Combinatorial observation ionospheric characteristics during tropical cyclone Debbie passing eastern Australia in 2017 using GPS and ion sounder

The ionospheric morphology responses to tropical cyclone passing over eastern Australia, named as DEBBIE in 2017, is investigated using Global Positioning System (GPS) Slant Total Electron Content (STEC), GPS ionospheric 15 scintillation S4 index and ionospheric characteristics by High Frequency (HF) radio. Based on the data analysis in this study, some significant morphological characteristics of ionospheric responses to tropical cyclone Debbie are identified as follows: a) As the GPS satellites PRN01 and PRN11 were passing above typhoon center, their ROTI (Rate of STEC index) is obviously increased. b) The S4 intensity of the GPS ionospheric scintillations is significantly enhanced on March 27, which mostly concentrate around tropical cyclone center and distribute over the region within 18°S-25°S. c) The stronger 20 enhancement of f0F1 and f0F2 was observed by High Frequency at Townsville on March 28, when the distance between Townsville and the center of tropical cyclone Debbie was shorter. Regarding the coupling mechanism between the ionospheric disturbance and the tropical cyclone, it is suspected that the electric field perturbations due to turbulent top movement from tropical cyclones might generate ionospheric irregularity and disturbance. To a certain extent, some ionospheric irregularities can further produce bubbles. 25


Introduction
It is acknowledged that the ionosphere is an important and indispensable part of atmosphere for earth.The significant characteristic of ionosphere is high variation and complex physical system.The ionospheric irregularity and disturbance subject to the exterior impact and influence can degrade the operation reliability of ground-based radio and GPS or even lead to their failure due to loss of signal lock.Normally, major ionospheric disturbances are caused by the strong solar and https://doi.org/10.5194/angeo-2019-72Preprint.Discussion started: 12 June 2019 c Author(s) 2019.CC BY 4.0 License.geomagnetic activity.It has been indicated that ionospheric dynamics behaviors at low, mid and high latitude are dominated by solar tides and horizontal geomagnetic field lines, the inner magnetosphere and neutral winds, and the solar wind and electron precipitation, respectively (Skone et al., 2001).On the other hand, the lower layer of the ionosphere is connected to the neutral atmosphere.In the 1950s, Beynon et al. (1953) pointed out that ionospheric disturbance is related to the atmospheric activity in the troposphere.Moreover, the lower-level atmospheric activity not only can cause the ionospheric structure and physical variation, but also trigger the small and medium scale ionospheric disturbances (Forbes, 1996).
The tropical cyclone is a typical and strong convective atmospheric activity.Bauer (1958) firstly discovered one phenomenon of ionospheric response to hurricane passage that the critical frequency f0F2 in the F2 ionospheric layer observed by High Frequency Radio (HF) began to increase as hurricane approaching to HF observation station.Additionally, the total electron content (TEC) and ionospheric scintillation can be achieved by ground GPS station to study the morphological characteristic of ionospheric response to tropical cyclone (Yang and Liu, 2016).Some previous studies have shown that the increasing TEC anomalies were detected during typhoon Matsa in 2005 (Mao et al., 2010), Aili in 2004 (Cheng et al., 2013), Nakri in 2008 (Lin, 2012), Tembin in 2012 (Yang and Liu, 2016) and Typhoon Meranti in 2016 (Chou et al., 2017).However, another study demonstrated that the TEC around the equatorial area decreased during typhoon Mahasen in 2013 and Hudhud in 2014 passing over the Indian sector (Guha et al., 2016).Meanwhile, the previous results showed that the ionospheric disturbances were emanating outward and lasting for more than 10 h before Super Typhoon Meranti in 2016 landfall (Chou et al., 2017), and that the number of radio occultation scintillation arriving at peak as typhoon Tembin in 2012 closest to Hong Kong (Yang and Liu, 2016).Using HF and Very Low (VL) Doppler radar, the results of statistical analysis showed that ionospheric disturbance percentage of 24 strong typhoons in China (Xiao et al., 2007), 41 tropical cyclones in the Atlantic Ocean (Nina et al., 2017), and 25 hurricanes in western and central part of the Czech Republic were 92%, 88% and 8%, respectively.Moreover, the characteristics of ionospheric disturbance intense were not the same.The results (Xiao et al., 2007;Rice et al., 2012;Yu et al., 2010;Ke et al., 2018) showed that the f0F2 was increased during studied cyclones, but the studies (Liu et al., 2006;Rozhnoi et al., 2014) demonstrated that it is decreased.
Using the sounder instruments on board the Cosmos 1809 satellite (Isaev et al., 2010), it is indicated that the pressure of the electron gas, electric field and scintillation intense increased in some specific zones, and plasma density and pressure above typhoon eye sharply decreased during typhoon intensification.
Thus, there are uncertainty and some controversies about morphological characteristics of ionospheric disturbances caused by tropical cyclones (Perevalova et al., 2011;Zakharov & Kunitsyn, 2012).Moreover, though there were also much many tropical cyclones in the southern hemisphere, rare tropical cyclones were used as cases to study the ionospheric response to them.Therefore, the previous studies were still limited not only in ionospheric observation instrument but also in the representative of the tropical cyclone cases studied worldwide.In the southern hemisphere, there are several ion sounder radio and GPS continuously operating reference stations (CORS) distributed around Australia for detecting the ionospheric morphological parameter.In March 2017, tropical cyclone Debbie was the strongest tropical cyclone in the Australian region since Cyclone Quang in 2015, which was branded the most dangerous cyclone to impact Queensland since Cyclone Yasi in 2011.The combination observation of HF Doppler radar, GPS STEC and GPS ionospheric scintillation will be utilized to study the morphological characteristics of ionospheric response to tropical cyclone Debbie.This study will be a representative case of the ionospheric response to tropical cyclone in the southern hemisphere.It is also valuable to real the coupling mechanism between ionospheric disturbances and tropical cyclones.
In the following section, we will introduce dataset and methodology used in this study.Then, the ionospheric response to cyclone Debbie in 2017 will be analyzed for demonstrating the morphological characteristics of ionospheric response to tropical cyclone Debbie in the southern hemisphere.Meanwhile, the possible coupling mechanism of reaction between ionospheric disturbance and tropical cyclone will be discussed.The ionospheric activity is mainly dominated and affected by the solar and geomagnetic activities.Therefore, the influence from the solar and geomagnetic field should be firstly analyzed before studying the ionospheric response to tropical cyclone.

