task 3 p2 ionosphere complete

8/2/2019 Task 3 P2 Ionosphere Complete

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Task 3 P2.3 Describe the structure and nature of the ionosphere withreference to daily, seasonal and long-term changes

In your own words briefly describe how the nature and structure of the ionosphere varieswith time.

To answer this question, I will explain the current theory surrounding the ionosphere, and conclude withexamples of real-time data and forecasting.

The ionosphere is series of concentric ionised layers in the upper region of the Earths atmosphere in whichthe number of electrically charged particles ions and electrons are large enough to affect the propagation ofradio waves. It is best described in terms of separated and distinct layers, which vary in size andcompactness depending on altitude, day/night time, prevailing solar wind, ion density.

The charged particles are created by the action of mainly cosmic and solar radiation on neutral atoms andmolecules of air. The ionosphere begins at a height of about 30 miles (50 Km) above the mean sea level, butit is most distinct and important above 50 miles (80 Km). In the upper regions of the ionosphere, beginningseveral hundred miles above Earths surface and extending tens of thousands ofmiles into space is the

magnetosphere, a region where the behaviour of charged particles is strongly affected by the magnetic fieldsof Earth and the Sun.

The level of ionisation varies over the extent of the ionosphere, and are very variable. One reason is thatradiation reduces with decreasing altitude. Also the density of the gases varies and there is a variation in theproportions of atomic and molecular forms of the gases, the single atoms forms of gases being far greater athigher altitudes. These and a variety of other atmospheric issues mean that there are variations in the levelof ionisation with altitude.

An illustration to show the concentric ring layers of the Ionosphere

The level of ionisation changes with the time of day, time of year, and according to many other externalinfluences. One of the main reasons why the electron density varies is that the Sun, which gives rise to theionisation is only visible during the day. While the radiation from the Sun causes the atoms and molecules tosplit into free electrons and positive ions. The reverse effect also occurs. When a negative electron meets a

positive ion, the fact that dissimilar charges attract means that they will be pulled towards one another andthey may combine. This means that two opposite effects of splitting and recombination are taking place. This


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is known as a state of dynamic symmetry. Accordingly the level of ionisation is dependent upon the rate ofionisation and recombination. This has a significant effect on radio communications.

Other effects like the season and the state of the Sun also have a major effect. Sunspots and solardisturbances have a major impact on the level of radiation received, and these effects are covered in otherarticles on this website on Sunspots and Solar Disturbances

Frequencies belowabout 10 MHz (wavelengths longer than 30 metres), including broadcasts in the mediumwave and shortwave bands (and to some extent long wave), propagate most efficiently by sky wave at night.Frequencies above10 MHz typically propagate most efficiently during the day.

Because the lower-altitude layers of the ionosphere largely disappear at night, the refractive layer of theionosphere is much higher above the surface of the Earth at night. This leads to an increase in the "skip" or"hop" distance of the sky wave at night.

Because of the losses in both the refractive and reflective processes, the less hops that you can use to get toyour destination the better. To get fewer hops, you need a lower angle of incidence at the ionosphere,making each hop longer.

There is sometimes a dead zone where the surface wave has run out and the ionisation levels and

frequency in use will not allow the wave to return to earth. On a band like 10m this may mean thatcommunications over relatively short ranges is virtually impossible. For example it will be rare to hear signalson 10m from stations between 20km away (where the surface wave runs out) and 1500km away where thefirst ground reflection occurs.

An illustration to show how the ionosphere layers refract at different frequencies.

Layers of the ionosphere

Historically, the ionosphere was thought to be composed of a number of relatively distinct layers that wereidentified by the letters D, E, and F. The F layer was subsequently divided into regions F1 and F2. It is nowknown that all these layers are not particularly distinct, but the original naming scheme persists.

D region

The D region is the lowest ionospheric region, at altitudes of about 70 to 90 km (40 to 55 miles). The Dregion differs from the E and F regions in that its free electrons almost totally disappear during the night,because they recombine with oxygen ions to form electrically neutral oxygen molecules. At this time, radio

waves pass through to the strongly reflecting E and F layers above. During the day some reflection can beobtained from the D region, but the strength of radio waves is reduced; this is the cause of the markedreduction in the range of radio transmissions in daytime. At its upper boundary the D region merges with theE region.


