Ham Radio operators,
shortwave radio enthusiasts often talk about propagation
index numbers, the status of the solar cycle and geomagnetic
conditions. Why? What do the numbers mean? On many ham
radio frequencies, especially on HF, these factors determine
whether worldwide contacts can be made with very little
effort (low power and a modest antenna) or radio blackout
conditions exist (no contacts possible). The following
information, adapted from NOAA, can be helpful in these
numbers.
The National Oceanic and
Atmospheric Administration (NOAA) uses WWV and WWVH to
broadcast geophysical alert messages that provide
information about solar terrestrial conditions. Geophysical
alerts are broadcast from WWV at 18 minutes after the hour
and from WWVH at 45 minutes after the hour. The messages are
less than 45 seconds in length and are routinely updated
every 3 hours (typically at 0000, 0300, 0600, 0900, 1200,
1500, 1800, and 2100 UTC). Updates are more frequent when
activity warrants.
The geophysical alerts
provide information about the current and predicted solar
terrestrial conditions found useful for long distance HF
radio communications and other applications. The alerts use
a standardized format and terminology that requires some
explanation. The terms used in the announcements are
defined below:
Solar flux is a
measurement of the intensity of solar radio emissions with a
wavelength of 10.7 cm (a frequency of about 2800 MHz. The
daily solar flux measurement is recorded at 2000 UTC by the
Dominion Radio Astrophysical Observatory of the Canadian
National Research Council located at Penticton, B.C.,
Canada. The value broadcast is in solar flux units that
range from a theoretical minimum of about 50 to numbers
larger than 300. During the early part of the 11-year
sunspot cycle, the flux numbers are low; but they rise
and fall as the cycle proceeds. The numbers will remain
high for extended periods around sunspot maximum.
The A and K indices are a
measurement of the behavior of the magnetic field
in and around the Earth. The K index uses a scale
from 0 to 9 to measure the change in the horizontal
component of the geomagnetic field. A new K index is
determined and added to the broadcast every 3 hours based on
magnetometer measurements made at the Table Mountain
Observatory, north of Boulder, Colorado, or an alternate
middle latitude observatory. The A index is a daily
value on a scale from 0 to 400 to express the range of
disturbance of the geomagnetic field. It is obtained by
converting and averaging the eight, 3-hour K index values.
An estimate of the A index is first announced
at 2100 UTC, based on 7 measurements and 1 estimated value.
At 0000 UTC, the announced A index consists entirely of
known measurements, and the word “estimated” is dropped from
the announcement.
Space Weather
describes the conditions in space that affect earth and its
technological systems. Space weather is a consequence of the
behavior of the sun, the nature of Earth’s magnetic field
and atmosphere, and our location in the solar system.
Space Weather storms observed
and expected are characterized using the NOAA Space Weather
scales. The abbreviated table below shows the levels of
activity that are included in the announcements and the
associated terminology. The descriptor used to identify
observed or expected conditions is the maximum level reached
or predicted. The
NOAA Space Weather Scales
are further described on the Space Environment Center web
site.
NOAA Space Weather Scales
|
|
Geomagnetic Storms |
Solar Radiation Storms |
Radio Blackouts |
Descriptor |
|
G5 |
S5 |
R5 |
Extreme |
|
G4 |
S4 |
R4 |
Severe |
|
G3 |
S3 |
R3 |
Strong |
|
G2 |
S2 |
R2 |
Moderate |
|
G1 |
S1 |
R1 |
Minor |
Geomagnetic storm
levels are determined by the estimated 3-hourly
Planetary K-indices which are that are derived in real
time from a network of western hemisphere ground-based
magnetometers.
|
Geomagnetic Storm levels |
|
Planetary K indices |
Geomagnetic storm level |
|
K
= 5 |
G1 |
|
K
= 6 |
G2 |
|
K
= 7 |
G3 |
|
K
= 8 |
G4 |
|
K=
9 |
G5 |
Solar Radiation
storms levels are determined by the proton flux
measurements made by the primary GOES satellite.
|
Solar
Radiation Storm levels |
|
Flux level of > 10 MeV particles |
Solar Radiation Storm level |
|
10 |
S1 |
|
102 |
S2 |
|
103 |
S3 |
|
104 |
S4 |
|
105 |
S5 |
Radio Blackouts
are determined by the x-ray level measured by the
primary GOES satellite.
|
Radio Blackouts |
|
Peak x-ray level and flux |
Radio Blackout level |
|
M1
and (10-5) |
R1 |
|
M5
and (5 x 10-5) |
R2 |
|
X1
and (10-4) |
R3 |
|
X10 and (10-3) |
R4 |
|
X20 and (2 x 10-3) |
R5 |
Every geophysical alert consists of
three parts as shown in tables 3.4 and 3.5. Table 3.4
describes the information contained in the geophysical
alert. Table 3.5 provides example text from an actual
message.
|
Section |
Table 3.4 -
Information in Voice Message |
|
1 |
The solar-terrestrial
indices for the day: specifically the solar flux,
the A index, and the K index. |
|
2 |
Space Weather storms
observed during the previous 24 hours. Includes all
observed geomagnetic storms, solar radiation storms
(proton events) and Radio blackouts (class M1 and
greater flares). |
|
3 |
Space Weather expected
during the following 24 hours. |
|
Section |
Table 3.5 - Example
of Actual Geophysical Alert Message |
|
1 |
Solar-terrestrial
indices for 08 November follow.
Solar flux 173 and Mid-Latitude A-index 14
The Mid-latitude K-index at 1500 UTC on 08 November
was 3. |
|
2 |
Space Weather for the
past 24 hours has been severe.
Solar radiation storm(s) reaching the S4 level is in
progress.
Radio blackouts(s) reaching the R2 level occurred. |
|
Alternate
section 2 |
No Space Weather storms
have been observed during the past 24 hours. |
|
3 |
Space Weather for the
next 24 hours is expected to be severe.
Solar radiation storms reaching the S4 level are
expected to continue.
Radio blackouts reaching the R2 level are expected. |
|
Alternate
section 3 |
No Space Weather storms
are expected during the next 24 hours. |
The announcements include the
descriptor of the largest space weather event observed (2)
or expected (3) in the first line of each section. The
remaining lines give the type of events and the level
observed for each one. In the example above, no geomagnetic
storm information is included because none was observed or
is expected during the period. In the case where none of the
three types of events are observed or expected, the
announcement would contain section 1, plus alternate section
2 and alternate section 3.
