2017 ARTEMIS SCIENCE NUGGETS
Growth Rate Measurement of ULF Waves in the Ion Foreshock
by Seth Dorfman
UCLA Dept. of Physics & Astronomy
Dept. Earth, Planetary, and Space Sciences
Introduction
The Earth's magnetic field makes life on our planet possible by protecting us from ions and electrons ("plasma") streaming outward from the sun. The first contact of this "solar wind" of streaming plasma particles with the Earth's magnetic field creates a region known as the "bow shock," much like the bow wave of a boat moving through water. At the bow shock, most of the plasma is deflected, but a small number of energetic plasma ions may be reflected back towards the sun. This reflected, high-energy ion beam can in turn generate Ultra Low Frequency plasma waves ("ULF waves") upstream of the bow shock. These ULF waves will be carried downstream by the solar wind towards the Earth where they can influence space weather disturbances all the way to the ground.
The two ARTEMIS spacecraft orbiting the moon are uniquely positioned to measure the growth of these ULF waves. Because ARTEMIS is twice as far from Earth as prior spacecraft measurements, interference between ULF waves from multiple ion beam sources, which may occur close to the bow shock, is minimized. Furthermore, the spacecraft separation is comparable to the expected wavelength of the ULF waves, making growth rate measurements possible.
|
| Figure 1. (left) shows the position of the two ARTEMIS spacecraft during the selected event. Fig. 1 (right) shows how the waves evolve in time after generation by the high-energy ion beam. |
Observations/Results
Consider Wave A in Fig. 1 which is generated by the ion beam at a time t0 at the location of the upstream ARTEMIS spacecraft and begins to grow. By time t1, Wave A has been carried downstream by the solar wind to the location of the downstream ARTEMIS spacecraft. If the spacecraft are well aligned along the flow direction, we can use the measurements of Wave A on each spacecraft to determine the rate at which the wave grows.
Waves are measured on both spacecraft – the horizontal axes in Fig. 2(a) are shifted to line up the waves between the spacecraft. Now, by measuring the ratio of the amplitudes during the initial growth period (shaded gray), we can measure the growth rate of the waves. We compare this to the expected value of the growth rate for the concurrently measured ion beam shown in Fig. 2 (b) and find an excellent match.
|
| Figure 2. (a) shows wave data from the two ARTEMIS spacecraft (us=upstream, ds=downstream). The gray shaded period is the time used to measure the growth of the waves. Fig. 2 (b) shows an example of an ion beam measured during this period. |
Conclusion
The growth rate of ULF waves upstream from the Earth's bow shock is measured for the first time and found to match expected values computed from concurrently observed ion beams. This result represents an important building block towards a full understanding of space weather disturbances that may damage sensitive infrastructure.
References
Dorfman, H. Hietala, P. Astfalk, and V. Angelopoulos, "Growth Rate Measurement of ULF Waves in the Ion Foreshock," Geophys. Res. Let. 44 (2017).Biographical Note
Dr. Seth Dorfman maintains a broad interest in fundamental plasma physics, including the use of laboratory experiments and satellite observations to explore key physical processes in our Sun-Earth system.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu