A study of long-term climatology of ionospheric irregularities by using GPS phase fluctuations at the Brazilian longitudes

Transcription

1 Advances in Space Research xxx (2007) xxx xxx A study of long-term climatology of ionospheric irregularities by using GPS phase fluctuations at the Brazilian longitudes F.D. Chu a,b, *, C.C. Lee c, W.S. Chen a, J.Y. Liu a,d a Institute of Space Science, National Central University, No. 300, Jhongda Road, Jhongli, Taoyuan 320, Taiwan b National Standard Time and Frequency Laboratory, Telecommunication Laboratories, Chunghwa Telecom Co. Ltd., No. 12, Lane 551, Min-Tsu Road Sec. 5, Yangmei, Taoyuan 326, Taiwan c General Education Center, Ching-Yun University, No. 229, Chienshin Road, Jhongli, Taoyuan 320, Taiwan d Center for Space and Remote Sensing Research, National Central University, No. 300, Jhongda Road, Jhongli, Taoyuan 320, Taiwan Received 1 November 2006; received in revised form 14 April 2007; accepted 2 May 2007 Abstract We have examined the ionospheric plasma irregularities that were recorded by using three ground-based receivers of the global positioning system (GPS) located at Brazilian longitudes during the period of a complete solar cycle, The statistic results show that ionospheric irregularities are very easy to occur in December solstice months but rare to occur in June solstice months. Besides, the occurrence rates of irregularities in both December and June solstice months are little dependent on solar activity. However, in equinoctial months, the development of irregularities is obviously dependent on solar activity. There is a new finding in this study that if strong irregularities are distinguished from moderate ones, their occurrence rates would increase with solar activity during the December solstice months. Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Equatorial ionosphere; Ionospheric disturbances; Ionospheric irregularities 1. Introduction * Corresponding author. Address: National Standard Time and Frequency Laboratory, Telecommunication Laboratories, Chunghwa Telecom Co. Ltd., No. 12, Lane 551, Min-Tsu Road Sec. 5, Yangmei, Taoyuan 326, Taiwan. address: cfonda@cht.com.tw (F.D. Chu). Over the past two decades, observations of the satellite signals have been used to derive phase and amplitude scintillation at various latitudes (see the papers listed in Aarons, 1993). A spaced-antenna scintillation system has been demonstrated that is able to establish climatology of zonal drift (Valladares et al., 1996). Furthermore, the establishment of the global positioning system (GPS) provides an alternative way to investigate the ionospheric irregularities. GPS observations are sensitive to the irregularities of scale size about several kilometers. Some applications of GPS observations to equatorial aeronomy in South America have been reported (Aarons et al., 1996, 1997; Mendillo et al., 2000). Aarons et al. (1997) used GPS phase fluctuation, which is practically the time variation of the total electron content (TEC), as an indicator of the occurrence of ionospheric irregularities to study the occurrence of irregularities during the storm periods of Mendillo et al. (2000) further developed two indices to quantitatively examine the GPS phase fluctuations in various months during The relationships of observations with the GPS and other instruments (e.g., the incoherent radar and the airglow imager) are interesting and have been examined (Aarons et al., 1996; Chu et al., 2005). Chu et al. (2005) made a comparison between the ionospheric climatology obtained by using GPS phase fluctuations and by using photometer/ imager measurements in Brazilian sector, and showed the two results are very similar, although the GPS database /$30 Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi: /j.asr

