Statistical properties of X-class flares and their relationship with super active regions during solar cycles 21–23

We investigate the statistical distribution of X-class flares and their relationship with super active regions (SARs) during solar cycles 21–23. Analysis results show that X1.0–X1.9 flares accounted for 52.71 % of all X-class flares, with X2.0–X2.9 flares at 20.59 %, X3.0–X4.9 at 13.57 %, X5–X9.9 at 8.37 % and ≥X10 at 4.75 %. All X-class flares occurred around the solar maximum during solar cycle 22, while in solar cycle 23, X-class flares were scattered in distribution. In solar cycle 21, X-class flares were distributed neither in a concentrated manner like cycle 22 nor in a scattered manner as cycle 23. During solar cycles 21–23, 32.2 % of the X1.0–X1.9 flares, 31.9 % of the X2.0–X2.9 flares, 43.3 % of the X3.0–X4.9 flares, 81.08 % of the X5.0–X9.9 flares, and 95.2 % of the ≥X10 flares were produced by SARs.     

Solar activity during first six years of solar cycle 24 and 23: a comparative study

Attempt to look into the nature of solar activity and variability have increased importance in recent days because of their terrestrial relationships. In the present work we have attempted to compare the solar activity events during first six years (2008–2013) of the ongoing solar cycle 24 with first six years (1996–2001) of solar cycle 23. To that end, we have considered sunspot numbers, F10.7 cm solar flux, halo CMEs and geomagnetic storms as comparison parameters. Sunspot number during the year 2008–2013 varied from 0 to 96.7 while during the year 1996 to 2001 it was observed from 0.9 to 170.1. Solar radio flux (F10.7 cm index) varied from 65 to 190 during the years 2008–2013 while it was observed from 65 to 283 during the years 1996–2001. 197 cases of halo CMEs (width=360^°) in solar cycle 23 (1996–2001) and 177 cases of halo CMEs (width=360^°) in solar cycle 24 (2008–2013) are investigated. 287 and 104 geomagnetic storm cases (Dst varies between ?50 and ?350 nT) are analysed during the half period of solar cycle 23 and 24 respectively. Comparative results indicate that solar cycle 23 was more pronounced in comparison of solar cycle 24.     

Resolving Differences in Absolute Irradiance Measurements Between the SOHO/CELIAS/SEM and the SDO/EVE

The Solar EUV Monitor (SEM) onboard SOHO has measured absolute extreme ultraviolet (EUV) and soft X-ray solar irradiance nearly continuously since January 1996. The EUV Variability Experiment (EVE) on SDO, in operation since April of 2010, measures solar irradiance in a wide spectral range that encompasses the band passes (26?–?34 nm and 0.1?–?50 nm) measured by SOHO/SEM. However, throughout the mission overlap, irradiance values from these two instruments have differed by more than the combined stated uncertainties of the measurements. In an effort to identify the sources of these differences and eliminate them, we investigate in this work the effect of reprocessing the SEM data using a more accurate SEM response function (obtained from synchrotron measurements with a SEM sounding-rocket clone instrument taken after SOHO was already in orbit) and time-dependent, measured solar spectral distributions – i.e., solar reference spectra that were unavailable prior to the launch of the SDO. We find that recalculating the SEM data with these improved parameters reduces mean differences with the EVE measurements from about 20 % to less than 5 % in the 26?–?34 nm band, and from about 35 % to about 15 % for irradiances in the 0.1?–?7 nm band extracted from the SEM 0.1?–?50 nm channel.     

The Interaction Between Coronal Mass Ejections and Streamers: A Statistical View over 15 Years (1996 – 2010)

We report on the statistical analysis of the interaction between coronal mass ejections (CMEs) and streamers based on 15 years (from 1996 to 2010 inclusive) of observation of the solar corona with the LASCO-C2 coronagraph. We used synoptic maps and improved the method of analysis of past investigations by implementing an automatic detection of both CMEs and streamers. We identified five categories of interaction based on photometric and geometric variations between the pre- and post-CME streamers: “brightening”, “dimming”, “emergence”, “disappearance”, and “deviation”. A sixth category, “no change”, included all cases where none of the above variations is observed. A “global set” of 21?242 CMEs was considered as well as a subset of the 10 % brightest CMEs (denoted “top-ten”) and three typical periods of solar activity: minimum, intermediate, and maximum. We found that about half of the global population of CMEs are not associated with streamers, whereas 93 % of the 10 % brightest CMEs are associated. When there is a CME-streamer association, approximately 95 % of the streamers experience a change, either geometric or radiometric. The “no change” category therefore amounts to approximately 5 %, but this percentage varies from 1?–?2 % during minimum to 7?–?8 % during intermediate periods of activity; values of 3?–?5 % are recorded during maximum. Emergences and disappearances of streamers are not dominant processes; they constitute 16?–?17 % of the global set and 23 % (emergence) and 28 % (disappearance) of the “top-ten” set. Streamer deviations are observed for 57 % and 70 % of, respectively, the global set and “top-ten” CMEs. The cases of dimming and brightening are roughly equally present and each case constitutes approximately 30?–?35 % of either set, global or “top-ten”.     

