Solar Physics最新文献

The solar corona is the prototypical example of a low-density environment heated to high temperatures by external sources. The plasma cools radiatively, and because it is optically thin to this radiation, it becomes possible to model the density, velocity, and temperature structure of the system by modifying the MHD equations to include an energy source term that approximates the local heating and cooling rates. The solutions can be highly inhomogeneous and even multiphase because the well-known linear instability associated with this source term, thermal instability, leads to a catastrophic heating and cooling of the plasma in the nonlinear regime. Here we show that there is a separate, much simpler linear instability accompanying this source term that can rival thermal instability in dynamical importance. The stability criterion is the isochoric one identified by Parker (1953), and we demonstrate that cooling functions derived from collisional ionization equilibrium are highly prone to violating this criterion. If catastrophic cooling instability can act locally in global simulations, then it is an alternative mechanism for forming condensations, and due to its nonequilibrium character, it may be relevant to explaining a host of phenomena associated with the production of cooler gas in hot, low density plasmas.

Hydrogen is the main chemical component of the solar plasma, and H-ionization determines basic properties of the first adiabatic exponent ({Gamma _{1}}). Its ionization significantly differs from the ionization of other chemicals. Due to the large number concentration, H-ionization causes a pronounced lowering of ({Gamma _{1}}), with a strongly asymmetric and extending across almost the entire solar convective zone. The excited states in the hydrogen atom are modeled using a partition function, which accounts for the internal degrees of freedom of the composite particle. A temperature-dependent partition function with an asymptotic cut-off tail is derived from the quantum mechanical solution for the hydrogen atom in the plasma. We present numerical simulations of hydrogen ionization, calculated using two partition function models: Planck-Larkin (PL) and Starostin-Roerich (SR). In the SR model, the hydrogen ionization shifts to higher temperatures than in the PL model. Different models for excited states of the hydrogen atom may change ({Gamma _{1}}) by as much as (10^{-2}). The ({Gamma _{1}}) profiles for pure hydrogen exhibit a “twisted rope” structure for the two models, significantly affecting the helium ionization and the position of the helium hump. This entanglement of H and He effect provides a valuable opportunity to investigate the role of excited states in the solar plasma.

The Metis coronagraph onboard Solar Orbiter and the LASCO-C2 coronagraph onboard SoHO both acquire white light polarized brightness (pB) images of the solar corona. When the Sun–Solar Orbiter distance is less than 0.85 AU, i.e., outside orbital segments around aphelia, the range of elongations covered by the fields-of-view of the two instruments overlap significantly, allowing a quantitative comparison of their images. We report on such a comparison during September 2022, with images taken during a superior conjunction of the two spacecraft with the Sun, as well as close to that event. In each comparison, the two instruments observed the corona from opposite viewpoints, within (approx 1^{circ }) in both Carrington longitude and latitude, with Metis at a distance of about half an astronomical unit from the Sun. We find that the Metis measurements are systematically larger than those of LASCO-C2 throughout the corona, with the Metis-to-C2 ratio of pB exhibiting a median value of (approx 1.6). The discrepancy is observed comparing essentially simultaneous observations, so it cannot be explained as an effect of coronal dynamics. Synthetic images of the solar corona computed from a stationary three-dimensional magneto-hydrodynamic model, replicating the geometry of the observations, are photometrically consistent. This rules out the small departure of the two instruments from observing from opposite viewpoints, or their different distance to the Sun, as the cause of their discrepant measurements. We conclude that the reported discrepancy has its root in the calibration methods of the two instruments, which should be further investigated.

Solar flares commonly have a “hot onset precursor event” (HOPE), detectable from soft X-ray observations. To detect this requires subtraction of pre-flare fluxes from the non-flaring Sun prior to the event, fitting an isothermal emission model to the flare excess fluxes by comparing the GOES passbands at 1 – 8 Å and 0.5 – 4 Å, and plotting the timewise evolution of the flare emission in a diagram of temperature vs. emission measure. The HOPE then appears as an initial “horizontal branch” in this diagram. It precedes the nonthermal impulsive phase of the flare and thus the flare peak in soft X-rays as well. We use this property to define a “flare anticipation index” (FAI), which can serve as an alert for observational programs aimed at solar flares based on near-real-time soft X-ray observations. This FAI gives lead times of a few minutes and produces very few false positive alerts, even for flare brightenings that are too weak to merit NOAA classification.

Solar energetic particles (SEPs) associated with shocks driven by fast coronal mass ejections (CMEs) or shocks developed by corotating interaction regions (CIRs) often extend to high energies, and are thus key elements of space weather. The PUNCH mission, set to be launched in 2025, is equipped with a photometric instrument that enables 3D tracking of solar wind structures in the interplanetary space through polarized light. Tracking techniques are used to estimate speeds and speed gradients of solar structures, including speed jumps at fast shocks. We report on a strong and robust relation between the shock speed jump magnitude at CME and CIR shocks and the peak fluxes of associated energetic particles from the analysis of 59 CME-driven shocks and 74 CIRs observed by Wind/STEP between 1997 – 2023. We demonstrate that this relation, along with PUNCH anticipated observations of solar structures, can be used to forecast shock-associated particle events close to the Sun, thus advancing and providing a crucial input to forecasting of SEP fluxes in the heliosphere.

