Dynamic processes in the magnetic field and in the ionosphere during the 30 August–2 September, 2019 geospace storm

Back at the end of the last century, L. F. Chernogor validated the concept that geospace storms are comprised of synergistically coupled magnetic storms, ionospheric storms, atmospheric storms, and storms in the electric field originating 10 in the magnetosphere, the ionosphere and the atmosphere (i.e., electric storms). Their joint studies require the employment of multiple-method approach to the Sun–interplanetary medium–magnetosphere–ionosphere–atmosphere–Earth system. This study provides general analysis of the 30 August–2 September 2019 geospace storm, the analysis of disturbances in the geomagnetic field and in the ionosphere, as well as the influence of the ionospheric storm on the characteristics of HF radio waves over the People's Republic of China. A unique feature of the geospace storm under study is its duration, of up to four 15 days. The main results of the study are as follows. The energy and power of the geospace storm have been estimated to be 1.5  10 J and 1.5  10 W, and thus this storm is weak. The energy and power of the magnetic storm have been estimated to be 1.5  10 J and 9  10 W, i.e., this storm is moderate, and a unique feature of this storm is the duration of the main phase, of up to two days. The recovery phase also was lengthy, no less than two days. On 31 August 2019 and on 1 September 2019, the variations in the H and D components attained 60–70 nT, while the Z-component variations did not 20 exceed 20 nT. On 31 August 2019 and on 1 September 2019, the level of fluctuations in the geomagnetic field in the 100– 1000 s period range increased from 0.2–0.3 nT to 2–4 nT, while the energy of the oscillations showed a maximum in the 300–400 s to 700–900 s period range. The geospace storm was accompanied by a moderate to strong negative ionospheric storm. During 31 August 2019 and 1 September 2019, the electron density in the ionospheric F region reduced by a factor of 1.4 to 2.4 times as compared to the values on the reference day. The geospace storm gave rise to appreciable disturbances 25 also in the ionospheric E region, as well as in the Es layer. In the course of the ionospheric storm, the altitude of reflection of radiowaves could sharply increase from 150 km to 300–310 km. The geospace storm was accompanied by the generation of atmospheric gravity waves modulating the ionospheric electron density. For the 30 min period oscillation, the amplitude of the electron density disturbances could attain 40 %, while it did not exceed 6 % for the 15 min period. The results obtained have made a contribution to understanding of the geospace storm physics, to developing theoretical and empirical 30 models of geospace storms, to the acquisition of detailed understanding of the adverse effects that geospace storms have on radiowave propagation and to applying that knowledge to effective forecasting these adverse influences.

Storms have the potential to harm humans on the ground or in the near-Earth space environment. Modern society and human well-being become reliant more and more on space-based technologies, and consequently, on the state of space weather and geospace storms. The manifestations of geospace storms vary over the solar cycle, and depend on season, local time, latitude, longitude, and observational facilities. Therefore, there is an urgent need to study each sufficiently large geospace storm. Such an investigation reveals both general storm properties and its specific features. 70 The purpose of this paper is to present a general analysis of the 30 August-2 September, 2019 geospace storm, to analyze disturbances in the ionosphere and in the geomagnetic field, and to examine the influence of the ionospheric storm on the characteristics of the HF radio wave propagating over the People's Republic of China area. The main feature of this geospace storm is its duration, of up to four days.
In this paper, a brief description of the instrumentation and the techniques employed is presented first. This is 75 followed by a general analysis of the space weather state, the magnetic and ionospheric storms. Next, a description of the results of radio observations obtained at oblique incidence on the reference day and in the course of the geomagnetic storm is examined in detail. Finally, the results of analysis of the geomagnetic storm features are discussed, and the main results are listed.

