Radio remote sensing of the corona and the solar wind

AbstractModern radio telescopes are extremely sensitive to plasma on the line of sight from a radio source to the antenna. Plasmas in the corona and solar wind produce measurable changes in the radio wave amplitude and phase, and the phase difference between wave fields of opposite circular polarization. Such measurements can be made of radio waves from spacecraft transmitters and extragalactic radio sources, using radio telescopes and spacecraft tracking antennas. Data have been taken at frequencies from about 80 MHz to 8000 MHz. Lower frequencies probe plasma at greater heliocentric distances. Analysis of these data yields information on the plasma density, density fluctuations, and plasma flow speeds in the corona and solar wind, and on the magnetic field in the solar corona. This paper will concentrate on the information that can be obtained from measurements of Faraday rotation through the corona and inner solar wind. The magnitude of Faraday rotation is proportional to the line of sight integral of the plasma density and the line-of-sight component of the magnetic field. Faraday rotation provides an almost unique means of estimating the magnetic field in this part of space. This technique has contributed to measurement of the large scale coronal magnetic field, the properties of electromagnetic turbulence in the corona, possible detection of electrical currents in the corona, and probing of the internal structure of coronal mass ejections (CMEs). This paper concentrates on the search for small-scale coronal turbulence and remote sensing of the structure of CMEs. Future investigations with the Expanded Very Large Array (EVLA) or Murchison Widefield Array (MWA) could provide unique observational input on the astrophysics of CMEs.

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Solar Wind Measurements from the Planetary Magnetometer Onboard the BepiColombo Spacecraft

10.5194/egusphere-egu2020-10707 ◽

2020 ◽

Author(s):

Daniel Heyner ◽

Ingo Richter ◽

Ferdinand Plaschke ◽

David Fischer ◽

Johannes Mieth ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Propulsion ◽

Small Scale ◽

Solar Electric Propulsion ◽

Scientific Analysis ◽

Wind Measurements ◽

Propulsion Phase ◽

The Magnetic Field ◽

Magnetic Disturbances

BepiColombo is en-route to Mercury. The boom carrying the planetary magnetometers (MPO-MAG instrument) was deployed in space on 25th of October in 2018. After the deployment, the magnetic disturbances arising from the spacecraft have been greatly decreased. Since the deployment, the fluxgate sensors have been monitoring the magnetic field continuously except for the solar electric propulsion phase. Extensive calibration and data processing activities have since enabled us to greatly decrease spacecraft-generated disturbances in the magnetic field observations; these activities constitute a key step towards making the data suitable for scientific analysis. We present a few cases of identified magnetic disturbances, discuss the challenges they pose, and compare methods to clean the data. We also compare MPO-MAG measurements to observations by the Advanced Composition Explorer (ACE) solar wind monitor, thereby highlighting the small-scale nature and rapid evolution of interplanetary magnetic field (IMF) variations. We conclude with an overview of the scientific goals of the instrument team for the in-orbit mission phase.

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Fluctuation dynamos and their Faraday rotation signatures

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314011697 ◽

2012 ◽

Vol 10 (H16) ◽

pp. 400-400

Author(s):

Pallavi Bhat ◽

Kandaswamy Subramanian

Keyword(s):

