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SCAR Report 23
Development of TEC Fluctuations in Antarctic Ionosphere during storm using GPS observations
I.I. Shagimuratov (1), A. Krankowski (2), L.W. Baran (2), J.
Cisak (3), I.I.
Ephishov (1)
(1) WD IZMIRAN, Kaliningrad, RUSSIA
(2) Institute of Geodesy, Warmia and Mazury University in Olsztyn, POLAND,
kand@moskit.uwm.edu.pl;
(3) Institute of Geodesy and Cartography, WARSAW, POLAND
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Abstract.
GPS observations
of the Antarctic stations belonging to IGS network were used to study TEC fluctuations
on high-latitude ionosphere during storms. Dual-frequency GPS phase measurements
along individual satellite passes with 30 sec sampling interval were served
as raw data. The ionospheric irregularities of different scale develop in
the auroral,
polar ionosphere. It is a common phenomenon which causes the phase fluctuations
of GPS signals. We distinguished the variations of TEC related with the ionospheric
structures of the spatial scale bigger than 200-300 km. At diagram of temporal
variations of TEC along satellite pass the structure of TEC corresponds to
the time scale bigger than 15-30min on the 4-6-th hour duration of tracking
satellite
by individual stations. We attribute the variations of the time scale smaller
than 15-30 min to TEC fluctuations related to small scale ionospheric irregularities.
We used the rate of TEC index (ROTI) expressed in TECU/min as the measure
of TEC fluctuations. The large scale ionospheric structures cause the increase
of
the horizontal gradients and difficulties of the carrier phase ambiguity
resolution in relative GPS positioning. In turn the phase fluctuations can
cause the cycle
slips. At polar stations: MCM4, CAS1, DAV1 we detected the ionospheric structures
of enhanced TEC bigger than 3-5 time relative background, while the TEC increased
to 10-30 TECU in about 10-15min. The structures were observed during a storm
as well as during moderate geomagnetic activity. The structures probably
can be attributed to polar cap patches. At lower latitude station: OHIG during
the
storm can essentially increase the horizontal gradient which we attribute
to the occurrence of the ionospheric trough and it storm-time dynamics.
The ROTI data was used to study the developments of phase fluctuations over the Antarctic ionosphere during geomagnetic disturbances. During storms the intensity of phase fluctuations increased. The occurrence of phase fluctuations was even detected during the active storm period of 31 March 2001 at middle latitude station OHIG located at 49∞ corrected geomagnetic latitude. The storm-time features in longitude and latitude development of phase fluctuations were obtained for the Antarctic region. The correlation between activity of phase fluctuations and magnetic field variation of Mowsen station was established.
Key words:
Ionosphere, TEC, modeling of ionosphere
1 Introduction
The structure of the high-latitude ionosphere is very complicated
and varied. Strong changes of the ionosphere occur during geomagnetic disturbances.
The
dramatic changes took place very frequently in the auroral and polar ionosphere.
In this
region the irregularities of differential scales are developed commonly which
cause the fluctuations of total electron content. In the paper we distinguished
two types of TEC fluctuations. In the first, the large scale fluctuations
(LSF) of TEC which are caused by the ionospheric irregularities with scale
bigger than
100-300 km. These ionospheric structures occurred as the deep spatial variations
of TEC. The second type of fluctuations is the irregularities with size about
ten kilometers which cause the phase fluctuations GPS signals. The small
irregularities can co-exist with large scale structures. In the report we
present the analysis
of the development of TEC fluctuations in March 2001. Two great geomagnetic
storms took place on 20 and 31 March. The storm on March 31st was the severest
one in
the last decade. The Kp index reached maximal value of 9 and 
Kp made up
60. The Dst index reached maximum magnitudes with extremely high value about
-360
nT. The geomagnetic conditions during March 2001 are presented in Figure
1 as variation of sum Kp index.
Fig.1. Variations of sum Kp during March 2001
2 The TEC data base
The GPS observations of the Antarctic IGS stations were
used to study the development of TEC fluctuations in high-latitude ionosphere.
The standard
GPS measurements
with 30 sec sampling provide the detection of the irregularities with size
bigger than 6-10 km. In the Table 1 there are geographic and corrected geomagnetic
coordinates
of the stations. The broad longitudinal area of Antarctic stations enables
to study the time development of TEC fluctuations.
The dynamics of the high-latitude ionosphere are controlled by the geomagnetic
field. In the Table 1 you can see that geomagnetic latitudes are essentially
different from geographic one. So in Antarctic region we can choose from
middle-latitude station – OHIG, auroral - SYOG, MAW1 to polar stations
- MCM4, CAS1, DAV1.
