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Night−day imprints of ionospheric slab thickness during geomagnetic storm
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Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1307–1314 www.elsevier.com/locate/jastp
Nightday imprints of ionospheric slab thickness during geomagnetic storm Tamara Gulyaevaa,, Iwona Stanislawskab a
IZMIRAN, 142190 Troitsk, Moscow Region, Russian Fderation Space Research Center, Bartycka 18-A, 00-716 Warsaw, Poland
b
Received 3 February 2004; received in revised form 11 April 2005; accepted 7 July 2005 Available online 2 September 2005
Abstract Spatial maps of the ionosphere–plasmasphere slab thickness (t) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 11 spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 351 to 701N, and longitudes 101 to 401E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS–TEC and ionosonde observations at solar minimum (1995–1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor (w) is introduced as a ratio of disturbance to quietness representing area mean TEC, foF2, and t for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and t maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but t enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS–TEC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300 km by night. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Plasmasphere; Total electron content
1. Introduction In recent years many techniques have been used to construct instantaneous maps from spatially distributed measurement data; many mapping models have been created. In particular, Kriging is a statistical mapping Corresponding author. Tel.: +7 95 3340284.
E-mail address: [email protected] (T. Gulyaeva).
technique based on the characteristic variability demonstrated by the variogram, i.e. a function that illustrates the differentiation of a given parameter value, depending on the distance between different measurements, which contains the variation form—its size and spatial scale. The value at a studied point situated between the measurement points is interpolated by giving it the appropriate weight. This weight depends on the distance between the measurements and the studied point and on
1364-6826/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2005.07.006
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the degree of correlation with measured variations nearby; it is determined by the variogram. The weight is different for the F2 layer critical frequency, foF2, and the total electron content, TEC (Stanislawska et al., 2000, 2002). Indirect meaurements of TEC are available lately from a GPS-based ionospheric monitoring network of stations. Combining TEC, m2, with ionosonde-derived F2 layer peak electron density NmF2, m3, (proportional to the F2 layer critical frequency foF2, MHz: NmF2 ¼ 1.24 1010 (foF2)2) one obtains a measure of the shape of electron density profile, the ionospheric slab thickness t, km (Davies and Liu, 1991). It is the equivalent thickness of an ionosphere, having constant uniform density equal to that at the F-region peak electron density NmF2. t ¼ TEC=NmF2
(1)
Studies of ionospheric storms using the F region electron density and total electron content have been conducted by many authors (e.g., Rishbeth et al., 1987; Pro¨lss, 1993; Field and Rishbeth, 1997; Danilov, 2001, Belehaki et al., 2003). Idealized storm-time model shows a ‘positive phase’ with enhanced peak electron density and electron content, followed by the ‘negative phase’ of depressed NmF2 and TEC below their mean values, with gradual ‘recovery phase’ afterwards (Anderson, 1976; Araujo-Pradere et al., 2002; Kutiev and Muhtarov, 2003). These features could be smoothed or distorted depending on season and local time of storm onset. Since the TEC parameter comes from GPS measurements from the altitude of 20 000 km over the Earth, the ionosphere and plasmasphere contributions in TEC and slab thickness should be distinguished. In particular, opposite movements of the ionosphere F2 layer peak height and the plasmapause is observed at the main phase of the magnetic storm when the hmF2 is growing (Deminova et al., 1998) while the plasmapause altitude is reduced (Gallagher et al., 2000). The transition from the ionosphere to plasmasphere is regarded to be located at altitude of about 1000 km over the Earth (Carpenter and Park, 1973). We assume that this height might be also affected by the storm according to the slab thickness variations, which proved to be growing during the magnetic storm while TEC and foF2 are both depressed. In particular, during the nighttime the slab thickness could exceed 1000 km towards the high latitudes (Gulyaeva and Jayachandran, 2004) which suggests re-examination of the upper boundary of the ionosphere. For validation of such hypothesis, the International Reference Ionosphere (Bilitza, 2001) extended towards the plasmapause (Gulyaeva, 2003a) has been modified to give vertical electron density profile consistent with the ionosonde F2 layer peak and GPS–TEC observations. This allows decomposing the slab thickness in three
parts referring to the bottomside and topside ionosphere and the plasmasphere. The results are used for estimates of the topside boundary of the ionosphere. For the physical explanation of the ionospheric storm the composition bulge of increased mean molecular mass has been suggested, i.e. the ratio of molecular gas concentration of (N2+O2) to the atomic oxygen concentration is increased (Pro¨lss, 1993; Fuller-Rowell et al., 1994). After its origin in nighttime auroral oval by input of the magnetospheric energy, the composition bulge is moving towards the mid- and low-latitudes by the nightside equatorward winds and then brought into the dayside by Earth’s rotation. The higher recombination rate within the bulge yields to a depletion of ionization and therefore to a negative disturbance. The spatial mapping of the slab thickness is undertaken below to identify relevant large-scale structures in the ionosphere and plasmasphere during a storm. The slab thickness derivation requires availability of both TEC and foF2 parameters measured simultaneously at the same site. However, there are well-known limitations on numerical estimate of the foF2 critical frequency during magnetic storms due to ionospheric absorption, sporadic E layer appearance, spread–F phenomena and other physical or technical reasons. Besides the current network of GPS–TEC observation is much denser than that of the ionosonde network; therefore, it is not possible in many cases to make a straightforward estimate of the slab thickness t for some of the GPS ground-based stations. Similarly, there are also sites of ionosonde observations where TEC observations are missing. To avoid the above limitations, the present study is based on instantaneous mapping of TEC and foF2 from independent ionosonde and GPS–TEC networks of observations using Kriging technique from which instantaneous maps of t are calculated. This approach is used to investigate spatial response of the ionospheric slab thickness to geomagnetic storms during the solar maximum and minimum phases of the present solar cycle.
