This study presents an investigation on the occurrence of fast flows in the
magnetotail using the complete available data set of the THEMIS spacecraft
for the years 2007 to 2015. The fast flow events (times of enhanced ion
velocity) are detected through the use of a velocity criterion, therefore
making the resulting database as large as almost 16 000 events. First, basic
statistical findings concerning velocity distributions, occurrence rates,
group structures are presented. Second, Superposed Epoch Analysis is utilized
to account for average profiles of selected plasma quantities. The data
reveal representative time series in near and far tail of the Earth with
typical timescales of the order of 1–2

Fast flow events or flow bursts as part of bursty bulk
flows (BBFs)

Since numerous statistical studies have already been performed, the aim of this work is not to reinvent the wheel but rather to provide a more comprehensive database of events using THEMIS data. Partial differences in previous findings further motivate revisiting available databases. Due to the orbital configuration, different science phases, and the uplift of two of the five spacecraft to a lunar orbit, various events are available in a total of 9 years of operation. Therefore, a comprehensive list of events can be compiled to be input to statistical methods. The goal is to present the database and the first findings on statistical properties. Detailed interpretation and further analysis will be postponed to future studies. Still, the results will be very useful for both theoretical considerations as well as computer simulations.

The paper is organized as follows: Sect. 2 presents the database and event selection method. Section 3 treats main statistical findings concerning velocity distributions, activity relations, group structure, while Sect. 4 treats results of superposed epoch analysis on selected plasma quantities. Section 5 is concerned with properties of magnetic field variations during the fast flows, and Sect. 6 will summarize the results.

The Time History of Events and Macroscale Interactions During Substorms
(THEMIS) mission consists of five identical spacecraft launched in 2007

To identify fast flow events in the Earth's magnetotail region, first, a tail
box is defined for a total data interval spanning the years 2007–2014 using

In a second step, the available ion bulk flow velocity is retrieved from the
THEMIS database in Geocentric Solar Magnetic coordinates (GSM). Following
earlier studies (see, e.g.,

Original example GSM ion velocity time series and smoothed and
averaged reference time series for event detection. Detected events are
marked with vertical dashed lines. The velocity threshold is at
200 km s

For the detection of fast flows a simple criterion on the GSM–

An example time series is shown in Fig.

Figure

Orbital distribution of the events found in the data set. Due to the bimodal nature, the data are grouped into near-earth (yellow) and downtail (blue) regions. The color code will be kept throughout this paper.

The peak velocity distribution for all three components of the two regions is
shown in Fig.

Taking this into account, the average magnetotail GSM–

Velocity distribution of all database events for all three components in the GSM frame.

Following earlier studies

Relation of Fast Flow events and their respective AE index at event
time. Events above 100

Bursty bulk flows are expected to be groups of fast flow events with
durations in the order of tens of minutes

Occurrence of groups in the database. Successive events are considered to belong together when their separation time does not exceed 10 min.

Results of superposed epoch analysis for various plasma quantities.
Left column displays near-Earth events, right column shows downtail events.
From top to bottom: ion velocity in

Having a database of 15 000 events, it is favorable to look for recurring
patterns in the data. For the purpose of finding a typical flow burst
signature superposed epoch analysis (SEA) is applied to the data

Constituents to fast flow events for

Having a look at the velocity profiles again, the question arises of how such
profiles can be modeled (e.g., to be used as input to numerical simulations).
While on first sight it resembles a Gaussian impulse function, the flanks of
the profile appear too soft. To quantify such an impulse, near-Earth and
downtail events are divided according to Fig.

Obviously, the data can be represented very well by the model suggested
above. In both cases, the two curves consist of a flow enhancement of similar
amplitude, while the background flow is very low for the near-Earth events
and much faster for the downtail region. The most striking observation is
that the two flow burst components (of downtail and near-Earth,
respectively), though of different amplitude, show almost the exact same
characteristic timescale (

Fitting parameters for isolated fast flow events. The values represent the 95 % confident intervals.

