Electric Currents and Transmission Lines in Space

 The plasma universe consists of swirling streams of electrons and ions flowing in filaments. Where pairs of these spaghettilike structures interact, the particle gain kinetic energy and at narrow 'pinch' regions produce the entire range of galaxy types as well as the full spectrum of cosmic electromagnetic radiation. Thus galaxies must lie 'like pearl beads on a necklace' along filaments, much as they are observed to do on a large scale. The bulk of the filaments are invisible from a distance, much like the Birkeland currents that circle the Earth but are invisible from its surface. In space, these currents are called Birkeland Currents, in honor of the 19th century physicist who suggested their existence. In the laboratory, they are called Bennett-pinches, Z-pinches, or 'Zed' pinches. In 1934 W. H. Bennett discovered that streams of electrons flowing in the axial or Z-direction, self pinch from the magnetic field they generate around themselves

Dynamical Characteristics of Plasmas

It is the global dynamics and systematic interactions of astrophysical plasmas that allow energy to be conveyed over great distances. The evolution of cosmic plasma that includes its structuring into cells results in a relative motion, however slow, of plasma clouds whose dimensions may be measured in hundreds or megaparsecs or gigaparsecs. All plasma clouds may be considered a system: they are coupled by electrical currents (charged particles beams) they induce in each other. These beams are the source of energy transfer from large, slow moving plasma to smaller plasma regions that may release the energy abruptly or cause local plasmas to pinch to the condense state.

Power Generation and Transmission

On earth, power is generated by nuclear and nonnuclear fuels, hydro and solar energy, and to a much lesser extent, by geothermal sources and magnetohydrodynamic generators. Always, the location of the supply is not the location of major power usage or dissipation. Transmission lines are used to convey the power generated to the load region. As an example, abundant hydroelectric resources in the Pacific Northwest of the United States produce power ($\sim$1,500 MW) that is then transmitted to Los Angeles, 1,330 km away, via 800 kV high-efficiency dc transmission lines. In optical and infrared emission, only the load region, Los Angeles, is visible from the light and heat it dissipates in power usage. The transmission line is invisible.

This situation is also true in space. With the coming of the space age and the subsequent discovery of magnetospheric-ionospheric electrical circuits, Kirchoff's circuit laws were suddenly catapulted to dimensions eight orders of magnitude larger than that previously investigated in the laboratory and nearly four orders of magnitude greater than that associated with the longest power distribution systems on earth.

On earth, transmission lines consist of metallic conductors or waveguides in which energy is made to flow via the motion of free electrons (currents) in the metal or in displacement currents in a time varying electric field. Often strong currents within the line allow the transmission of power many orders of magnitude stronger than that possible with weak currents. This is because a current associated with the flow of electrons produces a self-magnetic field that helps to confine or pinch the particle flow. Magnetic-insulation is commonly used in pulsed-power technology to transmit large amounts of power from the generator to the load without suffering a breakdown due to leakage currents caused by high electric potentials.

There is a tendency for charged particles to follow magnetic lines of force and this forms the basis of transmission lines in space. In the magnetosphere-ionosphere, a transmission line 7-8 earth radii in length ($R_e$ = 6,350 km) can convey tens of terawatts of power, that derives from the solar wind-magnetosphere coupling. The transmission line is the earth's dipole magnetic field lines along which electrons and ions are constrained to flow. The driving potential is solar-wind induced plasma moving across the magnetic field lines at large radii. The result is an electrical circuit in which electric currents cause the formation of auroras at high latitude in the upper atmosphere on earth. This aurora mechanism is observed on Jupiter, Io, Saturn, Uranus, and is thought to have been detected on Neptune and perhaps, Venus.

Only the aurora discharge is visible at optical wavelengths to an observer. The source and transmission line are invisible. Before the coming of space probes, in situ measurement was impossible and exotic explanations were often given of auroras. This is probably true of other non in situ cosmic plasmas today. The existence of a megaampere flux tube of current, connecting the Jovian satellite Io to its mother planet, was verified with the passage of the Voyager spacecraft.

Electrical Discharges in Cosmic Plasma

An electrical discharge is a sudden release of electric or magnetic stored energy. This generally occurs when the electromagnetic stress exceeds some threshold for breakdown that is usually determined by small scale properties of the energy transmission medium. As such, discharges are local phenomena and are usually accompanied by violent processes such as rapid heating, ionization, the creation of pinched and filamentary conduction channels, particle acceleration [Melrose 1997], and the generation of prodigious amounts of electromagnetic radiation.

