K. K. Mahajan

Atmospheric greenhouse effect and ionospheric trends

Theoretical calculations predict that CO2 doubling would produce a 50 K decrease in the thermospheric temperature which can result in about 20 km decrease in the F2 peak height (hmF2) and a minor decrease in the F2 layer critical frequency (foF2) [Rishbeth and Roble 1992]. In this paper we analyze ionosonde data for some 31 stations to study the long term trends in hmF2 and foF2. Regression coefficients for hmF2 and foF2 as a function of solar activity, are obtained for each station and departures (anomalies) from expected values derived for both these parameters. An analysis of hmF2 and foF2 anomalies indicates negative trends for some stations and positive trends for others. These varied between +29 to −20 km for hmF2 and +0.7 to −0.6 MHz for foF2 in the 34 year period since 1957, the International Geophysical Year (IGY). We conclude that the present data do not provide a definitive evidence of any global long term trend in the ionosphere.

Observations of X‐ray and EUV fluxes during X‐class solar flares and response of upper ionosphere

Most studies dealing with solar flare effects in the upper ionosphere, where ionization is caused by EUV photons, have been based upon X-ray fluxes measured by the SOLRAD and GOES series of satellites. To check the validity of such studies, we compare simultaneous observations of GOES X-ray fluxes and SOHO EUV fluxes for 10 X-class solar flares which occurred during the maximum phase of sunspot cycle 23. These include the greatest flare of 4 November 2003, the fourth greatest flare of 28 October 2003 and the 14 July 2000 Bastille Day flare. We find that the peak intensities of the X-ray and EUV fluxes for these flares are poorly correlated, and this poor correlation is again seen when larger data containing 70 X-class flares, which occurred during the period January 1996 to December 2006, are examined. However, this correlation improves vastly when the central meridian distance (CMD) of the flare location is taken into account. We also study the response of the upper ionosphere to these fluxes by using the midday total electron content (TEC), observed for these flares by Liu et al. (2006). We find that peak enhancement in TEC is highly correlated with peak enhancement in EUV flux. The correlation, though poor with the X-ray flux, improves greatly when the CMD of flare location is considered.

First VHF radar observations of tropical latitude E-region field aligned irregularities

Indian MST radar at Gadanki (13.5°N, 79.2°E, 12.5° dip) was operated during July/August 1994, to observe the 3-m scale size field aligned irregularities associated with the lower E region. Irregularity structure was studied by using height-time variation of the echo intensity and weighted mean Doppler velocity. In this paper results of three diurnal cycles of observation are presented. Field perpendicular echoes were observed both during daytime and nighttime. A layered irregularity structure extending down to altitude below 86 Km was seen during the nighttime. The daytime structure showed a narrow echoing region with significant downward slope. Doppler velocity was in the range of 20–50 ms−1, both during day and night and was, in general, consistent with the slope of scattering structure observed in the height-time-intensity plots.

Comparative study of TEC with IRI model for solar minimum period at low latitude

Measurements of electron density profiles carried out at the low latitude station Arecibo, with the incoherent scatter radar, are used to derive total electron content (TEC). Comparisons are made with the IRI-95 model. Some discrepancies are found between the observed and the IRI predicted TEC and these discrepancies are maximum in equinox and summer months and minimum in winter months. However, for winter, a fair agreement is found between the IRI and the Arecibo data. We attribute these discrepancies to the profile shapes in the IRI model.

Dependence of F2- peak height on solar activity: A study with incoherent scatter measurements

Abstract We have used about 16000 high resolution electron density profiles from Arecibo (18.4 N, 66.7 W, dip 50°) incoherent scatter radar for the years 1974–1977 and 1989–1990 to study the control of solar activity on the F2 peak height (hmF2). Although the general trend of diurnal variations in hmF2 are nearly identical (i.e. maximum around midnight and minimum around sunrise) during low as well as high solar activity, the median hmF2 increases by about 100 km from solar minimum to solar maximum. Further, there is a considerable day-to-day variability in hmF2. Comparison of measured hmF2 with the IRI (derived from numerical map of M(3000)F2) reveals that IRI generally overestimates this parameter during low solar activity and underestimates during high solar activity.

