The search for extended air showers at the Jicamarca Radio Observatory

Transcription

1 The search for extended air showers at the Jicamarca Radio Observatory D. Wahl*'^ J. Chau*, F. Galindo*, A. Huaman* and C. J. Solano^'** *Radio Observatorio de Jicamarca, Instituto Geofisico del Peru ^Instituto de Fisica, Universidad Nacional de Ingenieria, Lima, Peru **Instituto Nacional de Investigacion y Capacitacion de Telecomunicaciones, UNI, Lima, Peru Abstract. This paper presents the status of the project to detect extended air showers at the Jicamarca Radio Observatory. We report on detected anomalous signals and present a toy model to estimate at what altitudes we might expect to see air shower signals. According to this model, a significant number of high altitude horizontal air showers could be observed by radar techniques. Keywords: Cosmic rays, Radar, Horizontal air showers PACS: S-, sd, Za INTRODUCTION The most common method to study ultra-high energy (UHE) cosmic rays is to detect the extended air showers (EAS) produced when the cosmic rays interact with the atmosphere. Usually, either the secondary particles are detected at ground level (scintillator [], water Cerenkov [2]) or the fluorescence / Cerenkov light is detected from processes in the atmosphere [3]. A further method proposed recently is to use the radio pulse produced by geo-synchroton emission [4]. The development in radio detection methods has also prompted a renewal in interest in radar detection of EAS [5]. The Jicamarca Radio Observatory (JRO) is one of the largest radar facilities in the world and is located close to Lima, Peru. It has been proposed that the JRO main antenna could be used for the detection of EAS [6]. One of the main motivating factors is the experience Jicamarca have developed in detecting micro-meteors, which have kinetic energies of the same order of magnitude as the highest energy cosmic rays (-0 J). The following paper reviews some of the studies that have been performed in meteor and EAS detection. The following section introduces the main properties of the radar and reviews some meteor shower studies performed at Jicamarca. The subsequent section presents preliminary results obtained at Jicamarca and proposes future directions for research. METEOR STUDIES AT THE JICAMARCA RADIO OBSERVATORY The Jicamarca Radio Observatory (JRO) is the equatorial station of the chain of incoherent scatter radar (ISR) observatories extending from Lima, Peru to S0ndre Str0mfjord, Greenland. The Observatory is located about 20 km inland (east) from Lima, at.95'' CP23, Cosmic Rays and Astrophysics, edited by C. J. Solano Salinas, D. Wahl, J. Bellido, and 6. Saavedra 2009 American Institute of Physics /09/$

2 south, 6.S" west, the altitude of JRO is 520 m.a.s.l. and the magnetic dip angle is about I", and varies slightly with signal altitude and year. The 300x300m^ MHz incoherent scatter radar is the principal facility of the observatory. The radar antenna consists of a large square array of 8,432 half-wave dipoles arranged into 64 separate modules. The transmitters can deliver a peak power of 2 MW, with a maximum duty cycle of 5%. The JRO currently performs a variety of studies of the atmosphere, as well as detection of micro-meteors. Jicamarca has two modes of operation: the first mode is the incoherent scatter radar mode. In ISR mode, densities [7], temperature [8], composition [9] and electric field [0] of the ionosphere can be computed. The second mode is the coherent scatter radar mode. In this mode, ionospheric irregularities, like Spread F [] (See Fig. ), and neutral atmospheric dynamics and meteorology can be studied. SNR (db) 30:3 0 SI:DO abac as:[ic ss:30 a3:0d 33:30 Lotal Time (2C-Oct-L993 (a93» FIGURE. Coherent echoes over Jicamarca of an Equatorial Spread F (ESF). In CSR mode, radars can track meteors as the plasma generated by the collisions of the meteor with molecules in the atmosphere reflects the transmitted radio waves. Jicamarca started its meteor studies in 998 to coincide with the expected Leonid meteor shower. A typical meteor detection experiment will be made using the whole area of the antenna for the transmission and all four quarters section of the antenna for the reception, and a north-east linear polarization mode for the transmission (Fig. 2, left). The trajectory of the meteor can be reconstituted using interferometry and the simplified geometry shown in Fig. 2, right. Readers are referred to [2, 3] for further details of the experimental setup used in meteor studies. In these studies, the range resolution is km depending on the experiment with a lower limit of 8km due to reflections from surrounding mountains. Some of the main parameters that can be obtained in this way are: initial range, zenith angle, geocentric velocity, radial velocity, deceleration, inclination, eccentricity, mass etc. Results of the meteor properties are usually shown in graphical form, as presented in Fig

