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Chapter 1

Review of Earlier Works and Scope for Present Investigation

1.1 Introduction

Radio Astronomy, a field that has strongly evolved since the end of World War II, has become one of the most important tools of astronomical observations [1, 2]. Radio

astronomy has been responsible for a great part of our understanding of the universe, its

formation, composition, interactions, and even pre-dictions about its future path. This

article intends to inform the public about the history of radio astronomy, its evolution,

connection with solar studies, and the contribution of the spacecraft will have on the study

of this field [3, 4]. It is almost impossible to depict the most important facts in the history

of radio astronomy without presenting a sneak peak where everything started, the

development and understanding of the electromagnetic spectrum. Even though scientists like Faraday and Volta performed experiments with electricity and magnetism, it was not

until many years later that a scientist was able to relate both as two aspects of the same

force. James Clerk Maxwell developed the theory of electricity and magnetism by the

coherent integration of four equations. These equations not only summarized the relationship between electric and magnetic forces, but also predicted that there is a form of

radiation involved (known as electro-magnetic waves). Nevertheless, it was Oliver

Heaviside who in conjunction with Willard Gibbs in 1884 modified the equations and put them into modern vector notation [5, 6].

A few years later, Heinrich Hertz demonstrated the existence of electromagnetic waves by

constructing a device that had the ability to transmit and receive electromagnetic waves of about 5m wavelength. This was actually the first radio wave transmitter, which is what we

call today an LC oscillator. Just like Maxwell’s theory predicted, the waves were polarized. The radiation emissions were detected using a 1mm thin circle of copper wire [7].

Now that there is evidence of electromagnetic waves, the physicist Max Planck was

responsible for a breakthrough in physics that later developed into the quantum theory,

which suggests that energy had to be emitted or absorbed in small packets or “quanta” of energy. Quantum physics is the primary field for the in depth study of electromagnetic

radiation [8]. Other contributors to this field are Albert Einstein with his quantum theory-

photoelectric effect, Louis de Broglie and “particle wave duality”, and Erwin Schrodinger and his quantum physics wave equations, among others.

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After all these discoveries, scientists were able to apply their studies on electromagnetism and radio waves to develop ways of communication. In 1901, G. Marconi was the first to

send and receive signals across an ocean from Newfoundland to Cornwall. He improved

radio transmissions and, as a result of his contribution, commercial radiotelephone service became available in later years [9].

1.2 Development of Radio Astronomy

The electromagnetic spectrum outside the optical frequency largely improves astronomical

observations. Radio observations were found to be the most productive means of

astronomical research. In the twentieth century radio astronomy expanded all around [10].

Attempts for finding solar radio waves by using radio frequencies were in vain at first. The

creation of long distance radio communication was the first important application of radio

waves that began in the twentieth century. Further research on radio communication led to

the invention of radio waves from the Milky Way. The Bell Telephone Company was having problem with the working of their transatlantic service, because of static of some

sort in the late 1930. The company asked Karl Jansky to find such interference source [11].

To identify and track the source of static, a big revolving antenna, given the name of

“Jansky’s merry-go-round” was built by Jansky [12, 13]. The antenna could receive radio waves of 20.5 MHz frequency, and it was designed to locate the direction of any radio

signal by its rotational ability [14].

After studying several months of studying about such static, Jansky classified antenna into three separate types. The origin of the first two was nearby and distant thunderstorms.

However, the third source of static was quite different. He realized that there was a pattern

which characterizing these wave signals. It was very similar to the location of the Sun, but

more accurate measurements (signals repeated every 23 hours and 56 seconds) Jansky concluded that the radiation came from the constellation Sagittarius in the Milky Way

Galaxy [15, 16]. This is the fundamental discovery of radio astronomy. Grote Reber was

motivated by Jansky’s discovery. He was interested to find out the process that make the development of radio waves in space and verified whether the waves were coming from the

different celestial objects. Bell Labs were not so successful at that time of great depression.

Reber had the determination to achieve his goals, even if he did it all from his back yard.

On his own investigation Reber constructed a telescope in 1937 which had a parabolic dish

reflector and 3 receiving frequencies: 3300MHz, 900MHz and 160MHz [17]. In 1938, the

last receiver gave him the most wanted galactic radio waves. Reber’s data was shown as contour maps which represented the Milky Way with bright areas [18]. Reber was one of

the pioneers of modern radio astronomy. Depending on his work, after World War II, many scientists were able to build huge and better antennas or studying the universe [19].

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Figure1.1 Arecibo Radio telescope in Puerto Rico [20]

In these days, we have big radio telescopes like the Arecibo Radio telescope in Puerto Rico

(Figure1.1). It has 305m diameter and 167 feet depth and can cover an area of about twenty

acres [20]. A technique like radio interferometry was available in 1946. Using multiple

antennas Very Long Baseline Interferometry (VLBI) and the Space Very Long Baseline Interferometry (SVLBI) are some sophisticated techniques to record radio data. Projects

like the JPL SVLBI use space antenna technique to provide ―4 to 10 times the resolution of VLBI‖ [21]. Improved radio astronomy field can detect the radio emissions from planet Jupiter [22], observations of energetic objects like quasars, pulsars, and radio galaxies made

possible [23].

