Observation Northern Lights

The aurora's nature –- the fact that the lights are electromagnetic and respond to solar activity –- was only realized in the last 150 years. The visible light produced in the atmosphere as aurora is the last step of a chain of processes connecting the solar wind to the atmosphere. After a trip toward Earth that can last three days, the solar energetic particles and magnetic fields cause the release of particles already trapped near Earth, which in turn trigger reactions in the upper atmosphere in which oxygen and nitrogen molecules release photons of light. Nitric oxide is created during a aurora. Auroras are also heating up the gases in Earth's upper atmosphere. Auroras on Earth are linked with solar coronal mass ejections. The collision of solar particles and pressure into our planet’s magnetosphere accelerates particles trapped in the space around Earth like in the radiation belts and sent crashing down into Earth’s upper atmosphere at altitudes of 60 to 250 miles (100 to 400 kilometers) where they excite oxygen and nitrogen molecules and release photons of light. Auroras -or northern lights- are caused by geomagnetic storms. A geomagnetic storm is a kind of space weather event when a solar coronal mass ejection comes to compress and release the magnetic fields surrounding Earth. When the tail of the magnetosphere stretches and elongates under the force of the slamming solar wind send from the solar-exposed side of the Earth, it forces oppositely directed magnetic fields close together that join in an explosive process called magnetic reconnection. Like a stretched rubber band suddenly breaking, these magnetic field lines then snap back toward Earth, carrying charged particles along for the ride, which slam into the upper atmosphere, causing it to glow and generating the aurora. Auroras occur globally in an oval shape, last hours and appear primarily in greens, blues and reds. Energetic particles driven from the magnetosphere crash into the ionosphere below it which passes them further down. Moving magnetic fields cause an unstable environment, setting charged particles moving and initiating electric currents. A geomagnetic storm takes about 24 hours to pass at Earth and causes the release of particles already trapped near Earth which, in turn, triggers reactions in the upper atmosphere in which oxygen and nitrogen molecules release photons of light, causing the aurora. Solar wind, with its charged particles (ions), is managing to make its way, over the poles, into the Earth's magnetic field where the latter accelerate it and sent it towards the poles where it's accelerating electrically charged particles which are trapped there (electrons, protons). The both kinds of such high-speed particles crash onto Earth's atmosphere upperlayers' neutral atoms, causing the atmosphere to glow. Different solar events may trigger auroras like coronal mass ejections (CMEs) or coronal holes. CMEs are energetic events occurring at Sun and projecting increased amounts of plasma into the solar wind as coronal holes are places at Sun's surface where a looser magnetic field is letting some plasma to flow out more gently. Two kinds of waves concern auroras. Alfvén waves, named after Swedish Nobel laureate Hannes Alfvén who first predicted their existence in 1942, are thought to accelerate the electrons. They are measuring tens to hundreds of miles long from peak to peak and propagating along Earth’s magnetic field lines. On the other side Langmuir waves are generated by the electrons themselves, a process that steals some of the electrons’ energy and slows them down. A more technical -although usual- way to explain auroras is to say that both the solar interplanetary magnetic field and the Earth's magnetosphere are cancelling each other. Magnetic lines of the magnetosphere are seen directly linked to the magnetic lines of the solar wind. It is along such magnetic lines that the solar particles are reaching the upper atmosphere. Such a connection occur when both Sun's and Earth's magnetic field have opposed polarities. The polarity of the Earth's magnetosphere is always North (positive) as Sun' may be North (positive) or South (negative) as tangled bundles of solar wind and particles have various polarities. The polarity of Sun's magnetic field at Earth is named 'B_z'. Major auroras events occur when remainings of a particularly violent solar event manage to reach Earth. A diffuse, weak aurora is always present near the poles, but can't always be seen with the naked eye. The 'continuous' aurora is the faint continually produced by the solar wind, the 'diffuse' one is a relatively featureless glow as the 'discrete' aurora are the most intense auroras with a twisting aspect and a luminosity strong enough to read a newspaper. Brighter auroras require thus a influx of energy.The conversion of magnetic energy to particle energy, which is powering the aurora, has been further explained and gives a better explanation of