Meteor head and terminal flare echoes observed with the Gadanki MST radar

Accepted Manuscript Meteor head and terminal flare echoes observed with the Gadanki MST radar K. Chenna Reddy, B. Premkumar PII:

S1364-6826(17)30582-5

DOI:

https://doi.org/10.1016/j.jastp.2018.11.006

Reference:

ATP 4952

To appear in:

Journal of Atmospheric and Solar-Terrestrial Physics

Received Date: 11 October 2017 Revised Date:

5 November 2018

Accepted Date: 9 November 2018

Please cite this article as: Reddy, K.C., Premkumar, B., Meteor head and terminal flare echoes observed with the Gadanki MST radar, Journal of Atmospheric and Solar-Terrestrial Physics (2018), doi: https:// doi.org/10.1016/j.jastp.2018.11.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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K. Chenna Reddy, B. Premkumar

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Meteor head and terminal flare echoes observed with the Gadanki MST radar

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Department of Astronomy, Osmania University, Hyderabad, India - 500 007

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Abstract

Meteor observations with Gadanki MST radar usually detect variety of meteor echoes that includes head echoes, specular and non-specular trail echoes. Sometimes, but not always head echoes are followed by a sudden increase in signal strength amounting to many decibels at terminal end point of the trail, known as terminal flare echoes - a feature mostly observed with optical and high power large aperture (HPLA) radar systems. In this study, we report some examples of

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terminal flare echoes observed with Gadanki MST radar. Because these echoes provides valuable insight into the role of diffusion and plasma instabilities in the formation and evolution of meteor trail. From the observations, it has been noticed that the head echoes at higher altitudes are generating non-specular trail

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echoes, whereas they disintegrate as terminal flares associated with meteoroid fragmentation in lower altitudes. Although meteoroid fragmentation is a common phenomenon, but terminal flaring is a rare feature observed with Gadanki

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radar. A small, but non-negligible fraction of meteor events (∼2.5% of all head echo events) showed flaring apparently produced by terminal destruction of a meteoroid fragmentation along with the insights into the fragmentation and terminal flaring process. Keywords: Meteoroid Fragmentation, Head echoes, Terminal flare echoes.

∗ K.

Chenna Reddy Email address: [email protected] (K. Chenna Reddy, B. Premkumar)

Preprint submitted to Journal of LATEX Templates

November 12, 2018

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1. Introduction When a meteoroid of interplanetary origin enters the Earth’s orbit, its sur-

face gets heated up and the surface particles start to vapourize due to interaction with the atmospheric molecules. As a result of inelastic collisions, the vapour-

ized atoms forms a high-density ionized plasma column, known as meteor trail

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(Bronshten, 1983). This ionized plasma trail makes the meteor detectable with

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radar techniques, as plasma reflects radio waves (Ceplecha et al., 1998).

The radar meteor head echoes are caused by radio waves scattered from localized spherical dense region of plasma in the immediate vicinity of meteoroid itself and moving along with its atmospheric flight (Janches et al., 2003). The plasma trail left behind in the wake of meteoroid is often detected as the meteor trail echo, known as specular trail echo. When the pointing direction of the radar lies perpendicular to the ionized plasma trail, head echoes are often fol-

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lowed by long lasting trail echoes of duration few seconds to minutes (Dyrud et al., 2005). These echoes are known as non-specular trail echoes, and as they occur simultaneously over multiple radar range gates, hence also known as range spread trail echoes (RSTEs) by many authors, in order to differentiate them

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from the specular trail echoes (Zhou et al., 2001). These echoes were believed to be overdense, where the plasma frequency of the trail exceeding the exploring radar frequency. The head echoes were often followed by RSTE with time

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delay of few milliseconds to several seconds between head echo and enduring RSTE. The classical VHF radar of low power can detect specular trail echoes. On the other hand, the high power radars detect meteor head and trail echoes even when the specular condition is not satisfied. Instead, these radars detect meteor trail induced plasma instabilities.