1.3 Region of Emission of the Sun

Solar corona is a kind of plasma atmosphere of the Sun, which extends to millions of

kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph [24, 25]. Across the surface of the Sun, the distribution of corona is not always

even [26, 27]. The corona is nearly confined to the equatorial regions during periods of

quiet with coronal holes covering the Polar Regions whereas it is evenly distributed over

the equatorial and Polar Regions during Sun‘s active periods [28-31]. It is most prominent in areas with sunspot activity. The time span of one solar cycle is eleven years

approximately. Due to a differential rotation at the solar equator; the equator of the Sun

rotates quicker than the poles. Sunspot activity occurs more at solar maximum due to twisting of magnetic field to a maximum [32, 33]. The dark spots are created when the

cooler plasma below the photosphere is exposed due to the pushing of the photosphere

aside by magnetic flux [34-37]. Solar coronal structure is shown in Figure 1.2. The

mechanism of this structure is very complex and the exact mechanism of high heating of

solar corona is a matter of investigation [38, 39].

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Figure 1.2 Solar corona and solar wind diagram [40]

At time of Coronal mass ejections, the strong magnetic field lines emanating from the

sunspots become so strong that hot burning gasses from the sun are suddenly sucked out of

the interior of the Sun and carried along the magnetic field lines of the disturbance in a

violent explosion [41]. While the interior of the sun is exposed at the flare site, gamma and x-rays are allowed to escape, travelling outward at a speed equal to the speed of light [42-

44]. This explosion creates a shockwave which carries some of the burning solar mass out

into space [45]. The mass ejected is plasma containing electrons and protons as main

constituent and also may contain heavier elements like helium, oxygen, and even iron in small amount [46]. They travel at speeds greater than 1000 km/sec [47-50]. They are

closely studied because they can produce potentially harmful geomagnetic storms when the

charged particles rain down Earth's magnetic field lines. In addition to generating stronger than normal displays of Earth's auroras, geomagnetic storms aimed directly at our planet

can also disrupt satellites in orbit, cause widespread communications interference and

damage other electronic infrastructures [51-53]. The magnetic field strength in the vicinity

of CME is shown in Figure 1.3.

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Figure 1.3 Coronal mass ejections and variation of magnetic field strength [54]

Sunspot is the region on the sun‘s surface that appears dark because it is cooler than the surrounding areas. These are regions where the local magnetic field is very strong. Three

dimensional structures of magnetic field lines emanating from the sunspot is shown in the

Figure 1.4.

Figure 1.4 Three dimensional diagram of magnetic field structure inside sunspot

Sunspots have been divided into three groups called Alpha, Beta and Delta groups [55].

These active regions are carefully observed, as they could be possible indication flare activity [56, 57]. The solar cycle (or solar magnetic activity cycle) is the periodic change in

the sun's activity which includes changes in the levels of solar radiation, ejection of solar

material, changes number of sunspots, flares, etc. Solar cycles have an average duration of

about 11 years [58-61]. When bipolar magnetic fields develop between sunspots then the group is called a Beta group. When a Beta group becomes intense, with strong, bipolar

magnetic fields between sunspots, then the group is called a Delta group [62]. Alpha group

are sunspots with no bipolar magnetic fields, and are with little threat for the occurrence of

a flare. Beta groups have potential of causing C and M class flare, and Delta groups have

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high potential for causing large M and X class flare [63]. The development of the solar polar field strength throughout a solar sunspot cycle can be used to predict the magnitude

of the next cycle and the peak of the current cycle. Polar field reversals typically occur

within a year of sunspot maximum [64, 65]. Within the body of the Sun, currents of hot plasma circulate in an immense system called the great conveyor belt. Each of the belts two

branches – north and south takes about 40 earth years to complete a single cycle [66].

1.4 Radio Waves from the Sun

Scientists have proposed and determined that the emitted radio waves are the result of

different solar activities. Though the mechanism of various solar activities still remains unknown, the results of them are quite significant [67, 68]. Some mysterious solar flares

causes as a result of a magnetic sheer, for the kinking in the magnetic fields of the sun that

in turn crosses each other. Radiations through the whole electromagnetic spectrum,

including radio waves are emitted, when this happens. We are able to detect the waves lying in the electromagnetic spectrum‘s radio area. They would appear as a peak with a brief duration and significant amplitude in our data collection [69].

One kind of solar emission, Type III, happens when electrons are accelerated through the

sun‘s corona that leads to both solar flares and solar bursts. Consisting of helium, protons, electrons, oxygen and the zero-charge, solar winds like plasma generate electromagnetic

radiation when protons and electrons collide, reaching a steadier state and releasing energy that are in the form of radio waves [70, 71]. A solar flare is generated when pent up energy

in magnetic field of the sun is suddenly emitted in an intense burst of brightness, and forms

emissions ranging in the electromagnetic spectrum. Solar bursts, caused due to the violent

nature of solar flares, are happened when the given off electrons by a solar flare can interfere with radio waves and the plasma (less dense) of the sun‘s outer corona, generating radio waves with frequencies near 20 MHz [72-75]. Figure 1.5 show how Magnetic field

and particle ejection of the Sun and its effect the earth‘s magnetosphere [76, 77]. The sun‘s radio bursts are produced from the interaction between free electrons that travel at speeds about 0.ηc, the sun‘s corona and solar winds, whose magnetic field and heat are perfect for electromagnetic emissions [78-81]. Type II bursts is called solar corpuscular radiation,

which occurs due to interference of electrons with solar winds [82-86].

