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The Role of Bz in Medium and Long Range HF Propagation Conjecture
The following material is based on observation. The material published is an answer to a reader's question: "how does Bz fit into propagation forecasts?" Bz in virtually all reports that give the current solar weather. ANSWER Bz has little or no use in forecasts of high frequency (HF) radio wave propagation beyond a three hour period.
The direction of the north-south component of the Sun's Interplanetary Magnetic Field ( IMF ) can go from pointing in one direction to pointing in another direction over periods of less than an hour. These changes can be large. A change in Bz from -5 nT to 5 nT has been observed within the period of an hour. In our opinion it would be very difficult for extremes such as these to indicate the direction of the main portion of a solar wind stream. At times changes in HF propagation lags well behind reported shifts of solar wind direction (as represented by Bz). At other times observation has shown rapid changes in solar wind direction quickly parallel changes in propagation. This observed variability adds to the list of reason of why Bz should not be included in forecasts. We propose the idea that Bz may not be representative of the overall flow of the main solar wind stream. Bz could at times represent side currents in the solar wind stream. If this be the case, Bz should not enter into forecasts of propagation because the Bz value is not an indication of the true direction of a solar wind stream. The uncertanty of what Bz is actually measuring further makes it a poor choice for propagation forecasts. The value for Bz is not used in any of the formulas that we use for predicting HF conditions from 3-hours out to 72-hours. Bz can give an indication concerning the stability of Earth's magnetic field for the past 24-hours to the present time. Bz is primarily a 15-minute by 15- minute measurement used to describe, in part, Earth's magnetic field . Fact: the north - south component of the Interplanetary Magnetic Field is referred to as Bz. Bz is a "vector value" used in the Geocentric Solar Magnetic Coordinate System (GSM). We do use Bz to state conditions as they are now. Nowcasting. The following short article stands alone and does NOT require an understanding of GSM. GSM is described elsewhere on the Web Site (glossary). We will go into GSM from a different perspective at a later time. At an earlier point in time we stated that the Bz index is not useful in long or short term "forecasts" of HF propagation. This is not quite true. The term "forecast" refers to stating what propagation will be in the future. The term "forecast " does not have the same meaning as "NOWCAST" ( meaning conditions at this instant ). Bz is primarily used in nowcasting. Bz should have some usefulness or we would
not make such a great fuss by constantly reporting its value. Bz takes on some
value when it is studied along side of data that describe the conditions of
Earth's magnetic field over longer periods of time. One data that describes long
term behavior of Earth's magnetic field is Kp. The Kp index is usually still an
estimated value at the time that it most often used. Forecast time.* Kp describes, in part,
behavior of Earth's magnetic field averaged over the entire globe, for a
given 3-hour period. Kp does not take on a firm value until data is collected
from numerous data collecting stations around the globe and analyzed. Most of
the time that we use the Kp value is now - at the present moment
of time. It is still an estimated index when used in this fashion. The Kp value
is used in some forecasting formulas. One index that describes, in part, the behavior of Earth's magnetic field over even a longer period of time than does the Kp Index, is the Ap Index. This index is an estimated value during the times that we use it the most. Firming up the Ap Index is more complicated than deriving the Kp index - and it takes a longer time period to arrive at the final value for Ap. A very abbreviated description of what Ap measures is : Ap indicates the state of Earth's magnetic field, again in part, averaged over the entire globe for a 24-hour period. The "K", "Kp", "A", and "Ap" Indices are described in much greater detail in other parts of the Web Site. Those descriptions primarily deal with how the indices are collected and the relationship of the various indices with each other. At a later time, these indices will be described using a different approach. The Kp and Ap indices represent geomagnetic behavior and have places in formulas used for short term propagation forecast. In Nowcasting, the north - south component of the Interplanetary Magnetic Field is referred to as Bz. Solar flares, most of the time, are not smooth and symmetrical in their shape, in the speed of solar wind particles as they depart from the Sun, in the manner in which flare gasses and solar wind particle joint with the Sun's Interplanetary Magnetic Field ( IMF ), or in density of solar particle emissions, x-rays emitted, etc. The above mentioned rapid changes in the direction and intensity of Bz indicates that Bz probably is measuring something other than, or in addition to, a smooth and symmetrical solar wind stream. Because of the rapid changes possible in Bz values, Bz must be measuring something other than the main portion of a solar wind stream. Bz may say little about the main solar wind stream itself Bz values may reflect measurements of solar wind swirls and cavitations rather than indicating the true behavior of the main solar wind stream body. Bz values may reflect the portion of the solar wind stream that is analogous to a quickly moving stream of water pushing past obstructions and deviations of flow caused by a river bank. This could account for rapid changes in Bz as compared to the much smoother changes in Kp and Ap. We can only wonder if the bank of a stream is analogous to the magnetic forces of the IMF on the solar wind stream.
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