The Atmosphere and Radio Waves | Ionosphere The Atmosphere and Radio Waves View More The atmosphere plays a vital role in the way in which radio waves travel around the earth. Without its action it would not be possible for signals to travel around the globe on the short wave bands, or travel greater than only the line of sight distance at higher frequencies. with an interest in the topic. Copyright: Attribution Non-Commercial (BY-NC) THE ATMOSPHERE AND RADIO WAVES The atmosphere plays a vital role in the way in which radio waves travel around the earth.Without its action it would not be possible for signals to travel around the globe on the shortwave bands, or travel greater than only the line of sight distance at higher frequencies. Infact the way in which the atmosphere affects radio is of tremendous importance for anyonewith an interest in the topic.In view of the importance of the atmosphere an overview of its make-up is given here. Layers of the Atmosphere The atmosphere can be split up into a variety of different layers according to their properties. As different aspects of science look at different properties there is no singlenomenclature for the layers.

The system that is most widely used is that associated with.Lowest is the troposphere that extends to a height of 10 km. Above this at altitudesbetween 10 and 50 km is found the stratosphere. This contains the ozone layer at a heightof around 20 km. Above the stratosphere, there is the mesosphere extending from analtitude of 50 km to 80 km, and above this is the thermosphere whereThere are two main layers that are of interest from a radio viewpoint. The first is thetroposphere that tends to affect frequencies above 30 MHz. The second is the ionosphere.This a region which crosses over the boundaries of the meteorological layers and extendsfrom around 60 km up to 700 km. Here the air becomes ionised, producing ions and freeelectrons. The free electrons affect radio waves at certain frequencies, often bending themback to earth so that they can be heard over vast distances around the world. Troposphere The lowest of the layers of the atmosphere is the troposphere. This extends from groundlevel to an altitude of 10 km. It is within this region that the effects that govern our weather occur.

To give an idea of the altitudes involved it is found that low clouds occur at altitudesof up to 2 km whereas medium level clouds extend to about 4 km. The highest clouds arefound at altitudes up to 10 km whereas modern jet airliners fly above this at altitudes of upto 15 km.Within the troposphere there is generally a steady fall in temperature with height and thishas a distinct bearing on some propagation modes which occur in this region. The fall intemperature continues in the troposphere until the tropopause is reached. This is the areawhere the temperature gradient levels out and then the temperature starts to rise. At thispoint the temperature is around -50 ºC.The refractive index of the air in the troposphere plays a dominant role in radio signalpropagation. This depends on the temperature, pressure and humidity. When radio signalsare affected this often occurs at altitudes up to 2 km. The ionosphere The ionosphere is an area where there is a very high level of free electrons and ions. It isfound that the free electrons affect radio waves.

Although there are low levels of ions andelectrons at all altitudes, the number starts to rise noticeably at an altitude of around 30 km.However it is not until an altitude of approximately 60 km is reached that the it rises to asufficient degree to have a major effect on radio signals.The overall way in which the ionosphere is very complicated. It involves radiation from thesun striking the molecules in the upper atmosphere. This radiation is sufficiently intensethat when it strikes the gas molecules some electrons are given sufficient energy to leavethe molecular structure. This leaves a molecule with a deficit of one electron that is calledan ion, and a free electron. As might be expected the most common molecules to beionised are nitrogen and oxygen.Most of the ionisation is caused by radiation in the form of ultraviolet light. At very highaltitudes the gases are very thin and only low levels of ionisation are created. As theradiation penetrates further into the atmosphere the density of the gases increases andaccordingly the numbers of molecules being ionised increase.

