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A relationship between the auroral absorption and the magnetic activity in the polar cap
Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
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Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
A relationship between the auroral absorption and the magnetic activity in the polar cap Alexander Frank-Kamenetsky, Oleg Troshichev n Arctic and Antarctic Research Institute, 199397 St. Petersburg, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 May 2011 Received in revised form 2 November 2011 Accepted 12 November 2011 Available online 30 November 2011
The auroral absorption is an absorption of the cosmic radio noise. It depends on the electron concentration in the ionospheric D layer, which is strongly affected by the precipitating flux of highenergy (E 4 30 keV) electrons. The data from Canadian chain of riometers were used to study the absorption as a function of magnetic local time (MLT) and geomagnetic latitude for different levels of geomagnetic activity, characterized by the Polar Cap (PC) index (Troshichev et al., 2006). The 1 min and hourly values of absorption were divided into two sets depending on the level of activity and examined as a function of MLT and geomagnetic latitude for quiet conditions (/PCS ¼ 0.56 mV/m) and a higher activity (/PCS¼ 1.87 mV/m). It is shown that the auroral absorption reaches distinct maxima at latitudes of 661–681 (equatorward zone) and 711–721 (poleward zone), both of them being dependent on level of activity determined by the PC-index. The maximum correlation between the absorption and PC index is observed in the after-midnight MLT sector. The results show that the PC index is a good indicator of the solar wind energy entry into the magnetosphere and can be useful for monitoring the state of low ionosphere in the auroral zone. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Auroral absorption PC index Particle precipitation D-layer of ionosphere
1. Introduction. PC index was introduced by Troshichev and Andrezen (1985) and Troshichev et al. (1988) as an indicator of the polar cap magnetic activity generated by the geoeffective solar wind. The PC index is calculated independently for the northern (PCN) and southern (PCS) polar caps, based on magnetic data from two nearpole stations Thule (Greenland) and Vostok (Antarctica), using the formula PC ¼ ðdFbÞ=a, where dF is a value of magnetic DP2 disturbance determined by data from Thule or Vostok, a and b are coefficients of regression (calibrating coefficients) linking the dF with the value of the coupling function EKL determined using the formula (Kan and Lee, 1979). The comprehensive investigations by Newell et al. (2007, 2008) revealed that a unique coupling function applicable for to any circumstances and conditions (if it exists) must involve solar wind velocity v to the first (or a little higher) power, transverse IMF component BT to the first (or a little lower) power, and sine of IMF clock angle yc to the second (or more) power. Just function EKL ¼vBTsin2(yc/2) taken as basic in deriving a PC index is consistent with these requirements. Using the calibrating coefficients, the PC n
Corresponding author. Fax: þ7 0812 3522688. E-mail address: [email protected] (O. Troshichev).
1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.11.007
index is defined as a value of the polar cap magnetic activity normalized for intensity of the geoffective electric field EKL with allowance for the magnetic activity dependence on season, UT and hemisphere. As a result, the PCN and PCS indices adequately respond to impact on the interplanetary electric field on magnetosphere regardless of the location of the station and the season. Initially the PC index was introduced only as a characteristic of the polar cap magnetic activity. The recent studies (Janzhura et al., 2007; Troshichev and Janzhura, 2009; Troshichev et al., 2011) showed that the PC index implication seems to be more significant: the index can be regarded as an adequate indicator of the solar wind energy that entered into the magnetosphere. Indeed, the magnetospheric storms and substorms start only if the PC index reaches the definite threshold value ( 2 mV/m for storms and 41.5 mV/m for substorms); the substorm growth phase duration and substorm intensity are determined by the PC growth rate and substorms are stopped as soon as PC index falls below 1–1.5 mV/m; the storm length is terminated by a duration of period, when PC 42 mV/m, the storm intensity being linearly related to the PC index averaged for the storm time interval, and so on. In this connection it is interesting to examine the relationship between the PC index behavior and evidences for substorm in auroral ionosphere, the auroral absorption being one of them. The auroral absorption is an indicator of the high-energy electrons intrusion in the lowest ionosphere layer D. Energies of the auroral particles are in a wide range: from several tens of eV to hundreds of keV. While penetrating into the ionosphere they
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
41
Table 1 List of stations whose data were used in the analysis. Geographic coordinates
Invariant geomagnetic coordinates
Magnetic shell
Local time at 00 UT
Station
Lat F
Long L
Lat F0
Long L0
L
MLT
Pinawa Island Lake Gillam Fort Churchill Eskimo Point Rankin Inlet Taloyoak
50.2 53.86 56.38 58.76 61.11 62.82 69.54
96.04 94.66 94.64 94.08 94.05 92.11 93.55
60.288 63.963 66.379 68.675 71.221 72.58 78.644
28.971 27.377 27.72 27.248 28.54 24.833 30.261
4.13 5.26 6.32 7.67 9.79 11.32 27.05
16.97 17 16.94 16.92 16.84 16.95 16.33
8 7 6
PC, mV/m
5 4 3 2 1 0 -1 0:00
2:00
4:00
6:00
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
0:00
UT, h Magnetic midnight Fig. 1. Variations of auroral absorption (upper panel) at meridian MLong E–301 and PC index (lower panel) in course of substorm on January 7, 2004.
