Statistical analysis of ionospheric mid-latitude trough over the Northern Hemisphere derived from GPS total electron content data
© Yang et al. 2015
Received: 20 August 2015
Accepted: 27 November 2015
Published: 8 December 2015
This study statistically investigated the seasonal variation, magnetic local time (MLT) variation, geomagnetic activity dependence, and solar activity dependence of the mid-latitude trough using GPS total electron content (TEC) data from 2000 to 2014. The daily median Kp index was used to characterize the daily geomagnetic activity level. The results showed that the trough minimum position depended primarily on the geomagnetic activity, MLT, and the season. The trough depth depended primarily on the solar flux index (F107) and, to a lesser degree, on MLT. The trough depth increased as F107 increased and as the incidence angle of solar flux decreased. The trough equatorward half-width decreased as the geomagnetic activity increased. These variations in the GPS-TEC trough minimum position were compared with the variations in the TEC trough derived from the International Reference Ionosphere (IRI)-2007 model. The GPS-TEC trough minimum position changed little with respect to F107, whereas the IRI-TEC trough minimum position showed a strong F107 dependence.
The ionospheric mid-latitude trough is a depleted region of ionospheric plasma density in the nightside F region and lies just equatorward of the auroral equatorward boundary. Throughout the paper, trough will refer specifically to the mid-latitude trough. The trough generally consists of three parts: an equatorward wall, a trough minimum, and a poleward wall. Because the large electron density gradient on either side of the trough affects radio wave propagation, the precise position of the trough is very important for many practical activities, such as trans-ionospheric communication and navigation.
Since Muldrew (1965) discovered the trough with a topside sounder, the formation and maintenance mechanisms of the trough have been studied extensively, but many properties of the trough remain incompletely understood. Several mechanisms have been proposed to explain the trough in the pre-midnight sector. The first mechanism is plasma stagnation and decay of ionization in darkness in a region where corotation and convection electric fields counteract each other (e.g., Knudsen 1974; Spiro et al. 1978; Collis and Häggström 1988; and Hedin et al. 2000). Another mechanism for the trough formation during periods of enhanced geomagnetic activity is related to large poleward electric field events, which are also called subauroral ion drifts (SAIDs). The enhanced electric field can cause depletions in electron concentration by enhancing the recombination rate (Schunk et al. 1976). In addition, westward plasma drift driven by the large poleward electric field also advects depleted plasma density to earlier magnetic times; as such, the high-density plasma in earlier magnetic times can be displaced by the depleted plasma density. Rodger et al. (1992) pointed out that the trough is the “normal” ionosphere between the poleward wall and the equatorward wall. The poleward wall is formed by precipitation or plasma transported from elsewhere (Rodger et al. 1986), and the equatorward wall is formed by corotating plasma that continues to decay with time and replenishment from the plasmasphere, but the maintenance mechanism is absent in the vicinity of the trough. Evans et al. (1983) pointed out that the trough formed in the dusk-side region may be a fossil corotating into the nightside sector toward dawn. It is worthy to note that a specific mechanism alone cannot account for density depletion, because the F region plasma is long-lived and a particular mechanism may not be at work throughout the observation period (Nilsson et al. 2005).
The trough has been investigated using various data and methods such as data from satellites, tomography, total electron content (TEC), incoherent scatter radar (ISR), and models (Muldrew 1965; Nilsson et al. 2005; Pryse et al. 2006; Middleton et al. 2008; He et al. 2011; Lee et al. 2011; Ishida et al. 2014). Currently, due to the fast-growing number of ground-based GPS receivers, GPS-TEC measurements have been increasingly used for upper atmospheric research. Horvath and Essex (2003) used TEC measurements from three stations to investigate the features of the Southern Hemisphere trough. Wielgosz et al. (2004) showed that the structure of the ionosphere trough can be regularly identified from latitudinal GPS-TEC plots. In addition, Krankowski et al. (2008) and Zou et al. (2011) utilized GPS-TEC data to study the trough over Europe and Alaska, respectively. These studies demonstrated that GPS-TEC can be used to routinely identify the ionosphere trough.