To analyze the relation between fast flows and corresponding, i.e.,
simultaneous magnetic field variations, minimum variance analysis is applied
to the vector magnetic field data. Originally used to determine normal
directions of thin discontinuity layers

While the determination of the minimum variance direction can be associated
with the propagation direction of (magneto-hydrodynamic) wave modes it should
be noted that propagation direction might not be the correct term to
describe the observed phenomena. Thus, the discussion does not intend to
interpret this direction as wave vector direction, but rather present the
findings made. Still, applying the technique to the velocity data of the
events will result in a maximum variance direction,

For the analysis of magnetic field and velocity variations 60

To ensure that only those events are used where the directions can be well
determined, two eigenvalue criteria are imposed on the analysis: (1) for the
ratio of median to minimum eigenvalues of magnetic field analysis

The distribution of the direct angle between flow direction and minimum variance direction of the magnetic field.

For downtail and near-Earth regions the two distributions look very similar,
showing a very prominent peak at around 90

Figure

Distribution of orientation angles of the minimum variance direction of magnetic field disturbances referring to GSM coordinates. The azimuthal angle is shown in the upper panel, the elevation can be seen below.

To find out more about the nature of the magnetic field disturbances, the
maximum variance directions of the magnetic field and the velocity are
compared, again using the technique applied before. The panels a and b of
Fig.

The two panels below have been produced by computing the maximum variance
direction of magnetic field data,

Based on the magnetic field measurements alone, and indeed assuming for the
moment that the minimum variance direction is an indicator for wave
propagation direction, the key characteristics (large amplitude, propagation
almost perpendicular to the background field, maximum variance direction
aligned with the background field) fit the criteria defined for mirror modes

The aim of the present work was to investigate the total available THEMIS
data to compile a list of flow events in Earth's magnetotail. A total of
almost 16 000 events are found, many of them embedded in a group of fast
flows, so-called bursty bulk flows. As to this date, this is the largest list
of fast flow events that has been produced. Due to the orbital distribution
of the events, two different regions were defined as near-Earth and downtail
region. While many statistical characteristics are similar for the two
distributions, some differences were found as well. The velocity distributions
are very similar, the average fast flow is directed in GSM–

Associated minimum variance directions of magnetic field disturbances seem to
be confined to a plane almost perpendicular to the main flow direction. This
is a feature that is persistent for the whole time of the event. While the
relative direction is therefore clearly defined, nor preferred orientation
can be observed in GSM coordinates. Maximum variance directions of magnetic
field and velocity data are aligned for the larger part of the database,
i.e., the maximum variance direction of the magnetic field fluctuations is
along GSM–

The nature of these fluctuations is not understood through observations alone. Especially, it is not clear, whether the inferred minimum variance directions can serve as a proxy for propagation directions, which is a key information necessary to develop a bigger picture of these phenomena.

These findings are contrary to many previous studies on the first look (see,
e.g.,

We summarize important observations as follows:

Magnetotail fast flows occur both during magnetospheric active and quiet times.

While downtail multiple burst events dominate, near-Earth isolated events are observed more frequently.

Superposed Epoch Analysis reveals mean properties of fast flows in both regions.
Characteristic timescales are of the order of 60

Minimum variance directions of the magnetic field are on average perpendicular to the main flow and background magnetic field directions for the total flow burst timescale.

The presentation of the database and first statistical findings is only the start of detailed investigations of fast flow characteristics in the THEMIS data set. The results of the Superposed Epoch Analysis are certainly useful as input to numerical MHD simulations of magnetotail regions. It would be interesting to see the observational findings concerning the magnetic field variations to be confirmed and understood. To achieve this, theoretical aspects need also to be considered in the future. Therefore, future work will include more detailed investigation of different aspects of the data set with a focus on magnetic field fluctuations, theoretical considerations of the observations, especially concerning current systems, drivers of these structures, questions on possible neutral lines, as well as MHD simulations to reproduce similar features in controlled surroundings.

Auroral Electrojet indices for all necessary events were obtained from the World Data Center in Kyoto, Japan. We acknowledge NASA contract NAS5-02099 and V. Angelopoulos for use of data from the THEMIS Mission. Specifically: C. W. Carlson and J. P. McFadden for use of ESA data. This project is financially supported by the German Ministerium für Wirtschaft und Energie and the Deutsches Zentrum für Luft und Raumfahrt under contract 50OC1403. The topical editor, C. Owen, thanks Z. Yao and one anonymous referee for help in evaluating this paper.