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49
CHAPTER 4
THE EFFECT OF CME ON THE TERRESTRIAL ATMOSPHERE
4.1 Introduction
Investigation of the effects of solar and interplanetary (hemispheric) events on near -Earth
space is one of the most important components of solar–terrestrial physics. In spite of the
large amount of experimental and theoretical data that has been accumulated, the prediction of
effects of the space weather faces serious difficulties. Although it has been appreciated for
some time that solar wind disturbances drive major geomagnetic activity, there is now a better
understanding of the solar causes of these disturbances. “The identification of Coronal Mass
Ejections in the sun as the primary source of the most geo effective disturbances has given
new impetus and inspiration to the space weather forecasting” (Luhmann, 1997). Only after
the advent of the coronagraphs on board the Solar and Heliosphe ric Observatory (SOHO)
mission (Domingo et al. 1995) the connection to Coronal Mass Ejections near the sun became
evident, especially those affecting earth‟s space environment.
Coronal mass ejection is a huge outbur st of solar mass into the interplanetary space.
Mostly Coronal Mass Ejections are associated with solar flares, but such a causal relationship
has not yet been established fully. Mostly Coronal Mass Ejections originate from
magnetically active region in the solar disk and the process of magnetic reconnection results
in the emission of huge amount of matter and electromagnetic radiation into space. Magnetic
reconnection is the process wherein two oppositely directed magnetic fields merge
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
and an associated rearrangement occurs. The energy stored in the earlier oppositely aligned
field lines is emitted out as intense electromagnetic waves and a portion of the sun‟s magnetic
field which is in the form of closely spaced loops may become totally unconnected forming
helical loops. These loops along with the material they contain expand violently to the outer
space forming the coronal mass ejections. If the Coronal Mass Ejections are directed towards
earth, then their influence on the earth‟s magnetic field will be tremendous. The Coronal Mass
Ejection associated particles like protons and electrons, whic h are carried by the solar wind,
enter into the earth‟s magnetosphere thereby setting up current systems and modifying the
ambient electric fields of the earth leading to modification of the existing various structures.
This phenomenon is called the geomagnetic storm. The solar energetic particles having
energies of the order of keV to MeV cause strong auroras in the polar region which is caused
due to the particle entry into the earth through polar cusps. Once CME is ejected by the sun,
ideally it takes one to five days to reach the earth depending on the velocity of streaming
particles. If the solar wind velocity is larger than CME velocity, it accelerates the CME
particles and if CME velocity is higher than solar wind velocity it is decelerated. Very fast
CMEs can drive a shock wave if their velocities are faster than the sonic speed.
Large CME driven interplanetary disturbances are usually preceded by strong shocks that are
effective accelerators of particles and sources of radio emission. The enhanced S EP
production at stronger or faster shocks driven by the fast CMEs, can exceed the Alfven and
flow speeds of the solar wind (Zank et al. , 2000). Early studies established that there is a
rough correlation between the logs of the CME speed and the logs of the SEP intensities. The
outermost structure of fast CMEs is an MHD shock, which will first interact with the
preceding CME. The shock has to pass through the heterogeneous multi thermal plasma
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Chapter4 The Effect of CME on the Terrestrial A tmosphere
(core, cavity, and frontal of the preceding CMEs). Thus, the shock has to accelerate the SEPs
from the solar wind "contaminated" by the preceding CMEs, rather than from the quiet solar
wind. The interaction rather than the speed of the preceding CMEs seems to be important for
the SEP production (Gopalswamy et al. , 2002). Gopalswamy (2002) concluded that the
efficiency of the CME-driven shocks is enhanced as they propagate through the preceding
CMEs and that they accelerate SEPs from the material of the preceding CMEs rather than
from the quiet solar wind. Thus the discovery of CMEs and their roles in driving
interplanetary shocks changed the focus of our understanding of SEP sources.
The relation between solar energetic protons and coronal mass ejections was studied
more directly by Kahler et al. (1978) who suggested that CMEs are required for proton events.
