Correlation between S4 index and Simulated ESF using SAMI3 model.

1. THE STUDY OF THE CORRELATION BETWEEN S4
INDEX AND SIMULATED ESF USING SAMI3 MODEL.
Bahir Dar University
College of Science
Department of Physics
Author:Getnet Tegenie
Advisor: Dr. Melessew Nigussie
February 19, 2019
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2. Overview
1 Introduction
2 Earth’s ionosphere
Dynamics of Equatorial ionosphere
2 Data Source and Analysis Method
2 Result and Discussion
2 Conclusion
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3. Introduction:
Background of the study
Ionosphere is the ionized part of the Earth ’s upper atmosphere. The
ionospheric electron density is the basic ionospheric parameter used to
characterize the ionosphere.It is
I is the ionized region of the Earth neutral atmosphere.
I The number density of plasma (the sum of ni and ne) affects the
propagation of radio waves
I divides into D, E, and F layers based on the composition and the
production rate with altitude
I ionization process start from the solar heating differences.
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4. Objectives and Motivation of the study
General Objectives
To investigate and characterize the correlation between S4 index and
simulated ESF using SAMI3 model.
Specific Objectives
The specific tasks of this work are:
I To determine the daily and monthly occurrence of ionospheric
scintillation, S4 index.
I To determine correlation of amplitude ionospheric scintillation (S4
index) and simulated equatorial spread F region (ESF).
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5. Motivation of the study
I Motivation of the study
The ionospheric F-region is an important medium for radio
communication. Communication is important tool around the
Earth-space environment for variety purpose.Among a variety of
parameters characterizing the F-region, the plasma density is one of
the most essential parameters that affects radio communication.
Equatorial F-region ionosphere :
has a complex dynamics
shows intense plasma density fluctuation
irregularities give rise to significant space weather effects for high
frequency radio propagation, satellite communication and navigation
systems.
Plasma instabilities by electron density gradients there
This is the reason that we are motivated to focus on the S4 index and
ESF ionosphere.
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6. Earth’s ionosphere
is the region of ionized part of the Earth’s atmosphere.
divides into D, E, and F layers based on the composition and the
production rate with altitude
ionization process start from the solar heating differences
Fore instance, the intensity of solar radiation received by the
equatorial region is maximum during the noon time than the morning
time. This means, in the equatorial region, more production of
plasma takes place at noon than morning time.
Solar radiations variations on the ionosphere affect plasma ionization
( production) rates which in turn leads to the ionospheric variations.
The ionospheric plasma density determines by the ion:
XProduction process
Xloss process
Xtransport process
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7. Ion production and loss of the Earth ionosphere
Ion production
Ionization is the process in which electrons, which are negatively
charged, are removed from a neutral atom or molecules to leave
positively charged ions and free electrons. It is the ions that give their
name to the ionosphere These are photo ionization (primary
ionization) and impact ionization (secondary ionization).
1. Primary ionization: is primarily due to solar EUV and X-ray
radiation through a process called photoionization.
1 O + photon(λ . 91nm) → O+
+ e
2 N2 + photon(λ . 90nm) → N+
2 + e
3 O2 + photon(λ . 103nm) → O+
2 + e
All the above three reactions are the main source of O+
, N+
2 , and O+
2
ions in the higher ionosphere.
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8. Ionization loss due to chemical process
Chemical reaction can cause the loss of ions and free electrons in the
ionosphere through the process of recombination [Michael, 2009].
Ionization loss due to the chemical reactions are mostly through the
processes of dissociative recombination, radiative recombination and
charge exchange.
Dissociative recombination is one of the most type that can cause
the loss of ionospheric plasma density.
Similarly, the dissociate recombination of O+
2 and NO+ in the
ionosphere are given respectively by
O+
2 + e
KO+
2
,e
−
−
−
→ O + O (1)
NO+
+ e
KNO+,e
−
−
−
−
→ N + O (2)
By far the most important loss process for molecular ions is the
so-called dissociative recombination. In the E and F regions
dissociative recombination is the most important loss process.
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9. Cont...
Radiative recombination Electrons may recombine directly with positive
ions to make neutral atoms or molecules in the reaction which leads to a
production of photon.
O+
+ e
KO+,e
−
−
−
→ O∗
+ Photon (3)
In the F2region ionosphere, atomic oxygen (O+) lost through chemical
radiative recombination reaction.
