We use the magnetic field measurements from four spacecraft of the Cluster-II
mission (three events from 2005 to 2015) for the analysis of turbulent
processes in the Earth's magnetotail. For this ...study we conduct the spectral,
wavelet and statistical analysis. In the framework of statistical
examination, we determine the kurtosis for selected events and conduct
extended self-similarity evaluation (analysis of distribution function
moments of magnetic field fluctuations on different scales). We compare the
high-order structure function of magnetic fluctuations during dipolarization
with the isotropic Kolmogorov model and three-dimensional log-Poisson model
with She–Leveque parameters. We obtain power-law scaling of the generalized
diffusion coefficient (the power index that varies within the range of
0.2–0.7). The obtained results show the presence of super-diffusion
processes. We find the significant difference of the spectral indices for the
intervals before and during the dipolarization. Before dipolarization the
spectral index lies in the range from -1.68±0.05 to -2.08±0.05
(∼5/3 according to the Kolmogorov model). During dipolarization the
type of turbulent motion changes: on large timescales the turbulent flow is
close to the homogeneous models of Kolmogorov and Iroshnikov–Kraichnan (the
spectral index lies in the range from −2.20 to −1.53), and at smaller
timescales the spectral index is in the range from −2.89 to −2.35 (the
Hall–MHD model). The kink frequency is less than or close to the average
value of the proton gyrofrequency. The wavelet analysis shows the presence of both direct and inverse cascade
processes, which indicates the possibility of self-organization processes, as
well as the presence of Pc pulsations.
The Cluster mission, launched in 2000, has produced a large database of electron flux intensity measurements in the Earth's magnetosphere by the Research with Adaptive Particle Imaging Detector ...(RAPID)/Imaging Electron Spectrometer (IES) instrument. However, due to background contamination of the data with high‐energy electrons (>400 keV) and inner zone protons (230–630 keV) in the radiation belts and ring current, the data have been rarely used for inner‐magnetospheric science. The current paper presents two algorithms for background correction. The first algorithm is based on the empirical contamination percentages by both protons and electrons. The second algorithm uses simultaneous proton observations. The efficiencies of these algorithms are demonstrated by comparison of the corrected Cluster/RAPID/IES data with Van Allen Probes/Magnetic Electron Ion Spectrometer measurements for 2012–2015. Both techniques improved the IES electron data in the radiation belts and ring current, as the yearly averaged flux intensities of the two missions show the ratio of measurements close to 1. We demonstrate a scientific application of the corrected IES electron data analyzing its evolution during solar cycle. Spin‐averaged yearly mean IES electron intensities in the outer belt for energies 40–400 keV at L‐shells between 4 and 6 showed high positive correlation with auroral electrojet index and solar wind dynamic pressure during 2001–2016. The relationship between solar wind dynamic pressure and IES electron measurements in the outer radiation belt was derived as a uniform linear‐logarithmic equation.
Plain Language Summary
Radiation belts of the Earth, which are the zones of charged energetic particles trapped by the geomagnetic field, comprise enormous and dynamic systems. While the inner radiation belt, composed mainly of high‐energy protons, is relatively stable, the outer belt, filled with energetic electrons, is highly variable and depends substantially on solar activity. Hence, extended reliable observations and the improved models of the electron intensities in the outer belt depending on solar wind parameters are necessary for prediction of their dynamics. The Cluster mission has been measuring electron flux intensities in the radiation belts since its launch in 2000, thus providing a huge dataset that can be used for radiation belts analysis. Using 16 years of electron measurements by the Cluster mission corrected for background contamination, we derived a uniform linear‐logarithmic dependence of electron fluxes in the outer belt on the solar wind dynamic pressure.