Solar and geomagnetic field activity
Normally, the solar radio flux (10.7cm/2800MHz,F10.7), geomagnetic activity indexes Dst and Kp were used to judge the solar and geomagnetic activity level, respectively.Their variations during tropical cyclone Debbie are shown in Figure 2.
Generally, the F10.7 ranges of [70sfu, 100sfu], [100sfu, 150sfu], and [150sfu, 250sfu] represent low, moderate, and high level of solar activity, respectively [Wang et al., 2015].The Dst ranges of −50 nT < Dst ≤ −30 nT , −100 nT < Dst ≤ −50 nT, −200 nT < Dst ≤ −100 nT and Dst ≤ −200 nT signify small, moderate, large and stronger and severe geomagnetic storms, respectively [Mao et al., 2010].The Kp index ranges of 0-1, 2-4, 5, 6, and 7-9 denote quiet geomagnetic field, unstable geomagnetic field, small geomagnetic storm, large geomagnetic storm and severe geomagnetic storm, respectively.It is shown that the solar activity during tropical cyclone Debbie was at low level with F10.7 less than 100 sfu.But the Dst and Kp indexes indicate there were some a small geomagnetic storm on 27 March 2017 before cyclone Debbie landed on Hook Island of Queensland at UT00:00 on 28 Mar 2017.Therefore, the influence on ionosphere from the small geomagnetic storm should not be ignored to studying the ionospheric response to tropical cyclone Debbie.

The Rate of GPS STEC
The rate of GPS slant TEC (ROT) is as a measure of GPS phase fluctuation, which can be used to monitor the ionospheric irregularity and disturbance.It represents the derivative of GPS slant TEC between two successive epochs, which can be calculated by the following equation (Yang and Liu, 2016): where i is the No of a GPS satellite; tk is an epoch time; the unit of ROT is TECU/min.
Rate of STEC index (ROTI) indicates the extent of the GPS phase fluctuations, which can be used to detect the occurrence of the ionospheric irregularities by the sharp TEC gradient.The ROTI represents the standard deviation of the ROT in a specific time interval (Yang and Liu, 2016): where the angle brackets denote the average taking over a 5-min time interval.These indices are calculated for each visible GPS satellite over a ground-based GPS station.