Solar proton events (SPE), also known as polar cap absorption (PCA) events in the history of
radio physics, begin as emission of electrons and ions from the surface of the Sun. The ions
are mostly protons (≈ 90%) but heavier particles are also emitted, the relative abundances
being similar to those in the solar corona. For the most energetic coronal mass ejections
(CME), particle energies can be up to MeV or even GeV level, thus far exceeding the normal
solar wind values,
The acceleration of emitted particles is driven by processes related to the solar flare
accompanying the CME, and/or by solar wind shock fronts (Cane and Erickson, 2003).Even
particles having GeV energies are guided by the interplanetary magnetic field (IMF) over the
sun-earth distance. Therefore, the emitted particles will follow the spiral field lines of IMF,
and the location of the CME on the solar surface will determine wheth er or not the released
particles will hit the earth‟s magnetosphere. Also, the guidance of IMF results in some
additional time delay between the CME and the arrival of particles on earth. Typically, a
delay of several hours is observed. Particles having different energies have different Larmor
radii, and the particles with lower energies are more sensitive to the form of IMF. As a result,
the bulk velocity of the particle “cloud” is much less than that of individual particles,
e.g. ∼ 1 keV for protons.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
which can be used to explain the duration of near-earth effects, typically of the order of days,
and the isotropic angular distribution of the particles near the earth(Verronen et al., 2002).
In order to reach the atmosphere of earth, the particles have to interact with the
magnetosphere, at various high energies. The magnetosphere trajectories of high -energy
particles can be calculated using the Störmer theory (Störmer, 1950). According to the Störmer
theory, every geomagnetic latitude has a cut-off limit which the rigidity of an incoming
particle (defined as momentum per charge) must exceed for it to reach that particular location.
Penetration to lower latitudes requires higher rigidities, and certain latitude is a ffected by
particles having rigidity equal to, or higher than the corresponding cutoff. The cut - off rigidity
varies spatially and also with time, being dependent on the IMF as well as on the earth‟s
internal magnetic field, and on timescales from minutes to years. The magnetic storms, for
example, tend to compress the magnetosphere and lower the cutoff rigidity for given latitude.
As a consequence of geomagnetic cutoff, the particles are able to affect atmosphere above
certain magnetic latitude, covering the polar cap regions in both hemisphere. Typically
latitudes above about 60◦ are affected more or less uniformly, although the effects have
sometimes been observed near the geomagnetic poles first, and later throughout the polar cap
(Buonsanto, 1999).
Ionospheric storms are closely associated with geomagnetic storms. The disturb ances,
when affecting the ionosphere are known as ionospheric storms; tend to generate large
disturbances in ionospheric density distribution, total electron content, and the ionospheric
current system. Ionospheric storms have important terrestrial consequences such as disrupting
satellite communications and interrupting the flow of electrical energy over power grids.
Buonosanto (1999) has indicated that ionospheric storms represent an extreme form of space
weather with im portant effects on ground and space-based technological systems. The
ionosphere is an open system that strongly couples with the magnetosphere and thermosphere.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
Various photochemical, chemical reactions, dynamical and electro dynamical processes in the
system exchange and transport mass, momentum and energy in a complex manner (Rees,
1989). The SPEs provide a direct connection between the sun and the earth‟s middle
atmosphere. Ionospheric disturbances during solar proton events have been observed almost
solely by radio techniques, especially by riometers and incoherent scatter radars.
The earth‟s ionosphere, responds markedly to varying solar and magnetospheric
energy inputs. During geomagnetic storms, the disturbed solar wind com presses the earth‟s
magnetosphere and intense electric fields are mapped along geomagnetic field lines to the
high latitude ionosphere. At times these penetrate to low latitudes, and at high latitudes they
produce a rapid convection of plasma which also drives the neutral winds via collisions. At
the same time energetic particles precipitate to the lower thermosphere and below, expanding
to the auroral zone, and increasing ionospheric conductivities. Intense electric currents couple
the high latitude ionosphere with the magnetosphere and the enhanced energy input causes
considerable heating of the ionized and neutral gases. The resulting uneven expansion of the
thermosphere produces pressure gradients which drive strong neutral winds. The disturbed
thermospheric circulation alters the neutral composition and moves the plasma up and down
magnetic field lines, changing rates of production and recombination of the ionized species.
