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By Ian Poole, G3YWX

One of the key skills for any HFDXer is to know how to judgewhat band conditions may be like.

Conditions may be excellent one day withmany stations audible from all over theworld, but a few days later it may be thatonly a few stations are audible. To be ableto gain an idea about conditions, threemain indices are used: solar flux, and the

Ap and Kp indices. A good workingknowledge of what these numbers repre-sent and what they mean is an advantageeven for the ham with most well-equippedstation.

Synopsis

The ionosphere can be visualized ascontaining a number of layers. In fact,there is ionization throughout the iono-sphere; the layers are really peaks in thelevels of ionization, as we can see from

Figure 1. The ionosphere affects radiowaves because according to the level ofionization, the signals are refracted, i.e.,bent away from traveling in a straight

line. Often the level of ionization is suf-ficiently high to enable the signals to be

returned to Earth.Conditions are continually varying on

the HF bands as a result of the varyinglevels of ionization in the ionosphere. Theradiation coming chiefly from the Sunhits the upper ionosphere, causing mol-

ecules to ionize, creating positive ionsand free electrons. A state of dynamicequilibrium exists. The free electronsthat affect radio waves recombine withthe positive ions to reform molecules.When levels of ionization are higher

(when there are more free electrons) the

UnderstandingSolar IndicesWhen someone tellsyou that the flux is upto 200 and the K is 3,do you know whatthey are talkingabout? You will afteryou read this article!

An ultra-close view of a sunspot taken by the NSO Sacramento Peak Vacuum TowerTelescope.

NASA

Figure 1Approximatelevels of ionization in theatmosphere

ionosphere is more capable of bendingback radio signals to Earth. Also, highlevels of ionization mean high maximumusable frequencies and better HF condi-tions.

The level of ionization at any givenpoint above the Earth is dependent upona number of factors including the time ofday, the season and most important of allthe sunspot cycle. It is found that the levelof radiation from the Sun increases as the

number of sunspots increases. Accord-ingly, the level of radiation received fromthe Sun peaks around the top of the sun-spot cycle. In fact, it is the bright areajust around the sunspot called the plage

that emits most of the extra radiation.It is not all good news, though. At the

sunspot peak the level of geomagneticactivity also rises. This happens as theSun emits vast quantities of particles.There is normally a steady flow of these

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September 2002 39

particles, but at times such as when thereare solar flares the level of these emis-sions greatly increases. When they hit theEarths magnetic field it becomes dis-turbed, creating a magnetic storm thatcan be detected at points around the

globe. Another effect is that the iono-sphere itself can be disturbed, giving riseto an ionospheric storm. This will de-grade HF communications and when par-

ticularly bad it can lead to a total HFblackout. For a more in-depth recap onradio propagation, get on the Web andgo to www.radio-electronics.com/info/

propagation/radio_prop_list.html, orjust vis itwww.radio-electronics.comand navigate from there.

Solar Flux

A measure known as the solar fluxisused as the basic indicator of solar activ-ity, and to determine the level or radia-tion being received from the Sun. Thesolar flux is measured in solar flux units

(SFU) and is the amount of radio noiseor flux that is emitted at a frequency of2800 MHz (10.7 cm). The PentictonRadio Observatory in British Columbia,Canada reports this measure daily. Thesolar flux is closely related to the amount

of ionization and hence the electron con-centration in the F2 region. As a result itgives a very good indication of conditionsfor long-distance communication.

The figure for the solar flux can varyfrom as low as 50 or so to as high as 300.

Low values indicate that the maximumuseable frequency will be low and over-all conditions will not be very good,particularly on the higher HF bands. Con-versely, high values generally indicatethere is sufficient ionization to supportlong-distance communication at higher-than-normal frequencies. However, re-member that it takes a few days of highvalues for conditions to improve. Typi-cally values in excess of 200 will be mea-sured during the peak of a sunspot cycle

with high values of up to 300 being ex-perienced for shorter periods.

Geomagnetic ActivityThere are two indices that are used todetermine the level of geomagnetic ac-tivity: theA indexand the K index. Thesegive indications of the severity of themagnetic fluctuations and hence the dis-turbance to the ionosphere.

The first of the two indices used tomeasure geomagnetic activity is the K

index. Each magnetic observatory cali-brates its magnetometer so that its K in-dex describes the same level of magneticdisturbance, no matter whether the ob-servatory is located in the auroral regions

or at the Earths equator. At three hourly

Figure 2Signals traveling through the ionosphere will be refracted and may be returned to Earth.

The DX Summit Webcluster at oh2aq.kolumbus.com/dxs/provides solar activityreports in the frame near the bottom of the window (framed in red).

intervals starting at 0000 UTC each day,the maximum deviations from the quietday curve at a particular observatory are

determined and the largest value is se-lected. This value is then manipulatedmathematically and the K index is calcu-lated for that location.