Observational instruments
Fluxmeter magnetometer. The magnetometer is located at the Kharkiv V. N. Karazin National University Magnetometer Observatory (49. 64N, 36.93E). It acquires measurements of variations in the horizontal (H, D) geomagnetic field components in the 1-1000 s period range with a 0.5 s temporal resolution delivering 1 pT-1 nT sensitivity. The fluxmeter magnetometer is described in detail by Chernogor (2014) and Chernogor and Domnin (2014). Multi-frequency multipath system involving the software-defined technology for the oblique incidence radio 90 sounding of the ionosphere. It is located at the Harbin Engineering University campus, the People's Republic of China (45.78N, 126.68E) (Chernogor et al., , b, c, 2020Guo et al., 2019aGuo et al., , b, c, 2020. The ionosphere is continuously monitored over fourteen radio paths utilizing emissions from broadcasting stations in the 5-10 MHz frequency range and located in Japan, the Russian Federation, Mongolia, the Republic of Korea, and the People's Republic of China (Fig. 1), the radio path lengths (Table 1) are found in the (1-2)  10 3 km distance range, and the signal reception and processing is 95 performed at the Harbin Engineering University.

Analysis techniques
The fluxmeter magnetometer data recorded initially on a relative scale have been converted into absolute values using the magnetometer transfer function. Then, temporal dependencies of the geomagnetic field have been subjected to the systems spectral analysis, which employs simultaneously the short-time Fourier transform, the wavelet transform using the Morlet 105 wavelet as a basis function, and the Fourier transform in a sliding window with a width adjusted to be equal to a fixed number of harmonic periods (Chernogor, 2008). Analysis of the obtained spectra follows. https://doi.org/10.5194/angeo-2020-57 Preprint. Discussion started: 3 September 2020 c Author(s) 2020. CC BY 4.0 License.

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The Radio Astronomy of the National Academy of Sciences of Ukraine three-axis fluxgate magnetomer has been used to control a general state of the geomagnetic field, and a specific signal processing procedure was not needed.
The data acquired by the multi-frequency multipath system for the oblique incidence radio sounding of the 110 ionosphere have been subjected to processing in detail, and the products included the universal time dependencies of the Doppler spectra, the main ray amplitude, A(t), and the Doppler shift of frequency, fD(t). Further, the fD(t) and A(t) were subjected to secondary processing to obtain the trends , and the spectra in the period range T  1-60 min and greater (Chernogor, 2008).

Analysis of the space weather state 115
The data retrieved from https://omniweb.gsfc.nasa.gov/form/dx1.html have been used to analyze the solar wind parameters.
On 29 August 2019, the proton density, nsw, exhibited an increase from 10 6 m -3 to 15  10 6 m -3 , and subsequently, a 5 decrease from 15  10 6 m -3 to 10 6 m -3 in the course of the next three days (Fig. 2). In the course of 28 and 29 August 2019 and of the first half of 30 August 2019, the solar wind bulk speed, Vsw, varied from 350 km s -1 to 500 km s -1 . After 12:00 UT on 30 August 2019 through about 01:00 UT on 1 September 2019, the Vsw value exhibited an increase from 400 km s -1 120 to 750 km s -1 . During almost four days, Vsw  600-750 km s -1 . Before 12:00 UT on 30 August 2019, the temperature, Tsw, of the solar wind particles was observed to be in the (1-2)  10 5 K range. After 12:00 UT on 30 August 2019, it showed an increase from 10 5 K to 4.4  10 5 K in the course of 24 h, and eventually, fluctuating, it exhibited a gradual decrease from 4.4  10 5 K to 10 5 K. As expected, the increases in nsw and Vsw gave rise to an increase in the solar wind dynamic pressure, from 0.2 nPa to 3 nPa. The East-West By and the North-South Bz components of the interplanetary magnetic field 125 exhibited fluctuations in the -3 nT to 8 nT and from -7 to 3 nT ranges, respectively. Since approximately 12:00 UT on 30 August 2019, the value of the Bz component remained predominantly negative. This indicated that the magnetic storm ensued. Over the following day (from 08:00 UT on 30 August 2019 to 07:00 UT on 3 September 2019), energy input per unit time, εA, from the solar wind into the Earth's magnetosphere occasionally increased to 14-15 GJ s -1 ; before the storm commencement, the εA value did not exceeded 1 GJ s -1 . 130 The Kp index values exhibited variations from 0 to 2 before the storm commencement, and from 2 to 5.7 over four days afterwards. Before the storm commencement, the Dst index was observed to fluctuate in the -10 nT to 6 nT range. At about approximately 12:00 UT on 30 August 2019, Dst  12 nT; from 10:00 UT to 14:00 UT, the storm commencement was observed to occur. After 20:00 UT on 30 August 2019, the Dst values began to show a gradual decrease to -55 nT, which was attained at about 06:00 UT on 1 September 2019; over this time period, the storm main phase was observed to occur. 135 After 06:00 UT on 1 September 2019, the storm transitioned to the recovery phase, which lasted for a few days. Thus, this magnetic storm had the longest duration observed over the last few years, but it was not the strongest, which is its main feature. A long duration ionospheric storm was expected to follow the longest duration magnetic storm. The geomagnetic and ionospheric storm features are described further in detail.  On 1 September 2019, approximately from 08:00 UT to 13:00 UT, a considerable, of up to 2-4 nT, increase in the level of fluctuations was also observed to occur. On 2 and 3 September 2019, the level of fluctuations also exhibited 165 occasional enhancements, of up to 1.5-2 nT, approximately 1 h in duration.

