A comparison of the equatorial spread F derived by the International Reference Ionosphere and the S4 index observed by FORMOSAT-3/COSMIC during the solar minimum period of 2007–2009
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The latest version of the International Reference Ionosphere (IRI-2007) model includes an option for spread-F occurrence prediction for the first-time. The IRI-2007 spread-F occurrence is a function of solar 10.7 cm radiation flux, F10.7. In this paper, an attempt is made to cross-examine the spread-F occurrence derived by the IRI-2007 and the ionospheric scintillations in terms of the maximum value of the S4 index (S4max) between 150–350-km altitudes, calculated from fluctuations of the signal-to-noise ratio (SNR) intensity in the L1 channel of GPS radio occultation signals using FORMOSAT-3/COSMIC (F3/C) satellites during the low solar activity years 2007–2009. It is found that S4max maintains a fairly good consistency with spread-F occurrence simulated by IRI-2007 in the Brazilian region. Thus, the global S4 index statistics can be considered as a viable source of database to be incorporated into the global IRI spread-F prediction scheme owing to fact that the F3/C satellites can provide an unprecedented global database including the S4 index.
Key wordsGPS RO technique S4 index IRI-2007 spread-F
The Earth’s ionosphere often becomes turbulent and develops electron density irregularities which manifest themselves as spread-F on ionograms, plume-like structures in the range-time-intensity images of HF radars, intensity bite-outs in airglow intensity measurements and scintillations on amplitude, as well as the phase of VHF and UHF signals from satellites, and are commonly referred as equatorial spread-F irregularities (ESF), spanning scale sizes from 10s of centimeters to 100s of kilometers.
There have been many studies on the occurrence characteristics of ionospheric density irregularities using ground-based measurements (Rastogi, 1980; Abdu, 2001), satellite-based in situ measurements (Watanabe and Oya, 1986; Huang et al., 2002; Burke et al., 2004), and topside sounders (Maruyama and Matuura, 1984), and many important features of these have been reported including temporal variations (Sahai et al., 2000; Huang et al., 2002), seasonal-longitudinal variability (Burke et al., 2004), several-day variation, and day-to-day variation (Basu et al., 1996) though several enigmatic features of them are yet to be revealed (Thampi et al., 2009). However, studies on continuous and global characteristic features of ionospheric plasma irregularities are very scanty due to the fact that the global coverage is difficult to obtain from ground-based observations. Moreover, the observational locations are limited around the world and though satellite measurements can cover all longitudes, observational areas are fixed in geographic latitude and the satellite altitudes.
On the other hand, with the aid of low earth orbit (LEO) satellites performing GPS radio occultation (RO) observations, it is possible to obtain global soundings of the atmospheric profiles in three dimensions from the Earth’s surface to the topside of the satellite altitude with a higher vertical resolution (Schreiner et al., 2007). For a better understanding of the global phenomena of ionospheric irregularities, a RO technique using multiple satellites that can provide dense spatial coverage of the world such as FORMOSAT-3/COSMIC (F3/C) is essential. The F3/C configuration consists of six microsatellites at about 800-km altitude which are placed in 72° inclination circular orbits with a separation angle between neighboring orbital planes of 30° longitude. In principle, an occultation takes place when a GPS sets or rises behind the Earth’s atmosphere as seen by the LEO satellite, then the GPS radio signals are received by a receiver on the LEO satellite and such an occultation lasts a few minutes. Once the GPS signal is received at LEO during an occultation, the on-board algorithm, which was implemented into the GPS RO receiver software by the Jet Propulsion Laboratory (JPL) (http://cosmic-io.cosmic.ucar.edu/cdaac/doc/documents/s4description.pdf), computes an average of the intensity fluctuations from the raw 50-Hz L1 amplitude measurements and records it at a 1-Hz rate. A low-pass filter is further applied to a time series of those averages to obtain a new average of the intensity each second. Based on the filtered average (FA), the RMS (root mean square) and S4 (=RMS/FA) index are obtained. On average, nearly 6000 to 7000 S4 index profiles are derived by F3/C per day covering an altitude range from the Earth’s surface to 800 km. For scintillation studies, the F3/C dataset specifically denotes the maximum value and associated location (altitude, latitude, longitude) of each S4 profile (hereafter denoted S4max). In this paper, to help understand the three-dimensional structure of the equatorial F-layer scintillation, spread-F occurrence percentages, derived from IRI-2007, and concurrent F3/C S4max, in the Brazilian region, during the low solar activity years 2007–2009, are investigated.
2. IRI Spread-F Occurrence and F3/C S4 Index at the Brazilian Sector
3. Global S4max Map
4. Discussion and Summary
It is generally accepted that the generalized Rayleigh-Taylor instability (RTI) mechanism is responsible for the generation and growth of equatorial spread-F irregularities (Zalesak and Ossakow, 1980; Kelley, 1989; Sultan, 1996), which occur over a wide range of scale sizes, as has been established from various diagnostic techniques. The occurrence of spread-F irregularities is associated with post sunset height rise in the F-region (Chandra and Rastogi, 1972) which is highly affected by the upward E ×B drifts in pre-reversal enhancement (PRE), and is strongly dependent on the solar activity level and season (Fejer et al., 1991, 2008; Scherliess and Fejer, 1999). Thus, it is conceivable to expect higher spread-F occurrence percentages by IRI during the sunspot maximum, which, in general, establishes a positive correlation between them (Fig. 2) (Abdu et al., 1992; Subba Rao and Krishna Murthy, 1994).
Tsunoda (1985) shows that the ionospheric irregularity occurrence is a maximum when sunsets in the conjugate E-region are simultaneous. Simultaneous sunset is expected to intensify the so-called PRE (Abdu et al., 1981, 1992, 2010; Batista et al., 1986), a favorable and sufficient condition for the initiation of RTI. The angle between the geomagnetic meridian and the sunset terminator line was introduced as a proxy of the simultaneous sunset in the conjugate points (Burke et al., 2004). Since, in the Brazilian longitude sector, the angle tends to be much closer during November–February than March–September, higher occurrence probabilities and greater S4max are more expected during the November–February (see Figs. 2 and 3).
The time lag in onset timings between spread-F and S4max (Fig. 3) is primarily due to the physical processes involved in the generation of different scale sizes of the irregularities responsible for producing spread-F and scintillation effects. It is recognized that the spread-F in ionograms is associated with large scale (100s of km) plasma depletions, that originate initially at the F-layer bottomside (by the RTI mechanism) and, in turn, rise nonlinearly to the topside. These rising bubbles with the passage of time can bifurcate to form small-scale irregularities by cascade, or two step, mechanisms (Ossakow, 1981). Such smaller-scale irregularities effectively produce scintillations at the L-band (Basu et al., 1978; DasGupta et al., 1982).
In summary, it is shown that the IRI-2007 spread-F prediction model based on the Brazilian region data reflects well the temporal, diurnal and seasonal variations, and such variations are also found in the S4 index data. Therefore, the global S4 index data obtained by the F3/C GPS RO technique might be usefully incorporated into the global IRI spread-F prediction scheme.
The S4 index data are obtained from TACC (Taiwan Analysis Center for COSMIC) and CDAAC (COSMIC Data Analysis and Archive Center). This research is supported by the National Science Council (NSC) grant, NSC 98-2111-M-008-MY3. Research works of G. Uma and P. S. Brahmanandam are supported by the National Space Program Office (NSPO) grants, 98-NSPO (B)-IC-FA07-01 (A) and 98-NSPO(B)-IC-FA07-01(L).
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