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Two types of development of the E2 layer at Ahmedabad
RESEARCH NOTE Two types of development of the E2 layer at Ahmedabad (Received 6 Madt
1956)
SOON after the installation of the ionospheric recorder at Ahmedabad in January 1953, it was noticed that the P’-j records showed kinks in the reflection traces, characteristic of a maximum of ionization between the normal E and Fl layers of the ionosphere. An examination of these records showed that they were due to two intermediate layers (called El and EZ), having heights of maximum ionization at about 125 km and 140 km. These layers showed the properties of normal ionospheric layers formed by photoionization by solar radiation. An account of them has been published (RASTO~I, 1954). BECKER and DIEMIN~ER (195&J) have. described the occurrence of an E2 layer at Lindau. They concluded that the E2 layer was a continuous thin homogeneous layer and that it might be caused either by short-wave radiation or neutral corpuscles from the sun. A different sequence of formation of an ionized layer at about 140 km has been described by MCNICOL and GIPPS (1951). This layer separated out from Fl and formed an “E2” layer having properties similar to those of E and Fl. MCNICOL and GIPPS designated this layer as Ess. SKINNER, BROWN, and WRIGHT (1954) described a similar formation of an E2 layer from a ridge in the Fl layer at Ibadan. Recently, SAHA and RAY (1955) have reported the occurrence of an E2 layer at Calcutta. According to them, the E2 layer first appears as a ridge in Fl. It then moves downwards and merges with the E layer. The phenomenon was most usually observed at about 1300 local mean time. As the features of the E2 layer have been carefully watched during the last three years at Ahmedabad, which has about the same latitude as Calcutta, a summary of our observations at Ahmedabad may be of interest. The E2 layer at Ahmedabad first appears in the morning before ground sunrise. It shows itself first as a group retardation in the F reflection, then weak echoes appear from a layer below F, and these subsequently get stronger. The E2 layer develops after the F2 is affected by sunrise, but before the appearance of the E layer. Both the ordinary and extraordinary components of reflection from E2 are regularly observed. With increasing alt,itude of the sun, the ionization of the layer increases. These facts show that the E2 layer is formed by photoionization by solar radiation just like the other normal ionospheric layers. However, the formation of E2 is often accompanied by the development of one more layer between it and the normal E. Fig. 1 illustrates the development of these intermediate layers in the morning hours on 6 November 1054. The times of sunrise (75” E.M.T.) at different heights on that day are given below : Time of sunrise Height 0520 200 km 0536 100 km 0621 Okm At 0545 there existed only the F2 layer; the record shows the extra-ordinary trace only. At 0600 the F2 critical frequencies had increased due to sunrise at these levels; both the ordinary and extra-ordinary traces are clearly recorded, and the extra-ordinary trace shows group retardation at 1.8 Mc/sec. At 0615 the extra-ordinary component of E2 reflection appears at a virtual height of 150 km, with a critical frequency of 2.2 Mc/sec. At 0630 both the ordinary and extra-ordinary traces of E2 are clear, j,E2 being 1.8 Mc/sec and h’E2 130 km. At 0645 the extra-ordinary component of E2 shows group retardation due to a 71
Research note lower layer at 2.2 Mc/sec. At 0700 the extra-ordinary component of El has become quite clear, and there is also evidence of group retardation at I.8 Mc/sec. At 0715 both the ordinary and ext,ra-ordinary components of El and E2 are clearly recorded. Occasionally, with the growth of the E layer, the virtual height of the E2 layer increases ti, join the Fl layer or form a ridge in it. This happens at about llhr L.M.T. It then becomes difficult to distinguish the E2 layer from the ridge in PI. The ridge in Fl later breaks from Fl towards the E layer, with the formation of a separate layer. The virtual height of t’his separated E2 continues to decrease aft,e? breaking from the Fl layer. The ordinary and extra-ordinary components can be seen separately. The critical frequencies genera,lly increase if the breaking occurs much before noon, and decrease if the phenomenon occurs at about noon or in the afternoon. This again indicates a solar control of the E2 layer. By evening the la’yer comes down to about 120 km and merges with E or Es. The ordinary and extra-ordinary critical frequencies can still be distinguished by the start of the FI trace. On evenings when there is not much Es, the E2 layer can be followed till it disappea,rs with decreasing solar altitude. The Calcutta observers say that the merging of the E2 layer is followed by the formation of sporadic E ionization. Fig. 2 illustrates the formation of the E2 layer from a ridge in FI on ‘0 October 1954. At 0800 E2 wm at 125 km and its critical frequency 2. 