- ID:
- ivo://CDS.VizieR/J/ApJ/775/121
- Title:
- X-ray events in Earth's magnetosphere
- Short Name:
- J/ApJ/775/121
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We found 217 X-ray brightening events in Earth's magnetosphere. These events occur in the high-energy band (0.5-4{AA}) of the Geostationary Operational Environmental Satellite (GOES) X-ray light curves, although GOES X-ray light curves are frequently used as indices of solar flare magnitudes. We found that (1) brightening events are absent in the low-energy band (1-8{AA}), unlike those associated with solar flares; and (2) the peak fluxes, durations, and onset times of these events depend on the magnetic local time (MLT). The events were detected in 2006, 2010, and 2011 at around 19-10MLT, that is, from night to morning. They typically lasted for 2-3hr. Their peak fluxes are less than 3x10^-8^W/m^2^ in the 0.5-4{AA} band and are maximized around 0-5MLT. From these MLT dependencies, we constructed an MLT time profile of X-ray brightening events. Because 0.5-4 and 1-8{AA} fluxes were observed and had the same order of magnitude when GOES 14 passed through Earth's shadow, we expected that X-ray brightening events in the 1-8{AA} band are obscured by high-background X-ray fluxes coming from the Sun. We also found coincidence between X-ray brightening events and aurora substorms. In the majority of our events, the minimum geomagnetic field values (AL index) are below -400nT. From these results and consideration of the GOES satellite orbit, we expect that these X-ray brightening events occur in the magnetosphere. We cannot, however, clarify the radiative process of the observed X-ray brightening events.
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- ID:
- ivo://CDS.VizieR/J/ApJ/855/75
- Title:
- 24 years monitoring of Sun and Sun-like stars
- Short Name:
- J/ApJ/855/75
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We compare patterns of variation for the Sun and 72 Sun-like stars by combining total and spectral solar irradiance measurements between 2003 and 2017 from the SORCE satellite, Stromgren b, y stellar photometry between 1993 and 2017 from Fairborn Observatory, and solar and stellar chromospheric CaII H+K emission observations between 1992 and 2016 from Lowell Observatory. The new data and their analysis strengthen the relationships found previously between chromospheric and brightness variability on the decadal timescale of the solar activity cycle. Both chromospheric H+K and photometric b, y variability among Sun-like stars are related to average chromospheric activity by power laws on this timescale. Young active stars become fainter as their H+K emission increases, and older, less active, more Sun-age stars tend to show a pattern of direct correlation between photometric and chromospheric emission variations. The directly correlated pattern between total solar irradiance and chromospheric Ca ii emission variations shown by the Sun appears to extend also to variations in the Stromgren b, y portion of the solar spectrum. Although the Sun does not differ strongly from its stellar age and spectral class mates in the activity and variability characteristics that we have now studied for over three decades, it may be somewhat unusual in two respects: (1) its comparatively smooth, regular activity cycle, and (2) its rather low photometric brightness variation relative to its chromospheric activity level and variation, perhaps indicating that facular emission and sunspot darkening are especially well-balanced on the Sun.
- ID:
- ivo://CDS.VizieR/J/A+A/649/A141
- Title:
- 1000-year sunspot series
- Short Name:
- J/A+A/649/A141
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- The 11-year solar cycle (Schwabe cycle) is the dominant pattern of solar magnetic activity reflecting the oscillatory dynamo mechanism in the Sun's convection zone. Solar cycles have been directly observed since 1700, while indirect proxy data suggest their existence over a much longer period of time but generally without resolving individual cycles and their continuity. Here we reconstruct individual solar cycles for the last millennium using recently obtained 14C data and state-of-the-art models. Starting with the ^14^C production rate determined from the so far most precise measurements of radiocarbon content in tree rings, solar activity is reconstructed in three physics-based steps: (1) Correction of the ^14^C production rate for the changing geomagnetic field; (2) Computation of the open solar magnetic flux; and (3) Conversion into sunspot numbers outside of grand minima. All known uncertainties, including both measurement and model uncertainties are straightforwardly accounted for by a Monte-Carlo method. Results: Cyclic solar activity is reconstructed for the period 971-1900 (85 individual cycles) along with its uncertainties. This more than doubles the number of solar cycles known from direct solar observations. We found that lengths and strengths of well-defined cycles outside grand minima are consistent with those obtained from the direct sunspot observations after 1750. The validity of the Waldmeier rule (cycles with fast rising phase tend to be stronger) is confirmed at a highly significant level. Solar activity is found to be in a deep grand minimum when the activity is mostly below the sunspot formation threshold, during about 250 years. Therefore, although considerable cyclic variability in ^14^C is seen even during grand minima, individual solar cycles can hardly be reliably resolved therein. Three potential solar particle events, ca. 994, 1052 and 1279 AD, are shown to occur around the maximum phases of solar cycles. A new about 1000-year long solar activity reconstruction, in the form of annual (pseudo) sunspot numbers with full assessment of all known uncertainties, is presented based on new high-precision D14C measurements and state-of-the-art models, more than doubling the number of individually resolved solar cycles. This forms a solid basis for new, more detailed studies of solar variability.
