Contacter l'auteur / Contact the author

Recherche dans ce site / Search in this site



Ionospheric Perturbations

Huge solar prominence recorded on Sept 14, 1999 in UV light at 304Å by SOHO.

Solar and geomagnetic indices (V)

If we want to anticipate ionospheric perturbations, foresee fadings or blackouts in the propagation, or on the contrary openings on some bands to some countries, there is only one object to monitor : the sun.

All concerned people, geophysicists, astronomers, operators or radio amateurs have thus taken their pilgrim stick or rather their telescope, their satellite or their favorite website to monitor the solar activity, the sunspot number, the density and speed of the solar wind, CMEs and other chromospheric eruption, so many events that indirectly impact our ionosphere at short or longer term.

In order to monitor propagation conditions, two main sources of disturbances must be considered:

- The solar flux

- The geomagnetic field

Both events directly determines the MUF and in a lesser extent the LUF, thus the range of frequencies between which you can work DX stations, two other parameters that we will review on the last page.

A. The solar flux

When they are properly oriented toward Earth, electromagnetic waves and particle solar emissions have chances to reach the ionosphere and affect shortwave propagation.

When the solar activity goes down... These plots show received signal strengths on HF bands recorded on January 1, 2002 (left, SSN=113) and 2004 (right, SSN=52) using a scanner with a 50 kHz resolution. One sees very well the effect of the low solar activity above 12 MHz in 2004; the low ionization level of upper bands prevents transmitters to work in good conditions. Doc Space Environment Corporation.

The value that interest the most radio amateurs is the level of ionization of ionospheric layers, especially E and F layers. Their status depends primarily on the radioelectrical solar flux index, SFI, which is correlated with the sunspot number over 97%. Remind that D-layer is only active at daytime and only affects low band propagation up to the 40-m band.

The average solar flux (SFI) is related to the average or smoothed sunspot number (SSN) by the next relation :

SFI = 63.7 + 0.727 SSN + 8.95x10-4 SSN2

For example, a SFI of 85 is equivalent to a SSN of 28.

However, during periods of high solar activity, median values can be very different from the daily real values while during low activity they don't differ much. 

If you run a propagation prediction program using these smoothed values by default, don't forget to replace them by near-real-time data or, working offline, to run the simulation with values up to 20% higher and lower than the average ones to see what happens. You will be surprised.

Each day the solar flux is calculated at 2800 MHz (10.7 cm). It is expressed in flux unit equivalent to 10-22 Wm2/Hz. 

FSI goes down to 50 or less during a quiet Sun to reach 400 at the paroxysm of its activity (with an SSN 250). This index is transmitted on the air through the U.S. WWV service on various frequencies, on 2.5, 10, 15 and 20 MHz, each 18 and 45 minutes past the hour. Outside the U.S.A. it is easier to read these data via the Internet. On some DX clusters like DXHeat the solar flux is named "SFI" or "I" as below :

Several times each day the WWV service publishes on the air the solar activity indices, SFI or I, A (Ap) and K (Kp). These values are mandatory to simulate and estimate the propagation conditions to DX countries in software like WinCAP or DX ToolBox. This document was extracted from DX Summit at the end of October month 2003 when radio blackouts occured over Northern Europe and Canada. We observed aurora during these events between 38-75° of latitude, including in Australia, creating large openings for aurora traffic and the study of VLF.

B. The geomagnetic field

The second component influencing directly propagation conditions is the intensity of the geomagnetic field, mainly its horizontal component (it couples easier with antennas horizontally polarized working at mid to low latitudes). Its activity is described by at least 16 different indices, among them planetary indices, Ap and Kp are the most important. Here are a short list of these indices :

- The ap index is a median 3-hourly "equivalent amplitude" value of the geomagnetic activity. It is based on K index data (below).

- The Ak index. is a daily index of the geomagnetic activity recorded at various stations (represented generically here by k) and derived as the average of the eight 3-hourly ak indexes recorded in a Universal Time day.

- The Ap index is a median value of the geomagnetic activity derivated from the last eight 3-hour Ak indexes recorded by various observatories at the end of the day. This value depends also on local conditions. With Kp they constitute the planetary indices.

- The K index is a 3-hour quasi-logarithmic local index of geomagnetic activity relative to an assumed quiet-day curve recorded by various observatories at the end of the day. It is ranging between 0 and 9. The K index measures the deviation of the most disturbed horizontal component (contrarily to Q index which is a measurement of the largest relative deviation).

K index is recorded by an international network of northern (11) and southern (2) hemisphere magnetic observatories located between the geomagnetic latitudes of 46-63° by the Institut für Geophysik der Göttingen Universitätt, in Germany

- The Kp index was introduced by J.Bartels in 1949. This is a short-time forecast describing the expected geomagnetic activity for the next 3 hours, derivated from the standard K indexes (Ks) observed at 13 magnetic stations primarily located in the Northern Hemisphere. The Kp index follows a logarithmic scale, and varies between 0 and 9. It is used to determine the Ap index. With Ap they constitute the planetary indices.
The Kp index is broadcasted heigth times a day. It is thus more interesting than the Ap index as it concerns conditions to come rather than past. Kp is also calculated by the Institut für Geophysik der Göttingen Universitätt, in Germany. The first records go back to 1932.

