Northern Lights

Auroras -or northern lights- are caused by geomagnetic storms. Solar wind, with its charged particles (ions), is managing to make its way, over the poles, into the Earth's magnetic field, 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 upper

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

layers, 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. 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. for more about the theory of solar energetic events, see the tutorial 'The Sun'

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 an '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 As their 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. 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. An asymmetry has been found lately about both auroral ovals' behavior as the southern one only is shifting to the dawn side of Earth

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°. Their origin is still in debate. see more

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

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. 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. As Alaska or Iceland are places of choice in the northern hemisphere, 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. 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! An advantage of that Sun is monitored is that such sites like SOHO's or the SEC/NOAA allow to better forecast the possibility of auroras. The SEC/NOAA site has a fine dedicated section to auroras (via 'Data and Products' and 'POES Satellite')
An overview of the solar activity is seen too on our site, with data dedicated to northern lights. see it at each 'Events, Moon, Planets' localized page

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. 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