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Thermal behaviour of the Antarctic thermosphere observed from Mawson
A~V. .spuc~n~s.Vol. 24, No. I I, pp. 1433-1446, 1999 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 027?- I 177199 $1-0.00 + 0.00
Pergamon www.elsevicr.nl/locate/aar
PII: so173-
I 177(99)00704-8
THERMAL BEHAVIOUR OF THE ANTARCTIC THERMOSPHERE
B.E. Booth’, P. L. Dyson’, P. A. Greet2, J. L. Innis
OBSERVED FROM MAWSON
and D. J. Murphy’.
‘La Trobe University, Bundoora, VIC 3083, Australia 2Australian Antarctic Division, Kingston, TAS 7030, Australia
ABSTRACT The behaviour of thermospheric temperature in the region of Mawson, Antarctica during 1992-96 is presented. Temperatures have been derived from Fabry-Perot spectrometer observations of the 630-nm 01 emission. A distinct nighttime temperature gradient is observed along the magnetic meridian with hotter temperatures towards the pole. Equatorward of Mawson the temperature data have been fitted by an equation based on diurnal and annual sinusoidal variations, sunspot number and a quadratic dependence on Kp. Temperature differences between observed temperatures and the mathematical description have a mean of 10K and a standard deviation of 102K. Ol’)qY COSPAR. Published by Elscvier Science Ltd. INTRODUCTION Mawson is a high-latitude station (67.6’S, 62.9”E, invariant latitude 70.5’S) so the dynamics and thermodynamics of the local thermosphere are affected by aurora1 and polar cap processes in addition to solar heating (Innis et al., 1996, Smith et al., 1998). A Fabry-Perot Spectrometer (FPS) developed by Jacka (1984) has operated at Mawson for over 10 years and a variety of emissions have been observed to determine temperatures and winds at different levels in the atmosphere (e.g. Conde and Dyson, 1995a, 1995b; Greet et al., 1994). Each year since 1992 an extensive set of observations has been obtained at 630 nm, providing an excellent data set of temperatures and winds in the upper thermosphere. Studies of variations with time scales of tens of minutes to hours have shown that during the night, Mawson typically moves from being equatorward of the aurora1 oval to being in the polar cap, where the thermosphere is hotter and contains regions of upwelling (Innis et al, 1996, 1997, 1998). This paper presents an investigation of the morphological behaviour of thermospheric temperature in the region of Mawson. DATA ANALYSIS
AND RESULTS
In routine operations, the Mawson FPS obtains a sequence of observations from the zenith and at 30” elevation looking geographic north, south, east and west. The data from each observing direction were analysed separately and preliminary analysis showed that, as expected, the thermospheric temperatures have dominant diurnal components and strong dependencies on solar activity. An annual effect and dependency on Kp are also important. These effects are simply illustrated by presenting fits to the data for each parameter after the effects of the other parameters have been removed. Figure 1 shows the temperature variations in each cardinal direction (labelled N, E, S, W), as a function of UT. Each datum point is the average of all 1992-96 measurements obtained within plus or minus half an hour of that universal time. The solid line is the least squares fit of a 24-hour sine wave to the N data. A major feature of these results is the similarity between the N and W data (NW data set) and the S and E data (SE data set). Magnetic south is almost directly southeast of Mawson. Hence the gross differences between the NW and SE data indicate that for much of the night, there is a distinct temperature gradient along the magnetic meridian with the region poleward of Mawson being on average 20-30 K hotter than the region equatorward. This is consistent with
1443
444
B. E. Booth et 01.
E 1050:
-
,025:LI.. &
lOOO:-
:_ ---
875?. 8.S
1 --
I ,"I 13.1
‘i ‘1 19.5
”
16.3
UT
‘i ". 22.7
8 252
(Hours)
”
2S.l
13.1
16.3
19.5
22.7
J
25.9
28.1
25.9
28.1
UT (Hours)
N
16.3
UT (Hours)
IS.5
22.7
UT (Hours)
Fig. 1. Average temperatures to the East, South, North and West of Mawson. Error bars indicate standard errors. The solid line is the best fit sinusoidal diurnal variation to the North data. Midnight in magnetic local time occurs at 19.8 UT.
