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MET radio occultation experiment
Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
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Global sounding of sporadic E layers by the GPS=MET radio occultation experiment K. Hockea; ∗ ,K. Igarashia , M. Nakamuraa , P. Wilkinsonb , J. Wuc , A. Pavelyevd , J. Wickerte a Communications
Research Laboratory (CRL), Upper Atmosphere Section, Space Science Division, 4-2-1, Nukui Kitamachi, Koganei, Tokyo 184-8795, Japan b IPS Radio & Space Services, P.O. Box 1386, Haymarket, NSW 1240, Australia c China Research Institute of Radio Wave Propagation (CRIRP), P.O. Box 6301, Beijing 102206, China d Institute of Radio Engineering and Electronics of Russian Academy of Sciences (IRE RAS), 1, Vvedenskogo Square, Fryazino, 141120, Russia e GeoForschungsZentrum Potsdam (GFZ), Division of Kinematics & Dynamics of the Earth, Telegrafenberg, 14473 Potsdam, Germany Received 28 February 2001; received in revised form 13 July 2001; accepted 17 July 2001
Abstract The GPS radio occultation technique is sensitive for layered structures with horizontal scales of around 100 km and with vertical scales of a few 100 m or more at the Earth’s limb. These structures cause strong
1. Introduction The GPS=MET experiment consists of a GPS
earth orbit at around 730 km measuring the L1 and L2 signals from Global positioning system (GPS) satellites (Rocken et al., 1997). The measurement conAguration is depicted in Fig. 1, together with a geographic map of the observation sites of the Asia=Australia ionosonde chain. Phase path excess and amplitude variation of the received GPS signals contain valuable information on the atmospheric refractivity Aeld. Various applications of GPS=MET data have been already carried out ranging from derivation of high resolution temperature proAles in the
c 2001 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 1 ) 0 0 0 6 3 - 3
1974
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
Fig. 1. Measurement conAgurations. (a) observation sites of the Asia=Australia ionosonde chain for long-term observation of interhemispheric diJerences of ionospheric irregularities, (b) radio occultation technique: a GPS receiver onboard of a low earth orbit (LEO) satellite measures the phase paths of L1 and L2 radio signals from a GPS satellite behind the horizon in around 20; 000 km distance. The diJerence of the phase paths is proportional to TEC along the radio link and is represented as function of tangent point height h.
troposphere=stratosphere (Rocken et al., 1997), electron density proAles (Schreiner et al., 1999; Hajj and Romans, 1998), global ionospheric tomography (HernandKez-Pajares et al., 1998), stratospheric gravity waves (Tsuda et al., 2000; Steiner and Kirchengast, 2000), to global maps of tropospheric water vapor and geopotential height (Kursinski et al., 1995; Kursinski, 1997; Leroy, 1997). In the following, we investigate the application of radio occultation data of GPS=MET for derivation of sporadic E layers in the mesosphere and lower thermosphere. According to Mathews (1998) “sporadic E” is a name convention of the science community, and actually the name does not At to the thin ionization layers which often occur in a periodic, regular manner in the D, E and F1 regions. For generation of a dense sporadic E layer, ionospheric plasma of a large volume is swept together to a thin layer. This is possible by the combined eJect of neutral wind shears of tides and gravity waves in the upper atmosphere, ion-neutral collisions and the geomagnetic Aeld B yielding a v × B plasma drift (Whitehead, 1960, 1989). Besides this “wind shear” mechanism, which is most eJective at geomagnetic mid latitudes, large-scale convective electric Aelds of magnetospheric origin can generate sporadic E layers at high latitudes via the E × B plasma drift (Nygren et al., 1984; Bristow and Watkins, 1991, 1994). In addition, particle precipitation, low recombination rate of metallic ions of meteors, ice clouds and other ionization sources have been considered for explanation of the great variety and the high ionization densities of the observed layers (Thomas, 1996). Thus, sporadic E contains unique information on neutral wind variations and electrodynamical processes in the mesosphere and lower thermosphere. By observing the spatial and temporal behavior of the global sporadic E dis-