2 2 F.D. Chu et al. / Advances in Space Research xxx (2007) xxx xxx used in that study is a short-term set only during The advantage of the GPS observation is that the indices of phase fluctuations can be continuously obtained. This further allows us to investigate the long-term variations of ionospheric irregularities in various solar activities. However, Chu et al. (2005) only showed the power of GPS phase fluctuations to study ionospheric climatology by using the short database of a single GPS site. In this paper, a long-term database of three GPS sites which located near the magnetic equator and both the south and north equatorial anomalies during a complete solar cycle ( ) is employed in order to obtain more reliable results of the investigation on the occurrence of ionospheric irregularities in various solar activities. 2. Observation and interpretation There are three GPS sites of the International GNSS Service for Geodynamics (IGS) located in the east side of South America. The first site FORT ( 3.88 N, E geographic; 4.22 N, E geomagnetic), which near the magnetic equator, is located at Fortaleza, Brazil. The second site BRAZ ( N, E geographic; 9.77 N, E geomagnetic), which near the south equatorial anomaly, is located at Brasilia, Brazil. Moreover, the third site KOUR (5.25 N, E geographic; 9.96 N, E geomagnetic), which near the north equatorial anomaly, is located at Kourou, French Guyana. Note that BRAZ and KOUR are almost located at the locations of a pair of conjugate points. Fig. 1 illustrates the locations of the three GPS receivers, as well as the associated view area on the map of South America. The superimposed dark lines are the projection of all the GPS satellite-receiver passes at the ionospheric-point height of Fig. 1. The locations of the three GPS receivers, as well as the associated view area on the map of South America. The superimposed dark lines are the projection of all the passes of GPS satellites at the height of 400 km over the three GPS sites and for the elevation angles greater than 15 during the whole year of km over the three GPS sites and for the elevation angles greater than 15 during the whole year of The GPS data used in this study are only the elevation angles greater than 15. To quantitatively characterize phase fluctuations, we derive the indices fp and Fp from the GPS total electron contents (TEC) (for details see, Mendillo et al., 2000). The index fp of a certain GPS satellite n is defined as the median value over a 15-min period of phase fluctuation data from a GPS satellite, fpðn; h; iþ ¼Median DTEC min ; ð1þ where DTEC/min is defined by first subtracting each TEC from its previous 1-min value and then filtering the residual with a high pass filter, and i denotes each quarter of the hour, h. These fp values are then averaged over a period of 1 h for all satellites available to a site, and multiplied by 1000, to give the integer index Fp, which represents the average level of irregularities in the hour above that specific site, Pnsat P k fpðn; h; iþ=k n i FpðhÞ ¼ 1000: ð2þ nsatðhþ Mendillo et al. (2000) suggested that the background noise level of irregularities has an Fp value of <50, an Fp value between 50 and 200 represents the existence of moderate irregularities, and Fp > 200 indicates the occurrence of very strong irregularity levels. For a detailed study here, the appearance proportions of Fp > 50, 100 and 200 (gray, dark, and black in Figs. 2 7) observed during nighttime (between 1800 and 0600 LT) are employed to evaluate the frequency of occurrence of the ionospheric irregularities during the observation period of , the high solar ( , maximum in 2000), and low solar ( , , minimum in 1996) activity periods. Figs. 2 4 show the monthly occurrence rate of ionospheric irregularities over Fortaleza, Brasilia, and Kourou, respectively, during (gray, dark, and black denote the occurrence rate of Fp > 50, 100, and 200, respectively). For the high solar activity period ( ) the occurrence rate of ionospheric irregularities produces greater values in September April but smaller values in May August (Figs. 2a, 3a and 4a). Similarly, for the low solar activity period the occurrence rate also produces greater values in October April but smaller values in May August (Figs. 2b, 3b and 4b). Note for the December solstice months (January February, November December) of peak occurrence rate that there is little difference in the occurrence rate between solar maximum and minimum for each site. Likewise, the occurrence rates for the June solstice months (May August) also show no clear dependence on the solar activity for each site. Figs. 5 7 show the seasonal occurrence rate of ionospheric irregularities over Fortaleza, Brasilia, and Kourou,

3 F.D. Chu et al. / Advances in Space Research xxx (2007) xxx xxx 3 Fig. 2. The monthly occurrence rate of ionospheric irregularities over Fortaleza during (gray, dark, and black denote the occurrence rate of Fp > 50, 100, and 200, respectively): (a) solar maximum years ( ), and (b) solar minimum years ( , ). Fig. 4. The monthly occurrence rate of ionospheric irregularities over Kourou during respectively, during For the equinoctial months (March April, September October), the notable dependence of irregularity development on solar activity is evident (Figs. 5a, 6a and 7a) for Fp > 50, 100, and 200. In general, the occurrence rate increases from 1996 to 2000 as solar activity increases, and decreases from 2002 to 2005 as solar activity decreases. However, the occurrence rate in the June solstice months is small and the solar dependence is not clear (Figs. 5b, 6b and 7b). The occurrence rate of moderate irregularities (Fp > 50) is high (>80%) for each year and shows little dependence on solar activity (Figs. 5c, 6c and 7c). Nevertheless, the solar dependence is not simple for the irregularities that caused higher GPS phase fluctuations (Fp > 100 and Fp > 200) in the December solstice months. In general, the occurrence rate of strong phase fluctuations (Fp > 200) appears dependence on solar activity during the complete solar cycle ( ), but the occurrence rate of high phase fluctuations (Fp > 100) seems dependence only during the period of the increase of solar activity ( ). 3. Discussion Fig. 3. The monthly occurrence rate of ionospheric irregularities over Brasilia during It can be found that the seasonal patterns of the irregularity occurrences are similar for both high and low solar activities during January February and November December of the December solstice months (see Figs. 2 4, 5c, 6c and 7c), and however, increase with solar activity during March April and September October of the equinoctial months (Figs. 2 4, 5a, 6a and 7a). Sahai et al. (1999), Sobral et al. (2002), and Chu et al. (2005) reported that during both high and low solar activities the maximum occurrences of plasma depletion and ionospheric irregular-