However when moleculesare ionised the energy in the radiation is reduced, and even though the gas density ishigher at lower altitudes the degree of ionisation becomes less because of the reduction of the level of ultraviolet light.At the lower levels of the ionosphere where the intensity of the ultraviolet light has beenreduced most of the ionisation is caused by x-rays and cosmic rays which are able topenetrate further into the atmosphere. In this way an area of maximum radiation exists withthe level of ionisation falling below and above it. 2 Often the ionosphere is thought of as a number of distinct layers. Whilst it is veryconvenient to think of the layers as separate, in reality this is not quite true. Each layer overlaps the others with the whole of the ionosphere having some level of ionisation. Thelayers are best thought of as peaks in the level of ionisation. These layers are givendesignations D, E, and F 1 Living With A Star: 3: Balloon/Rocket Mission: Layers of the Atmosphere Main Menu > 3: Balloon/Rocket Mission > Layers of the Atmosphere This drawing shows the different layers of the atmosphere. The ground is shown in brown at the bottom of the picture.

The altitude in kilometers is given on the right hand scale and is approximate. ( Click for a larger image .) © UC Regents The Different Levels of the Atmosphere are: Troposphere: This is the lowest atmospheric layer and is about seven miles (11 km) thick. Most clouds and weather are found in the troposphere. The troposphere is thinner at the poles (averaging about 8km thick) and thicker at the equator (averaging about 16km thick). The temperature decreases with altitude. Stratosphere: The stratosphere is found from about 7 to 30 miles (11-48 kilometers) above the Earths surface. In this region of the atmosphere is the ozone layer, which absorbs most of the harmful ultraviolet radiation from the Sun. The temperature increases slightly with altitude in the stratosphere. The highest temperature in this region is about 32 degrees Fahrenheit or 0 degrees Celsius. Mesosphere: The mesosphere is above the stratosphere. Here the atmosphere is very rarefied, that is, thin, and the temperature is decreasing with altitude, about 130 Fahrenheit (-90 Celsius) at the top.

Thermosphere: The thermosphere starts at about 55 kilometers. The temperature is quite hot; here temperature is not measured using a thermometer, but by looking at the motion and speed of the rarefied gases in this region, which are very energetic but would not affect a thermometer. Temperatures in this region may be as high as thousands of degrees. Exosphere: The exosphere is the region beyond the thermosphere. Ionosphere: The ionosphere overlaps the other atmospheric layers, from above the Earth. The air is ionized by the Suns ultraviolet light. These ionized layers affect the transmittance and reflectance of radio waves. Different ioniosphere layers are the D, E (Heaviside-Kennelly), and F (Appleton) regions. This drawing shows the penetration of different kinds of electromagnetic radiation into the atmosphere. Note also the different layers of the ionosphere (D, E, F) and the level at which auroras occur.( Click for a larger image .) © UC Regents Starting at the left: Lightning can generate whistlers, radio waves that can travel long distances. Very high frequency radio waves (VHF) can travel through the atmosphere to and from satellites and are used for communication.

The airglow is due to a chemical reaction in the upper atmosphere that emits light. Many radio waves are reflected at night by the ionosphere making it possible to hear AM radio and shortwave radio stations that are far away. Gamma rays and x-rays are absorbed by the upper atmosphere; visible light can penetrate to sea level. Many infrared wavelengths can also pass through the atmosphere. Related to chapter 3 in the print guide. See Scientific Ballooning to learn how scientists use balloons for research. Click for the definitions of the following words that are used on this page: (Definitions appear in a pop-up window.) RADIO WAVE TRANSMISSION There are two principal ways in which electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna. One way is by GROUND WAVES and the other is by SKY WAVES . Ground waves are radio waves that travel near the surface of the Earth (surface and space waves). Sky waves are radio waves that are reflected back to Earth from the ionosphere. (See figure 2-11.) Figure 2-11. - Ground waves and sky waves. Ground Waves The ground wave is actually composed of two separate component waves.

These are known as the SURFACE WAVE and the SPACE WAVE (fig. 2-11). The determining factor in whether a ground wave component is classified as a space wave or a surface wave is simple. A surface wave travels along the surface of the Earth. A space wave travels over the surface. SURFACE WAVE . - The surface wave reaches the receiving site by traveling along the surface of the ground as shown in figure 2-12. A surface wave can follow the contours of the Earth because of the process of diffraction . When a surface wave meets an object and the dimensions of the object do not exceed its wavelength , the wave tends to curve or bend around the object. The smaller the object, the more pronounced the diffractive action will be. Figure 2-12. - Surface wave propagation. As a surface wave passes over the ground, the wave induces a voltage in the Earth. The induced voltage takes energy away from the surface wave, thereby weakening, or attenuating, the wave as it moves away from the transmitting antenna . To reduce the attenuation, the amount of induced voltage must be reduced.