8 Mc/sec. By 0915 f,,EZ has increased to 3.2 Mc/sec and h’E2 to 165 km. After this stage, E2 almost joined up with the Fl trace and appeared as a ridge in the Fl layer. Later, this ridge in Fl again broke t,o form an EZ layer, and it’s virtual height decreased with time. At 1145 there again appeared a ridge (as a slight break in the virtual height, but more in the amplitude of the trace) in FI. At 1200 this ridge broke from Fl and formed an E2 layer. The virtual height of E2 continually decreased, till at 1600 it descended to the height of the normal E layer and merged with sporadic E. This phenomenon too is generally accompanied with the appearance of a layer El between E2 and the normal E layer. It has been also observed that on certain nights there exists a layer at about 150 km, showing characteristic group retardation near its critical frequencies. SUNMARY
OF THE
O BSERVATIONS
AT
A HMEDABAD
The sequence of phenomena leading to t,he separation of the E2 layer in the morning hours after high-level sunrise indicates the presence of certain atmospheric constituents in the neighbourhood of 150 km which are photo-ionized by the solar radiation. Further, the frequent appearance of a stratified ionized layer at 150 km during night indicates that there exists a sensitive and stable region for the formation of an ionized layer at about 150 km over the tropics. Acknowledgement-The‘ work described is pa,rt of the programme of ionospheric research carried out in this laboratory wit,h financial assistance from the Council of Scientific and Industrial Research (India). The author is indebt’ed to Professor K. R. RAMANATHAN for his keen int,erest and guidance. R. G. RA S T O G I
Physical Research Laboratory, Ahmdabad, India.
R EFERENCES
BECKER IV. and DIEMINGER IT. BECKER TV. and DIEMINGER IV.
1950 1950
MCSICOL R. IV. E. and GIPPS G. DE RASTO~I R. G. SAHA A. K. and R AY S. SRINNER S. J., B ROWS R. 1., and W RIGHT R. IV.
1951 1954 1955 1954
~atwrwissenschaften 27, 90.
l_T.R.S.I., Proc. Commission .on Ionosphere, Second Meeting held in Brussels, p. 126. J. Beophys. Res. 56, 24. Proc. Ind. Acad. Sci. 40, 158. J. Atmosph. Terr. Phys. 7, 107. J. Atmosph. Terr. Phys. 5, 95. 52
20 OCTOBER
1954
0915
0945
1245
1000
1300
1015
1315
1045
1400
1100
1500
j
b MC/S. FIG, 2.
6 NOVEMBER HOURS. 75’ E.M.T. OS45
0600
0615
0630
0645
0700
0715
FIG. 1
1954
1956)
SOON after the installation of the ionospheric recorder at Ahmedabad in January 1953, it was noticed that the P’-j records showed kinks in the reflection traces, characteristic of a maximum of ionization between the normal E and Fl layers of the ionosphere. An examination of these records showed that they were due to two intermediate layers (called El and EZ), having heights of maximum ionization at about 125 km and 140 km. These layers showed the properties of normal ionospheric layers formed by photoionization by solar radiation. An account of them has been published (RASTO~I, 1954). BECKER and DIEMIN~ER (195&J) have. described the occurrence of an E2 layer at Lindau. They concluded that the E2 layer was a continuous thin homogeneous layer and that it might be caused either by short-wave radiation or neutral corpuscles from the sun. A different sequence of formation of an ionized layer at about 140 km has been described by MCNICOL and GIPPS (1951). This layer separated out from Fl and formed an “E2” layer having properties similar to those of E and Fl. MCNICOL and GIPPS designated this layer as Ess. SKINNER, BROWN, and WRIGHT (1954) described a similar formation of an E2 layer from a ridge in the Fl layer at Ibadan. Recently, SAHA and RAY (1955) have reported the occurrence of an E2 layer at Calcutta. According to them, the E2 layer first appears as a ridge in Fl. It then moves downwards and merges with the E layer. The phenomenon was most usually observed at about 1300 local mean time. As the features of the E2 layer have been carefully watched during the last three years at Ahmedabad, which has about the same latitude as Calcutta, a summary of our observations at Ahmedabad may be of interest. The E2 layer at Ahmedabad first appears in the morning before ground sunrise. It shows itself first as a group retardation in the F reflection, then weak echoes appear from a layer below F, and these subsequently get stronger. The E2 layer develops after the F2 is affected by sunrise, but before the appearance of the E layer. Both the ordinary and extraordinary components of reflection from E2 are regularly observed. With increasing alt,itude of the sun, the ionization of the layer increases. These facts show that the E2 layer is formed by photoionization by solar radiation just like the other normal ionospheric