Atmospheric X-rays in the Southern hemisphere

Journal of Atmospheric

Pergamon

and S&r-Terrestrral

PII: S1364-6826(96)001?30-0

Physics,

Vol. 59, No. 12, pp 1381-I 390, 1997 0 1997 Elsevier Scvnce Ltd All rights reserved. Pnnted in Gnat Bntain 136+6826/97 %17.00+0 00

Atmospheric X-rays in the Southern hemisphere 0. Pinto Jr.,’ 0. Mendes,’ I. R. C. A. Pinto,’ W. D. Gonzalez,’

R. H. Holzworth2

and H. Hu3

‘Institute National de Pesquisas Espaciais-INPE, 12227-010 SLo Jos& dos Campos, SBo Paulo, Brazil; ‘University of Washington, Seattle, WA 98195-1650, U.S.A.; ‘Jet Propulsion Lab, m/s 300-323, 4800 Oak Grove Drive, Pasadena, CA 91109, U.S.A. (Received in finalform 28 April 1995; accepted 28 November

1996)

Abstract-Atmospheric X-rays in the energy range from 30 to 150 keV were measured in the Southern hemisphere extending from 53 to 81” magnetic latitude during two long-duration balloon flights. The measurements were obtained during the Extended Life Balloon Borne Observatories (ELBBO) experiment. The experiment consisted of five superpressure balloon flights launched from Dunedin, New Zealand, in November and December 1992. The ELBBO X-ray data can be considered the longest continuous data set ever obtained in the Southern hemisphere, and extend over 30”of magnetic latitude previously unmeasured. The X-ray measurements are compared to similar data obtained in the past by several groups in the Southern and Northern hemispheres, as well as with available model results. Most ELBBO results confirm earlier findings about the flux and spectra of atmospheric X-rays and are in general agreement with model results for higher energies. However, they indicate that the X-ray flux in the Southern hemisphere is almost constant from high latitudes up to 30 magnetic degrees, in contrast to the model results that indicate a drop off around 50 magnetic degrees. Whether such discrepancy should be attributed to the different energies involved or to the presence of the South Atlantic magnetic anomaly is a point that remains to be investigated. 0 1997 Elsevier Science Ltd

INTRODUCTION

Atmospheric X-rays are secondary cosmic ray photons, being produced as the result of electromagnetic cascade showers in the Earth’s atmosphere. The theory of the electromagnetic cascade processes has been used with success for a long time (Charakhchyan et al., 1978). Based on this theory, the origin of the atmospheric X-rays is attributed to the degradation process of gamma rays produced by neutral pions (Daniel and Stephens, 1974). The theory predicts that the flux of the secondary photon component is about ten times larger than that of charged particles around the Pfotzer maximum. The intensity of the atmospheric X-rays is known to depend on the atmospheric depth. The dependence can be obtained from the cascade theory and it has been confirmed extensively by experimental data. It has a maximum around 100 g/cm* atmospheric depth, called the Pfotzer maximum. Below the maximum, the

intensity decreases almost exponentially, reflecting the predominance of the absorption over the multiplication. Above, it is usually represented by a power law function of the atmospheric depth. Below the altitude corresponds to 700 g/cm2 and above the altitude corresponds to 10 g/cm2, the measurements show some small changes concerning the behavior described above, which should be attributed to the presence of other X-ray sources. In addition, the intensity of the atmospheric Xrays depends on magnetic latitude and solar activity. Concerning magnetic latitude, the intensity decreases by about a factor of three from high latitudes toward the equator because of the increase of the magnetic rigidity (Charakhchyan et al., 1978). The location of the Pfotzer maximum, in turn, shifts from about 60g/cm2 (approximately 19 km) at high latitudes to about 130g/cm* (approximately 14 km) at low latitudes. These characteristics are thought to be valid in both hemispheres (Bazilevskaya et al., 1991). Con1381

