Useful aspects of chromospheric magnetic field data

In time – distance helioseismology, wave travel times are measured from the cross-correlation between Doppler velocities recorded at any two locations on the solar surface. However, one of the main uncertainties associated with such measurements is how to interpret observations made in regions of strong magnetic field. Isolating the effects of the magnetic field from thermal or sound-speed perturbations has proved to be quite complex and has yet to yield reliable results when extracting travel times from the cross-correlation function. One possible way to decouple these effects is by using a 3D sunspot model based on observed surface magnetic-field profiles, with a surrounding stratified, quiet-Sun atmosphere to model the magneto-acoustic ray propagation, and analyse the resulting ray travel-time perturbations that will directly account for wave-speed variations produced by the magnetic field. These artificial travel-time perturbation profiles provide us with several related but distinct observations: i) that strong surface magnetic fields have a dual effect on helioseismic rays – increasing their skip distance while at the same time speeding them up considerably compared to their quiet-Sun counterparts, ii) there is a clear and significant frequency dependence of both skip-distance and travel-time perturbations across the simulated sunspot radius, iii) the negative sign and magnitude of these perturbations appears to be directly related to the sunspot magnetic-field strength and inclination, iv) by “switching off” the magnetic field inside the sunspot, we are able to completely isolate the thermal component of the travel-time perturbations observed, which is seen to be both opposite in sign and much smaller in magnitude than those measured when the magnetic field is present. These results tend to suggest that purely thermal perturbations are unlikely to be the main effect seen in travel times through sunspots, and that strong, near-surface magnetic fields may be directly and significantly altering the magnitude and lateral extent of sound-speed inversions of sunspots made by time – distance helioseismology.KeywordsHelioseismology, direct modellingSunspots, magnetic fieldsMagnetic fields, models

Continuous high-cadence and high spatial resolution Dopplergrams allow us to study subsurface dynamics that may be further extended to explore precursors of visible solar activity on the surface. Since the p-mode power is absorbed in the regions of high magnetic field, the inferences in these regions are often presumed to have large uncertainties. In this paper, using the Dopplergrams from space-borne Helioseismic Magnetic Imager, we compare horizontal flows in a shear layer below the surface and the photospheric layer in and around active regions. The photospheric flows are calculated using the local correlation tracking (LCT) method, while the ring-diagram technique of helioseismology is used to infer flows in the subphotospheric shear layer. We find a strong positive correlation between flows from both methods near the surface. This implies that despite the absorption of acoustic power in the regions of strong magnetic field, the flows inferred from the helioseismology are comparable to those from the surface measurements. However, the magnitudes are significantly different; the flows from the LCT method are smaller by a factor of 2 than the helioseismic measurements. Also, the median difference between the direction of corresponding vectors is 49°.

The light-shining-through-walls (LSW) method of searching for ultralight bosonic dark matter (UBDM) uses lasers and strong dipole magnets to probe the coupling between photons and UBDM in the presence of a magnetic field. Since these experiments take place entirely in the laboratory, they offer a unique opportunity to perform a model independent measurement of this interaction. This involves shining a high-power laser through a magnetic field toward a wall which blocks the light. The interaction between the laser and the magnetic field generates a beam of UBDM that passes through the wall. Beyond the wall is another region of strong magnetic field that reconverts the UBDM back to photons that can then be measured by a single photon detection system. The sensitivity of these kinds of experiments can be improved further by implementing optical cavities before and after the wall to amplify the power of the light propagating through the magnetic fields. This chapter gives an introduction to LSW experiments and discusses a number of interesting challenges associated with the technique.

