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Kiran Jain

Publications and source records attributed to Kiran Jain.

At least 37 records · Page 2Linked to original sources
arXiv2021

What Seismic Minimum Reveals About Solar Magnetism Below the Surface?

The Sun's magnetic field varies in multiple time scales. Observations show that the minimum between cycles 24 and 25 was the second consecutive minimum which was deeper and wider than several earlier minima. Since the active regions observed at the Sun's surface are manifestations of the magnetic field generated in the interior, it is crucial to investigate/understand the dynamics below the surface. In this context, we report, by probing the solar interior with helioseismic techniques applied to long-term oscillations data from the Global Oscillation Network Group (GONG), that the seismic minima in deeper layers have been occurring about a year earlier than that at the surface for the last two consecutive solar cycles. Our findings also demonstrate a decrease in strong magnetic fields at the base of the convection zone, the primary driver of the surface magnetic activity. We conclude that the magnetic fields located in the core and near-surface shear layers, in addition to the tachocline fields, play an important role in modifying the oscillation frequencies. This provides evidence that further strengthens the existence of a relic magnetic field in the Sun's core since its formation.

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arXiv2021

Continuous Solar Observations from the Ground -- Assessing Duty Cycle from GONG Observations

Continuous observations play an important role in the studies of solar variability. While such observations can be achieved from space with almost 100% duty cycle, it is difficult to accomplish very high duty cycle from the ground. In this context, we assess the duty cycle that has been achieved from the ground by analyzing the observations of a six station network of identical instruments, Global Oscillation Network Group (GONG). We provide a detailed analysis of the duty cycle using GONG observations spanning over 18 years. We also discuss duty cycle of individual sites and point out various factors that may impact individual site or network duty cycle. The mean duty cycle of the network is 93%, however it reduces by about 5% after all images pass through the stringent quality-control checks. The standard deviations in monthly and yearly duty cycle values are found to be 1.9% and 2.2%, respectively. These results provide a baseline that can be used in the planning of future ground-based networks.

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arXiv2021

Solar inertial modes: Observations, identification, and diagnostic promise

The oscillations of a slowly rotating star have long been classified into spheroidal and toroidal modes. The spheroidal modes include the well-known 5-min acoustic modes used in helioseismology. Here we report observations of the Sun's toroidal modes, for which the restoring force is the Coriolis force and whose periods are on the order of the solar rotation period. By comparing the observations with the normal modes of a differentially rotating spherical shell, we are able to identify many of the observed modes. These are the high-latitude inertial modes, the critical-latitude inertial modes, and the equatorial Rossby modes. In the model, the high-latitude and critical-latitude modes have maximum kinetic energy density at the base of the convection zone, and the high-latitude modes are baroclinically unstable due to the latitudinal entropy gradient. As a first application of inertial-mode helioseismology, we constrain the superadiabaticity and the turbulent viscosity in the deep convection zone.

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arXiv2018

Un-interrupted Sun-as-a-star Helioseismic Observations over Multiple Solar Cycles

We analyze Sun-as-a-star observations spanning over solar cycles 22 -- 24 from the ground-based network BiSON and solar cycles 23 -- 24 collected by the space-based VIRGO and GOLF instruments on board the {\it SoHO} satellite. Using simultaneous observations from all three instruments, our analysis suggests that the structural and magnetic changes responsible for modifying the frequencies remained comparable between cycle 23 and cycle 24 but differ from cycle 22. Thus we infer that the magnetic layer of the Sun has become thinner since the beginning of cycle 23 and continues during the current cycle.