Solar transients disturbing the terrestrial magnetic environment at higher latitudes

Geomagnetic field variations during five major Solar Energetic Particle (SEP) events of solar cycle 23 have been investigated in the present study. The SEP events of 1 October 2001, 4 November 2001, 22 November 2001, 21 April 2002 and 14 May 2005 have been selected to study the geomagnetic field variations at two high-latitude stations, Thule (77.5^° N, 69.2^° W) and Resolute Bay (74.4^° E, 94.5^° W) of the northern polar cap. We have used the GOES proton flux in seven different energy channels (0.8–4 MeV, 4–9 MeV, 9–15 MeV, 15–40 MeV, 40–80 MeV, 80–165 MeV, 165–500 MeV). All the proton events were associated with geoeffective or Earth directed CMEs that caused intense geomagnetic storms in response to geospace. We have taken high-latitude indices, AE and PC, under consideration and found fairly good correlation of these with the ground magnetic field records during the five proton events. The departures of the H component during the events were calculated from the quietest day of the month for each event and have been represented as ΔH _THL and ΔH _RES for Thule and Resolute Bay, respectively. The correspondence of spectral index, inferred from event integrated spectra, with ground magnetic signatures ΔH _THL and ΔH _RES along with Dst and PC indices have been brought out. From the correlation analysis we found a very strong correlation to exist between the geomagnetic field variation (ΔHs) and high-latitude indices AE and PC. To find the association of geomagnetic storm intensity with proton flux characteristics we derived the correspondence between the spectral indices and geomagnetic field variations (ΔHs) along with the Dst and AE index. We found a strong correlation (0.88) to exist between the spectral indices and ΔHs and also between spectral indices and AE and PC.     

Imaging Observations of X-Ray Quasi-periodic Oscillations at 3 – 6 keV in the 26 December 2002 Solar Flare

Quasi-periodic oscillations in soft X-rays (SXR) are not well known due to the instrument limitations, especially the absence of imaging observations of SXR oscillations. We explore the quasi-periodic oscillations of SXR at 3?–?6 keV in a solar flare observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 26 December 2002. This was a B8.1 class event and showed three X-ray sources (S₁, S₂, and S₃) at 3?–?6 keV and two sources (S₁ and S₂) at 12?–?25 keV. The light curves of the total fluxes display a two-minute oscillation at 3?–?6 keV, but not in the energy bands above 8 keV. To investigate imaging observations of the oscillations, we prepared CLEAN images at seven energy bands between 3 keV and 20 keV with an eight-second integration. The light curves of three sources were analyzed after integrating the flux of each source region. We used the Fourier method to decompose each source light curve into rapidly varying and slowly varying components. The rapidly varying components show seven individual peaks which are well fitted with a sine function. Then we used the wavelet method to analyze the periods in the rapidly varying component of each source. The results show that three sources display damped quasi-periodic oscillations with a similar two-minute period. The damped oscillations timescale varies between 2.5 to 6 minutes. Source S₁ oscillates with the same phase as S₃, but is almost in anti-phase with S₂. Analyzing the flaring images in more detail, we found that these oscillation peaks are well consistent with the appearance of S₃, which seems to split from or merge with S₂ with a period of two minutes. The flare images with a high cadence of one second at 3?–?6 keV show that source S₃ appears with a rapid period of 25 seconds. The two-minute oscillation shows the highest spectral power. Source S₃ seems to shift its position along the flare loop with a mean speed of 130 km?s^?1, which is of the same order as the local sound speed. This connection between the oscillation peaks and emission enhancement appears to be an observational constraint on the emission mechanism at 3?–?6 keV.     