The head echo observations have been using radar techniques for more than 70 years, however, the first ever observations were reported by Hey et al., (1947). Since head echo is a signal from radar reflective plasma region travelling with

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the meteoroid itself, to observe a reasonable number of head echoes, high power larger aperture (HPLA) radar systems are needed (Close et al., 2002). The HPLA radars frequently detect very fast moving meteor head echoes with a range-rate velocity that follows the meteoroid as it travels through the upper

atmosphere. The head echo detection allows a very precise determination of

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instantaneous meteor altitude, velocity and deceleration. On the other hand,

head echoes also provide the first hand information about meteor particles and offer the possibility to obtain astrophysical quantities like trajectory parameters,

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mass estimation and source radiant for selected meteor particle sizes, which are not measurable by other techniques (Close et al., 2002). From head echo observations, we can also deduce meteor decelerations and densities, independent of assumptions about ionization. Whereas classical VHF meteor radars, typically not calibrated to provide meteor size or mass information directly. However, with modified version of all-sky meteor radar of high power, the SAAMER can detect meteor particles with minimum masses of the order of 102 µg, if the par-

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ticle travels at speed of 60 km/s and 104 µg if it travels at 15 km/s (Janches et al., 2014). Using optical and MAARSY MST radar data, Brown et al., (2017) estimated the head echo limiting meteoroid mass, which lies in the range of 0.1 to 1 µg for the speed of 30 - 60 km/s. In any case, for the sake of comparison, it

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is clear that the minimum masses detected by classical VHF radars are greater than the maximum masses detected by HPLA radars as reported by various au-

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thors (Janches et al., 2015; Brown et al., 2017). The intermediate power radar such as Gadanki MST radar, can detect head echoes from far smaller meteoroids than that can be detected with classical VHF meteor radars, hence, allows us to study the meteor population which probably contributes the most material to the Earth’s upper atmosphere.

The head echo observations sometimes noticed an explosive enhancement in signal strength at the terminal point due to excess plasma disintegration. Such echoes are known as terminal flares, where the meteoroid mass completely evaporates as a sudden burst at the terminal end point, a feature mostly ob3

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served with video and photographic observations. The HPLA radars have much higher sensitivity than the MST and classical VHF meteor radars, hence can

able to detect the small transient volume of dense ionization produced by head echoes in the immediate neighbourhood of the meteoroid itself. Since more than

two decades, head echo observations have been conducted mostly using HPLA

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radar facilities around the world (e.g. Pellinen-Wannberg and Wannberg, 1994; Mathews et al., 1997; Close et al., 2002; Sato et al., 2000; Chau and Wood-

man, 2004; Chau and Galindo, 2008; Janches et al., 2014; Kero et al., 2011,

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2012, 2013). And some of the recent observations have also reported detection of terminal flare echoes (Mathews et al., 2008; Briczinski et al., 2009, Malhotra and Mathews, 2011). However, only few head echo observations from the MST radars have been published (e.g. Schult et al., 2013; Brown et al., 2017) and there are no reports on detection of terminal flare echoes. The present study provides some interesting examples of terminal flare echoes detected by using

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Gadanki MST radar. These echoes are very rare and undoubtedly quite differ from typical head, specular trail and non-specular trail echoes (RSTEs). The rarity and peculiarity are the two vital reasons for worth studying the morphology of the terminal flare echoes. The identification and classification of terminal flare echoes may be useful in understanding the dynamical nature of the mete-

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oroid bodies. In addition, the beam width of Gadanki MST radar (FWHM of ∼ 3◦ ) is slightly wider than that of other HPLA radar systems. The HPLA radars

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generally have very narrow beam width, which is ∼ 1◦ for 50 MHz Jicamarca radar (Malhotra and Mathews, 2009), ∼ 1/2◦ for 1290 MHz Sondrestrom (SRF) radar, ∼ 1/6◦ for Arecibo UHF radar (Mathews et al., 1997) and it is 1.1◦ for

EISCAT UHF radar (Pellinen-Wannberg and Wannberg, 1994). However, the extreme case, 46.5 MHz MU radar has a beam width of about 3.6◦ (Zhou et al., 2001). Comparatively with wide beam, Gadanki radar can give large observing volume and, thus, longer event durations. With relatively wider beam than most of HPLA radars, Gadanki radar is crucial to the meteor community, since this instrument can detect meteoroid sizes that bridge a gap between classical VHF meteor radars and HPLA radars. 4

