• R G Rastogi

      Articles written in Proceedings – Section A

    • Intermediate layers of ionisation between the E and F1 layers of the ionosphere over ahmedabad (23° N, 72° · 6 E)

      R G Rastogi

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      Between the main layers E and F1 of the ionosphere over Ahmedabad, two intermediate layers occur regularly having virtual heights of 125 and 140 km. They also show the characteristic magneto-ionic splitting. The critical frequencies of these layers follow the position of the sun according to the lawf0=k cosnZ,n neing near about 0·38. Minimum virtual heights of these layers have been shown to be very misleading due to the proximity of the lower layers. The true height and the thickness have been determined for some cases. The true heights of these layers correspond well with the heights of ionisation maxima in the ionosphere determined with the aid of rockets.

    • The occurrence of high multiple reflections from the F2 region of the ionosphere based on a study of the Ahmedabad records

      R G Rastogi

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      These observations show clearly that even at night, the F2 layer can by no means be considered as a simple plane reflecting surface and that there are dynamic changes going on in it most of the time. Determination of the reflection coefficient of the layer and of the absorption of the atmosphere below it is rendered particularly difficult because of these changes.

    • The great solar flare of february 23, 1956 and associated ionospheric effects at Ahmedabad

      K R Ramanathan R V Bhonsle K M Kotadia R G Rastogi

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    • Erratum

      K R Ramanathan R V Bhonsle K M Kotadia R G Rastogi

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    • Vertical ionospheric soundings at Ahmedabad during total solar eclipse on 30th June 1954

      R G Rastogi

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      The vertical ionospheric soundings at Ahmedabad during the solar eclipse on 30th June 1954 presented some interesting results. The ionisation of the lower layer E and F1 decreased during the eclipse. Half an hour before the eclipse, the F1 layer showed a sharp cusp with a number of multiple and M reflections. It is difficult to say that this was due to the eclipse. The F2 critical frequencies, as well as the total ion content of the F2 layer during the eclipse, were above normal. The semi-thickness of the F2 layer increased during the eclipse. This was due to a lowering of the base of the F2 layer. It seems that the lower portion of the F2 layer, was more affected by the withdrawal of solar radiation than at the level of maximum ionisation. Multiple splitting, range duplication and other effects of the curvature of the ionos-pheric surface were observed during the eclipse. Maximum Es frequencies were higher during the eclipse period.

    • Vertical soundings at Ahmedabad during partial solar eclipse on 14th December 1955

      R G Rastogi

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      The results of vertical ionospheric soundings at Ahmedabad during the partial (magnitude 47%) solar eclipse on 14th December 1955 are discussed. The critical frequency of the E layer on control days varied according to the lawfoE=kcosnX, k andn having the mean values 4·06 Mc./s. and 0·37 respectively. The variation offoE during eclipse period was roughly consistent with that of Chapman Layer with a recombination coefficient of about 0·8 × 10−8 cm.3 sec.−1 The critical frequencies of E1, E2 and F1 layers decreased during eclipse, though the variations were irregular. The percentage deviation off0F2 on the eclipse day from the control day values showed a minimum at about the middle of the eclipse period. There were indications of the formation off1·5 during the latter part of the eclipse. Semi-thickness of the F2 layer had increased during the eclipse period. Sporadic E layer was not affected by the eclipse.

    • Some effects of geomagnetic activity on the F2 region of the ionosphere over leopoldville (4·4°S, 15·3°E)

      R G Rastogi

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    • Seasonal variation of the lunar tidal effects in the F2 layer of the ionosphere over Indian stations

      R G Rastogi

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      The lunar semi-diurnal oscillations in the midday values of the critical frequency (f0F2) and the height of maximum electron density (hpF2) of the F2 layer are computed for all Indian ionospheric stations separately for each season of the year. The amplitude of oscillation inf0F2 is found to be larger in winter than in summer at each of the stations. There is a reversal in the phase of the oscillation inf0F2 between the equatorial and tropical latitudes and this is most evident in the winter months and is almost absent in summer. The annual average oscillation inf0F2 is in agreement with that found in a previous paper (Rastogi, 1961). The phase has a large seasonal variation of about 180° at an equatorial or a tropical latitude station. The phase and amplitude of the lunar tide inhpF2 do not vary significantly with latitude or with season.

