Solar Physics

While it may appear that our Sun is a relatively quiet star, in fact the surface of the Sun can be violent. As we enter a period of increased solar activity (known as solar maximum), the violence on the Sun's surface is increasing. In some cases, huge quantities of plasma are ejected through the corona at speeds ranging from ~10 to ~2000 km/s. These events are known as coronal mass ejections, or CMEs. Due to driven shocks and reconfigurations of the solar magnetic field structure, these events are often associated with energetic particle acceleration. When the charged particles reach the earth they interact high in our ionosphere and can wreak havoc on our power grid, communications, and satellites. While the exact mechanism is not completely understood, the particles that accompany a CME can be very energetic. To date the highest energy particles observed from the Sun have had energies of several GeV (billion electron-Volts). Follow this link for a slideshow of a CME by P. Charbonneau and O.R. White of the High Altitude Observatory (NCAR) in Boulder Colorado. This series of pictures below of a CME is taken from their slide show.

Operating in a different mode than that normally used to detect air showers (Milagro can and does operate in both modes simultaneously) Milagro can have significant sensitivity to protons and neutrons with energies as low as 4 GeV. We call this mode of operation "scaler mode" Here we simply count how many times all of the PMTs in the pond are struck by light each second. Thus, even if only a single particle from an air shower reaches the pond it can be detected. This is how we gain sensitivity at such low energies. The drawback of this technique is that we can not reconstruct the direction from which the particle arrived. So our "background" (the constant data rate over which signal must be observed) rate is quite high, being integrated over the entire overhead sky. Nonetheless, Milagro is the world's most sensitive detector of multi-GeV particles from the Sun.

On November 6, 1997 the Climax neutron monitor in Colorado detected the passage of energetic particles from a coronal mass ejection. Below is a graph showing data from the Climax detector in green, data recorded in "scaler" mode by Milagrito in blue, and "triggered" data from Milagrito in red. The unit on the y-axis is the rate (the number of counts per second) measured by each of the instruments. (The data from each experiment has been rescaled to fit on the same plot.) The coincident increase in the scaler rate and the Climax data is a good indication that Milagrito detected multi-GeV particles from the CME. While the data from the triggered mode shows an interesting increase at roughly the same time, the complete analysis of this data has yet to be preformed. The observed long-term modulations in both the scaler data and the triggered data are due to changes in the atmospheric pressure and the temperature profile of the atmosphere.