why a aurora is varying in shape and luminosity in the sky! Auroral substorms are dynamic phenomena that occur in the upper atmosphere at night, and caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating auroras are much dimmer and less common. As far as the usual aurora is concerned, once inside the magnetosphere, solar particles and the energy they carry are stored on the nightside of it until a substorm releases the energy, as the electrons are then sent speeding down into Earth’s upper atmosphere. The pulsating aurora is due to whistler mode chorus waves which efficiently disturb some of the electrons. During a 'substorm' -- those small, intermittent solar disturbances occurring several times a day at the Earth -- when solar material impacts the magnetosphere, the day side contracts inward, while the back end, or the magnetotail, stretches out. When the stretched magnetotail finally snaps back, it starts to vibrate. In that unstable environment, electrons in near-Earth space rapidly stream down magnetic field lines. The aurora then itself moves in a harmony of a six-minute cycle with the vibrating field line, brightening when the wave of electrons slams into the upper atmosphere, and dimming when it ricochets off. The aurora creates nitric oxide. Electric fields drive the ionosphere and are predicted to set up enhanced neutral winds within a aurora arc. Aurora arcs are the familiar, slow-moving green curtains of light that can extend from horizon to horizon as they are yielded by 'Birkeland currents.' Height-dependent coupling processes exist that create localized neutral jets within the aurora with their associated heating and neutral structuring. Inverted-V arc and a dynamic Alfenic curtain also are linked with the aurora. Pulsating auroras are beautiful emission patterns in which the aurora appear to blink correspond to high-altitude chorus waves. They are quasiperiodic, blinking patches of light tens to hundreds of miles across, appearing at altitudes of about 62 miles in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. Rythm of the pulsation is from some to a dozen seconds. Substorms which occurs when the solar wind rips off Earth's magnetic field lines from the day side, pulling them around to the night side, where they pile up, storing vast amounts of energy until they release in explosive bursts of magnetic reconnection create surge of radiation and magnetism that rebounds toward Earth with its attendant aurora. Pulsating auroras are caused by low levels of low-energy secondary electrons originating into electrons which yield the usual auroras as colliding with the atoms and molecules of the upper atmosphere. Those electrons travel back and forth from a pole to a other through magnetic lines. Such a intermittent precipitation of low-energy energetic electrons arriving from the magnetosphere likely is due to that they are scattered by whistler-mode chorus waves, those electromagnetic waves. A 'cusp aurora' is a particular subset of the Northern Lights in which energetic particles are accelerated downward into the atmosphere directly from the solar wind. The magnetic cusps are where energy from the solar wind can come directly down into Earth’s atmosphere. This energy heats the atmosphere by hundreds of degrees, inflating it and driving fierce winds of both neutral atmospheric and ionized gases. During polar night, the cusp is visible to the naked eye. Cusp auroras although not particularly rare, occur during the day only. Because the magnetic North Pole is offset from the geographic North Pole, it’s often possible to see cusp auroras in Northern Europe, for example, near the winter solstice. Strong Thermal Emission Velocity Enhancement (STEVE) hints to there are unknown chemical processes taking place in the sub auroral zone that can lead to a light emission. That produces a kind of aurora, green and purple-hued and more South than normal, like a very narrow arc, aligned east-west, and extending for hundreds or thousands of miles. A STEVE can last from 20 minutes to one hour as its appears only along with a usual aurora as it might appear in some seasons only. 'Auroral wind' is a strong but intermittent stream of oxygen atoms flowing from Earth’s atmosphere into outer space during a aurora. Electric currents also exist in and around the region where aurora occur as they are invisible and due to the solar wind. They are heating the thin air of the upper atmosphere of Earth through the Joule heating process, expanding the thermosphere. Auroras also emit X-rays, generated as incoming particles decelerate. Electrons raining into Earth’s upper atmosphere reveal interactions between the solar wind and the magnetosphere as vertical winds in the E and F regions create a tumultuous particle soup that re-distributes the energy, momentum and chemical constituents of the atmosphere, likely contributing to so-called 'auroral winds.' for more about the theory of solar energetic events, see the tutorial 'The Sun'

