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Results of radio occultation measurement of polar ionosphere at satellite-to-satellite paths during strong flare solar activity
ARTICLE IN PRESS Acta Astronautica 67 (2010) 315–323
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Results of radio occultation measurement of polar ionosphere at satellite-to-satellite paths during strong flare solar activity O.I. Yakovlev a,, J. Wickert b, A.G. Pavelyev a, G.P. Cherkunova a, V.A. Anufriev a a b
Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, Russian Federation GeoForschugsZentrum, Potsdam, Germany
a r t i c l e in fo
abstract
Article history: Received 25 September 2009 Received in revised form 24 December 2009 Accepted 15 February 2010 Available online 1 April 2010
Features in the amplitude and phase variations of the radio wave propagating through the lower polar ionosphere in radio communication links between CHAMP and GPS satellites are observed. Parameters of sporadic layers and altitude profiles of electron density in the lower polar ionosphere during intensive solar flares are analyzed. It is shown, that the radio occultation technique is effective to find a relationship in a chain of following phenomena: the arrival of a shock wave of solar wind; the protons and electrons precipitation from the radiation belt; the small-scale plasma irregularities excitation in the F region of ionosphere; the intensive sporadic ES layers appearance in the lower ionosphere. & 2010 Published by Elsevier Ltd.
Keywords: Satellite Ionosphere Radio occultation method
1. Introduction Efficiency of the ionospheric monitoring by the radio occultation technique using the radio links satellite-tosatellite has been shown elsewhere (e.g. [1,2] and references therein). However investigation of the lower ionosphere is difficult because the radio ray twice time intersects the upper ionosphere which influence should be accounted for. It is known that the lower polar ionosphere is often strongly modified due to influence of different kinds of the solar activity leading to occurrence of sporadic ES layers, generation of the plasma irregularities in a wide spatial range, and to significant changes in the electron density. The aim of this paper is to reveal changes in the night-side of the lower polar ionosphere caused by influence of the solar flare by use of the data of radio occultation measurements. To achieve this aim it is necessary to determine the altitude profiles of the electron density Ne(h), to find the characteristics of sporadic structures and plasma irregularities. We used
Corresponding author.
E-mail address: [email protected] (O.I. Yakovlev). 0094-5765/$ - see front matter & 2010 Published by Elsevier Ltd. doi:10.1016/j.actaastro.2010.02.017
the results of 327 radio occultation events relevant to the latitudes higher 611 for the period from October 25 till November 9, 2003 when intensive solar activity has been observed. The lower ionosphere in the selected areas was not illuminated and the effects of ultraviolet and X-ray component of solar radiation were strongly reduced. The GPS satellites have near-circular orbit with an altitude of about 20 180 km and with an inclination to the equator of 551. The CHAMP satellite has circular orbit with a height of about 420 km and an inclination 871. The minimal altitude of the radio ray perigee H decreases from 130 km (the beginning of measurements) till HE30 km (the end of analysis of ionospheric effects) due to the movement of satellite CHAMP. The registration of the amplitude E and the phase excess path c of two coherent signals at the L-band frequencies f1 =1.575 GHz and f2 = 1.228 GHz was carried out during the radio occultation investigation of the ionosphere. The conditions of measurements, the satellite’s orbits, the technique of the analysis of ionospheric amplitude variations and the excess phase path of radio wave, the method of determining of the altitude profiles of electron density in the lower ionosphere have been described in papers [3–5]. Therefore we will derive those features of the lower polar
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relationship s2A ¼ s2 -s2min and carried out smoothing the temporal dependence s2A by means of the running window technique with time lag 6 h. The results of such processing are shown by curve in Fig. 1. This curve separates more clearly the time intervals between the amplitude fluctuations (and, consequently, the intensity of small-scale plasma irregularities) decrease and increase. In this figure the vertical dotted lines show the time of arrival to the Earth of the five intensive shock waves of the solar wind obtained from data [6,7]. The frequency spectra of amplitude fluctuations G(F) give an additional information about the plasma irregularities. In the interval 0.2C18 Hz of the fluctuation frequency F the experimental spectra G(F) can be approximated by a power law dependence G F n and also a spectral index n can be defined. From the theory of radio wave propagation in statistically irregular media it follows the connection between the index n and the index of power law in spatial spectrum of the small scale plasma irregularities p =n + 1 [8]. According to our data the spectral index of the plasma irregularities p =3.5 70.3 during strong increase in the amplitude fluctuations of radio wave. Fluctuation frequency F, the scale of the plasma irregularities L and the crossing speed of radio ray V are connected by the relationship L = VF–1. Therefore our experimental data correspond to the scales from Lmax E40 km till Lmin E0.2 km.
ionosphere which are most sensitive to influence of the solar flare activity.