42
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
give rise to electron density allowing the formation of anomalous ionization at altitudes from 60 to 300 km. Precipitation of high-energy electrons (Ee430 keV) enhances the electron concentration at altitudes 60–90 km that leads to absorption of the regular cosmic radio-emission known as an auroral radio absorption (Mitra and Shain, 1953). Auroral absorption (AA) is highly structured in both space and time. Statistics of auroral absorption has been studied for many latitudes and longitudes (Holt et al., 1961; Hartz et al., 1963; Driatsky, 1966; Hargreaves and Cowley, 1967; Foppiano and Bradley, 1984, 1985). Usually the diurnal occurrence shows two peaks, one close to magnetic midnight and the other before noon. The absorption zone covers several degrees of latitude centered between 641 and 681 magnetic latitude, a few degrees equatorward of the auroral oval (Hartz et al., 1963; Basler, 1963; Driatsky, 1966; Hook, 1968). It is mostly associated with the diffuse aurora but often occurs close to auroral arcs. A notable effect is the deep minimum in the afternoon (Hargreaves and Cowley, 1967). The most complete picture of the evolution of disturbances in the auroral absorption is given in Berkey et al. (1974). It is well known that the enhanced auroral absorption is one of the main features of magnetospheric substorm, however, there is still no clear idea of the relationship between the PC index behavior and the main parameters of absorption. In this study we examine the auroral absorption response to the PC index changes. The aim of this study is to ascertain the potentiality of use of the PC index, which is considered as an indicator of the solar wind energy that entered into the magnetosphere, to monitor the state of low ionosphere in the auroral zone.
To make allowance for varying delay times between the PC growth and auroral particle precipitation, the following procedure has been applied: the maximal values of absorption observed at stations in each 1 h was identified and these quantities were examined in relation to the appropriate hourly averaged values of the PC-index. Correlation between the PC indices and auroral absorption has been analyzed using 1 min and hourly data sets.
3. Main results The isolated events are usually examined to investigate the auroral absorption characteristics. As an example Fig. 1 shows the contours of the equal auroral absorption, obtained from the Canadian riometer chain for the 301 magnetic meridian (on the top) and variation of the PC index (on the bottom) as a function of universal time (UT) for the magnetic disturbance on January 7, 2004. One can see that almost all increases in PC-index observed after the midnight (00 MLT¼ 07 UT) were accompanied by an increase in auroral absorption, whereas the absorptions observed in the morning and midday hours are essentially ( Z1 h) delayed relative to the PC-index run. Auroral absorption occurring before the midnight seems to be unrelated with the PC index changes. Fig. 2 shows distribution of the monthly averaged absorption (Amax_aver) at latitudes F0 ¼601–801 for two winter months with low (PCaver ¼0.68 mV/m, December 2007) and enhanced (PCaver¼ 1.87 mV/m, January 2004) magnetic activity. Values (Amax_aver) were derived by data on the maximal values of absorption observed at the Canadian network of stations for each 1 h of examined months, and then the plots of flattening
2. Data description and analysis To analyze the relationships between the PC index and auroral absorption the data from the Canadian chain of riometers were used /http://www.dsc.lancs.ac.uk/iono/dataS. This meridian chain covering the interval of invariant latitudes from 601 to 781 (see Table 1) provides the most full information about the auroral absorption variations in course of magnetospheric substorms. The analysis is carried out for periods of low (February, June, August and December of 2007) and rather high (January and February of 2004) solar activity. The 1 min average values of absorption were compared with the 1 min PCN index derived for the northward polar cap /http://web.dmi.dk/projects/wdcc1/pcn/pcn.htmlS. As results (Janzhura et al., 2007; Troshichev and Janzhura, 2009) showed, the PC index growth precedes the substorm onset irrespective of a substorm type (isolated or sawtooth) and intensity, the substorm growth phase duration being dependent on the PC growth rate and PC value: the higher the rate and value, the shorter is the substorm growth phase. Magnetic disturbances in the auroral zone (AL index) better relate to the polar cap magnetic activity PC index than to coupling function EKL, even though the PC index was derived as a proxy of field EKL. Owing to high ionospheric conductivity in the sunlit polar cap, the summer PC index starts to grow in response to EKL increase changes, well before (from few minutes to 1 h) the substorm sudden onset. Because of low ionospheric conductivity in the dark polar cap, the winter PC index starts to increase with a delay with respect to the summer PC index and demonstrates a wholly satisfactorily correlation with the AL index in substorm growth and expansion phases. The latter means that during winter season the polar cap magnetic activity determined by intensity of Region 1 fieldaligned currents is regulated by ionospheric conductivity in the auroral zone. Thus the auroral absorption responds better to the winter PC index considering a delay between them, which depends on the value of the PC index value and its growth rate.
