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Register a new userObservational Quantification of the Energy Dissipated by Alfven Waves in a Polar Coronal Hole: Evidence that Waves Drive the Fast Solar Wind
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We present a measurement of the energy carried and dissipated by Alfven waves in a polar coronal hole. Alfven waves have been proposed as the energy source that heats the corona and drives the solar wind. Previous work has shown that line widths decrease with height in coronal holes, which is a signature of wave damping, but have been unable to quantify the energy lost by the waves. This is because line widths depend on both the non-thermal velocity v_nt and the ion temperature T_i. We have implemented a means to separate the T_i and v_nt contributions using the observation that at low heights the waves are undamped and the ion temperatures do not change with height. This enables us to determine the amount of energy carried by the waves at low heights, which is proportional to v_nt. We find the initial energy flux density present was 6.7 +/- 0.7 x 10^5 erg cm^-2 s^-1, which is sufficient to heat the coronal hole and acccelerate the solar wind during the 2007 - 2009 solar minimum. Additionally, we find that about 85% of this energy is dissipated below 1.5 R_sun, sufficiently low that thermal conduction can transport the energy throughout the coronal hole, heating it and driving the fast solar wind. The remaining energy is roughly consistent with what models show is needed to provide the extended heating above the sonic point for the fast solar wind. We have also studied T_i, which we found to be in the range of 1 - 2 MK, depending on the ion species.
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Alfven waves are responsible for the transfer of magnetic energy in the magnetized plasma. They are involved in heating solar atmosphere and driving solar wind through various nonlinear processes. Since the magnetic field configurations directly affect the nonlinearity of Alfven waves, it is important to investigate how they relate to the solar atmosphere and wind structure through the nonlinear propagation of Alfven waves. In this study, we carried out the one-dimensional magnetohydrodynamic simulations to realize the above relation. The results show that when the nonlinearity of Alfven waves in the chromosphere exceeds a critical value, the dynamics of the solar chromosphere (e.g., spicule) and the mass loss rate of solar wind tend to be independent of the energy input from the photosphere. In a situation where the Alfven waves are highly nonlinear, the strong shear torsional flow generated in the chromosphere ``fractures the magnetic flux tube. This corresponds to the formation of chromospheric intermediate shocks, which limit the transmission of the Poynting flux into the corona by Alfven waves and also inhibits the propagation of chromospheric slow shock.
We carry out two-dimensional magnetohydrodynamic (MHD) simulations of an ensemble of Alfvenic fluctuations propagating in a structured, expanding solar wind including the presence of fast and slow solar wind streams. Using an appropriate expanding box model, the simulations incorporate the effects of fast-slow stream shear and compression and rarefaction self-consistently. We investigate the radial and longitudinal evolution of the cross-helicity, the total and residual energies and the power spectra of outward and inward Alfvenic fluctuations. The stream interaction is found to strongly affect the radial evolution of Alfvenic turbulence. The total energy in the Alfven waves is depleted within the velocity shear regions, accompanied by the decrease of the normalized cross-helicity. The presence of stream-compression facilitates this process. Residual energy fluctuates around zero due to the correlation and de-correlation between the inward/outward waves but no net growth or decrease of the residual energy is observed. The radial power spectra of the inward/outward Alfven waves show significant longitudinal variations. Kolmogorov-like spectra are developed only inside the fast and slow streams and when both the compression and shear are present. On the other hand, the spectra along the longitudinal direction show clear Kolmogorov-like inertial ranges in all cases.
A growing body of evidence suggests that the solar wind is powered to a large extent by an Alfven-wave (AW) energy flux. AWs energize the solar wind via two mechanisms: heating and work. We use high-resolution direct numerical simulations of reflection-driven AW turbulence (RDAWT) in a fast-solar-wind stream emanating from a coronal hole to investigate both mechanisms. In particular, we compute the fraction of the AW power at the coronal base ($P_{rm AWb}$) that is transferred to solar-wind particles via heating between the coronal base and heliocentric distance $r$, which we denote $chi_{rm H}(r)$, and the fraction that is transferred via work, which we denote $chi_{rm W}(r)$. We find that $chi_{rm W}(r_{rm A})$ ranges from 0.15 to 0.3, where $r_{rm A}$ is the Alfven critical point. This value is small compared to~one because the Alfven speed $v_{rm A} $ exceeds the outflow velocity $U$ at $r<r_{rm A}$, so the AWs race through the plasma without doing much work. At $r>r_{rm A}$, where $v_{rm A} < U$, the AWs are in an approximate sense stuck to the plasma, which helps them do pressure work as the plasma expands. However, much of the AW power has dissipated by the time the AWs reach $r=r_{rm A}$, so the total rate at which AWs do work on the plasma at $r>r_{rm A}$ is a modest fraction of $P_{rm AWb}$. We find that heating is more effective than work at $r<r_{rm A}$, with $chi_{rm H}(r_{rm A})$ ranging from 0.5 to 0.7. The reason that $chi_{rm H} geq 0.5$ in our simulations is that an appreciable fraction of the local AW power dissipates within each Alfven-speed scale height in RDAWT, and there are a few Alfven-speed scale heights between the coronal base and $r_{rm A}$.
We report the detection of oscillatory phenomena associated with a large bright-point group that is 430,000 square kilometers in area and located near the solar disk center. Wavelet analysis reveals full-width half-maximum oscillations with periodicities ranging from 126 to 700 seconds originating above the bright point and significance levels exceeding 99%. These oscillations, 2.6 kilometers per second in amplitude, are coupled with chromospheric line-of-sight Doppler velocities with an average blue shift of 23 kilometers per second. A lack of cospatial intensity oscillations and transversal displacements rules out the presence of magneto-acoustic wave modes. The oscillations are a signature of Alfven waves produced by a torsional twist of +/-22 degrees. A phase shift of 180 degrees across the diameter of the bright point suggests that these torsional Alfven oscillations are induced globally throughout the entire brightening. The energy flux associated with this wave mode is sufficient to heat the solar corona.
We used our newly developed magnetohydrodynamic (MHD) code to perform 2.5D simulations of a fast-mode MHD wave interacting with coronal holes (CH) of varying Alfven speed which result from assuming different CH densities. We find that this interaction leads to effects like reflection, transmission, stationary fronts at the CH boundary and the formation of a density depletion that moves in the opposite direction to the incoming wave. We compare these effects with regard to the different CH densities and present a comprehensive analysis of morphology and kinematics of the associated secondary waves. We find that the density value inside the CH influences the phase speed as well as the amplitude values of density and magnetic field for all different secondary waves. Moreover, we observe a correlation between the CH density and the peak values of the stationary fronts at the CH boundary. The findings of reflection and transmission on the one hand and the formation of stationary fronts caused by the interaction of MHD waves with CHs on the other hand, strongly support the theory that large scale disturbances in the corona are fast-mode MHD waves.
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