Topic:
Pi2 pulsations
Main objectives:
What are the energetics of Pi2 pulsations in the near-Earth plasma sheet?
How are Pi2 pulsations generated?
What determines the characteristic Pi2 frequency?
Sub-objectives:
Where exactly is the mode conversion region of compressional and Alfven waves?
What is the 3-D picture of the coupling process?
General problem:
An important substorm phenomenon and a widely used substorm indicator (Saito, 1969) are ground Pi2 pulsations. In spite of the progress in Pi2 research made over decades, new results have put into question existing Pi2 generation scenarios and opened up new explanations. There are observational differences between Pi2s occurring at higher latitudes and those occurring at midlatitudes and low-latitudes; typically the frequency is lower and the amplitude larger for high-latitude Pi2s. It is thus thought that different mechanisms generate Pi2s including the cavity model (Sutcliffe and Yumoto, 1989), transient response model (Baumjohann and Glassmeier, 1984), local kinetic/MHD instability (Cheng, 2004), BBF model (Kepko and Kivelson, 1999), and a reconnection-driven model (Keiling et al., 2006). We note that none of these models has conclusively been confirmed. The last two models assume that the characteristic Pi2 frequency is controlled in the magnetotail whereas the others either assume as the controlling region the plasmasphere or the inner region of the plasma sheet. In spite of these differences, all models have in common that the near-Earth plasma sheet (8-15 RE) plays a key role in the mode conversion of Pi2 energy as it propagates from the magnetotail to the ground (Figure X). Hence, an understanding of the energetics of this region is crucial in obtaining an improved understanding of Pi2 pulsations and how they couple to the ionosphere.
Opportunities:
A successful investigation of the mode conversion region for Pi2 pulsations requires several conditions to be in place:
1. Instrumentation and Spacecraft Constellation
An understanding of the energetics (i.e., the energy transfer processes) in the mode conversion region requires measurements of the 3-D Poynting vector which has not been measured in this region before. Furthermore, a dynamic 3-D picture of the energy transfer processes can only be achieved with several spacecraft being simultaneously in this region in order to follow flow direction and development of the Poynting flux. Full 3-D vectors of the electric and magnetic field are also crucial in identifying the wave and propagation mode of the space Pi2. The three (or four?) THEMIS spacecraft are equipped with 3-D electric and magnetic field instruments, and thus capable of making the required measurements. We note that the mode conversion region will already be studied during the first two years of operation where we will also be able to identify the role of BBFs and reconnection for the Pi2 generation using probe P2 (~20 RE apogee) together with the three inner probes (P3, 4, 5). However, whereas during the first two years (T1 and T2), we will investigate the energy flow and its timing in the x-y plane of the mode conversion region, during T3 and T4 it will be the x-z plane. (The ideal constellation would be a 4-spacecraft tetrahedral which allows simultaneous measurements of both planes).
2. Ground coverage
The study of Pi2 pulsations requires the simultaneous observation of in-situ Pi2 and ground Pi2. Many ULF waves exist in the plasma sheet but they can only be accurately classified into Pi2 pulsations by simultaneous ground confirmation. THEMIS has both extensive ground coverage (magnetically and optically) and multi-spacecraft configuration to investigate the coupling between the mode conversion region and the ground. Hence, this important ground coverage requirement which has to be met in addition to the instrumentation and spacecraft constellation requirements is also met by THEMIS.
Figure X: Sketch of possible channels of signal generation and penetration from the magnetotail into the nightside polar ionosphere. The letters A, M, and BBF denote Alfvén type, magnetosonic type, and bursty bulk flow type disturbances, respectively (adapted from Engebretson et al., 2006).
References
Baumjohann, W. and Glassmeier, K.-H.: The transient response mechanism and Pi2 pulsations at substorm onset: Review and outlook, Planet. Space Sci., 32, 1361–1370, 1984.
Cheng, C.Z., Physics of substorm growth phase, onset, and dipolarization, Space Sci. Rev., 113, 207-270, 2004.
Engebretson, M.J., et al., ULF waves at very high latitudes, Geophysical Monograph 169, p.137-156, 2006.
Keiling, A., et al., Association of Pi2 pulsations and pulsed reconnection: ground and
Cluster observations in the tail lobe at 16 RE, Ann. Geophys., 24, 3433–3449, 2006.
Kepko, L. and Kivelson, M. G, Generation of Pi2 pulsations by bursty bulk flows, J. Geophys. Res., 104, 25 021–25 034, 1999.
Saito, T., Geomagnetic pulsations, Space Sci. Rev., 10, 319, 1969.
Sutcliffe, P.R., and K. Yumoto, On the cavity mode nature of low-latitudes, Geophys. Res. Lett., 96, 1543, 1991.