Table 1. Antarctic IGS stations
3 Large scale fluctuation of TEC
For the analysis of spatial and temporal changes of TEC during storm we used the high precision dual-frequency GPS phase measurements that provide more precise measurements of TEC than group delay ones. Phase ambiguities were removed by fitting phase measurements to the code data. After the above procedure the phase measurements contained satellite-receiver biases only. The absolute TEC and the instrumental biases were estimated using the single site algorithm (Baran et al., 1997). The biases were determined for every individual station using the GPS measurements for all satellite passes over station during 24 hour period. Using this procedure an absolute TEC for all satellite passes observed over single station during 24 hour period is calculated.
The spatial and temporal variations of TEC are clearly seen on the time variations of TEC along individual satellite passes. Figures 2a and 2b give an example of the TEC variations for individual satellite passes as observed from different stations during storm (dashed line) and quiet period (solid line). The vertical TEC in units of 1016 el/m2 is plotted as a function of universal time (UT). This represents a part of diurnal TEC pattern sampled by satellites at these times. The spatial positions of satellites on figures also are presented (cross). Because the GPS satellites are on 12 sidereal hour orbits, the tracks repeat day by day (only the satellites arrive 4 min earlier each day). The plot approximately indicates that the satellites ionospheric trace for two days under consideration.
The series of large scale fluctuations (LSF) as enhancement of TEC are clearly shown on temporal patterns (Figures 2a and 2b). At the polar stations (CAS1, MCM4, DAW1) the TEC patterns demonstrate the great variability during disturbed as well as quiet days. During storm the intensity of the fluctuations dramatically increased. The TEC increase of a factor of 2-8, the enhancement of TEC can exceed 10-20 TECU to relative phone. At lower latitudes the intensity LSF decreased. We attribute the TEC enhancement as occurrence of the polar cap ionospheric patches. These responses to F region plasma patch as the GPS ray encounter the patches structures (Weber et al., 1984). Polar cap patches are large regions of enhanced F region plasma density observed traveling trough the ionospheric polar cap under the influence of the high-latitude convection (Pederson et al., 2000). Discrete F region electron density is enhancement of a factor 2 or more. Patches are typically considered to be of the order of 100-1000 km in horizontal extent. The travelling speed of the patch is between 300-900 m-3 (Rodger et al., 1999). Thus in the temporary pattern shown the variations of TEC along satellite passes the duration of occurrence of the patches can be 10 min or more.
The patterns of TEC fluctuation demonstrate the similar structures at spaced stations. It is well seen in Figure 2a for DAV1 and MAW1. Very similar portrait of the patch structures shows the TEC variation for PRN 15 and PRN 17 at CAS1 stations (top panel on Figure 2a) for March 2001. The time delays between the similar discrete structures correspond to a propagation velocity of the patch, and it is about 700 ms-1.
Fig.2a. TEC variations for satellites observed at different stations for during the storm (dashed line) and quiet period (solid line).
The deep variations of TEC are observed very frequently at polar stations. Analyses of data of MCM4 stations show that patch-like structures (about 90% cases) were registered during March-April 2001 period. Over auroral station - VESL during quiet day the TEC demonstrates smooth run, during storm the LSF are often observed. The amplitudes of the TEC fluctuations in this region are smaller than on polar stations. At middle latitude station (OHIG) the large structures can be detected during the storm which we attribute to the occurrence of main ionospheric trough. In the time the horizontal gradients in the ionosphere over OHIG station increased. The sign storm time gradients can even be opposite to quiet geomagnetic conditions (Figure 2b).
Fig.2b. TEC variations for satellites observed at different stations for during the storm (dashed line) and quiet period (solid line).
4 Phase fluctuations of GPS signals
The TEC fluctuations, also called phase fluctuations, are caused by the presence of medium and small æ scale irregularities in the ionosphere. To estimate the phase fluctuations the dual-frequency phase measurements with 30 sec sample interval usually are used. The rate of TEC changes 1 min apart (ROT) is the source of the intensity of phase fluctuations study. The use of these relatively infrequent samples enables to study irregularity structures in the order of kilometers. When using ROT we avoid the problem of phase ambiguities.
Fig.3. The Magnetic activity at Mawson and the phase fluctuations occurrence at different stations on March 28-31.
The plots show the phase fluctuations for individual satellites,
the mark points out location of geomagnetic midnight
As a measure of ionospheric activity we used also the Rate of TEC Index (ROTI)
based on standard deviation of ROT (Pi et al., 1997)
ROTI has been estimated in 10-min interval.
