Introduction:

1. Identification and Analysis of Magnetic SubstormsPatricia Gavin1, Sandra Brogl1, Ramon Lopez2, Hamid Rassoul11. Florida Institute of Technology, Physics and Space Science Department, 2. University of Texas at Arlington, Department of Physics Abstract: Using AE data from [1], we have identified 218 isolated substorms whose initial onset is over North America (between 0300UT and 0800UT). We constructed a data table that contains each substorms’ onset time, strength in AE, duration, and whether or not the event was a multiple- or single-onset substorm. We have examined the statistics of these events, in particular comparing single-onset and multiple-onset substorms. Preliminary results indicate that the strengths of both single- and multiple-onset substorms are very similar. The investigations done here determined the weak relationships between aspects of the substorms, such as substorm’s maximum strength and duration. We have collected data about the substorms from satellites orbiting Earth and hope to put this data into a computer model to help further understand these events. Introduction: Substorms are an important tool in understanding the interactions between Earth’s magnetic field and solar wind. A substorm is a magnetic disturbance lasting only a few hours in Earth’s magnetotail. The life of a substorm consists of three phases: the growth phase, the expansion phase and the recovery phase. During the growth phase, when the Bz (vertical) component of the interplanetary magnetic field (IMF), carried by the plasma of the incoming solar wind, points southward, it connects with Earth’s northward pointing magnetic field lines on Earth’s day side, creating open field lines. An open field line in this case would indicate a magnetic field line whose one end is connected to the IMF and other end is connected to the Earth’s magnetic field. On Earth’s night side, however, the IMF lines reconnect, creating closed field lines (a loop of magnetic field lines) that elongate radially outward from the sun (Figure 1) storing energy in the magnetotail. Both ends of these lines are connected to Earth’s poles where they deposit charged plasma particles into Earth’s ionosphere [2]. Tools and Methods: The intensity of a substorm is measured in the Auroral Electrojet (AE) index. AE is simply the difference between the upper auroral (AU) index and the lower auroral (AL) index. It is a normalized value derived from the H-component from several observatories along the auroral zone. It measures the amount of energy being inputted into the ionosphere from Earth’s magnetotail [4],[5]. Substorms are divided into two categories: single-onset and multiple-onset. Figures 3 and 4 show examples of single- and multiple-onset substorms, respectively. They are identified by a sharp increase in AE and a gradual recovery afterward. Results(cont’d) Duration Distribution: Figure 6 shows a comparison of the duration of the two types of substorms. The average duration of the single-onset substorms was 1.36 hours and the average of the multiple-onset substorms was 1.90 hours. Thus, on average, multiple-onset substorms lasted longer than single-onset substorms by about 0.5 hours. Figure 6: A comparison of the duration of both types of substorms. The average duration of a multiple-onset substorm was about 0.5 hours longer than that of a single-onset substorm. Figure 3: February 7th, 1998 Onset: 0513 UT Strength: 179 nT Duration: 0.48 hrs The arrow indicates the onset of the substorm [1]. Multiple-onset Substorms: It was determined that for most of the multiple-onset substorms investigated, the first onset was the strongest. Most multiple-onset substorms reached their peak AE between 200 and 599 nT. There seemed to be a weak correlation between the number of onsets in a multiple-onset substorm and its maximum AE. There was also a weak relationship between the multiple-onset substorms’ maximum AE and their duration. Figure 4: February 11th, 1999 1st Onset: 0537 UT 1st Onset Strength: 496 nT Max AE: 622 nT Duration: 2.65 hrs The arrows indicate the first and second onsets (five total) [1]. Current and Future Work: The final goal of this project is to collect data about substorms and plug it into a computer model. This will help scientists further understand the physics behind substorms and possibly be able to predict these phenomena. Currently we are collecting data from satellites orbiting the Earth that take data on things like the strength and direction of the magnetic field and the solar wind velocity. We are looking for pairs of satellites, one in the magnetotail and one in the plasmasheet (See Figure 1). Figure 7 shows an example of such an orbital orientation from April 30th to May 4th, 2000. In this case, we will take data from Wind (black) and Imp-8 (blue) on day 124. Figure 1: A schematic of the Earth’s magnetic field Results and Conclusions: Strength Distribution: Figure 5 shows a comparison of the strength of the single-onset substorms that were investigated and the strength of the first onset of the multiple-onset substorms that were investigated. Both plots peak in the 300-399 nT range indicating a similarity of the two. During the expansion phase, the onset of the substorm current wedge occurs. When the reconnection of the IMF lines occurs rapidly, the closed field line region snaps back into place, releasing a plasmoid (loop of closed field lines) down the magnetotail (Figure 2). The resulting release of high-energy plasma particles into the ionosphere is called a substorm. These incoming energetic particles increase the intensity of and expand the auroral arc on Earth’s night side. During the recovery phase, the substorm current wedge stops and the magnetic field and auroral bulge gradually return to their original state [3]. Figure 7: An example of the orientation of satellites from which we will take data to plug into a computer model of substorms. Earth lies at (0,0) in both plots. The z-axis goes through the Earth’s poles and Earth’s equator lies in the x-y plane. References: [1] http://swdcwww.kugi.kyoto-u.ac.jp/ae_realtime/index.html, December 2007. [2] Baker, D. N. (1996). Magnetic Reconnection During Magnetospheric Substorms, NASA Astrophysics Data System, 365 – 372. [3] Lopez, R. E. (1990). Magnetospheric Substorms, Johns Hopkins APL Technical Digest 11, 264 – 271. [4] Kisabeth, J. L., and Rostoker, G. (1974). The Expansive Phase of Magnetospheric Substorms – Development of the Auroral Electrojects and Auroral Arc Configuration During a Substorm, JGR 79, 972 – 984. [5] Lopez, R. E., and von Rosenvinge, T. (1993). A Statistical Relationship Between the Geosynchronous Magnetic Field and Substorm Electrojet Magnitude, JGR 98, 3851 – 3857. Figure 5: A comparison of the strength distribution of single-onset substorms and multiple-onset substorms. Both plots peak in the 300-399 nT range indicating a similarity between the two types of substoms. Figure 2: The release of a plasmoid down the Earth’s magnetotail. (1) shows where the energy is stored. (2) shows the release of the plasmoid. (3) shows the magnetotail returning to normal.