Space Weather

There is a clear link between solar activity and effects observed in the space directly around the Earth. This space weather is the result of dynamic events within the Sun’s atmosphere. 

This essay first explores the mechanisms that cause the dynamic events in the Sun’s atmosphere before exploring how they affect space around Earth. Finally, the affects that solar activity has on humans is explored. 

The Solar Dynamo and Phenomena 

The 22-year solar cycle is a function of the dynamic magnetic field of the Sun (Freedman and Kauffman, 2008, p 424.) Many of the observed solar phenomena are explained by the magnetic-dynamo model first developed by Babcock. Prior to the 1960s a sound foundation had been laid from which Babcock could construct a model to explain solar observations.  

Babcock’s (1961) model had to explain a number of important phenomena observed on the Sun. It had to account for the: 

-       reversal of the main magnetic field. 

-       development of sunspots at 30⁰ north and south before migrating towards the equator. 

-       discovery that sunspots have strong magnetic dipolar fields in which the preceding and following members are opposite in the north and south hemispheres. Preceding members tend to be stronger and slightly closer to the equator. 

-       development of bipolar magnetic fields (BMRs,) that give rise to sunspots and disappear by expanding and their migration towards the pole in their hemisphere. 

A global model also has to take into account that the motion of conducting plasma that would affect the imbedded magnetic force lines. It had already been discovered that magnetic pole reversals occur approximately every 11 years at sunspot maxima. 

An important observation that held the key for a solar cycle model was differential rotation within the Sun. This provides both the probable source of the Sun’s magnetic field and the solar cycle. The Sun’s magnetic field is thought to be generated in the zone between the radiative and convection zones (Weiss and Thompson, 2009.) Differential rotation between the two zones provides the mechanism to produce the magnetic field. Also, differential rotation within the convective zone between the equator and the poles give rise to the solar cycle (Babcock, 1961.) 

In Babcock’s model the magnetic field is at its simplest at the start of a cycle.  The magnetic field lines emanate from the north pole and loop around to the south pole. Only latitudes greater than 55 are affected by the field. This is the state three years before the onset of a new sunspot cycle. 

With the passage of time the magnetic field lines submerged in the convection zone become drawn out due to differential rotation. The magnetic field becomes wrapped and approaches a near east-west orientation. This wrapping causes amplification of the field. After three years a critical magnetic field limit is reached at latitudes 30⁰ north and south of the equator. The critical limit will be met at lower latitudes later in the cycle. The wrapping of the magnetic field will have local irregularities leading to distortions and the formation of ‘flux ropes.’ The instabilities result in the formation of loops (see fig. 1.)

Figure 1: The development of the wrapping of the magnetic field lines and the creation of loops in flux ropes from the surface of the photosphere into the Suns atmosphere. Note that the preceding members in each hemisphere are opposite in polarity (Adapted from Freedman and Kauffman, 2008 p 425)

As distortion continues buoyancy effects due to the strengthening magnetic field results in upward lifting of the flux rope. They may break the photosphere surface to form BMRs. The magnetic field lines (defined by plasma being drawn along magnetic field lines) arc into the higher atmosphere forming coronal loops. Sunspots occur in BMRs when they are young and compact when the magnetic field is strong enough to inhibit convection. Thousands of BMRs may be formed during a single sunspot cycle. The magnetic field associated with these zones can be hundreds of times that of the main field (SPACEweba.) 

Throughout the sunspot cycle the magnetic field is partly dissipated by coronal loops. During the sunspot cycle sunspots are dissipated as the preceding members expand, are drawn out and migrate towards the equator. Like wise the following members expand, are drawn out and migrate towards the pole. During the dissipation of the BMRs magnetic flux loops are liberated into the corona. As a result of this process the magnetic field becomes weaker. 

As the coronal loops from the BMRs expand towards each other and towards the flux loops from the north and south poles they are realigned. As the field lines rise they may pinch and reconnect releasing energy and ejecting solar material from the corona (Freedman and Kauffman, 2008. p 426 – 427.) This is important for the coronal mass ejections. 

As this process proceeds the magnetic field is dissipated leading to the formation of a new global, opposite polarity magnetic field (Babcock, 1961.) A new cycle begins. Once the polarity again changes a complete 22 year cycle is concluded. 

