Amateur Radio Propagation Studies


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Solar-terrestrial realtime data

A guided tour from the Sun towards the Earth

There are many webpages providing solar-terrestrial realtime data compiled from various resources in the internet. You may wonder what data is most important, what type of information is displayed, what does it mean and what data is relevant for predicting and identifying Auroral backscatter on very high frequencies?

This webpage therefore provides a different approach, i.e. a guided tour by visiting and discussing selected sources of solar-terrestrial realtime data. However, it's you sitting in the driver seat. Fasten your seat belt and start an impressive journey from outer space back to Earth by following the below step-by-step procedure which only takes a couple of minutes for completion.

Arriving at the botton of this webpage, you will have latest information relevant to VHF radio amateurs. It is recommended to travel this tour at least once a day, or even in shorter periods of time depending on actual solar-terrestrial conditions.


See also: Analysis of the March 31, 2001 geomagnetic storm


Please follow this steps:

The tour starts at a distance of 150 million kilometers from Earth, i.e. on the Sun. Note that solar wind shock fronts travel approx. two days before reaching the Earth environment, i.e. spotting a new region of activity on the Sun isn't relevant to the present situation of Auroral backscatter. However, how do you know it is a new region? Perhaps it isn't new at all and a massive shock front is already on the way. This is the reason, item 1.2 is recommended as a permanent stop whenever you inspect solar-terrestrial data.

1.1 Sun spot number (optional). To obtain a general overview, let's have a quick look at the actual Sun Spot Number (SSN) and the behaviour of the Solar Cycle. Click the button and view the "ISES Solar Clycle Sunspot Number Progression".

NOAA's Space Environment Center

1.2 Activity Reports. Many solar activity reports are available in the internet, we will select the ones distributed by BBSO, i.e. the Big Bear Solar Observatory. Click button and read the Daily Solar Activity Reports section.

Big Bear Solar Observatory

1.3 Solar Disk. Another BBSO webpage provides images of the solar disk taken at different wavelengths. This images indicate the presence of sunspots on the Sun's disk, i.e. big sunspots close to the disk's center should in particular attract our attention because there is a higher chance for coronal mass ejections (CME) impacting the Earth's magnetosphere.

Big Bear Solar Observatory

Travelling in a spaceship close to the Sun, the Sun's gravitation is the dominating gravitational force affecting your spaceship. Travelling close to the Earth, on the other hand, your spaceship will consider the terrestrial gravitation the dominating force, of course. Thus, "somewhere in between" there is a point of Sun-Earth gravitational equilibrium, i.e. the so-called Lagrange point L1 (there are four others). Here, Human mankind placed a spacecraft orbiting L1: the Advanced Composition Explorer (ACE) which permanently monitors the solar wind. "Somewhere in between" means, by the way, 1.5 million kilometers from Earth, i.e. four times the Earth-Moon distance (a short distance though compared to the distance of the Sun, i.e. 150 million kilometers). Let's visit the Human mankind's sentry in space, i.e. let's visit the ACE spacecraft, now.

The ACE spacecraft

Please follow this steps:

NOAA Space Environment Center (SEC)

2.1 Bz component. The expanding solar wind drags also the solar magnetic field outward, forming the so-called interplanetary magnetic field (IMF). Its field lines are said to be "frozen in" to the solar wind plasma because the field lines cannot contact back to the Sun. At the magnetopause, the Earth's magnetic field and the IMF come into contact. If the IMF points south ("southward Bz"), then the IMF can partially cancel the Earth's magnetic field at the point of contact, i.e. the two fields link up. In this situation, a field line from Earth connects directly into the solar wind, i.e. energy may be injected into the Earth's magnetosphere which may cause widespread Auroras and other type of phenomena.

Please note the red curve on top of the diagrams which is labeled "Bz". If the actual data point on the right hand side of the graphics is located below the dotted line, the IMF shows a southward component - this is what we are looking for in Aurora dx openings.

2.2 Solar wind density. On Earth, we would consider a density of 150 to 200 particles per cubic centimeter a perfect vacuum, more or less. However, the same amount of charged particles would represent a large solar wind shock front.

Please note the orange curve in the third panel. Densities exceeding, say, 10 cubic centimeters may attract our interest. If density drops below 1 particle per cubic centimeter, there is a special indicator placed along the UTC axis on the page botton.

2.3 Solar wind speed. Typical solar wind speeds vary between, say, 300 and 700 kilometers per second. High speed solar wind shock fronts show a step-like increase of the solar wind speed. With high solar wind density and southward Bz, we will definitely have an Aurora band opening, guaranteed.

Please note the yellow curve in the diagrams. Is there any indication for an abrupt increase in solar wind speed or is there any indication for high-speed shock fronts? Abrupt changes may also result from spacecraft maneuvers, in this case you may find special indicators placed along the UTC axis.

