Magnetron

All cavity magnetrons consist of a  cathode with a high (continuous or pulsed) negative potential created by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path, a consequence of the Lorentz force. Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. This principle of cavity resonator is very similar to blowing a stream of air across the open top of a glass pop bottle. A portion of the field is extracted with a short antenna that is connected to a waveguide .The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar.

The magnetron is called a "crossed-field" device in the industry because both magnetic and electric fields are employed in its operation, and they are produced in perpendicular directions so that they cross. The applied magnetic field is constant and applied along the axis of the circular device illustrated. The power to the device is applied to the center cathode which is heated to supply energetic electrons which would, in the absence of the magnetic field, tend to move radially outward to the ring anode which surrounds it.

  

Electrons are released at the center  cathode by the process of thermionic emission and have an accelerating field which moves them outward toward the anode. The axial magnetic field exerts a magnetic force on these charges which is perpendicular to their initially radial motion, and they tend to be swept around the circle. In this way, work is done on the charges and therefore energy from the power supply is given to them. As these electrons sweep toward a point where there is excess negative charge, that charge tends to be pushed back around the cavity, imparting energy to the oscillation at the natural frequency of the cavity. This driven oscillation of the charges around the cavities leads to radiation of electromagnetic waves, the output of the magnetron.

The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube. This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices such as the klystron are used.

The magnetron is a self-oscillating device requiring no external elements other than a power supply. A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be synchronized with the build-up of oscillator output.

  

The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1 kilowatt input will generally create about 700 watts of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.).

Mag Flowmeter

Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the flow of liquid in a pipe. In a magnetic flowmeter, a magnetic field is generated and channeled into the liquid flowing through the pipe. Following Faraday’s Law, flow of a conductive liquid through the magnetic field will cause a voltage signal to be sensed by electrodes located on the flow tube walls. When the fluid moves faster, more voltage is generated. Faraday’s Law states that the voltage generated is proportional to the movement of the flowing liquid. The electronic transmitter processes the voltage signal to determine liquid flow.

A magnetic flow meter (mag flowmeter) is a volumetric flow meter which does not have any moving parts and is ideal for wastewater applications or any dirty liquid which is conductive or water based. Magnetic flowmeters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions). Magnetic flowmeters are also ideal for applications where low pressure drop and low maintenance are required.

Principle of Operation

The operation of a magnetic flowmeter or mag meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.

Faraday's Formula:

E is proportional to V x B x D where:

E = The voltage generated in a conductor

V = The velocity of the conductor

B = The magnetic field strength

D = The length of the conductor

 To apply this principle to flow measurement with a magnetic flowmeter, it is necessary first to state that the fluid being measured must be electrically conductive for the Faraday principle to apply. As applied to the

design of magnetic flowmeters, Faraday's Law indicates that signal voltage (E) is dependent on the average liquid velocity (V) the magnetic field strength (B) and the length of the conductor (D) (which in this instance is the distance between the electrodes). 

Magnetic flowmeters measure the velocity of conductive liquids in pipes, such as water, acids, caustic, and slurries.However fluids with low conductivity, such as de-ionized water, boiler feed water, or hydrocarbons, can cause the flowmeter to turn off and measure zero flow.

This flowmeter does not obstruct flow, so it can be applied to clean, sanitary, dirty, corrosive and abrasive liquids. Magnetic flowmeters can be applied to the flow of liquids that are conductive, so hydrocarbons and gases cannot be measured with this technology due to their non-conductive nature and gaseous state respectively.

Ion Thruster

An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions. Note that the term "ion thruster" frequently denotes the electrostatic or gridded ion thrusters only.

The thrust created in ion thrusters is very small compared to conventional chemical rockets, but a very high specific impulse, or propellant efficiency, is obtained. This high propellant efficiency is achieved through the very frugal propellant consumption of the ion thruster propulsion system. They do, however, use a large amount of power. Given the practical weight of suitable power sources, the accelerations given by these types of thrusters is of the order of one thousandth of standard gravity.

Due to their relatively high power needs, given the specific power of power supplies, and the requirement of an environment void of other ionized particles, ion thrust propulsion is currently only practical beyond planetary atmosphere (in space).

