Combined in situ ion measurements and remote sensing of energetic neutral atoms are used to determine the geocoronal Hydrogen density at large (∼10 RE) distances from the Earth. This method for ...determining the geocoronal density requires global magnetospheric modeling. Observations in the Earth's subsolar magnetosheath from the Magnetospheric Multiscale mission are used to determine the accuracy of using global models to predict the geocoronal density. On average, gas dynamic and magnetohydrodynamic (MHD) models and observations are in reasonable agreement, with differences <25%. In addition, the MHD model subsolar magnetopause is about 0.5 RE sunward of the observed location. However, variations around averages are large (up to a factor of 2), indicating that global models introduce relatively large uncertainties in geocoronal density estimates. Finally, the critical ion flux in the Interstellar Boundary Explorer IBEX‐Hi energy range is often minimally affected by fluctuations of a factor of 2 in the density.
Plain Language Summary
Scientists use a combination of in situ measurements and remote sensing of energetic neutral atoms to determine the density of hydrogen in the geocorona, which a very tenuous neutral atmosphere surrounding the Earth. This method relies on global models of the magnetosphere to determine the geocoronal density accurately. To validate these models, they are compared with measurements taken by the Magnetospheric Multiscale mission in the Earth's subsolar magnetosheath. The magnetosheath is the sheath that encompasses the Earth's magnetosphere. On average, the models and observations match reasonably well, with differences of less than 25%. However, there are significant variations around these averages, sometimes reaching twice the average value, indicating that the global models have relatively large uncertainties in the ultimate estimate of the geocoronal density. Interestingly, fluctuations in density by a factor of 2 have minimal impact on the critical ion flux in the Interstellar Boundary Explorer IBEX‐Hi energy range.
Key Points
Magnetohydrodynamic modeling tends to overestimate the subsolar magnetopause standoff distance and underestimate the magnetosheath density
The subsolar magnetopause moves constantly, even under quasi‐steady solar wind pressure, with displacements at least as large as ∼1 RE
Models for magnetosheath line‐of‐sight ion fluxes introduce uncertainties of up to a factor of 2 in the geocoronal density estimate at 10 RE
Observations near comet 67P by Rosetta show evidence of charged nanograins. Specifically, the ion and electron sensor (IES) observed negative particles with energy/charge (E/q) consistent with ...charged nanograins. However, theory predicts that both polarities should be present. On 19 September 2014, IES detected positively and negatively charged particles with E/q up to the maximum detectable by IES (~17 keV/q) coming roughly from the Sun. The arrival directions of the negative and positive particles were different and consistent with their different charges. This observation is the first simultaneous measurement of positively and negatively charged nanograins by a plasma detector in a cometary environment. As observed previously for negative nanograins coming from the comet, we find similar peaks for the picked up negative nanograins, but only one peak for the positive ones. These E/q peaks are interpreted as different charge states since they occur at discrete multiples (1:3 in this event).
Plain Language Summary
Negatively and positively charged particles are observed simultaneously by the ion and electron sensor (IES) onboard the Rosetta spacecraft soon after encountering comet 67P/Churyumov‐Gerasimenko. The two signals are shifted in opposite directions consistent with their charge sign (+/−). These signal features, together with their energies extending much higher than solar wind energies, strongly suggest that they are charged small dust grains. While negatively charged small dust particles have been detected near a comet, this is the first observation of their positively charged counterparts.
Key Points
Positively charged cometary nanograins are observed for the first time
Positive and negative nanograins are observed together but from different directions as predicted by the pickup process
Multiple peaks in the energy spectrum of picked up negative nanograins suggest multiple charge states
Reconnection X‐Line Orientations at the Earth's Magnetopause Fuselier, S. A.; Webster, J. M.; Trattner, K. J. ...
Journal of geophysical research. Space physics,
December 2021, 2021-12-00, 20211201, Letnik:
126, Številka:
12
Journal Article
Recenzirano
Observations from the magnetospheric multiscale mission in or near electron diffusion regions (EDRs) at the Earth's magnetopause are used to determine the orientation of reconnection X‐lines. The ...results highlight cross‐scale coupling of magnetic reconnection and the differences between component and anti‐parallel reconnection. These observations are consistent with a model that has a continuous, component reconnection X‐line that extends many Earth Radii (RE) across the dayside magnetopause when the interplanetary magnetic field (IMF) is southward and |BY| ∼ |BZ|. Encounters anywhere along the X‐line have similar cross‐section structure in the direction normal to the magnetopause, indicating that component reconnection is quasi‐two‐dimensional, at some scale larger than the electron scale. EDR encounters far from this primary component X‐line may be associated with transient or spatially limited reconnection structures. On the magnetopause flanks on either side of the component X‐line, there are regions where anti‐parallel reconnection occurs. These regions dominate the entire dayside magnetopause when the IMF is southward and |BY| << |BZ|. In parts of these regions, observations in or near EDRs are consistent with reconnection occurring at a series of short, anti‐parallel reconnection X‐lines. These X‐lines may be quasi‐stationary or propagating along an anti‐parallel ridge at the magnetopause. In other parts of these regions, X‐lines may be much longer.