We calculate the structure of a relativistic shock wave in which the internal energy of the shocked fluid is radiated away on a time scale much shorter than the characteristic shock propagation time. The shock is assumed to move through a uniformly magnetized, neutral plasma consisting of protons, electrons, and positrons, and allowance is made for the possible production of electron/positron pairs in the shock itself. The radiation mechanism is taken to be synchrotron and inverse-Compton emission (the latter involving both synchrotron-produced and externally supplied seed photons) by the electrons and positrons. We simplify the discussion by considering a shock in which the magnetic field is transverse to the direction of propagation and focus attention on the properties of the radiative zone that forms behind the shock transition. In particular, we investigate the possibility that the compression induced by the cooling of the gas amplifies the magnetic pressure until it reaches (and ultimately exceeds) equipartition with the thermal pressure (which, in turn, limits the overall compression). We show that, if a significant fraction of the postshock thermal energy is deposited in the electron/positron component, then a considerable portion of the emitted radiation will come from regions of strong magnetic field even if the field immediately behind the shock transition is well below equipartition. This field amplification mechanism may be relevant to the production of synchrotron flares in blazars, miniquasars, and gamma-ray burst sources. We consider the latter application in some detail and show that this process may play a role in the prompt $\gamma$-ray and possibly also the optical ``flash'' and radio ``flare'' emission, but probably not in the afterglow.

Parallel thermal conduction along stochastic magnetic field lines may be reduced because the heat conducting electrons become trapped and detrapped between regions of strong magnetic field (magnetic mirrors). The problem reduces to a simple but realistic model for diffusion of mono-energetic electrons based on the fact that when there is a reduction of diffusion, it is controlled by a subset of the mirrors, the principle mirrors. The diffusion reduction can be considered as equivalent to an enhancement of the pitch angle scattering rate. Therefore, in deriving the collision integral, the authors modify the pitch angle scattering term. They take into account the full perturbed electron-electron collision integral, as well as the electron-proton collision term. Finally, they obtain the four plasma transport coefficients and the effective thermal conductivity. They express them as reductions from the classical values. They present these reductions as functions of the ratio of the magnetic field decorrelation length to the electron mean free path at the thermal speed V{sub T} = {radical}2kT/m{sub e}. They briefly discuss an application of the results to clusters of galaxies.

Oscillations have long been observed in the sunspot umbral chromosphere and transition region, connected to global p-mode oscillations. These p-modes are thought to undergo mode conversion to slow magnetoacoustic waves in regions of strong magnetic field. More recently, propagating oscillations have also been observed in solar coronal loops. Using new spectroscopic imaging data at transition-region temperatures, combined with coronal imaging, we present direct observations of the propagation of these slow magnetoacoustic p-modes through the transition region and into the solar corona, along the magnetic field. The waves are observed as oscillations in the chromosphere/transition region and propagations in the corona due to the emission scale height of the different temperature lines combined with the magnetic field geometry.

Small cyclic variations in the frequencies of acoustic modes are expected to be a common phenomenon in solar‐like pulsators, as a result of stellar magnetic activity cycles. The frequency variations observed throughout the solar and stellar cycles contain information about structural changes that take place inside the stars as well as about variations in magnetic field structure and intensity. The task of inferring and disentangling that information is, however, not a trivial one. In the sun and solar‐like pulsators, the direct effect of the magnetic field on the oscillations might be significantly important in regions of strong magnetic field (such as solar/stellar spots), where the Lorentz force can be comparable to the gas‐pressure gradient. Our aim is to determine the sun‐/starspot effect on the oscillation frequencies and attempt to understand if this effect contributes strongly to the frequency changes observed along the magnetic cycle. The total contribution of the spots to the frequency shifts results from a combination of direct and indirect effects of the magnetic field on the oscillations. In this first work we considered only the indirect effect associated with changes in the stratification within the starspot. Based on the solution of the wave equation and the variational principal we estimated the impact of these stratification changes on the oscillation frequencies of global modes in the sun and found that the induced frequency shifts are about two orders of magnitude smaller than the frequency shifts observed over the solar cycle (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the Sun, magnetic activity is essentially restricted to latitudes within 45° of the equator. However, active stars, such as RS CVn systems, and T Tauri stars generally show polar activity. I model the evolution of a toroidal magnetic flux tube as it rises from the base of the convection zone in various lower main-sequence stars and show that, on the main sequence, polar activity is more likely for less massive stars. For example, a 1 M☉ star must rotate with ω > 5 ω☉ in order to develop significant polar magnetic flux, while a 0.4 M☉ star will generate predominantly polar flux for ω > 0.5 ω☉. This leads to the expectation that essentially all M stars should display polar, rather than equatorial, magnetic activity. A similar effect is expected for RS CVn and T Tauri systems, and for giant stars. An important consequence of this polar activity is the inhibition of angular momentum loss from these systems. On the lower main sequence, angular momentum is lost via the interaction of the stellar magnetic field with the outflowing stellar wind. However, if regions of strong magnetic field are confined to the poles, stellar spin-down rates can be reduced dramatically, by up to a factor of 3 or more for early M stars. Examination of open clusters should show the presence of rapidly rotating M stars at ages beyond those previously expected.

