Size of a plasma cloud matters

Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field.Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth’s ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field.Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed.Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field.Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.

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Cometary ion dynamics observed in the close vicinity of comet 67P/Churyumov–Gerasimenko during the intermediate activity period

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732082 ◽

2018 ◽

Vol 613 ◽

pp. A57 ◽

Cited By ~ 13

Author(s):

L. Berčič ◽

E. Behar ◽

H. Nilsson ◽

G. Nicolaou ◽

G. Stenberg Wieser ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Field ◽

Field Measurements ◽

Distribution Functions ◽

Activity Period ◽

Ion Density ◽

Ion Flow ◽

Velocity Distribution Functions ◽

Comet 67P

Aims. Cometary ions are constantly produced in the coma, and once produced they are accelerated and eventually escape the coma. We describe and interpret the dynamics of the cometary ion flow, of an intermediate active comet, very close to the nucleus and in the terminator plane. Methods. We analysed in situ ion and magnetic field measurements, and characterise the velocity distribution functions (mostly using plasma moments). We propose a statistical approach over a period of one month. Results. On average, two populations were observed, separated in phase space. The motion of the first is governed by its interaction with the solar wind farther upstream, while the second one is accelerated in the inner coma and displays characteristics compatible with an ambipolar electric field. Both populations display a consistent anti-sunward velocity component. Conclusions. Cometary ions born in different regions of the coma are seen close to the nucleus of comet 67P/Churyumov–Gerasimenko with distinct motions governed in one case by the solar wind electric field and in the other case by the position relative to the nucleus. A consistent anti-sunward component is observed for all cometary ions. An asymmetry is found in the average cometary ion density in a solar wind electric field reference frame, with higher density in the negative (south) electric field hemisphere. There is no corresponding signature in the average magnetic field strength.

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The root of a comet tail: Rosetta ion observations at comet 67P/Churyumov–Gerasimenko

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832842 ◽

2018 ◽

Vol 616 ◽

pp. A21 ◽

Cited By ~ 10

Author(s):

E. Behar ◽

H. Nilsson ◽

P. Henri ◽

L. Berčič ◽

G. Nicolaou ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Field ◽

Outer Region ◽

Field Measurements ◽

Night Side ◽

Combined Action ◽

Single Night ◽

Inner Region ◽

Comet 67P

Context. The first 1000 km of the ion tail of comet 67P/Churyumov–Gerasimenko were explored by the European Rosetta spacecraft, 2.7 au away from the Sun. Aims. We characterised the dynamics of both the solar wind and the cometary ions on the night-side of the comet’s atmosphere. Methods. We analysed in situ ion and magnetic field measurements and compared the data to a semi-analytical model. Results. The cometary ions are observed flowing close to radially away from the nucleus during the entire excursion. The solar wind is deflected by its interaction with the new-born cometary ions. Two concentric regions appear, an inner region dominated by the expanding cometary ions and an outer region dominated by the solar wind particles. Conclusions. The single night-side excursion operated by Rosetta revealed that the near radial flow of the cometary ions can be explained by the combined action of three different electric field components, resulting from the ion motion, the electron pressure gradients, and the magnetic field draping. The observed solar wind deflection is governed mostly by the motional electric field −uion × B.

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Steepening of magnetosonic waves in the inner coma of comet 67P/Churyumov–Gerasimenko

Annales Geophysicae ◽

10.5194/angeo-39-721-2021 ◽

2021 ◽

Vol 39 (4) ◽

pp. 721-742

Author(s):

Katharina Ostaszewski ◽

Karl-Heinz Glassmeier ◽

Charlotte Goetz ◽

Philip Heinisch ◽

Pierre Henri ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Interaction Region ◽

Occurrence Rate ◽

Solar Wind Interaction ◽

Background Magnetic Field ◽

The Waves ◽

Comet 67P ◽

The Magnetic Field ◽

Field Enhancements

Abstract. We present a statistical survey of large-amplitude, asymmetric plasma and magnetic field enhancements detected outside the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko from December 2014 to June 2016. Based on the concurrent observations of plasma and magnetic field enhancements, we interpret them to be magnetosonic waves. The aim is to provide a general overview of these waves' properties over the mission duration. As the first mission of its kind, the ESA Rosetta mission was able to study the plasma properties of the inner coma for a prolonged time and during different stages of activity. This enables us to study the temporal evolution of these waves and their characteristics. In total, we identified ∼ 70 000 steepened waves in the magnetic field data by means of machine learning. We observe that the occurrence of these steepened waves is linked to the activity of the comet, where steepened waves are primarily observed at high outgassing rates. No clear indications of a relationship between the occurrence rate and solar wind conditions were found. The waves are found to propagate predominantly perpendicular to the background magnetic field, which indicates their compressional nature. Characteristics like amplitude, skewness, and width of the waves were extracted by fitting a skew normal distribution to the magnetic field magnitude of individual steepened waves. With increasing mass loading, the average amplitude of the waves decreases, while the skewness increases. Using a modified 1D magnetohydrodynamic (MHD) model, we investigated if the waves can be described by the combination of nonlinear and dissipative effects. By combining the model with observations of amplitude, width and skewness, we obtain an estimate of the effective plasma diffusivity in the comet–solar wind interaction region and compare it with suitable reference values as a consistency check. At 67P/Churyumov–Gerasimenko, these steepened waves are of particular importance as they dominate the innermost interaction region for intermediate to high activity.