Fig. 2. Distribution of maximal absorption (Amax_aver) observed at the Canadian chain of stations in the winter months with low (PCaver ¼ 0.68 mV/m, December 2007) and enhanced (PCaver¼ 1.87 mV/m, January 2004) levels of magnetic activity.
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
distribution of values (Amax_aver) were presented as a function of UT (axis of abscissa) and geomagnetic latitude (axis of ordinate). It is seen that during the period of low activity (PCaver¼0.68 mV/m) only one zone of absorption is observed at latitude F0 ¼701–741 with maximum in the after-midnight sector (07–13 UT). The maximum of absorption reaches the value of 0.5 dB. The same regularity was revealed for averaged auroral distribution in summer months. In periods of enhanced activity (PCaver¼1.87 mV/m) two zones of absorption, the main one at latitudes 651–671 and the secondary one at latitude 701–721 are observed consistently. The morning (14–18 UT) maximum of absorption ( 2.6 dB) becomes dominant and considerably prevailing over the midnight absorption in the both zones. Regularity
43
of the diurnal changes of the auroral absorption intensity characterizing by one maximum in the absorption level near the midnight in quiet conditions and by two absorption maximums in the night time and morning sector under disturbed conditions is in good accordance with the well-known results obtained earlier (Hargreaves and Cowley, 1967). Relationship between the PC index and auroral distribution was examined with use of 1 min and hourly maximum data on auroral absorption, in the latter case the value of absorption was compared with hourly averaged PC values. As one would expect, correlation between 1 min values of PC and absorption turned out to be lower than that between the hourly quantities, as a result of varying delay time between the PC growth beginning
December 2007, PCaver = 0.68 80
R 78 0.9
76
0.8 74 0.7
MLat, deg
72
0.6
70
0.5 0.4
68
0.3 66 0.2 64
0.1
62
0
60 0
2
4
6
8
10
12
14
16
18
20
22
24
UT, h
Fig. 3. Correlation between the 1 min values of PC and absorption as a function of UT (axis of abscissa) and geomagnetic latitude (axis of ordinate) for the winter months with low (a) and enhanced (b) levels of magnetic activity.
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
Φ' = 63.9°, UT = 8-16, R = 0.57
7 6 5 4 3 2 1 0 -1 -1
1
3
7
5
-1
Φ' = 63.9,UT = 8-16, R = 0.67
8 7 6 5 4 3 2 1 0 -1 -1
1
3 5 PC, mV/m
7
3.5 3 2.5 2 1.5 1 0.5 0 -0.5 -1
9
A, dB
A, dB
Φ' = 68.7°,UT = 8-12, R = 0.64
A, dB
A, dB
44
9
1
5
3
7
9
Φ' = 68.7, UT = 8-12, R = 0.66
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 -0.5 0
2
4 PC, mV/m
6
8
Fig. 4. Relationship between the PC index and auroral absorption at latitudes 63.91 (left column) and 68.71 (right column) derived for time intervals of their best correspondence. The upper panel is for 1 min data set, the lower panel is for set of hourly averaged PC and hourly maximal absorption data set.