The GPS-TEC data used in this study were downloaded from MIT Haystack’s Madrigal database mirror site at the Institute of Geology and Geophysics, Chinese Academy Sciences (http://madrigal.iggcas.ac.cn/madrigal/). There are more than 2000 GPS receivers distributed across the globe. MIT Haystack has automated the process of downloading and processing GPS data (Rideout and Coster 2006) to produce globally gridded TEC data with a resolution of 5 min. The geomagnetic activity index Kp and solar flux index F107 data were downloaded from the National Geophysical Data Center (ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/INDICES/KP_AP/). In this study, we primarily investigated the trough structure averaged longitudinally over a day. As such, the daily median Kp index was used to characterize the daily geomagnetic activity level. In order to study the seasonal variation of the mid-latitude trough, the data were divided into three seasons: summer (May–July), equinox (February–April, August–October), and winter (November–January).
The TEC maps were used to create latitudinal TEC profiles (between 45° and 70° N) with intervals of 2 h and 1° steps in latitude. At first, the trough signature in the latitudinal TEC profile was recognized visually. Then, an automated procedure was used to estimate the trough parameters that formed the dataset used in this study. Although visual identification always involves some subjective judgment, the larger data sample was expected to smooth out such deficiencies.
Results and discussion
The trough minimum position depends primarily on geomagnetic activity, MLT, and the season. The trough shifts toward low latitudes with increasing geomagnetic activity and MLT. The seasonal variation in trough minimum position differs strongly with different MLTs.
The trough depth depends primarily on F107 and, to a lesser degree, on MLT. The trough depth increases with increasing F107 and with decreasing incidence angle of solar flux.
The trough equatorward half-width decreases with increasing geomagnetic activity.
The diurnal variation in the IRI-TEC trough minimum position is similar to that of the GPS-TEC trough minimum position. The GPS-TEC trough minimum position changes little with F107, whereas the IRI-TEC trough minimum position shows a strong F107 dependence.
In this study, we examined the trough structure averaged longitudinally over the course of a day. The longitudinal average of the trough structure depends not only on the longitude effect of the trough occurrence rate (He et al. 2011) but also on the longitude effect of the trough structure (Karpachev 2003). The occurrence rate of the trough shows significant variations depending on the season, solar activity, and MLT (Ishida et al. 2014). The longitude effect of the trough minimum position primarily depends on the Kp (Karpachev et al. 1996). These factors can lead to the large scatter of the data and make the interpretation of the data in terms of physical processes more difficult. Although these limitations exist, our results are consistent with those found in many earlier studies. During disturbed conditions, the temperature associated with the disturbed electric field increases in the trough region, and the plasma density at a trough minimum decreases. However, in this study, the TEC at the trough minimum remained constant with increasing Kp. Pryse et al. (2006) investigated the TEC trough in the United Kingdom and found the same feature. Therefore, the relationship between the TEC value at the trough minimum and the temperature at the trough minimum is needed to study at different geomagnetic activities, which can help us understand the physical mechanism of trough formation. In addition, a quantitative test of the wind effect in the trough is also needed in the future.
The mid-latitude trough is a typical feature of the F region. It has been demonstrated that GPS-TEC data can be used to investigate the time and space evolution of the trough, although the data provide no altitudinal information.
This research was supported by the Chinese Academy of Sciences project (KZZD-EW-01-3), National Key Basic Research Program of China (2012CB825604), and National Natural Science Foundation of China (41374162, 41231065, and 41321003). The GPS-TEC data used in this study were downloaded from MIT Haystack’s Madrigal database mirror site at the Institute of Geology and Geophysics, Chinese Academy Sciences (http://madrigal.iggcas.ac.cn/madrigal/).
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