During propagation, Magnetic Clouds (MC) interact with their environment and, in particular, may reconnect with the solar wind around it, eroding away part of its initial magnetic flux. Here we ...quantitatively analyze such an interaction using combined, multipoint observations of the same MC flux rope by STEREO A, B, ACE, WIND and THEMIS on November 19–20, 2007. Observation of azimuthal magnetic flux imbalance inside a MC flux rope has been argued to stem from erosion due to magnetic reconnection at its front boundary. The present study adds to such analysis a large set of signatures expected from this erosion process. (1) Comparison of azimuthal flux imbalance for the same MC at widely separated points precludes the crossing of the MC leg as a source of bias in flux imbalance estimates. (2) The use of different methods, associated errors and parametric analyses show that only an unexpectedly large error in MC axis orientation could explain the azimuthal flux imbalance. (3) Reconnection signatures are observed at the MC front at all spacecraft, consistent with an ongoing erosion process. (4) Signatures in suprathermal electrons suggest that the trailing part of the MC has a different large‐scale magnetic topology, as expected. The azimuthal magnetic flux erosion estimated at ACE and STEREO A corresponds respectively to 44% and 49% of the inferred initial azimuthal magnetic flux before MC erosion upon propagation. The corresponding average reconnection rate during transit is estimated to be in the range 0.12–0.22 mV/m, suggesting most of the erosion occurs in the inner parts of the heliosphere. Future studies ought to quantify the influence of such an erosion process on geo‐effectiveness.
Key Points
Demonstrate the occurrence of magnetic cloud erosion during propagation
Investigate all expected signatures of this mechanism
Highlight the implications in terms of impact in the Heliosphere and at Earth
Several recent studies suggest that magnetic reconnection is able to erode substantial amounts of the outer magnetic flux of interplanetary magnetic clouds (MCs) as they propagate in the heliosphere. ...We quantify and provide a broader context to this process, starting from 263 tabulated interplanetary coronal mass ejections, including MCs, observed over a time period covering 17 years and at a distance of 1 AU from the Sun with Wind (1995–2008) and the two STEREO (2009–2012) spacecraft. Based on several quality factors, including careful determination of the MC boundaries and main magnetic flux rope axes, an analysis of the azimuthal flux imbalance expected from erosion by magnetic reconnection was performed on a subset of 50 MCs. The results suggest that MCs may be eroded at the front or at rear and in similar proportions, with a significant average erosion of about 40% of the total azimuthal magnetic flux. We also searched for in situ signatures of magnetic reconnection causing erosion at the front and rear boundaries of these MCs. Nearly ~30% of the selected MC boundaries show reconnection signatures. Given that observations were acquired only at 1 AU and that MCs are large‐scale structures, this finding is also consistent with the idea that erosion is a common process. Finally, we studied potential correlations between the amount of eroded azimuthal magnetic flux and various parameters such as local magnetic shear, Alfvén speed, and leading and trailing ambient solar wind speeds. However, no significant correlations were found, suggesting that the locally observed parameters at 1 AU are not likely to be representative of the conditions that prevailed during the erosion which occurred during propagation from the Sun to 1 AU. Future heliospheric missions, and in particular Solar Orbiter or Solar Probe Plus, will be fully geared to answer such questions.
Key Points
MCs are frequently eroded at the front or at the rear in similar proportion
Nearly 30% of selected MC boundaries show reconnection signatures
The amount of eroded MCs and solar wind parameters do not seem to be correlated
The MAVEN spacecraft launched in November 2013, arrived at Mars in September 2014, and completed commissioning and began its one-Earth-year primary science mission in November 2014. The orbiter’s ...science objectives are to explore the interactions of the Sun and the solar wind with the Mars magnetosphere and upper atmosphere, to determine the structure of the upper atmosphere and ionosphere and the processes controlling it, to determine the escape rates from the upper atmosphere to space at the present epoch, and to measure properties that allow us to extrapolate these escape rates into the past to determine the total loss of atmospheric gas to space through time. These results will allow us to determine the importance of loss to space in changing the Mars climate and atmosphere through time, thereby providing important boundary conditions on the history of the habitability of Mars. The MAVEN spacecraft contains eight science instruments (with nine sensors) that measure the energy and particle input from the Sun into the Mars upper atmosphere, the response of the upper atmosphere to that input, and the resulting escape of gas to space. In addition, it contains an Electra relay that will allow it to relay commands and data between spacecraft on the surface and Earth.
