In free space, all electromagnetic waves (radio, light, X-rays, etc) obey the inverse-square law which states that the power density of an electromagnetic wave is proportional to the inverse of the square of the distance from the source or:

Doubling the distance from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.

The power density per surface unit is proportional to the product of the electric and magnetic field strengths. Thus, doubling the propagation path distance from the transmitter reduces each of their received field strengths over a free-space path by one-half.

Surface modes (Surface wave)

In this mode the radio wave propagates by interacting with the semi-conductive surface of the earth. The wave "clings" to the surface and thus follows the curvature of the earth. Vertical polarization is used to alleviate short circuiting the electric field through the conductivity of the ground. Since the ground is not a perfect electrical conductor, ground waves are attenuated rapidly as they follow the earth’s surface. Attenuation is proportional to the frequency making this mode mainly useful for LF and VLF frequencies.

Today LF and VLF are mostly used for time signals, and for military communications, especially with ships and submarines. Early commercial and professional radio services relied exclusively on long wave, low frequencies and ground-wave propagation. To prevent interference with these services, amateur and experimental transmitters were restricted to the higher (HF) frequencies, felt to be useless since their ground-wave range was limited. Upon discovery of the other propagation modes possible at medium wave and short wave frequencies, the advantages of HF for commercial and military purposes became apparent. Amateur experimentation was then confined only to authorized frequency segments in the range.

Direct modes (line-of-sight)

Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes at VHF and higher frequencies. Because radio signals can travel through many non-metallic objects, radio can be picked up through walls. This is still line-of-sight propagation. Examples would include propagation between a satellite and a ground antenna or reception of television signals from a local TV transmitter.

Ground plane reflection effects are an important factor in VHF line of sight propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power law for ground-plane limited radiation.

Ionospheric modes (skywave)

Skywave propagation, also referred to as skip, is any of the modes that rely on refraction of radio waves in the ionosphere, which is made up of one or more ionized layers in the upper atmosphere. F2-layer is the most important ionospheric layer for HF propagation, though F1, E, and D-layers also play some role. These layers are directly affected by the sun on a daily cycle, the seasons and the 11-year sunspot cycle determines the utility of these modes. During solar maxima, the whole HF range up to 30 MHz can be used and F2 propagation up to 50 MHz are observed frequently depending upon daily solar flux values. During solar minima, propagation of higher frequencies is generally worse.

Forecasting of skywave modes is of considerable interest to amateur radio operators and commercial marine and aircraft communications, and also to shortwave broadcasters.

Meteor scattering

Meteor scattering relies on reflecting radio waves off the intensely ionized columns of air generated by meteors. While this mode is very short duration, often only from a fraction of second to couple of seconds per event, digital Meteor burst communications allows remote stations to communicate to a station that may be hundreds of miles up to over 1,000 miles (1,600 km) away, without the expense required for a satellite link. This mode is most generally useful on VHF frequencies between 30 and 250 MHz.

Auroral reflection

Intense columns of Auroral ionization at 100 km altitudes within the auroral oval reflect radio waves, perhaps most notably on HF and VHF. The reflection is angle-sensitive - incident ray vs. magnetic field line of the column must be very close to right-angle. Random motions of electrons spiraling around the field lines create a Doppler-spread that broadens the spectra of the emission to more or less noise-like—depending on how high radio frequency is used. The radio-aurora is observed mostly at high latitudes and rarely extend down to middle latitudes. The occurrences of radio-auroras depends on solar activity (flares, coronal holes, CMEs) and annually the events are more numerous during solar cycle maximas.

Radio aurora includes the so-called afternoon radio aurora which produces stronger but more distorted signals and after the Harang-minima, the late-night radio aurora (sub-storming phase) returns with variable signal strength and lesser doppler spread. The propagation range for this predominantly back-scatter mode extends up to about 2000 km in east-west plane, but strongest signals are observed most frequently from north at nearby sites on same latitudes.

Rarely, a strong radio-aurora is followed by Auroral-E, which resembles both propagation types in some ways.