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arXiv2018

Sun's Seismic Radius as Measured from the Fundamental Modes of Oscillations and its Implications for the TSI Variations

In this letter, we explore the relationship between the solar seismic radius and total solar irradiance (TSI) during last two solar cycles using the uninterrupted data from space-borne instruments onboard {\it SoHO} and {\it SDO}. The seismic radius is calculated from the fundamental ({\it f}) modes of solar oscillations utilizing the observations from {\it SoHO}/MDI and {\it SDO}/HMI, and the total solar irradiance measurements are obtained from {\it SoHO}/VIRGO. Our study suggests that the major contribution to the TSI variation arises from the changes in magnetic field while the radius variation plays a secondary role. We find that the solar irradiance increases with decreasing seismic radius, however the anti-correlation between them is moderately weak. The estimated maximum change in seismic radius during a solar cycle is about 5 kilometers, and is consistent in both solar cycles 23 and 24. Previous studies suggest a radius change at the surface of the order of 0.06 arcsecond to explain the 0.1\% variation in the TSI values during the solar cycle, however our inferred seismic radius change is significantly smaller, hence the TSI variations can not be fully explained by the temporal changes in seismic radius.

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arXiv2017

Probing Subsurface Flows in Active Region NOAA 12192 - Comparison with NOAA 10486

Active Region (AR) 12192 is the biggest AR observed in solar cycle 24 so far. This was a long-lived AR which survived for four Carrington rotations (CR) and exhibited several unusual phenomena. We measure the horizontal subsurface flows in this active region in multiple rotation using the ring-diagram technique of local helioseismology and the Global Oscillation Network Group (GONG+) Dopplergrams, and investigate how different was the plasma flow in AR 12192 from that in AR 10486. Both regions produced several high M- and X-class flares but had different CME productivity. Our analysis suggests that these ARs had unusually large horizontal flow amplitude with distinctly different directions. While meridional flow in AR 12192 was poleward that supports the flux transport to poles, it was equatorward in AR 10486. Furthermore, there was a sudden increase in the magnitude of estimated zonal flow in shallow layers in AR 12192 during the X3.1 flare, however, it reversed direction in AR 10486 with X17.2 flare. These flow patterns produced strong twists in horizontal velocity with depth in AR 10486 that persisted throughout the disk passage as opposed to AR 12192, which produced a twist only after the eruption of the X3.1 flare that disappeared soon after. Our study indicates that the sunspot rotation combined with the re-organization of magnetic field in AR 10486 was not sufficient to decrease the flow energy even after several large flares that might have triggered CMEs. Furthermore, in the absence of sunspot rotation in AR 12192, this re-organization of magnetic field contributed significantly to the substantial release of flow energy after the X3.1 flare.

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arXiv2016

GONG ClassicMerge: Pipeline and Product

A recent processing effort has been undertaken in order to extend the range-of-coverage of the GONG merged dopplergrams. The GONG-Classic-era observations have now been merged to provide, albeit at lower resolution, mrvzi data as far back as May of 1995. The contents of this document provide an overview of what these data look like, the processing steps used to generate them from the original site observations, and the outcomes of a few initial quality-assurance tests designed to validate the final merged images. Based on these tests, the GONG project is releasing this data product to the user community (http://nisp.nso.edu/data).

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arXiv2015

Horizontal Flows in Active Regions from Ring-diagram and Local Correlation Tracking Methods

Continuous high-cadence and high-spatial resolution Dopplergrams allow us to study sub-surface dynamics that may be further extended to explore precursors of visible solar activity on the surface. Since the p-mode power is absorbed in the regions of high magnetic field, the inferences in these regions are often presumed to have large uncertainties. In this paper, using the Dopplergrams from space-borne Helioseismic Magnetic Imager (HMI), we compare horizontal flows in a shear layer below the surface and the photospheric layer in and around active regions. The photospheric flows are calculated using local correlation tracking (LCT) method while the ring-diagram (RD) technique of helioseismology is used to infer flows in the sub-photospheric shear layer. We find a strong positive correlation between flows from both methods near the surface. This implies that despite the absorption of acoustic power in the regions of strong magnetic field, the flows inferred from the helioseismology are comparable to those from the surface measurements. However, the magnitudes are significantly different; the flows from the LCT method are smaller by a factor of 2 than the helioseismic measurements. Also, the median difference between direction of corresponding vectors is 49 degree.

