The College News, 1929-11-20, Vol. 16, No. 07

Bryn Mawr College student newspaper. Merged with The Haverford News in 1968 to form the Bi-college News (with various titles from 1968 on). Published weekly (except holidays) during the academic year

The dynamics and observability of circularly polarized kink waves

Context. Kink waves are routinely observed in coronal loops. Resonant absorption is a well-accepted mechanism that extracts energy from kink waves. Nonlinear kink waves are know to be affected by the Kelvin-Helmholtz instability. However, all previous numerical studies consider linearly polarized kink waves. Aims. We study the properties of circularly polarized kink waves on straight plasma cylinders, for both standing and propagating waves, and we compare them to the properties of linearly polarized kink waves. Methods. We used the code MPI-AMRVAC to solve the full 3D magnetohydrodynamic equations for a straight magnetic cylinder, excited by both standing and propagating circularly polarized kink (m = 1) modes. Results. The damping due to resonant absorption is independent of the polarization state. The morphology or appearance of the induced resonant flow is different for the two polarizations; however, there are essentially no differences in the forward-modeled Doppler signals. For nonlinear oscillations, the growth rate of small scales is determined by the total energy of the oscillation rather than the perturbation amplitude. We discuss possible implications and seismological relevance

A Novel Approach to Resonant Absorption of the Fast MHD Eigenmodes of a Coronal Arcade

The arched eld lines forming coronal arcades are often observed to undulate as magne- tohydrodynamic (MHD) waves propagate both across and along the magnetic eld. These waves are most likely a combination of resonantly coupled fast magnetoacoustic waves and Alfv\' en waves. The coupling results in resonant absorption of the fast waves, converting fast wave energy into Alfv\' en waves. The fast eigenmodes of the arcade have proven difficult to compute or derive analytically, largely because of the mathematical complexity that the coupling introduces. When a traditional spectral decomposition is employed, the discrete spectrum associated with the fast eigenmodes is often subsumed into the continuous Alfv \'en spectrum. Thus fast eigenmodes, become collective modes or quasi-modes. Here we present a spectral decomposition that treats the eigenmodes as having real frequencies but complex wavenumbers. Using this procedure we derive dispersion relations, spatial damping rates, and eigenfunctions for the resonant, fast eigenmodes of the arcade. We demonstrate that resonant absorption introduces a fast mode that would not exist otherwise. This new mode is heavily damped by resonant absorption, only travelling a few wavelengths before losing most of its energy

The effect of the magnetic field on the damping of slow waves in the solar corona

Context. Slow magnetoacoustic waves are routinely observed in astrophysical plasma systems such as the solar corona, and they are usually seen to damp rapidly. As a slow wave propagates through a plasma, it modifies the equilibrium quantities of density, temperature, and the magnetic field. In the corona and other plasma systems, the thermal equilibrium is comprised of a balance between continuous heating and cooling processes, the magnitudes of which vary with density, temperature and the magnetic field. Thus the wave may induce a misbalance between these competing processes. Its back reaction on the wave has been shown to lead to dispersion, and amplification or damping, of the wave. Aims. This effect of heating and cooling misbalance has previously been studied in the infinite magnetic field approximation in a plasma whose thermal equilibrium is comprised of optically thin radiative losses and field-aligned thermal conduction, balanced by an (unspecified) heating process. In this work we extend this analysis by considering a non-zero β plasma. The importance of the effect of the magnetic field in the rapid damping of slow waves in the solar corona is evaluated and compared to the effects of thermal conduction. Methods. A linear perturbation under the thin flux tube approximation is considered, and a dispersion relation describing the slow magnetoacoustic modes is found. The dispersion relation’s limits of strong non-adiabaticity and weak non-adiabaticity are studied. The characteristic timescales were calculated for plasma systems with a range of typical coronal densities, temperatures, and magnetic field strengths. Results. The number of timescales characterising the effect of the misbalance is found to remain at two, as with the infinite magnetic field case. In the non-zero β case, these two timescales correspond to the partial derivatives of the combined heating and cooling function with respect to constant gas pressure and with respect to constant magnetic pressure. The predicted damping times of slow waves from thermal misbalance in the solar corona are found to be of the order of 10–100 min, coinciding with the wave periods and damping times observed. Moreover, the slow wave damping by thermal misbalance is found to be comparable to the damping by field-aligned thermal conduction. The change in damping with plasma-β is complex and depends on the coronal heating function’s dependence on the magnetic field in particular. Nonetheless, we show that in the infinite field limit, the wave dynamics is insensitive to the dependence of the heating function on the magnetic field, and this approximation is found to be valid in the corona so long as the magnetic field strength is greater than approximately 10 G for quiescent loops and plumes, and 100 G for hot and dense loops. Conclusions. A thermal misbalance may damp slow magnetoacoustic waves rapidly in much of the corona, and its inclusion in our understanding of slow mode damping may resolve discrepancies between the observations and theory relying on compressive viscosity and thermal conduction alone

