Introduction
The solar wind output from the Sun can sometimes 'reconnect' with the Earth's magnetic field, allowing plasma and energy to enter our magnetosphere. The magnetosphere responds to this input in different ways. One way is a substorm, which builds up energy and then quickly releases it, similar to a short evening thunderstorm. These events last around 2-3 hours. Another way is called steady magnetospheric convection, or SMC. It is analogous to a constant, steady rain throughout the day. These intervals can be many hours long, and are thought to result from balanced reconnection between the day and night sides of the magnetosphere.
SMC events typically occur when the solar wind is slow (V ~350 km/s) and steady. There happen to be repeating structures in the solar wind, called stream interfaces, in which slow (V ~300-400 km/s) solar wind occurs before the stream interface hits the Earth, and fast (500-700 km/s) solar wind occurs after. Therefore, we might expect a large number of SMC events in the days before the stream interface, during the slow solar wind. In fact, it turns out the opposite is true!
Results
In Figure 1, we compare the number of SMC events that occur relative to a stream interface hitting the Earth. The x-axis plots the number of days before (left) and after (right) of a stream interface (0 days). The y-axis plots the probability that an SMC occurs at that relative time. Just as the sunspot cycle controls activity on the Sun, it also affects activity at the Earth. The two colored lines are broken up by solar cycle phase: blue during the rising phase, or when solar activity is increasing, and red during the declining phase, when solar activity is decreasing.
First, we can see that there is a correlation between SMC and stream interfaces. That is, the probability goes up, peaking at around 0.08 for the red line. Although we expected this increase to occur before the stream interface, in the region of slow solar wind, it actually occurs just after the stream interface, when the solar wind velocity dramatically increases! This peak happens to occur half a day to one day after a stream interface hits the Earth. Furthermore, there's a clear difference in behavior between the two solar cycle phases. The declining phase (red) has a very strong peak, while the rising phase (blue) is very small.
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| Figure 1. SMC intervals are compared with stream interfaces. On the x-axis, 0 marks a stream interface (thick vertical dashed black line). The probability of SMCs occurring in the days before and after an interface is plotted in 12-hour intervals. The rising phase of the solar cycle, years 1997-2003, is plotted in the blue line and the declining phase, 2004-2008, is the red line. There is a clear peak in SMC occurrence just after the stream interface, from 0.5-1 days after, but only in the declining phase.
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Click each image to enlarge.
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The precise reason for the SMC-SI correlation in the declining phase is still unknown, and the correlation remains perplexing. However, several factors were found to contribute to the difference. The first is how 'effective' the Earth's dipole and the solar wind's magnetic field are to allow reconnection. The solar wind's orientation is fluctuating all the time, and the Earth's orientation depends on the tilt of the Earth. SMCs occur more during the 'effective' orientations in the declining phase, which means the solar wind inputs more energy into the magnetosphere. Second, the number of substorms increases after a stream interface in the declining phase, and it has been shown that substorms are correlated with the beginning of an SMC. Finally, the efficiency of reconnection (coupling) increases in the declining phase.
We identify the SMCs that occur after stream interfaces (between 0-2 days after) as "Associated SMCs." They occur during very different solar wind conditions than regular SMCs. Figure 2 is a representation of the different solar wind conditions: velocity (top left), magnetic field (top right), electric field (bottom left) and how steady the electric field is (bottom right). The lines correspond to different activity: green is all SMCs, red is Associated SMCs only, blue is regular SMCs only, and black is for all other data. If one line is to the right of another, it matches the word written to the right. For example, in the Velocity plot, the red line (Associated SMCs) is to the right of the blue line (regular SMCs), and thus they occur during faster solar wind velocity. So, Associated SMCs occur during faster velocity, weaker magnetic field, and less steady electric field than regular SMCs.
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| Figure 2. Solar wind conditions during SMCs: velocity, magnetic field (Bz), electric field (Ey) and steadiness of electric field (Ey). The green line shows conditions during all SMCs. The red line is conditions only during Associated SMCs, and the blue line is only during regular SMCs. The black thin line is for all other times.
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Click each image to enlarge.
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The last parameter, electric field (Ey), is very important. It represents the 'driver' of activity- it is the solar wind Ey parameter that results in reconnection with the Earth's field. We can see that there is no difference between the green, blue, or red lines. This means that all SMCs, no matter what solar wind velocity or magnetic field, are caused by the same 'driver,' or same levels of Ey. Therefore, it is the electric field, Ey, parameter that is important for causing SMCs.
Conclusions
We have shown that SMCs do occur predictably with respect to stream interfaces, but they occur afterwards, not before as expected. Also, this correlation is only significant during the declining phase of solar cycle 23. The difference between phases is due to the geo-effectiveness of the stream, increased solar wind-magnetospheric coupling in the declining phase, and larger number of substorms in the declining phase after interfaces.
We also identified for the first time the sub-population of associated SMCs. They occur during much higher solar wind velocity than previously reported, due to their occurrence in the high-speed stream after the interface. However, the 'driver' of activity (Ey) is the same for both types of SMCs.
Source
Kissinger, J., R. L. McPherron, T.-S. Hsu, V. Angelopoulos (2011), Steady magnetospheric convection and stream interfaces: Relationship over a solar cycle, J. Geophys. Res., 116, A00I19, doi:10.1029/2010JA015763.
Biographical Note
Jenni Kissinger is a graduate student in the Department of Earth and Space Sciences at UCLA.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
