The Origin of the Aurora

 

 

Electric currents originating in such fashion apparently give auroral electrons their energy. The magnetospheric plasma has an abundance of electrons: some are magnetically trapped, some reside in the magnetotail, and some exists in the upwards extension of the ionosphere, which may extend (with diminishing density) some 25,000 km around the Earth.

 

The convergence of magnetic field lines near Earth creates a “mirror effect” which turns back most of the down-flowing electrons (where currents flow upwards), inhibiting current-carrying capacity.

 

Some O+ ions (“conics”) also seem accelerated in different ways by plasma processes associated with the aurora.

 

In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR, discovered in 1972).

 

These “parallel voltages” accelerate electrons to auroral energies and seem to be a major source of aurora.

 

Other processes are also involved in the aurora, and much remains to be learned. Auroral electrons created by large geomagnetic storms often seem to have energies below 1 keV, and are stopped higher up, near 200 km. Such low energies excite mainly the red line of oxygen, so that often such auroras are red. On the other hand, positive ions also reach the ionosphere at such time, with energies of 20-30 keV.

Frequency of occurrence

 

 

Large magnetic storms are most common during the peak of the 11-year sunspot cycle, or during the 3 years after that peak.

 

Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoxes.During spring and autumn, the earth’s and the interplanetary magnetic field link up. At the magnetopause, Earth’s magnetic field points north. When Bz becomes large and negative (i.e., the IMF tilts south) it can partially cancel Earth’s magnetic field at the point of contact. South-pointing Bz’s open a door through which energy from the solar wind can reach Earth’s inner magnetosphere.

 

However, Bz is not the only influence on geomagnetic activity. The Sun’s rotation axis is tilted 7 degrees with respect to the plane of Earth’s orbit. Because the solar wind blows more rapidly from the Sun’s poles than from its equator, the average speed of particles buffeting Earth’s magnetosphere waxes and wanes every six months. The solar wind speed is greatest — by about 50 km/s, on average — around Sept. 5th and March 5th when Earth lies at its highest heliographic latitude.

 

Still, neither Bz nor the solar wind can fully explain the seasonal behaviour of geomagnetic storms. Those factors together contribute only about one-third of the observed semi-annual variation.

The solar wind and magnetosphere

 

The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cc and magnetic field intensity around 2–5 nT. The interplanetary magnetic field (IMF) may also be much stronger.

Southern vertical component of the Interplanetary Magnetic Field (IMF Bz < 0) is usually regarded as a predictor of the magnetospheric electromagnetic state disturbance. If IMF Bz < -5 nT for 3 hours consecutively, this component indicates the occurrence of a magnetic storm and its development.

Corona

 

 

 

 

 

Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881) established that the aurora appeared mainly in the “auroral zone”, a ring-shaped region of approx. radius 2500 km around the magnetic pole of the Earth, not its geographic one. It was hardly ever seen near that pole itself. The instantaneous distribution of auroras (“auroral oval”, Yasha Feldstein 1963) is slightly different, centred about 3-5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest equator-ward around midnight.

Graphs

 

 

 

 

Phi (Blue line)

Phi is the angle of the interplanetary magnetic field that is being carried out by the solar wind. Phi is measured in the GSM (geocentric solar magnetospheric) coordinate system.Sudden and rapid changes in the Phi angle in conjunction with increased solar wind speeds and Bz fluctuations is common during a CME impact.

 

Temperature (Green line)

The temperature of the solar wind is measured in Kelvin units. A rise in solar wind plasma temperature is likely during an incoming interplanetary shockwave.

 

Plasma Density (Orange line)

When a Coronal Mass Ejection (CME) is directed towards our planet, the solar wind may carry a dense cloud of energetic protons past Earth and this can help contribute to increased geomagnetic activty around our polar regions. The denser the plasma, the more energetic it is said to be.

 

Earth also has a magnetic field which forms a bubble around our planet. This is called the Magnetosphere. This bubble deflects the solar wind.

CMEs

 

 

 

 

 

CMEs can fling into space billions of tons of solar material, called plasma, as well as embedded magnetic fields. The ejected material hurdles into space at speeds up to several million miles per hour, creating an interplanetary shock wave

 

CMEs are thought to arise from large-scale magnetic instabilities. The solar atmosphere is contained by magnetic fields that can suddenly rearrange, releasing an enormous bubble of matter—a coronal mass ejection. CMEs are sometimes (but not always) associated with solar flares.

 

We are able to see CMEs by a coronagraph. You can spot CMEs on a coronagraph image as a large white tongue, blob, or halo that erupts from the corona. CMEs that are pointed toward earth are called halo events, because the approaching matter seems to surround the sun like a halo.

 

You probably already know that the earth has a magnetic field. You may not realize this field stretches way out into space—at least 37,000 miles (60,000 kilometers)—to form a protective bubble known as the magnetosphere.

 

The magnetosphere is important because it shields us from interplanetary space weather. The earth’s magnetic field and the IMF connect at the polar caps, and it’s here that energy and particles can and do enter the magnetosphere. South-pointing magnetic fields can spell trouble, while north-pointing fields usually coincide with calmer conditions.

The solar wind

 

 

 

The solar wind is a stream of charged particles which are ejected from the upper atmosphere of the sun. It consists mostly of high-energy electrons and protons that are able to escape the sun’s gravity in part because of the high temperature of the corona and the high kinetic energy particles gain through a process that is not well understood at this time.

 

The aurora is a glow observed in the night sky, usually in the polar zone. In northern latitudes, it is known as “aurora borealis”. The aurora borealis is also called the “northern lights”. The aurora borealis most often occurs from September to October and March to April.

 

The aurora is now known to be caused by electrons of typical energy of 1-15 keV, i.e. the energy obtained by the electrons passing through a voltage difference of 1,000-15,000 volts. The light is produced when they collide with atoms of the upper atmosphere, typically at altitudes of 80-150 km. It tends to be dominated by emissions of atomic oxygen–the greenish line at 557.7 nm and (especially with electrons of lower energy and higher altitude) the dark-red line at 630.0 nm.

 

The two most commonly referenced data points in some graphs are the Solar Wind (yellow line) and Bz component (red line). During times of relaxed solar activity, the solar wind usually streams past Earth at a speed of 250 km/s to 400 km/s. When a solar flare takes place, it can sometimes eject material into space and towards Earth also known as a Coronal Mass Ejection. The solar wind speeds carried past Earth by these shockwaves can sometimes exceed 700-800 km/s or even higher. The greater the increase, the stronger an impact to Earths geomagnetic field can be.

 

The Bz component represented by the red line is the current condition of the Sun’s magnetic field, also known as the interplanetary magnetic field (IMF).