This is done by using vertically polarized waves that minimize the extent to which the electric field of the wave is in contact with the Earth. When a surface wave is horizontally polarized, the electric field of the wave is parallel with the surface of the Earth and, therefore, is constantly in contact with it. The wave is then completely attenuated within a short distance from the transmitting site. On the other hand, when the surface wave is vertically polarized, the electric field is vertical to the Earth and merely dips into and out of the Earth's surface. For this reason, vertical polarization is vastly superior to horizontal polarization for surface wave propagation. The attenuation that a surface wave undergoes because of induced voltage also depends on the electrical properties of the terrain over which the wave travels. The best type of surface is one that has good electrical conductivity . The better the conductivity, the less the attenuation. Table 2-2 gives the relative conductivity of various surfaces of the Earth. Table 2-2. - Surface Conductivity Jungle Unusable Another major factor in the attenuation of surface waves is frequency . Recall from earlier discussions on wavelength that the higher the frequency of a radio wave, the shorter its wavelength will be.

These high frequencies, with their shorter wavelengths, are not normally diffracted but are absorbed by the Earth at points relatively close to the transmitting site. You can assume, therefore, that as the frequency of a surface wave is increased, the more rapidly the surface wave will be absorbed, or attenuated, by the Earth. Because of this loss by attenuation, the surface wave is impractical for long-distance transmissions at frequencies above 2 megahertz. On the other hand, when the frequency of a surface wave is low enough to have a very long wavelength, the Earth appears to be very small, and diffraction is sufficient for propagation well beyond the horizon. In fact, by lowering the transmitting frequency into the very low frequency (vlf) range and using very high-powered transmitters , the surface wave can be propagated great distances. The Navy's extremely high-powered vlf transmitters are actually capable of transmitting surface wave signals around the Earth and can provide coverage to naval units operating anywhere at sea. SPACE WAVE . - The space wave follows two distinct paths from the transmitting antenna to the receiving antenna - one through the air directly to the receiving antenna , the other reflected from the ground to the receiving antenna. This is illustrated in figure 2-13.

The primary path of the space wave is directly from the transmitting antenna to the receiving antenna. So, the receiving antenna must be located within the radio horizon of the transmitting antenna. Because space waves are refracted slightly, even when propagated through the troposphere , the radio horizon is actually about one-third farther than the line-of-sight or natural horizon. Figure 2-13. - Space wave propagation. Although space waves suffer little ground attenuation, they nevertheless are susceptible to fading. This is because space waves actually follow two paths of different lengths (direct path and ground reflected path) to the receiving site and, therefore, may arrive in or out of phase. If these two component waves are received in phase, the result is a reinforced or stronger signal. Likewise, if they are received out of phase, they tend to cancel one another, which results in a weak or fading signal. Q.13 What is the determining factor in classifying whether a radio wave is a ground wave or a space wave? Q.14 What is the best type of surface or terrain to use for radio wave transmission? Q.15 What is the primary difference between the radio horizon and the natural horizon?

Q.16 What three factors must be considered in the transmission of a surface wave to reduce attenuation? Which layer of the atmosphere reflects radio waves? | Reference.com Which layer of the atmosphere reflects radio waves? A: Quick Answer The ionosphere is the layer of the atmosphere that reflects radio waves. This effect is caused by the ionization, or electrical charging, of the gas particles present in the upper atmosphere by X-rays and ultraviolet rays emitted by the sun. Full Answer The ionization of gas particles in the ionosphere aids in the transmission of radio waves by reflecting waves back to Earth, rather than allowing them to escape the atmosphere. Solar flares, the spontaneous eruption of high-energy radiation from the surface of the sun, can cause a sharp increase in the number of ionized particles in the upper atmosphere and can interfere with radio wave transmissions. Radiowave Propagation & Atmosphere | Radio-Electronics.Com Radio Wave Propagation &Atmosphere - overview of radio signals and radio-wave propagation and how different areas of the atmosphere affect radio communications. In this section Radio propagation & atmosphere The way that radio signals propagate, or travel from the radio transmitter to the radio receiver is of great importance when planning a radio communications network or system.

