2016 THEMIS SCIENCE NUGGETS
Statistical analysis of magnetotail fast flows and related magnetic disturbances
by Dennis Frühauff
Technical University of Braunschweig, Germany
Introduction
The Earth's magnetotail is a highly dynamic region of the magnetosphere. The permanent impingement of the solar wind upon Earth is cause to constant reconfiguration and convection processes of the magnetic field and plasma regions. While the steady convection of the magnetosphere is usually referred to as the Dungey cycle [Dungey, 1961], more sudden and erratic reconfigurations take place during magnetospheric storms and substorms [Ohtani, 2004]. Whether or not these convection cycles are brought to life by calm or more active solar wind conditions, in any case they are accompanied by actual plasma movement, i.e., (fast) flows of the bulk plasma. Fast flows in the magnetotail have been observed for many years. They are considered to be occasions of plasma flows exceeding bulk velocities of 400 km/s and last typically about 1 minute. They usually occur in groups of 10 minute duration which are termed bursty bulk flows (BBFs) [Angelopoulos, 1992].
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| Figure 1. Simplified model of the Earth's magnetosphere. Dynamic solar wind impingement leads to constant reconfiguration of the surrounding plasma. While there will be plasma movement (flows) everywhere in the magnetosphere, the magnetotail serves as a very populated laboratory to investigate fast bulk plasma flows. |
THEMIS, being in space for now almost 10 years, has been able to observe a great number of fast flows during its tail science seasons. In this study, fast flows are detected in a pre-defined orbital box (X≤-8 RE, |Y|≤15 RE, |Z|≤10 RE, RE=6371km) whenever the bulk ion velocity exceeds 200 "km/s" and is directed towards Earth. In the years 2007-2015 almost 16 000 events are detected. Statistical findings of this database are presented below. The orbital placement of the spacecraft also enables us to examine differences in fast flows occurring close to Earth (~-10 RE) and far down the tail (~-60 RE).
Results
Velocity distributions. Figure 2 shows histograms (i.e., number of occurrences) of the fast flows' peak velocities in the x and y directions. The observations made from these plots can be summarized as follows:
- In case of the Vx velocities (direction towards Earth) the two distributions are very similar. Consequently, near Earth and downtail the average velocities to be found are similar as well.
- The Vy distributions (direction approximately perpendicular to the Earth-Sun line and the Earth's magnetic dipole axis) look different. While the downtail distribution is slightly narrower, it is also shifted to negative velocities peaking at around Vy≈-11km/s. Together with a typical Vx velocity from the upper panel, we can estimate that the far tail of the Earth is aberrating from the Sun-Earth line by an angle of approximately α≈tan-1((-11km/s)/(250km/s))≈2.5°. The reason for this phenomenon can be found in the fact that the solar wind is indeed not hitting the Earth perfectly face-on, but with a slight angle due to the Earth's relative sideways motion on its orbit around the sun [Fairfield, 1996].
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| Figure 2. The velocity distributions in the x and y directions for all events found in near-Earth and downtail regions. |
Group structure. It is possible to determine whether the events found occur as single fast flows or in groups of consecutive events (BBFs) by analyzing the time that passes between single events (i.e., happening within 10 minutes). Figure 3 summarizes that for the two regions, the group structure indeed looks different. In short, it is very likely to observe only a single event near-Earth, while on the other hand, groups of several fast flows are more frequently observed downtail. At this point it is difficult to determine the exact reason for this. Two explanations can be proposed: (1) We see fast flows in the regions that are generated by different processes. One that tends to create several fast flows, and one that releases preferably single structures. (2) We basically observe the same fast flows, but at different times. Structure that are separately (but close together) created downtail merge into a single fast flow while they propagate towards Earth.
