It shields our planet from harmful solar wind

     Solar Wind

                 Space Weather

   Van Allen Radiation Belts

The Magnetopause

               Plasma Sheet  

         Bowshock

  Will a weakenend magnetic field render the magnetosphere less effective?

                      Ring current

The magnetosphere is an oddly shaped invisible shield around Earth, extending tenth of thousands of kilometers into space. It is the region in which the magnetic field interacts with space matter. Because of the force of the incoming high-energy solar wind (ionized particles from the sun also called plasma), it is condensed at the sunlit side and elongated at the other forming a streamlined extension out into the night termed the magnetotail.

Within the magnetosphere are several layers and boundaries that are governed by the specifics of the magnetic field and the chemical composition of matter contained. Some of the solar particles are slowed so much by the magnetosphere that they wind up inside of it via the tail end or the polar cusps above the ionosphere. As the particles spiral along the magnetic field lines, they accumulate inside a layer in the tail section called the plasma sheet. The portion that is guided towards more immediate space surrounding Earth, enters the plasma-sphere. Here it remains trapped inside of the van Allen Belts until it decays. This is why the van Allen Belts, two wing-like half circles around our planet, are also called radiation belts. Because these moving charged particles cross and move along the geomagnetic field lines carrying their own magnetic and electric fields, large curents flow within the magnetosphere. When these currents are near the ionosphere, upper atmospheric gases are ionized and excited. Enhanced electromagnetic interactions with the geomagnetic field in times of strong solar emissions cause the disturbances and fluctuations recorded for the external field. When the strength of the surface field decreases by about 50 nanoTesla or more, we speak of a geomagnetic storm (Russell & Luhmann, 1997). Such storms compromise the readings of navigational equipment, interrupt communication systems and power distributions and adversely affect spacecrafts, satellites and astronauts.

The magnetosphere protects the atmosphere and thus all life on Earth from the corrosive, radioactive solar wind. During a major geomagnetic storm, the atmosphere takes a beating mostly in the polar regions, where radiation in the atmosphere is therefore slightly higher. The effectiveness of the magnetosphere is connected to the properties of the geomagnetic field. But, a temporarily weakened surface field does not diminish the total field strength. The field's intensity recovers within hours after a storm because the magnetic field is continuously recreated (Glatzmaier, 1997).

source:  http://www.hao.ucar.edu/public/education/slides/slide13.html     Courtesy of High Altitude Observatory

The clouds of ionized particles released into space consist mostly of protons, ions and Helium nuclei (Burch, 2001). At times the stream of solar particles is so dense and forceful that people speak of a shockwave. These very energetic particles arrive with supersonic speed (Russell& Luhmann, 1997) at the magnetosphere where they encounter a force that redirects their path. This first impact boundary of the magnetic shield is called bowshock. It averts most of the interplanetary plasma. The next layer inward is the magnetopause drawn in the image below as a purple line. The magnetopause is in a region in which the geomagnetic forces form a balance with the energies and pressures of the solar particles within the magnetosheath (Backus et al., 1996; Stern, 2001). It outlines the entire magnetosphere and guides the solar wind around the outer periphery. These boundaries vary in proximity to Earth as they are shaped by the density and forces of the incoming solar wind.

source: http://www.windows.ucar.edu/tour/link=/earth/images/earth_magneto_gif_image.html&edu=high Courtesy of Windows to the Universe, http://www.windows.ucar.edu

Inside the magnetosphere is trapped plasma, which accumulates in the plasmasphere around the ionosphere and also in the plasma sheet in the tail section. According to Russell and Luhmann (1997), because these particles have their own magnetic and electric field directions, they induce a complex field-aligned and counter-field current system along the magnetopause, around the plasmasphere and around and across the tail. The opposing currents and magnetic fields divide the tail into a northern and southern lobe and render the plasma sheet in the center neutral. Although less energetic than the incoming solar wind, the plasma accelerates rapidly as it responds to energies transmitted across the outer boundaries (Burch, 2001; Russell & Luhmann, 1997). From time to time, the magnetosphere deforms under the force of high-speed solar wind (Burch, 2001), and solar particles pass through the magnetopause. This happens in particular when the interplanetary magnetic field (that of the solar wind) takes on a polarity opposite to that of the Earth (Burch, 2001; Russell & Luhmann, 1997). According to these writers, the magnetosphere then experiences a downward force, flattens in reponse, and becomes pervious. Magnetic fields link up on the sunlit side and the solar wind field assumes the direction of the geomagnetic field. This is the first part of a process called reconnection. Now solar wind and associated energies flow into the ionosphere via the polar cusp (see diagram below). A second reconnection occurs inside the tail where the transfer of magnetic plasma along the now flattened and extended field lines leads to the buildup of magnetic flux. The tail section elongates under such pressures, and the plasma sheet in the center thins. When the magnetic flux is too strong, the sheet snaps, and the normally oppositely directed field lines above and below reconnect also.

                   source:  http://geomag.usgs.gov/intro.html     Courtesy of USGS

A magnetospheric substorm. The diagram shows how a southward directed interplanetary magnetic field links up with the geomagnetic field in a process called reconnection. Caused by energy transfers from the interplanetary field, this process can result in plasma convection and large current flows inside the magnetosphere. The forces released propels the magnetized plasma towards Earth. The thus accelerated particles carry large currents into the magnetospheric current systems. The swelling of the ring current, a counter-aligned current flow nearest to Earth, takes away from the strength of geomagnetic field. During great magnetic storms, these substorms become more violent and can occur several times (Burch, 2001; Russell & Luhmann, 1997).

Magnetic storms are rare according to Burch, and the intensity depends not so much on the solar event, but on the direction, speed, magnetization and duration of the incoming plasma (2001). Substorms on the other hand are more regular; they are the cause of normal auroral activity. But when a large shock wave compromises the magnetosphere, and the interplanetary field has a downward direction, that's bad weather prognosis. Then magnetic storms are often accompanied or followed by one or more very potent substorms which add to the severity of the perturbations (Chapman, 1964; Stern, 2001; Burch, 2001; ESA, 2004). Discharge of stored, magnetized plasma inside the magnetosphere can be very violent if the magnetic flux is high, and this long theoretically predicted phenomenon of substorm physics was affirmed in recent years by combined data and images send home from the European Cluster and the NASA IMAGE spacecrafts cruising deep inside the magnetosphere. The energies measured during one the most forceful storms reached as much as 10 MeV or more (ESA, 2004). In this visible imaging movie, courtesy of the Berkeley Space Physics Research Group, the auroral electrojet became obvious as high energy protons gushed into the northern cusp during the "Bastille" event (Odenwald, 2003). This very large magnetic storm was caused by a coronal mass ejection in July 2000 (Burch, 2001). Notice how the lights sway back and forth and across the entire polar cap. The image was produced from the NASA IMAGER Spacecraft (NASA IMAGE Discoveries, 2001).