This section gives details of the visualization and function of the interactive tool by demonstrating a previously published event of nongyrotropic velocity distribution (Ieda et al. 2003).
Figure 1 shows magnetic field and plasma data from the magnetic field (MGF) experiment (Kokubun et al. 1994) and the LEP experiment (Mukai et al. 1994b) on board the Geotail spacecraft (Nishida 1994) for a period of 1330–1400 UT on December 12, 1997. Geotail observed a fast, earthward plasma flow at 1345–1353 UT, when it approached the current sheet, as shown by a decrease in the total magnetic field (top panel) close to zero. The energy–time spectrograms of electrons and ions (bottom two panels) show significant changes in energy spectra, compared to the intervals when Geotail was away from the current sheet (before 1345 UT and after 1353 UT). The ion velocity distributions were not gyrotropic during the 1345–1353 UT interval. In the following, we show the capabilities of our visualization tool, using the ion data for the interval, specifically at 1348:00 UT.
When the tool is run by users, it automatically loads data or a pop-up window requests them to choose a data file. Next, the main window opens, as shown in Fig. 2. It allows users to choose a time interval of interest and a variety of visualization modes, initial settings, and options in the setting part (i.e., left part of the main window). The visualization part (i.e., the right part) displays a velocity distribution in velocity space for the designated time interval. Users can rotate the distribution toward any viewing direction by using the mouse. Three independent arrows can be drawn: the magnetic field direction, the bulk velocity vector, and a user-defined vector. All the vectors are centered at [X, Y, Z] = [0, 0, 0]. Using the slider below the box for determining the time interval, users can forward or backward the plot interval by a step defined in the “Step” box. The window displays data averaged over the number of spins of a spacecraft defined in the “Spin sum” box.
The scatter mode displays color-coded physical quantities (particle count or phase space density) at each data point of observations, as shown in Fig. 2. The 3D distribution data are provided in a spherical coordinate system in most of the plasma observations. The scatter mode shows small cubes at the data points in the spherical coordinates with colors representing the data values (i.e., physical quantities). The range of the physical quantity to be displayed can be changed on the “Scatter” tab, as shown in Fig. 3. This mode with the feature of limiting the range helps scientists to see how the original data are distributed in the velocity space, before any interpolation and/or gridding is made.
Figure 4a, b shows examples of the volume mode and the isosurface mode, respectively. The isosurface mode provides two layers, “Isosurface1” and “Isosurface2,” both of which can be independently defined. On the “Isosurface” tab, the range (minimum and maximum), color, and the type of visualization (solid or mesh) of each surface layer can be changed. The mesh option allows the view of a surface inside the other surface, as illustrated in Fig. 4b. The three modes can be chosen simultaneously by clicking the checkboxes on the top left. Figure 4c shows a combination of the scatter and isosurface modes. This helps scientists understand, for example, how widely or narrowly the high value of a physical quantity is distributed, and the exact location in velocity space where the particle data are actually obtained.
A two-dimensional (2D) slice of velocity distributions at a specific particle velocity or kinetic energy is one of the most widely used and helpful visualization methods. The tool, with settings in “Slice” tab, enables the display of a distribution slice of any level of the X–Y, Y–Z, or X–Z plane at any visualization mode or even with no modes selected, as shown in Fig. 5a. Vectors can also be displayed, depending on on/off settings of the vector in the “Vector” tab. With the “Contour” on/off checkbox in the “Slice” tab, contours can be added on the slice. Two or three slices can be displayed simultaneously, as shown in Fig. 5b. Figure 5c shows an example of a combination of two slices and the volume mode from a different viewpoint. The “Reset view” button on the “View” tab can reset the view direction to the default view. Clicking the XY-, XZ-, or YZ-plane buttons enables the jump to the viewpoint facing the selected plane at a right angle.
The tool also provides another function for the 2D slices. A right click on the visualization part allows users to open a new pop-up window, which displays a 2D distribution (X–Y, Y–Z, or X–Z slice) on the left, along with two one-dimensional (1D) distribution cuts on the right. Figure 6 shows a snapshot of a new window, which provides an interactive interface for visualizing the two slices. With the mouse pointer, users can choose between the two axes on the 2D slice, along which the 1D cuts are drawn.