13.81 Kp + 4.888Kp’
,DoD90.28 ’ cor(2pifs85~r
+ 0.04488) ;-““_ r= 0%
(c) 9%
Day of Year
Fig. 2. Fits to temperature residuals after other dependencies have been removed showing dependency on (a) Kp, (b) sunspot number and (c) season. (d) Histogram of differences between observed temperature and Eq. 1 values.
I445
Antarctic Thermospheric Thermal Behaviour
previous work that studied variations on individual nights where it was found that the polar cap often moves over Mawson during the night. Figure 1 indicates that from approximately 20 MLT, hot polar cap conditions are typical some 350 km poleward of Mawson. Interestingly the NW data show a well-defined, sinusoidal, diurnal temperature variation even though examination of the 630-nm intensity data shows evidence of aurora1 influence. McCormac et al. (1988) successfully modelled the hot polar cap thermosphere observed from Thule near solar maximum but the feature at Mawson is very persistent and will be investigated in detail in a separate study. The clear diurnal variation evident in the NW data set suggests that the thermosphere equatorward of Mawson is less complex thermodynamically than that poleward. Hence the remainder of the analysis concentrates on this equatorward region and uses the N data set. The thermosphere is heated by solar EW which varies with solar activity, However the EW flux is rarely measured and in studies of this type solar variability is usually accounted for using the sunspot number or F10.7 flux (Hemandez, 1982; Yagi and Dyson, 1985). In examining dependency of the Mawson temperature data on both parameters, it has been found that there is a clear linear relationship with sunspot number (Figure 2(a)) so this parameter is used here. It is to be expected that the temperature of the thermosphere near Mawson will be affected by magnetic activity because of the associated movement of the aurora1 oval and hot polar cap. Kp is a readily available index but since it is derived from observatories distiibuted around the world, it would not be surprising if it proved to be of limited value as a parameter in this study. However, Kp proves to be an acceptable parameter, with the thermospheric temperature exhibiting a clear quadratic dependence on Kp (Figure 2(b)). Finally, as expected, there is an annual effect causing hotter temperatures during summer (Figure 2(c)). Of course these parameters do not describe the data completely and from Figures 1 and 2(c) it is evident that the local time and seasonal dependencies are not purely diurnal and annual variations. Least-squares fitting the above dependencies to the N data set of over 6000 individual temperatures leads to the temperatures between 11.5 UT and 04.0 UT being described by the following equation: T(K)=880.0+1.775SS+13.31Kp+4.868(Kp)2
+92.97cos(;UT+4.039)+90.36~0~(
g
D + 0.04496)
(1)
where SS is sunspot number, Kp imagnetic index, UT universal time in hours, and D day number of the year. Figure 2(d) is a histogram of the differences between the measured temperatures and the corresponding temperatures given by Eq.1. The mean of the differences is -10.2K (i.e. with Eq.1 predicting lower temperatures) and the standard deviation of the differences is 102K. DISCUSSION Assuming the observations of the 630-nm emission relate to a height of 230 km, Eq.l refers to the location with geographic coordinates of 64.1”s and 62.9”E. In future work the temperature dependency on physical parameters described by Eq.1 will be used to aid investigations of the processes causing the higher temperature poleward of Mawson. It will also be used to assess how well existing models describe the temporal and spatial temperature variations which occur, particularly in passing from a region equatorward of the auroral oval, through the oval, and into the polar cap. As a preliminary investigation, a six day period from 28 May to 2 June 1995, chosen at random has been examined. Figure 3 shows the measured temperatures, those predicted using Eq.1 and the MSIS model (Hedin 1991). Near midnight marking the end of 29 May, there was a large increase in magnetic activity causing Kp to jump from 2+ to 4’ and the thermospheric temperature to increase by - 150K. Each night’s observations show considerable variability in the temperature. Daily parameters have been used in the MSIS model as is evident from the abrupt day to day changes. This helps highlight the variability within a night that is in addition to the diurnal variation, obtained using Eq.1. This reflects the dependence on the three-hourly parameter Kp. However even this dependence does not enable the real temperature variability to be described completely. Over the six day period there is good overall agreement between the measured temperatures and those derived using Hq. 1 although the predicted temperatures tend to be higher. The mean of the differences is 17K with a standard deviation of 59K. The large temperature rise at the end of 29 May (near 48 hours in Figure 3) is well described, as is the more gentle decrease over the next four days.
