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kilometers, initial, results, 6807, ja078i028p06802, 1973jgr, 6802a, 6802, freeman, dessler, 256, ja074i001p00247, 1969jgr, 247s, simultaneous, isolated, schroeder, howes, kletzing, 3103, 34099653, 8184961, 23377, 2021natco, 3103s, laboratory, lewis, iowa, physicists, phd, thesis, dartmouth, college, 1349, ddlp, 3332, coupling, theories, longer, borovsky, birn, fujita, otto, tanaka, 214002762, 0619, generator, reviewed, karlsson, rankin, 220509575, 00715, 2020ssrv, 92l, courtier, johnstone, dispersion, 5096, 4173665, 215045a0, 1967natur, 45b, reiff, craven, winningham, shelley, frank, friedman, 1988, determination, potentials, distributions, 7441, ja093ia07p07441, 1988jgr, 7441r, mcilwain, measurement, 2727

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on feeds strong outbursts of auroras mainly around midnight unloading process a geomagnetic storm resulting from greater interaction adds many more particles to the plasma trapped around earth also producing enhancement of the ring current occasionally the resulting modification of earth s magnetic field can be so strong that it produces auroras visible at middle latitudes on field lines much closer to the equator than those of the auroral zone moon and aurora acceleration of auroral charged particles invariably accompanies a magnetospheric disturbance that causes an aurora this mechanism which is believed to predominantly arise from strong electric fields along the magnetic field or wave particle interactions raises the velocity of a particle in the direction of the guiding magnetic field the pitch angle is thereby decreased and increases the chance of it being precipitated into the atmosphere both electromagnetic and electrostatic waves produced at the time of greater geomagnetic disturbances make a significant contribution to the energizing processes that sustain an aurora particle acceleration provides a complex intermediate process for transferring energy from the solar wind indirectly into the atmosphere aurora australis 11 september 2005 as captured by nasa s image satellite digitally overlaid onto the blue marble composite image an animation created using the same satellite data is also available the details of these phenomena are not fully understood however it is clear that the prime source of auroral particles is the solar wind feeding the magnetosphere the reservoir containing the radiation zones and temporarily magnetically trapped particles confined by the geomagnetic field coupled with particle acceleration processes 64 auroral particles edit the immediate cause of the ionization and excitation of atmospheric constituents leading to auroral emissions was discovered in 1960 when a pioneering rocket flight from fort churchill in canada revealed a flux of electrons entering the atmosphere from above 65 since then an extensive collection of measurements has been acquired painstakingly and with steadily improving resolution since the 1960s by many research teams using rockets and satellites to traverse the auroral zone the main findings have been that auroral arcs and other bright forms are due to electrons that have been accelerated during the final few 10 000 km or so of their plunge into the atmosphere 66 these electrons often but not always exhibit a peak in their energy distribution and are preferentially aligned along the local direction of the magnetic field electrons are mainly responsible for diffuse and pulsating auroras have in contrast a smoothly falling energy distribution and an angular pitch angle distribution favouring directions perpendicular to the local magnetic field pulsations were discovered to originate at or close to the equatorial crossing point of auroral zone magnetic field lines 67 protons are also associated with auroras both discrete and diffuse auroral particle acceleration edit just as there are many types of aurora there are many different mechanisms that accelerate auroral particles into the atmosphere electron aurora in earth s auroral zone i e commonly visible aurora can be split into two main categories with different immediate causes diffuse and discrete aurora diffuse aurora appear relatively structureless to an observer on the ground with indistinct edges and amorphous forms discrete aurora are structured into distinct features with well defined edges such as arcs rays and coronas they also tend to be much brighter than diffuse aurora in both cases the electrons that eventually cause the aurora start out as electrons trapped by the magnetic field in earth s magnetosphere these trapped particles bounce back and forth along magnetic field lines and are prevented from hitting the atmosphere by the magnetic mirror formed by the increasing magnetic field strength closer to earth the magnetic mirror s ability to trap a particle depends on the particle s pitch angle the angle between its direction of motion and the local magnetic field an aurora is created by processes that decrease the pitch angle of many individual electrons freeing them from the magnetic trap and causing them to hit the atmosphere in the