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2. Radar System and Observations The 53 MHz Gadanki (13.5◦ N, 79.2◦ E) MST radar is a highly sensitive monostatic pulse-coded phase coherent Doppler radar and it has a peak transmitted

power of 2.5 MW. The antenna system consists of 32 x 32 array of three-element

Yagi-Uda aerials with inter-elemental spacing of ∼ 0.7 times the radar wave-

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length, covering an area of 130 × 130 m. The same antenna system is used for both transmitting and receiving, and it radiates linear polarization in E-W and

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N-S planes. Conceptually, the main beam of antenna system can be oriented at any different look angle within ±20◦ off-zenith in two major planes (E-W and N-S). The antenna system generates a radiation pattern where the horizontal extent of measuring volume is defined by its beam width. The half power opening angle of Gadanki radar is ∼ 3◦ , which is equivalent to ∼ 532 m horizontal distance at a range of 100 km, corresponds to an altitude of 93.9 km with the given beam geometry. Furthermore, system description and technical details

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of Gadanki MST radar can be found in Rao et al., (1995) and experimental specifications chosen for meteor observations for the present study are outlined in table 1.

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The experimental data presented here were collected on two different Geminid meteor shower observational schedules (12-15 December 2010 and 12 -14 December 2013). Both the observational schedules are operated during 20:00 -

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06:00 hrs LT (LT=UT+05:30 hrs). For optimal detection of meteors, uniform beam of radar (k) sequentially pointed in four oblique directions, one inclined towards north at an angle of 13◦ from zenith (i.e., North-13), transverse direction to the earth’s magnetic field (B) and gives k ⊥ B pointing geometry. And the other three beams inclined towards east, west and south at an angle of 20◦ from zenith (i.e., East-20, West-20 and South-20). For this study, we have transmitted uncoded pulses of 8 µs sequence with an inter-pulse period (IPP) of 1 ms, which gives a range resolution of 1.2 km. The received data were stored as 34 range values sampled in four sequential beam orientations that covered an

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altitude range from 80 -120 km. The time series data as a sequence of In-phase (I) & Quadrature (Q) components of voltage of the returned signal for each

range gate along with header information can be recorded into data acquisition computer. Successive I & Q - data samples from each range-bin were coherently

averaged over 1024 FFT points, making the effective sampling period of 0.004

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s; hence, each data frame has continuous data for 4.096 s. As a whole, the

radar records both real (I) & imaginary (Q) part of the incident signal that can be processed by converting into Range Time Intensity (RTI) plots, and these

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plots were examined for identification of head and trail echoes through manual inspection of the signal to noise ratio (SNR).

The term signal to noise ratio (SNR) refers to the ratio of the integrated signal power to the integrated noise power. Here the signal is intensity of the radar return when the meteor is present and the noise is intensity of the radar

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return from the back ground noise. For each frame of real (I) & imaginary (Q) part of the incident signal, first, the DC component was removed by Hilbert transform method, and then SNR was calculated based on the method of Hildeband and Sekhon (1974). We have applied a threshold condition of SNR > 0 dB for a range bin to ensure the detection of true meteor echoes. For determination

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of SNR, it is assumed that the antenna has a uniform radiation pattern, and the resulting SNR will be independent of where the meteor is detected within

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the radar beam. Only the echo region of the meteor in range and time is selected from the RTI. When the echo in one or more range bins exceeded SNR of a specific threshold (SNR > 0 dB), then the frame is identified to contain backscattered echo from the meteor trail and is subsequently archived for further analysis.

On manual inspection of SNR, in most of the cases, there exists a clear temporal gap of few tens of ms between head and trail echo. However, in some rare cases, it has been noticed a sudden increase of signal strength due to intensive ionization of meteoroid at terminal point of the head echo, without 6

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Table 1: The Gadanki MST radar specifications for meteor observations

Specifications

Transmitting frequency

53 MHz

Peak transmitting power

2.5 MW

Antenna aperture area

16,900 m2 (32 × 32 crossed Yagi’s in 130 × 130 m area)

Polarization

Linear Polarisation in E-W and N-S planes

Pulse width

8 µs

Inter pulse period (IPP)

1 ms

Beam width

3◦

Range resolution

1.2 km

Number of FFT points

1024

Beam positions

±20◦ off-Zenith in East (E-20), West (W-20), South (S-20)

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Parameter

and ±13◦ in North (N-13)