Gridded electrostatic ion thrusters :

Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from a hot cathode filament and when accelerated in the electrical field of the cathode, fall to the anode (Kaufman type ion thruster). Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radio frequency ion thruster).

The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system via the plasma sheath the ions are accelerated due to the potential difference between the first and second grid (named screen and accelerator grid) to the final ion energy of typically 1-2 keV, thereby generating the thrust.

Ion thrusters emit a beam of positive charged xenon ions only. To avoid charging-up the spacecraft, another cathode is placed near the engine, which emits electrons (basically the electron current is the same as the ion current) into the ion beam. This also prevents the beam of ions from returning to the spacecraft and thereby cancelling the thrust.

Electromagnetic Wave

The electromagnetic field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell's equations and the Lorentz force law.It is further classified as near field and far field.

The near field and far field and the transition zone are regions of time-varying electromagnetic field around any object that serves as a source for the field. The different terms for these regions describe the way characteristics of an electromagnetic (EM) field change with distance from the charges and currents in the object that are the sources of the changing EM field. The more distant parts of the far-field are identified with classical electromagnetic radiation.

The basic reason an EM field changes in character with distance from its source is that Maxwell's equations prescribe different behaviors for each of the two source-terms of electric fields and also the two source-terms for magnetic fields. Electric fields produced by charge distributions have a different character than those produced by changing magnetic fields. Similarly, Maxwell's equations show a differing behavior for the magnetic fields produced by electric currents, versus magnetic fields produced by changing electric fields. For these reasons, in the region very close to currents and charge-separations, the EM field is dominated by electric and magnetic components produced directly by currents and charge-separations, and these effects together produce the EM "near field." However, at distances far from charge-separations and currents, the EM field becomes dominated by the electric and magnetic fields indirectly produced by the change in the other type of field, and thus the EM field is no longer affected (or much affected) by the charges and currents at the EM source. This more distant part of the EM field is the "radiative" field or "far-field," and it is the familiar type of electromagnetic radiation seen in "free space," far from any EM field sources (origins).

The far-field thus includes radio waves and microwaves several wavelengths from most types of antennas etc.

Schumann resonance

Lightning discharges are considered to be the primary natural source of Schumann resonance excitation; lightning channels (e.g clouds) behave like huge antennas that radiate electromagnetic energy at frequencies below 100 hz.These signals are very weak at large distances from the lightning source, but the Earth–ionosphere waveguide behaves like a resonator at ELF frequencies and amplifies the spectral signals from lightning at the resonance frequency.

Waveguide:

Waves in open space propagate in all directions, as spherical waves. In this way they lose their power proportionally to the square of the distance; that is, at a distance R from the source, the power is the source power divided by R2. The waveguide confines the wave to propagation in one dimension, so that (under ideal conditions) the wave loses no power while propagating.Conductors used in waveguides have small skin depth and hence large surface resistance.

Waves are confined inside the waveguide due to total reflection from the waveguide wall, so that the propagation inside the waveguide can be described approximately as a "zigzag" between the walls. This description is exact for electromagnetic waves in a hollow metal tube with a rectangular or circular cross-section. 

The real Earth–ionosphere waveguide is not a perfect electromagnetic resonant cavity.

Cavity resonators:

A cavity resonator is a hollow conductor blocked at both ends and along which an electromagnetic wave can be supported. It can be viewed as a waveguide short-circuited at both ends .

The cavity's interior surfaces reflect a wave of a specific frequency. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low loss (due to standing waves which are formed due to interaction of waves with different velocities). As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity.

Losses due to finite ionosphere electrical conductivity lower the propagation speed of electromagnetic signals in the cavity, resulting in a resonance frequency that is lower than would be expected in an ideal case, and the observed peaks are wide (rather sharp).In addition, there are a number of horizontal asymmetries – day-night difference in the height of the ionosphere, latitudinal changes in the Earth magnetic field, sudden ionospheric disturbances, polar cap absorption, variation in the Earth radius of +/- 11 km from equator to geographic poles, etc. that produce other effects in the Schumann resonance power specification.