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E region

The E region is also known as the Kennelly-Heaviside layer. It extends from an altitude of 60 miles (60Km) toabout 100 miles (160 Km). Unlike that of the D region, the ionisation of the E region remains at night, thoughit is considerably diminished. The ionisation density is typically 10

5electrons per cubic centimetre during the

day, though intermittent patches of stronger ionisation are sometimes seen.

Instead of attenuating radio communications signals this layer chiefly refracts them, often to a degree wherethey are returned to earth. As such they appear to have been reflected by this layer. However this layer stillacts as an attenuator to a certain degree.

The ionisation in this region results from a number of types of radiation. Soft X-Rays produce much of theionisation, although extreme ultra-violet (EUV) rays (very short wavelength ultra-violet light) also contribute.The degree to which all of the constituents contribute depends upon the state of the Sun and the latitude atwhich the observations are made.

F region

The F region extends upward from an altitude of about 100 miles (160 Km). This region has the greatestconcentration of free electrons. Although its degree of ionisation persists with little change through the night,

there is a change in the ion distribution. During the day, two layers can be distinguished: a small layer knownas F1 and above it a more highly ionised dominant layer called F2.

At night they merge at about the level of the F2 layer, which is also known as the Appleton layer. This regionreflects radio waves with frequencies up to about 30 MHz; the exact value depends on the peak amount ofthe electron concentration, typically 10

6electrons per cubic centimetre, though with large variations caused

by the sunspot cycle.

An illustration to show the ionosphere, and how at night the D layer disappears and the two Flayers merge.

The most important region in the ionosphere for long distance HF radio communications is the F region.During the daytime when radiation is being received from the Sun, it often splits into two, the lower one beingthe F1 region and the higher one, the F2 region. Of these the F1 region is more of an inflection point in theelectron density curve and it generally only exists in the summer.

Mechanisms of ionisation

Photoionisation

Most of the electrical activity in the ionosphere is produced by ionisation caused by light energy(photoionisation). Photons of short wavelength (that is, of high frequency) are absorbed by atmosphericgases. A portion of the energy is used to eject an electron, converting a neutral atom or molecule to a pair of
http://www.britannica.com/EBchecked/topic/457697/photo-ionizationhttp://www.britannica.com/EBchecked/topic/457697/photo-ionization


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charged atoms or molecules which are an electron, which is negatively charged, and a companion positiveion.

Ionization in the F1 region is produced mainly by ejection of electrons from molecular oxygen (O2), atomicoxygen (O), and molecular nitrogen (N2).

Positive ions in turn can react with neutral gases. There is a tendency for these reactions to favourproduction of more-stable ions.

Recombination

The electron density in the D, E, and F1 regions reflects for the most part a local balance between productionand loss. Electrons are removed mainly by, a process in which electrons attach to positively chargedmolecular ions and form highly energetic, unstable neutral molecules. These molecules decomposespontaneously.

Diffusion

Ions and electrons produced at high altitude are free to diffuse downward, guided by Earths magnetic field.

Photon absorption

Ionisation at any given level depends on three factors;

1. the availability of photons of a wavelength capable of effecting ionisation,2. a supply of atoms and molecules necessary to intercept this radiation,3. and the efficiency with which the atoms and molecules are able to do so

How the data is produced

An ionosonde, or chirp-sounder, is a special-radar for the study of the ionosphere. An ionosonde consistsof:

A high frequency (HF) transmitter, automatically tunable over a wide range. Typically the frequencycoverage is 0.523 MHz or 140 MHz, though normally sweeps are confined to approximately 1.612 MHz.

A tracking HF receiver which can automatically track the frequency of the transmitter.

An antenna with a suitable radiation pattern, which transmits well vertically upwards and is efficient overthe whole frequency range used.

Digital control and data analysis circuits.

An ionosonde measures the time for a wave to go up, and to be reflected around, and to come back down.Thus it measures the time, and not height. This translates to virtual height assuming the speed of light andmirror-like reflection. The real wave does not get as high as the virtual height

A real-time example of an Ionogram, which was recorded over Australia (26/01/12)


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An ionogram is a display of the data produced by an ionosonde. It is a graph of the virtual height of theionosphere plotted against frequency. Ionograms are often converted into electron density profiles. Datafrom ionograms may be used to measure changes in the Earth's ionosphere due to space weather events.

