- Email: [email protected]
Global ionospheric maps with a high temporal resolution
Results in Physics 16 (2020) 102927
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Global ionospheric maps with a high temporal resolution
T
Cheng Wang Research Institute for Frontier Science, Beihang University, Beijing, China School of Electronic Information Engineering, Beihang University, Beijing, China
ARTICLE INFO
ABSTRACT
Keywords: Ionosphere Total electron content GIMs High temporal resolution
A new approach is proposed for the global ionospheric modeling using epoch-by-epoch estimation. The GIMs with a high temporal resolution are greatly useful for studying the climatology and weather of the ionosphere, including ionospheric variations, such as noon-time bite-outs and nighttime enhancements.
The total electron content (TEC) is one of the most important parameters of the ionosphere. The International GNSS Service (IGS) has been providing global ionospheric maps (GIMs) by using Global Navigation Satellite Systems (GNSS) observations for over two decades [1]. However, the GIMs basically have 1-h or 2-h temporal resolution. This severely limits the scientific applications of GIMs in ionospheric physics studies. To obtain the GIMs with a high temporal resolution, we have developed the following algorithms: (1) To perform the global ionospheric modeling with spherical harmonics (SH) functions on a daily basis; (2) To extract the estimated SH coefficients, differential code biases (DCBs) and covariance matrix as the priori information; (3) To introduce the priori information into the epoch-by-epoch least square estimation; (4) To carry out the epoch-by-epoch estimation based on random walk model with priori information; (5) To generate the GIMs with a high temporal resolution (usually 30 s). We use the following random walk model to perform the modeling with priori information [2]:
X = (AT PA +
X
1) 1 (AT PL
Xt = Xt
1
+
X
+ et
1X )
(1)
where A is the design matrix; P is the weight matrix; L is the observations; X contains the prior SH parameters and DCB parameters; X
is the prior covariance matrix; X is the unknown parameters; t is the epoch; and e is random error. Fig. 1 shows the flow processing for epoch-by-epoch estimation of global ionospheric TEC. According to this approach, we use dual-frequency GNSS observations from approximately 300 IGS stations (in the year 2009 and 2014) for modeling the global ionospheric TEC and generating GIMs (named BEEG) with high temporal resolution of 30 s. We select several GIMs on the day of year (DOY) 126, 127 and 128 in 2009 and 2014 to present the detailed variation of TEC. Also, we collect the corresponding GIMs (named CODG) from the Center for Orbit Determination in Europe (CODE) as a reference data. Fig. 2 shows the vertical TEC (VTEC) derived from BEEG and CODG at 15°N latitude, 0° longitude during the three consecutive days in 2009 and 2014, respectively. Table 1 presents the days of noon-time bite-outs and nighttime enhancements observed by GIMs in 2009 and 2014. By comparison of the two kinds of VTEC time series, it is obviously that the GIMs with high temporal resolution BEEG presents more details of ionospheric VTEC during both low and high solar activity. The CODG with 1-h resolution is unable to present
E-mail address: [email protected]. https://doi.org/10.1016/j.rinp.2020.102927 Received 11 September 2019; Received in revised form 20 December 2019; Accepted 4 January 2020 Available online 07 January 2020 2211-3797/ © 2020 The Author. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 16 (2020) 102927
C. Wang
Table 1 Days of noon-time bite-outs and nighttime enhancements observed by GIMs.
BEEG CODG
Noon-time bite-outs
Nighttime enhancements
2009
2014
2009
2014
150 12
106 20
150 0
87 5
Acknowledgments The GIMs with 30 s resolution BEEG are publicly available via FTP (ftp://pub.ionosphere.cn/product/). The IGS CODG products were retrieved from the FTP (ftp://cddis.gsfc.nasa.gov/pub/gps/products/ ionex/). The author would like to thank IGS for openly providing GNSS observations. This study has been funded by the National Natural Science Foundation of China (No. 41804026 and No. 41931075).
