Solar soft X-rays and solar activity. II
Soft X ray bursts involving solar flares from satellite observations, discussing thermal nature and total activity related H-alpha emission rates
Teske, R. G.↗
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Evidence from Galactic Cosmic Rays That the Sun Has Likely Entered A Secular Minimum in Solar Activity
Since the beginning of the space age, the Sun has been in a multi-cycle period of elevated activity (secular maximum). This secular maximum is the longest in the last 9300 years. Since the end of solar cycle 21 (SC21), however, the Sun has shown a decline in over-all activity, which has remarkably increased the fluxes of galactic cosmic rays (GCRs). Here, we investigate the correlation between the modulation of GCRs, the heliospheric magnetic field, and solar wind speed for the last 24 solar cycles to find trends that can potentially be used to predict future solar activity. Specifically, we develop a tool for predicting future magnetic field intensity, based on the hysteresis in the GCR variation, during the last phases of the current cycle. This method estimates that SC25 will be as weak as or weaker than SC24. This would mean that the Sun has likely entered a secular minimum, which, according to historical records, should last for another two cycles (SC2527and SC26).
Solar cycles↗
Solar Active Region Coronal Jets: Hidden-onset Jets
Solar quiet- and coronal-hole region coronal jets frequently clearly originate from erupting minifilaments, but active-region jets often lack an obvious erupting-minifilament source. We observe a coronal-jet-productive active region (AR), AR12824, over 2021 May 22 0–8 UT, primarily using Solar Dynamics Observatory (SDO) Atmospheric Imaging Array (AIA) EUV images and SDO/Helioseismic and Magnetic Imager magnetograms. Jets were concentrated in two locations in the AR: on the south side and on the northwest side of the AR’s lone large sunspot. The south-location jets are oriented so that we have a clear view of the jets’ origin low in the atmosphere: their source is clearly minifilaments erupting from locations showing magnetic flux changes/cancelations. After erupting a projected distance ~5ʹʹ away from their origin site, the minifilaments erupt outward onto far-reaching field as part of the jet’s spire, quickly losing their minifilament character. In contrast, the northwest-location jets show no clear erupting minifilament, but the source site of those jets are obscured along our line of sight by absorbing chromospheric material. EUV and magnetic data indicate that the likely source sites were ~15ʹʹ from where the we first see the jet spire; thus, an erupting minifilament would likely lose its minifilament character before we first see the spire. We conclude that such AR jets could work like non-AR jets, but the erupting-minifilament jet source is often hidden by obscuring material. Another factor is that magnetic eruptions making some AR jets carry only a harder-to-detect comparatively thin (∼1ʹʹ–2ʹʹ) minifilament “strand.” A full report appears in Sterling, Moore, & Panesar (2024, ApJ, 960, 109).
Alphonse C. Sterling↗
THE ORIGIN OF NET ELECTRIC CURRENTS IN SOLAR ACTIVE REGIONS
There is a recurring question in solar physics regarding whether or not electric currents are neutralized in active regions (ARs). This question was recently revisited using three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulations of magnetic flux emergence into the solar atmosphere. Such simulations showed that flux emergence can generate a substantial net current in ARs. Other sources of AR currents are photospheric horizontal flows. Our aim is to determine the conditions for the occurrence of net versus neutralized currents with this second mechanism. Using 3D MHD simulations, we systematically impose line-tied, quasi-static, photospheric twisting and shearing motions to a bipolar potential magnetic field. We find that such flows: (1) produce both direct and return currents, (2) induce very weak compression currents—not observed in 2.5D—in the ambient field present in the close vicinity of the current-carrying field, and (3) can generate force-free magnetic fields with a net current. We demonstrate that neutralized currents are in general produced only in the absence of magnetic shear at the photospheric polarity inversion line—a special condition that is rarely observed. We conclude that photospheric flows, as magnetic flux emergence, can build up net currents in the solar atmosphere, in agreement with recent observations. These results thus provide support for eruption models based on pre-eruption magnetic fields that possess a net coronal current.
79 ASTRONOMY AND ASTROPHYSICS↗
Determination of solar source regions of the solar wind sampled by ulysses and ACE near solar activity maximum
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solar source solar wind Ulysses ACE near solar act↗
The distant future of solar activity: A case study of Beta Hydri. III - Transition region, corona, and stellar wind
The paper investigates the secular decay of solar-type activity through a detailed comparison of the present sun with the very old solar-type star, Beta Hyi, taken as a proxy of the future sun. Analyses of successive atmospheric layers are presented, with emphasis of the outermost parts. The FUV emission lines for the transition zone are among the faintest so far seen in any solar-type star. The coronal soft X-ray spectrum was measured through different filters on EXOSAT and compared to simulated X-ray observations of the sun seen as a star. The flux from Beta Hyi is weaker than that from the solar corona and has a different spectrum. It is inferred that a thermally driven stellar wind can no longer be supported, which removes the mechanism from further rotational braking of the star through a magnetic stellar wind.