Analysis of ionospheric state
The state of the ionosphere has been analyzed in general using the data from two ionosondes. The first of these is located in the vicinity of the propagation paths used for obliquely sounding the ionosphere, viz, near the City Wakkanai (45.16N, 141.25E), Japan. To assess the ionospheric storm on the global scale, ionosonde data from the City of Moscow (55.47N, 170 37.30E), the Russian Federation, have been used (Fig. 5).

Data from ionosonde in Japan
Since 29 August 2019 to 3 September 2019, the minimum frequency, fmin, showed insignificant variations, from 1.4 MHz to 1.5 MHz. Only on 1 September 2019, the fmin was observed to exhibit spikes of up to 1.7-2 MHz.
The behavior of the E-layer critical frequency, foE(t), was observed to be approximately the same on all the days. 175 During the daytime, this frequency attained 2.9-3.2 MHz; in the local evening, it decreased to 1.8 MHz; during night, the foE was not observed, and in the course of three hours in the morning, it showed an increase from 1.8 MHz to 3 MHz.
The sporadic-E critical frequency, foEs, exhibited variations in a broad range of frequencies, from 3 MHz to 12- showed an increase from 100 km to 120 km. 185 The sporadic Es layer virtual height exhibited considerable fluctuations, from 80 km to 160-170 km.
We have not succeeded in obtaining reliable data on the virtual height, , of the F2 layer. Most likely, it varied from 200 km to 300 km.

