Spotless days and geomagnetic index as the predictors of solar cycle 25

We study the sunspot activity in relation to spotless days(SLDs)during the descending phase of solar cycles 11-24 to predict the amplitude of sunspot cycle 25.For this purpose,in addition to SLD,we also consider the geomagnetic activity(aa index)during the descending phase of a given cycle.A very strong correlation of the SLD(0.68)and aa index(0.86)during the descending phase of a given cycle with the maximum amplitude of next solar cycle has been estimated.The empirical relationship led us to deduce the amplitude of cycle 25 to be 99.13±14.97 and 104.23±17.35 using SLD and aa index,respectively as predictors.Both the predictors provide comparable amplitude for solar cycle 25 and reveal that solar cycle 25 will be weaker than cycle 24.Further,we predict that the maximum of cycle 25 is likely to occur between February and March 2024.While the aa index has been utilized extensively in the past,this work establishes SLDs as another potential candidate for predicting the characteristics of the next cycle.

The energetic relationship among geoeffective solar flares, associated CMEs and SEPs

Major solar eruptions （flares, coronal mass ejections （CMEs） and solar energetic particles （SEPs）） strongly influence geospace and space weather. Currently, the mechanism of their influence on space weather is not well understood and requires a detailed study of the energetic relationship among these eruptive phenomena. From this perspective, we investigate 30 flares （observed by RHESSI）, followed by weak to strong geomagnetic storms. Spectral analysis of these flares suggests a new power-law relationship （r - 0.79） between the hard X-ray （HXR） spectral index （before flarepeak） and linear speed of the associated CME observed by LASCO/SOHO. For 12 flares which were followed by SEP enhancement near Earth, HXR and SEP spectral analysis reveals a new scaling law （r - 0.9） between the hardest X-ray flare spectrum and the hardest SEP spectrum. Furthermore, a strong correlation is obtained between the linear speed of the CME and the hardest spectrum of the corresponding SEP event （r - 0.96）. We propose that the potentially geoeffective flare and associated CME and SEP are well-connected through a possible feedback mechanism, and should be regarded within the framework of a solar eruption. Owing to their space weather effects, these new results will help improve our current understanding of the Sun-Earth relationship, which is a major goal of research programs in heliophysics.

Study of temporal and spectral characteristics of the X-ray emission from solar flares

Temporal and spectral characteristics of X-ray emission from 60 flares of intensity ≥C class observed by the Solar X-ray Spectrometer （SOXS） during 2003-2011 are presented. We analyze the X-ray emission observed in four and three energy bands by the Si and Cadmium-Zinc-Telluride （CZT） detectors, respectively. The number of peaks in the intensity profile of the flares varies between 1 and 3. We find moderate correlation （R ~=0.2） between the rise time and the peak flux of the first peak of the flare irrespective of energy band, which is indicative of its energy-independent nature. Moreover, the magnetic field complexity of the flaring region is found to be highly anti-correlated （R = 0.61） with the rise time of the flares while positively correlated （R = 0.28） with the peak flux of the flare. The time delay between the peak of the X-ray emission in a given energy band and that in 25-30keV decreases with increasing energy, suggesting conduction cooling is dominant in the lower energies. Analysis of 340 spectra from 14 flares reveals that the peak of differential emission measure （DEM） evolution is delayed by 60-360 s relative to that of the temperature, and this time delay is inversely proportional to the peak flux of the flare. We conclude that temporal and intensity characteristics of flares are dependent on energy as well as the magnetic field configuration of the active region.

Physical properties of lunar craters

The surface of the Moon is highly cratered due to impacts of meteorites, asteroids, comets and other celestial objects. The origin, size, structure, age and composition vary among craters. We study a total of 339 craters observed by the Lunar Reconnaissance Orbiter Camera （LROC）. Out of these 339 craters, 214 craters are known （named craters included in the IAU Gazetteer of Planetary Nomenclature） and 125 craters are unknown （craters that are not named and objects that are absent in the IAU Gazetteer）. We employ images taken by LROC at the North and South Poles and near side of the Moon. We report for the first time the study of unknown craters, while we also review the study of known craters conducted earlier by previous researchers. Our study is focused on measurements of diameter, depth, latitude and longitude of each crater for both known and unknown craters. The diameter measurements are based on considering the Moon to be a spherical body. The LROC website also provides a plot which enables us to measure the depth and diameter. We found that out of 214 known craters, 161 craters follow a linear relationship between depth （d） and diameter （D）, but 53 craters do not follow this linear relationship. We study physical dimensions of these 53 craters and found that either the depth does not change significantly with diameter or the depths are extremely high relative to diameter （conical）. Similarly, out of 125 unknown craters, 78 craters follow the linear relationship between depth （d） and diameter （D） but 47 craters do not follow the linear relationship. We propose that the craters following the scaling law of depth and diameter, also popularly known as the linear relationship between d and D, are formed by the impact of meteorites having heavy metals with larger dimension, while those with larger diameter but less depth are formed by meteorites/celestial objects having low density material but larger diameter. The craters with very high depth and with very small diameter are perhaps formed by the impact of meteorites that have very high density but small diameter with a conical shape. Based on analysis of the data selected for the current investigation, we further found that out of 339 craters, 100 （29.5%） craters exist near the equator, 131 （38.6%） are in the northern hemisphere and 108 （31.80%） are in the southern hemisphere. This suggests the Moon is heavily cratered at higher latitudes and near the equatorial zone.

Relationship between CME dynamics and solar flare plasma

The relationship between the velocity of CMEs and the plasma temperature of the associated X-ray solar flares is investigated. The velocity of CMEs increases with plasma temperature （R = 0.82） and photon index below the break energy （R = 0.60） of X-ray flares. The heating of the coronal plasma appears to be significant with respect to the kinetics of a CME from the reconnection region where the flare also occurs. We propose that the initiation and velocity of CMEs perhaps depend upon the dominant process of conversion of the magnetic field energy of the active region to heating/accelerating the coronal plasma in the reconnected loops. Results show that a flare and the associated CME are two components of one energy release system, perhaps, magnetic field free energy.