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Our Modern Eyes on the Sun


Tomas Hood illustrates the importance of space weather for the radio enthusiast and provides an overview of the various wavelengths of light


Tomas Hood illustrates the importance of space weather for the radio enthusiast and provides an overview of the various wavelengths of light, in which our Star is being observed by the latest spacecraft-based instruments


Governments and private businesses are spending vast sums of money and energy researching space weather and radio wave propagation and the field is an area of science gaining traction in institutions of higher learning.

However, how can the radio enthusiast benefit from these resources and why is this area important, you may ask. While it is, no doubt, fascinating to view ultra-high definition images of our Sun or watch in awe at spectacular events like a coronal mass ejection or a solar prominence breaking away from our local Star, does this scrutiny have any value to those of us who use magical boxes of electronics and a stretch of wire to communicate world-wide? In this and future columns, I aim to show you that the answer is a resounding ‘yes’ and I will explain why.

Those who follow this column know that radio communication is directly affected by our variable Sun. Sunspots, coronal mass ejections, the Earth’s geomagnetic field, the ionosphere, magnetosphere and even terrestrial weather all affect how our radio signals get from transmitter to receiver.


SIDs and Blackouts

For example, powerful ‘explosions’ near sunspot regions can cause sudden ionospheric disturbances (SID) or radio blackouts. At the speed of light, the powerful burst of X-rays, extreme ultraviolet energy and other radiation takes about eight minutes to reach Earth. When this radiation penetrates the ionosphere, it energizes each layer.

That is good news at the highest ionospheric layer, the F-region.

At the same time, the lowest layer that affects ionospheric radio signal propagation (the D-region) also becomes highly energized. In fact, the energy from the flare can cause the D-region to become so ionized that all signals in the short wave spectrum are absorbed, countering the positive ionizing of the F-region. This will result in a completely quiet spectrum, devoid of any signals. Such radio blackouts often caused radio operators to wonder whether their antenna had come down or the coax was cut.

All of this occurs on the sunlit side of the Earth because only the illuminated region of the ionosphere is exposed to the flare’s energy.


Coronal Mass Ejections

Another space weather event is a coronal mass ejection (CME). These phenomena sometimes accompany a solar flare. When the flare erupts, it releases a huge cloud of solar plasma from the Sun’s corona. The CME, if directed earthward, crashes into our magnetosphere anywhere from two to four days after it is ejected by the flare.

This, in turn, can cause long periods, sometimes days, of ionospheric depression, making short wave communications much more difficult than under normal conditions.

It stands to reason, then, that knowing about current space weather conditions – or even knowing what the space weather is going to be like in the future (within minutes, hours or days) – allows you to plan your TX and RX/ DX activities more quickly and effectively. For those who rely on short wave to accomplish their mission, be it in the military, emergency relief, international broadcasts or amateur radio, working with or around the effects of space weather is the key to successful communications.


Key Satellites and Observatories

On February 11th, 2010, NASA launched a United Launch Alliance Atlas V-401 rocket with a new spacecraft, charged with observing the Sun and solar dynamics (space weather) (Fig. 1).

The payload spacecraft was the Solar Dynamics Observatory (SDO). The SDO is the first satellite under the Living with a Star (LWS) and it is the most advanced spacecraft ever designed to study our Sun (Fig. 2). During its five-year mission, it will examine the Sun's magnetic field and provide a better understanding of the role the Sun plays in Earth's atmospheric chemistry and climate. Since the launch, engineers have been conducting a plethora of testing and verification procedures on the spacecraft’s components.

Now fully operational, the SDO will provide images with a clarity and resolution 10 times better than high-definition television and it will return more comprehensive science data faster than any other solar observation spacecraft (Fig. 3).

The satellite is designed to fly for five years but other spacecraft often keep working long past their initial mission life. The SOHO (Solar and Heliospheric Observatory) spacecraft, for example, which was built to fly for five years, celebrates its 23rd anniversary in 2018!

The SDO is unlike any other satellite. It collects huge amounts of data every day. In fact, the SDO produces enough data to fill a single CD every 36 seconds! Many satellites share a ground system (a location on Earth, where they send data and photographs) and have recording systems to save the data collected until they can talk to their ground station. Because the SDO has no recording system and collects so much data, the SDO mission had to build its very own ground station. For this to be possible, the SDO was placed in a geosynchronous orbit (GEO). This means that it revolves with the Earth and at the same speed as our planet. It will always be directly above – and in constant communication with – its ground station in New Mexico.

Every day, the SDO is returning stunning images of our Star (Fig. 4). Some of these show never-before-seen detail of material streaming outward and away from sunspots. Others show extreme close-ups of activity on the Sun’s surface. The spacecraft has also made the first high-resolution measurements of solar flares in a broad range of extreme ultraviolet wavelengths.