Key Points
We present two algorithms for background correction of RAPID electron measurements at energies 40‐400 keV
Corrected RAPID data coincide well with Van Allen Probes electron measurements
The generalized relationship between IES electron flux intensities and solar wind dynamic pressure was obtained
—
Magnetic field pulsations in the magnetosphere and the time of their detection and location on the Earth’s surface are analyzed. Measurements of magnetic field fluctuations from fluxgate ...magnetometers of the Cluster II satellites and measurements from ground-based magnetometers in the auroral oval region are used. The substorms on August 13, 2019, are examined. In particular, two substorms and flapping motions of the magnetotail current sheet are analyzed. The measurements from ground-based observatories are selected using the 3DView software, a tool for the visualization of spacecraft position with associated geomagnetic tail field lines. A continuous wavelet transform is used to identify geomagnetic pulsations, and an integrated representation in two frequency bands, 45–150 s (Pc4/Pi2) and 150–600 s (Pc5/Pi3), is considered to determine the pulsation type and estimate the observed shifts between the pulsations recorded in the Earth’s magnetotail and in the auroral oval region. Correlated Pi2 and Pc5 pulsations in the auroral region and in the magnetotail are detected. The magnitude of detected pulsations depends on the relative position of ground-based magnetometers and the projection of the field line on which the spacecraft are located. Based on the time delay between the maxima of geomagnetic pulsations at the Earth’s surface in relation to disturbances in the magnetosphere, the velocity of disturbance propagation along the magnetic field line is estimated.
Magnetic reconnection in planetary magnetospheres plays important roles in energy and mass transfer in the steady state, and also possibly in transient large‐scale disturbances. In this paper we ...report observations of a reconnection event in the Jovian magnetotail by the Galileo spacecraft on 17 June 1997. In addition to the tailward retreat of a main X‐line, signatures of recurrent X‐line formations are found by close examination of energetic particle anisotropies. Furthermore, detailed analyses of multi‐instrumental data for this period provide various spatiotemporal features in the plasma sheet. A significant density decrease was detected in the central plasma sheet, indicative of the transition to lobe (open field line) reconnection from plasma sheet (closed field line) reconnection. When Galileo vertically swept through the plasma sheet, a velocity layer structure was observed. We also analyze a strong southward magnetic field which is similar to dipolarization fronts observed in the terrestrial magnetotail: the ion flow (∼450 km s−1) was observed behind the magnetic front, whose thickness of 10000–20000 km was of the order of ion inertial length. The electron anisotropy in this period suggests an anomalously high‐speed electron jet, implying ion‐electron decoupling behind the magnetic front. Particle energization was also seen associated with these structures. These observations suggest that X‐line evolution and consequent plasma sheet structures are similar to those in the terrestrial magnetosphere, whereas their generality in the Jovian magnetosphere and influence on the magnetospheric/ionospheric dynamics including transient auroral events need to be further investigated with more events.
Key Points
Multi‐instrumental data analysis on a reconnection event in the Jovian tail
Transition to lobe reconnection and retreat of an X‐line are found
Plasma sheet structures resembling those in the terrestrial tail are generated
The Jovian magnetosphere undergoes periodic reconfiguration processes mainly driven by the fast planetary rotation and mass loading from the moon Io. These reconfiguration processes of the Jovian ...magnetosphere are associated with the release of plasmoids discernible as ion flow bursts associated with bipolar magnetic signatures. We investigate these plasma flows statistically using data from the Energetic Particles Detector and from the magnetometer on board Galileo. The plasma flows are observed in different magnetospheric regions: the current sheet center, the plasma sheet boundary layers, and the lobe. We show that the bulk velocity of all species is the same for most of the magnetic field bipolar signatures associated with these plasma flows. The average speed of the observed plasmoids in the plasma sheet associated with the ion flow bursts is between 350 and 500 km s−1, and the duration of the events is between 10 and 20 min. The associated plasmoid length is correspondingly ∼9 RJ. The plasmoids are moving approximately with Alfvénic speed. The convection electric field during the plasmoid release is about an order of magnitude higher than the ambient value of the Jovian convection electric field.
In this work we present an analysis of the dynamics of
suprathermal ions of different masses (H+, He+, O+) during
prolonged dipolarizations in the near-Earth magnetotail (X>-17RE) according to ...Cluster/RAPID observations in 2001–2005. All
dipolarizations from our database were associated with fast flow braking
and consisted of multiple dipolarization fronts (DFs). We found
statistically that fluxes of suprathermal ions started to increase
∼1 min before the dipolarization onset and continued to grow
for ∼1 min after the onset. The start of flux growth
coincided with the beginning of a decrease in the spectral index γ.
The decrease in γ was observed for protons for ∼1 min after the dipolarization onset, and for He+ and O+ ions for ∼3 and ∼5 min after the onset
respectively. The negative variations of γ for O+ ions were ∼2.5 times larger than for light ions. This demonstrates more
efficient acceleration for heavy ions. The strong negative variations of
γ were observed in finite energy ranges for all ion components. This
indicates the possibility of nonadiabatic resonant acceleration of ions in
the course of their interaction with multiple DFs during dipolarizations.