At the same time the disturbed neutral winds produce polarized electric f ields by a dynamo
effect, as they collide with the plasma in the presence of the earth‟s magnetic field. These
electric fields in turn affect the neutral and plasma alike, illustrating that the ionized and
neutral species in the upper atmosphere are closely coupled, so it is not possible to attain a
physical understanding of geomagnetic storm effects on ionospheric electron density without
considering the effects on the neutral thermosphere (Buonsanto, 1999; Gopalaswamy,
2011).This chapter however concerns with the effect of CMEs and geomagnetic storms in the
terrestrial ionosphere over equatorial la titudes. The ionospheric response to solar flares has
been widely studied with multiple instruments since the 1960s.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
This work presented in the following sections investigates the effects of CMEs on the
equatorial ionosphere through analysis of the correlation between CME parameters (linear
speed and angular width) with ionospher ic parameters (h‟F i.e. ionospheric base height as
inferred from ground based ionosonde measurements and electron density). The correlation of
CME parameters (linear speed and angular width) with the solar wind speed is also discussed.
4.2 Data and Methodology
The severest of geomagnetic storms and the largest of solar energetic particle (SEP)
events have been shown to be caused by energetic CMEs. The October – November 2003
period produced a large number of energetic CMEs, two of which were of historica l
importance. So study concentrated on the effect of CME parameters (linear speed and angular
width) observed by the LASCO coronagraph on the Solar and Heliospheric Observatory
(SOHO) during the months of October and November 2003 with the F region equato rial
ionospheric parameters.
These equatorial F-region ionospheric parameters were obtained from the ionospheric
measurements done using ground based ionosonde located at Thiruvananthapuram, a dip
equatorial station in India. The rationale for using the ionosonde data into the present thesis is
the following:
The terrestrial ionosphere is the direct outcome of the interaction of incoming solar
radiation with the various neutral species present in earth‟s upper atmosphere. These neutral
species get ionized as an outcome of this interaction and lead to the formation of ionosphere.
The distribution of ionosphere with altitude, latitude and longitude depends not only on the
intensity of the solar radiation and abundance of neutral constituents, but also on the
orientation of the geomagnetic field configuration. Typically, the altitudinal distribution of
ionospheric plasma exhibits the presence of two layers namely E -region and F-region, the
former lying in the ~80-140km altitude region while the latter above it. The ionospheric
altitudes and corresponding densities are obtained through the ionosonde measurements. In
making these measurements, radio waves in a range of frequencies are sequentially
55
transmitted vertically up from ground who‟s reflected echoes from ionosphere are received
after a time delay. The time delay gives ionospheric altitude and frequency of the wave
electron density at that altitude. The base height of the F -region is typically reffered to as h‟F.
These ionosonde measurements of the equatorial ionosphere are being made systematically
from Trivandrum. In view of the above, the terrestrial ionosphere is known to respond
immediately to the sudden radiation increase during solar flares manifesting as enhancement
in the overall ionization. As the CMEs are usually followed by the onset of geomagnetic
storms, the ionosphere responds to CMEs therefore after a time delay.
In this context , the ionospheric F -region height (h‟F) and electron density for the
same period were obtained from the ground-based ionosonde at Trivandrum, a dip equatorial
station in India, and solar wind speed data (hourly averages) from SOHO CELIAS Proton
Monitor. For comparison June (northern solstice), September (southward equinoxes) &
October 2004 data were also used for analysis.
The most familiar measure of dependence between two quantities is the "Pearson's
correlation coefficient”. CMEs typically reach Earth one to five days after leaving the sun. So
correlation coefficients were calculated at different time delays ranging from 3 to 96 hours
with the following CME parameters (linear speed and angular width) against ionospheric
parameters (h‟F, Electron density) and solar wind speed.
4.3 CME Link to Severe Geomagnetic Storms
It has already been established that there exists a good relationship between monthly averaged
maximum CME speeds and sunspot numbers, Ap and Dst indices (correlation coefficients are
0.76, 0.68, -0.53 respectively) (Gopalswamy, 2011). Statistical studies have shown that the
severest of geomagnetic storms (Dst < -150 nT) are always caused by CMEs. The CME link
to the geomagnetic storms is evident from the empirical relationship between Dst and the
product of Bs (nT) and the speed V (km/s) of the ICME structure (Gopalswamy, 2011).
Dst = - 0:01VBs – 32
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
CMEs and associated shocks arrive on earth as magnetic clouds. It was observed that
magnetic clouds and their shocks have large effects on magnetosphere activity, and they are
one of the main causes of intense geomagnetic storms (Echer et al., 2005). CME- CME
interactions are expected to be a frequent phenomenon in the Sun-Earth space during solar
maximum. The typical transit time of CMEs from the SUN to 1 AU is a few days, much
longer than the time within which multiple CMEs occur at solar maxim um(Ying et al., 2013).