Dravins, D.↗
The response of ozone to solar activity variations - A review
The effects of solar UV variations on the ozone concentration in the atmosphere are studied with respect to determinations of the global mean ozone concentrations as measured by satellite instrumentation. The Nimbus 4 satellite revealed an NO presence, which catalytically destroys ozone, during a solar proton event. An 11-year UV variability has been detected at increasing wavelengths above 1500 A, an energy band which causes photolysis of O2 into O3, N2O into NO, and NO production in the stratosphere. The latter two reactions decrease ozone destruction. IR interferometer spectrometer (IRIS) data is cited to show an ozone variation of about 3%, which is possible with a 20% solar UV variability at 1800 A; the variability has been confirmed to exist by measurements taken from the Atmospheric Explorer E satellite.
Keating, G. M.↗
Neon isotope studies of Fayetteville and Kapoeta meteorites and clues to ancient solar activity
Under the assumption that the solar-flare bombardment of the irradiated grains of gas-rich meteorites occurred about 4.5 b.y. ago on the parent body regoliths at 3 A.U., an estimate of the solar cosmic ray-produced Ne-21 is made by studying etched pyroxene minera separates from both light and dark portions of the gas-rich meteorites Fayetteville and Kapoeta. Excesses of solar cosmic ray Ne-21 were observed in dark portions of these meteorites, after accounting for their galactic cosmic ray Ne-21 production and solar flare Ne-21. In order to produce the estimated solar cosmic ray Ne-21 in the present samples, highly enhanced solar cosmic ray proton fluxes from the ancient sun are required.
Padia, J. T.↗
Solar irradiance measurements - Minimum through maximum solar activity
The Earth Radiation Budget Satellite (ERBS) and the NOAA-9 spacecraft solar monitors were used to measure the total solar irradiance during the period October 1984 to December 1989. Decreasing trends in the irradiance measurements were observed as sunspot activity decreased to minimum levels in 1986; after 1986, increasing trends were observed as sunspot activity increased. The magnitude of the irradiance variability was found to be approximately 0.1 percent between sunspot minimum and maximum (late 1989). When compared with the 1984 to 1989 indices of solar magnetic activity, the irradiance trends appear to be in phase with the 11-year sunspot cycle. Both irradiance series yielded 1,365/sq Wm as the mean value of the solar irradiance, normalized to the mean earth/sun distance. The monitors are electrical substitution, active-cavity radiometers with estimated measurement precisions and accuracies of less than 0.02 and 0.2 percent, respectively.
Lee, R. B., III↗
Invisibility of Solar Active Region Umbra-to-Umbra Coronal Loops: New Evidence that Magnetoconvection Drives Solar-Stellar Coronal Heating
Coronal heating generally increases with increasing magnetic field strength: the EUV/X-ray corona in active regions is 10--100 times more luminous and 2--4 times hotter than that in quiet regions and coronal holes, which are heated to only about 1.5 MK, and have fields that are 10--100 times weaker than that in active regions. From a comparison of a nonlinear force-free model of the three-dimensional active region coronal field to observed extreme-ultraviolet loops, we find that (1) umbra-to-umbra coronal loops, despite being rooted in the strongest magnetic flux, are invisible, and (2) the brightest loops have one foot in an umbra or penumbra and the other foot in another sunspot's penumbra or in unipolar or mixed-polarity plage. The invisibility of umbra-to-umbra loops is new evidence that magnetoconvection drives solar-stellar coronal heating: evidently, the strong umbral field at both ends quenches the magnetoconvection and hence the heating. Our results from EUV observations and nonlinear force-free modeling of coronal magnetic field imply that, for any coronal loop on the Sun or on any other convective star, as long as the field can be braided by convection in at least one loop foot, the stronger the field in the loop, the stronger the coronal heating.
Tiwari, Sanjiv K.↗
Characteristics of solar activity at 4 and 8.6 millimeters Final report, May 1, 1965 - Jul. 1, 1966
Average solar flux intensity and burst activity measurements
ASTROPHYSICS↗
Effects of long-period solar activity fluctuation on temperature and pressure of the terrestrial atmosphere
The present state of research on the influence of solar sunspot activity on tropospheric temperature and pressure is reviewed. The existence of an 11-year temperature cycle of 5 different types is affirmed. A cyclic change in atmospheric pressure, deducing characteristic changes between 11-year cycles is discussed. The existence of 80-year and 5-to-6-year cycles of temperature is established, and physical causes for birth are suggested.
Rubashev, B. M.↗
Multi-Scale Response of Global Sea Surface Temperature to Solar Activity
The variation in the sun output between the maximum and minimum activity over a 11-yr solar cycle is only about 0.1% of the average. Some people believe that even such a small variability in the sun's output is enough to cause our climate change. However, some others have thrown doubts upon such a relationship. Some of the arguments related to this controversial issue can be found in an earlier report by National Research Council (1982). There is a new wave of study about possible solar-climate relationship in the past decade, and the controversial discussion continues. One group still believes in the importance of the sun's role in climate change, while another still discount it with different degrees.