    • Lunar tidal variations in f0F2 in the American zone during period of low solar activity - I. Equatorial station, Huancayo

      R G Rastogi S K Alurkar

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      The coefficients of lunar monthly (L1) and semi-monthly (L2) oscillations in f0F2 values at Huancayo for each solar hour are calculated for different seasons of a year of low solar activity. The amplitude of L1 does not have a distinct variation with the time of the solar day and is about 0.1–0.2 Mc./s. for any particular solar hour; when the variation of f0F2 with local lunar time is averaged for all solar hours the amplitude of L1 oscillation becomes negligibly small. The amplitude of L2 oscillation is significantly larger for the daytime than for the night time hours. The L2 amplitude for midday hours is largest during December solstice and least during June solstice. The solar daily variation of the L2 amplitude is shown to be closely connected with the development of the abnormal belt of the F2 region over the magnetic equator during the daytime. A close relationship is found to exist between the lunar tides in f0F2 and the horizontal magnetic intensity H at the magnetic equator.

    • Comparison of ionospheric drifts over thumba by the method of similar fades and correlation method

      M R Deshpande R G Rastogi

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      The apparent speed and direction of the ionospheric drift by the simple similar fades method are not significantly altered by more detailed calculations of the time shift by optimum cross-correlation method between the records from pairs of antenna. The predominantly westward direction of drift at Thumba and large speeds may be taken as real.

    • Lunar tidal variations inf0F2 in the american zone during the years of low solar activity at huancayo, panama and buenos aires—A geomagnetic anomaly effect

      R G Rastogi

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      Chapman’s Phase Rule$$L = \sum\limits_{n = 1}^4 { c_n } sin \left\{ {n_\tau + (n - 2)\nu + a_n } \right\}$$ shown by him to be true for geomagnetic data is shown to be valid for the lunar oscillations inf0F2 also at the tropical latitude stations Panama and Buenos Aires. Lunar semi-monthly (M2) oscillations inf0F2 at Panama are significantly larger during the day hours with a maximum amplitude near noon, while at Buenos Aires the tidal amplitudes are significant for all hours of the day and night and the maximum amplitude occurs in the afternoon around 15 hours. Regarding seasonal variation, the amplitude of lunar semi-diurnal or semi-monthly tides is largest during December solstices and least during June solstices.

    • Lunar tidal variations inf0F2 in the American zone during periods of low solar activity - III. Middle latitude stations: Puerto Rico and Falkland

      R G Rastogi

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      Lunar semi-diurnal (L2) or lunar semi-monthly (M2) tides in f0F2 at Puerto Rico and Falkland are shown to the maximum during D-months (November to February) and least during J-months (May to August). The lunar daily variations of f0F2 at different lunar ages conform to the Chapman’s Phase law derived from the atmospheric dynamo theory for geomagnetism. The M2 tide at different solar hours indicate two maxima around 12 and 21 solar hours. At Puerto Rico the M2 tide is controlled by lunar time during D- and E-months and by lunar age during J-months. At Falkland, during D- and E-months, the M2 tide for daytime hours is controlled by lunar time while that for the night time hours is controlled by the lunar age. Thus the tidal movements in the F2 layer have control depending on the absolute position of the moon as well as on the relative positions of the sun and moon, the relative importance of the two changes with the season or between day and night hours.

    • Upward moving ionospheric irregularities over kodaikanal

      R G Rastogi

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      The occurrence of upward moving kinks first found on ionograms at Thumba (Rastogi, 1970) has been confirmed to occur at another equatorial station, Kodaikanal. These kinks have been found in many records and they occur mostly during local summer months. The occurrence of the kink is shown to be closely associated with horizontal F-region drifts, occurrence of intermediate cusp between F1 and F2 layers, bite-out effects of ƒ0F2 and rise of hPF2, all being most pronounced around 10 hr. The upward movement of the kink is due to$$\vec E \times \vec B$$ drift, while its initiation is probably due to a sudden change in the electrostatic field of the equatorial electrojet.