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Statistical analysis of the accelerated H2O ions above 1 keV: the comet 67P/Churyumov–Gerasimenko observed by the Rosetta spacecraft.

10.5194/egusphere-egu21-378 ◽

2021 ◽

Author(s):

Tsubasa Kotani ◽

Masatoshi Yamauchi ◽

Hans Nilsson ◽

Gabriella Stenberg-Wieser ◽

Martin Wieser ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Statistical Analysis ◽

The Sun ◽

Ion Composition ◽

Solar Winds ◽

Comet Activity ◽

Parallel Acceleration ◽

Comet 67P ◽

The Magnetic Field

The ESA/Rosetta spacecraft has studied the comet 67P/Churyumov-Gerasimenko for two years. Rosetta Plasma Consortium's Ion Composition Analyser (RPC/ICA) detected comet-origin water ions that are accelerated to > 100 eV.&#160; Majority of them are interpreted as ordinary pick-up acceleration&#160; by the solar wind electric field perpendicular to the magnetic field during low comet activity [1,2]. As the comet approaches the sun, a comet magnetosphere is formed, where solar winds cannot intrude.However,&#160; some water ions are accelerated to > 1 keV even in the magnetosphere [3]. Using RPC/ICA data during two years [4], we investigate the acceleration events > 1 keV where solar winds are not observed, and classify dispersion events with respect to the directions of the sun, the comet, and the magnetic field.&#160; Majority of these water ions show reversed energy-angle dispersion. Results of the investigation also show that these ions are flowing along the (enhanced) magnetic field, indicating that the parallel acceleration occurs in the magnetosphere.In this meeting, we show a statistical analysis and discuss a possible acceleration mechanism.References[1] H. Nilsson et al., MNRAS 469, 252 (2017), doi:10.1093/mnras/stx1491[2] G. Nicolau et al., MNRAS 469, 339 (2017), doi:10.1093/mnras/stx1621[3] T. Kotani et al., EPSC, EPSC2020-576 (2020), https://doi.org/10.5194/epsc2020-576[4] H. Nilsson et al., Space Sci. Rev., 128, 671 (2007), DOI: 10.1007/s11214-006-9031-z&#160;

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Using Compressibility and Electric Field to Characterize Circularly-Polarized Waves Near the Proton Cyclotron Frequency Observed by Solar Orbiter

10.5194/egusphere-egu21-15746 ◽

2021 ◽

Author(s):

Yuri Khotyaintsev ◽

Daniel B Graham ◽

Konrad Steinvall ◽

Andris Vaivads ◽

Milan Maksimovic ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Field ◽

Electromagnetic Waves ◽

Cyclotron Frequency ◽

Density Fluctuations ◽

Background Magnetic Field ◽

The Waves ◽

The Magnetic Field ◽

Proton Cyclotron

We report Solar Orbiter observations of electromagnetic waves near the proton cyclotron frequency during the first perihelion. The waves have polarization close to circular and have wave vectors closely aligned with the background magnetic field. Such waves are potentially important for heating of the solar wind as their frequency and polarization allows effective energy exchange with solar wind protons. The Radio and Plasma Waves (RPW) instrument provides a high-cadence measurement of plasma density and electric field which we use together with the magnetic field measured by MAG to characterize these waves. In particular we compute the compressibility and the phase between the density fluctuations and the parallel component of the magnetic field, and show that these have a distinct behavior for the waves compared to the Alfv&#233;nic turbulence. We compare the observations to multi-fluid plasma dispersion and identify the waves modes corresponding to the observed waves. We discuss the importance of the waves for solar wind heating.

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Voyager Observations of the Magnetic Field, Interstellar Pickup Ions and Solar Wind in the Distant Heliosphere

The Heliosphere in the Local Interstellar Medium ◽

10.1007/978-94-009-1782-8_5 ◽

1996 ◽

pp. 33-42

Author(s):

L. F. Burlaga ◽

N. F. Ness ◽

J. W. Belcher ◽

A. J. Lazarus ◽

J. D. Richardson

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Pickup Ions ◽

The Magnetic Field

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Magnetospheric response to solar wind driving

10.5194/egusphere-egu21-8951 ◽

2021 ◽

Author(s):