The solar events of October and November 2003 were extraordinary, with the largest so lar
flare ever recorded, solar wind speeds at earth approaching 2000km/s. Twelve X - Class flares
were observed from 19 October to 4 November from 90 0E to 830 W on the solar surface
(Richardson et al., 2005; Malandraki et al., 2005).
4.4 Observed Ionospheric Variability
October-November 2003
Figure 4.1 shows the time variations of the geomagnetic indices such as Solar wind speed,
Dst, and F10.7 solar flux for various months considered in this investigation during 2003 and
2004.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
Figure 4.1 shows the variations of solar wind velocity (top panels), Dst (middile panels) and F10.7cm solar flux
(bottom panels ) for different months during the year 2003 and 2004. The label on X -axis indicates the
period/months
As can be seen from the Dst variability, the -100nT which continued with a slow recovery till
October 28 only to mark the beginning of a great storm through a sudden and significant
decrease to about -400nT in Dst around October 30-31.
Figure 4.2 presents the variability of ionospheric F -region base height (Panels b and c), the
color scale indicating altitude in kilometers, and the peak electron density (Panels a and d)
from October to November 2003, the color scale indicating the frequency corresponding to
peak ionospheric density of F-region.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
It can be seen very clearly that the ionospheric density has undergone an overall
Figure 4.2: The figure in panels a and d depicts the variability of peak electron density of Ionospheric F -region
for months November and October 2003 respectively. The panels b and c show the ionospheric base height
variations for November and October 2003 respectively. The white patches indicate no data
enhancement, especially in the afternoon hours, beyond October 13 which persisted till
November 10. However, the density enhanced once again beyond November 20. While the
corresponding base height changes are observed to be small during the noon hours, there
appears to be significant day-to-day changes in them in the post evening hours around 20:00
hrs. Figure 4.1 shows the time variations of the geomagnetic indices such as Solar wind speed,
Dst, and F10.7 solar flux during the June, September and October months of 2004. As can be
seen from the Dst variability, the ring current variability had been moderate (around -20-
30nT) throughout with no major decreases in it indicating the onset of geomagnetic storm.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
September - October 2004
This indirectly indicates that the solar wind conditions had not changed very dramatically
during the above mentioned period. The prevailing ionospheric variability also ref lects this
aspect of prevailing quiet space weather conditions.
Figure 4.3: The figure in panels a, f and e depicts the variability of peak electron density of Ionospheric F -region
for months October, September and June 2004 respectively. The color scale represents frequency corresponding
to peak F-region ionospheric density. The panels b, c and d show the ionospheric base height variations for
October, September and June 2004 respectively. The color scales here represent the base height in kilometer of
F-region.The white patches indicate no data
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
Figure 4.3 depicts the variability of ionospheric electron density (Panels a, f, e) and base
height for (Panels b, c, d) for the June, September and October months of year 2004. It can be
seen very clearly that the ionospheric density on the whole are smaller than that observed
during Oct-Nov 2003. An enhancement is seen on a day-to-day basis in the morning and
evening hours with a minimum around noon. The corresponding base height changes are
observed to be small on the whole during the noon hours. However, as seen during Oct -Nov
2003 period, there appears to be significant day-to-day changes in them in the post evening
hours around 20:00 hrs during September-October while an absence of such a height change
during month of June is quite conspicuous.
It is clear from the comparison of ionospheric variability during 2003 and 2004 that during the
latter year the observed electron density changes are primarily governed by the large scale
process called Equatorial Ionization Anomaly (EIA) wherein the equatorial F -region
ionospheric density exhibits enhancement in the morning and evening hours with a minima
during noon (Rishbeth, 2000). The magnetospheric influences are not observed in the response
of the ionosphere. However, during the former year, the EIA appeared to have been
influenced by the ongoing magnetospheric/geomagnetic activity. Since, the geomagnetic
perturbations as inferred through the Dst had their origin in the activity on the sun i. e. the
CMEs from the sun, it was expected that the CMEs would have a bearing on the ionospheric
variations during the year 2003 which would appear as a correlation between parameters
representing CME and the ionosphere. As is known, the geomagnetic storms have a delayed
effect in the equatorial ionosphere; a time delayed correlation analysis was performed using
CME linear speed & angular width and base height of F -region of ionosphere and electron
density. It is observed that the inferred correlation coefficient varies significantly with time
delays. Table-1 to 4 summarizes the observed time-delays and the correlation coefficients for
various months used in this study. The variations of the inferred correlations with various time
delays are presented and discussed in the following sections. The inferred time delays would
indicate the time taken by the terrestrial ionosphere-magnetosphere system to respond to the
CME forcing so as to reflect in the electron densities through changes in the ionospheric
processes.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
4.5 Correlation of CME linear speed with Ionospheric height and Electron density
Coronal mass ejections reach velocities between 20km /s to 3200 km/s with an
average speed of 399 km/s, based on SOHO/LASCO measurements between 1996 and
2008. W ith following table we can determine how long the CME will take to travel from
the sun to earth providing it does not slow down along the way.