->Several Terms for a Same Thing
'Aurora', 'Northern lights', 'Southern lights' are equivalent terms to speak about this phenomenon. 'Aurora borealis' and 'Aurora australis' (northern and southern auroras, in Latin) may be also encountered

northern and southern auroral ovals. picture site 'Amateur Astronomy' based on data SEC/NOAA

As solar particles are reaching the atmosphere above the poles this yields a 'auroral oval' which is centered on Earth's magnetic poles. Earth is rotating below as the oval is at about 62 miles of altitude (100 km) or more and lying at around 65–70 degrees of northern -or southern- latitude (when the interplanetary magnetic field points northward, auroras can occur at even higher latitudes, sometimes resulting in theta aurora due to their shape in a Greek letter theta, a oval with a line crossing through the center; such auroras might be due to plasma trapped inside the magnetosphere from the reconnection phenomenon, and funnled into near-Earth space). Altitudes of where auroras occur range from 62 to 310 miles high. Like the name says northern lights are northernmost latitudes phenomenons. The same is true for southernmost latitudes and both the northern and southern lights are produced by the same solar events. Particularly strong solar events make the auroras appear at more moderate latitudes as, generally, it is powering up the aurora events (a X class flare can power the Northern Lights and make them visible as far south as Washington State, central Idaho, northern Wyoming, the Dakotas and east to Chicago, Detroit, NYC and Boston, in the USA). A asymmetry has been found lately about both auroral ovals' behavior as the southern one only is shifting to the dawn side of Earth relative to the magnetic pole. This is likely caused by the fact that the Earth's magnetic field is not a perfect 'dipole,' that is the magnetosphere's electric charges of opposite signs (negative and positive) are possibly not of equal strength. On the other hand, both ovals are usually seen deflected towards Earth's night side -relative to the magnetic poles, due to the part of the magnetosphere facing the Sun being compressed by the solar wind while the opposite part is stretched away. Auroras come in various shapes and aspects as they may be still or affected by various movements. In terms of brightness they range from the luminosity of the Milky Way to the one of brightly moonlit clouds. The brighter the aurora, the more the colors are clearly seen. The most common color in an aurora is green. But displays that occur extremely high in the sky may be red or purple due to nitrogen atoms. Red auroras are not as common as green auroras, however they can happen during strong solar activity and they occur a little more often at low latitudes. They are yielded when energetic electrons escape the Van Allen Belts and fall into the Earth's atmosphere. Red auroras may reach up to 250 miles (400km) above the Earth. As far as the full range of aurora colors -red, green, and many shades of purple- are concerned, these hues correspond to different quantum transitions in excited atoms of oxygen and nitrogen. The precise color at any altitude depends on the temperature and density of the local atmosphere. Most auroras occur about 50 to 200 miles above the Earth. Some extend lengthwise across the sky for thousands of miles. When the charged particles strike atoms and molecules in the atmosphere, energy is released as this interaction when increased at the time of a solar maximum, produces extremely bright auroras. The late THEMIS satellites spotted that ropes of magnetic lines are linking directly the Sun to the upper layers of the Earth's atmosphere. As seen by the satellites in the ultra-violet, auroras are seen occurring continuously above Earth's poles. The sound of a aurora, on a other hand, are not a legend as, barely louder than the surrounding noice and with a metallic tone, it was recorded by 2011 in Finland. The source looked like hanging by 230 feet above the ground as it is still ill-known how such sounds are caused and the link between the upper atmosphere and the altitude of the sound

->What About Noctilucent Clouds
Noctilucent clouds are extremely high altitude clouds, electric blue in color, mostly seen during summer months, after sunset, at latitudes above 40°. see more