tribution, solar-terrestrial relationships, coupling of magnetosphere, ionosphere and neutral atmosphere can be studied as well as the energy and momentum exchange between lower, middle, and upper atmosphere by atmospheric waves. It is expected that sporadic E layers are a sensitive indicator for the impact of solar variability, decrease of geomagnetic Aeld and anthropogenic eJects on the upper atmosphere (e.g. cooling of upper atmosphere because of greenhouse eJect) (Rishbeth et al., 1996; Roble and Dickinson, 1989). On the other hand, global monitoring of sporadic E layers enables a better selection of perturbation-free radio links for communication and navigation (Whitehead, 1989; Aarons, 1997). The most detailed world maps of sporadic E have been possibly obtained by the huge ionosonde network operated during the International Geophysical Year in 1958. Taguchi and Shibata (1961) present average world maps of foEs for noon=midnight and for each month. These early maps may have some artifacts (e.g., over oceanic areas) due to insuOcient global coverage of ionosondes, but they are still a valuable orientation for worldwide sporadic E occurrence. Because of the increased knowledge on atmospheric processes and solar-terrestrial relationships, since 1961 we may And some explanations for the observed sporadic E distributions in 1958. 2. Data analysis The data analysis of this initial study on electron density layers requires 50 Hz L1, L2 phase data and orbit data of GPS and LEO satellite during radio occultations. The geometrical optic approach and single ray propagation are assumed. More elaborated data retrievals based on wave
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
optics are currently under construction (Hocke et al., 1999; Igarashi et al., 2000; Sokolovskiy, 2000; Gorbunov et al., 2000). It has been shown that wave optical solutions provide enhanced height resolution (reaching 100 m) and avoids errors due to multipath=diJraction eJects (Marouf et al., 1986; Mortensen et al., 1999; Karayel and Hinson, 1997). A simulation of GPS radio wave propagation through an ionosphere with arbitrary medium and large-scale electron density
1975
The diJerence of the L1 and L2 phase paths, S1 − S2 , is closely related to the total electron content (TEC) accumulated along the ray path 2 2 1 f 1 f2 TEC = (S1 − S2 ): (1) 40:3 f12 − f22 Units are (e=m2 ) for TEC, (Hz) for GPS L1 and L2 carrier frequencies f1 = 1:57542 GHz and f2 = 1:22760 GHz; and (m) for phase paths S1 and S2 . The GPS receiver measures changes of L1 and L2 phase paths by phase correlation adjustment of a reference wave (generated by a local oscillator) to the GPS L1 and L2 signal wave (phase-lock technique). Contrary to this simple measurement method, it is also possible to record the complete GPS signal spectrum by means of open-loop GPS receivers, which are now in development (e.g., Sokolovskiy, 2001). The advantages are that the shape of the signal line proAle can be analysed and multipath ray propagation do not cause failures in signal tracking (phase cycle slips and loss of signal). During a radio occultation, the ray tangent point moves downward with a geocentric radial velocity of about 2:5–3 km=s. A proAle of the lower ionosphere is taken within a short time of around 20 s. Radio occultations are not continuously recorded. A LEO satellite may collect a few hundred events (atmospheric proAles) per day, each time when one of the 24 GPS satellites rises or sets at the Earth’s horizon as viewed by the LEO satellite. Microlab-1 recorded only setting occultations, since it has just one GPS antenna in backward direction. In the following, the interesting small-scale
1976
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
Fig. 2. Average number of available GPS=MET occultation measurements (50 Hz high rate, 10◦ latitudinal box) as function of height (left-hand side). Average number of occultation measurements as function of geographic latitude (right-hand side). The occultations have been observed in 2–16 February 1997.
