from Fig. 4, at the height of 1000 km, in contrast to the GSM TIP model results. In fact, the EIA should disappear with height – with increase in height the anomaly crests must approach to each other and then merge into a single maximum at the geomagnetic equator. This point is also noted by Heelis et al. (2009). As for quantitative differences, they may be related to the overestimation of the electron density in the EIA crests obtained using IRI model, which is also mentioned in (Bilitza, 2004; Heelis et al., 2009). It should be noted that the IRI model overestimates the observed values of the O+/H+ transition height by 10
over 200 km for the deep solar minimum (Heelis et al., 2009). At the same time the GSM TIP
calculation results of O+/H+ transition height and electron density in the outer ionosphere are consistent with the observation data of the C/NOFS satellite, presented by Heelis et al. (2009). 4. Conclusions In our study we present results of the global first-principal model/data estimation of PEC contribution to the TEC values, as well as for the first time performed estimation of how different altitudinal parts of the plasmasphere can contribute into TEC under solar minimum condition (2009 winter month). It is concluded that: −
At night the contribution of the plasmasphere in TEC has maximum at the equator (up to 85%) and can exceed consequently the contribution of the ionosphere in TEC.
−
During daytime the PEC contribution into the TEC does not exceed ~ 40%.
−
The value of IEC cannot be considered as the sole significant parameter to calculate the value of TEC, as TEC is the factor depending on the ionosphere as well as the plasmasphere. Therefore, to predict the TEC values one must consider the plasmasphere parameters in addition to the ionosphere, especially during solar minimum conditions.
−
We have found some shortcomings in empirical IRI model, related to the overestimation of the electron density in the EIA region and the presence of the EIA at too high altitudes.
Acknowledgments. The authors acknowledge the University Corporation for Atmospheric Research (UCAR) for providing the COSMIC Data. We are grateful to International GNSS Service (IGS) for GPS data and products. These investigations were partially supported by the Russian Federation President Grant MK-4866.2014.5 and Russian Foundation for Basic Research (RFBR) – Grant №14-05-00788.
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Namgaladze, A.A., Koren’kov, Yu.N., Klimenko, V.V. et al. Global model of the thermosphereionosphere-protonosphere system. Pure and Applied Geophysics (PAGEOPH) 127(2/3), 219–254, 1988. Nanan, B., Chen, C.Y., Rajesh, P.K. et al. Modeling and observations of the low latitude ionosphere-plasmasphere system at long deep solar minimum. J. Geophys. Res. 117, A08316, doi:10.1029/2012JA017846, 2012. Thompson, D.C., Scherliess, L., Sojka, J.J. et al. Plasmasphere and upper ionosphere contributions and corrections during the assimilation of GPS slant TEC. Radio Sci. 44, RS0A02, doi:10.1029/2008RS004016, 2009. Titheridge, J.E. Ion transition heights from topside electron density profiles. Planet. Space Sci. 24, 229–245, doi:10.1016/0032-0633(76)90020-9, 1976. Třísková, L., Truhlík, V., Šmilauer, J. et al. Comparison of O+/H+ and O+/(H+ + He+) Transition Levels. Adv. Space Res. 22(6), 895–898, doi: 10.1016/S0273-1177(98)00119-7, 1998. Třísková, L., Truhlík, V., Šmilauer, J. Manifestation of solar activity in the global topside ion composition – a study based on satellite data. Ann. Geophys. 23, 2511–2517, SRef-ID: 1432-0576/ag/2005-23-2511, 2005. Yizengaw, E., Moldwin, M.B., Galvan, D. et al. Global plasmaspheric TEC and its relative contribution to GPS TEC. J. Atmosph. Solar-Terr. Phys. 70, 1541–1548, 2008. Zakharenkova I.E., Cherniak, Iu.V., Krankowski, A. et al. Analysis of electron content variations over Japan during solar minimum: Observations and modeling. Adv. Space Res. 52(10), 1827–1836, doi:10.1016/j.asr.2012.09.043, 2013. Zhang, Y., Paxton, L.J. An empirical Kp-dependent global auroral model based on TIMED/GUVI FUV data. J. Atmos. Solar-Terr. Phys. 70, 1231–1242, 2008.