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Figure 1.5 Magnetic field and particle ejection of the Sun and its associated effect

on the Earth [87]

1.5 Solar Radio Observations

Mentioned earlier, solar radio data capturing begins long years ago in the radio astronomy science. The Sun was the first target as a source for radio waves, from this idea that it is the

nearest energetic body to Earth. Yet, many of these early investigations were not successful

[88, 89]. The first recorded radio signals from the Sun was captured by Thomas Alva

Edison in 1890 [90-94]. Kennelly, the laboratory assistant of Edison sent a letter to Lick Observatory explaining the construction of a detector made by wrapping a number of

cables around of iron core [95, 96]. But, there is no further proof of this effort. However,

the detection of solar radio signals would not have been made since the ionosphere shields

the long waves the only waves the instruments could detect from the Earth [97].

Sir Oliver J. Lodge around 1895 built a more versatile solar signal detector than Edison did.

Still, it was not highly sensitive to have detected the solar radio waves [98]. After this

attempt, Johannes Wilsing and Julius Scheiner made a device and conducted the experiment for about eight days, but they were also failed to capture radio signal. Beside

these, they were the first persons to write up formally and publish their work to detect solar

radiation data. They concluded that the surroundings were absorbing the radio signals [99]. In 1900, to solve problems of previous attempts Charles Norman made a long wire antenna

and installed on a glacier on the mountain Mont Blanc at a height of 3100m. He explained

that if Wilsing and Scheiner were correct, the only solution was to collect data at a higher

altitude. It was very closer approach to detecting low frequency radio bursts. However, the experiment was performed in solar minimum.

Solar observations were ignored for many years. In 1920s Oliver Heaviside explained the

existence of the ionosphere and many questions about solar signal data were answered. Astronomers understood that they had to develop radio receivers of high frequency (above

20MHz) in order for the waves to enter through the ionosphere [100].

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World War II had a direct effect on the history of solar radio astronomy. In 1942, an

English radar station detected a strong noise signal thought to be an interference created by

enemy [101]. But observation exhibits that radio wave emissions from the Sun related with a group of sunspots that detected at that time. In the same year, G.C. Southworth detected

solar microwaves at of 1 and 10cm wavelengths at the Bell Tele-phone Laboratories [102].

Few years later (1944) these observations were published, but Grote Reber continued to

record radio observations and he was the first person to publish solar radio signal observations [103, 104]. After the war was over, scientists started to observe the Sun and at

that time they began to discover many features of the Sun, such as classes of radio bursts,

storms, and to establish the relation between solar flares and radio bursts. Countries like

Australia, Canada and Great Britain joined in the study of the solar radio data. This became

popular after the International Geophysical Year (1958) [105].

Since then, so many satellites have been employed to study the Sun and its effect on Earth. Receiving radio data from the Sun is important for the understanding of the space weather,

astronomers decided to construct satellites for detecting solar radio waves. Few of these

satellites are: RAE-1 and 2, ISEE-1, 2 and 3, Voyager- 1 and 2, Galileo, Ulysses and

Cassini [106]. At this time, the new satellites created to observe the Sun are called STEREO launched on October 25th, 2006 [107]. STEREO employs two closely identical

space related observatories: one ahead of Earth in its orbit and the other trailing behind to

support continuous stereoscopic measurements to observe the Sun and the properties of its

CME [108, 109]. STEREO provides stereo viewing of the Sun from every point along Earth‘s orbit. The satellite is able to take photograph and track disturbances of space-weather from Sun to Earth and to form image of solar activity measurements of energetic

particles at 1AU [110]. The STEREO mission is the first to carry radio triangulation with two types of satellites to determine the region of interplanetary shocks [111].

Radio astronomy had its start with the application and discovery of electromagnetic waves

and it has slowly improved since then. History supported that radio observations enlarged astronomy‘s horizons. Primarily it was responsible for the identification of objects like quasars pulsars, and radio galaxies [112, 113]. Also it is partly responsible for the idea that

dark matter acts as an important part of this universe; the rotation of galaxies by radio measurements suggest that there is more mass in galaxies that has been observed directly.

Overall, it gives a better understanding of the interactions and components of the earth

[114]. We expect that radio astronomy must continue to improve in the coming years,

making its techniques perfect and bringing a lot of astronomical discoveries.

1.6 Flare Detection using VLF Radio Signals

The detection of Solar Flares can be achieved by continuously recording the signals from

some of the Military radio transmitters on VLF (Very Low Frequency) radio [115]. In the radio field these events were given the name Sudden Ionospheric Disturbances (SID) early

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on, and Flare detection is sometimes referred to as SID detection [116]. The effect depends upon the response of the ionosphere to the burst of solar radiation, and the mechanics of the

radio propagation mechanism [117].

The ionosphere is a region of the Earth‘s atmosphere where the gas density is low enough for atoms that become ionized to exist for a significant period of time before meeting and

colliding with another atom and becoming neutralized again [118]. This region is at an

altitude of between 50 km and about 600 km above sea level. The ionizing energy arises mainly from the Sun in the form of particles and electromagnetic radiation from the visible

spectrum right through to gamma rays [119, 120].