3 (a) B (E) ~47m Final cantsct We t(w) Souti{S) -47m FIGURE 2. (a) Channels configuration of the Cosmic Rays experiment. Each square represents the quarters used in the reception process, (b) Geometric representation of a meteor trajectory. C-> V, = km A = 6,59 bm/a ^pip = km/e & - r S rirrte (5) Q.ia4 23-Nav-20C5 (327^ 64:32: ms FIGURE 3. The left panel shows the meteor RTI (Range Time Intensity). The second panel (middle) shows the movement in the radar plane and right panel shows the Signal to Noise Rate (SNR) and Doppler velocity. RADAR DETECTION OF AIR SHOWERS Extended air showers (EAS) can be studied indirectly through an analysis of the ionisation they produce. In this sense, studying air showers should be very similar to the study of meteors. The study of EAS using radars was first proposed by Lovell and Blackett in 940 [4]. Following the development of other methods of detection of EAS, this proposition fell out of fashion, but was revived recently by the renewed interest for complementary detection techniques for cross calibration of experiments [6, 5,6]. The key to whether this is possible or not is the density of electrons produced in the air shower 206

4 (for detailed calculation, readers are referred to [5]). The electron density determines the plasma frequency (Vp) of the ionization column: Vr ripe" Time 8.98x03 V«^//z () wherewg is the electron density (cm~^). If the radar frequency is less than the plasma frequency (over dense regime), the ionization column presents metal-like reflection of the incident radiation (coherent scattering). For typical electron densities expected in EAS, the range of radar frequencies to use is of the order of tens of MHz, making Jicamarca (50 MHz) a good candidate to perform such studies. Of course, there are many factors which can mitigate this idealised behaviour, such as the radial density profile of the EAS and noise levels, so further research is necessary to provide theoretical guidance as to the magnitude of signal that can be expected. Failing a clear theoretical framework, we adopted a search strategy to look for any anomalous signals in meteor data. Potencia com binada E i 80 0 S ().495 ",,, 0.5,,,!,, [,,!,,,,!, ,,,, 0.53,,,,,,, Time [s] FIGURE 4. Typical "anomalous" signals in RTI H7OOOO H60000 H50000 H40000 H3OOOO These searches unveiled a recurring type of anomalous event, which so far has escaped explanation [7]. For a signal to be considered as an event, we require coincidence in all aquisition channels of the radar with at least a 5(7 deviation from background noise. We also require the signal to be persistent for at least three consecutive samples (^ 'S\is). A typical event meeting these requirements is shown in RTI form in Fig. 4. The distribution in time and altitude of these events during a run in May 2007 is shown in Fig. 5. For vertical air showers, we would not expect to see signals above 5km. However, Fig. 5 shows the anomalous events are spread throughout the 8-50km range. In an attempt to rule out the possibility that these events are due to cosmic rays, we developed a simple toy model to determine what the flux for EAS might be at such altitudes. In this model, we assume that primary cosmic rays interact with a mean free path of 25 g/cnp' [8]. In practice, such depth can can be achieved for horizontal or slightly upward traveling cosmic rays, even at high altitudes. A full calculation will be