1446
B. E. Booth L’I ul
Time Fig. 3. Temperature
(Hours)
variations equatorward
of Mawson, 28 May - 2 June 1995.
The temperatures obtained from the MSIS model also describes the rapid temperature increase at the end of 29 May and the subsequent decrease over successive days. However the variations are less and overall the MSIS temperatures are lower than the measurements. The mean of the differences is -77K and the standard deviation is 59K. The MSIS temperatures have been calculated for a height of 230 km. The temperature difference is largely removed using a height of 300 km however this is unexpectedly high for observations that include the effects of aurora1 emissions.
ACKNOWLEDGEMENTS We gratefully acknowledge support by Australian Research Council, Antarctic Science Advisory Committee, Australian National Antarctic Research Expeditions and Expeditioners who operated the Mawson FPS. REFERENCES Conde, M. and P. L. Dyson. Thermospheric
Vertical Winds Above Mawson, Antarctica,
J. Atmos. Terr. Phys., 57,
589,1995a.
Conde, M. and P. L. Dyson. Thermospheric horizontal winds above Mawson, Antarctica, Adv. Space Rex, 16, (5) 41,1995b. Greet, P., J. Innis and P. L. Dyson. High-resolution Fabry-Perot observations of mesospheric OH(6-2) emissions, Geophysical Research Letters, 21, 1153, 1994. Hedin, A., M. Biondi, R. Burnside, G. Hernandez, R. Johnson, T, Killeen, C. Mazaudier, J. Meriwether, J. Salah, R. Sica, R. Smith, N. Spencer, V. Wickwar and T. Virdi, Revised global model of thermospheric winds using satellite and ground-based observations, J. Geophys. Res., 96,7657, 1991. Hernandez, G. Mid-latitude thermospheric neutral kinetic temperatures, 1, Solar, geomagnetic, and long-term effects, J. Geophys. Res., 87, No A5, 1623, 1982. Innis, J. L., P. A. Greet and P. L. Dyson. Fabry-Perot Spectrometer Observations of the Aurora1 Oval/Polar Cap Boundary above Mawson, Antarctica, J. Atmos. Terr. Phys., 58, 1973, 1996. Innis J. L., P. L. Dyson and P. A. Greet. The Vertical Wind at the Auroral Oval/Polar Cap Boundary: Further Observations, and Consequences for Thermospheric Wind and Temperature Measurements at Auroral-latitude sites, J. Atmos. Terr. Phys., 59,2009, 1997. Innis, J. L., P. A. Greet and P. L. Dyson. Are Polar Cap Gravity Waves a Heat Source for the High-latitude Thermosphere? Geophys. Res. Let., 25, 1487, 1998. Jacka, F., Application of Fabry-Perot spectrometers for measurement of upper atmosphere temperature and winds, Ch 2 in Middle Atmosphere Handbook for MAP, Ed. R. A. Vincent, 1984. McCormac, F. G., T. L. Killeen, A. G. Bums, J. W. Meriwether, R. G. Roble, L. E. Wharton and N. W. Spencer. Polar Cap Diurnal Temperature Variations: Observations and Modelling, J. Geophys. Res., 93,7466, 1988. Smith R. W., G. Hemandez, R. G. Roble, P. L. Dyson, M. Conde, R. Crickmore and M. Jarvis. Observation and Simulation of Winds and Temperatures in the Antarctic Thermosphere for August 2 - 10, 1992, J. Geophys. Res., 103,9473, 1998. Yagi, T. and P. L. Dyson. Measurement of thermospheric temperatures at a mid-latitude station., Planet. space sci., 33,203, 1985.