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The critical frequency, foF2(t), of the F2 layer for the ordinary wave showed a decrease to 3 MHz during the 28/29 August 2019 night, which was followed by an increase to 4.5 MHz during the daytime, and even by an increase up to 5 MHz on 30 August 2019. During almost all local daytime on 31 August 2019, the foF2(t) was observed to be 0.7-1.1 MHz lower 205 than on 29 August 2019. On 31 August 2019, from 09:00 UT to 11:00 UT and from 12:00 UT to 15:00 UT, an increase in foF2(t) was observed to be 0.7-0.8 MHz. During night and in the morning on 1 September 2019, the foF2 values were observed to be 0.5-0.6 MHz lower than those observed on 2 September 2019; during the daytime, the difference between these frequencies did not exceeded 0. The radio station operating at 5,000 kHz is located in the People's Republic of China at a great-circle propagation path range, R, of 1,875 km from the receiver.
Approximately from 00:00 UT to 07:00 UT on 29 August 2019, i.e., during sunlit hours on the reference day, the 225 signal amplitude, A, was observed to be ~-70 dBV, and the Doppler shift of frequency in the main ray signal, fD(t), to be ~0. 0 Hz, as can be seen in Fig. 6. After sunset at ~07:00 UT, i.e., in the evening hours, the A showed a gradual increase of up to -40 dBV. The fD(t) values gradually decreased from 0 Hz to -(0.5-1) Hz. Approximately from 09:00 UT to 16:00 UT, the Doppler spectra were observed to significantly broaden, from -2.5 Hz to 2 Hz. On 30 August 2019, the fD(t) exhibited considerable, from -0.3 Hz to 0.4 Hz, variations during the 18:00 UT to 22:00 UT period. 230 On 31 August 2019, the fD(t) changed from -0.3 Hz to 0.3 Hz over the 12:00-18:00 UT period when quasi-periodic variations in the fD(t) took place with ~40 min period, T, and ~0.20-0.25 Hz amplitude, fDa. From 17:00 UT to 22:00 UT, the amplitude A(t) exhibited considerable, up to 15-20 dBV, variations.
On 1 September 2019, the fD(t) showed significant increase, from -1.8 Hz to 1.4 Hz, in the course of sunset in the ionosphere. The ionospheric storm effect was observed to occur from at least 10:00 UT to 19:00 UT. The amplitude A(t) was 235 observed to exhibit considerable, up to 20 dBV, variations during the 11:30-21:00 UT period. On 2 and 3 September 2019, the behavior of the Doppler spectra almost did not differ from that on the undisturbed day.

Hwaseong to Harbin radiowave propagation path
The 6,015 kHz transmitter is located in the Republic of Korea at an ~950 km distance from the receiver, and it did not operate from 00:00 UT to 03:40 UT. On 2 and 3 September 2019, the Doppler spectra and signal amplitudes did not exhibit considerable variations.

Chiba/Nagara to Harbin radiowave propagation path 255
The radio station operating at 6,055 kHz is located in Japan at an ~1,610 km range from the receiver. The signal transmissions were absent from 15:00 UT to 22:00 UT.
The Doppler spectra exhibited similar behavior on 29, 30, and 31 August 2019 (Fig. 8). From 06:00 UT to 15:00 UT, the spectra were observed to be spread; they occupied the -1.5 Hz to 1.5 Hz frequency range.
On 1 September 2019, the Doppler spectra exhibited behavior sharply different from that observed on the preceding 260 day. The spread was evident weakly; from 10:00 UT to 15:00 UT, the Doppler shifts of frequency exhibited sharp changes from -1.5 Hz to 1.3 Hz; the quasi-periodic process with the ~60 min and greater period, T, and the ~0.2 Hz and greater amplitude, fDa, became evident. On this day, the signal amplitude also exhibited considerable (up to 20 dBV) fluctuations.
On 3 September 2019, the Doppler spectrum spread was insignificant. The Doppler shift of frequency, fD(t), was 265 observed to be close to zero level most of the time.

Beijing to Harbin radiowave propagation path
The 6,175 kHz transmitter is located in the People's Republic of China at approximately 1,050 km range from the receiver.
On 29 and 30 August 2019, the Doppler spectra were characteristic of the single ray propagation; the second ray 270 appeared only sporadically (Fig. 9). The Doppler shift of frequency, fD(t), was observed to be close to zero level almost all the time, and the signal amplitude A(t)  -(30-40) dBV.
On 2 and 3 September, 2019, the Doppler spectra exhibited the behavior characteristic of the quiet ionosphere.