5 E ViiEIK ftr UTCT FIGURE 5. Distribution in space and time of anomalous events recorded at Jicamarca. described elsewhere (D. Wahl et al., Using the Jicamarca radar to detect high altitude horizontal air showers, In preparation), but we reproduce some of the main steps herein. The main task is to estimate the relative flux (j){h,eo) a.s, a. function of altitude (h) and cosmic ray energy (EQ). We proceed by numerical integration of the following equation: fdmaxih) (l>{h,eo)oca{h) Pd{v,h,Eo)d{cosv] (2) Jo The term A (/i) is a factor proportional to the area illuminated by the radar at altitude h. A good approximation for Jicamarca is: A{h) A{\.2km) h > n.2km otherwise The factor P^ represents the probability that a shower of energy EQ has been initiated (and not terminated) before the depth (d) corresponding to {h,6). The depth d can be obtained as a function of the path of the cosmic ray by using atmospheric models such as the Linsley parametrisation [9]. In the simplest model for the shower development, we assume a symmetrical shower development centred at a depth of dmax after the interaction point, where dmax is given by the NKG parametrisation (i.e. shower age = ). The maximum depth of the shower is hence given by dgnd = '^dmax- In the case that d < dend the only probability needed is that the shower is initialised. Otherwise, we must calculate the probability that the shower has been initialised and that it has not terminated by depth d. In this case Pd is given in eq. 4. (3) Pd{v,h,Eo) -d{v,h) exp -{d{v,h)-dendieo))\ exp exp -d{v,h) u < dend Otherwise (4) 208

6 The final term in eq. 2 that needs calculating is the limit Omax{h)- 0 is the angle between the cosmic ray main axis and the normal to the earth's surface above the radar. Omax{h) is the maximum physics value 6 can take (i.e. the cosmic ray does not pass through the earth). The result of the numerical integration of eq. 2 are shown in Fig ^ Norm. ^ '- os ^ i Altitude [km] Altitude km] a FIGURE 6. (a) Flux of cosmic rays as a function of altitude (excluding the A(h) term of equation 2) (b) Measurable cosmic ray flux as a function of altitude. As can be seen, even though the flux of EAS is significantly smaller at high altitudes (Fig. 6a), the larger area swept by the radar pulse means that under ideal conditions, more echoes would be expected at high altitudes (Fig. 6b). In practice however, the number of echoes observed will depend on the sensitivity of the radar to echoes which we will be the object of further studies at Jicamarca. b CONCLUSIONS The Jicamarca Radio Observatory is ideally suited to test if it is possible to detect EAS with radar. Jicamarca has been routinely detecting micro-meteors since 998, and has started searches for EAS in Though thus far no conclusive signals have been observed, anomalous signals have been observed with require explanation. The development of a simple model to predict the relative flux of high altitude horizontal air showers does not exclude these signals from being originated by cosmic rays. We plan to continue EAS searches at these altitudes which could be particularly interesting as only accesible to radar and satellite experiments. At the same time, other candidate events with different profiles have been observed at Jicamarca and are to be analysed for potential match to EAS. ACKNOWLEDGMENTS The authors would like to thank Dr. J. Bellido of the organization committee for having kindly accepted to present part of this work on our behalf. 209

7 REFERENCES. T. Antoni et al. [Kascade Collaboration], Nucl. Instr. Meth.A, 53 (2003) J. Abraham et al. [Pierre Auger Collaboration], Nucl. Instr. Meth. A, 523 (2004) K. Bernlohr et al., Astropart. Phys., 20 (2003) 4. T. Huege et al., Astron. and Astrophys., 42 (2003) 9 5. PW. Gorham, Astropart. Phys., 5 (200) T. Vinogradova et al., AIP Conf. Proc, 579 (200) D.T. Farley, Radio Science, 4 (969) D.T. Farley et al.. Journal Geophys. Research, 72 (967) I.N. Aponte et al., Geophys. Res. Lett., Vol 24 (992) R.F Woodman, Journal of Geophys. Research, 75 (970) B. Fejer et al., Phys. Space Plasmas, 5 (998) 2. J. Chau et al., Atmos. Chem. and Phys., 4 (2004) J. Chau et al. Icarus, 88 (2007) PM.S. Blackett et al., Proc. Royal Soc. (London) A, 77 (94) A. lyono et al., Proc. 28''' ICRC (2003), Tsukuba, Japan 6. D.O. Damazio. Nucl. Phys. B (Proc Suppl), 34 (2004) D. Wahl et al., Proc. 30''' ICRC (2007), Merida, Mexico 8. R. M. Baltrusaitis et al., Phys. Rev. Lett., 52 (984) B. Keilhauer et al., Astropart. Phys. 22 (2004)