Substorm effects of ionosphere and HF propagation

Transcription

1 Radio Science, Volume 35, Number 5, Pages , September-October 00 Substorm effects of ionosphere and HF propagation D. V. B lagoveshchensky State University of Aerospace Instrumentation, St. Petersburg, Russia T. D. Borisova Arctic and Antarctic Scientific Research Institute, St. Petersburg, Russia Abstract. Four subauroral HF radio paths are investigateduring substorms of various intensities. The principal substorm effects are both an increase of the signal strength 1-2 hours before the substorm expansion phase and a change in the azimuth of the received signal. The latter is explained by increasing the intensities of irregularities caused by particle precipitation on the poleward edge of the main ionospheric trough. Model calculation of the signal amplitude variations during the substorm growth phase has been carried out. Excellent agreement between the model and observations is achieved when the critical frequency of the F2 layer is increased by several megacycles and the height of the F2 layer maximum is reduced by -40 km at the reflection point the nearesto the receiving center. Similar investigations may be useful for quantitative description of any ionospheric model within disturbed periods. 1. Introduction arrival angle, refraction, mode structure, and Doppler shift. Nonstationary processes on the Sun are the causes of The goal of this paper is to study the influences of disturbances in the auroral and subauroral ionosphere of some of the factors mentioned above during isolated the Earth, which, in turn, lead to irregular conditions of substorms (AEmax>_ 250 nt) on several signal parameters decametric radio wave propagation at the high latitudes. from a network of HF subauroral radio paths of Radio propagation conditions are not well understood different lengths and directions. The study concentrates because the situation is complicated by a large number on periods when the interplanetary magnetic field (IMF) of influencing factors. In particular, the influences component B < 0 in premidnight hours of winter of include (1) the main ionospheric trough (MIT), (2) The substorms have a growth phase duration of irregularities of ionosphere during magnetospheric 1-2 hours before the sharp beginning of the expansion storms or substorms, (3) changing gradients of phase (breakup), defined as T = 0 for this study. The ionization, and (4) day-to-day variability of the ionization distribution. All these processes affect the character of decametric radio wave propagation on the paths located in the range of invariant latitudes 50 ø- 75 ø. In doing so, the structure and location of the MIT and ionization gradients essentially depend on the intensity of the substorm or storm. According to results of some researchers [Basler et al., 1988; Cannon, 1989; Hunsucker, 1991, 1992; Blagoveshchensky et al., 1992; Milan et al., 1996, 1997, 1998], strong variations of signal parameters take place on HF radio paths during substormtime. Key radio wave parameters include signal strength, distortions, elevation and azimuth of duration of the substorms is typically an hour, followed by the recovery phase of 1-2 hours duration [Akasofu, 1968]. The moment T = 0 is determined with +5 min accuracy at a level of ( ) AEmax when sharp growth of AE index occurs up to maximum. Two related areas are addressed in the paper. First there is a presentation of levels of received electromagnetic field intensity and azimuthal angles of arrival from experimental data. Then the experimental results are compared with those of a computer model for the same conditions of radio wave propagation. The results are interpreted in terms of the physical mechanisms. Copyright 00 by the American Geophysical Union. Paper number 1998RS /00/1998RS Experiment Description Figure 1 shows the layout of the radio paths chosen specially for monitoring the substorm effects in the 1165