2. Database and technique of analysis This study of the ionospheric slab thickness during geomagnetic storms has been made using data from the network of 23 stations in Europe and Russia for the solar minimum (1995–1996) and solar maximum (2002) given in Table 1. The foF2 values were measured using ionosonde, and the vertical TEC values were derived using the measurements with ground-based receivers from global positioning system GPS (providing electron content up to an altitude of about 20 000 km above the Earth). Spatial interpolation of the multi-site ionosonde observations of the critical frequency foF2 has been
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Table 1 Geodetic and geomagnetic coordinates of observatories providing foF2 critical frequency and GPS-TEC data at the period selected for the present study Station
Geodetic coordinates
Geomagnetic coordinates
1995–1996
2002
1 Kiruna 2 Sodankyla 3 Sankt-Petersburg 4 Uppsala 5 Onsala 6 Tomsk 7 Zwenigorod 8 Moscow 9 Julius-Rugen 10 Potsdam 11 Warsaw-Josefoslaw 12 Slough-Chilton 13 Hailsham 14 Pruhonice 15 Lannion 16 Rostov-on-Don 17 Graz 18 Sofia 19 Rome 20 Matera 21 Tortosa 22 Athens 23 El Arenosillo
67.81N 67.4 60.0 59.8 57.4 56.5 55.7 55.5 54.6 52.4 52.1 51.6 50.9 50.0 48.5 47.2 47.1 42.7 41.8 40.6 40.4 38.0 37.1
65.01N 63.6 56.1 58.4 57.3 46.0 51.1 50.4 54.3 52.3 50.5 54.1 53.4 49.8 52.0 42.4 46.8 41.0 42.3 40.3 43.6 36.4 41.4
TEC, TEC, TEC, TEC,
TEC foF2 foF2
020.41E 026.6 030.7 017.6 011.9 084.9 036.7 037.3 013.4 013.1 021.1 358.7 000.3 014.6 356.7 039.7 015.5 023.4 012.5 016.7 000.3 023.6 353.3
116.51E 120.8 118.3 106.5 099.6 160.6 120.6 123.2 099.7 097.8 105.1 083.2 084.5 098.0 080.1 120.3 097.6 103.9 093.2 096.3 080.9 102.5 072.3
foF2 foF2 foF2 foF2
foF2 TEC, foF2 TEC, foF2 TEC, foF2 TEC, foF2 TEC, foF2 TEC, TEC, foF2 TEC,
TEC foF2 TEC foF2 foF2 TEC TEC foF2 TEC
foF2 foF2 foF2 foF2
foF2 TEC, foF2
foF2 foF2 TEC foF2 foF2 TEC foF2 foF2 foF2
Table 2 Comparison of TEC netweight factor (w) from GPS observations and maps. Date and time of storm onset and maximum value of sub-auroral am index are given Date_hrs, UT of storm onset
07.04.1995_00 27.09.1995_09 18.10.1995_09 13.01.1996_03 22.10.1996_09 10.01.2002_12 05.02.2002_12 24.03.2002_00 17.04.2002_09 23.05.2002_09 Delta
am, nT
192 125 163 71 138 101 67 95 139 211
00 UT
12 UT
Obs.
Map
Obs.
Map
1.343 1.125 1.182 1.167 0.952 0.687 1.235 1.207 0.686 0.522
1.339 1.135 1.166 1.152 0.948 0.706 1.233 1.169 0.660 0.498 0.016
1.071 0.839 0.791 1.332 0.844 1.015 0.635 0.994 0.622 0.848
1.094 0.846 0.805 1.349 0.865 1.015 0.639 1.023 0.630 0.827 0.014
The mean difference between TEC-map and observation Delta ¼ 0.1S|w(Map)–w(Obs.)|.
made using instantaneous mapping techniques (Stanislawska et al., 2000). For TEC instantaneous mapping Kriging technique has been applied (Stanislawska et al. 2002). The foF2 and TEC instantaneous maps are produced from the multi-station data at geodetic latitudes of 351 to 701N, geodetic longitudes of 101 to 401E with 11 grid step in both directions. The resulting slab thickness
map is produced by applying Eq. (1) at each grid point of TEC and foF2 maps. Five magnetic storms have been selected at solar minimum, and five storms at solar maximum. Date and time of onset of the selected storms, and the maximum value of sub-auroral amplitude am index are given in Table 2. The time of the storm onset has been estimated for the period preceding the maximum amplitude index
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when running average of am exceeded half of its maximum value. Data for two successive days following the onset of magnetic storm have been analyzed starting at 00 hU T or 12 hU T depending on time of onset of magnetic storm. The maps have been produced for 00, 12, 23 h UT corresponding to local nighttime and daytime conditions for the selected area. To characterize the degree of disturbance-to-quietness for the selected area, a net-weight factor is introduced representing the ratio of the area mean daily-hourly parameter normalized by the relevant quiet median area mean value as follows: w ¼ Sk Yi=Sk Ymed
the real thickness of these areas. Since the real thickness of the bottomside ionosphere is readily available and equal to the height step between the F2 layer peak height and the lower boundary of the ionosphere (around 65–80 km over the Earth), we assume that the same ratio of the slab thickness to the real thickness is at least preserved in the topside ionosphere. This assumption allows obtain a first-order estimate of the topside boundary of the ionosphere illustrated below.
