50 100 150 200 250 300 350 2 4 6 8 10 12 14 16 18 20 22 24 Day number UT foF2, MHz 2 3 4 5 6 7 8 9 10 Characteristics of TIDs in Antarctic Peninsula region from HF and GNSS observations Vadym Paznukhov* (1) , Keith Groves (1) , Yuri Yampolski (2) , Andrey Sopin (2) , Andrey Zalizovski (2) , and Sergei B Kascheev (2) , Anton Kashcheyev (2,3) , Kathleen Kraemer (1) 1. Institute for Scientific Research, Boston College Chestnut Hill, MA 02467, USA 2. Institute of Radio Astronomy Kharkiv, Ukraine 3. Abdus Salam International Center for Theoretical Physics Abstract Data-acquisition system for coherent HF sounding of the ionosphere has been operating in Antarctic Peninsula since the summer of 2015. HF radar with sounding frequencies from 2 to 6 MHz operates in oblique and vertical sounding modes between Palmer (USA) and Vernadsky (Ukraine) Antarctic stations. The system is built on software defined radio USRP N210. Temporal variations of the ionospherically reflected HF signal parameters on this quasi- vertical radio paths are used for deriving TID characteristics with Frequency and Angular (FAS) technique. The observed climatology of ionospheric disturbances in Antarctic Peninsula region varies significantly through the analyzed period of 2015-2016 and appears to be mainly controlled by background plasma density and neutral wind direction. The most frequently observed periods of the disturbances range from about 20 min to almost an hour, with typical velocities of the order of 100- 300 m/s, and spatial scales of several hundreds of kilometers. Analysis of the data shows that during the nighttime, TIDs are observed only about 30%, while during the daytime they were typically observed 70-80% of the time. The intensities of the daytime TIDs are also higher by almost a factor of 2. During the winter period disturbances are present mostly during the day time. During the summer part of the year, disturbances are present for the most part of the day, characterized by somewhat lower velocities and are absent near the minimum of the local plasma density of the ionosphere. The exact mechanism for such pattern and the role of the solar terminator needs further investigation, but it is clear that the main controlling factor is the background plasma density. The first results of the TID propagation direction analysis indicate that during the geomagnetically quiet time propagation direction varies through the day and follows the direction opposite to the background neutral wind flow. This is most likely the effect of the wind filtering of the gravity waves in the lower atmosphere which is the main source of the disturbances during the geomagnetically quiet periods. TID observations in Antarctic Peninsula region Drake Passage in Antarctica generates severe tropospheric waves, i.e., It is associated with cyclones, convective plumes, enhanced zonal winds, orographic waves, etc. Bistatic HF System • Three-channel HF receive system based on software-defined radio built and installed at Palmer Station to measure parameters of HF signals reflected from ionosphere. • Transmitter is installed at Vernadsky station (50 km due south). • Raw data (decimated IQ samples) shipped by sea to Boston College USRP N210 The measured TID wavelengths and periods allow classifying them as MSTIDs. Overall the observed climatology of ionospheric disturbances in Antarctic Peninsula region vary significantly through the analyzed period and is mainly controlled by neutral wind and background plasma density. During the Antarctic summer period, disturbances are present mainly during the night time and morning hours, when background plasma density is at maximum (Weddell Sea Anomaly). In general, TID intensity and frequency of occurrence is directly proportional to the background plasma density, suggesting that low density plasma is not capable of supporting propagation of TID waves. TID direction of propagation are consistent with the antiwinward direction, indicating wind filtering effect of the their original AGW source. Software defined technology made it possible to build inexpensive HF radar that is used to measure TIDs in the Antarctic Peninsula Region. Bistatic HF sounder has been operating routinely round-the-clock with most of the measurements (~80%) producing the data suitable for TID analysis. During the rest of the time, ionospheric conditions make it impossible to detect ionospherically reflected signal. Most reliable results are achieved when the measurements made with HF and GNSS techniques are combined. This research is supported by NSF Project # 1341557 Summary TID Observations Single Antenna (Palmer) Figure 7. Left: Distribution of the observed vertical displacement velocities (normalized Doppler frequency shifts) for the Antarctic summer of 2015-2016, showing the maximum near 10 m/s. Right: Distribution of the observed TID periods for the same period of time. Parameter Value Comments Operating frequency 2-10 MHz appropriate for the location Radiated power < 50 W CW Output sample rate 100 Hz-1 kHz reduces data storage requirements Input Rx Impedance 50 Ohm compatible standard Operating system Linux real-time system Transmit Antenna (Vernadsky) Receive Antenna Array (Palmer) Receive Loop Antenna (Palmer) Figure 1. Left: Dipole transmit antenna at