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arXiv2015

Divergent Horizontal Sub-surface Flows within Active Region 11158

We measure the horizontal subsurface flow in a fast emerging active region (NOAA 11158) using the ring-diagram technique and the HMI high-spatial resolution Dopplergrams. This active region had a complex magnetic structure and displayed significant changes in the morphology during its disk passage. Over the period of six days from 2011 February 11 to 16, the temporal variation in the magnitude of total velocity is found to follow the trend of magnetic field strength. We further analyze regions of individual magnetic polarity within AR 11158 and find that the horizontal velocity components in these sub-regions have significant variation with time and depth. The leading and trailing polarity regions move faster than the mixed-polarity region. Further, both zonal and meridional components have opposite signs for trailing and leading polarity regions at all depths showing divergent flows within the active region. We also find a sharp decrease in the magnitude of total horizontal velocity in deeper layer around major flares. It is suggested that the re-organization of magnetic fields during flares combined with the sunspot rotation decreases the magnitude of horizontal flows or that the flow kinetic energy has been converted into the energy released by flares. After the decline in flare activity and the sunspot rotation, the flows tend to follow the pattern of the magnetic activity.We also observe less variation in the velocity components near the surface but these tend to increase with depth, further demonstrating that the deeper layers are more affected by the topology of active regions.

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arXiv2013

Temporal Variations of High-Degree Solar p-Modes using Ring-Diagram Analysis

We study temporal variations in the amplitudes and widths of high-degree acoustic modes by applying the ring-diagram technique to the GONG+, MDI and HMI Dopplergrams during the declining phase of cycle 23 and rising phase of cycle 24. The mode parameters from all three instruments respond similarly to the varying magnetic activity. The mode amplitudes and widths show consistently lower variation due to smaller magnetic activity in cycle 24 as compared to the previous solar cycle.

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arXiv2012

Two-Dimensional Helioseismic Power, Phase, and Coherence Spectra of {\it Solar Dynamics Observatory} Photospheric and Chromospheric Observables

While the {\it Helioseismic and Magnetic Imager} (HMI) onboard the {\it Solar Dynamics Observatory} (SDO) provides Doppler velocity [$V$], continuum intensity [$I_C$], and line-depth [$Ld$] observations, each of which is sensitive to the five-minute acoustic spectrum, the {\it Atmospheric Imaging Array} (AIA) also observes at wavelengths -- specifically the 1600 and 1700 Angstrom bands -- that are partly formed in the upper photosphere and have good sensitivity to acoustic modes. In this article we consider the characteristics of the spatio--temporal Fourier spectra in AIA and HMI observables for a 15-degree region around NOAA Active Region 11072. We map the spatio--temporal-power distribution for the different observables and the HMI Line Core [$I_L$], or Continuum minus Line Depth, and the phase and coherence functions for selected observable pairs, as a function of position and frequency. Five-minute oscillation power in all observables is suppressed in the sunspot and also in plage areas. Above the acoustic cut-off frequency, the behaviour is more complicated: power in HMI $I_C$ is still suppressed in the presence of surface magnetic fields, while power in HMI $I_L$ and the AIA bands is suppressed in areas of surface field but enhanced in an extended area around the active region, and power in HMI $V$ is enhanced in a narrow zone around strong-field concentrations and suppressed in a wider surrounding area. The relative phase of the observables, and their cross-coherence functions, are also altered around the active region. These effects may help us to understand the interaction of waves and magnetic fields in the different layers of the photosphere, and will need to be taken into account in multi-wavelength local helioseismic analysis of active regions.

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arXiv2011

Subsurface flows associated with rotating sunspots

In this paper, we compare components of the horizontal flow below the solar surface in and around regions consisting of rotating and non-rotating sunspots. Our analysis suggests that there is a significant variation in both components of the horizontal flow at the beginning of sunspot rotation as compared to the non-rotating sunspot. In most cases, the flows in surrounding areas are relatively small. However, there is a significant influence of the motion on flows in an area closest to the sunspot rotation.