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cerning solar activity, the intensity of atmospheric Xrays is primarily modulated by the 11 yr solar cycle. Charakhchyan et al. (1978) have found variations in the intensity of X-rays at the Pfotzer maximum from solar minimum to solar maximum, which amount to 20% at high latitudes and 5% at middle latitudes. In a given atmospheric depth, the energy spectrum of the atmospheric X-rays is believed to represent an average equilibrium spectrum resulting of a large number of superimposed cascades (Charakhchyan et al., 1978). As a result, the spectrum is almost independent on atmospheric depth and magnetic latitude. This statement has been confirmed experimentally, in spite of small differences found in comparing different measurements. Such differences are generally attributed to the performance of the experiment itself or to statistical uncertainty related to the rapid ascent of the experiment in the atmosphere (Pinto and Gonzalez, 1986a). The situation is quite different at atmospheric depths less than log/cm’, where the X-ray energy spectrum tends to become softer as a result of the presence of other X-ray sources (Pinto and Gonzalez, 1986a, b). In this article we present the results of the analysis of the atmospheric X-ray data obtained at 26 km of altitude by two long-duration balloon flights launched from Dunedin, New Zealand, in November 1992, as part of the Extended Life Balloon Borne Observatories (ELBBO) experiment. The experiment is an international program involving the University of Washington, Seattle, U.S.A., the University of Otago, Dunedin, New Zealand, and the National Institute of Space Research (INPE), Sao Paulo, Brazil (Holzworth et al., 1993). The program consisted of the launch of five balloon-borne payloads on 5 000 m3 superpressure balloons into the stratosphere to study the atmospheric electrodynamics. Three groups provided the instrumentation for the following measurements: vector electric field and conductivity (University of Washington), VLF hiss (University of Otago) and X-rays (INPE). The design and fabrication of the basic payloads and nearly all other ‘engineering’ requirements were made by and on behalf of the Washington group (see Holzworth et al., 1993). The results are compared to similar ones obtained in the past by several groups in the Southern and Northern hemispheres, as well as with available model results.

EXPERIMENTAL

SET UP AND MEASUREMENTS

Each X-ray detector consisted with dimensions 3 xi inches

of a NaI(T1) crystal (effective area of

30.4cm2), set up to look upward. The measurements were sorted in three differential energy channels: 3050 keV, 50-70 keV and 7&150 keV. The detectors were calibrated so as to give a response as similar as possible to those used at low latitudes in the past (Pinto and Gonzalez, 1986a, b). Even in this case, flux differences between ELBBO and low latitude flights as large as 30%40% can be expected for the low energy channel, because of small differences in the geometric factor and payload design. For the whole energy range, however, such differences are less than 10%. More details about the detectors can be found elsewhere (Pinto and Gonzalez, 1986a, b; Pinto et al., 1996). The data used in this article were obtained by the ARGOS telemetry system. As a result of the very low data rates available through this system, they consisted of 10 min average of 20 s integrated counts. Each 10min of data are stored on board for about 4 h and transmitted repeatedly every 200 s during the storage period, so as to increase the opportunity for data to be received by the polar orbiting satellites. This procedure greatly reduced the data gaps. Besides the 10min averaged data, real time 20s data were transmitted when a satellite was overhead. These real time data permitted us to evaluate the instruments and verify the onboard calculations. A complete description of the data acquisition and processing can be found in Hu (1994). The ELBBO campaign consisted of five flights. Flights 1 to 3 gave good X-ray data. Flight 4 had problems in the X-ray package just after the launch and Flight 5 had no X-ray instrument on board. However, in Flight 1 the X-ray data were influenced by a large external day-to-night temperature variation. As such a variation can affect the long-time scale changes associated with the atmospheric X-rays (see below), these data were not used in this article. In flights 2 and 3, this problem was reduced by increasing the thermal isolation of the X-ray package. Figure 1 shows the trajectories of the ELBBO flights 2 and 3 in the Southern hemisphere, together with the launch times. Also indicated in this figure are the approximate location of the South Atlantic magnetic anomaly (SAMA) and the aurora1 zone. The trajectories go from New Zealand to the edge of the polar cap, crossing the aurora1 region and covering magnetic latitudes from 53 to 81”. The trajectories shown in Fig. 1 correspond just to the period in which X-ray data exist, about 10 days for Flight 2 and 4.5 days for Flight 3. In Flight 2, after this period, the Xray counts dropped to zero because of a malfunction in the high voltage power supply. In Flight 3 a balloon malfunction, immediately apparent after launching,

1383

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Fig. 1. Trajectories of ELBBO flights in the Southern hemisphere. The approximate location of the South Atlantic magnetic anomaly (SAMA) and the aurora1 zone are indicated.

caused premature termination of the flight. However, as will be shown later, the balloon malfunction in Flight 3 gave a unique opportunity to study the dependence of the atmospheric X-ray energy spectrum on atmospheric depth and magnetic latitude. In spite of the technical problems pointed out above, the ELBBO data set is the longest continuous X-ray data set ever obtained in the Southern hemisphere. Figure 2 shows the 10min averages of 20 s integrated X-ray counts obtained at 26 km altitude during Flight 2. The data are shown in three different energy channels. Throughout this article AVXl corresponds to 30-50 keV range, AVX2 to 5C-70 keV and AVX3

to 7&l 50 keV. The magnetic rigidity (R) is also shown in the bottom panel of Fig. 2, based on the results published by Gustafsson et al. (1992). The X-ray data in this figure show a variation anticorrelated with the magnetic rigidity. The variation is most pronounced in AVX3. Such variation indicates that the flux measured most likely corresponds to atmospheric X-rays and not to aurora1 X-rays because the variation in AVX3 occurs at 26 km where aurora1 fluxes are strongly attenuated by the overlying atmosphere (Vij et al., 1980). Also the average background fluxes are continuous during the flight, even when the balloon crossed the aurora1 region where some short X-ray

