Stability Of Coronal Loops Research Articles

Solar Wind Helium Abundance as a Function of Speed and Heliographic Latitude: Variation through a Solar Cycle

We present a study of the variation of the relative abundance of helium to hydrogen in the solar wind as a function of solar wind speed and heliographic latitude over the previous solar cycle. The average values of A(sub He), the ratio of helium to hydrogen number densities, are calculated in 25 speed intervals over 27-day Carrington rotations using Faraday Cup observations from the Wind spacecraft between 1995 and 2005. The higher speed and time resolution of this study compared to an earlier work with the Wind observations has led to the discovery of three new aspects of A(sub He), modulation during solar minimum from mid-1995 to mid-1997. First, we find that for solar wind speeds between 350 and 415 km/s, A(sub He), varies with a clear six-month periodicity, with a minimum value at the heliographic equatorial plane and a typical gradient of 0.01 per degree in latitude. For the slow wind this is a 30% effect. We suggest that the latitudinal gradient may be due to an additional dependence of coronal proton flux on coronal field strength or the stability of coronal loops. Second, once the gradient is subtracted, we find that A(sub He), is a remarkably linear function of solar wind speed. Finally, we identify a vanishing speed, at which A(sub He), is zero, is 259 km/s and note that this speed corresponds to the minimum solar wind speed observed at one AU. The vanishing speed may be related to previous theoretical work in which enhancements of coronal helium lead to stagnation of the escaping proton flux. During solar maximum the A(sub He), dependences on speed and latitude disappear, and we interpret this as evidence of two source regions for slow solar wind in the ecliptic plane, one being the solar minimum streamer belt and the other likely being active regions.

Non-inductive current driven by Alfvén waves in solar coronal loops

It has been shown that Alfven waves can drive non-inductive current in solar coronal loops via collisional or collisionless damping. Assuming that all the coronal-loop density of dissipated wave power (W= 10−3 erg cm−3 s−1), which is necessary to keep the plasma hot, is due to Alfven wave electron heating, we have estimated the axial current density driven by Alfven waves to be 〈jz〉 ≈ 103–105 statA cm−2. This current can indeed support the quasi-stationary equilibrium and stability of coronal loops and create the poloidal magnetic field up to Bθ≈1−5 G.

The evolution of twisted coronal loops

The evolution of coronal loops in response to slow photospheric twisting motions is investigated using a variety of methods. Firstly, by solving the time-dependent equations it is shown that the field essentially evolves through a sequence of 2-D equilibria with no evidence of rapid dynamic evolution. Secondly, a sequence of 1-D equilibria are shown to provide a remarkably good approximation to the 2-D time-dependent results using a fraction of the computer time. Thus, a substantial investigation of parameter space is now possible. Finally, simple bounds on the 3-D stability of coronal loops are obtained. Exact stability bounds can be found by using these bounds to reduce the region of parameter space requiring further investigation. Twisting the loop too much shows that a 3-D instability must be triggered.

Resistive ballooning modes in line-tied coronal fields

The resistive stability of coronal loops to perturbations with short wavelength across the magnetic field is analysed, taking full account of the line tying effect due to the presence of the photosphere. The results presented are similar to those previously obtained for arcades: configurations with a pressure profile decreasing with distance from the loop axis at some point are found to be always unstable, the growth rate γ increasing monotonically with the wavenumber (n) and scaling approximately as γ ∼ (n2Dr)1/3 in the limit of large n.

Comments on the MHD stability of coronal plasmas with line-tying

view Abstract Citations (15) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Comments on the MHD stability of coronal plasmas with line-tying An, C. -H. Abstract The effect of line-tying in the photosphere on the magnetohydrodynamic (MHD) stability of coronal loops has been investigated by a number of authors. The author comments on the various boundary conditions, plasma displacement functions, and other assumptions used by a number of investigators. Then he studies kink instabilities in cylindrical plasmas. The results are very different from those of a previous study which used a nonhelical test function, which indicates the importance of a helical test function. It is found that the effects of line-tying on force-free equilibria are not the same as on non-force-free equilibria. Publication: The Astrophysical Journal Pub Date: June 1984 DOI: 10.1086/162112 Bibcode: 1984ApJ...281..419A Keywords: Coronal Loops; Magnetohydrodynamic Stability; Solar Corona; Boundary Conditions; Boundary Value Problems; Hydrodynamic Equations; Plasma Equilibrium; Solar Physics full text sources ADS |

The stability of coronal loops: Finite-length and pressure-profile limits

Results are described from a quickly converging, necessary-and-sufficient, MHD-stability test for coronal-loop models. The primary stabilizing influence arises from magnetic line tying at the photosphere, and this end conditions requires a series expansion of possible loop excitations. The stability boundary is shown to quickly approach a limit as the number of terms increases, providing a critical length for the loop in proportion to its transverse magnetic scale. Several models of force-free-field profiles are tested and the stability behavior of a localized current channel, embedded in an external current-free region, is shown to be superior to that of other, broader, current profiles. Pressure-gradient effects, leading to increased or decreased stability, are shown to be amplified by line tying. Long loops must either conduct low net current, or exhibit an axial-field reversal coexisting with a low-pressure core. The limits on stability depend on the magnetic aspect ratio, the plasma-to-magnetic pressure ratio, and the field orientation at the loop edge. Applications of these results to the structure of coronal loops are described.

Effect of pressure gradients and line-tying on the magnetic stability of coronal loops

Abstract In this paper we study the stability of an idealised magnetostatic coronal loop, incorporating both the effect of line-tying, due to the dense photosphere, and of pressure gradients. The stability equations may be solved analytically for our particular equilibrium. From the marginally stable case, the critical conditions separating instability from stability are derived. It is found that stretching or twisting a loop eventually makes it kink unstable.

The stability and uniqueness of coronal loops

A hydrodynamic model of high resolution is used to examine the stability of coronal loops to finite amplitude perturbations. The loop is heated by means of a low-amplitude energy input and its subsequent dynamic relaxation is followed.

Stability of coronal loops heated by direct current dissipation

The stability of coronal loop plasmas heated by direct current dissipation is studied. The current contributes not only to the heating but also to the energy transport. Although the latter point was stressed by Hinata, it has been neglected in previous stability analyses. We obtain the dispersion relation for a current-carrying plasma with these two effects taken into account.