Earth’s Ionosphere

1.1 Ionosphere:

Ionosphere is a partially ionized gas which envelope the earth from height varies from 60 kms to thousand kms. Appleton in 1994 experimentally send the radio waves of low frequency which are returned back to the ground when he increased the frequency of radio wave he found that different frequencies are reflected from different height thus it was observed that ionosphere has layered structure and after the invention of rocket and satellite in 1960 the complete structure of ionosphere was identified.

1.2 Structure of ionosphere:

There are mainly four layer of ionosphere D, E, F1, F2 layers, and in the night time a weak ionospheric layer is identified as C layer, which is shown in Fig 1.1.

Fig 1.1: Structure of ionosphere [source: https://www.google.com)

1

It is observed that different layer at different height ranges it structure varies in day and night also in night time D layer appears. F1 and F2 layer merge to form single layer F. The electron density as well as different ion densities is different in different layers (Table 1.1). Temperature, different ion species are also tabulated in Table 1.1.

Table 1.1: Ionospheric parameters

Layer D E F1 F2 Parameters

Altitude 60-85 85-140 140-200 200-1500

Electron 103 1 -2× 105 1 -5×105 0.5 × 106 density

+ + + + + + + + Ion species O2 , NO NO , O2 O2 , O O , He+ ,H

Causes of Lyman α Lyman β He I, He -II Some soft X- ionisation rays

1.3 Chemistry of ionosphere

The causes of formation of ionosphere is solar extremely ultra violet radiation

푋 + ℎ휗 X+ + e-

It is found that ionosphere is highly variable. There is a diurnal variation due to which day and night has different structures and height profile. There is a seasonal variation. It has also variation to season to season due to the rotation of earth and its position from sun this is known as seasonal variation. There is variation of in the structure of ionosphere due to the solar maximum and minimum periods thus density is different during solar maximum period compare to solar minimum period. Inspire of this variation there are short term variations depending upon the different space weather event like solar flares, geomagnetic storm, coronal mass ejection.

1.4 Formation of ionosphere:

2

Ionosphere is formed due to photoionization due to extremely UV radiations are coming from Sun and radiations.

Equation

The photon energy of the incident solar beam which is greater than the ionization potential of the different species of the atmosphere, ionize the earth atmosphere. It is found that the ionization potential of different species of atmosphere lies between 10 eV to 20 eV. It means the photoionization requires that the photon wavelength is less than 100 nm. Thus extremely ultra violet radiation and X-radiation which varies 100 nm to 1260 Å is sufficient to ionize the earth ionosphere.

The most important radiations that is the line spectra for the ionization of ionosphere are

He-I: 584Å

He-II: 304Å

Lyman α: 1216Å - D-Layer

Lyman β: 1026Å – E -Layer

Due to photo-ionization there are some other reactions like recombination, dissociation etc. which produces different types of ions in the ionosphere.

1.5 Photochemistry of ionosphere:

+ - N2+h휗 N2 + e

O2 +h휗 O2 + e-

O +h휗 O+ + e-

+ + + Dominant ions are N2 , O2 , O

+ + + Due to the production of different species N2 ,O2 ,O are formed in the lower ionosphere that is + + + the D layer the peak ionization occur near 150Km of N2 and O2 where as 180 Km for O (Fig 1.2)

3

Fig 1.2: Photochemistry of ionosphere (source :https://www.google.com)

We define a continuity equation for the net source of production as well as different loss processes:

Ss = Ps - Ls

There are several loss processes due to ion-neutral reaction and due to electron -ion + + recombination. Thus, the major ion produced in D region (N2 , O2 ) again react by ion-neutral reaction.

+ + N2 + O2 O2 +N2

+ + N2 +O NO +N

+ + The rate reaction of ion-neutral reaction is fast. Thus N2 , O2 are produced in D and E layers as + + a major ion .Both O2 and NO further react by electrons-ion dissociative recombination.

+ - O2 + e O + O

4

NO+ + e- N + O

N + h휗 N+ + e-

+ The ion-neutral reactions are comparatively very fast in the E region due to which almost all N2 + + + ions become either O2 or NO .This is the region where NO density dominates in the E region and at higher altitudes it decreases very sharply .In the higher altitudes it decreases in F layer the O+ density is maximum and it becomes comparable to electron density in F layer .The upper portion of the F layer is dominated by the H2 ion and He ion due to their production and scale height.

1.6 Solar Ionosphere Magnetosphere coupling: when a solar activity occurs like solar flares coronal mass ejection ,large amount of ions and electrons are accelerated towards earth form Sun .They compresses the day side magnetosphere and change earth magnetic field which generate geomagnetic intensity this energetic particle deflected from day side and enter into the magnetosphere from magnetotail region where they are again accelerated and starts gyrate along geomagnetic field lines in helical motion. Thus, the particles are trapped along geomagnetic field lines and known as radiation belt particles. When these particles are more energetic they overcome magnetic mirror effect and enter deep into the ionosphere where they interact with atoms ,ions and molecules to give energy to them to rise them up to higher state Thus the energetic particles then precipitate into ionospheric particles and the ionospheric atom and ions produce auroral emission on different wave length. Thus we see that there is a coupling between the solar wind particles with the magnetosphere particles and again into the ionosphere (Fig 1.3).