2 1166 BLAGOVESHCHENSKY AND BORISOVA: SUBSTORM EFFECTS OF THE IONOSPHERE LONO1TUDE (degrees) -70 -ljo O 4O 2O Figure 1. Scheme of radio paths with common receiving center in St. Petersburg; the dashed lines show the position of the main ionospheric trough by model [Halcrow and Nisbet, 1977] for Kp = 2; t = 30 LT and winter season. ionosphere. Their common receiving center is St. Petersburg (St.P), Russia (geographicoordinates 60øN, 30.7øE, geomagnetic coordinates 55.7øN, 108.1øE). Four paths are considered: Ottawa-St.P (a distance D of 6600 km and an operating frequency f of MHz), Havana-St.P (D = 8900 km, f= MHz), Quito- St.P (D-- 11,100 km, f= MHz), and London- St.P (D = 00 km, f= MHz). Figure 1 also shows the location of the main ionospheric trough calculated according to the model [Halcrow and Nisbet, 1977] for winter conditions, Kp = 2 (Kp is the planetary index of magnetic activity for 3 hours), and for Moscow local time t = 30 LT (LT = UT + 3 hours). This is about the center of the premidnight hours. Later local times were not considered because the radio propagation paths normally get into the MIT fully after midnight and the propagation conditions get worse. Calculations of the MIT location by the model [Halcrow and Nisbet, 1977] here satisfactorily coincide with calculations by means of the formula L/= Kp - 0.5t, where L/is the invariant latitude (trough minimum) and t is local time with respecto midnight [Rodger et al., 1992]. On all paths the azimuthal angles of radio wave arrival and signalevel were measured simultaneously every 3 min. Azimuthal deviations of arrival angles A ø (off-azimuthal) were determined from the great circle arcs, with positive A ø values corresponding to wave arrivals from northern directions. 3. Results of Observations on the HF Radio Paths Analysis of experimental data shows that the paths Ottawa-St.P and Havana-St.P are very similar in their temporal variations (Figure 2). Signal level E and A ø on the paths Quito-St.P and London-St.P (Figure 3) differ substantially from the Ottawa and Havana paths. In the lower parts of Figures 2 and 3 there are curves of AE index characterizing a substorm intensity for 3 days: the quiet day (q), January 14, 1982, AEm, nt; the weak disturbance (w), January 6, 1982, AEm, nt; and the strong disturbance (s), January 8, 1982, AEm,x nt. The difficulty in choosing these days was that (1) they must be near each other, (2) they must have substorms with different intensities, and (3) substorms must occur when St.P is in the premidnight

3 BLAGOVESHCHENSKY AND BORISOVA: SUBSTORM EFFECTS OF THE IONOSPHERE 1167, m - o- 3 - ' - o I I I I I Iii ; I I I 0 18 I _ I _. l I :: 18 19! _., T= T=0 Time (LT) (h) Figure 2. Data of measurements: E, signal level; A, azimuthal deviations of arrival angle; AE, index of geomagnetic activity. Results of numerical calculations: squares, signal level for January 6, 1982, weak disturbance (w); circles, azimuthal deviations for January 8, 1982, strong disturbance (s); asterisks, azimuthal deviations for January 6, 1982 (w); triangles, azimuthal deviations for January 14, 1982, quiet day (q). sector. The vertical lines in Figures 2 and 3 show T = 0 starts of the substorm expansion phases. The three selected examples are the best illustrations from a wider data set studied by Blagoveshchensky and Borisova [1995] as the substorms have sharp onsets, different intensities, and different values of the planetary index Kp of magnetic activity. Referring to Figure 2, the following patterns can be identified. First the signalevels E on the paths Ottawa- St.P and Havana-St.P increase by + 30 db shortly before T = 0 in comparison with a quiet period without substorms. This is particularly clear for the Ottawa-St.P and the Havana-St.P paths on January 6, The signal level rises from 2130 tmtil 22 LT, which coincides with the onset of substorm expansion phase T The growth of the signal level on January 8 (s) takes place from 1900 till 15 LT, i.e., T = 0. On January 8 (s) the signal level is raised over the quiet level for the path Quito-St.P from 1930 till 2115 LT and for the path London-St.P from 1950 till 2130 LT (see Figure 3), i.e., until T = 0. As a rule, the increase in signalevel is observeduring 1-2 hours

