Plumes, forming at the plasmapause and released outwards, constitute a well-established mode for plasmaspheric material release to the Earth's magnetosphere. They are associated to active periods and ...the related electric field change. In 1992, Lemaire and Shunk proposed the existence of an additional mode for plasmaspheric material release to the Earth's magnetosphere: a plasmaspheric wind, steadily transporting cold plasmaspheric plasma outwards across the geomagnetic field lines, even during prolonged periods of quiet geomagnetic conditions. This has been proposed on a theoretical basis. Direct detection of this wind has, however, eluded observation in the past. Analysis of ion measurements, acquired in the outer plasmasphere by the CIS experiment onboard the four Cluster spacecraft, provide now an experimental confirmation of the plasmaspheric wind. This wind has been systematically detected in the outer plasmasphere during quiet and moderately active conditions, and calculations show that it could provide a substantial contribution to the magnetospheric plasma populations outside the Earth's plasmasphere. Similar winds should also exist on other planets, or astrophysical objects, quickly rotating and having an atmosphere and a magnetic field.
We analyze the radial distribution of electron populations inside 20 Rs in Saturn's magnetosphere, and we calculate moments for these populations by a forward modeling method using composite spectra ...produced by the CAPS/ELS (0.6 eV to 26 keV) and the MIMI/LEMMS (15 keV to 10 MeV) instruments on board Cassini. We first calculate and harmonize both data sets in physical units and apply corrections taking into account biases introduced by spacecraft interaction with the magnetospheric environment. We then test different bimodal isotropic electron distribution models, deciding on a model with two kappa distributions. We adjust our isotropic model to the flux composite spectra with a least square method to produce three sets of fluid parameters (density, temperature, spectral index) per electron population. The radial profiles are then analyzed, revealing a relevant boundary at 9 Rs in both thermal and suprathermal electron populations. Observed discontinuities in the moment profiles (sudden drop‐off in cold density profile outside 9 Rs, hot electrons drop‐off inside 9 Rs) coincide with the known outer edge of Saturn's neutral OH cloud. Farther out, thermal electrons disappear completely beyond 15 Rs while suprathermal electrons are still observed in the middle and outer magnetosphere.
Due to its polar orbit Cluster spacecraft crossed plasmaspheric plumes out of the magnetic equatorial plane. We study the occurrence of broadband, narrowband, and rising tone emissions in the plume ...vicinity, below the local proton gyrofrequency. Based on a database of 935 Cluster plumes crossings, reduced to 189 unique plumes, we find that broadband activity is the most common case. We confirm result from a previous study showing that plume vicinity is not a preferred place for observing narrowband emissions. Rising tones are the less frequently observed of these three kinds of emissions. Nevertheless, ElectroMagnetic Ion Cyclotron (EMIC) rising tone occurrence rate is high compared to the narrowband one: Tones are seen in six of 30 plume events (20%) when narrowband emissions are observed. Rising tones are observed at absolute magnetic latitudes larger than 17° and up to 35° . We detail the 16 August 2005 plume crossing when a rising tone is observed. Results of a ray tracing analysis agree with a tone triggering process taking place above 15° of magnetic latitude.
Key Points
Coherent rising tones are found in 20% (6/30) of the plume (and vicinity) events with EMIC emissions
EMIC rising tones are observed at magnetic latitudes larger than 17 degrees and up to 35 degrees
Emissions below H+ gyrofrequency in the vicinity of plasmaspheric plumes are mainly broadband
Approaches regarding how to turn the instrument background counts into scientifically valuable data are presented in this Technical Report on Methods. The background counts due to penetrating ...energetic particles of radiation belts detected on Cluster CIS HIA and CODIF instruments and the Double Star HIA instrument are used in these approaches. In HIA spectrograms, the background counts are seen simultaneously in all energy channels marking the entry and exit of the radiation belts by the spacecraft, therefore, the locations of the boundaries of the outer and inner belts can be determined. In the case when HIA measurements are not readily available, a new method is proposed in which supplementary data streams within the CODIF telemetry is exploited. It employs separate counts that register “start,” “stop,” and “non‐valid” signals increasing in the presence of penetrating particles even when no corresponding increase are shown in the energy‐time spectrograms. The locations of the radiation belt boundaries are defined by following the changes in counts gradients with time and visual inspection of all the available measurements. The July–August 2007 and September–October 2012 time periods are analyzed for method demonstration on a presence of a third radiation belt, or storage ring.
