Effect of Geomagetic Activity on Cosmic Ray Muon Rate Mendon High School Regents Physics Honors Period 9 Class A group of Honors physics students plotted the frequency of muons detected at the University of Rochester in the months of October and December 2006 versus the time of day and the KP Index. The purpose of this lab was to determine whether there is a correlation between Kp index and the rate of muon detection. To accomplish this goal muon detection data was taken from the University of Rochester muon detection laboratory as well as the Kp indexes at the relevant times. After collecting data and analysis was performed with the creation of several graphs and discussion. The Kp Index is numerical value that ranges on a scale form 1 to 9 that indicates the severity of geomagnetic activity. Muons are charged particles and because of this it is known that magnetism will have some type of effect on them. Cosmic rays may reach the upper layers of our atmosphere where they interact with atmospheric nuclei. Those interactions, like most high energy particle interactions, produce pions and many of those pions decay to muons which then penetrate the atmosphere. K Index is a measure of the thunderstorm potential. The K- index scale has a range from 0 to 9 and is directly related to the maximum amount of fluctuation (relative to a quiet day) in the geomagnetic field over a three-hour interval. The K-index is quasi-logarithmic local index of the 3-hourly range in magnetic activity relative to an assumed quiet- day curve for a single geomagnetic observatory site. First introduced by J. Bartels in 1938, it consists of a single-digit 0 thru 9 for each 3-hour interval of the universal time day (UT). The disturbance of the geomagnetic field is measured by an instrument called a magnetometer. The Kp index is derived through by an algorithm that essentially averages the K-indices from several stations. The planetary 3-hour-range index Kp is the mean standardized K-index from 13 geomagnetic observatories between 44 degrees and 60 degrees northern or southern geomagnetic latitude. The scale is O to 9 expressed in thirds of a unit, e.g. 5- is 4 2/3, 5 is 5 and 5+ is 5 1/3. This planetary index is designed to measure solar particle radiation by its magnetic effects. Fluctuations in the solar wind plasma and magnetic field are the natural consequences of unstable particle distributions. One example is the streaming electrons that escape from the corona at the time of a solar flare. The axis of the magnetic field is tipped with respect to the rotation axis of the Earth. Thus, true north (defined by the direction to the north rotational pole) does not coincide with magnetic north (defined by the direction to the north magnetic pole) and compass directions must be corrected by fixed amounts at given points on the surface of the Earth to yield true directions.K Index The muon detector possesses two scintillator panels. A scintillator is a material which “scintillates” or gives off small flashes of light in the form of photons when charged particles pass through it. The panels of the detector are made of plastic with a small fraction of their composition being an organic scintillator material. When a charged particle such as a muon enters the detector, photons bounce around the panel and a fraction of them emerge and only enter the light guide. The scintillator panels are oriented horizontally. They are shielded with aluminum foil, black plastic sheets and black tape. The muons penetrating these panels will scintillate the chemicals within the panel, and the resulting photons of light will pass to a light guide. From the light guide the light passes to a photomultiplier tube which amplifies the signal. The base of the tube is connected to a circuit board which registers one count per signal from one pair of scintillator panels. The digital readout shows the number of muons counted. The muon data was initially corrected for pressure using the nifty formula pcor =rate + (pres-1013)*(slope) (since from the previous lab we became aware of the anticorrelation of muon rate and pressure). Next we averaged the rate of muons at each K Index and plotted those average rate of muons versus the KP Index. Lastly, we assigned error bars using the formula sigma = (rate/time)^1/2. The rate muons in Hertz for the days in October and December were averaged for the time of day and plotted for a twenty four hour cycle. This was accomplished using the ingenious Excel program. Since the root cause of rapid changes in cosmic ray intensity is solar activity which disrupts the solar wind, rapid changes in muon intensity usually occur some time, of the order of days, after a violent solar event. At that time, solar plasma has propagated to the vicinity of the Earth. The site of the solar event is often observable on the visible side of the Sun and can usually be found in sunspot or visual solar flare data, often occurring close to the foot of the solar magnetic field spiral which joins the Earth to the solar disk. That event will also cause solar radio activity. A further, direct, consequence of such events and their effect on the solar wind, is changes to the state of our magnetosphere with consequent effects on the magnetic field of the Earth. Those effects can be seen in the various heliospheric and terrestrial magnetic indices, K index and Kp. The fluctuation of the rate of muons illustrates that our Sun can exhibit extremely violent and rapid variability. Using the Forbush decrease in January 1999 as an example of how the muon rate fluctuates with the K index, there should be a rapid intensity decrease when a storm occurs (a high K index) followed by a characteristic extended recovery. Our October 2006 graph of the average muon rate versus KP Index showed no correlation although in past analysis there has been an anticorrelation, yet the December 2006 graph showed a strong anticorrelation. The Earth’s magnetic field can bend the path of cosmic particles and have a facilitating effect.