Skip to main content

Reducing Data Gathered by VLF Monitoring Systems

AAVSO Website Being Upgraded on Tuesday, 31 May. More details here!


Solar Program, Reducing Data and Email Format

Reducing Data Gathered by VLF Monitoring Systems

(Extracted from SID Technical Bulletin Vol. 3, Number 4, Oct. 1992)

The reduction process is quite simple, and consists of the steps which follow. First, the universal time for each of three event-phases must be measured for each SID. Examples are indicated on the recording at the end of this report.

Event Start: The moment when an event begins. Be aware that the amplitude of different events varies considerably, and some may appear in inverted form. If it is obvious that an event began before the first definable start time, an 'E' (before) is appended to the recorded time.

Event End: Of the three times, the end time is the most difficult to determine. It is defined as the moment when the trace returns to the diurnal trend line, or is interrupted by the onset of a new event. In the latter situation, the letter 'D' (after) is appended to that time, which in turn becomes the start time for the following event.

Event Maximum: The moment when the ascending (descending in the case of inverted events) branch slows its sharp rise. Note that this generally does not coincide with the event's peak amplitude.

If the trace goes off-scale as maximum is approached, or is interfered with in some other manner such as the onset of sunset, device failure, etc., we append a 'U' (uncertain) to the last identifiable time. For example, if the SID maximum occurs somewhere off-scale and the last determinable time is fourteen hours, ten minutes (1410), the time is recorded as 141OU.


Duration Importance
< 19 minutes 1-
19-25 1
26-32 1+
33-45 2
46-85 2+
86-125 3
>125 3+

After each of these times have been recorded, the event's 'Importance' and 'Definiteness' ratings can be determined. Importance generally refers to the length of an event, found by subtracting the start time from the end time. The table at right associates the Importance rating with event duration.

With respect to the observer's typical daily trace, a class 1 SID is an event with small intensity change and a relatively short duration. Class 2 is a moderate intensity event with a fairly long duration, and a class 3 describes a SID with a 'great' change in intensity and long duration.


Confidence Definition
Questionable 0
Possible 1
Fair 2
Reasonable 3
Definite 5

Definition is a subjective estimate of an observer's confidence in the event. Normally, the events which are included in our reports have Definitions equal to 3 or greater. Use the right-hand scale as a guide to Definition. Careful study and experience teaches observers how to recognize false SID events caused by man-made interference. Beginners should not hesitate to assign high Definition values to those events which are not correlated with known noise sources. Other observers' results will be compared to eliminate suspect events before a final report is submitted to the NGDC.

Email Format for SID Event Reports

The format prescribed by the NGDC always begins with the '40' NOAA SID identifier code. The AAVSO SID Coordinator assigns observer codes after the station is operating correctly and data is received regularly from that observer.

Data code; always 40
Start time; UT hours and minutes event began
Start time qualifier; D = after, E = before, U = uncertain
End time; UT hours and minutes event ended
End time qualifier; D, E, U
Maximum time; UT hours and minutes of event maximum
Maximum time qualifier; D, E, U
45-46               SID Importance; sign in column 46
Code for monitored transmitter. The field consists of the final two station call letters and the two numbers which represent the frequency (kHz). The latter is rounded to the nearest integer. Example; the code for NSS at 21.4 kHz is recorded as SS21
Observer code; Example A82
AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484