and the corresponding absorption development. Fig. 3 shows the plots of flattening distributions for coefficients of correlation between 1 min values of PC and absorption as a function of UT (axis of abscissa) and geomagnetic latitude (axis of ordinate) for two levels of activity (a) PCaver ¼0.68 mV/m and (b) PCaver ¼1.87 mV/m. One can see that the meaningful correlation between PC and auroral absorption is observed generally in areas with high absorption. Under conditions of low activity the correlation is highest near the midnight and in the after-midnight sectors at latitudes 681–721, however the distinct spotty manner of distribution is indicative of rather accidental character of correlation. Under conditions of enhanced activity two zones of the high correlation are observed in the after-midnight sector (07–16 UT): the first zone is at latitudes 601–651 and the secondary one is at latitudes 681–721. So, the region of high correlation within the main zone turned out to be displaced equatorward relative to the region of the maximal absorption. Most likely, this inconsistency is connected with delayed development of absorption at higher latitudes. Indeed, as Fig. 1 demonstrates, the coincidence of bursts in PC and absorption values is typical only for equatorward part of the main zone, the higher latitude, the later maximum of absorption is observed. Fig. 4 shows a relationship between the PC index and auroral absorption at latitudes 63.91 (the main zone) and 68.71 (the secondary zone) during the time intervals for which the best correlation between the absorption and the PC index has been observed, namely between 8 and 16 UT for the main absorption zone, and 8 and 12 UT for the secondary zone, the upper panel being for set of 1 min data, the lower panel being for set of hourly averaged PC and hourly maximal absorption data. It is seen that the auroral absorption quickly enlarges with the PC growth in the entire auroral zone, irrespective of data choice, but correlation is improved when passing from 1 min to the hourly data. As it was noted above, in the latter case the effect of varying delay times is withdrawn from the examination.
4. Discussion and conclusions The results of the analysis demonstrate existence of two distinct maxima in the latitudinal distribution of the absorption intensity, the first one being located at latitudes 631–681 and the second one being located at 711–721. The presence of two latitudinal zones of auroral absorption was shown previously by Samsonov and Sokolov (1990) and Samsonov et al. (1991) who revealed, with use of data from the Yakut riometer chain, two regions of the maximal absorption at latitudes of 661–671 and 711 before noon. It was concluded that there are different mechanisms of energetic electrons precipitation in low and high latitudinal zones: at latitudes of 651–671 precipitation occurs in the regime of weak, but at 71 1 in the regime of strong pitch-angle diffusion. Satellite observations also confirm the existence of two zones of energetic electrons precipitation. According to Shepherd et al. (1990), the equatorward, stable peak is located at a latitude of 651, whereas the poleward, stretched along the auroral oval, peak is located at a latitude 701. The latter is identified by Shepherd et al. (1990) with the particle precipitation from the boundary plasma layer. As our analysis shows, the magnitude of auroral absorption in both zones is evidently dependent on the activity level, which is characterized by PC-index. It is known that auroral absorption initially starts at about midnight hours at the latitude of the auroral oval and subsequently absorption region expanded primarily to the pole and to the east. Namely for this reason, the maximum correlation coefficients between the absorption and PC index are observed in the after-midnight MLT sector. The auroral absorption quickly enlarges with the PC growth in the entire auroral zone.
References Basler, R.R., 1963. Radio wave absorption in the auroral ionosphere. Journal of Geophysical Research 68, 4665. Berkey, F.T., Driatsky, V.M., Henriksen, K., Hultqvist, B., Jelly, D.H., Schuka, T.I., Theander, A., Yliniemi, J., 1974. A synoptic investigation of particle precipitation
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
dynamics for 60 substorms in IQSY (1964–65) and IASY (1969). Planetary Space Science 22, 265–307. Driatsky, V.M., 1966. Study of the space and time distribution of auroral absorption according to observations of the riometer network in the Arctic. Geomagnetism and Aeronomy 6, 828–832. Foppiano, A.J., Bradley, P.A., 1984. Day-to-day variability of riometer absorption. Journal of Atmospheric and Terrestrial Physics 46, 689–696. Foppiano, A.J., Bradley, P.A., 1985. Morphology of background auroral absorption. Journal of Atmospheric and Terrestrial Physics 47, 663–674. Hargreaves, J.K., Cowley, F.C., 1967. Studies of radio absorption at three magnetic latitudes 1. Occurrence and statistical properties of the events. Planetary Space Science 15, 1571–1583. Hartz, T.R., Montbriand, L.E., Vogan, E.L., 1963. A study of auroral absorption at 30 Mc/s. Canadian Journal of Physics 41, 581–595. Holt, O., Landmark, B., Lied, F., 1961. Analysis of riometer observations obtained during polar radio blackouts. Journal of Atmospheric and Terrestrial Physics 23, 229–243. Hook, J.L., 1968. Morphology of auroral zone radiowave absorption in the Alaska sector. Journal of Atmospheric and Terrestrial Physics 30, 1341. Janzhura, A., Troshichev, O., Stauning, P., 2007. Unified PC indices: relation to the isolated magnetic substorms. Journal of Geophysical Research 112, A09207. doi:10.1029/2006JA012132. Kan, J.R., Lee, L.C., 1979. Energy coupling function and solar wind-magnetosphere dynamo. Geophysical Research Letters 6, 577. Mitra, A.P., Shain, C.A., 1953. The measurement of ionospheric absorption using observations of 18.3 mc/s cosmic radio noise. Journal of Atmospheric and Terrestrial Physics 3, 203.