1.7 Origin of Solar Radio Bursts

Scientists now realize that the magnetic field emitting from the sun is directly responsible

for sunspots number and other phenomena like flares, geomagnetic storm, coronal mass ejections (CME‘s) etc. Radio antenna receives and detects the radiation due to these solar events. When charged particles are trapped in the curved magnetic field lines, solar bursts

occur [121, 122]. Magnetic field lines are reconnected due to pinched off of plasma cloud,

sending the cloud into surrounding space like thermal energy at a fraction of the speed of light as shown in Figure 1.6, where HXR represents hard x-rays which is very high

energetic, SXR represents soft x-rays of low energy, and the direction of the electron beams

are represented by the arrows. The upward electron beams result in the Type III solar bursts

in general that we receive with radio antennas [123].

Figure 1.6 Magnetic reconnections and electron acceleration

The temperature near the region of a solar burst is about millions of Kelvin, and the solar flare releases about 1014 Joule from the high energetic magnetic field around a sunspot

[124, 125]. The energies of CME are even greater than flares and can emit gas at hundreds

of Km/sec. Astronomers believed that CME‘s are a result of sudden changes in the solar

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magnetic field, further it is found that solar bursts can occur in the region of complex sunspot groups [126]. Figure 1.6 clearly shows how electrons are accelerated due to

magnetic force in the reconnection region.

Solar bursts are generated by twisting of magnetic field, which produces due to differential

rotation [127, 128]. Because the magnetic field lines get twisted more, more is the kink and

tangle occur, which poke out from the corona of the sun. These protrusions are the regions

where sunspots create, resulting the plasma to travel in the direction of the protrusions which pushes the solar surface to become colder and darker in this region [129, 130]. Solar

bursts create when these magnetic protrusions cross over and reconnect, as illustrated in

Figure 1.7, frequencies that are emitting all across the electromagnetic radio spectrum

[131].

Figure 1.7 Babcock Magnetic Dynamo Models [132]

The 1st picture in Figure 1.7 shows the starting of the cycle where magnetic fields are

affected by differential rotation; the 2nd image exhibits the result of differential rotation

after many rotations; the 3rd picture shows the sunspots, each having its own poles, which

are a result of the kinks; the last image shows the cycle progressing the sunspots displace toward the equatorial region where their mini-poles change the overall polarity of magnetic

field lines of the Sun [133].

The study and observation of solar bursts at a frequency of 50 MHz to 300 MHz corresponds to a wavelength between 6 meter and 1 meter, which lies in the VHF portion

of the electromagnetic spectrum. The corona is the main region of strong solar radio wave

emits [134]. When charged particles are influenced of magnetic fields, their velocity increases along helical path, and show periodic behavior, resulting radiation in cyclotron or

synchrotron radiation patterns [135, 136]. We consider the cyclotron and synchrotron

radiation, and a brief description of magnetic reconnection [137, 138].

1.7.1 Cyclotron and Synchrotron Radiation

If we consider a particle moving in an electromagnetic field, the total force it would

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experience by it, expressed by the Lorentz force equation, in Gaussian units: F = q [ E+ v×B ] (1.1)

Where the magnetic force is: FB =q(v×B) (1.2)

which is normal to both v and B. Relative to B, the velocity can be decomposed into its

translational (vǁ) and rotational (v┴ ) components (Figure 1.8), for which the rotational is: = v sinθ (1.3)

Figure 1.8 Particle of charge q is moving with velocity v and making angle with B Thus we know that the accelerating particle emits radiation. For an electron, the force

equation is:

mea = ˗ (v×B) (1.4)

This becomes

me ┴ = ┴ B = (1.5)

Where, r is the radius of the cyclic path of the electron. Hence the cyclotron frequency

given by [139]

ωcyl = (1.6)

Combining (1.5) and (1.6), we have

ωcyl = = 1.8× 107 ( ) (1.7)

Here magnetic field is the only variable. Power emitted from the acceleration of charged

particles [140]

Pcyl = ω2cyl v2 sin2θ = = ω2cyl ┴ (1.8)

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Monochromatic radiation is due to path traversed by the particle is circular in the v|| frame.

Figure 1.9 exhibits the angular distribution of the cyclotron radiation, emitted by the

particle.

Figure 1.9 Radiation pattern and their angular distribution, without relativistic corrections

When magnetic field is very strong equation (1.8) becomes inaccurate, since for an electron traversing with a speed v tends to c the effects of length contraction and time dilation

cannot be neglected [139, 140]. The path of electron will no longer be circular and emit a

band of frequencies instead of just one. Also, instead of having a symmetric dipole pattern as clearly seen in Figure 1.9, the radiation pattern is growing up in the direction of motion,

resulting of Doppler blue-shift. These changes from Figure 1.10 are illustrated in Figure

1.11.

Figure 1.10 Illustration of the radiation beam as a result of the relativistic dilation

The equations of motion for the particle change for relativistic kinematics:

FB = = q (v×B) /c (1.9)

FE = = qE.v = 0 (1.10)

Where -1/2

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So the magnetic force equation becomes

FB = = q (v×B) /c (1.11) From above, the acceleration is due to the perpendicular component: ┴ = ┴ , vǁ ) = 0 (1.12) Using the same method of calculation of cyclotron frequency, it can obtain the synchrotron

frequency:

ωsyn =

(1.13) We get power emitted by the synchrotron radiation (Larmor equation) [141]:

Pcyl = v2 sin2 = = ω2syn ┴ (1.14)

Its radiation pattern with respect to laboratory frame is shown in Figure 1.11.