Table 4.1: Travel time from the sun to earth for CMEs with different velocity
CME speed (km/s) Travel time (hours)
500 83.33
1000 41.67
1500 27.78
2000 20.83
2200 18.94
Gopalswamy et al. (2005b) studied the violent solar eruptions of 2003 October and
November, invoking fast CMEs, X-class solar flares, interplanetary shocks, intense
geomagnetic storms, and SEPs. These violent solar eruptions of October/ November 2003
period can be regarded as extreme events in terms of their origin as well as their
heliospheric consequences. The two earth-directed full halo CMEs produced intense
geomagnetic storm (Dst index of -363 and -401 nT for the 28 and 29 october CMEs,
respectively. Two of the halo CMEs resulted in shocks that a rrived at earth in <24 hrs.
Bhatt et a l. (2013) investigated the spectral behavior (hardness parameter β) of an SEP
event and its relation with the dynamics of the associated CME and arrived at a power-law
with a correlation coefficient of ~0.96. Tripathi et al. (2013) made a statistical study on the
evolution of solar proton events simultaneously with the dynamics of associated CMEs
and found the linear relationship between the solar proton spectral index and initial CME
velocity with fairly good correlation coefficiel (0.71). So a time delayed correlation is
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Chapter4 The Effect of CME on the Terrestrial Atmospher
carried out between CME parameters (linear speed and angular width) observed by the
LASCO coronagraph on the Solar and Heliospheric Observatory (SOHO) during the
months of October and November 2003 with the F region equatorial ionospheric
parameters. For comparison June, September and October 2004 are also analysed.
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Figure 4.4: Results of linear correlation between CME linear speed and ionospheric base height by giving
different time delays. The left column of panels show results of October, November 2003 and the right column
of panels show results for June, September and October 2004.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
Fig.4.4 shows the change in correlation coefficient between the CME linear speed and
ionospheric base height for varying time delays for five months. Table 4.2 summarizes the
observed time-delays and the correlation coefficients for various months used in this study.
The highest correlation coefficient obtained is 0.6 for a time-delay of about ~8 hrs for
October 2003. During the m onth of October 2003, most of the CMEs are fast with an average
speed of 749 km/s. The relation between the SEP event a nd the dynamics of the CME are
considered as responsible for the greater correlation in October 2003. Though the average
speed of CME is 841 km/s, the overall proton flux was substantially small during the Solar
Proton Event on November 3. In June, September and October 2004 the average speed of
CME decreased and no Solar Proton Events were recorded.
Table 4.2 : Correlation analysis between CME linear speed and Ionospheric height
Month & year Maximumcorrelation coefficient Average CME
Speed(km/S)
SPE Proton
Flux
(pfu>10MeV)
October 2003 0.5971 for ~ 8 & 72 hrs 713 29500
November 2003 0.2485 for ~ 8 & 24 hrs 841 1570
June 2004 0.1886 for ~ 85 hrs 425 Nil
September 2004 0.2805 for ~ 24 hrs 463 273
October 2004 0.2543 for ~ 42 hrs 305 Nil
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
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Figure 4.5: Results of linear correlation between CME linear speed and electron density by giving different time
delays. The left column of panels show results of October, November 2003 and the right column of panels show
results for June,September and October 2004.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
Figure 4.5 depicts the variation of observed correlations between CME linear speed and
electron density for different months. The observed correlations have been found to be lying
in the range -.2 to .3. These correlations are found to be insignificant.