Sporadic-E propagation

Sporadic E (Es) propagation can be observed on HF and VHF bands. It must not be confused with ordinary HF E-layer propagation. Sporadic-E at mid-latitudes occurs mostly during summer season, from May to August in the northern hemisphere and from November to February in the southern hemisphere. There is no single cause for this mysterious propagation mode. The reflection takes place in a thin sheet of ionisation around 90 km height. The ionisation patches drift westwards at speeds of few hundred km per hour. There is a weak periodicity noted during the season and typically Es is observed on 1 to 3 successive days and remains absent for a few days to reoccur again. Es do not occur during small hours, the events usually begin at dawn, there is a peak in the afternoon and a second peak in the evening. Es propagation is usually gone by local midnight.

Maximum observed frequency (MOF) for Es is found to be lurking around 30 MHz on most days during the summer season, but sometimes MOF may shoot up to 100 MHz or even more in ten minutes to decline slowly during the next few hours. The peak-phase includes oscillation of MOF with periodicity of approximately 5...10 minutes. The propagation range for Es single-hop is typically 1000 to 2000 km, but with multi-hop, double range is observed. The signals are very strong but also with slow deep fading.

Thomas F. Giella, a noted retired Meteorologist, Space Plasma Physicist and Amateur Radio Operator, KN4LF cites the following from his professional research.

Just as the E layer is the main refraction medium for medium frequency (300–3000 kHz) signal propagation within approximately 5000 km (3000 mi), so is a Sporadic-E (Es) cloud. Sporadic-E (Es) clouds occur at approximately 100 km (60 mi) in altitude and generally move from ESE to WNW. Like Stratosphere level warming and Troposphere level temperature and moisture discontinuities, Sporadic-E (Es) clouds can depending on the circumstances absorb, block or refract medium, high and very high frequency RF signals in an unpredictable manner.

The main source for "high latitude" Sporadic E (Es) clouds is geomagnetic storming induced radio aurora activity.

The main source for "mid latitude" Sporadic-E (Es) clouds is wind shear produced by internal buoyancy/gravity waves (IBGW's), that create traveling ionosphere disturbances (TID's), most of which are produced by severe thunderstorm cell complexes with overshooting tops that penetrate into the Stratosphere. Another tie in between Sporadic-E (Es) and a severe thunderstorm is the Elve.

The main sources for "low latitude" Sporadic-E (Es) clouds is wind shear produced by internal buoyancy/gravity waves (IBGW's), that create traveling ionosphere disturbances, most of which are produced by severe thunderstorm cell complexes tied to tropical cyclones. High electron content in the Equatorial Ring Current also plays a role.

The forecasting of Sporadic-E (Es) clouds has long been considered to be impossible. However it is possible to identify certain troposphere level meteorological conditions that can lead to the formation of Sporadic E (Es) clouds. One is as mentioned above the severe thunderstorm cell complex.

Sporadic-E (Es) clouds have been observed to initially occur within approximately 150 km (90 mi) to the right of a severe thunderstorm cell complex in the northern hemisphere, with the opposite being observed in the southern hemisphere. To complicate matters is the fact that Sporadic-E (Es) clouds that initially form to the right of a severe thunderstorm complex in the northern hemisphere, then move from ESE-WNW and end up to the left of the severe thunderstorm complex in the northern hemisphere. So one has to look for Sporadic-E (Es) clouds on either side of a severe thunderstorm cell complex. Things get even more complicated when two severe thunderstorm cell complexes exist approximately 1000–2000 miles apart.

Not all thunderstorm cell complexes reach severe levels and not all severe thunderstorm cell complexes produce Sporadic-E (Es). This is where knowledge in tropospheric physics and weather analyses/forecasting is necessary.

Some of the key elements in identifying which severe thunderstorm cell complexes have the potential to produce Sporadic-E (Es) via wind shear, from internal buoyancy/gravity waves, that produce traveling ionosphere disturbances include:

1.) Negative tilted mid and upper level long wave troughs.

