OBSERVATION OF GEMINID METEOR SHOWER 2004 USING VLF SFERICS AT 21.5 kHz Soma Banik (1) and Barin Kumar De (2) (1)Department of Physics, Tripura University Suryamaninagar-799130, West Tripura, India e-mail: [email protected] (2)As (1) above, but e-mail: [email protected] ABSTRACT The signature of the strong Geminid meteor shower occurs as sudden rise and falls in the Atmospheric Radio Noise Field Strength (ARNFS) level during the predicted period. Total no. of rise and falls observed are 29 and 17 respectively from 12th to 14th December at frequency 21.5 kHz monitored in diurnal basis. The average values of durations of rise and falls are 11min and 8 min respectively. We have calculated the value of βe Nm for both the -1 -1 daytime and night time. The day and night time values of βe Nm lies in the range 0.01-0.11 s and 0.023-0.134 s respectively. INTRODUCTION Predictions had been announced regarding the activity of the annual Geminid Meteor shower 2004 to be occurred during 7th to 17th December expecting a peak activity during 13th and 14th. The viewing conditions for enjoying this celestial event was also expected to be an ideal one as the moon would only be two days past new phase. The velocity of the shower particles is 35 km/hour as usual The IMO, the International Meteor Organization, lists their Zenithal Hourly Rate (ZHR) as 120 meteors per hour - the highest on their list. The Geminid meteor particles are reported to penetrate up to a depth of 75 to 85 km. For the year 2004 this is the brightest annual shower. Geminids stand apart from the other meteor showers in that they seem to have been spawned not by a comet, but by 3200 Phaeton, an Earth-crossing asteroid. There's a cloud of dust trailing the asteroid and Earth plows through it every year in mid-December. Studies show that the Geminids are rich in slow, bright, graceful meteors and bright fireballs, as well as faint meteors, with relatively fewer objects of medium brightness. According to meteor specialist Neil Bone [1], at 2 grams per cubic centimeter on average, Geminid meteoroids are several times denser than the cometary dust flakes that supply most meteor showers, so they burn up less quickly. According to the IMO, the maximum activity will be at about 22h 20m UT +/- 2.3h, on December 13th. The reports of IMO about the shower have shown a high value of ZHR from 05:59 UT of 13th to 09:50 of 14th December. VLF atmospherics at frequency 21.5 kHz for the time being is used as a general tool for the study of this terrestrial phenomenon on the ionosphere. Because of small electron densities in D region (109m-3) and high electron-neutral collision frequencies (106sec-1) the D region can not be studied directly [2]. VLF signal that propagates through earth-ionosphere wave guide can provide sufficient information about the lower boundary of the ionosphere- the D region. On the predicted period of shower there has been no solar flare occurred and moreover the sky was clear and no reports of local thunderstorms have been obtained. Our observations highly correlate the effect of meteor shower on sferics. It also as a consequence shows the changes in ionization in the upper atmosphere due to meteor shower. INSTRUMENTATION An inverted L- type has been used to receive vertically polarized atmospheric sin the VLF band from near and far sources. The induced voltage in the antenna is fed to the input buffer and filtered through in the next stage using a low pass filter. The filtered output is then amplified accordingly and the output of this stage is fed to the high Q tuned circuit having a resonant frequency 21.5 kHz. The gain of the amplified is so adjusted so that maximum voltage induced in the antenna should not bring it to the saturation level. Then output envelope is detected using a diode detector. The time constant of the detector circuit is taken to be 0.22s so as to get records of even small variations in sferics level. In the next stage of the receiver a logarithmic amplifier is used to squeeze the voltage and care is taken of to maximize the dynamic range of the signal. At the output a potential divider is used so that final output fed into the Data acquisition system safely lies with in 5 volt. A capacitor is used at the output to cutoff very rapid insignificant fluctuations in dc level resulting in a total time constant of about 10s. ANTENNA INPUT LOW PASS AC TUNED BUFFER FILTER AMPLIFIER CIRCUIT