SCINTILLATION MONITORING RESULTS DURING THE 30 OCTOBER 2003 SPACE WEATHER EVENT

1. SCINTILLATION MONITORING RESULTS DURING THE 30 OCTOBER 2003 SPACE WEATHER EVENT Marcio Aquino, Alan Dodson Institute of Engineering Surveying and Space Geodesy (IESSG), University of Nottingham, Nottingham, UK Lucilla Alfonsi, Giorgiana De Franceschi, Vincenzo Romano Instituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy Cathryn Mitchell Department of Physics and Astronomy, University of Bath, Bath, UK

2. ABSTRACT Ionospheric scintillation may present significant effects in equatorial and auroral regions, especially during times of high solar flux. In the auroral regions scintillation events are often associated with higher levels of geomagnetic activity and can affect GNSS users at sub-auroral (and potentially mid-latitude) regions whose satellite to ground links cross the resulting plasma instability patches, with impact ranging from degradation of accuracy to loss of signal tracking. An attempt to investigate the impact of ionospheric scintillation and Total Electron Content (TEC) gradients on GNSS user accuracy, availability and integrity was carried out at the IESSG (Aquino et al, 2005). Analyses of the occurrence of high levels of scintillation disturbing a number of satellites simultaneously showed that, on a day of enhanced geomagnetic activity for nearly 2% of the time two satellites could be concurrently affected at high latitudes (Rodrigues et al, 2004). If those scintillation levels can lead to receiver loss of lock on satellites then this may prove crucial during periods when only a few satellites are in view. In a recent paper, Mitchell et al (2005) studied the severe ionospheric storm that occurred at the end of October 2003, with emphasis on the events of the evening of 30 October. In particular they used a GPS scintillation monitor receiver deployed by INGV and located at Ny Alesund (lat 79oN, long 12oE geographic) to analyse the mechanism for the generation of the irregularities causing scintillation on the L band, their results indicating that GPS scintillation in that region can originate from ionospheric plasma structures from the American sector. These findings have implications for ionospheric scintillation forecasting during storms, of considerable relevance to GNSS users in Northern Europe. The IESSG was also able to register scintillation levels during the October 2003 storm with their network of similar receivers, set up at geographic latitudes varying from 53oN to 71oN, and longitudes from 1oW to 24oE, potentially expanding the scintillation data availability for analysis. Introducing this data it was possible to see a pattern of irregularity movement in agreement with what was found by Mitchell et al.

3. Ny Alesund BACKGROUND In this paper we analyse scintillation and TEC data obtained on 30 October 2003. The scintillation data originates from 5 GPS scintillation monitor receivers, deployed at the locations shown in figure 1.The Ny Alesund receiver is run by the INGV, whereas the other 4 receivers were in place at the time as part of the IESSG scintillation project. The analyses that follow build on the findings of Mitchell et al (2005), with complementary insight obtained from the IESSG data, recently included in the study, and further investigations undertaken by the authors, with particular interest in the period of time between 21:00 and 22:00 UT, when a coinciding TEC gradient and phase and amplitude scintillation event had been observed. The scintillation monitor receivers are designed to record phase and amplitude scintillation on the L1 signals, given by the widely used indices S4 and Sigma Phi. The S4 index is defined as the standard deviation of the received signal power normalized to the average signal power. S4 is calculated every 1 minute based on 50Hz sampling rate data (3000 data samples). The receivers also compute the S4 due to ambient noise in a way that a corrected S4 (without noise effects) can be calculated. The phase scintillation data is estimated from measurements of phase variance at every 60 seconds interval and, similarly to the amplitude scintillation measurement, is based on 50Hz sampling rate measurements of L1 carrier phase. The standard deviation of the phase is usually referred to as , an index commonly used for phase scintillation measurements. The receiver computes this index over 1, 3, 10, 30 and 60 seconds. The 60 seconds phase scintillation index, also referred to as Phi60, was used throughout the analyses. The receivers also track the L2 signal and are able to compute TEC from combined L1 and L2 pseudorange and carrier phase measurements. The large scale distribution of TEC values were obtained from a network of geodetic receivers. MIDAS (Multi Instrument Data Analysis System), developed at the University of Bath, was used to calibrate the satellite and receiver interfrequency biases and the individual slant TEC values were then converted to equivalent vertical TEC using a geometrical correction factor.