We present a broad review of (1/f) noise observations in the heliosphere, and discuss and complement the theoretical background of generic (1/f) models as relevant to NASA’s Polarimeter to UNify the Corona and Heliosphere (PUNCH) mission. First observed in the voltage fluctuations of vacuum tubes, the scale-invariant (1/f) spectrum has since been identified across a wide array of natural and artificial systems, including heart rate fluctuations and loudness patterns in musical compositions. In the solar wind the interplanetary magnetic field trace spectrum exhibits (1/f) scaling within the frequency range from around (unit[2 times 10^{-6}]{Hz}) to around (unit[10^{-3}]{{Hz}}) at 1 au. One compelling mechanism for the generation of (1/f) noise is the superposition principle, where a composite (1/f) spectrum arises from the superposition of a collection of individual power-law spectra characterized by a scale-invariant distribution of correlation times. In the context of the solar wind, such a superposition could originate from scale-invariant reconnection processes in the corona. Further observations have detected (1/f) signatures in the photosphere and corona at frequency ranges compatible with those observed at 1 au, suggesting an even lower altitude origin of (1/f) spectrum in the solar dynamo itself. This hypothesis is bolstered by dynamo experiments and simulations that indicate inverse cascade activities, which can be linked to successive flux tube reconnections beneath the corona, and are known to generate (1/f) noise possibly through nonlocal interactions at the largest scales. Conversely, models positing in situ generation of (1/f) signals face causality issues in explaining the low-frequency portion of the (1/f) spectrum. Understanding (1/f) noise in the solar wind may inform central problems in heliospheric physics, such as the solar dynamo, coronal heating, the origin of the solar wind, and the nature of interplanetary turbulence.

We investigate the fundamental mode and overtones in the transverse oscillations of coronal loops associated with an active region using intensity observations taken by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The fundamental periods of the two selected coronal loops are found to be 17.0 minutes and 15.2 minutes, respectively. The first loop oscillated in the first and second overtones, with periods of around 6.9 minutes and 4.3 minutes, respectively. However, the second loop was detected only with the first overtone of approximately 7.7 minutes period. The period ratios of the fundamental to the first overtones of these loops are 1.24 and 0.99, respectively, while the fundamental-to-second-overtone period ratio of the first loop is 1.33. Thus, the deviation of period ratios from unity helps estimate the density scale height and the loop expansion factor. We obtained a density scale height of 11 Mm for the second loop and a loop expansion factor of 1.5 for the first coronal loop, considering that coronal loops have a greater effect on the loop expansion factor than on longitudinal density stratification associated with a sigmoidal active region. Using their lengths and periods of oscillations, we estimated a reasonable average magnetic field strength within a range of (20-30) G in the coronal loops.

The present study utilizes the planetary magnetic activity A(_{rm p}) index and the sunspot numbers as geomagnetic precursor pair for predicting the strength of ongoing Cycle 25. The monthly smoothed sunspot number (SSN) and disturbed days (A(_{rm p} geq 25)), during the post-peak segments of Sunspot Cycles 17 to 24 are processed through regression analysis and the obtained analytical results are validated by comparing with the observed SSN. Hind casting results show close agreement between predicted and observed maximum amplitudes within a confidence limit of up to 10 percent. The obtained results suggest the maximum sunspot number for Solar Cycle 25 to be (approx 112 pm 18). The probable peak time of Cycle 25 may appear within (48pm 3) months after the commencement of the cycle, i.e., between October 2023 and April 2024.

Ca ii K observations of the Sun have a great potential for probing the Sun’s magnetism and activity, as well as for reconstructing solar irradiance. The Kodaikanal Solar Observatory (KoSO) in India, houses one of the most prominent Ca ii K archives, spanning from 1904 to 2007, obtained under the same experimental conditions over a century, a feat very few other sites have achieved. However, the KoSO Ca ii K archive suffers from several inconsistencies (e.g., missing/incorrect timestamps of observations and orientation of some images) which have limited the use of the archive. This study is a step towards bringing the KoSO archive to its full potential. We did this by developing an automatic method to orient the images more accurately than in previous studies. Furthermore, we included more data than in earlier studies (considering images that could not previously be analyzed by other techniques, as well as 2845 newly digitized images), while also accounting for mistakes in the observational date/time. These images were accurately processed to identify plage regions along with their locations, enabling us to construct the butterfly diagram of plage areas from the entire KoSO Ca ii K archive covering 1904 – 2007. Our butterfly diagram shows significantly fewer data gaps compared to earlier versions due to the larger set of data used in this study. Moreover, our butterfly diagram is consistent with Spörer’s law for sunspots, validating our automatic image orientation method. Additionally, we found that the mean latitude of plage areas calculated over the entire period is (20.5%pm 2.0) higher than that of sunspots, irrespective of the phase or the strength of the solar cycle. We also studied the north–south asymmetry showing that the northern hemisphere dominated plage areas during solar cycles 19 and 20, while the southern hemisphere dominated during Solar Cycles 21 – 23.