3. Results
(2)
Here Y stands for foF2, TEC or t, Yi refers to data for a particular day and hour, Ymed refers to the monthly median values. Monthly median values of foF2, TEC and t for observing sites have been calculated based on the daily–hourly values of the respective parameter. Index k presents either the number of grid points for a map, or the number of available measurements from the network of observations so that net-weights can be compared for a map with the observations. The netweight w41 means positive phase of storm-time deviation for the given parameter, 0owo1 means negative phase. So the defined index allows to compare the storm-time behavior of different parameters. Comparisons of nighttime (00 h UT) and daytime (12 h UT) net-weights for GPS–TEC observations with relevant Kriging TEC maps are presented in Table 2. It is evident from Table 2 that mapping results are well consistent with observations. So we proceed further with Kriging mapping technique for TEC and foF2 parameters yielding the slab thickness map by applying Eq. (1) at each grid point of the source maps. To estimate the deformation of the ionosphere during storm-time, we have used model-assisted analysis of the slab thickness results. The IRI model extended towards the plasmasphere (IRI*) has been used for inversion of results of observation to electron density profile. Input of ionosonde-derived critical frequency foF2 and peak height hmF2 ensures IRI* true electron density maximum at the F2 layer peak. The IRI* code makes use of one more anchor point in the topside ionosphere at the half peak density 0.5 NmF2 (Gulyaeva, 2003b). The latter topside shape parameter is varied in the present study to conform the electron density profile to GPS–TEC observations. So the IRI* code providing electron content separately below and above hmF2 in the ionosphere and that for the plasmasphere allows to distinguish the separate parts of the slab thickness in the bottomside ionosphere (tbot), topside ionosphere (ttop) and the plasmasphere (tpl) using Eq. (1). Eliminating the plasmaspheric slab thickness component from total t, we can deal with the residual proportions of the slab thickness in the topside and bottomside ionosphere to
Examples of instantaneous maps of TEC, foF2, and slab thickness t for the nighttime negative ionospheric storm at solar maximum, 10 January 2002, are given in Fig. 1. The high-latitude trough (decrease) of electron content TEC (upper section) is located at lower latitudes than the F2 layer peak ionization trough (middle section). Calculations of t (bottom section) yields the slab thickness bulge (increased t) centered over the F2 layer trough of peak electron density. Table 3 presents mapping results of the area mean and maximal values of t for nighttime and daytime for 10 perturbed days. The area mean value represents the
Fig. 1. TEC, foF2 and slab thickness maps during the negative nighttime ionosphere storm.
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Table 3 Slab thickness area average and maximum values of t, km, from instantaneous maps for disturbed days Date
07.04.95 28.09.95 19.10.95 13.01.96 23.10.96 10.01.02 06.02.02 24.03.02 18.04.02 24.05.02
Night: UT ¼ 00 h
Day: UT ¼ 12 h
t ave
t max
F1N
L1E
t ave
t max
F1N
L1E
782 480 345 824 428 1069 514 365 698 852
1725 859 537 1157 741 1713 876 622 2332 2229
57.2 54.6 40.7 64.9 65.4 57.8 54.6 56.9 59.4 56.3
118.0 100.1 70.1 81.0 115.9 100.1 125.6 99.4 123.4 117.3
387 355 341 447 399 244 256 328 540 314
550 520 690 1080 1085 440 504 611 1759 413
72.8 72.8 72.8 63.9 63.9 72.8 72.8 72.8 72.8 63.9
93.3 93.3 93.3 132.9 132.9 93.3 93.3 93.3 93.3 132.9
Geomagnetic coordinates of tmax are given.
Fig. 2. Map-produced latitudinal variation of the ionospheric slab thickness under monthly median quiet (Q—circles) conditions and disturbed (D—asterisks) conditions at nighttime and daytime for solar minimum (1995) and maximum (2002) at longitude of 301E.
average of the parameter from all grid points of the map. The mean values of t are greater by night than by day. While normal average slab thickness does not exceed 800 km (Goodwin et al., 1995), very large values of tmax exceeding 1000 km require further analysis of the ionosphere–plasmasphere interactions during the storm. Nighttime and daytime latitudinal profiles of slab thickness at selected longitude of 301E for quiet monthly median conditions and two storms at equinox (April) for low- and high-solar activity are plotted on Fig. 2. Increase of t during the storm is most effective at night both at solar minimum and maximum. The slab
Fig. 3. Evolution of net-weight factor derived from TEC (crosses), foF2 (circles) and slab thickness t (asterisks) maps during 48 h since the onset of magnetic storm averaged separately for nighttime and daytime onset of magnetic storm from the data of 10 storms at solar minimum and maximum.
thickness bulge (over the trough of ionization) is observed under the storm conditions. More general outcome from mapping results is plotted on Fig. 3. Here average evolution of net-weight parameters for foF2, TEC, and t during the time since onset of 10 magnetic storms (Table 2) are presented grouped in respect to the level of solar activity, nighttime or daytime onset of the magnetic storm defined by the period preceding the peak of storm with running average of am index reached its half-maximum value, and applying a superposed epoch analysis to demonstrate the time evolution of each ionospheric parameter after the storm onset (0 h in Fig. 3). Positive phase of the critical frequency foF2 and TEC (w41) is gradually transformed into the negative phase (wo1).