4. INVESTIGATIONS AND DISCUSSION From measurements made at Ny Alesund, it had been observed by Mitchell et al (2005) thatregions of both phase and amplitude scintillation occurred in coincidence with increasing TEC values in the evening of 30 October 2003. This was exemplified in particular for the GPS satellite PRN31, during the period of time between 21:00 and 22:00 UT, when noticeable TEC gradients were observed. In figure 2 we show an extended picture of what was presented in that paper, now with the addition of data from the IESSG receivers. Each time series shows the Phi60 scintillation index and the corresponding values of non-calibrated vertical TEC for the link between each of 5 receivers and PRN31 (here we use the terminology ‘non-calibrated’ because the TEC values have not been corrected for the receiver interfrequency bias, which is in this case irrelevant given the relative nature of the analyses). It can be seen that the peaks in the Phi60 values occur in coincidence mostly with the steepest parts of the vertical TEC time series, indicating the presence of high TEC gradients. A close look at the time axis of each diagram also allows to assess the timing of the event, as observed at the various stations. Figure 3 gives a graphical representation of the time/space movement of the maximum Phi60 values experienced at the pierce points of the links to PRN31 from each of the stations of the monitoring network.

5. Non-calibrated TEC Phi60 Bergen Nottingham Ny Alesund Loss of L2 lock Hammerfest Bronnoysund Loss of L2 lock Figure 2. Peak in Phi60 values coinciding with steep TEC gradients as observed at the monitoring network (link to PRN31), on 30 October 2003, 21:00 to 22:00 UT)

6. Ny Alesund (21:23 UT) Hammerfest (21:56 UT) Geographic Latitude (degrees) Bronnoysund (21:39 UT) Bergen (21:22 UT) Nottingham (21:25 UT) Geographic Longitude (degrees) Figure 3. Time/space movement of maximum Phi60 experienced at pierce point of link to PRN31 from monitoring network

7. Figure 4. Equivalent vertical TEC. Black irregular line encloses the high TEC values Figure 4 shows the equivalent vertical TEC computed from MIDAS for 15 minute windows during the period under investigation (21:00 to 22:00 UT, on the 30 October 2003). The black irregular lines enclose the high TEC values observed in the region.

8. Figure 5. Highest sigma phi per satellite and receiver over a 15 minute period. Elevated sigma phi seems to be associated with high TEC values. Next we show, in figure 5, the highest values of the Phi60 index, as observed by the 5 receivers of the monitoring network during the corresponding 15 minute sessions. The black irregular line of figure 4 was superimposed. It can be seen that elevated values of the phase scintillation index seem to be geographically associated with the higher TEC values.

9. Figure 6. Highest S4 per satellite and receiver over a 15 minute period. The largest values of S4 seem to be associated with the EDGES of high TEC values. In a similar fashion, in figure 6 we analysed the highest values of the amplitude scintillation index S4. It seems that the largest values of S4 lie on the edges of the region of high TEC values. These findings confirm that the irregularities associated with scintillation during this storm are likely to be associated with the large-scale plasma that has convected across the polar cap from the USA. A possible mechanism, as suggested in Mitchell et al (2005) is the gradient drift instability.

10. Between 21:00 and 22:00 UT the orientation of the Interplanetary Magnetic Field (IMF) is southward (Bz<0), i.e. the reconnection between the IMF and the geomagnetic field is favoured on the subsolar magnetopause with a configuration of two convection cells. This should imply the plasma across the polar cap coming from the American sector as it had already been found by Mitchell at al (2005). From the literature (e.g. Ruhoniemi and Greenwald, 2005 and references therein),  the sign of the IMF By component should give information on the dimension of the two convection cells and on the shift of the site of reconnection. Figure 7 shows the convection pattern based on the Weimer model (Weimer, 2005) generated for 21:16 UT using IMF components as measured by the ACE satellite. Since the ACE solar wind speed and density data are not available for the period under consideration, we assumed a reasonable value of 1400 km/s and 10 cm-3, respectively. Following Mitchell et al (2005), we have drawn on the Weimer ionospheric convection pattern the TEC enhancement contour already plotted in figures 4, 5 and 6 for the frame 21:15/21:30 UT. During this particular time interval By < 0. This means that the shift of the site of reconnection should be dawnward as confirmed by figure 7 that shows the TEC enhancement tongue moving from south west to northeast. This reconstruction of the general movement of the plasma causing scintillations is also confirmed in figure 3. Figure 7. Convection pattern (Weimer model) generated for 30 October, 21:16 UT