15
Figure captions Fig. 1. Map of the O+/H+ transition height obtained in GSM TIP model for 12:00 UT on January 15, 2009. Fig. 2. From top to bottom: the maps of IEC, TEC, PEC in TECU and PEC in %. Left panel – GSM TIP model calculation results, right panel – observation data of IEC from COSMIC and TEC from IONEX. Fig. 3. Maps of PEC contribution into the TEC values from the different parts of plasmasphere in TECU (left panel) and in % (right panel). Fig. 4. Electron density global maps obtained in GSM TIP model (left panel) and IRI model (right panel) for 03:00 UT on December 22, 2009 at heights of 300 km (top panel) and 1000 km (bottom panel).
16
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S0273-1177(14)00389-5 http://dx.doi.org/10.1016/j.asr.2014.06.027 JASR 11848
To appear in:
Advances in Space Research
Received Date: Revised Date: Accepted Date:
19 February 2014 7 June 2014 18 June 2014
Please cite this article as: Klimenko, M.V., Klimenko, V.V., Zakharenkova, I.E., Cherniak, Iu.V., The global morphology of the plasmaspheric electron content during Northern winter 2009 based on GPS/COSMIC observation and GSM TIP model results, Advances in Space Research (2014), doi: http://dx.doi.org/10.1016/j.asr.2014.06.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The global morphology of the plasmaspheric electron content during Northern winter 2009 based on GPS/COSMIC observation and GSM TIP model results
M.V. Klimenko *, V.V. Klimenko, I.E. Zakharenkova, Iu.V. Cherniak
West Department of Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation RAS, Kaliningrad, 236017, Russia, e-mail: [email protected], [email protected], [email protected], [email protected] Correspondence to: M.V. Klimenko ([email protected])
Abstract We studied the contribution of the global plasmaspheric and ionospheric electron content (PEC and IEC) into total electron content (TEC). The experimental PEC was estimated by comparison of GPS TEC observations and FORMOSAT-3/COSMIC radio occultation IEC measurements. Results are retrieved for the winter solstice (January and December 2009) conditions. Global maps of COSMIC-derived IEC, PEC and GPS TEC were compared with Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) results. In addition, we used GSM TIP model results in order to estimate the contribution of plasmaspheric electron content into TEC value at the different altitudinal regions. The advantages and problems of the outer ionospheric/plasmaspheric parameters (O+/H+ transition height, TEC and electron density at height above F2 layer peak) representation by the IRI (International Reference Ionosphere) model are discussed.
Keywords: total electron content; plasmasphere; GPS; FORMOSAT-3/COSMIC; numerical modeling.
1. Introduction Total Electron Content (TEC) is one of the most important parameters used in the ionospheric studies. Nowadays the dense network of the Global Positioning System (GPS) receivers allows the simultaneous coverage of TEC values in global scale with high temporal resolution. However, one of the main limitations of the GPS technique is that the value of GPS TEC has an integral character and it is difficult to determine the precise contribution of the ionosphere to GPS TEC based on GPS measurements only. In fact, the TEC value consists of the ionospheric electron content (IEC) and plasmaspheric electron content (PEC). The plasmasphere is often ignored at analysis and estimation of GPS TEC data due to the arguments that (1) the electron density at plasmasphere are several orders of magnitude less than F region electron density (Gallagher et al., 2000); (2) the TEC and foF2 highly correlated (Liu et al., 1996). Are these arguments right for all longitudes, latitudes and local times? How much does the PEC contribute to TEC at different locations and for different local times? Previous studies show that the plasmaspheric contribution to the TEC is approximately 55–65% during the nighttime and 10–20% during the day-time (Balan et al., 2002; Belehaki et al., 2004; Gulyaeva and Gallagher, 2007; Manju et al., 2008), with the higher percentages occurring near the equator during the winter months (Balan et al., 2002). Most part of the previous investigations, estimated contribution of PEC to the TEC values, were made for the very limited geographic locations by: (1) taking the difference between the electron content measured by the differential Doppler and the Faraday rotation technique using radio beacons on geostationary satellites (Kersley and Klobuchar, 1978); (2) comparisons of GPS TEC data and modeled IEC (Lunt et al., 1999; Balan et al., 2002); (3) comparative analysis of GPS TEC data and IEC derived from the observation of a ground-based ionosonde (Belehaki et al., 2004; Mozert et al., 2007), coherent radio beacon experiment (Manju et al., 2008), and FORMOSAT3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) Radio Occultation (RO) measurements (Zakharenkova et al., 2013). There were only three studies that 2
try to resolve this issue in global scale (Yizengaw et al., 2008; Cherniak et al., 2012; Lee et al., 2013). However there was not even one investigation that analyzes the global first-principal model/data comparisons of PEC contribution to the TEC values. This paper presents first results of the joint analysis based on GPS TEC observations, FORMOSAT-3/COSMIC radio occultation measurements, and results derived by the global numerical model of ionosphere/plasmasphere system. In addition, the comparisons of F region and topside ionospheric electron density derived from numerical model with the IRI (International Refference Ionosphere) output are presented. 2. Observations and brief model description 2.1. Experimental Data – global TEC maps As a source of vertical TEC data we used IGS (the International GNSS (Global Navigation System Service) Service) Global Ionospheric Maps (GIMs) of TEC in the IONEX (IONosphere map
EXchange)
format.