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2000
2000
1500
1500 Height (km)
Height (km)
1312
1000
1000 500
500 40
0 70
60
Lati
tude
50 (de
g)
40
0 -10
40
0 70
30 20 ) g de 10 e( itud t a L
60
Lati
tude
50 (de g)
40
0 -10
30 20 ) g de 10 e( itud t a L
2000 2000 Height (km)
1500 Height (km)
1500 1000
1000 500
500 0 70
40
Lat
60 itud
50 e (d
eg)
40
0 -10
30 20 ) g de 10 e( itud t a L
40
0 70
60 Lati
tude
50 (de g)
40
0 -10
30 20 ) g de 10 e( itud t a L
Fig. 4. Nighttime and daytime surface maps of the transition height between the ionosphere and plasmasphere (the topside boundary of the ionosphere) under quiet monthly median conditions and storm on 24 March 2002.
TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 h during the magnetic storm at solar maximum but t enhancement is shorter and delayed by 12–24 h regarding the storm onset at solar minimum. Fig. 4 presents night–day imprints of the surface maps of the upper boundary of the ionosphere under quiet (monthly median) conditions and during the storm on 24 March 2002 as revealed by IRI model-assissted analysis of the above results. It is evident from this Figure that the ionosphere–plasmasphere transition height is growing during the perturbed conditions both by daytime and nighttime (upto 2000 km altitude over the Earth) exceeding normal boundary of the ionosphere under quiet median conditions. Formation of the ionosphere bulge over the trough of the F2 layer peak ionization is observed the most pronounced by nighttime (see Fig. 1 for a comparison). Summary of the temporal–spatial mean values of the upper boundary of the ionosphere and its standard
deviation determined for quiet and disturbed conditions are presented in Table 4. From this table the upper boundary is greater by nighttime (1200–1600 km) that by daytime (600–900 km) increasing from quiet to disturbed conditions and from solar minimum to maximum. The individual values of the upper boundary height are changing from 500 km by day to 2300 km by night with standard deviation of results comprising 10–30% of the mean values representing the RMS error of model evaluation of the upper boundary of the ionosphere.
4. Discussion and conclusions Results of slab thickness evolution during the magnetic storm depend on relative variations of TEC and NmF2, as well as on the storm intensity and the time passed since the storm onset. Growing slab thickness during a storm (‘positive phase’ of slab
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Table 4 Area mean and standard deviation (s) of the upper boundary of ionosphere, km, averaged for all selected periods for night and day, quiet and disturbed conditions, solar minimum and maximum Year
1995–1996 2002
Night UT ¼ 00 h
Day UT ¼ 12 h
Conditions
Mean
s
Mean
s
1540 1244 1516 1259
370 152 312 171
791 647 917 830
243 62 150 102
thickness storm) may serve as a manifestation of electric fields that produce a vertical drift, driving plasma upwards to regions of lower loss (Pro¨lss, 1991; Rishbeth, 1991). Enhancement of the slab thickness (area mean net-weight w41) is observed for the negative phase of the F region peak and electron content TEC for solar maximum (during more than 48 h since storm onset) and solar minimum (delayed by 12–24 h since storm onset). Growth of the slab thickness accompanied by uplift of the topside boundary of the ionosphere requires reevaluation of earlier results of the F layer peak density analysis which should be considered jointly with the ionosphere thickness variations (see e.g. Field and Rishbeth, 1997). Our extreme maximal values of t (see Table 3) suggest that definition of the upper boundary of the ionosphere should be re-examined. The upper boundary of the ionosphere is regarded to be at altitudes about 1000 km over the Earth, gradually transformed into plasmasphere/magnetosphere above this height. Eliminating the plasmasphere contribution to the slab thickness, the growing slab thickness during a magnetic storm exceeding 1000 km implies expanding of an actual ionosphere thickness which is directly related with the slab thickness (Gulyaeva et al., 2004). It is shown that the upper boundary of the ionosphere is shifted upward during the storm forming, something like the ionosphere-tail over the nighttime high-latitude trough of ionization. Modelassissted approach for GPS–TEC profile inversion allows to estimate the ionospheric upper boundary, assuming that the ratio of slab thickness to real thickness in the topside ionosphere conforms to the relevant ratio in the bottom ionosphere. It is found that the real thickness of the ionosphere is greater than the ionospheric slab thickness, both being increased significantly during the magnetic storms. The spatial–temporal average of the upper ionosphere boundary varies from 600 to 1600 km with nighttime and daytime dispersion of 10–30%. The ionosphere–plasmasphere (IP) boundary altitude has been related from the earlier studies with the O+/H+ transition height assumed to occur near 1000 km above the Earth (Carpenter and Bowhill, 1971; Carpenter and Park,
Disturbed Quiet Disturbed Quiet
1973). The availability of the satellite data base allowed to construct a model of O+/H+ transition height (Kutiev et al., 1994). Our solar cycle dependence of the IP boundary follows variations of O+/H+ transition height, but the diurnal variations of our estimates of the IP boundary differ from the O+/H+ model which shows dominant daytime values. The reduced daytime values of the IP boundary height are similar to magnetosphere configuration compressed at dayside due to impringing solar wind. Independent techniques for measurements of the plasma mass density in the region where the ionosphere mixes with the plasmasphere (Price et al., 1999) might shed light on the processes governing the position of the IP boundary zone.
Acknowledgments The authors are grateful to Norbert Jakowski, DLR, Germany, and Liljana Cander, RAL, UK, for providing GPS-TEC data. Magnetic indices and ionosonde data have been provided through the World Data Centers: Boulder, CO, USA; Oxford, UK; Moscow, Russia; Meudon, France; Copenhagen, Denmark; Kyoto, Japan; Warsaw, Poland; Sodankyla, Finland; Rome, Italy. The authors are indebted to important comments of two unknown referees stimulating improvements of the paper.
