Inter-hemispheric imaging of the ionosphere with the upgraded IRI-Plas model during the space weather storms
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 24 November 2010
Accepted: 25 April 2011
Published: 29 December 2011
International Reference Ionosphere extended to the plasmasphere (IRI-Plas) is upgraded analytically for assimilative mode of operation using GPS-derived Total Electron Content (TEC) for reconstruction of instantaneous ionospheric critical frequency and topside scale height at magnetic conjugate hemisphere. The performance of IRI-Plas code is examined with TECgps retrieved from Global Ionospheric Maps compared with the F2-layer critical frequency at eight ionosonde locations in East Asia region on both hemispheres during the space weather storms at solar maximum (2000) and solar minimum (2006). Missing ionosonde data are completed by cloning of critical frequency. Decomposition of TECgps in electron density profile with IRI-Plas code reveals the opposite relative changes of critical frequency and the topside scale height depending on solar activity. The ionospheric weather W index is computed for the desired locations in conjugate hemispheres and consistent results are obtained indicating the departure of instantaneous values of ionospheric parameters from their respective median varying from quiet state to intense storm.
The history of investigations into inter-hemispheric conjugate effects in the ionosphere can be traced back to early 1960s. Though the monthly median values of the F2-layer critical frequency at conjugate low-latitude stations showed poor correlations, deviations from the median exhibited positive conjugate correlations in all seasons for both geomagnetically quiet and disturbed days (Matsushita, 1968). In Rotwell (1962), the relations of the noon F2-layer critical frequency have been investigated for the magnetic conjugate locations at the ends of the magnetic line of force assuming that the daytime temperature of the neutral gas is proportional to the cosine of the Sun’s zenith angle at local noon. The correction factor deduced for this relation, the so-called magnetic “M-factor”, has been expanded for the diurnal, seasonal, solar cycle and spatial variations, and is used for constructing the empirical model of the foF2 critical frequency using the data from a global network of ionosondes (Besprozvannaya, 1987, 1991, 1995; Chasovitin et al., 1987). The M -based model is used in the present study as a quiet reference for the F2-layer critical frequency at magnetic conjugate locations.
Renewed interest in the conjugate effects in the Earth’s environment has been raised by the ICESTAR Program which focuses on the “Inter-hemispheric Similarities and Asymmetries in Geospace Phenomena” (http://www.scar.icestar.org/). The primary goal of the ICESTAR Program is to create an integrated, quantitative description of the upper atmosphere over Antarctica, and its coupling to the global atmosphere and the geospace environment. It is found that there exists a systematic displacement for the sub-storm onset locations in one hemisphere compared to the other (Ostgaard et al., 2007). At the same time, no evidence of geomagnetic conjugacy in pulsating aurora is found, as discussed in Watanabe et al. (2007). The authors report two types of non-conjugacy: (i) pulsating auroras can appear in both hemispheres but their spatial appearance and period are different and (ii) pulsating auroras appear only in one hemisphere.
Recent studies of conjugate effects are not restricted to auroral phenomena. The existence of a global asymmetry in the neutral upper-atmospheric density between the Northern and Southern hemispheres has been indicated (Illes-Almar and Almar, 2006). The authors assume that the phenomenon can be related to the asymmetric distribution of the continents and oceans between the hemispheres and/or with the asymmetry of the geographic field to the geomagnetic field. At the same time, the special campaign of mid-latitude airglow observations at the conjugate locations in Japan and Australia has revealed a one-to-one correspondence of wave structures between the Northern and Southern hemispheres, indicating strong electro-dynamic coupling between the two hemispheres through the geomagnetic field line (Shiokawa et al., 2005).
The simultaneous observations of the F3-layer at the low-latitudinal meridional network located in Southern Asia confirmed the plasma-diffusion effects along the magnetic field lines in the low-latitude region (Uemoto et al., 2007). A more sophisticated picture of the ionospheric E - and F2 -region parameters has been obtained in special near-equatorial empirical conjugate studies in Brazil (McNamara et al., 2008; MacDougall et al., 2009). Considerations of the ambient conditions at the conjugate locations indicate that relatively small differences in parameters such as tidal winds, electric fields, and background density gradients can significantly affect the structuring of the Es layer. Variations of the F2-layer peak density and height appear to result from a reshaping of the F2 -layer manifested by the scale height around the peak.
