K. N. Iyer

Differential rotation measurement of soft X-ray corona

The aim of this paper is to study the latitudinal variation in the solar rotation in soft X-ray corona. The time series bins are formed on different latitude regions of the solar full disc (SFD) images that extend from 80°S to 80°N. These SFD images are obtained with the soft X-ray telescope (SXT) on board the Yohkoh solar observatory. The autocorrelation analyses are performed with the time series that track the SXR flux modulations in the solar corona. Then for each year, extending from 1992 to 2001, we obtain the coronal sidereal rotation rate as a function of the latitude. The present analysis from SXR radiation reveals that: (i) the equatorial rotation rate of the corona is comparable to the rotation rate of the photosphere and the chromosphere, (ii) the differential profile with respect to the latitude varies throughout the period of the study; it was more in the year 1999 and least in 1994, and (iii) the equatorial rotation period varies systematically with sunspot numbers and indicates its dependence on the phases of the solar activity cycle.

Interplanetary scintillation observations for the solar wind disappearance event of May 1999

In this article we present ground-based interplanetary scintillation (IPS) measurements at 103 and 327 MHz for the period of the solar wind disappearance event of May 1999 as seen by various space probes. The solar wind velocity measurements at 327 MHz showed a variable solar wind velocity during this period at a distance of ∼0.5 AU from the Sun. The average solar wind velocity from three radio sources varied in the range of 200–300 km s−1. The scintillation index measurements at 103 MHz indicate that plasma density was very low in the interplanetary medium closer to the Earth and that the density was normal away from it during May 11–13. The scintillation index was enhanced significantly on May 14 after the disappearance event. The comparison with the in situ observations shows that the effect is dramatic in IPS observations. IPS and in situ measurements show that a large, tenuous, and slow plasma cloud engulfed our planet around this time, which could be because of a corotating low-density narrow stream. From the source (Sun) point of view, this was mostly a normal plasma flow in most of the interplanetary medium.

Equatorial spread F: Statistical comparison between ionosonde and scintillation observations and longitude dependence

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Space Weather Effects of Coronal Mass Ejection

This paper describes the space weather effects of a major CME which was accompanied by extremely violent events on the Sun. The signatures of the event in the interplanetary medium (IPM) sensed by Ooty Radio Telescope, the solar observations by LASCO coronagraph onboard SOHO, GOES X-ray measurements, satellite measurements of the interplanetary parameters, GPS based ionospheric measurements, the geomagnetic storm parameter Dst and ground based ionosonde data are used in the study to understand the space weather effects in the different regions of the solar-terrestrial environment. The effects of this event are compared and possible explanations attempted.

Interplanetary and terrestrial observations of an Earth‐directed coronal mass ejection

In this article we report interplanetary scintillation observations at 103 and 327 MHz of an Earth-directed coronal mass ejection (CME) which occurred near the center of the solar disk at 0435 UT on May 12, 1997. The disturbance was found to have plasma density ∼4 times more than that of the ambient plasma at a distance of ∼ 0.5 AU from the Sun. The most peculiar aspect of this CME is that it appears that the disturbance moved slightly slower than the ambient medium. Solar and Heliospheric Observatory (SOHO) and interplanetary scintillation (IPS) estimates of solar wind are quite different; it appears that the difference could be due to the projection effect of the SOHO image.Though the disturbance was not very severe, its impact on Earths environment produced a geomagnetic storm. This event was associated with a two-ribbon flare.The ionospheric effects of soft X rays from this solar flare were observed by a digital ionosonde at Ahmedabad in the form of excess ionization (∼1200 el cm−3) in the D region of the ionosphere.

Variability of the solar coronal rotation

Abstract Helioseismology measurements have begun to provide a picture of Suns angular velocity as a function of both depth and latitude. The rate of improvement of our knowledge has been fast and is likely to be more so for the near future. This has the potential of providing a nearly complete picture of the solar rotation throughout solar interior. The rotation of the solar photosphere is usually measured by tracking tracers e.g., sunspots, faculae, low-level magnetic features, Little is known about the solar coronal rotation etc. We recently attempted a fractal analysis of radio emission from the solar corona and found that the time series of radio emission at several frequencies can be easily be used for determining the rotation of the solar corona. The study reveals several interesting features about the rotation of the solar corona. The solar corona rotates faster than the photosphere. It is not possible to investigate latitudinal variations of the solar coronal rotation by this method, however, it is certainly possible to investigate temporal changes of this rotation. It is found that the coronal rotation period varies during different phases of the solar cycle. Here, the results of an auto correlation analysis for the solar cycles 21 and 22 will be presented and compared with those obtained by other methods.

Solar radio burst (type III)

This article describes the observations of a type III radio burst observed at 103 MHz simultaneously by the two radio telescopes situated at Rajkot (22.3°N, 70.7°E) and Thaltej (23°N, 72.4°E). This event occurred on September 30, 1993 at about 0430 UT and lasted for only half a minute. The event consisted of several sharp spikes in a group. The rise and fall time of these are comparable, however the peaks of individual spikes varied by a factor of four. The comparison of these observations with the data of solar radio spectrograph HiRAS indicates that this was a metric radio burst giving highest emission at about 103 MHz.

Coronal mass ejections and interplanetary scintillation

Abstract Here we report the Interplanetary Scintillation (IPS) observations of three events on April 2000, May 1999 and May 1997. The April 2000 observations are the interplanetary consequences of a halo Coronal Mass Ejection (CME). The May 1999 event is termed a solar wind disappearance event; it appears to be due the passage of a large void. The May 1997 event is that due to an Earth directed CME and the IPS observations of a radio source 3C48 show that the interplanetry disturbance due to this CME had density ∼4 times more than the ambient, but moved slower than the ambient medium.