auroras australes
What is a polar aurora?
It occurs when a solar mass ejection collides with the north and south poles of the Earth's magnetosphere, producing a diffuse but predominantly light projected into the Earth's atmosphere.
Auroras appear in two ovals centered above the Earth's magnetic poles, which do not coincide with the geographic poles. It occurs when charged particles (protons and electrons) from the sun are guided by the Earth's magnetic field and impinge on the atmosphere. near the poles. When these particles collide with oxygen and nitrogen atoms and molecules, which are the most abundant components of the air, some of the energy from the collision disturbs those atoms and molecules, driving them into excited states of energy. After a very short time, on the order of millionths of a second or even less, the atoms and molecules return to the fundamental level and return energy in the form of light. That light is what we see from the ground and we call “auroras”.
The sun, located 150 million kilometers from Earth, is continuously emitting charged particles: protons, with a positive charge, and electrons, with a negative charge. That is the flow of particles constitutes the called solar wind. Charged particles found in the sun's atmosphere tend to escape and are accelerated and channeled by the Sun's magnetic field, reaching Earth's orbit and beyond. Particles from the solar wind travel at speeds of 300 to 1000 kilometers per second, so that they travel the Sun-Earth distance in about two days. In the vicinity of the Earth, the solar wind is deflected by the Earth's magnetic field or magnetosphere. The particles flow in the magnetosphere in the same way that a river flows around a stone or a bridge pillar. The solar wind also pushes the magnetosphere and deforms it so that instead of a uniform beam of magnetic field lines such as an imaginary magnet placed in a north-south direction inside the Earth would show, what you have is an elongated comet-shaped structure with a long tail in the opposite direction to the Sun. Charged particles have the priority of being trapped and traveling along the magnetic field lines, so they will follow the path that they mark. Auroras typically occur between 95 and 1000 kilometers in elevation. Auroras stay above 95 km because at that altitude the atmosphere is so dense and collisions with charged particles occur so frequently that atoms and molecules are practically at rest. On the other hand, auroras cannot be higher than 500-1000 km because at that height the atmosphere is too thin - not very dense - for the few collisions that do occur to have a significant effect.
The auroras have different names depending on where they are observed. This is how it is called aurora borealis when this phenomenon is observed in the northern hemisphere and aurora australis when it is observed in the southern hemisphere. But there is no difference between them.
The colors and shapes of the auroras
Auroras have very diverse shapes, structures, and colors that also change rapidly over time.
During one night, the aurora can begin as a very elongated isolated arc that extends on the horizon, usually in an east-west direction. Around midnight, the arc may begin to brighten. Waves or curls begin to form along the arc and also vertical structures that look like very elongated and thin rays of light. Suddenly the entire sky can last from a few minutes to hours. As dawn approaches the whole process seems to calm down and only a few small areas of the sky appear bright until morning. The colors that we see in the auroras depend on the atomic or molecular species that the particles of the solar wind excite and the level of energy that those atoms or molecules reach. Oxygen is responsible for the two primary colors of auroras, green / yellow, while the redder color is produced by a less frequent transition. Nitrogen, from which a collision can remove some of its outermost electrons, produces a bluish light, while nitrogen molecules are very often responsible for the red / purple coloration of the lower edges of the auroras and the outermost curved parts.
Auroras on other planets
This phenomenon is not restricted to Earth. Other planets in the Solar System show similar phenomena, such as Jupiter and Saturn, which have stronger magnetic fields than Earth (Uranus, Mercury, and Neptune also have magnetic fields), and both have wide radiation belts. The auroras have been observed on both planets with the Hubble telescope. These auroras appear to be caused by the solar wind; Furthermore, the moons of Jupiter, especially Io, are important sources of auroras. This occurs due to electrical currents along lines, generated by a dynamo mechanism caused by the relative motion between the planet and its moons. Io, which has active volcanoes and an ionosphere, is a particularly strong source, and its currents generate, in turn, radio emissions, studied since 1955. Auroras have also been detected on Mars by the Mars Express spacecraft, during observations made in 2004 and published a year later. Mars lacks a magnetic field similar to Earth's, but it does have local fields associated with its crust. It seems that these are responsible for the auroras on this planet.
When do the polar auroras occur? When is it possible to observe this phenomenon? Since the particles of the solar wind continually reach the Earth, there are always auroras both during the day and at night, although, obviously, during the day the sunlight is much more intense and we cannot see them.
The following factors favor the chances of observing an aurora.
Time of day: since the intensity of the brightness of an aurora is very low, it can only be observed at night. In fact, the brightest and most active auroras normally occur around midnight, so the best times to observe them are between 11 p.m. and 2 a.m.
Season: at latitudes where auroras are more common, in summer there is sunlight practically all day. Autumn and spring are very suitable periods, due to the number of night hours available and the temperatures, which are not too low. In most polar regions the weather tends to be good and clear in the middle of winter, so observations can also be made during this time of year.
The cycle of solar activity: every 11 years the Sun has a maximum of magnetic activity, so the greater the solar activity, the more frequent the auroras are and it is possible that the range of latitudes at which they are observed extends somewhat more towards the north in the southern hemisphere and vice versa in the northern hemisphere. It must be said, however, that bright and intense auroras are observed at any time in the solar cycle.
The rotation of the Sun: It is very likely that within 27 days of having occurred a polar aurora, you will be lucky enough to observe another. This is because the Sun takes 27 days to rotate on its own axis, so it is very possible that after that period of time we will be able to see an aurora but more weakened than the first.
Moon phase: avoid, as far as possible, observing auroras on a full moon night or near this phase and also if the Moon is very high above the horizon.
Location: the further north or further south, the greater the probability of seeing a polar aurora. However, it must be taken into account that the positions near the North and South magnetic poles are not suitable. Greenland, Northern Canada and Alaska are prime locations in the North, and Antarctica is perfect in the South. The same occurs in southern Australia and New Zealand, where the phenomenon is also seen with some frequency.