The Ionosphere Irregularities Modeling on the base of ROTI Mapping
Iurii Cherniak, Irina Zakharenkova, Andrzej Krankowski, Irk Shagimuratov
West Department of IZMIRAN, University of Warmia and Mazury in Olsztyn, Institut de Physique du Globe de Paris, France
email: tcherniak@ukr.net
The ionosphere plays an important role in GNSS applications because it influences on the radio wave propagation through out. The ionosphere delay is the biggest error source for satellite navigation systems, but it can be directly measured and
mitigated with using dual frequency GNSS receivers. However GNSS signal fading due to electron density gradients and irregularities in the ionosphere can decrease the operational availability of navigation system. There were developed several
models in order to represent ionospheric fluctuations and scintillation activity under different geophysical conditions, but they were calibrated with data sets, that did not include GNSS derived data. It is very actual to develop empirical model based on
GNSS derived measurements which can represent strong ionosphere fluctuation events. In this work we use the data provided by the existing permanent GNSS network in order to make the empirical model of ionospheric irregularities over the Northern
hemisphere. As initial data we used the daily dependences of the ROTI index as a function of geomagnetic Local Time on the specific grid. The ROTI maps allow to estimate the overall fluctuation activity and auroral oval evolutions. With ROTI index
maps it was determined the irregularities oval border and averaging parameter – semi-hemisphere fluctuation index. It was investigated the statistical and correlation dependences between Kp geomagnetic index and parameters that characterized the
ionosphere irregularities activity for period of 2010 – 2014 years
Measurements and Database
In order to analyze TEC fluctuation activity there were considered the Northern hemisphere polar, subauroral and midlatitude regions geographically located from 45°N geomagnetic latitude to the North Pole.
Monitoring the time-derivative of TEC (ROT, rate of TEC change) is useful for tracing the presence of the ionospheric irregularities. ROT, as a measure of phase fluctuation activity, is calculated using the algorithm, that was proposed by Pi et al. [1997].
ROT is calculated in units of TECU/min for each visible satellite over GNSS station. The ROT values are calculated and then detrended for all individual satellite tracks for elevation angles over 20 degrees. ROTI is defined as the standard deviation (taken
over 5 minutes) of the detrended rate of change of TEC (ROT) [Pi et al., 1997]. Based on the retrieved values of ROT, the ROTI values are calculated over 5-minute periods with running window for each station.
To observe the spatial behavior of the ionospheric fluctuations over the North Pole, we process ROTI data from considered multi-site database and visualize result in the form of ROTI map. There are more than 700 permanent stations (from IGS,
UNAVCO and EUREF databases) have been involved into processing for the ionosphere fluctuation service. Such number of stations provides enough data for representation a detailed structure of the ionospheric irregularities pattern.
Due to strong connections between the Earth’s magnetic field and the ionosphere, the behavior of the fluctuation occurrence is represented as a function of the magnetic
local time (MLT) and of the corrected magnetic latitude. The grid of ROTI maps in polar coordinates with cell size 2 degree (magnetic local time) and 2 degree
(geomagnetic latitude).
Each map, as a daily map, demonstrates ROTI variation with geomagnetic local time (00-24 MLT). The value in every cell is calculated by averaging of all ROTI values
cover by this cell area and it is proportional to the fluctuation event probability in the current sector. If in the cell there are only few ROTI values (less than 30), this grid cell
is marked as a blank. This approach will allow to avoid frequent errors of interpolation techniques, related with unrealistic interpolation values over areas with data gaps.
It is developed and tested service for automatically download RINEX observational and navigational files, unzip and process data and generate ionospheric products.
The locations of the stations in the North
Hemisphere used for ROTI map construction and
quadrant of ROTI map grid.
