9 research outputs found
Observation of a New Type of Low Frequency Waves at Comet 67P/Churyumov-Gerasimenko
We report on magnetic field measurements made in the innermost coma of
67P/Churyumov-Gerasimenko in its low activity state. Quasi-coherent,
large-amplitude (), compressional magnetic field
oscillations at 40 mHz dominate the immediate plasma environment of the
nucleus. This differs from previously studied comet-interaction regions where
waves at the cometary ion gyro-frequencies are the main feature. Thus classical
pick-up ion driven instabilities are unable to explain the observations. We
propose a cross-field current instability associated with newborn cometary ion
currents as a possible source mechanism.Comment: 6 pages, 3 Figure
Statistical analysis of magnetotail fast flows and related magnetic disturbances
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 min, corresponding to scale
sizes of 3 <i>R</i><sub>E</sub>. Last, related magnetic field disturbances are
analyzed. It is found that the minimum variance direction is essentially
confined to a plane almost perpendicular to the main flow direction while, at
the same time, the maximum variance direction is aligned with flow and
background field directions. The presentation of the database and first
statistical findings will prove useful both as input for
magneto-hydrodynamical simulations and theoretical considerations of fast
flows
Statistical analysis of severe magnetic fluctuations in the near-Earth plasma sheet observed by THEMIS-E
We statistically analyzed severe magnetic fluctuations in the nightside
near-Earth plasma sheet at 6–12 RE (Earth radii; 1 RE = 6371 km), because they are important for non-magnetohydrodynamics
(non-MHD) effects in the magnetotail and are considered to be necessary for
current disruption in the inside-out substorm model. We used magnetic field
data from 2013 and 2014 obtained by the Time History of Events and Macroscale
Interactions during Substorms E (THEMIS-E) satellite (sampling rate: 4 Hz).
A total of 1283 severe magnetic fluctuation events were identified that
satisfied the criteria σB∕B > 0. 5, where σB and
B are the standard deviation and the average value of magnetic field
intensity during the time interval of the local proton gyroperiod, respectively.
We found that the occurrence rates of severe fluctuation events are 0.00118,
0.00899, and 0.0238 % at 6–8, 8–10, and 10–12 RE,
respectively, and most events last for no more than 15 s. From these
occurrence rates, we estimated the possible scale sizes of current disruption
by severe magnetic fluctuations as 3.83Â RE3 by assuming that
four substorms with 5 min intervals of current disruption occur every day.
The fluctuation events occurred most frequently at the ZGSM (Z
distance in the geocentric solar magnetospheric coordinate system) close to
the model neutral sheet within 0.2Â RE. Most events occur in
association with sudden decreases in the auroral electrojet lower (AL) index and magnetic field
dipolarization, indicating that they are related to substorms. Sixty-two
percent of magnetic fluctuation events were accompanied by ion flow with
velocity V > 100 km s−1, indicating that the violation of ion
gyromotion tends to occur during high-speed flow in the near-Earth plasma
sheet. The superposed epoch analysis also indicated that the flow speed
increases before the severe magnetic fluctuations. We discuss how both the
inside-out and outside-in substorm models can explain this increase in flow
speeds before magnetic fluctuation events
Spin axis offset calibration on THEMIS using mirror modes
A newly developed method for determining spin axis offsets of magnetic field
instruments on spacecraft is applied to THEMIS. The formerly used
determination method, relying on solar wind Alfvénic fluctuations, was
rarely applicable due to the orbital restrictions of the mission. With the
new procedure, based on magnetic field observation of mirror modes in the
magnetosheath, updated spin axis offsets can be estimated approximately once
per year. Retrospective calibration of all THEMIS magnetic field measurements
is thereby made possible. Since, up to this point, spin axis offsets could
hardly ever be calculated due to the mission's orbits, this update represents
a substantial improvement to the data. The approximate offset stability is
estimated to be < 0.75 nT year−1 for the complete course of the
mission
Average plasma sheet polytropic index as observed by THEMIS
Multi-spacecraft data from the years 2008 to 2015 of the THEMIS mission particularly in the near-Earth plasma sheet are used in order to empirically determine the polytropic index in the quiet and active time magnetotail. The results of a number of previous studies in the 1990s can be confirmed. An analysis of the total database, although showing poor correlation, results in an average polytropic index of γ = 1. 72. The active time plasma sheet is well correlated with an average γ = 1. 49. However, the data scattering suggests that the analysis of the data in total is not adequate. In order to reduce the timescales, individual spacecraft orbits are analyzed, giving a broad distribution of polytropic indices throughout the plasma sheet. The major part of the distribution falls in a range between γ = 0. 67 and γ = 2. Our results indicate a variety of thermodynamic processes in the magnetotail and an all-time presence of heat exchange of the plasma. A description of the plasma sheet using an equation of state with a single γ is probably inadequate. This necessitates the application of more sophisticated approaches, such as a parametrization of the heat flux vector in magnetohydrodynamic equations or a superposition of polytropic indices
Structure and evolution of the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko
International audienceThe long duration of the Rosetta mission allows us to study the evolution of the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko in detail. From 2015 April to 2016 February 665 intervals could be identified where Rosetta was located in a zero-magnetic-field region. We study the temporal and spatial distribution of this cavity and its boundary and conclude that the cavity properties depend on the long-term trend of the outgassing rate, but do not respond to transient events at the spacecraft location, such as outbursts or high neutral densities. Using an empirical model of the outgassing rate, we find a functional relationship between the outgassing rate and the distance of the cavity to the nucleus. There is also no indication that this unexpectedly large distance is related to unusual solar wind conditions. Because the deduced shape of the cavity boundary is roughly elliptical on small scales and the distances of the boundary from the nucleus are much larger than expected we conclude that the events observed by Rosetta are due to a moving instability of the cavity boundary itself