2. Fluctuations of radio wave and ionospheric plasma irregularities To determine the intensity of small-scale irregularities in the ionospheric plasma the random amplitude fluctuations of radio wave dE were analyzed. Fluctuations of dE were processed by use of a standard statistical procedure to obtain the root mean square of amplitude deviations s. It is necessary to note that the fluctuations dE are caused mainly due to the plasma irregularities located in the ionospheric F layer with altitudes hE200C300 km. For analysis of fluctuations dE the results of amplitude registration were used in the H=50C70 km interval of the ray perigee altitudes. The analysis has shown that s has variations with the minimal and maximal values smin E 1.5% and smax E20%, respectively. The minimal value smin is related to the receiver noise influence. Values of the root mean square of amplitude fluctuations s corresponding to the period from October 26 till November 9, 2003 are shown by points in Fig. 1; dates and times for each of the measurements (indicated by points) are shown on the horizontal axis. Radio occultation events have different positions of the radio ray path relative to the Pole. At the beginning of this period the fluctuation intensity was s E2% and later at night October 27 the initial increase in fluctuations was observed. The fluctuations seemed the largest about 10%C14% during October 29–31. From November 1 the fluctuation intensity decreased and from November 4 once again the increase of the value s was registered. For a more objective determination of the amplitude fluctuations s an additional data processing was carried out. We excluded a minor influence of always present small fluctuations with magnitude smin = 1.5% by use of the
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3. Sporadic structures In the polar regions the sporadic ES structures are characterized by significant vertical gradients of electron density on their boundaries. The boundaries of ES are clearly seen in the dependence of the amplitude E and variations of the phase path Dc on the minimum of the ray perigee altitude H. In Fig. 2A an example of variations E(H) and Dc(H) due to the influence of thin ES layer is
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given. The dotted lines show the heights of upper H1 and lower H2 boundaries of the sporadic layer, the thickness DH = H1–H2 of such structures is usually about of 3C4 km. Such thin ES layers are similar to those ones for the equatorial ionosphere which were described in [5]. If the ES have an increased electron density and a considerable thickness DH= 15C20 km then the changes of E(H) and Dc(H) become similar to the same one presented in Fig. 2B. Such layers are specified by strong changes of E and Dc, they often have a complex structure. The vertical and horizontal resolution in the direction perpendicular to the line of sight GPS-CHAMP satellites is determined by the size of the first Fresnel zone scale, comparable with the Fresnel’s zone size of about 1.5 km. The rough estimate for the spatial resolution along the line of sight is of about 7350 km. Histograms of distributions H1 and H2 are shown in Fig. 3A and B, respectively. In 29% of the cases of detection of the ES structures their upper boundary was located in a
narrow range H1 = 9672 km, and in 71% of the cases, the altitude H1 varied over wide limits from 94 to 114 km. In 30% of the cases of detection of the ES structures their lower boundary H2 was located in an interval 84 72 km and in 70% of the cases it is located in an interval from 82 to 98 km. Note, that significant horizontal inhomogeneity of the sporadic ES structures small values of the altitude H2 may be connected with absence of the spherical symmetry of the ionosphere. In Fig. 3 for 8% of cases the small values H2 in the interval H2 =76C80 km are indicated. These values are not reliable because influence of the horizontal gradients in the ionosphere. ‘‘The thickness of a layer’’ defined as the difference DH=H1–H2 gives an obvious idea of the degree of perturbation in the polar ionosphere. This difference is a simple quantitative characteristic of ES structures. The histogram of distribution DH is shown in Fig. 3C. Thin layered structures with thickness DHE4 km were observed in 29% of the cases. In the altitude dependence of the amplitude of radio wave a
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characteristic change maximum–minimum–maximum is observed when the height of the ray perigee H is decreasing. Sporadic formations occupying a broad height interval with DH=10C26 km are present in 58% of cases. In some radio occultations almost all lower ionosphere from the height 80 km was disturbed, with formally determined values DH reaching 30C40 km. In Fig. 4 gives the data on the variability of the layer thickness DH over the studied period. The vertical dotted lines (similar to Figs. 1 and 7) and the digits 1–5 show the time of detection of five intense shock waves of the solar wind. It is seen that after the arrival of a shock wave, perturbations, whose thickness DH increases up to 8C19 km, appear in the altitude profile of the electron density and then intense ES structures with DHZ8 km are observed for 20C50 h. It should be emphasized that according to occultation measurements, the ES structures were not observed in the same period in other polar regions, or their thickness was less than 2 km.
4. Electron density For determining the Ne(h) profile for ho130 km by the two-frequency radio occultation method, one should
know the model dependence of the electron density Nm(h) at altitudes h4130 km, which was specified using the IRI-2001 ionospheric model [9]. It is important that the choice of the Nm(h) model influences the determination of the experimental dependence Ne(h) in the upper range of altitudes at h= 120C130 km, and this factor becomes insignificant at ho120 km. For reliable determination of the dependence Ne(h), it is important that two conditions are fulfilled. During radio occultation, the radio wave pass through the entire ionosphere, and a variation in its characteristics in any part of the ray line influences the possibility and accuracy of reconstruction of the altitude profile Ne(h) in the investigated region. The variability of the ionospheric altitude profile Ne(h) along the entire link manifests itself, first of all, as variations in the eikonal, which is proportional to the integral electron density and, therefore, as the appearance of false features of the experimental profile Ne(h). To avoid such inaccuracies, it was important to ensure that for the ray perigee altitude H=55C75 km the experimental dependence Ne(h) converges to zero on the average and has small fluctuations. This is the first condition of reliable determination of the dependence Ne(h). We assumed that the found dependences Ne(h) were qualitative if the variations in Ne(h) in the mentioned interval were a factor of 4C6 less than Ne(h) for h =100 km. The second important condition is that a local spherically symmetric distribution of the electron density is realized in a particular radio occultation session. It is known that the polar ionosphere is strongly inhomogeneous and has significant horizontal gradients and that the mentioned condition can be fulfilled only approximately. For analysis of experimental data, we used a simplified procedure of selection of the ionosphere sounding sessions in which the condition of a