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Samsonov, S.N., Sokolov, V.D., 1990. Two-zone of precipitation of energetic electrons. Geomagnetism and Aeronomy 30 (2), 216–221. Samsonov, S.N., SokolovV, D., Ranta, H., 1991. Features of the day riometric absorption. Research on Geomagnetism, Aeronomy and Solar Physics 93, 69–72. Shepherd, G.G., Steen, A., Murphree, J.S., 1990. Auroral boundary dynamics observed simultaneously from the VIKING spacecraft and from the ground. Journal of Geophysical Research 95 (A5), 5845–5865. Troshichev, O.A., Andrezen, V.G., 15, 1985. The relationship between interplanetary quantities and magnetic activity in the southern polar cap. Planetary Space Science 33, 4. Troshichev, O., Janzhura, A., 2009. Relationship between the PC and AL indices during repetitive bay-like magnetic disturbances in the auroral zone. Journal of Atmospheric and Solar-Terrestrial Physics 71, 1340–1352. Troshichev, O.A., Andrezen, V.G., Vennerstrøm, S., Friis-Christensen, E., 1988. Magnetic activity in the polar cap—a new index. Planetary Space Science 36, 1095. Troshichev, O.A., Janzhura, A., Stauning, P., 2006. Unified PCN and PCS indices: method of calculation, physical sense and dependence on the IMF azimuthal and northward components. Journal of Geophysical Research 111, A05208. doi:10.1029/2005JA011402. Troshichev, O., Sormakov, D., Janzhura, A., 2011. Relation of PC index to the geomagnetic storm Dst variation. Journal of Atmospheric and Solar-Terrestrial Physics 73, 611–622. /http://www.dsc.lancs.ac.uk/iono/dataS. /http://web.dmi.dk/projects/wdcc1/pcn/pcn.htmlS.
Contents lists available at SciVerse ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
A relationship between the auroral absorption and the magnetic activity in the polar cap Alexander Frank-Kamenetsky, Oleg Troshichev n Arctic and Antarctic Research Institute, 199397 St. Petersburg, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 May 2011 Received in revised form 2 November 2011 Accepted 12 November 2011 Available online 30 November 2011
The auroral absorption is an absorption of the cosmic radio noise. It depends on the electron concentration in the ionospheric D layer, which is strongly affected by the precipitating flux of highenergy (E 4 30 keV) electrons. The data from Canadian chain of riometers were used to study the absorption as a function of magnetic local time (MLT) and geomagnetic latitude for different levels of geomagnetic activity, characterized by the Polar Cap (PC) index (Troshichev et al., 2006). The 1 min and hourly values of absorption were divided into two sets depending on the level of activity and examined as a function of MLT and geomagnetic latitude for quiet conditions (/PCS ¼ 0.56 mV/m) and a higher activity (/PCS¼ 1.87 mV/m). It is shown that the auroral absorption reaches distinct maxima at latitudes of 661–681 (equatorward zone) and 711–721 (poleward zone), both of them being dependent on level of activity determined by the PC-index. The maximum correlation between the absorption and PC index is observed in the after-midnight MLT sector. The results show that the PC index is a good indicator of the solar wind energy entry into the magnetosphere and can be useful for monitoring the state of low ionosphere in the auroral zone. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Auroral absorption PC index Particle precipitation D-layer of ionosphere
1. Introduction. PC index was introduced by Troshichev and Andrezen (1985) and Troshichev et al. (1988) as an indicator of the polar cap magnetic activity generated by the geoeffective solar wind. The PC index is calculated independently for the northern (PCN) and southern (PCS) polar caps, based on magnetic data from two nearpole stations Thule (Greenland) and Vostok (Antarctica), using the formula PC ¼ ðdFbÞ=a, where dF is a value of magnetic DP2 disturbance determined by data from Thule or Vostok, a and b are coefficients of regression (calibrating coefficients) linking the dF with the value of the coupling function EKL determined using the formula (Kan and Lee, 1979). The comprehensive investigations by Newell et al. (2007, 2008) revealed that a unique coupling function applicable for to any circumstances and conditions (if it exists) must involve solar wind velocity v to the first (or a little higher) power, transverse IMF component BT to the first (or a little lower) power, and sine of IMF clock angle yc to the second (or more) power. Just function EKL ¼vBTsin2(yc/2) taken as basic in deriving a PC index is consistent with these requirements. Using the calibrating coefficients, the PC n
Corresponding author. Fax: þ7 0812 3522688. E-mail address: [email protected] (O. Troshichev).