Plain Language Summary
Magnetic reconnection is the process that couples the solar wind and the Earth's magnetosphere. This process produces long reconnection lines that stretch across the Earth's magnetopause. This paper describes the orientation of these reconnection lines.
Key Points
Magnetospheric multiscale observations are used to determine the orientation of reconnection X‐lines at the Earth's magnetopause
For southward interplanetary magnetic field and BY ∼ BZ, there is a long, continuous, component reconnection X‐line extending many RE across the dayside magnetopause
In high shear regions, there may be many, short, anti‐parallel reconnection X‐lines or a long X‐line along a high magnetic shear ridge
Tracking Magnetopause Motion Using Cold Plasmaspheric Ions LLera, K.; Fuselier, S. A.; Petrinec, S. M. ...
Journal of geophysical research. Space physics,
November 2023, 2023-11-00, 20231101, Letnik:
128, Številka:
11
Journal Article
Recenzirano
We demonstrate that any plasmaspheric/cold ions accelerated in the vicinity of the magnetopause boundary, can proxy the local magnetopause motion over many minutes. The timeseries of this motion ...capture local structures such as waves on the boundary. We determine cold ion velocities normal to full magnetopause boundary crossings for three events with varying distances to the predicted reconnection X‐line, thus, providing a proof‐of‐concept study demonstrating the potential for using cold ion velocities to track magnetopause motion over a long period of time. Obtaining the time history of the (local) motion of the magnetopause relative to the spacecraft is determined by integrating the bulk (<100 eV for H+) ion velocities normal to the boundary. Timeseries of these tracked cold ion accelerations may be used to investigate boundary layer thicknesses, potential wave structures on the magnetopause, and their evolution beyond the boundary crossing. This method generally tracks magnetopause motion out to distances of ∼1–2RE away from the spacecraft during quasi‐steady space weather conditions.
Key Points
Energized plasmaspheric cold ions effectively track the magnetopause motion continuously over many minutes
The magnetopause location and velocity are tracked reliably out to distances of ∼1–2 RE from the spacecraft given fairly consistent cold ion detectability
One of the 3 events shows quasi‐periodic magnetopause motion suggesting that this technique reveals wave propagation along the boundary
On 6–8 June 2015, the Ion and Electron Sensor on board Rosetta observed keV‐range water‐group pickup ions arriving from the solar direction. Based on magnetic field intensification and variations, ...the appearance of the ions was likely to have been caused by a coronal mass ejection. During the 3‐day period when Rosetta was 200 km from the comet, peak ion energy/charge (E/q) varied over a range from 50 eV to 1 keV in concert with neutral gas density variations caused by the rotation of the comet and its variable solar illumination. Thermal ion densities showed the same variations. The neutral density variations provided a unique opportunity to observe the repeated slowing of the solar wind by mass loading caused by charge exchange between energetic water‐group ions and thermal water‐group molecules. Such solar wind slowing was observed previously only by flyby missions that provided single events.
Plain Language Summary
The Rosetta orbiter carried a number of instruments to measure the properties of the neutral and ionized gas surrounding the nucleus. Included in these were plasma instruments to measure the characteristics of the charged particles. The Ion and Electron Sensor was one of them. Also on board were the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, the Ion Composition Analyzer, and the Langmuir Probe. This paper discusses some of the results of measurements by these instruments and their relation to each other. It was found that the neutral gas emitted by the comet nucleus and the resulting positively charged ions interact in such a way to produce slowing down of the solar wind as a result of what is called “charge exchange”, in which an electron is transferred from a neutral molecule to an ion.
