|Electromagnetic Sounds of our Solar System
Symphonies of the Planets and our Sun
The Symphonies of the Planets series, a collection of works by NASA and Brain/Mind Research in which planetary electromagnetic waves are captured by the Voyager unmanned space probes and converted into audible sound, can also be considered an organic manifestation of dark ambient.
'Share the journey of a 5 billion mile trek to the outer limits of our
solar system. Hear the beautiful songs of the planets. The complex
interactions of the cosmic plasma of the universe, charged
electromagnetic particles from the solar wind, planetary magnetosphere,
rings and moons create vibration "soundscapes" which are at once utterly
alien and deeply familiar to the ear.
Some of these sounds are
hauntingly like human voices singing, giant Tibetan bowls, wind, waves,
birds and dolphins. Many are familiar in a way unique for each
"Voyager has left our Solar System forever. The sounds on this recording will never be made again in our lifetime."
The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum.
The Sun's magnetic field leads to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System.
Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power.
Solar activity is thought to have played a large role in the formation and evolution of the Solar System.
Solar activity changes the structure of Earth's outer atmosphere. All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles).
The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences (see magnetic reconnection).
This twisting action creates the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years. The solar magnetic field extends well beyond the Sun itself. The magnetized solar wind plasma carries Sun's magnetic field into the space forming what is called the interplanetary magnetic field.
Since the plasma can only move along the magnetic field lines, the interplanetary magnetic field is initially stretched radially away from the Sun. Because the fields above and below the solar equator have different polarities pointing towards and away from the Sun, there exists a thin current layer in the solar equatorial plane, which is called the heliospheric current sheet.
At the large distances the rotation of the Sun twists the magnetic field and the current sheet into the Archimedean spiral like structure called the Parker spiral. The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field.
The Sun's 50–400 μT (in the photosphere) magnetic dipole field reduces with the cube of the distance to about 0.1 nT at the distance of the Earth. However, according to spacecraft observations the interplanetary field at the Earth's location is about 100 times greater at around 5 nT.
Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field.
According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s.
The magnetic field strength at the Mercurian equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar.
Unlike Earth, however, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.
It is likely that this magnetic field is generated by way of a dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.
Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within the Earth, is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface.
Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles were detected in the planet's magnetotail, which indicates a dynamic quality to the planet's magnetosphere.
During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury’s magnetic field can be extremely "leaky." The spacecraft encountered magnetic "tornadoes" – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 800 km wide or a third of the radius of the planet.
These 'tornadoes' form when magnetic fields carried by the solar wind connect to Mercury's magnetic field. As the solar wind blows past Mercury's field, these joined magnetic fields are carried with it and twist up into vortex-like structures.
These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface. The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos.
It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. However, the MESSENGER observations show the reconnection rate is ten times higher at Mercury. Mercury's proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER.
In 1967, Venera-4 found that the Venusian magnetic field is much weaker than that of Earth.
This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo in the core like the one inside the Earth.
Venus' small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges.
The lack of an intrinsic magnetic field at Venus was surprising given that it is similar to Earth in size, and was expected also to contain a dynamo at its core.
A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, while its rotation is often thought to be too slow, simulations show that it is adequate to produce a dynamo. This implies that the dynamo is missing because of a lack of convection in the Venusian core.
On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust.
This caused the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, there is not an internal geodynamo that can drive a magnetic field. Instead the heat energy from the core is being used to reheat the crust.
One possibility is that Venus has no solid inner core, or its core is not currently cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already completely solidified. The state of the core is highly dependent on the concentration of sulfur, which is unknown at present.
The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles.
At the equator of the magnetic field, the magnetic field strength at the planet's surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3.
According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents.
These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment.
This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago. The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth.
The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.
Although Mars has no evidence of a current structured global magnetic field, observations show that parts of the planet's crust have been magnetized, and that alternating polarity reversals of its dipole field have occurred in the past.
This paleomagnetism of magnetically susceptible minerals has properties that are very similar to the alternating bands found on the ocean floors of Earth.
One theory, published in 1999 and re-examined in October 2005 (with the help of the Mars Global Surveyor), is that these bands demonstrate plate tectonics on Mars four billion years ago, before the planetary dynamo ceased to function and caused the planet's magnetic field to fade away.
Current models of the planet's interior imply a core region about 1,480 km in radius, consisting primarily of iron with about 14–17% sulfur. This iron sulfide core is partially fluid, and has twice the concentration of the lighter elements than exist at Earth's core.
The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but now appears to be inactive.
The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km. Earth's crust, averaging 40 km, is only one third as thick as Mars’ crust, relative to the sizes of the two planets.
Jupiter's broad magnetic field is 14 times as strong as the Earth's, ranging from 4.2 gauss (0.42 mT) at the equator to 10–14 gauss (1.0–1.4 mT) at the poles, making it the strongest in the Solar System (except for sunspots).
This field is believed to be generated by eddy currents—swirling movements of conducting materials—within the metallic hydrogen core.
The volcanoes on the moon Io emit large amounts of sulfur dioxide forming a gas torus along the moon's orbit. The gas is ionized in the magnetosphere producing sulfur and oxygen ions.
They, together with hydrogen ions originating from the atmosphere of Jupiter, form a plasma sheet in Jupiter's equatorial plane.