The TEC fluctuation service operation diagram
The evolution of ROTI maps
Ionospheric irregularities modeling: approaches
During last decades there were developed several models in order to represent ionospheric fluctuations and scintillation
activity under different geophysical conditions. The WBMOD model describes a worldwide climatology of the ionospheric
plasma density irregularities. The parameters of ionospheric irregularities are modeled on the basis of experimental data. The
model provides the intensity scintillation index S4 and the phase scintillation index, computed by means of the propagation
model under the pre-specified geophysical conditions. The Global Ionospheric Scintillation Model (GISM) provides the
statistical characteristics of the transmitted signals, in particular scintillation indices. Maps of scintillation indices can easily be
obtained (Beniguel, 2002). The main limitation of these scintillation models is that those models are based on the weak
scattering theory and are limited to low scintillation values. WBMOD and GISM are theoretical models calibrated on the global
morphology of scintillation activity derived from combination of punctual experimental data on VHF and L band links,
calibration datasets do not include GNSS derived data. The most severe limitation in the comparison of scintillation models
with GNSS derived experimental data is focused on very high scintillation activity which is responsible for loss of signal lock
and consequently degrading of GPS positioning and navigation operations. So, it is very important to involve GNSS based
fluctuation data to existing theoretical model by new calibration and to develop new empirical or semi-empirical model based
on GNSS derived measurements of the ionospheric fluctuations and scintillation.
The scatter plot of HROTI
index with sum Kp. R is the
correlation coefficient.
The red line corresponds to
the best fit line.
The analysis of the ionospheric irregularities manifestation on base of daily ROTI maps for periods 2010-2013, that include
the peak of 24th solar cycle, but characterized by the moderate geomagnetic activity. The obtained results reveal that the main
features of the irregularity pattern are the ROTI index intensity and position of the irregularities oval southern border (Cherniak
et al., 2014). These parameters strongly depend on the space weather conditions such as solar wind parameters and
geomagnetic indices. For estimation of the day-to day fluctuation activity dependences (and diurnal ROTI maps) from
geomagnetic conditions, there is most convenient to use the sum of Kp index values during considered day.
As a measure of the overall fluctuation activity for selected region we proposed to use the Hemisphere ROTI index (HROTI,
daily values) that taking into account all fluctuation events from mid-latitude to auroral regions. The idea of HROTI index for
ionospheric irregularities is similar to GEC index (Global Electron Content) proposed by Afraimovich et al. (2008), when GEC
is calculated by summation of the TEC values multiplied by cell’s area. The correlation between HROTIij with daily sum of Kp
(SKp) geomagnetic index is presented on the Fig. 2. It is clearly seen the strong dependence between fluctuation activity index
and Kp. It was revealed the strong correlation (R=0.79) between SKp and HROTI, and HROTI values can be modeled using
linear predictor function (linear regression model).
In order to specify the position of the irregularities oval southern (equatorward) border we developed algorithms for
determination shape and position for southern border of the oval. These algorithms are based on the ROTI values analysis
over the whole ROTI map along selected set of cells from “South” to “North” directions. IIt was analyzed the dependences of
position of the Southern border of the ionospheric irregularities oval (SBIR) for period 2010-2014. The bottom figure presents
the SBIR position for different values of the daily sum of geomagnetic index Kp. The SBIR positions (indicated by red line)
constructed for sum Kp less than 10; from 10 to 20; from 20 to 30 and more than 30. The solid black lines indicate the
standard deviations of calculated values. These graphs illustrate that for non-disturbed conditions equal to sum Kp less than
10 all irregularities were located at the night sector and near cusp region and oval border, not crossing 70 degree, oval was
expanded to the morning sector. The averaged HROTI index for such conditions did not exceed the value of 2000. When the
sum Kp values was within 10 to 20 limits the SBIR expands to 75 degree in night sector. With increase of geomagnetic activity
the fluctuation activity is also raised (the index value is more than 4000 for Σ Kp>30) and SBIR expanded to 65º - 60º MLAT
and can cover sector from 23 to 8 MLT. For geomagnetic conditions when Σ Kp>20 the SBIR can expand to 75º - 70º MLAT
even at the day-time sector.