Figure 1.11 The radiation pattern of Figure 1.9 being increase in the direction of motion

1.7.2 Magnetic Reconnection

In order to clarify what happens during the time of magnetic reconnection, we need to

understand the properties of the magnetic field [141, 142]. A current sheet is formed when

electric current flowing parallel to a surface [143]. This behavior of the magnetic field,

which is interacting with the current sheet, can result in magnetic reconnection [144]. From Faraday‘s Law relate the electric field to a magnetic field that changes with timeμ ×E = (1.15) In order to have a relationship with the total current density, J, we use Ampere‘s Lawμ ×B = J (1.16)

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τhm‘s Law states J = E = (v×B) (1.17)

where E = (v×B) results from the Lorentz transformation and v denotes the relative velocity

of the particles. Connecting these relations through τhm‘s Law, we can rewrite the electric field as:

E = (v×B) = =

×B ) (1.18)

E = (v×B)

By virtue of (1.18) we can rewrite (1.15) in two different ways: = ×(v×B) (1.19) And then = [ × ( ×B)] (1.20)

From the vector identity, [ × ( ×B)] = B (1.21)

From Gauss law or magnetism, = 0 So using equation (1.20) and (1.21), we get (1.22)

Where, = . In (1.19), ×(v×B) describes how the particles attached with the magnetic

field, known as the advective term, and in (1.22) represents how the particles spread out with time, known as diffusive term. If the diffusive term gets stronger the particles

spread out more over time, thus current density decrease and thickness of current sheet gets

lower, in which case Faraday‘s Law is best way written in the form of Eq. (1.22). For example the latter case occurs when two fields of opposite orientation start to diffuse

toward one another and reconnect, as exhibited in Figure 1.12.

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Figure 1.12 Illustration of two oppositely oriented magnetic fields diffusing together

and causing reconnection

1.8 Geomagnetic Behavior of the Earth

Solar flares are energy-outbursts of from the Sun occurring suddenly, which are the biggest explosions of the Solar System. It is realized that they arise in the corona of the Sun, which

is the hottest- temperature region. Solar flares are arisen during magnetic reconnection of

the magnetosphere of the sun in the field- fluctuating areas [145]. These fluctuations bring weak points in the magnetosphere from where the large amount of energy comes out.

Mainly Solar flares comprise energy in the form of ultraviolet radiation, high frequency X-

rays, and large amounts of light [146, 147]. Due to this, it is rarely found that a flare is

strong enough to be seen against the spectrum of white light of the Sun, which means that flares most often should be studied by high frequency specific detecting instruments. The

radiation from dense solar flares can come to the Earth in a very little time (around 499

seconds). With this radiation, largely energized particles can take times less than an hour

for arriving Earth [148]. Those particles can have great influence on the Earth‘s magnetic field and atmosphere and can also destroy satellite electronics and affect noticeably our

own electrical grid [149, 150].

Earth‘s magnetic field is dipolar structured such that there was a bar magnet in the core region of the Earth. The dipole axis is tilted 11 ½° from the rotational axis of the earth. As a

result, the geographic poles and north and south magnetic poles are in separate locations

[151, 152]. This field is divided into three components. These components are the Bx directed to the north, By towards the east, and Bz to the positive downwards. The

components of this magnetic field are taken vectors in the sense that when we calculate the

square root of their squared sum, you find the total B field [153]. To demonstrate: Btot

=√ . The mean measurement of the total B field is nearly 50,000 nT. The most frequently used scale to distinguish solar flares is the NOAA GOES scale. It

calculates brightness of X-ray of flares in one to eight Angstroms wavelength range. Then

these measurements are put onto a scale that classify flares in a descending order of X, M,

C, B and A. There are nine divisions such as M1, M2, M3H and M9 on each letter of the scale. Decimal numbers which follow the first digit are common for more accurate

measurement [154]. C flares take place most frequently of the C, M, and X level flares and

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pose with no threat to any Earth-based or Earth-orbiting systems. M and X flares take place not nearly as frequently, but pose a greater threat and create radio blackouts and radiation

storms differing in strength [155]. It was studied to search evidence to support or discredit

the view that the GOES classification of solar flares may be correlated to the strength of Bz component of the geomagnetic field [156]. This was found by considering the strength of

the Bz component in response to impacting solar flares.

In order to find the potential impact of the solar flares on the geomagnetic field, a new data reduction system was developed [157]. This system was known as the GVIS (Geomagnetic

Variational Intensity Scale). The GVIS was designed to accurately and evenly calculate the

intensity of geomagnetic variations due to solar flares [158]. The calculations can give a

scale which is able to show a level of fluctuation in the magnetic field strength at the time

which the number is measured.

The ACE mission was firstly proposed in 1986 that was a part of the Explorer Concept Study Program to measure coordinately the isotopic and elemental composition of

accelerated nuclei in the solar phenomena. It was chosen for development in 1989 and

construction started on it in 1994 [159]. ACE was launched on 25. 08. 1997 on a Delta II

rocket from Cape Canaveral Air Station. Its scientific goals are studying the composition of the distinct samples of matter like interplanetary medium, the solar corona, local interstellar

medium, and the galactic matter etc. It will accomplish this by studying the composition of

the coronal mass ejections, solar wind and the solar energetic particles in the solar flares

[160]. Its collected data consists of the strength of IMF (Interplanetary Magnetic Field), velocity of the solar wind, and the particles‘ density in the solar wind [161].