DATA POTENTIAL LOG ENVELOPE ACQUISITION DIVIDER AMPLIFIER DETECTOR SYSTEM RADIO SKY PIPE Fig.1. Block diagram of receiver circuit tuned at 21.5 kHz OBSERVATIONS DURING SHOWER ACTIVITY: The strong meteor shower effects have been observed as sudden enhancements and decrements in ARNFS. The total no of enhancements and decrements found to be on 12th, 13th and 14th of December’04 are 29 and 17 respectively. The average values of durations of rise and falls are 11min and 8min respectively. The range of enhancement lies between 1 dB to 10 dB whereas the range of decrement lies between 1 dB to 6 dB. 2.4 PEAK ACTIVITY OF GEM INID SHOW ER 1.6 0.8 ARNFS IN ARBITRARY SCALE IN ARBITRARY ARNFS 0.0 11:30 13:25 15:20 17:15 19:10 TIME IN IST(Hr) Fig.1. Effect of Geminid Shower on VLF ARNFS at 21.5 kHz Table1. Distribution of events in different durations Duration of events in Number of decrements in Number of enhancements in minutes sferics level sferics level 3-6 6 7 6-9 5 6 9-12 3 8 12-15 3 4 >15 - 4 Table2. Distributions of decrements in different dB ranges Date Number of Number of occurrence between occurrence 1-3dB 3-5dB >5dB 12.12.04 6 5 1 - 13.12.04 8 7 - 1 14.12.04 3 3 - - Total 17 15 1 1 Table3. Distributions of increments in different dB ranges Date Number of Number of occurrence between occurrence 1-3dB 3-5dB >5dB 12.12.04 13 12 1 - 13.12.04 10 10 - - 14.12.04 6 5 - 1 Total 29 27 1 1 CALCULATION OF βemN The events of enhancements and decrements are due to extra ionization in the D and E regions of the ionosphere forming overdense and underdense trails. The duration of each enhancement or decrease are controlled by removal - of extra ionization via ambipolar diffusion and mainly attachment of electrons with neutral O2 molecules. ΛAm2/3 4 The line density of ionized trail formed is [3] q=τ ()ρ V (where A=a dimensionless shape factor, q a 4ζη ρm m=mass and ρm = the effective density of the meteor, ρa = air density, ζ = heat of ablation of the meteoroid, Λ = a dimensionless heat transfer coefficient,η = the mean ionization potential per atom involved, τ q = dimensionless ionization efficiency factor, V= velocity of meteor) and it is independent of the radius of the trail, although the volume density at a radial distance r from the axis of the cylindrical trail at time t after its formation is given by λ r 2 Nrt(,)=−+0 exp[{β Nt }], where D= the ionic diffusion coefficient, λ = initial line e 22em 0 π (4Dt++ r00 ) 4 Dt r density, βe =attachment coefficient of electrons with neutral molecules and Nm = the neutral molecular density participating in attachment process. The VLF ARNFS once changed from its normal value returns to the ambient level after the time (T) when electron density of the ionized trail comes to the normal value. Considering both the ambipolar diffusion and attachment processes, from the expression of volume density, Ne (r, t), substituting the time T, after which the electron density at the axis returns to the normal value, Ne (0, T), the value of βeNm is obtained 1 λ as β N = ln[0 ] . em T π 4(0,)DTNe T The initial radius of the trail and rate of diffusion of electrons depends upon atmospheric pressure and hence on height, whereas the initial line density depends upon the height as well as the velocity of meteors. Log r0= 0.075H-7.9 Log D= 0.067H-5.6 4.4Log λ = 82-H+49LogV 0 Here height is in km,r in m, D in m2s-1, λ in m-1 and V is in kms-1.Daytime enhancement of VLF ARNFS supports 0 0 that the ionized are formed at height of 70 km. Here we assume that the height of ionization is invariant with respect to day and night. For Geminid particles the value of λ calculated is 8.4 *10^19 m-1. 0 STATISTICAL DISTRIBUTION OF ATTACHMENT 20 COEFFECIENT DURING DAY 16 12 8 NO. OF EVENTS 4 0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 ATTACHMENT COEFFICIENT(S-1) Fig.2. Statistical distribution of βe Taking the daytime and nighttime normal normal electron density at 70 km to be 108 m-3 and 107 m-3 respectively, -1 -1 the ranges of βe are found to be 0.01-0.11 s and 0.023-0.134 s respectively. DISCUSSION The propagation of VLF radio wave from cloud discharge occurs through the wave guide formed between earth and lower surface of d region of ionosphere. The change of field strength during the formation of ionized trail of meteor is due to the change of relative phase of different modes which interfere to give the resultant field at the receiving antenna.