2 Channels (I & Q - components)

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Receiver

any time delay. For this study, the minimum time delay considered to judge a given event is terminal flare or not is less than 10 ms. The visual inspection of RTI plots or SNR profiles were used to verify the increase of signal strength at terminal point. In case of RTI plot, intensity represent the signal strength,

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which changes from blue to red, and in case of SNR profiles, the signal strength increase to many decibels within few ms followed by a head echo. Thus, these

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echoes are identified as terminal flares, which constitute nearly 2.5 % of the total head echo population. Our result consists of head echoes, trail echoes of both specular and non-specular and terminal flare echoes. As stated earlier, however, in this study, we focus only on terminal flare echoes followed by head echoes.

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Table 2: The beam position-wise distribution of the head-, trail- and terminal flare echoes in 2010 and 2013 at Gadanki MST radar

2010

Date Beam Position

12/13

13/14

14/15

E20

W20

S20

N13

E20

W20

S20

N13

E20

Total No. of echoes

93

102

104

65

119

144

138

116

124

179

Head echoes

13

21

19

15

17

30

25

22

17

21

Trail echoes

80

81

85

50

102

114

113

% of trail echoes RSTE trails

52

32

41

26

66

38

30

% of RSTE trails Head echo flaring

03

04

00

00

07

06

03

39 01

107 58 04

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% of head echo flaring

94

158 34 05

2013 12/13

13/14

W20

S20

Total

N13

E20

W20

S20

N13

E20

W20

S20

Total

175

117

1476

124

138

184

112

119

146

189

109

1121

28

23

251

20

24

32

23

18

27

35

25

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N13

% of heads

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Year

147 40 02

17.1 94

1225

104

105

146

72

121

130

162

77

82.9 35

491 39 2.6

917 81.8

56

41

37

40

61

48

43

39

33.2 04

204 18.2

365 32.5

06

03

00

00

05

08

05

01

28 2.4

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3. Head and terminal flares with Gadanki MST radar The figure 1 is a typical example of RTI plot showing a head and RSTE

echo pair of an individual meteor event detected on 14-12-2010 at 02:07:21 in

East-20 beam. Initially, head echo occurred at high altitude, out of detection

range of radar, and slowly receded down with development of trail echo at about

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110 km of altitude. The head echo receding down as low as about 104 km of altitude with a 50 km/s line of sight velocity of the meteoroid entry into the

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earth’s atmosphere. Here the line of sight velocity is calculated from the time delay between the occurrence of head echo in successive range bins (i.e., Doppler shift of signal along range bins). On close examination, there exists a clear gap with reduced SNR between head and RSTE. Moreover, we have also adopted a Doppler threshold filtering method similar to Sugar et al., (2010) to identify head- and trail- echoes and to filter out secondary head- and trail-echoes which

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usually occur within long enduring echoes.

In our data set, it is noticed that nearly 32% of head echoes exhibit an appreciable RSTE formation and the number and percentage of occurrence of head, trail and terminal flare echoes are given in table 2. It can be noticed that

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the percentage of formation of RSTE echoes with Gadanki radar is about 50 % lower than the same at the 46.5 MHz MU radar observations (Zhou et al., 2001). It should be noted that the MU radar is a HPLA radar with ∼ 3.6◦ beam

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width, which is comparatively wider than the other radars of similar category. Also, the observations presented in Zhou et al., (2001) were conducted on two non-shower schedules between 00:00 and 08:30 LT. On first observational night, the radar beam was pointed to zenith and on the other observational night, the radar beam (k) was pointed alternatively parallel (k) and perpendicular (⊥) to the earth’s magnetic field (B). In the k ⊥ B pointing geometry, Zhou et al., (2001) noticed numerous long-lasting RSTEs, which were mostly absent in other two pointing directions.

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Figure 1: The Range Time Intensity (RTI) plot of a typical head echo with RSTE trail formation detected on 14-12-2010 at 02:07:21 in East-20 beam. The color label shows SNR in dB scale.

