Resonance maps for 3D Alfvén waves in a compressed dipole field

Funding: Science and Technology Facilities Council (Grant Number(s): ST/N000609/1); National Natural Science Foundation of China (Grant Number(s): 41774172); Leverhulme Trust (Grant Number(s): ECF-2019-155).Resonance Maps depict the possible locations and polarisations of resonant Alfvén waves (Field Line Resonances – FLRs) for a given equilibrium and driving frequency. Previously the use of Resonance Maps has been developed for gaining insight into the behavior of Alfvén waves in basic potential magnetic fields that allow the use of an orthogonal field aligned coordinate system. In more general magnetic fields these coordinates do not exist. In this paper we explore the application of Resonance Maps to such equilibria. A number of simulations of resonant Alfvén waves are presented and interpreted using the Maps. We find that Resonance Maps remain useful when some of the constructions used previously are generalised to accommodate the properties of more general magnetic fields. For example, Resonance Maps are able to predict the location and polarisation of Alfvén waves which are driven strongly by fast mode waves using a “tangential alignment condition”. Unusual properties, such as Alfvén waves crossing flux surfaces persist in the more general magnetic fields we consider.Publisher PDFPeer reviewe

Polarization of magnetospheric ULF waves excited by an interplanetary shock on 27 February 2014

Funding: KT was supported by NASA Grants NNX17AD34G, 80NSSC19K0259, and 80NSSC21K0453. TE was partially funded by Leverhulme Early Career Fellowship ECF-2019-155. ANW was partially funded by the Science and Technology Facilities Council (STFC) Grant (ST/N000609/1). AWD was supported by NSFC Grants 42225405 and NAF\R1\19“1047”.Many previous studies have reported that magnetospheric ultralow frequency waves excited by interplanetary shocks exhibit a strong toroidal component, corresponding to azimuthal displacement of magnetic field lines. However, the toroidal oscillations excited by an interplanetary shock on 27 February 2014 and observed on the dayside by multiple spacecraft were accompanied by a strong poloidal component (radial field line displacement). The frequency of the toroidal oscillations changed with the radial distance of the spacecraft as expected for standing Alfvén waves. We run a 3-D linear numerical simulation of this wave event using a model magnetosphere with a realistic radial variation of the Alfvén velocity. The simulated wave fields, when sampled at a radial distance comparable to those of the observations in the real magnetosphere, exhibit polarization similar to the observations. In the simulation, the poloidal component comes from radially standing fast mode waves and the toroidal component results from a field line resonance driven by the fast mode waves. As a consequence, the relative amplitude and phase of the toroidal and poloidal components change with radial distance.Publisher PDFPeer reviewe

Auroral Morphological Changes to the Formation of Auroral Spiral during the Late Substorm Recovery Phase: Polar UVI and Ground All-Sky Camera Observations

The ultraviolet imager (UVI) of the Polar spacecraft and an all-sky camera at Longyearbyen contemporaneously detected an auroral vortex structure (so-called "auroral spiral") on 10 January 1997. From space, the auroral spiral was observed as a "small spot" (one of an azimuthally-aligned chain of similar spots) in the poleward region of the main auroral oval from 18 h to 24 h magnetic local time. These auroral spots were formed while the substorm-associated auroral bulge was subsiding and several poleward-elongated auroral streak-like structures appeared during the late substorm recovery phase. During the spiral interval, the geomagnetically north-south and east-west components of the geomagnetic field, which were observed at several ground magnetic stations around Svalbard island, showed significant negative and positive bays caused by the field-aligned currents related with the aurora spiral appearance. The negative bays were reflected in the variations of local geomagnetic activity index (SML) which was provided from the SuperMAG magnetometer network at high latitudes. To pursue the spiral source region in the magnetotail, we trace each UVI image along field lines to the magnetic equatorial plane of the nightside magnetosphere using an empirical magnetic field model. Interestingly, the magnetotail region corresponding to the auroral spiral covered a broad region from Xgsm ~ -40 to -70 RE at Ygsm ~ 8 to 12 RE. The appearance of this auroral spiral suggests that extensive areas of the magnetotail (but local regions in the ionosphere) remain active even when the substorm almost ceases, and geomagnetic conditions are almost stable.Comment: 39 Pages, 6 Figures (8 pages), 1 Table, and Supporting Information file (including 2 Figures (8 pages) and 1 Movie

North-south Asymmetric Nightside Distorted Transpolar Arcs within A Framework of Deformed Magnetosphere-Ionosphere Coupling:IMF-By Dependence, Ionospheric Currents, and Magnetotail Reconnection

The terrestrial magnetosphere is perpetually exposed to and significantly deformed by the interplanetary magnetic field (IMF) in the solar wind. This deformation is typically detected at discrete locations by space‐ and ground‐based observations. Earth's aurora, on the other hand, is a globally distributed phenomenon that may be used to elucidate magnetospheric deformations caused by IMF variations, as well as plasma supply from the deformed magnetotail to the high‐latitude atmosphere. We report the utilization of an auroral form known as the transpolar arc (TPA) to diagnose the plasma dynamics of the globally deformed magnetosphere. Nine TPAs examined in this study have two types of a newly identified morphology, which are designated as “J”‐ and “L”‐shaped TPAs from their shapes and are shown to have antisymmetric morphologies in the Northern Hemisphere and Southern Hemisphere, depending on the IMF polarity. The TPA‐associated ionospheric current profiles suggest that electric currents flowing along the magnetic field lines (field‐aligned currents [FACs]), connecting the magnetotail and the ionosphere, may be related to the “J”‐ and “L”‐shaped TPA formations. The FACs can be generated by velocity shear between fast plasma flows associated with nightside magnetic reconnection and slower background magnetotail plasma flows. Complex large‐scale TPA FAC structures, previously unraveled by an magnetohydrodynamic (MHD) simulation, cannot be elucidated by our observations. However, our interpretation of TPA features in a global context facilitates the usage of TPA as a diagnostic tool to effectively remote sense globally deformed terrestrial and planetary magnetospheric processes in response to the IMF and solar wind plasma conditions

