Investigation of Terrestrial Bow Shock Using Cluster Spacecraft Data

1. Investigation of Terrestrial Bow Shock Using Cluster Spacecraft Data Dr. Harald Kucharek Dr. Pamela Puhl-Quinn Cayle Castor Megan Schulz 1. The sun is constantly releasing plasma into the space around it. A portion of this plasma is constantly flowing towards the earth. Because plasma consists of positively charged ions and electrons it interacts with the earth’s natural magnetic field, the magnetosphere, when it reaches it. Charged particles strictly move along magnetic field lines and require an immense amount of energy to transfer from one magnetic field line to another. Because of this, the magnetosphere acts as an obstacle in the path of plasma particles coming from the sun. The place where the drastic changes in particle and field parameters occur is called the Bow Shock. Using pre-existing data from the Cluster satellites, ion density, ion velocity and magnetic field strength changes were investigated at key points at the bow shock. These points are at the front of the bow shock (the nose), and the two flanks of the bow shock (the parallel and perpendicular sides). 2. The Cluster spacecrafts are situated in a tetrahedral position and placed about 1000 kilometers apart in order to get a large sampling of the bow shock at any particular crossing so that scientists can get an accurate idea of what’s happening at these points. If one were to look only at a small portion of the ocean, for example, one might see a tsunami or a thunderstorm, and assume that the whole ocean is violent, when there is a large unseen portion of the ocean that acts very differently. Scientists want to make sure they are not just seeing a small portion of the magnetosphere and getting the wrong idea about the big picture. 4. The sun's magnetic field lines do not extend straight outward due to its rotation. As depicted in the picture above, the magnetic field lines are slightly curved. Because charged particles follow magnetic field lines, the paths of the plasma is also curved. This phenomenon leads to the solar wind contacting the magnetosphere at an angle. This angle results in the nose contact points receiving plasma approximately a 45 degree angle. The perpendicular flank receives plasma at approximately a 90 degree angle. The parallel side receives plasma at approximately 180 degree angle. 3. As an object moves through a fluid, or a fluid moves past an object, information about the movement propagates in all directions around the object, including the direction of the object's movement. This speed that information propagates defines when a shock will occur. For atmosphere, the information speed is the speed of sound. If an object moves faster then the speed of sound, it catches up with is own propagating information waves and it piles them up creating a large shock. This is illustrated by the large v shape around the bullet. In general, if an object accelerates or decelerates past the speed of information a shock is formed. In a plasma, like the plasma ejected from the sun, the speed of information is the Alfven wave speed. The plasma is moving faster than the speed of information before it hits the magnetosphere (super Alfvenic speeds) and decelerates past the speed of Alfven waves and creates the bow shock of the earth after it contacts the magnetosphere. 5. At the bow shock, the total pressure on the solar wind side (side 1, or upstream) equals the total pressure on the magnetospheric side (side 2, or downstream). The total pressure on either side is the sum of thermal, dynamic and magnetic pressures. These terms involve the magnetic field (B), the density (n), the temperature (T) and the velocity (V) and the pressure balance has the form: n1kT1 + mn1V12 + B12/20 = n2kT2 + mn2V22 + B22/20 (thermal) (dynamic) (magnetic) (thermal) (dynamic) (magnetic) where m is the particle mass, and k and 0 are constants. An increase in the upstream pressure (assuming no change in the downstream pressure) will cause the bow shock position to move inward. Alternatively, a decrease in the upstream pressure would cause the bow shock to move outward. This is one of the reasons for such a large spread in our bow shock position data. 6. Using the Cluster Quicklook data for each crossing, we examined several compression ratios at the crossings. We plot changes in ion velocity, change in ion density, change in magnetic field strength, the distance of the bow shock position, and a comparison of the bow shock position to a model relying on approximate coordinates. *From the graph showing the distance of the bow shock, we find: 1.) The nose is the closest to the earth and the flanks (perpendicular and parallel sides) are farther away. 2.) The shock profile becomes less identifiable on the parallel side, because of the increased ion reflection. *Comparing density, magentic field strength, and velocity, we find: 1.) After entering the bow shock, the density and magnetic field strength increase, while the velocity decreases. 2.) Compression ratios for all the values stay relatively constant, which is an excellent example of conservation of energy and momentum at work. 7. Conclusions General Properties of the Bow Shock: * It's has a symmetric shape * The nose position is closer to the earth than the flanks * It has 3 distinct crossing regions: Nose, perpendicular, parallel Microstructure of crossings: * The perpendicular crossing is very “clean” and transitions in plasma and field parameters are easy to identify. * The parallel crossing is more difficult to identify due to increased back-scattering. Physics of Pressure Balance and Conservation * Position of bow shock is determined by balancing thermal, dynamic and magnetic pressures on upstream and downstream sides * Compression ratios of B, N and V are related to one another such that mass, energy and momentum are conserved. Acknowledgements:Dr. Pamela Puhl-QuinnDr. Harald Kucharek Dan Seaton Cluster Science Data System(c)2000, European Space Agency Visuals Unlimited.com ESA (Cluster) Science and Technology