4-D observation of traveling ionosphericdisturbances using a dense GPS
receiver array
By
Nicholas Ssessanga, Yong Ha Kim, Eunsol Kim
Chungnam National University
Dept. of Astronomy and Space Science
Daejeon, South Korea

Introduction
 Travelling Ionospheric Disturbances (TIDs) are wave-likestructures observed in the Earth’s plasma density.
 Signatures in the ionosphere of: Atmospheric gravity waves (AGW) due to geomagnetic
activity at high latitude, (as a result of Joule heating,Lorentz forces or particle precipitations, .e.g.,
Valladares et al., 2009; Ding et al., 2007).
 Atmospheric gravity waves propagating from low –upper atmosphere (associated with atmospheric tides,
tropospheric weather, volcanic explosions,earthquakes, rocket launches).
P. statement

Classification:
 Small-Scale (SS TIDs)
 Period: 1-10 min
 Horizontal wavelength 0.1 to 1 km
 Medium-Scale TIDs (MS TIDs)
 velocities from 50 to 300 m/s
 Period: 20 - 60 min (e.g. Hunsucker, 1982, Tsugawa et al. 2007)
 Horizontal wavelength: < 1000 km.
 They are confined to mid and high-latitude zones (Bruinsma and Forbes,2008; Mayr et al., 1990).
 Large Scale TIDs (LS TIDs)
 400 – 1000 m/s
 Period: > 1 hr
 Order of thousands of kilometers in horizontal scale.

• Focus is on MSTIDs that are characteristic
wave phenomenon in the ionospheric F-region
at mid-latitudes.

Problem Most studies have been limited twodimensional imaging (2-D Total ElectronContent, TEC, maps that infer information on
the horizontal structuring of the electrondensity)
An example of a 2D TEC map.

Mainly:Due to the paucity of Global
Positioning System (GPS) receiversand limited projection angles.
• Where, projection angle is the line
integral along a given view.

Luckily, Japan has a GPS Earth Observation Network(GEONET) of more than 1000 receivers (with averagedistance between two neighbouring points being 2530 km).

Investigation(Tomography)
The possibility of using a MART andCalibrated IRI model to observed TIDs

 With multiple ground stations each being able to “see” 6-8 GPS satellites at time,
tomography is possible.
To do this, we set up a three-dimensional grid of voxels (i.e. volume pixels), eachbounded in latitude, longitude and altitude.

12...

Ionospheredivided into voxels

Earth Compute the length of each element of a satellite-to-receiver signal
propagation path though each intersected voxel.

continued
The contribution to the line integrals in eachvoxel is decomposed into a matrix, A.
Given the measurements of these line
integrals, m (Slant-TEC), the problem
becomes one of inverting A to solve for theunknown electron concentration, x.
Ax = mHowever, the inverse problem is ill-posed andill-conditioned.

To solve the inverse problem
we use the MART (Multiplicative Algebraic
Reconstruction Technique)λk is the relaxation parameter and controls theconvergence of the algorithm, is bounded between 0 and 1(Pryse et al., 1998; Raymund et la., 1990).
Advantage: Low memory requirements , non negativeelectron density values.
However, MART is sensitive to the initial guess (xo),>>>> IRI-2012 modelCalibrated IRI-2012(Cal-IRI-2012 )The most recent version, IRI-2012, is used.
Two input indices, Sun Spot number (SSN)and Ionospheric index (IG12), are adjusted inrelation to derived GPS vertical TEC (VTEC).
Only a few stations within the grid are need toreach the optimal solution.

Algorithm

foF2 analysis
 Use TEC to adjust foF2values

Percentage improvement = 50.5%*Regional optimization of IRI-2012 output (TEC, foF2) using derived GPS-TEC (Ssessanga
Nicholas and Yong Ha Kim., In press)We use Cal-IRI-2012 (Xo) as the initial guess.
Hence:
Quick convergenceA better solutions for resolving small scalestructures.

SetupA plot of utilised stations
Grid size
Latitude: 20o N : 1o : 50o N
Longitude: 120o E : 1o : 150o E
Height: 100: 10 :1000 Km
Time resolution: (15 min)utilised stations: 700

Day of Analysis

2D dTEC Maps, 14-16, July 2012

The selection was done through a visual examination of these sequences of maps. If there
were TIDs passing by, there would be regularly moving band-like structures.
The perturbation components of TEC values were derived by subtracting a trend of theTEC values that is a 30-min running average.
 The data from satellite receiver paths with low elevation angles below 30o were notincluded. (using a program a from NICT)Results

Spatial representation
NESW
 Slice a vertical plane with most number of data points

Smooth Ionosphere - Cal-IRI
2012
SWResults from algorithmNE 15 minute resolution , spatial representation
The ionosphere is quite moderately modulated by the TIDs

SWSW

NE

NE

SWNE

• In local time = UT + 9

SWNE• In local time = UT + 9

In local time(LT) = UT+9In UT

In local time = UT + 9

Spatial period ≈ 333.6 Km

Time series variation ateach point

Choose two points along
the planeNE37.8 N, 140 E

34.3 N, 135 ESW
 Distance between the two points = 594.4 km
 Use cross correlation to determine time lag = 3436.6s
 Velocity of TID = [ Distance/ time lag ] = 173 m/s

Take FFT of the two signals
and determine average period
Period = 48 minutes

Period = 51 minutes

Average period ≈ 49.6minutes

Medium-Scale TIDs (MS TIDs)
velocities from 50 to 300 m/s (≈ 173 m/s)
Period: 20 - 60 min (≈ 49 min)
Horizontal wavelength: < 1000 km (≈ 333.6 Km)

• Summary: This investigative procedure has provided
promising results,
 that a dense network of GPS receivers could be
used to infer further information about thevertical structure of TIDs.

THANKS