The ionospheric behavior in conjugate hemispheres during an annular solar eclipse has clearly indicated that due to the eclipse, fewer photoelectrons travel along the magnetic field lines from the eclipse region to the conjugate region resulting in reduced photoelectron heating in the conjugate hemisphere causing a drop in the electron temperature and a subsequent positive disturbance effect in the peak electron density, peak height and Total Electron Content (Le et al., 2009).
One of the most acknowledged models of the ionosphere suitable for investigations of conjugate phenomena is the International Reference Ionosphere (IRI) (Rawer, 1988, 1990; Bilitza et al., 1993; Bilitza, 2001). The IRI model has been traditionally based on observations of monthly medians of ionospheric parameters obtained from ionosondes mostly located in the northern hemisphere. The temporal and spatial sparsity of data that has been included in the development of the IRI model can be compensated for either by assimilation of new ionosonde results, or by upgrading the IRI model to include situations like space weather storms. When the first option is not feasible, it is better to approach the problem with a cost-effective improvement in the IRI model. The International Reference Ionosphere extended to the plasmasphere (IRI-Plas) (Gulyaeva et al., 2002) is a recent version of IRI where the region of interest can include the plasmasphere up to a height of 20,200 km. The Global Positioning System (GPS) derived Total Electron Content (TEC) can be incorporated into IRI-Plas for a better representation of temporal variations in the ionosphere, and the interaction between magnetic conjugate hemispheres can be used to model the ionosphere in regions where ionosonde data are not available.
In the International Reference Ionosphere (IRI) model, the topside and bottomside electron density descriptions use a ‘relative layer shape’ formula depending on vertical coordinate adapting the absolute values to those at the peak (Rawer, 1988, 1990; Bilitza et al., 1993). Since plasma interchange in the F-region is field aligned, it is not possible to describe adequately the spatial structures occurring at low latitudes when admitting that they depend exclusively on the vertical coordinate as is supposed in IRI (Rawer, 1990). The reference peak electron density and height in the IRI system are provided by the ITU-R (former CCIR) or URSI maps. For the conjugate F2 peak reconstruction, the IRI driven quiet reference-critical-frequency is replaced in the present study by an M -based empirical ionospheric model (Besprozvannaya, 1987, 1991, 1995) which accounts for the magnetic field geometry. Since the IRI F2-layer peak-height yields inadequate results in some regions of the Southern hemisphere, the model peak height hmF2 is improved (Chasovitin et al., 1987) as derived from the ITU-R (former CCIR) maps of M3000F2 using the formulations of Bilitza et al. (1979) for the Northern hemisphere and supposed to be symmetric in the Southern hemisphere.
The term ‘Imaging of the ionosphere’ is well-recognized in the tomographic decomposition of the GPS-derived Total Electron Content (TECgps) into the electron density distribution (Bust and Mitchell, 2008). In their review, an introduction and the history of ionospheric imaging is presented, beginning with computerized ionospheric tomography. The ability of imaging algorithms to incorporate multiple types of data and use advanced inverse techniques borrowed from meteorological data assimilation to produce four-dimensional images of electron density is discussed therein. The technique implemented in the present paper is based on a sort of data assimilation scheme of TECgps values obtained using the Global Ionospheric Maps (GIM) (Manucci et al., 1998; Nayir et al., 2007). While TEC measurements at GPS-receivers sites are more accurate compared with spatially- and temporally-smoothed GIM TEC products, the GIM data are readily available for extracting (interpolation) the TEC value at any given location, in particular, in the conjugate hemisphere.
The International Reference Ionosphere model extended to GPS heights in the plasmasphere, IRI-Plas, (Gulyaeva et al., 2002) is used for retrieving the electron density height profile from TECgps input (Gulyaeva, 2011). Guided by the knowledge gained from previous data analysis, the IRI-Plas algorithm is complemented with a feedback loop in order to update two model parameters: (1) an instantaneous peak electron density that is proportional to the square of the F2-layer critical frequency, and (2) the electron density scale height at the lower topside.