5

. Fig 1.3: Solar ionosphere magnetosphere coupling (Source: https://www.google.com)

1.7 Ionospheric Variation

(i). Quiet time variation:

I. Equatorial ionisation anomaly:

Due to the large production of electron density in the equatorial region and due to Lorentz force this electron density rises up and they align themselves along geomagnetic field line thus the electron density along equator is not maximum and it is observed maximum along +15°and - 15° geomagnetic equators which is known as equatorial ionisation anomaly.

II. Diurnal variation:

The effect of natural disturbance on the earth’s magnetic field at any one place is at least two fold: (i) to introduce a regular variation (Sd) periodic within the day and additional to, as well as different in type from (except in a limited region round the magnetic axis pole), the variation associated with quiet days (Sq); and (ii) to suppose on Sd irregular changes which may either be of the distinctive type peculiar to large storms especially in low latitudes and generally preceded by the particular type of perturbation known as a sudden commencement, or the changes in the field may be of the apparently nondescript class which comprises an unlimited variety of short- period irregular oscillations. Of these effects of disturbances Sd is definitely a local time

6 phenomenon: the sudden commencement with subsequent depression in the horizontal component of the field as definitely follows universal time.

III. Seasonal and Semi-annual variation: The global distribution of variations in the behaviour of the electron density of the F2-layer at midday is examined for different levels of solar activity. It is found that the variations in Nmax can be divided into three major components: winter maximum (seasonal), equinoctial maxima (semi-annual) and a component which peaks in December–January (annual). At solar maximum, the winter maximum prevails over much of the northern hemisphere and appears to be due to an increase in the [O][N2] ratio caused by convection of the O from the summer to winter hemisphere. The winter maximum also occurs in the Australia-Indian Ocean section of the southern hemisphere. The equinoctial maxima prevail over the remainder of the globe. We suggest that this is due to the semi-annual variations in neutral densities associated with geomagnetic and auroral activity. In the South Pacific-South Atlantic regions summer noon peak electron densities are greater than those in winter. We suggest that this is due to particle precipitation in these regions which surround the geomagnetic anomaly. As solar activity declines, the extent of the winter maximum decreases and only that in the northern hemisphere in the longitudes of the magnetic anomaly remains. The evidence points to an energy input imbalance between northern and southern hemispheres which can be attributed to the lower magnetic intensities which prevail over a large area of the southern hemisphere. As a result, meridional convection from the southern summer to the northern winter regions is enhanced in these longitudes, and the reverse flow from the northern summer to southern winter is decreased so that the equinoctial or summer maximum predominates. The semi-annual component decreases with solar activity and the summer maximum regions become more predominant. (Source: https://www.sciencedirect.com/science/article/pii/0021916973901402)

1.8 Irregularities in Ionosphere:

The electron density of ionosphere has layered structure and within these layers the ions and electrons are not evenly distributed. There are a irregular distribution of electron density in the form of electron bubble and polar patches. These irregular distributions of electron density are

7 known as ionospheric irregularities. There are two regions in the whole globe where the irregularities are dominant.

I. Equatorial irregularity:

In the equatorial region the solar radiations are dominant which produces enhance ionization in the day time and just after Sun set the different ions and electrons start to recombine due to which there is electron density. Thus in the equatorial region of the ionosphere plasma bubbles are formed just after the Sun set.

II. Polar patches.

In the equatorial region the solar radiations are dominant which produces enhance ionization in the day time and just after sunset the different ions and electrons start to recombine due to which there is a depletion in electron density thus in the equatorial region of the ionosphere plasma bubbles are generated just after sunset.

In the polar regions 70°- 80° north and south poles the radiation belt plasma particles interact with the ionospheric constituents and give them energy to produce aurora and they are precipitated to the ionosphere which produces irregular distribution of electron density in small patches which is known as polar patches.

Depending upon the generation of irregularity in different layers of ionosphere. We can classify mainly into two types:

1. Sporadic-E irregularities.

2. Spread-F irregularities.

1. Sporadic E irregularities:

The irregularities generate in E region of ionosphere which are generated in small sizes just after noon time are known as sporadic E irregularities. They mostly occur in day time in afternoon hours in varieties of shape and sizes. The general generation mechanism is two stream instability.

2. Spread F irregularities:

8

The plasma irregularities generated in F layer of ionosphere mostly just after sunset is known as spread F irregularity it is generated by plasma instability in the equatorial region of the ionosphere mainly known as Rayleigh Taylor instability.