Magnetic Field ◽

Simple Model ◽

Faraday Rotation ◽

High Field ◽

Line Of Sight ◽

Simulation Data ◽

Thermal Electron ◽

Volume Filling ◽

Rotation Measure ◽

The Magnetic Field

We study fluctuation dynamo (FD) action in turbulent systems like galaxy-clusters focusing on the Faraday rotation signature. This is defined as RM = K ∫LneB ⋅ dl where ne is the thermal electron density, B is the magnetic field, the integration is along the line of sight from the source to the observer, and K = 0.81 rad m−2 cm−3 μG−1 pc−1. We directly compute, using the simulation data, ∫ B ⋅ dl, and hence the Faraday rotation measure (RM) over 3N2 lines of sight, along each x, y and z-directions. We normalise the RM by the rms value expected in a simple model, where a field of strength Brms fills each turbulent cell but is randomly oriented from one turbulent cell to another. This normalised RM is expected to have a nearly zero mean but a non-zero dispersion, σRM. We show in Fig. 1a and 1b, that a suite of simulations, on saturation, obtain the value of σRM = 0.4−0.5, and this is independent of PM, RM and the resolution of the run. This is a fairly large value for an intermittent random field; as it is of order 40%–50%, of that expected in a model where Brms strength fields volume fill each turbulent cell, but are randomly oriented from one cell to another. We also find that the regions with a field strength larger than 2Brms contribute only 15–20% to the total RM (see Fig. 1a). This shows that it is the general ‘sea’ of volume filling fluctuating fields that contribute dominantly to the RM produced, rather than the the high field regions.

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Diamagnetic structures as a basis of quasi-stationary slow solar wind

Solnechno-Zemnaya Fizika ◽

10.12737/szf-53201904 ◽

2019 ◽

Vol 5 (3) ◽

pp. 36-49

Author(s):

Виктор Еселевич ◽

Viktor Eselevich

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Plasma Density ◽

Neutral Line ◽

Angular Size ◽

Radial Component ◽

Slow Solar Wind ◽

Small Scale ◽

The Sun ◽

Streamer Belt

The results presented in this review reflect the fundamentals of the modern understanding of the nature of the structure of the slow solar wind (SW) along the entire length from the Sun to the Earth's orbit. It is known that the source of the slow quasi-stationary SW on the Sun is the belt and the chains of coronal streamers The streamer belt encircles the entire Sun as a wave-like surface (skirt), representing a sequence of pairs of rays with increased brightness (plasma density) or two lines of rays located close to each other. Neutral line of the radial component of the solar global magnetic field goes along the belt between the rays of each of these pairs. The streamer belt extends in the heliosphere is as the heliospheric plasma sheet (HPS). Detailed analysis of data from Wind and IMP-8 satellites showed that HPS sections on the Earth orbit are registered as a sequence of diamagnetic tubes with high density plasma and low interplanetary magnetic field. They represent an extension of rays with increased brightness of the streamer belt near the Sun. Their angular size remains the same over the entire way from the Sun to the Earth's orbit. Each HPS diamagnetic tube has a fine internal structure on several scales, or fractality. In other words, diamagnetic tube is a set of nested diamagnetic tubes, whose angular size can vary by almost two orders of magnitude. These sequences of diamagnetic tubes that form the base of slow SW on the Earth's orbit has a more general name — diamagnetic structures (DS). In the final part of this article, a comparative analysis of several events was made, based on the results of this review. He made it possible to find out the morphology and nature of the origin of the new term “diamagnetic plasmoids” SW (local amplifications of plasma density), which appeared in several articles published during 2012–2018. The analysis carried out at the end of this article, for the first time, showed that the diamagnetic plasmoids SW are the small-scale component of the fractal diamagnetic structures of the slow SW, considered in this review.

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Synthetic observations of spiral arm tracers of a simulated Milky Way analog

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037690 ◽

2020 ◽

Vol 642 ◽

pp. A201 ◽

Cited By ~ 1

Author(s):

S. Reissl ◽

J. M. Stil ◽

E. Chen ◽

R. G. Treß ◽

M. C. Sormani ◽

...

Keyword(s):

