Natural Radio wave Emissions: Sferics

There are different kinds of sferics-activity to observe: the large lightning strokes between cloud and earth, emitting a broad electromagnetic spectrum with very high power and a spectral maximum at 10 kHz. Less powerful lightning strokes occur between clouds. At least discharges within a cloud happen, emitting only little power with spectral maximum at long- and medium frequencies.

Monitoring was performed from 05.10.2011, 20:30 to 08.10.2011, 08:15 local time at the location of Algermissen, northern Germany (52° 15’ 9’’ N 9° 58’ 43’’ E). Antenna is the DX500 manufactured by RF-Systems. Conditions of space weather show no spectacular irregularities (taken from www.solen.info/solar):

Date

Solar sun spot number (NOAA)

Solar flux (NOAA)

Planetary K-indices

05.10.2011

115

126.7

22235533

06.10.2011

99

123.9

12112322

07.10.2011

88

122.0

22221112

08.10.2011

61

118.4

11112233

The AR5000RA software was used to set the two vfos A and B of the AR5000 receiver as following:

VFO

Frequency

Mode

IF-Bandwidth

AGC

HF-Attenuation

A

32 kHz

AM

6 kHz

fast

-10 dB

B

460 kHz

AM

6 kHz

fast

-10 dB

Then the feature “Monitor” > “Sferics” was chosen with the two VFOs employed. Responsivity was set to 7. The resulting counts of lightning strokes at 32 and 460 kHz are shown here:

Plot: counts of lightning strokes per min

The amount of lightning strokes per minute vary strongly. Proceeding in 1 minute steps, the course of sferics-activity can be smoothed by calculating a 60 min mean each time. To get an idea how well the gained data represent the real sferics activity, I took the results of the non commercial project blitzortung.org for comparison. blitzortung.org is an European network of private stations registering sferics. The following graph shows the smoothed data lying over the data of blitzortung.org:

Plot: Overlay of counts of lightning strokes

The underlying graph is taken with kindly permission from the private german weather station Rosstal, which is part of the project blitzortung.org. The filled blue curve show the counted lightning strokes by the weather station Rosstal itself, the filled light blue curve is the result of the combined counts of all european members of blitzortung.org. You will find the localization of the sferics here: 5. Oct., 6. Oct, 7. Oct. and 8. Oct. 2011.

What are the findings? During the night-times (18:42 - 07:30 local time) the absolute counts of lightning strokes at 32 kHz are higher than the registrations of blitzortung.org. But the relative course is close to the results of blitzortung.org. During the daytimes the 32 kHz counts well correspond with the data of blitzortung.org with exception of 6. Oct. 08:00 - 14:30 local time. Within this span of time the 32 kHz counts deviate not only absolutely but also relatively from the registrations of blitzortung.org.

Whereas the nightly raised counts of sferics at 32 kHz can easily be explained by the increased ionospheric propagation condition during night, the deviations at forenoon of 6. Oct. are not clear. Looking at the counts at 460 kHz one can see a rise during this period of time also. This tells us, the source of these counts must be within a radius of about 250 km (the range of the ground wave of medium frequency waves). Maybe a local thunderstorm with only cloud to cloud discharges produced the counts. blitzortung.org registers only the strong cloud - earth lightning strokes. Also man made noise must be taken in account. However a sudden rise in ionosperic propagation can be excluded, the above shown table of space weather conditions give no indication for such a phenomenon.