GPS ionospheric Scintillation
The intense of GPS ionospheric scintillation is typically quantified by the S4 index, which is calculated using the following equation (Kintner et al., 2007): where I is the intense of GPS signal, which is output from the GPS receiver tracking loop./ 0 ̅̅̅̅̅̅̅ is the average Signal-to-Noise Ratio of satellite L1 Band in an observation period.If S4 > 0.2, it indicates strong GPS ionospheric scintillation (Muella et al., 2008).

Ionospheric characteristics by HF radio
The ionospheric parameters of f0E, f0F1, and f0F2 can be used to analyze the ionospheric response to cyclone in vertical direction.Accordingly, the electron density   in each layer can be calculated using the frequency of reflection radio wave by the follow equation: where f is the critical frequency f0E, f0F1 and f0F2 for the corresponding plasma with electron density  .The units of f and   are MHz and m -3 , respectively.The electron density   is linear dependent on f 2 .Therefore, the parameters of f0E, f0F1, f0F2 can reflect the electron density variation of ionosphere in vertical layers in response to tropical cyclone.GPS STEC extracted by other GPS satellites were not anomalous.Therefore, it can be inferred that the anomaly of ROTI and ROT extracted by GPS PRN01 and PRN11 above tropical cyclone center on UT12:00 of 27 and 28 March were more likely triggered by cyclone Debbie.To further determine the correlation between GPS ionospheric scintillation and tropical cyclone Debbie, the spatial relations between GPS ionospheric scintillation points puncturing into ionosphere and tropical cyclone Debbie are shown in Figure 5.

GPS ionospheric scintillation response
It is obvious that more points of S4>0.2 are mainly distributed around the tropical cyclone center.Moreover, the intensity and number of the points of S4>0.2 above the area of 18 ° − 25 ° in the latitude and 150 ° − 155 ° in the longitude around tropical cyclone center (B = 19.6 °,  = 149.8°) was stronger and larger than those above the other area.Normally, the occurrence of strong ionospheric scintillations is more frequent in low geomagnetic latitude (±15 °) and high geomagnetic latitude (±70 °).However, the area of these GPS ionospheric scintillations with S4>0.2 doesn't belong to the area of frequent ionospheric scintillation.The evidence further verified that the strong ionospheric scintillations above the area around tropical cyclone Debbie might be triggered by tropical cyclone Debbie.

The ionospheric parameters in E/F1/F2 layer by ion sounder
The ionosphere is divided into D, E, F1 and F2 layers.There are differences in the characteristics of the ionosphere in each layer.However, it is difficult to distinguish the characteristics of the ionosphere in each layer in response to tropical cyclone     The above results reveal that tropical cyclone Debbie can cause ionospheric irregularity and disturbance, which can further produce ionospheric scintillations.However, the coupling mechanism between tropical cyclone and ionosphere disturbances is still indefinite (Shen, 1982).So far, there are following proposed coupling mechanisms, such as gravity waves generated by tropical cyclone (Xiao, et al. 2007), an atmosphere divergence/convergence model and dynamic coupling (Shen, 1982), a disturbed electric field (Isaev, et al. 2002), turbulent top moving (Shen, 1982;Liu, et al. 2006;Wang, et al. 2005) and lighting discharge (Shao, et al. 2013).According to the morphological characteristics of ionospheric response to tropical cyclone Debbie, the coupling mechanism will be discussed based on turbulent top moving, the structure of atmosphere and typhoon (Figure 9), and electron photochemical reactions.
Tropical cyclone belongs to airflow system in troposphere with a height of 20 km or less, which might not directly affect the ionosphere with a height of 50 km or more in the left of Figure 9 (Shen, 1982).But the strong airflow of lower layer in troposphere can lead to the structure change of stratosphere and mesosphere (Shen, 1982;Liu, et al. 2006).The airflow direction of the tropopause around tropical cyclone center in the right of Figure 9 is downward.The convergence of airflow will increase the height of the stratosphere upward above tropical cyclone (Liu, et al. 2006).Then the upward airflow will continue to develop upward due to the temperature structure of the middle layer and elevate the height of the turbulence layer, whose height is about 100km (Shen, 1982).According to atmospheric turbulence layer movement theory (Liu, et al. https://doi.org/10.5194/angeo-2019-72Preprint.Discussion started: 12 June 2019 c Author(s) 2019.CC BY 4.0 License.