In many instances, terrestrial radio propagation is governed to a great degree by the regions of the atmosphere through which the signals pass. Without the action of the atmosphere it would not be possible for radio communications signals to travel around the globe on the short wave bands, or travel greater than only the line of sight distance at higher frequencies. In fact the way in which the atmosphere affects radio communications is of tremendous importance for anyone associated with radio communications, whether they are for two way radio communications links, mobile radio communications, radio broadcasting, point to point radio communications or any other radio. In view of the importance of the atmosphere to radio communications, an overview of its make-up is given here. Atmospheric layers The atmosphere can be split up into a variety of different layers according to their properties. Although there are is a number of different ways of classifying the different atmospheric regions - typically different scientific displaces may have their own nomenclature as a result of their interest in different properties. The lowest area in the meteorological system is referred to as the Troposphere. This extends to altitudes of around 10km above the Earth's surface. Above this is the Stratosphere that extends from altitudes around 10 to 50km.

Above this at altitudes between 50 and 80 km is the Mesosphere and above this is the Themosphere: named because of the dramatic rise in temperatures here. From the viewpoint of radio propagation, there are two main areas of interest: Troposphere: As a very approximate rule of thumb, this area of the atmosphere tends to affect signals more above 30 MHz or so. Ionosphere: The ionosphere is the area that enables signals on the short wave bands to traverse major distances. It crosses over the meteorological boundaries and extends from altitudes around 60 km to 700 km. The region gains its name because the air in this region becomes ionised by radiation primarily from the sun. Free electrons in this region have affect radio signals and may be able to refract them back to Earth dependent upon a variety of factors. Troposphere The lowest of the layers of the atmosphere is called the troposphere. The troposphere extends from ground level to an altitude of 10 km. It is within the tropospheric region that what we term the weather, occurs. Low clouds occur at altitudes of up to 2 km and medium level clouds extend to about 4 km.

The highest clouds are found at altitudes up to 10 km whereas modern jet airliners fly above this at altitudes of up to 12 km. Within this region of the atmosphere there is generally a steady fall in temperature with height. This affects radio propagation because it affects the refractive index of the air. This plays a dominant role in radio signal propagation and the radio communications applications that use tropospheric radio-wave propagation. This depends on the temperature, pressure and humidity. When radio communications signals are affected this often occurs at altitudes up to 2 km. The ionosphere The ionosphere is the area that is traditionally thought of as providing the means by which long distance communications can be made. It has a major effect on what are normally thought of as the short wave bands, providing a means by which signals appear to be reflected back to earth from layers high above the ground. The ionosphere has a high level of free electrons and ions - hence the name ionosphere. It is found that the level of electrons sharply increases at altitudes of around 30 km, but it is not until altitudes of around 60km are reached that the free electrons are sufficiently dense to significantly affect radio signals.

The ionisation occurs as a result of radiation, mainly from the sun, striking molecules of air with sufficient energy to release electrons and leave positive ions. Obviously when ions and free electrons meet, then they are likely to recombine, so a state of dynamic equilibrium is set up, but the higher the level of radiation, the more electrons will be freed. Much of the ionisation is caused by ultraviolet light. As it reaches the higher reaches of the atmosphere it will be at its strongest, but as it hits molecules in there upper reaches where the air is very thin, it will ionise much of the gas. In doing this, the intensity of the radiation is reduced At the lower levels of the ionosphere, the intensity of the ultraviolet light his much reduced and more penetrating radiation including x-rays and cosmic rays gives rise to much of the ionisation. As a result of many factors it is found that the level of free electrons varies over the ionosphere and there are areas that affect radio signals more than others. These are often referred to as layers, but are possibly more correctly thought of a regions as they are quite indistinct in many respects. These layers are given designations D, E, and F1 and F2.