There are a number of problems with the dynamo model (Freedman and Kauffman (2008. p 425 - 426) First of all the reversal of the Sun’s magnetic field is not fully understood. Also it doesn’t explain why sunspot activity can disappear for many years. An example of this is from 1645 to 1715. During this same period there were climatic changes in Europe and USA. Also in the eleventh and twelfth centuries higher temperatures appears to have been associated with increased sunspot activity.

Solar Weather 

Solar phenomena give rise to changes in the physical conditions in space that affects human technology and life on Earth (Gopalswamy, 2007.)  The change in the conditions near Earth is also called space weather. Three solar phenomena give rise to adverse space weather: coronal mass ejections, flares and coronal holes (Hochedez et al, 2005.) Before considering the effects of space weather on Earth the source of the influences on space weather should be detailed in reference to how they are linked to the solar cycle. 

Coronal Mass Ejections 

Coronal mass ejections (CMEs) are large eruptions that eject mass and the Sum’s magnetic field into interplanetary space (Gopalswamy, 2007.) CMEs result in the production of solar energetic particles (SEPs) and geomagnetic storms in the Earth’s magnetosphere. They are largest single event triggers for adverse space weather. 

CMEs originate from the closed magnetic fields associated with active regions and quiescent filament regions (Gopalswamy et al, 2009.) Typically they are more prevalent while the sunspots areas are on the increase and decrease than at sunspot maximum. During sunspot maximum periods CMEs are found to originate from higher latitude non-active regions of the Sun. 

CMEs commonly result from the reconnection of coronal flux loops. As with flares CMEs form shock waves in the interplanetary medium. CMEs result a larger release of energy from flares with increased effects to space weather. 

Solar Flares 

Solar flares are the result of the release of twisted magnetic fields above or near sunspots (SPACEwebb.) The build up of the magnetic field may occur over several days and release in one minute. This release produces a radiation burst in a range from radio waves to gamma-rays. The amount of energy released may be equivalent to millions of 100-megaton hydrogen bombs simultaneously exploding (HESPweb.) The release accelerates electrons, protons and heavy nuclei. Flares extend through the chromosphere and into the corona. As they are associated with sunspots it stands to reason that solar flares are most common at the height of sunspot activity. 

Coronal Holes 

Coronal holes have a significant effect on solar weather (Vršnak, Temmar and Veronig, 2007.) During quiet periods these features are found at the solar poles. During active periods they may also form at lower latitudes where they are more likely to allow emissions towards Earth. Coronal holes are where there are dark areas in the corona where open magnetic fields are present and have a reduced electron density (Navarro-Peralta and Sanchez-Ibara, 1994.) They are a source of fast components of the solar wind. 

As the fast solar wind interacts with the slower component of the solar wind increases in density and the magnetic field develop resulting in a shockwave. Also kinetic energy of the fast component is converted to heat. 

The changes in the interplanetary magnetic field cause by coronal holes can cause long lasting geomagnetic storms (Vršnak, Temmar and Veronig, 2007.) The storms may last for several days. The storms are less severe than those caused by coronal mass ejections but are more stable. Coronal hole events may prolong the effects of coronal mass ejections. 

Effect on the Earth and Humans 

The Sun emits a steady stream of solar wind that is mostly deflected by the Earth’s magnetosphere. Energetic events in the Sun’s atmosphere as described above can cause adverse space weather conditions that can damage equipment and endanger life. 

Space weather can be affected by three different factors: magnetic storms, SEPs and electromagnetic radiation (Hochedez et al, 2005.) Geomagnetic storms are commonly caused by the shockwaves created by CMEs that change the interplanetary magnetic field (IMF.) The solar wind carries the Sun’s magnetic field to form the IMF. However, magnetic storms can also occur due to changes in the solar wind due to high speed flows from coronal holes. The IMF interacts and disturbs Earth’s magnetic field. Flares tend to cause x-ray and UV radiation that causes disturbances in the ionosphere. CMEs and flares create SEPs that can damage electronic equipment. 

Under normal conditions the magnetosphere deflects solar winds. The magnetosphere and interplanetary magnetic field are in contact at the magnetopause (SPACEweba.) The two fields can link up across the magnetopause allowing for the solar wind to enter Earth’s atmosphere. This allows for ‘solar wind gusts,’ flares and CMEs that are emitted towards the Earth to inject matter and energy into the magnetosphere. It is these injections that cause magnetic storms. 