2–3, and the localization height can be wrong in individual case. Averaging of many
3. Results and discussion In Fig. 3, four examples are presented for phase path diJerences as function of height. The proAles are proportional to Qne . Geographic coordinates and time [UT ] are given in the viewgraph titles. In the upper left viewgraph an unusual smooth proAle is shown with a surprising peak at around 60 km height. Since the electron density is too low at these heights for formation of intense ionization layers, it is more likely that the GPS rays (with tangent point at around 60 km height) traversed an isolated small-scale perturbation at higher altitudes in the E- or F-region. In the upper right viewgraph, a double layer proAle is depicted while the lower left proAle gives some evidence for electron density bite outs (sudden decrease of electron density) at low
Fig. 3. Some proAles of atmospheric phase path
latitudes. The lower right proAle is typical for high latitudes. It seems that there are many irregularities=layers probably caused by particle precipitation, wind shears, and electric Aelds. As already mentioned, the error of individual proAles can be large, and further discussion of individual proAles is not meaningful since one cannot be sure enough for an individual case if the ionospheric inhomogeneity occurs at the tangent point or maybe at another part of the ray trajectory. Sokolovskiy (2000) has an appropriate method for localization of ionospheric irregularities in individual cases, but implementation of this method for automatic data processing seems to be diOcult. The average Aeld of sporadic E (Qne ) has been calculated as function of height (60–140 km) and geographic latitude and is depicted in Fig. 4 for the prime time from 19 June to 10 July 1995, having 1900 radio occultation events. In the present study, we use only data of those three prime times of GPS=MET which have the most radio occultation events. Figs. 5 and 6 shows the average sporadic E distribution along the meridian for the prime times 10 –25 October 1995, (1540 events), and 2–16 February 1997, (2690 events). A representation of sporadic E as function of geographic latitude has been favored rather than of geomagnetic latitude, since neutral dynamics is often the driving force of the layer formation process and the background ionization of the lower ionosphere mainly depends on solar radiation. Some features of sporadic E depending on geomagnetic inclination and magnetosphere–ionosphere interaction are clearer if sporadic E is shown as function of geomagnetic latitude. However, the essential characteristics of Figs. 4 – 6 do not change if geomagnetic latitude is taken instead of geographic latitude. It is obvious in the GPS=MET observations that sporadic E is in particular a phenomenon of the summer hemisphere as noted in a review paper by Whitehead (1989). Fig. 4
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
1977
Fig. 4. Sporadic E occurrence for 19 June to 10 July 1995 derived by using 1900 GPS=MET radio occultations. The average of electron density
Fig. 5. Same as Fig. 4, but for 10 –25 October 1995 (1540 radio occultation events).
shows enhanced sporadic E in June=July 1995 when summer was in the northern hemisphere, and in Fig. 6, sporadic E is increased in late summer 1997 of the southern hemisphere. Indeed ionosondes and radars also observe a dramatic seasonal dependence of sporadic E with a maximum in the summer months (e.g., Taguchi and Shibata, 1961; Haldoupis and Schlegel, 1996). According to Figs. 4 – 6, the maximal occurrence of sporadic E is at heights 90–110 km. At geo-
graphic high latitudes, layers frequently occur at lower altitudes around 90 km height and may correspond to polar mesospheric summer echoes (PMSE). There is also some evidence for intermediate layers beyond 110 km; which are best seen in Fig. 4 at 20–30◦ latitude. The distance of these layers is around 6 km and may indicate the vertical wavelength of the atmospheric wave forming these ionization layers.
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K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
Fig. 6. Same as Fig. 4, but for 2–16 February 1997 (2690 radio occultation events).
The enhanced occurrence of sporadic E at the equator with a maximum at around 100 km height is in agreement with other observations and the theory that sporadic E and plasma irregularities are probably associated to the equatorial electrojet and the gradient drift plasma instability (Whitehead, 1989). In summary, the GPS=MET data give a reasonable pattern of global occurrence of sporadic E with many interesting details and for diJerent seasons. Since our knowledge and experience of limb sounding of the ionosphere have just started, there is a need for comparison between results of the radio occultation technique and results of the well-approved ground-based radio techniques. Appropriate ground-based data are certainly the data of the Asia=Australia ionosonde chain. In Figs. 7 and 8 the average magnitude (foEs) and virtual height (h’Es) of sporadic E are depicted for February 1997 and June 1995. The viewgraphs can be compared to the GPS=MET observations in Figs. 4 and 6 by keeping in mind that the GPS=MET slices are averages over all longitude sectors. World maps of sporadic E shown by Taguchi and Shibata (1961) and Hocke and Tsuda (2001a) indicate that during southern hemisphere summer, sporadic E is mainly concentrated over South America. The main characteristic (enhanced sporadic E in the summer hemisphere) is present in the ground- and space-based observations. In particular, we And agreement for strong sporadic E in the northern hemisphere at around 30◦ latitude in June=July 1995 (Figs. 4 and 8). The virtual height measurement of ionosonde observations may be calibrated in future by using radio occultation observations. Ground-based observations are most useful for description of the tem-
Fig. 7. Ground-based observation of (a) the average magnitude foEs and (b) the average virtual height h’Es of sporadic E by the Asia=Australia ionosonde chain during February 1997 (please compare with Fig. 6).