case of diffuse auroras the electron pitch angles are altered by their interaction with various plasma waves each interaction is essentially wave particle scattering the electron energy after interacting with the wave is similar to its energy before interaction but the direction of motion is altered if the final direction of motion after scattering is close to the field line specifically if it falls within the loss cone then the electron will hit the atmosphere diffuse auroras are caused by the collective effect of many such scattered electrons hitting the atmosphere the process is mediated by the plasma waves which become stronger during periods of high geomagnetic activity leading to increased diffuse aurora at those times in the case of discrete auroras the trapped electrons are accelerated toward earth by electric fields that form at an altitude of about 4000 12000 km in the auroral acceleration region the electric fields point away from earth i e upward along the magnetic field line 68 electrons moving downward through these fields gain a substantial amount of energy on the order of a few kev in the direction along the magnetic field line toward earth this field aligned acceleration decreases the pitch angle for all of the electrons passing through the region causing many of them to hit the upper atmosphere in contrast to the scattering process leading to diffuse auroras the electric field increases the kinetic energy of all of the electrons transiting downward through the acceleration region by the same amount this accelerates electrons starting from the magnetosphere with initially low energies tens of ev or less to energies required to create an aurora 100s of ev or greater allowing that large source of particles to contribute to creating auroral light the accelerated electrons carry an electric current along the magnetic field lines a birkeland current since the electric field points in the same direction as the current there is a net conversion of electromagnetic energy into particle energy in the auroral acceleration region an electric load the energy to power this load is eventually supplied by the magnetized solar wind flowing around the obstacle of earth s magnetic field although exactly how that power flows through the magnetosphere is still an active area of research 69 while the energy to power the aurora is ultimately derived from the solar wind the electrons themselves do not travel directly from the solar wind into earth s auroral zone magnetic field lines from these regions do not connect to the solar wind so there is no direct access for solar wind electrons some auroral features are also created by electrons accelerated by dispersive alfvén waves at small wavelengths transverse to the background magnetic field comparable to the electron inertial length or ion gyroradius alfvén waves develop a significant electric field parallel to the background magnetic field this electric field can accelerate electrons to kev energies sufficient to produce auroral arcs 70 if the electrons have a speed close to that of the wave s phase velocity they are accelerated in a manner analogous to a surfer catching an ocean wave 71 72 this constantly changing wave electric field can accelerate electrons along the field line causing some of them to hit the atmosphere electrons accelerated by this mechanism tend to have a broad energy spectrum in contrast to the sharply peaked energy spectrum typical of electrons accelerated by quasi static electric fields in addition to the discrete and diffuse electron aurora proton aurora is caused when magnetospheric protons collide with the upper atmosphere the proton gains an electron in the interaction and the resulting neutral hydrogen atom emits photons the resulting light is too dim to be seen with the naked eye other aurora not covered by the above discussion include transpolar arcs formed poleward of the auroral zone cusp aurora formed in two small high latitude areas on the dayside and some non terrestrial auroras atmosphere edit auroras result from emissions of photons in earth s upper atmosphere above 80 km 50 mi from ionized nitrogen atoms regaining an electron and oxygen atoms and nitrogen based molecules returning from an excited state to the ground state 73 they are ionized or excited by the collision of particles precipitated into the atmosphere both incoming electrons and protons may be involved excitation energy is lost within the atmosphere by the emission of a photon or by collision with another atom or molecule oxygen emissions green or orange red depending on the amount of energy absorbed nitrogen emissions blue purple or red blue and purple if the molecule 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 0 7 seconds to emit the 557 7 nm green light and up to two minutes for the red 630 0 nm emission collisions with other atoms or molecules absorb the excitation energy and prevent emission this process is called collisional quenching because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities such collisions are rare enough to allow time for oxygen to emit red light collisions become more frequent