The K index is a quasi logarithmic

number and as such cannot be averagedto give a longer-term view of the state ofthe Earths magnetic field. Thus was bornthe A index, a daily average. At each 3-hour increment the K index at an obser-vatory is converted to an equivalent aindex using Table 1, and the 8 a-indexvalues are averaged to produce the A in-dex for that day. It can vary up to valuesaround 100. During very severe geomag-netic storms it can reach values of up to200 and very occasionally more. The A-index reading varies from one observatory

to the next, since magnetic disturbances

can be local. To overcome this, the indi-

ces are averaged over the globe to pro-

vide the Ap index, the planetary value.Similarly, the Kp index is the plan-

etary average of all the K indices at ob-servatories around the globe. Valuesbetween 0 and 1 represent quiet magneticconditions and this would indicate goodHF band conditions, subject to a suffi-

cient level of solar flux. Values between2 and 4 indicate unsettled or even activemagnetic conditions, and are likely to bereflected in a degradation of HF condi-tions. Moving up the scale, 5 represents

a minor storm, 6 a larger storm and 7through 9 represents a very major stormthat would result in a blackout of HFcommunications.

Although geomagnetic and iono-spheric storms are interrelated, it is worthnoting that they are different. A geomag-netic storm is a disturbance of the Earthsmagnetic field and an ionospheric storm

is a disturbance of the ionosphere.

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Interpreting the Figures

The easiest way to use these figures isto enter them into a propagation predic-

tion program. This will provide the mostaccurate prediction of what might be hap-pening. These programs will take into

account factors such as signal paths be-cause some will cross the poles and theywill be far more affected by storms thanwill those across the equator.

If you dont own propagation soft-ware, it is still possible to gain a goodinsight into what the figures mean purelyby assessing them mentally. Obviously,high levels of solar flux are good news.

Generally the higher the flux the betterthe conditions will be for the higher HFfrequencies and even 6 meters. However,the levels need to be maintained for somedays. In this way the overall level of ion-ization in the F2 layer will build up. Typi-

cally values of 150 and more will ensuregood HF band conditions, although lev-els of 200 and more will ensure they areat their peak. In this way the maximumusable frequencies will rise, thereby pro-viding good conditions.

The level of geomagnetic activity hasan adverse affect, depressing the maximumusable frequencies. The higher the level ofactivity as reflected in higher Ap and Kpindices, the greater the depression of theMUFs. The actual amount of depression

will depend not only on the severity of thestorm, but also its duration.

Summary

As a broad rule of thumb, check outthe levels of solar flux and the K index.These figures can be found at a varietyof places, including on the Internet at avariety of sites including www.eham.net,www.qrz.com/,DX Summit at oh2aq.kolumbus.com/dxs/and in the K7VVVSolar Updates that are posted regularly

on ARRLWeb at www.arrl.org. If youconnect by radio or telnet to a DX spot-

ting network, you can obtain this infor-

Table 1

The General Relationship betweenA and K Values

A K Comments

0 0 Quiet2 1 Quiet3 1 Quiet4 1 Quiet to unsettled7 2 Unsettled

15 3 Active

27 4 Active48 5 Minor storm80 6 Major storm

132 7 Severe storm208 8 Very major storm400 9 Very major storm

Glossary of Solar Index Terms

ap index:A measure of the general level of geomagnetic activity over the globe fora given day. A mean, 3-hourly equivalent amplitude of magnetic activity based onK index data from 11 Northern and 2 Southern Hemisphere magnetic observatoriesbetween the geomagnetic latitudes of 46 and 63 degrees.

Ap index:A daily index determined from eight ap index values.

Geomagnetic activity:Natural variations in the geomagnetic field classifiedinto quiet, unsettled, active and geomagnetic storm levels.

Geomagnetic storm:A worldwide disturbance of the Earths magnetic field,distinct from regular diurnal variations. A storm occurs when the Ap > 29, a

minor storm when 29 < Ap < 50, a major storm when 50 = 100.

K indexA quasi-logarithmic local index of the 3-hourly range in magneticactivity relative to an assumed quiet-day curve for a single geomagnetic obser-vatory site. First introduced by J. Bartels in 1938, it consists of a single-digit 0through 9 for each 3-hour interval of the universal time day (UT).

Kp indexThe planetary 3-hour-range index Kp is the mean standardized K-index from 13 geomagnetic observatories between 44 degrees and 60 degreesnorthern or southern geomagnetic latitude. The scale is 0 to 9 expressed inthirds of a unit; e.g., 5 is 42/3, 5 is 5 and 5+ is 5 1/3. This planetary index is de-signed to measure solar particle radiation by its magnetic effects. The 3-hourlyap (equivalent range) index is derived from the Kp index.

Note: Kp, Ap and other indices can be downloaded via FTP at ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/INDICES/KP_AP/. Indices can also be downloaded fromwww.sec.noaa.gov/Data/alldata.html .

From A Glossary of Space Weather Terms (www.irfl.lu.se/HeliosHome/spacew9.htmlw

)and the National Geophysical Data Center Web site ( ww.ngdc.noaa.gov/stp/GEOMAG/kp_ap.html).

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mation by sending the command SHOW/

WWV. Please note that the A and K indi-ces broadcast by WWV represent themid-latitude values for Boulder, Colo-rado, and may not be representative ofconditions around the whole world.

For best conditions, the solar flux should

remain above about 150 for a few days withthe K index below 2. When these condi-tions have been met, check out the bands

and expect some good DX to be about!

Ian Poole has been licensed as a radio hamfor over 30 years and has been active onmany bands favoring HF SSB and CW. Heis also author of several books includingYour Guide to Propagation (and VHF/UHFAntennas) that is available through the ARRL bookstore. You can contact Ian at5 Meadway, Staines, TW18 2PW, UnitedKingdom;ian_poole@lineone.net .

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