Sporadic Aurora near Geomagnetic Equator: In the Philippines, on 27 October 1856

While low latitude auroral displays are normally considered to be a manifestation of magnetic storms of considerable size, Silverman (2003, JGR, 108, A4) reported numerous "sporadic auroras" which appear locally at relatively low magnetic latitudes during times of just moderate magnetic activity. Here, a case study is presented of an aurora near the geomagnetic equator based on a report from the Philippine Islands on 27 October 1856. An analysis of this report shows it to be consistent with the known cases of sporadic aurorae except for its considerably low magnetic latitude. The record also suggests that extremely low-latitude aurora is not always accompanied with large magnetic storms. The description of its brief appearance leads to a possible physical explanation based on an ephemeral magnetospheric disturbance provoking this sporadic aurora.Comment: 15 pages, 3 figures, accepted for publication in Annales Geophysicae on 18 August 201

How do auroral substorms depend on Earth's dipole magnetic moment?

Earth's dipole magnetic moment M is known to decrease by ∼9% over the past 150 years. It has been argued that the decrease in M makes the near-Earth space environment different. We investigated how the change in M affects the development of an auroral substorm by increasing and decreasing M by a factor of 1.5 in global magnetohydrodynamics simulation. The ionospheric conductivity decreases with increasing M, in accordance with the aid of empirical relations. When we imposed the southward interplanetary magnetic field, an auroral substorm took place regardless of M, but its development depends largely on M. When M is lower, (1) the expansion onset takes place later, (2) the auroral electrojet develops slowly, and (3) the maximum auroral electrojet increases. The first two consequences are probably associated with the slow magnetospheric convection as manifested by the polar cap potential drop. The third consequence is associated with the nonlinear dependence of substorm-associated field-aligned currents (FACs) on the ionospheric conductivity. The maximum values of the westward auroral electrojet and the net FACs increase with decreasing M, whereas the incident magnetic energy into the magnetosphere decreases with decreasing M. This implies that the efficiency of the generation of the substorm-associated FACs increases with decreasing M. It is also found that, for the lower M-value, the auroral oval shifts equatorward during the growth phase and expands more equatorward and poleward during the expansion phase. Evolution of substorms depends largely on the value of Earth's dipole moment and the ionospheric conductivity

Generation of Field-Aligned Currents During Substorm Expansion: An Update

We investigated generation processes of field-aligned currents (FACs) that are abruptly intensified at the beginning of the substorm expansion phase by tracing a packet of the Alfvén wave backward in time from the onset position in the ionosphere in the global magnetohydrodynamics (MHD) simulation. The generation region is found in the near-Earth plasma sheet, in which (a) azimuthally moving plasma pulls the magnetic field line, and performs negative work against the magnetic tension force to excite the Alfvén waves, (b) FACs are generated from the requirement of Ampère and Faraday laws, and (c) field-perpendicular current is converted to FACs. We call this near-Earth FAC dynamo. The plasma involved originates in the tail lobe region. When near-Earth reconnection occurs in the plasma sheet, the plasma is accelerated earthward by the Lorentz force, and decelerated by the plasma pressure gradient force, followed by the Lorentz force. The flow is deflected to the west and east directions by the plasma pressure gradient force and the Lorentz force, resulting in the excitation of Alfvén waves and FACs. The Alfvén waves propagate along the magnetic field in the rest frame of the moving plasma. When it arrives at the ionosphere, the auroral electrojet starts developing and the substorm expansion phase begins. The near-Earth FAC dynamo can be distinguished from the near-Earth dynamo (J · E < 0, where J is the current density and E is the electric field). We suggest that the evolution of the substorm can be understood in terms of the development of FACs

Quantitative ring current model: Overview and comparison with observations

This paper describes a new quantitative ring current model that solves temporal evolution of the ion distribution in the magnetosphere by tracing the ion drift motion. The plasma sheet density as a boundary condition of our model depends on the solar wind density. The tracing is performed under a dipole magnetic field and a time-dependent convection electric field depending on the solar wind parameters. The ions are lost by two processes; the charge exchange with neutral hydrogen and the convection outflow due to encounter with the dayside magnetopause. Magnetic disturbance is directly derived from the calculated current density with the three- dimensional Biot-Savart integral; this is a new simulation method. Using this model, we have examined the physical mechanism of the storm-time ring current responding to the interplanetary parameters. We simulated three successive storms which occurred in April 1997 as a case study. The following subjects concerned with dynamics of the ring current were examined; (1) the causes of the ring current development, (2) the electric current distribution, (3) the effects of the charge exchange loss, (4) the energy composition of the plasma pressure, (5) the response time lag of the plasma sheet density variation to the solar wind density and (6) the diamagnetic effect

