ngVLA Plasma Astrophysics Science Working Group

Welcome to the Plasma Astrophysics wiki page. The goal of this science working group is to elaborate science cases and to define requirements
for a future interferometric array.

Participants

  • Tim Bastian (NRAO)
  • Arnold Benz (ETH)
  • Paul Cassak (WVU)
  • Dale Gary (NJIT)
  • Jean Eilek (NMT/NRAO)
  • Sam Krucker (UCB)
  • Hui Li (LANL)
  • Maxim Lyutikov (Purdue)
  • Tom Jones (U Minn)
  • Gregg Hallinan (Caltech)
  • Rachel Osten (STScI)
  • Frazer Owen (NRAO)
  • Steve Spangler (U Iowa)
  • Stephen White (AFRL)
  • Phillipe Zarka (Obs Paris)
  • Ellen Zweibel (U Wisc)

Science topics/drivers

from JE: Magnetic reconnection

Commonly found in solar, space & lab context, where it is well understood as source of plasma heating (through release of stored magnetic energy). It is less familiar on extra-solar-system scales, but has recently been invoked as possible source of nonthermal, relativistic particles (e.g. simulations by Gou, Daughton, Uzdensky, others), and suggested to be relevant to settings such as radio jets and Crab Nebula flares. However, clear proof of reconnection acceleration outside of the solar system is lacking. One attractive test is the radio spectrum: relativistic particles accelerated by shocks are generally limited to relatively steep spectra, while reconnection acceleration is capable of producing flatter particle spectra. Thus, a flat radio spectrum from regions within an extended object (such as features within the Crab Nebula) would be a strong discriminant between reconnection acceleration and the "standard picture" of shock acceleration. But robust demonstration of a flat power-law spectrum requires good measurements -- from high quality, resolved images (of extended objects) iacross a broad frequency range (say a factor of ten). This is stretching what JVLA can do, but could be well suited for the extended frequency coverage and imaging capability of NGVLA.

from JE: Filaments in synchrotron sources :

Ubiquitous structures whose origin is not understood. Examples: bright radio galaxies (e.g. M87, Cygnus A), and some relics in galaxy clusters (e.g. Abell 2256). These filaments are reminiscent of magnetic flux ropes (as seen in space & lab plasmas), possibly carrying currents supported by collisional E fields. If so, they provide a new option for particle acceleration, as particles on trapped Speiser orbits fall through the potential drop. However, this idea needs testing: what is internal structure of these filaments? Are they uniform & homogeneous, or do they contain smaller-scale features (e.g. are they aggregations of many thinner pinches)? How do their spectra and (projected) magnetic fields behave on small scales? To answer these questions we need high quality, high resolution NGVLA images, with spectral and polarization information, of diffuse extended objects.

from FO: Shocks and SZ:

This is the result of some ideas stimulated by the first telecom:

ngVLA is capable of measuring the SZ effect in galaxies under ideal circumstances. That means detecting nT~10^6 with a pathlength through a hypothetical galaxy of 10kpc. However, it strikes me that a much easier observation might be detecting the SZ increase through a cluster shock. Basically the path length might be much larger and nT might increase by 10^6 or more. One should be able to see these signals in places where gas is too hot to detect the X-rays. SZ cares about nT, not T, as long as T is in the thermal range.

The key is that ngVLA can increase the spatial resolution without losing surface brightness sensitivity, assuming it has a significant part of the collecting area, like 50%, in a compact configuration. The ability to use a larger bandwidth also helps. The spatial resolution could be ~1 arcsec at the high end of the proposed band, basically 70-110 GHz. We are used to thinking about the SZ effect at 1 arcminute resolution. Thus ngVLA might well be an ideal instrument for detecting shocks in clusters with or without any non-thermal or X-ray thermal emission.

In more detail, SZ is proportional to the integral of nT through the medium doing the scattering, although calculated entirely correctly, one would need to include the relativistic particles, which scatter much less as their energy increases. This is potentially another powerful observable, i.e. the integrated thermal pressure. As I understand it, naively, one might expect for shock seen edge on to get a factor of (5M^2-1)/4)xL in the scattering brightness temperature if one could observe with a resolution equal to the width of the shock. Here M is the Mach number and L is the pathlength along the line of sight. If one saw that much signal, that would be a very big effect. In practice, referencing to simulations, one must get a more complex situation. How would we expect the brightness to change as we increase the linear resolution ? Mostly now the resolution for SZ is ~1 arcmin. That must smear out any small-scale structure in the integral of P. I am picturing a thin line in the sky with a pathlength of perhaps 100 kpc. In that case if one could resolve the line, SZ effect should be quite large. The SZ effect doesn't care about T, just P, so one should be able to see shocks where one can see no X-ray or radio emission.