Charge exchanger: Atom interchange followed by dissociative
recombination is the principal loss in the E and F regions. For example,
atomic oxygen is lost through charge exchanging of N2 and O2 as:
O+
+ N2
KO+,N2
−
−
−
−
→ NO+
+ N (4)
O+
+ O2
KO+,O2
−
−
−
−
→ O+
2 + O (5)
Since particles exchange simply charges, the ionization density does not
changes. It increase the collision rate between atoms (molecules) and
plasma, which in turn increase the recombination rate.
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10. Dynamics of Equatorial ionosphere
The equatorial region of the ionosphere :
is strongly affected by electromagnetic (EM) forces.
the orientation of geomagnetic field is horizontal Due this ionospheric
effects on GPS signals are more pronounced in the low latitudes than
in the other latitudes.
the dual effect of ~
E and ~
B causes the occurrence of ionospheric
irregularities.
E×B drift is up ward during the daytime and down ward at night
after the plasma is lifted to greater heights and it’s structure is
fountain effect.
the plasma instability occur under the influence of gravity and
pressure gradient forces over the magnetic equator.
is highly dynamic and affects communication and navigation systems.
is Electro dynamical processes in the E-and F-regions
[Sridhara,etal.;1999].
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11. Ionospheric irregularities and ESF
The F-region irregularities are associated with equatorial
Spread-F(ESF).
Instabilities can occur in the post-sunset hours at the equatorial
F-region.
The vertical plasma drift changes the height of the equatorial F layer.
This prereversal enhancement (PRE) is associated with an enhanced
eastward electric field when the E region conductivity decreases
rapidly immediately after sunset.
ESF occurs at the bottom of the F-region due to the gravitational
Rayleigh-Taylor (GRT) instability.
The condition for GRT develops after sunset at the geomagnetic
equator when the bottom-side of the F-layer recombines with the
dense neutral atmosphere. Simultaneously, the pre-reversal
enhancement raises the F-region to higher altitude. As a result, a
sharp vertical gadient in electron density exists.
This condition of high-density plasma on top of low-density plasma is
very unstable.
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12. cont...
Figure: RT instability initiation mechanism adopted from Nigussie, M.
The density is n1 above the interface and zero below. The gravitational
force is downward which is anti-parallel to the density gradient. Jx is along
the g × B that is horizontal. This implies that Jx will be larger when n is
larger and small when n is small. Which means Jx has divergence and
charges will pile up on the edges of the small initial perturbation.
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13. E region irregularities
Figure: Schematic of the formation of the latitude variation of ionization density
in the Equatorial F region, known as the equatorial anomaly or the Appleton
anomaly.
F2-region plasma rises until it loses momentum.
Kassa et al. describe that the daytime ionosphere is characterized by
an F region electron density trough at the geomagnetic equator in
2014.
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14. Ionospheric scintillation
I is rapid temporal fluctuations in both amplitude and phase of signals.
I is caused by the scattering of irregularities.
I has large day-to-day variability.
I degrades the radio signal if ionospheric density suddenly increases or
decreases.
I can reduce the accuracy and the confidence of positioning results
[Dubey etal.;2006].
I indexes quantified as S4 for amplitude scintillation and σφ for phase
scintillation.
I indexes reflect the variability of the signal.
I The higher the S4 index, the more severe the scintillation [Doherty
etal.,2000].
I occurs in equatorial and high latitude regions of the Earth [Aquino et
al., 2009]
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15. Data Source and Analysis Method
In this study, we use two years data 2012 and 2016 from GPS receiver and
SAMI3 model. Scintillation data are stored under a file extension .scn.
The data with a file extension .scn are extracted and processed using
Rinex-TEC. In addition, We categorize the amplitude scintillation index
(S4 index) into weak, moderate and severe scintillation as follow.
0 < S4 < 0.3 ...................Weak scintillation
0.3 < S4 < 0.6 .................Moderate scintillation
0.6< S4 < 1 ....................Severe scintillation
We also use local time(LT) and universal time(UT) related as
LT = UT + 2.5Hr → for 37.40 longitude sector. The magnitude of
scintillation index can be described as follow
S4 =
r
< I >2 − < I2 >
< I >2
(6)
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16. Model Description
SAMI3 is three-dimensional global ionospheric model based on the
two-dimensional model SAMI2 [Huba et al., 2000, 2008]. The SAMI3
calculates the plasma and chemical evolution of seven ion species (H+,
He+, N+, O+, NO+, N2
+
and O2
+
) in the altitude range 85 km to
20,000 km.