World-wide information gathered to produce the above graph, can be used to produce a general ionosphereprofile, as illustrated below.

A graphic to show the various layers of the ionosphere,their ion populations,and their respectiveheights above ground. The density in the ionosphere varies considerably.

Below a global data produced from;http://www.swpc.noaa.gov/drap/global.html

Real TimeData of amap to illustrate the D region frequency absorption
http://www.swpc.noaa.gov/drap/global.htmlhttp://www.swpc.noaa.gov/drap/global.htmlhttp://www.swpc.noaa.gov/drap/global.htmlhttp://www.swpc.noaa.gov/drap/global.html


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The map, above illustrates conditions in the D region of the ionosphere which is having a powerful effect onhigh frequency (HF) communications and low frequency (LF) navigation systems. The global D RegionAbsorption Predictions (D-RAP) depicts the D region at high latitudes where it is driven by particles as wellas low latitudes, where photons cause the prompt changes. (note, the purple region is day light, the blackregion is night, and the effects on high latitudes).

Below a global data produced from ;http://www.ips.gov.au/HF_Systems/6/5

Real Time Data of amap to illustrate the F2 region frequency absorption

References

Physics of the Upper Atmosphere (1960)...................................................................................By J.A. Ratcliffe

Physics of the Earths Upper Atmosphere (1965).........................................................................By C.O. Hines

An introduction to the ionosphere and magnetosphere............................................By John Ashworth Ratcliffe

The ionosphere:its significance for geophysics and radio communications................................By Karl Rawer

Models of the atmosphere and ionosphere: proceedings of Workshop VIII and X of the COSPAR twenty-fifthPlenary Meeting...........................................................................................................................By Karl Rawer

Path toward improved ionosphere specification and forecast models, Volume 33, Issue 6.......................................................................................................by D. Bilitza, Karl Rawer, Bodo W. Reinisch, COSPAR

D. Bilitza, International Reference Ionosphere 1990. (ccmc.gsfc.nasa.gov/modelweb/ionos/iri/IRI1990pp0-84.pdf)

D. Bilitza, International Reference Ionosphere 2000, Radio Science 36, 2001.(ccmc.gsfc.nasa.gov/modelweb/ionos/iri/iri_2000_rs.pdf)
http://www.ips.gov.au/HF_Systems/6/5http://www.ips.gov.au/HF_Systems/6/5http://www.ips.gov.au/HF_Systems/6/5http://www.ips.gov.au/HF_Systems/6/5


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Internet references

http://twistedphysics.typepad.com/cocktail_party_physics/ionosphere/

http://www.ips.gov.au/Educational/5/1

http://www.frankswebspace.org.uk/ScienceAndMaths/physics/physicsGCE/radioComms.htm

http://www.amateur-radio-wiki.net/index.php?title=Propagation

http://aprs.mountainlake.k12.mn.us/

http://prop.hfradio.org/

http://ccmc.gsfc.nasa.gov/modelweb/ionos/iri.html

http://iono.jpl.nasa.gov/gim.html

http://iono.jpl.nasa.gov/sitemap.html

http://www.rsgb.org/committees/spectrumforum/band-plans.php

http://www.encyclopedia.com/searchresults.aspx?q=ionosphere

http://www.britannica.com/EBchecked/topic/1369043/ionosphere-and-magnetosphere

http://www.britannica.com/EBchecked/topic/1262240/radio-technology/25125/The-ionosphere#ref189670


http://www.britannica.com/EBchecked/topic/199594/F-region


http://www.encyclopedia.com/searchresults.aspx?q=ionosphere

http://www.encyclopedia.com/topic/ionosphere.aspx

http://www.radio-electronics.com/info/propagation/ionospheric/ionosphere.php

http://www.radio-electronics.com/info/propagation/ionospheric/hf-propagation-basics.php

http://www.vlba.nrao.edu/memos/sci/gps_ion/node3.html

http://www.cnofs.org/Handbook_of_Geophysics_1985/Chptr10.pdf

http://physics.info/em-waves/

http://myplace.frontier.com/~k9la/The_Structure_of_the_Ionosphere.pdf

http://ecjones.org/physics.html

http://www.swpc.noaa.gov/info/Iono.pdf

http://www.ips.gov.au/HF_Systems/7/1/12

task 3 p2 ionosphere complete

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