4.6 Correlation of CME angular width with ionospheric height and Electron density
One of the basic requirem ents for CMEs to produce a geomagnetic storm is that they should
hit and interact with earth‟s magnetosphere. Statistical studies have shown that CMEs
generally propagate radially above the source regions. As a consequence, the CMEs need to
originate close to the centre of the solar disk in order to arrive on earth (Gopalswamy
2011).The width of CMEs causing geomagnetic storms typically exceed 600. So the solar
sources need to be located within a central meridian distance (CMD) ~ 300. When the CME
originates beyond ±300
meridian, only a small section of the CME might arrive on earth or
none at all, depending on the angular extent of the CMEs. Thus the CME angular width also
contributes a major role. In the present study, the time-delayed correlation analysis between
CME angular width and ionospheric height has also been carried out. Figure 6 shows the
inferred correlations and corresponding time-delays for different months.
Table 4.3 : Correlation analysis between CME linear speed and Electron density
Month & year maximum correlation coefficient
October 2003 0.1520 for ~ 42 hrs
November 2003 0.2867 for ~ 22 hrs
June 2004 0.2853 for ~ 72 hrs
September 2003 0.2946 for ~ 32 hrs
October 2004 0.3341 for ~ 24 hrs
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
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Figure 4.6: Results of linear correlation between CME angular width and ionospheric base height by giving
different time delays. The left column of panels show results of October, November 2003 and the right column
of panels show results for June, September and October 2004.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
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0.30.40.5
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Figure 4.7: Results of linear correlation between CME angular width and electron density by giving different
time delays. The left column of panels show results of October, November 2003 and the right column of panels
show results for June, September and October 2004.
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
From the figure, it is clear that there exists a correlation between CME angular w idth and
ionospheric h‟F though the overall correlation coefficients are small. Maximum correlations
of .3 to .5 were observed for October 2003 for time delays of ~ 8 hrs and ~ 70 hrs. Table 4.4
summarizes maximum correlation coefficients for various months. The variation of the
observed correlation coefficients between CME angular width and electron density is
presented in figure 4. 7. Interestingly, the observed trends are found to be more or less similar
for all the months. Nonetheless, the observed correlations are found to be not more than 0.3
for any month indicating that the correlations may be insignificant.
Table 4.4 : Correlation analysis between CME Angular width and Ionospheric height
Month & year maximum correlation coefficient
October 2003 0.6559 for ~ 4 hrs
November 2003 0.2421 for ~ 32 hrs
June 2004 0.2211 for ~ 72 hrs
September 2004 0.4274 for ~ 32 hrs
October 2004 0.3275 for ~ 55 hrs
Table 4.5 : Correlation analysis between CME Angular width and Electron density
Month & year maximum correlation coefficient
October 2003 0.2216 for ~ 24 hrs
November 2003 0.2150 for ~ 22 hrs
June 2004 0.1792 for ~ 24 hrs
September 2004 0.1711 for ~ 10 hrs
October 2004 0.2893 for ~ 24 hrs
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
4.7 Correlation of CME Linear Speed and Angular Width with Solar W ind Speed
Solar wind is a stream of charged particles (a plasma) released from the sun. This
stream constantly varies in speed, density and temperature. The most dramatic difference in
these three parameters occurs when the solar wind escapes from a coronal hole or during a
coronal mass ejection. A stream originating from a coronal hole can be seen as a steady high -
speed stream of solar wind as where a CME is more like an enormous fast moving cloud of
solar plasma. During their propagation CMEs interact with solar wind and the interplanetary
magnetic field. The speed of the solar wind is an important factor. Particles with a higher
speed hit earth‟s magnetosphere harder and have a higher chance of causing disturbed
geomagnetic conditions. The solar wind speed at earth normally lies around 300km/s but
increases when a high speed Coronal Mass Ejection arrives. During a coronal mass ejection
impact, the solar wind speed can jump suddenly to 500, or even more than 1000km/s. As a
consequence slow CMEs are accelerated towards the speed of the Solar wind and the fast
CMEs are decelerated toward the speed of the Solar wind. Figures 4.8.& 4.9 clearly indicate
that there existed a good correlation between CME parameters and solar wind speed during
October/November2003.
0 10 20 30 40 50
0.0
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Oct 2004
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Oct 2003
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Nov 2003
Figure 4.8: Results of linear correlation between CME linear speed and Solar wind speed .The left column of
panels show results of October, November 2003 and the right column of panels show results for June and
October 2004.