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Journal of Atmospheric and Solar-Terrestrial Physics 67 (2005) 1307–1314 www.elsevier.com/locate/jastp
Nightday imprints of ionospheric slab thickness during geomagnetic storm Tamara Gulyaevaa,, Iwona Stanislawskab a
IZMIRAN, 142190 Troitsk, Moscow Region, Russian Fderation Space Research Center, Bartycka 18-A, 00-716 Warsaw, Poland
b
Received 3 February 2004; received in revised form 11 April 2005; accepted 7 July 2005 Available online 2 September 2005
Abstract Spatial maps of the ionosphere–plasmasphere slab thickness (t) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 11 spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 351 to 701N, and longitudes 101 to 401E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS–TEC and ionosonde observations at solar minimum (1995–1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor (w) is introduced as a ratio of disturbance to quietness representing area mean TEC, foF2, and t for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and t maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but t enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS–TEC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300 km by night. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Plasmasphere; Total electron content
1. Introduction In recent years many techniques have been used to construct instantaneous maps from spatially distributed measurement data; many mapping models have been created. In particular, Kriging is a statistical mapping Corresponding author. Tel.: +7 95 3340284.
E-mail address: [email protected] (T. Gulyaeva).
technique based on the characteristic variability demonstrated by the variogram, i.e. a function that illustrates the differentiation of a given parameter value, depending on the distance between different measurements, which contains the variation form—its size and spatial scale. The value at a studied point situated between the measurement points is interpolated by giving it the appropriate weight. This weight depends on the distance between the measurements and the studied point and on
1364-6826/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2005.07.006
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the degree of correlation with measured variations nearby; it is determined by the variogram. The weight is different for the F2 layer critical frequency, foF2, and the total electron content, TEC (Stanislawska et al., 2000, 2002). Indirect meaurements of TEC are available lately from a GPS-based ionospheric monitoring network of stations. Combining TEC, m2, with ionosonde-derived F2 layer peak electron density NmF2, m3, (proportional to the F2 layer critical frequency foF2, MHz: NmF2 ¼ 1.24 1010 (foF2)2) one obtains a measure of the shape of electron density profile, the ionospheric slab thickness t, km (Davies and Liu, 1991). It is the equivalent thickness of an ionosphere, having constant uniform density equal to that at the F-region peak electron density NmF2. t ¼ TEC=NmF2
(1)
Studies of ionospheric storms using the F region electron density and total electron content have been conducted by many authors (e.g., Rishbeth et al., 1987; Pro¨lss, 1993; Field and Rishbeth, 1997; Danilov, 2001, Belehaki et al., 2003). Idealized storm-time model shows a ‘positive phase’ with enhanced peak electron density and electron content, followed by the ‘negative phase’ of depressed NmF2 and TEC below their mean values, with gradual ‘recovery phase’ afterwards (Anderson, 1976; Araujo-Pradere et al., 2002; Kutiev and Muhtarov, 2003). These features could be smoothed or distorted depending on season and local time of storm onset. Since the TEC parameter comes from GPS measurements from the altitude of 20 000 km over the Earth, the ionosphere and plasmasphere contributions in TEC and slab thickness should be distinguished. In particular, opposite movements of the ionosphere F2 layer peak height and the plasmapause is observed at the main phase of the magnetic storm when the hmF2 is growing (Deminova et al., 1998) while the plasmapause altitude is reduced (Gallagher et al., 2000). The transition from the ionosphere to plasmasphere is regarded to be located at altitude of about 1000 km over the Earth (Carpenter and Park, 1973). We assume that this height might be also affected by the storm according to the slab thickness variations, which proved to be growing during the magnetic storm while TEC and foF2 are both depressed. In particular, during the nighttime the slab thickness could exceed 1000 km towards the high latitudes (Gulyaeva and Jayachandran, 2004) which suggests re-examination of the upper boundary of the ionosphere. For validation of such hypothesis, the International Reference Ionosphere (Bilitza, 2001) extended towards the plasmapause (Gulyaeva, 2003a) has been modified to give vertical electron density profile consistent with the ionosonde F2 layer peak and GPS–TEC observations. This allows decomposing the slab thickness in three
parts referring to the bottomside and topside ionosphere and the plasmasphere. The results are used for estimates of the topside boundary of the ionosphere. For the physical explanation of the ionospheric storm the composition bulge of increased mean molecular mass has been suggested, i.e. the ratio of molecular gas concentration of (N2+O2) to the atomic oxygen concentration is increased (Pro¨lss, 1993; Fuller-Rowell et al., 1994). After its origin in nighttime auroral oval by input of the magnetospheric energy, the composition bulge is moving towards the mid- and low-latitudes by the nightside equatorward winds and then brought into the dayside by Earth’s rotation. The higher recombination rate within the bulge yields to a depletion of ionization and therefore to a negative disturbance. The spatial mapping of the slab thickness is undertaken below to identify relevant large-scale structures in the ionosphere and plasmasphere during a storm. The slab thickness derivation requires availability of both TEC and foF2 parameters measured simultaneously at the same site. However, there are well-known limitations on numerical estimate of the foF2 critical frequency during magnetic storms due to ionospheric absorption, sporadic E layer appearance, spread–F phenomena and other physical or technical reasons. Besides the current network of GPS–TEC observation is much denser than that of the ionosonde network; therefore, it is not possible in many cases to make a straightforward estimate of the slab thickness t for some of the GPS ground-based stations. Similarly, there are also sites of ionosonde observations where TEC observations are missing. To avoid the above limitations, the present study is based on instantaneous mapping of TEC and foF2 from independent ionosonde and GPS–TEC networks of observations using Kriging technique from which instantaneous maps of t are calculated. This approach is used to investigate spatial response of the ionospheric slab thickness to geomagnetic storms during the solar maximum and minimum phases of the present solar cycle.
