2.10.1 The global electrical circuit The Earth's surface — ocean and solid — and the ionosphere are highly conductive. The atmosphere conducts electricity because of the presence of positive and negative ions plus free electrons. Conductivity is poor near sea level but increases rapidly with height up to the ionosphere; also it is greater at polar latitudes than equatorial. The conductivity near sea level is low because there are fewer ions, and those ions tend to become attached to the larger aerosol particles that are more common near the surface. Refer to section '1.5 Atmospheric moisture'. During fair weather there is an electric potential difference of 250 000 to 500 000 volts between the ionosphere and the Earth's surface, the surface being negative relative to the ionosphere. This gives rise to the fair weather current, which is a steady flow of electrons from the surface at about one microwatt per square metre. The three main generators in the global electrical circuit are the solar wind entering the magnetosphere, the ionospheric wind and thunderstorms. The average CB generates a current of about one amp during its active period. With an estimated 1000 to 2000 thunderstorms continually active around the globe, emitting possibly 5000 lightning strokes per minute, there is an electrical current of 1000 to 2000 amps continually transferring a negative charge to the surface, and an equal and opposite charge to the upper atmosphere. The electrical charge continually flowing into the stratosphere/ionosphere from the CBs maintains the fair weather current flowing to the surface. 2.10.2 Static charge and discharge Apart from the CB clouds, the atmosphere carries a net positive charge and the electric potential increases with height, and in cloud and fog. Strong electrical forces also exist in and around rain showers, which can transfer a charge of either polarity to the surface, or to an aircraft. Static electricity is the imbalance of negative and positive charge. Aircraft accumulate electrical charges in two ways. The most substantial is from flying through the extremely high voltage electrical fields associated with CB, or potential CB development. The static charge can pervade the whole aircraft, internally and externally, and render navaids useless. The rapid discharge of this charge — a single-channel spark discharge rather than a slow bleed-off from the airframe — may happen in any conditions, but the chances are more probable in temperatures between 10 °C and –10 °C, and where flying in rain mixed with snow. The other lesser type is precipitation static. The aircraft charge accumulates from the charge carried by precipitation particles, particularly snow crystals, and separates when the particles break up against the aircraft. Maximum build-up occurs in temperatures a few degrees either side of 0 °C. Static charges imparted to antennae will affect communications, particularly navaids where the effect on signal-to-noise ratio may be considerable. The built-up static charge is usually slowly bled off into the atmosphere, or as a quiet, non-luminous point discharge. In extreme build-ups, the consequent corona discharge streamers or brush discharge are manifested as St Elmo's fire, which is usually not visible in daylight but visible at night as a continuous, luminous blue-green discharge from wing tips, propellers and protuberances. 2.10.3 Lightning The electrostatic structure within CB, or CU CON, is such that pockets of different charge exist throughout the cloud. Generally, the main net positive charge resides on the ice crystals in the upper part of the cloud and the main net negative charge of similar magnitude is centred near the middle or lower part of the cloud at the sub-freezing level. That charge mainly resides on supercooled droplets. A smaller positive charge centre may exist at the bottom of the cloud where temperatures are above freezing. The electrostatic forces of repulsion and attraction induce secondary charge accumulations outside the cloud, a positive region accumulates on the Earth's surface directly below the cloud. Above the cloud, positive ions are transferred away from, and negative ions are transferred toward, the cloud. One favoured theory for the charge separation mechanism is the 'precipitation' theory. This suggests that the disintegration of large raindrops, and the interaction between the smaller cloud particles and the larger precipitation particles in the updrafts and downdrafts, causes the separation of electrical charge — with downward motion of negatively charged cloud and precipitation particles, and upward motion of positively charged cloud particles. Discharge channels Lightning is a flow of current, or discharge, along an ionised channel that equalises the charge difference between two regions of opposite