Goyang to Harbin radiowave propagation path
The radio station operating at 6,600 kHz is located in the Republic of Korea at a range, R, of ~910 km from the receiver.
From 05:00 UT to 08:50 UT, the Doppler measurements were not possible.
On 29 August 2019, the Doppler spectra represented the undisturbed state of the ionosphere. For the main ray, the Doppler shift of frequency fD(t)  0 Hz (Fig. 10). On 1 September 2019, over the 08:30-13:00 UT period, the fD(t) also showed significant variations, from -1.5 Hz to 0.7 Hz. The signal amplitude, A(t), fluctuated wildly, up to 30 dBV.

On 2 and 3 September 2019, the fD(t) and A(t)
showed virtually no change. The state of the ionosphere along the propagation path was quiet. 300

Ulaanbaatar to Harbin radiowave propagation path
The radio station operating at 7,260 kHz is located in Mongolia at an ~1,496 km range from the receiver. It was switched off from 05:00 UT to 07:00 UT and from 18:00 UT to 20:30 UT.
On 29 August 2019, the Doppler spectra showed that the propagation was more likely to occur along a single ray, the fD(t) varied virtually monotonically (Fig. 11). 305 On 30 August 2019, from 12:00 UT to 15:00 UT, the fD(t) exhibited quasi-periodic variations with 20 and 40 min periods, T, and with an ~0.1 Hz amplitude, fDa, for T  20 min and with fDa  0.3 Hz for T  40 min.
On 31 August 2019, the fD(t) fluctuated wildly and varied quasi-periodically with an ~20 min period, T, and an ~0.1 Hz amplitude, fDa, almost all the time; from 13:30 UT to 14:00 UT, it exhibited a sharp decrease from 0 Hz to -1.5 Hz, which was followed by a subsequent increase from -1.5 Hz to 0 Hz. 310 On 1 September 2019, during the 09:00-12:30 UT period, sharp changes in fD(t) became evident, from 0 Hz to -1.5 Hz and conversely.
Since 30 August 2019 through 2 September 2019, an increase in the frequency and level of fluctuations in signal amplitude were noted.

Yakutsk to Harbin radiowave propagation path
The 7,350 kHz transmitter is located in the Russian Federation at a range, R, of ~1,845 km from the receiver. Unfortunately, 320 the transmitter operated only over the 11:00-18:00 UT and 20:15-24:00 UT periods.

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On 31 August 2019, the Doppler spectra occupied the -1.5 Hz to 1.5 Hz range. The fD(t) varied quasi-periodically with an ~24 min period, T, and ~0.2 Hz amplitude, fDa. From 13:40 UT to 14:50 UT, the fD(t) exhibited a decrease in fD(t) from 0 Hz to -1.5 Hz, which was followed by an increase from -1.5 Hz to 0 Hz, while the amplitude showed a decrease by 330 10 dBV. From 15:00 UT to 16:00 UT, the excursion of fluctuations in A(t) attained 20 dBV.
On 2 and 3 September 2019, the behavior of fD(t) and A(t) represented the behavior of the quiet ionosphere. 335

Shijiazhuang to Harbin radiowave propagation path
The radio station operating at 9,500 kHz is located in the People's Republic of China at an ~1,310 km range, R, from the receiver.
On 29 and 30 August 2019, the behaviors of the Doppler spectra and signal amplitudes were similar. The ionosphere did not experience appreciable disturbances (Fig. 13). 340 On 31 August 2019, the Doppler spectra showed that the propagation is more likely to occur along a single ray. The fD(t) exhibited significant variations, from -1 Hz to 0.8 Hz. Quasi-periodic variations in fD(t) with an ~30 min period, T, and an ~0.3-0.5 Hz amplitude, fDa, became evident. From 17:00 UT to 20:25 UT, A(t)  -70 dBV, the signal amplitude was observed to be at the noise level. On 1 September 2019, the signal amplitude was also observed to be at the noise level during the 09:10-11:50 UT and 17:00-21:40 UT periods; during the rest of the time, fD(t)  0 Hz. 345 The behavior of the Doppler spectra and the signal amplitudes on 2 and 3 September, 2019 was characteristic of the undisturbed state of the ionosphere. Since fD(t)  0 Hz all the time, the radio wave was apparently reflected from the Es layer screening the ionospheric F region.