Evolution of auroral substorm as viewed from MHD simulations: dynamics, energy transfer and energy conversion

An auroral substorm is a visual manifestation of large-scale, transient disturbances taking place in space surrounding the Earth, and is one of the central issues in the space plasma physics. While a number of studies have been conducted, a unified picture of the overall evolution of the auroral substorm has not been drawn. This paper is aimed to overview the recently obtained results of global magnetohydrodynamics (MHD) simulations in a context of a priori presence of anomalous resistivity leading to magnetic reconnection, and to illuminate what the global MHD simulation can sufficiently reproduce the auroral transients during the auroral substorm. Some auroral transients are found to be seamlessly reproduced by the MHD simulation, including complicated auroral structures moving equatorward during the growth phase, auroral brightening starting to appear near the equatorward border of the preexisting auroral arc, and an auroral surge traveling westward. Possible energy transfer and conversion from the solar wind to the Earth are also overviewed on the basis of the MHD simulation. At least, 4 dynamo regions appear sequentially in the course of the development of the auroral substorm. Although the MHD simulation reproduces some transients, further studies are needed to investigate the role of kinetic processes

Three‐Dimensional Closure of Field‐Aligned Currents in the Polar Ionosphere

Using a simplified three-dimensional Hall-magnetohydrodynamics simulation, we investigated the current closure of field-aligned currents (FACs) in the polar ionosphere. Ion-neutral collision was taken into consideration. To excite a pair of FACs, an electric field perturbation is applied to the upper boundary of the simulation box. The flow shear propagated downward accompanied with the FACs. When the electron density was initially uniform, most of the FACs are connected with the Pedersen current due to electrostatic processes. Some of them are connected with the Hall current due to inductive processes. When the density was initially enhanced in a longitudinally elongated region (high-density band), overflow of the Hall current takes place near the edge of the high-density band. Additionally, localized FACs appear to bridge between the Pedersen current layer (high altitude) and the Hall current layer (low altitude). The formation of the additional FACs is closely associated with the field-aligned gradient of ∇∙E, where E is the electric field. We compared the current lines with those evaluated by the traditional thin-layer assumption. The current closure obtained by the thin-layer assumption is fully different from that obtained by the three-dimensional model. In the full three-dimensional simulation, a current line flowing in the Hall layer can pass underneath the current flowing in the Pedersen layer. Such “intersection” is not allowed in the thin-layer assumption. We believe that the three-dimensional model offers advantages for fully understanding the closure of the FACs together with the current closure on the magnetospheric side

Prediction of geomagnetically induced currents (GICs) flowing in Japanese power grid for Carrington-class magnetic storms

Large-amplitude geomagnetically induced currents (GICs) are the natural consequences of the solar–terrestrial connection triggered by solar eruptions. The threat of severe damage of power grids due to the GICs is a major concern, in particular, at high latitudes, but is not well understood as for low-latitude power grids. The purpose of this study is to evaluate the lower limit of the GICs that could flow in the Japanese power grid against a Carrington-class severe magnetic storm. On the basis of the geomagnetic disturbances (GMDs) observed at Colaba, India, during the Carrington event in 1859, we calculated the geoelectric disturbances (GEDs) by a convolution theory, and calculated GICs flowing through transformers at 3 substations in the Japanese extra-high-voltage (500-kV) power grid by a linear combination of the GEDs. The estimated GEDs could reach ~ 2.5 V/km at Kakioka, and the GICs could reach, at least, 89 ± 30 A near the storm maximum. These values are several times larger than those estimated for the 13–14 March 1989 storm (in which power blackout occurred in Canada), and the 29–31 October 2003 storm (in which power blackout occurred in Sweden). The GICs estimated here are the lower limits, and there is a probability of stronger GICs at other substations. The method introduced here will be immediately applicable for benchmark evaluation of low-latitude GICs against the Carrington-class magnetic storms if one assumes electrical parameters, such as resistance of transmission lines, with sufficient accuracy