Nonlocal interactions between electrons and Alfvén waves on auroral field lines

2005 ◽  
Vol 110 (A10) ◽  
Author(s):  
Robert L. Lysak
Keyword(s):  
Alfven Waves ◽  
Alfvén Waves ◽  
Nonlocal Interactions ◽  
Field Lines

scholarly journals Wave particle interactions in the high-altitude polar cusp: a Cluster case study

2005 ◽  
Vol 23 (12) ◽  
pp. 3699-3713 ◽  
Author(s):  
B. Grison ◽  
F. Sahraoui ◽  
B. Lavraud ◽  
T. Chust ◽  
N. Cornilleau-Wehrlin ◽  
...  
Keyword(s):  
Electromagnetic Wave ◽  
High Altitude ◽  
Wave Spectrum ◽  
Alfven Waves ◽  
Wave Activity ◽  
Alfven Wave ◽  
Alfvén Waves ◽  
Plasma Convection ◽  
Field Lines ◽  
Kinetic Alfvén Waves

Abstract. On 23 March 2002, the four Cluster spacecraft crossed in close configuration (~100 km separation) the high-altitude (10 RE) cusp region. During a large part of the crossing, the STAFF and EFW instruments have detected strong electromagnetic wave activity at low frequencies, especially when intense field-aligned proton fluxes were detected by the CIS/HIA instrument. In all likelihood, such fluxes correspond to newly-reconnected field lines. A focus on one of these ion injection periods highlights the interaction between waves and protons. The wave activity has been investigated using the k-filtering technique. Experimental dispersion relations have been built in the plasma frame for the two most energetic wave modes. Results show that kinetic Alfvén waves dominate the electromagnetic wave spectrum up to 1 Hz (in the spacecraft frame). Above 0.8 Hz, intense Bernstein waves are also observed. The close simultaneity observed between the wave and particle events is discussed as an evidence for local wave generation. A mechanism based on current instabilities is consistent with the observations of the kinetic Alfvén waves. A weak ion heating along the recently-opened field lines is also suggested from the examination of the ion distribution functions. During an injection event, a large plasma convection motion, indicative of a reconnection site location, is shown to be consistent with the velocity perturbation induced by the large-scale Alfvén wave simultaneously detected.

scholarly journals Investigating the damping rate of phase-mixed Alfvén waves

2019 ◽  
Vol 632 ◽  
pp. A93 ◽  
Author(s):  
A. P. K. Prokopyszyn ◽  
A. W. Hood
Keyword(s):  
Wave Energy ◽  
Alfven Waves ◽  
Alfvén Waves ◽  
Coronal Loops ◽  
Phase Mixing ◽  
Heating Mechanism ◽  
At Resonance ◽  
Damping Rate ◽  
Field Lines ◽  
The Waves

Context. This paper investigates the effectiveness of phase mixing as a coronal heating mechanism. A key quantity is the wave damping rate, γ, defined as the ratio of the heating rate to the wave energy. Aims. We investigate whether or not laminar phase-mixed Alfvén waves can have a large enough value of γ to heat the corona. We also investigate the degree to which the γ of standing Alfvén waves which have reached steady-state can be approximated with a relatively simple equation. Further foci of this study are the cause of the reduction of γ in response to leakage of waves out of a loop, the quantity of this reduction, and how increasing the number of excited harmonics affects γ. Methods. We calculated an upper bound for γ and compared this with the γ required to heat the corona. Analytic results were verified numerically. Results. We find that at observed frequencies γ is too small to heat the corona by approximately three orders of magnitude. Therefore, we believe that laminar phase mixing is not a viable stand-alone heating mechanism for coronal loops. To arrive at this conclusion, several assumptions were made. The assumptions are discussed in Sect. 2. A key assumption is that we model the waves as strictly laminar. We show that γ is largest at resonance. Equation (37) provides a good estimate for the damping rate (within approximately 10% accuracy) for resonant field lines. However, away from resonance, the equation provides a poor estimate, predicting γ to be orders of magnitude too large. We find that leakage acts to reduce γ but plays a negligible role if γ is of the order required to heat the corona. If the wave energy follows a power spectrum with slope −5/3 then γ grows logarithmically with the number of excited harmonics. If the number of excited harmonics is increased by much more than 100, then the heating is mainly caused by gradients that are parallel to the field rather than perpendicular to it. Therefore, in this case, the system is not heated mainly by phase mixing.