Key Points
Instrument background counts reflect radiation belt features including temporal phenomena such as the storage ring
Telemetry counts register signals which increase due to penetrating particles and can be used for radiation belt boundaries
Data with indirect effects from energetic particles become of high value when no particle measurements are available
We examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the ...Earth's magnetotail by applying multi‐scale multipoint analysis methods. In order to study the large‐scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X‐line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter‐rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi‐point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in‐ and outflow structure, consistent with previous simulations on strong guide‐field reconnection events. This study shows that magnetic reconnection may not only take place at large‐scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field.
Plain Language Summary
Magnetic Reconnection is a key energy conversion process, where magnetic energy is converted into kinetic energy of plasma particles. During this process the magnetic field topology changes and the plasma particles decouple from the magnetic field in the so‐called diffusion region and get accelerated, forming a fast outflow jet. Over the last decades, hints arise that reconnection can take place at many different places in the magnetosphere and also very locally and intermittently. Fast plasma flows in the Magnetotail, moving toward the Earth, are assumed to be a consequence of magnetic reconnection, and are often accompanied by dipolar‐shaped magnetic flux bundles, embedded into them. The leading edges of such flux bundles are called dipolarization fronts (DF). In this work, we investigate a DF event, which hosts a diffusion region. First, we study the large‐scale characteristics of the DF, by utilizing data from both the Magnetospheric Multiscale (MMS) and the Cluster mission, that observe different regions of the event almost simultaneously. Second, we performed a 3D magnetic field reconstruction technique and compared the results to MMS data, to investigate the event on small scales.
Key Points
A thin current sheet inside a dipolarization front, embedded in a diverging flow is analyzed using a polynomial reconstruction technique
Transient reconnection event is detected in a high magnetic shear region, where the magnetic field is deflected due to duskward fast plasma flow
The reconstructed current sheet has a guide field of ∼1.8 the reconnecting component with normalized reconnection rate between 0.16 and 0.18
The megaelectron volt proton radiation belts of Saturn are isolated from the middle and outer magnetosphere, and the source of these high‐energy protons is thought to be linked to the access of ...galactic cosmic rays (GCRs) in the system. To validate this hypothesis, it is first of all necessary to determine the realistic spectrum of GCRs at Saturn. Previously, only analytical attempts were performed in order to calculate the GCR spectra. In this letter we provide for the first time the numerical solution for the determination of the GCR access to the upper atmosphere and rings of Saturn. The proposed method is based on the charged particle tracing technique. As a result the GCRs access energies to the atmosphere and to the equatorial plane of the magnetosphere as a function of radial distance to the planet were obtained and the GCRs spectra were reconstructed. Dependencies of the spectral parameters such as the time or the incidence direction were also obtained offering all necessary information for simulating the interaction of GCRs with the Saturnian system during different phases of the Cassini mission.
Plain Language Summary
In this research letter we propose the numerical solution for the calculation of the galactic cosmic rays (GCRs) access energies to Saturn and its magnetosphere. Previously, only analytical approach was used to address this problem in the Saturnian system providing uncertain results. The new numerical approach on the basis of particle tracing brings much more accurate access energies, GCR spectra, and integrated GCR flux to the planet and across the magnetosphere and allows to estimate the Cosmic Ray Albedo Neutron Decay process on Saturn, calculate its contribution to the Saturnian radiation belts, and evaluate the impact of GCRs on the atmosphere, rings and moons of Saturn.
Key Points
We provide GCR access energies to the Saturnian atmosphere and across the magnetosphere
GCR spectra and integrated flux to Saturn and to the equatorial plane of the magnetosphere are calculated
Advantages of a particle tracing approach over the analytical solution are demonstrated
Thin Current Sheet Behind the Dipolarization Front Nakamura, R.; Baumjohann, W.; Nakamura, T. K. M. ...
Journal of geophysical research. Space physics,
October 2021, Letnik:
126, Številka:
10
Journal Article
Recenzirano
Odprti dostop
We report a unique conjugate observation of fast flows and associated current sheet disturbances in the near‐Earth magnetotail by MMS (Magnetospheric Multiscale) and Cluster preceding a positive bay ...onset of a small substorm at ∼14:10 UT, September 8, 2018. MMS and Cluster were located both at X ∼ −14 RE. A dipolarization front (DF) of a localized fast flow was detected by Cluster and MMS, separated in the dawn‐dusk direction by ∼4 RE, almost simultaneously. Adiabatic electron acceleration signatures revealed from the comparison of the energy spectra confirm that both spacecraft encounter the same DF. We analyzed the change in the current sheet structure based on multi‐scale multi‐point data analysis. The current sheet thickened during the passage of DF, yet, temporally thinned subsequently associated with another flow enhancement centered more on the dawnward side of the initial flow. MMS and Cluster observed intense perpendicular and parallel current in the off‐equatorial region mainly during this interval of the current sheet thinning. Maximum field‐aligned currents both at MMS and Cluster are directed tailward. Detailed analysis of MMS data showed that the intense field‐aligned currents consisted of multiple small‐scale intense current layers accompanied by enhanced Hall‐currents in the dawn‐dusk flow‐shear region. We suggest that the current sheet thinning is related to the flow bouncing process and/or to the expansion/activation of reconnection. Based on these mesoscale and small‐scale multipoint observations, 3D evolution of the flow and current‐sheet disturbances was inferred preceding the development of a substorm current wedge.
