Total Electron Content (TEC) Variability of Low Latitude Ionosphere and Role of Dynamical Coupling: Quiet and Storm-Time Characteristics
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Date
2014-01
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Addis Ababa University
Abstract
The low latitude ionosphere and its coupling to lower atmosphere such as (mesosphere,
stratosphere and thermosphere)and high latitude ionosphere are investigated using
observational and model data. The dominant modes of variability of low latitude
ionosphere are linked to Equatorial Electrojet (EEJ), Counter Electrojet(CEJ) and
Prereversal Enhancement(PRE). These are investigated using magnetometer data and
EEJ models. The results of the analysis reveal that EEJ undergoes day and seasonal
variations. From the daily variation analysis, the EEJ value at Addis Ababa sharply
increases to its peak value from 9.00-13.00 LT and dies out around 18.00 LT. EEJ
versus time graphs shows that the H-components of the magnetic eld at Addis Ababa
location varies by a factor of 2-4 times larger compared to Adigrat. Seasonally as
both measurements and model data revealed, EEJ has signi cantly increased during
E season compared to D and J seasons. Our observation also shows that counter CEJ
are more frequent on solstices than equinoxes and on the contrary Morning Counter
Electrojets (MCEJ) occurs more frequently on equinoxes than solstices. Apart from
that the localized EEJ, CEJ and PRE, low latitude ionosphere is dynamically coupled
to lower atmosphere and high latitude ionosphere that control both low latitude
ionospheric dynamics and chemistry during both quiet and storm-time. In this regard,
the storm-time low latitude ionospheric total electron content (TEC) variability and
the in
uences of lower and high latitude ionsophere are investigated.
The TEC variation is characterized in relation to diurnal, seasonal, temporal, spatial
and longitudinal behaviors. The diurnal characteristics of TEC can be explained
in terms of the Diurnal, Semidiurnal,Terdiurnal and Quterdiurnal tides. The TEC
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climatology from NeQuick and ionosonde is also compared during both quiet and geomagnetically
disturbed days. Day time enhanced TEC value followed by night time
depilation is a typical behavior of the ionosphere.
A geomagnetic storm occurred during January 22-25, 2012. The Disturbance
storm time (Dst) values for the period shows that it is a moderate intensity geomagnetic
storm. The geomagnetic storm is characterized by di erent indices and
indicators. The Symmetric H component of ring current (SYMH) value on January
22 shows a sudden increase to more than 50 nT at the Sudden Storm Commencement
(SSC) and followed by sharp decrease to a value of -100 nT after which a
recovering started. A second SSC on January 24 followed by a shock on January
25. These SSCs before the main storms on January 22 and 25 are evidences for the
occurrence of Coronal Mass Ejection (CME). Moreover, the short recovery period,
unlike that of Co-rotating Interaction Regions (CIR) driven geomagnetic storms, implies
the geomagnetic storm is a CME-driven. The sudden jump of the solar wind
dynamic pressure and the The southward Interplanetary Magnetic Field (IMF Bz)
are also consistent with occurrence of CME. Our observational evidences of Proton
uxes of high energy ranges and increase in proton density are also other indicators
for occurrence of CME. The high values Auroral Electrojet (AE) index on these
dates implies ionospheric perturbation in response to the storm. This is also re
ected
in the Total Electron Content (TEC) change during the storm relative to quiet day
TEC over the polar regions. The response of the ionospheric to geomagnetic storms
is also investigated from amplitude modulation of wave components that account for
the majority of TEC variance during the period. The diurnal and semidiurnal TEC
variances account upto a maximum of 83% and 30% of the TEC variance over fairly
exclusive ionospheric regions respectively. The diurnal variability dominates the subtropical
latitude where the solar cycle is an important factor while the semidiurnal
variance is limited to higher latitudes. The diurnal and semidiurnal TEC variances
show hemispheric asymmetries. The stationary planetary waves also account for TEC
variances that exceed semidiurnal TEC variance and exhibit hemispheric asymmetry
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of opposite sign to diurnal TEC variance. These features of TEC variance are climatology
of TEC variability irrespective of the storms. However, the impact of the
geomagnetic storms are distinctly marked in the daily time series of amplitudes of
diurnal and semidiurnal migrating tides and stationary planetary waves. The abrupt
changes in amplitudes of diurnal (upto 5 TECU) and semidiurnal (upto 2 TECU)
migrating tides are observed within the 20oS-20oN latitude band and along 20oN respectively
while that of stationary planetary wavenumber 1 is in the order of 3 TECU
and is mainly localized along 20oS
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Keywords
Role of Dynamical Coupling