Key Points
Charge exchange of energetic water group pickup ions with coma water molecules is observed within the solar wind cavity of Comet 67P Churyumov‐Gerasimenko
Rotation of the comet produced variations in the neutral density at the spacecraft causing strong charge‐exchange deceleration of the incoming ions within the density peaks
Recovery to the initial water‐group ion beam as minimum neutral densities, again appeared, was shown to result from pickup of thermal ions in the coma
The Miniaturized Electron pRoton Telescope, MERiT, is a low‐mass, low‐power, compact instrument using an innovative combination of particle detectors, sensor electronics, and onboard processing. ...MERiT is flying on the Compact Radiation belt Explorer, CeREs, a 3U CubeSat launched into a low earth orbit of 500‐km altitude and inclination of 85° on 16 December 2018. The primary and secondary science goals of CeREs are to investigate electron microbursts and to study solar particles. MERiT comprises a stack of solid state detectors (SSD) behind space facing avalanche photo diodes (APDs) surrounded by W‐Al shielding to reduce side‐penetrating particle background. The APD‐SSD combination enables measurement of electrons from 5 to 200 keV and 1 to 8 MeV; protons from 200–400 keV and 7–100 MeV in differential channels with energy resolution ΔE/E≈30% for both electrons and protons. MERiT measures microbursts with a high time resolution ranging from 4 to 16 ms and solar particles with a cadence of 1 s. MERiT energy channels and cadences are software configurable via algorithms and lookup tables residing on a field‐programmable gate array. The lookup tables can be changed via ground commands. MERiT geometry factor is 31 sq.cm‐sr and optimized to measure microbursts with the instrument viewing the local zenith in orbit. MERiT enables investigation of dynamical processes of radiation belt electron energization and loss, solar electron and proton transport, and their access to the Earth's polar caps. We describe the MERiT sensor design, calibration, operational modes, data products, and science goals.
We present partial ring distributions of solar wind protons observed by the Rosetta spacecraft at comet 67P/Churyumov‐Gerasimenko. The formation of ring distributions is usually associated with high ...activity comets, where the spatial scales are larger than multiple ion gyroradii. Our observations are made at a low‐activity comet at a heliocentric distance of 2.8 AU on 19 April 2016, and the partial rings occur at a spatial scale comparable to the ion gyroradius. We use a new visualization method to simultaneously show the angular distribution of median energy and differential flux. A fitting procedure extracts the bulk speed of the solar wind protons, separated into components parallel and perpendicular to the gyration plane, as well as the gyration velocity. The results are compared with models and put into context of the global comet environment. We find that the formation mechanism of these partial rings of solar wind protons is entirely different from the well‐known partial rings of cometary pickup ions at high‐activity comets. A density enhancement layer of solar wind protons around the comet is a focal point for proton trajectories originating from different regions of the upstream solar wind. If the spacecraft location coincides with this density enhancement layer, the different trajectories are observed as an energy‐angle dispersion and manifest as partial rings in velocity space.
Plain Language Summary
Particles of solar origin, called the “solar wind,” flow straight from the Sun in interplanetary space. When this solar wind meets an obstacle, such as a planet, it gets deflected around it. At comet 67P/Churyumov‐Gerasimenko, visited by the Rosetta spacecraft from 2014 to 2016, our instrument Rosetta Plasma Consortium (RPC)‐Ion Composition Analyzer (ICA) measured the main constituents of this solar wind: protons and alpha particles. When the comet is far away from the Sun, the solar wind protons are usually observed coming from the sunward direction with only slight deflection and constant velocities. On 19 April 2016, the main case for our study, we measure solar wind protons arriving in a wide range of directions. The velocity of these protons depends on how much they have been deflected. This creates partial ring distributions, which we visualize and quantify using a method specifically developed for this purpose. We show that these partial rings are a rare observation of a spatially confined region where solar wind protons from different regions of the solar wind are observed simultaneously.