Fig. 1. Flow diagram for global ionospheric modeling by epoch-by-epoch estimation.
several details of VTEC variation, such as noon-time bite-outs and nighttime enhancements. Some researchers have studied on the noon-time bite-outs by using foF2 data at different regions [3–5]. Due to the limited geographic coverage of ionosondes and radars, it is difficult to study further about ionospheric anomalies on global scale. Also, the traditional GIMs with 1-h or 2-h resolution cannot present these anomalies in smaller time scale. As shown in Fig. 2, the noon-time bite-out phenomena are clearly seen in the BEEG, but not observed in the CODG. Similarly, nighttime enhancements can also be presented by BEEG. Additionally, looking from another view, the traditional GIMs have lower accuracy during noon-time and nighttime periods, especially in near midnight at high solar activity.
Declaration of Competing Interest The authors declare no conflict of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2020.102926.
(a) 2009
(b) 2014
Fig. 2. The VTEC with 30 s resolution at 15°N latitude, 0° longitude on DOY 126, 127 and 128 in 2009 and 2014. The blue and red curves depict the VTEC derived from BEEG and CODG, respectively. The pink lines show the probable time horizon of noon-time bite-outs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2
Results in Physics 16 (2020) 102927
C. Wang
References
electron content. J Geophys Res Space Phys 2012;117:A09303. https://doi.org/10. 1029/2012JA017909. [4] Liu L, et al. Comparative study of the equatorial ionosphere over Jicamarca during recent two solar minima. J Geophys Res Space Phys 2012;117(A1). https://doi.org/ 10.1029/2011JA017215. [5] Lynn KJW, Gardiner-Garden RS, Heitmann A. The spatial and temporal structure of twin peaks and mid-day bite out in foF2 (with associated height changes) in the Australian and South Pacific low mid-latitude ionosphere. J Geophys Res Space Phys 2014;119(12):10294–304. https://doi.org/10.1002/2014JA020617.
[1] Roma-Dollase D, et al. Consistency of seven different GNSS global ionospheric mapping techniques during one solar cycle. J Geod 2018;92(6):691–706. https://doi. org/10.1007/s00190-017-1088-9. [2] Cheng W, et al. Improved modeling of global ionospheric total electron content using prior information. Remote Sens 2018;10(1):63. https://doi.org/10.3390/ rs10010063. [3] Lee Chien-Chih. Examination of the absence of noontime bite-out in equatorial total
3
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Global ionospheric maps with a high temporal resolution
T
Cheng Wang Research Institute for Frontier Science, Beihang University, Beijing, China School of Electronic Information Engineering, Beihang University, Beijing, China
ARTICLE INFO
ABSTRACT
Keywords: Ionosphere Total electron content GIMs High temporal resolution
A new approach is proposed for the global ionospheric modeling using epoch-by-epoch estimation. The GIMs with a high temporal resolution are greatly useful for studying the climatology and weather of the ionosphere, including ionospheric variations, such as noon-time bite-outs and nighttime enhancements.
The total electron content (TEC) is one of the most important parameters of the ionosphere. The International GNSS Service (IGS) has been providing global ionospheric maps (GIMs) by using Global Navigation Satellite Systems (GNSS) observations for over two decades [1]. However, the GIMs basically have 1-h or 2-h temporal resolution. This severely limits the scientific applications of GIMs in ionospheric physics studies. To obtain the GIMs with a high temporal resolution, we have developed the following algorithms: (1) To perform the global ionospheric modeling with spherical harmonics (SH) functions on a daily basis; (2) To extract the estimated SH coefficients, differential code biases (DCBs) and covariance matrix as the priori information; (3) To introduce the priori information into the epoch-by-epoch least square estimation; (4) To carry out the epoch-by-epoch estimation based on random walk model with priori information; (5) To generate the GIMs with a high temporal resolution (usually 30 s). We use the following random walk model to perform the modeling with priori information [2]:
X = (AT PA +
X
1) 1 (AT PL
Xt = Xt
1
+
X
+ et
1X )
(1)
where A is the design matrix; P is the weight matrix; L is the observations; X contains the prior SH parameters and DCB parameters; X
is the prior covariance matrix; X is the unknown parameters; t is the epoch; and e is random error. Fig. 1 shows the flow processing for epoch-by-epoch estimation of global ionospheric TEC. According to this approach, we use dual-frequency GNSS observations from approximately 300 IGS stations (in the year 2009 and 2014) for modeling the global ionospheric TEC and generating GIMs (named BEEG) with high temporal resolution of 30 s. We select several GIMs on the day of year (DOY) 126, 127 and 128 in 2009 and 2014 to present the detailed variation of TEC. Also, we collect the corresponding GIMs (named CODG) from the Center for Orbit Determination in Europe (CODE) as a reference data. Fig. 2 shows the vertical TEC (VTEC) derived from BEEG and CODG at 15°N latitude, 0° longitude during the three consecutive days in 2009 and 2014, respectively. Table 1 presents the days of noon-time bite-outs and nighttime enhancements observed by GIMs in 2009 and 2014. By comparison of the two kinds of VTEC time series, it is obviously that the GIMs with high temporal resolution BEEG presents more details of ionospheric VTEC during both low and high solar activity. The CODG with 1-h resolution is unable to present
E-mail address: [email protected]. https://doi.org/10.1016/j.rinp.2020.102927 Received 11 September 2019; Received in revised form 20 December 2019; Accepted 4 January 2020 Available online 07 January 2020 2211-3797/ © 2020 The Author. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 16 (2020) 102927
C. Wang
Table 1 Days of noon-time bite-outs and nighttime enhancements observed by GIMs.