The Southern border of the ionospheric irregularities oval for different levels of geomagnetic activity.
The case study. It is known that structure and dynamics of the ionosphere are highly sensitive to the geomagnetic disturbance activity that occurs regularly within the Earth’s magnetosphere. Occurrence of the ionospheric irregularities is difficult to
predict and to model, because of the temporal and spatial variability of the ionosphere and solar activities that are driving forces of the space weather phenomena. The constructed ROTI maps allow to estimate the overall fluctuation activity and auroral
oval evolutions. For the case study analysis, we selected two geomagnetic storms of the new 24th solar cycle with similar intensity described by Dst index but different initial and recovery conditions.
Occurrence of the ionospheric irregularities is driven by forces of the space weather phenomena and considered case-studies demonstrate the ability to retrieve information about the ionospheric irregularities structure, position and duration. Obtained
results show clear dependence of the ionospheric fluctuation activity, described by ROT and ROTI, on solar wind and IMF parameters variations. The occurrence and magnitude of TEC fluctuations, measured using GNSS networks, increase dramatically
during space weather events. The irregularities oval expands considerably equator-ward with simultaneous increase of the fluctuation intensity. We found that during quiet ionospheric conditions the fluctuation events are mainly observed during geographic
or magnetic local night time within the circle of 70º MLat and practically are not registered in the dayside sector. The calculated position of the Southern border of the ionospheric irregularities oval indicated by black lines.
Summary
Analysis and specification of the ionospheric irregularities at the mid- and high latitudes using GNSS measurements represent very
actual task for both scientific point of view and GNSS applications, as the presence of the ionospheric irregularities can impact on different
communication and navigation systems. The spatial and temporal dynamics of the ionosphere irregularities strongly depends on geophysical
conditions. Usually GPS positioning requires a minimum of four simultaneously tracked satellites with good geometry (Misra and Enge, 2006). If
electron density irregularities cover a big area above receiver, there is a high probability that a receiver can lose more than one satellite signal
simultaneously. Simultaneous loss of a significant number of satellites signal discontinues GNSS navigation or degrades the navigation accuracy.
Therefore, strong ionosphere TEC gradients and resulted fluctuations and signal scintillations could be hazardous in terms of continuity and
availability for GNSS navigation (Forte and Radicella, 2004).
So the knowledge about morphology and dynamics of the ionospheric irregularities, dependences on geophysical factors is very
important for mitigation of the space weather effects on GNSS. The approaches based on the ionospheric irregularities mapping can effectively
solve the task of estimation of fluctuation activity levels and locations of the large scale irregularities structures. The indices and maps, based on
TEC changes, can be effective and very perspective indicator of the presence of phase fluctuations at the high and mid-latitude ionosphere.
The results of our study demonstrate that it is possible to use current network of GNSS permanent stations to reveal the ionospheric
irregularities intensity, that described by ROTI index (corresponded ROTI maps and HROTI index) and position of the irregularities oval southern
border. It was established the correlation dependences and linear regression coefficients between these parameters and geomagnetic index Kp
(daily sum Kp).
The obtained results are usable for retrospective estimation of space weather impact on positioning and for development of the new
ionospheric irregularities models.
References:
Pi, X., A. J. Mannucci, U. J. Lindqwister and C. M. Ho (1997), Monitoring of global ionospheric irregularities using the worldwide GPS network,
Geophys. Res. Lett., 24, 2283.
Beniguel, Y. (2002), GISM: a propagation model for scintillations of transmitted signals, Radio Sci., 37 (3), doi:10.129/2000RS002393.
Cherniak Iu, Krankowski A, Zakharenkova I. (2014) Observation of the ionospheric irregularities over the Northern Hemisphere:
Methodology and Service. Radio Sci. DOI: 10.1002/2014RS005433
Cherniak Iu, Zakharenkova I., Krankowski A, (2014) The approaches for ionosphere irregularities modeling on base of ROTI mapping. Earth,Planets and Space (EPS)
journal, in press.