These
data
are
accessible
at
the
ftp
server:
ftp://cddis.gsfc.nasa.gov/pub/gps/products/ionex. The GIMs are generated routinely by the IGS community with resolution of 5° longitude and 2.5° latitude and temporal interval of 2 hours; one TEC unit (TECU) is equal to 10 16 electrons/m2. Currently, there are three types of IGS GIMs: the final, rapid and predicted respectively. There are four IGS Associate Analysis Centres (IAACs) for the final and rapid ionospheric products: CODE, ESA/ESOC, JPL and gAGE/UPC. Detailed description of IGS GIMs computation and validation can be found in HernandezPajares et al. (2009). The IAACs provide ionosphere maps computed with independent methodologies that use GNSS data from different set of GPS stations. These maps are uploaded to the IGS Ionosphere Product Coordinator, who computes the official IGS combined products. During the period of more than 10 years of continuous IGS ionosphere operation, the techniques used by the IAACs and the strategies of combination have improved in such a way that the combined IGS GIMs are now significantly more accurate and robust. The combined product generated from GIMs maps, provided by CODE, JPL, ESA and UPC, are now very accurate over 3
regions with regular GNSS permanent networks, but of course it is less accurate over oceans, deserts and at the high latitudes. In this study, the final product of IGS combined GIMs produced by GRL/UWM were used to calculate the global maps of monthly median of GPS TEC values. We calculate median TEC maps with 2 h resolution for January 2009, based on 31 days observations. 2.2. Experimental Data – COSMIC We used COSMIC RO measurements and products that are provided by the COSMIC Data Analysis and Archive Center (CDAAC, http://www.cosmic.ucar.edu/cdaac/). At CDAAC, the ionospheric profiles are retrieved by use of the Abel inversion technique from TEC along Low Earth Orbiting (LEO) satellites-GPS satellites rays. Detailed description of CDAAC data processing and the Ne profile retrieval method can be found in Kuo et al. (2004). We used second level data provided by CDAAC – “ionprf” files containing information about ionospheric electron density profiles (EDPs). It should be mentioned that there is a need to analyze the data quality and to fix and remove bad or questionable data – electron density profiles with large and irregular spikes and data gaps. Special processing routine analyses the shape of each profile, rate of Ne change, its comparison with confidence limit. All unsuitable RO profiles were removed from our analysis. In order to estimate the ionospheric electron content (IEC) the integration of COSMIC RO data was done. For the present study the upper limit of the ionosphere has been taken to be at 700 km (altitude of COSMIC satellites). All selected COSMIC RO EDPs were integrated up to the height of 700 km, in that way estimates of IEC, which correspond to the altitudinal range of 100–700 km, were retrieved. For each month IEC values were accumulated and then they were divided into 12 data sets by intervals with 2 h duration. For the global representation of IEC estimates a spherical harmonics expansion up to degree and order 15 was carried out. The resulted maps illustrated monthly median distribution of IEC; the final outputs are at the same of IONEX format. So, for the considered month 12 IEC median maps (2 h resolution) with spatial 4
resolution similar to IGS GIMs (2.5/5.0 deg in latitude/longitude) were calculated. In order to retrieve monthly median estimates for COSMIC IEC, we used more than 58000 EDPs that were obtained during January 2009. Joint analysis of GPS TEC and COSMIC IEC data can be resulted in the quantitative differences PEC = TEC – IEC as a measure of the plasmasphere contribution (h > 700 km) to GPS TEC. To compare the global morphology of the IEC, TEC and PEC, the global maps of COSMIC IEC, GPS TEC and derived PEC estimates were constructed in magnetic latitude and local time coordinates. 