On the outflow of O2 + ions at Mars

The ASPERA (automatic space plasma experiment with a rotating analyzer) instrument aboard Phobos 2 has detected a large outflow of molecular ions (possibly O2+) from the Martian environment. We suggest that this molecular outflow results from horizontal transport of O2+ ions from the dayside. We demonstrate that the in situ profiles of O2+ ions as observed by the Viking 1 and 2 landers at Mars are eroded by the solar wind interaction down to an altitude of about 150–160 km. The profiles are considerably depleted compared with the corresponding diffusive equilibrium profiles. The difference between the diffusive equilibrium and the observed profile is the likely amount of O2+ ions which flow over to the nightside, a part of which would escape the gravity of the planet. However, the estimated escape rate exceeds the observed total ionospheric escape rate by about an order of magnitude and that of the molecular ions by 1 to 2 orders of magnitude. This suggests that there is possibly a missing component in the observed molecular ion outflow from Mars.

Mars Global Surveyor radio science electron density profiles: Some anomalous features in the Martian ionosphere

We have analyzed some 807 Mars Global Surveyor electron density profiles that are confined to the northern high latitudes and thus are relatively free of the effects of crustal magnetic fields. These profiles have shown some anomalous features in the Martian ionosphere, and one of these is the noticeable variability in number density (N m ) and height (h m ) of the primary ionospheric peak on the same day when solar conditions and solar zenith angle have remained the same, a feature not expected from a photochemically controlled layer. We study this feature by generating longitudinal plots of N m and h m for the 807 profiles and by applying a least squares spectral fit consisting of wave number 1, 2, and 3 components to these data sets. We find some significant relationship between the two parameters, with the troughs in N m coinciding with the ridges in h m (and vice versa) on the longitudinal scale. An examination at fixed solar zenith angles shows a significant anticorrelation between the two parameters recorded over a period of about 3 months. However, theoretical considerations would support a positive correlation expected in response to changes in the EUV flux that occurred during this period. Further, we observe a large variability in electron density at 160 and 180 km, altitudes in the topside ionosphere, where photochemistry is expected to dominate. This is an additional anomalous feature. No such variability is observed in the topside ionosphere of Venus. We discuss plausible mechanisms like neutral atmosphere dynamics and solar wind interaction to explain some of the features.

Variability of the F-region parameter h0.5

Abstract High resolution electron density profiles, measured with the Arecibo incoherent scatter radar have been used to derive h0.5, the height of the half density point. More than 2400 profiles, for the period August 1974 – May 1977, have been used to study the behaviour of this parameter. Our analysis shows that during the night, h0.5 varies mostly between 200 and 350 km, but during the day the excursion is larger and values lower than 150 km are also seen. These lower values are often coincident with the presence of a layer (F1) between h0.5 and hmF2. We have therefore, divided the daytime data into two classes, class ‘A’ where h0.5 is more than hmF1 and class ‘B’ where h0.5 is less than hmF1. We find that in both the classes, most of the variability in h0.5 is due to the variability in hmF2 and a linear relationship between h0.5 and hmF2 is seen, although the dispersion in class ‘B’ is larger. Within the same class, no difference is seen in the relationship between day and night. We also compare the parameter Y0.5 hmF2 obtained from the Arecibo measurements with that calculated from the IRI-90 model, based upon Gulyaevas (1987) formula. In class ‘A’ the median values of this parameter vary between 0.15 and 2.0 with no major seasonal differences. The IRI values, however, are significantly larger. On the other hand, class ‘B’ shows larger seasonal differences, particularly between summer and winter, but the discrepancy with the IRI is somewhat smaller.

Detection of Kelvin‐Helmholtz instability with the Indian mesosphere‐stratosphere‐troposphere radar: A case study

Measurements with the Indian mesosphere-stratosphere-troposphere (MST) radar at Gadanki (13.5°N, 79.2°E) on May 26, 1994, revealed the presence of a Kelvin-Helmholtz instability (KHI) in between two stratified shear layers. The KHI occurred at the shear maximum, at 17.7 km, just above the meteorological tropopause. The characteristic period for the KH wave was found to be 12 min with a billow wavelength of 7.2 km. At the critical level, 17.7 km, the wave showed a sudden change of phase of the order of 160°. In addition to the characteristic high-signal power bursts, it also exhibited enhanced Doppler width during the occurrence of KHI. This indicates the generation of 3 m scale turbulence in the region where KHI is observed. A detailed investigation of the Doppler width structure reveals that the KH billows undergo breakdown in between the shear layers, causing peaked structure in the Doppler width profile.

Variability of foF2 at low and middle latitudes

Applying classical statistical methods to data of 14 low- and mid-latitude locations the reduced standard deviation r.s.d. of monthly foF2 is shown to depend on hour, season and solar cycle phase. With daily percentage departures of foF2 from monthly mean exceeding r.s.d. a comparable analysis applied to individual days shows magnetically disturbed days clearly outside. As experience shows this holds no more when reduction with respect to the true monthly mean is replaced by one to a predicted median e.g. the URSI numerical map.