You can watch a few captivating videos of the many dynamic activities on our Sun at the following websites:


The Solar Magnetic Field

From the very beginning of the SDO mission, the great excitement about it was shared: “These initial images show a dynamic Sun I had never seen in more than 40 years of solar research,” said Richard Fisher, director of the Heliophysics Division at NASA Headquarters in Washington. He continued, "The SDO will change our understanding of the Sun and its processes, which affect our lives and society. This mission will have a huge impact on science, similar to the impact of the Hubble Space Telescope on modern astrophysics.”

The SDO helps scientists to explore how the Sun's magnetic field is generated, structured and converted into violent solar events such as the turbulent solar wind, solar flares and coronal mass ejections.

Even more exciting than this is the fact that the SDO is providing critical data, which are improving the ability to predict these space weather events.

NASA's Goddard Space Flight Center in Greenbelt, Maryland, built, operates and manages the SDO spacecraft for the agency’s Science Mission Directorate in Washington.

“I’m so proud of our brilliant workforce at Goddard, which is rewriting science textbooks once again,” said Barbara Mikulski, the chairwoman of the Commerce, Justice and Science Appropriations Subcommittee that funds NASA. “This time Goddard is shedding new light on our closest star, the Sun, discovering new information about powerful solar flares that affect us here on Earth by damaging communication satellites and temporarily knocking out power grids. Better data means more accurate solar storm warnings.”

The instrument sends 1.5 terabytes (TB) of data back to Earth each day, which is equivalent to a daily download of half a million songs onto an MP3 player.

The observatory carries three important instruments for solar research:

First, the Helio-seismic and Magnetic Imager (HMI) maps solar magnetic fields and looks beneath the Sun’s opaque surface (Fig. 5). This experiment deciphers the physics of the Sun’s activity, taking pictures in several very narrow bands of visible light. With the instrument, scientists produce ultrasound images of the Sun and study active regions, in ways similar to watching sand shift in a desert dune.

Second, the Atmospheric Imaging Assembly (AIA) is an array of four telescopes designed to photograph the Sun’s surface and atmosphere. This instrument covers ten different wavelength bands (or ‘colours’) selected to reveal key aspects of solar activity. These types of images will show details never before seen by scientists.

Finally, the Extreme Ultraviolet Variability Experiment (EVE) measures fluctuations in the Sun’s radiant emissions. These emissions have a direct and powerful effect on Earth’s upper atmosphere, heating it, puffing it up and breaking apart atoms and molecules. Researchers are not sure about how fast the Sun can vary at many of these wavelengths. Therefore, they expect to make discoveries about flare events.

“These amazing images, which show our dynamic sun in a new level of detail, are only the beginning of SDO's contribution to our understanding of the Sun," said SDO Project Scientist Dean Pesnell of Goddard.

In April 2018, NASA released an incredible video that shows a massive solar prominence erupting on March 30, 2010. You can watch it here:

“We've seen solar prominences before—but never quite like this," stated Alan Title of Lockheed Martin, the principal investigator of the AIA; "some of my colleagues say they've learned new things about prominences just by watching this one movie."


The SDO and the Radio Hobby

The successful launch and deployment of the SDO is great news for radio hobbyists, on many levels: “The SDO is our 'Hubble Space Telescope’ for the Sun,” claimed Program scientist Lika Guhathakurta at NASA headquarters and she added, “It promises to transform solar physics in the same way the Hubble has transformed astronomy and cosmology."

“No solar telescope has ever come close to the combined spatial, temporal and spectral resolution of SDO," Alan Title added. “This is possible because of the combination of 4096 x 4096-pixel CCDs with huge dynamic range and a geosynchronous orbit which allows the SDO to observe the sun and communicate with the ground around the clock.”

Solar telescopes such as those aboard the SDO make use of this wavelength information. Instruments known as spectrometers, observe many wavelengths of light simultaneously and can measure how much of each wavelength of light is present. This helps create a composite understanding of what temperature ranges are exhibited in the material around the Sun. Spectrographs do not look like an ordinary picture. Instead, they are graphs that categorize the amount of each kind of light.

SDO scientists chose ten different wavelengths at which to observe the Sun, as filtered by the instruments in the SDO’s AIA instrument. Each wavelength is largely based on a single (or, perhaps, two) types of ions. Slightly longer and shorter wavelengths produced by other ions are, invariably, also part of the picture. However, the main detail revealed by a specific wavelength is specific to just one or two types of ions. Each wavelength is chosen to highlight a particular part of the Sun's atmosphere (Fig. 6).


Wavelengths of Solar Light

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Moving from the Sun's surface in an outward direction, the wavelengths, at which the SDO can observe are measured in Ångström, after the Swedish physicist Anders Jonas Ångström (1814 to 1874), one of the pioneers of optical spectroscopy.

The ångström (æŋstrəm) is a unit of length equal to 10-10m or 0.1 nanometers (nm). Its symbol is Å.

Table 1 shows the various wavelengths of observable sunlight and the varying temperatures in the various layers and regions of the Sun.

At each wavelength, unique processes and features of the Sun’s activity are revealed. To help scientists categorize and observe what is occurring, they artificially colour the images of the particular wavelength in a standardised colour format.