THEMIS (Time History of Events and Macro-scale Interactions During Substorms) mission was started originally to know the magnetotail and the serial of events that create

auroral substorms in this atmosphere. Charged electrons in the atmosphere cause Auroras

from the solar winds which are enhanced. The electrons move along magnetic field lines

and also interact with gases in the atmosphere which makes the gases to raise temperature and emit light. In order to search the order of events which initiate the substorms, five

identically instrumented spacecrafts were sent into space where each align once in every

four days over the array of the ground observatories which are situated in Canada and the

Northern USA. Eleven magnetometers were then installed in schools over the US as part of

this THEMIS public outreach program. The magnetometers measured the magnetic field

strength across time of two measurements per second. The data from the Remus, MI

ground-based magnetometer can be used for comparing to the ACE data at corresponding times [162].

NOAA SWPC (Space Weather Prediction Center) is one of the parts of the National

Weather Service. The SPWC collects data from GOES (Geostationary Operational Environmental Satellites) to calculate x-ray flux of solar flares and give data of when they

take place. The satellites are the parts of the στAA‘s mission to earn knowledge about the Earth‘s weather patterns. They monitor continuously the Earth in a geosynchronous orbit

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which is above the Earth‘s equatorial plane with a speed matching the rotation of the Earth. This indicates that they will however over one spot on the Earth from 35, 800 km away.

Attached to GOES 12 through 15 are sophisticated solar X-ray Imagers that monitor the

Sun‘s X-rays to detect solar flares, CMEs, and other events which impact the geospace environment [163].

1.9 Signals from Jovian Planets

In 1955 mysterious signals from space were discovered by radio astronomers at the

Carnegie Institution of Washington, DC. Some thought the signals were local interference,

perhaps a noisy ignition system of a pickup truck whose driver was returning home from a

late night date. However, analysis revealed that the planet Jupiter was in the beam of the

Mills Cross antenna each time that signals were heard [164]. Unlike many radio astronomy

dish antennas, the huge Mills Cross comprised over 100 dipoles strung between wooden

poles planted in a Maryland field [165]. The dipoles were phased to produce a narrow, steerable, pencil-thin beam some 2.5º in width. That is an amazingly narrow beam

considering the operating frequency was 22.2 MHz ever since this accidental discovery;

researchers have aimed shortwave antennas at Jupiter as they attempted to understand the

source of these powerful signals [166].

The so-called decametric radio signals from Jupiter are not on the air all the time but seem

to be linked to three longitude regions around the planet, cleverly named the A, B, and C

source regions [167-169]. If one of these source regions is facing Earth, we have an increased probability of receiving signals. If the Jovian moon Io is in the right place in its

orbit, the probability of receiving signals is greatly enhanced [170]. The moon Io happens

to be within the tidal force‘s limit of Jupiter and it is literally being torn apart by gravitational forces with tides as large as 100 meters. Io crosses the magnetic field of

Jupiter and is thus able to release charged particles into the field. These charges are

accelerated to very high speed and spiral along magnetic field lines and generate

synchrotron radiation, which manifests itself as the radio signals detected here on Earth [171]. There is additional data that suggests that Ganymede and Europa may also contribute

to the radio emissions [172, 173]. Earth‘s ionosphere limits ground based reception below about 15 MHz and Jupiter itself does not emit these signals above 39.5 MHz - an upper

limit determined by the strength of the Jovian magnetic field [174, 175].

So what do signals from the Jupiter sounds like? There are two distinct types: L-bursts

sound like ocean waves breaking up on a beach, and S-bursts, which can occur at rates of

tens of bursts per second, sound like popcorn pop-ping or a handful of gravel thrown onto a tin roof [176]. Late at night is the best time, when the ionosphere has become transparent

and most terrestrial signals have disappeared on the 15 meter band. The quiet hiss in your

headphones comes mostly from relativistic electrons spiraling in the galactic magnetic field. L-bursts and S-bursts are heard above this background noise. A radio noise storm of

L or S-bursts can last from a few minutes to a couple of hours [177]. During a good storm,

Jovian signals can be easily heard [178], often several dB above the background noise. Of

18

course, the bigger the antenna the stronger the signals, the 640-dipole, 26.3 MHz, phased array antenna at the University of Florida would yield signals well over 20 dB above the

background [179, 180].

1.10 Detecting Radio Waves from Jupiter

Jupiter‘s distance is 5.2 times long from the Sun in comparison to Earth, and its equatorial radius is about 71492 km, which is 11 times (in volume 1400 times) the radius of the Earth [181]. Jovian atmosphere is mainly made of the simple molecules of helium and hydrogen

with sulfur, nitrogen and oxygen in small amount. Jupiter has magnetic moment of nearly

4.3 Gauss-RJ 3, that is 20,000 times greater than that of our Earth, with magnetic field in

direction opposite to that on Earth and inclined by 9.6 degree , which is very close to 11

degree tilt of the Earth [182, 183]. Generally the form of Jupiter‘s magnetosphere is like that of Earth but about 1200 times greater in dimensions, as at 5.2 AU the solar wind

pressure is only 4% of its value of that at 1 AU. It is notable that though Jupiter is bigger than Saturn slightly, its magnetic field is 4 times large in each dimension [184]. While

Earth‘s magnetic field is produced by the iron core, the Jovian magnetosphere is produced by the motion of the magnetic material that is inside the liquid metallic shell. At nearly

1000 km lower than top of the cloud the hydrogen atmosphere begins to thick and finally changes phase to be liquid hydrogen [185, 186]. Under this liquid hydrogen layer a metallic

Hydrogen layer exists which creates the Jovian magnetic field. Unlike a dipole at the

Earth‘s core, an octupole and quadrupole also contribute to generate the Jovian magnetic field, which explains the nature and shape of its magnetosphere. Jupiter is the fastest rotator among all the planets with a 10-hour day [187, 188]. The power maintaining and for

populating the magnetosphere of Jupiter comes mostly from the rotational energy of Jupiter

and the orbital energy of its moon Io, whereas the source of power for Earth‘s magnetosphere is mainly the solar wind [189].