In this study, the instantaneous ionospheric critical frequency will be reconstructed using a new technique for conjugate hemispheres. The validation will be achieved by a comparison of reconstructed critical frequency values with the ionosonde source station (SS) data. The proposed data assimilation technique makes use of the TEC maps provided by GIM for the estimate of TECgps. The TECgps data representing an integral of plasma density characteristics in the Earth’s environment, which embraces the plasma transfer between the hemispheres along the magnetic lines of force, are the most suitable for magnetic conjugacy studies by providing an independent source of information at the conjugate points, CP. TECgps provides an estimate of the total number of free electrons inside the cylinder with 1-m2 cross-section area in the column from the bottom of the ionosphere (65 km) to 20,200 km (GPS orbit). The TECgps also exhibits the temporal variability of the ionosphere and the plasmasphere. Therefore, the assimilation of TECgps into the IRI-Plas code provides a more accurate representation of critical ionospheric parameters especially at CP points, both under quiet, and under storm, conditions. In Section 2, the new technique of the assimilation of TECgps data into IRI-Plas is discussed. Section 3 includes the validation of the modified IRI-Plas during storm conditions.
2. Data Processing
Geographic, and corrected geomagnetic, coordinates of the source ionosonde stations (SS) and geographic coordinates of the magnetic conjugate points (CP) used for the analysis.
The gaps in SS ionosonde observations are filled up using the cloning procedure discussed in Gulyaeva et al. (2008). The variations in foF2 at a ‘parent’ station are used for the reconstruction of a measurement missed at another location. The transform is applied to the proxy for the F2-layer critical frequency reduced by the solar zenith angle which improves the correlation between the data at different locations (Gulyaeva, 2009). This procedure assumes the availability of the daily-hourly values of critical frequency measured at a particular parent station and the median for the preceding 27 days at both stations.
The electron-density profile and the F2-layer critical frequency at the conjugate locations are obtained using the 3-D ionosphere-plasmasphere model, IRI-Plas. The IRIPlas code provides a three-dimensional and time-dependent global model of electron-density, Ne(h), and temperature, Te(h), profiles through the ionosphere and plasmasphere. The Ne(h) profile is fitted to anchor points at the F2-layer peak electron density, NeF2, and height, hmF2, which can be modeled using ITU-R (former CCIR) maps of foF2 and M3000F2, or they can be derived experimentally. The model predicts the Total Electron Content, TECiri, by numerical integration of the Ne(h) profile from an altitude of 65 km to a few Earth’s radii (specified currently at 20,000 km near the GPS orbit) for the desired location, date and time.
The IRI-Plas code is updated for the assimilation of the TECgps as discussed in Gulyaeva (2011). The TECgps data for SS and CP sites are extracted from the GIM maps produced by the Jet Propulsion Laboratory, provided online at ftp://cddis.gsfc.nasa.gov/pub/gps/ products/ionex/. The GIMs provided with a temporal resolution of 2 hours are linearly interpolated for a 1-h resolution. In the literature, there are various techniques providing the assimilation of TECgps data by an updated IRI code, or other profilers (Komjathy et al., 1998; Stankov et al., 2003; Yizengaw et al., 2006). In our approach, the topside IRI electron-density profile is linked with the plasmasphere model, IRI-Plas, (Gulyaeva et al., 2002) at an altitude of one basis-scale-height above the F2 peak (Gulyaeva, 2011) close to the O+ /H+ transition height, where the concentration of hydrogen becomes comparable with that of oxygen. The topside basis-scale-height, Hsc, presents the distance in km above the peak height at which the peak plasma density, NeF2, decays by a factor of e (∼2.718).
In the present study, the algorithm applied in the IRI-Plas model for obtaining the topside scale height, Hsc, when the F2-peak parameters are available from observation (Gulyaeva, 2011) is updated using the input of GPS-derived TEC to obtain both the instantaneous peak electron density and the topside scale height by fitting the model result TECiri to TECgps.
The results obtained with the above analysis are presented in the next section.
The current investigation for the magnetic conjugate points is carried out using the eight ionosonde source stations of Table 1. The ionospheric parameters obtained with the above technique at the conjugate hemisphere are compared with SS data obtained from ionosondes at the same hemisphere. The TEC values are obtained from the GIM. The space weather storms that are highly effective in the ionosphere and plasmasphere can be identified and graded using the planetary ionosphere-plasmasphere storm index, Wp (Gulyaeva and Stanislawska, 2008, 2010). The Wp index is derived using the GIM-TEC maps, and they are provided online from 1999 up to the present at http://www.izmiran.ru/services/iweather/.