Depending  on geophysical conditions, a Kp index of 9 can represents 300 gammas at lower latitudes or ten times this value near the Arctic circle in the auroral area. Its mean value is 2. When we need a Kp index of 4 to see aurora over the States, Europe requests a Kp index of 9 due to the tilt of the geomagnetic pole towards Canada.

At last the solar activity interferes with the chemical constitution of the atmosphere too. Together these conditions can favour or not contacts with DX or local stations.

Classification of geomagnetic activity

Ap Index

Kp index


0 - 7

0 - 1


8 - 15

2 - 3


16 - 29



30 - 49


Minor Storm

50 - 99


Major Storm

100 - 400

7 - 9

Severe Storm

Note that there are some other indexes that are used in simulation programs like ICEPAC that comes with the VOACAP package. One of them is the "Quietest day" index, Q-days.

Like the "Most disturbed days" index (D-days), Q-days is deduced each month from K indices in the basis of three criteria for each day:

- The sum of the eight Kp values

- The sum of squares of the eight Kp value

- The maximum of the eight Kp values.

At last, there is the Q index. It is a 15-minute index of geomagnetic activity intended for high-latitude stations. It is thus related to auroral activity. After quiet diurnal variations are removed, Q is the largest deviation scaled from the undisturbed level for the two horizontal components (and thus it differs from the K index, which is scaled from the largest relative deviation). Q-index values are measured each 15-minute past the hour. Q is ranging between 0 and 11 and can be aligned with the goemagnetic field intensity using next approximation expressed in nanoteslas :

Relationship between Q index and its expression in nanoteslas



























According to each of these criteria, a relative order number is then assigned to each day of the month, the three order numbers are averaged and the days with the lowest and the highest mean order numbers are selected as the five (respectively ten) quietest and the five most disturbed days.

Note however, that a selected quiet day is considered as "not really quiet" and is marked by the letter A if the Ap > 6, or marked by the letter K is Ap £ 6, and either one Kp > 3 or two Kp > 2+.

At last a selected disturbed day is considered as "not really disturbed" and marked by an asterisk (*) if Ap < 20.

Interactions between SFI, Ap and Kp

In short, we can say that the solar flux impact is opposite to the one of the solar indices but they are linked each others. Good propagation conditions appreciated by DXers occur when the solar flux is high enough to ionize the upper level of the ionosphere. But in the same time the sun has to display a relative quiet activity in order to not disturb too much the geomagnetic field what should increase values of Ap and Kp indices.

When they reach high values, Ap and Kp indices have a negative effect on HF propagation, reducing your chances to contact or hear a far country to almost null. They both alter shortwaves because as these values increase with the increasing of solar radiations, particles from the ionosphere absorb more and more signals around the polar caps. At the end all signals passing near polar latitudes are transformed into noise, creating a huge QRN on all bands. Conversely, this gradually closing of HF bands, can produce enhanced propagation on the 6m or the 2m band. We will come back on these indices when we will deal with ionospheric models and the impact of geomagnetic indices on forecasts.

Relationship between Ap and Kp indices























A way to be warned of these perturbations is to listen to broadcast stations located in northern latitudes (for users in the northern hemisphere) like the U.S.A., Canada, Russia, Japan, etc or beacons (in CW mode) located near these areas (e.g. OH2B, 4U1UN, VE3AT, RR9O, JA2IGY). If their signal weaken or is subject to fading for hours, you can be sure to be face to a geomagnetic storm some thousands kilometres norther. You can confirm your suspicious by connecting you to various websites studying the earth environment physics as the one listed at the bottom of the next page.

As will confirm DX hunters, if you had only two things to remember from these indices, know that your chance of working a DX station will be better when the solar flux (SFI) is high associated to a low geomagnetic activity (Kp). If furthermore the sunspot cycle is peaking during these up and down periods, all conditions are met to work new DX stations, even late in the night.

Solar Flux

Ap index

Kp index


> 180

< 8

< 3

E-W opening

> 180

< 8

> 3

N-S opening

> 250

> 30

> 3


Blackouts and extra openings

When conditions are deeply degraded, the only possibilities to work on HF is using the gray line and looking for trans-equatorial communications where bands are always a bit less disturbed. The explanation is the following. While signals disappear over mid-latitudes countries, the ionization level increases over the equatorial region enhancing all signals emitting in that area. So a geomagnetic storm can be a good opportunity to work DX located beyond the equator, like Chile, South Africa, Polynesia or Australia if you live at northern latitudes and vice-versa. But usually the countries the most concerned by this effect are located between the equator and the tropics (0°-30°N/S). At higher latitudes it is more efficient to work with the gray line. In such conditions, although critical, these stations arrive stronger to you than usual, with a difference than reach several S-points. These openings are regularly accessible until the evening up to the 17m band. 

This explain why during such blackouts Patrice, LA0HF, hears very well DX stations located in the southern hemisphere and works them sometimes easier than in normal circumstances. On my side, located at mid-latitude, these bands are closed and I work best these DX stations when the gray line is covering Western Europe.

Last chapter


Page 1 - 2 - 3 - 4 - 5 - 6 -

Back to:


Copyright & FAQ