J. R. Kan

A model for thinning of the plasma sheet

Abstract A one-dimensional model for thinning of the plasma sheet is developed on the basis of launching a fast mode MHD rarefaction wave propagating in the tailward direction along the plasma sheet. Behind the rarefaction wave the pressure is reduced, leading to thinning of the plasma sheet and also to an Earthward plasma flow with a speed on the order of the sound speed a 0 . The plasma sheet thickness is reduced by a factor of 2 if an Earthward plasma flow speed of 0.8 a 0 is induced. The predictions of the model are in reasonable agreement with observations.

Generation of auroral kilometric radiation and the structure of auroral acceleration region

Abstract Generation of auroral kilometric radiation (AKR) in the auroral acceleration region is studied. It is shown that auroral kilometric radiation can be generated by the backscattered electrons trapped in the acceleration region via a cyclotron maser process. The parallel electric field in the acceleration region is required to be distributed over 1–2 RE. The observed AKR frequency spectrum can be used to estimate the altitude range of the auroral acceleration region. The altitudes of the lower and upper boundaries of the acceleration region determined from the AKR data are respectively ∼2000 and ∼9000 km.

Geospace magnetic field responses to interplanetary shocks

We perform a statistical survey of geospace magnetic field responses, including the geosynchronous magnetic field and the sudden impulses on the ground, to interplanetary shocks (IP shocks) between 1998 and 2005. The magnitude of the geosynchronous magnetic field (dB(z)) responses to IP shocks depends strongly on local time, which peaks near the noon meridian; however, the relative magnitude of the responses depends only weakly on local time. These results are similar to those obtained from the statical study of the responses to solar wind dynamic pressure pulses. However, negative responses (where dBz is negative) were sometimes observed in the nightside of the magnetosphere even though the IP shocks always caused increases in the solar wind dynamic pressure, a new phenomenon not widely reported in the literature. Our analysis shows that similar to 75% of negative responses in the midnight sector are associated with southward interplanetary magnetic field. For a moderately compressed magnetosphere, the amplitude of the geosynchronous response dBz could be determined by the average value of the background local magnetic field. As the magnitude of the upstream solar wind dynamic pressure increases, the rate of response increases correspondingly. The dBz at the geosynchronous orbit near local noon and the amplitude of sudden impulses (dSYM-H) on the ground are highly correlated.

A dynamo theory of solar flares

It is proposed that the solar flare phenomenon can be understood as a manifestation of the electrodynamic coupling process of the photosphere-chromosphere-corona system as a whole. The system is coupled by electric currents, flowing along (both upward and downward) and across the magnetic field lines, powered by the dynamo process driven by the neutral wind in the photosphere and the lower chromosphere. A self-consistent formulation of the proposed coupling system is given. It is shown in particular that the coupling system can generate and dissipate the power of 1029 erg s#X2212;1 and the total energy of 1032 erg during a typical life time (103 s) of solar flares. The energy consumptions include Joule heat production, acceleration of current-carrying particles along field lines, magnetic energy storage and kinetic energy of plasma convection. The particle acceleration arises from the development of field-aligned potential drops of 10–150 kV due to the loss-cone constriction effect along the upward field-aligned currents, causing optical, X-ray and radio emissions. The total number of precipitating electrons during a flare is shown to be of order 1037–1038.

The energy coupling function and the power generated by the solar wind-magnetosphere dynamo

Abstract A solar wind parameter e , known as the energy coupling function, has been shown to correlate with the power consumption in the magnetosphere. It is shown in the present paper that the parameter e can be identified semi-quantitatively as the dynamo power delivered from the solar wind to an open magnetosphere. This identification not only provides a theoretical basis for the energy coupling function, but also constitutes an observational verification of the solar wind-magnetosphere dynamo along the magnetotail. Moreover, one can now conclude that a substorm results when the dynamo power exceeds 10 18 ergs −1 .