2.3. GSM TIP Model Description In this work the results of numerical modeling, derived with the use of Global Selfconsistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) are presented. The Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) (Namgaladze et al., 1988; Korenkov et al., 1998) was developed in the WD IZMIRAN (West Department of Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio wave propagation of the Russian Academy of Sciences). This model calculates time-dependent global three-dimensional structure of the near-Earth space environment from 80 km to 15 Earth radii, in particular: (1) the distributions of temperature, composition and velocity vector of neutral gas; (2) the density, temperature, and velocity vectors of atomic and molecular ions and electrons; (3) the two-dimensional distribution of electric field potential both of a dynamo and magnetospheric origin. The GSM TIP model consists of three main blocks: thermospheric block, ionospheric block, and the block of electric fields. In the thermospheric block the global distribution of the neutral gas temperature, the N2, O2, O, NO, N(4S), and N(2D) concentrations as well as the threedimensional circulation of the neutral gas are calculated in the range from 80 to 526 km in a spherical geomagnetic coordinate system. In the vertical dimension, the thermospheric code uses 30 layers, with each layer approximately equal to a half thickness of scale height. The minimum distance between knots is 3 km near the lower boundary and increases to 40 km at 526 km 5
altitude. Ionospheric block consists of two modules. In the first module (the D, E, F1 region module), the three-dimensional density of the N2+, O2+, and NO+, ion temperature and velocities are calculated in the range from 80 to 526 km in the spherical geomagnetic coordinate system. In the second module (the F2 region and protonospheric module), the densities, temperatures, and vector velocities of atomic (O+, H+) ions and electrons are calculated in the magnetic dipole coordinate system from a base altitude 175 km in the northern hemisphere to 175 km in the southern hemisphere or to a maximum distance of 15 Earth’s radii (in case of open geomagnetic field lines). In this case, the ionosphere code for atomic ions does not require the upper boundary condition. This ionospheric part of the code has variable spatial steps along the magnetic field lines. Additionally, the model also provides the two-dimensional electric field potential distribution for dynamo and magnetospheric origin. The calculation of electric fields in the GSM TIP model has recently been modified by Klimenko et al. (2006, 2007). All model equations are solved by the finite difference method. The Earth’s magnetic field is approximated by the central dipole. Thus, discrepancy of geographical and geomagnetic axes is taken into account. The solution of the full system of equations of the model is performed numerically on a global grid with resolutions of 5° in latitude and 15° in longitude as specified in the spherical geomagnetic coordinate system; the time step is 2 min. The transformations between all coordinate systems in the model are given by standard equations. The model inputs are (1) the solar EUV and UV spectra (10–1760 Ǻ); (2) the precipitating electron fluxes; (3) the amplitudes and spatial distribution of Region 1 field aligned currents or a cross-polar cap potential difference and Region 2 field aligned currents. The cross-polar cap potential difference was set at geomagnetic latitudes ±75°, and Region 2 Field-Aligned Currents (R2 FAC), j2, at geomagnetic latitudes ±70°. The cross-polar cap potential difference was set equal to 38 kV according to Feshchenko and Maltsev (2003) and R2 FAC was set equal to 3.5⋅10 -9, A/m2 according to Klimenko et al. (2011) which corresponds to quiet geomagnetic conditions. In the current simulation, the empirical model by Zhang and Paxton (2008) was used for high-energy 6
particle precipitation. In this model, the energy and energy flux of precipitating electrons depend on a three-hour Kp-index. The GSM TIP model was described in detail by Namgaladze et al. (1988), and its recent modifications were presented in Klimenko et al. (2006, 2007, 2011). A modified GSM TIP model has already been used to study the F2 region and topside ionospheric structure in quiet conditions and during different space weather events (Klimenko et al., 2008, 2011, 2012, Korenkov et al., 2012). 