In reality (or in simulation at least), what one might see is probably complex. For the purposes of the ngVLA, I think one could assume just the angular resolution and scale to a given linear resolution assming z. There is not much to simulate with the instrument. I don't think dynamic range should be an issue. To first order SZ is just another observable but only the integral of nT goes into the sum. Maybe there are simulations that already calculated the integral of P ? What we want is not quite that since we are only interested in P_thermal but something like that is what seems interesting from a simulation.

The Sun

The Sun serves as an ideal laboratory for studying plasma astrophysical phenomena and as a touchstone for similar processes in other astrophysical contexts. Below are a few examples that the ngVLA should be designed to address.

Particle acceleration

The Sun is the most powerful particle accelerator in the solar system, promptly accelerating electrons to energies >100 MeV and ions to energies in excess of 1 GeV /nuc. The two most prominent types of energetic phenomena on the Sun that accelerate electrons and ions to high energies are solar flares and coronal mass ejections (CMEs) although low-level particle acceleration occurs more or less continuously in the Sun’s corona. It is widely believed that solar flares are powered by magnetic reconnection and that the bulk of the energy released is deposited in non-thermal electrons although for some events the ions can also contain comparable amounts of energy. CMEs result from the destabilization of a magnetic flux rope and its ejection from the Sun. Fast CMEs drive a shock far into the interplanetary medium (IPM). These are believed to produce Solar Energetic Particle (SEP) events. Several broad classes of particle acceleration mechanism may be relevant to flares and CMEs. These include quasi-static electric fields, stochastic acceleration, shock drift acceleration, and diffusive shock acceleration. Energetic electrons emit copiously at radio wavelengths via coherent emission mechanisms (e.g., plasma radiation) and incoherent mechanisms (free-free and gyrosynchrotron emission) and offer a wealth of diagnostics for studying the spatiotemporal evolution of the electron distribution function, thereby imposing detailed constraints on the particle acceleration mechanism(s) involved. Measurements of the electron distribution function (as well as the ambient density and magnetic field) require time resolved, broadband, imaging spectroscopic observations of the (polarized) radio spectrum for flare/CME emissions.

Particle and energy transport

While energetic particles are highly mobile a variety of factors affect their transport in the solar atmosphere and as a result, the transport of energy within and from the environment in which they are accelerated. The bulk of energetic particles accelerated in a flare remain on the Sun. They are accelerated and injected into coronal magnetic loops. Those particles with sufficiently large pitch angles mirror and remain trapped in the coronal loop for a time; those with small pitch angles “precipitate” from the loop and collide with dense material near the foot points of the loop where they liberate their energy. Trapped particles suffer Coulomb collisions with the ambient plasma and eventually scatter to small pitch angles (weak diffusion) at which point they, too, are lost from the trap. The magnetic reconnection process driving the flare may drive a turbulent cascade; stochastic acceleration may play an important role. Alternatively, or in addition, beamed distributions of particles are unstable to the production of plasma waves. Wave-particle interactions subsequently cause particles to diffuse in momentum and pitch angle. Consequently, scattering on turbulence may strongly affect the transport of fast particles, possibly confining them. Indeed, acceleration and transport are closely intertwined in these circumstances. Thermal conduction, mass motions, and other processes must also be considered. Again, detailed observations of the evolution of the radio spectrum and polarization in time and space impose powerful constraints on transport mechanisms as well as acceleration mechanisms.

Plasma heating

Most nonthermal electrons accelerating in flares eventually lose their energy to Coulomb collisions with the relatively cool chromospheric plasma. Since only a small fraction of the energy is emitted as nonthermal HXR radiation (~10-5), most of the energy in nonthermal electrons goes toward plasma heating. The chromospheric plasma responds dynamically, a process given the misnomer “chromospheric evaporation”, which fills the magnetic loops with hot plasma (~20 MK), which emits copious thermal SXRs. Therefore, a significant fraction of the energy going into accelerated particles in flares is ultimately radiated away in the SXR band although mass motions are also important components of the energy budget. The question of plasma heating extends beyond flares - the general problem of plasma heating in the chromosphere and corona remains an outstanding problem.