Io-Jupiter system carries a unique distinction in this solar system. While Io is the most

volcanically active among all the planetary bodies, the Jupiter is the biggest planet, has the strongest magnetic field, the fastest spin, the biggest and the most powerful magnetosphere,

and the densest among all the planetary atmospheres [190]. The study of this unique moon-

planet system is very important as it is different from our Lunar-Earth relationship, it helps

us to understand the basic plasma-neutral-surface interactions, and also it has implications

to know many similar processes happening at other places in this solar system and in extra-

solar planetary systems too [191].

1.10.1 Extraterrestrial Radio Waves: The Discovery

Apart from the Sun, for receiving radio waves from other planet, constructing an antenna

and receiver and to set their position according to the motion of the sun are required. Even if the receivers used are not much sophisticated, radio wave data may be used accurately to

study the properties of distant planets, like shape, size and properties of the magnetic field

[192, 193]. Data received can also be helpful to map the activity of the Sun and other

19

planets, the rising and setting of known heavenly bodies [194]. Radios tuned to 20.1 MHz are able to pick up signals from certain planets, comets, meteors, and the sun. From

Comparison between several days of data, we can make many important conclusions about

the behavior and nature of the sun and planet like Jupiter [195-198].

1.10.2 Source of Radio Waves: Magnetospheres

It was later established that the recorded radio waves by Reber and Jansky were due to the

magnetospheres of big planets in space [199, 200]. A magnetosphere is a very large electromagnetic field. Planets and other celestial bodies are surrounded by it [201-205]. If a

planet has sufficient magnetic materials, which has the ability of carrying a substantial

amount of current, a magnetosphere may form around it [205-209]. Magnetosphere of

Jupiter is particularly large that extends up to Saturn. This very large magnetosphere of

Jupiter can appear in the sky being large as full moon, if it is possible to watch the entire

magnetic field surrounding Jupiter from Earth [210-214].

1.10.3 Formation of the Jovian Magnetosphere

The magnetosphere surrounding Jupiter is actually much different from those around rocky

planets similar to the Earth, in its characteristics and size. Like many magnetospheres, Jupiter‘s magnetosphere is created by the magnetic field of the planet [214-218]. The magnetic material necessary for forming this field is situated in a special area of Jupiter

named the liquid metallic cell [219].

Magnetosphere of Jupiter is much extensive, and for this it drapes over every moon of it

and also bits of celestial matter moving around it. As Jupiter‘s moons, especially Io, move around the planet, clouds of particles are left behind them [220, 221]. (Earth‘s moon is not situated within the magnetosphere of this earth, so it does not do this.) This gathering of charged particles, called as a torus, spread more in magnitude as the Jupiter‘s moons leave behind further ions and when the Io‘s volcanic atmosphere is bombarded with electrons and atoms (radiation) from the center of the planet [222, 223]. As there is an electromagnetic field around all Jupiter‘s moons, those clouds of particles charged electrically move along the orbits of them [224-228]. Eventually, the volume and magnitude of these ions‘ clouds become large enough to influence significantly the behavior of magnetosphere of Jupiter as

a whole, by forming currents. Thus, when a magnetosphere surrounding a rocky planet would create the necessary spherical extension of this magnetic field, then the metallic cell

can produce a field which is not so regular and is pointed at one end, like bullet [229, 230].

The special property of the magnetosphere of Jupiter which is of main interest to the experiment is the emission of radio waves by Jupiter‘s magnetosphere [231-235]. Still scientists are researching into this topic as it is not so simple [236, 237]. It is thought that

the torus, heated ring of the charged particles, has very powerful electric field, and then this field creates radio emissions when it comes in touch with the intrinsic magnetic field of

20

Jupiter. Certain particles ―pushed‖ into the ion-current (also known as the aural flow), produce radio noises called DAM (decametric) [238-242].

1.10.4 Anatomy of Jupiter’s Magnetosphere

Magnetosphere of Jupiter has many distinct parts. Figure 1.13 shows some of the major

components of the Jupiter‘s magnetosphere [243, 244]. The bow shock serves to deflect different solar winds on a tangential curve which are directed toward the planet [245].

Figure 1.13 Different Parts of a Magnetosphere of Jupiter [246]

Streams of protons and electrons, solar winds, emanating from the sun, are produced when

the trapped particles in the magnetic field can drag and overcome the field in a certain

direction. The magneto-sheath is the effectively empty space which exists between the magnetopause and the bow shock. The magnetopause is simply the direct magnetic

boundary between the solar winds and Jupiter‘s field that try to penetrate it [247]. Within the area known as the neutral sheet, the magnetic fields from the southern and northern

regions of Jupiter cancel out each other, making the area relatively ―neutral‖ compared to other parts of the magnetosphere [248]. The lobes are an integral part of the magneto-tail,

which are located between the neutral sheet and the magnetopause [249]. These lobes are

characterized by directly opposite magnetic direction of them, allowing the circular current to flow. The flow of solar wind and magnetic field lines around Jupiter atmosphere are

shown in Figure 1.14.