4 1168 BLAGOVESHCHENSKY AND BORISOVA: SUBSTORM EFFECTS OF THE IONOSPHERE on all paths. One possible explanation is that magnetospheric electric field penetrates to subauroral latitudes, thereby lifting the F2 layer and increasing the F2 layer ionization [Aarons and Rodger, 1991 ]. The sharp reduction of signal level observed at T-- 0 is connected with the transition from radio wave reflection to backscatter from ionospheric irregularities and not with signal absorption. This conclusion is made from analysis of riometer data as well as from experimental data [Blagoveshchensky and Borisova, 1995; Borisova et al., 1991; Gal erin et al., 1990], which showed that backscatter occurs chiefly on irregularities of the poleward edge of the main ionospheric trough (PET). This can be seen in Figure 2, where the rapid change of AO values takes place at the moment of sharp descent of the signal level: LT (s), 2100 LT (w), and 2145 LT (q). Also the relative A variations have mainly positive values; the scattered signals are received from the north directions, consistent with scatter from the PET [Blagoveshchensky and Borisova, 1995]. The deviations of AO values have the tendency to decrease with time on the northernmost 40- II '!l In i IllIn Ii I In Ill i Il] I ii I'" _ 411 IIIII IIIII III IIIII!. I I I I ot 0 <! - 18 o o o,.,.,, :..:, in- - - d i, i, i 19 2'0 :il 2'2,i i %... X / OO loo '0 T=o Time (LT) (h) Figure 3. Data of measurements: E, signalevel; Aft), azimuthal deviations of arrival angle; AE, index of geomagnetic activity. Results of numerical calculations: circles, azimuthal deviations for January 8, 1982, strong disturbance (s); triangles, azimuthal deviations for January 14, 1982, quiet day (q).

5 BLAGOVESHCHENSKY AND BORISOVA: SUBSTORM EFFECTS OF THE IONOSPHERE 1169 paths, Ottawa-St.P and Havana-St.P, as shown in Figure 2. This effect is explained by the PET displacement equatorwarduring the premidnight time. With increasing substorm intensification (growing Kp), as referring to Figure 2, the transition moment from the reflection to the backscatter, and jump of A, moves to the earlier hours as the PET removes equatorward with increasing activity [Halcrow and Nisbet, 1977; Gal?9erin et al., 1990]. For the paths Ottawa-St.P and Havana-St.P these changes recur at 2145 LT (q), 2100 LT (w), and LT (s). On the paths Quito-St.P and London-St.P (Figure 3), similar regularities are not found because the paths are not affected by the PET. With growth of the Kp index the A deviations northward have a more irregular character, indicating a growth of ionospheric irregularities during the substorm period. 4. Modeling the Effects of the Substorm layer in the ionosphere reflection area adjust to the receiving center [Blagoveshchensky et al., 1992]. The parameters used were an increase of the maximum of the critical frequency of the F2 layer, fof2, of Afo = MHz, and a height fall of the layer maximum, hmf2, of AhmF2 = - 40 km [Blagoveshchensky et al., 1992; Lvova, 1987], 1-2 hours before T = 0 [Blagoveshchensky et al., 1992; Bates, 1970]. Figure 2 demonstrates the calculated values of signal level on January 6 (w) for the radio path Ottawa-St.P with squares. Here during the substorm growth phase from 21 till 22 LT the additional increase of the fof2 frequency is described by a smooth function Afo (t, D) with maximum of 2 MHz at t = 22 LT at a distance of 1500 km away from the receiving center. Simultaneously, the decrease of hmf2 is given by a function AhmF2 (t, D) with the extremity of AhmF2 = -40km. It is shown that within these changes on the path Ottawa-St.P the propagation modes 2F2 and 3F2 are possible. For the regular ionospheric parameters for that time of day no solution of the wave propagation task is possible. As may be seen in Figure 2, the introduced changes in the F2 layer parameters give a high signal level at the receiving center before the substorm expansion phase that matches well to observed ones. During the substorm growth phase both model and To simulate the radio wave propagation