Pergamon www.elsevicr.nl/locate/aar
PII: so173-
I 177(99)00704-8
THERMAL BEHAVIOUR OF THE ANTARCTIC THERMOSPHERE
B.E. Booth’, P. L. Dyson’, P. A. Greet2, J. L. Innis
OBSERVED FROM MAWSON
and D. J. Murphy’.
‘La Trobe University, Bundoora, VIC 3083, Australia 2Australian Antarctic Division, Kingston, TAS 7030, Australia
ABSTRACT The behaviour of thermospheric temperature in the region of Mawson, Antarctica during 1992-96 is presented. Temperatures have been derived from Fabry-Perot spectrometer observations of the 630-nm 01 emission. A distinct nighttime temperature gradient is observed along the magnetic meridian with hotter temperatures towards the pole. Equatorward of Mawson the temperature data have been fitted by an equation based on diurnal and annual sinusoidal variations, sunspot number and a quadratic dependence on Kp. Temperature differences between observed temperatures and the mathematical description have a mean of 10K and a standard deviation of 102K. Ol’)qY COSPAR. Published by Elscvier Science Ltd. INTRODUCTION Mawson is a high-latitude station (67.6’S, 62.9”E, invariant latitude 70.5’S) so the dynamics and thermodynamics of the local thermosphere are affected by aurora1 and polar cap processes in addition to solar heating (Innis et al., 1996, Smith et al., 1998). A Fabry-Perot Spectrometer (FPS) developed by Jacka (1984) has operated at Mawson for over 10 years and a variety of emissions have been observed to determine temperatures and winds at different levels in the atmosphere (e.g. Conde and Dyson, 1995a, 1995b; Greet et al., 1994). Each year since 1992 an extensive set of observations has been obtained at 630 nm, providing an excellent data set of temperatures and winds in the upper thermosphere. Studies of variations with time scales of tens of minutes to hours have shown that during the night, Mawson typically moves from being equatorward of the aurora1 oval to being in the polar cap, where the thermosphere is hotter and contains regions of upwelling (Innis et al, 1996, 1997, 1998). This paper presents an investigation of the morphological behaviour of thermospheric temperature in the region of Mawson. DATA ANALYSIS
AND RESULTS
In routine operations, the Mawson FPS obtains a sequence of observations from the zenith and at 30” elevation looking geographic north, south, east and west. The data from each observing direction were analysed separately and preliminary analysis showed that, as expected, the thermospheric temperatures have dominant diurnal components and strong dependencies on solar activity. An annual effect and dependency on Kp are also important. These effects are simply illustrated by presenting fits to the data for each parameter after the effects of the other parameters have been removed. Figure 1 shows the temperature variations in each cardinal direction (labelled N, E, S, W), as a function of UT. Each datum point is the average of all 1992-96 measurements obtained within plus or minus half an hour of that universal time. The solid line is the least squares fit of a 24-hour sine wave to the N data. A major feature of these results is the similarity between the N and W data (NW data set) and the S and E data (SE data set). Magnetic south is almost directly southeast of Mawson. Hence the gross differences between the NW and SE data indicate that for much of the night, there is a distinct temperature gradient along the magnetic meridian with the region poleward of Mawson being on average 20-30 K hotter than the region equatorward. This is consistent with
1443
444
B. E. Booth et 01.
E 1050:
-
,025:LI.. &
lOOO:-
:_ ---
875?. 8.S
1 --
I ,"I 13.1
‘i ‘1 19.5
”
16.3
UT
‘i ". 22.7
8 252
(Hours)
”
2S.l
13.1
16.3
19.5
22.7
J
25.9
28.1
25.9
28.1
UT (Hours)
N
16.3
UT (Hours)
IS.5
22.7
UT (Hours)
Fig. 1. Average temperatures to the East, South, North and West of Mawson. Error bars indicate standard errors. The solid line is the best fit sinusoidal diurnal variation to the North data. Midnight in magnetic local time occurs at 19.8 UT.