2. Database and technique of analysis This study of the ionospheric slab thickness during geomagnetic storms has been made using data from the network of 23 stations in Europe and Russia for the solar minimum (1995–1996) and solar maximum (2002) given in Table 1. The foF2 values were measured using ionosonde, and the vertical TEC values were derived using the measurements with ground-based receivers from global positioning system GPS (providing electron content up to an altitude of about 20 000 km above the Earth). Spatial interpolation of the multi-site ionosonde observations of the critical frequency foF2 has been
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Table 1 Geodetic and geomagnetic coordinates of observatories providing foF2 critical frequency and GPS-TEC data at the period selected for the present study Station
Geodetic coordinates
Geomagnetic coordinates
1995–1996
2002
1 Kiruna 2 Sodankyla 3 Sankt-Petersburg 4 Uppsala 5 Onsala 6 Tomsk 7 Zwenigorod 8 Moscow 9 Julius-Rugen 10 Potsdam 11 Warsaw-Josefoslaw 12 Slough-Chilton 13 Hailsham 14 Pruhonice 15 Lannion 16 Rostov-on-Don 17 Graz 18 Sofia 19 Rome 20 Matera 21 Tortosa 22 Athens 23 El Arenosillo
67.81N 67.4 60.0 59.8 57.4 56.5 55.7 55.5 54.6 52.4 52.1 51.6 50.9 50.0 48.5 47.2 47.1 42.7 41.8 40.6 40.4 38.0 37.1
65.01N 63.6 56.1 58.4 57.3 46.0 51.1 50.4 54.3 52.3 50.5 54.1 53.4 49.8 52.0 42.4 46.8 41.0 42.3 40.3 43.6 36.4 41.4
TEC, TEC, TEC, TEC,
TEC foF2 foF2
020.41E 026.6 030.7 017.6 011.9 084.9 036.7 037.3 013.4 013.1 021.1 358.7 000.3 014.6 356.7 039.7 015.5 023.4 012.5 016.7 000.3 023.6 353.3
116.51E 120.8 118.3 106.5 099.6 160.6 120.6 123.2 099.7 097.8 105.1 083.2 084.5 098.0 080.1 120.3 097.6 103.9 093.2 096.3 080.9 102.5 072.3
foF2 foF2 foF2 foF2
foF2 TEC, foF2 TEC, foF2 TEC, foF2 TEC, foF2 TEC, foF2 TEC, TEC, foF2 TEC,
TEC foF2 TEC foF2 foF2 TEC TEC foF2 TEC
foF2 foF2 foF2 foF2
foF2 TEC, foF2
foF2 foF2 TEC foF2 foF2 TEC foF2 foF2 foF2
Table 2 Comparison of TEC netweight factor (w) from GPS observations and maps. Date and time of storm onset and maximum value of sub-auroral am index are given Date_hrs, UT of storm onset
07.04.1995_00 27.09.1995_09 18.10.1995_09 13.01.1996_03 22.10.1996_09 10.01.2002_12 05.02.2002_12 24.03.2002_00 17.04.2002_09 23.05.2002_09 Delta
am, nT
192 125 163 71 138 101 67 95 139 211
00 UT
12 UT
Obs.
Map
Obs.
Map
1.343 1.125 1.182 1.167 0.952 0.687 1.235 1.207 0.686 0.522
1.339 1.135 1.166 1.152 0.948 0.706 1.233 1.169 0.660 0.498 0.016
1.071 0.839 0.791 1.332 0.844 1.015 0.635 0.994 0.622 0.848
1.094 0.846 0.805 1.349 0.865 1.015 0.639 1.023 0.630 0.827 0.014
The mean difference between TEC-map and observation Delta ¼ 0.1S|w(Map)–w(Obs.)|.
made using instantaneous mapping techniques (Stanislawska et al., 2000). For TEC instantaneous mapping Kriging technique has been applied (Stanislawska et al. 2002). The foF2 and TEC instantaneous maps are produced from the multi-station data at geodetic latitudes of 351 to 701N, geodetic longitudes of 101 to 401E with 11 grid step in both directions. The resulting slab thickness
map is produced by applying Eq. (1) at each grid point of TEC and foF2 maps. Five magnetic storms have been selected at solar minimum, and five storms at solar maximum. Date and time of onset of the selected storms, and the maximum value of sub-auroral amplitude am index are given in Table 2. The time of the storm onset has been estimated for the period preceding the maximum amplitude index
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when running average of am exceeded half of its maximum value. Data for two successive days following the onset of magnetic storm have been analyzed starting at 00 hU T or 12 hU T depending on time of onset of magnetic storm. The maps have been produced for 00, 12, 23 h UT corresponding to local nighttime and daytime conditions for the selected area. To characterize the degree of disturbance-to-quietness for the selected area, a net-weight factor is introduced representing the ratio of the area mean daily-hourly parameter normalized by the relevant quiet median area mean value as follows: w ¼ Sk Yi=Sk Ymed
the real thickness of these areas. Since the real thickness of the bottomside ionosphere is readily available and equal to the height step between the F2 layer peak height and the lower boundary of the ionosphere (around 65–80 km over the Earth), we assume that the same ratio of the slab thickness to the real thickness is at least preserved in the topside ionosphere. This assumption allows obtain a first-order estimate of the topside boundary of the ionosphere illustrated below.