Key Points
Broad energy spectra in our observations are due to solar wind protons forming partial ring distributions
The partial ring distributions form due to solar wind proton trajectories focusing at a density enhancement layer
From the partial ring distributions we estimate the average upstream magnetic field direction and the average bulk plasma drift velocity
We present cross‐scale magnetospheric observations of the 17 March 2015 (St. Patrick's Day) storm, by Time History of Events and Macroscale Interactions during Substorms (THEMIS), Van Allen Probes ...(Radiation Belt Storm Probes), and Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS), plus upstream ACE/Wind solar wind data. THEMIS crossed the bow shock or magnetopause 22 times and observed the magnetospheric compression that initiated the storm. Empirical models reproduce these boundary locations within 0.7 RE. Van Allen Probes crossed the plasmapause 13 times; test particle simulations reproduce these encounters within 0.5 RE. Before the storm, Van Allen Probes measured quiet double‐nose proton spectra in the region of corotating cold plasma. About 15 min after a 0605 UT dayside southward turning, Van Allen Probes captured the onset of inner magnetospheric convection, as a density decrease at the moving corotation‐convection boundary (CCB) and a steep increase in ring current (RC) proton flux. During the first several hours of the storm, Van Allen Probes measured highly dynamic ion signatures (numerous injections and multiple spectral peaks). Sustained convection after ∼1200 UT initiated a major buildup of the midnight‐sector ring current (measured by RBSP A), with much weaker duskside fluxes (measured by RBSP B, THEMIS a and THEMIS d). A close conjunction of THEMIS d, RBSP A, and TWINS 1 at 1631 UT shows good three‐way agreement in the shapes of two‐peak spectra from the center of the partial RC. A midstorm injection, observed by Van Allen Probes and TWINS at 1740 UT, brought in fresh ions with lower average energies (leading to globally less energetic spectra in precipitating ions) but increased the total pressure. The cross‐scale measurements of 17 March 2015 contain significant spatial, spectral, and temporal structure.
Key Points
Observations by THEMIS, Van Allen Probes, and TWINS contain much spatial, spectral, and temporal variation
During main phase, all three missions measured two‐peak ion spectrum in center of partial ring current
Encounters with bow shock, magnetopause (by THEMIS) and plasmapause (RBSP) reproduced by models
Low‐altitude emissions (LAEs) are the energetic neutral atom (ENA) signature of ring current ions precipitating along the magnetic field to an altitude of 200–800 km. This altitude region is ...considered to be “optically thick” because ring current ions undergo multiple charge changing interactions (MCCIs) with Earth's dense oxygen exosphere. While each interaction involves an energy loss of ~36 eV, no prior study has determined the accumulated energy lost by 1–100 keV H+ emerging as LAEs. We have developed a 2‐D model with a geomagnetic dipole that captures the net effects in energy loss and pitch angle evolution as a result of MCCIs without the computational requirements of a full Monte Carlo simulation. Dependent on the amount of latitudinal migration, the energy loss is greater than 20% for ions below 60 keV for equatorward moving particles (30 keV for poleward). Since the ENA travels ballistically across a geomagnetic dipole, upon reionization, ion velocity along the local field increases (antiparallel in the northern hemisphere). Redirecting the particle upward through MCCIs is most effective during poleward ENA motion. The net effect is to redirect precipitating ions (below 2,500 km) to eventually emerge from the optically thick region either as an ion or ENA. Precipitation is a joint ion‐neutral process, affecting both the energy and pitch angle distribution through the transverse motion of ENA segments in a converging field. For particles that enter the MCCI regime, the energy loss and evolution of the pitch angle distribution must be considered within a realistic magnetic field.
Plain Language Summary
The ring current is composed of charged energetic particles, trapped by the Earth's magnetic field (~Earth‐sized bar magnet). About 2–6 Earth radii away, these trapped particles bounce along the magnetic field line they are bound to. As they approach Earth, these energetic particles transition to higher neutral densities and lose energy switching between charged and neutral states. Although the individual energy loss is 1% or less of the original ring current energy, the accumulated energy loss is unknown. During the neutral state, the particle is no longer influenced by the magnetic field but moves in a straight path until it becomes charged again. The new location changes the charged particle's motion along a new magnetic field line. We modeled the particle's path (both charged and neutral states) and found that the energy loss is significant, specifically for particles migrating equatorward. Modeling the shape and the strength of the Earth's “bar magnet” is crucial to the process of charged particles reaching near Earth and escaping back to outer space. Emerging energetic neutrals provide an indirect way to globally measure the seemingly invisible ring current population. But an energy correction is needed to interpret observations and to model the global ring current population.
Key Points
Low‐altitude ENAs sustain significant energy loss from multiple interactions
ENAs crossing dipole field are redirected upward in altitude
Pitch angle evolution is significantly affected by inclusion of dipole field
As Rosetta was orbiting comet 67P/Churyumov‐Gerasimenko, the Ion and Electron Sensor detected negative particles with angular distributions like those of the concurrently measured solar wind protons ...but with fluxes of only about 10% of the proton fluxes and energies of about 90% of the proton energies. Using well‐known cross sections and energy‐loss data, it is determined that the fluxes and energies of the negative particles are consistent with the production of H− ions in the solar wind by double charge exchange with molecules in the coma.
Key Points
Double charge exchange of protons produces negative H ions in the solar wind
The measurements agree with published laboratory cross sections and energy deficits
The cross sections and energy deficits are estimated for the first time in the space environment
PDF