Description of ionospheric regions D region: The D layer or D region is the lowest of the regions that affects radio signals. It exists at altitudes between about 60 and 90 km. It is present during the day when radiation is being received from the sun, but because of the density of molecules at this altitude, free electrons and ions quickly recombine after sunset when there is no radiation to retain the ionisation levels. The main effect of the D region is to attenuate signals that pass through it, although the level of attenuation decreases with increasing frequency. Accordingly its effects are very obvious on the medium wave broadcast band - during the day when the D region is present, few signals are heard beyond that provided by ground wave coverage. At night when the region is not present, signals are reflected from higher layers and signals are heard from much further afield. E region: Above the D region, the next region is the E region or E layer. This exists at an altitude of between 100 and 125 km. The main effect of this region is to reflect radio signals although they still undergo some attenuation. In view of its altitude and the density of the air, electrons and positive ions recombine relatively quickly.

This means that after sunset when the source of radiation is removed, the layer reduces in strength very considerably although some residual ionisation does remain. F region: The F region or F layer is higher than both the D and E regions and it the most important region for long distance HF communications. During the day it often splits into two regions known as the F1 and F2 regions, the F1 layer being the lower of the two. At night these two regions merge as a result of the reduction in level of radiation to give one region called the F region. The altitudes of the F regions vary considerably. Time of day, season and the state of the sun all have major effects on the F region. As a result any figures for altitude are very variable and the following figures should only be used as a very rough guide. Typical summer altitudes for the F1 region may be approximately 300 km with the F2 layer at about 400 km or even higher. Winter figures may see the altitudes reduced to about 200 km and 300 km. Night time altitudes may be around 250 to 300 km.

Like the D and E regions, the level of ionisation fort he F region falls at night, but in view of the much lower air density, the ions and electrons combine much more slowly and the F layer decays much more slowly. As a result it is able to support radio communications at night, although changes are experienced because of the lessening of the ionisation levels. The way in which the various regions in the atmosphere affect radiowave propagation and radio communications is a fascinating study. There are very many factors that influence radio propagation and the resulting radio communications links that can be established. Predicting the ways in which this occurs is complicated and difficult, however it is possible to gain a good idea of the likely radio communications conditions using some simple indicators. Further pages in this section of the website detail many of these aspects. Tracking Solar Flares Back to top Nighttime During the night (image below, right side), the ionosphere has only the F and E layers. A VLF wave from a transmitter reflects off the ions in the E layer and bounces back.

Back to top Daytime During the daytime (image above, left side), the Sun’s X-ray and UV light increase the ionization of the ionosphere, creating the D and enhancing the E layers, and splitting the F region into 2 layers. The D layer is normally not dense enough to reflect the radio waves. However, the E layer is, so the VLF signals go through the D layer, bounce off the E layer, and go back down through the D layer to the ground. The signals lose energy as they penetrate through the D layer and hence radios pick up weaker signals from the transmitter during the day. When a solar flare occurs, even the D layer becomes ionized, hence allowing signals to bounce off it. Back to top Sunrise and Sunset Effects The reflection height for VLF waves changes from about 70 km in the daytime to about 85 km at night (44-53 miles). During sunrise, sunlight strikes the ionosphere before the ground, and at sunset the light continues to strike the ionosphere after the Sun has set above the ground. The amount of time it takes for the Sun to ionize the ionosphere once it strikes it is virtually instantaneous.

So at sunrise and sunset, the signal your SID monitor picks up is basically the effect of the VLF waves bouncing off the ionosphere along the entire path from transmitter to receiver, which could be several thousand miles. That is, the monitor picks up this process of change in conditions as sunlight sweeps over the path between transmitter and receiver. The length of the effect is dependent upon the longitudinal separation between the two sites (because the sunrise/sunset terminator takes longer to sweep over the path). Hence if you look at primarily north/south paths between transmitter and receiver, the data will show a well-defined “daytime” and a well-defined nighttime, with a pretty quick transition. For paths far apart in longitude, however, the sunrise/set effect lasts much longer and does not feature as rapid changes. Latitude contributes too, since the equatorial daytime is the same length, but the higher latitude daytimes are highly seasonal in length. The Ionosphere 2.) Aluminum screen (can be purchased at home supply store such as Home Depot) Introduction The electromagnetic spectrum consists of waves of many wavelengths ranging from very long wavelength radio waves to very short wavelength gamma rays. Visible light, consisting of short wavelength waves, is placed near the middle of this spectrum.

