to the actual value of the latest 15 minute average of the B_z component of the magnetic field vector. The letters N and S refer to North and South of the magnetic equator. The color bar indicates green for positive values, yellow for small negative values , and red for large negative values of B_z.

Why B_z is so important:

When the interplanetary magnetic field turns south compared to the Earth's magnetic field, geomagnetic activity will increase. As the B_z (Southward pointed) value becomes more negative, the associated geomagnetic activity increases.

   2. Speed (V) dial:

Ranges from 200 to 1000 km s-1. The scale is linear over the full range. For values beyond the maximum and below the minimum, the arrow will stay pegged at the maximum or minimum. If the speed data are missing, the arrow will not appear. The arrow will move to the location on the scale corresponding to the actual value of the latest 15 minute average of the Speed V of the solar wind. The color bar indicates green for low, yellow for moderate, and red for high solar wind speed. The arrow moves to indicate speed similar to a car's speedometer.

   3. Dynamic Pressure Dial:

Ranges from 0.1 to 100 nPa. The scale is log10 over the full range. If the density or speed data are missing, the arrow will not appear. The arrow will move to the location on the scale corresponding to the actual value of the latest 15 minute average of the Dynamic Pressure P of the solar wind. Dynamic Pressure is a function of speed and density. The actual formula is

P = 1.6726e-6 * n * V²

where Pressure P is in nPa (nano Pascals), n is the density in particles cm-3 and V is the speed in km s-1 of the solar wind. The color bar indicates green for low, yellow for moderate, and red for high dynamic pressure.

Note:
In space physics there are two different definitions of dynamic pressure commonly used. The formula above is based on the more common definition in use in space physics,

P = density * V²

In this definition, P is the momentum flux of the solar wind. This definition has is origin the flow equations that describe conservation of momentum. The second definition is

P = 1/2 * density * V²

In this definition, P is the kinetic energy per unit volume in the wind. This definition seems to be used less often in space physics, but is the one usually found in aerodynamics and fluid dynamics; it comes from Bernoulli's Equation which is derived from the conservation of energy in a steady flow.