Weng, Hengyi↗
TWIST AND CONNECTIVITY OF MAGNETIC FIELD LINES IN THE SOLAR ACTIVE REGION NOAA 10930
Twist and connectivity of magnetic field lines in the flare-productive active region NOAA 10930 are investigated in terms of the vector magnetograms observed by the Solar Optical Telescope on board the Hinode satellite and the nonlinear force-free field (NLFFF) extrapolation. First, we show that the footpoints of magnetic field lines reconstructed by the NLFFF correspond well to the conjugate pair of highly sheared flare ribbons on the Ca II images, which were observed by Hinode as an X3.4 class flare on 2006 December 13. This demonstrates that the NLFFF extrapolation may be used to analyze the magnetic field connectivity. Second, we find that the twist of magnetic field lines anchored on the flare ribbons increased as the ribbons moved away from the magnetic polarity inversion line in the early phase of the flare. This suggests that magnetic reconnection might commence from a region located below the most strongly twisted field. Third, we reveal that the magnetic flux twisted more than a half turn and gradually increased during the last one day prior to the onset of the flare, and that it quickly decreased for two hours after the flare. This is consistent with the store-and-release scenario of magnetic helicity. However, within this active region, only a small fraction of the flux was twisted by more than one full turn and the field lines that reconnected first were twisted less than one turn. These results imply that the kink mode instability could hardly occur, at least before the onset of flare. Based on our results, we discuss the trigger process of solar flares.
79 ASTRONOMY AND ASTROPHYSICS↗
Heliocentric Distance and Solar Activity Dependence of Sustained Quasi-radial Interplanetary Magnetic Field Occurrence
Planets close to their stars experience an interplanetary magnetic field (IMF) that is dominantly quasi-radial. Our solar system serves as a laboratory to study how the occurrence of quasi-radial IMF varies away from the star and under different stellar activities. Furthermore, on time and spatial scales relevant to magnetospheric physics, solar wind variability prevails in the form of structures generated both at the Sun and locally in the interplanetary space. The stationary Parker spiral model only approximates the large-scale structure of the IMF. Deviations from the Parker spiral often result in strongly radial magnetic fields that give rise to kinetic foreshock turbulence, which in turn can impact planetary magnetospheres. The relative significance of this type of interaction can be estimated statistically based on the occurrence rate of cases where the IMF is directed along the radial direction, leading to the entire day-side magnetosphere being downstream of the ion foreshock. We use observations covering radial distances from 0.1 to 10 au and more than 2 solar cycles to quantify the prevalence of radial IMFs throughout the heliosphere. Near Earth's orbit, it is found that the occurrence rates of quasi-radial and southward IMF orientations are similar, and that the Pearson correlation coefficient is ${{ \mathcal R }}_{{xy}}\sim -0.7$ calculated between quasi-radial IMF occurrence rate and solar activity. A negative correlation is demonstrated for radial distances extending to at least Mars but not to Saturn.
Brandon L Burkholder↗
Resonant Rossby waves and solar activity
Large scale transient waves are an essential part of atmospheric dynamics. Some of these waves (like 27 day waves) could have a solar nature. The contribution of the 27 day planetary waves to a total long period spectrum of the atmospheric processes during one solar cycle was investigated. Ivanovsky and Krivolutsky proposed that the 27 day wave has a resonant nature. The real atmospheric processes were investigated. The method of 2-D wave analysis used is described by Krivolutsky. It was concluded that the resonant nature of the 27 day wave is not unicum. There are long periods waves (50 day wave) in stratosphere which belong to the resonant waves, too. It is a very interesting fact for the solar activity-weather problem.
Krivolutsky, A. A.↗
Solar magnetic structure and the solar activity cycle review of observational data
Data on solar magnetism that may offer clues for understanding stellar magnetism in general were reviewed. Magnetic phenomena in the photosphere and low chromosphere, where the magnetic structure can be studied in detail, are discussed. Properties of the discrete magnetic elements and their extensions through the chromosphere and transition zone up to the corona are described. The structure and evolution of active regions and other structured aggregates of magnetic elements are considered.
Zwaan, C.↗
Comments on filament-disintegration and its relation to other aspects of solar activity.
Studies of sudden disintegrations of filaments in solar cycles 19 and 20 (to 1969) indicate that such events occur frequently. Approximately 30% of all large filaments in these cycles disintegrated in the course of their transit across the solar disk. 'Major' flares occurred with above average frequency on the last day on which 141 large disappearing filaments were observed. Relationships between a disintegrating filament on July 10-11, 1959, a prior major flare, a newly formed spot, and concomitant growth of H-alpha plage are presented. Observation of prior descending prominence material apparently directed towards the location of the flare of July 15, 1959 is reported. The development of the filament-associated flare of Feb. 13, 1967 is described.
Dodson, H. W.↗