Characteristics of a GaAs/AlGaAs quantum well (QW) laser diode are studied for the first time under strong magnetic fields up to 30 Tesla. When field normal to QW plane is increased, the spontaneous emission spectrum is found to shift toward the higher photon energy, following the shift \\hbarωc/2 of the lowest Landau level in the region of strong magnetic fields. Moreover, the emission spectrum is shown to be very little affected when magnetic fields are applied parallel to the QW plane. These results give evidence of light emission from a fully-quantized zero-dimensional carrier system in GaAs QW structures with strong magnetic fields normal to the QW plane.

Observations have shown that electrons and protons are energized up to at least 1 MeV in the magnetotail during substorms. This magnitude cannot be explained by the cross‐tail electrostatic field, which typically has a modest potential difference of only 50 kV. A rotational electric field induced by a time‐dependent magnetic field does not have such a limitation. In order to evaluate its capabilities a simple model of a localized, growing disturbance in the neutral sheet current was used to calculate perturbation magnetic and electric fields; the model includes the formation of X and O type neutral lines. Plasma sheet test particles were followed in these time‐dependent fields by using the full relativistic equation of motion. The most efficient energizing mechanism is a two‐step process, with an initial linear acceleration along a neutral line up to moderate energies, followed by betatron acceleration. The former imparts a large magnetic moment µ = W⊥/B to the particle as it begins to gyrate and drift in the magnetic field, W⊥ being the transverse kinetic energy. During this drift into regions of stronger magnetic field there is a large increase in W⊥, the relativistic invariant γµ remaining constant. Because of the rotational property of the induced electric field it is possible for a particle to gain energy even when the drift motion by itself would cause a loss. The model also includes acceleration along magnetic field lines in the plasma sheet, with moderate energy gain. A cyclic pattern of electron and proton precipitation is predicted, such as is observed during auroral breakup.

We have compared the rates at which flux emerges in active and quiet solar regions within the sunspot belts. The emerging flux regions (EFRs) were identified by the appearance of arch filament structures in Hα. All EFRs in high-resolution films of active regions made at Big Bear in 1978 were counted. The comparable rate of flux emergence in quiet regions was obtained from SGD data and independently from EFRs detected outside the active region perimeter on the same films. The rate of flux emergence is 10 times higher in active regions than in quiet regions. A sample of all active regions in 31 days of 1983 gave a ratio of 7.5. We discuss possible mechanisms which might funnel new magnetic flux to regions of strong magnetic field.