Visible light can pass through window glass, but a solid wall will absorb a portion of the light and reflect the remaining portions. Scientists would say that glass is transparent to visible light, but a wall is opaque. Since the atmosphere is transparent to visible light (while absorbing some of the light), astronomers who use telescopes can see things from far away using visible light to form images. Earth's atmosphere, however, acts an opaque barrier to much of the electromagnetic spectrum. The atmosphere absorbs most of the wavelengths shorter than ultraviolet, most of the wavelengths between infrared and microwaves, and most of the longest radio waves. For radio astronomers this leaves only short wave radio to penetrate the atmosphere and bring information about the universe to our Earth-bound instruments. The main frequency ranges allowed to pass through the atmosphere are referred to as the radio window. The radio window consists of frequencies which range from about 5 MHz (5 million hertz) to 30 GHz (30 billion hertz). The low-frequency end of the window is limited by signals being reflected by the ionosphere back into space, while the upper limit is caused by absorption of the radio waves by water vapor and carbon dioxide in the atmosphere. As atmospheric conditons change the radio window can expand or shrink.

On clear days with perfect conditions signals as high as 300GHz have been detected. It is the effects of the ionosphere on the lower end of the radio spectrum that we will investigate in this exercise. The ionosphere The ionized part of the Earth's atmosphere is known as the ionosphere. Ultraviolet light from the sun collides with atoms in this region knocking electrons loose. This creates ions, or atoms with missing electrons. This is what gives the Ionosphere its name and it is the free electrons that cause the reflection and absorption of radio waves. How does this affect our observations of Jupiter? When the sun is overhead during the day, most of the ionosphere is ionized due to the large amount of ultraviolet light coming from the sun. As radio waves enter Earth's atmosphere from space some of the waves are absorbed by the electrons in the ionosphere while others pass through and are detectable to ground based observers. The frequency of each of these waves is what determines whether or not it is absorbed or able to pass through the atmosphere. Low frequency radio waves do not travel very far through the atmosphere and are absorbed rather quickly. Higher frequency waves are able to pass through the atmosphere entirely and reach the ground.

This process also works in reverse for radio waves produced on the earth. The high frequency waves pass through the ionosphere and escape into space while the low frequency waves reflect off the ionosphere and essentially "skip" around the earth. The diagram below will help illustrate this: What's all this talk about high frequency and low frequency radio waves? What types of things fall in each range? Astronomical radio sources emit over a wide range of frequencies. Their emission can be measured ranging from low frequencies to high frequencies. Jupiter for example emits radio waves from about 10 kHz up to about 300 GHz. This emission is broken into several groupings. The lowest being the kilometric emission which ranges from 10kHz up to 1000kHz. Other frequency groups include hectometric (1000KHz to 3MHz), decametric (3MHz to 40MHz), and decimetric (100MHz to 300GHz). It is the decametric emissions that we are concerned about with Radio Jove. The Radio Jove receiver is tuned to a frequency of 20.1MHz. Radio waves produced on Earth are mostly manmade and are often at one specific frequency.