Key Points
Evolution of localized fast flows and dipolarization front is obtained from multi‐scale multi‐point observations in near‐Earth magnetotail
Current sheet thinning accompanied by intense field‐aligned currents is detected following the passage of the dipolarization front
From signatures of adiabatic electron acceleration it is confirmed that the same flow front was detected by the multi‐point measurements
The structure of Earth's magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the ...magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high-latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previously, it has been suggested that closed flux can become trapped in the lobe and that this plasma-trapping process could explain another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the trapped closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs.
Baryonic matter in geospace is almost exclusively in a plasma state, with protons (H
+) and to some extent ionized helium (He) and oxygen (O) being the dominant ion species. But also other heavier ...ion species and even molecular ions are present in geospace. The Research with Adaptive Particle Imaging Detectors (RAPID) on board the Cluster satellites can identify and characterize some of these ions by utilizing their measured time of flight and energy. Usually, the measurements are then assigned into three discrete species channels; protons (H
+), helium (He), and a common channel for carbon, nitrogen, and oxygen (CNO), each with flux, energy, and angular information. But RAPID also has a Direct Event (DE) diagnostic mode in which the full time of flight and energy information for a limited number of incident particles are stored. With knowledge about energy losses in the various detector parts, it is then possible to derive the atomic mass of the incident particle. In this paper we report on results from a study of Cluster DE events during the years 2001–2018, with a particular emphasis of iron (Fe) ions. We show that suprathermal Fe ions can be found all over geospace covered by Cluster, and that the time variation is consistent with modulation by geomagnetic disturbances and solar activity. We do not find any clear correlations between detection of suprathermal Fe and meteor showers or sputtering off the moon.
Key Points
We present the first observations of (Fe
+/Fe
N+) from the Cluster mission
Suprathermal Fe is detected in all regions traversed by Cluster
Time variations and likely sources of the observed Fe ions are investigated
The foreshock, extending upstream of Earth's bow shock, is a region of intense electromagnetic wave activity and nonlinear phenomena, which can have global effects on geospace. It is also the first ...geophysical region encountered by solar wind disturbances journeying toward Earth. Here, we present the first observations of considerable modifications of the foreshock wave field during extreme events of solar origin called magnetic clouds. Cluster's multispacecraft data reveal that the typical quasi‐monochromatic foreshock waves can be completely replaced by a superposition of waves each with shorter correlation lengths. Global numerical simulations further confirm that the foreshock wave field is more intricate and organized at smaller scales. Ion measurements suggest that changes in shock‐reflected particle properties may cause these modifications of the wave field. This state of the foreshock is encountered only during extreme events at Earth, but intense magnetic fields are typical close to the Sun or other stars.
Plain Language Summary
Solar storms are giant clouds of particles ejected from the Sun into space during solar eruptions. When solar storms are directed toward Earth, they can cause large disturbances in near‐Earth space, for example, disrupting communications or damaging spacecraft electronics. Understanding in detail what happens when solar storms reach Earth is crucial to mitigate their effects. Using measurements from the Cluster spacecraft, we investigate how solar storms modify the properties of the very first region of near‐Earth space they encounter when journeying toward Earth. This region, called the foreshock, extends ahead of the protective bubble formed by the Earth's magnetic field, the magnetosphere. The foreshock is home to intense electromagnetic waves, and disturbances in this region can perturb the Earth's magnetosphere. Our study reveals that solar storms modify profoundly the foreshock, resulting in a more complex wave activity. Global numerical simulations performed with the Vlasiator code confirm our findings. These changes could affect the regions of space closer to Earth, for example, in modifying the wave properties or the amount of solar particles entering the Earth's magnetosphere. This needs to be taken into account to better anticipate the effects of solar storms at Earth.
Key Points
When reaching geospace, magnetic clouds modify significantly the properties of the first geophysical region they encounter, the foreshock
Typical quasi‐monochromatic foreshock waves are replaced by a superposition of waves at different periods with a shorter transverse extent
Multiple field‐aligned beams observed during one event suggest a link between the multiple wave periods and the suprathermal ion properties
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