The Bell-Like Magnetosphere

1996 ◽  
Author(s):  
Charles F. Kennel
Keyword(s):  
Low Frequency ◽  
Compressional Wave ◽  
Alfven Waves ◽  
Convincing Evidence ◽  
Alfvén Waves ◽  
Wave Form ◽  
Global Mode ◽  
Critical Issues ◽  
Space Age ◽  
Field Lines

The fact that the geomagnetic field “pulsates” was known a century before the space age opened. The century of ground-based observations did lead to an effective empirical classification of the pulsations based on period, wave form, and geographical distribution (Section 3.1), but why the magnetic field of an astronomical body should oscillate on short time scales was a first-class scientific puzzle that could only by solved in the space age. Low-frequency hydromagnetic waves were first observed in the distant magnetosphere on Explorer 6 (Judge and Coleman, 1962). The task for space research was to relate the oscillations of plasma and fields in deep space to the ground observations using the refined theoretical languages of magnetohydrodynamics and plasma physics. There have been two critical issues. The first was to understand how plasma instabilities generate some of the observed pulsations. The second, the subject of this chapter, has been to understand how motions of the magnetopause induced by the variability of the solar wind are communicated to the interior of the magnetosphere. The breakthrough came when it was understood that the MHD fast mode can cross field lines and couple resonantly to localized standing Alfven waves. What is seen on the ground is due primarily to the resonant Alfven waves (Section 3.3). In Section 3.4, we provide basic theoretical information about the eigenmodes of the “MHD box” as a conceptual framework for the observations of oscillating fields and particles in the magnetospheric cavity. Space observations provided convincing evidence for the existence of standing Alfven waves shortly after the fast-wave coupling theory was proposed (Section 3.5). The next issue was which standing wave harmonics are excited (Section 3.6). Multiharmonic excitations now seem to be a semipermanent feature of the dayside magnetosphere, attesting to the constant activity at the magnetopause. There have been a few observations of the “global mode,” the low-frequency, radially standing compressional wave that may be responsible for discrete frequency resonant oscillations (Section 3.7).

scholarly journals Phase mixing of nonlinear Alfvén waves

2019 ◽  
Vol 624 ◽  
pp. A90 ◽  
Author(s):  
A. P. K. Prokopyszyn ◽  
A. W. Hood ◽  
I. De Moortel
Keyword(s):  
Numerical Experiments ◽  
Magnitude Estimate ◽  
Alfven Waves ◽  
Alfven Wave ◽  
Alfvén Waves ◽  
Uniform Density ◽  
Phase Mixing ◽  
Poynting Flux ◽  
Magnetic Diffusivity ◽  
Field Lines

Aims. This paper presents 2.5D numerical experiments of Alfvén wave phase mixing and aims to assess the effects of nonlinearities on wave behaviour and dissipation. In addition, this paper aims to quantify how effective the model presented in this work is at providing energy to the coronal volume. Methods. The model is presented and explored through the use of several numerical experiments which were carried out using the Lare2D code. The experiments study footpoint driven Alfvén waves in the neighbourhood of a two-dimensional x-type null point with initially uniform density and plasma pressure. A continuous sinusoidal driver with a constant frequency is used. Each experiment uses different driver amplitudes to compare weakly nonlinear experiments with linear experiments. Results. We find that the wave trains phase-mix owing to variations in the length of each field line and variations in the field strength. The nonlinearities reduce the amount of energy entering the domain, as they reduce the effectiveness of the driver, but they have relatively little effect on the damping rate (for the range of amplitudes studied). The nonlinearities produce density structures which change the natural frequencies of the field lines and hence cause the resonant locations to move. The shifting of the resonant location causes the Poynting flux associated with the driver to decrease. Reducing the magnetic diffusivity increases the energy build-up on the resonant field lines, however, it has little effect on the total amount of energy entering the system. From an order of magnitude estimate, we show that the Poynting flux in our experiments is comparable to the energy requirements of the quiet Sun corona. However a (possibly unphysically) large amount of magnetic diffusion was used however and it remains unclear if the model is able to provide enough energy under actual coronal conditions.

scholarly journals Winds in late-type stars: Mechanisms of mass outflow

1981 ◽  
Vol 59 ◽  
pp. 187-212
Author(s):  
Jeffrey L. Linsky
Keyword(s):  
Shock Waves ◽  
Mass Loss ◽  
Alfven Waves ◽  
Alfvén Waves ◽  
Pressure Gradients ◽  
Late Type ◽  
Periodic Shock ◽  
Thermally Driven ◽  
Mass Outflow ◽  
Field Lines

AbstractFour basic mechanisms have been proposed to explain the acceleration of winds in late-type stars –– thermal pressure gradients, radiation pressure on circumstellar dust grains, momentum addition by Alfvén waves, and momentum addition by periodic shock waves. In this review I describe recent work in applying these mechanisms to stars, and consider whether these mechanisms can work even in principle and whether they are consistent with recent ultraviolet and X-ray data from the IUE and Einstein spacecraft. Thermally-driven winds are likely important for late-type dwarfs, where the mass loss rates are small, and perhaps also in G giants and supergiants, but they cannot operate alone in the K and M giants and supergiants. Radiatively-driven winds are probably unimportant for all cool stars, even the M supergiants with dusty circumstellar envelopes. In principle, Alfvén waves can accelerate winds to high speeds provided the field lines are initially open or forced open by some mechanism, but detailed calculations are needed. Magnetic reconnection is an interesting suggestion for an acceleration mechanism when the field lines are initially closed. For the Mras and semiregular variable supergiants, periodic shock waves provide a simple way of producing rapid mass loss. Thus we are making some progress in understanding mass loss mechanisms for the cool half of the H-R diagram.

Sign in / Sign up

Export Citation Format

Share Document