Magnetic Field ◽

Interstellar Medium ◽

Faraday Rotation ◽

Milky Way ◽

Line Of Sight ◽

Unit Distance ◽

Spiral Arms ◽

Rotation Measure ◽

The Magnetic Field ◽

Spiral Arm

Context. The Faraday rotation measure (RM) is often used to study the magnetic field strength and orientation within the ionized medium of the Milky Way. Recent observations indicate an RM magnitude in the spiral arms that exceeds the commonly assumed range. This raises the question of how and under what conditions spiral arms create such strong Faraday rotation. Aims. We investigate the effect of spiral arms on Galactic Faraday rotation through shock compression of the interstellar medium. It has recently been suggested that the Sagittarius spiral arm creates a strong peak in Faraday rotation where the line of sight is tangent to the arm, and that enhanced Faraday rotation follows along side lines which intersect the arm. Here our aim is to understand the physical conditions that may give rise to this effect and the role of viewing geometry. Methods. We apply a magnetohydrodynamic simulation of the multi-phase interstellar medium in a Milky Way-type spiral galaxy disk in combination with radiative transfer in order to evaluate different tracers of spiral arm structures. For observers embedded in the disk, dust intensity, synchrotron emission, and the kinematics of molecular gas observations are derived to identify which spiral arm tangents are observable. Faraday rotation measures are calculated through the disk and evaluated in the context of different observer positions. The observer’s perspectives are related to the parameters of the local bubbles surrounding the observer and their contribution to the total Faraday rotation measure along the line of sight. Results. We reproduce a scattering of tangent points for the different tracers of about 6° per spiral arm similar to the Milky Way. For the RM, the model shows that compression of the interstellar medium and associated amplification of the magnetic field in spiral arms enhances Faraday rotation by a few hundred rad m−2 in addition to the mean contribution of the disk. The arm–interarm contrast in Faraday rotation per unit distance along the line of sight is approximately ~10 in the inner Galaxy, fading to ~2 in the outer Galaxy in tandem with the waning contrast of other tracers of spiral arms. We identify a shark fin pattern in the RM Milky Way observations and in the synthetic data that is characteristic for a galaxy with spiral arms.

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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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Constraining magnetic fields in galaxy clusters

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319004459 ◽

2018 ◽

Vol 14 (A30) ◽

pp. 299-302

Author(s):

Annalisa Bonafede ◽

Chiara Stuardi ◽

Federica Savini ◽

Franco Vazza ◽

Marcus Brüggen

Keyword(s):

Magnetic Field ◽

Particle Acceleration ◽

Magnetic Fields ◽

Galaxy Clusters ◽

Faraday Rotation ◽

Small Scale ◽

Radio Sources ◽

Synchrotron Emission ◽

Combined Approach ◽

The Magnetic Field

AbstractMagnetic fields originate small-scale instabilities in the plasma of the intra-cluster medium, and may have a key role to understand particle acceleration mechanisms. Recent observations at low radio frequencies have revealed that synchrotron emission from galaxy clusters is more various and complicated than previously thought, and new types of radio sources have been observed. In the last decade, big steps forward have been done to constrain the magnetic field properties in clusters thanks to a combined approach of polarisation observations and numerical simulations that aim to reproduce Faraday Rotation measures of sources observed through the intra-cluster medium. In this contribution, I will review the results on magnetic fields reached in the last years, and I will discuss the assumptions that have been done so far in light of new results obtained from cosmological simulations. I will also discuss how the next generation of radio instruments, as the SKA, will help improving our knowledge of the magnetic field in the intra-cluster medium.

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Polarization study of the pulsars in the globular cluster 47 Tucanae

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317008389 ◽

2017 ◽

Vol 13 (S337) ◽

pp. 295-298

Author(s):

F. Abbate ◽

A. Possenti ◽

C. Tiburzi ◽

W. van Straten ◽

E. Barr ◽

...

Keyword(s):

Magnetic Field ◽

Globular Cluster ◽

Galactic Disk ◽

Line Of Sight ◽

Radio Telescopes ◽

Parallel Component ◽

Polarization Study ◽

Rotation Measure ◽

Linearly Polarized ◽

The Magnetic Field

AbstractThe linearly polarized component of a pulsar signal at different radio frequencies can help to constrain the parallel component of the magnetic field along the line of sight. In this work we measured the polarimetric properties of the pulsars in the globular cluster 47 Tucanae and we report the Rotation Measure (RM) for 13 of them. A gradient in the RM values of the pulsars across the cluster is detected suggesting the presence of significant variations in the magnetic field across the very small angular scales associated with the lines of sight to the pulsars in 47 Tucanae. Both magnetic fields located in the globular cluster or in the Galactic disk in the direction of the cluster are taken into consideration. However, more detailed modelling of the dynamics of the cluster and deeper observations with the MeerKAT and/or the SKA1 radio telescopes are necessary to discriminate among the models.