Our analysis showed that some fraction of light ions can be accelerated up
to energies ≥600 keV and some fraction of oxygen ions can be
accelerated up to ∼1.2 MeV. Such strong energy gains cannot
be explained by acceleration at a single propagating DF and suggest the
possibility of multistage ion acceleration in the course of their
interaction with multiple DFs during the prolonged dipolarizations.
Recent studies have shown that low energy ions constitute a significant part of the total ion population in the Earth's magnetosphere. In this study, we have used a comprehensive data set with ...measurements of cold (total energy less than 70 eV) ion velocity and density to determine their source. This data set is derived from Cluster satellite measurements combined with solar wind and interplanetary magnetic field measurements and geomagnetic indices. By using the guiding center equation of motion, we were able to calculate the trajectories and thus determine the source region of the cold ions. Our results show that the polar cap region is the primary source for cold ions. We also found that the expansion and contraction of the polar cap as a consequence of changes in solar wind parameters were correlated with the source region size and intensity of the cold ion outflow. Elevated outflow fluxes near the nightside auroral zone and the dayside cusps during disturbed conditions suggest that energy and particle precipitation from the magnetosphere or directly from the solar wind can enhance the outflow of cold ions from the ionosphere.
Key Points
The primary source of the cold ions in magnetotail is the polar cap regions
Elevated fluxes on polar cap during disturbed geomagnetic periods are found
Size and intensity of the source region varies with geomagnetic activity
Most regions of the Jovian magnetosphere covered by the Galileo spacecraft measurements undergo quasi‐periodic modulations with a time period of several Earth days. These modulations appear in ...various field and particle properties. Most prominent are periodically recurring ion flow bursts associated with disturbances in the meridional component of the magnetic field in the Jovian magnetotail or variations of the energy spectral shape of the particle distribution associated with the stretching and dipolarization of the magnetic field. Each individual cycle of these modulations is believed to represent a global reconfiguration of the Jovian magnetosphere. We present a simple conceptual model for these periodic processes assuming (1) ion mass loading from internal plasma sources and (2) fast planetary rotation causing magnetotail field line stretching due to centrifugal forces. This leads to a magnetotail configuration favoring magnetic reconnection. Magnetic reconnection causes plasmoid formation and release as well as dipolarization of field lines connected to the planet. Continued mass loading leads again to a stretching of the tail field lines. Our model shows that the suggested intrinsic mechanism can explain the observed periodicities of several days in Jovian substorm‐like processes.
The dynamics of high-energy electron fluxes (with energies over 40 keV) is analyzed in 13 events of magnetic field dipolization observed by the Cluster satellites in the near-tail of the Earth ...magnetosphere. In all of the events, the observed energetic electron fluxes are enhanced simultaneously with initial dipolization. Good correlation (correlation coefficient >0.6) is observed between the dynamics of the energetic electron fluxes with energies up to 90 keV and the
B
Z
component of the magnetic field. Electron fluxes with higher energies display a decline of correlation with the magnetic field. The increase in electron fluxes with energies up to 90 keV during dipolization development is shown to be mainly due to the mechanism of betatron acceleration. The dynamics of electron fluxes with higher energies is poorly described by the betatron scenario and requires consideration of other, probably nonadiabatic, mechanisms.
The dynamics of fluxes of thermal and suprathermal H
+
and O
+
ions and the pressure variations of these components during 11 events of magnetic field dipolarization in the plasma sheet of the ...geomagnetic tail are analyzed based on Cluster satellite observations
.
It was found that the energy of H
+
and O
+
ions corresponding to the maximum flux approached or exceeded the upper energy threshold of CODIF thermal plasma mass spectrometers (~40 keV) in all of the events. During such periods, the determined values of the density, temperature, and pressure in an energy range up to 40 keV are underestimated as compared to the actual values. To determine the pressure during such intervals, we used measurements of the ion-component fluxes in an energy range up to 1.3 MeV based on a joint analysis of observations of CODIF and RAPID energy-mass spectrometers. As a result, it was found that the ion pressure during dipolarization with the high-energy component of the spectrum taken into account can be several times higher than the pressure determined in the range up to 40 keV. Using the superposed epoch analysis, we showed that the error associated with underestimation of the pressure of the H
+
and O
+
ion components is largest during the dipolarization growth phase. The underestimation is most significant for the O
+
ion pressure.
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