1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.11.007
index is defined as a value of the polar cap magnetic activity normalized for intensity of the geoffective electric field EKL with allowance for the magnetic activity dependence on season, UT and hemisphere. As a result, the PCN and PCS indices adequately respond to impact on the interplanetary electric field on magnetosphere regardless of the location of the station and the season. Initially the PC index was introduced only as a characteristic of the polar cap magnetic activity. The recent studies (Janzhura et al., 2007; Troshichev and Janzhura, 2009; Troshichev et al., 2011) showed that the PC index implication seems to be more significant: the index can be regarded as an adequate indicator of the solar wind energy that entered into the magnetosphere. Indeed, the magnetospheric storms and substorms start only if the PC index reaches the definite threshold value ( 2 mV/m for storms and 41.5 mV/m for substorms); the substorm growth phase duration and substorm intensity are determined by the PC growth rate and substorms are stopped as soon as PC index falls below 1–1.5 mV/m; the storm length is terminated by a duration of period, when PC 42 mV/m, the storm intensity being linearly related to the PC index averaged for the storm time interval, and so on. In this connection it is interesting to examine the relationship between the PC index behavior and evidences for substorm in auroral ionosphere, the auroral absorption being one of them. The auroral absorption is an indicator of the high-energy electrons intrusion in the lowest ionosphere layer D. Energies of the auroral particles are in a wide range: from several tens of eV to hundreds of keV. While penetrating into the ionosphere they
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
41
Table 1 List of stations whose data were used in the analysis. Geographic coordinates
Invariant geomagnetic coordinates
Magnetic shell
Local time at 00 UT
Station
Lat F
Long L
Lat F0
Long L0
L
MLT
Pinawa Island Lake Gillam Fort Churchill Eskimo Point Rankin Inlet Taloyoak
50.2 53.86 56.38 58.76 61.11 62.82 69.54
96.04 94.66 94.64 94.08 94.05 92.11 93.55
60.288 63.963 66.379 68.675 71.221 72.58 78.644
28.971 27.377 27.72 27.248 28.54 24.833 30.261
4.13 5.26 6.32 7.67 9.79 11.32 27.05
16.97 17 16.94 16.92 16.84 16.95 16.33
8 7 6
PC, mV/m
5 4 3 2 1 0 -1 0:00
2:00
4:00
6:00
8:00
10:00
12:00
14:00
16:00
18:00
20:00
22:00
0:00
UT, h Magnetic midnight Fig. 1. Variations of auroral absorption (upper panel) at meridian MLong E–301 and PC index (lower panel) in course of substorm on January 7, 2004.
42
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
give rise to electron density allowing the formation of anomalous ionization at altitudes from 60 to 300 km. Precipitation of high-energy electrons (Ee430 keV) enhances the electron concentration at altitudes 60–90 km that leads to absorption of the regular cosmic radio-emission known as an auroral radio absorption (Mitra and Shain, 1953). Auroral absorption (AA) is highly structured in both space and time. Statistics of auroral absorption has been studied for many latitudes and longitudes (Holt et al., 1961; Hartz et al., 1963; Driatsky, 1966; Hargreaves and Cowley, 1967; Foppiano and Bradley, 1984, 1985). Usually the diurnal occurrence shows two peaks, one close to magnetic midnight and the other before noon. The absorption zone covers several degrees of latitude centered between 641 and 681 magnetic latitude, a few degrees equatorward of the auroral oval (Hartz et al., 1963; Basler, 1963; Driatsky, 1966; Hook, 1968). It is mostly associated with the diffuse aurora but often occurs close to auroral arcs. A notable effect is the deep minimum in the afternoon (Hargreaves and Cowley, 1967). The most complete picture of the evolution of disturbances in the auroral absorption is given in Berkey et al. (1974). It is well known that the enhanced auroral absorption is one of the main features of magnetospheric substorm, however, there is still no clear idea of the relationship between the PC index behavior and the main parameters of absorption. In this study we examine the auroral absorption response to the PC index changes. The aim of this study is to ascertain the potentiality of use of the PC index, which is considered as an indicator of the solar wind energy that entered into the magnetosphere, to monitor the state of low ionosphere in the auroral zone.
To make allowance for varying delay times between the PC growth and auroral particle precipitation, the following procedure has been applied: the maximal values of absorption observed at stations in each 1 h was identified and these quantities were examined in relation to the appropriate hourly averaged values of the PC-index. Correlation between the PC indices and auroral absorption has been analyzed using 1 min and hourly data sets.