during substorm events, it is necessary to have a realistic ionospheric model. For the undisturbed ionosphere we take the global ionospheric model constructed by unification of known models of separate ionospheric E, F1, and F2 layers and an interlayer valley [Chernyshov and Vasil'eva, 1975; Anufrieva and Shapiro, 1976; Rawer et a1.,1978]. Modeling the radio wave propagation parameters is performed within the scope of the approximation of two-scale resolution of the geometrical optics method. This method takes into observed levels exceed the regular signal level of the multihop mechanism up to db. Analysis of calculation results of the radio wave trajectories also has shown that on the considered paths the basic propagation mechanism during the daytime account the smooth horizontal ionospheric was a multihop 1 with reflection from the F2 layer. In inhomogeneities [Borisova et al., 1986]. Input the evening, wave capture into an ionospheric parameters for the model are (1) the geographical waveguide on the regular longitudinal gradients takes coordinates of the transmitter and receiver, (2) the radiation patterns of receiving and transmitting antennas, (3) the date and time, and (4) a measure of the prevailing geophysical conditions using Wolf number and magnetic activity level with given Kp index [Blagoveshchensky and Borisova, 1989]. From the raytracing calculation the mode types, arrival angles in the vertical plane, the group and phase paths, and the electromagnetic field strength have been determined. Calculations of the radio wave propagation are carried out for all paths under the geophysical conditions corresponding to those of the given experimento allow interpretion of the experimental data. The substorm growth phase influence on the place. When the terminator crosses the paths, signal reception after 00 LT is possible only because some radio waves escape from the waveguide on the night side according to our calculations. This leaking mechanism has been discussed earlier [Borisova et al., 1991]. The leakage arises either by refraction of the radio waves on the horizontal gradients of the electron concentratio near the PET or by scattering the radio waves by field-aligned auroral irregularities of the E and F layers. In the calculation of the azimuthal deviations for the period from 1900 till 2300 LT, the F2 layer parameters in a region of supposed field-aligned scattering are changed in conformity with the initial model of the propagation parameters was included in the model by ionosphere: hmf krn, fof2 = assuming that changes of plasma distribution of the F2 MHz. The range of possibl elevation angles according

6 ß 1170 BLAGOVESHCHENSKY AND BORISOVA: SUBSTORM EFFECTS OF THE IONOSPHERE to the trajectory calculations becomes narrow as time increases from 1900 till 2300 LT. Figure 2 shows the results of the A calculations together with the experimental measured values of A on the paths Ottawa-St.P and Havana-St.P for January 8 (s) and January 14 (q). There is a close concurrence of the observed and calculated data. Similar calculations of the azimuthal deviations were fulfilled for the path Quito- St.P with regard to field-aligned scattering on the PET irregularities. Referring to Figure 3, good agreement is again achieved. 5. Conclusions Decametric radio wave propagation during magnetospheric substorms was considered at four subauroral paths on the basis of experimental data and modeling results. The substorm effects manifest themselves, in the case of most observations, on all radio paths. As a rule there is a growth of signal level at the receiving center 1-2 hours before the substorm intensification (expansion phase). For description of signal amplitude variations during substorm growth phase, the increase of the J,F2 value by MHz and decreasing the F2 layer maximum height hmf2 by -40 km in the wave reflection point of the F region closesto the receiving center are assigned in the ionospheric model. During the expansion phase there is a northern bias in the azimuth of arrival of the signals, i.e., increased deviation