In fact, this is one way astronomers can tell a signal created on Earth apart from an astronomical signal. If they are able to tune their receivers to a slightly higher or lower frequency and the signal disappears it is most likely an Earth based signal. Radio waves fall into three main categories with a variety of uses: H.F. (High Frequency 3 to 30MHz) Long Range communications. Shipping, Aircraft, World Broadcast Communications, Radio Amateurs. Use involves reflecting the signal off the ionosphere back down to waiting recieving stations. Prone to atmospheric changes causing fading and noise. Range from 500 to thousands of Kilometres. V.H.F. (Very High Frequency 30 - 300 MHz) Medium range communications. Fleet vehicles, mobile, coastal shipping and air to tower communications. Range 70-100km (aircraft several hundred km). U.H.F (Ultra High Frequency 300-3000 MHz) This is the domain of such things as Police handheld radios, cell-phones, T.V., and spacecraft to ground communications. In the high U.H.F. range the signal can "bounce" off buildings and reflect until it is detected by a receiver.

Activities We can use the aluminum screens to mimic the effects of the ionosphere. To do this follow the following steps: 1.) Take the AM radio outside and tune in an AM station. 2.) Using the aluminum screen construct a small cage which you can place over the AM receiver. After placing the cage over the receiver observe how the station is weaker or disappears. 3.) Remove the cage and the station returns. The screen acts as a solid object to the incoming radio waves. We can imagine the radio as an Earth based receiving station. The screen, like the ionosphere, reflects the low energy AM radio waves and they are not detectable. Below is an animation comparing the ionospheric conditions during a typical day with that of a day containing an ionospheric storm. An ionospheric storm is caused by a coronal mass ejection from the sun that strikes the Earth's atmosphere. These mass ejections contain large amounts of particles that smash into the ionosphere and knock electrons loose from atoms. As discussed above the loose electrons reflect radio waves from astronomical sources back into space. The addition of loose electrons as a result of a mass ejection makes observations and communications difficult. The dark blue and purple areas are the areas where the number of loose electrons is low.

In these areas there are few electrons to reflect radio waves and thus lower frequency waves are able to reach the ground. As can be seen from the animations the night time and early morning hours are best for observations due to the fact that the sun is not in the sky and its ultraviolet light is not reaching the atmosphere at this time. The density of electrons (how many electrons there are per every cubic centimeter) is represented by the varying colors. Bands of high density that appear at high latitudes during the storm but disappear rapidly as it subsides are due to the high velocity particles smashing into the atoms in the atmosphere and knocking electrons free. These same high velocity particles produce the auroral lights. We can use these maps and the varying colors to find the lowest frequency that is detectable from the ground. The lowest frequency detectable, known as the critical frequency, is related to the density of electrons by the equation: f = 9x10^-3 x sqrt(N) MHz. In this equation f is the critical frequency and N is the electron density, sqrt means to take the square root of the electron density.

In the maps above the electron density ranges from 33300 electrons/cm^3 (dark blue) to 249750 electrons/cm^3 (green) to 552780 electrons/cm^3 (red). Exercise Use these values to calculate the critical frequency for the dark blue, green and red areas on the maps. dark blue = 1.8 MHz, green = 5 MHz and red = 7.2 MHz Questions and Problems The speed of all electromagnetic waves (the speed of light, or c) is 300,000,000 meters per second (3 x 10^8 m/s). The distance traveled (d) by an electromagnetic wave in time (t) is given by the equation: d = c x t. The frequency and wavelength of an electromagnetic wave are related by the equation: c = x f where f is the frequency of the wave, c is the speed of light, and is the wavelength in meters. Example 1.) How long would it take a radio wave to travel to Earth from the moon? The moon is 400,000 kilometers from Earth.

(Note: 400,000 kilometers = 4x10^8 meters.) d = c x t (500x10^3) = 600m 1.) If radio astronomers are studying 20MHz radio signals from the sun which is 1.5x10^11 meters away from the Earth, how long did take for the radio signals to reach Earth? t = 500 seconds 2.) 3MHz is the lowest frequency which will pass through the ionosphere. Calculate the wavelength of these waves. = 100 meters 3.) A general rule is that a spacing on the order of 1/10 of a wavelength will seem solid to a radio wave. What size would the spacings in our mesh screen have to be in order to block the 20MHz solar radio waves discussed in problem 1? = 15 meters, spacing = 1.5 meters 4.) As shown above, the ionosphere can act as a solid object to radio waves. Using this idea do you think it is possible to build a radio dish that is not solid? If so, what are the benefits and advantages of a mesh dish?