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| Figure 3. Distribution of the group sizes of fast flows within the database. |
Superposed Epoch. To infer characteristics of fast flow time profiles, Superposed Epoch Analysis [Chree, 1913] can be utilized. It averages over all data points that are associated with the same instant in time. Through this, average time series for a given phenomenon can be produced. Figure 4 summarizes the results for velocity and density profiles of fast flows as observed by THEMIS:
- While the peak velocities for both regions are very similar, the downtail fast flow is embedded in a very large background flow. Since, here, grouped events are included in the computation, this background is even larger than the initial velocity threshold.
- While in the average near-Earth fast flow the density decreases during the event, in the downtail region, the density tends to increase within the fast flow.
- From the velocity profiles we can infer transit times of ~100s (near-Earth) and ~70s (downtail) and scale sizes of ~3 RE for the average magnetotail fast flow.
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| Figure 4. Superposed Epoch time series for fast flows in both regions. Upper panels show velocity profiles, lower panels show particle density variations. The shaded areas are an uncertainty estimate. |
Conclusion
The long-term availability of THEMIS observations in the magnetotail makes it possible to assemble comprehensive statistical databases of various key phenomena of the plasma. Using very simple identification criteria the study of magnetotail fast flows can be revisited using almost 10 years of data.
The database reveals basic velocity distribution functions and group sized of fast flows and BBFs. Using Superposed Epoch Analysis average time profiles for near-Earth and downtail regions are revealed. A closer inspection of the velocity profiles shows that flow bursts can be expected to consist of three different components: (1) The background flow (V0), (2) a flow large scale enhancement (VFE) of typically ~3 RE size, and, (3) the actual flow burst (VFB) with a timescale of ~60s and sizes of 0.5-1.4 RE (see Fig. 5).
When analyzing magnetic field variations of fast flows it can be shown that for the larger part of the database, the strongest variations are aligned with the main flow direction, indicating mirror mode-like generation of waves along the fast flows. For a more comprehensive description of the results, see [Frühauff and Glassmeier, 2016].
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| Figure 5. Superposed Epoch Analysis of single fast flows in the magnetotail. Each fast flow seems to consist of background, flow enhancement, and, flow burst component. |
Reference
Angelopoulos, V., Baumjohann, W., Kennel, C. F., Coronti, F. V., Kivelson, M. G., Pellat, R., Walker, R. J., Luehr, H., and Paschmann, G.: Bursty bulk flows in the inner central plasma sheet, J. Geophys. Res., 97, 4027–4039, 1992.Chree, C.: Some Phenomena of Sunspots and of Terrestrial Magnetism at Kew Observatory, Philos. T. R. Soc. Lond., 212, 75–116, 1913. Dungey, J. W.: Interplanetary Magnetic Field and the Auroral Zones, Phys. Rev. Lett., 6, 47–48, 1961.
Fairfield, D. H., Lepping, R. P., Frank, L. A., Ackerson, K. L., Paterson, W. R., Kokubun, S., Yamamoto, T., Tsuruda, K., and Nakamura, M.: Geotail Observations of an Unusual Magnetotail under Very Northward IMF Conditions, J. Geomagn. Geoelectr., 48, 473–487, 1996.
Frühauff, D. and Glassmeier, K.-H.: Statistical analysis of magnetotail fast flows and related magnetic disturbances, Ann. Geophys., 34, 399-409, doi:10.5194/angeo-34-399-2016, 2016.
Ohtani, S.-I., Shay, M. A., and Mukai, T.: Temporal structure of the fast convective flow in the plasma sheet: Comparison between observations and two-fluid simulations, J. Geophys. Res.-Space, 109, A03210, doi:10.1029/2003JA010002, 2004.
Biographical Note
Dennis Frühauff is a PhD student at the Institute of Geophysical and Extra-Terrestrial Physics at Technical University of Braunschweig, Germany, under supervision of Karl-Heinz Glassmeier. While being responsible for the big part of the offset calibrations of THEMIS' fluxgate magnetometers FGM, his scientific research focuses on data mining of the THEMIS magnetic field database. Aided by MHD simulations of simple plasma sheet models, the studies analyze fast flows and dipolarization flux bundles in the magnetotail.
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Emmanuel Masongsong / emasongsong @ igpp.ucla.edu