We performed three-dimensional numerical simulations of the solar surface acoustic wave field for the quiet Sun and for three models with different localized sound-speed perturbations in the interior with deep, shallow, and two-layer structures. We used the simulated data generated by two solar acoustics codes that employ the same standard solar model as a background model, but utilize different integration techniques and different models of stochastic wave excitation. Acoustic travel times were measured using a time–distance helioseismology technique, and compared with predictions from ray theory frequently used for helioseismic travel-time inversions. It is found that the measured travel-time shifts agree well with the helioseismic theory for sound-speed perturbations, and for the measurement procedure with and without phase-speed filtering of the oscillation signals. This testing verifies the whole measuring–filtering–inversion procedure for static sound-speed anomalies with small amplitude inside the Sun outside regions of strong magnetic field. It is shown that the phase-speed filtering, frequently used to extract specific wave packets and improve the signal-to-noise ratio, does not introduce significant systematic errors. Results of the sound-speed inversion procedure show good agreement with the perturbation models in all cases. Due to its smoothing nature, the inversion procedure may overestimate sound-speed variations in regions with sharp gradients of the sound-speed profile.

We have studied the correlation between the flare ribbon separation speed and the magnetic flux density in a two ribbon flare. The peak separation speed is up to about 260 km s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in a weak magnetic field region, while it is about 30 km s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in a strong magnetic field region. The separation speed is negatively correlated with the longitudinal magnetic field, and the negative correlation is the strongest at the early stage of the flare, and the weakest at position of the HXR source.

Encounter of Voyager with Saturn’s environment revealed the presence of electromagnetic ion-cyclotron waves (EMIC) in Saturnian magnetosphere. Cassini provided the evidence of dynamic particle injections in inner magnetosphere of Saturn. Also inner magnetosphere of Saturn has highest rotational flow shear as compared to any other planet in our solar system. Hence during these injections, electrons and ions are transported to regions of stronger magnetic field, thus gaining energy. The dynamics of the inner magnetosphere of Saturn are governed by wave-particle interaction. In present paper we have investigated those EMIC waves pertaining in background plasma which propagates obliquely with respect to the magnetic field of Saturn. Applying kinetic approach, the expression for dispersion relation and growth rate has been derived. Magnetic field model has been used to incorporate magnetic field strength at different latitudes for radial distance of $6.18~R_{{s}}$ ( $1~R_{{s}}= 60{,}268~\mbox{km}$ ). Various parameters affecting the growth of EMIC waves in cold bi-Maxwellian background and after the hot injections has been studied. Parametric analysis inferred that after hot injections, growth rate of EMIC waves increases till $10^{\circ}$ and decreases eventually with increase in latitude due to ion density distribution in near-equatorial region. Also, growth rate of EMIC waves increases with increasing value of temperature anisotropy and AC frequency, but the growth rate decreases as the angle of propagation with respect to $B_{0}$ (Magnetic field at equator) increases. The injection events which assume the Loss-cone distribution of particles, affect the lower wave numbers of the spectra.

A new method is presented to forecast the solar irradiance of selected wavelength ranges within the extreme ultraviolet (EUV) and far ultraviolet (FUV) bands. The technique is similar to a method recently published by Henney et al. (2012) to predict solar 10.7 cm (2.8 GHz) radio flux, abbreviated F10.7, utilizing advanced predictions of the global solar magnetic field generated by a flux transport model. In this and the previous study, we find good correlation between the absolute value of the observed photospheric magnetic field and selected EUV/FUV spectral bands. By evolving solar magnetic maps forward 1 to 7 days with a flux transport model, estimations of the Earth side solar magnetic field distribution are generated and used to forecast irradiance. For example, Pearson correlation coefficient values of 0.99, 0.99, and 0.98 are found for 1 day, 3 day, and 7 day predictions, respectively, of the EUV band from 29 to 32 nm. In the FUV, for example, the 160 to 165 nm spectral band, correlation values of 0.98, 0.97, and 0.96 are found for 1 day, 3 day, and 7 day predictions, respectively. In the previous study, the observed F10.7 signal is found to correlate well with strong magnetic field (i.e., sunspot) regions. Here we find that solar EUV and FUV signals are significantly correlated with the weaker magnetic fields associated with plage regions, suggesting that solar magnetic indices may provide an improved indicator (relative to the widely used F10.7 signal) of EUV and FUV nonflaring irradiance variability as input to ionospheric and thermospheric models.