Schumann resonances are recorded at many separate research stations around the world. The sensors used to measure Schumann resonances typically consist of two horizontal magnetic inductive coils for measuring the north-south and east-west components of the magnetic field, and a vertical electric dipole antenna for measuring the vertical component of the electric field. A typical passband of the instruments is 3–100 Hz. The Schumann resonance electric field amplitude (~300 microvolts per meter) is much smaller than the static fair-weather electric field (~150 V/m) in the atmosphere. Similarly, the amplitude of the Schumann resonance magnetic field (~1 picotesla) is many orders of magnitude smaller than the Earth magnetic field (~30–50 microteslas. Specialized receivers and antennas are needed to detect and record Schumann resonances. 

CCD Vs CMOS sensors

Digital cameras have become extremely common as the prices have come down. One of the drivers behind the falling prices has been the introduction of CMOS image sensors. CMOS sensors are much less expensive to manufacture than CCD sensors. 

Both CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor) image sensors start at the same point -- they have to convert light into electrons. One simplified way to think about the sensor used in a digital camera (or camcorder) is to think of it as having a 2-D array of thousands or millions of tiny solar cells, each of which transforms the light from one small portion of the image into electrons. Both CCD and CMOS devices perform this task using a variety of technologies. 

In a CCD device, the charge is actually transported across the chip and read at one corner of the array. An analog-to-digital converter turns each pixel's value into a digital value. In most CMOS devices, there are several transistors at each pixel that amplify and move the charge using more traditional wires. The CMOS approach is more flexible because each pixel can be read individually. 

CCDs use a special manufacturing process to create the ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CMOS chips, on the other hand, use traditional manufacturing processes to create the chip -- the same processes used to make most microprocessors. Because of the manufacturing differences, there have been some noticeable differences between CCD and CMOS sensors.

CCD sensors, as mentioned above, create high-quality, low-noise images. CMOS sensors, traditionally, are more susceptible to noise.

Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip tends to be lower. Many of the photons hitting the chip hit the transistors instead of the photodiode.

CMOS traditionally consumes little power. Implementing a sensor in CMOS yields a low-power sensor.

CCDs use a process that consumes lots of power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.

CMOS chips can be fabricated on just about any standard silicon production line, so they tend to be extremely inexpensive compared to CCD sensors.

CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality and more pixels.

CMOS sensors are just now improving to the point where they reach near parity with CCD devices in some applications. CMOS cameras are usually less expensive and have great battery life.

Pyroelectricity

Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled.The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to a voltage across the crystal. If the temperature stays constant at its new value, the pyroelectric voltage gradually disappears due to leakage current (the leakage can be due to electrons moving through the crystal, ions moving through the air, current leaking through a voltmeter attached across the crystal, etc.).

Pyroelectricity should not be confused with thermoelectricity: In a typical demonstration of pyroelectricity, the whole crystal is changed from one temperature to another, and the result is a temporary voltage across the crystal. In a typical demonstration of thermoelectricity, one side of the material is kept at one temperature and the other side at a different temperature, and the result is a permanent voltage across the crystal.

Pyroelectricity can be visualized as one side of a triangle, where each corner represents energy states in the crystal: kinetic, electrical and thermal energy. The side between electrical and thermal corners represents the pyroelectric effect and produces no kinetic energy. The side between kinetic and electrical corners represents the piezoelectric effect and produces no heat.

Although artificial pyroelectric materials have been engineered,effect was first discovered in minerals such as tourmaline. The pyroelectric effect is also present in both bone and tendon.

Pyroelectric charge in minerals develops on the opposite faces of asymmetric crystals. The direction in which the propagation of the charge tends toward is usually constant throughout a pyroelectric material, but in some materials this direction can be changed by a nearby electric field. These materials are said to exhibit ferroelectricity. All pyroelectric materials are also piezoelectric, the two properties being closely related. However, note that some piezoelectric materials have a crystal symmetry that does not allow pyroelectricity.

Very small changes in temperature can produce an electric potential due to a materials' pyroelectricity. Passive infrared sensors are often designed around pyroelectric materials, as the heat of a human or animal from several feet away is enough to generate a difference in charge.