Planetary ionosphere-plasmasphere storms deduced from IGS-TEC maps during solar maximum (2000) and solar minimum (2006) and extreme space weather indices used in the present study at the conjugate hemispheres. Root-mean-square RMS deviation, in MHz, between the disturbed and quiet F-region critical frequency is averaged over a number of sites (SS or CP) in the Southern (S) or Northern (N) hemispheres.
Storm onset date_UT, h
Storm end date_UT, h
Duration, Σ h
W p max
A p −Api max, nT
RMS at S SS_CP
RMS at N SS_CP
Solar maximum, 2000
Solar minimum, 2006
The Root-Mean-Square (RMS) deviation, in MHz, between the disturbed and quiet F-region critical frequency averaged over a number of sites (SS or CP) particularly for the Southern (S) and Northern (N) hemispheres (Table 2) demonstrates a measure of disturbance of the peak electron density which is consistent for the observed foF2 (SS) and the reconstructed critical frequency (CP) with the proposed algorithm.
RMS deviation of TECiri from TECgps, in TECU, at the main phase of the ionospheric storms at solar maximum (2000) and solar minimum (2006). Results of IRI-Plas model for two options: (1) ionosonde-derived inputof foF2 and hmF2 combined with input of TECgps, (2) input of foF2 and hmF2 from ionosonde alone. Instantaneous results (i) and monthly median results (m) are shown. Data are missing for two stations for 2000.
The RMS difference between TECiri and TECgps, obtained with the data of the source stations, is listed in Table 3 for both the solar maximum and the solar minimum years. It is natural that a low RMS value is obtained with the parameters derived from the TECgps (option 1) because the TECgps values were used for computing these parameters. Differences in the RMS obtained for the instantaneous and median post-fit TECiri are considered in more detail in the ‘Discussion and Conclusions’ section. Results of reshaping the electron density profile with IRI-Plas code using option 1 and option 2 suggest that incorporation of only the F2-layer peak parameters observed with an ionosonde can yield a model TECiri which differs from TECgps data.
The ionosonde observations of foF2 are shown for the source stations in Fig. 3(a), and their conjugate counterparts retrieved with Eqs. (2)–(3) are given in Fig. 3(b). The solid curve denotes the 27-day median fmF2 at SS (a), and the solid line indicates the M-based model at CP (b). The corrected magnetic latitudes of SS and CP (equal in absolute value but opposite in sign in the conjugate hemispheres) are indicated at the right border of each panel. The crosses denote the instantaneous critical frequency, foF2, which is the ionogram derived values at SS (a) and retrieved from TECgps values for CP (b). The negative phase of the ionospheric storm is evident at the four ionosonde SS where foF2 values are depleted with respect to the median, excluding the few hours of positive enhancement signatures at Darwin during the first day. Similar dominating effects of the negative storm are obtained at the conjugate locations, where the positive storm signatures at the conjugate magnetic latitude of −31° for Kokubunji are very similar to those of Darwin station located at the low magnetic latitude of −25° in the winter Southern hemisphere. We remind that all these features are retrieved due to TECgps changes. Thus, the similarity of CP foF2 variability to those of SS observations in the relevant hemisphere is very promising.
The ionospheric plasma density near the equator and its structure are determined by the processes that dictate its production, loss, and transport (see, e.g., Rishbeth (2000)). Production of plasma occurs during daylight hours when the sun illuminates the atmosphere at both conjugate hemispheres while the night-time production is small. For the daytime, with combined SS and CP data, the equatorial anomaly of enhanced peak plasma density at both sides of the magnetic equator in Fig. 5(b) has a stronger symmetry compared to that of Fig. 5(a). The night-time electron density distribution in the Northern and Southern hemispheres is asymmetric, which is indicated with the enhanced peak electron density at the Northern summer night and the reduced peak electron density at the Southern summer night map, as rightfully observed with combined SS and CP data in Fig. 6(b). This is a well-known effect of winter anomaly in the peak ionization enhanced (superimposed) by plasma exchange along the field lines during the peak of the space weather storm.
4. Discussion and Conclusions
The investigation of ionospheric irregularities along the magnetic field line and through magnetic conjugate phenomena has proven to be highly advantageous. Along with the introduction of Earth-based GPS receivers, a cost-effective method of producing GIM and the observation of ionospheric irregularities and conjugate phenomena have been made possible since the last solar maximum of 1999–2001. In this study, a new technique for the reconstruction of the peak electron density and the topside scale height at the conjugate hemisphere is proposed based on recent developments in the International Reference Ionosphere, extended to the plasmasphere, the IRI-Plas code, and the availability of TECgps data through GIM.