      The study of the upward moving kink gives a direct measure of the height variation of the vertical upward drift of ionization over the magnetic equator.

    • The disappearance of equatorial Es and the reversal of electrojet current

      R G Rastogi H Chandra S C Chakravarty

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      Based on simultaneous observations of the horizontal geomagnetic field component H, sporadic E (Es) and E-W electron drifts at stations close to the dip equator within the equatorial electrojet region, it has been found that on quiet days and sometimes on disturbed days, when there is an abnormal large decrease in H during daytime, there is a simultaneous disappearance of Es and a reversal of the direction of drift of electrons from westward to eastward. This suggests that the disappearance of equatorial Es during day-time is due to a temporary reversal of the electrojet current, which is caused by the imposition of an additional electrostatic field opposite in direction to that of normal Sa field.

    • Equatorial sporadic E and cross-field instability

      R G Rastogi

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      The occurrence of sporadic E at an equatorial station during magnetically quiet daytime conditions corresponds almost exactly to the time during which the horizontal component of the earth’s magnetic field is above the mean night time level. Any large decrease of H below the night time level is accompanied by the disappearance of equatorial Esq reflections precisely for the period when the value of H remains below its night time level.

      Such disappearance of Esq can be attributed to the reversal of the current equal to, or greater than, the normal eastward equatorial electrojet current. During magnetically disturbed conditions, however, the depressions in H are not always accompanied by the disappearance of Esq.

      Whenever the normal E and sporadic E reflections can be resolved on the equatorial ionograms, the minimum virtual height of the normal E is seen to be clearly greater than that of the sporadic E layer.

      The creation of E region irregularities at equatorial latitudes giving the appearance of an Esq layer in daytime ionograms is suggested to be due to cross-field (plasma gradient) instability. The horizontal magnetic field and the upward Hall polarisation (electric) field produce irregularities in the lower E-region where the rate of increase of ambient electron density is large and directed upward.

      A temporary reversal of the electrojet current indicated by a decrease in H below the night time level and the disappearance of Esq are due to the temporary reversal of the vertical Hall polarisation field making it downward instead of upward which being opposite to the direction of the gradient of plasma density inhibits the cross-field instabilities.

    • The effect of polar magnetic sub-storms on the equatorial sporadic E

      R G Rastogi

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      It is known that equatorial sporadic E disappears at night when dynamo field is east to west. During some DP2 type magnetic sub-storms, which cause a depression of the geomagnetic horizontal field at the equator, theq type of sporadic E is found to disappear at the equatorial stations Huancayo and Kodaikanal. This suggests that one of the mechanisms causing the temporary disappearance of Esq during daytime in equatorial ionograms is the replacement of the east to west dynamo electric field by a west to east electric field due to the imposition of an electric field opposing the normal daytime Sq field.

    • Some remarks on the equatorial sporadicE layer at kodaikanal

      R G Rastogi

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      It is shown that the sporadicE layer at an equatorial station is not always of the commonly known equatorial (q) type Es. Broadly there occur two types of Es at a station near the magnetic equator,viz., (i) diffuseq type and (ii) multiple or blanketing (b) type. The latter may be of diffuse type or of thin layer type. The Es-b is due to highly dense cloud of ions which are localised, whereas Es-q occurs over an extended area at any time.

      The effect of counter-electrojet is seen on both the types of Es layers. The disappearance of sporadicE during counter-electrojet is valid for only theq type of Es, but sometimes Es-b may occur during periods of very weak or reversed electrojet. It is suggested that published equatorial Es data should be used with caution unless the type of Es is also indicated. The plausible causes of the sudden disappearance of Es-q on some occasions and the appearance of Es-b during others are suggested.