poral behavior of sporadic E (e.g., periodicities caused by tides and planetary waves) and for Anding physical mechanisms of layer formation by multi-instrument campaigns (Mathews, 1998). 4. Concluding remarks The study of sporadic E layers observed by GPS=MET gives an impression of the high application potential of
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
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Acknowledgements We are grateful to C. Rocken and the GPS=MET team of University Corporation for Atmospheric Research (UCAR=Boulder) for providing the GPS=MET data. T. Koizumi (CRL) and the WDC for Ionosphere are acknowledged for the ionosonde data. T. Tsuda (RASC=Kyoto), M. Yamamoto (RASC) and T. Ogawa (STEL=Nagoya) are thanked for helpful discussions. We thank the reviewers for numerous improvements. The study has been partly performed while one of the authors (Klemens Hocke) has been at DLR (Neustrelitz), GFZ (Potsdam), and RASC (Kyoto). The Science and Technology Agency of Japan is thanked for providing a fellowship. Fig. 8. Ground-based observation of (a) the average magnitude foEs and (b) the average virtual height h’Es of sporadic E by the Asia=Australia ionosonde chain during June 1995 (please compare with Fig. 4).
GPS radio occultation measurements for research and monitoring of the mesosphere–thermosphere–ionosphere system. The measurements are complementary to vertical soundings of radars and ionosondes. The latter techniques are appropriate for observing the dynamic behavior of sporadic E layers (e.g., layer formation process, vertical layer descend with time, plasma instabilities), while GPS radio occultation provides global maps derived from snap shots of the atmosphere, enabling a new view on sporadic E by horizontal sounding. Analysis of small-scale
References Aarons, J., 1997. Global positioning system phase
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K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
atmosphere and retrieving meteorological parameters from radio occultation data. Earth, Planets and Space 52, 893–899. Karayel, E.T., Hinson, D.P., 1997. Sub-Fresnel-scale vertical resolution in atmospheric proAles from radio occultation. Radio Science 32, 411–423. Kursinski, E.R., 1997. The GPS radio occultation concept: theoretical performance and initial results. Ph.D. Thesis, Caltech, Pasadena. Kursinski, E.R., Hajj, G.A., Hardy, K.R., Romans, L.R., SchoAeld, J.T., 1995. Observing tropospheric water vapor by radio occultation using the global positioning system. Geophysical Research Letters 22, 2365–2368. Leroy, S.S., 1997. Measurement of geopotential heights by GPS radio occultation. Journal of Geophysical Research 102, 6971–6986. Marouf, E., Tyler, G.L., Rosen, P.A., 1986. ProAling Saturn’s rings by radio occultation. Icarus 68, 120–166. Mathews, J.D., 1998. Sporadic E: current views and recent progress. Journal of Atmospheric and Solar-Terrestrial Physics 60, 413–435. Mortensen, M.D., LinAeld, R.P., Kursinski, E.R., 1999. Vertical resolution approaching 100 m for GPS occultations of the Earth’s atmosphere. Radio Science 34, 1475–1484. Nygren, T., Jalonen, L., Oksman, J., Taurunen, T., 1984. The role of electric Aeld and neutral wind direction in the formation of sporadic E-layers. Journal of Atmospheric and Solar-Terrestrial Physics 46, 373–381. Rishbeth, H., Kohl, H., Barclay, L.W., 1996. A history of ionospheric physics and radio communications. In: Kohl, H., RUuster, R., Schlegel, K. (Eds.), Modern Ionospheric Science. European Geophysical Society, Katlenburg-Lindau, pp. 4–31. Roble, R.G., Dickinson, R.E., 1989. How will changes in carbon dioxide modify the mean structure of the mesosphere and thermosphere?. Geophysical Research Letters 16, 1441–1444.