Particle dynamics in reconnection field configurations

Abstract Trajectories of test particles are studied numerically in two types of reconnection magnetic field configurations, a single X-line magnetic field configuration and a tearing magnetic field configuration. Both adiabatic and nonadiabatic motions are examined, with special emphasis on net energy gain and time spent in the neutral line regions. They spend typically one characteristic gyroperiod in the X-line region and are ejected predominantly along field lines in the vicinity of the separatrix. Both adiabatic and nonadiabatic test particles in the tearing-type field configuration are channelled into and accelerated along the O-line region. It may be inferred from these test particle results that particle energizations are significant along the O-line region, but not along the X-line region. These results are in qualitative agreement with those obtained by a self-consistent particle simulation.

Power transmission from the solar wind-magnetosphere dynamo to the magnetosphere and to the ionosphere: Analysis of the IMS Alaska meridian chain data

Abstract It is shown that the power e generated by the solar wind-magnetosphere dynamo is transmitted to the convective motion of magnetospheric plasma. This convective motion generates what we may call the “Pedersen counterpart currents” in the magnetosphere and drives a large part of the “region 1 and 2” field-aligned currents which are closed by the Pedersen currents in the ionosphere. These results are based on a self-consistent set of the ionospheric current and potential distribution patterns obtained from a study of the International Magnetosphere Study Alaska meridian chain data.

Dynamo process governing solar wind-magnetosphere energy coupling

Abstract Based on the method of dimensional analysis, the energy transfer rate from the solar wind into the magnetosphere can be characterized by a magnetic coupling parameter α on open field lines and by a viscous coupling parameter β on closed field lines. By assuming that the energy transfer rate can be monitored by the total energy dissipation rate of the magnetosphere, the histogram of α is constructed and is found to peak around −0.1 α

Variations of the polar cap potential measured during magnetospheric substorms

Measurements of the polar cap potential drop and size have been obtained during magnetospheric substorms. Using double-probe electric field measurements on the DE 2 satellite, 148 measurements have been obtained at random times preceding, during, and after 64 substorms. The polar cap potentials are graphed as a function of the difference between the time of the polar cap measurement and the time of the expansion onset of the corresponding substorm. The ratios of the auroral electrojet (AE) indices and the potential are also determined. The results show that on the average the polar cap potential starts to increase at 1.5 hours before onset, reaches a level of 70 kV in the 30-min period before onset, and starts to decline at 1.5 hours after onset. However, on a case-by-case basis there are substantial variations from the average, as polar cap potentials over 120 kV were measured as early as 1 hour before substorm onset and values as low as 40 kV were observed during the expansion phase. The size of the polar cap ranged from 23° to 38° invariant latitude at the time of onset, and had an average value of 31°. The AE/ΦPC ratio is nearly constant before and after substorms, but decreases slightly during the substorm growth phase and increases greatly during the expansion phase. This increase is most likely due to a higher conductivity and westward electric field within the electrojet during expansion, which causes AE to increase without a corresponding change in the polar cap potential.

Bow shock contributions to region 1 field‐aligned current: A new result from global MHD simulations

We present a new result, based on a global MHD simulation model, that the bow shock contributes significantly to the region 1 field-aligned current (FAC) under strong southward interplanetary magnetic field conditions. More than 50 percent of the total region 1 FAC may originate from the bow shock in certain circumstances. Stronger southward interplanetary magnetic field, higher solar wind speed, or larger ionospheric Pedersen conductance, leads to greater contribution from the bow shock to the region 1 current. This new result quantifies the interaction between the bow shock and the ionosphere through the region 1 current. The methodology presented in the present paper should be applicable to quantify the bow shock contributions to the substorms through the region 1 current, as well as to the magnetic storms through the ring current and the region 2 current, leading to understanding the space weather as magnetic storms in the solar-terrestrial environment. Citation: Guo, X. C., C. Wang, Y. Q. Hu, and J. R. Kan (2008), Bow shock contributions to region 1 field-aligned current: A new result from global MHD simulations.