Shocks

Impulsive energy release, CMEs, and other mass ejecta can produce shocks in the corona and the interplanetary medium (IPM) that subsequently accelerate particles to high energies. The initiation and propagation of shocks, as well as their role in the acceleration of particles, is of keen interest for solar and space physics, as well as a for other astrophysical phenomena (e.g., supernovae and supernova remnants). At radio wavelengths, shocks are known to drive type II radio bursts in both the corona and IPM. While their radio spectroscopic signatures have been studied for decades, imaging observations across the relevant bandwidth have been sparse (coronal type IIs) to non-existent (IPM type IIs). Since the emission from type II bursts occurs at the fundamental and/or harmonic of the local electron plasma frequency, coronal type IIs typically occur at frequencies less than a few hundred MHz, and interplanetary type IIs occur at frequencies below ~10 MHz. Imaging spectroscopy of type II radio bursts offers fundamentally new diagnostics of coronal/IPM shocks, allowing shock properties to be deduced in space and time.

Turbulence

A particularly fertile medium for the study of MHD turbulence is the solar wind. The solar wind is a supersonic outflow from the Sun carrying mass and magnetic fields. Many generations of spacecraft have studied the solar wind in situ in order to study solar wind turbulence structure and evolution, its role in energy transport and dissipation, its role in mediating particle transport, and its intermittency. Remote sensing observations at radio wavelengths also play an important and complementary role. A variety of propagation phenomena – amplitude and phase scintillations, spectral and angular broadening – have been exploited over the years to measure fundamental properties of the solar wind: its speed as a function of radius and position angle, the power spectrum of solar wind turbulence, and 3D tomography of macroscopic structures in the solar wind such as CMEs and co-rotating interaction regions. With the launch of Solar Orbiter and Solar Probe Plus in 2018, the 2020s will open a new era of solar wind studies in which ground based radio instrumentation can be positioned to play a significant role. Ground based radio instrumentation will need to provide sensitive, time-resolved, high-angular-resolution of myriad background sources to effectively exploit these techniques.

Magnetic fields and magnetic energy release

At the heart of solar activity and heating phenomena are magnetic fields. Of particular interest are coronal magnetic fields because it is in the corona that magnetic free energy is released to power flares and CMEs. Quantitative measurements of coronal magnetic fields have been nearly impossible to date because of a lack of appropriate instrumentation. It has been known for some time, however, that such measurements are possible at infrared wavelengths (using the Zeeman and Hanle effects) and at radio wavelengths using a wide variety of diagnostics. All such diagnostics have been demonstrated as “proofs of concept” but have not yet entered mainstream usage – again, because of a lack of appropriate instrumentation. Three such diagnostics will be mentioned here: 1) gyroresonance aborption at low harmonics of the electron gyrofrequency renders the corona optically thick above active regions, yielding direct measurements of the magnetic field; 2) free-free absorption in a magnetized plasma throughout the solar atmosphere can be used to measure the longitudinal component of the magnetic field in the chromosphere and corona; 3) gyrosynchrotron emission from solar flares can be used to make coronal magnetic field measurements wherever non-thermal electrons are present in the flaring source.

Implications for the ngVLA

While details are not presented here, the above types of solar and space physics observations and diagnostics require the following (specifics pending):
  • Excellent snapshot imaging
    • Dense uv coverage out to baselines of ~4 km (for solar studies)
    • Sparser coverage to several 10s to 100s of km (for solar wind studies)
  • Large instantaneous frequency bandwidth
    • 1-20 GHz for active region coronal magnetography
    • 1-50+ GHz for flare physics
  • High time resolution
    • 1-10 ms for coherent emission processes
    • 100 ms for incoherent emission and scintillations
  • High dynamic range for input signals (solar emissions can range up to 100 megaJy)
  • Excellent amplitude/flux calibration for incoherent emissions
  • Excellent polarimetry (emphasis on circularly polarized signals)
The notional ngVLA covers frequencies >1 GHz. The upper frequency limit may be as high as 100 GHz. The amount of bandwidth that is instantaneously available is TBD. Note that the discussion of shocks pointed out that coronal and interplanetary type II radio bursts occur at frequencies <1 GHz. It is unlikely that the ngVLA will extend to frequencies significantly lower than 1 GHz and so the ngVLA may not be able to contribute much to coronal shock studies. However, it is known that analogs to certain types of solar radio bursts are seen on other stars (e.g., dMe stars). Type-III-like bursts have been observed (driven by electron beams) and type-II-like bursts are actively being sought. Many of these stars have significantly denser and hotter coronae. It is quite possible that stellar type II radio bursts can be observed with the ngVLA, providing an exciting new diagnostics of shocks on other stars.

Watch this space

Telecon notes

-- TimBastian - 2015-08-05



Topic revision: r5 - 2015-09-25, TimBastian
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