4 4 F.D. Chu et al. / Advances in Space Research xxx (2007) xxx xxx Fig. 5. The seasonal occurrence rate of ionospheric irregularities over Fortaleza during (gray, dark, and black denote the occurrence rate of Fp > 50, 100, and 200, respectively): (a) equinoctial months (March April, September October), (b) June solstice months (May August), and (c) December solstice months (January February, November December). ities are observed in December solstice months over Cachoeira Paulista, Brazil ( 22.7 N, 45.0 E geographic; 16.7 N, 22.1 E, geomagnetic) and Brasilia, Brazil, respectively. It is interesting to find that based on ionograms over Cachoeira Paulista recorded in October months of Abdu et al. (1985) concluded that the occurrence of spread-f is proportional to solar activity. Moreover, Sobral et al. (2002), and Chu et al. (2005) reported that the occurrences of plasma depletion and ionospheric irregularities depend positively on solar activity. Their results are in agreement with the results reached in this paper. Note there is no previous study that reported the occurrence of F layer spreading could depend on solar activity in December solstice months at the Brazilian longitudes. Therefore, it may be a new finding in this study that the occurrence rate of strong irregularities (Fp > 200) appears Fig. 6. The seasonal occurrence rate of ionospheric irregularities over Brasilia during The cross symbols denote not available data. dependence on solar activity during the complete solar cycle ( ) in December solstice months. The results can be confirmed by the cross comparisons among the patterns of irregularity occurrences obtained at the three GPS sites. The departure from the patterns of irregularity occurrences, for example, years of 1995 and 1998 in Fig. 6a, and that of Fp > 100 and Fp > 200 for the year 1996 in Fig. 6c, may be due to too few observation days (only 41, 55, and 35 days, respectively). Besides, the departure of that of Fp > 200 for years of 2003 and 2004 in Fig. 6c may be due to missing some GPS data during LT in which hours ionospheric irregularities often occur over Brasilia. 4. Conclusions The accumulated raw database of GPS observations has now been longer than a complete solar cycle of 11 years, This work is the first attempt to investigate

5 F.D. Chu et al. / Advances in Space Research xxx (2007) xxx xxx 5 following two conclusions. (1) The occurrence rates of ionospheric irregularities yield maximum values in December solstice months and minimum values in June solstice months, and show no clear dependence on solar activity; but the occurrence rates increase with solar activity in equinoctial months. (2) If strong irregularities are distinguished from moderate ones, their occurrence rates would increase with solar activity during the December solstice months. Acknowledgements The GPS data are obtained from the International GNSS Service for Geodynamics (IGS). This research was supported by the Ministry of Economic Affairs under Grant for The Maintenance and Enhancement of National Standard Time and Frequency project to National Standard Time and Frequency Lab, Telecommunication Laboratories, Chunghwa Telecom Co., Ltd. References Fig. 7. The seasonal occurrence rate of ionospheric irregularities over Kourou during the climatology of ionospheric irregularities during the complete solar cycle by using the GPS data recorded at the three GPS sites at the Brazilian longitudes. It can be expected that we would obtain more reliable results of the climatology of ionospheric irregularities by using the phase fluctuations derived from the long-term database and by cross comparing the patterns of irregularity occurrences observed at the three sites. The results lead to the Aarons, J. The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence. Space Sci. Rev. 63, , Aarons, J., Mendillo, M., Yantosca, R. GPS phase fluctuations in the equatorial region during the MISETA 1994 campaign. J. Geophys. Res. 101, 26,851 26,862, Aarons, J., Mendillo, M., Yantosca, R. GPS phase fluctuations in the equatorial region during sunspot minimum. Radio Sci. 32, , Abdu, M.A., Sobral, J.H.A., Nelson, O.R., et al. Solar cycle related range type spread-f occurrence characteristics over equatorial and low latitude station in Brazil. J. Atmos. Terr. Phys. 47, , Chu, F.D., Liu, J.Y., Takahashi, H., et al. The Climatology of ionospheric plasma bubbles and irregularities over Brazil. Ann. Geophys. 23, , Mendillo, M., Lin, B., Aarons, J. The application of GPS observations to equatorial aeronomy. Radio Sci. 35, , Sahai, Y., Fagundes, P.R., Bittencourt, J.A. Solar cycle effects on large scale equatorial F-region plasma depletions. Adv. Space Res. 24, , Sobral, J.H.A., Abdu, M.A., Takahashi, H., et al. Ionospheric plasma bubble climatology over Brazil based on 22 years ( ) of 630 nm airglow observations. J. Atmos. Solar Terr. Phys. 64, , Valladares, C.E., Sheehan, R., Basu, S., et al. The multi-instrumented studies of equatorial thermosphere aeronomy scintillation system: Climatology of zonal drifts. J. Geophys. Res. 101, 26,839 26,850, 1996.