2.) Approximate 150 knot (170 mph, 280 km/h) jet stream jet maxes that produce divergence and therefore create a sucking vacuum effect above thunderstorm cells, that assist thunderstorm cells in reaching and penetrating the tropopause into the stratosphere.

3.) 500 mb (50 kPa) temperatures of −20 °C or colder, which produce numerous positive and negative lightning bolts and inter-related Sprites and Elves.

4.) Approximate 150–175 knot (170–200 mph) updrafts within thunderstorm cells complexes that create overshooting tops that penetrate the Tropopause into the Stratosphere, launching upwardly propagating internal buoyancy/gravity waves, which create traveling ionosphere disturbances and then wind shear.

Tropospheric modes

Tropospheric scattering

At VHF and higher frequencies, small variation (turbulence) in the density of the atmosphere at a height of around 6 miles (10 km) can scatter some of the normally line-of-sight beam of radio frequency energy back toward the ground, allowing over-the-horizon communication between stations as far as 500 miles (800 km) apart. The military developed the White Alice communications system covering all of Alaska, on these principles.

Tropospheric ducting and enhancement or refraction via inversion layer

Sudden changes in the atmosphere's vertical moisture content and temperature profiles can on random occasions make microwave and UHF & VHF signals propagate hundreds of kilometers up to about 2,000 kilometers (1,300 mi)—and for ducting mode even farther—beyond the normal radio-horizon. The inversion layer is mostly observed over high pressure regions, but there are several tropospheric weather conditions which create these randomly occurring propagation modes. Inversion layer's altitude for non-ducting is typically found between 100 meters (300 ft) to about 1 kilometer (3,000 ft) and for ducting about 500 meters to 3 kilometers (1,600 to 10,000 ft), and the duration of the events are typically from several hours up to several days. Higher frequencies experience the most dramatic increase of signal strengths, while on low-VHF and HF the effect is negligible. Propagation path attenuation may be below free-space loss. Some of the lesser inversion types related to warm ground and cooler air moisture content occur regularly at certain times of the year and time of day. A typical example could be the late summer, early morning tropospheric enhancements that bring in signals from distances up to few hundred kilometers for a couple of hours, until undone by the Sun's warming effect.

Rain scattering

Rain scattering is purely a microwave propagation mode and is best observed around 10 GHz, but extends down to a few gigahertz—the limit being the size of the scattering particle size vs. wavelength. This mode scatters signals mostly forwards and backwards when using horizontal polarization and side-scattering with vertical polarization. Forward-scattering typically yields propagation ranges of 800 km. Scattering from snowflakes and ice pellets also occurs, but scattering from ice without watery surface is less effective. The most common application for this phenomenon is microwave rain radar, but rain scatter propagation can be a nuisance causing unwanted signals to intermittently propagate where they are not anticipated or desired. Similar reflections may also occur from insects though at lower altitudes and shorter range. Rain also causes attenuation of point-to-point and satellite microwave links. Attenuation values up to 30 dB have been observed on 30 GHz during heavy tropical rain.

Aeroplane scattering

Aeroplane scattering (or most often reflection) is observed on VHF through microwaves and besides back-scattering, yields momentary propagation up to 500 km even in a mountain-type terrain. The most common back-scatter application is air-traffic radar and bistatic forward-scatter guided-missile and aeroplane detecting trip-wire radar and the US space radar.

Lightning scattering

Lightning scattering has sometimes been observed on VHF and UHF over distance of about 500 km. The hot lightning channel scatters radiowaves for a fraction of a second. The RF noise burst from the lightning makes the initial part of the open channel unusable and the ionisation disappears soon because of combination at low altitude high atmospheric pressure. Although the hot lightning channel is briefly observable with microwave radar, this mode has no practical use for communications.

Other effects

Diffraction

Knife-Edge diffraction is the propagation mode where radio waves are bent around sharp edges. For example, this mode is used to send radio signals over a mountain range when a line-of-sight path is not available. However, the angle cannot be too sharp or the signal will not diffract. The diffraction mode requires increased signal strength, so higher power or better antennas will be needed than for an equivalent line-of-sight path.