3. Results The amount of the ionospheric and plasmaspheric contribution into the Total Electron Content (TEC) depends on O+/H+ transition height that is the boundary between ionosphere and plasmasphere (protonosphere). Integral electron content in the column from the lower boundary of the ionosphere up to O+/H+ transition height represents the Ionospheric Electron Content (IEC) and from O+/H+ transition height up to GPS satellite altitude (~20000 km) – the Plasmaspheric Electron Content (PEC). The O+/H+ transition height has been already investigated by means of satellite observations (Miyazaki, 1979; Kutiev et al., 1980; Triskova et al., 1998). Empirical models of O+/H+ transition height have been also developed (Titheridge, 1976; Miyazaki, 1979; Kutiev et al., 2006). For high solar activity the O+/H+ transition height lies much higher than for low solar activity (e.g. MacPherson et al., 1998; Třísková et al., 1998; Truhlík et al., 2005). It is shown that during 2008/2009 the O+/H+ transition height dropped to it lowest ~450 km at night and ~750 km during daytime (Heelis et al., 2009; Hyssel et al., 2009; Balan et al., 2012; Nanan et al., 2012; Hazra et al., 2012; Aponte et al., 2013), that was not observed in the previous minima of solar activity. It follows the changes in the PEC contribution to the TEC during this period. To confirm or deny this statement it is necessary to use the model calculations and measurements of the TEC, PEC and IEC in the same place at the same time. Note that the PEC always contributes to the TEC. This contribution depends on solar activity level, season and local time. At that, the relative PEC contribution into TEC is the most significant at night and decreases during daytime. 7
Figure 1 presents the map of the O+/H+ transition height obtained in GSM TIP model for 12:00 UT on January 15, 2009. It is evident that the transition height: is less in the winter hemisphere than in the summer one; drops below 500 km at mid-latitudes of the Northern Hemisphere in the American longitudinal sector in the morning that is consistent with the observations and model calculations of other authors (Heelis et al., 2009; Balan et al., 2012; Nanan et al., 2012; Hazra et al., 2012; Aponte et al., 2013); reaches the maximum values of ~ 700 km near the equator and about 1000 km at middle latitudes in the afternoon. Thus, the average value of the transition height for the selected date and UT moment amounts to ~ 700 km. In such a way, we can approximately assume that for the given UT moment the ionospheric (IEC) and plasmaspheric (PEC) contribution to the TEC determined at the altitude range below 700 km and from 700 km up to 20000 km, respectively. Figure 2 presents the TEC, IEC and PEC maps (for the chosen moment of time corresponding to
12:00 UT), constructed in magnetic latitude and local time coordinates for January 2009. These results were derived from observations and model results. We limit these maps by magnetic latitude of 50 degree, as both COSMIC EDPs and global TEC GIMs can reveal some problems with data accuracy at the high latitudes.
The main morphological features of observed and modeled TEC, IEC and PEC maps are very similar. It is evident that the PEC contribution to the TEC values has a maximum near to geomagnetic equator. It follows that the equatorial ionization anomaly in TEC is weaker than in IEC due to reduction of the depth of the equatorial trough. The percentage contribution of the plasmasphere in TEC has maximum at the equator and reaches 85% at night and about 40% during the daytime. It is necessary to emphasize that our results revealed higher values of PEC contribution in comparison to all previous studies (e.g., Balan et al., 2002; Belehaki et al., 2004; Gulyaeva and Gallagher, 2007; Manju et al., 2008) that might be due the fact that our PEC estimates are related to the greater height interval (700–20200 km) then above-mentioned studies (1100–20200 km)
8
due to the lowest O+/H+ transition height during 2008-2009 deep minimum of solar activity. Another contributing factor is the small value of TEC during very deep solar minimum that leads to the fact that the relative contribution of the plasmasphere to GPS TEC is the largest, especially during night-time. The obtained results are in a good agreement with plasmasphere contribution estimates corresponded to the Northern Hemisphere mid-latitudes and low solar activity (Cherniak et al., 2012). Another interesting objective of our paper is the estimation of which altitudinal intervals of plasmasphere have the most significant contribution to the TEC value. Why this is important? Remember that most of empirical and first-principle models have the upper boundary lower than GPS satellite orbits. Therefore, these models are usually underestimating TEC values observed using GPS satellites. For examples: IRI model has the upper boundary at the height of 2000 km; Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) has the upper boundary at the height of ~ 700–800 km (Roble and Ridley, 1994); a whole Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) has the upper boundary at the height of ~ 3000 km (Jin et al., 2012). The GSM TIP model provides the unique opportunity to