S. R. Prabhakaran Nayar

Short-Period Features of the Interplanetary Plasma and Their Evolution

The solar wind plasma exhibits many features of the solar surface passed on to the interplanetary medium as temporal variations due to the solar rotation. The yearly average values of solar wind velocity, and geomagnetic index Ap during 1965–1999 were found to exhibit long period evolution. They were found to peak around the declining phase of each solar cycle. While the solar wind velocity peaks around the second half of the declining phase, the IMF field strength increases around the first half of the declining phase of each solar cycle. The power spectrum of these parameters shows peaks around 37-day, 30-day, 27-day, 13.5-day, 9-day, and 7-day periods. The temporal evolution of the power spectrum of the solar wind plasma parameters and the geomagnetic activity index Ap are also studied in detail and presented with the help of contour graphs. These studies indicate that the strength of the quasi-periodicities in the interplanetary medium evolves with time.

Galaxy classification using fractal signature

Fractal geometry is becoming increasingly important in the study of image characteristics. For recognition of regions and objects in natural scenes, there is always a need for features that are invariant and they provide a good set of descriptive values for the region. There are many fractal features that can be generated from an image. In this paper, fractal signatures of nearby galaxies are studied with the aim of classifying them. The fractal signature over a range of scales proved to be an efficient feature set with good discriminating power. Classifiers were designed using nearest neighbour method and neural network technique. Using the nearest distance approach, classification rate was found to be 92%. By the neural network method it has been found to increase to 95%.

Height gradient of F region vertical drift in the evening equatorial ionosphere

Simultaneous measurements of F-region vertical drift are made in the evening hours (1700-2100 IST) at Trivandrum (dip 0.6°N) and Kodaikanal (dip 4°N) on fifteen days during December 1993-January 1994 using the HF phase path technique on two different probing frequencies. The data are used to study the height dependence of vertical plasma drift in the bottomside F-region in the dusk sector after correcting the drifts (at Kodaikanal) for meridional wind effects and chemical loss. It is found that growth and decay of a positive height gradient in vertical drift occurs fairly regularly in the dusk period. On the average the vertical velocity gradient is positive in in the interval 1815-1925 IST and is preceded by negative values. The positive height gradient of vertical plasma drift below the F layer peak is interpreted in terms of altitude dependence of the relative contributions of E and F region dynamos to the electric fields responsible for plasma drifts (vertical and zonal) of the dusktime equatorial F-region. These results are for winter solstice solar minimum conditions.

Ionospheric response to a geomagnetic storm during November 8-10, 2004

This paper investigates the response of the equatorial, and near equatorial, ionosphere to geomagnetic disturbances during the period November 8–10, 2004. Ionosonde data from Trivandrum (8.5°N 77°E and dip 0.5°N) and SHAR (13.5°N, 80.2°E, dip ~5.5°N), magnetic field data from Tirunelveli (8.7°N, 76.9°E, dip latitude 0.5°S) and Alibag (18.64°N, 72.87°E), and GUVI O/N2 data in the Indian longitude sector, are used for the study. The behavior of interplanetary parameters is also examined in conjunction with the ionospheric data. On 8 November, the EIA around noontime is not fully inhibited even though the electrojet strength an indicates inhibition of EIA due to a disturbance dynamo electric field effect. It is the enhanced O/N2 over TRV and SHAR, with a larger increase over SHAR, which results in a larger (than expected) value of the EIA proxy parameter. On 9 November, the comparable values of foF2 at TRV and SHAR around noon time is due to the combined effect of a weakened anomaly in the presence disturbance dynamo electric field effects leading to the EIA crest being near SHAR, and increased O/N2 values at TRV and SHAR with a larger increase at TRV. On 10 November, the very strong values of the EIA proxy-SHAR parameter is attributed to the combined effects of prompt penetration electric field related modulations of EIA, and significant O/N2 changes at the equatorial, and near equatorial, latitude. Thus, the study reveals the important role of storm-induced O/N2 changes, along with prompt penetration electric fields and disturbance dynamo electric fields in modulating the ionization distribution in the equatorial ionization anomaly (EIA) region during this period.