The number and percentage of RSTEs detected at Gadanki and MU radar are comparable, particularly in k ⊥ B pointing geometry. Essentially, all head

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echoes resulted in accompanying RSTEs at MU radar, about 100 RSTEs per hour were detected, especially early morning hours. At Gadanki radar, N-13 beam gives k ⊥ B pointing geometry, where only half of the head echoes resulting in RSTEs. This discrepancy in data from both the radars possibly arises

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because of distinct radar detection techniques used and data analysis procedure adopted. However, Zhou et al., (2001) neither had given details of detection technique nor statistical information on detected head and trail echoes. With-

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out such information, it is difficult to determine the reason for differences in both the observations. Yet, differences in the observed echo percentage depend on frequency, transmitting power and latitude of the radar. The echo detection rate also depends on the number of FFT points and beam width of the radar.

In the process of manual inspection of RTI plots, it has been observed atypical sudden increase in signal strength by several orders of magnitude at terminal point of head echoes. Figure 2 shows a typical down-the-beam head echo detected on 14-12-2013 at 02:39:24 in W-20 beam. The head echo was ini-

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Figure 2: The Range Time Intensity (RTI) plot of a typical terminal flaring head echo with RSTE trail formation detected on 14-12-2013 at 02:39:24 in West-20 beam. The color label

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shows SNR in dB scale.

Figure 3: The Range Time Intensity (RTI) plot of a typical multiple flaring head echo with RSTE trail formation detected on 11-12-2010 at 20:38:29 in East-20 beam. The color label shows SNR in dB scale.

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Figure 4: The Range Time Intensity (RTI) plot of a typical flaring head echo followed by RSTE trail formation with a strong sporadic-E layer detected on 12-12-2013 at 23:20:59 in North-13 beam. The color label shows SNR in dB scale.

tially identified at about 108 km of altitude but due to fragmentation, terminal flaring is identified at about 86 km of altitude, and it has descended down to

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about 83 km of altitude, before signal strength gets invisible in the background floor noise. Initially, head echo developed a trail echo (RSTE) almost at the same height of head echo followed by another RSTE at height of 95 km. The head echo spread as down as 81 km of altitude and moving with a line of sight

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radial velocity of about 28.2 km/s, which was estimated by fitting a line through range of maximum power, towards the radar beam and therefore, it is evident over several range gates. This is likely to be a terminal flare echo similar to those

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events reported at the 1290 MHz Sondrestrom Research Facility (Mathews et al., 2008), which is believed to be overdense echo. The terminal flaring feature occurs on time scale of order of 1 ms or less. While figure 2 shows the most convincing example of all terminal flare echoes and there are sizeable number of such echoes (∼ 2.5%) during the observational period as given in table 2.

Figure 3 shows another example of a down-the-beam head echo detected on 11-12-2010 at 20:38:29 in E-20 beam. This head echo event is very complex because it includes multiple trail-echoes as well as multiple terminal flaring. The

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head echo begins at about 2.3 s at an altitude of 107 km followed by multiple trail-echoes at different heights that receded down as low as about 96 km

of altitude. Of these, multiple terminal flaring is due to successive meteoroid

fragmentation, one detected at about 90 km of altitude and the other at lower altitudes (<85 km). In each case, head echo is associated with meteoroid frag-

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mentation that is undergoing multiple flaring at different terminal heights until it gets buried in the background noise. Malhotra and Mathews (2009) by us-

ing 50 MHz Jicamarca Radio Observatory (JRO) HPLA radar also observed

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these kind of echoes, but completely below 90 km of altitude and classified as low-altitude trail echoes (LATE). The terminal flare echo shown in figure 3, detected below 85 km of altitude is in the same category of echoes observed by Malhotra and Mathews (2009). However, in contrary to the above, two distinct population of flare trails, one maximizing at 90 km and the other at 112 km were reported by Sugar et al., (2010) based on their observations with the same

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50 MHz JRO radar. Sugar et al., (2010) also argued that the multiple streak flares usually occurred at higher altitudes, although a few were observed at the lower altitude range too. However, our observations by using 53 MHz Gadanki radar revealed that no high altitude flares were observed and also multiple flaring is dominant at lower altitudes (below 85 km). This discrepancy in detection

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altitudes in both the data sets may be because of different operating powers and

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beam widths of two radars.