spherically symmetric distribution Ne(h) is fulfilled. To this end, we compared the dependences of the signal amplitude E(H), which was normalized to the level for free propagation of radio wave, and the phase-path increment variations Dc(H), which were found after the exclusion of the regular trend in the dependence c(H). The regular trend was determined by smoothing the temporal dependence c(t) by 51-points running window, which corresponded to averaging over a temporal interval of about 1 s. The smoothed out values of c(H) were subtracted from the measured ones, and in such a way we extracted the phase-path excess Dc(H) stipulated by the ionospheric structures with small vertical scales. If after such a processing the dependence Dc(H) was similar to amplitude variations, then we assumed that the condition of local spherical symmetry of the distribution Ne(h) in the given measurement session is fulfilled and the altitude profile Ne(h) can be determined with the use of the Abel transform. For determining the dependences Ne(h), we used the ionospheric radio occultation sessions in which two above-mentioned conditions were fulfilled. When the amplitude and phase variations are compared, the location of the upper H1 and lower H2 boundaries of the ionospheric sporadic structures can also be determined reliably. We now analyze electron density variations in the period in which intense shock waves of the solar wind
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came to the Earth. Patterns of variations in the altitude profile Ne(h) during the action of several extremely intense shock waves of the solar wind are shown in Fig. 5 and 6. The day, month, universal time, latitude and longitude of the region are specified in Figs. 5 and 6. At first (see Fig. 5), from 17:00 UT in October 29, 2003 to 14:00 UT in October 30, 2003 the quiet state of the ionosphere was recorded. In that period, a weak peak with the electron density Ne(h)E(2C3) 104 cm–3 was observed at altitudes 86C95 km, and the electron
density amounted to about 104 cm–3 at h =110 km, which is close to measurement errors for Ne(h) under quiet ionospheric conditions. Then, at 14:35 UT in October 30, 2003, an abrupt increase in Ne(h) was recorded in the ionosphere at h483 km, and the electron density reached 2.2 105 and 1.6 105 cm–3 at altitudes 90 and 100 km, respectively. The maximum values of Ne(h), according to Fig. 5, were recorded at 06:40 UT in October 31, 2003 when the electron density reached 4.5 105 cm–3 at altitudes 95C100 km. High values of Ne(h) were
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recorded till 23:10 UT in October 31, 2003 (see Fig. 6). Intense fluctuations of the phase path were observed, which led to the impairment of the accuracy of determination of Ne(h). Namely, the error could reach 3 104 cm–3. According to Fig. 6, by 05:46 UT in November 1, 2003 the electron density at altitudes 90C100 km decreased down to (2C6) 104 cm–3 and remained low till 08:51 UT in those days. For the quiet state of the ionosphere and a low electron density, the errors of determination of Ne amounted to about 104 cm–3. The similar pattern was obtained when the next shock wave came to the Earth. At 06:13 UT in November 4, 2003, the quiet state of the ionosphere was recorded, for which the electron density Ne at altitudes 90 and 100 km amounted to 3 104 and 2 104 cm–3, respectively. Then, at 14:00 UT, the electron density at altitudes 100C110 km was slightly elevated and at 18:00 UT in November 4, 2003 Ne increased drastically. The electron density reached 2 105 cm–3 at these altitudes. By 10:54 UT in November 5, 2003, the ionosphere reached the quiet state, and Ne decreased down to 2 104 cm–3 at altitudes h= 90C100 km, which is close to the limit of accuracy of its determination. About one hour later, at 11:00 UT in November 5, 2003, a weak sporadic layer appeared at an altitude of 95 km, and Ne amounted to 3 104 cm–3 at an altitude of 100 km. The data analysis showed that a strong increase in the electron density was observed in close time, both in the northern and southern polar regions, as is seen in Figs. 5 and 6. Note that in the period of strong increase in Ne(h), low values of the electron density were recorded sometimes in close polar regions, which is
related to the spatial inhomogeneity and rapid variability of the polar ionosphere. It was much more difficult to define an approximate characteristic of ionization ‘‘intensity’’ of the lower ionosphere. As a conventional ‘‘intensity’’ we elected the difference I(90)= i(90)–i(70) where i(90) and i(70) are values of the total electron content along the link satellite-to-satellite at the altitude of the ray perigee H= 90 and 70 km. We express the difference of total electron content in units of TECU, 1 TECU= 1016 m–2. The results of the definition of ‘‘intensity’’ I(90) in the period in question are shown in Fig. 7, where one can see that the powerful ionization increase in the lower ionosphere was observed in October 30, 31 and November 4, 6, 2003. In 20% of the cases small values I(90)=(0.6C1)TECU were observed, in 74% of the cases values I(90)= (2C12)TECU were observed and in 6% of the cases very big values I(90)= (16C36)TECU were registered. The errors of determination of I(90) amounted to about 71 TECU.
5. Connection nightside polar ionosphere with the manifestations of solar activity Let us consider the connection between variations of the characteristics of the lower polar ionosphere with the manifestations of solar activity. For this aim we will use the information given in [6,7,10] about speed and density of solar wind, on polar lights, on flows of charged particles and riometric absorption and will compare them with the dependencies shown in Figs. 1, 4 and 7. It is seen from these
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figures that the high-latitude ionosphere is characterized by small values of a s, DH and I(90) till 05 UT October 28. According to the data from the works mentioned above a small shock wave was noticed in 05 UT October 28 2003 and this event is noted by number 1 in Figs. 1, 4 and 7. The arrival of this shock wave brought about practically simultaneous increase of s, DHN I(90) lasting till 13 UT. The event 1 was the cause of the increase of riometric absorption of radio wave till 2.5 dB which was observed in the Kulpisiarvy observatory in Finland (69.051N, 20.791E) from 13.30 till 14.20 UT. The period from 29 till 31 October is characterized by high values s, DH and I(90) that are, in our opinion, due to the appearance of two successive strong shock waves (events 2, 3). According to the data [6,7,10] the shock wave 2 which speed exceeded 2000 km s 1 and the plasma density reach 15 cm 3 came to the Earth at 05:58 UT October 29. With effect of this wave we connect the increase of the total electron content I(90), the increase of radio wave fluctuations s and the increase of thickness of ES formations DH which began soon after the arrival of the shock wave and continued till the end of October 29. This wave caused the magnetic storm covering all the high-latitude ionosphere and strong increase of riometric absorption in different riometer sites. In the observatory Kulpisiarvy the riometric absorption began growing practically simultaneously with coming of the shock wave, it reached the maximum value 14 dB and continued till 11 UT with the following smooth fall up to 16 UT. In 06 UT the maximum riometric absorption about 16 dB with decrease to the values typical for the quiet ionosphere till 14 UT with the following quick increase in absorption about 20 UT of 30 October was observed in the observatory of Tiksi (71.651N, 128.81E) in Russia [10]. Further long-time increase in absorption at the beginning of the October 30 was observed in this observatory. Aurora [10] was registered simultaneously at 14:33 UT October 29 in Zhigansk (66.791N, 123.361E) and Maimage (62.01N, 129.731E) by Russian observatories. At 16:19 UT a shock wave 3 came with the speed 1950 km c–1 and density 7 cm–3. In Figs. 1, 4 and 7 it is