21

Figure 1.14 Solar wind bent around Jovian magnetosphere

The anatomy of the magnetosphere of Jupiter essentially works to deflect solar winds effectively. The system acts as an enormous magnetic field itself, and the byproduct of the different processes that occur within the magnetosphere are radio waves which can be detected here on earth [250].

1.11 Discussion

According to electromagnetic interaction theory, there is a strong correlation exists between the numbers of sunspots and solar burst activity, which was to be expected from theory.

Our experiment involves a long term monitoring of the Sun at VHF frequency range and

that is for the Jupiter at 20.1 MHz [251]. These signal results from the cyclotron

mechanism, which occurs when a charged particle travels at non-relativistic speeds along a magnetic field line, causing it to spiral and emit radiation. Radiation consists of higher

frequencies results from the highly relativistic synchrotron mechanism. These energies are

a result of extremely strong magnetic fields and fast-moving charged particles. The faster

the particle moves, the narrower and focused the beam becomes. This narrowing of the beam is a result of the Lorentzian time dilation [252].

Both solar burst activity and sunspot activity show a long term cyclic behavior. It then stands to reason that solar burst activity at a different frequency should be the same. The

relationship between solar radio bursts and sunspot number should become more obvious

as we go through the solar maximum [253]. So it is the appropriate time to monitoring solar

radio observation and its effect on the geomagnetic conditions of the earth [254].

Jupiter can produce very strong radio bursts at decametric wavelengths from regions of

temporary radio emissions in its magnetosphere [255]. This magnetosphere is so large that

despite the distance between the Earth and Jupiter, it would appear in the sky as large as a

22

full moon, if it were possible to look the whole magnetic field around Jupiter from Earth. Our present work reports identification and characterization of Jovian signal at Kalyani

(West Bengal) at a frequency of 20.1 MHz besides the listening of Jupiter songs by audio

monitoring arrangement. The present study has special importance from the point of view

of radio astronomical observations on Jupiter over a tropical station in Indian subcontinent.

1.12 The Scheme of Presentation

Chapter 1: In the early section of this chapter a brief review of the work done in this field

is presented.

Chapter 2: The techniques and instruments developed for the study has been presented in

this chapter.

Chapter 3: Continuous data recording (using Log Periodic Dipole Antenna) methodologies

are thoroughly described in this chapter. Recorded solar radio bursts are classified into

three different types and compared the recorded data with the data of other observatories

situated different parts of the world.

Chapter 4: Radio emissions due to interaction with the magnetic field lines of the sun have

been considered in this chapter. Solar signal variation data due to the received bursts

obtained during 2013 have been analyzed with reference to the geomagnetic conditions for

different values of Kp and AP indices in particular. Variations of some other parameter have also been simultaneously analyzed. Also variation of Ap indices in the years of solar

minima and solar maxima are examined with a view to measure the condition of the earth‘s magnetic field at such times.

Chapter 5: Radio emissions from Jupiter and its moons have been taken into consideration

emphasizing radiation output and magnetic field. Spectrum of Jovian radio emissions compared with spectra of four other magnetized planets in kilometric radiation are

examined and idealized radio frequency spectrum of the two types of decametric radio

noise bursts received from Jupiter over Kalyani are presented and discussed in this chapter.

Chapter 6: The atmosphere of Jupiter is mainly made of molecular hydrogen and helium.

The cloud pattern as found at different Jupiter belts and the vertical structure of its atmosphere are examined indicating how the pressure drops with altitude. The elemental

abundances relative to hydrogen for Sun and those for Jupiter to Sun have been taken into

account as well as the unique characteristics of belts and zones that divide the atmosphere of Jupiter have been considered starting from the North and South Polar Regions including

the Jupiter's cloud bands. Taking data from NASA's New Horizons spacecraft and two

telescopes at Earth, Jupiter's Little Red Spot (LRS) has been considered showing there the

highest wind which can be taken as the precursor of storms. Photographs, taken by the Galileo spacecraft suggest that Jupiter's storms are caused by heat deep in the planet.

Jupiter produces vortices of circular rotating structures similar to the Earth's atmosphere

and can be classified as cyclones and anticyclones. The properties of vortices formed in

Jupiter are critically examined and the results reported by computer simulation of the

http://en.wikipedia.org/wiki/Molecular_hydrogenhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Vortexhttp://en.wikipedia.org/wiki/Cyclonehttp://en.wikipedia.org/wiki/Anticyclone

23

merger of two Jovian White Ovals have been analyzed and compared with the circulation of Earth. Jupiter & Earth similarities have examined and the mystery of some observations

have pointed out.

Chapter 7: In this chapter changing characteristics of little and great red spots of Jupiter

have considered using available data and published results as derived from various spacecrafts from time to time. Theoretical approach for determining eccentricity, relative

vorticity and Rossby number is first pointed out. The appearances and disappearances of

the spots are categorized with illustrations with their scatter plots. Distribution of their

lifetimes are critically examined and the zonal wind profile of Jupiter as obtained from Cassini by applying two different methods are considered including a comparison between

the latitude distribution of spots and the zonal wind profile. Finally, relative velocity vs.

mean zonal velocity is critically analyzed and a multi-wavelength comparison of the LRS

and GRS is made.

Chapter 8: This chapter gives a summary of the important results obtained from the present

study. The scope of further investigation is also outlined.

24

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