Diffraction depends on the relationship between the wavelength and the size of the obstacle. In other words, the size of the obstacle in wavelengths. Lower frequencies diffract around large smooth obstacles such as hills more easily. For example, in many cases where VHF (or higher frequency) communication is not possible due to shadowing by a hill, one finds that it is still possible to communicate using the upper part of the HF band where the surface wave is of little use.

Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony tend to be dominated by ground-plane effects as they travel over the rooftops of the urban environment. They then diffract over roof edges into the street, where multipath propagation, absorption and diffraction phenomena dominate.

Absorption

Low-frequency radio waves travel easily through brick and stone and VLF even penetrates sea-water. As the frequency rises, absorption effects become more important. At microwave or higher frequencies, absorption by molecular resonance in the atmosphere (mostly water, H₂O and oxygen, O₂) is a major factor in radio propagation. For example, in the 58–60 GHz band, there is a major absorption peak which makes this band useless for long-distance use. This phenomenon was first discovered during radar research during World War II. Beyond around 400 GHz, the Earth's atmosphere blocks some segments of spectra while still passes some—this is true up to UV light, which is blocked by ozone, but visible light and some of the NIR is transmitted.

Heavy rain and snow also affect microwave reception.

Amateur Radio Band Characteristics

Medium frequency

• 160 meters – 1.8-2 MHz(1800–2000 kHz) – Often taken up as a technical challenge in a manner similar to 6m. Most useful at night, though notoriously noisy. In many locations, a separate specialized receive-only antenna (such as a shielded loop) is necessary for successful operation on the band. Also known as the "top band" and the "Gentlemen's Band", in apparent contrast to the supposedly freewheeling 80m allocation. Allocations in this band vary widely from country to country.

High frequency

• 80 meters – 3.5-4 MHz (3500–4000 kHz) – Best at night, with significant daytime signal absorption. Works best in winter due to atmospheric noise in summer. Only countries in the Americas and few others have access to all of this band, in other parts of the world amateurs are limited to the bottom 300 kHz or less. In the US and Canada the upper end of the subband from 3600–4000 kHz, which permits use of single-sideband voice, is often referred to as 75 meters. Operators in this sub-band have a reputation for rowdiness similar to CB operators.
• 60 meters – 5 MHz region – A relatively new allocation and only available in a small number of countries such as the United States, United Kingdom, Norway and Iceland. In most countries, the allocation is channelized, and in the USA it is mandatory to operate in upper sideband mode.
• 40 meters – 7.0–7.3 MHz – Considered the most reliable all-season DX band, and most popular at night, and extremely useful for medium distance contacts during the day. Much of this band is shared with broadcasters, and in most countries only the bottom 100 kHz or 200 kHz are available to amateurs.
• 30 meters – 10.1–10.15 MHz – a very narrow band, which is shared with non-amateur services. It is recommended that only Morse Code and data transmissions be used here, and in some countries amateur voice transmission is actually prohibited. Not released for amateur use in a small number of countries. Due to its location in the centre of the shortwave spectrum, provides significant opportunities for long-distance communication at all points of the solar cycle. 30 meters is a WARC band.
• 20 meters – 14.0–14.35 MHz – Considered the most popular DX band; usually most popular during daytime. QRP operators recognize 14.060 MHz as their primary calling frequency in that band. Users of the PSK31 data mode tend to congregate around 14.071 MHz. Analog SSTV activity is centered around 14.230 MHz.
• 17 meters – 18.068–18.168 MHz – Similar to 20m, but more sensitive to solar conditions. By unofficial agreement, this band is not used for amateur contesting, which makes it a fairly quiet place. It is often used for extended, informal chats known as "ragchews". 17 meters is a WARC band.
• 15 meters – 21–21.45 MHz – Most useful during solar maximum, and generally a daytime band.

"WARC" bands are so called due to the special World Amateur Radio Conference allocation of these newer bands to amateur radio use. almost all radio amateur contests are not allowed on all three WARC bands.

