Space Weather Dials Interpretation Guide
This page provides explanatory text for the dials shown on the Rice
Current Solar Wind Conditions
and Current Solar
Wind Conditions (Dials Only) webpage. These dials use the last good
received set of measurements from the
Real Time Solar Wind package on the
Advanced Composition Explorer (ACE) spacecraft. These data are relayed
through the Space Environment Center of the
National Oceanic and Atmospheric
Administration (NOAA), United States
Department of Commerce. The ACE spacecraft is stationed
near the Earth-Sun First Lagrange Point (L1), so it can stay constantly between
the Earth and the Sun without using too much fuel. This L1 point is about
45 minutes upwind of the Earth in normal solar wind conditions.
On the dial background color schemes, green indicates that values in this
range are unlikely to disturb the near-Earth space environment. Yellow
indicates that values in this range may contribute to disturbances, and Red
indicates that values in this range are
likely to drive disturbances. This scheme
is valid for all dials except the "Log[Beta]" dial for which a different scheme
Quantities Shown on the Dials:
Solar Wind Density:(Measured) This quantity is the number of solar
wind protons per unit volume as measured by the ACE
Solar Wind Electron
Proton Alpha Monitor (SWEPAM).
Solar Wind Speed:(Measured) This quantity is the average
("bulk") speed of solar wind protons as measured by ACE/SWEPAM. This is
the solar wind speed just as the bulk speed of air molecules is the
"wind speed" we know here on the surface of the Earth.
Solar Wind Pressure:(Derived) This quantity is the solar wind
ram pressure, the force per unit area required to stop the solar wind
flow. This is similar in concept to the force a surface wind exerts on a sail.
The solar wind ram pressure depends on the solar wind speed and density.
Solar Wind Temperature:(Measured) This quantity is the temperature
of protons in the solar wind. It is measured by ACE/SWEPAM.
Interplanetary Magnetic Field Magnitude:(Measured) This quantity
is the strength of the interplanetary magnetic field (IMF) as measured by the
Interplanetary Magnetic Field Polar Angle:(Derived) This quantity
is the angle between the IMF and the geomagnetic axis. When the IMF is
southward, antiparallel fields near the magnetospheric subsolar point allow
merging between the IMF and geomagnetic fields. This process increases the
transport of solar wind mass, momentum, and energy into the Earth's
magnetosphere. This process can also open the magnetosphere to solar energetic
particle radiation. In severe conditions this radiation can threaten
high altitude aircraft in high latitude and polar regions. Under less severe
conditions this radiation can still threaten polar orbiting spacecraft.
This quantity depends on IMF components measured by ACE/MAG.
Interplanetary Magnetic Field Azimuth:(Derived) This quantity
is the direction of the IMF perpendicular to the geomagnetic axis. This
affects the details of solar wind-magnetosphere interactions; however, this is
of tertiary importance compared to the IMF magnitude and polar angle. This
quantity also depends on IMF components measured by ACE/MAG.
Voltage Across the Polar Cap / Convection Potential:(Derived)
This quantity measures the solar wind energy input to the magnetosphere that
drives magnetospheric convection. It appears as an electric potential imposed
across the polar ionosphere. The quantity shown here is an estimate of the
asymptotic convection potential based on ACE/SWEPAM and ACE/MAG measurements
as well as the work of Boyle, et al. (Journal of Geophysical Research 102,
111, 1997.) This estimate is asymptotic because it does not
account for the time delays such as those imposed by friction between the
ionosphere and the neutral atmosphere.
Quantities Shown on the Dials (Dials Only Page)
Alfven Speed:(Derived) This quantity is the propagation speed
of shear Alfven (intermediate mode) magnetohydrodynamic waves in the solar
Sound Speed:(Derived) This quantity is the propagation speed
of gasdynamic (sound) waves in the solar wind. Although collision rates are
generally so low in the solar wind that classic sound waves do not travel
effectively, this quantity is necessary for calculating the propagation speeds
of compressional Alfven (fast and slow mode) magnetohydrodynamic waves in the
solar wind. The magnetohydrodynamic waves together allow the use of
gasdynamic approximations to describe portions of the solar wind-magnetosphere
Thermal Energy Density:(Derived) This quantity is the heat
content of the solar wind. It takes into account both solar wind density
and temperature and can be translated into solar wind thermal pressure.
This quantity is generally less important than either the solar wind ram
pressure or the solar wind (IMF) magnetic pressure.
Log[Beta]:(Derived) "Beta" is the ratio between the thermal and
magnetic energy densities in the solar wind; this ratio controls whether
particle thermal processes or magnetic processes dominate the behavior of the
plasma. The base-10 logarithm of Beta is shown on the dial. The blue portion
of the dial shows when magnetic processes govern solar wind structures; the
purple portion shows when thermal processes govern these structures.
Alfven Mach Number:(Derived) This quantity is the ratio between
the solar wind speed and the Alfven speed. This normally controls the type
of bow shock required to divert the solar wind around the magnetosphere. This
bow shock is similar to the shock in front of a supersonic airplane that
generates a "sonic boom" when the shock passes an observer.
Mach Number:(Derived) This quantity is the classic Mach number,
the ratio between the solar wind speed and the gasdynamic sound speed. This
quantity controls the bow shock when the solar wind plasma is in a high-Beta
This page was last revised on 04/20/11.