investigate in details the PEC contributions into TEC values in order to estimate the TEC underestimation obtained by different models, associated with the neglect of the PEC values from the model’s upper boundary up to the GPS satellite orbits. We divided PEC into four different altitudinal parts: (1) 700 – 2000 km, (2) 2000 – 3300 km, (3) 3300 – 10000 km, (4) 10000 – 20000 km and calculated the contribution of each PEC parts into the TEC. The results of these calculations are shown in Fig. 3. It is seen that the PEC contribution in TEC decreases with increasing altitude. Estimation of contribution of (1), (2) and (3) regions into TEC reveals rather close values to each other – each region has about 12-15% of TEC at daytime and ~ 2030% at night. At that the electron content in region (1) gives a largest contribution to TEC. The contribution to TEC decreases with altitude that is evident from comparison between contributions of (1) and (2) regions which have the same altitudinal scale. The electron content 9
contribution of (3) region is just a little greater than the same one of (2) region due to a larger altitudinal range of this region. Region (4) represents the smallest contribution of its electron content to TEC - about 3% at night and 2% at daytime. This information is of the high importance for interpretation of differences between TEC maps derived with use of the different ionospheric models that have different plasmaspheric extensions. Knowledge of separate contribution of the ionosphere and plasmasphere to TEC is essential for improvement of the ionospheric data assimilation (e.g. Thompson et al. (2009)), empirical (IRI) and first principles (TIME-GCM and GAIA) models and to correct interpretation of TEC variability during quiet and disturbed conditions. Figure 4 presents the global maps of electron density obtained in GSM TIP and IRI models for 03:00 UT on December 22, 2009 at heights of 300 and 1000 km. The selected date and UT moment correspond to the most favorable conditions for the formation of such large-scale ionospheric features of the electron density distribution as Main Ionospheric Trough (MIT) in the northern hemisphere (local winter) and Weddell Sea Anomaly (WSA) in the southern hemisphere (local summer). In general, Fig. 4 shows a good qualitative agreement between the calculated results obtained using the GSM TIP and IRI models. In particular, this agreement refers to the Equatorial Ionization Anomaly (EIA), WSA in the Southern Hemisphere and MIT in the Northern Hemisphere at a height of 300 km and to manifestations of WSA and MIT at a height of 1000 km. The EIA in the IRI model results appears on all altitudes above the F2 layer peak and, in particular, as it is clear from Fig. 4, at the height of 1000 km, in contrast to the GSM TIP model results. In fact, the EIA should disappear with height – with increase in height the anomaly crests must approach to each other and then merge into a single maximum at the geomagnetic equator. This point is also noted by Heelis et al. (2009). As for quantitative differences, they may be related to the overestimation of the electron density in the EIA crests obtained using IRI model, which is also mentioned in (Bilitza, 2004; Heelis et al., 2009). It should be noted that the IRI model overestimates the observed values of the O+/H+ transition height by 10
over 200 km for the deep solar minimum (Heelis et al., 2009). At the same time the GSM TIP
calculation results of O+/H+ transition height and electron density in the outer ionosphere are consistent with the observation data of the C/NOFS satellite, presented by Heelis et al. (2009). 4. Conclusions In our study we present results of the global first-principal model/data estimation of PEC contribution to the TEC values, as well as for the first time performed estimation of how different altitudinal parts of the plasmasphere can contribute into TEC under solar minimum condition (2009 winter month). It is concluded that: −
At night the contribution of the plasmasphere in TEC has maximum at the equator (up to 85%) and can exceed consequently the contribution of the ionosphere in TEC.
−
During daytime the PEC contribution into the TEC does not exceed ~ 40%.
−
The value of IEC cannot be considered as the sole significant parameter to calculate the value of TEC, as TEC is the factor depending on the ionosphere as well as the plasmasphere. Therefore, to predict the TEC values one must consider the plasmasphere parameters in addition to the ionosphere, especially during solar minimum conditions.