• 12 meters – 24.89–24.99 MHz – Mostly useful during daytime, but opens up for DX activity at night during solar maximum. 12 meters is a WARC band.
• 10 meters – 28–29.7 MHz – Best activity is during solar maximum; during periods of moderate solar activity the best activity is found at low latitudes. The band offers useful short- to medium-range groundwave propagation, day or night. Also the site of frequent illegal unlicensed operation ("bootlegging") and freeband activity by operators using modified Citizen's Band equipment.

Characteristics above HF

While "line of sight" propagation is a primary factor for range calculation, much of the interest in the bands above HF comes from use of other propagation modes. A VHF signal transmitted from a hand-held walkie-talkie will typically travel about 5-10 km depending on terrain. With a low power home station and a simple antenna range would be around 50 km. With a large antenna system like a long yagi, and higher power (typically 100 or more watts) contacts of around 1000 km are common. Ham operators seek to exploit the limits of the frequencies' usual characteristics looking to learn and experiment with radio technology. They also seek to take advantage of "band openings" where due to various natural occurrences, radio emissions can travel well over their normal characteristics. There are numerous causes for these band openings and many hams listen for hours to take advantage of their rare manifestations, which may be of fleeting duration.

Some openings are caused by intense ionization of the upper atmosphere, known as the ionosphere. Other band openings are caused by a weather phenomenon known as an inversion layer, where warm air traps colder air beneath it, which forces the radio emission to travel over long weather layers. Radio signals can travel hundreds or even thousands of kilometres due to these weather layers.

For example, the longest distance contact reported due to tropospheric refraction on 2 meters is 4754 km between Hawaii and a ship south of Mexico, with reports of one way reception of signals from Réunion to Western Australia, a distance of more than 6000 km.

Ionospheric, or skywave propagaton also occurs on 6, 4 and 2 meters, and very occasionally on higher frequencies. The longest terrestrial contact ever reported on 2 meters was between a station in Italy and one in South Africa, at a distance of 7784 km, using anomalous enhancement of the inonosphere over the geomagnetic equator.

Amateur television

Amateur television (ATV) is the hobby of transporting broadcast- compatible video and audio by amateur radio. It also includes the study and building of such transmitters and receivers and the propagation between these two.

In NTSC countries, ATV operation requires the ability to use a 6 MHz wide channel. All bands at VHF or lower are less than 6 MHz wide, so ATV operation is confined to UHF and up. Bandwidth requirements will vary from this for PAL and SECAM transmissions.

ATV operation in the 70 cm band is particularly popular, because the signals can be received on any cable-ready television. Operation in the 33 cm and 23 cm bands is easily augmented by the availability of various varieties of consumer-grade wireless video devices that exist and operate in unlicenced frequencies coincident to these bands.

ATV operation may be enhanced by using specially-equipped repeaters.

Understanding HF Radio Propagation Forecasts

Ham Radio operators, shortwave radio enthusiasts often talk about propagation index numbers, the status of the solar cycle and geomagnetic conditions. Why? What do the numbers mean?  On many ham radio frequencies, especially on HF, these factors determine whether worldwide contacts can be made with very little effort (low power and a modest antenna) or radio blackout conditions exist (no contacts possible). The following information, adapted from NOAA, can be helpful in these numbers.

The National Oceanic and Atmospheric Administration (NOAA) uses WWV and WWVH to broadcast geophysical alert messages that provide information about solar terrestrial conditions.  Geophysical alerts are broadcast from WWV at 18 minutes after the hour and from WWVH at 45 minutes after the hour. The messages are less than 45 seconds in length and are routinely updated every 3 hours (typically at 0000, 0300, 0600, 0900, 1200, 1500, 1800, and 2100 UTC).  Updates are more frequent when activity warrants.

