Ionospheric Research with the LOFAR Telescope

Transcription

1 Ionospheric Research with the LOFAR Telescope Leszek P. Błaszkiewicz Faculty of Mathematics and Computer Science, UWM Olsztyn

2 LOFAR - The LOw Frequency ARray The LOFAR interferometer consist of a large number dipole antennas (~ high-frequency MHz and ~5000 low-frequency MHz), arranged in over 40 stations. The antennas in each station are combined to mimic a single telescope dish, which is electronically steered into the desired direction. The outputs of the stations are split into narrow frequency bins, correlated, averaged over short intervals, and stored for offline processing. Calibration of LOFAR is essential, and as described in has several components: calibration of the station beamshapes, and calibration of the refraction in the ionosphere. At low frequencies the effect of the ionosphere is stronger than at the higher frequencies used by most current telescopes, because the phase shift caused by the ionosphere scales with wavelength. 2

3 LOFAR Network 3

4 LOFAR is the first fully digital radio telescope (do not have any moving parts). Observations can be carried out in many directions simultaneously with the split of the band on to 256 channels. Left: LBA antennas to receive waves In range 4 10 meters [10 90 MHz] (48/96/station) Down: The HBA antennas collected in pannels waves 1 3 meters[ MHz] (48/96/station) 4

5 LOFAR Computing the data Data collection and processing takes a supercomputer Blue Gene / P at the University of Groningen. The speed of data processing is about 50 Gb/s Computer architecture cores - IBM PowerPC 450 (850 MHz) RAM Memory > 1 Petabyte a Peak efficiency ~ 270 Tflops 5

6 LOFAR Core - Exloo 6

7 One of the stations (Exloo, NL) HBA LBA Up: One of the stations - Effelsberg, Max Planck Institut für Astrophysik 7

8 POLFAR The Polish part of The LOFAR

9 LOFAR Station in Bałdy (UWM) - 20 km from Olsztyn

10 What is LOFAR for? Astronomy: Cosmology epoch of reionisation (21 cm H line from z=11.4 (115 MHz) to z=6 (180 MHz) can be probed) Surveys forming of the galaxies (z>6), Star formation processes in the early Universe using starburst galaxies as probes Ultra High Energy Cosmic Rays ( ev ) Solar Physics and Space Weather Transients sources (sources with jets, Planets in Solar System and egzoplanets, chromospheric active stars, neutron stars and stellar-mass black holes, pulsars) Magnetic Fields study of magnetic fields in the universe by observing polarized radio synchrotron emission. 10

11 What is LOFAR for? Geophysics (additional hardware) Reveal the structure of the earth s crust (until some 30 kilometers), Characterize earth tremors, and Detect changes in the subsurface over time. Study of Ionosphere!! Agriculture (network of additional detectors) 11

12 Cassiopeia A The view of the sky from the single LOFAR Station. Cygnus A Sun Center of MW 12

13 Low frequency telescopes and Ionosphere At low frequencies (LF, <300 MHz), the dominant effects are refraction, propagation delay and Faraday rotation caused by the ionosphere. For a groundbased interferometer (array from here on) observing a LF cosmic source, the ionosphere is the main source of phase errors in the visibilities. Amplitude errors may also arise under severe ionospheric conditions due to diffraction or focussing. 13

14 Measured in TEC Integrated total electron content along propagation path 1 TEC unit = 10 m ~5-50 TEC 16-2 TEC impact varies as a linear function of wavelenght, and so signals at low frequencies are most affected. 14

15 Low frequency telescopes and Ionosphere One of the challenges in the design and work of the LOFAR radio telescope is the calibration of the ionosphere which, at low frequencies, is not uniform and can change within minutes timescales shorter than the lenght of observations. The VTEC across Europe for March 23rd, 2012 at 00:00 UT. The square indicates the LOFAR core stations and the triangles represent the locations of the international stations. Sotomayor-Beltran et al., A&A 2013 Calibrating for ionospheric Faraday rotation is complicated because the free electron content of the ionosphere varies depending on the time of day, season and level of solar activity. 15

16 Raraday Rotation Assuming a typical observing frequency of 150MHz (LOFAR high band) and an ionospheric Faraday depth of 1 rad m^ 2, the additional rotation of the polarization angle imparted by the ionosphere will be Although the rotation of the polarization angle is less pronounced at higher frequencies, the Faraday depth of the source will still be systematically affected by the ionosphere. Due to the direction of the geomagnetic field, ionospheric Faraday rotation has a positive or negative contribution to the total Faraday depth of a source when observing from the northern or southern hemispheres, respectively. For instance, the contribution from the ionosphere should be corrected for in order to derive reliable Faraday depths due to the ISM alone when observing Galactic pulsars. 16

17 Low frequency telescopes and Ionosphere Phase error Removing this radio distortion will provide a measurement of the TEC along the line of sight of each LOFAR antenna. 17

18 Low frequency telescope and Ionosphere The ionosphere changes within the station beam and direction-dependent calibration is required. Also the ionosphere is different for different stations during the observation. van der Tool and van der Veen,

19 Different calibration regimes V V S V S A S S V A A baseline V beam S scale of ionosphetric fluctuations A Intema et al A 19

20 Different calibration regimes The ionosphere causes propagation delay differences between array elements, resulting in phase errors in the visibilities. The delay per array element (antenna from here on) depends on the line-of-sight through the ionosphere, and therefore on antenna position and viewing direction. The calibration of LF observations requires phase corrections that vary over the field-of-view of each antenna. Calibration methods that determine just one phase correction for the full of each antenna (like self-calibration) are therefore insufficient. So, special methods of calibration are required and and these methods can give us the ionospheric details at the time of observation as an additional result. 20

21 LOFAR as the ionospheric probe Field of view over 1000 km Temporal resolution - <1s Horizontal resolution 2 m Vertical resolution 2m Relative accuracy < TECU Tomographic techniques can be used to invert the thousands of changing and independent total electron content (TEC) measurements produced by LOFAR into three-dimensional electron density specifications above the array. These specifications will measure spatial and time scales significantly smaller and faster than anything currently available. These specifications will be used to investigate small-scale ionospheric irregularities, equatorial plasma structures, and ionospheric waves. In addition, LOFAR will improve the understanding of the solar drivers of the ionosphere by simultaneously measuring the solar radio bursts and the TEC. 21

22 Ionospheric feedback At present, the most numerous and easily accessible ionospheric data come from the international network of ground-based GPS receivers. However, the spatial and temporal sampling of available GPS data is sufficient for neither complete calibration of radio astronomical measurements nor reliable ionospheric modeling; however, linking real-time GPS data to the data reduction of telescopes such as LOFAR, as well as linking ionospheric monitoring using known radio sources to tomographic inversion of the ionosphere from GPS measurements will provide advantages to both disciplines. 22

23 Thank you for your attention 23