Tatiana Výbošťoková ◽

Zdeněk Němeček ◽

Jana Šafránková

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Field ◽

Geomagnetic Storms ◽

Interplanetary Space ◽

Time Lags ◽

Interplanetary Shocks ◽

Geomagnetic Indices ◽

The Earth ◽

The Magnetic Field

Interaction of solar events propagating throughout the interplanetary space with the magnetic field of the Earth may result in disruption of the magnetosphere. Disruption of the magnetic field is followed by the formation of the time-varying electric field and thus electric current is induced in Earth-bound structures such as transmission networks, pipelines or railways. In that case, it is necessary to be able to predict a future state of the magnetosphere and magnetic field of the Earth. The most straightforward way would use geomagnetic indices. Several studies are investigating the relationship of the response of the magnetosphere to changes in the solar wind with motivation to give a more accurate prediction of geomagnetic indices during geomagnetic storms. To forecast these indices, different approaches have been attempted--from simple correlation studies to neural networks.We study the effects of interplanetary shocks observed at L1 on the Earth's magnetosphere with a database of tens of shocks between 2009 and 2019. Driving the magnetosphere is described as integral of reconnection electric field for each shock. The response of the geomagnetic field is described with the SYM-H index. We created an algorithm in Python for prediction of the magnetosphere state based on the correlation of solar wind driving and magnetospheric response and found that typical time-lags range between tens of minutes to maximum 2 hours. The results are documented by a large statistical study.

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Steepening of magnetosonic waves in the inner coma of comet 67P/Churyumov-Gerasimenko

10.5194/angeo-2020-84 ◽

2020 ◽

Author(s):

Katharina Ostaszewski ◽

Karl-Heinz Glassmeier ◽

Charlotte Goetz ◽

Philip Heinisch ◽

Pierre Henri ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Interaction Region ◽

Occurrence Rate ◽

Solar Wind Interaction ◽

Plasma Properties ◽

Background Magnetic Field ◽

The Waves ◽

Comet 67P ◽

The Magnetic Field

Abstract. We present a statistical survey of large amplitude, asymmetric plasma, and magnetic field enhancements at comet 67P/Churyumov-Gerasimenko from December 2014 to June 2016. The aim is to provide a general overview of these structures' properties over the mission duration. At comets, nonlinear wave evolution plays an integral part in the development of turbulence and in particular facilitates the transfer of energy and momentum. As the first mission of its kind, the ESA Rosetta mission was able to study the plasma properties of the inner coma for a prolonged time and during different stages of activity. This enables us to study the temporal evolution of steepened waves and their characteristics. In total, we identified ~70000 events in the magnetic field data by means of machine learning. We observe that the occurrence of wave events is linked to the activity of the comet, where events are primarily observed at high outgassing rates. No clear indications of a relationship between the occurrence rate and solar wind conditions were found. The waves are found to propagate predominantly perpendicular to the background magnetic field, which indicates their compressive nature. Characteristics like amplitude, skewness, and width of the waves were extracted by fitting a skew normal distribution to the magnetic field magnitude of individual events. With increasing massloading the average amplitude of steepened waves decreases while the skewness increases. Using a modified 1D MHD model it was possible to show that such solitary structures can be described by the combination of nonlinear, dispersive, and dissipative effects. By combining the model with observations of amplitude, width, and skewness we obtain an estimate of the effective plasma viscosity in the comet-solar wind interaction region. At 67P/Churyumov-Gerasimenko steepened waves are of particular importance as they dominate the innermost interaction region for intermediate to high activity.

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Solar wind protons in the diamagnetic cavity at comet 67P/Churyumov-Gerasimenko

10.5194/epsc2021-344 ◽

2021 ◽

Author(s):

Charlotte Goetz ◽

Lucie Scharre ◽

Cyril Simon-Wedlund ◽

Hans Nilsson ◽

Elias Odelstad ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Gas Production ◽

Alpha Particles ◽

Cometary Plasma ◽

Plasma Environment ◽

Rosetta Mission ◽

Show Evidence ◽

Solar Wind Origin ◽

Comet 67P

Against expectations, the Rosetta spacecraft was able to observe protons of solar wind origin in the diamagnetic cavity at comet 67P/Churyumov-Gerasimenko. This study investigates these unexpected observations and gives a working hypothesis on what could be the underlying cause. The cometary plasma environment of a comet is shaped by two distinct plasma populations: the solar wind, consisting of protons, alpha particles, electrons and a magnetic field, and the cometary plasma, consisting of heavy ions such as water ions or carbon dioxide ions and electrons.&#160; As the comet follows its orbit through the solar system, the amount of cometary ions that is produced varies significantly. This means that the plasma environment of the comet and the boundaries that form there are also dependent on the comet's heliocentric distance.&#160; For example, at sufficiently high gas production rates (close to the Sun) the protons from the solar wind are prevented from entering the inner coma entirely. The region where no protons (and other solar wind origin ions) can be detected is referred to as the solar wind ion cavity.&#160; A second example is the diamagnetic cavity, a region very close to the nucleus of the comet, where the interplanetary magnetic field, which is carried by the solar wind electrons, cannot penetrate the densest part of the cometary plasma.&#160; The Rosetta mission clearly showed that the solar wind ion cavity is larger than the diamagnetic cavity at a comet such as 67P/Churyumov-Gerasimenko. However, this new study finds that in isolated incidences this order can be reversed and ions of solar wind origin (mostly protons, but also helium) can be detected inside the diamagnetic cavity. We present the observations pertaining to these events and list and discard possible mechanisms that could lead to such a configuration. Only one mechanism cannot be discarded: that of a solar wind configuration where the solar wind velocity is aligned with the magnetic field. We show evidence that fits this hypothesis as well as solar wind models in support.&#160;

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