The pyroelectric coefficient may be described as the change in the spontaneous polarization vector with temperature.The total pyroelectric coefficient measured at constant stress is the sum of the pyroelectric coefficients at constant strain (primary pyroelectric effect) and the piezoelectric contribution from thermal expansion (secondary pyroelectric effect). Thermal expansion is due to heating and cooling.

Artificial pyroelectric materials, usually in the form of a thin film, are of gallium nitride (GaN), caesium nitrate (CsNO3), polyvinyl fluorides, derivatives of phenylpyridine, and cobalt phthalocyanine.

LED

The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages.
When a light-emitting diode is switched on, electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.
The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light. LED development began with infrared and red devices made with gallium arsenide.

In semiconductor physics, the band gap of a semiconductor is always one of two types, a direct band gap or
an indirect band gap. The band gap is called "direct" if the momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice. The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum  (k-vector) in the Brillouin zone. If the k-vectors are the same, it is called a "direct gap". If they are different, it is called an "indirect gap".
In case of LED display (not to be confused with LED backlid LCD display) cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution.

Tesla Coil

The Tesla coil is one of Nikola Tesla's most famous inventions. It is essentially a high-frequency air-core transformer. It takes the output from a 120v AC to several kilovolt transformer & driver circuit and steps it up to an extremely high voltage. Voltages can get to be well above 1,000,000 volts and are discharged in the form of electrical arcs. Tesla himself got arcs up to 100,000,000 volts. Tesla coils are unique in the fact that they create extremely powerful electrical fields. Large coils have been known to wirelessly light up florescent lights up to 50 feet away, and because of the fact that it is an electric field that goes directly into the light and doesn't use the electrodes, even burned-out florescent lights will glow.

A Tesla coil transformer operates in a significantly different fashion from a conventional (i.e., iron core) transformer. In a conventional transformer, the windings are very tightly coupled and voltage gain is determined by the ratio of the numbers of turns in the windings. This works well at normal voltages but, at high voltages, the insulation between the two sets of windings is easily broken down and this prevents iron cored transformers from running at extremely high voltages without damage.

Unlike those of a conventional transformer (which may couple 97%+ of the fields between windings), a Tesla coil's windings are "loosely" coupled, with a large air gap, and thus the primary and secondary typically share only 10–20% of their respective magnetic fields. Instead of a tight coupling, the coil transfers energy (via loose coupling) from one oscillating resonant circuit (the primary) to the other (the secondary) over a number of RF cycles.

In each circuit, the AC supply transformer charges the tank capacitor until its voltage is sufficient to break down the spark gap. The gap suddenly fires, allowing the charged tank capacitor to discharge into the primary winding.

When the spark gap fires, the charged capacitor discharges into the primary winding, causing the primary circuit to oscillate. The oscillating primary current creates a magnetic field that couples to the secondary winding, transferring energy into the secondary side of the transformer and causing it to oscillate with the toroid capacitance. The energy transfer occurs over a number of cycles, and most of the energy that was originally in the primary side is transferred into the secondary side. The greater the magnetic coupling between windings, the shorter the time required to complete the energy transfer. As the primary energy transfers to the secondary, the secondary's output voltage increases until all of the available primary energy has been transferred to the secondary (less losses). Even with significant spark gap losses, a well designed Tesla coil can transfer over 85% of the energy initially stored in the primary capacitor to the secondary circuit.

As the secondary coil's energy (and output voltage) continue to increase, larger pulses of displacement current further ionize and heat the air at the point of initial breakdown. This forms a very conductive "root" of hotter plasma, called a leader, that projects outward from the toroid. The plasma within the leader is considerably hotter than a corona discharge, and is considerably more conductive. In fact, it has properties that are similar to an electric arc. The leader tapers and branches into thousands of thinner, cooler, hairlike discharges (called streamers). The streamers look like a bluish 'haze' at the ends of the more luminous leaders, and transfer charge between the leaders and toroid to nearby space charge regions. The displacement currents from countless streamers all feed into the leader, helping to keep it hot and electrically conductive. 

