Natural Sources of Electromagnetic Radiation

Solar Activity Description

Solar activity is described according to the number of flares which occur during the day. The status or solar activity is braodcast hourly on shortwave and VHF on radio station WWV and WWVH. WWV broadcasts on 2.5, 5, 10, 15 20 MHz. These stations can be heard easily, even inside a classroom, with a modest portable shortwave radio.

Activity is basically classified as follows:

Very Low : X-ray events less than class C.

Low : C-class x-ray events.

Moderate : Isolated (one to 4) M-class x-ray events.

High : Several (5 or more) M-class x-ray events or isolated (1 to 4) M5 or greater x-ray events.

Very High : Several M5 or greater x-ray events.

Flare Classifications

Flares are classified using one of two different systems.

System One

The first classification ranks the event by measuring its peak x-ray intensity in the 0.1-0.8 nanometre band as measured by the GOES satellites. This x-ray classification offers at least two distinct advantages compared with the second system of classification (optical): it gives a better measure of the geophysical significance of the event and it provides an objective means of classifying geophysically significant activity regardless of its location on the solar disk or near the solar limb. This scheme classes flares as

Class Peak Flux (0.1-0.8 nanometres in Wm-2)

A < 10-7

B < 10-6 but > class A

C < 10-5 but > class B

M < 10-4 but > class C

X > 10-4

The letter designates the order of magnitude of the peak value. Following the letter the measured peak value is given. For descriptive purposes, a number from 1.0 to 9.9 is appended to the letter designation. The number acts as a multiplier. For example, a C3.2 event indicates an x-ray burst with a peak flux of 3.2 x 10-6 Wm-2. Since x-ray bursts are observed as a full-Sun value, bursts below the x-ray background level are not discernable. The background drops to class A level during solar minimum; only bursts that exceed B1.0 are classified as x-ray events. During solar maximum, the background is often at the class M level, and therefore class A, B and C x-ray bursts cannot be seen. Bursts greater than 1.2 x 10-3 Wm-2 may saturate the GOES detectors. If saturation occurs, the estimated peak flux value is reported as "yikes!!!!" or "holycowmama"

System Two

The second system of classification involves a purely optical method of observation. A flare event is observed optically (in H-alpha light) and is both measured for size and brightness. This classification therefore includes two items of information: a descriptor defining the size of the flare and a descriptor defining the peak brightness of the flare. They are listed below:


S - Subflare area less than = 2.0 square degrees.

1 - between 2.1 and 5.1 square degrees.

2 - between 5.2 and 12.4 square degrees.

3 - between 12.5 and 24.7 square degrees.

4 - more than 24.8 square degrees.


F - Faint.

N - Normal.

B - Brilliant.

Example: A major flare rated as a class M7.4/2B event indicates that the flare attained a maximum x-ray intensity of 7.4 x 10-5 Wm-2. The "2B" portion of this specification indicates that the flare was an importance 2 flare (>= 5.2 and <= 12.4 square degrees) and was optically Brilliant. This sample flare is a powerful event. Flares that reach x-ray levels in excess of class M4 can begin to have an impact on the Earth. Likewise, flares rated 2B or greater are generally capable of influencing the Earth, particularly if accompanied by Type II and IV radio sweeps (discussed below).

Sweep Frequency Events (Type II, III, IV and V events

Energetic solar events often produce characteristic radio "bursts". These bursts are generated by solar material plunging through the solar corona. Type III and type V events are caused by particles being ejected from the solar environment at near relativistic speeds. Type II and IV events are caused by slower-moving solar material propagating outward at speeds varying between approximately 800 and 1600 kilometers per second. Type II and IV radio bursts are of particular importance.

These sweep frequency radio events are signatures of potentially dense solar material which has been ejected from the solar surface. If the region responsible for these events is well positioned, the expelled solar material may succeed in impacting with the Earth. Such an impact often causes an SSC followed by Minor to Major geomagnetic storm conditions and significantly degraded radio propagation conditions. It is therefore interesting to pay attention to events which cause Type II and/or IV radio sweep events, since they may indicate the potential for increased magnetic activity (and decreased propagation quality) within 48 hours. It should be noted, however, that predicting degraded terrestrial conditions is significantly more complex than simply observing whether the energetic event had an associated Type II or IV radio sweep. Flare position, proton spectra, flare size, event duration, event intensity and a host of other variables must be analysed before a qualitative judgement can be made.

It should also be noted that sweep frequency radio events are capable of producing Short Wave Fades (SWFs) and Sudden Ionospheric Disturbances (SIDs). Depending on the severity of the event, the duration of SWFs and SIDs may last in excess of several hours with typical values being approximately 30 minutes. SWFs and SIDs cause absorption of radio signals (due to intense ionization) at frequencies up to and well in excess of 500 MHz. Microwave continuum bursts can affect frequencies up to 30 GHz. Frequencies in the HF region can be completely blacked out for a period of time during intense energetic events.


Prepared by the YES I Can! Science Team,