Vortex Generation and Auroral Response to a Solar Wind Dynamic Pressure Increase: Event Analyses

In this study, we investigate ionospheric responses, including currents and aurorae, to solar wind dynamic pressure (SW Pdyn) sudden increases, which are critical for understanding solar wind‐magnetosphere‐ionosphere coupling. We focus on two similar SW Pdyn pulse events that occurred on 24 January 2012 and 12 November 2010. In both cases, equivalent ionospheric currents (EIC) vortices were generated within about ten minutes after the pressure pulse arrival, with a counter‐clockwise rotating vortex (viewed from above) observed on the dusk side in the former case, and a clockwise vortex observed on the dawn side in the latter. Simultaneous ground‐based All‐Sky Imager (ASI) observations in the vicinity of the observed EIC vortex in each case showed that aurorae intensified on the dusk side, and diminished on the dawn side. These observations provide direct evidence of the scenario proposed byShi et al. (2014) that magnetospheric flow vortices generated by a solar wind pressure pulse carry field‐aligned currents into the ionosphere and thereby modulate auroral activity. The dawn/dusk asymmetry in the auroral intensification is a direct result of the opposite sense of vortex rotation on the dawn and dusk sides, which generate oppositely directed field‐aligned currents into/out of the ionosphere

Evidence for lunar tide effects in Earth’s plasmasphere

Tides are universal and affect spatially distributed systems, ranging from planetary to galactic scales. In the Earth–Moon system, effects caused by lunar tides were reported in the Earth’s crust, oceans, neutral gas-dominated atmosphere (including the ionosphere) and near-ground geomagnetic field. However, whether a lunar tide effect exists in the plasma-dominated regions has not been explored yet. Here we show evidence of a lunar tide-induced signal in the plasmasphere, the inner region of the magnetosphere, which is filled with cold plasma. We obtain these results by analysing variations in the plasmasphere’s boundary location over the past four decades from multisatellite observations. The signal possesses distinct diurnal (and monthly) periodicities, which are different from the semidiurnal (and semimonthly) variations dominant in the previously observed lunar tide effects in other regions. These results demonstrate the importance of lunar tidal effects in plasma-dominated regions, influencing understanding of the coupling between the Moon, atmosphere and magnetosphere system through gravity and electromagnetic forces. Furthermore, these findings may have implications for tidal interactions in other two-body celestial systems

Model for Relaxation Oscillations in a Helicon Discharge

Relaxation oscillations observed in the large-volume, helicon plasma experiment WOMBAT (Waves on Magnetized Beams and Turbulence) [R. W. Boswell and R. K. Porteous, Appl. Phys. Lett. 50, 1130 (1987)] are modeled. These oscillations have a period of several milliseconds and have been identified as transitions between a low-density, inductive discharge and a high-density, helicon-wave discharge. In the model, it is assumed that the mode transitions are triggered by variations in the neutral density in the source region. The neutral density decreases due to ionization augmented by ion pumping and increases due to refilling of the source chamber from the much larger diffusion chamber. The system is modeled using two, coupled, nonlinear, ordinary differential equations that describe the neutral and plasma densities in the source chamber. Ionization by inductively-coupled fields and ionization due to electrons accelerated by helicon waves with phase velocities near the threshold electron velocity for ionization are considered. The model is found to reproduce experimentally measured variations of the plasma density and helicon wave phase velocity with rf power, neutral pressure and magnetic field. The negative impedance needed for the existence of a relaxation oscillation is provided by the helicon-wave coupling mechanism

Intense On-Axis Plasma Production and Associated Relaxation Oscillation in a Large-Volume Helicon Source

A helicon wave mode with a peak downstream density of greater than 1018 m-3 in argon that exhibits bright ArII emission along the axis has been characterized. The experimental conditions are: Ar gas pressure of 1-5 mTorr, external magnetic field of 70-150 G and radio frequency (rf) power input between 2 and 4 kW a 13.56 MHz using a double half-turn antenna into a source of 9 cm inner radius and 50 cm length that opens into a diffusion chamber 45 cm radius and 200 cm length. Radial profiles of the density in the source and downstream show that plasma production is strongly concentrated on axis, B-dot probe measurements indicate that the wave phase velocity in this discharge mode is between 2 and 2.5 × 106 m/s, which has been shown previously to be the optimum velocity for resonant wave heating of electrons to increase the ionization rate. An interesting property of the high-density mode is that it is unstable on timescales of a few milliseconds and that a relaxation oscillation occurs between the high- and low-density modes. It is believed that this is driven by the depletion of neutrals in the source region due to ionization and momentum exchange with ions leaving the source

Absolute measurements and modeling of radio frequency electric fields using a retarding field energy analyzer

Measurements of the rf electric field have been made along the z axis of a helicon reactor using a retarding field energy analyzer. A fluid code and a simple analytical model have been developed to analyze the ion energy distribution functions, especially in the case of bimodal distributions where the ion transit time through the sheath in front of the analyzer is comparable to the rf period. A generalized curve (and an analytical approximation to that curve) has been developed from the analytical model and confirmed by the self-consistent fluid model for high, low, and intermediate ion transit time, which can be used by experimenters to quickly convert the experimental results (energy peak separation, plasma potential and density, electron temperature), which are related to rf sheath oscillations, to absolute values of the rf electric field. An analysis of the errors involved in the derivation of the field is given. The results agree qualitatively with rf pickup measured with a floating Langmuir probe