3. Results
(2)
Here Y stands for foF2, TEC or t, Yi refers to data for a particular day and hour, Ymed refers to the monthly median values. Monthly median values of foF2, TEC and t for observing sites have been calculated based on the daily–hourly values of the respective parameter. Index k presents either the number of grid points for a map, or the number of available measurements from the network of observations so that net-weights can be compared for a map with the observations. The netweight w41 means positive phase of storm-time deviation for the given parameter, 0owo1 means negative phase. So the defined index allows to compare the storm-time behavior of different parameters. Comparisons of nighttime (00 h UT) and daytime (12 h UT) net-weights for GPS–TEC observations with relevant Kriging TEC maps are presented in Table 2. It is evident from Table 2 that mapping results are well consistent with observations. So we proceed further with Kriging mapping technique for TEC and foF2 parameters yielding the slab thickness map by applying Eq. (1) at each grid point of the source maps. To estimate the deformation of the ionosphere during storm-time, we have used model-assisted analysis of the slab thickness results. The IRI model extended towards the plasmasphere (IRI*) has been used for inversion of results of observation to electron density profile. Input of ionosonde-derived critical frequency foF2 and peak height hmF2 ensures IRI* true electron density maximum at the F2 layer peak. The IRI* code makes use of one more anchor point in the topside ionosphere at the half peak density 0.5 NmF2 (Gulyaeva, 2003b). The latter topside shape parameter is varied in the present study to conform the electron density profile to GPS–TEC observations. So the IRI* code providing electron content separately below and above hmF2 in the ionosphere and that for the plasmasphere allows to distinguish the separate parts of the slab thickness in the bottomside ionosphere (tbot), topside ionosphere (ttop) and the plasmasphere (tpl) using Eq. (1). Eliminating the plasmaspheric slab thickness component from total t, we can deal with the residual proportions of the slab thickness in the topside and bottomside ionosphere to
Examples of instantaneous maps of TEC, foF2, and slab thickness t for the nighttime negative ionospheric storm at solar maximum, 10 January 2002, are given in Fig. 1. The high-latitude trough (decrease) of electron content TEC (upper section) is located at lower latitudes than the F2 layer peak ionization trough (middle section). Calculations of t (bottom section) yields the slab thickness bulge (increased t) centered over the F2 layer trough of peak electron density. Table 3 presents mapping results of the area mean and maximal values of t for nighttime and daytime for 10 perturbed days. The area mean value represents the
Fig. 1. TEC, foF2 and slab thickness maps during the negative nighttime ionosphere storm.
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Table 3 Slab thickness area average and maximum values of t, km, from instantaneous maps for disturbed days Date
07.04.95 28.09.95 19.10.95 13.01.96 23.10.96 10.01.02 06.02.02 24.03.02 18.04.02 24.05.02
Night: UT ¼ 00 h
Day: UT ¼ 12 h
t ave
t max
F1N
L1E
t ave
t max
F1N
L1E
782 480 345 824 428 1069 514 365 698 852
1725 859 537 1157 741 1713 876 622 2332 2229
57.2 54.6 40.7 64.9 65.4 57.8 54.6 56.9 59.4 56.3
118.0 100.1 70.1 81.0 115.9 100.1 125.6 99.4 123.4 117.3
387 355 341 447 399 244 256 328 540 314
550 520 690 1080 1085 440 504 611 1759 413
72.8 72.8 72.8 63.9 63.9 72.8 72.8 72.8 72.8 63.9
93.3 93.3 93.3 132.9 132.9 93.3 93.3 93.3 93.3 132.9
Geomagnetic coordinates of tmax are given.
Fig. 2. Map-produced latitudinal variation of the ionospheric slab thickness under monthly median quiet (Q—circles) conditions and disturbed (D—asterisks) conditions at nighttime and daytime for solar minimum (1995) and maximum (2002) at longitude of 301E.
average of the parameter from all grid points of the map. The mean values of t are greater by night than by day. While normal average slab thickness does not exceed 800 km (Goodwin et al., 1995), very large values of tmax exceeding 1000 km require further analysis of the ionosphere–plasmasphere interactions during the storm. Nighttime and daytime latitudinal profiles of slab thickness at selected longitude of 301E for quiet monthly median conditions and two storms at equinox (April) for low- and high-solar activity are plotted on Fig. 2. Increase of t during the storm is most effective at night both at solar minimum and maximum. The slab
Fig. 3. Evolution of net-weight factor derived from TEC (crosses), foF2 (circles) and slab thickness t (asterisks) maps during 48 h since the onset of magnetic storm averaged separately for nighttime and daytime onset of magnetic storm from the data of 10 storms at solar minimum and maximum.
thickness bulge (over the trough of ionization) is observed under the storm conditions. More general outcome from mapping results is plotted on Fig. 3. Here average evolution of net-weight parameters for foF2, TEC, and t during the time since onset of 10 magnetic storms (Table 2) are presented grouped in respect to the level of solar activity, nighttime or daytime onset of the magnetic storm defined by the period preceding the peak of storm with running average of am index reached its half-maximum value, and applying a superposed epoch analysis to demonstrate the time evolution of each ionospheric parameter after the storm onset (0 h in Fig. 3). Positive phase of the critical frequency foF2 and TEC (w41) is gradually transformed into the negative phase (wo1).