seen that the amplitude fluctuations of the radio wave increased and then the total electron content I(90) and the thickness of ES increase abruptly after some hours after this events. Increase of the riometric absorption was registered in 20:00 UT in Kilpisjarve and in 17:20 UT in Tiksi during this event. The aurora in Maimage started in 18:30 UT and were followed by two bright bursts in the interval 19:30C21:10 UT. According to the data [6] the increase of the riometric absorption corresponds to almost every increase of luminescence of polar lights testifying precipitation of the electrons with energy 10 keV and higher. Let us compare our materials with the ionosondes data. In work [10] the data of changing of the critical frequency at vertical sounding of the ionosphere are given for the period from October 28, 2003 till November 3, 2003. These data were obtained during ground-based observations in Norilsk (69.31N, 88.21E) and Jakutsk (62.01N, 129.71E). The authors notice that in Norilsk in the evening October 28 the reflections disappeared due to a total absorption which lasted almost three days. Sometimes ES layers of the auroral type with large frequencies of radio wave reflection appear for a short time. From the work mentioned it follows that from about 16:00 UT October 28, 2003 till 2:00 UT November 01, 2003 the absorption of radio wave was so strong that the radiosonde in Norilsk did not give information on the ionosphere. There were only two temporal intervals (from 16 to 18 UT in October 29, 2003 and from 14 to 20 UT in October 31, 2003) when reflections from the ES layers with a critical frequency of 6C8 MHz were observed. These critical frequencies correspond to the electron density values (4.4C8.0) 105 cm–3. It follows from Figs. 5 and 6 of our paper that in about the same period, a strong increase in the electron density, which reached 4.5 105 cm–3, took place and the appearance of altitude-extended ES structures of thickness DH= 8C16 km was recorded at altitudes 95C110 km using the radiooccultation method. The existence of connection between
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arrival of the shock wave, increasing of the root mean square of the amplitude fluctuations s and total electron content I(90), and appearing ES is observed even after October 30. The arrival of a weak shock wave (event 4) is registered according to the data [6,7] at 06:00 UT November 4. Its effect heightened the electron density in the lower ionosphere and led to increase of radio wave fluctuations till the end November 4. Event 4 was accompanied by the rise in the riometric absorption in 10:00 UT in the observatory Kulpisiarvy. In 20:20 UT November 6 a more weak shock wave reached the Earth (event 5). Its arrival has also been accompanied by increase of s, DH and I(90) and the burst of riometric absorption. These phenomena have been observed practically simultaneously. Comparison of these data indicates the efficiency of the radio occultation method for determining ionospheric characteristics in the periods of strong solar flares. It follows from comparison of this work with results published in [6,7,10] that there exists a chain of events: the arrival of the shock wave-particles precipitation from the radiation belts-increase of the intensity of small-scale plasma irregularities in F region and the electron density and appearance of the extended sporadic ES in the lower ionosphere.
6. Conclusions In this work and in publications [3–5] it is shown that radio occultation technique at links satellite-to-satellite are effective for the investigation of the lower ionosphere and small-scale plasma irregularity in the F region. The method allows to define the parameters the sporadic formations: the height of lower and upper boundaries, the thickness of the disturbed ES layer and to find altitude profiles of electron density. It has a good resolution on the height: boundaries and the thickness of ES being observed in error of the order of 71 km. According to our data two types of ES are seen in the polar regions. Thin layered formations with thickness of 2C3 km with small values of the electron density change similar to those observed in the equatorial ionosphere and were investigated in work [5] are characteristic for the first type. They appear to be related to wind shear. ES of the second type occupy a large region of altitudes and have thicknesses DH= 8C25 km, a strong increase of the electron density is its characteristic. These structures appear almost after arrival of intensive shock waves of the solar wind accompanied by precipitation of particles with high energy in the ionosphere. From our data it follows that there is a stable linkage between the solar wind shock wave coming to the Earth and the appearance of sporadic formations, and the increase of the electron density in the low night ionosphere and the intensity of the small-scale irregularities of F region. Radio occultation of the ionosphere along the satellite-tosatellite links is effective for finding out the next phenomena: the arrival of the shock wave—particles precipitation from the radiation belts—increase of the intensity of small-scale plasma irregularities in F region and the electron density, and appearance of the extended sporadic ES in the lower ionosphere.
During period of our investigations CHAMP satellite provided about of 200 radio occultation sessions in different regions. This is a cause of relatively rare attendance in the investigated night polar region. In the present time there are several radio occultation missions: FORMOSAT-3/COSMIC-GPS, GRACE-GPS, METOP-GPS, which produced about of 3000 radio occultations per day. This allows one to construct the map of the geographical distribution of the ES structures [11,12]. We intend to use these new possibilities for more detailed investigations of the sporadic ES layers with a global coverage. During investigations of such kind it is necessary to separate two different phenomena: the wind shear effect which produces thin ES structures and influence of the precipitation of energetic particles that increase the ionization of the lower ionosphere and produces intense and extended in the vertical direction ES layers. It is necessary also to account for the strong horizontal plasma inhomogeneity in the ES structures which leads to violation of the spherical symmetry in the electron density distribution Ne(h). This may produce systematic errors in the retrieved vertical distribution Ne(h) and in determination of the top and bottom boundaries of layers. This effect of horizontal inhomogeneity has been analyzed in [13,14] and it was shown that the actual height of the sporadic layers should higher than the ray perigee altitude. So the radio occultation data in the presence of the horizontal ionospheric gradients give systematically lower values for height of layers.
