Ionospheric response to the St. Patrick's Day space weather events in March 2012, 2013, and 2015 at Southern Low and Middle Latitudes

The changes in critical frequency of the F2 layer (foF2) and foF2 deviation (ΔfoF2) have been determined for three geomagnetic storms in March of the years 2012, 2013, and 2015 at low‐latitude stations, Darwin (geomag. lat. 21.96°S) and Townsville (28.95°S), and midlatitude stations, Brisbane (36.73°S), Canberra (45.65°S), and Hobart (54.17°S). The moderate storm during 15–16 March 2012 (Dst = −87 nT) showed a decrease in foF2 at midlatitude and no effect at low‐latitude stations. For the intense storm of 17–18 March 2013 (Dst = −132 nT) and the super storm of 17–18 March 2015 (Dst = −222 nT), some middle‐ to low‐latitude stations showed a short‐duration increase in foF2, but all stations showed a long‐duration decrease in foF2 during the recovery phases with ΔfoF2% varying from 26% (Darwin) to 36.6% at Hobart for the March 2013 storm and above 40% for the March 2015 storm at all of the stations. Short‐duration (~2–4 hr) increase in foF2 seems to be associated with the prompt penetrating electric fields. Long‐duration (>6 hr) decrease in foF2 is mainly accounted to the decrease in thermospheric O/N2 density ratio because of storm‐induced high‐latitude circulation of gas with depleted O/N2 density ratio to lower latitudes and partly due to disturbance dynamo electric fields. A comparison of ionosonde given foF2 for equinoctial storms (March 2013 and 2015) with similar strength Southern Hemisphere winter storms (July 2012 and June 2015) has been made with the IRI‐2016 model foF2 for Darwin, Brisbane, and Canberra stations

Large Scale Traveling Ionospheric Disturbances during geomagnetic storms in the Australian region

Large-scale travelling ionospheric disturbances (LSTIDs) are detected using the critical frequency of the F2 layer (foF2) of the ionosphere. The HF interferometry (HF-Int) technique is applied to the network of ionosondes in the Australian region to detect and estimate propagation parameters. Here we present traveling ionospheric disturbances (TIDs) characteristics (period, velocity, and intensity) associated with selected geomagnetic storms. The TID response during geomagnetic storms varies significantly owing to the differing storm evolution patterns. Results show the morphology of parameters detected continually changes with time corresponding to highly dynamic response effects on the ionosphere. TIDs detected had a wide range of velocity magnitudes (mean ± standard deviation) such as 687 ± 16 m/s during onset phase, 516 ± 125 m/s during main phase and 569 ± 91 m/s during the recovery phase of the storm presented here. Propagation directions are predominantly equatorward with east and west deviations owing to higher latitude source generations associated with storm induced intensifications. The HF-Int system uses spectral techniques to estimate the dominant period and outputs the spectral energy contribution (SEC) which is a measure of the contribution of the TID to the total variability of the time series. TID activity levels are based on SEC criteria mostly indicated moderate and weak events with a range of periods 60 – 140 min

Preliminary findings of the effect of some atmospheric parameters on Ku-band satellite link

A four-month study of the attenuation measurement on satellite TV transmission down link is reported. The time percentage distributions of the attenuation show a fairly large month-to month variation. However, such variations seem to be closely related to the variation of the rain-rate distribution. Comparison of exceedance and the cumulative rainfall during these four months with those of the ten -year data indicates a similar variation this year. Measurements on a “cloudy” day without any rain indicate that the attenuation by cloud is small

Modified rain attenuation model for tropical regions for Ku-band signals

Attenuation measurement on Ku- band satellite signal in a tropical site, Fiji is presented. Rain-attenuation prediction by ITU-R and the Crane Global models showed noticeable deviation to the measured values. Unlike the monotonic decrease predicted by these models, exceedance of rain-rate and attenuation in Fiji and other tropical regions showed the presence of breakpoints. For Suva, the breakpoint in rain-rate and attenuation were at 58 mm/h and 9.4 dB with exceedances of 0.009 and 0.018%, respectively. Modifications to the ITU-R model are proposed in this paper, for adopting it in the tropics. These modifications are based on the properties that in the tropics (i) the accumulation time factor at the breakpoints is an invariant (ii) for elevation angles5608 and at high rain rates multiple rain cells intersect the slant path. The attenuation exceedance is predicted by two expressions similar to the ITU-R model, one for rain-rates lower than the breakpoint rain-rate and the other above it. The modified prediction model show remarkable agreement with the measured Ku-band attenuation in seven tropical sites

Rain attenuation measurement on Ku-band satellite TV downlink in small Island countries

Analysis of 13-year rain-rate measurements in Fiji shows that the rain exceeded 50 mm/h for 0.01% of a year (R0.01), which is significantly lower than those predicted by existing models. Simultaneous measurements of Ku-band attenuation and rain-rate give A0.01 and R0.01 as 9.7 dB and 54 mm/h, respectively. The ratio of attenuation exceedence for the worst month and yearly (at A=A0.01) was 1.8, which is lower than that predicted by the ITU-R study group. The attenuation exceeds the Ku-band link margin for about 15.6 h in a year

Total rain accumulation and rain-rate analysis for small tropical Pacific islands: a case study of Suva, Fiji