A run model is found at https://ccmc.gsfc.nasa.gov website. The
inputs in the registration are email address, first name,last name,keyword,
year of study and date, AP
index(quiet(0-15),moderate(15-80),strong(80-240),extremely
strong(240-400)),radio flux, field line(150-20,000),max E ×
B(5m/s-30m/s) respectively, then finally click submit and confirm. After
here, a run code number sent to my email address. The website published
and set my run model at ionosphere/thermosphere model as
Getnet Tegenie 082118 IT 10; then click it to update plots.The inputs to
update the plots are elevation angle,azimuthal
angle,height,longitude,latitude and the quantities(number density of
electron(N e).
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17. Result and Discussion:
Temporal variation
The temporal variation of scintillation shows as how scintillation activity
varies for short period of time. In addition, we can see how scintillation is
correlated with total electron content in this short time periods.
Figure: Occurrence probability of S4 index in ranges from 0-0.3(top
left),0.3-0.6(top right),0.6-1(bottom left) and 0.3-1(bottom right) versus local
time on 8 April 2012.
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18. cont...
Figure: Occurrence probability of S4 index in ranges from 0-0.3(top
left),0.3-0.6(top right),0.6-1(bottom left) and 0.3-1(bottom right) versus local
time on 17 April 2016.
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19. Correlation between S4 index and vertical total electron
content
Figure: Relation between S4 index and VTEC on March 26 in 2016.
Based on the figure, S4 index is more dependent on the extent of the
fluctuation of the vertical total electron content rather than it’s
magnitude. Top panel show the variation of vertical total electron content
as a function of time.By the time TEC shows smooth variation between 5
and 15LT. The bottom panel show us the variation of S4 index as a
function of time.
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20. The dependence of S4 index on elevation angle
Figure: Graph of S4 index versus elevation angle on April 6 in 2016.
S4 index is high when the satellite elevation is low and vice versa. These
may be because of a signal from a satellite at low elevation angle covers
long distance in the ionosphere. The probability of the signal to pass
through many irregular regions is higher in this case than a signal that
comes from a satellite at high elevation angle. For most low elevation
angles below 200, S4 index is greater than 0.5. This indicate us that the
probability of the occurrence of S4 index is increased as the elevation angle
extremely decreased.
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21. Correlation between S4 index and simulated ESF
Figure: (Top panels, from left to right) shows Simulated ESF at 10:00:10 UT and
17:13:31 UT respectively and the bottom panels,from left to right shows
simulated ESF at 18:13:08UT and S4 index as a function of Local time
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22. Comparison of S4 index occurrence in 2012 and 2016
1. Comparison of S4 index in January 2012 and 2016
Figure: Severe S4 index occurrence in January 2012 and 2016 .
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23. 2. Comparison of S4 index in March 2012 and 2016
The occurrence of scintillation index is quite different between 2012 and
2016 in March. However, their maximum occurrence percentage is closer
each other. That is observed around 21.17LT on March 6/2016 and from
18LT to 21LT in March 2012 with the same level of color bar around 35.
Severe scintillation is a one-day activity in March 2016.
Figure: Severe S4 index occurrence in March 2012 and 2016.
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24. 3. Comparison of 2012 and 2016 in April month
Figure: Severe S4 index occurrence in April 2012 and 2016.
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25. 4. Comparison of 2012 and 2016 in June month
Figure: Severe S4 index occurrence in June 2012 and 2016
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26. Conclusions of this study
1 Most of the time, Severe scintillation index is observed after sunset in
both 2012 and 2016 at Bahir Dar statation.
2 The maximum electron number density lifted to a higher altitude near
post sunset time than a day time as soon as S4 index is increased. It
might be due to the interaction between vertical upward density
gradient and force of gravity. These interactions may create electron
density instability which may in turn affect S4 index.
3 The dual effect of electric field and magnetic field play a prominent
role for the upward motion of plasma to higher altitude. This in turn
may cause plasma instability.
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27. Cont...
1 the occurrence probability of S4 index is dependent on electron
density irregularity(fluctuation).
2 But S4 index does not depend on the magnitude of electron density.
3 S4 index increases as on the altitude of maximum electron density
increases.This implies that S4 index and ESF are positively correlated.
4 Electron density irregularity(fluctuation) is high after sunset. This
might be due
I). the E region conductivity decreased rapidly immediately after
sunset.
II). gravitational growth rate develops after sunset at the geomagnetic
equator.
III). recombination reaction with neutral atmosphere increased below
F-region.
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28. The End...
Presented By: GETNET TEGENIE
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