Acknowledgments We would like to thank Stanislav Sergeevich Matyugov for participation in our investigation and helping in preparation of this paper. References [1] N. Jakowski, R. Leitinger, M. Angling, Annals of Geophysics Supplement to 47, no. 2/3 (2004) 1049–1057. [2] P. Spalla, N. Jakowski, A. Wehrenpfennig, P. Spencer, C. Reigber (Eds.), First CHAMP Mission Results, Springer, 2003, pp. 545–556. [3] O.I. Yakovlev, J. Wickert, S.S. Matyugov, V.A. Anufriev, Radiowave fluctuations in the polar ionosphere on the satellite-to-satellite paths under high solar activity, Radiophysics and Quantum Electronics 49 (2006) 167–174. [4] O.I. Yakovlev, V.A. Anufriev, J. Wickert, S.S. Matyugov, Potentialities of radio-occultation monitoring of the lower ionosphere on satellite-to-satellite paths, Journal of Communication Technology and Electronics 53 (2008) 155–162. [5] O.I. Yakovlev, J. Wickert, A.G. Pavelyev, S.S. Matyugov, V.A. Anufriev, Sporadic structures in equatorial ionosphere as revealed from GPS occultation data, Acta Astronautica 63 (2008) 1350–1359. [6] I.S. Veselovsky, M.I. Panasyuk, S.I. Avdyushin, et al., Solar and heliospheric phenomena in October–November 2003: causes and effects, Cosmic Research 42 (5) (2004) 435–488. [7] M.V. Eselevich, V.G. Eselevich, Sporadic plasma streams and their sources in the period of extraordinary solar activity from October 26 to November 6, 2003, Cosmic Research 42 (6) (2004) 571–583. [8] A. Ishimaru, Wave Propagation and Scattering in Random Media, vol. 2, Academic press, 1978, P. 312. [9] D. Bilitza, International reference ionosphere, Radio Science 36 (2) (2001) 261–275. [10] G.A. Zherebtsov, O.M. Pirog, N.M. Polekh, et al., The ionospheric situation in the Eastern Asian longitudinal sector during the
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geoactive period October–November 2003, Geomagnetism and Aeronomy 45 (1) (2005) 101–108. [11] J. Wickert, A.G. Pavelyev, Y.A. Liou, T. Schmidt, Ch. Reigber K. Igarashi, A.A. Pavelyev, S.S. Matyugov, Amplitude variations in the GPS signals as a possible indicator of the ionospheric structures, Geophysical Research Letters 31 (24) (2004) 1–4 L24801. [12] A.G. Pavelyev, Y.A. Liou, J. Wickert, T. Schmidt, A.A. Pavelyev, S.F. Liu, Effects of the ionosphere and solar activity on radio occultation signals: application to CHAllenging Minisatellite Payload satellite
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observations, Journal of Geophysical Research 112 (A06326) (2007) 1–14. [13] A.G. Pavelyev, Y.A. Liou, J. Wickert, T. Schmidt, A.A. Pavelyev, S.S. Matyugov, Phase acceleration: a new important parameter in GPS occultation technology, GPS Solutions 14 (1) (2010) 3–14, doi:10.1007/s10291-009-0128-1. [14] A.G. Pavelyev, Y.A. Liou, J. Wickert, A.L. Gavrik, C.C. Lee, Eikonal acceleration technique for studying of the Earth and planetary atmospheres by radio occultation method, Geophysical Research Letters 36 (2009) 1–5.
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Results of radio occultation measurement of polar ionosphere at satellite-to-satellite paths during strong flare solar activity O.I. Yakovlev a,, J. Wickert b, A.G. Pavelyev a, G.P. Cherkunova a, V.A. Anufriev a a b
Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, Russian Federation GeoForschugsZentrum, Potsdam, Germany
a r t i c l e in fo
abstract
Article history: Received 25 September 2009 Received in revised form 24 December 2009 Accepted 15 February 2010 Available online 1 April 2010
Features in the amplitude and phase variations of the radio wave propagating through the lower polar ionosphere in radio communication links between CHAMP and GPS satellites are observed. Parameters of sporadic layers and altitude profiles of electron density in the lower polar ionosphere during intensive solar flares are analyzed. It is shown, that the radio occultation technique is effective to find a relationship in a chain of following phenomena: the arrival of a shock wave of solar wind; the protons and electrons precipitation from the radiation belt; the small-scale plasma irregularities excitation in the F region of ionosphere; the intensive sporadic ES layers appearance in the lower ionosphere. & 2010 Published by Elsevier Ltd.
Keywords: Satellite Ionosphere Radio occultation method
1. Introduction Efficiency of the ionospheric monitoring by the radio occultation technique using the radio links satellite-tosatellite has been shown elsewhere (e.g. [1,2] and references therein). However investigation of the lower ionosphere is difficult because the radio ray twice time intersects the upper ionosphere which influence should be accounted for. It is known that the lower polar ionosphere is often strongly modified due to influence of different kinds of the solar activity leading to occurrence of sporadic ES layers, generation of the plasma irregularities in a wide spatial range, and to significant changes in the electron density. The aim of this paper is to reveal changes in the night-side of the lower polar ionosphere caused by influence of the solar flare by use of the data of radio occultation measurements. To achieve this aim it is necessary to determine the altitude profiles of the electron density Ne(h), to find the characteristics of sporadic structures and plasma irregularities. We used
Corresponding author.
E-mail address: [email protected] (O.I. Yakovlev). 0094-5765/$ - see front matter & 2010 Published by Elsevier Ltd. doi:10.1016/j.actaastro.2010.02.017
the results of 327 radio occultation events relevant to the latitudes higher 611 for the period from October 25 till November 9, 2003 when intensive solar activity has been observed. The lower ionosphere in the selected areas was not illuminated and the effects of ultraviolet and X-ray component of solar radiation were strongly reduced. The GPS satellites have near-circular orbit with an altitude of about 20 180 km and with an inclination to the equator of 551. The CHAMP satellite has circular orbit with a height of about 420 km and an inclination 871. The minimal altitude of the radio ray perigee H decreases from 130 km (the beginning of measurements) till HE30 km (the end of analysis of ionospheric effects) due to the movement of satellite CHAMP. The registration of the amplitude E and the phase excess path c of two coherent signals at the L-band frequencies f1 =1.575 GHz and f2 = 1.228 GHz was carried out during the radio occultation investigation of the ionosphere. The conditions of measurements, the satellite’s orbits, the technique of the analysis of ionospheric amplitude variations and the excess phase path of radio wave, the method of determining of the altitude profiles of electron density in the lower ionosphere have been described in papers [3–5]. Therefore we will derive those features of the lower polar
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relationship s2A ¼ s2 -s2min and carried out smoothing the temporal dependence s2A by means of the running window technique with time lag 6 h. The results of such processing are shown by curve in Fig. 1. This curve separates more clearly the time intervals between the amplitude fluctuations (and, consequently, the intensity of small-scale plasma irregularities) decrease and increase. In this figure the vertical dotted lines show the time of arrival to the Earth of the five intensive shock waves of the solar wind obtained from data [6,7]. The frequency spectra of amplitude fluctuations G(F) give an additional information about the plasma irregularities. In the interval 0.2C18 Hz of the fluctuation frequency F the experimental spectra G(F) can be approximated by a power law dependence G F n and also a spectral index n can be defined. From the theory of radio wave propagation in statistically irregular media it follows the connection between the index n and the index of power law in spatial spectrum of the small scale plasma irregularities p =n + 1 [8]. According to our data the spectral index of the plasma irregularities p =3.5 70.3 during strong increase in the amplitude fluctuations of radio wave. Fluctuation frequency F, the scale of the plasma irregularities L and the crossing speed of radio ray V are connected by the relationship L = VF–1. Therefore our experimental data correspond to the scales from Lmax E40 km till Lmin E0.2 km.
ionosphere which are most sensitive to influence of the solar flare activity.