3. Main results The isolated events are usually examined to investigate the auroral absorption characteristics. As an example Fig. 1 shows the contours of the equal auroral absorption, obtained from the Canadian riometer chain for the 301 magnetic meridian (on the top) and variation of the PC index (on the bottom) as a function of universal time (UT) for the magnetic disturbance on January 7, 2004. One can see that almost all increases in PC-index observed after the midnight (00 MLT¼ 07 UT) were accompanied by an increase in auroral absorption, whereas the absorptions observed in the morning and midday hours are essentially ( Z1 h) delayed relative to the PC-index run. Auroral absorption occurring before the midnight seems to be unrelated with the PC index changes. Fig. 2 shows distribution of the monthly averaged absorption (Amax_aver) at latitudes F0 ¼601–801 for two winter months with low (PCaver ¼0.68 mV/m, December 2007) and enhanced (PCaver¼ 1.87 mV/m, January 2004) magnetic activity. Values (Amax_aver) were derived by data on the maximal values of absorption observed at the Canadian network of stations for each 1 h of examined months, and then the plots of flattening
2. Data description and analysis To analyze the relationships between the PC index and auroral absorption the data from the Canadian chain of riometers were used /http://www.dsc.lancs.ac.uk/iono/dataS. This meridian chain covering the interval of invariant latitudes from 601 to 781 (see Table 1) provides the most full information about the auroral absorption variations in course of magnetospheric substorms. The analysis is carried out for periods of low (February, June, August and December of 2007) and rather high (January and February of 2004) solar activity. The 1 min average values of absorption were compared with the 1 min PCN index derived for the northward polar cap /http://web.dmi.dk/projects/wdcc1/pcn/pcn.htmlS. As results (Janzhura et al., 2007; Troshichev and Janzhura, 2009) showed, the PC index growth precedes the substorm onset irrespective of a substorm type (isolated or sawtooth) and intensity, the substorm growth phase duration being dependent on the PC growth rate and PC value: the higher the rate and value, the shorter is the substorm growth phase. Magnetic disturbances in the auroral zone (AL index) better relate to the polar cap magnetic activity PC index than to coupling function EKL, even though the PC index was derived as a proxy of field EKL. Owing to high ionospheric conductivity in the sunlit polar cap, the summer PC index starts to grow in response to EKL increase changes, well before (from few minutes to 1 h) the substorm sudden onset. Because of low ionospheric conductivity in the dark polar cap, the winter PC index starts to increase with a delay with respect to the summer PC index and demonstrates a wholly satisfactorily correlation with the AL index in substorm growth and expansion phases. The latter means that during winter season the polar cap magnetic activity determined by intensity of Region 1 fieldaligned currents is regulated by ionospheric conductivity in the auroral zone. Thus the auroral absorption responds better to the winter PC index considering a delay between them, which depends on the value of the PC index value and its growth rate.
Fig. 2. Distribution of maximal absorption (Amax_aver) observed at the Canadian chain of stations in the winter months with low (PCaver ¼ 0.68 mV/m, December 2007) and enhanced (PCaver¼ 1.87 mV/m, January 2004) levels of magnetic activity.
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
distribution of values (Amax_aver) were presented as a function of UT (axis of abscissa) and geomagnetic latitude (axis of ordinate). It is seen that during the period of low activity (PCaver¼0.68 mV/m) only one zone of absorption is observed at latitude F0 ¼701–741 with maximum in the after-midnight sector (07–13 UT). The maximum of absorption reaches the value of 0.5 dB. The same regularity was revealed for averaged auroral distribution in summer months. In periods of enhanced activity (PCaver¼1.87 mV/m) two zones of absorption, the main one at latitudes 651–671 and the secondary one at latitude 701–721 are observed consistently. The morning (14–18 UT) maximum of absorption ( 2.6 dB) becomes dominant and considerably prevailing over the midnight absorption in the both zones. Regularity
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of the diurnal changes of the auroral absorption intensity characterizing by one maximum in the absorption level near the midnight in quiet conditions and by two absorption maximums in the night time and morning sector under disturbed conditions is in good accordance with the well-known results obtained earlier (Hargreaves and Cowley, 1967). Relationship between the PC index and auroral distribution was examined with use of 1 min and hourly maximum data on auroral absorption, in the latter case the value of absorption was compared with hourly averaged PC values. As one would expect, correlation between 1 min values of PC and absorption turned out to be lower than that between the hourly quantities, as a result of varying delay time between the PC growth beginning
December 2007, PCaver = 0.68 80
R 78 0.9
76
0.8 74 0.7
MLat, deg
72
0.6
70
0.5 0.4
68
0.3 66 0.2 64
0.1
62
0
60 0
2
4
6
8
10
12
14
16
18
20
22
24
UT, h
Fig. 3. Correlation between the 1 min values of PC and absorption as a function of UT (axis of abscissa) and geomagnetic latitude (axis of ordinate) for the winter months with low (a) and enhanced (b) levels of magnetic activity.