11. CONCLUSIONS Comparatively analysing figures 3 and 4 it is possible to confirm that the events causing highest values of phase scintillation on the link between PRN31 and the GPS monitor receivers present a time/space dynamics which seems to be compatible with the movement observed for the high TEC values from MIDAS. It is likely that during the 15 minute session from 21:15 to 21:30 UT the irregularities causing highest scintillation move in a southwesterly direction as sensed at pierce points observed by the monitor receivers in Bergen, Ny Alesund and Nottingham (figure 3), in agreement with the second diagram of figure 4. Immediately afterwards, during the 21:30/21:45 UT session, the subsequent movement is now to the northeast (pierce points of Nottingham and Bronnoysund in figure 3). And finally later, during the 21:45/22:00 UT session, the most severe scintillation events advance further to the east/northeast (pierce points of Bronnoysund and Hammerfest in figure 3). These seem to agree quite clearly with the last two 15 minute sessions of figure 4, where the high TEC values expand following respectively similar patterns. This type of analysis seems to present a potential tool for validating the possible deployment of a monitoring network that could be used to forecast scintillation events in Northern Europe. The chain of GPS receivers here considered has allowed to follow the temporal/spatial behaviour of the scintillations pattern, also confirming theoretical achievements as those related to the influence of Bz and By on the two convection cells configuration, here represented by the Weimer model. ACKNOWLEDGEMENTS The authors thank the Polarnet project (CNR), the CNR SRC Dept., NOAA/SEC (Boulder, CO USA), and the PNRA (Italian National Program for Antarctic Research) project. LA thanks Dan Weimer for making available the IDL code for his electric potential model. Thanks are also due to the Engineering and Physical Sciences Research Council in the UK for funding the ionospheric scintillation research at Bath and Nottingham Universities.

12. REFERENCES Aquino M, T Moore, A Dodson, S Waugh, J Souter and F S Rodrigues, Implications of Ionospheric Scintillation for GNSS Users in Northern Europe – The Journal of Navigation, Royal Institute of Navigation, 58, pp 241-256, 2005. De Franceschi G, V Romano, L Alfonsi, L Perrone, M Pezzopane, B Zolesi, ISACCO (Ionospheric Scintillations Arctic Campaign Coordinated Observations) project at Ny-Ålesund, proceedings of “Atmospheric Remote Sensing using Satellite Navigation Systems, Special Symposium of the URSI Joint Working Group FG“, Matera, Italy, October 2003. Mitchell, C N, L Alfonsi, G De Franceschi, M Lester, V Romano and A W Wernik, GPS TEC and Scintillation Measurements from the Polar Ionosphere during the October 2003 Storm, Geophysical Research Letters, Vol. 32, No. 12, L12S03 10.1029/2004GL021644, 2005. Rodrigues F S, M Aquino, A Dodson, T Moore and S Waugh, Statistical Analysis of GPS Ionospheric Scintillation and Short-Time TEC Variations over Northern Europe - Journal of the Institute of Navigation, Vol. 51, No. 1, pp 59-75, Spring 2004. Ruohoniemi J M, R A Greenwald, Dependencies of High-latitude Plasma Convection: Consideration of Interplanetary Magnetic Field, Seasonal, and Universal Time Factors in Statistical Patterns, Journal of Geophysical Research, 110, A09204, doi:10.1029/2004JA010815, 2005. Weimer, D. R., Improved Ionospheric Electrodynamic Models and Application to Calculating Joule Heating Rates, Journal of Geophysical Research., Vol. 110, No. A5, A05306, 2005. contact: marcio.aquino@nottingham.ac.uk, alan.dodson@nottingham.ac.uk,defranceschi@ingv.it,c.n.mitchell@bath.ac.uk,lucilla.alfonsi@ingv.it, romano@ingv.it