BEEG CODG
Noon-time bite-outs
Nighttime enhancements
2009
2014
2009
2014
150 12
106 20
150 0
87 5
Acknowledgments The GIMs with 30 s resolution BEEG are publicly available via FTP (ftp://pub.ionosphere.cn/product/). The IGS CODG products were retrieved from the FTP (ftp://cddis.gsfc.nasa.gov/pub/gps/products/ ionex/). The author would like to thank IGS for openly providing GNSS observations. This study has been funded by the National Natural Science Foundation of China (No. 41804026 and No. 41931075).
Fig. 1. Flow diagram for global ionospheric modeling by epoch-by-epoch estimation.
several details of VTEC variation, such as noon-time bite-outs and nighttime enhancements. Some researchers have studied on the noon-time bite-outs by using foF2 data at different regions [3–5]. Due to the limited geographic coverage of ionosondes and radars, it is difficult to study further about ionospheric anomalies on global scale. Also, the traditional GIMs with 1-h or 2-h resolution cannot present these anomalies in smaller time scale. As shown in Fig. 2, the noon-time bite-out phenomena are clearly seen in the BEEG, but not observed in the CODG. Similarly, nighttime enhancements can also be presented by BEEG. Additionally, looking from another view, the traditional GIMs have lower accuracy during noon-time and nighttime periods, especially in near midnight at high solar activity.
Declaration of Competing Interest The authors declare no conflict of interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.rinp.2020.102926.
(a) 2009
(b) 2014
Fig. 2. The VTEC with 30 s resolution at 15°N latitude, 0° longitude on DOY 126, 127 and 128 in 2009 and 2014. The blue and red curves depict the VTEC derived from BEEG and CODG, respectively. The pink lines show the probable time horizon of noon-time bite-outs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2
Results in Physics 16 (2020) 102927
C. Wang
References
electron content. J Geophys Res Space Phys 2012;117:A09303. https://doi.org/10. 1029/2012JA017909. [4] Liu L, et al. Comparative study of the equatorial ionosphere over Jicamarca during recent two solar minima. J Geophys Res Space Phys 2012;117(A1). https://doi.org/ 10.1029/2011JA017215. [5] Lynn KJW, Gardiner-Garden RS, Heitmann A. The spatial and temporal structure of twin peaks and mid-day bite out in foF2 (with associated height changes) in the Australian and South Pacific low mid-latitude ionosphere. J Geophys Res Space Phys 2014;119(12):10294–304. https://doi.org/10.1002/2014JA020617.
[1] Roma-Dollase D, et al. Consistency of seven different GNSS global ionospheric mapping techniques during one solar cycle. J Geod 2018;92(6):691–706. https://doi. org/10.1007/s00190-017-1088-9. [2] Cheng W, et al. Improved modeling of global ionospheric total electron content using prior information. Remote Sens 2018;10(1):63. https://doi.org/10.3390/ rs10010063. [3] Lee Chien-Chih. Examination of the absence of noontime bite-out in equatorial total
3