    • On the simultaneous existence of eastward and westward flowing equatorial electrojet currents

      R G Rastogi

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      The counter-electrojet currents are evidenced by the disappearance of theq type ofEs layer (Es-q) or the appearance of the blanketing type ofEs (Es-b) at Kodaikanal, associated with the depression of the geomagneticH field and the reveisal of ionospheric drift at Thumba. The necessary condition for such an event is not the decrease of theH field below the night level but that the difference of theH field between an equatorial and a non-equatorial station should decrease below its night level.

      The different kinds of association between the disappearance ofEs-q and the depression in theH field are suggested due to superimposition over theSq current system (at about 107 km) of a separate westward current system at a lower level (about 100 km).

      The source of the reversed current over the dip equator during the daytime hours is sought in the current system generated by the lunar tides or in various magnetospheric processes generating the polar substorms.

      Large day-to-day variations of the solar daily range ofH at the equator independent ofSq variation at tropical latitudes are suggested to be due to superimposition at the equator of the two rather independent current systems.

    • Effect of interplanetary magnetic field on ionosphere over the magnetic equator

      R G Rastogi V L Patel

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      Large and quick changes of the latitude of the interplanetary magnetic field from its southward to northward direction is shown to be associated with the disappearance of theq type of sporadicE layer at the equatorial ionosphere during the daytime or with the reversal ofE region horizontal andF region vertical electron drifts during the hours of daytime as well as nighttime.

      This phenomenon is suggested as the imposition of an electric field in the ionosphere from the evening to the morning sector of the earth and is thus in a direction opposite to that of the Sq electric field. The resultant electrostatic field on the equatorial ionosphere would be decreased or even reversed from its normal direction. This would result in the reduction of electron drift velocity. On some occasions when the normal Sq field is over-compensated by the magnetospheric electric field, the electron drifts are reversed and the irregularities in theE region due to the cross-field instabilities are inhibited resulting in the sudden disappearance of theq type ofEs layers.

    • The equatorial anomaly in ionospheric total electron content and the equatorial electrojet current strength

      K N Iyer M R Deshpande R G Rastogi

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      Faraday Rotation of 40 and 41 MHz signals from the satellite BE-B (Explorer 22) recorded simultaneously at Ahmedabad (dip 34° N) and Kodaikanal (dip 3·4° N) during the years 1964–69 are used to derive the latitudinal profiles of Total Electron Content (TEC) over the Indian equatorial anomaly region. From these profiles the diurnal development of the equatorial anomaly and its correlation with equatorial electrojet strength are studied. The anomaly is found to maximise around 1400 LT,i.e., two-three hours after the electrojet attains its peak. The anomaly parameters such as the dip latitude of the anomaly peak,φ, the normalised depth,d, of the anomaly and the strength of the anomaly defined asSxd are found to be well correlated with the electrojet strength.

    • Equatorial range spreadF and high multiple echoes from theF region

      R G Rastogi

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      It has been shown that the post-sunset rise of the minimum virtual height (h′F) of theF region of the ionosphere near the magnetic equator during the high sunspot years is an effect of the overall rising of the completeF region. This feature of theF region produces very high order multiple reflections from theF regions. On some occasions strong spreadF is observed simultaneously with the high multiples suggesting theF region to be smooth and devoid of irregularities inside it. The irregularities causing the spreadF during the early stages of its development are suggested to be at heights below theF region.

    • Ionospheric total electron content and slab-thickness at low latitudes in Indian zone

      R G Rastogi K N Iyer R P Sharma

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      Observations of Faraday rotation of beacon signals from low orbiting satellite BE-B recorded at one station near the dip equator (Kodaikanal, dip 3·4° N) and at another station near the peak of the equatorial anomaly (Ahmedabad, dip 34° N) give a complete coverage of the equatorial anomaly belt in Indian zone. Contours of total elctron content (TEC) are obtained on a grid of latitudeversus local time for the different seasons of low (1964–66) and high (1967–69) solar activity epochs in the latitude belt 10° S to 26° N dip latitude. The development of the equatorial anomaly and its dependence on season and solar activity are discussed. Using similar contours of F2 layer critical frequency, f0F2 contours of equivalent slab-thickness, τ are also constructed. The dependence of τ on season and solar activity and its implications on temperature are discussed.

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