As an example, multi-terawatt pulsed-power generators on earth rely on strong electrical discharges to produce intense particle beams, X rays, and microwaves. Megajoules of energy are electrically stored in capacitor banks, whose volume may encompass 250 m$^3$. This energy is then transferred to a discharge region, located many meters from the source, via a transmission line. The discharge region, or load, encompasses at most a few cubic centimeters of space, and is the site of high-variability, intense, electromagnetic radiation.

On earth, lightning is another example of the discharge mechanism at work where electrostatic energy is stored in clouds whose volume may be of the order of 3,000 km$^3$. This energy is released in a few cubic meters of the discharge channel.

The aurora is a discharge caused by the bombardment of atoms in the upper atmosphere by 1-20 keV electrons and 200 keV ions spiraling down the earth's magnetic field lines at high latitudes. Here, the electric field accelerating the charged particles derives from plasma moving across the earth's dipole magnetic field lines many earth radii into the magnetosphere. The potential energy generated by the plasma motion is fed to the upper atmosphere by multi-megaampere Birkeland currents that comprise a transmission line, 50,000 kilometers in length, as they flow into and out of the discharge regions at the polar horns. The generator region may encompass $10^{12}-10^{13}$ km$^3$ while the total discharge volume can be $10^9-10^{10}$ km$^3$.

The flickering of a light in Los Angeles does not mean that the the supply source, a waterfall or hydroelectric dam in the Pacific Northwest, has abruptly changed dimensions or any other physical property. The flickering comes from electrical changes at the observed load or {\it radiative source}, such as the formation of instabilities or virtual anodes or cathodes in charged particle beams that are orders of magnitude smaller than the supply. Bizarre and interesting non-physical interpretations are obtained if the flickering light is interpreted by a distant observer to be both the source and supply. This also holds true for astrophysical plasmas. As discussed earlier, space is not vacuum but rather filled with plasma whose properties, in volume, differ little from those in the laboratory or magnetospheres. And plasmas exhibit large {\it system} global properties, such as the transfer of energy over great distances to smaller regions where it may be systematically or catastrophically released.

Filamentation: The Signature of Electrical Currents in Astrophysical Plasmas

Left: In the laboratory filaments are produced when a pulse-power generator delivers 10 trillion watts to a plasma only a few centimeters long, heating it to a temperature of 8,000,000 degrees Kelvin. Center: Similar structure is seen in solar prominences, but in this case the lengths are measured in jundreds of thousands of kilometers. Right: Long, thin structures near the center of the Milky Way stretch out over roughly 120 light-years. Another jump to a scale a few million times larger would bring us to the size of filaments that plasma cosmology needs to form galaxies.

As far as we know, most cosmic low density plasmas also depict a filamentary structure. For example, filamentary structures are found in the following cosmic plasmas, all of which are observed or are likely to be associated with electric currents:

•In the aurora, filaments parallel to the magnetic field are often observed. These can sometimes have dimensions down to about 100 m. \item Inverted V events and the in-situ measurements of strong electric fields in the magnetosphere ($10^5-10^6$ A, $10^8$ m) demonstrate the existence of filamentary structures.

•In the ionosphere of Venus, flux ropes", whose filamentary diameters are typically 20 km, are observed. \item In the sun, prominences ($10^{11}$ A), spicules, coronal streamers, polar plumes, etc., show filamentary structure whose dimensions are of the order $10^7-10^8$ m.

•Cometary tails often have a pronounced filamentary structure.

•In the interstellar medium and in interstellar clouds there is an abundance of filamentary structures [e.g., the Veil nebula, the Lagoon nebula, the Orion nebula, and the Crab nebula].

•The center of the Galaxy, where twisting plasma filaments, apparently held together by a magnetic field possessing both azimuthal and poloidal components, extend for nearly 60 pc ($10^{18}$ m).

•Within the radio bright lobes of double radio galaxies, where filament lengths may exceed 20 kpc ($6 \times 10^{20}$ m).

•In extended radio sources and synchrotron emitting jets.

Regardless of scale, the motion of charged particles produces a self-magnetic field that can act on other collections of particles or plasmas, internally or externally. Plasmas in relative motion are coupled via currents that they drive through each other. Currents are therefore expected in a universe of inhomogeneous astrophysical plasmas of all sizes.