Particle acceleration at astrophysical shocks: A theory of cosmic ray origin

The central idea of diffusive shock acceleration is presented from microscopic and macroscopic viewpoints; applied to reactionless test particles in a steady plane shock the mechanism is shown to produce a power law spectrum in momentum with a slope which, to lowest order in the ratio of plasma to particle speed, depends only on the compression in the shock. The associated time scale is found (also by a macroscopic and a microscopic method) and the problems of spherical shocks, as exemplified by a point explosion and a stellar-wind terminator, are treated by singular perturbation theory. The effect of including the particle reaction is then studied. It is shown that if the scattering is due to resonant waves these can rapidly grow with unknown consequences. The possible steady modified shock structures are classified and generalised Rankine-Hugoniot conditions found. Modifications of the spectrum are discussed on the basis of an exact, if rather artificial, solution, a high-energy asymptotic expansion and a perturbation expansion due to Blandford. It is pointed out that no steady solution can exist for very strong shocks; the possible time dependence is briefly discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/46/8/002/pdf

An introduction to the theory of diffusive shock acceleration of energetic particles in tenuous plasmas

The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft provides simultaneous spectra and images of the photosphere, chromosphere, transition region, and corona with 0.33 – 0.4 arcsec spatial resolution, two-second temporal resolution, and 1 km s−1 velocity resolution over a field-of-view of up to 175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains spectra in passbands from 1332 – 1358 A, 1389 – 1407 A, and 2783 – 2834 A, including bright spectral lines formed in the chromosphere (Mg ii h 2803 A and Mg ii k 2796 A) and transition region (C ii 1334/1335 A and Si iv 1394/1403 A). Slit-jaw images in four different passbands (C ii 1330, Si iv 1400, Mg ii k 2796, and Mg ii wing 2830 A) can be taken simultaneously with spectral rasters that sample regions up to 130 arcsec × 175 arcsec at a variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to emission from plasma at temperatures between 5000 K and 10 MK and will advance our understanding of the flow of mass and energy through an interface region, formed by the chromosphere and transition region, between the photosphere and corona. This highly structured and dynamic region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona and solar wind combined. The IRIS investigation includes a strong numerical modeling component based on advanced radiative–MHD codes to facilitate interpretation of observations of this complex region. Approximately eight Gbytes of data (after compression) are acquired by IRIS each day and made available for unrestricted use within a few days of the observation.

/pdf/the-interface-region-imaging-spectrograph-iris-52t0ocd17t.pdf

The Interface Region Imaging Spectrograph (IRIS)

The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft provides simultaneous spectra and images of the photosphere, chromosphere, transition region, and corona with 0.33-0.4 arcsec spatial resolution, 2 s temporal resolution and 1 km/s velocity resolution over a field-of-view of up to 175 arcsec x 175 arcsec. IRIS was launched into a Sun-synchronous orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains spectra in passbands from 1332-1358, 1389-1407 and 2783-2834 Angstrom including bright spectral lines formed in the chromosphere (Mg II h 2803 Angstrom and Mg II k 2796 Angstrom) and transition region (C II 1334/1335 Angstrom and Si IV 1394/1403 Angstrom). Slit-jaw images in four different passbands (C II 1330, Si IV 1400, Mg II k 2796 and Mg II wing 2830 Angstrom) can be taken simultaneously with spectral rasters that sample regions up to 130 arcsec x 175 arcsec at a variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to emission from plasma at temperatures between 5000 K and 10 MK and will advance our understanding of the flow of mass and energy through an interface region, formed by the chromosphere and transition region, between the photosphere and corona. This highly structured and dynamic region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona and solar wind combined. The IRIS investigation includes a strong numerical modeling component based on advanced radiative-MHD codes to facilitate interpretation of observations of this complex region. Approximately eight Gbytes of data (after compression) are acquired by IRIS each day and made available for unrestricted use within a few days of the observation.