Observations of Plasma Blobs by OI 630 nm Using ASI and Photometer over Kolhapur,India

This paper presents observations of plasma blobs by nightglow OI 630.0 nm emissions using ground-based techniques, all sky imager and photometer from Kolhapur. The nightglow observations have been made at low latitude station, Kolhapur (16.42°N, 74.2°E, and 10.6°N dip lat.) during clear moonless nights for period of October 2011–April 2012. Generally, these occur 3 h after sunset (18:00 IST). Herein we have calculated velocities of plasma blobs using scanning method, introduced by Pimenta et al. (Adv Space Res 27:1219–1224, 2001). The average zonal drift velocity (eastward) of the plasma blobs were found to be 133 ms^?1 and vary between 100 and 200 ms^?1. The width (east–west expansion) and length (north–south expansion) of plasma blobs is calculated by recently developed method of Sharma et al. (Curr Sci 106(08):1085–1093, 2014b). Their mean width and length were in the range of 70–180 and 500–950 km respectively. The study shows that localized eastward polarization electric field plays an important role in the generation of plasma blobs.     

Cycle 23 Variation in Solar Flare Productivity

The NOAA listings of solar flares in cycles 21?–?24, including the GOES soft X-ray magnitudes, enable a simple determination of the number of flares each flaring active region produces over its lifetime. We have studied this measure of flare productivity over the interval 1975?–?2012. The annual averages of flare productivity remained approximately constant during cycles 21 and 22, at about two reported M- or X-flares per region, but then increased significantly in the declining phase of cycle 23 (the years 2004?–?2005). We have confirmed this by using the independent RHESSI flare catalog to check the NOAA events listings where possible. We note that this measure of solar activity does not correlate with the solar cycle. The anomalous peak in flare productivity immediately preceded the long solar minimum between cycles 23 and 24.     

Incoherent Scatter Radar Studies of Daytime Plasma Lines

First results from wideband (electron phase energies of 5–51 eV), high-resolution (0.1 eV) spectral measurements of photoelectron–enhanced plasma lines made with the 430 MHz radar at Arecibo Observatory are presented. In the F region, photoelectrons produced by solar EUV line emissions (He II and Mg IX) give rise to plasma line spectral peaks/valleys. These and other structures occur within an enhancement zone extending from electron phase energies of 14–27 eV in both the bottomside and topside ionosphere. However, photoelectron–thermal electron Coulomb energy losses can lead to a broadened spectral structure with no resolved peaks in the topside ionosphere. The plasma line energy spectra obtained in the enhancement zone exhibit a unique relation in that phase energy is dependent on pitch angle; this relation does not exist in any other part of the energy spectrum. Moreover, large fluctuations in the difference frequency between the upshifted and downshifted plasma lines are evident in the 14–27 eV energy interval. At high phase energies near 51 eV the absolute intensities of photoelectron-excited Langmuir waves are much larger than those predicted by existing theory. The new measurements call for a revision/improvement of plasma line theory in several key areas.     

THERMAP: a mid-infrared spectro-imager for space missions to small bodies in the inner solar system

We present THERMAP, a mid-infrared spectro-imager for space missions to small bodies in the inner solar system, developed in the framework of the MarcoPolo-R asteroid sample return mission. THERMAP is very well suited to characterize the surface thermal environment of a NEO and to map its surface composition. The instrument has two channels, one for imaging and one for spectroscopy: it is both a thermal camera with full 2D imaging capabilities and a slit spectrometer. THERMAP takes advantage of the recent technological developments of uncooled microbolometer arrays, sensitive in the mid-infrared spectral range. THERMAP can acquire thermal images (8–18 μm) of the surface and perform absolute temperature measurements with a precision better than 3.5 K above 200 K. THERMAP can acquire mid-infrared spectra (8–16 μm) of the surface with a spectral resolution Δλ of 0.3 μm. For surface temperatures above 350 K, spectra have a signal-to-noise ratio >60 in the spectral range 9–13 μm where most emission features occur.     

Photospheric and Chromospheric Oscillations in Solar Faculae

The differences between physical conditions in solar faculae and those in sunspots and quiet photosphere (increased temperature and different magnetic field topology) suggest that oscillation characteristics in facula areas may also have different properties. The analysis of 28 time series of simultaneous spectropolarimetric observations in facula photosphere (Fe?i 6569 Å, 8538 Å) and chromosphere (Hα, Ca?ii 8542 Å) yields the following results. The amplitude of five-minute oscillations of line-of-sight (LOS) velocity decreases by 20?–?40% in facula photosphere. There are only some cases revealing the inverse effect. The amplitude of four- to five-minute LOS velocity oscillations increases significantly in the chromosphere above faculae, and power spectra fairly often show pronounced peaks in a frequency range of 1.3?–?2.5 mHz. Evidence of propagating oscillations can be seen from space?–?time diagrams. We have found oscillations of the longitudinal magnetic field (1.5?–?2 mHz and 5.2 mHz) inside faculae.     