The geophysical alerts provide information about the current and predicted solar terrestrial conditions found useful for long distance HF radio communications and other applications.  The alerts use a standardized format and terminology that requires some explanation.  The terms used in the announcements are defined below:

Solar flux is a measurement of the intensity of solar radio emissions with a wavelength of 10.7 cm (a frequency of about 2800 MHz.  The daily solar flux measurement is recorded at 2000 UTC by the Dominion Radio Astrophysical Observatory of the Canadian National Research Council located at Penticton, B.C., Canada.  The value broadcast is in solar flux units that range from a theoretical minimum of about 50 to numbers larger than 300.  During the early part of the 11-year sunspot cycle, the flux numbers are low; but they rise and fall as the cycle proceeds.  The numbers will remain high for extended periods around sunspot maximum.

The A and K indices are a measurement of the behavior of the magnetic field in and around the Earth. 

The K index uses a scale from 0 to 9 to measure the change in the horizontal component of the geomagnetic field.  A new K index is determined and added to the broadcast every 3 hours based on magnetometer measurements made at the Table Mountain Observatory, north of Boulder, Colorado, or an alternate middle latitude observatory. 

The A index is a daily value on a scale from 0 to 400 to express the range of disturbance of the geomagnetic field.  It is obtained by converting and averaging the eight, 3-hour K index values.  An estimate of the A index is first announced at 2100 UTC, based on 7 measurements and 1 estimated value. At 0000 UTC, the announced A index consists entirely of known measurements, and the word “estimated” is dropped from the announcement.

Space Weather describes the conditions in space that affect earth and its technological systems. Space weather is a consequence of the behavior of the sun, the nature of Earth’s magnetic field and atmosphere, and our location in the solar system.

Space Weather storms observed and expected are characterized using the NOAA Space Weather scales. The abbreviated table below shows the levels of activity that are included in the announcements and the associated terminology. The descriptor used to identify observed or expected conditions is the maximum level reached or predicted. The NOAA Space Weather Scales are further described on the Space Environment Center web site.

Geomagnetic storm levels are determined by the estimated 3-hourly Planetary K-indices which are that are derived in real time from a network of western hemisphere ground-based magnetometers.

Solar Radiation stormslevels are determined by the proton flux measurements made by the primary GOES satellite. 

Radio Blackouts are determined by the x-ray level measured by the primary GOES satellite.

Every geophysical alert consists of three parts as shown in tables 3.4 and 3.5.  Table 3.4 describes the information contained in the geophysical alert.  Table 3.5 provides example text from an actual message.

The announcements include the descriptor of the largest space weather event observed (2) or expected (3) in the first line of each section. The remaining lines give the type of events and the level observed for each one. In the example above, no geomagnetic storm information is included because none was observed or is expected during the period. In the case where none of the three types of events are observed or expected, the announcement would contain section 1, plus alternate section 2 and alternate section 3.

The above section is from the Space Environment Center web site.

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Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, and chemical composition) of celestial objects such as stars, galaxies, and the interstellar medium, as well as their interactions. The study of cosmology is theoretical astrophysics at the largest scales where Albert Einstein's general theory of relativity plays a major role.

Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics electromagnitism,statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics. In practice, modern astronomical research involves a substantial amount of physics. The name of a university's department ("astrophysics" or "astronomy") often has to do more with the department's history than with the contents of the programs. Astrophysics can be studied at the bachelors, masters, and Ph.D.levels in aerospace engineering, physics, or astronomy departments at many universities.

Related Topics

Solar Wind

The solar wind is a stream of charged particles (i.e., a plasma) which are ejected from the upper atmosphere of the sun. It consists mostly of high-energy electrons and protons (about 1 keV) that are able to escape the sun's gravity in part because of the high temperature of the corona and the high kinetic energy particles gain through a process that is not well understood at this time.

Many phenomena are directly related to the solar wind, including geomagnetic storms that can knock out power grids on Earth, aurorae (e.g., Northern Lights) and the plasma tail of a comet always pointing away from the sun. While early models of the solar wind used primarily thermal energy to accelerate the material, by the 1960s it was clear that thermal acceleration alone cannot account for the high speed solar wind. Some additional acceleration mechanism is required, but is not currently known, but most likely relates to magnetic fields in the solar atmosphere.