2. Fluctuations of radio wave and ionospheric plasma irregularities To determine the intensity of small-scale irregularities in the ionospheric plasma the random amplitude fluctuations of radio wave dE were analyzed. Fluctuations of dE were processed by use of a standard statistical procedure to obtain the root mean square of amplitude deviations s. It is necessary to note that the fluctuations dE are caused mainly due to the plasma irregularities located in the ionospheric F layer with altitudes hE200C300 km. For analysis of fluctuations dE the results of amplitude registration were used in the H=50C70 km interval of the ray perigee altitudes. The analysis has shown that s has variations with the minimal and maximal values smin E 1.5% and smax E20%, respectively. The minimal value smin is related to the receiver noise influence. Values of the root mean square of amplitude fluctuations s corresponding to the period from October 26 till November 9, 2003 are shown by points in Fig. 1; dates and times for each of the measurements (indicated by points) are shown on the horizontal axis. Radio occultation events have different positions of the radio ray path relative to the Pole. At the beginning of this period the fluctuation intensity was s E2% and later at night October 27 the initial increase in fluctuations was observed. The fluctuations seemed the largest about 10%C14% during October 29–31. From November 1 the fluctuation intensity decreased and from November 4 once again the increase of the value s was registered. For a more objective determination of the amplitude fluctuations s an additional data processing was carried out. We excluded a minor influence of always present small fluctuations with magnitude smin = 1.5% by use of the
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3. Sporadic structures In the polar regions the sporadic ES structures are characterized by significant vertical gradients of electron density on their boundaries. The boundaries of ES are clearly seen in the dependence of the amplitude E and variations of the phase path Dc on the minimum of the ray perigee altitude H. In Fig. 2A an example of variations E(H) and Dc(H) due to the influence of thin ES layer is
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given. The dotted lines show the heights of upper H1 and lower H2 boundaries of the sporadic layer, the thickness DH = H1–H2 of such structures is usually about of 3C4 km. Such thin ES layers are similar to those ones for the equatorial ionosphere which were described in [5]. If the ES have an increased electron density and a considerable thickness DH= 15C20 km then the changes of E(H) and Dc(H) become similar to the same one presented in Fig. 2B. Such layers are specified by strong changes of E and Dc, they often have a complex structure. The vertical and horizontal resolution in the direction perpendicular to the line of sight GPS-CHAMP satellites is determined by the size of the first Fresnel zone scale, comparable with the Fresnel’s zone size of about 1.5 km. The rough estimate for the spatial resolution along the line of sight is of about 7350 km. Histograms of distributions H1 and H2 are shown in Fig. 3A and B, respectively. In 29% of the cases of detection of the ES structures their upper boundary was located in a
narrow range H1 = 9672 km, and in 71% of the cases, the altitude H1 varied over wide limits from 94 to 114 km. In 30% of the cases of detection of the ES structures their lower boundary H2 was located in an interval 84 72 km and in 70% of the cases it is located in an interval from 82 to 98 km. Note, that significant horizontal inhomogeneity of the sporadic ES structures small values of the altitude H2 may be connected with absence of the spherical symmetry of the ionosphere. In Fig. 3 for 8% of cases the small values H2 in the interval H2 =76C80 km are indicated. These values are not reliable because influence of the horizontal gradients in the ionosphere. ‘‘The thickness of a layer’’ defined as the difference DH=H1–H2 gives an obvious idea of the degree of perturbation in the polar ionosphere. This difference is a simple quantitative characteristic of ES structures. The histogram of distribution DH is shown in Fig. 3C. Thin layered structures with thickness DHE4 km were observed in 29% of the cases. In the altitude dependence of the amplitude of radio wave a
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characteristic change maximum–minimum–maximum is observed when the height of the ray perigee H is decreasing. Sporadic formations occupying a broad height interval with DH=10C26 km are present in 58% of cases. In some radio occultations almost all lower ionosphere from the height 80 km was disturbed, with formally determined values DH reaching 30C40 km. In Fig. 4 gives the data on the variability of the layer thickness DH over the studied period. The vertical dotted lines (similar to Figs. 1 and 7) and the digits 1–5 show the time of detection of five intense shock waves of the solar wind. It is seen that after the arrival of a shock wave, perturbations, whose thickness DH increases up to 8C19 km, appear in the altitude profile of the electron density and then intense ES structures with DHZ8 km are observed for 20C50 h. It should be emphasized that according to occultation measurements, the ES structures were not observed in the same period in other polar regions, or their thickness was less than 2 km.