Armed with such rich views of the sun, as well as the wealth of new space weather data, radio hobbyists are well-equipped, indeed, to understand current conditions and plan contacts and DXing activity.

Consider this: during the last twenty-five years, during Sunspot Cycles 23 and 24, amazing progress has been made in solar and terrestrial science.

Imagine what we could discover during this next solar cycle minimum and the next cycle (Cycle 25).

This column will continue to explore how space weather affects your radio communications activities.


Short Wave Conditions

Solar activity is expected to be at about the same level as observed this time last year. This results in low maximum usable frequencies. Even so, expect fair openings into most areas of the world throughout the day on 22, 19, and 16m.

Throughout the summer, you can expect propagation between north and south regions during daylight hours. Nineteen and 16 meters will be the strong daytime bands, with 19m remaining popular throughout the year.

Reception of stations located in tropical or equatorial areas may be possible well into the hours of darkness. For distances between 800 to several thousand miles, expect exceptionally strong signals.

Multi-hop signals will be observed.

The 25 and 22m bands will remain open from just before sunrise to a few hours past sunset. From late afternoon to well into darkness, expect these to offer worldwide coverage.

The 31m band is a year-round band, offering outstanding domestic and international paths around the clock.

During periods of low geomagnetic activity, this band may offer long distance DXing through the night.

The 41and 49m bands offer domestic propagation by day and sometimes at night.

The tropical bands (60, 75, 90, and 120m) are not noticeably affected by the solar flux but will be degraded during geomagnetic turbulence. Through the summer, expect these bands to be more challenging, though less so this year than last year.

Overall, daytime bands will open just before sunlight, and last a few hours after dark. If you are DXing or making amateur radio contacts, look higher in frequency during the day, as these frequencies will be less affected by any solar storms occurring, and more broadcasters will have transmissions in these upper bands at this time.


VHF Conditions

The summertime Sporadic-E (‘Es’) season for the Northern Hemisphere will be quite active through July. Usually, these Es openings are single-hop events, with paths of up to 1500 miles. However, July's Es events, like June's, are often of a double-hop nature. Look for HF openings on the higher frequencies as well as on low-VHF during the day and do not forget to check during night hours, too.


Solar Cycle 24 Today

The Royal Observatory of Belgium, the world’s official keeper of sunspot records, reported a monthly mean sunspot number of 5.3 for April 2018. This was up a few points from 1.5 for March 2018, yet lower than 6.4 in February. The highest daily sunspot count was 28 on April 28th, while the lowest was 0 (zero) on April 2nd to 9th, 11th, 18th, and from 28th to 30th (a total of 13 days without any sunspots). The twelve-month running smoothed sunspot number anchored on the month of October 2017 was 10.0. Following the curve of the 13-month running smoothed values, a smoothed sunspot level of 10 is expected for July 2018, plus or minus 14 points.

The Dominion Radio Astrophysical Observatory at Penticton, BC, Canada, reported a 10.7-cm observed monthly mean solar flux of 70.0 for April 2018. The twelve-month smoothed 10.7-cm flux, positioned around October 2017, was 75.1. A smoothed 10.7-cm solar flux of about 70 is predicted for July 2018.

The geomagnetic activity as measured by the Planetary-A index (Ap) for April 2018 was 7. The twelve-month smoothed Ap index, pivoting around October 2017 was 9.8.

Geomagnetic activity this month should be mostly quiet with fair to good propagation conditions. You can find a last-minute forecast at this URL:  



Editor’s Reading Tips:

Golub, L. and Pasachoff, J.M. (2014) Nearest Star – The Surprising Science of Our Sun (CUP: Cambridge)

Nichols, E.P. KL7AJ (2015) Propagation and Radio Science (ARRL)

The Space Weather Quarterly









Table 1: The Sun observed at ten different Wavelengths



Wavelength (Å)

Temperature (°K))

Solar Region

Comments/ Features



The Sun’s ‘surface’ of Photosphere

White-light images (‘Intensitygrams’)



The Photosphere and Chromosphere

The temperature begins rising, as the altitude above the Photosphere increases.



The Photosphere and Transition Region, between the Chromosphere and the uppermost layer of the solar atmosphere (The Corona)

In the Transition Region, the Sun’s temperature rises rapidly.


2.5 Million

This wavelength also shows hotter, magnetically active, regions in the Corona.




Light is being emitted from both Chromosphere and the Transition Region



2 Million

This wavelength shows hotter, magnetically active, regions in the Sun's Corona.




1 Million

Hotter region of the Corona, and also the much hotter material of a solar flare

This wavelength also reveals coronal holes, when present.



The Sun's Atmosphere (Corona) – when it is quiet – and large magnetic arcs coronal loops

Images from this wavelength are often included with this column, as many radio hobbyists are interested in the magnetic structures at play on the Sun.


10 Million

The hottest material in a flare.



6 Million

This highlights regions of the corona during a solar flare.


Source, and with kind permission of: NASA/ SDO Solar Mission.


This article was featured in the July 2018 issue of Radio User