vernadsky. Center: Three antenna array installed at Palmer station. Distance between the antenna pairs is approximately 25 meters, and antennas are leveled to 1-2 cm accuracy. Right: Antenna #2 close-up photo showing the active loop and supporting structure. System Diagram Main System Characteristics 20:00 20:15 20:30 20:45 21:00 4.2 4.4 4.6 4.8 5.0 5.2 5.4 20:00 20:15 20:30 20:45 21:00 -140 -120 -100 -80 -60 20:00 20:15 20:30 20:45 21:00 65 70 75 80 85 p 1,2 =0.0277 p 1,3 =0.03 p 2,3 =0.00 Doppler, Hz UT Azimuth, deg Elevation, deg 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 0 20 40 60 80 4.949181 0.039075 -0.122293 2.821947 Doppler, Hz -140 -130 -120 -110 -100 -90 -80 -70 -60 0 20 40 60 80 -102.242609 193.106115 -0.232696 2.937156 Azimuth, deg 66 68 70 72 74 76 78 80 82 84 86 0 50 100 76.502319 15.028425 0.080206 2.634675 Elevation, deg 6/16/2015 7/11/2015 8/5/2015 8/30/2015 9/24/2015 04 06 08 10 12 14 16 18 Nighttime Daytime Date UT TID Intensity, a.u. 0 0.1125 0.2250 0.3375 0.4500 0.5625 0.6750 0.7875 0.9000 12/25/2015 1/14/2016 2/3/2016 2/23/2016 20 15 10 5 0 Date UT 0 5 10 15 20 25 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 UT TID amplitude December 2015-March 2016 0 200 400 600 800 1000 0 200 400 600 800 1000 December 2015 - March 2016 Count Wavelength, km Figure 2. Example of the measured signal characteristics. Left: Doppler shift and AoA signal variations recorded in the oblique sounding mode on December 1, 2015. Center: Calculations of the statistical distributions of the measured AoAs and Doppler shift make it possible to implement several quality controls (correlation between azimuth and elevation; symmetry of the AoA histograms). Right: Simultaneous measurements at Vernadsky in a vertical mode allow calculation of the north-south propagation speed using Doppler shift measurements only. Disturbances in the ionosphere (e.g., TIDs) affect the propagation of the HF radio waves. By measuring the parameters of the ionospherically reflected HF signal it is possible to monitor and to measure the characteristics of TIDs. Here the Frequency and Angular Sounding (FAS) method is used, e.g., Paznukhov et al., ASR, 49, p.700-710,2012. Figure 8. Left: Distribution of the measured TID wavelengths during the quiet times shows that the observed disturbances are classified as MSTIDs. Center: TID amplitude distribution calculated with the FAS technique (hourly averaged). Note that this is in agreement with the results in Figure 4. Right: Directions of TID propagation for the period of the strongest TIDs (0-10 UT). The results from HF measurements (white circles) are overlapped on the azimuthal distribution calculated from GNSS TEC measurements. Black dots show direction opposite to the thermospheric wind flow calculated with TIEGCM. SA21A-2497 00:10 00:20 00:30 00:40 00:50 01:00 -2 0 2 4 6 8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 1.5 Magnetic field Doppler shift nT UT Doppler shift, Hz Figure 3. Left: Spectrogram of HF signal at Palmer together with H, D, Z and HD (horizontal) component of Earth magnetic field measured at Vernadsky magnetic observatory. Right: Comparison of the magnetic HD component variations to the Doppler shift of the maximum in the signal spectrum. Variations show the same periodicity and time of occurrence indicating that HF signal propagation is affected by the ULF geomagnetic field disturbances (Pc 3-5 band). Figure 5. Climatology of the observed TIDs for 2015-2017, 0-12 UT. Left: Distribution of the TID events; Right: critical plasma frequency variations measured by the ionosonde at Vernadsky station. Similarly to the results in Figure 4, distribution of TID events throughout the period of observations follows the background plasma density distribution. These results indicate that low density plasma typically is not capable of supporting propagation of TID waves. Figure 4. Climatology of the observed TIDs. Left: Intensity of the TIDs measured with the HF sounder during the Antarctic summer of 2015-2016. Center: Typical plasma density distribution (in terms of foF2) during the year (2012) measured with the ionosonde at Vernadsky. Right: TID climatology measured with GNSS TEC technique during 2011-2013. In agreement with the GNSS TEC measurements, HF observations show that during the Antarctic summer time, TIDs are predominantly observed during the night time, when the local peak density is at its maximum (Weddell Sea Anomaly). Such plasma distribution is illustrated with the data from Vernadsky ionosonde. Therefore, TID intensity is directly proportional to the background plasma density. It is also worth noting that HF measurements show more disturbances during the night time. Figure 6. Left: Left: TID intensities measured with a single channel system in 2015. Intensity was determined from the amplitude of the Doppler shift variations. Right: intensity of TIDs as measured by bistatic HF sounder at different frequencies. Note that measurements at the two highest frequencies have approximately same intensities, indicating a possible presence of the maximum of in the vertical profile of TID distribution. More frequent presence of the disturbances during the daytime indicates that the presence of the TIDs is also controlled by the background plasma density. HD component