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Topic Name: UCLA Scientists Identify Origin of Plasmaspheric Hiss in Upper Atmosphere that Control the Dynamics of the Van Allen Radiation Belts

Category: STAR (Space, Telecommunications & Radioscience)

Research persons:

Location: Department of Atmospheric and Oceanic Sciences, University of California - Los Angeles, United States

Details

Scientists have solved a 40-year-old puzzle by identifying the origin of the intense radio waves in the Earth's upper atmosphere that control the dynamics of the Van Allen radiation belts — belts consisting of high-energy electrons that can damage satellites and spacecraft and pose a risk to astronauts performing activities outside their spacecraft.

The source of these low-frequency radio waves, which are known as plasmaspheric hiss, turns out to be not lightning or instabilities from a plasma, as previously proposed, but an intense electromagnetic wave type called "chorus," which energizes electrons and was initially thought to be unrelated to hiss, said Jacob Bortnik, a researcher with the UCLA Department of Atmospheric and Oceanic Sciences.

The findings appear March 6 in the journal Nature.

"That chorus waves are the dominant source of plasmaspheric hiss was a complete surprise," said Bortnik, whose research was federally funded by the National Science Foundation.

"Numerous theories to explain the origin of hiss have been proposed over the past four decades, but none have been able to account fully for its observed properties," Bortnik said. "Here, we show that a different wave type, called chorus, can propagate into the plasmasphere from tens of thousands of kilometers away and evolve into hiss. Our new model naturally accounts for the observed frequency band of hiss, its incoherent nature, its day-night asymmetry in intensity, its association with solar activity and its spatial distribution.

"The connection between chorus and hiss is very interesting because chorus is instrumental in the formation of high-energy electrons outside the plasmasphere, while hiss depletes these electrons at lower equatorial altitudes," he said.

Beginning in the late 1960s, spacecraft observations of wideband electromagnetic noise at frequencies below a few kilohertz established the presence of a steady, incoherent noise band in the frequency range between 200 Hz and 1 kHz. This emission was dubbed plasmaspheric hiss because of its unstructured nature, its spectral resemblance to audible hiss and its confinement to the plasmasphere, a dense plasma region around the Earth.

Bortnik was initially studying chorus, not hiss, when he made the discovery — one of many examples of serendipity in science.

Hiss tends to be confined inside of the plasmasphere, and chorus outside of it. Bortnik was modeling chorus because he knew it was important in creating high-energy electrons in space. While chorus occurs outside the plasmasphere, it leaks inside of it.

A better understanding of plasmaspheric hiss will help scientists to more accurately model the behavior of the high-energy electrons in the Van Allen radiation belts and thus improve their forecasts of space conditions, Bortnik said.

Note for Radio waves
Radio waves are electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum. A common use is to transport information through the atmosphere or outer space without wires. Radio waves are distinguished from other kinds of electromagnetic waves by their wavelength, a relatively long wavelength in the electromagnetic spectrum.
Radio waves are usually produced by electric current alternating at radio frequency flowing in a special purpose conductor, called an antenna. Antenna dimensions must generally be comparable to wavelength to work efficiently. Very long waves are not practical because of the enormous antennas needed to produce them, although they are sometimes produced by lightning. Radio waves are also produced by cosmic phenomena in deep space. Actually, any kind of reciprocating motion of electric charges or magnets can produce radio waves if it is fast enough. Although very impractical, even a person waving a charged stick very fast can produce faint radio waves.
Propagation is a term that describes the travel of electromagnetic waves, there being three main modes of propagation. The first is a straight line travel: the manner that radio waves travel through deep space (ignoring the slight deviations caused by gravity under the theory of relativity). A second way is skip, which is bouncing between the surface of the earth and the ionosphere. Frequencies between 3 MHz and 30 MHz are most reliable for this kind of propagation, called High Frequency. The third way is to hug the surface of the earth as it curves around. Radio waves of very low frequency most often travel this way.
Radio signals can also enter two ionospheric layers of differing electron densities and duct between them. The image at the right illustrates this. Two radio signals of differing elevation angles are broadcast into the ionosphere, where they split into ordinary (red) and extraordinary (green) components. In this example, the ordinary component began ducting between the E and F ionospheric regions.
Although this mode of radio wave propagation is less common than the skip mode, it is nonetheless an important mode because it permits radio signals to travel significant distances with little attenuation.