It is also observed that there is a clear distinction between RSTE and termi-

nal events. In the case of terminal flares, there is no time delay in signal, between the head and terminal flares, however in case of RSTE, there is an appreciable time delay, sometimes of the order of several seconds. A speculative reason for lack of time gap between the head and terminal flares is that the plasma may become turbulent much faster than usual because of rapid non-uniform release of material as a result of meteoroid fragmentation (Malhotra and Mathews, 2009). Whatever may be the reason, flares are clearly different from non-specular trails (RSTEs) and very rare. By using the Arecibo Observatory UHF radar, Briczin13

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ski et al. (2009) statistically estimated the percentage of terminal flare events and found that it constitutes up to 15% of all the events. However, in our case flare events are found to be a small fraction of total events, which is in

comparable with Sugar et al., (2010) observations. Only about 2.5% of heads have appreciable flare echoes, as opposed to the 32% that produce non-specular

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trails (RSTEs). The rarity of these events suggests that something unusual is

happening. The immediacy of the signal following the head echo implies that something initiates the plasma irregularities and that they do not grow from a

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smooth column, as we believe is the case for typical non-specular trails.

Figure 4 shows a pair of complex meteor event which includes head and multiple trail echoes, as well as an altitude-narrow impulsive terminal flaring. The head echo of this event detected at an altitude of 94 km, its initial atmospheric trajectory occurred outside the main beam of antenna and as the head echo

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becomes visible, an immediate RSTE developed. The head echo intensifies until 88 km of altitude and at that point, the scattering efficiency decreases abruptly in a single IPP apparently because of terminal flaring. The line of sight velocity of this event is ∼ 27.5 km/s. The terminal flare trails often display interference fading between/among the head and trail echo components. Of particular

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interest is the strong sporadic-E layer centred near 100 km of altitude, similar to the observations of Malhotra et al., (2008) using 50 MHz JRO radar. Over

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the years, wind shear theory is widely accepted mechanism responsible for the formation of sporadic-E layer (Mathews, 1998). The equatorial region sporadicE is observed particularly in k ⊥ B pointing geometry of the radar whereas

at mid-latitudes, incoherent scatter radars detect sporadic-E layer at almost all the time. Further, the observational history and detailed theories of sporadic-E formation can be found in Mathews (1998).

At Gadanki radar, N-13 beam gives k ⊥ B geometry, sporadic-E layer is observed mostly in N-13 beam orientation and there are few terminal flare events also observed in this pointing geometry. It has been well established that long14

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lived metallic ions are essential for the development of sporadic-E layer. Over the years, there have been many studies of high to low correlation between

meteor shower activity and formation of sporadic-E layer (Chandra et al., 2001; Baggaley and Steel, 1984). The time of observations also has a major impact

on the occurrence of sporadic-E. In equatorial regions, it is closely associated

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with the electrojet and primarily a daytime phenomenon with little difference

the year round. The occurrence of sporadic-E is enhanced with the presence of wind-shear and meteor deposition. Malhotra et al., (2008) emphasized on the

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effect of meteor induced metallic ionization on sporadic-E formation observed at Jicamarca radar. At equatorial region, even though the sporadic-E occurrence is a daytime aspect, occasional night time occurrence at Jicamarca (Malhotra et al., 2008) and also at Gadanki as shown in figure 4 is mostly due to enhancement of sporadic-E induced by the meteor metallic deposition.

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4. Discussions

The primary aim of the observations presented here is to understand the behaviour of meteoroid in the atmosphere, particularly meteor head and terminal flare echoes using Gadanki MST radar. Results comprise of vast majority of

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meteor events that exhibit complex radar head echoes, trail echoes (both specular and non-specular) and flare echoes associated with the meteoroid terminal phase. However, the terminal flares are different in their nature and occurrence.

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In contrary to RSTE, terminal flares are unusual in a way that there is a sudden streak of high SNR at terminal phase, amounting to few decibels immediately after head echo. The rise of signal strength depends on many different parameters like size of the meteoroid, its velocity, fragmentation process and porosity of the material, and it varies from event to event. These unusual echoes are strongly altitude dependent without any time delay between head and trail echoes. The maximum time delay that has been considered to know whether the detected event is terminal flare or not is less than few ms.