North-south asymmetry in the heliospheric current sheet and the IMF sector structure

It is shown that in a heliomagnetic field the presence of a magnetic quadrupole in addition to a magnetic dipole introduces a north-south asymmetry in the heliospheric current sheet (HCS) about the heliographic equator. The dominant polarity of the interplanetary magnetic field (IMF) for the above type of current sheet reverses sign at a transition latitude θT, which lies in a heliohemisphere opposite to the one in which the HCS has more heliolatitudinal extension. The position of θT in the heliosphere and the north-south asymmetry introduced in the HCS change with the relative phase of the dipole and quadrupole components present in the solar magnetic field. The effect of the above type of asymmetric HCS in the IMF ‘mean sector width’ is evaluated and the results are in agreement with the observations during the minima of solar cycle 21.

Hale sector boundary and geomagnetic activity

Abstract The variation of the geomagnetic activity index, Ap, at the IMF sector boundaries (+ to − and − to +) has been studied for three solar cycles, separating the data into vernal and autumnal equinoxes. It has been found that at the Hale boundary, geomagnetic activity index, Ap, shows a sharp increase compared to that around the non-Hale boundary.

On the cause of geomagnetic activity at Hale sector boundary

Abstract The solar wind velocity and density have been studied around the interplanetary magnetic field sector boundaries (+ to − and − to +) during 1965–1974, separating the data into autumnal and vernal periods. It is noticed that the solar wind velocity shows a sharp increase around the Hale type of sector boundary in both northern and southern heliosphere indicating a more favourable condition for the high speed stream after Hale type of sector boundary crossing than non-Hale boundary.

Evolution and nature of north-south asymmetry in the heliospheric current sheet

The nature and evolution of north-south asymmetry in the heliospheric current sheet (HCS) has been investigated using solar and interplanetary magnetic field (IMF) observations for the past few solar cycles. The mean heliographic latitude of the HCS (averaged over the solar longitude) ‘a0’ is found to be non-zero during many solar rotations indicating that the large-scale solar magnetic field is more ordered in a system where the origin is shifted away from the centre of the Sun. We have shown that the asymmetry in HCS manifests in different forms depending on the transition heliographic latitude of the reversal of dominant polarity of the IMF (θT) and the difference in the maximum latitudinal extension of the HCS in the two solar hemispheres (Δ). The classification of the observed asymmetry during 1971–1985 and its effect on IMF observations near Earth has been studied. We have also inferred the sign of θT during 1947–1971 using inferred IMF polarity data. The observed sign reversals of θT suggest the importance of periodicities less than the solar cycle period to be associated with the evolution of asymmetry in HCS. Asymmetry in sunspot activity about the solar equator does not seem to relate consistently well with the asymmetry in HCS about the heliographic equator.

Role of periodic fluctuations in solar wind-magnetosphere coupling

In this paper, the behaviour of the fluctuating component in the solar wind parameters (V, Bz), the auroral electrojet indices (AU, AL), the ring current index (Dst) and the interplanetary electric field (V × B2) during 10 magnetic storms is analysed to understand the solar wind-magnetosphere coupling. It is found that during the moderate storms (Dst > −100 nT), the fluctuating component of 3–4 h periodicity is clearly discernible in all the parameters, and during the intense storms (Dst < −100 nT) the periodic fluctuations are not well defined.

On the cause of observed solar wind velocity variation around Hale type sector boundary

Abstract The concept of positive gradient in solar wind velocity with respect to the heliomagnetic latitude and the change in heliomagnetic latitude of Earth has been utilized to explain the differences in solar wind velocity variation around − and −/+ IMF sector boundaries in northern and southern heliolatitudes. It is shown that, using a sinusoidal symmetric heliospheric current sheet, one can explain the observed solar wind changes around the Hale and non-Hale types of sector boundaries.