Armature Reaction

Armature reaction is the reaction or intereaction between the magnetic field produced by the flowing of current in the coil wound on the armature and the main magnetic field of the D.C. genarator or the motor.

The conversion of mechanical energy to the electrical energy in the electric dc motor or generator is due to this interaction.

Armature reaction causes the neutral plane, a plane parallel to armature windings with the magnetic flux, to shift in the direction of rotation. Thus armature reaction distort the magnetic flux (cross magnetisation) and lead the applied field not radial and reduction in magnetic field (de magnetisation ) which changes the efficiency of the output of the dc generator or dc motor.

In order to minimise the armature reaction, two simple methods can be taken up :

1. Shifting of the position of the brushes such that its plane are in the neutral plane (but this is not feasible as single machine is used as both generator and motor).

The brushes of a generator must be set in the neutral plane; that is, they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the neutral plane, they would short-circuit "live" coils and cause arcing and loss of power.

2. Installation of interpoles in the generator or motor to nullify the effect of armature reaction although it reduces available flux.
Compensating windings or intrrpoles are used for this purpose.Their function is to neutralize the cross magnetizing effect of armature reaction. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature in such a way that the current in them flows in opposite direction to that flowing in armature conductors directly below the pole shoes.
The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field

Armature reaction is response generated by armature of motor or generator to the change in the flux linked with it.This is the main cause for back emf in motors.
The armature reaction generates eddy currents in the armature which may result in losses in the machine.

FAQs of Battery

The amount of charge provided by battery is its capacity. Since charge is the product of current and time , therefore in order to increase capacity , current should be increased. This is done by connecting more number of batteries in parallel. This will provide more charge at same terminal voltage, hence capacity increases.

A household deep cycle  12V  (12.6V on fully charged) battery generally contains six 2V batteries in series  internally.

Each cell performs electrolysis  It has two electrodes , of lead oxide and other of lead. The electrolyte is a mixture of water and sulphuric acid.

                   FULLY CHARGED                                                                

                FULLY DISCHARGED

During complete charging, full amount of sulphuric acid is left as electrolyte along water. With the help of this electrolyte , movement of charge takes place between anode and cathode. As discharging begans, the sulphuric acid starts accumulating over electrodes and at complete discharge only water is left as electrolyte. Thus transfer of charge stops and we say that battery is down.

During these processes , some unwanted phenomenon also takes place which are as follows:

Sulphonation:

During charging there should complete transfer of sulphate from electrodes to electrolyte but it is not practacally observed. Some amount of sulphate remains on electrodes . This is sulphonation. It reduces the cross section area of plate and hences reduces capacity. To avoid this , desulphonation process is performed to remove layers of sulphates from electrodes.

Stratification :

Since water and sulphuric acid both are present in electrolyte  and water is lighter. lighter therefore after some time sulphuric acid settles down. Due to this, corrosion takes place in the lower section of electrodes. To avoid this, it is advised to discharge completely the battery after long cycles or if possible shake the battery frequently.

Explosion:

Due to over chaging , electrolysis of water takes place and hydrogen gas is formed. Since it is a supporter of cumbustion and temperature is also high it explodes.

To avoid this , some ventillations are provided.

        

Aurora

Aurora  is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere. Most aurorae occur in a band known as the auroral zone

It is basically an electrical phenomenon.

Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state.  They are ionized or excited by the collision of solar wind (it is basically a photon wave. Photon wave consistes of electrons and protons with high energy.) and magnetospheric particles (this includes charged ions of enviornmental gasses) :The Earth's magnetic field traps these particles, many of which travel toward the poles (since poles have strongest magnetic field) where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle (i.e. during solar storms) when coronal mass ejections increase the intensity of the solar wind.

Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that auroras are shaped by Earth's magnetic field. Indeed, satellites show that electrons are guided by magnetic field lines, spiraling around them while moving toward Earth.

It is observed that large electric currents were associated with the aurora and such currents  flowing from the dayside toward (approximately) midnight were later named "auroral electrojets". 

The colour of aurora is due to following reasons:

Oxygen emissions green or brownish-red, depending on the amount of energy absorbed. nitrogen emissions blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state. Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red.