Magnetosphere

A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune. Mercury and Jupiter's moon ganymede are magnetized, but too weakly to trap plasma. Mars has patchy surface magnetization. The term "magnetosphere" has also been used to describe regions dominated by the magnetic fields of celestial objects, e.g. pulsar magnetospheres.

Ionsphere

The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the Thermosphere.

Auroras

Auroras (Polar Lights; or aurorae: aurora) are natural different colored light displays, which are usually observed in the night sky, particularly in the polar zone. Some scientists therefore call them "polar auroras" (or "aurorae polaris"). In northern latitudes, it is known as the aurora borealis, named after the Roman goddess of the dawn, Aurora, and the Greek name for north wind, Boreas. It often appears as a greenish glow (or sometimes a faint red), as if the sun were rising from an unusual direction. The aurora borealis is also called the northern [polar] lights, as it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April.

The Solar Wind and Magnetosphere

The earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the sun in all directions, a result of the million-degree heat of the sun's outermost layer, the. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cc and magnetic field intensity around 2–5 nT (nanoteslas; the earth's surface field is typically 30,000–50,000 nT). These are typical values. During magnetic storms, in particular, flows can be several times faster; the interplanetary magnetic field (IMF) may also be much stronger.

The IMF originates on the sun, related to the field of sunspots, and its field lines are dragged out by the solar wind. That alone would tend to line them up in the sun-earth direction, but the rotation of the sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge ("limb") of the visible sun.

The earth's magnetosphere is the space region dominated by its magnetic field. It forms an obstacle in the path of the solar wind, causing it to be diverted around it, at a distance of about 70,000 km (before it reaches that boundary, typically 12,000–15,000 km upstream, a bow shock forms). The width of the magnetospheric obstacle, abreast of Earth, is typically 190,000 km, and on the night side a long "magnetotail" of stretched field lines extends to great distances.

When the solar wind is perturbed, it easily transfers energy and material into the magnetosphere. The electrons and ions in the magnetosphere that are thus energized move along the magnetic field lines to the polar regions of the atmosphere.

Cosmic Rays

Cosmic rays are energetic particles originating from space that impinge on Earth's Atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons. The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.

The variety of particle energies reflects the wide variety of sources. The origins of these particles range from energetic processes on the Sun all the way to as yet unknown events in the farthest reaches of the visible universe. Cosmic rays can have energies of over 1020 eV, far higher than the 1012 to 1013 eV that man-made particle accelerators can produce. (See Ultra-High energy rays for a description of the detection of a single particle with an energy of about 50 J, the same as a well-hit tennis ball at 42 m/s [about 94 mph].) There has been interest in investigating cosmic rays of even greater energies.

Synchrotron radiation

Synchrotron radiation is electromagnetic radiation, similar to cyclotron radiation, but generated by the acceleration of ultrarelevitistic(i.e., moving near the speed of light) charged particles through magnetic fields. This may be achieved artificially by storage rings in a synchotron, or naturally by fast moving electrons moving through magnetic fields in space. The radiation typically includes radio waves, infrared light, visible light, ultraviolet light, and x-rays

The radiation was named after its discovery in a General Electric synchrotron accelerator built in 1946 and announced in May 1947 by Frank Elder, Anatole Gurewitsch, Robert Langmuir, and Herb Pollock in a letter entitled "Radiation from Electrons in a Synchrotron". Pollock recounts:

"On April 24, Langmuir and I were running the machine and as usual were trying to push the electron gun and its associated pulse transformer to the limit. Some intermittent sparking had occurred and we asked the technician to observe with a mirror around the protective concrete wall. He immediately signaled to turn off the synchrotron as "he saw an arc in the tube." The vacuum was still excellent, so Langmuir and I came to the end of the wall and observed. At first we thought it might be due to Cherenkov Radiaton, but it soon became clearer that we were seeing lvanenko and Pomeranchuk radiation.

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