lightweight, saves building costs, not much gathers inside (rain, snow, leaves, etc.), in cases of large antennas a mesh can allow sunlight to reach the ground below and help prevent erosion of soil by allowing growth of grass and plant life. 5.) If spacings of 1/10 of a wavelength seem solid to electromagnetic waves would imperfections in a solid radio dish (holes or knicks) of this size matter? Radio studies deal with waves ranging from about 100m to 1mm. If we built a solid dish for each of these extremes (100m and 1mm) how small could the imperfections in our antenna be? 10 meters and .1mm (or .0001m) Credits The H.F/V.H.F./U.H.F. image located in the section describing high and low frequency radio waves as well as the Earth image under the ionosphere section are courtesy of Elbate Engineering LTD. ~elbate/propo.htm What is the structure of the atmosphere? The atmosphere has several different layers The Earth's atmosphere has several distinct layers, some of which are shown in the image above. The layer nearest the Earth's surface is called the troposphere.

The troposphere is usually approximately 16 km (ten miles) thick and is the layer that contains most of the weather. Above the troposphere is the stratosphere, which stretches from ten miles to 30 miles high. The stratosphere contains the ozone layer, which protects the Earth's surface from dangerous ultraviolet radiation from the Sun. The ionosphere is found above the stratosphere. This layer contains many free electrons and ions, which reflect radio waves. This reflection allows radio waves to travel much farther than they would without the ionosphere. The highest layer of the atmosphere is the exosphere. Here the air is extremely thin and some of the air molecules are lost to space. Credit: NASA Goddard DAAC Earth's Atmosphere, Atmosphere Information, Facts, News, Photos -- National Geographic More » Protective Buffer The Earth's atmosphere is more than just the air we breathe. It's also a buffer that keeps us from being peppered by meteorites, a screen against deadly radiation, and the reason radio waves can be bounced for long distances around the planet. The air that accomplishes all of this is composed of five major layers. The lowest is the troposphere, which is the layer that provides most of our weather.

It contains about four-fifths of the Earth's air, but extends only to a height of about 11 miles (17 kilometers) at the Equator and somewhat less at the Poles. The name comes from a Greek word that refers to mixing. And mixing is exactly what happens within the troposphere, as warm air rises to form clouds, rain falls, and winds stir the lands below. Typically, the higher you go in the troposphere, the colder it gets. Above the troposphere is the stratosphere. It extends to a height of about 30 miles (50 kilometers) and includes the ozone layer, which blocks much of the sun's harmful ultraviolet rays. The stratosphere is warmer than the troposphere because of the energy from the ultraviolet light absorbed by the ozone. At its base, the stratosphere is extremely cold, about -110 degrees Fahrenheit (-80 degrees Celsius). At its top, the temperature has risen back nearly to freezing. Next comes the mesosphere. In this layer, the air temperature drops again, down to nearly -180 degrees Fahrenheit (-120 degrees Celsius) at the top.

Meteors generally burn up in the mesosphere, which extends to a height of about 52 miles (85 kilometers). This is why the Earth's surface isn't pocked with meteor craters, like the moon's. Entering Outer Space Above the mesosphere is the ionosphere. It extends to about 430 miles (690 kilometers) and is so thin it's generally considered part of outer space. The International Space Station and many satellites orbit within the ionosphere. The ionosphere is named for the ions created within this layer by energetic particles from sunlight and outer space. These ions create an electrical layer that reflects radio waves, allowing radio messages to be sent across oceans in the days before communication satellites. Electrical displays in the ionosphere also create the auroras called the Northern and Southern Lights. Beyond the ionosphere lies the exosphere. This tenuous portion of the Earth's atmosphere extends outward until it interacts with the solar wind. Solar storms compress the exosphere. When the sun is tranquil, this layer extends further outward.

Its top ranges from 620 miles (1,000 kilometers) to 6,214 miles (10,000 kilometers) above the surface, where it merges with interplanetary space.

Question: Which of the Earth's atmospheric layers reflects radio waves?
Answer: Ionosphere