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arXiv2011

How peculiar was the recent extended minimum - A hint towards double minima

In this paper, we address the controversy regarding the recent extended solar minimum as seen in helioseismic low- and intermediate-degree mode frequencies: studies from different instruments identify different epochs of seismic minima. Here we use mode frequencies from a network of six identical instruments, Global Oscillation Network Group, continuously collecting data for more than 15 years, to investigate the epoch of minimum in solar oscillation frequencies prior to the beginning of solar cycle 24. We include both low- and intermediate-degree modes in the $\ell$ range of 0 -- 120 and frequency range of 2.0 -- 3.5 mHz. In this analysis, we demonstrate that there were indeed two minima in oscillation frequencies, depending upon the degree of modes, or more precisely the lower turning point radius of the propagating wave. We also analyze frequencies as a function of latitude to identify the beginning of solar cycle 24. We observe two minima at high latitudes and a single minimum at mid/low latitudes. This scenario is in contrast to cycle 23 where the epoch of seismic minimum did not change with latitude or depth. Our results also hint towards a possible role of the relic magnetic field in modifying the oscillation frequencies of modes sampling deeper layers.

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arXiv2010

Inhomogeneous Power Distribution in Magnetic Oscillations

We apply ring-diagram analysis and spherical harmonic decomposition methods to compute 3-dimensional power spectra of magnetograms obtained by the Global Oscillation Network Group (GONG) during quiet periods of solar activity. This allows us to investigate the power distribution in acoustic waves propagating in localized directions on the solar disk. We find evidence of the presence of five-minute oscillations in magnetic signals that suggests a non-homogeneous distribution of acoustic power. In this paper, we present our results on the asymmetry in oscillatory power and its behaviour as a function of frequency, time and magnetic field strength. These characteristics are compared with simultaneous velocity measurements.

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arXiv2010

What Solar Oscillation Tell us About the Solar Minimum

The availability of continuous helioseismic data for two consecutive solar minima has provided a unique opportunity to study the changes in the solar interior that might have led to this unusual minimum. We present preliminary analysis of inter mediate-degree mode frequencies in the 3 mHz band during the current period of minimal solar activity and show that the mode frequencies are significantly lower than those during the previous activity minimum. Our analysis do not show any signature of the beginning of cycle 24 till the end of 2008. In addition, the zonal and meridional flow patterns inferred from inverting frequencies also hint for a delayed onset of a new cycle. The estimates of travel time are higher than the previous minimum confirming a relatively weak solar activity during the current minimum.

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arXiv2009

Correlation Analysis of Mode Frequencies with Activity Proxies at Different Phases of the Solar Cycle

We analyze intermediate degree p- and f-mode eigenfrequencies measured by GONG and MDI/SOHO for a complete solar cycle to study their correlation with solar activity. We demonstrate that the frequencies do vary linearly with the activity, however the degree of correlation differs from phase to phase of the cycle. During the rising and the declining phases, the mode frequencies are strongly correlated with the activity proxies whereas during the low- and high-activity periods, the frequencies have significantly lower correlation with all the activity proxies considered here.

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arXiv2009

Solar Activity Phases and Intermediate-degree Mode Frequencies

We analyze intermediate degree p-mode eigenfrequencies measured by GONG and MDI/SOHO over a solar cycle to study the source of their variability. We carry out a correlation analysis between the change in frequencies and several measures of the Sun's magnetic activity that are sensitive to changes at different levels in the solar atmosphere. The observations span a period of about 12 years starting from mid-1996 (the minimum of cycle 23) to early-2008 (near minimum of cycle 24), corresponding to a nearly complete solar activity cycle. We demonstrate that the frequencies do vary in phase with the solar activity indices, however the degree of correlation differs from phase to phase of the cycle. During the rising and declining phases, the mode frequency shifts are strongly correlated with the activity proxies whereas during the high-activity period, the shifts have significantly lower correlation with all activity proxies, except for the 10.7-cm radio flux. In particular, the proxies that are only influenced by the variation of the strong component of the magnetic field in the photosphere have a much lower correlation at the high-activity period. On the other hand, the shifts are better correlated with the proxies sensitive to changes in the weak component of the magnetic field. Our correlation analysis suggests that more than 90% of the variation in the oscillation frequencies in all activity phases can be explained by changes in both components of the magnetic field. Further, the slopes obtained from the linear regression analysis also differ from phase to phase and show a strong correlation with the correlation coefficients between frequency shifts and solar activity.

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