The Figure 3 demonstrates the occurrence of the phase fluctuations (ROT) during 28-31 March 2001 with the most disturbed storm day - 31 March. The plot shows the variations of raw phase fluctuations for all satellites observed from Antarctic stations during 24 hour period. The top panel demonstrates the behaviour of variations of geomagnetic field at Mawson station. In the pictures the mark points out the location of the geomagnetic midnight. The maximum occurrence of phase fluctuations took place usually around local midnight (MN) (Aarons et al., 2000), it is clearly seen at auroral stations (SYOG, VESL). In the same time the picture demonstrates that the developments of phase fluctuations are controlled by the geomagnetic activity. During the storm over Mawson station the development of phase fluctuation correlate with variations of geomagnetic field.
On polar stations the phase fluctuations all day are observed. On middle-latitude station - OHIG the fluctuations occurred only during storm day of 31 March. It is evident that during 31 March auroral oval for irregularities until middle latitudes was expanded.
The latitudinal occurrence of intensity of phase fluctuations depended on time is presented in Figure 4 (The polar coordinates – Corrected Geomagnetic Latitude (CGL) and Magnetic Local Time (MLT)). The intensity of the fluctuations with correspondence to the symbols is indicated. Figure 4a illustrates the exhibition of phase fluctuation over polar station - CAS1 and auroral one - MAV1. As Aarons showed, a dominant factor in the development of phase fluctuations during quiet period is the location of station relative to auroral oval. During quiet day of 26 March the maximal intensity is observed around magnetic local midnight. The geomagnetic storms modify the diurnal patterns of phase fluctuations extending the time of development and increasing their intensity. At polar site of CAS1 the weak intensity during storm increased more than at auroral station of MAV1. The developments of fluctuations over DAV1 station are more clearly controlled by geomagnetic activity. Figure 4b illustrates the exhibition of phase fluctuation over middle latitude station - OHIG and polar station - MCM4. In quiet and moderate magnetic activity at OHIG phase fluctuations are very weak and only in the most disturbed day 31 March the fluctuations occurred. It appeared during the greatest storms when the oval irregularities until middle latitudes are developed.
Fig.4a. Location of TEC fluctuations derived from GPS measurements in Geomagnetic local time and Corrected geomagnetic latitude for 26.03.2001, 30.03.2001 and 31.03.2001 on the different stations in south hemisphere. Intensity of fluctuations is indicated with following symbols: crosses represent fluctuations between 0.3 and 0.5 TEC/min, rhombs – bigger than 1.5 TEC/min.
Fig.4b. Location of TEC fluctuations derived from GPS measurements in Geomagnetic local time and Corrected geomagnetic latitude for 26.03.2001, 30.03.2001 and 31.03.2001 on the different stations in south hemisphere. Intensity of fluctuations is indicated with following symbols: crosses represent fluctuations between 0.3 and 0.5 TEC/min, rhombs – bigger than 1.5 TEC/min.
Fig.4c. Location of TEC fluctuations derived from GPS measurements in Geomagnetic local time and Corrected geomagnetic latitude for 26.03.2001, 30.03.2001 and 31.03.2001 on the different stations in south hemisphere. Intensity of fluctuations is indicated with following symbols: crosses represent fluctuations between 0.3 and 0.5 TEC/min, rhombs – bigger than 1.5 TEC/min.
Figure 4c demonstrates the occurrence of phase fluctuations at lower (SYOG) and higher latitude edge of auroral oval. The intensity of fluctuations is essentially lower over SYOG than over DAV1 station. During storm day the fluctuations are registered the whole time, their intensity also markedly increased in the storm time.5 Conclusion
The occurrence of TEC fluctuations depends on the geomagnetic latitude of a site. In the Antarctic region the difference between geomagnetic and geographic coordinates of site can be bigger than 10 degrees. So in the Antarctic region we can distinguish midlatitude, subauroral, auroral and polar stations.
Maximal TEC fluctuations took place at polar stations. The variations of TEC during storm reached 10-40 TEC. The enhancement of TEC exceeded 2-8 times a relative phone. Deep variations of TEC observed along individual satellite passes related to polar patches. They are transferred across line-of-sight of the receiver-satellite. The speed of the patches obtained from GPS observations was bigger than 700 m s-1.
At lower latitudes the fluctuations of GPS signals are attributed to small and middle-scale irregularities. The intensity of phase fluctuations depends on geomagnetic activity. During maximal phase of storm on 31 March 2001 the fluctuations of moderate intensity were observed at middle latitude station of OHIG. The development of TEC strongly correlates with the geomagnetic field variations of Mawson station. The ionospheric gradients increased essentially during the storm. The irregular gradients sometimes exceed the regular ones. During the storm time it can cause the increasing of errors in determining phase ambiguities of GPS observations in the Antarctic region.
References
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Baran L.W., Shagimuratov I.I., Tepenitsina, The use of GPS for ionospheric studies, Artificial Satellites, Vol. 32, No 1, pp.49-60, 1997
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