D. J. Wu

Solitary kinetic Alfvén waves in the two‐fluid model

Employing the two‐fluid model, a generalized Sagdeev equation governing solitary kinetic Alfven waves (SKAWs) and the criterion for the existence of SKAWs, which are valid for different ranges of plasma pressure parameter β, are presented. In the limit cases of β≫me/mi and β≪me/mi, the present results correspond, respectively, with conclusions obtained by Hasegawa et al. [Phys. Rev. Lett. 37, 690 (1976)] and by Shukla et al. [J. Plasma Phys. 28, 125 (1982)], that is, SKAWs accompanied by, respectively, hump and dip density solitons for β≫me/mi and β≪me/mi. However, for the case of β∼me/mi, the present results show that SKAWs accompanied by both hump and dip density solitons are possible, and lead to KdV solitons in the small amplitude limit. In addition, the possibility for applying these results to electromagnetic spikes observed by the Freja scientific satellite is discussed [detailed information about the Freja satellite experiments can be found in serial papers presented in Space Sci. Rev. 70, Nos. 3/...

Magnetic flux cancellation associated with the major solar event on 2000 July 14

The major solar event on 2000 July 14 is characterized by the simultaneous occurrence of a giant filament eruption, a great flare, and an extended Earth-directed coronal mass ejection. We examined in detail the magnetic evolution in its source active region, NOAA 9077, and found that the only obvious magnetic change in the course of the event is magnetic flux cancellation at many sites in the vicinity of the filament. Moreover, all the initial disturbance in the filament and the initial brightening around the filament took place at the cancellation sites. It is clearly indicated that the slow magnetic reconnection in the lower atmosphere, which is manifested as observed flux cancellation, is of overwhelming importance in leading to the global instability responsible for the major magnetic activity.

Size and energy distributions of interplanetary magnetic flux ropes

In observations from 1995 to 2001 from the Wind spacecraft, 144 interplanetary magnetic flux ropes were identified in the solar wind around 1 AU. Their durations vary from tens of minutes to tens of hours. These magnetic flux ropes include many small- and intermediate-sized structures and display a continuous distribution in size. Energies of these flux ropes are estimated and it is found that the distribution of their energies is a good power law spectrum with an index similar to - 0.87. The possible relationship between them and solar eruptions is discussed. It is suggested that like interplanetary magnetic clouds are interplanetary coronal mass ejections, the small- and intermediate-sized interplanetary magnetic flux ropes are the interplanetary manifestations of small coronal mass ejections produced in small solar eruptions. However, these small coronal mass ejections are too weak to appear clearly in the coronagraph observations as an ordinary coronal mass ejection.

Comparative study of bow shock models using Wind and Geotail observations

Wind and Geotail observed bow shock (BS) crossings were selected from the 1998 to 2001 ISTP database. We analyzed 625 case events containing 4381 Geotail BS crossings and 130 case events containing 917 Wind BS crossings. The location of the BS crossings, in the aberrated GSE coordinate system, varied over a wide range from -85 Re to 45 Re along the X-GSE axis, up to 90 Re in the perpendicular direction. ACE, Wind, and Geotail measurements were used to determine the upstream solar wind conditions in the interplanetary medium. These conditions were determined for the BS crossings in each case event by using the delay time of direct solar wind propagation from an upstream monitor to the probe satellite (Wind or Geotail). The solar wind conditions for the BS crossings varied over a wide range of dynamic pressures Pd (from 0.02 nPa to 49 nPa), IMF Bzs (from -26 nT to 23 nT), thermal/magnetic pressure ratios beta (from 0.002 to 50), and magnetosonic Mach numbers M(ms) (from 1.02 to 29). Such a wide spatial and dynamic range of BS crossings permits us to consider the different parameters that control the BS size and shape, such as the radius of curvature of the magnetopause which depends on Pd and Bz, the Alfven, sonic, and magnetosonic Mach numbers, and the IMF orientation. To study the dependence on these parameters, we compared the accuracy of the BS models formulated by Peredo et al. [1995], Russell and Petrinec [1996], Verigin et al. [2001b], and Chao et al. [2002] for the prediction of selected BS crossings observed in different bow shock regions and with various upstream solar wind conditions. It was found that the Chao et al. [2002] model had the best capability for predicting the BS crossings. The solar wind dynamic pressure and magnetosonic Mach number were determined to be the most important parameters controlling the BS size and shape. The important role of the dawn-dusk asymmetry of the bow shock tail region is emphasized. The effect of the southward IMF influence on the dayside magnetosheath thickness is revealed and discussed.

OBSERVATIONS OF ANISOTROPIC SCALING OF SOLAR WIND TURBULENCE

Using high-speed solar wind data recorded by the Ulysses spacecraft, we investigate and estimate the anisotropic inertial range scaling of the interplanetary magnetic field. We apply the method of the magnetic structure function (MSF), S-n(tau) = proportional to tau(zeta(n)), to analyze the scaling of solar wind turbulence over the range from 1 s to 10(4) s. By sorting the fluctuations according to the direction of the local mean magnetic field, we obtain a second-order structure function in (r, Theta) coordinates that reveals the scale-dependent anisotropy of the power spectrum. The scale-dependent anisotropy of the MSF indicates that the fluctuation energy tends to cascade toward the direction perpendicular to the local field. The dependence of the MSF scaling index zeta on the direction of the local field is found to be similar to that reported in Horbury et al. and Podesta, with zeta(perpendicular to) = 0.53 +/- 0.18 and zeta(parallel to) = 1.00 +/- 0.14. Furthermore, we estimate and find the scaling law between the perpendicular and parallel scales r(parallel to) proportional to r(perpendicular to)(0.614), which implies the elongation along the parallel direction as the turbulence eddy evolves toward the small lengthscales. These results are in agreement with the predictions of magnetohydrodynamic turbulence theory.

