P. A. Dalin, G. N. Zastenker, and M. N. Nozdrachev
Institute of Space Research, Moscow, Russia
I. S. Veselovsky
Institute of Nuclear Physics, Moscow State University, Moscow, Russia
Received 5 April 2001, published online 22 January 2002
There are many sharp and large disturbances in the solar wind plasma density (and in the ion flux). They present the impulses of increasing and decreasing in the solar wind density although the first ones are observed more often than the seconds. The typical time duration of such events is about 1-3 hours. The value of the density impulse can exceed the average level in 3-5 times. Several tens of such events in the solar wind plasma were studied by Shodhan et al. . They associated the origin of these high-density structures with the solar events (coronal mass ejections (CMEs)) and the processes in the interplanetary medium (corotating interaction regions (CIRs)). Usually, these variations and their interaction with the magnetosphere are investigated in the timescale of the order of hours or minutes [Borodkova et al., 1995; Gosling et al., 1987; Sibeck et al., 1991]. In this paper, large and sharp fronts of such events are studied on the basis of high-resolution (1 s) measurements obtained on Interball 1 spacecraft.
This work focuses on a detailed study of the sudden and sharp fronts of large disturbances in the solar wind ion flux. For this study we used Interball 1 ion flux measurements with the 1-s resolution data for 1996 and 1998. These measurements were performed by Faraday cup instrument VDP (see its description in the work of [Safrankova et al., 1997]). We found more than 200 cases where the solar wind ion flux or density had large and sharp variations, that is, the absolute value of density changed on 20% or more, and the duration of the front was less than 15 min. We have concentrated on the examination of the statistical properties of these fronts: the absolute and relative values of the flux variation, durations, and steepness of the sharp change.
The relation between the front duration dT and the absolute magnitude of plasma front dF is shown in Table 1. We can see that most events (45) are observed when the duration ranges from 10 to 60 s and the magnitude of the front varies from 3 to 6 108 cm-2 s-1. The absolute values of more than 6 108 cm-2 s-1 are observed in 37% of the events. As the average value of the ion flux is about 5 108 cm-2 s-1, it means that variations in plasma density appear as the enhancements of a factor of ~2 in one third of the examined events. Very short fronts (less than 10 s) were found in 30 cases (15% of the total number) and in 5 of them, we observed very fast increases with the very large magnitude, more than 10 108 cm-2 s-1. It is necessary to mention that the duration of fronts, as small as several seconds, means that spatial dimension of large solar wind disturbances is less than about 10 proton gyroradii.
We have studied about 20 cases of large (about a factor of 2) solar wind density enhancements observed by two spatially separated spacecraft, Interball 1 (near the Earth) and Wind (near the L1 point, about 1,500,000 km sunward from the Earth). For all events we used measurements of velocity components and ion temperature (with about 1.5 min resolution) by Wind (shifted by solar wind propagation time to the Interball 1 position and interpolated to the moments of Interball 1 measurements) and data series of the ion flux and the magnetic field [Nozdrachev et al., 1998] with a time resolution 1 s by Interball 1. The plasma density value was obtained from Interball 1 ion flux divided by Wind velocity value. Up to now, we did not classify these events by discontinuity analysis, but we investigated qualitatively the temporal and spatial dynamics of solar wind plasma enhancements propagating from L1 point to the Earth.
The characteristic feature of selected events is the rather small change in the bulk velocity and in the velocity components, no more than 5%. At the same time, the variations of the ion thermal speed (in the same cases) were rather strong, more then 50%.
The time behaviour of the magnetic field vector in each case was very different. Sometimes, strong synchronous plasma and magnetic field variations are observed. In some cases they can be interpreted as shock waves, rotational or tangential discontinuities. The magnetic field intensity drops up to very low magnitudes ("magnetic holes") were also observed. Approximately in the half of the selected cases there are no appreciable variations in the magnetic field vector components and in the strength of the field. However, the magnetic field intensity variations when it was observed has the clear tendency to anticorrelate with the solar wind density changes and to form sometimes the pressure balanced structures (i.e., the structures in the solar wind where the sum of plasma thermal pressure and magnetic field pressure across the variation is a constant).
The inhomogeneities of the large-scale structures in the solar wind can be determined by measurements on spatially separated spacecraft in a plane perpendicular to the line of solar wind propagation. As was shown in our previous works [Zelenyi et al., 2000], the solar wind plasma structures do not undergo essential changes on distances up to 100 RE perpendicular to the Sun-Earth line. The solar wind plasma correlation lengths are very large (on average, from 600 to 800 RE ) in the plane perpendicular to its propagation. Thus the solar wind plasma structures have the average dimension exceeding the magnetosphere cross size of about 20 times. So, it seems that the inhomogeneity of the solar wind structures could not be the reason of the event shown in Figure 5.
The origins of the observed impulses and their fronts remain unclear. They could be produced by the dynamical processes near the Sun or in the interplanetary space as was attributed in the paper [Shodhan et al., 1999].
From the other side it seems that the very sharp solar wind ion flux (or density) fronts that we observed cannot survive on the way from the solar corona to the Earth orbit and have to be created in the interplanetary space not far from the point of observation.
The formation processes of these sharp fronts may be strongly dependent on the solar wind conditions. Such dependence is not evident up to now, but it is clear that the enhanced solar wind ion fluxes during the solar minimum periods are usually associated with coronal streamers producing slow, dense, and cold plasma flows [Gosling et al., 1981]. The faster, hotter, and lower density solar wind originates from coronal holes [Nolte et al., 1976].
The solar wind and IMF are remarkably variable in the scale of days, even during the solar minimum. It is mainly because of the streamer belt and adjacent polar coronal holes dynamics, continuously producing plasma outbursts, which are clearly seen in the SOHO-LASCO movies [Sheeley et al., 2000] and in our solar wind measurements near the Earth [Paularena et al., 1998]. During the May-June 1996 period the Sun is near the minimum of the solar activity cycle. The heliospheric current sheet is flat and the corona is rather tenuous. The Earth is immersed mainly in the heliospheric plasma sheet. Because of this, the observed impulses are presumably formed from the coronal streamer material.
It is necessary to mention that for long-scale intervals (about one solar rotation) a rather good correspondence between observed interplanetary structures and their parent solar sources was successfully demonstrated, especially regarding the streamer belt and the coronal hole associations [Galvin and Kohl, 1999]. Nevertheless, the interplanetary medium was still nonstationary in a scale of days, hours, and even minutes showing large variations that would be difficult to compare with rather slow observations of the solar corona and solar magnetic field structures.
1. Many cases of large and abrupt impulses are observed in the solar wind plasma. The most probable durations of increasing/decreasing plasma fronts are about 10-50 s. Shorter fronts are also observed. Most of the plasma density impulses appear as enhancements on about 2 times.
2. The physical nature of these large events is sometimes related to the convective "frozen-in" pressure balance structures in the plasma and the magnetic field. The remaining part is represented by nonlinear MHD perturbations probably generated locally in the interplanetary space. Hence both the "frozen-in" and the evolutionary structures are presented in the solar wind plasma presumably associated with the coronal streamer conditions.
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