Figure 1 :
Figure 1: GPS stations of ISMs (Red triangles: Willi Island), Ionosonde stations (Blue triangles, L: Learmonth, T: Townsville, B: Brisbane), paths of tropical cyclone DEBBIE (Red line), the tropical cyclone moving directions (arrows) and the places with the largest wind velocity (Red pentagrams) Tropical cyclone Debbie was formed above the south of Solomon Sea of South Pacific at Universal Time (UT) 12:00 of 21 Mar 2017.Then it landed on Hook Island of Queensland at UT00:00 on 28 Mar 2017 at the wind speed of 105m/s.Tropical cyclone Debbie left the Australian continent from Brisbane at 12:00 on 30 Mar 2017.As shown in Figure 1, tropical cyclone Debbie moved from north to south and its impact zone was in medium geomagnetic latitude from 15 ° S to 60 °S in the southern hemisphere.Tropical cyclone Debbie center passed above Willi GPS ionospheric scintillation station for S4 index, Townsville and Brisbane HF Doppler Radar stations for ionospheric parameters operated by Ionospheric Prediction Services (IPS) Radio and Space Weather Services (SWS) of Australia, TOW2 and MCHL IGS GPS stations for GPS STEC belonging to IGS Network.When tropical cyclone Debbie landed on Hook Island of Queensland at UT00:00 on 28 Mar 2017 at the

Figure 3 :
Figure 3: The variation of ROT for GPS PRN01 and PRN11 and ROTI (5min) for all satellites during March 26-29 over the TOW2 IGS station.The GPS STEC response to passing tropical cyclone Debbie from 26-29 March 2017 is firstly analyzed.The GPS STEC was extracted from TOW2 IGS station, which tropical cyclone Debbie passed above.The variation of ROT for PRN01 and PRN11 and ROTI (5min) for all satellites during March 26-29 are shown in figure 3. It shows that ROTI has an evident increment on UT 12:00 (Local Time (LT) =UT+8h) March 27, the day before tropical cyclone Debbie center landed on Hook Island.When tropical cyclone Debbie center landed on Hook Island of Queensland on March 28, the increasing of ROTI became slight.As shown in the top two of Figure 3, the variation of ROT for PRN01 and PRN11 were obviously increasing as the cyclone Debbie landing.The IPP traces of GPS PRN01 and PRN11 satellites over TOW2 station were above the impact area of tropical cyclone Debbie on 28 March.Although there was a small geomagnetic storm on 27 March,

Figure 4 :
Figure 4: The GPS ionospheric scintillation S4 variations of GPS PRN01 and PRN11 satellites during 22-29 March 2017.The dotted red line is the threshold of the strong GPS ionospheric scintillation.The magenta vertical line denotes the time point when cyclone Debbie center was the closest to GPS stationAlthough ionospheric scintillation can degrade the GPS signal quality or even cause failure of the signal lock, GPS also provides a new tool for detecting ionospheric irregularity and scintillation.GPS scintillation amplitude index S4 more than 0.2 indicates strong ionospheric scintillation.Figure4shows that the variation of GPS ionospheric scintillation of PRN01 and PRN11 above Willi Island GPS station during the period of 22-29 March 2017.When the distance from tropical cyclone center to Willis station was 370 km and the center wind speed was 105 m/s at midnight of 27 March, the number and intensity of S4>0.2 observed by GPS satellite G01 and G11 were more and larger than those at the other time.It shows that the geomagnetic activity was with a small storm in Figure2.Under the geomagnetic condition, the anomaly of ionospheric scintillation of GPS PRN01 and PRN11 might be related to the small geomagnetic storm.But the S4 values of the other GPS satellites are all smaller than the S4 threshold.Hence, it can be deduced that the number increment of GPS ionospheric scintillations might be triggered by tropical cyclone Debbie on March 27.

