Low-frequency Alfvén Waves Produced by Magnetic Reconnection in the Sun’s Magnetic Carpet

Abstract

The solar corona is a hot, dynamic, and highly magnetized plasma environment whose source of energy is not yet well understood. One leading contender for that energy source is the dissipation of magnetohydrodynamic (MHD) waves or turbulent fluctuations. Many wave-heating models for the corona and the solar wind presume that these fluctuations originate at or below the Sun's photosphere. However, this paper investigates the idea that magnetic reconnection may generate an additional source of MHD waves over a gradual range of heights in the low corona. A time-dependent Monte Carlo simulation of the mixed-polarity magnetic field is used to predict the properties of reconnection-driven coronal MHD waves. The total power in these waves is typically small in comparison to that of photosphere-driven waves, but their frequencies are much lower. Reconnectiondriven waves begin to dominate the total power spectrum at periods longer than about 30 minutes. Thus, they may need to be taken into account in order to understand the low-frequency power-law spectra observed by both coronal spectropolarimetry and in situ particle/field instruments. These low-frequency Alfvén waves should carry more magnetic energy than kinetic energy, and thus they may produce less nonthermal Doppler broadening (in comparison to photosphere-driven high-frequency waves) in emission lines observed above the solar limb.

Transverse motions in sunspot super-penumbral fibrils

“…Moreover, it is not evident how the near-surface convection leads to significant energy input at the larger time scales found in the solar wind. Although, recently Cranmer [16] discussed how magnetic reconnection could provide a contribution to wave generation in the corona from the evolution of the magnetic carpet.…”

On the Origin of Long Period Alfven Waves in the Solar Corona

“…На наш взгляд, генерация долгопериодических альфвеновских волн скорее происходит в нижней короне Солнца. При этом важную роль может играть процесс пересоединения магнитных силовых линий [2]. Однако этот вопрос требует отдельного рассмотрения.…”

“…Альфвеновские возмущения в солнечном ветре с периодами от нескольких минут до нескольких десятков часов известны уже около 50 лет [1]. Между тем происхождение долгопериодических мод в короне Солнца все еще вызывает много вопросов [2]. Между тем как показывают наблюдения, долгопериодические волны альфвеновского типа могут генерироваться в нижней атмосфере [3,4].…”

Assessing the Quality of Models of the Ambient Solar Wind

“…In the photosphere all open field lines are close to closed loops that comprise the magnetic carpet and it has been suggested that some of the slow wind is transferred from the closed carpet field lines to the open field lines through a constant process of local magnetic reconnection (e.g., Cranmer, ). Finally, the S‐web concept can encompass all of these ideas.…”