13.81 Kp + 4.888Kp’
,DoD90.28 ’ cor(2pifs85~r
+ 0.04488) ;-““_ r= 0%
(c) 9%
Day of Year
Fig. 2. Fits to temperature residuals after other dependencies have been removed showing dependency on (a) Kp, (b) sunspot number and (c) season. (d) Histogram of differences between observed temperature and Eq. 1 values.
I445
Antarctic Thermospheric Thermal Behaviour
previous work that studied variations on individual nights where it was found that the polar cap often moves over Mawson during the night. Figure 1 indicates that from approximately 20 MLT, hot polar cap conditions are typical some 350 km poleward of Mawson. Interestingly the NW data show a well-defined, sinusoidal, diurnal temperature variation even though examination of the 630-nm intensity data shows evidence of aurora1 influence. McCormac et al. (1988) successfully modelled the hot polar cap thermosphere observed from Thule near solar maximum but the feature at Mawson is very persistent and will be investigated in detail in a separate study. The clear diurnal variation evident in the NW data set suggests that the thermosphere equatorward of Mawson is less complex thermodynamically than that poleward. Hence the remainder of the analysis concentrates on this equatorward region and uses the N data set. The thermosphere is heated by solar EW which varies with solar activity, However the EW flux is rarely measured and in studies of this type solar variability is usually accounted for using the sunspot number or F10.7 flux (Hemandez, 1982; Yagi and Dyson, 1985). In examining dependency of the Mawson temperature data on both parameters, it has been found that there is a clear linear relationship with sunspot number (Figure 2(a)) so this parameter is used here. It is to be expected that the temperature of the thermosphere near Mawson will be affected by magnetic activity because of the associated movement of the aurora1 oval and hot polar cap. Kp is a readily available index but since it is derived from observatories distiibuted around the world, it would not be surprising if it proved to be of limited value as a parameter in this study. However, Kp proves to be an acceptable parameter, with the thermospheric temperature exhibiting a clear quadratic dependence on Kp (Figure 2(b)). Finally, as expected, there is an annual effect causing hotter temperatures during summer (Figure 2(c)). Of course these parameters do not describe the data completely and from Figures 1 and 2(c) it is evident that the local time and seasonal dependencies are not purely diurnal and annual variations. Least-squares fitting the above dependencies to the N data set of over 6000 individual temperatures leads to the temperatures between 11.5 UT and 04.0 UT being described by the following equation: T(K)=880.0+1.775SS+13.31Kp+4.868(Kp)2
+92.97cos(;UT+4.039)+90.36~0~(
g
D + 0.04496)
(1)
where SS is sunspot number, Kp imagnetic index, UT universal time in hours, and D day number of the year. Figure 2(d) is a histogram of the differences between the measured temperatures and the corresponding temperatures given by Eq.1. The mean of the differences is -10.2K (i.e. with Eq.1 predicting lower temperatures) and the standard deviation of the differences is 102K. DISCUSSION Assuming the observations of the 630-nm emission relate to a height of 230 km, Eq.l refers to the location with geographic coordinates of 64.1”s and 62.9”E. In future work the temperature dependency on physical parameters described by Eq.1 will be used to aid investigations of the processes causing the higher temperature poleward of Mawson. It will also be used to assess how well existing models describe the temporal and spatial temperature variations which occur, particularly in passing from a region equatorward of the auroral oval, through the oval, and into the polar cap. As a preliminary investigation, a six day period from 28 May to 2 June 1995, chosen at random has been examined. Figure 3 shows the measured temperatures, those predicted using Eq.1 and the MSIS model (Hedin 1991). Near midnight marking the end of 29 May, there was a large increase in magnetic activity causing Kp to jump from 2+ to 4’ and the thermospheric temperature to increase by - 150K. Each night’s observations show considerable variability in the temperature. Daily parameters have been used in the MSIS model as is evident from the abrupt day to day changes. This helps highlight the variability within a night that is in addition to the diurnal variation, obtained using Eq.1. This reflects the dependence on the three-hourly parameter Kp. However even this dependence does not enable the real temperature variability to be described completely. Over the six day period there is good overall agreement between the measured temperatures and those derived using Hq. 1 although the predicted temperatures tend to be higher. The mean of the differences is 17K with a standard deviation of 59K. The large temperature rise at the end of 29 May (near 48 hours in Figure 3) is well described, as is the more gentle decrease over the next four days.