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Based on 50 MHz Jicamarca radar observations, Malhotra and Mathews (2009) offered a speculative reasoning for lack of time delay. The authors ar-

gued that due to meteoroid fragmentation, there will be a rapid non-uniform release of material and hence the plasma surrounding the head echo may be-

come turbulent much faster than usual. The mechanism of terminal flaring is

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not connected with any critical values of the pressure or specific energy flow. Near the terminal phase, meteoroid flaring takes place whereby a large density of plasma is deposited at a particular range and remains visible for longer than

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normal. The terminal flaring is probably due to severe fracturing of meteoroid in to thousands of microscopic fragments at the final stage of the echo by the rapid change in deceleration. The flaring at the terminal point may be due to a real explosion of meteoroid into many small fragments (partial, gross or discrete fragmentation) or sudden change in physical circumstances that give rise to more evaporation and excitation of ionization (Ceplecha, et al., 1998). There-

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fore, it is possible that the interaction between the gas layers of the non-zero tangential velocity and the ‘dust-ball’ body, a porous meteoroid model proposed by Hawkes and Jones (1975) may be the cause of the flaring phenomena. Alternatively, volatile material in the meteor might have been exposed suddenly

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and allowed to ablate causing an unexpected increase in brightness.

5. Conclusions

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With 53 MHz frequency, Gadanki MST radar, which is intermediate in power

and aperture to high power large aperture (HPLA) and traditional VHF radars, is also good enough for detection of head, specular and non-specular trail echoes (RSTEs). Occasionally, some of the head echoes are followed by flaring at terminal phase, a rare feature that is mostly observed with optical and HPLA radar systems. The meteor terminal flaring is unusual in a way that a streak of high SNR is immediately followed by a head echo without any time gap. These echoes constitute about 2.5% of total head echo population, and quite different from non-specular echoes (RSTEs). The percentage of terminal flares

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detected with 53 MHz Gadanki radar are comparable with the same at 50 MHz Jicamarca Radio Observatory (JRO) radar, but not with 430 MHz Arecibo Ob-

servatory (AO) radar. The AO radar is more sensitive than JRO and Gadanki radar, hence the number of events detected and detection height distribution is

higher and also recorded greater proportion of high deceleration events relative

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to the other two radars. At Gadanki radar, the percentage of terminal flares are nearly 2.5% of total head echo population and the same at 50 MHz JRO is less than 1%. But at 430 MHz AO radar, the terminal echoes are 15% of head

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echo population (Briczinski et al., 2009). This concludes that the AO radar detect a complete different class of meteor events than the other two, hence not comparable.

From close examination of RTI plots, it has been noticed that the head echoes at higher altitudes (i.e., 90 - 120 km) are generating RSTEs whereas at

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lower altitudes (i.e., 80 - 90 km) it is causing terminal flare events. The flares are generally associated with fragmentation that can be eroded out totally or partially. However, in spite of observing the meteor fragmentation by different techniques over 70 years, the physical process of micrometeoroid fragmentation is poorly known. These observational studies will form the basis for future mod-

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elling efforts for meteoroid mass loss and fragmentation mechanism. Currently, unavailability of interferometric facility at Gadanki MST radar limits our study

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to determination of many other meteoroid parameters like acceleration, deceleration, dynamical meteor masses and computation of radar cross section. Future upgradation will allow these parameters to be studied in much more detailed manner.

Acknowledgements: The authors thankful to the Director and Staff of

NARL, Gadanki for their support in conducting experiment. One of the authors, KCR, acknowledge DST, India for financial support through DST-PURSE-II Programme to Osmania University, Hyderabad.

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Meteor head and terminal are echoes observed with the Gadanki MST radar K. Chenna Reddy, B. Premkumar

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Highlights:

In this study, we report on the head and terminal flare echo observations from the Gadanki MST radar, intermediate power radar with beam-width wider than that of most HPLA radar systems.

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The observations from these radars are crucial to the meteor community, since these instruments can detect meteoroid sizes that bridge a gap between traditional specular meteor radars (SMRs) and HPLA radars.

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These echoes are very rare and undoubtedly quite differ from typical head and nonspecular range spread trail echoes (RSTEs). Identification and classification of terminal echoes may useful in knowing the dynamical nature of the meteoroid body.

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From the observations it has been noticed that the head echoes at higher altitude are generating non-specular trail echoes, whereas at lower altitudes the echo disintegrate as a terminal flaring associated with meteoroid fragmentation.