charge; this occurs when the charge potentials exceed the electrical resistance of the intervening air. These discharges can be between the charged regions of the same cloud (intra-cloud), between the cloud and the ground (cloud-to-ground), between separate clouds (cloud-to-cloud) or between the base of a cloud and a charge centre in the atmosphere underneath it (cloud-to-air). The discharge channels, or streamers, propagate themselves through the air by establishing, and maintaining, an avalanche effect of free electrons that ionise atoms in their path. Lightning rates, particularly intra-cloud strokes, increase greatly with increase in the depth of clouds. Cloud-to-cloud and cloud-to-air discharges are rare but tend to be more common in the high-base CB found in the drier areas of Australia. Discharges above the CB anvil into the stratosphere and mesosphere also occur. When intra-cloud lightning — the most common discharge — occurs, it is most often between the upper positive and the middle negative centres. The discharge path is established by a 'stepped leader', the initial lightning streamer that grows in stages and splits into more and more branches, as it moves forward seeking an optimal path between the charge centres. The second, and subsequent, lightning strokes in a composite flash are initiated by 'dart leaders', streamers that generally follow the optimum ionised channel established by the stepped leader. The associated electrical current probably peaks at a few thousand amperes. A distant observer cannot see the streamers but sees a portion of the cloud become luminous, for maybe less than 0.5 seconds, hence 'sheet lightning'. Cloud-to-ground discharges Most cloud-to-ground discharges occur between the main negatively charged region and the surface — initially by a stepped leader from the region, which usually exhibits branching channels as it seeks an optimal path. When the stepped leader makes contact, directly with the surface or with a 'ground streamer' (which is another electrical breakdown initiated from the surface positive charge region and which rises a short distance from the surface), the cloud is short-circuited to ground; to complete each lightning stroke, a 'return streamer', or return stroke, propagates upwards. (The return streamer starts as positive ions that capture the free electrons flowing down the channel and emit photons. The streamer carries more positive ions upward, and their interaction with the free-flowing electrons gives the impression of upwards movement.) The charge on the branches of the stepped leader that have not been grounded flow into the return streamer. Subsequent strokes in the composite flash are initiated by dart leaders, with a return streamer following each contact. The return streamer, lasting 20–40 microseconds, propagates a current-carrying core a few centimetres in diameter with a current density of 1000 amperes per cm² and a total current typically 20 000 amps, but peaks could be much greater. A charged sheath or corona, a few metres in diameter, exists around the core. The stroke sequence of dart leader–return streamer occurs several times in each flash to ground, giving it a flickering appearance. Each stroke draws charge from successively higher regions of the CB and transfers a negative charge to the surface. Return streamers occur only in cloud-to-ground discharges and are so intense because of the Earth's high conductivity. Some rare discharges between cloud and ground are initiated from high surface structures or mountain peaks, by an upward-moving stepped leader and referred to as a ground-to-cloud discharge. Rather rarely an overhanging anvil-to-ground discharge can be triggered by heavy charge accumulation in the anvil, and the high-magnitude strike can move many kilometres from the storm — a 'bolt from the blue', but another reason for recreational pilots to give large storm cells a very wide berth. The temperature of the ionised plasma in the return streamer is at least 30 000 °C and the pressure is greater than 10 atmospheres. This causes supersonic expansion of the channel, which absorbs most of the dissipated energy in the flash. The shock wave lasts for 10–20 microseconds and moves out several hundred metres before decaying into the sound wave — thunder — with maximum energy at about 50 hertz. The shock wave can damage objects in its path. The channel length is typically 5 km. Channel length can be roughly determined by timing the thunder rumble after the initial clap; e.g. a rumble lasting for 10 seconds x 335 m/sec = 3.3 km channel length. When a lightning stroke occurs within 150 m or so, the observer hears the shock wave as a single, high-pitched bang. Effect on aircraft instruments The lightning discharges emit radio waves — atmospherics or 'sferics — at the low