An analytical solution of finite‐amplitude solitary kinetic Alfvén waves

An analytical solution of finite‐amplitude solitary kinetic Alfven waves (SKAWs) in a low‐β (β≪me/mi≪1) plasma is presented. This solution has been compared with the solution of the Korteweg–de Vries (KdV) equation in the small‐amplitude limit. It is found that the KdV soliton solution is valid only for the maximum relative density perturbation Nm<0.1. For the larger Nm, the exact analytical solution shows that the SKAWs have a much wider structure and much stronger perturbed fields than the KdV solitons with the same Nm. Moreover, the relations between the width and the amplitude of SKAWs are also considerably different from that of the KdV solitons. In addition, the possibility for applying these results to some events observed from the Freja scientific satellite is discussed. (The Freja is a Swedish–German scientific project for the investigation of ionospheric and magnetospheric plasmas, and the Freja satellite was launched on a Long‐March II rocket of China on October 6, 1992.)

Interplanetary small‐ and intermediate‐sized magnetic flux ropes during 1995–2005

We present a comprehensive survey of 125 small- and intermediate-sized interplanetary magnetic flux ropes during solar cycle 23 (1995-2005) using Wind in situ observations near 1 AU. As a result, we found the following: (1) The annual number of small- and intermediate-sized interplanetary magnetic flux ropes is not very sensitive to the solar cycle, but its trend is very similar to that of magnetic clouds (MCs). (2) Average speeds of the individual small- and intermediate-sized interplanetary magnetic flux ropes varied from 289 to 790 km/s with a mean value of 420 +/- 86 km/s. Most small- and intermediate-sized interplanetary magnetic flux ropes were found to have a propagation speed similar to typical slow speed solar wind speed, and only a few small- and intermediate-sized interplanetary magnetic flux ropes had speeds comparable to the typically high speed solar wind. (3) Average magnetic field strength for small- and intermediate-sized interplanetary magnetic flux ropes is less than the average magnetic field strengths of MCs, while it is larger than that of background solar wind. (4) The distributions of the axial orientations for small- and intermediate-sized interplanetary magnetic flux ropes are also similar to that of MCs. The results show that small- and intermediate-sized interplanetary magnetic flux ropes and MCs have many similar (or relative) characters. So we suggest that both MCs and small- and intermediate-sized interplanetary magnetic flux ropes originate from solar eruptions.

Two-fluid motion of plasma in Alfven waves and the heating of solar coronal loops

Taking the effect of two-fluid motion of plasma in Alfven waves into account, a nonzero parallel electric field can rise within the wave itself under collisionless conditions. This leads to a kinetic dissipation of Alfven waves by the wave-particle resonant interaction and electron heating along the ambient magnetic field lines. Employing the drift kinetic equation, we investigated this electron-heating mechanism under the cool ion approximation. The result shows that the damping rate and the heating rate obviously depend on the strength distribution of the ambient magnetic field, and that they reach their maximum values at B-0 = B-m and B-0 = (1 + 2 alpha)B-1/2(m), respectively, for the perturbed field of delta B proportional to B-0(alpha), where B-m is the ambient magnetic field strength when upsilon(A) = upsilon(Te). Finally, we propose that this heating mechanism can be applied to explain the brightness distribution of solar soft X-ray coronal loops.

Model of auroral electron acceleration by dissipative nonlinear inertial Alfven wave

In a recent work [Wu, 2003a, 2003b], a dissipative nonlinear inertial Alfven wave (DNIAW) were proposed as the physical explanation for the formation of the strong electric spikes often observed in the auroral ionosphere and the magnetosphere. DNIAW can also lead to the field_aligned electron acceleration. In the present paper, dynamical characteristics of DNIAW acceleration are discussed and its possible role in auroral electron acceleration is further investigated. The effective acceleration region for auroral electrons with energies of the order of keV produced by DNIAW acceleration is between 0.5 and 2.5 R-E above the ionosphere, and the most efficient acceleration occurs around 0.8 R-E where both the Alfven velocity and the produced auroral electron energy peak, and the peak energy is around 10 keV. We suggest that this could explain the precipitous decrease of the auroral electron energy spectrum toward energies above 10 keV, which can be inferred from measurements of energy distribution of precipitating auroral electrons. Typical widths of auroral arcs caused by the DNIAW acceleration are in scales of the order of 1 km.

Dissipative solitary kinetic Alfvén wave and electron acceleration

Some recent studies of observations in situ by space satellites show that low frequency electromagnetic fluctuations in the auroral ionosphere and magnetosphere can often be identified as soliatry kinetic Alfven waves (SKAWs), and further analyses of the data reveal clearly that electron collisional dissipation can considerably affect the structure and evolution of SKAWs. In this paper, a model of nonlinear kinetic Alfven waves, called a dissipative SKAW (DSKAW), is presented, in which the effect of electron collisional dissipation has been taken into account. The results show that DSKAW can produce a local shock-like structure with a net parallel electric potential drop, in which the associated parallel electric field is primarily caused by nonlinear electron inertia. In particular, it is argued that DSKAW can accelerate electrons efficiently to the order of the local Alfven velocity. This suggests that DSKAW can provide an efficient acceleration mechanism for energetic electrons, which can frequently be encountered in various space and cosmic plasma environments