Cyclic and Long-Term Variation of Sunspot Magnetic Fields

Measurements from the Mount Wilson Observatory (MWO) were used to study the long-term variations of sunspot field strengths from 1920 to 1958. Following a modified approach similar to that presented in Pevtsov et al. (Astrophys. J. Lett. 742, L36, 2011), we selected the sunspot with the strongest measured field strength for each observing week and computed monthly averages of these weekly maximum field strengths. The data show the solar cycle variation of the peak field strengths with an amplitude of about 500?–?700 gauss (G), but no statistically significant long-term trends. Next, we used the sunspot observations from the Royal Greenwich Observatory (RGO) to establish a relationship between the sunspot areas and the sunspot field strengths for cycles 15?–?19. This relationship was used to create a proxy of the peak magnetic field strength based on sunspot areas from the RGO and the USAF/NOAA network for the period from 1874 to early 2012. Over this interval, the magnetic field proxy shows a clear solar cycle variation with an amplitude of 500?–?700 G and a weaker long-term trend. From 1874 to around 1920, the mean value of magnetic field proxy increases by about 300?–?350 G, and, following a broad maximum in 1920?–?1960, it decreases by about 300 G. Using the proxy for the magnetic field strength as the reference, we scaled the MWO field measurements to the measurements of the magnetic fields in Pevtsov et al. (2011) to construct a combined data set of maximum sunspot field strengths extending from 1920 to early 2012. This combined data set shows strong solar cycle variations and no significant long-term trend (the linear fit to the data yields a slope of ??0.2±0.8 G?year^?1). On the other hand, the peak sunspot field strengths observed at the minimum of the solar cycle show a gradual decline over the last three minima (corresponding to cycles 21?–?23) with a mean downward trend of ≈?15 G?year^?1.     

WSO/UV spectrographs: the expected performance of HIRDES

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the UV range. WSO/UV consists of a single UV telescope with a primary mirror of 1.7 m diameter feeding the UV spectrometer and UV imagers. The spectrometer comprises three different spectrographs, two high-resolution echelle spectrographs (the High-Resolution Double-Echelle Spectrograph, HIRDES) and a low-dispersion long-slit instrument. Within HIRDES the 102–310 nm spectral band is split to feed two echelle spectrographs covering the UV range 174–310 nm and the vacuum-UV range 102–176 nm with high spectral resolution (R>55000). The technical concept is based on the heritage of two previous ORFEUS SPAS missions. The phase-B1 development activities are described in this paper considering the performance of the instruments.     

Thin magnetic hydrogen atmospheres and the neutron star RX J1856.5–3754

RX J1856.5–3754 is one of the brightest nearby isolated neutron stars, and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5–3754, within the uncertainties. In our simplest model, the best-fit parameters are an interstellar column density N _H≈1×10²⁰ cm^?2 and an emitting area with R ^∞≈17 km (assuming a distance to RX J1856.5–3754 of 140 pc), temperature T ^∞≈4.3×10⁵ K, gravitational redshift z _g ～0.22, atmospheric hydrogen column y _H≈1 g cm^?2, and magnetic field B≈(3–4)×10¹² G; the values for the temperature and magnetic field indicate an effective average over the surface.     

Evidence of recent changes in the local Galactic IMF of stars and substars

This paper calculates the stellar–substellar initial mass function (IMF) based on data from low-luminosity stars and substars. We find the mass spectrum for the stars and substars less than 10 million years in age, and we estimate the low-mass part of the stellar–substellar IMF for 10 million years ago.     

Visibility and Origin of Compact Interplanetary Radio Type IV Bursts

We have analyzed radio type IV bursts in the interplanetary (IP) space at decameter–hectometer (DH) wavelengths to determine their source origin and a reason for the observed directivity. We used radio dynamic spectra from the instruments on three different spacecraft, STEREO-A, Wind, and STEREO-B, which were located approximately 90 degrees apart from each other in 2011?–?2012, and thus gave a 360 degree view of the Sun. The radio data were compared to white-light and extreme ultraviolet (EUV) observations of flares, EUV waves, and coronal mass ejections (CMEs) in five solar events. We find that the reason that compact and intense DH type IV burst emission is observed from only one spacecraft at a time is the absorption of emission in one direction and that the emission is blocked by the solar disk and dense corona in the other direction. The geometry also makes it possible to observe metric type IV bursts in the low corona from a direction where the higher-located DH type IV emission is not detectable. In the absorbed direction we found streamers, and they were estimated to be the locations of type II bursts, caused by shocks at the CME flanks. The high-density plasma was therefore most probably formed by shock–streamer interaction. In some cases, the type II-emitting region was also capable of stopping later-accelerated electron beams, which were visible as type III bursts that ended near the type II burst lanes.     