A. Frank-Kamenetsky, O. Troshichev / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 40–45
Φ' = 63.9°, UT = 8-16, R = 0.57
7 6 5 4 3 2 1 0 -1 -1
1
3
7
5
-1
Φ' = 63.9,UT = 8-16, R = 0.67
8 7 6 5 4 3 2 1 0 -1 -1
1
3 5 PC, mV/m
7
3.5 3 2.5 2 1.5 1 0.5 0 -0.5 -1
9
A, dB
A, dB
Φ' = 68.7°,UT = 8-12, R = 0.64
A, dB
A, dB
44
9
1
5
3
7
9
Φ' = 68.7, UT = 8-12, R = 0.66
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 -0.5 0
2
4 PC, mV/m
6
8
Fig. 4. Relationship between the PC index and auroral absorption at latitudes 63.91 (left column) and 68.71 (right column) derived for time intervals of their best correspondence. The upper panel is for 1 min data set, the lower panel is for set of hourly averaged PC and hourly maximal absorption data set.
and the corresponding absorption development. Fig. 3 shows the plots of flattening distributions for coefficients of correlation between 1 min values of PC and absorption as a function of UT (axis of abscissa) and geomagnetic latitude (axis of ordinate) for two levels of activity (a) PCaver ¼0.68 mV/m and (b) PCaver ¼1.87 mV/m. One can see that the meaningful correlation between PC and auroral absorption is observed generally in areas with high absorption. Under conditions of low activity the correlation is highest near the midnight and in the after-midnight sectors at latitudes 681–721, however the distinct spotty manner of distribution is indicative of rather accidental character of correlation. Under conditions of enhanced activity two zones of the high correlation are observed in the after-midnight sector (07–16 UT): the first zone is at latitudes 601–651 and the secondary one is at latitudes 681–721. So, the region of high correlation within the main zone turned out to be displaced equatorward relative to the region of the maximal absorption. Most likely, this inconsistency is connected with delayed development of absorption at higher latitudes. Indeed, as Fig. 1 demonstrates, the coincidence of bursts in PC and absorption values is typical only for equatorward part of the main zone, the higher latitude, the later maximum of absorption is observed. Fig. 4 shows a relationship between the PC index and auroral absorption at latitudes 63.91 (the main zone) and 68.71 (the secondary zone) during the time intervals for which the best correlation between the absorption and the PC index has been observed, namely between 8 and 16 UT for the main absorption zone, and 8 and 12 UT for the secondary zone, the upper panel being for set of 1 min data, the lower panel being for set of hourly averaged PC and hourly maximal absorption data. It is seen that the auroral absorption quickly enlarges with the PC growth in the entire auroral zone, irrespective of data choice, but correlation is improved when passing from 1 min to the hourly data. As it was noted above, in the latter case the effect of varying delay times is withdrawn from the examination.
4. Discussion and conclusions The results of the analysis demonstrate existence of two distinct maxima in the latitudinal distribution of the absorption intensity, the first one being located at latitudes 631–681 and the second one being located at 711–721. The presence of two latitudinal zones of auroral absorption was shown previously by Samsonov and Sokolov (1990) and Samsonov et al. (1991) who revealed, with use of data from the Yakut riometer chain, two regions of the maximal absorption at latitudes of 661–671 and 711 before noon. It was concluded that there are different mechanisms of energetic electrons precipitation in low and high latitudinal zones: at latitudes of 651–671 precipitation occurs in the regime of weak, but at 71 1 in the regime of strong pitch-angle diffusion. Satellite observations also confirm the existence of two zones of energetic electrons precipitation. According to Shepherd et al. (1990), the equatorward, stable peak is located at a latitude of 651, whereas the poleward, stretched along the auroral oval, peak is located at a latitude 701. The latter is identified by Shepherd et al. (1990) with the particle precipitation from the boundary plasma layer. As our analysis shows, the magnitude of auroral absorption in both zones is evidently dependent on the activity level, which is characterized by PC-index. It is known that auroral absorption initially starts at about midnight hours at the latitude of the auroral oval and subsequently absorption region expanded primarily to the pole and to the east. Namely for this reason, the maximum correlation coefficients between the absorption and PC index are observed in the after-midnight MLT sector. The auroral absorption quickly enlarges with the PC growth in the entire auroral zone.
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