Rocken, C., Anthes, R., Exner, M., Hunt, D., Sokolovskiy, S., Ware, R., Gorbunov, M., Schreiner, W., Feng, D., Herman, B., Kuo, Y.-H., Zou, X., 1997. Analysis and validation of GPS=MET data in the neutral atmosphere. Journal of Geophysical Research 102, 29,849–29,866. Schreiner, W.S., Sokolovskiy, S.V., Rocken, C., Hunt, D.C., 1999. Analysis and validation of GPS=MET radio occultation data in the ionosphere. Radio Science 34, 949–966. Sokolovskiy, S.V., 2000. Inversions of radio-occultation amplitude data. Radio Science 35, 95–106. Sokolovskiy, S.V., 2001. Modeling and inverting radio occultation signals in the moist troposphere. Radio Science 36, 441–458. Steiner, A., Kirchengast, G., 2000. Gravity wave spectra from GPS=MET occultation observations. Journal of Atmospheric and Oceanic Technology 17, 495–503. Taguchi, S., Shibata, H., 1961. World maps of foEs. Journal of the Radio Research Laboratories, Tokyo 8 (38=39), 355–386. Thomas, L., 1996. The coupling of the lower ionosphere with the mesosphere and lower thermosphere. In: Kohl, H., RUuster, R., Schlegel, K. (Eds.), Modern Ionospheric Science. European Geophysical Society, Katlenburg-Lindau, pp. 67–101. Tsuda, T., Nishida, M., Rocken, C., Ware, R.H., 2000. A global morphology of gravity wave activity in the stratosphere revealed by the GPS occultation data (GPS=MET). Journal of Geophysical Research 105, 7257–7267. Whitehead, J.D., 1960. Formation of the sporadic E layer in the temperate zones. Nature 188, 567. Whitehead, J.D., 1989. Recent work on mid-latitude and equatorial sporadic E. Journal of Atmospheric and Solar-Terrestrial Physics 51, 401–424.
www.elsevier.com/locate/jastp
Global sounding of sporadic E layers by the GPS=MET radio occultation experiment K. Hockea; ∗ ,K. Igarashia , M. Nakamuraa , P. Wilkinsonb , J. Wuc , A. Pavelyevd , J. Wickerte a Communications
Research Laboratory (CRL), Upper Atmosphere Section, Space Science Division, 4-2-1, Nukui Kitamachi, Koganei, Tokyo 184-8795, Japan b IPS Radio & Space Services, P.O. Box 1386, Haymarket, NSW 1240, Australia c China Research Institute of Radio Wave Propagation (CRIRP), P.O. Box 6301, Beijing 102206, China d Institute of Radio Engineering and Electronics of Russian Academy of Sciences (IRE RAS), 1, Vvedenskogo Square, Fryazino, 141120, Russia e GeoForschungsZentrum Potsdam (GFZ), Division of Kinematics & Dynamics of the Earth, Telegrafenberg, 14473 Potsdam, Germany Received 28 February 2001; received in revised form 13 July 2001; accepted 17 July 2001
Abstract The GPS radio occultation technique is sensitive for layered structures with horizontal scales of around 100 km and with vertical scales of a few 100 m or more at the Earth’s limb. These structures cause strong
1. Introduction The GPS=MET experiment consists of a GPS
earth orbit at around 730 km measuring the L1 and L2 signals from Global positioning system (GPS) satellites (Rocken et al., 1997). The measurement conAguration is depicted in Fig. 1, together with a geographic map of the observation sites of the Asia=Australia ionosonde chain. Phase path excess and amplitude variation of the received GPS signals contain valuable information on the atmospheric refractivity Aeld. Various applications of GPS=MET data have been already carried out ranging from derivation of high resolution temperature proAles in the
c 2001 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 1 ) 0 0 0 6 3 - 3
1974
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
Fig. 1. Measurement conAgurations. (a) observation sites of the Asia=Australia ionosonde chain for long-term observation of interhemispheric diJerences of ionospheric irregularities, (b) radio occultation technique: a GPS receiver onboard of a low earth orbit (LEO) satellite measures the phase paths of L1 and L2 radio signals from a GPS satellite behind the horizon in around 20; 000 km distance. The diJerence of the phase paths is proportional to TEC along the radio link and is represented as function of tangent point height h.
