The Solar Wind Space physics 7,5hp Teknisk fysik '07 1
Contents History... 3 Introduction... 3 Two types of solar winds... 4 Effects of the solar wind... 5 Magnetospheres... 5 Atmospheres... 6 Solar storms... 6 References... 7 2
History The interest in an explanation on how the geomagnetic field could undergo large and rapid variations began as early as 1722, when an English scientist Graham invented a compass sensitive enough to react to these variations. In the 1740's Swedish scientists Celsius and Hiorter noted that when aurorae lit up the sky, there was disturbances in Grahams compass needle. Together with Graham, Celsius and Hiorter determined that these disturbances were not caused by local phenomena. But, the explanation came to linger for another 100 years before the connection to the solar winds could be made. In the early 1800's an amateur astronomer in Germany, S. Heinrich Schwabe began observing the sun, and making counts of sunspots. His later discovery of the decennial sunspot cycle, and it's reflection in terrestrial magnetism, was the very start of what we today call the Solar- Terrestrial physics. In 1859 an English astronomer, R. C. Carrington, observed a solar flare while studying sunspots. He compared his observations with the magnetic records from Key Observatory in London, and noted a short lived, but definite disturbance at the time of the flare. The connection had been made, but were of many others deflected as chance coincidence. Not until 1871, when an Italian astronomer Secchi, observed that solar eruptions occasionally displayed velocities equal to or greater than the escape velocity of the sun, came the idea that the origin of the magnetic storms was a result from solar disturbances propagating towards earth as a cloud of charged particles. The idea was not officially accepted as of then, but from then on undisputable proof began to gather. [1] Introduction The solar wind is a continuous stream of charged particles flowing from the sun, travelling at the speed of about 450 km/s. It takes about four days for the wind to reach the earth's magnetic field (see Figure 1). This region is referred to as the magnetosphere. [2] In the past the solar winds did not affect us directly, but lately as we've become more and more dependent on technology, we begin to see the effects in blackouts and broken technological equipment. The sun is consisting of hot gas shaped by magnetic fields, which also create bright spots on the sun's surface. Using a device called "Hinode's Extreme Ultraviolet Imaging Spectrometer" (EIS) it is possible to measure the speed at which material flows out from the sun. By doing this scientists have discovered that at the edges of the bright spots on the sun, hot gas spurts out into space. The magnetic fields links the different regions on the sun together. When the magnetic fields from two regions collide they allow hot gas to escape, which becomes the solar winds. [3] The solar winds varies routinely over a 27-day period connected to the rotation of the sun, as a result of eruptions in the corona. [4] 3
l Figure 1: The solar winds travelling towards earth's magnetic field. [7] Two types of solar winds The solar winds can have low or high speeds. The fast solar wind (the high speed wind) is moving at a pace of 750 km/s and has a temperature of about 8 10 5 K, while the slow solar winds move at a speed of 400 km/s with a temperature of 1.4 1.6 10 6 K. The slow solar wind is twice as dense, has a more complex structure, and vary more in intensity than the fast solar winds. The slow solar winds originate from the "streamer belt" stretching around the sun's equatorial belt. The slow winds occur between latitudes of 30-35 degrees during the solar minimum (the time at which the solar activity is at its lowest), and then expand to the poles as the solar activity moves towards its maximum. The fast solar winds originate from coronal holes, which are more frequent at the suns magnetic poles. These coronal holes are regions of open field lines. The plasma is at first confined in convection cells (areas where different temperatures and/or pressure create density variations in the plasma) in the solar atmosphere. The plasma is then released into narrow necks of coronal hole streams into the photosphere (see figure 2), which give rise to high plasma velocities. [5] [6] 4
Figure 2: An overview of the layers of the sun. [8] Effects of the solar wind The sun's rotation rate has decreased significantly over the lifetime of the sun due to loss of mass from escaping solar winds, but the solar winds do not affect only the sun. Magnetospheres When the solar wind approaches a planet with a strong magnetic field, the particles in the plasma are deflected in the magnetosphere due to the Lorentz force. It causes the particles not to hit the atmosphere of the planet directly, but rather be transported around it. However, a small number of particles are able to pass through to earth's upper atmosphere due to the Van Allen radiation belt (a torus of plasma around the earth which is compressed on the sunward side of the earth due to the solar winds, see Figure 3). When these particles hit the upper atmosphere of the earth, they fluoresce and create an aurora. [6] [7] 5
Figure 3: An illustration of the Van Allen radiation belts. It is not uniformly formed in reality though, because of a compression on the sunward side from the solar winds. [9] Atmospheres Without the magnetic field to protect the Earth, our atmosphere would be stripped by the solar wind. It is believed that atmospheric stripping is gas being trapped in magnetic field "bubbles", which are then ripped off by solar winds. Mars (which barely has a magnetic field in comparison with the Earth), is located four times the distance away from the sun compared to the Earth, and yet it has only 1/100th of the atmosphere due to the solar wind stripping of the atmosphere. [6] Solar storms Solar storms, or interplanetary coronal mass ejections (ICMEs) are fast-moving outbursts of plasma from the sun (see figure 4). These are often caused by release of magnetic energy from the sun. Due to their high velocities ICMEs cause shock waves in the thin plasma of the heliosphere. These shock waves launches electromagnetic waves, and accelerates charged particles towards the earth. When they hit the magnetosphere, it temporarily deforms Earth's magnetic field, causing a large counter induced ground current in the earth and changes the direction of compass needles. This is called a geomagnetic storm. [6] These geomagnetic storms can damage satellites and disrupt electrical power systems and communications. [3] 6
Figure 4: A large coronal mass ejection. This is a classical representation of its shape. [10] References [1] E.W. Cliver " Eos, Transactions, American Geophysical Union, Vol. 75, No. 49" http://www-ssc.igpp.ucla.edu/spa/papers/eos_40yrs/ December 6, 1994, Pages 569, 574-575 [2] National Maritime Museum " Solar weather" http://www.nmm.ac.uk/gcse-astronomy/sun-and-moon/solar-weather/ [3] Eurpoean Space Agency " Source of the slow solar wind" http://www.esa.int/esasc/semjqk5qgef_index_0.html [4] NASA " Solar Wind" http://helios.gsfc.nasa.gov/sw.html [5] Wikipedia "Convection cell" http://en.wikipedia.org/wiki/convection_cell [6] Wikipedia "Solar Wind" http://en.wikipedia.org/wiki/solar_wind [7] Wikipedia "Van Allen radiation belt" http://en.wikipedia.org/wiki/van_allen_radiation_belt 7
Pictures [7] NASA Figure 1: http://www.newhomewindpower.com/images/solar-wind-nasa.jpg [8] NASA Figure 2: http://www.nasa.gov/images/content/171926main_heliolayers_label_lg.jpg [9] Wikipedia Figure 3: http://en.wikipedia.org/wiki/file:van_allen_radiation_belt.svg [10] SOHO - Solar and Heliospheric Observatory Figure 4: http://sohowww.nascom.nasa.gov/gallery/images/large/20021202c2cme_prev.jpg 8