1. Infrasonic detection of meteoroid entries
Ludwik Liszka
Swedish Institute of Space Physics
Umeå, Sweden

3. SIN

4. Subject of this presentation: meteoroid entries

5. Link to order the report

6. Meteor impacts during the last century in Northern Scandinavia

7. Impact crater in Kvavisträsk caused by the event of May 20, 1900

8. Major meteoroid entries recorded during recent years
Jokkmokk
Bygdsiljum
Karelia
Tromsö
Oulu

14. Fine structure of the infrasonic signal from the Jokkmokk-event recorded in Kiruna

15. Procedure to determine the impact orbit
The sequence of computed data describing the signal (time-of-arrival, T, angle-of-arrival, A, and trace velocity, V) are sorted with respect to the trace velocity and three outermost outliers on each side of the distribution are removed.
The distribution is divided into two equal parts, low, L, and high, H, one on each side of the median value.
New median values are calculated for all three variables and for both partial distributions:
TL1, AL1, VL1 and TH1, AH1, VH1. AL1 and AH1 are the lower and upper quartiles of the original distribution, respective.
The same procedure is repeated for the same event data recorded at another array and a new set of values: TL2, AL2, VL2 and TH2, AH2, VH2 is obtained.

16. Procedure to determine the impact orbit (cont.)
The procedure marks two points, P1 and P2, on the acoustic active part of the source trajectory as seen at both locations. If there is a significant variation in the trace velocity, V, at both arrays, the order of events may be determined. For a meteoroid entry, the event will start with a high trace velocity (highest speed and altitude). Observations from both arrays may now be matched together, assuming that marked points with high trace velocity, P1, correspond to the upper part of the trajectory. In the same way, points with lower trace velocity, P2, will be attributed to the lower part of the trajectory. The present procedure may be useful when extracting orbital information from the data.
The procedure may be used up to distances of the order of 100 km.
At larger distances propagation effects dominate the signal properties.
The procedure is repeated at different frequencies corresponding to different altitudes.

17. Jokkmokk-event

18. Tromsö-event

19. Search for unknown meteor events
Multiple indicators for known meteor events are extracted and used as a training set.
All infrasonic data are converted into multiple indicators (MIM-method)
A Neural Network model is constructed using the training set.
All data are passed through the model and the events which are most similar to the reference data are found.
These possible event are validated.

20. MIM of infrasound data

21. NN-model

22. A sample of model’s response

23. A plot of all responses

24. Responses that repeat on the same day of the year

25. Visual meteors (-6 - -3 mag)

26. Geographical distribution of meteor-like signals