Probing the density fine structuring of the solar corona with comet Lovejoy

The passage of sungrazing comets in the solar corona can be a powerful tool to probe the local plasma properties. Here, we carry out a study of the striae pattern appearing in the tail of sungrazing Comet Lovejoy, as observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) during the inbound and outbound phases of the comet’s orbit. We consider the images in EUV in the 171 Å bandpass, where emission from oxygen ions O4+ and O5+ is found. The striae are described as due to a beam of ions injected along the local magnetic field, with the initial beam velocity decaying because of collisions. Also, ion collisional diffusion contributes to ion propagation. Both the collision time for velocity decay and the diffusion coefficient for spatial spreading depend on the ambient plasma density. A probabilistic description of the ion beam density along the magnetic field is developed, where the beam position is given by the velocity decay and the spreading of diffusing ions is described by a Gaussian probability distribution. Profiles of emission intensity along the magnetic field are computed and compared with the profiles along the striae observed by AIA, showing a good agreement for most considered striae. The inferred coronal densities are then compared with a hydrostatic model of the solar corona. The results confirm that the coronal density is strongly spatially structured

Two naked emperors? Concerns about the Stability & Growth Pact and second thoughts about central bank independence

This 2003 Institute for Fiscal Studies Lecture addresses two sets of issues relevant to current and prospective future E(M)U members: the consequences of the Stability and Growth Pact for fiscal-financial sustainability and macroeconomic stability, and some risks associated with operational independence of the central bank. The relevance of the second issue is not restricted to E(M)U members. Poor communication, co-operation and co-ordination between the fiscal and monetary authorities can be costly in two contingencies. The first of these occurs when the central bank’s role as the lender of last resort needs to be backed up by the willingness of the Treasury to recapitalise the central bank, should the need arise. The second contingency occurs when unwanted deflation needs to be prevented or combated, but the central bank’s conventional monetary arsenal is exhausted. Friedman’s helicopter drop of money, a temporary tax cut or transfer payment increase financed through the issuance of base money will always stimulate demand provided it is not expected to be reversed, in present value terms, in the future. In most real-world institutional/legal settings, – the implementation of a helicopter drop of base money requires co-ordinated actions by the central bank and Treasury. Central bank independence is unlikely to survive if either or both of these contingencies occur, if there is an ineffective response by the fiscal and monetary authorities and if this is blamed on lack of communication, co-operation or co-ordination

Bundesbank-government relations in Germany in the 1990s: from GEMU to EMU

The Bundesbank's widely-discussed independence ascribes it only discretionary power in the realm of monetary policy, but its influence can extend into other areas of economic policy. Since the government retains the initiative in these policy realms, the Bundesbank's influence consists of being able to mold the form rather than the direction of government policy. To exercise this influence, however, the Bundesbank must have public opinion on its side. An examination of the government-Bundesbank relationship as it touched upon the cases of Economic and Monetary Union in Germany (1990) and Europe (1990-98) reveals the extent and limitations of the Bundesbank's influence over economic policy. It concludes by exploring the consequences of these findings on the influence of the European Central Bank (ECB)

Tracking and Seismological Analysis of Multiple Coronal Loops in an Active Region

We present a new method to track the position and evolution of coronal loops designed for observations such as active regions in which multiple loops appear in close proximity or overlap with each other along the observational line of sight. The method is based on modeling a time–distance map containing one or more loops and fitting the modeled map to observational data, as opposed to the commonly used technique of analyzing each frame independently. This allows us to control the variability of the model, informed by our physical interpretation, and use the trends present to help constrain the model parameters. We apply our method to an observation of a bundle of coronal loops previously investigated using a spatiotemporal autocorrelation method and compare our results. A benefit of our method is that it provides the time series for the position of the loops that may be used for further analysis using established seismological techniques. We demonstrate this by modeling the oscillation of several loops in response to flaring energy releases that occur during the observation, and we find evidence of loop evolution consistent with the Kelvin–Helmholtz instabilit