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Magnetic topology of the north solar pole

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832751 ◽

2018 ◽

Vol 616 ◽

pp. A46 ◽

Cited By ~ 3

Author(s):

A. Pastor Yabar ◽

M. J. Martínez González ◽

M. Collados

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Field Vector ◽

Line Of Sight ◽

Small Scale ◽

Quiet Sun ◽

The North ◽

Field Distributions ◽

Disc Centre ◽

The Magnetic Field

The magnetism at the poles is similar to that of the quiet Sun in the sense that no active regions are present there. However, the polar quiet Sun is somewhat different from that at the activity belt as it has a global polarity that is clearly modulated by the solar cycle. We study the polar magnetism near an activity maximum when these regions change their polarity, from which it is expected that its magnetism should be less affected by the global field. To fully characterise the magnetic field vector, we use deep full Stokes polarimetric observations of the 15 648.5 and 15 652.8 Å FeI lines. We observe the north pole as well as a quiet region at disc centre to compare their field distributions. In order to calibrate the projection effects, we observe an additional quiet region at the east limb. We find that the two limb datasets share similar magnetic field vector distributions. This means that close to a maximum, the poles look like typical limb, quiet-Sun regions. However, the magnetic field distributions at the limbs are different from the distribution inferred at disc centre. At the limbs, we infer a new population of magnetic fields with relatively strong intensities (~600−800 G), inclined by ~30° with respect to the line of sight, and with an azimuth aligned with the solar disc radial direction. This line-of-sight orientation interpreted as a single magnetic field gives rise to non-vertical fields in the local reference frame and aligned towards disc centre. This peculiar topology is very unlikely for such strong fields according to theoretical considerations. We propose that this new population at the limbs is due to the observation of unresolved magnetic loops as seen close to the limb. These loops have typical granular sizes as measured in the disc centre. At the limbs, where the spatial resolution decreases, we observe them spatially unresolved, which explains the new population of magnetic fields that is inferred. This is the first (indirect) evidence of small-scale magnetic loops outside the disc centre and would imply that these small-scale structures are ubiquitous on the entire solar surface. This result has profound implications for the energetics not only of the photosphere, but also of the outer layers since these loops have been reported to reach the chromosphere and the low corona.

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Hα Structures and Small-Scale Magnetic Field Configurations

Symposium - International Astronomical Union ◽

10.1017/s0074180900022762 ◽

1971 ◽

Vol 43 ◽

pp. 323-328 ◽

Cited By ~ 1

Author(s):

Sara F. Smith

Keyword(s):

Magnetic Field ◽

Fine Structure ◽

Line Of Sight ◽

Small Scale ◽

Field Observations ◽

Birefringent Filter ◽

Mixed Polarity ◽

Polarity Change ◽

The Magnetic Field

Magnetic field observations made utilizing a 1/7 Å birefringent filter for the FeI line at 5324 Å resolve magnetic field features comparable in scale to the Hα fine structure. Without exception, low lying filaments in the center of the disk coincide with boundaries of polarity change in the line of sight component of the magnetic field. Small filaments can be identified on a scale comparable to the Hα fibril structures. The arch filament systems are associated with areas of mixed polarity. ‘Satellite spots’ may sometimes be identified as small localized enhancements of magnetic field of both polarities around the perimeter of a sunspot.

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Size of a plasma cloud matters

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833199 ◽

2018 ◽

Vol 616 ◽

pp. A50 ◽

Cited By ~ 13

Author(s):

H. Nilsson ◽

H. Gunell ◽

T. Karlsson ◽

N. Brenning ◽

P. Henri ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Field ◽

Plasma Density ◽

Cloud Model ◽

Parameter Range ◽

Pickup Ions ◽

Rosetta Mission ◽

Comet 67P ◽

The Magnetic Field

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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