Coronal mass ejections (CMEs) occur when long-lived magnetic flux ropes (MFRs) anchored to the solar surface destabilize and erupt away from the Sun. This destabilization is often described in terms of an ideal magnetohydrodynamic instability called the torus instability. It occurs when the external magnetic field decreases sufficiently fast such that its decay index, , is larger than a critical value, , where for a full, large aspect ratio torus. However, when this is applied to solar MFRs, a range of conflicting values for is found in the literature. To investigate this discrepancy, we have conducted laboratory experiments on arched, line-tied flux ropes and applied a theoretical model of the torus instability. Our model describes an MFR as a partial torus with foot points anchored in a conducting surface and numerically calculates various magnetic forces on it. This calculation yields better predictions of that take into account the specific parameters of the MFR. We describe a systematic methodology to properly translate laboratory results to their solar counterparts, provided that the MFRs have a sufficiently small edge safety factor or, equivalently, a large enough twist. After this translation, our model predicts that in solar conditions falls near and within a larger range of , depending on the parameters. The methodology of translating laboratory MFRs to their solar counterparts enables quantitative investigations of CME initiation through laboratory experiments. These experiments allow for new physics insights that are required for better predictions of space weather events but are difficult to obtain otherwise.
Generation and propagation of lower hybrid drift wave (LHDW) near the electron diffusion region (EDR) during guide field reconnection at the magnetopause is studied with data from the Magnetospheric Multiscale mission and a theoretical model. Inside the current sheet, the electron beta (β e) determines which type of LHDW is excited. Inside the EDR, where the electron beta is high (β e ∼ 5), the long-wavelength electromagnetic LHDW is observed propagating obliquely to the local magnetic field. In contrast, the short-wavelength electrostatic LHDW, propagating nearly perpendicular to the magnetic field, is observed slightly away from the EDR, where β e is small (∼0.6). These observed LHDW features are explained by a local theoretical model, including effects from the electron temperature anisotropy, finite electron heat flux, electrostatics, and parallel current. The short-wavelength LHDW is capable of generating significant drag force between electrons and ions. Plain Language Summary The lower hybrid drift wave (LHDW) is generated inside the current sheet by the electric current perpendicular to the local magnetic field. With data from the Magnetospheric Multiscale (MMS) mission, we confirm that two types of LHDW are excited in the current sheet during magnetic reconnection with guide field, depending on the local plasma parameter, called beta (ratio between the magnetic field pressure and plasma pressure). One is the short-wavelength, quasi-electrostatic LHDW excited just outside the central reconnection site, called the electron diffusion region. The other is the long-wavelength, electromagnetic LHDW excited in the electron diffusion region. A local theoretical model is developed to explain the excitation of both types of the LHDW in the current sheet. Results from the model agree with MMS observations. The short-wavelength LHDW is capable of generating additional friction between electrons and ions, indicating possible importance of this LHDW on reconnection and electron dynamics.
In a magnetized, collisionless plasma, the magnetic moment of the constituent particles is an adiabatic invariant. An increase in the magnetic-field strength in such a plasma thus leads to an increase in the thermal pressure perpendicular to the field lines. Above a β-dependent threshold (where β is the ratio of thermal to magnetic pressure), this pressure anisotropy drives the mirror instability, producing strong distortions in the field lines on ion-Larmor scales. The impact of this instability on magnetic reconnection is investigated using a simple analytical model for the formation of a current sheet (CS) and the associated production of pressure anisotropy. The difficulty in maintaining an isotropic, Maxwellian particle distribution during the formation and subsequent thinning of a CS in a collisionless plasma, coupled with the low threshold for the mirror instability in a high-β plasma, imply that the geometry of reconnecting magnetic fields can differ radically from the standard Harris-sheet profile often used in simulations of collisionless reconnection. As a result, depending on the rate of CS formation and the initial CS thickness, tearing modes whose growth rates and wavenumbers are boosted by this difference may disrupt the mirror-infested CS before standard tearing modes can develop. A quantitative theory is developed to illustrate this process, which may find application in the tearing-mediated disruption of kinetic magnetorotational "channel" modes. †
Two magnetopause reconnection events of the Magnetospheric Multiscale mission with whistler wave activity are presented. The whistler mode around half of the electron cyclotron frequency is excited near the magnetospheric separatrix. In both events, there are positive correlations between the whistler wave and the lower hybrid drift instability (LHDI) activities, indicating a possible role of LHDI in the whistler wave generation. A sudden change in the electron pitch angle distribution (PAD) function of energetic electrons is observed right after intense LHDI activity. This change in the PAD leads to temperature anisotropy in energetic electrons which is responsible for the whistler wave excitation. The measured dispersion relation demonstrates that the whistler wave propagates toward the X line nearly parallel to the magnetic field line. Finally, a linear analysis with the measured distribution function verifies that the whistler mode is excited by the temperature anisotropy in energetic electrons.
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