POES energetic particles | How the comparison is made | Time of observation
Belt Indices and the Solar Cycle | L-Value Curves

Comparison of Energetic Particle Observations
with the One-Year Baseline


The display above shows the responses of the 90° electron sensor to >30 keV electrons throughout a recent day compared with the median responses of that detector over the past year. The red box in the plot shows the satellite location at the beginning of this day's observations. The red triangle shows its location when the last measurements were downloaded. Data gaps or missing orbits appear as gaps in the satellite track around the earth. The 90°, >30 KeV electron sensor is one of 22 sensor channels in the NOAA POES second generation Space Environment Monitor, SEM-2.

POES Energetic Particles

The second-generation Space Environment Monitor (SEM-2) onboard the NOAA Polar Orbiting Environmental Satellite (POES) orbits the Earth in a high-inclination (polar), sun-synchronous orbit at about 800 km altitude. In addition to the Total Energy Detector (TED) that provides the data used to determine auroral activity, it contains a Medium Energy Proton and Electron Detector (MEPED) that monitors the intensities of charged particle radiation at higher energies extending up to cosmic rays. These higher energy particles can produce ionization deep within the Earth's atmosphere that can degrade radio communications (occasionally making short wave radio communication impossible in the polar regions), can occasionally disrupt the proper operation of satellite systems, and, when intensities are high, increase the radiation dose to astronauts in space.

The Space Weather Prediction Center processes the particle counts received from the POES MEPED sensors into color-coded plots showing the intensity of recently measured counts for the current day and also for the three previous days, in comparison to a one-year median of similar counts from the same sensor taken at the same geographic location. This type of plot provides an instant estimate of whether the current particle environment is more, or less, intense than usual.

How the Comparison is Made

The sensor response at each of the 2° by 5° latitude-longitude locations visited by the satellite during the day, as it completes about 14 orbits around the earth, is divided by the year-long median sensor response at the same location--taking into account whether the satellite was northbound or southbound. This division yields a ratio ranging from less than 1.0 (the particle intensities below the median) to larger than 1.0 (the particle intensities higher than the median).

The value of the ratio is color coded and plotted at the geographic location of the measurement. The color bar indicates that very large ratios, signifying unusually intense particle fluxes, are plotted towards the red; ratios near 1.0, signifying nominal conditions (equal to the median), are plotted as yellow or blue. Very low ratios, signifying particle intensities far below the median, are plotted as white, although white is also used when the particle fluxes observed at that location are insignificant or at background levels -- regardless of how they compared to the one-year median value.

Time of Observation

The actual universal time of each observation in this display is not explicitly given; knowledge of the characteristics of the NOAA satellite orbit, however, permits rough estimates to be made of the universal time of the observations. The NOAA satellites are in a 98° inclination, sun-synchronous orbit. This means that the satellite always moves from the east towards the west, and always crosses the equator northbound at the same local time. The NOAA POES orbits cross the equator northbound at 7:30 PM local time. An orbit that crosses the equator northbound at the Greenwich meridian (0° east longitude) does so at 1930 universal time, and an orbit that crosses the equator northbound at 15° east longitude does so at 1830 universal time -- for every 15° further east in longitude the universal time of equator crossing is one hour earlier. By noting the longitude of northbound equator crossing of a given orbit, one can estimate the universal time of the start of that orbit, and, knowing that an orbital period is about 105 minutes, the universal time of a given observation during that orbit can be estimated.

A Word of Caution About Belt Indices and the Solar Cycle

The belt indices are a measure of the integrated difference, or departure, of individual sensor responses observed on a given day from the median responses of those sensors over the previous year's observations.

It must be stressed that the one-year median data, that form the basis for the comparisons and calculation of belt indices, are recalculated from time to time (about every three to six months) and do not represent observations from a fixed one-year interval, for example, at solar cycle minimum.

This was done for two reasons. First, several of the solid state detectors in the Space Environment Monitor experience normal radiation damage over time that reduces their sensitivity and make misleading a comparison of their current reponses with their responses early in their life, even for the same radiation environment. Second, it was believed that most operational users were interested in the departure of the current radiation environment from the environment that the user's systems had normally experienced over the more recent past as opposed to the radiation environment many years ago.

In the past the use of mean sensor responses in creating the baseline data sets had the potential of introducing significant changes from year to year as solar and geophysical activity changed. However the change to using the median values of sensor responses in generating the baseline data sets should significantly reduce any year to year changes.

L-Value Curves

At the request of a user, these POES Energetic Particles plots were upgraded to display a set of McIlwain 'L-value' curves plotted as white dotted lines stretching from left to right across the geographic map. Six curves have been added, three in the southern hemisphere and three in the northern corresponding to the geographic location, at the altitude of the satellite, for L-values of 2.0, 4.0, and 6.0.

The McIlwain L-value is effectively a method of labeling magnetic field lines. The numerical value of the 'L' assigned to a magnetic field line corresponds approximately to the distance from the center of the earth, in units of earth radii (6370 km), to the point in space where that field line crosses the magnetic equator. For example, a field line with an L-value of 6.0 will cross the magnetic equator at a distance of about 6.0 earth radii from Earth center or, alternatively, at an altitude of about 5.0 earth radii (32,000 km) above Earth's surface.

Including the L-value curves on the plots facilitates comparing particle observations made by the POES satellite at low altitude with simultaneous particle observations made from other satellites at points further away from the earth.