troposphere=stratosphere (Rocken et al., 1997), electron density proAles (Schreiner et al., 1999; Hajj and Romans, 1998), global ionospheric tomography (HernandKez-Pajares et al., 1998), stratospheric gravity waves (Tsuda et al., 2000; Steiner and Kirchengast, 2000), to global maps of tropospheric water vapor and geopotential height (Kursinski et al., 1995; Kursinski, 1997; Leroy, 1997). In the following, we investigate the application of radio occultation data of GPS=MET for derivation of sporadic E layers in the mesosphere and lower thermosphere. According to Mathews (1998) “sporadic E” is a name convention of the science community, and actually the name does not At to the thin ionization layers which often occur in a periodic, regular manner in the D, E and F1 regions. For generation of a dense sporadic E layer, ionospheric plasma of a large volume is swept together to a thin layer. This is possible by the combined eJect of neutral wind shears of tides and gravity waves in the upper atmosphere, ion-neutral collisions and the geomagnetic Aeld B yielding a v × B plasma drift (Whitehead, 1960, 1989). Besides this “wind shear” mechanism, which is most eJective at geomagnetic mid latitudes, large-scale convective electric Aelds of magnetospheric origin can generate sporadic E layers at high latitudes via the E × B plasma drift (Nygren et al., 1984; Bristow and Watkins, 1991, 1994). In addition, particle precipitation, low recombination rate of metallic ions of meteors, ice clouds and other ionization sources have been considered for explanation of the great variety and the high ionization densities of the observed layers (Thomas, 1996). Thus, sporadic E contains unique information on neutral wind variations and electrodynamical processes in the mesosphere and lower thermosphere. By observing the spatial and temporal behavior of the global sporadic E dis-
tribution, solar-terrestrial relationships, coupling of magnetosphere, ionosphere and neutral atmosphere can be studied as well as the energy and momentum exchange between lower, middle, and upper atmosphere by atmospheric waves. It is expected that sporadic E layers are a sensitive indicator for the impact of solar variability, decrease of geomagnetic Aeld and anthropogenic eJects on the upper atmosphere (e.g. cooling of upper atmosphere because of greenhouse eJect) (Rishbeth et al., 1996; Roble and Dickinson, 1989). On the other hand, global monitoring of sporadic E layers enables a better selection of perturbation-free radio links for communication and navigation (Whitehead, 1989; Aarons, 1997). The most detailed world maps of sporadic E have been possibly obtained by the huge ionosonde network operated during the International Geophysical Year in 1958. Taguchi and Shibata (1961) present average world maps of foEs for noon=midnight and for each month. These early maps may have some artifacts (e.g., over oceanic areas) due to insuOcient global coverage of ionosondes, but they are still a valuable orientation for worldwide sporadic E occurrence. Because of the increased knowledge on atmospheric processes and solar-terrestrial relationships, since 1961 we may And some explanations for the observed sporadic E distributions in 1958. 2. Data analysis The data analysis of this initial study on electron density layers requires 50 Hz L1, L2 phase data and orbit data of GPS and LEO satellite during radio occultations. The geometrical optic approach and single ray propagation are assumed. More elaborated data retrievals based on wave
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
optics are currently under construction (Hocke et al., 1999; Igarashi et al., 2000; Sokolovskiy, 2000; Gorbunov et al., 2000). It has been shown that wave optical solutions provide enhanced height resolution (reaching 100 m) and avoids errors due to multipath=diJraction eJects (Marouf et al., 1986; Mortensen et al., 1999; Karayel and Hinson, 1997). A simulation of GPS radio wave propagation through an ionosphere with arbitrary medium and large-scale electron density
1975
