Introduction
This page outlines my research on forward scattered head echoes off radio meteors. All radio signal recording was done in USB reception mode with a Funcube Dongle Pro+ fed by a HB9CV antenna. Deployed transmitter was the GRAVES radar in southern France at 143.050 MHz. To perform such measurements a radio meteor detecting system is required that is able to pick up meteor head echoes and their frequency change in the audio stream with a sufficient time and frequency resolution. For this purpose the software Meteor Logger was written. Its raw data output afterwards was processed and analysed by the software Process Data, see Download page.
Radio detectability of forward scattered head echoes
1. An observed concentration of head echoes exhibiting a Doppler shift crossing zero Hz
Object of study was the forward scatter radio observation of sporadic meteors providing a wide spread range of mass, speed and trajectories. A broad distribution of head echo Doppler shifts over the frequency range of the receiving system was anticipated but not found. Instead a noticeable accumulation of head echoes passing zero Doppler shift were observed in a forward scatter setup.
Fig. 1: Frequency distribution of 4434 head echo frequencies at the maximum of their power curves. This is the result of a 6 week lasting monitoring session from Jan-Feb. 2018 in Algermissen, Northern Germany. Zero Hz Doppler shift is reached at about 1195 Hz audio frequency (which is determined by the setting of the beat frequency oscillator of the usb-reception mode). Included is a picture showing an arbitrary collection of audio spectrograms of head echoes. The dashed line indicates zero Hz Doppler shift. This picture is for illustration of the distribution graph only.
The overwhelming number of observed head echo power curves culminate near zero Hz Doppler shift, see Fig. 1. Their power maxima are by many orders of magnitude larger than the power maxima of head echo power curves received at frequencies off zero Hz Doppler shift. It appears that head echoes reaching zero Doppler shift during the ablation process of the meteoroid have a significantly enhanced chance of reception in a forward scatter set up. Consequently, only the less numerous larger meteoroids with higher radar cross sections could be detected off zero Hz Doppler shift. An explanation of this phenomenon is pending. A detailed description of this analysis was published in the Journal of the International Meteor Organization: Kaufmann W. (2020): "Enhanced radio detectability of forward scattered head echoes passing zero Doppler shift". WGN, The Journal of the IMO, 48:4, 108-111. You can read the article here.
2. An restricted observability of head echoes due to high Doppler shifts
During the maximum activity of the Quadrantids at about 22 hours UTC, 2018-01-03, a forward scatter radio observation at Algermissen, Northern Germany, involving GRAVES radar as transmitter failed to detect meteor head echoes of this stream for more than one hour.
Fig. 2: A Stellarium-simulation of the sky in northern direction at 22 hours UTC, 2018-01-03, seen from Algermissen, Northern Germany. The radiant of the Quadrantids (in yellow) is 15° above the horizon.
A simulation of head echo Doppler shifts was performed on the basis of fixed receiver-transmitter-geometry with varying meteoroid trajectories in the height of 120-80 km by means of the software MDopplerShift, see Download page. It could be shown that the ascertained head echo blackout coincided with a radiant position of the Quadrantids (see Figure 2) that resulted in trajectories producing very large Doppler shifts. From forward scatter radio observations it was found that the signal strength of a head echo declines rapidly with increasing Doppler shift. Simple forward scatter radio systems usually are too insensitive to detect head echoes Doppler shifts above a few kHz when meteoroid masses are small. This caused the blackout. A detailed description of this analysis is published in the Journal of the International Meteor Organization: Kaufmann W. (2020): "Limitations of the observability of radio meteor head echoes in a forward scatter setup". WGN, The Journal of the IMO, 48:1, 12-16. You can read the article here.
Analysis of Head Echoes
1. Visualising Meteor Showers
The outcome of radio meteor observations of a basic monitoring system based on forward scattering can be enhanced by additionally analysing meteor head echoes. Meteoroids of common origin (radiant) are characterised by similiar geocentric velocities and parallel trajectories. They produce in a circumscribed area of reflection a bunch of head echoes with equally Doppler shifted frequency gradients. The observed frequency gradients are a result of velocity, angle of incidence and bearing of the meteors. So different radiants produce different groups of frequency gradients. These groups can be made visible as hot spots in a kernel density map. In this way spatial different sources of meteoric activity in the sky can be detected. E.g. Fig. 3 shows the kernel density map of the frequency slopes of all received head echoes during the outburst of the Draconids in 2018.
Fig. 3: Kernel density map of the frequency slopes of the head echoes (dots) registered in the days around the outburst of the Draconids in 2018. The Draconids (DRA) depict as well distinguishable hot spot on October 08th. Beyond the Draconids sporadic meteors are visible also. Especially the antihelion- (AH), apex- (AP) and helion-source (H) as regions of increased activity are discernable. The dotted lines mark 00h, 06h and 12h UTC each day.
A detailed description of the procedure is published in the Journal of the International Meteor Organization: Kaufmann W. (2018): "Visualizing meteor streams by radio forward scattering on the basis of meteor head echoes". WGN, The Journal of the IMO, 46:1, 39-44. You can read the article here, its Figures 4-7 can be viewed in the supplement in high resolution.
2. Visualising Sporadic Meteor Radiants
Sporadic meteors are considered as meteors not being part of a recognized shower. The distribution of the radiants of sporadic meteors within the celestial sphere were subject of a number of surveys. Since 1956 it is known that the sources of sporadic meteors are non-homogeneously distributed in space. 6 regions with increased activity are established meanwhile: north and south toroidal source, antihelion (AH) and helion (H) source and north and south apex (AP). Fig. 4 shows the result of an radio observation of sporadic meteors based on the above described technique and started at the beginning of 2018 shortly after the Quadrantids maximum. It was extended to mid-February. This time span was free of major meteor showers which allowed for an unbiased observation of the sporadic meteors.
Fig. 4: Kernel density map of the frequency slopes of head echoes from 40 x 24h of continuous observation (Jan 5th-Feb 14th). The cumulated head echo counts per hour are added as bar chart (Maximum 247 Head echoes/h). Daily time span is 18h-17h59m UTC (= 19h-18h59m local time). The positions of high density hotspots are marked by lower cases.
The bar plot of the cumulated hourly count rates of the head echoes (see Figure 2) exhibits three peaks. They occur at daytimes that coincide with the highest radiant positions in the sky of the AH, AP and H source, respectively. It can be assumed that these peaks are mainly the manifestation of the activity of the AH, AP and H source with minor contribution of the northern toroidal source as well as sporadic meteors that do not belong to one of these sources. In Figure 2 the hot spots exhibiting higher density are marked with lowercases a-n. They can be clustered into three groups by daytime which can be assigned to the AH (a-c), the AP (d-i) and to the H (j-n) peak of the head echo count rates. So for the period 2018 January 05th-February 14th at least 14 different major radiants could be distinguished basically within the AH, AP and H source by this technique. In detail this approach is published in the Journal of the International Meteor Organization: Kaufmann W. (2018): "Visualizing sporadic meteor radiants and their dynamics by radio forward scattering". WGN, The Journal of the IMO, 46:6, 201-204. You can read the article here.