end of the AM broadcast band and at TV band 1. These radio waves are the basis for airborne storm mapping instruments such as Stormscope and Strikefinder. The NDB/ADF navigation aids also operate near the low end of the AM band, so that the tremendous radio frequency energy of the storm will divert the radio compass needle. Weather radars map storms from the associated precipitation. Strike effect on aircraft When most aeroplanes, excluding ultralights, are struck by lightning the streamer attaches initially to an extremity such as the nose or wing tip, then reattaches itself to the fuselage at other locations as the aircraft moves through the channel. The current is conducted through the electrically bonded aluminium skin and structures of the aircraft, and exits from an extremity such as the tail. If an ultralight is struck by lightning, the consequences cannot be determined but are likely to be very unpleasant. Ultralights particularly should give all CBs a wide berth; supercells and line squalls should be cleared by 25–30 nm at least. Although a basic level of protection is provided in most light aeroplanes for the airframe, fuel system and engines, there may be damage to wing tips, propellers and navigation lights, and the current has the potential to induce transients into electrical cables or electronic equipment. The other main area of concern is the fuel tanks, lines, vents, filler caps and their supporting structure, where extra design precautions prevent sparking or burn-through. In heavier aircraft, radomes constructed of non-conductive material are at risk. 2.10.4 Red sprites and blue jets When large cloud-to-ground lightning discharges occur below an extensive CB cluster with a spreading stratiform anvil, other discharges are generated above the anvil. These discharges are in the form of flashes of light lasting just a few milliseconds and probably not observable by the untrained, naked eye but readily recorded on low-light video. Red sprites are very large but weak flashes of light emitted by excited nitrogen atoms and equivalent in intensity to a moderate auroral arc. They extend from the anvil to the mesopause at an altitude up to 90 km. The brightest parts exist between 60–75 km, red in colour and with a faint red glow extending above. Blue filaments may appear below the brightest region. Sprites usually occur in clusters that may extend 50 km horizontally. Blue jets are ejected above the CB core and flash upward in narrow cones, which fade out at about 50 km. These optical emissions are not aligned with the local magnetic field. Images and further information are available at the University of Alaska site. 2.10.5 Auroral displays The Aurora Australis is usually only seen from latitudes higher than 60° south but may sometimes be seen from the Australian mainland. The displays, or aurora storms, take place at altitudes of 100–300 km. The auroral glow is caused by an increase in the number of high-energy, charged particles in the solar wind (separated hydrogen protons and electrons) associated with increased solar flare activity. Some of these particles, captured by the magnetosphere, are accelerated along the Earth's open magnetic field lines (which are only open in the polar regions) and penetrate to the inner Van Allen belt, overloading it and causing a discharge of the charged particles into the ionosphere. The discharges extend in narrow belts 20–25° or so from each magnetic pole. The excitation of oxygen and nitrogen atoms by collision with the particles causes them to emit visible radiation — forming moving patches, bands and columns of limited colours. The display colour depends on the gas and the altitude. Oxygen atoms emit a red glow at high levels, orange at medium levels and pale green at low levels. Nitrogen emits blue and violet at high levels and red at low levels. The major forms of auroral display, and typical sequence of appearance, are: glow — a faint glow near the horizon, usually the first indication of an aurora arch — a bow-shaped arc running east to west, usually with a well-defined base and small waves or curls rays — vertical rays or streaks, often signifying the start of an aurora substorm and forming into bands band — a broad, folded curtain moving in waves and curves, and indicating maximum activity is near corona — rays appear to converge near the zenith veil — a weak, even light across a large part of the sky often preceding the end of the display patch — an indistinct nebulous cloud-like area which may appear to pulsate. Extensive auroral displays, which are associated with high sunspot activity, are accompanied by disturbances in radio communications. The period of maximum and minimum intensity of the aurora follows the 11-year sunspot cycle. STRICT COPYRIGHT JOHN BRANDON AND RECREATIONAL FLYING (.com)