/pdf/the-interface-region-imaging-spectrograph-iris-2piw9mvbd7.pdf

Alfven waves have been invoked as a possible mechanism for the heating of the Sun's outer atmosphere, or corona, to millions of degrees and for the acceleration of the solar wind to hundreds of kilometers per second. However, Alfven waves of sufficient strength have not been unambiguously observed in the solar atmosphere. We used images of high temporal and spatial resolution obtained with the Solar Optical Telescope onboard the Japanese Hinode satellite to reveal that the chromosphere, the region sandwiched between the solar surface and the corona, is permeated by Alfven waves with strong amplitudes on the order of 10 to 25 kilometers per second and periods of 100 to 500 seconds. Estimates of the energy flux carried by these waves and comparisons with advanced radiative magnetohydrodynamic simulations indicate that such Alfven waves are energetic enough to accelerate the solar wind and possibly to heat the quiet corona.

https://science.sciencemag.org/content/sci/318/5856/1574.full.pdf

Chromospheric alfvenic waves strong enough to power the solar wind.

Standard coronal heating mechanisms such as dissipation of acoustic waves generated in turbulent photospheric layers, or dissipative heating by current sheets and resonant absorption (see proceedings of Heidelberg conference /1/) cannot produce high speed solar wind streams We propose that the source of energy in these streams lies in the regions of strong magnetic field which define the boundaries of the chromospheric supergranulation network pattern If the magnetic field in these “trunks” is not strictly unipolar but instead also contains closed loops then the available free energy can be readily released in impulsive reconnection events (“microflares”) which give rise to high frequency hydromagnetic waves with periods of the order of or less than one second It appears necessary to invoke a strong source of relatively high frequency waves to allow substantial dissipation in the corona itself and also to heat selectively species of differing charge to mass ratios by cyclotron interactions Moreover such wave-particle interactions are also necessary to account for the detailed properties of high speed streams in which the perpendicular temperature exceeds the parallel temperature, ionic species tend to have the same temperature per unit mass and heavier species tend to move faster than the protons by about the Alfven speed It is noteworthy that the same source mechanism for generating the high frequency waves may also be responsible for an enhancement of low FIP ions if the required closed loops of magnetic flux emerge in the transition region where the temperature is between 104 and 105 °K

The origin of high speed solar wind streams

The structure of cosmic ray shocks

A two-fluid model for the fast solar wind is described which attempts to account for observations at large distances and close to the Sun in coronal holes, allowing for anisotropic proton temperature, Alfven wave pressure and a realistic magnetic field, but neglecting the effects of proton heat conduction and viscosity. The resulting acceleration of the wind near the Sun is quite rapid, although somewhat less so than in our previous models where the wave pressure was neglected. The proton temperatures in the corona remain relatively high and exceed the associated wave “temperature” out to a heliocentric distance of about 5 solar radii. The proton temperature anisotropy may become very large if the effective particle scattering rate is small and it is suggested that in fact there may be significant particle-wave scattering in the corona at least.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GL02097

The fast solar wind

We describe a simple analytic model for the mag- netic field in the solar corona and interplanetary space which is appropriate to solar minimum conditions. The model com- bines an azimuthal current sheet in the equatorial plane with an axisymmetric multipole field representing the internal mag- netic field of the Sun. The radial component of the field filling interplanetary space is approximately monopolar at large helio- centric distances as observed. These open field lines connect to the polar regions of the Sun and define the polar coronal holes which are prevalent at solar minimum and which are the source of the fast solar wind. By including both dipole and a quadrupole terms at the origin it is possible to construct a good representa- tion of the coronal magnetic field in such conditions. We also note that the Parker spiral will be underwound relative to the case of the monopole because the open field lines emanate from solar latitudes in excess of 60.

http://aa.springer.de/papers/8337003/2300940.pdf

An analytic solar magnetic field model

We outline a theory for the origin and acceleration of the fast solar wind as a consequence of network microflares releasing a spectrum of high frequency Alfven waves which heat (by cyclotron absorption) the corona close to the Sun. The significant features of our model of the fast wind are that the acceleration is rapid with the sonic point at around two solar radii, the proton temperatures are high (~ 5 million degrees) and the minor ions are correspondingly hotter, roughly in proportion to their mass. Moreover we argue that since the energy flux needed to power the quiet corona in closed field regions is about the same as that needed to drive the fast solar wind, and also because at deeper levels (< 2 × 105 K) there is no great difference in the properties of supergranules and network in closed and open field regions, the heating process (i.e., dissipation of high frequency waves) must be the same in both cases.

J. F. McKenzie

Papers

The origin of high speed solar wind streams

The structure of cosmic ray shocks

The fast solar wind

An analytic solar magnetic field model

Acceleration of the High Speed Solar Wind in Coronal Holes