Hourly ionosonde observations for eight source stations, SS, in the Northern and Southern hemispheres at the Asia-Far East region are included in the investigation. The performance of the IRI-Plas code is evaluated with the data from ionosonde stations during the storm events listed in Table 2 that occurred in the years of both solar maximum (2000) and solar minimum (2006). It is observed that TECiri is estimated with the higher accuracy if the F2-layer peak parameters and TECgps are available to the program at the same time.
An analytical expression to represent the opposite changes of the F2-layer critical frequency and the topside scale height is also derived including the term of solar activity retrieved with the IRI-Plas model incorporating TECgps. Though there are more relevant solar proxy indices for TEC modeling and decomposition than sunspot numbers (Maruyama, 2010), the sunspot number is used in the present study because it remains the IRI driving parameter, in particular, due to well-established predictions of the 12-months smoothed sunspot numbers a few months in advance.
From Table 3, the RMS error for the median values are significantly less than those for the instantaneous parameter values. This fact may be due to the more smoothed median behavior than the instantaneous storm-time values. The difference in the instant parameters can be explained by different processes governing plasma at the different altitude ranges over the Earth. It is well known that the ionosphere, a conductive, ionized region of the Earth’s atmosphere, exhibits significant variations in the peak plasma density residing typically in a relatively thin layer (200–400 km) in the F-region. On the other hand, the storm-time effects on TECgps including both the ionosphere and the plasma-sphere at altitudes up to a few Earth’s radii is controlled by an electric field induced by the Earth’s rotation which forces the plasma to co-rotate with the Earth, while, at the higher L shells, the plasma motion is driven by a convection electric field caused by the solar wind interaction with the Earth’s magnetosphere. Also, the input parameters of the IRI-Plas code (foF2, hmF2, TECgps) are not measured directly neither by the ionosonde nor the GPS receiver. These parameters are derived quantities that include modeling, computational and measurement errors due to a sophisticated pre-processing and signal manipulation (Piggot and Rawer, 1972; Arikan et al., 2002, 2007; Reinisch et al., 2005; Smith et al., 2008; Anghel et al., 2009). Therefore, this inherited controversy of input parameters may affect IRI-Plas results and yield greater RMS errors with an instantaneous input than with the more-smoothed monthly median values.
As seen in the examples provided in Figs. 3, 4, 7, and 8, the storm-time effects in the peak electron density of the conjugate hemisphere retrieved with the proposed model anchored to TECgps tend to display a congruent behavior with the ionosonde data in the same hemisphere. At the same time, either symmetry or asymmetry of the peak plasma density variations can occur in SS-CP pairs of data depending on the F2-layer storm time dominant signatures at a particular region, season and local time. However, ambiguities in the symmetry or asymmetry of the ionosphere changes in the conjugate hemispheres will likely remain, because processes in the ionosphere are governed not only by plasma interchange along field lines but are also greatly affected by forcing from above and below on local and regional scales.
The proposed analysis method indicates that the IRI-Plas code can yield better estimates of the ionospheric parameters at the conjugate hemispheres along with the input of TECgps. This is very important for the investigation of ionospheric variability where ionosonde stations are sparse or lacking.
This study is made possible by a joint grant from TUBITAK EEEAG 110E296 and RFBR 11-02-91370-CT_a. The ionosonde data used in the present study are provided by NGDC, NOAA, Boulder, CO, USA; NICT, Tokyo, Japan. The IRI source code is available online from NASA’s National Space Science Data Center (http://IRI.gsfc.nasa.gov/). The demo software of the IRI-Plas model is presented at the Internet site of IZMIRAN (http://ftp.izmiran.ru/pub/izmiran/SPIM/). Relevant vertical TEC data for the ionosonde locations are extracted from GPS-derived global ionospheric maps GIM-JPL provided by the Jet Propulsion Laboratory online at ftp://cddis.gsfc.nasa.gov/pub/gps/products/ionex/. The planetary ionosphere-plasmasphere storms are provided online at http://www.izmiran.ru/services/iweather/storm/. The authors are grateful to two referees for their useful comments and suggestions for further improvement of the submitted manuscript.
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