Space weather is a term applied to conditions in space as a result of solar activity. The Sun’s corona generates a stream of electrically charged particles (plasma) called the solar wind, which expands far beyond the planets. The composition of the solar wind mostly consists of hydrogen protons and electrons, as well as a small amount of ionized helium atoms. The terms electrically charged and ionized are interchangeable. The atoms become ionized in the Sun’s corona. The extreme heat of the corona gives the electrons sufficient energy to remove from orbit around the nucleus. This separation of the electrons and the protons, which are located in the nucleus, gives the particles an electric charge since electrons have a negative charge and protons have a positive charge. An atom in its normal state has no charge since the number of electrons equals the number of protons, canceling the positive and negative charges.
The solar wind is tenuous, with an average density of one to ten particles per cubic centimeter around the area of the Earth. The average velocity of the solar wind is 450 km/s (it can travel from London to New York in twelve seconds), although it can vary between 200 and 900 km/s. As the Sun rotates, the solar wind emanates from the Sun very much in the same fashion water does from a sprinkler. An image of this effect is below.
The solar wind has a direct effect on the shape of the Earth’s magnetic field. Much in the same way that the solar wind directs a comet’s tail away from the Sun, it pushes the Earth’s magnetic field downstream from the Sun. This pushes the magnetic field towards the Earth on the day side and pulls the field away from the Earth on the night side. The tail of the Earth’s magnetic field is about 300,000 kilometers long on average.
Disturbances in the solar wind can have consequences to us on Earth. To understand this, it is necessary to understand the following three principles regarding electricity and magnetism:
A changing or moving magnetic field creates an electric current.
An electric current creates a magnetic field.
A particle with an electric charge will travel in a path along a magnetic field line.
As electrically charged particles surf the solar wind and hit the Earth’s magnetic field, they will move along the Earth’s magnetic field lines. In the picture above, these field lines connect the Earth’s magnetic poles, which are close to the Earth’s geographic poles. As the field lines converge towards the poles, they enter an area called the polar cusp. The solar wind particles follow the field lines into the cusp entering the Earth’s atmosphere. The collisions between the solar wind particles and atoms in the upper atmosphere creates the aurora. This is why the aurora is generally only seen near the poles. The creation of the aurora is a two-step process. The solar wind particles collide with atoms in the upper atmosphere and transfer their kinetic energy to these atoms. As the atom absorbs the energy, electrons are excited to a higher energy state in a higher orbit around the nucleus. As the atom relaxes back to its normal lower energy state, it releases the excess energy in the form of a light photon.
Below is an image of the aurora from the International Space Station.
A disturbance in the solar wind can be caused by an event such as a Coronal Mass Ejection (CME). A CME is an ejection of a large amount of mass from the Sun. The CME produces a large shock wave striking the Earth’s magnetic field. This produces a change in the Earth’s magnetic field, referred to as a magnetic storm, as it reaches down to the surface towards the poles. This causes an electric current to be produced in a conductor. Consequently, electrical surges are generated in power grids. An example of this was the 1989 Quebec blackout caused by a magnetic storm. Space satellites are also at risk as a current can be generated overloading circuits. Thus, space weather forecasting has become an important area of research. If a magnetic storms are predicted accurately, electrical systems can be put in a protective mode to prevent damage. The video below demonstrates how computer models show us the manner in which CME events disrupt the Earth’s magnetic field including the historic 1859 Carrington Event.
NASA’s MMS mission has allowed us for the first time to directly observe a magnetic reconnection event. These events on Earth’s day side transfer mass and energy from the solar magnetic field to Earth’s magnetic field. The CME event distorts the Earth’s magnetic field compressing field lines on the day side and stretching back away from the night side. On the night side, the field lines reconnect again and like rubber bands stretched too far, snap back and release their storage of charged particles towards the Earth’s poles. This phenomena is responsible for massive aurora eruptions that can often be seen farther away from the usual polar position of the aurora.
To obtain the current status of the solar wind and aurora, you may visit the Space Weather Prediction Center website.
*Image atop post credit: NASA/SDO