−
We have found some shortcomings in empirical IRI model, related to the overestimation of the electron density in the EIA region and the presence of the EIA at too high altitudes.
Acknowledgments. The authors acknowledge the University Corporation for Atmospheric Research (UCAR) for providing the COSMIC Data. We are grateful to International GNSS Service (IGS) for GPS data and products. These investigations were partially supported by the Russian Federation President Grant MK-4866.2014.5 and Russian Foundation for Basic Research (RFBR) – Grant №14-05-00788.
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Figure captions Fig. 1. Map of the O+/H+ transition height obtained in GSM TIP model for 12:00 UT on January 15, 2009. Fig. 2. From top to bottom: the maps of IEC, TEC, PEC in TECU and PEC in %. Left panel – GSM TIP model calculation results, right panel – observation data of IEC from COSMIC and TEC from IONEX. Fig. 3. Maps of PEC contribution into the TEC values from the different parts of plasmasphere in TECU (left panel) and in % (right panel). Fig. 4. Electron density global maps obtained in GSM TIP model (left panel) and IRI model (right panel) for 03:00 UT on December 22, 2009 at heights of 300 km (top panel) and 1000 km (bottom panel).
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Fig. 1. Map of the O+/H+ transition height obtained in GSM TIP model for 12:00 UT on January 15, 2009.
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2.8 2.6 2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6
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34 32
90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90
Geographic Longitude (deg)
2.8
Geographic Longitude (deg)
10
0
3
30 60 90 120 150 180 210 240 270 300 330 360
14 12
PEC (%) 3300-10000 km 12:00 UT 15.01.2009
3.2
0
16
0
3.6 3.4
2 1.8
PEC (TECU) 10000-20000 km 12:00 UT 15.01.2009
20 18
Geographic Longitude (deg)
2.4 2.2
Geographic Longitude (deg)
22
0
2.8 2.6
30 60 90 120 150 180 210 240 270 300 330 360
24
PEC (%) 2000-3300 km 12:00 UT 15.01.2009
0.4 0.2
3.2 3
0
28 26
Geographic Longitude (deg)
2.8 2.6
0
30
0
3.6 3.4
36 34 32
90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90
0
3.2 3
Geogtaphic Longitude (deg)
90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90
0.4 0.2
Geographic Latitude (deg)
30 60 90 120 150 180 210 240 270 300 330 360
Geographic Latitude (deg)
3.2 3
0
PEC (%) 700-2000 km 12:00 UT 15.01.2009
3.6 3.4
Geographic Latitude (deg)
90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90
Geographic Latitude (deg)
Geographic Latitude (deg)
PEC (TECU) 700-2000 km 12:00 UT 15.01.2009
4 2 0
Fig. 3. Maps of PEC contribution into the TEC values from the different parts of plasmasphere in TECU (left panel) and in % (right panel).
21
0
30 60 90 120 150 180 210 240 270 300 330 360
Geographic Longitude (deg)
Geographic Latitude (deg)
Ne (1.e4 cm-3) h=1000 km 03 UT 22.12.2009 90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90
Ne (1.e5 cm-3) h=300 km 03 UT 22.12.2009
2.6 2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
Geographic Latitude (deg)
90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90
1 0.8 0.6 0.4
Geographic Longitude (deg)
10 9 8 7 6 5 4 3 2 30 60 90 120 150 180 210 240 270 300 330 360
1 0
Ne (1.e4 cm-3) h=1000 km 03 UT 22.12.2009
1.6
1.2
30 60 90 120 150 180 210 240 270 300 330 360
12 11
Geographic Longitude (deg)
1.4
0
90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90 0
Geographic Latitude (deg)
Geographic Latitude (deg)
Ne (1.e5 cm-3) h=300 km 03 UT 22.12.2009
0.2
90 75 60 45 30 15 0 -15 -30 -45 -60 -75 -90
3 2.5 2 1.5 1 0.5 0
0
30 60 90 120 150 180 210 240 270 300 330 360
Geographic Longitude (deg)
Fig. 4. Electron density global maps obtained in GSM TIP model (left panel) and IRI model (right panel) for 03:00 UT on December 22, 2009 at heights of 300 km (top panel) and 1000 km (bottom panel).
22
0