Hohhot to Harbin radiowave propagation path
The 9,520 kHz transmitter is located in the People's Republic of China at an ~1,340 km range from the receiver. The radio 350 station usually does not broadcast from 16:00 UT to 21:40 UT.
On 29 August 2019, considerable variations in the Doppler spectra, fD(t), and the signal amplitude, A(t), were observed to occur near the dusk and dawn terminators in the ionosphere (Fig. 14).
On 30 August 2019, significant variations in the Doppler spectra became evident from 14:00 UT to 16:00 UT.
On 2 and 3 September 2019, the ionosphere did not experience considerable disturbances.

Yamata to Harbin radiowave propagation path 360
The 9,750 kHz transmitter is located in Japan at an ~1,570 km range, R, from the receiver. The transmissions are usually absent from 16:00 UT to 22:00 UT.

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On 31 August 2019, a considerable, from -0.4 Hz to 0.8 Hz, increase of variations in fD(t) was observed to occur from 12:00 UT to 16:00 UT, while the fluctuations in the signal amplitude, A(t), were small, in the 10-15 dBV range.
On 2 and 3 September 2019, the fD(t) and A(t) exhibited behavior characteristic of the quiet days. 375

Beijing to Harbin radiowave propagation path
The radio station broadcasting at 9,830 kHz over an interval shorter than half of a day is located in the People's Republic of China at an ~1,050 km range, R, from the receiver. where εAmin = 10 GJ s -1 , have been introduced in (Chernogor and Domnin, 2014) and is used to measure the storm strength.
Substituting εAmax  15 GJ s -1 for the storm under study gives Gst  1.8. According to the classification of Chernogor and 390 Domnin (2014), this storm is minor. Assuming the storm length to be Δt  10 5 s, the energy entering the magnetosphere is found to be Est  1.5  10 15 J. Such a storm falls into the Geospace Storm Index 1 (GSSI1) type (Chernogor and Domnin, 2014).

Geomagnetic field effects
The effects in the geomagnetic field began to appear after 12:00 UT on 30 August 2019. Considerable effects in the 395 geomagnetic field occurred during the main phase of the magnetic storm, i. e., on 31 August 2019 and 1 September 2019.
Let us estimate the magnetic storm energy Ems and the power Pms, using the relation of Gonzalez et al. (1994): In accordance with the NOAA Space Weather Scale [http://www.sec.noaa.gov], this storm is classified as moderate.
In accordance with the classification system of Chernogor and Domnin (2014), magnetic storms with Kp = 5.0-5.9 are 410 classified as moderate, and their energy and power lie within the Ems  (1-5)  10 15 J and Pms  (6-22)  10 10 W limits, respectively.

Effects in geomagnetic field fluctuations
The universal time dependences of the horizontal components of the geomagnetic field in the 100-1000 s period range were subjected to the systems spectral analysis in the 100-1000 s period range. 415 The results of the spectral analysis for 29 August 2019, which could be considered as reference date, are presented in Fig. 17. The H-and D-component levels did not exceed 2-3 nT, while the spectra exhibited predominantly 600-900 s period oscillations.
On 31 August 2019, the day when the storm's main phase was observed, the H-and D-components attained 5-10 nT (Fig. 18). The spectra of the H-and D-components showed predominantly 300-400 s, 700-900 s and 400-600 s, 700-420 900 s period oscillations, respectively.
On 1 September 2019, the levels of the components remained the same as those on 31 August 2019. The 800-1000 s period oscillations were predominant in both components.