A very active northern aurora as seen in January 2012 from the International Space Station as the station was flying at about 240 miles above Winnipeg, in the state of Manitoba, Canada. picture site 'Amateur Astronomy' based upon a picture NASA

The best time to see the aurora with the naked eye is during a clear night near just after midnight around the equinox and through the winter. But aurora, however dim and quiescent, is always present day and night. Auroral activity usually occurs around 60 magnetic latitude near midnight. Some give a 5 p.m.-2 a.m. like a good time window. A aurora usually last 15-30 minutes (some hours with some luck) as the aurora blends into the sky before reappearing some later. The northern or southernmost you live, the more chance you have of seing auroras. Precisely, the more North or South you are in terms of magnetic latitudes, the more chance you have. Where about 50 percent of the nights are seing auroras, year round, in the northern hemisphere, is immediately North of Anchorage, Alaska, over the James Bay -south of Hudson Bay, Canada, and in northern Sweden and northernmost Finland. Most continental USA, South of the Great Lakes, have a 1-5 percent of the nights year round -alike to northern Europe (like northern France or northern Germany), as southern Europe has a mere 0.05 to 1 percent opportunities. Source give the Norwegian Svalbard like the Mecca of northern lights, as some places in Norway, Lapland, Canada, Alaska or Iceland are also places of choice. There are few lands of the auroras South as most favored places like southeastern Australia or New Zealand are still 10° of magnetic latitude behind the above said locations. The Arctic bases only are favored ;-) The midnight sun is beginning to occur in April in northernmost latitudes like Alaska. At such intermediary periods it's possible to see the auroras mixed with the twilight, as, further into summer, the night-long day is interrupting the auroras viewing season. It doesn't start back until August. Northern Lights are best seen in northern autumn and winter, with dark often present (and chances of clear skies highest towards the end of the period). 'Shoulder periods,' from late August to late September, and from late March until mid-April may also see you lucky enough but the nights then are shorter. As the auroral ovals are oval :-) and as they are fixed relative to the magnetic poles, Earth is rotating under them. This means for example for the northern hemisphere where the 'acute' spike of the oval is pointing to about the geographic meridian of St. Louis, in the USA, that Alaska, each night, passes from outside the oval to just under it, then to inside it and back! Latest studies are showing that autumn is producing almost twice more auroras than the annual average as spring is another good season for auroras. Winter and summer are poorer. This is still badly known. It might be linked to the annual variation of the Earth' axis hence of the axis of Earth's magnetic field. In summer and winter the Earth's magnetic field axis is at an angle with the axis of Sun's magnetic field as it is not in spring and autumn. It's at these seasons that the opposition between both axis is the greatest. On the other hand, the auroral activity is linked to the 11-year solar cycle. The nearer the maximum the more auroras are seen. Such sites like the one of SOHO or the one of the NOAA (the American weather agency) with the 'Space Weather Enthusiasts Dashboard', allow to better forecast the possibility of auroras. A good index to the possibility of auroras is the 'Kp' one, a number from 0 to 9, which is found at various official sites dealing with solar activity values, and which give the intensity of the geomagnetic activity. It's not until a Kp of 4 that the aurora boundary is found at the magnetic latitude (latitude refering to the magnetic poles) of 58.3. The other index is the value 'Bz' which is indicating whether the Sun's magnetic field at Earth is pointing North or South. A South direction is another sure hint to the occurrence of the aurora. When auroral conditions (solar wind, orientation of Sun's magnetic field) are met, the aurora is seen in the Northern hemisphere as well as in the Southern one. Latest studies are showing that autumn is producing almost twice more auroras than the annual average as spring is another good season for auroras. Winter and summer are poorer. This is still badly known. It might be linked to the annual variation of the Earth' axis hence of the axis of Earth's magnetic field. In summer and winter the Earth's magnetic field axis is at an angle with the axis of Sun's magnetic field as it is not in spring and autumn. It 's at these seasons that the opposition between both axis is the greatest. On the other hand, the auroral activity is linked to the 11-year solar cycle. The nearer the maximum the more auroras are seen