Anita Joshi

Asymmetries during the maximum phase of solar cycle 22

This paper presents the results of studies of the asymmetries (N-S and E-W) for different manifestations of solar activity events (sunspot groups, Hα flares and active prominences/filaments) during the maximum-phase (1989–1991) of solar cycle 22. During the period considered, the results obtained show the existence of a real N-S asymmetry, whereas the E-W asymmetry may exist only for Hα flares. There is no definite relationship between the asymmetries and the occurrence of events; however, around low activity sometimes we find enhanced asymmetry, and low asymmetry around high activity. Our study suggests a good agreement with similar studies made by others.

INTERMEDIATE TERM PERIODICITIES IN SOFT X-RAY FLARE INDEX DURING SOLAR CYCLES 21, 22 AND 23

We have analyzed the intermediate-term periodicities in soft X-ray flare index (FISXR) during solar cycles 21, 22 and 23. Power-spectral analysis of daily FISXR reveals a significant period of 161 days in cycle 21 which is absent during cycles 22 and 23. We have found that in cycle 22 periodicities of 74 and 83 days are in operation. A 123-day periodicity has been found to be statistically significant during part of the current solar cycle 23. The existence of these periodicities has been discussed in the light of earlier results.

Multi-Wavelength Observations of an Unusual Impulsive Flare Associated with Cme

We present the results of a detailed analysis of multi-wavelength observations of a very impulsive solar flare 1B/M6.7, which occurred on 10 March, 2001 in NOAA AR 9368 (N27 W42). The observations show that the flare is very impulsive with a very hard spectrum in HXR that reveal that non-thermal emission was most dominant. On the other hand, this flare also produced a type II radio burst and coronal mass ejections (CME), which are not general characteristics for impulsive flares. In Hα we observed bright mass ejecta (BME) followed by dark mass ejecta (DME). Based on the consistency of the onset times and directions of BME and CME, we conclude that these two phenomena are closely associated. It is inferred that the energy build-up took place due to photospheric reconnection between emerging positive parasitic polarity and predominant negative polarity, which resulted as a consequence of flux cancellation. The shear increased to >80○ due to further emergence of positive parasitic polarity causing strongly enhanced cancellation of flux. It appears that such enhanced magnetic flux cancellation in a strongly sheared region triggered the impulsive flare.

Energetics and Dynamics of an Impulsive Flare on March 10, 2001

We present Hα observations from ARIES (Nainital) of a compact and impulsive solar flare that occurred on March 10, 2001 and which was associated with a CME. We have also analyzed HXT, SXT/Yohkoh observations as well as radio observations from the Nobeyama Radio Observatory to derive the energetics and dynamics of this impulsive flare. We coalign the Hα, SXR, HXR, MW, and magnetogram images within the instrumental spatial-resolution limit. We detect a single HXR source in this flare, which is found spatially associated with one of the Hα bright kernels. The unusual feature of HXR and Hα sources, observed for the first time, is the rotation during the impulsive phase in a clockwise direction. We propose that the rotation may be due to asymmetric progress of the magnetic reconnection site or may be due to the change of the peak point of the electric field. In MW emission we found two sources. The main source is at the main flare site and another is in the southwest direction. It appears that the remote source is formed by the impact of accelerated energetic electrons from the main flare site. From the spatial correlation of multiwavelength images of the different sources, we conclude that this flare has a three-legged structure.

Superactive region AR 5395 of solar cycle 22

During the ascending phase of solar cycle 22, the NOAA active region AR 5395 of March 1989 was responsible for a sudden jump in solar activity. The evolution of the active region during its three successive rotations (viz., AR 5354, AR 5395, and AR 5441) across the solar disk manifests the dominant role of the above active region; in other words, the active region was excitingly active (superactive region). To illustrate this, a brief description of the flare activity and the associated phenomena from the region is given on the basis of worldwide ground-based and satellite observations. Magnetic characteristics of the group have also been discussed to show the development of flare activity with respect to the twisting index of the magnetic neutral line.

Analysis of the 9th November 1990 flare

In this paper we present complete two-dimensional measurements of the observed brightness of the 9th November 1990Hα flare, using a PDS microdensitometer scanner and image processing software MIDAS. The resulting isophotal contour maps, were used to describe morphological-cum-temporal behaviour of the flare and also the kernels of the flare. Correlation of theHα flare with SXR and MW radiations were also studied.