In the Earth's inner magnetosphere, the distribution of energetic electrons is controlled by pitch‐angle scattering by waves. A category of Whistler waves originates from powerful ground‐based VLF ...transmitter signals in the frequency range 10–25 kHz. These transmissions are observed in space as waves of very narrow bandwidth. Here we examine the significance of the VLF transmitter NWC on the inner radiation belt using DEMETER satellite global observations at low altitudes. We find that enhancements in the ∼100–600 keV drift‐loss cone electron fluxes at L values between 1.4 and 1.7 are linked to NWC operation and to ionospheric absorption. Waves and particles interact in the vicinity of the magnetic equatorial plane. Using Demeter passes across the drifting cloud of electrons caused by the transmitter; we find that ∼300 times more 200 keV electrons are driven into the drift‐loss cone during NWC transmission periods than during non‐transmission periods. The correlation between the flux of resonant electrons and the Dst index shows that the electron source intensity is controlled by magnetic storm activity.
We explore the mechanism of MeV and sub‐MeV electron precipitations into the atmosphere in the outer radiation belt, through quasi‐linear pitch‐angle scattering by electromagnetic ion cyclotron ...(EMIC) waves, when strong compressional Pc4–Pc5 ultralow‐frequency (ULF) waves are simultaneously present. Theoretically, the opposite magnetic field and density modulations produced by such ULF waves can significantly reduce the minimum electron energy for cyclotron resonance with EMIC waves, and this could potentially lead to the loss of lower energy (MeV and sub‐MeV) electrons. Statistical satellite observations of simultaneous, intense EMIC and ULF waves reveal the parameter domains most conducive to such lower energy electron losses, which are shown to be mostly located near the geosynchronous orbit. Selected events further suggest that such a mechanism could be efficient in the outer radiation belt and that even larger effects might occur during strong injections from the plasma sheet.
Key Points
Occurrence rates of simultaneous, intense EMIC and Pc4–Pc5 ULF waves in the outer radiation belt are provided
The minimum energy of electrons precipitated by EMIC waves could theoretically decrease by 25% due to ULF waves
We conjecture that EMIC waves could cause sub‐MeV electron loss in the presence of intense Pc5 ULF waves near L=6
Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma ...experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma.
The MAVEN Solar Wind Electron Analyzer Mitchell, D. L.; Mazelle, C.; Sauvaud, J.-A. ...
Space science reviews,
04/2016, Letnik:
200, Številka:
1-4
Journal Article
Recenzirano
The MAVEN Solar Wind Electron Analyzer (SWEA) is a symmetric hemispheric electrostatic analyzer with deflectors that is designed to measure the energy and angular distributions of 3-4600-eV electrons ...in the Mars environment. This energy range is important for impact ionization of planetary atmospheric species, and encompasses the solar wind core and halo populations, shock-energized electrons, auroral electrons, and ionospheric primary photoelectrons. The instrument is mounted at the end of a 1.5-meter boom to provide a clear field of view that spans nearly 80 % of the sky with ∼20° resolution. With an energy resolution of 17 % (
Δ
E
/
E
), SWEA readily distinguishes electrons of solar wind and ionospheric origin. Combined with a 2-second measurement cadence and on-board real-time pitch angle mapping, SWEA determines magnetic topology with high (∼8-km) spatial resolution, so that local measurements of the plasma and magnetic field can be placed into global context.