layers. However, the formation of E2 is often accompanied by the development of one more layer between it and the normal E. Fig. 1 illustrates the development of these intermediate layers in the morning hours on 6 November 1054. The times of sunrise (75” E.M.T.) at different heights on that day are given below : Time of sunrise Height 0520 200 km 0536 100 km 0621 Okm At 0545 there existed only the F2 layer; the record shows the extra-ordinary trace only. At 0600 the F2 critical frequencies had increased due to sunrise at these levels; both the ordinary and extra-ordinary traces are clearly recorded, and the extra-ordinary trace shows group retardation at 1.8 Mc/sec. At 0615 the extra-ordinary component of E2 reflection appears at a virtual height of 150 km, with a critical frequency of 2.2 Mc/sec. At 0630 both the ordinary and extra-ordinary traces of E2 are clear, j,E2 being 1.8 Mc/sec and h’E2 130 km. At 0645 the extra-ordinary component of E2 shows group retardation due to a 71
Research note lower layer at 2.2 Mc/sec. At 0700 the extra-ordinary component of El has become quite clear, and there is also evidence of group retardation at I.8 Mc/sec. At 0715 both the ordinary and ext,ra-ordinary components of El and E2 are clearly recorded. Occasionally, with the growth of the E layer, the virtual height of the E2 layer increases ti, join the Fl layer or form a ridge in it. This happens at about llhr L.M.T. It then becomes difficult to distinguish the E2 layer from the ridge in PI. The ridge in Fl later breaks from Fl towards the E layer, with the formation of a separate layer. The virtual height of t’his separated E2 continues to decrease aft,e? breaking from the Fl layer. The ordinary and extra-ordinary components can be seen separately. The critical frequencies genera,lly increase if the breaking occurs much before noon, and decrease if the phenomenon occurs at about noon or in the afternoon. This again indicates a solar control of the E2 layer. By evening the la’yer comes down to about 120 km and merges with E or Es. The ordinary and extra-ordinary critical frequencies can still be distinguished by the start of the FI trace. On evenings when there is not much Es, the E2 layer can be followed till it disappea,rs with decreasing solar altitude. The Calcutta observers say that the merging of the E2 layer is followed by the formation of sporadic E ionization. Fig. 2 illustrates the formation of the E2 layer from a ridge in FI on ‘0 October 1954. At 0800 E2 wm at 125 km and its critical frequency 2. 8 Mc/sec. By 0915 f,,EZ has increased to 3.2 Mc/sec and h’E2 to 165 km. After this stage, E2 almost joined up with the Fl trace and appeared as a ridge in the Fl layer. Later, this ridge in Fl again broke t,o form an EZ layer, and it’s virtual height decreased with time. At 1145 there again appeared a ridge (as a slight break in the virtual height, but more in the amplitude of the trace) in FI. At 1200 this ridge broke from Fl and formed an E2 layer. The virtual height of E2 continually decreased, till at 1600 it descended to the height of the normal E layer and merged with sporadic E. This phenomenon too is generally accompanied with the appearance of a layer El between E2 and the normal E layer. It has been also observed that on certain nights there exists a layer at about 150 km, showing characteristic group retardation near its critical frequencies. SUNMARY
OF THE
O BSERVATIONS
AT
A HMEDABAD
The sequence of phenomena leading to t,he separation of the E2 layer in the morning hours after high-level sunrise indicates the presence of certain atmospheric constituents in the neighbourhood of 150 km which are photo-ionized by the solar radiation. Further, the frequent appearance of a stratified ionized layer at 150 km during night indicates that there exists a sensitive and stable region for the formation of an ionized layer at about 150 km over the tropics. Acknowledgement-The‘ work described is pa,rt of the programme of ionospheric research carried out in this laboratory wit,h financial assistance from the Council of Scientific and Industrial Research (India). The author is indebt’ed to Professor K. R. RAMANATHAN for his keen int,erest and guidance. R. G. RA S T O G I
Physical Research Laboratory, Ahmdabad, India.
R EFERENCES
BECKER IV. and DIEMINGER IT. BECKER TV. and DIEMINGER IV.
1950 1950
MCSICOL R. IV. E. and GIPPS G. DE RASTO~I R. G. SAHA A. K. and R AY S. SRINNER S. J., B ROWS R. 1., and W RIGHT R. IV.
1951 1954 1955 1954
~atwrwissenschaften 27, 90.
l_T.R.S.I., Proc. Commission .on Ionosphere, Second Meeting held in Brussels, p. 126. J. Beophys. Res. 56, 24. Proc. Ind. Acad. Sci. 40, 158. J. Atmosph. Terr. Phys. 7, 107. J. Atmosph. Terr. Phys. 5, 95. 52
20 OCTOBER
1954
0915
0945
1245
1000
1300
1015
1315
1045
1400
1100
1500
j
b MC/S. FIG, 2.
6 NOVEMBER HOURS. 75’ E.M.T. OS45
0600
0615
0630
0645
0700
0715
FIG. 1
1954