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events were seen. The rigidity related variation of AVXl and AVX2 is superimposed on a daily variation which is a detector effect related to temperature changes. The temperature change from nighttime to daytime condition produces small (a few keV) changes in the energy band widths causing a variation in the Xray flux. This variation is larger in the low energy channels. For the whole energy range (30-150 keV), however, it is not significant. Short duration X-ray perturbations can also be seen in Fig. 2 on November 23 and 24 (between hours I10 and 130). These perturbations are probably associated with local time aurora1 precipitation. Figure 3 shows in detail the X-ray data of Flight 2 on November 23, when the balloon was inside the aurora1 region. For reference, the magnetic activity is indicated by the ‘up’ index in the bottom panel of this figure. The ap values shown in Fig. 3 indicate global activity in the aurora1 region on November 23. The activity remained on November 24, though at a lower level. The ratio

(see text for

AVX3/AVX2 is also shown in Fig. 3. This ratio indicates that during the perturbation there was a softening of the energy spectrum. Such behavior is consistent with aurora1 precipitation. Similar events appear in the other flights and will be the subject of a separate article. Figure 4 shows the X-ray data for Flight 3 in the same format as in Fig. 2. Also included in this figure is the atmospheric pressure, shown in the bottom panel. During this flight we can see from the pressure data that the balloon had a slow movement up and down in the atmosphere (caused by the balloon having lost superpressure when the gas cooled at night-see also Quinn and Holzworth, 1987). The balloon crossed the Pfotzer maximum six times while the balloon altitude varied between about 10 km and 26 km. In contrast to the normal rapid ascent of a balloon in the atmosphere, the slow vertical movement of the balloon in this flight provides a unique opportunity to measure the X-ray energy spectrum at the Pfotzer maximum

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at different magnetic latitudes. The bottom of Fig. 4 indicates times when the Sun was on the balloon so that the movement of the balloon as it follows the night and day pattern may be identified. RESULTS Figure from 30 Flight 2 Southern

5 shows the X-ray flux in the energy range to 150 keV measured at 20 mbar during the (ELBBO 2) versus magnetic latitude in the hemisphere. The flux is in units of photons

of 36150

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1385

cm-* s-’ keV_‘, assuming that the efficiency is equal to one (1) for the whole energy range (all data presented in this section take this assumption into account). Also included in this figure are the results obtained in the Southern hemisphere by Ghielmetti et al. (1964), Charakhchyan et al. (1978) and Pinto and Gonzalez (1986a), as well as the model results obtained by Daniel and Stephens (1974) for 1 MeV (the lower energy considered in the model). The Daniel and Stephens model is the unique global model of atmospheric X-rays ever published. We can see in this figure that the X-ray flux near 30” is almost equal to that measured from about 30 to 80” magnetic latitude. Below 30” the flux decreases sharply. Such behavior is in reasonable agreement with the model results, in spite of the difference in the energy involved. However, in the model (dashed line) the flux begins to decrease sharply around 50”, instead of 30” as in the data. The same dependence on latitude described above can be seen in Fig. 6, where we have plotted the 30150 keV X-ray flux measured by ELBBO 3 (Flight 3) and the 1 MeV flux obtained by the model of Daniel and Stephens (1974) at the Pfotzer maximum versus magnetic latitude. The comparison between Figs 5 and 6 shows that the X-ray flux has a slower variation with latitude in the Pfotzer maximum than in 20mbar. This feature can also be seen in the model results of Daniel and Stephens (1974). We have included in Fig. 6 some measurements made in the Northern hemisphere by Anderson (1960), Anderson (1961), Vette et al. (1978). All measure(1962) and Charakhchyan ments shown in this figure were made during or near the solar maximum. The ELBBO 3 fluxes are slightly lower than the Northern hemisphere ones. We believe that this fact is probably because of small differences in the design of the experiments by different groups. Figure 7 shows the X-ray energy spectra obtained by ELBBO 3 at the Pfotzer maximum at 53 and 69” magnetic latitude in the Southern hemisphere. The spectra obtained by Pinto and Gonzalez (1986b) inside the SAMA region and by Barcus and Rosenberg (1966) in the Northern hemisphere during solar minimum are also shown. At the right top of the figure are indicated the e-folding energies that are obtained by fitting an exponential function to the observed spectra. We can see in this figure that the spectrum of the atmospheric X-rays is almost independent of the magnetic latitude. Figure 7 also seems to indicate (see efolding values in the right top of the figure) that the spectrum tends to become slightly softer from solar maximum to solar minimum condition. In addition, the ELBBO 3 data confirm, with a very good statistical accuracy, that the spectrum between the Pfotzer