Rain-rate analyses using hourly data from 1990 to 2002 predict rain occurrence for ~15% of the year while minute data for the period Apr ‘02–Mar ‘03 resolve rain-rate more accurately and show rain occurrence of ~2% of the year. It is estimated that <1% of the rain at the site is mainly convective. Rainfall analysis confirms its dependence on El Nino Southern Oscillation and convergence zones

Statistics of Drop Size Distribution Parameters and Rain Rates for Stratiform and Convective Precipitation during the North Australian Wet Season

International audienceC-band polarimetric radar measurements spanning two wet seasons are used to study the effects of the large-scale environment on the statistical properties of stratiform and convective rainfall around Darwin, Australia. The rainfall physical properties presented herein are the reflectivity fields, daily rainfall accumulations and raining area, rain rates, and drop size distribution (DSD) parameters (median volume diameter and “normalized” intercept parameter). Each of these properties is then analyzed according to five different atmospheric regimes and further separated into stratiform or convective rain categories following a DSD-based approach. The regimes, objectively identified by radiosonde thermodynamic and wind measurements, represent typical wet-season atmospheric conditions: the active monsoon regime, the “break” periods, the “buildup” regime, the trade wind regime, and a mixture of inactive/break periods. The large-scale context is found to strongly modulate rainfall and cloud microphysical properties. For example, during the active monsoon regime, the daily rain accumulation is higher than in the other regimes, while this regime is associated with the lowest rain rates. Precipitation in this active monsoon regime is found to be widespread and mainly composed of small particles in high concentration compared to the other regimes. Vertical profiles of reflectivity and DSD parameters suggest that warm rain processes are dominant during this regime. In contrast, rainfall properties in the drier regimes (trade wind/buildup regimes) are mostly of continental origin, with rain rates higher than in the moister regimes. In these drier regimes, precipitation is mainly formed of large raindrops in relatively low concentration due to a larger contribution of the ice microphysical processes on the rainfall formation

Surface fair-weather potential gradient measurements from a small tropical Island station Suva, Fiji

Systematic analysis of the surface fair-weather potential gradient (PG) measured for 13 months (July 2005-July 2006) at 10 s resolution over a small tropical island station Suva (18.08°S, 178.45°E), Fiji is presented. Based on the solar radiation (>800 W m-2) and surface wind speed (<4.5 m s-1) conditions, 63 fair-weather days were selected. After sorting the data into a range of 0-1000 V m-1, the average PG was ∼139 V m-1. The measured fair-weather PG had a semi-diurnal structure, with a more pronounced peak at 0730 LT (1930 UT) and a reduced peak at 2200 LT (1000 UT). The time of occurrence of the morning peak and the noontime minima agreed well with the Carnegie curve. However, the variation about the global mean in the measured PG was 150% whereas for the Carnegie curve it was only 35%. The effects of the local meteorological parameters on the PG measurements were found to be small. On average, the PG during the dry season (May-October) was always greater than in the wet season (November-April). In contrast, analysis of regional (0-60°S and 100°E-160°W) lightning activity on fair-weather days showed a peak at ∼2000 LT and higher lightning activity during the wet season. These results indicate that the regional thunderstorm activity has no direct connection with the local fair-weather PG at the site

Occurrence rate and duration of space weather impacts on high-frequency radio communication used by aviation

High frequency (HF) radio wave propagation is sensitive to space weather-induced ionospheric disturbances that result from enhanced photoionization and energetic particle precipitation. Recognizing the potential risk to HF radio communication systems used by the aviation industry, as well as potential impacts on GNSS navigation and the risk of elevated radiation levels, the International Civil Aviation Organization (ICAO) initiated the development of a space weather advisory service. For HF systems, this service specifically identifies shortwave fadeout, auroral absorption, polar cap absorption, and post-storm maximum useable frequency depression (PSD) as phenomena impacting HF radio communication and specifies moderate and severe event thresholds to describe event severity. This paper examines the occurrence rate and duration of events crossing the moderate and severe thresholds. Shortwave fadeout was evaluated based on thresholds in the solar X-ray flux. Analysis of 40-years of solar X-ray flux data showed that moderate and severe level solar X-ray flares were observed, on average, 123 and 5 times per 11-year solar cycle, respectively. The mean event duration was 68 min for moderate level events and 132 min for severe level events. Auroral absorption events crossed the moderate threshold for 40 events per solar cycle, with a mean event duration of 5.1 h. The severe threshold was crossed for 3 events per solar cycle with a mean event duration of 12 h. Polar cap absorption had the longest mean duration at ~8 h for moderate events and 1.6 days for severe events; on average, 24 moderate and 13 severe events were observed per solar cycle. Moderate and severe thresholds for shortwave fadeout, auroral absorption, and polar cap absorption were used to determine the expected impacts on HF radio communication. Results for polar cap absorption and shortwave fadeout were consistent with each other, but the expected impact for auroral absorption was shown to be 2–3 times higher. Analysis of 22 years of ionosonde data showed moderate, and severe PSD events occurred, on average, 200 and 56 times per 11-year solar cycle, respectively. The mean event duration was 5.5 h for moderate-level events and 8.5 h for severe-level events. During solar cycles 22 and 23, HF radio communication was expected to experience moderate or severe impacts due to the ionospheric disturbances caused by space weather, a maximum of 163 and 78 days per year, respectively, due to the combined effect of absorption and PSD. The distribution of events is highly non-uniform with respect to the solar cycle: 70% of moderate or severe events were observed during solar maximum compared to solar minimum