1.9 Importance of ionosphere

It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere (Wolf et al., 2007). Although people have learned to use the properties of the ionosphere in many beneficial ways over the last century, most notably for long distance propagation of radio signals, there is still a great deal to learn about its physics, chemical makeup and dynamic response to influences from the Sun (Gurevich and Tsodilina, 1985). The upper portions of the ionosphere can be studied to some extent with satellites, but the lower levels are below orbital altitudes while still too high to be studied using instruments carried by balloons or high-flying aircraft. Much of current theory is inferred by observing the ionosphere effect on communication systems (Goodman and Aarons, 1990). Incoherent scatter radars located on the ground can study small scale structures in the ionosphere nearly as well as an instrument in the ionospheric layer.

The ionosphere affects our modern society in many ways. International broadcasters such as the Voice of America and the British Broadcasting Corporation still use the ionosphere to reflect radio signals back towards the Earth so that their programs can be heard around the world. The ionosphere provides long range capabilities for commercial ship-to-shore communications, for trans-oceanic aircraft links, and for military communication and surveillance systems. The Sun has a major effect on the ionosphere; solar events such as flares or coronal mass ejections can lead to worldwide communication blackouts on the short wave bands (Lanzerotti, 2001).

Signals transmitted to and from satellites for communication and navigation purposes must pass through the ionosphere. Ionospheric irregularities, most common at equatorial latitudes (although they can occur anywhere), can have a major impact on system performance and reliability, and commercial satellite designers must account for their effects (Wernik et al., 2004).

The ionosphere is a plasma, the most common form of matter in the universe, often called the fourth state of matter. Plasmas do not exist naturally on the Earth's surface, and they are difficult

9 to contain for laboratory study. Many current active ionospheric research programs are efforts to improve our understanding of this type of matter by studying the ionosphere, which is not feasible in the laboratory.

Recently, it has become possible to produce computer simulations of ionospheric processes (Pavlov, 2003). The development of computer visualizations have allowed us to see and appreciate the enormous variability and turbulence that occurs in the ionosphere during a major solar geomagnetic storm and the resultant effects that can affect radio communication and navigation systems.

1.10 Equatorial ionization anomaly (EIA) and fountain effect:

At the geomagnetic equator, the magnetic field lines are horizontal and the primary E-region field is eastward during the daytime. Due to 푬 × 푩 drift, the plasma tends to move vertically upward. However, ion-neutral collision frequency being very large, ions are left behind setting up a vertical polarization field. This field in associated with geomagnetic field produces enhanced flow of electrons towards the west in the daytime. This enhanced flow of current is known as the equatorial electrojet. This is simplified picture of the equatorial electrojet in which the flow of vertical currents is inhibited. It would be more correct if the condition, 풅풊풗. 푱 = ퟎ is included in the theory.

India is one of the countries in the world, through which the geomagnetic equator passes. Hence, the phenomenon of the equatorial electrojet and associated plasma density irregularities, neutral wind ionosphere interaction and many other associated phenomena have been extensively studied using ground based, satellite and rocket born, and numerical simulation techniques by Indian scientist from different institution includeing, Physical Research Laboratory, Ahmedabad; Indian institute of geomagnetic, Mumbai; National Atmospheric Research Laboratory, Gadanki.

The dynamical responses of different region of Earth’s atmosphere is different and depends upon the variable energy inputs from the Sun. The upper atmosphere is coupled with different regions by electro-dynamical processes within the thermosphere-ionosphere-system (TIS) and magnetosphere-thermosphere ionosphere-system (MITS), etc. The equatorial vertical drift of ionization has large impact on the equatorial TIS and vice-versa. As the eastward electric field increases during the course of the day, due to intensification of global scale dynamo action,

10

푬 × 푩 drift also increases. The plasma is lifted to the higher altitudes, sometimes up to 1200 km (Tsunoda et al., 1982) or more. The high lifted altitude plasma is then eventually transported to poleward latitudes by diffusion along the magnetic field lines (Martyn, 1947). The latitudinal extent, to which this plasma can diffuse depends upon the altitude attained over the equator. The plasma is moved to lower latitude ionosphere along the field lines which have foot points in lower latitudes. The uplift of plasma over the equator and eventual diffusion of it down to the low latitudes, is analogous to a fountain. Therefore, this process is called the equatorial fountain effect. When the extra ionization reaches from above the equator to low latitudes in both sides of the equator, the density over the low latitude zones is enhanced. As a matter of fact, the ionization at the equator should be maximum due to maximum solar flux received there, but due to the fountain effect, the situation is reversed and the maximum density is observed on both sides of the equator in low latitudes. While the highest ion production rates in the ionosphere are found at the sub-solar point, the peak ion densities are located several degrees to either side of the magnetic equator. This causes an anomaly in the latitudes on both sides of the dip equator. This ionization anomaly is called equatorial ionization anomaly (EIA) or Appleton anomaly (Appleton, 1946; Hanson and Moffett, 1966; Lin et al., 2007; Sagawa et al., 2005). The EIA in terms of TEC in Indian zone and its relation with electrojet is described by Rastogi et al., (1975). The location of peak density or the crest of EIA moves away from the dip equator during the day and return towards the equator during the night, owing to the reversal in the polarity of the zonal electric field (Chatterjee et al., 2014).

11