The plasma in the sheet co-rotates with the planet causing deformation of the dipole magnetic field into that of magnetodisk. Electrons within the plasma sheet generate a strong radio signature that produces bursts in the range of 0.6–30 MHz.
At about 75 Jupiter radii from the planet, the interaction of the magnetosphere with the solar wind generates a bow shock. Surrounding Jupiter's magnetosphere is a magnetopause, located at the inner edge of a magnetosheath—a region between it and the bow shock. The solar wind interacts with these regions, elongating the magnetosphere on Jupiter's lee side and extending it outward until it nearly reaches the orbit of Saturn.
The four largest moons of Jupiter all orbit within the magnetosphere, which protects them from the solar wind. The magnetosphere of Jupiter is responsible for intense episodes of radio emission from the planet's polar regions. Volcanic activity on the Jovian moon Io (see below) injects gas into Jupiter's magnetosphere, producing a torus of particles about the planet.
As Io moves through this torus, the interaction generates Alfvén waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When the Earth intersects this cone, the radio emissions from Jupiter can exceed the solar radio output.
The magnetosphere of Saturn is the cavity created in the flow of the solar wind by the planet's internally generated magnetic field.
Discovered in 1979 by the Pioneer 11 spacecraft, Saturn's magnetosphere is the second largest of any planet in the Solar System after Jupiter.
The magnetopause, the boundary between Saturn's magnetosphere and the solar wind, is located at a distance of about 20 Saturn radii from the planet's center, while its magnetotail stretches hundreds of radii behind it.
Saturn's magnetosphere is filled with plasmas originating from both the planet and its moons.
The main source is the small moon Enceladus, which ejects as much as 1,000 kg/s of water vapor from the geysers on its south pole, a portion of which is ionized and forced to co-rotate with the Saturn’s magnetic field.
This loads the field with as much as 100 kg of water group ions per second. This plasma gradually moves out from the inner magnetosphere via the interchange instability mechanism and then escapes through the magnetotail. The interaction between Saturn's magnetosphere and the solar wind generates bright oval aurorae around the planet's poles observed in visible, infrared and ultraviolet light.
The aurorae are related to the powerful saturnian kilometric radiation (SKR), which spans the frequency interval between 100 kHz to 1300 kHz and was once thought to modulate with a period equal to the planet's rotation. However, later measurements showed that the periodicity of the SKR's modulation varies by as much as 1%, and so probably does not exactly coincide with Saturn’s true rotational period, which as of 2010 remains unknown.
Inside the magnetosphere there are radiation belt, which house particle with energy as high as tens Megaelectronvolts. The energetic particle have significant influence on the surfaces of inner icy moons of Saturn. In 1980–1981 the magnetosphere of Saturn was studied by the Voyager spacecraft. As of 2010 it is a subject of the ongoing investigation by Cassini mission, which arrived in 2004.
Before the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery.
Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, since it would then align with the planet's poles that lie in the ecliptic.
Voyager's observations revealed that the magnetic field is peculiar, both because it does not originate from the planet's geometric center, and because it is tilted at 59° from the axis of rotation.
In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by as much as one third of the planetary radius.
This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), whereas in the northern hemisphere it can be as high as 1.1 gauss (110 µT).
The average field at the surface is 0.23 gauss (23 µT). In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its geographical equator. The dipole moment of Uranus is 50 times that of Earth. Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.
One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giant planets, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean. Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock located at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail and radiation belts.
Overall, the structure of Uranus's magnetosphere is different from Jupiter's and more similar to Saturn's. Uranus's magnetotail trails behind the planet into space for millions of kilometers and is twisted by the planet's sideways rotation into a long corkscrew. Uranus's magnetosphere contains charged particles: protons and electrons with small amount of H2+ ions.
No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona. The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively. The density of low energy (below 1 kiloelectronvolt) ions in the inner magnetosphere is about 2 cm−3.
The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps. The particle flux is high enough to cause darkening or space weathering of the moon’s surfaces on an astronomically rapid timescale of 100,000 years.
This may be the cause of the uniformly dark colouration of the moons and rings. Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles. Unlike Jupiter's, Uranus's aurorae seem to be insignificant for the energy balance of the planetary thermosphere.
Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii, or about 13500 km from the planet's physical centre.
Before Voyager 2's arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation.
In comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors.
This field may be generated by convective fluid motions in a thin spherical shell of electrically conducting liquids (probably a combination of ammonia, methane and water) resulting in a dynamo action.
The dipole component of the magnetic field at the magnetic equator of Neptune is about 14 microteslas (0.14 G). The dipole magnetic moment of Neptune is about 2.2 × 1017 T·m3 (14 μT·RN3, where RN is the radius of Neptune). Neptune's magnetic field has a complex geometry that includes relatively large contributions from non-dipolar components, including a strong quadrupole moment that may exceed the dipole moment in strength.
By contrast, Earth, Jupiter and Saturn have only relatively small quadrupole moments, and their fields are less tilted from the polar axis. The large quadrupole moment of Neptune may be the result of offset from the planet's center and geometrical constraints of the field's dynamo generator.
Neptune's bow shock, where the magnetosphere begins to slow the solar wind, occurs at a distance of 34.9 times the radius of the planet. The magnetopause, where the pressure of the magnetosphere counterbalances the solar wind, lies at a distance of 23–26.5 times the radius of Neptune. The tail of the magnetosphere extends out to at least 72 times the radius of Neptune, and very likely much farther.