We report the first direct measurements of the Venusian atmospheric erosion rate due to the interaction with the solar wind. The erosion through the ion escape is determined during the period of the ...minimum solar activity from 24 May 2006 to 12 December 2007. The ion fluxes are measured in the energy range 10 eV to 25 keV by an ion mass spectrometer on board the Venus Express spacecraft and sampled statistically dense in the volume in the Venusian wake. The rates are Q(H+) = 7.1 · 1024 s−1, Q(He+) = 7.9 · 1022 s−1, and Q(O+) = 2.7 · 1024 s−1. The reported escape rates measured for the solar minimum are close to the rates estimated for the solar maximum from the Pioneer Venus Orbiter observations. We may thus propose that the atmospheric loss due to solar wind interaction depends weakly on the solar conditions. The paper also presents in detail how the global escape rates are deduced from the in situ measurements.
Key Points
First direct measurements of the ion escape rate from Venus
The ion escape rates for solar minimum are close to ones for solar maximum
The technique for how to deduce the global escape rates from the local measurements
During the time period of 1 March 2001–1 April 2008, the Composition and Distribution Function (CODIF) Analyzer on board Cluster observed 41 prolonged He+ energization events, lasting for 1.10–4.97 ...h, on average, 3.18 ± 0.91 h. These He+ heating events occurred predominantly at low/middle magnetic latitudes (MLAT = −4.3°–51.7°) in the afternoon sector (MLT = 11:32–19:06) in the outer magnetosphere (L = 7.9–14.6). During the events, the He+ ions resonantly interacted with electromagnetic ion cyclotron (EMIC) waves and were perpendicularly energized to energies up to 1 keV. Their contribution to total ion density in the energy range of 0.04–1 keV was elevated on average up to 51%. A superposed epoch analysis of the plasma data measured by Cluster during these events indicates the presence of the two EMIC wave‐controlling factors: hot anisotropic H+ (the wave free‐energy provider) and cold dense plasma (the wave generation catalyst). In addition, it is common in the events that the density of the energetic H+ is elevated and electron plasma/gyrofrequency ratio (fpe/fce) reaches values higher than 10. Quiet solar wind and geomagnetic activity appear to be favorable conditions for the generation of the EMIC waves and thus the resultant He+ energization in the outer‐magnetospheric region. The reason is that, under quiet solar wind and geomagnetic conditions, an overlap of hot anisotropic H+ from the plasma sheet and cold dense plasma from a plasmaspheric plume or plume‐like region could exist in the afternoon sector of the outer magnetosphere.
Key Points
Cluster observed 41 prolonged He+ energization events in the outer magnetosphere
Hot anisotropic H+ and cold dense plasma were present during the events
Quiet geomagnetic activity is favorable for the generation of the events
The occurrence of spatially and temporally variable reconnection at the Earth's magnetopause leads to the complex interaction of magnetic fields from the magnetosphere and magnetosheath. Flux ...transfer events (FTEs) constitute one such type of interaction. Their main characteristics are (1) an enhanced core magnetic field magnitude and (2) a bipolar magnetic field signature in the component normal to the magnetopause, reminiscent of a large‐scale helicoidal flux tube magnetic configuration. However, other geometrical configurations which do not fit this classical picture have also been observed. Using high‐resolution measurements from the Magnetospheric Multiscale mission, we investigate an event in the vicinity of the Earth's magnetopause on 7 November 2015. Despite signatures that, at first glance, appear consistent with a classic FTE, based on detailed geometrical and dynamical analyses as well as on topological signatures revealed by suprathermal electron properties, we demonstrate that this event is not consistent with a single, homogenous helicoidal structure. Our analysis rather suggests that it consists of the interaction of two separate sets of magnetic field lines with different connectivities. This complex three‐dimensional interaction constructively conspires to produce signatures partially consistent with that of an FTE. We also show that, at the interface between the two sets of field lines, where the observed magnetic pileup occurs, a thin and strong current sheet forms with a large ion jet, which may be consistent with magnetic flux dissipation through magnetic reconnection in the interaction region.
Key Points
We characterized the scale, geometry, and propagation of an ion scale current structure resulting from the interaction between interlaced flux tubes
Some signatures of magnetic reconnection are found at the interaction interface
The intrinsic properties of this event are inconsistent with a single, homogenous helicoidal magnetic structure as expected from a typical flux transfer event (FTE)
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