My name is Jesse F. Leaman.  I am a junior at the East Stroudsburg University of Pennsylvania (ESU), majoring in Physics.  Next to me is my assistant Christina Lugo, a nursing student at ESU.  During the summer of '99 I thoroughly enjoyed a ten week internship in the Space Physics Data Facility (SPDF) of the Goddard Space Flight Center (GSFC),  sponsored by the ACCESS Entry Point program of the American Association for the Advancement of Science (AAAS).
Throughout my stay I worked for Dr. Mona Kessel, a Space Physicist.  My primary task was to analyze real satellite data from the Geotail spacecraft, which orbits Earth in an elliptical orbit of about 30 by eight Earth radii.

Space physics is the branch of physics that deals primarily with Sun-Earth interactions.  It is a relatively new field with almost all knowledge dating from the 1960s and the start of the Space Age.  Of particular interest to my mentor is the interaction between the solar wind and Earth's magnetic field.  Our planet is surrounded and permeated by a magnetic field,  generated because Earth has a molten iron core and is continually rotating about its axis.  The magnetic field has a north and south pole, similar to a bar magnet, but the field lines are compressed on the day side and drawn out into a long tail on the night side, due to the pressure created by the Sun's solar wind (See Figure).  The solar wind is a plasma, or ionized gas, coming from the Sun.  It is very diffuse, which means there are rarely any collisions between particles on their way towards Earth.
The outer boundary of Earth's magnetic field is called the magnitopause.  Ahead of this on the sunward side is the bow shock, created because Earth's magnetosphere is moving through the solar wind.  This is analogous to a ship moving through water and creating a standing bow wave upstream.  In fact, there are so many similarities between dynamic magnetic fields and hydro dynamics, that magneto-hydro-dynamics (MHD) was developed.
This magnetic bow shock is the first thing the solar wind encounters on its way towards Earth.  Here the solar wind particles, mostly protons and electrons, are slowed, heated, and redirected around Earth's magnetosphere.  Very few particles actually cross the magnitopause, which is very fortunate for us since these energetic particles would destroy our atmosphere.

During its five day orbit around Earth Geotail will encounter the bow shock, but instead of just crossing once as would be expected, Geotail often crosses the bow shock several times in a matter of minutes.  Of particular interest are times when Geotail crosses the bow shock during high speed solar wind streams.  At those times Geotail records more than 15 crossings, as high as 30, during a few hours.  The data from Geotail that was of most relevance for this study was the magnetic field and the solar wind density and velocity.

My job was to look at these individual bow shock crossings Geotail observed, and determine the shock normal direction, that is the direction normal to a tangent at a location on the bow shock surface.  NASA has its own X-Files, which are actually a set of Unix-based computer platforms where data and computer programs can be shared by all NASA scientists.  The particular computer program I used was written by Dr. Kessel and produced a least squares fit of the magnetic field and plasma data, upstream and downstream of the shock, to conservation equations defined by MHD theory.

1995 was a very interesting year in the study of the bow shock.  There were a large number of high speed solar wind streams, because it was the declining phase of the Sun’s eleven year solar cycle (Figure 3).  The vertical lines indicate times when Geotail crossed the bow shock and arrows indicate crossings I analyzed.  There is a clear correlation between high speed streams and an increased number of crossings, but before any conclusions can be drawn there has to be a detailed analysis of as many of those individual crossings as possible.

The first step in finding a bow shock normal is to plot ten minutes worth of data centered about the crossing.  The times of crossings had previously been determined and were available in a bow shock database.  The data plotted consisted of the three components of the magnetic field, the three vectors components of solar wind velocity, the density of the solar wind, and the absolute value of the magnetic field.  The vectors were in the Geocentric-Solar-Ecliptic (GSE) coordinates and the absolute magnetic field value was the primary indicator of a crossing.  Upstream the magnetic field magnitude is low and steady, but once past the bow shock the magnetic field magnitude becomes larger and more turbulent (Figure 4).
 

The next step is to select two intervals, of approximately equal lengths, upstream and downstream of the crossing.  Then run that data through a computer program and plot a contour-surface graph of the results.  From the surface graph one can determine the lowest, most stable, physically possible result. Figure 5 is a good example of a surface graph where all three of these criteria were put to the test.  The first priority is to find a stable area, three of which are highlighted in the figure.  The lowest area is not physically possible, because values for theta and phi result in a negative pressure change.  The highest area does not work either, because values for theta and phi indicate that the solar wind actually increased its velocity after crossing the bow shock, which is not possible.  Therefore the best possible result, for this particular example, is achieved where the theta and phi correspond to the minimum of the middle area.  Then to find out what the actual bow shock normal is you run that theta-phi combination through a similar computer program.   If the results are satisfactory the whole procedure has to be repeated for the same crossing, using different upstream and downstream intervals.  Finally, if the results of the second analysis were close enough to the first, the shock normals could be entered into the bow shock database.

So far there are a few things we understand about the turbulent behavior of the bow shock.  As mentioned above there seems to be a direct correlation between high speed solar wind streams and an increased number of crossings.  We have also learned that the shock normals are less turbulent on the dusk side than on the dawn side (see Figure).
This can be explained in great detail, but in short it is due to the “frozen in” magnetic field of the solar wind.  On the dawn side this magnetic field is almost parallel to the bow shock normal, while on the dusk side it is almost perpendicular.

There are still many questions about the bow shock that had to be left unanswered.  The next step is to write a computer program that will take those shock normals and the location of Geotail at the exact times of those particular crossings, and try to shed more light onto the true nature of the bow shock.  Furthermore, there is another interesting correlation that will have to be studied in the future.  During high speed solar wind streams there is also an increase in the number of energetic electrons detected in Earth’s lower magnetosphere (see Figure), which could be a result of in situ acceleration by waves produced as high speed solar wind streams encounter the bow shock.

Since this was my first exposure to real physics research it was quite an eye opener that our universe, in this case Earth’s magnetic bow shock, is very dynamic and can not possibly be completely understood in ten weeks.  I do want to pursue a career in research though, so I really appreciated that experience and now know what it takes to split up a big problem into smaller, more manageable ones.

Finally, I would like to thank NASA for giving me the opportunity to work with some of the brightest people in the world for the second summer in a row.  This summer I want to thank Dr. Mona Kessel for being a great mentor, the Space Physics Data Facility for providing a comfortable working environment, and the Equal Opportunity Office for making sure my special needs were taken care of.

Dr. Robert McGuire
McGuire@nssdca.gsfc.nasa.gov, (301) 286-7794
Space Physics Data Facility, Head
NASA Goddard Space Flight Center
Greenbelt, MD 20771, USA

Tami Kovalick,
kovalick@bolero.gsfc.nasa.gov, (301) 286-9422
Raytheon ITSS
Space Physics Data Facility
NASA Goddard Space Flight Center
Greenbelt, MD 20771, USA

Last Modified:Thursday, 5-August-1999 16:06:03 EDT