Figure 5 :
Figure 5: Ionospheric puncturing point traces and GPS ionospheric scintillation S4 intensity of GPS PRN01 and PRN11 satellites through cyclone Debbie on March 27.The red dashed cycles indicate the area affected by cyclone Debbie.The colorful solid circles are GPS ionospheric scintillations and their intensity Debbie only by GPS.Therefore, HF radios installed in Townsville and Brisbane are used to detect and analyze the characteristic parameters of f0E, f0F1 and f0F2 in E, F1 and F2 layer for ionospheric response to cyclone Debbie.With regard to characteristic parameters f0E, f0F1 and f0F2, the best fits for their mean value from 24 to 31 March were referred as their normal values to judge electron density response to tropical cyclone in vertical layers.https://doi.org/10.5194/angeo-2019-72Preprint.Discussion started: 12 June 2019 c Author(s) 2019.CC BY 4.0 License.

Figure 6 :
Figure 6: The f0E variation in the ionospheric E layer at Learmonth, Townsville and Brisbane HF radar during the period of 24 to 30 March in 2017 as tropical cyclone Debbie moving.The f0E is the ordinary wave critical frequency of the lowest thick layer which causes a discontinuity.The red lines denote the best fit for the mean value of f0E from 24 to 30 March The characteristic parameters f0E in the ionospheric E layer from Townsville and Brisbane HF radio stations as tropical cyclone Debbie moving from 24 to 30 March are shown in Figure 6.Although f0E values of Townsville were small enhanced and elevated at midday of 29 and 30 March after tropical cyclone Debbie landfall, the f0E values of Learmonth, Townsville and Brisbane on landfall day and other days were no significant anomaly relatively to their reference values.The f0E of Learmonth, Townsville and Brisbane were all approximately equal.The ranges of f0E were respectively from 1.7 to 4.0 MHz and from 85 km to 100 km.The phenomenon in the ionospheric E layer indicates that the ionosphere in E layer is not disturbed by tropical cyclone Debbie.

Figure 7 :
Figure 7: The f0F1 variation in the ionospheric F1 layer at Learmonth, Townsville and Brisbane HF radar stations during the period from 24 to 30 March in 2017 as tropical cyclone Debbie moving.The f0F1 is the ordinary wave F1 critical frequency.The red lines denote the best fit for the mean value of f0F1 from 24 to 30 March

Figure 8 :
Figure 8: The f0F2 variation in the ionospheric F2 layer at Learmonth, Townsville and Brisbane HF radar stations during the period of 24 to 30 March in 2017 as tropical cyclone Debbie moving.The f0F2 is ordinary wave critical frequency of the highest stratification in the F region.The red lines denote the best fit for the mean value of f0F2 values from 24 to 30 March The ionospheric parameters f0F1 and f0F2 on Learmonth, Townsville and Brisbane HF radio stations as tropical cyclone Debbie moving from 24 to 30 March are shown in Figure 7 and Figure 8.It is obvious that the f0F1 and f0F2 intensity on Townsville station on 28 March were significantly stronger than the best fit value.However, the f0F1 and f0F2 intensity on Learmonth and Brisbane are approximately equal to the best fit values under the same solar and geomagnetic activity.Theoretically, the influences on f0F1 and f0F2 of Learmonth and Townsville in the same latitude from the small geomagnetic storm on 27 and 27 March should be almost the same.However, the anomaly extent of f0F1 and f0F2 on Townsville station are significantly larger than those on Learmonth on 28 March.The largest f0F1 deviation on Townsville station relative to their best fits was 1.0MHz -1.5MHz.Additionally, the largest f0F2 deviation on Townsville station relative to their best fits was 5.0MHz.The electron density   is linear related to f0F1 and f0F2.Thus, it can be inferred that the electron density of F1 and F2 significantly increased on the tropical cyclone Debbie landfall day.Moreover, the enhancement f0F1 and f0F2 on Townsville station on landfall day were stronger than those of Brisbane, when the ellipsoidal distances from tropical cyclone center to Townsville and Brisbane were 230 km and 925 km, respectively.Therefore, the stronger f0F1 and f0F2 enhancement of Townsville station on 28 March should be attributed to tropical cyclone Debbie.