Optical spectroscopy of the radio pulsar PSR B0656+14

We have obtained the spectrum of a middle-aged PSR B0656+14 in the 4300–9000 Å range with the ESO/VLT/FORS2. Preliminary results show that at 4600–7000 Å the spectrum is almost featureless and flat with a spectral index α _ν ??0.2 that undergoes a change to a positive value at longer wavelengths. Combining with available multiwavelength data suggests two wide, red and blue, flux depressions whose frequency ratio is about 2 and which could be the 1st and 2nd harmonics of electron/positron cyclotron absorption formed at magnetic fields ～10⁸ G in upper magnetosphere of the pulsar.     

Increase of the mean Sun–Earth distance caused by a secular mass accumulation

Based on many planetary observations between the years 1971 and 2003, Krasinsky and Brumberg (Celest. Mech. Dyn. Astron. 90:267–288, 2004) have estimated a rate of increase in the mean Sun-Earth distance of (15±4) m per century. Together with other anomalous observations in the solar system, this increase appears to be unexplained (Lämmerzahl et al. in Astrophys. Space Sci. Lib., vol. 349, pp. 75–101, 2008). We explain these findings by invoking a recently proposed gravitational impact model (Wilhelm et al. in Astrophys. Space Sci. 343:135–144, 2013) that implies a secular mass increase of all massive bodies. This allows us to formulate a quantitative understanding of the effect within the parameter range of the model with a mass accumulation rate of the Sun of (6.4±1.7)×10¹⁰ kg?s^?1.     

Multi-height Scattering Phase Shifts from Sunspots and Phase Differences Between Photospheric Heights with SDO/HMI and AIA Data

Following Couvidat (Solar Phys. 282, 15, 2013), we analyze data from the Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) instruments onboard the Solar Dynamics Observatory. Doppler velocity and continuum intensity at 6173 Å as well as intensity maps at 1600 Å and 1700 Å are studied on 14 active regions and four quiet-Sun regions at four heights in the solar photosphere. A Hankel–Fourier analysis is performed around these regions of interest. Outgoing–ingoing scattering phase shifts at a given atmospheric height are computed, as well as ingoing–ingoing and outgoing–outgoing phase differences between two atmospheric heights. The outgoing–ingoing phase shifts produced by sunspots show little dependence on measurement height, unlike the acoustic power reduction measured in Couvidat (2013). Phenomena happening above the continuum level, like acoustic glories, appear not to have a significant effect on the phases. However, there is a strong dependence on sunspot size, line-of-sight magnetic flux, and intensity contrast. As previously suggested by other groups, the region of wave scattering appears both horizontally smaller and vertically less extended than the region of acoustic power suppression, and occurs closer to the surface. Results presented here should help refine theoretical models of acoustic wave scattering by magnetic fields. Ingoing–ingoing phase differences between two measurement heights are also computed and show a complex pattern highly dependent on atmospheric height. In particular, a significant sensitivity of AIA signals to lower chromospheric layers is shown. Finally, ingoing–ingoing minus outgoing–outgoing phase differences between various measurement heights are discussed.     

Astronomical spectroscopy in the last four decades: survival of the fittest

Spectroscopic astronomical instrumentation has much evolved in the last 40 years. Long-slit grating spectrographs with a photographic plate as the detector working in the 0.3–1 μm range were prevalent up to the early 1970s. The replacement of photographic plates by two-dimensional digital detectors provided gains in sensitivity of two orders of magnitude and much better photometric and radial velocity precision, and opened the 1 to 25 μm infrared domain. Another gain in speed by up to two orders of magnitude was then obtained through the development of various spectroscopic systems, each optimized for a subset of astronomical objects. This development was underpinned by a number of technological advances, in particular the development of automatic data reduction pipelines using sophisticated algorithms. With ever larger and more complex instrument systems for the present 8–10 m diameter telescopes—and soon even more for the next generation of Extremely Large Telescopes, the development of an instrument is now a big enterprise, ranging all the way from long-term enabling technology efforts to management of large teams for construction and deployment over typically 7–8 years.