Solar flares emit x-rays and UV that can interfere with the ionosphere and cause communication problems (Gopalswamy, 2007.) 

A change in the unrelenting solar wind by active solar events leading to adverse space weather can result in a number of effects on humans: (Gopalswamy, 2007) 

-        Satellite drag caused by atmospheric density changes. 

-        Satellites sensor damage by charged particles. 

-        Geomagnetically induced currents in power grids and pipelines 

-        Increased radiation threat for passengers on high flying aircraft and spacecraft. 

-        High frequency communications blackouts in polar regions. 

The largest known magnetic storm occurred in 1859 (NASAweb.) It has been named the Carrington Event after Richard Carrington who observed the solar flare that gave rise to the storm. The associated aurora (resulting from charged particles cascading through the atmosphere (Freedman and Kauffman, 2008. p. 222)) was observed as far south as Cuba. The magnetic storm caused damage to communication equipment. It is estimated that if a similar event took place today the repair costs would amount to two trillion dollars in the USA. 

Power grids are also vulnerable to induced currents from magnetic storms. In 1989 a large magnetic storm left 6 million people with no electricity for 9 hours in Quebec, Canada (UCARweba.) The magnetic storm was triggered by a CME. The storm induced currents which caused a transformer to fail. 

Satellites are particularly vulnerable to space weather. In 1994 a magnetic storm affected three communication satellites (FLIGHTweb.) The two Canadian satellites Anik E1 and E2 were affected by induced currents that disrupted their guidance circuitry. Only one of the satellites was recovered. The cost was $228 million for the lost satellite and $3 billion in lost revenue. The same storm also affected the Inelsat K satellite. 

Low altitude satellites may be affected by atmospheric drag. During magnetic storms the atmosphere expands (UCARwebb.). Increased density results in unpredicted drag on satellites in low orbit. This can result in an earlier re-entry than expected reducing the life of satellites as occurred with the Skylab.  

X-ray and ultra-violet radiation from flares is able to increase ionisation of the ionosphere leading to high frequency communication problems (Gopalswamy, 2007.) This can have an impact on trans-polar flights leading to added costs to airlines due to diversion of flights to avoid polar regions during magnetic storms. 

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References 

Babcock, H.W., 1961., Astro-phys. J., 133, 572. 

FLIGHTweb: Flight Global: Space knockout webpage, http://www.flightglobal.com/articles/1996/10/02/9397/space-knockout.html

Freedman, R. A. & Kaufmann, W. J, III. 2007, Universe, 8^th ed.;  New York: W.H. Freeman & Co. 

Gopalswamy, N Coronal mass ejections and space weather Climate and Weather of the Sun-Earth System (CAWSES.) Selected Papers from the 2007 Kyoto Symposium, p.77–120. 

Gopalswamy, N., Akiyama, S, Yashiro,S and Mäkelä, P., 2009 arXiv:0903.1087v1[Astro-phSR] 

HESPweb: Goodard Space Flight Center: What is a Solar Flare? webpage, http://hesperia.gsfc.nasa.gov/sftheory/flare.htm

Hochedez et al, 2005, Annales Geophysicae, 23, 3149 

NASAweb: Science @ NASA: Severe Space Weather webpage, http://science.nasa.gov/headlines/y2009/21jan_severespaceweather.htm

Navarro-Peralta, P and Sanchez-Ibara, A, 1994, Solar Phys., 153, 169 

SPACEweba: Space Weather: The Interplanetary Magnetic Field webpage http://spaceweather.com/glossary/imf.html

SPACEwebb: Space Weather: The Classification of X-ray Solar Flares webpage, http://spaceweather.com/glossary/flareclasses.html

UCARweba: Windows to the Universe: Blackout - Massive Power Grid Failure webpage, http://www.windows.ucar.edu/spaceweather/blackout.html

UCARwebb: Windows to the Universe: Atmospheric Drag webpage, http://www.windows.ucar.edu/spaceweather/sat_drag.html

Vršnak, B, Temmer, M and Veronig, A.M.,  2007, Solar Phys., 240, 315.

Weiss, N.O. and Thompson, M.J., 2009, Space Sci. Rev, 144, 53.