4. Electron density For determining the Ne(h) profile for ho130 km by the two-frequency radio occultation method, one should
know the model dependence of the electron density Nm(h) at altitudes h4130 km, which was specified using the IRI-2001 ionospheric model [9]. It is important that the choice of the Nm(h) model influences the determination of the experimental dependence Ne(h) in the upper range of altitudes at h= 120C130 km, and this factor becomes insignificant at ho120 km. For reliable determination of the dependence Ne(h), it is important that two conditions are fulfilled. During radio occultation, the radio wave pass through the entire ionosphere, and a variation in its characteristics in any part of the ray line influences the possibility and accuracy of reconstruction of the altitude profile Ne(h) in the investigated region. The variability of the ionospheric altitude profile Ne(h) along the entire link manifests itself, first of all, as variations in the eikonal, which is proportional to the integral electron density and, therefore, as the appearance of false features of the experimental profile Ne(h). To avoid such inaccuracies, it was important to ensure that for the ray perigee altitude H=55C75 km the experimental dependence Ne(h) converges to zero on the average and has small fluctuations. This is the first condition of reliable determination of the dependence Ne(h). We assumed that the found dependences Ne(h) were qualitative if the variations in Ne(h) in the mentioned interval were a factor of 4C6 less than Ne(h) for h =100 km. The second important condition is that a local spherically symmetric distribution of the electron density is realized in a particular radio occultation session. It is known that the polar ionosphere is strongly inhomogeneous and has significant horizontal gradients and that the mentioned condition can be fulfilled only approximately. For analysis of experimental data, we used a simplified procedure of selection of the ionosphere sounding sessions in which the condition of a spherically symmetric distribution Ne(h) is fulfilled. To this end, we compared the dependences of the signal amplitude E(H), which was normalized to the level for free propagation of radio wave, and the phase-path increment variations Dc(H), which were found after the exclusion of the regular trend in the dependence c(H). The regular trend was determined by smoothing the temporal dependence c(t) by 51-points running window, which corresponded to averaging over a temporal interval of about 1 s. The smoothed out values of c(H) were subtracted from the measured ones, and in such a way we extracted the phase-path excess Dc(H) stipulated by the ionospheric structures with small vertical scales. If after such a processing the dependence Dc(H) was similar to amplitude variations, then we assumed that the condition of local spherical symmetry of the distribution Ne(h) in the given measurement session is fulfilled and the altitude profile Ne(h) can be determined with the use of the Abel transform. For determining the dependences Ne(h), we used the ionospheric radio occultation sessions in which two above-mentioned conditions were fulfilled. When the amplitude and phase variations are compared, the location of the upper H1 and lower H2 boundaries of the ionospheric sporadic structures can also be determined reliably. We now analyze electron density variations in the period in which intense shock waves of the solar wind
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came to the Earth. Patterns of variations in the altitude profile Ne(h) during the action of several extremely intense shock waves of the solar wind are shown in Fig. 5 and 6. The day, month, universal time, latitude and longitude of the region are specified in Figs. 5 and 6. At first (see Fig. 5), from 17:00 UT in October 29, 2003 to 14:00 UT in October 30, 2003 the quiet state of the ionosphere was recorded. In that period, a weak peak with the electron density Ne(h)E(2C3) 104 cm–3 was observed at altitudes 86C95 km, and the electron
density amounted to about 104 cm–3 at h =110 km, which is close to measurement errors for Ne(h) under quiet ionospheric conditions. Then, at 14:35 UT in October 30, 2003, an abrupt increase in Ne(h) was recorded in the ionosphere at h483 km, and the electron density reached 2.2 105 and 1.6 105 cm–3 at altitudes 90 and 100 km, respectively. The maximum values of Ne(h), according to Fig. 5, were recorded at 06:40 UT in October 31, 2003 when the electron density reached 4.5 105 cm–3 at altitudes 95C100 km. High values of Ne(h) were
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recorded till 23:10 UT in October 31, 2003 (see Fig. 6). Intense fluctuations of the phase path were observed, which led to the impairment of the accuracy of determination of Ne(h). Namely, the error could reach 3 104 cm–3. According to Fig. 6, by 05:46 UT in November 1, 2003 the electron density at altitudes 90C100 km decreased down to (2C6) 104 cm–3 and remained low till 08:51 UT in those days. For the quiet state of the ionosphere and a low electron density, the errors of determination of Ne amounted to about 104 cm–3. The similar pattern was obtained when the next shock wave came to the Earth. At 06:13 UT in November 4, 2003, the quiet state of the ionosphere was recorded, for which the electron density Ne at altitudes 90 and 100 km amounted to 3 104 and 2 104 cm–3, respectively. Then, at 14:00 UT, the electron density at altitudes 100C110 km was slightly elevated and at 18:00 UT in November 4, 2003 Ne increased drastically. The electron density reached 2 105 cm–3 at these altitudes. By 10:54 UT in November 5, 2003, the ionosphere reached the quiet state, and Ne decreased down to 2 104 cm–3 at altitudes h= 90C100 km, which is close to the limit of accuracy of its determination. About one hour later, at 11:00 UT in November 5, 2003, a weak sporadic layer appeared at an altitude of 95 km, and Ne amounted to 3 104 cm–3 at an altitude of 100 km. The data analysis showed that a strong increase in the electron density was observed in close time, both in the northern and southern polar regions, as is seen in Figs. 5 and 6. Note that in the period of strong increase in Ne(h), low values of the electron density were recorded sometimes in close polar regions, which is
related to the spatial inhomogeneity and rapid variability of the polar ionosphere. It was much more difficult to define an approximate characteristic of ionization ‘‘intensity’’ of the lower ionosphere. As a conventional ‘‘intensity’’ we elected the difference I(90)= i(90)–i(70) where i(90) and i(70) are values of the total electron content along the link satellite-to-satellite at the altitude of the ray perigee H= 90 and 70 km. We express the difference of total electron content in units of TECU, 1 TECU= 1016 m–2. The results of the definition of ‘‘intensity’’ I(90) in the period in question are shown in Fig. 7, where one can see that the powerful ionization increase in the lower ionosphere was observed in October 30, 31 and November 4, 6, 2003. In 20% of the cases small values I(90)=(0.6C1)TECU were observed, in 74% of the cases values I(90)= (2C12)TECU were observed and in 6% of the cases very big values I(90)= (16C36)TECU were registered. The errors of determination of I(90) amounted to about 71 TECU.