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Fig. 5. X-ray flux in the energy range from 30 to 150 keV measured during Flight 2 at 20mbar compared with other measurements made in the Southern hemisphere (black dots) and the model results for 1 MeV obtained by Daniel and Stephens (1974) (dashed line).

Fig. 6. X-ray flux in the energy range from 30 to 150 keV measured from 53 to 89” magnetic latitude during Flight 3 at the Pfotzer maximum, compared with other measurements made in the Southern and Northern hemispheres and the model results for 1 MeV obtained by Daniel and Stephens (1974) (dashed line).

maximum

solar cycle, mainly above 50 of magnetic latitude. Three points are worth reporting from this figure: the very large multiplicity; the large ratio between photon and charged particle fluxes; and the different behavior of the photon and charged particle curves at middle latitudes. The charged particle curve shows a behavior similar to that for 1 MeV photons calculated by Daniel and Stephens (1974) and shown in Fig. 6. While the first two points agree with the cascade theory, the last point seems to indicate that other aspects not considered by the theory should be invoked.

and 20 mbar

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above confirm past evidences. It is worth noting, however, that the spectrum at the Pfotzer maximum remains unchanged even inside the SAMA region. Finally, Fig. 8 shows the total flux of photons and charged particles at the Pfotzer maximum in the Southern hemisphere versus magnetic latitude. The total flux of photons shown in Fig. 8 was obtained by integrating the flux measured in the energy range from 30 to 150 keV, assuming the spectrum shown in Fig. 7. The charged particle flux was taken from the work of Charakhchyan et al. (1978). Also shown is the primary cosmic-ray flux in the top of the atmosphere. All fluxes were obtained near the solar maximum. In contrast to the fluxes at the Pfotzer maximum, the primary cosmic ray flux changes considerably during the 11 yr depth.

In principle,

the

results

CONCLUSION

From the analysis of ELBBO measurements, we conclude that most results confirm earlier findings about the atmospheric X-rays, although now extended

Fig. 4. Integrated X-ray counts obtained during Flight 3 in the same format as in Fig. 2. The two bottom panels show the atmospheric pressure and the day-night monitor PVmon which equals 1 (zero) indicates daytime (nighttime) conditions.

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to previously Hemisphere.

1389

the available model results. ELBBO data agree with the expected softening of the X-ray spectrum at high latitudes under the aurora, the flux intensity measured at similar latitudes in the northern hemisphere, and with the energy spectra obtained at other latitudes at same solar activity conditions. However, comparing the ELBBO results with others obtained in the Southern hemisphere, we found that the X-ray flux in the Southern hemisphere seems to be almost constant from high latitudes to 30” magnetic latitude, in contrast to the model results that indicate a drop off around 50” magnetic latitude. Whether such discrepancy should be attributed to the different energies considered by the data and the model or to the energetic electron precipitation from the outer belt in the region of the South Atlantic magnetic anomaly is a point that remains to be investigated. Global X-ray models in the energy range of 3(r 150 keV should be developed and compared with the data presented in this work so as to evaluate the role of the South Atlantic magnetic anomaly on the atmospheric X-rays in the Southern hemisphere. Also we would like to suggest that more long duration flights should be done, in particular in the transition region between 30 and 50” south latitude, so that we can get a comprehensive view about the atmospheric X-rays in the Southern hemisphere.

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unmeasured latitudes in the Southern Our results are in general agreement with

Acknowledgements-The Brazilian participation in the ELBBO project was supported by the Conselho National de Desenvolvimento Cientifico e Tecnologico (CNPq) and the Funda@o de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP). At the University of Washington this research was supported by NASA grants NAGS-668 and NAGW4147 and by National Science Foundation grants ATM 8920428 and ATM 9402764. The authors would like to thank the University of Washington engineer John Chin, the ELBBO program manager Kent W. Norville (now at CH2M Hill, Portland, OR), and the INPE engineers Wanderli Kabata and Osvaldo Celso Pontieri for their technical support.

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