from great circle propagation compared with quiet times. This has been attributed to the effects of ionospheric irregularities on the poleward edge of the main ionospheric trough. Excellent (very good) agreement between the model calculation and the observations for both the signal intensity and the angle of azimuth is achieved. Acknowledgment. The authors wish to thank A. S. Rodger for his help in preparing and evaluating this paper. References Aarons, J., and A.S. Rodger, The effect of electric field and ring current energy increases on F layer irregularities at auroral and subauroralatitudes, Radio Sci., 26, , Akasofu, S.I., Polar and Magnetospheric Substorms, D. Reidel, Norwell, Mass., Anufrieva, T.A., and B.S. Shapiro, Geometrical Parameters of the Ionospheric F2 Layer (in Russian), Nauka, Moscow, Basler, R.P., P.B. Bentley, G.H. Price, R.T. Tsunoda, and T.L. Wong, Ionospheric distortion of HF signals, Radio Sci., 23, , Bates, H. F., HF propagation through the auroral curtain, J. Geophys. Res., 75 (1), , Blagoveshchensky, D.V., and T.D. Borisova, Correction of the HF radio channel model with allowance of the solar and magnetic activity, Geomagn. Aeron. (in Russian), 29, , Blagoveshchensky, D.V., and T.D. Borisova, Main ionization trough parameters for ionosphere modeling by HF radio network observations, Adv. Space Res., 16 1), 65-68, Blagoveshchensky, D.V., L.V. Egorova, and V.M. Lukashkin, High-latitude ionospheric phenomena diagnostics by highfrequency radio wave propagation observations, Radio Sci., 27, , Borisova, T.D., A.N. Baranets, and Y.N. Cherkashin, A method of calculation of ray-tracing and energy parameters of the radio wave propagation on the long paths, in Ionospheric Radio Wave Propagation (in Russian), pp.12-, Nauka, Moscow, Borisova, T.D., A.N. Baranets, V.A. Bubnov, and N.F. Blagoveshchenskaya, Non- great circle effects for HF propagation at high latitudes, Radiophysics (in Russian), 34, , Cannon, P.S., Morphology of the high-latitude ionosphere and its implication for HF communication systems, IEE Proc., Part I., 136, 1-10, Chemyshov, D.V., and T.N. Vasil'eva, Prediction of the Maximum Critical Frequencies for W=10,50,100,150,0 (in Russian), Nauka, Moscow, Gal'perin, Y. I., L.D. Sivtseva, V.M. Philippov, and V.L. Khalipov, Subauroral Upper Ionosphere (in Russian), Nauka, Novosibirsk, Halcrow, B.H., and J.S. Nisbet, A model of b-2 peak electron densities in the main trough region of the ionosphere, Radio Sci., 12, , Hunsucker, R.D., Radio Techniques for Probing the Terrestrial Ionosphere, Springer-Verlag, New York, Hunsucker, R.D., Auroral and polar- cap ionospheric effects on radio propagation: A mini-review, IEEE Trans. Antennas Propag., 40, , Lvova, A.A., Height variations of ionospheric F2 layer maximum under influence of electric fields, Geomagn. Aeron. (in Russian), 27, , Milan, S.E., T.B. Jones, M. Lester, E.M. Warrington, and G.D. Reeves, Substorm correlated absorption on a 30 km HF propagation path, Ann. Geophys., 14, , Milan, S.E., T.B. Jones, and E.M. Warrington, Enhanced MUF propagation of HF radio waves in the auroral zone, J. Atmos. Sol. Terr. Phys., 59, , Milan, S.E., M. Lester, T.B. Jones, and E.M. Warrington, Observed variations in the available HF bandwidth on four high latitude point-to-point communication circuits during

7 BLAGOVESHCHENSKY AND BORISOVA: SUBSTORM EFFECTS OF THE IONOSPHERE 1171 periods of geomagnetic disturbance, J. Atmos. Sol. Terr. Phys., 60, , Rawer, K., S. Ramakrishnan, and D. Bilitza, International Re rence Ionosphere, COSPAR, URSI, Brussels, Rodger, A.S., R.J. Moffett, and S. Quegan, The role of ion drift in the formation of ionization troughs in the mid- and high-latitude ionosphere: A review, J. Atmos. Terr. Phys., 54, 1-30, D. V. Blagoveshchensky, State University of Aerospace Instrumentation, 67 Bolshaya Morskaya Str., St. Petersburg, Russia. (dvb aanet.ru) T. D. Borisova, Arctic and Antarctic Scientific Research Institute, 38 Bering Str., St. Petersburg, Russia. (borisova aari.nw.ru) (Received June 8, 1998; revised December 28, 00; accepted May 5, 00.)