5. Connection nightside polar ionosphere with the manifestations of solar activity Let us consider the connection between variations of the characteristics of the lower polar ionosphere with the manifestations of solar activity. For this aim we will use the information given in [6,7,10] about speed and density of solar wind, on polar lights, on flows of charged particles and riometric absorption and will compare them with the dependencies shown in Figs. 1, 4 and 7. It is seen from these
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figures that the high-latitude ionosphere is characterized by small values of a s, DH and I(90) till 05 UT October 28. According to the data from the works mentioned above a small shock wave was noticed in 05 UT October 28 2003 and this event is noted by number 1 in Figs. 1, 4 and 7. The arrival of this shock wave brought about practically simultaneous increase of s, DHN I(90) lasting till 13 UT. The event 1 was the cause of the increase of riometric absorption of radio wave till 2.5 dB which was observed in the Kulpisiarvy observatory in Finland (69.051N, 20.791E) from 13.30 till 14.20 UT. The period from 29 till 31 October is characterized by high values s, DH and I(90) that are, in our opinion, due to the appearance of two successive strong shock waves (events 2, 3). According to the data [6,7,10] the shock wave 2 which speed exceeded 2000 km s 1 and the plasma density reach 15 cm 3 came to the Earth at 05:58 UT October 29. With effect of this wave we connect the increase of the total electron content I(90), the increase of radio wave fluctuations s and the increase of thickness of ES formations DH which began soon after the arrival of the shock wave and continued till the end of October 29. This wave caused the magnetic storm covering all the high-latitude ionosphere and strong increase of riometric absorption in different riometer sites. In the observatory Kulpisiarvy the riometric absorption began growing practically simultaneously with coming of the shock wave, it reached the maximum value 14 dB and continued till 11 UT with the following smooth fall up to 16 UT. In 06 UT the maximum riometric absorption about 16 dB with decrease to the values typical for the quiet ionosphere till 14 UT with the following quick increase in absorption about 20 UT of 30 October was observed in the observatory of Tiksi (71.651N, 128.81E) in Russia [10]. Further long-time increase in absorption at the beginning of the October 30 was observed in this observatory. Aurora [10] was registered simultaneously at 14:33 UT October 29 in Zhigansk (66.791N, 123.361E) and Maimage (62.01N, 129.731E) by Russian observatories. At 16:19 UT a shock wave 3 came with the speed 1950 km c–1 and density 7 cm–3. In Figs. 1, 4 and 7 it is
seen that the amplitude fluctuations of the radio wave increased and then the total electron content I(90) and the thickness of ES increase abruptly after some hours after this events. Increase of the riometric absorption was registered in 20:00 UT in Kilpisjarve and in 17:20 UT in Tiksi during this event. The aurora in Maimage started in 18:30 UT and were followed by two bright bursts in the interval 19:30C21:10 UT. According to the data [6] the increase of the riometric absorption corresponds to almost every increase of luminescence of polar lights testifying precipitation of the electrons with energy 10 keV and higher. Let us compare our materials with the ionosondes data. In work [10] the data of changing of the critical frequency at vertical sounding of the ionosphere are given for the period from October 28, 2003 till November 3, 2003. These data were obtained during ground-based observations in Norilsk (69.31N, 88.21E) and Jakutsk (62.01N, 129.71E). The authors notice that in Norilsk in the evening October 28 the reflections disappeared due to a total absorption which lasted almost three days. Sometimes ES layers of the auroral type with large frequencies of radio wave reflection appear for a short time. From the work mentioned it follows that from about 16:00 UT October 28, 2003 till 2:00 UT November 01, 2003 the absorption of radio wave was so strong that the radiosonde in Norilsk did not give information on the ionosphere. There were only two temporal intervals (from 16 to 18 UT in October 29, 2003 and from 14 to 20 UT in October 31, 2003) when reflections from the ES layers with a critical frequency of 6C8 MHz were observed. These critical frequencies correspond to the electron density values (4.4C8.0) 105 cm–3. It follows from Figs. 5 and 6 of our paper that in about the same period, a strong increase in the electron density, which reached 4.5 105 cm–3, took place and the appearance of altitude-extended ES structures of thickness DH= 8C16 km was recorded at altitudes 95C110 km using the radiooccultation method. The existence of connection between
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arrival of the shock wave, increasing of the root mean square of the amplitude fluctuations s and total electron content I(90), and appearing ES is observed even after October 30. The arrival of a weak shock wave (event 4) is registered according to the data [6,7] at 06:00 UT November 4. Its effect heightened the electron density in the lower ionosphere and led to increase of radio wave fluctuations till the end November 4. Event 4 was accompanied by the rise in the riometric absorption in 10:00 UT in the observatory Kulpisiarvy. In 20:20 UT November 6 a more weak shock wave reached the Earth (event 5). Its arrival has also been accompanied by increase of s, DH and I(90) and the burst of riometric absorption. These phenomena have been observed practically simultaneously. Comparison of these data indicates the efficiency of the radio occultation method for determining ionospheric characteristics in the periods of strong solar flares. It follows from comparison of this work with results published in [6,7,10] that there exists a chain of events: the arrival of the shock wave-particles precipitation from the radiation belts-increase of the intensity of small-scale plasma irregularities in F region and the electron density and appearance of the extended sporadic ES in the lower ionosphere.
6. Conclusions In this work and in publications [3–5] it is shown that radio occultation technique at links satellite-to-satellite are effective for the investigation of the lower ionosphere and small-scale plasma irregularity in the F region. The method allows to define the parameters the sporadic formations: the height of lower and upper boundaries, the thickness of the disturbed ES layer and to find altitude profiles of electron density. It has a good resolution on the height: boundaries and the thickness of ES being observed in error of the order of 71 km. According to our data two types of ES are seen in the polar regions. Thin layered formations with thickness of 2C3 km with small values of the electron density change similar to those observed in the equatorial ionosphere and were investigated in work [5] are characteristic for the first type. They appear to be related to wind shear. ES of the second type occupy a large region of altitudes and have thicknesses DH= 8C25 km, a strong increase of the electron density is its characteristic. These structures appear almost after arrival of intensive shock waves of the solar wind accompanied by precipitation of particles with high energy in the ionosphere. From our data it follows that there is a stable linkage between the solar wind shock wave coming to the Earth and the appearance of sporadic formations, and the increase of the electron density in the low night ionosphere and the intensity of the small-scale irregularities of F region. Radio occultation of the ionosphere along the satellite-tosatellite links is effective for finding out the next phenomena: the arrival of the shock wave—particles precipitation from the radiation belts—increase of the intensity of small-scale plasma irregularities in F region and the electron density, and appearance of the extended sporadic ES in the lower ionosphere.
During period of our investigations CHAMP satellite provided about of 200 radio occultation sessions in different regions. This is a cause of relatively rare attendance in the investigated night polar region. In the present time there are several radio occultation missions: FORMOSAT-3/COSMIC-GPS, GRACE-GPS, METOP-GPS, which produced about of 3000 radio occultations per day. This allows one to construct the map of the geographical distribution of the ES structures [11,12]. We intend to use these new possibilities for more detailed investigations of the sporadic ES layers with a global coverage. During investigations of such kind it is necessary to separate two different phenomena: the wind shear effect which produces thin ES structures and influence of the precipitation of energetic particles that increase the ionization of the lower ionosphere and produces intense and extended in the vertical direction ES layers. It is necessary also to account for the strong horizontal plasma inhomogeneity in the ES structures which leads to violation of the spherical symmetry in the electron density distribution Ne(h). This may produce systematic errors in the retrieved vertical distribution Ne(h) and in determination of the top and bottom boundaries of layers. This effect of horizontal inhomogeneity has been analyzed in [13,14] and it was shown that the actual height of the sporadic layers should higher than the ray perigee altitude. So the radio occultation data in the presence of the horizontal ionospheric gradients give systematically lower values for height of layers.
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