Mustafa Asfur Research website

Atmospheric electricity

Writing by mustafa on Saturday, 16 of February , 2008 at 10:07 am

The global electrical circuitThe Earth’s surface, ocean and solid, and the ionosphere are both 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.

  Atmospheric electricity

The global electrical circuitThe Earth’s surface, ocean and solid, and the ionosphere are both 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.

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 cloud generates a current of about one amp during its active period and 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.

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 separated 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 antennas will effect 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, but in extreme build-ups the consequent corona discharge streamers or brush discharge are manifested as St Elmo’s fire, usually not visible in daylight but visible at night as a continuous luminous blue-green discharge from wing tips, propellers and protuberances.

LightningThe electrostatic structure within Cb, or Cu con, is such that pockets of different charge exist throughout the cloud but, in 90% or more, with a main net positive charge residing on the cloud ice crystals in the upper part of the cloud and a main net negative charge, of similar magnitude, centred near the middle or lower part of the cloud at the sub-freezing level, the charge mainly residing 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 / attraction induce secondary charge accumulations outside the cloud, a positive region 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 which suggests that the disintegration of large raindrops and the interaction between the smaller cloud particles and the larger precipitation particles in the up / downdraughts 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 ionized channel that equalizes the charge difference between two regions of opposite charge, occurring 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 which ionize 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 based Cb found in the drier areas of Australia. Discharges above the Cb anvil into the stratosphere and mesosphere also occur, refer 11.4 below.

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 which 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 which generally follow the optimum ionized 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 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 earth 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 and to complete each lightning stroke a return streamer, or return stroke, propagates upwards. (The return streamer starts as positive ions which 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 cm 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’.

The temperature of the ionised plasma in the return streamer is at least 30 000 °C and the pressure is greater than 10 atmospheres, causing supersonic expansion of the channel which absorbs most of the dissipated energy in the flash. The shockwave 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 and channel length can be roughly determined by timing the thunder rumble after the initial clap, e.g. a rumble lasts 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 shockwave 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, which 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 re-attaches 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 but 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, damage to wing tips, propellers and navigation lights may occur 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, being constructed of non-conductive material, are at risk.

Red sprites and blue jetsWhen large cloud to ground lightning discharges occur below an extensive Cb cluster, which has 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 which 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. 

Auroral displays

The Aurora Australis is usually only seen from locations higher than 60° South but may sometimes be seen from the Australian mainland. (Check these photographs taken from 31° South). 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:

1. glow – a faint glow near the horizon, usually the first indication of an aurora
2. arch – a bow-shaped arc running east to west usually with a well defined base and small waves or curls
3. rays – vertical rays or streaks often signifying the start of an aurora substorm and forming into bands
4. band – a broad folded curtain moving in waves and curves and indicating maximum activity is near
5. corona – rays appear to converge near the zenith
6. veil – a weak, even light across a large part of the sky often preceding the end of the display
7. patch – an indistinct nebulous cloud-like area which may appear to pulsate

Extensive auroral displays, being 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.

Category: Articles

• Add this post to
• Del.icio.us -
• Meneame -
• Digg

No comments yet.