1. 23.06.2006Scattering and Scintillation in Radio Astronomy 1 The dual-frequency calibration of ionosphere influence in VLBA data processing Andrey Chuprikov Igor Guirin Astro Space Center Moscow Russia

2. 23.06.2006Scattering and Scintillation in Radio Astronomy 2 The ionized shell around the Earth is caused by the Sun UV radiation The electronic column density is peaked at an height of 350 km above the ground and spreads by several tens of kilometers around this altitude The TEC variation has time scale of several hours and induce a delay of about 1 ns at 8.4 GHz Such delay is 13 times larger at 2.3 GHz due to dispersion Several strategies have been used to calibrate the ionosphere in VLBI data. One of them is technique of Dual Frequency Observations

3. 23.06.2006Scattering and Scintillation in Radio Astronomy 3 The ionosphere can be calibrated out with simultaneous observations in two widely separated frequency bands. Usually, these are X Band (8.4 GHz) S Band (2.3 GHz) A linear combination of group delays in these bands yields straightforwardly the ionosphere-free delay :

4. 23.06.2006Scattering and Scintillation in Radio Astronomy 4 Description of the experiment We present some results of VLBA experiment titled RDV14. Observations had been carried out on 15/04/1999. FITS-file has been created with VLBA correlator and then placed into archive Later, these data have been kindly placed at our disposal by Dr. L. Petrov (Goddard Space Center)

5. 23.06.2006Scattering and Scintillation in Radio Astronomy 5 Frequency structure of initial data F0 (MHz)DF (MHz)nCh IF12221.22816 IF22241.22816 IF32331.22816 IF42361.22816 IF58406.22816 IF68476.22816 IF78791.22816 IF88896.22816

6. 23.06.2006Scattering and Scintillation in Radio Astronomy 6 List of sources observed SourceRA (2000)DEC (2000)Flux [Jy] S-bandX-band 2.3 GHz8.4 GHz 10003-06600:06:13.892894-06:23:35.335301.02.2 20106+01301:08:38.77107401:35:00.317141.72.0 30119+11501:21:41.59504311:49:50.413130.91.5 40229+13102:31:45.89405413:22:54.716271.11.0 50235+16402:38:38.93010816:36:59.274710.91.0 60336-01903:39:30.937787-01:46:35.803992.01.8 70528+13405:30:56.41674413:31:55.149552.02.3 80539-05705:41:38.083384-05:41:49.428391.01.0 90552+39805:55:30.80560839:48:49.165003.25.4 100718+79207:26:11.73517779:11:31.016241.01.0 110727-11507:30:19.112472-11:41:12.600482.02.3 120823+03308:25:50.33835503:09:24.520141.21.5 130851+20208:54:48.87492520:06:30.640881.41.8 140923+39209:27:03.01391639:02:20.851954.411.0 150953+25409:56:49.87536125:15:16.049771.11.1 160955+47609:58:19.67164847:25:07.842500.91.0 171032-19910:35:02.155274-20:11:34.359751.01.0 181044+71910:48:27.61991771:43:35.938281.31.0 191124-18611:27:04.392448-18:57:17.441661.42.2 201308+32613:10:28.66384532:20:43.782951.71.8 211418+54614:19:46.59740154:23:14.787211.01.0 221606+10616:08:46.20317910:29:07.775851.31.1 231638+39816:40:29.63277139:46:46.028541.21.0 241739+52217:40:36.97785052:11:43.407501.21.0 251741-03817:43:58.856137-3:50:04.616684.04.0 261749+09617:51:32.8185739:39:00.7285101.63.6 271803+78418:0:45.68391878:28:04.018511.71.6 281830+28518:32:50.18563128:33:35.955301.01.0 291908-20119:11:09.652893-20:6:55.109073.22.8 301958-17920:00:57.090448-17:48:57.672510.81.3 312128-12321:31:35.261758-12:07:04.795881.52.1 322145+06721:48:05.45867906:57:38.604222.67.0 332234+28222:36:22.47086028:28:57.413291.01.0

7. 23.06.2006Scattering and Scintillation in Radio Astronomy 7 Data processing The main idea was to use a brightest and compact source titled 0923+392 (No. 14) (alternative titles are J0927+3902 or 4C+39.25) as a Fringe Finder Then, due to phase stability of VLBA equipment, it is enough to interpolate the GAINs obtained and to apply them to any other source to reconstruct the phase of visibility function

8. 23.06.2006Scattering and Scintillation in Radio Astronomy 8 Data processing The a priori amplitude calibration is not a problem because there are calibration tables (TY, GC) in initial FITS-file The trick is to compensate the ionosphere- free delay before Self-Calibration with multiplication the Visibility by

9. 23.06.2006Scattering and Scintillation in Radio Astronomy 9 Results of data processing DA193 (J0555+3948) without usage of procedure

10. 23.06.2006Scattering and Scintillation in Radio Astronomy 10 Results of data processing DA193 (J0555+3948) with usage of procedure

11. 23.06.2006Scattering and Scintillation in Radio Astronomy 11 Results of data processing 1741-038 (J1743-0350) without usage of procedure

12. 23.06.2006Scattering and Scintillation in Radio Astronomy 12 Results of data processing 1741-038 (J1743-0350) with usage of procedure

13. 23.06.2006Scattering and Scintillation in Radio Astronomy 13 References 1.Fey, A.F., et al., 1996, ApJSS, 105, 299 - 330 1.Brisken, W.F., et al., 2000, ApJ, 541, 959 - 962

14. 23.06.2006Scattering and Scintillation in Radio Astronomy 14 Conclusions 1.Our experience demonstrates, that usage of dual- frequency calibration of VLBA data can essentially improve the source image even if it is a strong calibrator 2.If procedure has not been performed for any data, the final source image can be shifted from the phase center, and, additionally, image may have the false components in this case 3.The similar results have been published by other authors (Fey, A.L., 1996), (Brisken, W.F., 2000)

15. 23.06.2006Scattering and Scintillation in Radio Astronomy 15 The “Astro Space Locator” Software Please, contact with me : Andrey Chuprikov Astro Space Center, Moscow, Russia achupr@asc.rssi.ru