70
Chapter4 The Effect of CME on the Terrestrial Atmosphere
But this correlation decreased in 2004. When we approach solar minimum there is
lesser number of CMEs and the coupling between the CME linear speed and Solar wind spe ed
gets weakened.
0 10 20 30 40 50
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d e m o d e m o d e m o d e m o d e m o
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Oct 2004d e m o d e m o d e m o d e m o d e m o
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Oct 2003
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d e m o d e m o d e m o d e m o d e m o d e m o
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d e m o d e m o d e m o d e m o d e m o d e m o
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d e m o d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o d e m o
d e m o d e m o d e m o d e m o d e m o d e m o
Figure 4.9: Results of linear correlation between CME angular width and Solar wind speed .The left column of
panels show results of October, November 2003 and the right column of panels show results for June and
October 2004.
Table 4.6 : Correlation analysis between CME linear speed and Solar wind speed
Month & year maximum correlation coefficient
October 2003 0.3799
November 2003 0.3890
June 2004 0.2938
October 2004 0.1365
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
4.8 Discussions
As can be seen from the analysis, it is the ionospheric height, and not the Electron
density, which exhibits significant correlations with the CME linear speed and angular spread.
The observed maximum correlation of 0.5 or more was found to be for October 2003 month
for a time delay of ~ 8hrs and ~ 70 hrs. The rationale for this is given as follows:
Over a dip equatorial station like Trivandrum, owing to the unique geomagne tic field
configuration the ionospheric height variations are purely due to the vertical plasma drift
caused by prevailing dynamo electrical field alone. In the initial /main phase of a storm the
main magnetospheric forcing that affects the ionosphere are m agnetospheric compression,
increase in the auroral electrojet heating, variation in the polar cap potential etc (Catherine and
Blanc, 1984). All these processes result in a modulation of the prevailing electrodynamical
coupling. For instance, the initial compression appears as the sudden commencement and
manifests as the height rise over the equator. On the other hand, variations in the polar cap
potential also maps to equator as prompt penetration field in the initial phase of a storm
(Balan et al., 2008). In this context, the observed time-delay of 8 hrs for maximum correlation
for October 2003 month corroborates with above understanding. In this context, the observed
time delay is indicative of the arrival of shock and resulting compression of magnetopaus e.
Further as the time progresses beyond 10 hrs or so, the composition, wind dynamics,
Table 4.7: Correlation analysis between CME angular width and Solar wind speed
Month & year maximum correlation coefficient
October 2003 0.3960
November 2003 0.3257
June 2004 0.1043
October 2004 0.2624
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Chapter4 The Effect of CME on the Terrestrial Atmosphere
and waves starts playing a role. As a consequence, thermospheric -ionospheric energetics and
dynamics gets modified. For instance, the disturbance dynamo gets active after about 30 -40
hrs or so. As mentioned earlier, over equator, the net result of the modified
energetics/dynamics manifests as an overall lowering of the prevailing electric field. In t his
context, the time delay of ~70 hrs is attributable to the time taken by the terrestrial upper
atmosphere to respond to the CME forcing in order to manifest in terms of variation in the
prevailing fields such as lowering of dynamo electric field over equator.
The electron density over the equatorial regions on the other hand, is influenced by a
lot of processes during storms e.g. meridional wind circulation and composition changes,
EIA.diffusion etc. As a consequence, the ionospheric density is expected to exhibit a small, or
no, correlation at all with the CME parameters. Also there exists a good correlation between
CME parameters and Solar wind speed during October/ November 2003 .
4.9 Concluding remarks
In this chapter focus was on the effects of CMEs on the equatorial geomagnetic activity
based on the correlation between CME parameters (linear speed and angular width) with
ionospheric parameters (h‟F and electron density) and Solar wind speed. Although effects of
CMEs on ionosphere have been studied since the 1950s, this work is different in its approach.
We found that the CME parameters (linear speed & angular width) display significant
correlations primarily with ionospheric height h‟F. The highest correlation observed during
the month October 2003 for a time delay of about 8 hrs is due to the passage of the ICME and
the shock associated with it while time delay of ~72 hrs indicates the time taken by terrestrial
upper atmosphere over equator to respond to CME forcing. Our analysis reveals that CME
(linear speed and angular width) could play a major role in modulating the ionospheric height
variations over the equator for short periods (a few hours) as well as for a long time scale
(days).
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