progressing down into the atmosphere due to increasing density so red emissions do not have time to happen and eventually even green light emissions are prevented the change in auroral colour with altitude is therefore explained oxygen red is predominant at high altitudes followed by oxygen green and nitrogen blue purple red then finally other hues of nitrogen blue purple red where particle collisions prevent oxygen from emissions green is the most common colour then comes pink a mixture of light green and red followed by pure red then yellow a mixture of red and green and finally pure blue precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere proton auroras are usually observed at lower latitudes 74 ionosphere edit bright auroras are generally associated with birkeland currents schield et al 1969 75 zmuda and armstrong 1973 76 which flow down into the ionosphere on one side of the pole and out on the other in between some of the current connects directly through the ionospheric e layer 125 km the rest region 2 detours leaving again through field lines closer to the equator and closing through the partial ring current carried by magnetically trapped plasma the ionosphere is an ohmic conductor so some consider that such currents require a driving voltage which an as yet unspecified dynamo mechanism can supply electric field probes in orbit above the polar cap suggest voltages of the order of 40 000 volts rising up to more than 200 000 volts during intense magnetic storms in another interpretation the currents are the direct result of electron acceleration into the atmosphere by wave particle interactions ionospheric resistance has a complex nature and leads to a secondary hall current flow due to physics the magnetic disturbance on the ground due to the main current almost cancels out so most of the observed effect of auroras is due to a secondary current the auroral electrojet an auroral electrojet index measured in nanotesla is regularly derived from ground data and serves as a general measure of auroral activity kristian birkeland 77 deduced that the currents flowed in the east west directions along the auroral arc and such currents flowing from the dayside toward approximately midnight were later named auroral electrojets see also birkeland currents the ionosphere can contribute to the formation of auroral arcs via the feedback instability under high ionospheric resistance conditions observed at night time and in the dark winter hemisphere 78 interaction of the solar wind with earth edit earth is constantly immersed in the solar wind a flow of magnetized hot plasma a gas of free electrons and positive ions emitted by the sun in all directions a result of the two million degree temperature of the sun s outermost layer the corona the solar wind reaches earth with a velocity typically around 400 km s a density of around 5 ions cm 3 and a magnetic field intensity of around 2 5 nt for comparison earth s surface field is typically 30 000 50 000 nt during magnetic storms in particular flows can be several times faster the interplanetary magnetic field imf may also be much stronger joan feynman deduced in the 1970s that the long term averages of solar wind speed correlated with geomagnetic activity 79 her work resulted from data collected by the explorer 33 spacecraft the solar wind and magnetosphere consist of plasma ionized gas which conducts electricity it is well known since michael faraday s work around 1830 that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across or is cut by rather than along the lines of the magnetic field an electric current is induced within the conductor the strength of the current depends on a the rate of relative motion b the strength of the magnetic field c the number of conductors ganged together and d the distance between the conductor and the magnetic field while the direction of flow is dependent upon the direction of relative motion dynamos make use of this basic process the dynamo effect any and all conductors solid or otherwise are so affected including plasmas and other fluids the imf originates on the sun linked to the sunspots and its field lines lines of force are dragged out by the solar wind that alone would tend to line them up in the sun earth direction but the rotation of the sun angles them at earth by about 45 degrees forming a spiral in the ecliptic plane known as the parker spiral the field lines passing earth are therefore usually linked to those near the western edge limb of the visible sun at any time 80 the solar wind and the magnetosphere being two electrically conducting fluids in relative motion should be able in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind however this process is hampered by the fact that plasmas conduct readily along magnetic field lines but less readily perpendicular to them energy is more effectively transferred by the temporary magnetic connection between the field lines of the solar wind and those of the magnetosphere unsurprisingly this process is known as magnetic reconnection as already mentioned it happens most readily when the interplanetary field is...