Note for Van Allen Radiation Belt
The Van Allen Radiation Belt is a torus of energetic charged particles (plasma) around Earth, held in place by Earth's magnetic field. The Van Allen belts are closely related to the polar aurora where particles strike the upper atmosphere and fluoresce.
The presence of a radiation belt had been proposed by Immanuel Velikovsky and later by Nicholas Christofilos prior to the Space Age and was confirmed by the Explorer 1 on January 31, 1958, and Explorer 3 missions, under Dr. James Van Allen at the University of Iowa. The trapped radiation was first mapped out by Sputnik 3, Explorer 4, Pioneer 3 and Luna 1.
Energetic electrons form two distinct radiation belts, while protons form a single belt. Within these belts are particles capable of penetrating about 1 g/cm2  of shielding (e.g., 1 millimetre of lead).
The term Van Allen Belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts. The Earth's atmosphere limits the belts' particles to regions above 200-1,000 km, while the belts do not extend past 7 Earth radii RE. The belts are confined to an area which extends about 65° from the celestial equator.
An upcoming NASA mission, Radiation Belt Storm Probes will go further and gain scientific understanding (to the point of predictability) of how populations of relativistic electrons and ions in space form or change in response to changes in solar activity and the solar wind.
The large outer radiation belt extends from an altitude of about (3 to 10 Earth radii) above the Earth's surface, and its greatest intensity is usually around 4-5 RE. Outer electron radiation belt is mostly produced by the inward radial diffusion and local acceleration due to transfer of energy from whistler mode plasma waves to radiation belt electrons. Radiation belt electrons are also constantly removed by collisions with atmospheric neutrals, losses to magnetopause, and the outward radial diffusion. The outer belt consists mainly of high energy(0.1–10 MeV) electrons trapped by the Earth's magnetosphere. The gyroradii for energetic protons would be large enough to bring them into contact with the Earth's atmosphere. The electrons here have a high flux and at the outer edge (close to the magnetopause), where geomagnetic field lines open into the geomagnetic "tail", fluxes of energetic electrons can drop to the low interplanetary levels within about 100 km (a decrease by a factor of 1,000).
The trapped particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source.
The outer belt is larger than the inner belt, and its particle population fluctuates widely. Energetic (radiation) particle fluxes can increase and decrease dramatically as a consequence of geomagnetic storms, which are themselves triggered by magnetic field and plasma disturbances produced by the Sun. The increases are due to storm-related injections and acceleration of particles from the tail of the magnetosphere.
There is debate as to whether the outer belt was discovered by the US Explorer 4 or the USSR Sputnik 2/3.
The inner Van Allen Belt extends from an altitude of 700–10,000 km (0.1 to 1.5 Earth radii) above the Earth's surface, and contains high concentrations of energetic protons with energies exceeding 100 MeV and electrons in the range of hundreds of kiloelectronvolts, trapped by the strong (relative to the outer belts) magnetic fields in the region.
It is believed that protons of energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the magnetic field during geomagnetic storms.
Due to the slight offset of the belts from Earth's geometric center, the inner Van Allen belt makes its closest approach to the surface at the South Atlantic Anomaly.

Note for Earth's Atmosphere
The Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly (by molar content/volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, trace amounts of other gases, and a variable amount (average around 1%) of water vapor. This mixture of gases is commonly known as air. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.
There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades into space. Three quarters of the atmosphere's mass is within 11 km of the planetary surface. In the United States, people who travel above an altitude of 80.5 km (50 statute miles) are designated astronauts. An altitude of 120 km (~75 miles or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The Kármán line, at 100 km (62 miles or 328,000 ft), is also frequently regarded as the boundary between atmosphere and outer space.
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Each of these layers has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere (where the Earth's magnetic fields interact with the solar wind). An important part of the atmosphere for life on Earth is the ozone layer, a component of the stratosphere that partially shields the surface from ultraviolet light. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.
Due to thermal energy, some of the molecules at the outer edge of the Earth's atmosphere have their velocity increased to the point where they can escape from the planet's gravity. This results in a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate. For this reason, the Earth's current environment is oxidizing, rather than reducing, with consequences for the chemical nature of life which developed on the planet. The oxygen-rich atmosphere also preserves much of the surviving hydrogen by locking it up in water molecules.

Note for Plasmasphere
The plasmasphere, or inner magnetosphere is a region of the Earth's magnetosphere consisting of low energy (cool) plasma. It is located above the ionosphere. The outer boundary of the plasmasphere is known as the plasmapause, which is defined by an order of magnitude drop in plasma density.
The plasmasphere was discovered in 1963 by Don Carpenter from the analysis of VLF whistler wave data.
Traditionally, the plasmasphere has been regarded as a well behaved cold plasma with particle motion dominated entirely by the geomagnetic field and hence corotating with the Earth. In contrast, recent satellite observations have shown that density irregularities such as plumes or biteouts may form. It has also been shown that the plasmasphere does not always co-rotate with the Earth.

Note for Electromagnetic (EM) Radiation
Electromagnetic (EM) radiation, also called light even though it is not always visible, is a self-propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation, and are in phase with each other. Electromagnetic radiation is classified into types according to the frequency of the wave: these types include, in order of increasing frequency, radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
EM radiation carries energy and momentum, which may be imparted when it interacts with matter.
Electric and magnetic fields obey the properties of superposition, so fields due to particular particles or time-varying electric or magnetic fields contribute to the fields due to other causes. (As these fields are vector fields, all magnetic and electric field vectors add together according to vector addition.) These properties cause various phenomena including refraction and diffraction. For instance, a travelling EM wave incident on an atomic structure induces oscillation in the atoms, thereby causing them to emit their own EM waves. These emissions then alter the impinging wave through interference.
Since light is an oscillation, it is not affected by travelling through static electric or magnetic fields in a linear medium such as a vacuum. In nonlinear media such as some crystals, however, interactions can occur between light and static electric and magnetic fields - these interactions include the Faraday effect and the Kerr effect.
In refraction, a wave crossing from one medium to another of different density alters its speed and direction upon entering the new medium. The ratio of the refractive indices of the media determines the degree of refraction, and is summarized by Snell's law. Light disperses into a visible spectrum as light is shone through a prism because of refraction.
The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism.
EM radiation exhibits both wave properties and particle properties at the same time. The wave characteristics are more apparent when EM radiation is measured over relatively large timescales and over large distances, and the particle characteristics are more evident when measuring small distances and timescales. Both characteristics have been confirmed in a large number of experiments.
There are experiments in which the wave and particle natures of electromagnetic waves appear in the same experiment, such as the diffraction of a single photon. When a single photon is sent through two slits, it passes through both of them interfering with itself, as waves do, yet is detected by a photomultiplier or other sensitive detector only once. Similar self-interference is observed when a single photon is sent into a Michelson interferometer or other interferometers.

Co-authors of the research are Richard M. Thorne, University of California - Los Angeles professor of atmospheric and oceanic sciences, and Nigel P. Meredith at the British Antarctic Survey, Cambridge, U.K.

In figure 2, Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.

In figure 3, Laboratory simulation of the Van Allen belts' influence on the Solar Wind