The diJerence of the L1 and L2 phase paths, S1 − S2 , is closely related to the total electron content (TEC) accumulated along the ray path 2 2 1 f 1 f2 TEC = (S1 − S2 ): (1) 40:3 f12 − f22 Units are (e=m2 ) for TEC, (Hz) for GPS L1 and L2 carrier frequencies f1 = 1:57542 GHz and f2 = 1:22760 GHz; and (m) for phase paths S1 and S2 . The GPS receiver measures changes of L1 and L2 phase paths by phase correlation adjustment of a reference wave (generated by a local oscillator) to the GPS L1 and L2 signal wave (phase-lock technique). Contrary to this simple measurement method, it is also possible to record the complete GPS signal spectrum by means of open-loop GPS receivers, which are now in development (e.g., Sokolovskiy, 2001). The advantages are that the shape of the signal line proAle can be analysed and multipath ray propagation do not cause failures in signal tracking (phase cycle slips and loss of signal). During a radio occultation, the ray tangent point moves downward with a geocentric radial velocity of about 2:5–3 km=s. A proAle of the lower ionosphere is taken within a short time of around 20 s. Radio occultations are not continuously recorded. A LEO satellite may collect a few hundred events (atmospheric proAles) per day, each time when one of the 24 GPS satellites rises or sets at the Earth’s horizon as viewed by the LEO satellite. Microlab-1 recorded only setting occultations, since it has just one GPS antenna in backward direction. In the following, the interesting small-scale
1976
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
Fig. 2. Average number of available GPS=MET occultation measurements (50 Hz high rate, 10◦ latitudinal box) as function of height (left-hand side). Average number of occultation measurements as function of geographic latitude (right-hand side). The occultations have been observed in 2–16 February 1997.
2–3, and the localization height can be wrong in individual case. Averaging of many
3. Results and discussion In Fig. 3, four examples are presented for phase path diJerences as function of height. The proAles are proportional to Qne . Geographic coordinates and time [UT ] are given in the viewgraph titles. In the upper left viewgraph an unusual smooth proAle is shown with a surprising peak at around 60 km height. Since the electron density is too low at these heights for formation of intense ionization layers, it is more likely that the GPS rays (with tangent point at around 60 km height) traversed an isolated small-scale perturbation at higher altitudes in the E- or F-region. In the upper right viewgraph, a double layer proAle is depicted while the lower left proAle gives some evidence for electron density bite outs (sudden decrease of electron density) at low
Fig. 3. Some proAles of atmospheric phase path
latitudes. The lower right proAle is typical for high latitudes. It seems that there are many irregularities=layers probably caused by particle precipitation, wind shears, and electric Aelds. As already mentioned, the error of individual proAles can be large, and further discussion of individual proAles is not meaningful since one cannot be sure enough for an individual case if the ionospheric inhomogeneity occurs at the tangent point or maybe at another part of the ray trajectory. Sokolovskiy (2000) has an appropriate method for localization of ionospheric irregularities in individual cases, but implementation of this method for automatic data processing seems to be diOcult. The average Aeld of sporadic E (Qne ) has been calculated as function of height (60–140 km) and geographic latitude and is depicted in Fig. 4 for the prime time from 19 June to 10 July 1995, having 1900 radio occultation events. In the present study, we use only data of those three prime times of GPS=MET which have the most radio occultation events. Figs. 5 and 6 shows the average sporadic E distribution along the meridian for the prime times 10 –25 October 1995, (1540 events), and 2–16 February 1997, (2690 events). A representation of sporadic E as function of geographic latitude has been favored rather than of geomagnetic latitude, since neutral dynamics is often the driving force of the layer formation process and the background ionization of the lower ionosphere mainly depends on solar radiation. Some features of sporadic E depending on geomagnetic inclination and magnetosphere–ionosphere interaction are clearer if sporadic E is shown as function of geomagnetic latitude. However, the essential characteristics of Figs. 4 – 6 do not change if geomagnetic latitude is taken instead of geographic latitude. It is obvious in the GPS=MET observations that sporadic E is in particular a phenomenon of the summer hemisphere as noted in a review paper by Whitehead (1989). Fig. 4
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
1977
Fig. 4. Sporadic E occurrence for 19 June to 10 July 1995 derived by using 1900 GPS=MET radio occultations. The average of electron density
Fig. 5. Same as Fig. 4, but for 10 –25 October 1995 (1540 radio occultation events).