Disturbances in ionogram parameters 425
Variations in ionogram parameters observed with the Japan and Russian Federation ionosondes exhibit similar behaviors.
This suggests that the ionospheric storm under study occurred on a global scale.
The list of the main effects that accompanied the ionospheric storm include the following.
2. An increase in foEs from 3 MHz to 6-7 MHz from 05:00 UT to 08:00 UT on 31 August 2019. Estimation of a decrease in the electron density, N, during the ionospheric storm as compared to the electron density, N0, on the reference day has been made using the following relation: The dawn, daytime, and dusk N0/N ratio for 31 August 2019 were observed to be 1.8-2, 1.4, and 2.4, respectively. 450 The dawn and daytime N0/N ratio for 1 September 2019 was observed to be close to 1.56 and 1.16, respectively. Given the N0/N, the negative ionospheric index [Chernogor and Domnin, 2014]  manifested itself not only in the ionospheric F region, but also in the ionospheric E region, and in sporadic Es layer. decrease from 0 Hz to -(1-1.5) Hz, followed by an increase from the minimum value to 0 Hz. This duration of this effect was observed to be 50 to 60 min for different propagation paths. The sharp decrease in fD(t) followed by its increase to the initial value indicates that a rise in the reflection height occurred. A rise in the altitude can be estimated by using the following simplified relation:

Radio-wave reflection height variations
where c is the speed of light, ΔfDm is an fD maximum value, ΔT1 is the duration of a decrease in fD(t), ΔT is an overall duration of the variation in fD, 1 cos  , and 2 cos are values averaged over ΔT1 and ΔT-ΔT1, respectively, and  is an angle of incidence with respect to the vertical.
(2) that the altitude of reflection increases when ΔfDm < 0, and vice versa.
The expression in Eq.
The magnitudes of periods, of 15-60 min, and of the amplitudes Na suggest that the quasi-periodic variations in 495 fD(t) and N(t) launched atmospheric gravity waves (AGWs). It is well known that AGWs are generated in the auroral oval in the course of geospace storms and propagate to low latitudes (see, for example, Hajkowicz, 1991;Lei et al., 2008;Lyons et al., 2019). Thus, the generation of AGWs responsible for traveling ionospheric disturbances is also a manifestation of geospace storms.
The studies presented at this paper demonstrate conclusively that the multi-frequency multipath facility involving 500 the software-defined technology for sounding obliquely the ionosphere at the Harbin Engineering University is an effective means for investigating the influence of ionospheric storms on the characteristics of HF radio waves and the short-term variability of dynamic processes operating in the ionosphere.

Conclusions
1. The energy and power of the geospace storm have been estimated to be 1.5  10 15 J and 1.5  10 10 W, which means that 505 this storm is classified as weak. https://doi.org/10.5194/angeo-2020-57 Preprint. Discussion started: 3 September 2020 c Author(s) 2020. CC BY 4.0 License.
2. The energy and power of the magnetic storm have been estimated to be 1.5  10 15 J and 9  10 9 W, which means that this storm is classified as moderate. The storm's main feature is its main phase duration, of up to two days. The recovery phase was also long, no less than two days. 5. The geospace storm was accompanied by a moderate to strong negative ionospheric storm. In the course of the 31 515 August-1 September 2019 period, the electron density in the ionospheric F region exhibited a decrease by a factor of 1.4 to 2.4 times as compared to that on the reference day.
6. The geospace storm acted to notably disturb the ionospheric E region, as well as sporadic Es layer.
7. In the course of the ionospheric storm, the altitude of reflection of radio waves could exhibit sharp increases from 150 km to 300-310 km. 520 8. The geospace storm was accompanied by the generation of AGWs, which modulate the electron density in the ionosphere.
The amplitude of the disturbances in the electron density could attain 42 %, at 30 min period, while at 15 min period, it did not exceed 6 %.

Code availability 525
The doppler14.grc file contains the computer program code that generates the data from the raw data recorded by the multi-

Data availability
The raw data sets recorded by the multi-frequency multipath system at the Harbin Engineering University campus, the People's Republic of China (45.78 N,126.68 E) and discussed in this paper can be requested online at https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/86LHDC (Luo et al., 2020).