2.4 Estimating the onset of geomagnetic disturbances. You are currently watching ACE data corresponding to a distance of 1.5 million kilometers from Earth. Thus, whenever the ACE spacecraft detects a solar wind shock front, the front hasn't yet arrived at the near-Earth space environment. Assuming the solar wind speed is 700 km per second, the shock front travels 1.500.000 km divided by 700 km/s, i.e. 2143 seconds corresponding to 36 minutes before impacting the Earth's mangetosphere. Hence, a time delay needs to be considered between ACE and all phenomena which may result on Earth - the above estimation actually represents a minimum time delay, i.e. the real time delay may be longer than calculated. Note that the shock front may change its parameters while travelling towards Earth, i.e. ACE data cannot really predict the shock front's nature in close vicinity to Earth.

To estimate the time a solar wind shock front travels from the ACE spacecraft to Earth, you may apply this formula:

time delay in minutes = 25.000 divided by the corresponding wind speed in km/s.

Example: wind speed = 700 km/s, delay time = 36 minutes.


Still in space, we nevertheless arrived near Earth, i.e. onboard of the NOAA satellites in a height around 1.000 kilometers above ground. The POES instruments provide estimates of the power flux into the polar cap regions, displayed by color codes mapped onto the northern and southern hemisphere. The data is extrapolated from the most recent pass of the satellites, i.e. we are actually dealing with quasi-realtime data (the data updates approximately every 1.5 hours), i.e. you are advised to consider the time stamp on the actual display. Please also note the so-called normalization factor on the left hand side of the graphics which "takes into account how effective the satellite was in sampling the Aurora during its transit over the polar region." A large factor exceeding the value of 2.0 indicates lower confidence, factors below 2.0 denote a reasonable level of confidence in the estimate of power.

Please follow this steps:

NOAA Space Environment Center (SEC)

Click on the downloaded image displaying your hemisphere to obtain a larger version.

source: NOAA

3.1 Interpreting the color code. There is no direct relationship between the color code and the availability of Aurora dx QSOs, unfortunately. You may therefore experience Auroral backscatter in geographical regions even outside of the red and yellow areas. Nevertheless, the POES graphics does provide a brilliant overview on Auroral activity, which is evident from the above figures displaying examples of high and low activity.

Please view the actual oval of Aurora acivity. If you find low activity, our tour is finished, i.e. you may ignore all steps following further below.

3.2 Latitudinal extend and longitudial displacement by the rotation of Earth. During major geomagnetic disturbances, the oval of Aurora activity blows up, i.e. the region of activity moves from polar latitudes towards south into midlatitudes. The POES graphics therefore provides an important information to VHF radio amateurs, i.e. the actual latitudinal extend of the oval. Note that the most southern extend is opposite to the actual direction of the Sun, at the noon meridian (indicated by the red arrow in the POES graphics), the oval withdraws to the north (this explains, by the way, why midlatitude Aurora band openings is a rare phenomenon at noon). This red arrow indicating the Sun's direction rotates clockwise due to the rotation of Earth. So does the Aurora oval, i.e. comparing a series of recent POES images you will notice the oval is rotating around the geomagnetic pole.

Please estimate your home location in the POES graphics and imagine the oval would rotate clockwise. This gives you an idea what activity may be expected assuming the magnitude of Aurora activity does not change. Note that the oval rotates 15 degree in 1 hour (45 degree is easier to judge though, which corresponds to 3 hours).


Arriving on Earth, or tour ends in northern Sweden, i.e. at the Swedish Institute for Space Physics in Kiruna. The Kiruna magnetogram provides realtime data of the local Earth magnetic field at ground level. Note that all magnetograms only show field variation rather than the total field strength. The magnitude of the Earth magnetic field is around 40.000 Nanotesla (nT), major geomagnetic disturbances, on the other hand, may cause variations of 2.000 nT or so which corresponds to only five percent of the main field.

Please follow this steps:

Swedish Institute for Space Physics

4.1 Field variation. Reading magnetograms is a science, we are no experts though and must therefore accept a trivial approach: the more the three curves jiggle around, the higher is the geomagnetic disturbance. However, it is important to consider the magnetogram's vertical axis, i.e. the scaling labeled Deflection [nT]. A dramatic looking field variation may actually correspond to 30 nT only, i.e. this deflection is rather small and does not indicate any major geomagnetic disturbances at all.

Please check the actual field variation and try the following: refer to item 3.2 and locate northern Scandinavia on POES' map of the northern hemisphere. Watch the rotation of the Aurora oval by re-visiting the POES page every, say, 1 1/2 hours: how does the Kiruna magnetic field vary while the Aurora oval rotates?

source: V. Grassmann, DF5AI

source: Swedish Institute for Space Physics


Unless otherwise stated, all material is copyright of Volker Grassmann. All rights reserved. The material, or parts thereof, may not be reproduced in any form without prior written permission of the author.

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