shows enhanced sporadic E in June=July 1995 when summer was in the northern hemisphere, and in Fig. 6, sporadic E is increased in late summer 1997 of the southern hemisphere. Indeed ionosondes and radars also observe a dramatic seasonal dependence of sporadic E with a maximum in the summer months (e.g., Taguchi and Shibata, 1961; Haldoupis and Schlegel, 1996). According to Figs. 4 – 6, the maximal occurrence of sporadic E is at heights 90–110 km. At geo-
graphic high latitudes, layers frequently occur at lower altitudes around 90 km height and may correspond to polar mesospheric summer echoes (PMSE). There is also some evidence for intermediate layers beyond 110 km; which are best seen in Fig. 4 at 20–30◦ latitude. The distance of these layers is around 6 km and may indicate the vertical wavelength of the atmospheric wave forming these ionization layers.
1978
K. Hocke et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1973–1980
Fig. 6. Same as Fig. 4, but for 2–16 February 1997 (2690 radio occultation events).
The enhanced occurrence of sporadic E at the equator with a maximum at around 100 km height is in agreement with other observations and the theory that sporadic E and plasma irregularities are probably associated to the equatorial electrojet and the gradient drift plasma instability (Whitehead, 1989). In summary, the GPS=MET data give a reasonable pattern of global occurrence of sporadic E with many interesting details and for diJerent seasons. Since our knowledge and experience of limb sounding of the ionosphere have just started, there is a need for comparison between results of the radio occultation technique and results of the well-approved ground-based radio techniques. Appropriate ground-based data are certainly the data of the Asia=Australia ionosonde chain. In Figs. 7 and 8 the average magnitude (foEs) and virtual height (h’Es) of sporadic E are depicted for February 1997 and June 1995. The viewgraphs can be compared to the GPS=MET observations in Figs. 4 and 6 by keeping in mind that the GPS=MET slices are averages over all longitude sectors. World maps of sporadic E shown by Taguchi and Shibata (1961) and Hocke and Tsuda (2001a) indicate that during southern hemisphere summer, sporadic E is mainly concentrated over South America. The main characteristic (enhanced sporadic E in the summer hemisphere) is present in the ground- and space-based observations. In particular, we And agreement for strong sporadic E in the northern hemisphere at around 30◦ latitude in June=July 1995 (Figs. 4 and 8). The virtual height measurement of ionosonde observations may be calibrated in future by using radio occultation observations. Ground-based observations are most useful for description of the tem-
Fig. 7. Ground-based observation of (a) the average magnitude foEs and (b) the average virtual height h’Es of sporadic E by the Asia=Australia ionosonde chain during February 1997 (please compare with Fig. 6).
poral behavior of sporadic E (e.g., periodicities caused by tides and planetary waves) and for Anding physical mechanisms of layer formation by multi-instrument campaigns (Mathews, 1998). 4. Concluding remarks The study of sporadic E layers observed by GPS=MET gives an impression of the high application potential of
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Acknowledgements We are grateful to C. Rocken and the GPS=MET team of University Corporation for Atmospheric Research (UCAR=Boulder) for providing the GPS=MET data. T. Koizumi (CRL) and the WDC for Ionosphere are acknowledged for the ionosonde data. T. Tsuda (RASC=Kyoto), M. Yamamoto (RASC) and T. Ogawa (STEL=Nagoya) are thanked for helpful discussions. We thank the reviewers for numerous improvements. The study has been partly performed while one of the authors (Klemens Hocke) has been at DLR (Neustrelitz), GFZ (Potsdam), and RASC (Kyoto). The Science and Technology Agency of Japan is thanked for providing a fellowship. Fig. 8. Ground-based observation of (a) the average magnitude foEs and (b) the average virtual height h’Es of sporadic E by the Asia=Australia ionosonde chain during June 1995 (please compare with Fig. 4).
GPS radio occultation measurements for research and monitoring of the mesosphere–thermosphere–ionosphere system. The measurements are complementary to vertical soundings of radars and ionosondes. The latter techniques are appropriate for observing the dynamic behavior of sporadic E layers (e.g., layer formation process, vertical layer descend with time, plasma instabilities), while GPS radio occultation provides global maps derived from snap shots of the atmosphere, enabling a new view on sporadic E by horizontal sounding. Analysis of small-scale
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