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2000
2000
1500
1500 Height (km)
Height (km)
1312
1000
1000 500
500 40
0 70
60
Lati
tude
50 (de
g)
40
0 -10
40
0 70
30 20 ) g de 10 e( itud t a L
60
Lati
tude
50 (de g)
40
0 -10
30 20 ) g de 10 e( itud t a L
2000 2000 Height (km)
1500 Height (km)
1500 1000
1000 500
500 0 70
40
Lat
60 itud
50 e (d
eg)
40
0 -10
30 20 ) g de 10 e( itud t a L
40
0 70
60 Lati
tude
50 (de g)
40
0 -10
30 20 ) g de 10 e( itud t a L
Fig. 4. Nighttime and daytime surface maps of the transition height between the ionosphere and plasmasphere (the topside boundary of the ionosphere) under quiet monthly median conditions and storm on 24 March 2002.
TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 h during the magnetic storm at solar maximum but t enhancement is shorter and delayed by 12–24 h regarding the storm onset at solar minimum. Fig. 4 presents night–day imprints of the surface maps of the upper boundary of the ionosphere under quiet (monthly median) conditions and during the storm on 24 March 2002 as revealed by IRI model-assissted analysis of the above results. It is evident from this Figure that the ionosphere–plasmasphere transition height is growing during the perturbed conditions both by daytime and nighttime (upto 2000 km altitude over the Earth) exceeding normal boundary of the ionosphere under quiet median conditions. Formation of the ionosphere bulge over the trough of the F2 layer peak ionization is observed the most pronounced by nighttime (see Fig. 1 for a comparison). Summary of the temporal–spatial mean values of the upper boundary of the ionosphere and its standard
deviation determined for quiet and disturbed conditions are presented in Table 4. From this table the upper boundary is greater by nighttime (1200–1600 km) that by daytime (600–900 km) increasing from quiet to disturbed conditions and from solar minimum to maximum. The individual values of the upper boundary height are changing from 500 km by day to 2300 km by night with standard deviation of results comprising 10–30% of the mean values representing the RMS error of model evaluation of the upper boundary of the ionosphere.
4. Discussion and conclusions Results of slab thickness evolution during the magnetic storm depend on relative variations of TEC and NmF2, as well as on the storm intensity and the time passed since the storm onset. Growing slab thickness during a storm (‘positive phase’ of slab
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Table 4 Area mean and standard deviation (s) of the upper boundary of ionosphere, km, averaged for all selected periods for night and day, quiet and disturbed conditions, solar minimum and maximum Year
1995–1996 2002
Night UT ¼ 00 h
Day UT ¼ 12 h
Conditions
Mean
s
Mean
s
1540 1244 1516 1259
370 152 312 171
791 647 917 830
243 62 150 102
thickness storm) may serve as a manifestation of electric fields that produce a vertical drift, driving plasma upwards to regions of lower loss (Pro¨lss, 1991; Rishbeth, 1991). Enhancement of the slab thickness (area mean net-weight w41) is observed for the negative phase of the F region peak and electron content TEC for solar maximum (during more than 48 h since storm onset) and solar minimum (delayed by 12–24 h since storm onset). Growth of the slab thickness accompanied by uplift of the topside boundary of the ionosphere requires reevaluation of earlier results of the F layer peak density analysis which should be considered jointly with the ionosphere thickness variations (see e.g. Field and Rishbeth, 1997). Our extreme maximal values of t (see Table 3) suggest that definition of the upper boundary of the ionosphere should be re-examined. The upper boundary of the ionosphere is regarded to be at altitudes about 1000 km over the Earth, gradually transformed into plasmasphere/magnetosphere above this height. Eliminating the plasmasphere contribution to the slab thickness, the growing slab thickness during a magnetic storm exceeding 1000 km implies expanding of an actual ionosphere thickness which is directly related with the slab thickness (Gulyaeva et al., 2004). It is shown that the upper boundary of the ionosphere is shifted upward during the storm forming, something like the ionosphere-tail over the nighttime high-latitude trough of ionization. Modelassissted approach for GPS–TEC profile inversion allows to estimate the ionospheric upper boundary, assuming that the ratio of slab thickness to real thickness in the topside ionosphere conforms to the relevant ratio in the bottom ionosphere. It is found that the real thickness of the ionosphere is greater than the ionospheric slab thickness, both being increased significantly during the magnetic storms. The spatial–temporal average of the upper ionosphere boundary varies from 600 to 1600 km with nighttime and daytime dispersion of 10–30%. The ionosphere–plasmasphere (IP) boundary altitude has been related from the earlier studies with the O+/H+ transition height assumed to occur near 1000 km above the Earth (Carpenter and Bowhill, 1971; Carpenter and Park,
Disturbed Quiet Disturbed Quiet
1973). The availability of the satellite data base allowed to construct a model of O+/H+ transition height (Kutiev et al., 1994). Our solar cycle dependence of the IP boundary follows variations of O+/H+ transition height, but the diurnal variations of our estimates of the IP boundary differ from the O+/H+ model which shows dominant daytime values. The reduced daytime values of the IP boundary height are similar to magnetosphere configuration compressed at dayside due to impringing solar wind. Independent techniques for measurements of the plasma mass density in the region where the ionosphere mixes with the plasmasphere (Price et al., 1999) might shed light on the processes governing the position of the IP boundary zone.
Acknowledgments The authors are grateful to Norbert Jakowski, DLR, Germany, and Liljana Cander, RAL, UK, for providing GPS-TEC data. Magnetic indices and ionosonde data have been provided through the World Data Centers: Boulder, CO, USA; Oxford, UK; Moscow, Russia; Meudon, France; Copenhagen, Denmark; Kyoto, Japan; Warsaw, Poland; Sodankyla, Finland; Rome, Italy. The authors are indebted to important comments of two unknown referees stimulating improvements of the paper.
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