1446
B. E. Booth L’I ul
Time Fig. 3. Temperature
(Hours)
variations equatorward
of Mawson, 28 May - 2 June 1995.
The temperatures obtained from the MSIS model also describes the rapid temperature increase at the end of 29 May and the subsequent decrease over successive days. However the variations are less and overall the MSIS temperatures are lower than the measurements. The mean of the differences is -77K and the standard deviation is 59K. The MSIS temperatures have been calculated for a height of 230 km. The temperature difference is largely removed using a height of 300 km however this is unexpectedly high for observations that include the effects of aurora1 emissions.
ACKNOWLEDGEMENTS We gratefully acknowledge support by Australian Research Council, Antarctic Science Advisory Committee, Australian National Antarctic Research Expeditions and Expeditioners who operated the Mawson FPS. REFERENCES Conde, M. and P. L. Dyson. Thermospheric
Vertical Winds Above Mawson, Antarctica,
J. Atmos. Terr. Phys., 57,
589,1995a.
Conde, M. and P. L. Dyson. Thermospheric horizontal winds above Mawson, Antarctica, Adv. Space Rex, 16, (5) 41,1995b. Greet, P., J. Innis and P. L. Dyson. High-resolution Fabry-Perot observations of mesospheric OH(6-2) emissions, Geophysical Research Letters, 21, 1153, 1994. Hedin, A., M. Biondi, R. Burnside, G. Hernandez, R. Johnson, T, Killeen, C. Mazaudier, J. Meriwether, J. Salah, R. Sica, R. Smith, N. Spencer, V. Wickwar and T. Virdi, Revised global model of thermospheric winds using satellite and ground-based observations, J. Geophys. Res., 96,7657, 1991. Hernandez, G. Mid-latitude thermospheric neutral kinetic temperatures, 1, Solar, geomagnetic, and long-term effects, J. Geophys. Res., 87, No A5, 1623, 1982. Innis, J. L., P. A. Greet and P. L. Dyson. Fabry-Perot Spectrometer Observations of the Aurora1 Oval/Polar Cap Boundary above Mawson, Antarctica, J. Atmos. Terr. Phys., 58, 1973, 1996. Innis J. L., P. L. Dyson and P. A. Greet. The Vertical Wind at the Auroral Oval/Polar Cap Boundary: Further Observations, and Consequences for Thermospheric Wind and Temperature Measurements at Auroral-latitude sites, J. Atmos. Terr. Phys., 59,2009, 1997. Innis, J. L., P. A. Greet and P. L. Dyson. Are Polar Cap Gravity Waves a Heat Source for the High-latitude Thermosphere? Geophys. Res. Let., 25, 1487, 1998. Jacka, F., Application of Fabry-Perot spectrometers for measurement of upper atmosphere temperature and winds, Ch 2 in Middle Atmosphere Handbook for MAP, Ed. R. A. Vincent, 1984. McCormac, F. G., T. L. Killeen, A. G. Bums, J. W. Meriwether, R. G. Roble, L. E. Wharton and N. W. Spencer. Polar Cap Diurnal Temperature Variations: Observations and Modelling, J. Geophys. Res., 93,7466, 1988. Smith R. W., G. Hemandez, R. G. Roble, P. L. Dyson, M. Conde, R. Crickmore and M. Jarvis. Observation and Simulation of Winds and Temperatures in the Antarctic Thermosphere for August 2 - 10, 1992, J. Geophys. Res., 103,9473, 1998. Yagi, T. and P. L. Dyson. Measurement of thermospheric temperatures at a mid-latitude station., Planet. space sci., 33,203, 1985.